Stream Simulation:

An Ecological Approach 

to Providing Passage for 

Aquatic Organisms at 

Road-Stream Crossings

U.S. Department 

of Agriculture

Forest Service

National Technology 

and Development 

Program

7700—Transportation Mgmt

0877 1801—SDTDC

August 2008

DEPARTMENT OF TRANSPORTATION

UNITED STATES OFAMERICA

FOREST SERVICE

D E P A RTMENT OF AGRICU L T U R E

ost? Road access location OK?

Is road needed?

Wh

a

ROAD NETWORK

p r i o r i t i e s f o r

r e p l a c e m

e

n

t

Crossing

location OK?

ITY

Information contained in this document has been developed for the guidance of employees of the

U.S. Department of Agriculture (USDA) Forest Service, its contractors, and cooperating Federal

and State agencies. The USDA Forest Service assumes no responsibility for the interpretation

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Stream Simulation:

An Ecological 

Approach To Providing 

Passage for Aquatic 

Organisms at RoadStream Crossings

by

Forest Service Stream-Simulation Working Group

in partnership with

U.S. Department of Transportation, Federal Highway 

Administration Coordinated Federal Lands Highway 

Technology Implementation Program

National Technology and Development Program

San Dimas, CA 91773

August 2008

v

Table of Contents

Preface	.......................................................................................................................................... xv

Acknowledgements.............................................................................................................xvii

Introduction ............................................................................................................................ xxi

Chapter 1—Ecological Considerations for Crossing Design

1.1	 Ecological concepts............................................................................................. 1—1

1.1.1	 Habitat ......................................................................................................................... 1—2

1.1.2	 Aquatic Communities .................................................................................................. 1—2

1.1.3	 Ecosystem Processes ................................................................................................... 1—3

1.1.4 Viability and Persistence of Populations....................................................................... 1—4

1.2	 Animal Movement ................................................................................................... 1—7

1.2.1	 Importance of Movement for Individual Animals ....................................................... 1—7

1.2.2	 Ecological Functions of Movement ............................................................................. 1—9

1.2.3	 Movement Capabilities of Aquatic and Riparian Organisms ......................................... 1—10

1.2.4	 Barriers to Movement Providing Some Positive Benefit ........................................... 1—14

1.3	 Potential AdverseImpacts of Road-Stream Crossing Structures ...... 1—15

1.3.1	 Effects on Channel Processes and Aquatic Habitats .................................................. 1—15

1.3.2	 Effects on Aquatic Organism Passage ....................................................................... 1—19

1.3.3	 Effects on Individual Animals ................................................................................... 1—21

1.3.4	 Reduced Access to Vital Habitats .............................................................................. 1—21

1.3.5	 Population Fragmentation and Isolation .................................................................... 1—23

1.3.6	 Disruption of Processes That Maintain Regional Populations ......................................... 1—23

1.3.7	 Time and Geography ................................................................................................. 1—23

1.4	 An Ecosystems Approach .................................................................................. 1—30

Chapter 2—Managing Roads for Connectivity

2.1	 Review the Road Network ................................................................................. 2—1

2.2	 Optimize Road and Crossing Locations ...................................................... 2—2

2.3	 InventoryBarriersand SetPriorities ForPassage Restoration............. 2—4

2.4	 Set Project Objectiv es and Design To Achi eve Them....................................2—4

	 2.4.1	 Road Approaches to the Stream Crossing ................................................................... 2—9

2.5	 Construct and Maintain the Crossing ..................................................... 2—10

2.6	 Monitor the Crossing ........................................................................................ 2—11
vi

Stream Simulation

Chapter 3—Introduction to Stream Simulation 

3.1	 What Stream Simulation Is and What It Isn’t .......................................... 3—1

3.2	 Key Elements and Limitations of Stream Simulation.................................3—3

3.3	 How Complex Does It Need To Be? .................................................................... 3—5

3.4	 Roadmap for Stream-Simulation Design ................................................... 3—6

3.4.1 	 Initial Watershed and Reach Review............................................................................ 3—6

3.4.2	 Site Assessment ............................................................................................................ 3—7

3.4.3	 Stream-simulation Design ............................................................................................ 3—7

3.4.4	 Final Design and Contract Preparation......................................................................... 3—7

3.4.5	 Construction.................................................................................................................. 3—8

3.4.6	 Maintenance and Monitoring........................................................................................ 3—8

Chapter 4—Initial Watershed and Reach Review

4.1	 Review the Road Context ................................................................................... 4—2

4.2	 Review Resource Values...................................................................................... 4—2

4.3	 Evaluate Watershed Risk Factors.................................................................. 4—3

4.3.1	 Geomorphic Hazards ................................................................................................... 4—4

4.3.2 	 History and Location of Land Cover Changes and Watershed Events ........................ 4—5

4.3.3	 Offsite Channel Stability .............................................................................................. 4—6

4.4	 Conduct the Initial Site Reconnaissance.................................................. 4—6

4.4.1	 Construction Issues..................................................................................................... 4—17

4.5	 Assess Site Suitability ....................................................................................... 4—17

4.6	 Defining Project Objectives and Initial Design Concept ................ 4—19

4.7	 Document Your Findings................................................................................... 4—21

4.8	 Initial Review Example....................................................................................... 4—21

Chapter 5—Site Assessment

5.1.	Collecting site data ............................................................................................. 5—1

5.1.1.	 Sketch Map................................................................................................................... 5—2

5.1.2.	 Topographic Survey...................................................................................................... 5—3

5.1.3.	 Longitudinal Profile ..................................................................................................... 5—6

	 5.1.3.1. What and where to survey..................................................................................... 5—7

	 5.1.3.2. Length of the longitudinal profile ........................................................................ 5—8

	 5.1.3.3. Grade controls .................................................................................................... 5—10

5.1.4.	 Cross Sections ............................................................................................................ 5—10

	 	 5.1.4.1. Location and number of cross sections .............................................................. 5—11

	 	 5.1.4.2. Typical cross-section measuring points ............................................................. 5—13

5.1.5.	Channel Types and Bed Mobility ............................................................................... 5—16

5.1.6.	Channel-bed and Bank-material Characteristics ....................................................... 5—18
vii

Table of Contents

	 	 5.1.6.1. Sampling strategies and methods........................................................................ 5—19

	 5.1.6.2. Key features....................................................................................................... 5—24

	 5.1.6.3. Wood .................................................................................................................. 5—25

	 5.1.6.4. Bank materials and morphology ....................................................................... 5—26

5.1.7.	 Preliminary Geotechnical Investigation .................................................................... 5—27

5.1.8.	 Road Travel-way and Construction Considerations .................................................. 5—28

5.2.	Analyzing and Interpreting Site Data ...................................................... 5—29

5.2.1.	 Interpreting Sediment Processes and Mobility .......................................................... 5—29

5.2.2.	 Analyzing the Longitudinal Profile ............................................................................ 5—30

	 5.2.2.1. Identify longitudinal profile shape ...................................................................... 5—32

	 5.2.2.2. Determining vertical adjustment potential.......................................................... 5—40

5.3.	Project Site Risk Assessment........................................................................... 5—46

5.3.1.	 High Flood-plain Conveyance.................................................................................... 5—46

5.3.2.	 Lateral Adjustment Potential and Alignment.............................................................. 5—51

5.3.3.	 Headcutting Potential ................................................................................................ 5—51

5.3.4.	 Debris ......................................................................................................................... 5—54

	 5.3.5. Unstable Channels ................................................................................................ 5—55

5.4.	Document Key Design Considerations and Recommendations ..... 5—55

5.5.	Reference Reach: The Pattern for Stream-Simulation Design..... 5—56

5.5.1.	 Reference Reach Data Required for Stream Simulation Design ............................... 5—59

Chapter 6—Stream-Simulation Design

6.1 	Project Alignment and Profile ....................................................................... 6—1

6.1.1.	 Alignment .................................................................................................................... 6—2

	 6.1.1.1. Risks of longer culverts ....................................................................................... 6—3

	 6.1.1.2. Channels skewed to the road ................................................................................ 6—4

	 6.1.1.3. Culvert on a bend ................................................................................................. 6—6

	 6.1.1.4. Transitions ............................................................................................................ 6—8

6.1.2.	 Designing the Project Longitudinal Profile ................................................................. 6—9

	 6.1.2.1. Uniform channels with local scour and fill around an undersized culvert.......... 6—11

	 	 6.1.2.2. Steep channels with large key features .............................................................. 6—12

	 6.1.2.3. Concave slope transitions ................................................................................... 6—13

	 6.1.2.4 Convex slope transitions...................................................................................... 6—16

	 6.1.2.5 Incised channels................................................................................................... 6—16

6.1.3.	 Project Alignment and Profile Design: Two Examples .............................................. 6—21
viii

Stream Simulation

6.2.	 Design of the Stream-Simulation Channel Bed.................................... 6—31

6.2.1.	 General Procedures for Simulated Streambed Design .............................................. 6—31

	 6.2.1.1. Bed material size and gradation in armored channels ....................................... 6—31

	 6.2.1.2. Channel cross section ......................................................................................... 6—34

	 6.2.1.3. Bank and channel margin features...................................................................... 6—35

	 6.2.1.4. Key features 	6—37

	 6.2.2.	 Bed Design Considerations for Specific Channel Types ........................................... 6—39

	 6.2.2.1. Dune-ripple channels.......................................................................................... 6—39

	 6.2.2.2. Pool-riffle channels............................................................................................. 6—42

	 6.2.2.3. Plane-bed channels.............................................................................................. 6—44

	 6.2.2.4. Step-pool channels ............................................................................................. 6—44

	 6.2.2.5. Cascade channels ............................................................................................... 6—45

	 6.2.2.6. Bedrock channels................................................................................................ 6—46

	 6.2.2.7. Channels with cohesive bed material ................................................................. 6—47

6.3.	 Crossing Structure Dimensions and Elevation.................................... 6—47

6.3.1.	Culvert Width	 6—47

	 6.3.2.	Culvert Elevation and Height .................................................................................... 6—49

6.3.3.	Culvert Shape and Material ....................................................................................... 6—52

6.4.	 Bed-Mobility and Stability Analysis ......................................................... 6—52

6.4.1.	 When is a Bed-mobility Analysis Necessary? ........................................................... 6—53

6.4.2.	 What Particle Sizes Are Analyzed? ........................................................................... 6—53

6.4.3.	 What Flows Are Analyzed? ....................................................................................... 6—53

6.4.4.	 Bed Mobility Analysis Equations............................................................................... 6—54

6.4.5.	 Stability Analysis for Immobile Key Feature Rocks ................................................. 6—54

6.5.	 Managing Risk Factors....................................................................................... 6—55

6.5.1.	 Potential Culvert Failure Risks—Stream-simulation Culverts................................... 6—55

	 6.5.1.1. Flood-plain constriction ..................................................................................... 6—58

	 6.5.1.2. Rapid lateral channel migration.......................................................................... 6—61

	 6.5.1.3. Steepened channel .............................................................................................. 6—61

	 6.5.1.4. Downstream channel instability ......................................................................... 6—62

	 6.5.1.5. Inlet control with submerged inlet ...................................................................... 6—62

	 6.5.1.6. Long culvert 	6—63

	 	 6.5.1.7. Initial lack of bed consolidation ......................................................................... 6—63

	 6.5.1.8. Excessive infiltration into the streambed ............................................................ 6—64

6.5.2.	 Potential Culvert Failure Risks—All Culverts .......................................................... 6—64

	 6.5.2.1. Flow exceeds culvert capacity ........................................................................... 6—64

	 6.5.2.2. Debris or sediment blockage .............................................................................. 6—65

	 6.5.2.3.Stream diversion potential ................................................................................... 6—65

6.6.	 Design Documentation ....................................................................................... 6—66
ix

Table of Contents

Chapter 7—Final Design and Contract Preparation

7.1.	Phase Overview......................................................................................................... 7—1

7.2.	Crossing Structure Selection......................................................................... 7—6

7.2.1.	 Site Geometry............................................................................................................... 7—8

	 7.2.1.1. Dipping the road profile to prevent stream diversion .......................................... 7—8

	 7.2.1.2. Low embankment options .................................................................................... 7—8

7.2.2.	 Construction Considerations ...................................................................................... 7—10

7.2.3.	 Cost Considerations.................................................................................................... 7—11

	 7.2.4.	 Tips for Choosing Structures...................................................................................... 7—14

7.3.	Structural Design ............................................................................................... 7—14

7.3.1.	 The Crossing Structure .............................................................................................. 7—14

7.3.2.	 Footing Design ........................................................................................................... 7—15

7.3.3.	 Structure Backfill ....................................................................................................... 7—16

7.3.4.	 Existing Site Materials................................................................................................ 7—17

7.4.	Handling Traffic During Construction................................................... 7—18

7.5.	Developing Specifications................................................................................ 7—19

7.5.1	 Submittals................................................................................................................... 7—19

7.5.2.	 Supplemental Specification 251: Streambed Construction ........................................ 7—20

	 7.5.2.1. Description ......................................................................................................... 7—20

	 7.5.2.2. Materials ............................................................................................................. 7—20

	 7.5.2.3. Construction methods ......................................................................................... 7—21

7.5.3.	 Developing SPS 705: Specifying Rock Sizes............................................................. 7—23

7.6.	Designing for Flood and Debris Failure Prevention ........................... 7—27

7.7.	Planning for Erosion and Pollution Control .................................... 7—27

7.7.1. General Erosion Control During Construction ........................................................... 7—27

7.7.2. Permanent Erosion Control Measures ........................................................................ 7—29

	 7.7.2.1. Diversion-prevention dips................................................................................... 7—29

7.8.	 Dewatering, Bypass, and Water Treatment During Construction . 7—30

7.8.1. Bypass Dams............................................................................................................... 7—33

7.8.2. Bypass Design ............................................................................................................ 7—34

7.8.3.	 Sump Design............................................................................................................... 7—35

	 7.8.4.	 Sediment Treatment Methods..................................................................................... 7—36

7.8.5.	 Backwatered Sites....................................................................................................... 7—37

7.8.6.	 Deep Fills.................................................................................................................... 7—37

7.8.7.	 Large Streams ............................................................................................................ 7—38

7.8.8.	 Small Streams ............................................................................................................ 7—38
x

Stream Simulation

	 7.8.9.	 Bedrock Channels ...................................................................................................... 7—38

7.8.10. Field Modifications .................................................................................................. 7—38

7.8.11. Pollution Control ...................................................................................................... 7—38

7.9.	Special Contract Requirements................................................................... 7—39

Chapter 8—Stream-Simulation Construction

8.1.	Brief Introduction to Stream-simulation Construction................ 8—2

8.1.1.	 Roles............................................................................................................................. 8—2

8.1.2.	 Communications........................................................................................................... 8—2

8.1.3.	 Contact Administration Meetings................................................................................. 8—3

	 8.1.3.1. Prebid tour............................................................................................................. 8—3

	 8.1.3.2. Prework meeting ................................................................................................... 8—4

	 8.1.3.3. Prework field meeting........................................................................................... 8—4

	 8.1.3.4. Final inspection/post-construction meeting .......................................................... 8—4

8.1.4.	 Construction and Inspection ........................................................................................ 8—4

8.1.5.	 Construction BMPs....................................................................................................... 8—5

	 8.1.6.	 Construction Survey and Tolerances............................................................................ 8—5

8.1.7.	 Permits and Permit Requirements................................................................................ 8—5

8.1.8.	 Contract Modifications/Design Changes...................................................................... 8—5

8.1.9.	 As-built Drawings and Final Construction Report....................................................... 8—6

8.2.	Stream-Simulation Construction Topics................................................... 8—6

8.2.1.	 Safety ........................................................................................................................... 8—7

8.2.2.	Construction Survey .................................................................................................... 8—7

8.2.3.	 Special Contract Requirements (H Clauses) ............................................................... 8—9

8.2.4.	 Signs and Traffic Control Plans.................................................................................... 8—9

8.2.5.	 Erosion, Sediment, and Pollution Control.................................................................. 8—10

	 8.2.5.1. Reviewing erosion- and sediment-control plans................................................. 8—10

	 8.2.5.2. Pollution control and prevention plans .............................................................. 8—13

8.2.6.	 Dewatering and Sediment Removal .......................................................................... 8—13

	 8.2.6.1. Protection of aquatic organisms when dewatering ............................................. 8—14

	 8.2.6.2. Dewatering plan review...................................................................................... 8—15

	 8.2.6.3. Dewatering inspection recommendations........................................................... 8—16

	 8.2.6.4. Tips for collecting and treating sediment-laden water........................................ 8—19

8.2.7. Excavation................................................................................................................... 8—22

	 8.2.7.1. OSHA and excavation safety .............................................................................. 8—22

	 8.2.7.2. Excavation—what can go wrong? ...................................................................... 8—22
xi

	 8.2.8. Structural Excavation ................................................................................................. 8—24

	 8.2.8.1. Bedrock and blasting........................................................................................... 8—24

	 8.2.8.2. Settlement beneath foundations and pipes.......................................................... 8—25

	 8.2.9. Constructed Concrete Features.............................................................................. 8—26

	 8.2.9.1. Concrete form inspection.................................................................................... 8—26

	 8.2.9.2. Pouring concrete ................................................................................................. 8—26

	 8.2.9.3. Inspection recommendations for concrete placement......................................... 8—27

8.2.10 Culvert Installation ................................................................................................... 8—28

	 8.2.10.1. Closed-bottom culvert bedding......................................................................... 8—28

	 8.2.10.2. Open-bottom culvert attachment....................................................................... 8—28

	 8.2.10.3. Pipe assembly.................................................................................................... 8—29

	 8.2.10.4. Multiplate pipes ................................................................................................ 8—29

	 8.2.10.5. Backfill and embankments................................................................................ 8—30

8.2.11. Stream-simulation Bed Material Placement............................................................. 8—31

	 8.2.11.1. Size of streambed materials .............................................................................. 8—32

	 8.2.11.2. Constructing the simulated streambed .............................................................. 8—33

	 	 8.2.11.2.1. Recommendations for placing material in open-bottom arches............... 8—34

	 	 8.2.11.2.2. Recommendations for placing material in embedded pipes..................... 8—34

	 	 8.2.11.2.3. Placing channel rocks............................................................................... 8—36

8.2.12. Permanent Erosion Control Measures...................................................................... 8—36

	 8.2.12.1. Revegetation ..................................................................................................... 8—36

	 8.2.12.2. Riprap inspection ............................................................................................. 8—39

8.2.13. General Road Construction ...................................................................................... 8—39

	 8.2.13.1. Roadway drainage structures............................................................................ 8—39

8.2.14. Demobilization/Cleanup........................................................................................... 8—40

8.3.	 Post Construction................................................................................................ 8—40

	 8.3.1. Post-construction Project Review.......................................................................... 8—40

	 8.3.2. Post-construction Monitoring ................................................................................ 8—41

	 	 8.3.2.1. Physical monitoring of structure performance .............................................. 8—42

	 	 8.3.2.2. Physical monitoring of streambed performance............................................ 8—42

Glossary

References

Table of Contents
xii

Stream Simulation

Appendix A—Geomorphic Principles Applied in Stream Simulation

A.1.	 Why Consider Fluvi alProcesses In Crossing Design? .............................. A—1

A.2. The Watershed Context ...................................................................................... A—2

A.3. Channel Characteristics ................................................................................. A—5

A.3.1.	Streambed Material ..................................................................................................... A—5

A.3.2.	Channel Slope ............................................................................................................. A—8

A.3.3.	Channel Pattern............................................................................................................ A—9

A.3.4.	Channel Dimensions, Confinement, and Entrenchment ........................................... A—11

A.3.5.	Channel Bedforms .................................................................................................... A—13

A.3.6.	Flow Resistance or Channel Roughness ................................................................... A—14

A.4. Channel Stability and Equilibrium .......................................................... A—15

A.4.1.	Equilibrium and Bankfull Flow................................................................................. A—16

A.5. Fluvial Processes ............................................................................................... A—17

A.5.1.	Sediment Dynamics .................................................................................................. A—17

A.5.2.	Vertical Channel Adjustment..................................................................................... A—18

A.5.3.	Lateral Channel Adjustment ..................................................................................... A—19

A.5.4.	Flood-plain Inundation and Dynamics ..................................................................... A—21

A.6. Channel Classification Systems ................................................................. A—22

A.6.1.	Montgomery and Buffington Channel Classification ............................................... A—22

A.6.2.	Rosgen Channel Classification ................................................................................. A—27

A.7. Unstable Channels ............................................................................................ A—27

A.7.1.	Inherently Unstable Landforms and Channel Types ................................................ A—27

A.7.2.	Channels Responding to Disturbances ..................................................................... A—30

Appendix B—Other Culvert Design Methods for Fish Passage

B.1. Hydraulic Design Method ..................................................................................B—1

B.2. Hybrid Design and Roughened-Channel Design .....................................B—2

B.3. Velocity Simulation .............................................................................................B—3

B.4. “No-slope” Design.....................................................................................................B—3

Appendix C—Site Assessment Check List
xiii

Appendix D—Estimating Design Stream Flows at Road-Stream 

Crossings

D.1. INTRODUCTION............................................................................................................ D—1

D.2. Design Flow Estimates ......................................................................................... D—2

D.2.1.	Design Flow Estimates at Gauged Sites...................................................................... D—2

	 	 D.2.1.1. Weighted flood frequency ................................................................................... D—5

D.2.2.	Design-flow Estimates Near Gauged Sites.................................................................. D—6

	 	 D.2.2.1 Ungauged site on a gauged stream....................................................................... D—6

	 	 D.2.2.2. Ungauged site near a gauged stream................................................................... D—7

D.2.3.	Flow Estimates on Ungauged Streams........................................................................ D—8

D.3. Verifying Flow Estimates at Ungauged Streams.................................. D—9

Appendix E—Methods for Streambed Mobility/Stability Analysis

E.1	 Flow Hydraulics: Shear Stress and Unit Discharge ..............................E—2

E.1.1	 Models for Calculating Flow Hydraulics ....................................................................E—3

E.1.2	 What Flows to Analyze ................................................................................................E—5

E.2	 Particle Entrainment in Natural Channels ............................................E—5

E.2.1. Modified Critical Shear Stress Approach......................................................................E—8

E.2.2. Critical Unit Discharge Approach.................................................................................E—9

E.2.3.	 Uncertainty in Predicting Particle Entrainment ............................................................E—11

E.3	 Sediment Mobility/Stability Analysis Example: 

Schafer Creek Tributary ..............................................E—14

E.3.1.	Channel and Road-stream Crossing Background Information...................................E—14

E.3.2.	Modified Critical Shear Stress Approach ...................................................................E—16

E.3.3	 Critical Unit Discharge Approach ..............................................................................E—20

E.3.4.	Summary.....................................................................................................................E—24

E.4	 Sizing Immobile Key Pieces................................................................................E—24

Appendix F—Channel Grade Control Structures

Boulder Weirs............................................................................................................................F—1

Roughened channels............................................................................................................................. F—3

Rigid Weirs................................................................................................................................F—4

Table of Contents
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Stream Simulation

Appendix G—Additional Tools and Tips

G.1. Contract Preparation Checklist .................................................................. G—1

G.2. Sample Project Schedule ................................................................................... G—2

G.3. Estimated Project Costs: Comparing Different Structure 	 	

Types and Sizes ..................................................................... G—5

G.3.1.	12-foot and 18-foot Open-bottom Arch Multiplate Structures—20-foot Fill.............. G—5

G.3.2.	8-foot and 12-foot Embedded CMPs—20-foot Fill .................................................... G—7

G.3.3.	8-foot and 12-foot Embedded CMPs—12-foot Fill .................................................... G—9

G.4. Tips from Engineers and Biologists Experienced in 	 	

Stream-Simulation Construction .......................... G—11

G.4.1.	Diversion, Dewatering, and Water Treatment System Components ........................ G—11

	 	 G.4.1.1. Bypass and backwater dams ............................................................................. G—11

	 	 G.4.1.2. Pump types and characteristics......................................................................... G—16

	 	 G.4.1.3. Sediment removal methods............................................................................... G—17

G.4.2.	Foundation and Footing Design ............................................................................... G—19

	 	 G.4.2.1. Overturning forces ............................................................................................ G—20

	 	 G.4.2.2.Scour .................................................................................................................. G—20

	 	 G.4.2.3. Bedrock ............................................................................................................. G—20

	 	 G.4.2.4. Soil strength ...................................................................................................... G—21

G.4.3.	Revegetation and Erosion Control............................................................................. G—22

	 	 G.4.3.1. Salvaging and storing topsoil ........................................................................... G—22

	 	 G.4.3.2. Collecting seeds and cuttings for native species revegetation .......................... G—24

	 	 G.4.3.3. Water quality monitoring .................................................................................. G—25

	 	 G.4.3.4. Training and quality control.............................................................................. G—25

	 	 G.4.3.5. Temporary soil stabilization until vegetation is fully established..................... G—25

	 	 	 G.4.3.6. Miscellaneous ‘things that can go wrong’ during construction ................... G—26

	 	 	 G.4.3.7. Seasonal work shutdown and resumption of work...................................... G—27

	 	 	 G.4.3.8. Common problems with revegetation.......................................................... G—27

	 	 	 G.4.3.9. Resources for revegetation and erosion control........................................... G—29

	 	 G.4.4. Aquatic Organism Capture and Transport ........................................................... G—30

	 	 	 G.4.4.1. South Fork Desolation Creek....................................................................... G—31

	 	 	 G.4.4.2. Karnowski Creek habitat restoration ........................................................... G—32
xv

Appendix H—Sample Contract Provisions

SUPPLEMENTAL SPECIFICATION 157—SOIL EROSION CONTROL............................ H—3

SU PPLEMENTAL S PEC IFICAT ION 251—STREAM BED CONSTRUCT ION ............................ H—7

SUPPLEMENTAL SPECIFICATION 705—STREAMBED-SIMULATION MATERIALS . H—9

SPECIAL CONTRACT REQUIREMENTS (H-CLAUSES) ................................................ H—10

Table of Contents

xvii

Preface This is a guide to stream simulation—a method for designing and building 

road-stream crossings intended to permit free and unrestricted movements 

of any aquatic species. The guide aims to help national forests achieve 

their goal of maintaining the physical and biological integrity of the 

stream systems they manage, including existing populations of fish and

other wildlife species (see National Forest Management Act, 16 U.S.C. 

1600-1616). Habitat fragmentation is an important factor contributing to 

population declines of many fish, and crossing structures that are barriers are

a large part of the problem. Stream simulation provides continuity through 

crossing structures, allowing all aquatic species present to move freely 

through them to access habitats, avoid adverse conditions, and seek food and 

mates. Stream simulation applies to crossing structures on any transportation 

network, including roads, trails, and railroads. For brevity, the guide refers to 

all of these types of transportation infrastructure as ‘roads.’ 

 Whether culverts or bridges, stream-simulation structures have a continuous 

streambed that mimics the slope, structure, and dimensions of the natural 

streambed. The premise of stream simulation is that since the simulation 

has very similar physical characteristics to the natural channel, aquatic 

species should experience no greater difficulty moving through it. Water

depths and velocities are as diverse as those in a natural channel, providing 

passageways for all swimming or crawling aquatic species. 

 Work on this guide began in response to a set of project proposals from 

engineers and biologists concerned with designing culverts for anadromous 

fish passage in the Alaska, Pacific Northwest, and Northern Forest Service,

U.S. Department of Agriculture regions. During the initial project scoping 

process, it became apparent that many other fish and nonfish species across

the country are also harmed by passage barriers. At that point, the project’s 

focus expanded from anadromous fish to all aquatic organisms. Stream

simulation is the technology most likely to achieve the goal of aquatic 

organism passage.

 The idea of creating crossings that mimic the stream is not new (Katapodis 

2005), but the technique was developed in its now best-known form in 

the Washington Department of Fish and Wildlife’s 1999 “Fish Passage 

Guidelines” (Bates 2003). The present guide builds on that foundation, 

expanding our understanding of stream simulation and adding the results 

of several more years of design and construction experience, much of it by 

Forest Service engineers, biologists, and geomorphologists. The intent is to 

meet the needs of the Forest Service for a flexible design process for aquatic

organism passage at road-stream crossings. The guide is for project teams 

that include members from several disciplines. It aims to help each team 

Preface
xviii

Stream Simulation

member better understand the challenges and considerations pertinent to the 

other disciplines, as well as their own. Although organized to suit the project 

design, construction, and management processes of the Forest Service, the 

guidance should also be helpful for other groups.

 Stream-simulation technology is relatively new and changing rapidly. The 

bulk of the experience reflected in this guide’s content comes from Alaska,

and the Pacific Northwest coastal and inland States. The guide’s authors,

editors, and reviewers encourage practitioners in other landscapes to adapt 

the methods described here to local stream processes, and to contribute 

their findings to the expanding collection of experience and guidelines. We

anticipate great strides in our ability to effectively and efficiently simulate

streams through crossings, as forests apply, monitor, and modify the 

technology in vastly different areas. 
xix

Acknowledgements Many people have contributed to this guide in a variety of ways. What 

started as a relatively restricted task—to collect design recommendations 

from a small group of people experienced in stream simulation—grew 

into a more extensive guide that now includes topics on final engineering

design and construction. Because stream simulation is a relatively new 

way of handling the restoration of stream ecosystems during road-stream 

crossing projects, unknowns and debatable issues remain. As this guide 

evolved, these gray areas required clarification through lively discussions,

debates, critiques, and the patient support of too many people to name 

here. We, the editors and principal authors, thank all who participated in 

this process.

STREAM-SIMULATION

WORKING GROUP

Contributing Editor Kim Clarkin, hydrologist, Forest Service, San Dimas Technology and 

Development Center (SDTDC) project leader, San Dimas, CA.

Principal Authors Robert A. Gubernick, engineering geologist, Forest Service, Tongass 

National Forest, Alaska. 

 Daniel A. Cenderelli, geomorphologist, Forest Service, Stream Systems 

Technology Center, Fort Collins, CO. 

Kozmo Ken Bates, consulting fish passage engineer, Olympia, WA.

 David Kim Johansen, geotechnical engineer, Forest Service, Willamette 

and Siuslaw National Forests, Oregon. 

 Scott D. Jackson, herpetologist, and Director of the Natural Resources 

and Environmental Conservation Extension Program, University of 

Massachusetts, Amherst, MA.

Technical Advisors The technical advisors have considerable experience and expertise in fish 

passage design, and contributed significantly to the development of many 

of the techniques described in this guide:

 Mark R. Weinhold, hydrologist and civil engineer, Forest Service, White 

River National Forest, Colorado.

 Traci L. Sylte, hydrologist and civil engineer, Forest Service, Lolo 

National Forest, Montana. 

Acknowledgements
xx

Stream Simulation

Other Contributors The following people collaborated closely with the working group and 

made important contributions to the technical content and presentation of 

the guide: 

 William V. Crane, civil engineer, Forest Service (retired), SDTDC, and 

contributing editor for the final engineering design and construction

sections. 

 Mark A. Fedora, hydrologist, Forest Service, Eastern Region, Ironwood, 

MI. 

Michael J. Furniss, hydrologist, Forest Service, Pacific Northwest and

Pacific Southwest Research Stations, Arcata, CA.

Kathleen Moynan, fishery biologist, U.S. Department of the Interior, Fish

and Wildlife Service, Portland. OR

Brian W. Riggers, fishery biologist, Forest Service, Lolo National Forest,

Montana.

John Sanchez, fishery biologist, Forest Service, Siuslaw National Forest,

Oregon.

Other Assistance Thanks also to the following people who provided photos, graphics, 

examples, and expertise:

 All unattributed photos in this guide were contributed by Forest Service 

employees.

 Rowan Baker, U.S. Department of the Interior, Fish and Wildlife Service, 

Portland, OR.

 Jack N. Conyngham, U.S. Army Corps of Engineers, Research and 

Development Center, Vicksburg, MS.

 Kurt Fausch, Colorado State University, Fort Collins, CO. 

 Craig J. Fischenich, U.S. Army Corps of Engineers, Research and 

Development Center, Vicksburg, MS.

 Thomas Gillins, Forest Service, Intermountain Region, Ogden, UT.

Ben Hipple, Forest Service, Payette National Forest, McCall, ID.

 Jack Holcomb, Forest Service, Southern Region, Atlanta, GA.

 Mark Hudy, Forest Service, Eastern and Southern Regions, Harrisonburg, 

VA.

 Shawn Jones, Earthwork Consulting LLC, Gresham, OR.

 Gordon Keller, Forest Service, Plumas National Forest, Quincy, CA.

 Michael S. Kellett, Forest Service, Boise National Forest, Boise, ID.

 Michael Love, Love and Associates, Eureka, CA.
xxi

 Robert Newbury, Newbury Hydraulics, Inc., Okanagan Centre, BC, 

Canada. 

 Dan Rhodes, U.S. Department of the Interior, National Park Service, 

Yellowstone National Park.

 Alan Richmond, University of Massachusetts, Amherst, MA.

Brett Roper, Forest Service, Intermountain Research Station, Logan, UT.

Bill Shelmerdine, Forest Service, Olympic National Forest, Olympia, WA.

 Ross Taylor, Ross Taylor and Associates, McKinleyville, CA.

 Roger Thoma, Ohio State University, Columbus, OH.

Technical Editors Natalie Reid, consultant and editor, Albuquerque, NM. 

 Wendy Masri, consultant (systems engineering and technical 

documentation), and technical editor during the early stages of document 

development, Sierra Madre, CA. 

Graphic Artists Susan Christensen, Forest Service, Tongass National Forest, Alaska.

 Deborah Mucci, Forest Service, Missoula Technology and Development 

Center, Montana.

 George Toyama, Forest Service, San Dimas Technology and Development 

Center, California.

 L’Tanga Watson, Forest Service, Angeles National Forest, California. 

Paul Karr, Flathead, Idaho. 

Reviewers Bob Barnard, Washington Department of Fish and Wildlife, Olympia, WA.

Bart Bergendahl, U.S. Department of Transportation, Federal Highway 

Administration, Central Federal Lands Highways, Denver, CO.

 Mark Browning, U.S. Department of Transportation, Federal Highway 

Administration, Western Federal Lands Highways, Vancouver, WA.

 Kristin Bunte, Colorado State University, Fort Collins, CO.

Michael J. Furniss, Forest Service, Pacific Northwest and Pacific

Southwest Research Stations, Arcata, CA.

 Tom Gillins, Forest Service, Intermountain Region, Ogden, UT.

 John Kattel, Forest Service, Northern Region, Missoula, MT.

 Russ Lafayette, Forest Service, Eastern Region, Milwaukee, WI.

Tom Lisle, Forest Service, Pacific Southwest Research Station, Arcata,

CA.

Acknowledgements
xxii

Stream Simulation

 Michael Love, Love and Associates, Eureka, CA.

 Mark Miles, Alaska Department of Transportation, Anchorage, AK.

Bruce Reiman, Forest Service (retired), Intermountain Research Station, 

Boise, ID.

 Julianne Thompson, Forest Service, Tongass National Forest, Alaska.

 Marcin Whitman, California Department of Fish and Game, Sacramento, 

CA.
xxiii

Purpose of the Guide 

and Its Intended 

Audience The intent of this guide is to:

l Explain to land and road managers and a general audience:

 n Why providing stream continuity at road-stream crossings is 

critical for maintaining aquatic animal populations and habitats. 

 n How stream simulation works to provide stream continuity at 

road-stream crossings. 

l Guide practitioners working in multidisciplinary design teams 

through the assessment, design, and construction phases of a streamsimulation project.

Stream simulation is an approach to designing crossing structures 

(usually culverts), that creates a structure that is as similar as 

possible to the natural channel. When channel dimensions, slope, 

and streambed structure are similar, water velocities and depths 

also will be similar. Thus, the simulated channel should present no 

more of an obstacle to aquatic animals than the natural channel. 

The first part of the guide (chapters 1 and 2) builds the case for stream

continuity at crossings and gives a general overview of how to achieve 

continuity using stream-simulation methods. This part addresses a general 

audience, including managers responsible for roaded ecosystems. The 

remainder of the guide is for project teams responsible for either building 

a new crossing or replacing a crossing structure where full aquatic 

organism passage is a goal. This guide does not deal with the question of 

when full aquatic organism passage is necessary at a site. That decision 

depends on local policy and ecological needs. 

Figure 1—Project team at a crossing site in New Hampshire.

Introduction

Words shown in 

bold

throughout this 

document are 

defined in the glossary
xxiv

Stream Simulation

 The greatest challenge of stream simulation is that it requires expertise 

in different technical fields. This guide does not teach all the technical

concepts and methods needed for designing and constructing a streamsimulation crossing. Rather, it assumes that people skilled in engineering, 

contract administration, hydrology, geomorphology, and biology work 

together as a team throughout the process. The guide aims to help each 

member understand the challenges and considerations pertinent to the 

other disciplines, as well as to their own. Although different specialists 

may take the lead at different times, the whole team should be available 

for consultation throughout the project.

Background Streams and roads are long, linear networks whose functions include 

transporting material and organisms across the landscape. Being narrow 

and linear, both streams and roads are highly susceptible to blockages. 

The two systems frequently intersect, and at the junctions each can 

pose an obstacle to the other’s continuity. In the past, most road-stream 

crossing design has aimed at protecting the road and minimizing traffic

interruptions. Less attention has been given to protecting stream functions, 

such as sediment transport, fish and wildlife passage, or the movement of

woody debris. Not surprisingly, many culverts disrupt the movement of 

aquatic organisms and impair aquatic habitats. 

 The numbers of road-stream junctions are huge. On National Forest 

System lands in Washington and Oregon, there are over 6,250 road-stream 

crossings on fish-bearing streams—approximately one crossing per every

3.6 miles of stream. According to Dave Heller, fishery biologist for the

Pacific Northwest Region, in March 2004 about 90 percent of nonbridge

(mostly culvert) crossings were considered to be at least partial barriers to 

anadromous fish passage. These barriers blocked about 15 percent of fishbearing stream miles on national forest lands in the region (figure 2).

Until recently, where fish were a serious concern, designing culverts

for passage of a target species (the “design fish”) during its migration

season was considered best practice. This practice, however, often 

does not achieve the best ecological results. For example, considerable 

resources have gone into facilitating passage of adult salmon and steelhead 

migrating to their spawning grounds, only for fishery biologists to find that

accommodations made for adults did not even begin to cover the needs 

of juveniles of the same species. Sustaining a population demands that 

all life stages must succeed, and fry, juveniles, and adults have different 

movement needs and capabilities. 
xxv

Introduction

Figure 2—Forest Service Pacific Northwest Region map of road-stream crossing barrier status, 2005. Red dots indicate 

road-stream crossings that, at least partially, blocked passage of juvenile and/or adult anadromous salmonids.
xxvi

Stream Simulation

 As chapter 1 will show, focusing on a single desirable species is not 

enough: The entire aquatic ecosystem is linked, and all species depend 

on each other for food and other essential interactions. As survival of a 

“target species” depends on a healthy and diverse ecosystem, it is essential 

to focus on habitat quality and continuity for aquatic communities rather 

than for individual species. Without an ecosystem-based approach to roadstream crossings, we will be at risk of facilitating passage for particular 

fish species while at the same time undermining the ecological integrity of

the ecosystems on which these fish depend.

Figure 3—Culvert on the Boise National Forest prevents migration of kokanee 

salmon.

 Stream simulation supports the ecosystem-based approach to road-stream 

crossing design and aims to provide full aquatic organism passage; that 

is, all aquatic and semiaquatic species should be able to travel through the 

crossing structure with no greater impediment than the natural channel 

would offer. The crossing, therefore, acts as neither a barrier nor a filter

that passes only certain individuals, species, or age groups (life stages). 

Moreover, because a stream-simulation crossing accommodates the full 

channel width, it does not impede the downstream transport of floodwater,

sediment, or woody debris as much as narrower, traditional culverts do. 

Stream simulation thus provides for not only the long-term sustainability 

of the entire aquatic community, but also a more durable roadway that is 

less susceptible to damage by high flows and debris blockage.
xxvii

Introduction

Structure and Scope 

of the Guide	 The first two chapters of this guide summarize the ecological

consequences of habitat fragmentation caused by road-stream crossing 

barriers, and outline the steps necessary for restoring connectivity. These 

chapters answer the following two questions: Why is stream continuity 

at road-stream crossings important? and, How do we create it? Managers 

faced with making fiscally significant decisions about providing habitat

connectivity at crossings should find these chapters especially useful.

 Chapter 1, Ecological Cosiderations for Crossing Design, discusses when 

and why aquatic species need to move, what they require to be able to 

move, and what the consequences of barriers to individuals, populations, 

and communities are. Biologists should note that this guide does not 

describe how to determine where, when, or for which species passage 

is required. This guide also does not cover setting priorities for barrier 

removal.

 Chapter 2, Managing Roads for Continuity, is a very brief overview of 

the planning, design, construction, and monitoring practices that can 

solve road-stream crossing barrier problems, including best management 

practices (BMPs). This overview is intended for land managers who 

participate in setting project objectives and making policy decisions that 

affect crossing projects. The chapter places stream simulation in context 

within a range of crossing design approaches. 

 The next six chapters describe the steps or phases of a stream-simulation 

design project. The process is applicable to new and replacement 

crossings, and to crossing removals. The focus is on forest roads; however, 

the concepts and general approach are applicable to crossings on other 

parts of the transportation system such as trails, highways, and railroads. 

 Chapters 3 through 8 are addressed to members of multidisciplinary 

project teams responsible for the assessment, design, and construction 

of road-stream crossings. Readers who are unfamiliar with stream 

morphology and processes can refer to appendix A for a brief introduction 

to geomorphic terms and concepts used throughout the assessment and 

design process. 
xxviii

Stream Simulation

 Chapter 3, Introduction to Stream Simulation, provides an overview of 

the process of stream-simulation design and construction. It defines and

describes stream simulation and discusses limitations on its application. 

Since this guide is intended as a reference, the descriptions of each 

phase of a stream-simulation project are comprehensive, including 

many complicating circumstances that may or may not pertain to 

a specific project. On any actual project, only factors and issues 

relevant to that project need to be considered. The level of detail 

in the assessment and design process should depend on the size, 

complexity, and risk of the project. Once teams gain experience, they 

can tailor the design process to the needs of each site.

 Chapter 4, Initial Watershed and Reach Review, describes the large-scale 

assessments of watershed and aquatic resources and transportation needs 

that provide context for the project. At this stage, the project team takes a 

look at the “big picture.” The team also conducts a rapid reconnaissance of 

the project reach to verify that the road and crossing are well located, to 

identify risks, and to formulate preliminary project objectives. 

 Chapter 5, Site Assessment, describes the process of collecting and 

analyzing the geomorphic and other site data that are the basis for streamsimulation design. 

 Chapter 6, Stream-Simulation Design, shows practitioners how to use 

the assessment information in designing the simulated channel through 

the road-stream crossing. Note: To cover many road and stream settings 

with the design procedure, the authors have synthesized many years of 

experience in stream-simulation design and consulted experts throughout 

the country. Nonetheless, the guide primarily reflects experience in the

Inland and Pacific Northwest. The technology is still in development.

While culverts up to 15-percent slope have been constructed with these 

methods, such methods have not been used extensively on very lowgradient streams in fine sediments, cohesive soils, or densely vegetated

streambeds. 
xxix

Introduction

Chapters 7 and 8 describe the final engineering design and construction

phases. They are primarily directed to the project engineer and contract 

administrator, but all team members should find the material useful for

understanding the elements and process of final design and construction.

Consultation with the entire project team is essential in these final phases,

especially when contract changes become necessary. 

 Chapter 7, Final Design and Contract Preparation, discusses structural 

design and contract preparation. It includes making the final decision on

structure type, as well as on materials and contract requirements that are 

unique or that may need more emphasis in stream simulation projects. 

 Chapter 8, Stream-Simulation Construction, discusses the construction 

planning and implementation actions that are especially important to 

both the success of stream-simulation crossing construction projects and 

the protection of aquatic species and habitats. It offers field construction

experience on stream-simulation projects and aims to help new 

practitioners avoid common mistakes. 

 This guide does not deal in detail with the last phase of all road-stream 

crossing projects—maintenance and monitoring (a brief discussion is in 

section 8.3.2). Monitoring is especially important on stream-simulation 

projects, since it is the only way to collect the information necessary for 

continually improving crossing design and construction practices. This 

guide is not the last word in this rapidly evolving field, and the authors

anticipate with enthusiasm the growth of knowledge and experience that 

application of these principles in different environments will bring. 

 A glossary and a series of appendixes appear at the end of this guide. The 

glossary will be particularly useful for understanding terms used by a 

discipline in which the reader may not be well versed. As the material in 

certain chapters is directed towards team members with specific expertise,

definitions of terms common within the discipline under discussion may

not appear in the text. The glossary is therefore quite comprehensive, and 

readers should make good use of it.

1Ecological 

Considerations for 

Crossing Design

1.1 Ecological Concepts

1.2 Animal Movement

1.3 Potential Adverse Impacts of Road-stream 

Crosssing Structures

1.4 An Ecosystems Approach
Stream Simulation
Chapter 1—Ecological Considerations for Crossing Design

1.1 Ecological Concepts

Rivers and streams are more than mere conduits for water and fish.

They are long, linear ecosystems made up of the physical environment, 

communities of organisms, and a variety of ecological processes that 

shape and maintain these ecosystems over time (figure 1.1). The long-term

conservation of important aquatic resources (such as fish) requires the

maintenance of healthy and ecologically viable ecosystems. As this chapter 

will show, road crossings have the potential to undermine the ecological 

integrity of roaded river and stream systems in a number of ways. To 

ensure the productivity and viability of river and stream ecosystems, we 

must protect and restore the quality of the physical environment (habitat), 

maintain intact communities of aquatic organisms, and take care not to 

disrupt critical ecological processes.

Figure 1.1—Long-term conservation of aquatic resources requires the maintenance of healthy and ecologically viable ecosystems.

1.1.1 Habitat 

 To survive, an organism must have access to all habitats it needs for 

basic life functions. For many species, these needs for access occur 

throughout an organism’s life cycle. Habitat is a combination of physical 
1—2

Stream Simulation

and biological characteristics of an area or areas, which are essential 

for meeting the food and other metabolic needs, shelter, breeding, and 

overwintering requirements of a particular species. For some species, 

habitat can be as small as individual rocks or the spaces between pebbles 

in the streambed. For others, it can include many miles of rivers, streams, 

flood plains, wetlands, and ocean. 

 The size and distribution of sediment particles and pore spaces within the 

streambed is particularly important for small and sedentary organisms. 

Water depth and velocity, as well as the physical and chemical properties 

of water, are also important elements of habitat for aquatic organisms. 

Substrate and hydrological characteristics of rivers and streams often vary 

in predictable ways, depending on whether a particular area is a cascade, 

riffle, run, pool, side channel, backwater, or flood plain. The size and

complexity of these habitat types affect the abundance and diversity of 

organisms using those areas. The amount and distribution of habitat types 

within a river or stream reach will, in turn, determine whether the area 

serves as appropriate habitat for larger and more mobile species. The 

types, amount, and distribution of habitat types vary, depending on the size 

and gradient of a river or stream and its association with a significant flood

plain (figure 1.2).

Figure 1.2—The complexity of habitat types affects the abundance and diversity 

of organisms inhabiting the stream as well as the resilience and persistence of 

animal populations. Photo: Scott Jackson, University of Massachusetts.
1—3

Chapter 1—Ecological Considerations for Crossing Design

 At any of these scales—from individual rocks in a streambed to particular 

habitat types (riffles, pools, cascades) to an entire river system—the

particular area’s characteristics will determine what species are likely to 

be present. The tendency of areas to form structurally and functionally 

distinct portions of the landscape (for example, riffles, pools, runs, flood

plains, headwater streams, tidal rivers) means that organisms that inhabit 

these areas often form distinct assemblages of species called communities. 

These communities of organisms and the physical environmental they 

inhabit are what constitute ecosystems.

1.1.2 Aquatic Communities 

 Natural communities are more than mere collections of organisms. Species 

that make up communities are interconnected by a variety of ecological 

relationships, such as nutrient cycling and energy flow, predator-prey

relationships, competition, and species interdependency. For example, 

a single stream reach may support a variety of fish species competing

with each other for food and appropriate habitat. Diverse communities of 

invertebrates are essential for providing a food base for fish throughout the

year. Disease organisms, parasites, or predators may differentially affect 

species and thus can affect the balance of competition among these fish.

The presence or absence of fish can affect whether other species are able

to use river or stream habitats. Many amphibians, to breed successfully, 

require aquatic habitats that are fish free. These species may use floodplain pools or intermittent sections of streams as long as fish regularly

are not present. On the other hand, numerous species of North American 

freshwater mussels require specific fish hosts to complete reproduction

(figure 1.3). Larval stages (glochidia) of these mussels attach themselves

to the gills or fins of host fish (or in one case, host salamanders), a process

essential for proper development and dispersal. The nature of these 

interdependencies is such that freshwater mussels are unable to occupy 

otherwise appropriate habitat if their particular fish hosts are not present.

 Loss of species due to extirpation (extermination) of local populations 

or the exclusion of species due to migratory barriers (e.g., anadromous 

fish) has the potential to alter and undermine the sustainability of natural

communities. Similarly, the presence or introduction of nonnative species 

can seriously degrade natural communities. Nonnative species may prey 

upon, compete, or interbreed with native species, and may serve as vectors 

for disease transmission.
1—4

Stream Simulation

Figure 1.3—A broken-rays mussel uses a mantle-flap lure to attract host darter 

that it will infect with glochidia. Photo: Chris Barnhart, Missouri State University. 

1.1.3 Ecosystem Processes 

 Other ecosystem processes that affect the composition and balance 

of organisms within a community include hydrology; the movement 

of sediment, woody debris, and other organic material; and natural 

disturbances that can significantly change the physical and biological

characteristics of ecosystems. 

As the defining feature of aquatic systems, the amount, distribution,

movement, and timing of water is a critical factor in shaping aquatic 

communities. Many organisms time their life cycles or reproduction to 

take advantage of or avoid specific hydrological conditions. Flowing

waters also transport sediment downstream, changing the substrate 

characteristics of areas contributing and receiving the material. Sediment 

lost downstream is normally replaced by material transported from farther 

upstream. Woody debris is a habitat feature for many species and a factor 

that can significantly change the physical and biological characteristics

of streams. Debris dams or partial dams (deflectors) can create pools and

scour holes, and change patterns of sediment deposition within the stream 

channel (figure 1.4).
1—5

Chapter 1—Ecological Considerations for Crossing Design

Figure 1.4—Debris dams can create pools and scour holes, and change patterns 

of sediment deposition within the stream channel. Photo: Scott Jackson, University of Massachusetts. 

Natural disturbances, such as floods, drought, and ice scour can interrupt

more regular cycles of stream flow, sediment transport, and the amount

and distribution of woody debris. However, not only are these disturbances 

part of larger patterns of physical and biological change that help define

aquatic ecosystems, but they also are generally responsible for defining

channel characteristics.

 Organisms too, move through river and stream ecosystems. These 

movements range from regular movements necessary for accessing food, 

shelter, mates, nesting areas, or other resources, to significant shifts in

response to extreme conditions brought about by natural disturbances. 

1.1.4 Viability and Persistence of Populations 

Populations are groups of organisms that regularly interact and interbreed. 

Animal movements are necessary to maintain continuous populations, and 

constraints on movement often delineate one population from another. 

The ability of a population to remain genetically viable and to persist 

over time is related to both its size and its degree of interaction with other 

populations of the same species. 
1—6

Stream Simulation

 An important consideration for maintaining viable populations is 

maintaining sufficient genetic variability within populations. Small

populations are at risk of losing genetic variability due to genetic drift, 

and very small populations may be subject to the negative consequences 

of inbreeding depression. Over the short term—depending on a species’ 

life history characteristics—the minimum population size necessary to 

maintain genetic diversity ranges from 50 to 200 or more individuals 

(Franklin 1980; Soulé 1980). For longer-term genetic stability, estimates 

often range from 500 to 5,000 or more individuals (examples are provided 

in Lemkuhl 1984; Reiman and Allendorf 2001; Reiman and McIntyre 

1993; Fausch et al. 2006). 

 Fausch et al. (2006) provide an excellent synthesis of the literature on 

population size, viability, and population isolation for salmonids. Fausch 

et al. (2006) note that true “viability” (in the sense of sustainability of a 

population over time) also may require the ability of populations to adapt 

and evolve to changing environmental conditions. Long-term conservation 

of species and ecological functions may require greater numbers of 

individuals and amounts of genetic variability than that required for mere 

maintenance or “persistence” of small population isolates. Landscape 

attributes and the range or percentage of life history types present (e.g., 

migratory versus nonmigratory forms) also appear to strongly influence

persistence and viability of salmonids (Neville et al. 2006; Fausch et al. 

2006).

Given the narrow, linear configuration of streams and rivers, animal

movements are critical for maintaining populations large enough to remain 

viable. Smaller populations may be able to persist, despite their small 

size, if they are connected to larger, regional populations. Connections 

occur when individuals move from one population to another. For 

some species, dispersing juveniles are responsible for these movements 

between populations. For other species, dispersal occurs via adults. Such 

movements maintain gene flow among populations, helping to maintain

genetic health. They may also represent movements of surplus animals 

from one population to another, perhaps to one that could not support itself 

on its own reproduction. This supplementation of failing populations from 

“source” populations is referred to as “the rescue effect.” Finally, areas of 

appropriate habitat that may be temporarily vacant due to local extinction 

can be recolonized by individuals from nearby populations. Stochastic 

(random) risks such as catastrophic disturbances (landslides, debris flows,

toxic spills) even when localized can easily eradicate small isolated 

populations. Rieman and McIntyre (1993) provide additional background 

information on stochastic risks to small, isolated populations. 
1—7

Chapter 1—Ecological Considerations for Crossing Design

 As part of a long-term study of brook trout (Salvelinus fontinalis) in 

western Massachusetts, Letcher et al. (2007) used data on survival and 

fish movement within the population to model estimated time to extinction

under various scenarios. Under one scenario that simulated placement 

of barriers to upstream movement into two tributaries, local population 

extinction was predicted in two to six generations. These barriers also 

increased the probability of network-wide extinction in both tributaries 

and in a 1-kilometer section of the main stem. Once disconnected from 

the tributary populations the network-wide population could only be 

maintained via a large influx of individuals (7 to 46 percent of the total

population) immigrating into the population from downstream areas.
1—8

Stream Simulation

Understanding Ecosystems: A Case Study of Fragmentation 

The lack of population data over long periods of time—whether decades or hundreds of years—means 

that our understanding of population viability and vulnerability is largely based on theoretical concepts and 

population modeling. These theories and models predict that population extinction is more likely to occur in 

smaller populations and that the dispersal of individuals between populations is important for maintaining 

both genetic viability and local and regional populations in the face of population extinctions (Leigh 1981; 

Shaffer 1981; Fahrig and Merriam 1985; Shaffer and Samson 1985; Hanski and Gilpin 1991).

One recent study provides an excellent illustration of the impact of fragmentation in riverine systems. This 

study, by Kentaro Morita and Shoichiro Yamamoto (2002), focused on populations of white-spotted charr 

(Salvelinus leucomaenis) occupying mountain streams in Japan. The white-spotted charr is a salmonid fish 

that occurs as both large migrant individuals and small resident fish that normally interbreed in unaltered 

streams. Many of the mountain streams that charr use have been fragmented by small erosion-control 

dams that prevent fish from moving upstream. Above these dams, charr populations are sustained only by 

the smaller, resident fish. 

Morita and Yamamoto surveyed both dammed and undammed stream segments for the presence of charr 

in appropriate habitat. Based on habitat conditions, they concluded that charr should have been able to 

establish populations in all dammed sites. However, although charr populations were found in all surveyed 

undammed sites, charr were absent in 32.7 percent of dammed sites. The results indicated that the probability of charr occurring in dammed stream segments decreased with decreasing watershed area and 

increasing isolation period. Further, this study also found evidence of genetic deterioration in populations 

above dams (compared to populations below dams), including lower genetic diversity, higher morphological asymmetry, and genetically based lower growth rates. 

Results of this white-spotted charr study are consistent with predictions of increased vulnerability for smaller and more isolated populations. Genetic and population consequences resulting from fragmentation 

occurred over a relatively short period of time (30 to 35 years). That the probability of occurrence was related to watershed size suggests that the smallest populations were the most vulnerable. The relationship 

between isolation period and probability of occurrence suggests that additional populations may well be 

lost over time. 

The situation of small dams on headwater streams in Japan may be comparable to United States watersheds that contain road crossings with substandard culverts. Culverts that block the upstream movement 

of fish and other organisms effectively isolate populations above these crossings. Areas with relatively 

small amounts of habitat upstream of the crossing will be most vulnerable to population loss. Over time, the 

failure of more and more populations is expected, and the disruption of metapopulation dynamics is likely 

to keep these areas of suitable habitat unoccupied. 

Studies of other riverine species have yielded similar results. Genetic effects correlated with small habitat 

patches and isolation have been documented for Lahontan cutthroat trout (Neville et al. 2006). Habitat 

patch size (a surrogate for population size) and isolation have been found to be significantly correlated with 

the presence or absence of animal populations for bull trout (Dunham and Rieman 1999), cutthroat trout 

(Oncorhynchus clarki) (Dunham et al. 1997; Harig and Fausch 2002), and spring salamanders (Gyrinophilus porphyriticus) (Lowe and Bolger 2002). Harig and Fausch (2002) point out that large interconnected 

stream networks not only are likely to support larger populations of fish, but are likely to provide the complexity of habitat types required by these fish throughout their life cycles.
1—9

Chapter 1—Ecological Considerations for Crossing Design

1.2 Animal Movement

1.2.1 Importance of Movement for Individual Animals 

 Animals move through rivers and streams for a variety of reasons. Some 

movements are regular daily movements to find food and avoid predators.

It is not unusual for aquatic animals to forage at night and seek shelter 

during the day. Examples include juvenile bull trout and Atlantic salmon, 

American eel, hellbenders, and many other species of stream salamanders. 

The crayfish Orconectes virilis typically moves in the open at night,

ranging upstream or downstream as much as 82.5 feet or more before 

returning to the same daytime area (Hazlett et al. 1974). 

Changes in habitat conditions, such as temperature, water depth, or flow

velocity, may require organisms to move to areas with more favorable 

conditions. During the summer, for example, many salmonid species move 

up into cool headwater streams to avoid temperature stress in mainstem 

waterways. When conditions become too dry, these animals shift to areas 

with suitable water. Flood-plain side-channels and sidewall-channels fed 

by ground water also provide thermal refuges for fish and other aquatic

organisms. 

In many stream systems where natural disturbances cause significant

habitat variability, access to refuge habitat is especially important. 

Humans, too, can cause disturbances that require fish to seek refuge

habitats. For example, major highways parallel many streams, and toxic 

spills in streams are not uncommon. When these occur, fish must have the

ability to move to unaffected habitats. 

 Some animal movements are seasonal and therefore linked to the 

reproductive biology of the species. During the breeding season, animals 

move to find mates, and smaller individuals may have to move to avoid

areas dominated by larger, territorial adults. A common strategy among 

river and stream fish is to segregate habitats used by adults from those

used by juvenile fish. Adult fish typically use habitats in areas of deeper

water and more stable hydrology than those in which they spawn. 

They migrate to spawning areas that have higher productivity or fewer 

predators, such as flood plains and headwater streams. In these areas,

recently hatched fish can take advantage of decreased predation or higher

productivity, with the large number of juveniles compensating for the risks 

inherent in these more variable habitats (Hall 1972). 
1—10

Stream Simulation

 The most dramatic examples of breeding movements are the long-range 

migrations of anadromous fish, including various species of salmon, searun trout, shad and other herring species, sturgeons, and other fish. By

contrast, the common eel is a catadromous species—living as adults in 

freshwater and migrating to the ocean to breed. 

 Adult salmon live in the ocean until the breeding season, when they 

migrate long distances to reach spawning streams. As they become larger, 

juvenile salmon hatched in these streams make their way downstream 

to the ocean, where the large marine food base can support much higher 

growth rates than freshwater environments can provide. Other fish species

make similar but less dramatic migrations to reach spawning habitats. 

Pike and pickerel move into vegetated flood plains to spawn. Many

“nonmigratory” fish (for example, some species of trout, suckers, and

freshwater minnows) use headwater streams as spawning and nursery 

habitat. 

In contrast to fish, many stream salamanders use intermittent headwater

streams as adults but deposit their eggs in more perennial areas of the 

stream. The semiaquatic adults can readily move up into headwaters to 

exploit the productivity of these areas. The salamanders’ less mobile larvae 

are aquatic, needing areas of more reliable, year-round surface water. 

 As organisms move through their various life stages, they need access to 

areas that meet a variety of habitat requirements that may change as the 

organisms grow and develop. Sometimes spawning habitat doubles as 

nursery habitat for juvenile fish or larval amphibians. In other cases the

survival needs of eggs (for example, cool temperatures, specific substrates,

or well-oxygenated water) may greatly differ from those required by 

juveniles or larvae (appropriate cover, more persistent hydrology, lower 

flow velocities, or adequate food supplies). Adult fish may require deeper

water and larger cover objects. In Wisconsin, brown trout were observed to 

move more than 9.6 miles downstream to overwintering sites that were too 

warm for trout during the summer (Meyers et al. 1992). 

In dynamic environments like rivers and streams, the location and 

quality of habitats are everchanging. Large woody debris is an important 

component of many stream ecosystems. Large logs in the stream can 

dam up water or create plunge pools on the downstream side of the log. 

Accumulations of woody debris can change the local hydraulics of the 
1—11

Chapter 1—Ecological Considerations for Crossing Design

stream, scouring some areas and depositing the material in other places 

(figure 1.5). Woody debris that forms jams across the stream can create

large and relatively deep pools. These features (woody debris, scour holes, 

pools, deposited gravel) are important habitat characteristics. However, 

they are not permanent features; woody debris will eventually break up 

or move downstream. Flooding, substrate composition, and woody debris 

work together to shape river and stream channels, water depth, and flow

characteristics, creating a shifting mosaic of habitats within riverine 

systems. In these dynamic environments movement is critical for aquatic 

organisms to be able to avoid unfavorable habitat conditions and to find

and exploit areas of vacant habitat. 

Figure 1.5—Woody debris has altered the local hydraulic conditions in such a 

way that a deep hole has been scoured out beneath and just upstream of the 

‘deflector,’ with fresh gravel deposited on the downstream side. Photo: Scott 

Jackson, University of Massachusetts. 

In the intermittent Colorado plains streams that provide habitat for the 

Arkansas darter (figure 1.6), habitat changes seasonally with regular wet

and dry cycles. During dry periods, darters rely on ground-water-fed 

refuge pools. The number, distribution, and quality of these pools change 

in response to drought, winter conditions (pool freezing), and flooding

that occur every few years or decades on average. Occasional flash floods

scour out new pools and fill others. To persist in these streams in this

ever-changing landscape, Arkansas darters must rely on long-distance 

movements to locate and colonize pools (Labbe and Fausch 2000). 

Deep scoured

hole under

deflector

Sediment

accumulation
1—12

Stream Simulation

Figure 1.6—Arkansas darter.Photo: Kurt Fausch, Colorado State University.

For a time, fisheries biologists thought that fish species such as trout

generally stayed put, except for specific periods of movement for

breeding or avoiding unfavorable conditions. However, we now see that 

a significant proportion of these fish make regular and remarkably longrange movements (ranging behavior) that allow individuals to locate and 

exploit favorable habitat within these ever-shifting mosaics (Gowan et al. 

1994). For a detailed summary of salmonid fish movement within rivers

and streams see Northcote (1997).

1.2.2 Ecological Functions of Movement 

 Although movement and migration present obvious advantages for 

individual organisms, these movements are also important for maintenance 

of populations over time. Animal movement has several important 

ecological functions responsible for maintaining populations and 

ecosystems. 

 Survival of individual animals, facilitation of reproduction, and the 

maintenance of continuous populations (sufficient to prevent genetic

differentiation) are important functions of movement at a population level. 

Extreme events, such as floods, debris flows, and droughts, may force

entire populations to avoid unfavorable conditions by moving. Provided 

that no barriers prevent the movement of individual animals back into 
1—13

Chapter 1—Ecological Considerations for Crossing Design

the areas, populations will reoccupy the habitat once conditions have 

improved. Among aquatic communities, the movement of animals helps 

maintain the balance between predators and prey, and facilitates more 

efficient use of food-based energy within the system.

 Dispersal of individuals regulates population density. These dispersing 

individuals maintain gene flow among populations and may supplement

populations where recruitment is unable to keep pace with the loss of 

individuals. For many small species, especially invertebrates, dispersal of 

individuals provides a mechanism for colonizing habitat, allowing local 

populations to come and go as habitat is created or eliminated, while 

maintaining viable regional populations. 

 Movement is an important ecosystem process for upstream cycling of 

nutrients and organisms. Within aquatic ecosystems there is a tendency 

for organisms and nutrients to shift downstream. This tendency has been 

documented for a number of amphibians, including tailed frogs, boreal 

toads, and a variety of stream salamanders. The upstream movement of 

individuals counters this biological displacement and returns nutrients to 

upstream portions of these systems. When adult salmon migrate upstream 

and die, they transport essential nutrients to spawning streams, a process 

that can have an enormous impact on the productivity of those streams (for 

example, Levy 1997; Wipfli et al. 1999).

 Some streams on the Great Plains support a number of minnow species 

that produce semibuoyant eggs during high-flow conditions. This

buoyancy mechanism allows the spawn of adult fish inhabiting perennial

upstream areas to drift many miles downstream into intermittently flooded

portions of streams running through the plains. With this reproductive 

strategy, not only is downstream drift important, but unimpeded movement 

of young fish into more persistent upstream sections is also essential for

maintaining minnow populations. 

1.2.3 Movement Capabilities of Aquatic and Riparian Organisms

 The timing of animal movements varies by species and lifestages. 

Often this means that, at virtually all times of year, one or more species 

is moving (figure 1.7). Movements may be between areas of shallow

and deeper water or between the water’s edge and midstream. Animal 

movements may be downstream (intentionally or unintentionally) 

or upstream. For many organisms inhabiting small streams, lateral 

movements or movements between surface and deeper water within the 
1—14

Stream Simulation

stream channel are severely constrained. Under these circumstances, 

upstream and downstream movements become all the more important 

for these organisms. Also important are movements between the stream 

channel and adjacent flood plains, as well as upstream and downstream

through flood plains and riparian areas. For rivers with large flood plains,

these movements are especially important. 

 Some organisms are weak swimmers capable of moving only relatively 

short distances unless displaced by floods or attached to other animals or

woody debris. Others are strong swimmers with the capacity for longdistance movements and the ability to move upstream against strong 

currents. In between are a whole host of species: some with the capacity 

for strong bursts of swimming but with a tendency to stay put; and 

others—some crayfish, for example—that are capable of long-distance

movements but typically crawl rather than swim. 

Figure 1.7—Migration timing for a fish community in British Columbia or Alaska. 

There is virtually no time when migration barriers do not pose a problem for at 

least one species. Graphic: Brett Roper, Forest Service. Data from Scott and 

Crossman 1973.

For fish, swimming ability is highly variable among species. While terms

related to swimming ability do not have standardized meaning, most 

researchers use three categories to describe swimming ability (Beamish 

1978). These include (1) burst speed (relatively high speeds that can 

be maintained for only a few seconds), (2) prolonged swimming speed 

(including the range of speeds between burst and sustained), and (3) 

sustained speed (speeds that can be maintained for long periods without 

fatigue). Swimming speeds are significant factors affecting the ability of

Spawning migration timing: Cutthroat trout, Bull trout, Longnose sucker, Longnose 

dace, Redside shiner, Pygmy whitefish, Burbot

 Jan Mar May July Sept Nov

1

Relative spawning intensity

0

Note: At a relative spawning intensity of 1, peak numbers of fish are spawning.
1—15

Chapter 1—Ecological Considerations for Crossing Design

animals to move through river and stream ecosystems. Burst speed is most 

relevant for physical barriers that require jumping or short sections of 

relatively high water velocity. Prolonged speed is important for crossing 

longer sections of fast water. Long-distance movements of migratory fish

and the ability of fish to maintain position in the stream channel for long

periods of time depend on the sustained speed of fish.

 There are a number of uncertainties in using data on the swimming 

abilities of fish for hydraulic design of stream crossings. For several

reasons, the available data may not reflect how wild fish behave in real

streams: 

l Most swim-speed data currently available were developed by forcing 

fish to swim at a constant speed in a laboratory swimming tunnel.

Such conditions are not ideal for developing estimates of a fish’s

volitional swimming ability.

l Actual swim performance is affected by a host of environmental 

and physiological factors ranging from water quality (temperature, 

dissolved oxygen, toxins) to fish condition (disease, spawning status,

exercise history, body fat). 

l	Individual fish of the same species have widely varying swimming

capabilities. 

l Ordinary swim-performance tests do not include the effects of 

turbulence.

 Most swim-speed data are based on the assumption of a constant 

relationship between fish swim speed and water velocity. Peake (2004)

discovered that free-swimming fish increased their mean ground speed

(swimming speed minus water velocity) in response to higher water 

velocity. Due to their increase in ground speed, small mouth bass actually 

decreased their passage time as velocity increased.

The fact that swim speed data do not perfectly represent real fish

performance in the field does not mean the data are not useful for

designing crossing structures. On the contrary, hydraulic design has been 

used extensively to provide passage for spawning adult trout and salmon, 

and for other fish for which data exist. It is the best method in many

situations, such as retrofits, jacked pipes, and highly altered streams.

Nonetheless, we know very little about the majority of fish species,

especially small fish (including juveniles). We know even less about the

swimming abilities of nonfish species that inhabit rivers and streams.
1—16

Stream Simulation

 A number of relatively large aquatic animals that inhabit rivers and streams 

rarely are considered in terms of barriers to movement (figure 1.7). Much

of the United States supports large species of aquatic salamanders (species 

that rarely or never venture forth on land). Mudpuppies, waterdogs, 

hellbenders, sirens, and amphiumas are fully aquatic salamanders that 

range in adult size from about 1 foot to over 3 feet in length (figure 1.8).

The Oklahoma salamander and the Pacific giant salamanders of the West

Coast are other aquatic salamanders that are vulnerable to movement 

barriers. 

Figure 1.8—Mudpuppy. Photo: Alan Richmond, University of Massachusetts.

Significant portions of the United States support softshell and musk turtles

(figure 1.9)—aquatic reptiles that rarely travel overland. Movements of

spiny softshell turtles are almost exclusively aquatic, with the exception of 

nesting and basking. In Arkansas, these turtles moved on 85 percent of the 

days they were tracked, with average daily movements of 403 to 465 feet 

per day. Some individuals moved more than 2,970 feet per day. Annual 

home-range length for these animals averaged between 4,620 and 5,775 

feet (Plummer et al. 1997). 

 Although little is known about the swimming abilities of amphibians and 

reptiles, they are not believed to be strong swimmers, relative to migratory 

fish. Many species may rely more on crawling than swimming, yet

movement and population continuity are essential to the survival of their 

populations. When moving upstream, aquatic amphibians and turtles are 

thought to seek out lower velocity sections of streams and take advantage 

of boundary layers (low-velocity zones) along the stream bottom and bank 
1—17

Chapter 1—Ecological Considerations for Crossing Design

edges. Some salamanders may require relatively continuous cover on the 

stream bottom, moving from rock to rock to reduce exposure to predators 

or high velocities (figure 1.10).

Figure 1.9—Spiny softshell turtle. Photo: Gary Stolz, U.S. Fish and Wildlife Service (USFWS) digital image library (http://images.fws.gov/default.cfm)

Figure 1.10—Northern dusky salamander.Photo: Scott Jackson, University of 

Massachusetts.

Although some crayfish can travel overland, many species are fully

aquatic. Some have been documented moving long distances within 

streams, and all most likely depend on smaller scale movements to 
1—18

Stream Simulation

maintain continuous and interconnected populations. Crayfish are

dominant components of headwater stream systems of the Ozarks and 

southern Appalachians, rivaling aquatic insects in importance (figure

1.11). Some headwater populations have been isolated long enough (due 

to natural conditions) to become separate species. In these United States 

regions, headwater streams support many rare crayfish with very limited

distribution. Further population fragmentation could imperil entire species 

of crayfish.

Figure 1.11—The Grandfather Mountain crayfish (Cambarus eeseeohensis) is 

only found in the headwaters of the Linville River, North Carolina, upstream of 

the Linville River falls. This species does not leave the stream and cannot travel 

overland around a barrier. Photo: Roger Thoma, Ohio State University.
1—19

Chapter 1—Ecological Considerations for Crossing Design

 As a group, the most vulnerable animal species in the United States 

are freshwater mussels. Over 70 percent of the 297 species native to 

the United States and Canada are endangered, threatened, or of special 

concern (Williams et al. 1993). Although adult mussels have a very limited 

capacity for movement, typically dispersal occurs when larvae (glochidia) 

attach themselves to host fish or salamanders. Therefore, survival and

persistence of freshwater mussel populations depends on the capacity of 

the host fish or salamander to move through river and stream systems.

Many endangered mussels depend on small, sedentary host fish that are

typically weak swimmers and therefore highly vulnerable to movement 

barriers. 

 River and stream ecosystems contain many other species about which 

we know little except that they appear to have limited capacities for 

movement. These species include worms, flatworms, leeches, mites,

amphipods, isopods, and snails. Collectively, these often overlooked 

taxa account for a significant amount of the biomass and diversity of

river and stream ecosystems. For most, swimming ability is less relevant 

than the ability to move through streambed substrates. Although large 

numbers of invertebrates can often be supported in relatively small areas, 

appropriate habitats may be patchy and dynamic. In these situations, 

a regional population is generally maintained through cycles of local 

extinction and colonization in response to changes in habitat conditions. 

Scour and deposition related to flooding or changes in stream hydraulics

(for example, debris dams and deflectors) may destroy habitat in some

areas while creating suitable habitat in others. How these organisms move 

upstream any significant distance is unclear. That some mechanism must

exist is a reasonable assumption; otherwise, populations would continually 

shift downstream as upstream populations are lost to local extinctions. One 

possible mechanism for such movements is when larger animals transport 

small organisms or eggs, perhaps in association with adhered sediment or 

debris. 

 Many weak swimmers and crawling species take advantage of boundary 

zones along bank edges and the stream bottom where water velocities 

are much lower than in the water column. Under natural conditions, the 

movement of some stream organisms depends on the diversity of channel 

structure and hydraulics typically found in natural streams. This diversity 

creates alternate pathways throughout the channel bed and along the 

bankline; if any point in the channel is a barrier (high-velocity or highturbulence zones) other less strenuous pathways are generally available. 

Maintenance of unfragmented stream bottom and bank-edge habitats is the 

best strategy for maintaining continuous and interconnected populations 

for a variety of weak-swimming species. 
1—20

Stream Simulation

In addition to aquatic organisms, riparian wildlife use rivers and streams 

as travel corridors. These species include semiaquatic animals, such 

as muskrat, mink, otter, frogs, stream salamanders, turtles, and snakes 

(figures 1.12 through 14). Within the larger landscape, rivers and streams

provide vital links connecting wetland, aquatic, and terrestrial ecosystems. 

In developed areas, rivers and streams often represent the only available 

travel corridors for many wildlife species. In arid environments, stream 

channels and riparian corridors offer wet and humid conditions during 

extended dry periods, and serve as movement corridors for terrestrial and 

semiaquatic amphibians. 

Figure 1.12—River otters. Photo: Jim Leopold, USFWS digital image library.

Figure 1.13—Muskrat. Photo: R. Town, USFWS digital image library.
1—21

Chapter 1—Ecological Considerations for Crossing Design

Figure 1.14—Snapping turtle. Photo: Scott Jackson, University of Massachusetts.

1.2.4 Barriers to Movement Providing Some Positive Benefit 

In some circumstances, barriers to animal movement may serve a useful 

purpose. When natural barriers have been in place for long periods, 

isolated populations can become genetically distinct or evolve into 

separate species. For example, a population of brook trout in western 

Massachusetts isolated for more than 400 generations (approximately 910 

years) above a natural barrier has evolved demographic characteristics 

distinct from populations in neighboring tributaries (Letcher et al. 2007). 

Individuals in the isolated population have higher early survival rates and 

reproduce at smaller sizes, traits that may have been instrumental in the 

persistence of this isolated population. The loss of the natural barriers 

could result in the genetic swamping of a distinct population that has not 

yet fully differentiated into a separate species. Removal of natural barriers 

can also provide access for organisms that might successfully outcompete 

rare and geographically restricted species, or allow transmission of 

parasites and disease from one population to another.

Artificial barriers, such as road crossings, dams, and diversions, also can

have positive benefits. Where stocked or introduced strains of fish are

genetically different from native populations, movement barriers may 

protect the native fish from contamination by outside genotypes. Movement

barriers also can be important for containing the spread of exotic, invasive 

species, such as the zebra mussel, Asiatic clam, and rusty crayfish.
1—22

Stream Simulation

 Many populations of native trout in the inland West are vulnerable to the 

negative effects of introduced salmonids. Artificial barriers are viewed

as a potential tool for protecting native populations from the negative 

genetic and population effects of introduced species. However, the use of 

such barriers comes with risks. Native populations isolated above these 

barriers may not be large enough to persist. There also may be negative 

consequences for other, nontarget species. Fausch et al. (2006) offer a well 

thought-out framework for analyzing the risks and tradeoffs associated 

with constructing an artificial barrier to isolate a population and protect it

from invasive species.

 Relying on substandard road-stream crossings to prevent the spread of 

invasive species is unwise. While such structures may serve to inhibit 

movement of invasive species, they may not be complete barriers to 

passage. When exclusion of exotic species is the goal, structures should 

be designed with the specific objective of blocking movement of the target

(undesired) organisms. 

1.3 Potential Adverse Impacts of Road-Stream 

 Crossing Structures

 Traditional culverts can impact aquatic animals directly. However, they 

also can affect aquatic-animal habitats by means of their effects on stream 

channels and flood plains. These impacts are not universally adverse, but

beneficial effects are less common than detrimental ones.

1.3.1 Effects on Channel Processes and Aquatic Habitats 

 Streams do the vast majority of their habitat construction and valley 

modification work—mobilizing, sorting, and depositing sediments, woody

debris, and ice—at a range of higher flows. The highest flows approach or

exceed the conveyance capacity of many stream crossings on low-volume 

roads; therefore, the potential for stream crossings to alter the fundamental 

processes that create and renew physical geometry and habitat properties 

of the channel and valley bottom is high. 

Aggradation 

Upstream Road-stream crossings that are narrower than the incoming channel can 

cause upstream backwatering during high flows (figure 1.15). In many

cases, debris enhances this tendency by plugging the structure. The 

backwatering usually results in sediment deposition, which can extend a 

distance of several channel widths upstream of a narrow culvert. These 
1—23

Chapter 1—Ecological Considerations for Crossing Design

sediment and debris accumulations at the pipe inlet can constitute fish

passage barriers (figure 1.16). The accumulation steepens the local

gradient, sometimes accelerating flow at the inlet beyond the velocity

against which fish can swim, especially at the upstream end of the journey

through the pipe. 

Figure 1.15—Many crossing structures are narrower than the stream and block 

fluvial processes that maintain aquatic habitats. The structures also impede 

aquatic species passage. Photo: Scott Jackson, University of Massachusetts.

Figure 1.16—Debris and sediment at culvert inlet can be a fish barrier. Photo 

courtesy of Ross Taylor and Associates, McKinleyville, California.
1—24

Stream Simulation

 Aggradation also can be induced by a crossing structure that is skewed 

with respect to the stream. As a cost-efficiency measure to minimize

culvert length, culverts are sometimes installed perpendicular to the road 

and skewed relative to the stream channel. Where these pipes force flow

to turn abruptly at the inlet, they may induce sediment deposition (see 

skew discussion in section 6.1.1). Skewed-pipe outlets often aim flow at

one bank, causing it to erode. A skewed alignment is not always harmful; 

where the culvert width is nearly as wide as the channel, a mild skew can 

create an eddy that functions as a resting area for fish.

Degradation 

Downstream Because water speeds up inside a culvert, which is usually narrower and 

smoother than the natural channel, the water flowing out the downstream

end surges out as a jet at high flows, scouring (degrading) the streambed

(figure 1.17). The degradation usually occurs during the first few years

after construction. Scouring can create good habitat; the deepest pool 

in the affected reach may be the outlet plunge pool. However, it also 

creates a vertical discontinuity that often stops or impedes passage of 

aquatic animals. Because the scoured streambed is lower in elevation, the 

streambanks are taller and may be less stable. Plunge pools caused by local 

scour at culvert outlets usually do not extend further than 3- to 6-channel 

widths below the culvert.

Figure 1.17—High-velocity discharge from undersized culverts causes downstream scour. (a) Culvert was placed at grade in 1979. (b) By 1998, undersized 

culvert had caused over 1 foot of downstream scour.

(a)

(b)
1—25

Chapter 1—Ecological Considerations for Crossing Design

Plugged Culverts Debris-plugged inlets often are found to be responsible for crossing and 

fill failures due to overtopping during floods (Furniss et al. 1998) (figure

1.18). Plugged culverts act as small dams, and overtopping flows can cause

partial or complete fill failure. Alternatively, where the road slopes away

from the crossing, flow will divert down the road. If the flow then runs

across the road onto a hillslope, it may erode a gully that can contribute 

sediment to the stream (Furniss et al. 1997). The diverted flow may reach

another channel, increasing flow there and causing that channel to erode

and enlarge.

Figure 1.18—Culvert-crossing failure after flooding, Plumas National Forest, 

California.

Flood-plain Hydrology Almost all streams have an adjacent valley bottom of some width. The 

stream may inundate the valley bottom frequently (every 1 to 3 years) or 

infrequently (greater than 50-year recurrence interval). During floods,

water, sediment, and woody debris move down-valley across the flood plain

creating new habitats, such as side channels and debris accumulations. 

Roadfills approaching crossings are often raised above the flood-plain

surface, creating a bottleneck at flows higher than bankfull, and locally

changing the erosional and depositional processes that maintain the diverse 

flood-plain habitats. The extent and duration of upstream flood-plain

backwatering shown in figure 1.19 are unusual, but the photos demonstrate

the concept. 
1—26

Stream Simulation

Figure 1.19—Roadfill effects on flood-plain hydrology—Minnesota. (a) Meandering channel with half-mile-wide flood plain remains backwatered for several weeks 

during snowmelt runoff, and sediment deposition extends for thousands of feet 

upstream. High water tables have killed the flood-plain trees. (b) Downstream view 

from same point as (a).

 The channel itself can be affected when sediment transport into the 

downstream reach is reduced, as in figure 1.19. When overbank flows are 

funneled through the culvert, streambed scour tends to occur at the culvert 

outlet. Bank erosion can occur at both the inlet and outlet.

Direct Habitat Loss 

and Degradation	 Replacing the natural streambed and banks with an artificial crossing

structure usually results in direct loss of some habitat value. Culvert 

crossings provide very little habitat within the culvert. Some habitat can 

be provided if the culvert is sufficiently embedded with substrate that is

similar to the natural streambed. Open-bottom or arch culverts and bridge 

crossings often maintain natural streambeds, although some habitat may be 

lost to footings, piers, and abutments. Fords may or may not significantly

affect habitat near the crossing, depending on how much the ford alters the 

streambed, banks, and water-surface elevations (figure 1.20).

(a)

(b)
1—27

Chapter 1—Ecological Considerations for Crossing Design

Figure 1.20—Elevated concrete-slab ford eliminates aquatic habitat area directly 

underneath the structure and blocks fish passage at low flows. However, it may 

not significantly alter the character of aquatic habitats upstream and downstream.

Erosion and sedimentation are two significant impacts of road crossings.

They often occur during construction if BMPs are not used, but they 

also can occur even when BMPs are in place. Ongoing erosion of 

embankments, the road surface, and drainage ways are of more long-term 

concern. Excess sedimentation degrades river and stream habitats by 

increasing suspended solids in the water and altering downstream substrate 

and channel characteristics. Increased turbidity in the water can adversely 

affect visual predators and increase the amount of inorganic particles 

(relative to organic particles) available to filter feeders downstream.

1.3.2 Effects on Aquatic Organism Passage 

 There are a variety of ways by which crossing structures can impede or 

prevent the movement of animals: 

Inlet or Outlet Drop Elevation drops at the inlet or outlet or within a crossing structure can create 

physical barriers to many animal species. Not all stream-dwelling aquatic 

species have strong jumping capabilities, and many subadult life stages of 

strong jumpers are not well enough developed to navigate vertical drops 

associated with crossing structures. In addition, outlet pools often have 

insufficient depth to allow fish to jump into structures (figure 1.21).
1—28

Stream Simulation

Figure 1.21—Outlet drop formed by scour at the downstream end of an asphalt 

apron. Photo: Scott Jackson, University of Massachusetts. 

Physical Barriers Clogged or collapsed culverts and trash racks can block animal movement. 

Weirs or baffles, which are typically designed to facilitate fish passage by

increasing depth or decreasing local velocities within a crossing structure, 

can be barriers for nontarget weak-swimming or crawling species. 

Excessive Water 

Velocities Water velocities can be too high to pass fish or other organisms during

some or all of the year. As stream-discharge increases, velocities within 

culverts increase correspondingly. Average velocities can easily exceed 

10 feet per second, a speed far greater than the prolonged swim speed of 

most fish. In addition, culverts usually contain no resting areas for aquatic

species attempting to pass through them. The result is that the animal may 

have to swim the entire length of the structure at burst speeds, and may 

exhaust itself before reaching the end of the culvert.

In corrugated metal pipes, the corrugations moderate velocities near 

the culvert wall, and fish use those lower velocity areas. Depending 

on the flow, culvert average velocities can be much higher than water 

velocity in the swimming zone inside corrugated metal pipes (Behlke et 

al. 1991). Average velocity is more likely to represent the swimming zone 

in smooth-walled concrete box culverts and steep bare-metal pipes.
1—29

Chapter 1—Ecological Considerations for Crossing Design

Absence of 

Bank-edge Areas Because certain organisms utilize bank edges for movement in natural 

stream channels it is possible that the absence of those bank edges may 

at least inhibit, if not prevent, passage by weak-swimming or crawling 

organisms (figure 1.22). Constructing a crossing structure that allows

for bank-edge areas is often challenging, because of the increased cost 

associated with the larger structure needed. However, long-term costs 

to species may justify the additional cost of constructing a structure that 

provides bank-edge areas. 

Figure 1.22—This box culvert has a concrete floor and no shallow edges for 

crawling-species passage. Photo: Scott Jackson, University of Massachusetts. 

Excessive Turbulence When a culvert creates more turbulence than the natural channel, the 

associated aeration and chaotic flow pattern can disorient aquatic species,

inhibit their swimming ability, and block their passage. Turbulence barriers 

are common downstream of perched culverts; at some flows fish may not

even be able to approach culvert outlets. Baffles, riprap, or other roughness

elements designed to reduce the water velocity can also create turbulence 

that blocks movement. Turbulence at culvert inlets can also block passage.

Insufficient 

Water Depth	 Absence of a low-flow channel can result in water depths too shallow to

allow passage for fish or other organisms (figure 1.23). In streams with

highly variable flows, the challenge is constructing a structure capable

of passing high flows while still maintaining a defined low-flow channel
1—30

Stream Simulation

similar to the natural streambed. In these systems the most successful 

structures are often those that provide bank edges and a flood plain within

the structure. When designing these types of crossings, project teams need 

to pay particular attention to the size, location, and spacing of substrate 

within the structure to emulate the natural streambed as closely as possible. 

Figure 1.23—Lack of a low-flow channel results in insufficient water depth in 

these box culverts. Photo: Scott Jackson, University of Massachusetts. 

Discontinuity of 

Channel Substrate Crossing structures that lack any natural substrate or contain substrates 

(including riprap, baffles, or other armoring) that contrast with the natural

stream channel create discontinuities in streambed habitats. Many benthic 

(streambed-dwelling) organisms are confined to the streambed and

can only move through, or over the surface of, appropriate substrates. 

Hyporheic zones (saturated stream sediments below the surface of the 

streambed) typically support a host of invertebrate species including 

copepods, ostracods, amphipods, nematodes, tardigrades, rotifers, 

oligochaete worms, and early instars of aquatic insects. Fauna in the 

hyporheic zone are an important contributor to nutrient cycling and foodchain support in river and stream communities. 

 Much of the movement of benthic organisms is downstream as passive 

drift. However, rare upstream movements must also occur to compensate 

for this drift and ensure that upgradient sections of streams do not become 

depleted over time. The flying adult stage of most aquatic insects provides
1—31

Chapter 1—Ecological Considerations for Crossing Design

an obvious opportunity for upstream movement. However, noninsect 

invertebrates most likely require other mechanisms, such as movement 

through the streambed or attachment to larger organisms for upstream 

movement. There is some concern that streambed discontinuities caused 

by crossing structures may disrupt and fragment populations of these 

benthic organisms. Vaughan (2002) offers a thorough discussion of 

crossing effects on invertebrates. 

 Summary: How Crossing Structures Can Impede Movement

l Debris accumulation

l Inlet or outlet drops

l Physical barriers (weirs, collapsed culverts)

l Water velocities exceed swimming ability (too fast for too long)

l Absence of bank-edge areas

l Excessive turbulence

l Insufficient water depth

l Discontinuity of channel substrate

1.3.3 Effects on Individual Animals 

If not properly designed, road-stream crossings can block animal 

movements, delay migration (a process made worse where many crossings 

exist), and cause physiological stress as animals expend energy passing 

both natural and artificial obstacles (Fleming 1989) (figure 1.24). Delays

in movement also can result in overlap of individuals that typically occupy 

different stream reaches. For example, culverts often concentrate migrating 

fish in large pools at their outlets. These pools often provide resident fish

habitat, and residents can experience increased predation or competition 

from migrants when such overlap occurs. Increased susceptibility to 

fishing pressure and stress associated with overcrowding can also occur

when fish movements are delayed at crossings.
1—32

Stream Simulation

27-Jan

Discharge (cfs) 150

100

50

0

3-Feb 10-Feb 17-Feb 24-Feb 2-Mar 9-Mar

North fork

of W di e R vi er

Wide Canyon Hwy

Mill Creek

Mill Creek Road

Culvert #1

Culvert #2

Culvert #4

Culvert #3

Flow

1. Fish enters North Fork of Wide River and swims to mouth of Mill Creek. Culvert 1 is low-flow barrier, water too shallow.

2. After 2 weeks of waiting for sufficient water depth, the fish passes through culvert 1 as flow rises.

3. Fish reaches culvert 2 as flow begins to recede. Velocity in culvert too high. Fish repeatedly attempts to swim through 

 culvert. 

4. Fish successfully passes through culvert 2 after flow drops. Water depth in upstream channel has become insufficient. 

5. After waiting 7 days for flows to rise again, the fish is able to swim upstream to culverts 3 and 4 but oulets are 

 perched too high.

6. Finding no mates, the fish migrates further downstream to spawn, 4 weeks after first arriving at the mouth of Mill 

 Creek.

7. South Fork of Mill Creek remains unseeded, with young-of-the-year concentrated in the lower portion of the watershed.

1 4

3

2

5

Figure 1.24—Hypothetical example of cumulative effects of delaying spawning salmon at a series of culverts. 

Used by permision of Mike Love, Love and Associates, Eureka, CA.
1—33

Chapter 1—Ecological Considerations for Crossing Design

 Riparian wildlife may choose to cross over the road surface rather than 

pass through a crossing structure that does not have banks or other dry 

passage. However, if physical barriers, such as fencing or Jersey barriers 

are present, passage across the roadway may be blocked. Even where 

passage over the road is not blocked physically, if the road supports hightraffic volumes, individual animals are likely to be killed trying to cross.

For some long-lived species with low reproductive rates, such as turtles, 

roadkill can undermine the viability of populations significantly. Streamsimulation structures generally offer dry passage opportunities for ripariandependent species, since the structures are wide enough that the channel 

edges are dry much of the year.

1.3.4 Reduced Access to Vital Habitats 

 Crossing structures may be complete barriers—essentially blocking 

passage for all aquatic species—or they selectively may pass some species 

or lifestages while blocking others. Even for a particular species a partial 

barrier may allow passage for only the strongest swimming individuals in 

a population. Partial barriers are sometimes referred to as “filters” because

of their selective nature in facilitating passage. Other structures may be 

barriers at certain times of the year (high-flow or low-flow conditions)

but not others. For some species, the timing of movement is critical 

and temporary or seasonal barriers might seriously impact survival or 

reproduction within a population. 

 Crossings that are partial or complete barriers may reduce access to vital 

habitats. These vital habitats can be spawning areas, nursery habitat for 

juvenile fish, foraging areas, refuge from predators, deepwater refuges,

or other seasonal habitats. With restricted access to vital habitats, we 

would expect populations of affected fish or wildlife to be reduced or lost

altogether [figure 1.25 (a) through (c)]. For important fisheries, reduced

access to vital habitats can result in a significant reduction in productivity.

1.3.5 Population Fragmentation and Isolation 

 To the extent that road-stream crossings act as barriers to animal passage, 

they can fragment and isolate populations [figure 1.26 (a) through

(c)]. Smaller and more isolated populations are vulnerable to genetic

change and extinction from chance events. Genetic changes may result 

from genetic drift that occurs in small populations, or via inbreeding 

depression in very small populations. Local extinctions can result from 

demographic chance events (change in sex ratio), natural disturbances, 

or human impacts. As crossings contribute to population fragmentation 

and isolation, they undermine the viability of animal populations. (For 
1—34

Stream Simulation

examples of how this may have impacted riverine species, see: Dunham et 

al. 1997; Dunham and Rieman 1999; Harig and Fausch 2002; Letcher et 

al. 2007; Lowe and Bolger 2002; Morita and Yamamoto 2002; Neville et 

al. 2006).

1.3.6 Disruption of Processes That Maintain Regional Populations 

 Decreased animal movement can undermine processes that help maintain 

regional populations over time. Barriers to movement can block the 

exchange of individuals among populations, eliminating gene flow

and disrupting the ability of “source” populations to support declining 

populations nearby. Barriers to dispersing individuals also eliminate 

opportunities for recolonizing vacant habitat after local extinction events 

[figure 1.27 (a) through (f)]. (For examples affecting riverine species see

Cooper and Mangel 1999; Dunham and Rieman 1999; Letcher et al. 2007; 

Lowe and Bolger 2002; Morita and Yamamoto 2002).

1.3.7 Time and Geography 

 When road-stream crossings result in the loss or degradation of habitat, 

impacts, such as those caused by erosion and sedimentation, are 

immediately obvious. Portions of streams may no longer provide habitat 

for certain species. As a result, the abundance and diversity of aquatic 

organisms inhabiting those stream sections changes. By contrast, adverse 

impacts that result from the disruption of ecosystem processes, including 

the restriction of animal movement, are not as obvious and may take years 

to fully manifest themselves. 

 The loss or degradation in habitat conditions from changes in hydrology, 

sediment transport, or the movement of woody debris within a river or 

stream, may occur over many years. It may result in gradual changes that, 

over time, reduce the amount of suitable habitat for aquatic organisms. 

With less available habitat, populations will become smaller and more 

vulnerable to genetic changes or local extinctions. As these smaller areas 

of suitable habitat become separated by increasing amounts of unsuitable 

habitat, animal movements become even more important for maintaining 

the viability of populations. 

The problem of dams, culverts, and other barriers to fish passage is an

obvious concern for migratory fish, especially anadromous, adfluvial (lakedwelling fish that migrate to streams to spawn), and fluvial fish. Because

anadromous fish travel such long distances and must often pass many

potential barriers to reach their spawning grounds, barriers to passage can 

result in significant and immediate impacts on these species. Where barriers
1—35

Chapter 1—Ecological Considerations for Crossing Design

Figure 1.25 (a) through (c)—Hypothetical example of population effects of barrier culverts that reduce access

to spawning areas.

(a) For most of the year a population of brook 

trout occupies the mainstem of a stream network.

During spawning 

season, adult fish move 

into the headwater 

tributaries to mate and 

deposit eggs.

(b)

Construction of a road with 

substandard culverts blocks access to 

some of the spawning areas. With reduced 

access to these vital habitats, the stream 

network can support only a fraction of its 

previous population.

(c)
1—36

Stream Simulation

Figure 1.26 (a) through (c)—Hypothetical example of effects of barrier culverts that isolate populations.

This stream network supports a 

continuous population of Pacific Giant 

Salamanders, an aquatic species with limited 

swimming abilities (occupied area illustrated in 

purple).

(c)

(b)

(a)

Smaller and more isolated populations 

are more vulnerable to genetic changes and 

local extinctions due to chance events. Over 

time, as these smaller populations fail, the 

salamander is eliminated from a significant 

portion of the suitable habitat available in this 

drainage.

After 

construction of a road 

with substandard 

culverts the population 

is fragmented into 

five smaller and more 

isolated populations.
1—37

Chapter 1—Ecological Considerations for Crossing Design

(a)

(b)

(c)

The headwaters of this stream network 

support populations of the Appalachian Brook 

Crayfish.

In a period of extended drought it 

would not be unusual to lose one or more 

of the small crayfish populations. However, 

dispersal of individuals from populations 

nearby would recolonize some of the areas.

Although the 

mainstem is not suitable 

as habitat, crayfish are 

still able to move through 

the area to occasionally 

exchange individuals 

among populations. Such 

exchanges facilitate gene 

exchange and can allow 

source populations to 

supplement and maintain 

populations that would 

otherwise be declining.

Figure 1.27 (a) through (c)—Hypothetical example of population effects of barrier culverts that prevent 

recolonization after catastrophic disturbances.
1—38

Stream Simulation

(d)

(e)

Figure 1.27 (d) through (f)—Hypothetical example of population effects of barrier culverts that prevent 

recolonization after catastrophic disturbances.

Once these areas are recolonized, they 

can serve as a base to reestablish a population 

in the more distant tributary. Maintenance of a 

regional population structure eventually allows all 

suitable habitat in the area to be reoccupied after 

the drought.

The presence 

of a road with 

substandard culverts 

blocks movement of 

individuals among 

populations.

Tributaries that had supported 

populations that failed due to genetic effects 

of fragmentation or natural disturbance such 

as drought, can no longer be recolonized by 

dispersing individuals from nearby populations.

(f)
1—39

Chapter 1—Ecological Considerations for Crossing Design

prevent nonmigratory animals from accessing vital habitats, populations 

of certain species may quickly disappear from river and stream systems. 

These losses may or may not be noticed, depending on whether the species 

is closely monitored. As changes in habitat or barriers to movement 

cause populations to become smaller and more isolated, we can expect a 

gradual and continual loss of species over time. Because mechanisms for 

the recolonization of habitat made vacant by local extinctions have been 

disrupted, species loss is a cumulative process that can eventually undermine 

the stability of ecosystems. 

 Although the effects of population fragmentation and isolation may take 

years to occur, these effects are nonetheless important. A Canadian study 

found that the diversity of birds, reptiles, amphibians, and plants in 30 

Ontario wetlands was negatively correlated with the density of paved 

roads on land up to 1.2 miles from the wetlands (Findlay and Houlahan 

1997). The study calculated that an increase in hard-surface road density of 

less than 1-linear-mile per acre would have approximately the same impact 

on species richness as the loss of half the wetland area. Further analysis of 

the data, including data of the road network from 1944, revealed an even 

more significant negative relationship between roads and species richness

(Findlay and Bourdages 2000). The inference drawn from this was that 

lower species diversity today may be the result of roads and highways 

built many years ago. These studies suggest that, despite taking decades 

for the ultimate impact of roads to be apparent, the impacts can be quite 

significant. Thurow et al. (1997) concluded from a study of seven salmonid

fish in the Interior Columbia River and portions of the Klamath River

and Great Basin that the proportion of areas with healthy populations 

(strongholds) declined from 0.58 in roadless watersheds to 0.16 in 

watersheds that exceeded 4 kilometers of road per square kilometer.

 Another important consideration of scale is that of landscape position 

and the geographic extent of impacts. Culverts are the crossing structures 

most often used for small streams. Typically, little consideration is given 

to the ecology of these small streams, probably because they are perceived 

as being less important than larger streams and rivers. However, small 

streams are extremely important to the ecology of river and stream 

ecosystems and support species of fish and wildlife that are not found in

larger waterways (Meyer et al. 2007). A road network that crosses every 

tributary of a river could have a large effect on the entire system. 

Zero-, first- and second-order streams account for most of the total 

stream miles within any watershed. They cumulatively provide much more 

habitat area for aquatic organisms than large rivers. Small streams are 

also highly productive systems, owing to their relationships with adjacent 

upland habitats (figure 1.28). These areas of high productivity are often

used for spawning and nursery habitat by fish that normally inhabit larger

waterways as adults. 
1—40

Stream Simulation

 Even intermittent and very small perennial streams play an important role 

in transporting invertebrates, detritus, and other organic matter that fuel 

downstream food webs (Wipfli et al. 2007). One study in Alaska estimated

that fishless headwater streams export enough invertebrates downstream

to feed 100 to 2,000 young-of-the-year salmonids per kilometer (0.6 mile) 

of salmonid habitat (Wipfli and Gregovich 2002). In another study (of

Sagehen Creek in California), researchers estimated that 39 to 47 percent 

of rainbow trout in the population spawn in an intermittent tributary that 

flows for less than half the year (Erman and Hawthorne 1976). Bryant et

al. (2004) emphasized the importance of small, high-gradient streams to 

fish communities in southeast Alaska.

Figure 1.28—Headwater streams are important habitats for aquatic species. 

Photo: Scott Jackson, University of Massachusetts.

Small streams provide important summer habitat for cold-water fish that

move up into headwater streams to escape unfavorably warm conditions in 

ponds and rivers. Headwater streams also provide a significant amount of

woody debris input to mountainous stream systems. 
1—41

Chapter 1—Ecological Considerations for Crossing Design

In addition to providing critical habitat for fish, small streams support

many animals that do not occur in larger streams and rivers. These include 

species of stream salamanders, crayfish, and probably countless other

invertebrate species. Many rare species of crayfish are confined to a very

limited number of small streams. 

 When considering the impacts or potential impacts of a crossing, project 

teams should consider the cumulative effect of all barriers to movement, 

such as crossings, dams, and other significant discontinuities (channelized,

intermittent, dewatered, or piped sections) within the watershed (see figure

1.29). The greater the number of artificial barriers and discontinuities, the

more threatened the ecosystem. Because small streams make up the larger 

proportion of stream miles within a watershed, these headwater systems 

are particularly vulnerable to fragmentation by crossings. On the other 

hand, because stream systems are convergent, a passage barrier low in the 

watershed (close to confluence with an ocean or other important water

body) can block migratory fish access to entire stream networks. Setting

priorities for limited resources calls for a watershed perspective, evaluating 

restoration opportunities in terms of both habitat quality and river and 

stream continuity. 

Figure 1.29—Aquatic organism passage barriers in the 721-square mile Chicopee River watershed, Massachusetts, include 195 old small-scale industrial dams 

and 2,230 rail and road crossings.
1—42

Stream Simulation

1.4 An Ecosystems Approach

The impacts of substandard crossing structures on migratory fish affect

rivers and streams up and down the Atlantic, Pacific, and Gulf coasts of

the United States. The importance of migratory fish as fisheries resources

and the status of some as federally “threatened” or “endangered” species 

has focused much attention on fish passage for migratory species. A

large amount of time, money, and effort have been expended on the issue 

of passage barriers for migrating adults. Unfortunately, some efforts to 

promote upstream passage for adult fish have failed to provide passage

for the juvenile stages of the same species. Strategies that focus solely 

on adult fish but don’t address all life stages for a particular species are

unlikely to maintain populations over time. 

 As strategies are adjusted for passage issues for both adult and juvenile 

stages of migratory fish, we must avoid replacing one type of short-term

thinking with another. Even when a particular species is the primary target 

for management, management strategies that ignore the community and 

ecosystem context for that species cannot succeed. Conservation strategies 

that focus only on target species—without careful planning to maintain 

habitat quality, passage for the variety of aquatic organisms in the stream, 

and other ecosystem processes—may succeed in the short term, but they 

undermine long-term prospects for success. 

“If the biota, in the course of eons, has built something we like 

but do not understand, then who but a fool would discard 

seemingly useless parts? To keep every cog and wheel is the first 

precaution of intelligent tinkering.”

 — Aldo Leopold

 Given the large number of species that make up most river and stream 

communities and the lack of information about swimming abilities and 

passage requirements for most organisms, using a species-based design 

to meet the movement needs of an aquatic community is impractical 

in many cases. An ecosystems approach is the most practical way of 

maintaining both the viable populations of organisms that make up aquatic 

communities and the fundamental integrity of river and stream ecosystems. 

Such an approach focuses on maintaining the variety and quality of 

habitats, the connectivity of river and stream ecosystems, and the essential 

ecological processes that shape and maintain these ecosystems over time.
1—43

Chapter 1—Ecological Considerations for Crossing Design

 THREE GUIDING PRINCIPLES FOR THE DESIGN 

 OF ROAD-STREAM CROSSINGS 

To preserve or restore all important elements of aquatic ecosystems, 

crossing structures should be designed following these three 

principles: 

1. The design should fit both the stream and the road, not just the road. 

Crossing designs must accommodate the stream—the stream’s 

geomorphic processes and anticipated changes over the life of the 

structure—not simply road or transportation needs.project teams must 

factor both systems into the design.

2. Minimum intervention in the stream process results in the least risk. 

Crossings should present the least possible obstacle to stream processes. 

Streams move water, wood, sediment, and organisms. Crossings should 

be designed, constructed, and maintained to permit movement of these 

components to the greatest degree possible. 

3. Crossings should present no greater challenge to organism movement 

than the stream being crossed.

Crossings should not fragment aquatic habitats. Avoiding fragmentation 

means reproducing the natural conditions of the stream being crossed. 

The key is matching the structure to the stream, both in form and 

process. 

Stream simulation is one approach to road-stream crossings that protects 

habitats, maintains ecological processes, and sustains aquatic communities. 

The stream-simulation approach avoids flow constriction during normal

conditions by using structures at least as wide as the natural channel. 

The constructed stream channel within the culvert is designed to insure 

adequate water depth during low-flow conditions and resist scouring

during flood events. Well-designed stream-simulation culverts can

maintain the continuity of stream bottom and hydraulic conditions, thereby 

facilitating passage for aquatic organisms.

 Designing culverts to avoid channel constriction and maintain appropriate 

channel conditions within the structure is a relatively simple and effective 

approach for accommodating the normal movements of aquatic organisms and 

preserving (or restoring) ecosystem processes that maintain habitats and aquatic 

animal populations. Where passage for riparian and terrestrial wildlife is desired, 

stream-simulation structures can be adapted for wildlife preferences (see Forman 

et al. 2003). 

 Connectivity is key to the successful functioning of both roads and rivers. 

Ultimately, our goal should be to create a transportation infrastructure that does 

not fragment or undermine the essential ecological infrastructure of the land. 

2Managing Roads 

for Connectivity

2.1 Review the Road Network

2.2 Optimize Road and Crossing Locations

2.3 Inventory Barriers and Set Priorities For Passage 

Restoration

2.4 Set Project Objectives and Design To Achieve Them

2.5 Construct and Maintain the Crossing

2.6 Monitor the Crossing
Stream Simulation

Figure 2.1—General process for providing habitat connectivity at road-stream crossings begins with large-scale 

assessments and drills down to site-scale design and monitoring.

Is road worth maintenance cost? Road access location OK?

Is road needed?

Wh

at tra

f

fic level?

ROAD NETWORK

Soil, w

ater p

roblems

Critical/high-value habitats

Geologic hazards Expected land use and hydrologic changes

ASSESS WATERSHED

A s

s e s s

b a r r i e r s

a n d e s t a b l i s h p r i o r i t i e s f o r

r e p l a c e m

e

n

t

Monitor

&

maintain

Construct

using BMPs

Crossing

location OK?

Site 

reconnaissanceestablish crossing

objectives

Design for expected 

traffic and desired 

degree of stream

continuity

HABITAT

CONNECTIVITY

AT

CROSSING
Chapter 2—Managing Roads for Connectivity

Chapter 1 showed that to maintain or restore the long-term viability 

of stream ecosystems and aquatic populations, roads and road-stream 

crossings must protect stream connectivity. This chapter briefly describes 

the planning, design, and implementation work needed to provide for 

stability and continuity in both road and stream networks. The chapter is 

a summary overview for land managers and decisionmakers among other 

readers, highlighting actions that protect aquatic habitat. Setting project 

objectives is emphasized here because it is one of the most important 

actions that require managers’ participation. Chapters 4 and 5 provide 

more detail about formulating project objectives during the project 

development process.

Figure 2.1 shows the general sequence of steps required for constructing 

crossings that maintain or restore stream connectivity—from large-scale 

transportation system planning to project construction and site monitoring. 

The feedback loop from monitoring to planning and design is an essential 

step without which experience cannot improve the technology. Because 

crossing design is not a perfect science, project teams need to learn quickly 

from their mistakes if they are to avoid repeating them year after year. 

2.1 Review the Road Network

Before deciding on the location or design of any particular road or 

structure, project teams should review the area road network to ensure 

that it is as efficient and environmentally benign as possible. Creating a 

road system that is safe, efficient (that is, minimum length to meet access 

objectives), and protective of the aquatic and terrestrial environment calls 

for considering a variety of elements from a broad range of disciplines. 

For road systems on national forest lands, “Roads Analysis: Informing 

Decisions About Managing the National Forest Transportation System” 

(USDA Forest Service 1999) provides a framework for analysis supporting 

broad-scale, strategic planning. This framework includes a comprehensive 

set of questions that transportation-planning teams should ask about the 

areas and facilities they are evaluating. The procedure poses each question 

in the context of an overall analysis at several scales, citing resources for 

assistance in determining the relevance of each question. Planning for 

transportation needs and mitigating environmental effects is often referred 

to as “access and transportation management”—an application of roads 

analysis, with the goal of planning the development of the transportation 

system over a decade or more. 
2—2

Stream Simulation

The roads-analysis process should answer the first question in any roadcrossing planning effort: Is the road needed? Before going on to the 

next step in crossing planning, be sure this question has been answered. 

Compare the access benefit against the resources and other costs the road 

incurs, and then ask: Is it worth it? (figure 2.2). The process helps avoid 

the expensive mistake of retrofitting a crossing for organism passage on a 

road that may soon undergo decommissioning. 

Figure 2.2—Remove or Retain? The cost-risk analysis. From USDA Forest Service 1999.

2.2 Optimize Road and Crossing Locations

Many forest roads were originally constructed where access was 

easiest—in the valley bottom. Despite the damage they may have caused 

over the years, many of those roads are still maintained. Before doing 

any upgrade work on a road, check that it is located properly. As all 

crossings result in some impacts to streams, the first principle is locating 

roads to avoid stream crossings, wherever doing so is feasible and 

consistent with transportation and other environmental considerations. 

All options for locating roads should be explored, because the more roads 

that are near streams or cross streams, the greater the potential adverse 

cumulative effects (figure 2.3). Roads that either run along streams 

or have many crossings, or both, should be considered for relocation 

or decommissioning. Relocating roads is often the only approach to 

mitigating the impact of old roads built in streamside areas. Many roads 
2—3

Chapter 2—Managing Roads for Connectivity

have alternative routes that access the same places, and these are good 

candidates for decommissioning. Where stream crossings are unavoidable, 

their number should be the bare minimum. 

Figure 2.3—Road proximity to streams is usually strongly correlated with road failures, 

problems, and risks to aquatic ecosystems. From USDA Forest Service 1999.

Conduct a thorough geologic review of areas traversed by the road. If a 

road is in a high-hazard location, such as steep, wet, or unstable slopes, 

or streamside areas, consider removing, relocating, or modifying it to 

reduce its effects (figure 2.4). Also, identify critical or high-value habitats 

(wetlands, spawning grounds), and avoid them if possible. Road alignment 

and roadfills should avoid isolating flood plains, constricting or realigning 

channels, or constraining channel migration, so that riparian and aquatic 

habitats retain their natural character.

Figure 2.4—Road located on a geologically unstable slope causes massive 

landslides, Bolivia. 

0-25 26-50 51-75 76-100 101-125 126-150 151-175 176-200 201-225 226-250

1.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00

All failures (319)

Mass wasting (229)

Surface erosion (90)

Distance from crenulated-stream (meters)

Road failures per mile by distance from crenulated-stream channel

(Multiple watersheds)

Road failure/mile of road
2—4

Stream Simulation

Try to locate roads away from high-value areas that are sensitive to 

disturbances created by road users. Roads can provide access for poaching, 

introduce exotic and invasive organisms, contribute to declines in rare or 

unique native vertebrate populations, or otherwise increase the potential of 

damage to important habitats. 

As we will see later, crossing location is a critical element in streamsimulation design because location affects the risks associated with 

processes like shifting stream alignments, flood-plain constriction, and 

debris flows.

 2.3 Inventory Barriers and Set Priorities For Passage

 Restoration

There are several systems for evaluating culverts for their impacts on 

aquatic animal passage and other ecosystem processes (Taylor and Love 

2003; Clarkin et al. 2003; Coffman 2005). After these evaluations are 

done, a process for prioritizing barrier crossings for remediation is 

needed. Priority setting should take into account the habitat quality in 

the river or stream and surrounding areas, upstream and downstream 

conditions, as well as the number of barrier crossings and other barriers on 

and off national forest lands (resource and risk assessments are described 

in sections 4.2 and 4.3). In some cases, dealing with other problems, such 

as the impacts of water withdrawals, restoration of in-stream habitat, or 

control of exotic invasive species, may be a higher priority than upgrading 

substandard culverts. 

To maximize positive outcomes and avoid unintended consequences, 

using a watershed-scale approach to restoring connectivity is critical. 

The diversity and complexity of stream ecosystems impede the creation 

of precise formulas for weighing the various costs, benefits, and other 

factors that affect decisions about whether and how to replace substandardcrossing structures. Clearly, priorities for restoring connectivity depend 

in part on biological values in an area. High priority goes to areas with 

high biological diversity or productivity or with other special values, 

such as migration-route connectivity. However, because many other 

social, economic, logistical, and engineering elements go into prioritizing 

crossing replacement, the project team should weigh and balance them all 

before recommending priorities.
2—5

Chapter 2—Managing Roads for Connectivity

2.4 Set Project Objectives and Design To Achieve Them

The level of stream and flood-plain connectivity at a site has tremendous 

implications for transportation efficiency, safety, cost, fluvial changes, 

ecological effects, longevity, maintenance needs, and so on. Again, 

the most successful approaches to defining the appropriate degree 

of connectivity involve an active partnership between engineers, 

geomorphologists, hydrologists, and biologists, using an ecosystems 

approach for each case. At every site, the project team should analyze 

resource values, ecological risks and consequences, future management 

constraints, and access needs (see chapter 4). From that analysis, they can 

recommend what level of stream and valley continuity to aim for. 

Federal land managers should be aware of at least three Federal 

laws when making decisions about the degree of connectivity at a 

new or replacement crossing: 

l The 1973 Endangered Species Act 

[16 U.S.C. §§ 1531-1544]. 

l The Clean Water Act, 1977 amendment of the 1972 

Federal Water Pollution Control Act Amendments 

[33 U.S.C. §§ 1251-1387].

l The 1976 National Forest Management Act 

[16 U.S.C §§ 1600-1616].

All these laws contain provisions that apply at road-stream 

crossings. 

Ecologically speaking, crossing objectives can range from providing 

for full flood-plain functioning and large-animal passage to providing 

capacity for a certain flood, with no consideration of either animals or 

woody debris. 

A corresponding continuum of design approaches exists (figure 2.5). 

The degree of stream and habitat connectivity decreases as we move 

from crossings designed for minimum interference with flood plain and 

valley processes to those designed simply for passing a flood of a certain 

frequency. Stream simulation is in the middle of this continuum. The 

structure types shown on figure 2.5 are not the only ones that correspond 

to the stated objectives; they simply illustrate the degree of connectivity. In 

addition to ecological objectives, the design approach will vary according 

to many criteria, such as traffic volume and type.
2—6

Stream Simulation

bankfull 

width

Road surface

a. Span valley with bridge or viaduct. Full functioning of 

all valley and 

flood-plain processes;

large animal passage.

Pass design flood.

Fill

e. Hydraulic design for flood capacity.

Fill

Pass sediment, debris, all 

aquatic species and provide 

for some flood-plain

connectivity; may provide 

for terrestrial animal passage.

b. Stream simulation with flood-plain connectivity.

Fill

Pass 

target 

fish.

d. Hydraulic design for fish passage.

Pass sediment, 

debris, all aquatic 

species.

Fill

c. Stream simulation.

+

Range of connectivity

Reference 

channel

bankfull 

width

Road surface

Reference 

channel

bankfull width

Road surface

Reference channel

Road surface

bankfull width

Reference channel

Road surface

Figure 2.5—Range of crossing ecological objectives and examples of corresponding design approaches. 
2—7

Chapter 2—Managing Roads for Connectivity

Most sites will have a suite of biological, geomorphic, hydraulic, and/

or infrastructure objectives. Some of these may conflict. The goal is 

to balance all the objectives appropriately and design a structure that 

optimizes achievement of all of them. 

The team may have to modify objectives as the assessment and design 

process progresses. Site and other constraints that limit the degree to 

which certain objectives can be achieved may become evident as project 

planning progresses. Site conditions, public safety, land ownership, and 

cost are some of the many possible constraints. As the team learns more 

about the site, they are likely to engage in a healthy and challenging 

discussion about the achievability of objectives, feasibility of structure 

types, and best design approaches. An open and balanced discussion with 

due consideration for all aspects of the project is most likely to produce 

the best overall plan.

Following are examples of some of the ecological objectives and design 

approaches that a team might recommend for a site. There will be many 

other objectives related to, for example, local regulations, traffic safety, 

vehicle types, project footprint, associated infrastructure, etc. 

Full Valley and Flood 

Plain Functioning A team might recommend minimal interference with valley and flood 

plain processes where:

l The stream is shifting rapidly across a wide valley flat. 

l There are many side channels used by juvenile fish or other aquatic 

species.

l The valley flat is a migration corridor for large mammals and traffic 

is high on the road.

l The full range of riparian habitat diversity must be sustained as 

critical habitat.

This objective might guide the project toward a bridge and/or viaduct 

that spans the valley flat [figure 2.5(a)]. On very low volume roads where 

traffic interruptions are acceptable, other less expensive ways to maintain 

a high level of valley and channel connectivity may be appropriate, 

such as fords and dips. In some situations, well-designed fords can help 

maintain flood plain connectivity by keeping the approach road low across 

the flood plain. However, maintaining passage for aquatic organisms 

across fords is challenging, and requires designing the structure to fit the 

needs of the specific site (Clarkin et al. 2006).
2—8

Stream Simulation

Minimal Interference

with Flood Plain

Habitats and Transport

of Water, Sediment, and

Debris on Flood Plains Flood plains are extremely important components of the aquatic system. 

During floods, water, sediment, and woody debris may move across flood 

plains, constructing important and unique habitats. Flood-plain stability—

and channel stability—may depend on deposition of sediment and debris 

from upstream and on maintenance of the natural flooding regime. Floodplain continuity is therefore an important value in many locations. Side 

channels are often important habitats on active flood plains, calling for 

preservation of aquatic organism passage in these smaller channels, too. 

In figure 2.5(b), culverts are placed in side channels and swales to achieve 

this objective. In other situations, such as little-used roads, ephemeral flow, 

seasonal closure, simple rocked dips may offer adequate passage. 

Terrestrial 

Animal Passage Wildlife species primarily associated with stream ecosystems, and others 

that use riparian areas as movement corridors, may need passage through a 

crossing structure if the road has a high volume of traffic and/or very high, 

steep fillslopes. For some species of wildlife, such as muskrat and stream 

salamanders, maintaining streambed continuity (with a stream-simulation 

structure) may be adequate. Many other species prefer to use banks or 

dry streambed areas to cross through structures. Figure 2.5(b) shows a 

structure slightly wider than the bankfull channel that offers dry passage 

for some terrestrial animals.

Larger wildlife species are thought to have minimum requirements for 

the height of the structure (in many cases minimum requirements are 

not known). These species may be sensitive to the relative “openness” 

of the structure. [A structure’s openness ratio is defined as the crosssectional area of the crossing opening divided by the structure’s length, 

and is usually stated in meters.] A few studies of structure use by deer, for 

example, indicate that these species need openness ratios of at least 0.6, 

and that ratios of 1.0 or greater are preferred (Brudin 2003; Reed 1981). 

The Wildlife Crossing Toolkit (www.wildlifecrossings.info) provides 

information on terrestrial wildlife requirements.

Compared to other crossing structures, bridges are more likely to facilitate 

the passage of riparian and terrestrial wildlife, because they are more open 

and shorter in the along-stream direction. When sized properly, openbottom arches are similar to bridges; the arches maintain the continuity 

of the streambed, allow unrestricted flow during normal conditions, and 
2—9

Chapter 2—Managing Roads for Connectivity

typically allow the passage of some woody debris. Project teams may 

sometimes be tempted to rule out bridges or open-bottom structures in the 

beginning of the design process because of high cost. However, when the 

lifetime costs and resource effects are considered together, these structure 

types may sometimes be the best overall solution. 

Fish and Other 

Aquatic Organism 

Passage Ideally, crossing structures should constitute no greater restriction on 

movement for fish (including juvenile and relatively small resident fish) 

and other aquatic organisms, such as amphibians, reptiles, and crayfish, 

than the organisms confront in the stream itself. Unnatural physical 

barriers, such as inlet or outlet drops, debris racks, weirs, baffles, or other 

structures that would block movement of aquatic organisms should be 

avoided if at all possible. Keep in mind, however, that creating passage 

where there was none originally may be just as undesirable as creating a 

barrier (see Fausch et al. 2006). 

Stream-simulation design is appropriate where passage is desired 

for all aquatic organisms present in the channel. Structures include 

open- and closed-bottom structures, but in all cases the streambed is 

continuous through the structure. [Figures 2.5(b) and (c) show streamsimulation structures; (b) goes further and provides for partial flood-plain 

connectivity.] Since streambed width, slope, and composition are all 

similar to the natural channel, stream-simulation structures accommodate 

the normal movements of aquatic organisms and preserve (or restore) the 

transport processes that maintain habitats and aquatic animal populations. 

Weak-swimming and crawling species may need appropriate bank-edge 

habitat for movement. Again, where passage for riparian and terrestrial 

wildlife is desired, teams should adapt structures to meet minimum height 

and openness requirements. 

Hydraulic design [figure 2.5(d)] has been used for decades as the primary 

design tool for fish passage at road crossings all over the world. Hydraulic 

design optimizes the hydraulic effects of culvert size, slope, material, and 

length to create water depths and velocities suited to the swimming ability 

of a target fish. It can be appropriate when designing for a small number 

of target species with similar requirements, if the hydraulic requirements 

of those species are known. In current practice, the weakest-swimming 

species and lifestage of concern is usually selected to set velocity criteria, 

with the assumption that this also provides for the stronger swimmers. 

This design method and the uncertainties associated with it are covered in 

appendix B.
2—10

Stream Simulation

Passage of

Watershed Products Streams move water, sediment, and organic materials such as wood and 

detritus. Maintaining natural channel slope, width, and alignment through 

crossings is the best way to permit these stream functions to maintain the 

channel and flood plain downstream. Substantial decreases in slope or 

channel width will tend to restrict the movement of watershed products 

and contribute to higher maintenance costs and a risk of crossing failure. 

Minimal Risk of 

Crossing Failure Culvert failures usually do much more damage than bridge or ford failures 

because of the amount of fill that is mobilized within the channel. Teams 

will find many approaches to minimizing both the probability of failure 

and its consequences. Stream-simulation design reduces the probability of 

failure by matching channel width, which generally provides capacity for 

rare flood flows plus debris and sediment. Carefully designed transitions 

between structure and stream also minimize the probability of failure. 

Nonetheless, any crossing can fail, so where the risks and consequences 

of failure are high, designing for a “soft” failure is a wise strategy. Such a 

design strategy may mean providing a dip at the crossing to prevent stream 

diversion, and/or armoring a portion of the fill to sustain overtopping flow. 

Invasive Species 

Barrier In a world where exotic species are invading many aquatic habitats, 

managers sometimes may have to erect or maintain a barrier to protect a 

population. The value of protecting a population from invasives sometimes 

outweighs the increased risk to both target populations and other species 

when habitat is restricted. Fausch et al. 2006 present a framework for 

evaluating these tradeoffs that may help in making these decisions. 

Culvert barriers are often designed hydraulically [figure 2.5(d)] so that 

they are perched higher than the target fish can jump, or have faster water 

velocities than the fish can swim. Steep or perched crossing structures not 

specifically designed as barriers may not reliably block invaders because 

they may be passable at some flows or to some individual animals. 

Control Stream Bed 

Elevations on an 

Incising or Incised 

Channel Where a headcut is progressing upstream and the existing crossing is 

protecting the upstream channel from incision, the team may recommend 

maintaining the grade control function. This might happen, for example, 

where the roadfill backs up water and creates an unusually valuable wetland 

habitat. A full-bottom culvert or ford can function as a grade control, but 

to provide for aquatic species passage, the installation may require special 

measures, such as a specially designed side channel. 
2—11

Chapter 2—Managing Roads for Connectivity

Channel Restoration Where a channel has incised downstream of the existing culvert and 

degraded important habitat, the team may recommend restoring both 

passage and habitat. This work would involve restoring the channel to an 

elevation and sinuosity that makes the transition across the road crossing 

as close to seamless as possible. These projects may be more extensive and 

expensive than those in which only the crossing is treated. 

2.4.1 Road Approaches to the Stream Crossing 

The effectiveness of any structure depends on how well its design fits the 

site. Size, alignment, and provision for overbank flows and woody debris 

passage all influence the long-term sustainability and passage effectiveness 

of structures. Part of the challenge of fitting the structure into the site and 

minimizing ecological damage is designing the road approaches to the 

crossing and implementing needed BMPs. For example, where the road 

crosses an active flood plain, the continuity of water, sediment, and debris 

transport along the flood plain depends on drainage through the roadfill. 

Side-channel culverts, and culverts or dips on flood-plain swales and other 

locations across the flood plain might be necessary for maintaining floodplain habitats and passing aquatic species that use those features.

Other design BMPs act to hydrologically disconnect the road from the 

stream. Their purpose is to leave no continuous surface flow path from the 

approach road to the stream during runoff events, so that water quality is 

protected from road-derived pollutants. These BMPs include:

l Ensuring that drainage ditches discharge muddy storm runoff to a 

vegetated buffer area or a constructed sediment trap rather than the 

stream. 

l Stabilizing road fills effectively so that sediment production 

is minimized, not chronically disturbing road fills during road 

maintenance, and revegetating or rearmoring them for stability where 

needed. 

l Outsloping road surfaces for surface drainage dispersal wherever 

possible. (Outsloping minimizes needed excavation, hydrologic 

connectivity, drainage concentration, and maintenance needs. Backup 

cross-drainage may be necessary where outsloped running surfaces 

become rutted.) (http://www.stream.fs.fed.us/water-road/w-r-pdf/

surfaceshape.pdf)

l Armoring road surfaces where necessary to prevent erosion and 

sediment transport to the stream. 
2—12

Stream Simulation

l Ensuring that stream crossings do not have diversion potential. (http://

www.stream.fs.fed.us/water-road/w-r-pdf/diversionpntl.pdf)

l Anticipating and preventing maintenance problems, and disturbing 

well-cured roads and trails only when needed for safety or drainage. 

l Monitoring roads, trails, and crossings at regular intervals after large 

storms, and promptly remedying problems.

2.5 Construct and Maintain the Crossing

The next step is to build the new crossing, vensuring adequate protection 

of the aquatic ecosystem during construction. This step involves timing 

and sequencing of installation, appropriate construction methods, and use 

of BMPs for water quality and aquatic habitat protection. 

Timing is important for reducing the environmental impacts of crossing 

construction. Construction sites may be more vulnerable to erosion—and 

organisms that inhabit the stream or river may be especially sensitive to 

impacts—during certain times of the year. For example, many freshwater 

mussels shed their larvae directly into the water, where the larvae drift 

downstream until they encounter host fish. These releases occur at specific 

times of the year, varying according to species. During spawning season, 

fish may require natural flow conditions to reach headwater spawning 

areas. Likewise, some life stages (eggs, larvae, fry) cannot easily move 

to avoid unfavorable conditions, such as periods of higher-than-normal 

turbidity, or dewatering of the stream channel. Before determining the 

most favorable time for construction, therefore, teams should identify 

species using the stream or river and understand their specific life cycles 

and habitat requirements. Except where species are particularly vulnerable 

during low-flow conditions, timing construction during periods of low 

flow is usually best. In practice, the ‘work window’ is often specified in the 

State permit for in-channel work.

The best construction practices are those that reduce the amount of 

erosion and sedimentation; minimize the extent, abruptness, and duration 

of streamflow changes; and avoid the creation of physical barriers to 

animal passage (figure 2.6). Where tradeoffs need to be made among these 

considerations, knowledge of watershed conditions, the species present, 

and their ecological needs should guide decisionmaking. 
2—13

Chapter 2—Managing Roads for Connectivity

Figure 2.6—Isolating the construction area at a bridge reconstruction site in Yellowstone 

National Park. Photo: Dan Rhodes, National Park Service.

Water quality, channel integrity, and downstream habitats are always at 

risk in crossing construction and retrofit projects. Diligent attention to 

erosion and sedimentation controls and stormwater management during 

and after construction is essential. Common events such as summer 

thunderstorms can have important negative effects if teams do not 

anticipate them when planning for erosion control. 

Maintenance and restoration of riparian vegetation is another important 

BMP. Riparian vegetation helps anchor banks, maintains channel form, 

provides shade and temperature control, contributes nutrients essential 

for productivity in small streams, provides large woody debris that shapes 

stream channel environments, and is an important component of habitat 

for riparian wildlife. (See chapters 7 and 8 for descriptions of construction 

methods that protect aquatic and riparian resources.)
2—14

Stream Simulation

2.6 Monitor the Crossing

Only by monitoring can we know whether our methods meet our objectives. 

Before beginning, teams must clearly delineate monitoring objectives and 

determine what data they need to achieve the desired confidence in the 

results. Several types or levels of monitoring exist: 

Implementation monitoring occurs during and/or immediately after 

construction, when the project team checks whether construction BMPs are 

being implemented and determines whether the structure was installed as 

designed. Regardless of what further monitoring is planned, as-built surveys 

or the plans annotated by the contract administrator (with changes made 

during construction) should be permanently filed, so that future changes can 

be identified. 

Effectiveness monitoring answers the question: is the structure performing 

as intended? It does not need to be complex and time consuming, and can be 

as simple as the team visiting the site to see whether streambed continuity is 

being maintained over time. This monitoring can also be incorporated into 

regularly scheduled road safety checks. In an evolving technology such as 

stream simulation, this type of monitoring is essential for verifying whether 

design methods need modification. In some cases, installation problems 

may reduce a structure’s effectiveness, and team members need feedback 

so that they may correct for past mistakes or poor decisions and continue to 

improve the process. 

Validation monitoring (determining how well species can actually 

move through a structure) is more complex. It should be done as an 

administrative study, designed and conducted in cooperation with 

university or other researchers. Much has been learned from past 

experience, especially from detailed case studies that result from careful 

validation monitoring (see, for example, Lang et al. 2004). Continued 

monitoring of crossing structures—with particular attention to innovative 

designs and a broad range of species—will ensure that we know how well 

our efforts to protect stream ecosystems are succeeding and how we can 

improve those efforts.
3Introduction to 

Stream Simulation

3.1 What Stream Simulation Is and What It Isn’t

3.2 Key Elements and Limitations of Stream Simulation

3.3 How Complex Does It Need To Be?

3.4 Roadmap for Stream-Simulation Design
Stream Simulation

Figure 3.1—Roaring River crossing site before-and-after culvert replacement in 

2007, Boise National Forest, Idaho.
Chapter 3—Introduction to Stream Simulation

3.1 What Stream Simulation Is and What It Isn’t 

Stream simulation is a method of designing crossing structures 

(usually culverts), with the aim of creating within the structure a 

channel as similar as possible to the natural channel in both structure 

and function. The premise is that the simulated channel should 

present no more of an obstacle to aquatic animals than the adjacent 

natural channel.

Stream simulation developed when people began to realize how important 

it is to provide passage for the variety of aquatic species and lifestages 

present in most streams, and how difficult that is to accomplish in a bare 

or baffled culvert. To solve the passage problem simultaneously for many 

different species with different movement capabilities and timing needs, 

stream simulation takes a very different approach from hydraulic design. 

Stream simulation does not target specific fish or other species for passage, 

nor does the designer need to match species-specific water velocity, water 

depth, or crossing length criteria. Instead, a continuous streambed that 

simulates natural channel width, depth, and slope connects the reaches upand downstream of the crossing. The simulation creates the diverse water 

depths and velocities, hiding and resting areas, and moist-edge habitats 

that different species need for movement (figure 3.1). Given the similar 

conditions, we can safely presume that the simulated channel inside the 

crossing presents no more of an obstacle to movement than the adjacent 

natural channel. Stream simulation crossings are larger than traditional 

crossings, and therefore less prone to debris plugging. This can benefit the 

road by reducing any tendency for debris plugging to cause overtopping or 

flow diversion. 

The goal in stream simulation is to set the stage so that the simulated 

channel adjusts to accommodate a range of flood discharges and sediment/

debris inputs, without compromising aquatic organism passage and 

without having detrimental effects on up- or downstream reaches. For the 

simulated streambed to maintain itself through a broad range of flows, 

stream processes that control sediment and debris transport and maintain 

hydraulic diversity must function similarly to the natural channel. In 

other words, flows that transport sediment and debris and rework the 

channel bed should not be constrained or accelerated inside the crossing 

structure. Bankfull flow is widely recognized as a good estimator of the 

channel-forming flow in stable alluvial rivers (Wolman and Miller 1960; 
3—2

Stream Simulation

Andrews 1980; Hey 1997) (see appendix A.4.1). Therefore, as a working 

criterion, we ensure that the channel inside the structure is at least as wide 

as bankfull width in the reference reach. Although this criterion is by 

no means the only characteristic of a self-maintaining stream-simulation 

structure, it is an essential one. 

First, the simulated channel is designed. Then the crossing structure—

either a bridge or culvert—is fitted over and around it. Its width depends 

strongly on project objectives, and it may exceed reference reach bankfull 

width if necessary for achieving objectives such as bed stability or 

amphibian or terrestrial animal passage. 

Simulations are not exact replications of real stream channels. Features we 

cannot recreate inside crossing structures include: 

lNatural light. 

lCohesive soils. 

lChannel-spanning or embedded wood. 

lDebris jams.

lBankline vegetation.

lChannel bends.

lFlood-plain functions. 

Features that provide roughness in a stream channel are essential 

for stabilizing the bed and creating the depth and velocity variations 

needed for aquatic species passage. Though we cannot duplicate these 

characteristics, we can simulate some of them with large rock. For 

example, to simulate natural banklines, we can place immobile rock 

along the channel margin in various arrangements that mimic the natural 

streambank. We can also use rock to simulate the grade-stabilizing 

functions of embedded debris. 

For these and other reasons, the design is not a perfect simulation of the 

natural channel. Where to draw the boundaries of “stream simulation” is 

not always clear. Although stream simulation is most often described in 

terms of performance (providing passage for all aquatic organisms), since 

we are unable to verify free mobility for all aquatic organisms at a site, 

success is likely to remain somewhat subjective. 
3—3

Chapter 3—Introduction to Stream Simulation

Real stream channels are tremendously diverse and complex, with some 

degree of unpredictability in their response to runoff events and land 

management. Even using sophisticated quantitative methods for design, 

we cannot guarantee that a simulated streambed will sustain itself through 

the full range of flows it may experience. Moreover, our knowledge is 

continually expanding as we build more structures and as floods test those 

structures. While this guide synthesizes years of experience to date, the 

authors have tried throughout to make its limitations clear. 

“Always use the best data and methods available at the time.”

 —Dr. Charles Behlke

3.2 Key Elements and Limitations of Stream Simulation

The reference reach is the key element of a stream-simulation design. A 

natural stable reach, preferably upstream and near the project (see section 

5.4), becomes the design template. The reference reach must satisfy the 

physical conditions of the crossing site, especially the slope, and it must 

be self-sustainablity inside a confined structure. In other words, flows 

interacting with the bed and the structure walls will dynamically maintain 

the streambed within the structure. In high flows, although some features 

of the simulated bed may be immobile, other streambed materials should 

mobilize and restructure themselves similarly to the natural channel; 

sediment transported from upstream should replace eroded material. 

Setting the stage for self-sustainability in the simulated channel means 

establishing basic characteristics of the reference reach, such as gradient, 

cross-section shape, bank configuration, and bed material size and 

arrangement. The reference reach need not reflect the average conditions 

in the natural channel; however, the condition should not be extreme. 

We assume that if we can simulate a reach representative of the natural 

channel, passage will be as good as in the natural channel. This is a virtual 

certainty in the many cases where the reference reach is very near the 

project site and represents the project reach as it would be if the crossing 

had never been constructed. 
3—4

Stream Simulation

The ideal of simulating a stable reference reach inside the crossing 

structure may not be feasible in certain common situations. These 

situations include highly unstable channels that are rapidly changing, 

such as after a major flood, where no stable reference reach exists. 

Other examples are inherently unstable landforms subject to frequent 

disturbances, such as alluvial fans (figure 3.2) and debris torrent-prone 

channels. Even stable sites where channel changes occur frequently, such 

as active meandering streams, are undesirable sites for any rigid structure. 

The ideal solution is to relocate the crossing and/or the road. Where 

relocation is not feasible, the project team must predict potential channel

adjustments for the life of the structure and design for them. 

Figure 3.2—Active alluvial fan channel where flows have deposited gravel over 

the fan surface.

Channels in wide, active flood plains present a challenge to stream 

simulation if the structure has to accommodate a large amount of flow 

that normally spreads across the flood plain. Funneling flood-plain flows 

through the structure can exert the sort of pressure on the simulated 

streambed that a reference reach connected to the flood plain never sees. 

Chapter 6 (section 6.5.1.1) gives a detailed discussion of design solutions. 
3—5

Chapter 3—Introduction to Stream Simulation

On some occasions the crossing needs to maintain a steeper-than-natural 

grade. For example, where a long stream reach downstream of the road has 

incised, the crossing might be retained as a grade control to protect the 

upstream channel. For such sites, the project team may have to search the 

stream to find a reference reach of the desired (steeper) slope. How far a 

simulation can diverge from the natural slope of the project reach and still 

achieve full passage remains uncertain (see section 5.5). The key question 

is whether the channel immediately upstream of the crossing will be able 

to supply the size and volume of sediment that the simulated channel 

needs. Section 6.1.2.3 discusses designing simulations steeper than the 

natural channel.

Assuming downstream channel incision is not ongoing, the ideal way to 

handle crosssings with large elevation drops is channel restoration. Instead 

of steepening the culvert to tie the upstream and downstream elevations 

together, the design restores the incised segment to its natural elevation, 

sinuosity, and diversity. In some cases, to achieve sustainability, restoration 

of a long reach becomes necessary. 

Channel restoration can restore more than aquatic species passage at the 

crossing; it also can restore aquatic habitat where that habitat has been 

simplified or destabilized. Section 6.1.2.3 covers the channel restoration 

option, but details of channel design are beyond the scope of this guide. 

For more information on channel restoration, see Federal Interagency 

Stream Restoration Working Group (1998) and Saldi-Caromile et al. 

(2004). 

Many older culverts have caused sediment deposition upstream and local 

scour downstream (even when the channel has not incised), leaving an 

elevation difference that the replacement project must deal with. A simple 

method of handling this situation is to simply reconnect the streambed 

and allow it to regrade naturally. However, in some cases undesirable 

ecological effects could result. For example, a small wetland may have 

developed above the old culvert, and that wetland may now be providing 

valuable habitat to amphibians. Or an important spawning habitat may 

exist downstream, where sediment should be minimized. Section 5.3.3 

describes some of these considerations. 
3—6

Stream Simulation

What constitutes “stream simulation” in these less straightforward 

situations is not entirely clear: How far can the characteristics of the 

constructed channel diverge from the natural channel before some aquatic 

species is impeded? How much steeper than the surrounding reaches 

can the simulated bed be? We might find a short, steep natural reach 

somewhere upstream, and ask: Can we use this reach as a valid reference 

reach? To answer this, we should keep a couple of basic questions in mind: 

lDoes the natural reach impede movement of aquatic species?

lAre the local controls on sediment supply, transport, and bed stability 

similar to the culvert site?

If the reach passes these tests, most practitioners would consider it a valid 

reference reach. 

Where teams can find no reference reach steep enough to achieve site 

objectives, they can reasonably use a hybrid design procedure for the 

structure’s streambed. This technique simulates the streambed materials 

and structure that would be expected in nature at the desired slope. 

However, the major structural features of the bed are designed to be 

immobile because, if washed away, they would not be replaced by 

upstream rock of the same size (see appendix B). The structure may or 

may not pass all aquatic species at the site; the further the design departs 

from the characteristics of the natural channel, the less likely it is to pass 

all aquatic species that are present. To maximize the project’s resource 

benefits and minimize its natural resources costs, the project team and 

managers must weigh these compromises and trade-offs that some 

situations necessitate. 
3—7

Chapter 3—Introduction to Stream Simulation

3.3 How Complex Does It Need To Be?

All these factors may make the design method for stream simulation seem 

complex, but the key is to tailor the level of effort to the complexity of 

the site. Complicated sites, such as those listed below, require a careful, 

detailed design process. 

 CROSSING DESIGN IS MORE CHALLENGING WHEN 

 A CHANNEL IS:

l Unstable (laterally or vertically).

l Undergoing rapid meander shift or bank erosion.

l Severely incised below the crossing.

l Severely aggraded above the crossing.

l	 Subject to debris flows, hillslope erosion events, or other 		

large sediment inputs upstream of the crossing.

l Steeper than 6 percent.

l Made up of intermittent bedrock exposures in the streambed 

(see section 8.2.10).

Simple sites may not need detailed assessment, and their design is often 

straightforward. As teams gain experience, they can streamline the process 

appropriately for each site. 

Part of the reason why the stream-simulation process appears complex is 

that it is inherently multidisciplinary. It requires considerable expertise 

and experience in diverse disciplines. The project team should include 

members who understand aquatic wildlife biology and ecology, so that 

they can identify passage needs, participate in setting project objectives, 

and protect wildlife during construction. Fluvial geomorphology and 

hydrology are important to understanding the watershed processes that 

the design must accommodate and the fluvial processes and channel 

features that must be simulated through the crossing. Civil engineering and 

hydraulics are essential to designing a fixed structure that will withstand 

the dynamic stream and valley environment. As no single person can 

competently cover all these areas of scientific and engineering knowledge, 

stream-simulation projects always involve a team of people experienced 

in applying these sciences (figure 3.3). Sometimes other specialists will be 

needed at especially complicated sites. In all cases, good communications 

between disciplines is crucial throughout the project. 
3—8

Stream Simulation

Figure 3.3—Multidisciplinary project team on initial reconnaissance of a project site. 

3.4 Roadmap for Stream-Simulation Design

Figure 3.4 shows the phases of a stream-simulation project, somewhat 

modified from phases defined by Jim Doyle (former fishery biologist, 

Mount Baker-Snoqualmie National Forest). Except for stream-simulation 

design, the phases are essentially the same as for any crossing design 

project. Figures at the beginning of each of the following chapters will 

expand figure 3.4 to show details of the actions and considerations 

pertinent to each phase. It will function throughout the guide as a 

navigational “road map” to the project development process.

The project phases are identified primarily as a way of organizing this 

guide. The actual process of stream-simulation design is not linear. The 

phases overlap, and the team may have to go back and forth between 

phases when knowledge gained in a particular phase forces reevaluation of 

earlier conclusions. Often—especially at complex sites—a decision taken 

in one phase must be revisited in light of new information in later phases. 

The process starts with a broad view, and focuses down to smaller scales 

and more detail as the project develops. 
3—9

Chapter 3—Introduction to Stream Simulation

Figure 3.4—Steps in the stream-simulation design and construction process.

3.4.1 Initial Watershed and Reach Review 

Unless you are well versed in the field of fluvial geomorphology,

read appendix A before plunging into the stream-simulation process. 

Appendix A introduces geomorphic terms and concepts that are used 

throughout this guide and that are essential to understanding stream 

simulation. 

In this phase (discussed in chapter 4) the project team reviews the access 

and travel management plan to verify that the road is both necessary and 

well located. They collect existing biological and physical watershed-scale 

information as background for project planning and for helping to interpret 

observations from the site-assessment phase. Placing the crossing site in 

the context of the road network and the watershed helps ensure recognition 

of ‘big picture’ risks, consequences, and opportunities.

Additionally, the team does an initial site walk-through reconnaissance, 

looking at site-specific risks such as woody debris, sediment accumulation 

potential, and the elevation drop through the crossing. 

Assessment of site risks and suitability for stream simulation begins now, 

and continues through the site assessment and design phases. If risks are 

high, the team can plan for a higher level of detail in subsequent phases. 

This initial review should be done before replacing any crossing structure. 

INITIAL

ASSESSMENT CONSTRUCTION MAINTENANCE AND

MONITORING

SITE

ASSESSMENT

STREAM-SIMULATION

DESIGN

FINAL DESIGN AND

CONTRACT PREPARATION

Compile

watershed-scale

information

Conduct 

initial site

reconnaissance

Establish

crossing

objectives

Survey

project area

Survey 

reference reach

Design

simulated channel

Design 

structure and

road approaches

Prepare 

construction contract

Build

the crossing

Complete

“as-built”

documentation

Maintain

crossing 

as needed

Monitor

performance
3—10

Stream Simulation

3.4.2 Site Assessment

The site assessment (chapter 5) is a detailed survey and analysis of 

the project site, including channel and road longitudinal profiles, cross 

sections, and channel bed materials. It also includes a survey of the 

reference reach that will be the template for the simulated stream channel. 

From the results of the assessment, the project team develops a set of 

specific design objectives, and provides the information needed to design 

the simulated channel.

3.4.3 Stream-simulation Design 

Stream-simulation design (chapter 6) begins with establishing the crossing 

alignment and the longitudinal profile of the simulated channel. Assuming 

that stream simulation is feasible, the next steps are to: 

lDesign the simulated channel based on channel characteristics of the 

reference reach. 

lSize the crossing structure.

lVerify bed mobility and stability, where necessary.

At the end of this phase, the simulated stream-channel design is complete, 

and we know the area and depth the structure will have to cover. Although 

the focus in this guide is primarily on culverts, the same principles apply 

to bridges, and the team does not have to make a final choice of structure 

type until phase 4, final design. 
3—11

Chapter 3—Introduction to Stream Simulation

3.4.4 Final Design and Contract Preparation 

In this phase, final design and contract preparation (chapter 7), the 

engineer-designer completes the structural design and details of the overall 

installation. Contract drawings and specifications are prepared, including 

stream simulation bed construction details, as well as water quality, 

wildlife, and other environmental protections. The level of engineering 

expertise necessary in this phase of the project depends on site conditions 

and risk, but in all cases the engineer-designer is part of the project team. 

Working through the details, the engineer-designer may discover that 

certain design objectives cannot be met or that changes in the preliminary 

design are needed. In this case, he or she should communicate with other 

team members, who may be able to suggest alternate solutions and should 

review any changes. Communication with the Contracting Officer’s 

Representative (COR) is also crucial for predicting and solving problems 

that may arise during construction. 

At this point, if not before, the COR should become a member of 

the project team. The COR should review the design during contract 

preparation, to become familiar with the critical design elements and 

to comment on the practicality of contract specifications and special 

requirements. As he or she will have to deal with any contract changes 

or unforeseen site conditions, the COR should understand earlier design 

decisions thoroughly. Good communication and mutual trust among team 

members make it much easier to handle sudden challenges during the 

construction process. 

3.4.5 Construction 

Contracting officers, CORs, and inspectors take the lead in phase 5 

(construction, chapter 8), which begins when the solicitation is advertised. 

Again, to help manage changes in project design or unexpected conditions 

as they arise, the COR should keep other team members informed about 

progress, and make them aware of construction issues. For example, the 

biologist may need to be involved in trapping and moving the aquatic 

organisms at the site before construction and dewatering begin. The fluvial 

geomorphology specialist who participated in the design may also be able 

to advise on channel construction. Specialists’ continued involvement will 

help assure the design objectives are accomplished as intended.
3—12

Stream Simulation

3.4.6 Maintenance and Monitoring 

Road-maintenance personnel need to be informed about critical design 

elements that may not be obvious—especially any grade controls, bank 

stabilization, or sediment control measures that may require occasional 

maintenance. Over time, road maintenance staff may be not only the 

caretakers but also the most regular monitors of crossing condition. 

Stream simulations are expected to have lower maintenance needs, 

since their larger size decreases the probability of them plugging and 

overtopping. Nonetheless, some maintenance needs will undoubtedly 

arise. Unforeseen watershed or climatic events and channel adjustments 

may occur, perhaps changing the simulated streambed in ways that impair 

passage. Floods exceeding the structure’s capacity certainly will cause 

a need for maintenance. All stream-simulation projects should prepare 

for maintenance and emphasize both monitoring and sharing monitoring 

results as a way of improving these design methods as rapidly as possible. 

This guide covers maintenance and monitoring only briefly (section 8.3.2), 

despite their importance. Maintenance, continued monitoring observations 

over time, and documentation are essential to further development of 

stream-simulation technology. Early stream-simulation design replacement 

structures should be monitored intensely to improve our understanding and 

knowledge of the stream-simulation assessment, design, and construction 

process. Such monitoring will ensure that (1) mistakes are not repeated 

on future installations and (2) knowledge gained on techniques and 

interpretations is applied on future installations. 
4Initial Watershed 

and Reach Review

4.1 Review the Road Context

4.2 Review Resource Values

4.3 Evaluate Watershed Risk Factors

4.4 Conduct the Initial Site Reconnaissance

4.5 Assess Site Suitability

4.6 Defining Project Objectives and Initial Design 

Concept

4.7 Document Your Findings

4.8 Initial Review Example
Stream Simulation

Figure 4.1—Steps and considerations in initial watershed and reach review.

Review the road context

lAccess needs

lRoad location

lRoad management objectives

lLandownership and partnership potential

Review watershed and site resource values

l Aquatic species, habitats, and conditions

l Terrestrial animal passage needs

l Flood-plain values

l Water uses

Evaluate watershed-scale risk factors

l Geomorphic hazards

l Event history

l Past and projected land management

l Crossing maintenance history

l Channel stability

Evaluate site risk factors

lChannel stability

lPotential for blockage by debris, ice, and/or sediment

lFlood-plain constriction

lLarge elevation change across existing structure

lChannel sensitivity to change

Evaluate site suitability

Establish project objectives

l	Traffic access requirements

l Degree of stream continuity

l	Degree of flood-plain continuity

l Aquatic and terrestrial animal passage requirements

l Channel restoration

RESULTS

Site suitability evaluation

l Type of crossing

Broad project objectives 

l Full aquatic organism passage

l Terrestrial wildlife passage

l	Full flood-plain continuity

l Channel restoration, etc.

Steps and Considerations in Initial Watershed and Reach Review
Chapter 4—Initial Watershed and Reach Review

The first phase of the crossing-design project is the watershed-scale

review and site reconnaissance (figure 4.1). It can be completed quickly

at low-risk sites where stream and watershed conditions are well known.

The process applies to replacements, removals, and new installations, and

much of it applies to any crossing, whether or not it is a stream simulation.

The questions to answer in this phase are:

	l	Is the site suitable as a crossing location? Determining site suitability

is mostly a matter of weighing risks and consequences. The team

can learn a great deal about risks and environmental consequences

in this phase by synthesizing historical, management, and watershed

condition information. That information, along with a site walkthrough, is usually sufficient for identifying sites that are unsuitable

for any rigid structure and unsuitable for stream simulation.

l	What are we trying to achieve with this project? Setting realistic

project objectives requires knowledge of watershed and road network

conditions that only a broad-scale review can provide. Setting

realistic objectives also requires some understanding of the stream

reach, which you can get from a quick reconnaissance of the site.

Project objectives may later be validated, stated in more detail, or

changed in light of new information.

	l	Do site characteristics and project objectives lend themselves

to stream simulation? The feasibility of using stream simulation

depends on both project objectives and site conditions. In this rapid

initial review, you can identify some important site conditions

that might make stream simulation infeasible or complicated, and

decide whether to pursue stream simulation as an option. The broad

overview also will indicate how complex the project is likely to be.

4.1 Review the Road Context

Note: Because most Forest Service crossing projects today are on already

existing roads, this guide usually assumes the crossing-design project is

for a replacement. For new crossings and crossing removals, the steps and

considerations are essentially the same.
4—2

Stream Simulation

Before planning a crossing replacement, always ask the questions: 

Is the road necessary? Is there a better location for the road and/

or crossing?

Consult existing planning documents, such as the area roads analysis 

and pertinent watershed analyses. Those analytical efforts should 

show: 

l Location and type of the resources the road accesses. 

	 l Long-term access needs in the area. 

	 l Expected future development and its effects on road use and 

stability.

	 l Road standard needed. 

	 l Stability and appropriateness of the current road location. 

This information allows a reasonable evaluation of the long-term 

need for the road and whether it justifies expected maintenance

requirements. 

If a road analysis has been done (section 2.1), it will indicate whether the

road should remain at its current location or could be relocated. If not,

make those determinations before continuing.

Review road management objectives to identify traffic access

requirements—an important component of the crossing project objective.

What transportation needs are to be served, at what standard, for how long,

at what cost? For some seasonally closed roads on intermittent streams,

a ford or other low-water crossing may suffice. If a road is being closed

or put into long-term storage, removing crossing structures might be an

option until the road reopens. Roads that must stay open during all but the

largest floods will require a structure that reliably passes not only large

floods but also the sediment and debris they carry. Safety is a primary

consideration.

After reviewing land ownership in the area, identify potential partners for

passage and habitat restoration among downstream or upstream property

owners. Other interested parties—such as watershed councils, county road

departments, and wildlife interest groups—might be possible partners.
4—3

Chapter 4—Initial Watershed and Reach Review

4.2 Review Resource Values

To build an understanding of the degree of passage required at a site,

compile existing information on watershed- and site-resource values.

Background information might come from stream surveys, watershed

inventories, special uses databases, and the personal knowledge of

forest specialists, among other sources. Where the crossing is a passage

barrier, habitat value for upstream reaches is an especially critical piece

of information. It helps establish the context and priority of a possible

passage-restoration project. If existing information is not adequate, do the

necessary field investigations.

Examples of potential resources values might include:

l	Threatened or endangered aquatic species.

l	Excellent or rare aquatic habitats (both up- and downstream of the

crossing) that need protection from excessive sediment and other

pollutants at all costs.

l	Terrestrial animal travel routes (for example, the valley is an

important migration corridor for large mammals).

l	Specialized flood-plain habitats (for example, ground-water-fed

channels provide crucial cool-water refuges for fish).

l	Flood-plain water storage for flood attenuation, maintenance of base

flows, and maintenance of riparian habitats.

l	Domestic, municipal, or irrigation water supplies.

l	Cultural or archeological resources.

l	Recreation.

l	Aesthetics.

Where high-value or unique resources could be affected, the consequences

of partially blocking movement of animals, water, sediment, and/or

debris may be unacceptable. Where severe consequences combine with a

high risk of crossing failure, such as in areas subject to debris torrents,

consider relocating the crossing to a more suitable location. The value and

sensitivity of the resources at risk are also two of the factors that dictate

the level of effort that should go into the design and the degree of stream 

continuity the crossing should provide (see also section 4.6).
4—4

Stream Simulation

4.3 Evaluate Watershed Risk Factors

Take a “big-picture” look at large-scale watershed conditions and

processes that have or can influence the crossing reach. Some of them are:

l	Geologic or geomorphic hazards.

l	History of flooding and geologic/geomorphic events.

l	Past, current, and anticipated land management in the contributing

watershed.

l	Regional channel instability (for example, downstream channel 

incision; see appendix A.7.2)

Together with a field visit to the site, the watershed background

information provides a basis for understanding how the channel has

responded to watershed events in the past. This knowledge, in turn, helps

predict the direction and degree of future channel change. Predicting

future changes is critical because stream-simulation structures must

accommodate future streambed changes. Key questions include:

l	What events and processes led to the current channel form? Is the

channel stable, or is it still adjusting to past events?

l	What watershed changes are likely during the life of the structure?

How might they affect runoff and sediment loads?

l	What channel changes are likely during the life of the structure? How

will the stream respond to large floods?

To answer these questions, it helps to know what the watershed has

delivered in terms of floods, debris flows, droughts, etc., and how future

land use changes might change flows and sediment and debris loads. On

the site scale, it is important to know what current reach conditions are

and how responsive (sensitive) the reach is to changes in water, sediment,

and debris loads (see section 5.3). Depending on the complexity of the site

and the watershed, these interpretations can be hard to make. Someone

knowledgeable in watershed and channel processes should guide the team

in interpreting watershed and channel risk factors.

Note: Appendix A 

describes geomorphic 

concepts used in stream 

simulation.
4—5

Chapter 4—Initial Watershed and Reach Review

4.3.1 Geomorphic Hazards 

Research the geology, soil, vegetation, and hydrology of the general area.

Interpret these characteristics in terms of their likely effect on watershed

processes and site stability. If a watershed analysis has already been

completed, this information will be available. If not, tailor the detail of

the investigation to the apparent risks at the site. For example, a 3-footwide stream on a closed road may not require the same level of effort as a

20-foot-wide river on a highway.

Evaluate each site for its proximity to potentially unstable landforms that

could dramatically change sediment and debris loading to the crossing

reach (see sidebar “Information Sources”). Look for features such as:

l	Slope stability problems such as landslides and earthflows.

l	Snow-avalanche chutes.

l	Debris torrent-prone channels.

In addition, the site itself may be located on an inherently unstable

landform susceptible to sediment deposition or erosion (for example,

alluvial fans, deltas, coastal bluffs). Geologic materials may be highly

prone to erosion, such as unconsolidated glacial sands. These features raise

red flags about site stability.

Information Sources. Information sources commonly available 

on national forests are watershed analyses, access- and travelmanagement plans, aquatic-habitat inventories, geographic 

information systems layers, Infra (Forest Service database housing 

information about constructed features on national forests) and 

the Natural Resources Information Systems (NRIS) database. 

U.S. Geological Survey professional papers, water-supply papers, 

technical reports, and surface-geology maps are valuable resources 

for helping identify geologic hazards. In more populated areas, State 

and local agency maps and reports are often available. Land-type 

maps with descriptions of dominant geomorphic processes and 

hazards are available on some forests. Do not rely solely on published 

information. Field and aerial photo interpretations are essential in 

identifying geomorphic hazards.
4—6

Stream Simulation

 4.3.2 History and Location of Land Cover Changes and 

 Watershed Events 

Information needed includes:

l	Location of the reach in the watershed and in relation to landforms or

activities that could influence water, sediment, and wood input to the

channel such as: geomorphic hazards, in-channel gravel extraction

operations, large-scale riparian forest harvest, road and crossing

failures, dams, etc. 

l	History of watershed land use and road system.

l	Maintenance history at crossing site.

l	History of major hydrologic events such as fires, floods, mass 

wasting, and droughts.

l	Recent flood events.

l	Type and intensity of channel responses to those events.

l	Projected land use and road system changes in the watershed.

This historical information is the background needed to develop an

understanding of current reach condition as it relates to past events and

current watershed conditions (see figure 4.2 for an example). Is the reach

changing? How have past changes affected the existing crossing? What

is the direction of change? For excellent formal examples of this type of

historical watershed analysis, see Wissmar et al. (1994); McIntosh et al.

(1994); and Stillwater Sciences (2005).

Collect information on crossing maintenance and failure history to get

an idea of how well the existing structure has performed at the site. This

information will give an idea of channel processes that affect the crossing,

and help identify chronic problems that the new structure should solve.

In addition, analyze how runoff timing and amount and sediment loads

may change in the future as a result of expected watershed events such as

fires, landslides, or development. Project how the reach may respond to

those changes.
4—7

Chapter 4—Initial Watershed and Reach Review

Figure 4.2—Flood-damage surveys can provide historical context for stream 

condition. (a) On Gap Creek in northeastern Washington, extensive erosion 

occurred on a riparian road in unconsolidated glacial sands during a 1993 flood. 

(b) Sediment filled the channel for several years but this transport channel 

remained stable and the sediment progressively cleared out during subsequent 

high flows. 

4.3.3 Offsite Channel Stability

Instability elsewhere in the watershed can affect a crossing structure over

time. For example, a headcut could migrate upstream and undermine a

structure. (Refer to appendix A, section A.7.2 for a discussion of headcuts

and channel incision.) Alternatively, if an upstream reach is unstable, it

could dramatically increase sediment and debris loading to the site. Since

the crossing structure will have to accommodate any large, enduring

changes in the channel, it is important to predict the magnitude, direction,

and timing of likely channel changes.

(a)

(b)
4—8

Stream Simulation

Detecting significant channel instability in the watershed is not always

possible without field work. Where forest cover is not too dense, a

time series of aerial photographs can show changes in channel reach

planform and instability. Photos might show noticeable change in channel

width, rapid growth and movement of depositional bars, and growth of

alluvial fans at tributary mouths (Grant 1988). These changes frequently

are associated with observable land uses such as mining, agriculture,

subdivision and road development, or forest harvest. Channel incision is

a common type of regional instability caused by channel straightening,

gravel mining, or loss of an important grade control feature. Historical

accounts of stream and watershed conditions sometimes are available in

local libraries or from community elders.

4.4 Conduct the Initial Site Reconnaissance

With this background knowledge about the watershed and the road, the

project team should traverse the channel up- and downstream of the

crossing to (a) get a general overview of channel conditions in the project 

reach and (b) identify key geomorphic features and potential channel

stability concerns. The actual length of the reconnaissance depends in part

on how much information already exists about the stream. If good stream

surveys are not available, the reconnaissance may need to extend well

upstream from the crossing to evaluate the extent, accessibility, and quality

of habitat. If the team has confidence in the accuracy of the existing survey

information, walk the channel for at least 30- to 50-channel widths upand downstream of the crossing. The reconnaissance should be longer for

more responsive channels, such as where the streambed is more mobile, or

banks are sensitive to disturbance. Be sure to go far enough to confidently

assess channel conditions outside the existing structure’s area of influence.

“Read” the stream for clues about the magnitude of overbank floods and

channel-forming flows, the frequency and type of sediment transport

events, and other channel processes, such as debris transport, beaver

influences, bank erosion, streambed aggradation and degradation, and

general channel stability.(The sidebar provides a checklist of questions that 

might be a useful starting point.)

Identify unstable features that could affect the crossing, such as a sediment

wave progressing downstream, an unstable debris jam that could fail,

a potential landslide, or an active headcut. Consider how the crossing

is aligned relative to the stream and whether the alignment could be

improved. Be aware of recent large floods or other unique occurrences

that might affect interpretations of channel conditions. Observing how

the stream has responded to the existing crossing structure can help you

predict stream responses when the structure is replaced.
4—9

Chapter 4—Initial Watershed and Reach Review

 Initial Site Reconnaissance Tickler Checklist

Note: This checklist is not exhaustive. There are likely many other 

questions that should be answered in different environments. 

Modify it as needed. 

4 What effects has the existing crossing had on the stream? How 

high is the perch, if any?

4 How prevalent is woody debris? What role does it play in channel 

structure and stability? How stable is it? Does the riparian area 

provide a future supply of wood?

4	Is there a high-conveyance flood plain? Is there evidence of

scour, sediment, and wood deposition on the flood plain? Locate

side channels and swales. Are there culverts or dips at these 

locations?

4	What processes modify the channel (for example, debris flows,

meander shift, ice or debris jamming, beaver, etc.)?

4 Are the banks stable? 

4 What are the dominant streambed materials and how mobile are 

they?

4 Is culvert alignment creating stability problems (for example, 

with plugging, bank erosion)? Should alternative alignments be 

considered?

4 Is the channel a response or a transport reach? What channel 

type is it?

4 Are there natural or other barriers to aquatic species passage in 

the reach?

4 Are there solid grade controls (e.g., boulder weirs, bedrock 

outcrops, high-stability log weirs) in the reach? These locations 

can function as end points for the longitudinal profile surveyed in

the site assessment (chapter 5).

4 Is the downstream reach incised? If so, should the crossing be 

retained as a grade control? 

4 Is there a reach similar to the project site nearby that might be a 

potential reference reach?

4 What features might constrain construction activities at the site? 

4 Are there specialized habitats that require protection during 

construction?
4—10

Stream Simulation

During the site reconnaissance, think through the elements of streamsimulation design (described in chapter 6) to verify that stream simulation

is actually feasible at the site. Sketch a plan-view map of the channel

and adjacent flood plain or valley side slopes. Annotate the map with

observations, such as location of high flow marks, severe bank erosion,

and bedrock outcrops. (See section 5.1.1 for more discussion on sketch

maps.) Now is a good time to establish photo points. If multiple site visits

become necessary, there may be opportunity to photograph the site at

different flows. Locate the photo points on the sketch map, and mark them

in the field.

Most importantly, focus on the stability of the existing channel and

its responsiveness to water and sediment inputs from natural and

anthropogenic disturbances. Since a stream-simulation design must

accommodate the potential range of channel adjustments during

the service life of the replacement structure, channel stability and

responsiveness to disturbances strongly affect the design. In general,

response reaches are more sensitive than transport reaches. As described in

appendix A, section A.2, response reaches tend to have finer, more erodible

materials, and are more prone to sediment deposition, channel widening,

channel scouring, and channel migration. Knowledge of channel types

(appendix A.6) can often help with interpreting channel responsiveness.

During the site assessment (chapter 5), channel characteristics affecting

responsiveness and stability will be fully documented, but some channel

characteristics and geomorphic settings that can complicate design are

easily observable during the initial walk-through (see sidebar “Reach

Conditions”).
4—11

Chapter 4—Initial Watershed and Reach Review

 Reach Conditions Requiring Special Consideration 

l Existing structures with large elevation drops (perched).

l	High flood plain-conveyance.

l Active lateral channel migration. 

l Depositional reaches: alluvial fans, braided streams, concave 

stream reaches.

l Channels with large amounts of woody debris, especially 

channels prone to debris flows or within a debris-flow runout

zone. 

l Channels prone to icing.

l	Channels with unusual flow regimes, such as estuarine channels

with tidal influences, glacial-meltwater channels, palustrine

(wetland) channels where ground water and area flooding are

important influences, tributary channels backwatered by the

mainstem.

l Channels with intermittently exposed bedrock.

l Unstable channels (laterally or vertically unstable).

These channel characteristics and geomorphic settings are not 

universally or equally hazardous. In most situations, designs that 

mitigate risks to acceptable levels are feasible. Usually, mitigating 

designs will affect project costs to some degree, so be aware from 

the outset that these conditions may entail additional costs. 

Descriptions of channel characteristics and geomorphic settings requiring

special consideration along with some of their field indicators follow:

Where substantial aggradation above and/or incision below the existing

structure have occurred, the replacement structure design needs to address

the large change in streambed elevation. Such situations can compromise

the feasibility of stream simulation, and their implications are analyzed

in full detail during the site assessment and design phases (chapters 5 and

Existing structures 

with large elevation 

drops
4—12

Stream Simulation

6). Documenting the situation now alerts managers that the design may

require more than the usual care and effort. If the existing structure is

functioning as a grade control on an incising channel (see appendix A.7.2),

the team will need to consider whether to preserve the grade control.

High flood plainconveyance Overbank flows may transport large quantities of sediment and debris

on high-conveyance flood plains. These sites require special design

elements to avoid putting the simulated streambed at risk by concentrating

floodwaters through the crossing structure (see section 6.5.1.1).

Geomorphic evidence of substantial flow on the flood plain includes:

scoured channels or swales, slack-water sediment deposits, buried

vegetation, trees scarred by floating debris, and small debris accumulations

upstream of obstructions.

Active lateral 

channel migration 	 Rapid channel shifting across the valley floor may cause alignment

problems for the crossing and structure design will need to account for the

rate and extent of lateral migration (figure 6.4).

Estimate channel-migration rates from historical aerial photographs,

anecdotal information, and/or field observations, although the first

two techniques may be difficult to use in small channels obscured by

vegetation or located in remote areas. In meandering channels, consider

the following characteristics when evaluating the risk of channel migration

in the field:

l	Condition, type, and successional stage (age) of vegetation on

channel banks and bars. (These can sometimes indicate the rates of

shifting and heights of flooding; for example, age of vegetation on

existing point bars can indicate rate of bar growth. The root strength

of bank plants with dense and/or deep rooting habits can limit

channel shifting.)

l	Presence of a cutoff channel, abandoned channel, or swale along an

inner channel bend (on the point bar).

l	Composition and stratigraphy of bank materials. (Are bank sediments

cohesive or noncohesive? Are certain layers more resistant or

susceptible to erosion?)

l	Evidence of active bank scour on the outside of bends, such as pieces

of bank, exposed root masses, or fallen whole trees or shrubs lying

at the bank toe or in the stream. (Be careful not to confuse channel

migration with bank erosion resulting from sediment accumulation

above an undersized culvert that has forced flow against one or both

banks.)
4—13

Chapter 4—Initial Watershed and Reach Review

l	Recent sediment deposition on point bars that has partially buried

vegetation.

l	Large in-channel debris accumulations, with evidence of flow

diversion onto the adjacent flood plain or terrace surface.

l	Extreme angles of stream approach to a culvert inlet. (These may

indicate (1) that the stream has migrated since the existing structure

was built, (2) that sediment deposition upstream from an undersized

culvert initiated local bank erosion, changing the stream’s angle of

approach, or (3) the crossing was poorly aligned with the stream

when installed.)

Some channel shifting in the immediate vicinity of a crossing may have

been caused by the original crossing alignment. For example, where a

straight culvert replaced a meander bend, the stream may have responded

by eroding banks and developing new meanders to restore the original

channel length. The severity of this response depends on the amount of

channel shortening and the composition of streambed and streambank

material.

Channel migration is likely to be slower on moderately entrenched and

entrenched channels because the shifting channel must erode higher

banks. However, it can happen. For example, debris jams that backwater

the main channel can force water to overtop the adjacent terrace and incise

into the surface. If the process continues, it can lead to channel avulsion.

Depositional reaches 	 Braided streams, alluvial fans, and reaches where stream slope flattens

tend to experience lateral channel shifting due to aggradation or sediment

deposition on bars (figure 4.3). Review the aerial photos of the watershed

above the reach, looking for active sediment sources, areas prone to mass

wasting, etc. Consider how past land uses in the watershed affected erosion

and sedimentation rates, and how expected land-use changes may affect

them in future. Keep in mind that sediment deposition may be chronic

(for example, land use may increase upstream bank erosion and long-term

sediment supply) or episodic (for example, occasional landslides).
4—14

Stream Simulation

Figure 4.3—Depositional reach on Kiowa Creek, Colorado. The channel shifted 

location across the valley bottom during a flood several years before this 

photograph was taken, when aggradation put additional erosive pressure on 

banks. 

In general, it is far better to avoid locating a road on an alluvial fan. The

potential for sediment deposition and channel shift on fans makes for

severe maintenance headaches. If an alluvial fan location is unavoidable,

observe the upper, middle, and lower sections of the fan for recent

sediment deposition activity or active channel incision. Coarse sediment

from the watershed may be actively depositing during flood events near

the upper portion of the fan. The channel may split into poorly defined

distributaries as it flows down the fan, and their locations may change

as deposited sediment and/or debris jams block them. On some fans, the

stream may have incised through the fan deposits, so that deposition is

occurring further downstream. These observations help determine the least

active section of the fan—the best place to locate the road crossing in a

difficult geomorphic setting. However, this least active section of the fan

may still have the potential to become more active during the service life

of the structure.
4—15

Chapter 4—Initial Watershed and Reach Review

Channels with 

large amounts 

of woody debris 	 Observe the presence, stability, size, and accumulation potential of wood

in the project reach, especially upstream of the road crossing. If large

wood is abundant in or near the channel, wood may play an important role

in maintaining channel stability and controlling grade. It may also pose a

risk to the replacement structure.

The following questions help in evaluating woody debris risks and roles:

l	Are there individual wood pieces or large woody debris structures in

the channel? Is the woody debris well anchored, or is there evidence

of recent transport? Are most of the wood pieces generally longer

than channel bankfull width? (Pieces longer than bankfull width

typically have limited mobility.)

l	Is the wood mostly solid and likely to last, or is it decaying and

subject to being washed away?

l	If the watershed has a history of wood-dominated debris flows, is the

crossing within the projected debris-flow runout zone?

l	If steps in the channel are maintained by woody debris, how stable

are the steps? (see figure 4.4).

l	Are there low-gradient channel segments with unusually fine bed

material? (Check to see if these channel segments are controlled by

embedded pieces of wood. Especially in fine-grained channels, even

small pieces of wood can contribute to channel bed stability.)

l	Do trees border the downstream channel assuring continued wood

inputs to the channel? Do downstream channel conditions and

stability depend on upstream woody debris inputs? (If so, wood

transport through the crossing structure may be critical to the longterm stability of the whole reach.)

l	Has woody debris been previously removed from this stream for fish

habitat improvement, flood hazard mitigation, etc.?

Table 4.1 shows simple criteria for assessing the risk that woody debris

may plug a crossing structure. Reaches may have any or all of the

characteristics described for a particular class.
4—16

Stream Simulation

Table 4.1—Qualitative criteria for assessing the risk of plugging by woody debris at a road-stream crossing structure

Woody Debris Risk Description

LOW l Debris mostly absent or well anchored on banks and in channel. 

 l Debris dispersed uniformly along the reach (i.e., it has not moved).

 l Available wood is much larger than the stream’s ability to move it (i.e., large 

trees in small streams). 

 l Little or no wood available for local recruitment. 

 l Bed material not anchored by debris. 

 l Woody debris likely to remain at or near source area.

MODERATE l Most wood pieces anchored in the channel bed or channel banks. 

 l Potential for local recruitment of wood.

 l History of occasional maintenance to remove wood at the crossing. 

 l Small translational slides or undercut slopes adjacent to channel. 

HIGH l Unstable accumulations of woody debris present along banks, gravel bars, and 

channel constrictions. 

 l Most wood pieces not anchored to bed or banks.

 l Considerable wood available for local recruitment.

 l History of frequent maintenance to remove wood at the crossing.

 l Upstream watershed susceptible to debris flows.
4—17

Chapter 4—Initial Watershed and Reach Review

Figure 4.4—(a) A wood-controlled step exhibiting high stability. Note the largediameter logs embedded in the bank. (b) A wood-controlled step exhibiting 

moderate stability, Mitkof Island, Alaska. (c) A wood-controlled step exhibiting low 

stability, New Hampshire. Note the small-diameter pieces and lack of embedment 

in the bank. 

Channels prone

to icing In cold regions, ice can play havoc with crossing structures, especially

on low-gradient streams. During spring breakup, moving ice can hit and

damage a structure. Ice jams can also dam the channel, potentially causing

floodwaters to overtop the road. These problems are most common on

perennial streams and near lake outlets. In wetlands, ground water

seeping from streambanks can build thick layers of ice that sometimes

reduce the size of culvert openings.

(a) (b)

(c)
4—18

Stream Simulation

Field evidence that ice jams and accumulations may pose a risk includes:

l	Ice-impact scars on the upstream side of trees (on banks or

overhanging the stream). These can be several feet up the tree

because of ice dam break-out floods.

l	Isolated piles of gravel or cobbles on the banks or flood plain

before spring runoff. Sediments overlie snow, ice, or last year’s old

vegetation.

l	Blocks of ice present on banks after spring thaw, especially

near meander bends, on point bars, and above natural channel

constrictions.

l	Discontinuous scour holes or channels that begin on the flood plain

away from the stream bank, then join the main channel downstream.

l	Weeping cut banks or wetlands next to crossings.

	To determine winter-ice thickness in the area, see USACE (1999).

Channels with

unusual flow regimes	 Designing a stream-simulation crossing (a stable channel with streambed

characteristics similar to the natural channel) requires the flow regime

be well understood, whatever that regime may be. Some unusual flow

conditions make design more difficult because of their unpredictability

(for example, glacial meltwater, backwatered tributary). The fine-grained

bed materials common in palustrine and estuarine channels can limit the

feasibility of constructing an embedded culvert.

Channels with

intermittently

exposed bedrock 	 Many times intermittent bedrock is a design advantage, because it limits

the extent of vertical channel adjustment after placement of the new

crossing. However, it also can be a problem. For example, if undetected

until construction, bedrock can be a surprise obstruction to placing a

culvert at the correct elevation. Likewise, if a crossing happens to be

located just downstream of a natural bedrock barrier that is now buried

under the backwater sediment wedge, the new installation will exhume the

barrier.

The important thing is to notice the presence of shallow or intermittently

exposed bedrock during the walk through. The team can then plan to

determine its extent and design for it.
4—19

Chapter 4—Initial Watershed and Reach Review

Unstable channels	 Stable channels vary from nearly static and unchanging to highly dynamic

and adjustable. Distinguishing highly dynamic but stable channels

from unstable ones can be difficult (see appendix A, section A.4). Truly

unstable channels are undesirable locations for stream crossings. They

are particularly undesirable for stream-simulation crossings because of

the need to project the changes that are likely to occur over the crossing

lifetime, and design for them. There may be no stable reference reach for a

design template.

Assess overall channel stability outside the influence of the existing

crossing. A single indicator of instability is not necessarily conclusive

by itself. Look for other geomorphic evidence along the length of the

reach that confirms or challenges your conclusion of channel instability.

Indicators of stability or instability should be consistent throughout

the reach. In addition, use stable channels in nearby similar landscape

positions as benchmarks for comparison.

Recent sediment deposition may suggest a channel is unstable and

undergoing aggradation (Pfankuch 1978; Copeland et al. 2001) (figure

4.5). Field evidence can include the following:

l	Large, mid-channel bar deposits that have little or no vegetation.

l	Loose bed material with fresh surfaces.

l	Unusually high percentage of fine material on the streambed.

l	Little difference between surface and subsurface streambed materials;

poorly armored streambed.

l	Flood-plain vegetation buried by deposited sediment.

l	Upland dry-site vegetation located low on the bank or dead on the

flood plain (indicates recent channel filling).

Evaluating bank stability is often key to determining whether a channel is

stable or unstable. Field evidence can include:

l	Substantial and consistent bank caving, toppling, or slumping.

l	Irregular channel width and scalloped banks.

l	Unstable undercuts.

l Tension cracks at elevations above bankfull.

l	Shallow-rooted, sparse, or weak bank vegetation.

l	Artificial bank armoring (riprap) may indicate past bank instability.
4—20

Stream Simulation

High, unstable banks can also be associated with channel incision or

gullying (figure 4.6). If a headcut has reached the existing culvert, you

may find a distinct difference in bank height and stability between the

up- and downstream channels. (See appendix A.7.2 and section 5.3.4

for descriptions of typical channel type changes associated with incising

channels.)

Figure 4.5—Massive gully erosion upstream (figure 4.6) caused channel filling 

and flood-plain sedimentation in this depositional reach, eastern Colorado. 

Figure 4.6—Channel widening after recent incision, eastern Colorado. 
4—21

Chapter 4—Initial Watershed and Reach Review

One useful procedure for rapidly assessing channel stability in the vicinity

of road-stream crossings is by Johnson et al. (1999). Their procedure,

which builds on several earlier methods (Pfankuch 1978; Simon and

Downs 1995; Thorne et al. 1996; Rosgen 1996), is based on 13 qualitative

and quantitative indicators, each of which is rated with a point system

(table 4.2). These ratings are weighted and added, producing an overall

stability rating for the channel at the crossing. Some of the site variables

(11 through 13) help in evaluating channel response to the existing

structure. Johnson et al. (1999) provide guidance on interpreting the

results to identify the type of instability (lateral, vertical, large transport/

deposition of debris or sediment) and stabilization needs at the site. Any

reach-based assessment procedure like this should be interpreted in the

context of larger-scale stability issues, such as regional incision. The team

can then focus its efforts during the detailed site assessment on the major

risks at the site.
4—22

Stream Simulation

Table 4.2—Stability indicators, descriptions, and ratings (Johnson et al. 1999, used with permission of the American 

Society of Civil Engineers) 
4—23

Chapter 4—Initial Watershed and Reach Review

4.4.1 Construction Issues

During the initial review, identify features that might limit construction

access. Show them on the site sketch, and flag them to ensure that the site

assessment survey will include them. Such features include:

l	Utility corridors, buried utility lines.

l	Wetlands.

l	Soft soils.

l	Critical habitats.

l	Steep slopes.

l	Rights-of-way.

l	Property boundaries.

l	Existing landings, opportunities for storage and staging areas.

l	Roadway lines-of-sight.

4.5 Assess Site Suitability 

The team can now make a first assessment of site suitability for the

crossing. Again, if possible, avoid locations where rapid channel change

can be anticipated (figures 4.7 and 4.8). Crossings in dynamic reaches

have a higher potential for failure than a stable site. If the consequences of

failure would also be high, seriously consider relocating to a more stable

site. The cost of moving the road may be more than offset by the lower

risk of damage to the road or to high-value habitats and by the lower

maintenance requirements.

Although stream simulation is possible at many risky sites, special design

considerations are necessary. To mitigate such risks, make every effort to

thoroughly understand current stream conditions and potential changes

during the life of the project. Designing a structure that accommodates

those changes and minimizes the potential for and/or the consequences

of failure at such a site will take more effort and care. Both the design

process and the structure itself may be more expensive than at simpler

sites.
4—24

Stream Simulation

C

Longitudinal Profile View

Plan View

D

B C to D

Transitional zone from

C channel type to 

D channel type. Existing road

Optimal crossing

location

Optimal road Location

Brewster Creek Road Culvert Replacement, 

Lolo National Forest, Montana

Example provided by Traci Sylte

Where Brewster Creek exits its narrow valley onto a wider, flatter flood plain, it deposits sediment

and forms an alluvial fan (figure 4.7). The Brewster Creek road crosses near the head of the fan

where sediment begins to deposit as the grade flattens.

Figure 4.7—Brewster Creek 

crossing plan-view sketch. Original drawing by Traci Sylte.

The previous culvert, approximately half as wide as 

the bankfull channel, was full 

of sediment. As a result, the 

stream frequently overflowed

the road. The forest replaced 

the culvert with a new bottomless box culvert in the same 

location. The new structure, 

which spans the bankfull 

width, was designed for fish

passage. It was also designed 

to pass the 100-year flow, with

some free board under the 

deck.

Figure 4.8—Brewster Creek road replacement box culvert, filled to 85 percent of its 

rise after 1 year.

The year after construction, the new 

culvert also filled with sediment to

about 85 percent of its rise. The stream 

still overflows the road frequently. A

simple recognition that the crossing 

was located in a depositional zone, 

coupled with an easy road-location 

change to only 150 feet upstream 

(figure 4.7), could have avoided this

problem.
4—25

Chapter 4—Initial Watershed and Reach Review

 4.6 Defining Project Objectives and Initial Design

 Concept

Together with considerations of traffic access needs, maintenance

requirements, safety, and funding, the geomorphic hazards and ecological

values identified during the initial review provide the basis for defining

preliminary project objectives. These objectives are preliminary because

they may change as the team learns more about the site constraints

and opportunities during the site assessment (chapter 5). Throughout

the predesign phases of the project, the entire team—as well as the

manager—should be involved as objectives are set or revised in light of

new information. In cases where objectives conflict, priorities may be

reshuffled. To make sure the objectives and priorities are clear and that

all participants understand them in the same way, write objectives, and

document any changes as they occur.

Objectives should respond directly to the risks and resource values

associated with the project—by minimizing both the potential and

consequences of failure, in accordance with the importance of the

resources. For example, if conditions force a crossing to remain near highquality spawning habitat, an important objective would be to minimize the

risk of degrading that habitat; the project team might therefore consider a

lower-risk structure, such as a valley-spanning bridge. If regional channel

incision is occurring, one objective may be to preserve the crossing as a

local base-level control. To minimize the risk to aquatic populations, at

least partial passage could be provided by installing a bypass fishway or a

fish ladder.

Some examples of ecological project objectives follow. Refer back to

section 2.4 for a more detailed discussion of these objectives. [Road safety,

traffic interruptibility, and other transportation system objectives also enter

into a full objectives statement.]

l	Provide passage for aquatic organisms.

l	Minimize the risk of culvert plugging. On channels where the risk of

plugging by wood, sediment, or ice is very high, objectives might be

to minimize both the probability of plugging (by providing a large

opening) and the consequences (by designing the structure to sustain

overtopping flows and prevent stream diversion).

l 	 Maintain flood-plain functions and continuity. Where flood plains

have important habitats formed during overbank flows, maintaining

the natural flooding regime and providing for flood-water continuity

down the valley may be important.
4—26

Stream Simulation

l	Accommodate channel shifting. Where meanders are migrating

rapidly across the flood plain, design the structure to accommodate

channel movement as much as possible (see section 6.1.1.3).

l	Provide terrestrial wildlife passage. Accommodate animals that use

riparian areas for movement where traffic volume and/or fill height

make crossing the road infeasible.

l	Maintain grade control. Where a headcut is progressing upstream

and the existing crossing is protecting upstream habitats, you may

decide to maintain that protection. You might make the same decision

where an undersized culvert backs up water and sediment, creating

an unusually valuable wetland habitat. In cases like these, stream

simulation may not be feasible, so the installation may require special

measures, such as a fish ladder, ramp, or side channel, to provide for

passage of some or all aquatic species.

l	Restore a degraded channel. Where a channel has incised downstream

of the existing culvert and degraded important habitat, an objective

might be restoring both passage and habitat. This work would involve

restoring the channel such that the transition across the road crossing

is as nearly seamless as possible.

l	Maintain a barrier against invasive exotic species. With this objective,

stream simulation is not a design option. Undersized culverts

sometimes function as partial or full barriers. Culverts not specifically

designed for exclusion, however, may not be 100-percent effective,

because some individual animals may be able to negotiate them at

some flows.

Identifying preliminary objectives does not imply that the final design

must fully achieve them. New information may cause the team to

modify them, and more detailed project objectives will be formulated

after the detailed site assessment. By this time, though, some of the site

conditions or objectives that preclude stream simulation as a design option

(maintaining a barrier), or that call its feasibility into question (maintaining

a grade control) are known. The team probably has an initial idea of the

type of structure (culvert or bridge) necessary for achieving the objectives.

Another result of the initial assessment is that the project’s complexity

is now known, and the team can judge the appropriate level of detail

for the site assessment and design efforts (see box below). Stable and

straightforward sites do not require great detail for ensuring structure
4—27

Chapter 4—Initial Watershed and Reach Review

stability and aquatic organism passage. However, where the risk factors

or project objectives make the project more complex or where traffic can

only be briefly interrupted during construction, a higher level of effort is

justified.

 Factors Determining Level of Site Analysis

1. Site history: Has the crossing structure failed before? Has it been 

a continual maintenance problem? What is the channel condition 

(historic and existing)?

2. Watershed history: Are there known active or historic geohazards 

(earthflow, landslides, etc.) in the watershed or in adjacent

watersheds with similar characteristics (rock types, soils, 

vegetation, climate)?

3. Location: Where in the watershed is the site located, and on what 

type of landform) alluvial fan, glacial outwash plains, hillslope, 

etc.)?

	4. Design life, road management objective, project constraints: Is

this a highway or a logging road? What is the desired design life 

of a the structure? Are options at the site constrained by power 

lines, rights-of-way, property boundaries, or other infrastructures?

5. Channel type: What is the channel type? Is it sensitive to 

changes or fairly stable?

6. Is the channel incised or incising?

	7. Consequences of failure: What will occur if the structure fails?

What is the spatial relationship to sensitive resources (fish,

riparian, vegetation, property, etc.), and how would failure 

impact them? What are the consequences of failure in terms of 

resources, monetary costs, loss of access, public safety?

4.7 Document Your Findings

Summarize the important findings from the watershed and reach review

in a convenient format (narrative, map, form) for the project file. This

documentation will continue to provide large-scale context and reminders

of important offsite conditions throughout the project process, and will

help you verify the level of detail needed for assessment. Include a

complete set of photos taken from permanently marked photo points.
4—28

Stream Simulation

4.8 Initial Review Example

The following Mitkof Island, Alaska, example shows how a Tongass

National Forest team documented the initial review and used it for risk

assessment, site suitability determination, validation of project objectives,

and preliminary decisions on structure type and design method. [The

example uses the Rosgen (1994) channel classification system.]

For this example, information gathered in the office included:

l	Location.

l	Existing structure.

l	Access and travel management.

l	Area description.

l	Geology.

l	Soils.

l	Vegetation.

l	Site history.

l	Slope stability.

The project team performed the following local-reach-scale assessments

during their reconnaissance field visit:

l	Channel types.

l	Channel stability.

l	Large woody debris risk.

l	Risk of sediment retention.

l	Streambank sensitivity.

l	Site proximity to important or sensitive resources.
4—29

Chapter 4—Initial Watershed and Reach Review

Initial Geomorphic

Assessment for

Crossing 6235-17.59 (Information provided by Bob Gubernick)

Location: Mitkof Island, Southeast Alaska, Road 6235, milepost 17.59.

Existing Structure: The existing culvert does not pass spawning adults

or juvenile salmonids due to a 1.9-foot perch at the outlet. Beaver activity

occurs in the area, with a dam located in the culvert inlet (figure 4.9). This

culvert is scheduled for replacement.

Figure 4.9—Existing culvert on Road 6235, milepost 17.59 (Tongass National 

Forest). (a) Culvert inlet. (b) Culvert outlet.

Access and Travel Management: Road 6235 is a permanent, high-use

mainline arterial road (maintenance level 3), so traffic interruptions cannot

be tolerated. The road must be safely passable by low-clearance vehicles in

all weather conditions.

Area Description: The site is in a narrow valley bottom below a uniform

hillslope. Descending the hillslope, the channel is steep and moderately

incised. It enters the mainstem channel soon after reaching the broader,

flatter flood plain. The crossing site is located near the slope transition

between the hillslope and the wide flood plain.

(a)

(b)
4—30

Stream Simulation

Interpretation: The site is a response reach that may be subject to

sediment deposition at the transition to a flatter slope. Large vertical

adjustments can occur.

Geology: The area is composed of sedimentary deposits (marine

greywacke, mudstone, and conglomerates), andesitic-to-basaltic volcanic

rocks, and regionally metamorphosed equivalents of these strata (source:

Gerhels and Berg 1992). 

Interpretation: Sedimentary and metasedimentary materials can vary

greatly in durability and are usually platy in shape.

Soils: The hillslope soil is in the Kupreanof series (origin is weathered

sedimentary rock). The valley bottom soil is silty alluvium (source: forest

GIS layer).

Interpretation: Kupreanof series soils have high silt contents. On

steep slopes, they are susceptible to translational landslides, which can

initiate a debris flow or torrent. Check slope stability characteristics.

Vegetation: The hillslope is dominated by a mixed conifer series (Sitka

spruce, western and mountain hemlock, cedar). The valley bottom is a

sedge and bog plant community adjacent to the main channel. A mountain

hemlock/blueberry series lies further from the channel (source: forest

GIS layer). The area is primarily pristine (99+ percent), with only a small

managed section (source: air photos 1985 and 1998). The forest anticipates

no new management activities.

Interpretation: All plant series are composed of dense, deeply rooted

vegetation that stabilizes banks and limits lateral migration.

Site History: The original culvert was installed in the late 1960s. Periodic

beaver activity has caused continual maintenance problems (source:

maintenance records and personal communication from maintenance

foreman).

Interpretation: Beaver activity will limit options. To minimize longterm maintenance needs, consider structures with wide openings such

as bridges or embedded box culverts with removable lids (vented

fords). To avoid making the crossing more attractive to beavers,

design will have to minimize road elevation.

Slope Stability: Air photos (1963, 1979, 1985) show no indications of

slope instability (landslides, debris flows).

Hillslopes above the site range between 18- to 36-percent slope, decreasing

to 16 percent on the lower slopes. The moderate slopes, available lowerslope run-out length of 1,500 feet, and lack of activity in 40 years of the
4—31

Chapter 4—Initial Watershed and Reach Review

photo record indicate that the site has extremely low risk from debris flow

or landslides (figure 4.10).

Interpretation: Slope stability is not a concern. Vertical clearance (to

accommodate debris flows) is not an issue. 

Figure 4.10—Map of slope classes above crossing. Slopes are mostly moderate 

in the upper watershed, and the risk of slope instability is low. Tongass National 

Forest GIS layer.

crossing
4—32

Stream Simulation

Channel Types:

l	Hillslope: high-gradient, step-pool channels composed of bedrock,

boulders, and/or cobbles (Rosgen A1a to A3).

l	Valley bottom (above site): riparian wetland; low-gradient pool-riffle

channel composed of silt and clay, with beaver activity (E6).

l	Valley bottom (below site): moderately sloped pool-riffle channel

composed primarily of gravels (C4).

Figure 4.11—1985 aerial photo.

crossing
4—33

Chapter 4—Initial Watershed and Reach Review

Channel Stability: The channel above the site is not visible on the 1963,

1979, or 1985 aerial photos (figure 4.11). Below the site, the channel

appears stable, with no observable change in the photos. Neither the

sequence of aerial photos nor the reconnaissance field visit shows any

evidence of rapid channel change in either the tributary or the mainstem.

Interpretation: No system-wide base-level adjustments are visible or

anticipated. No major adjustments in design are needed.

Large Woody Debris Hazard: Wood in the steep section of the channel is

large (greater than 1-foot diameter) and is generally either well-embedded

or in stable debris jams. Little debris transport is anticipated, and the site

is far enough away from the edge of the valley bottom that the risk of

plugging by large wood transported from upslope is low. However, the risk

of plugging resulting from beaver activity is high.

Interpretation: Opening should be large, because of beaver activity.

Risk of Sediment Retention: Hillslope: low (transport channel). Valley

bottom: high (response channel).

Interpretation: The beaver pond is an aggradational zone. If the pond

is removed, the fine material also may need to be removed for waterquality protection.

Streambank Sensitivity: Sensitivity is low for both uplands and lowlands.

Deep-rooted vegetation holds banks together both on the hillslope (mixed

conifers) and on the flood plain (sedge, berry brush, and occasional

conifer). Sedge and berry brush are extremely deep rooted and dense in the

immediate up- and downstream reaches.

Interpretation: Banks can adjust to minor changes without

destabilizing. Minor alignment changes should not pose a problem.

Site Proximity to Important or Sensitive Resources: Immediately

adjacent to site (30 feet downstream) is high quality salmon-spawning

habitat.

Interpretation: Proximity to spawning habitat means that site design

should have a high safety factor. Sediment control is a major concern,

given close proximity of the upstream pond.
4—34

Stream Simulation

Overall Risk Assessment: Based on the stability of hillslopes, the channel

types in the area, and on the photo record, overall risk is low.

Project Objectives:

l	Provide free passage for aquatic species, sediment, and woody debris

(stream-simulation design).

l	Use culvert or low-profile bridge if cost effective. (Keep approach

fills low. If selecting a culvert, design road for overtopping and

minimize risk of sedimentation from beavers’ plugging the culvert.)

l	Minimize the installation’s attractiveness to beaver by using as large

an opening as possible.

l	Remove beaver dam, but try to maintain some water depth upstream

if possible.

l	Minimize sediment released to the downstream spawning area during

construction and over time.

l	Maximize flood-plain connectivity by installing additional culverts in

side channels and flood swales.
5Site Assessment

5.1 Collecting Site Data

5.2 Analyzing and Interpreting Site Data

5.3 Project Site Risk Assessment

5.4 Document Key Design Considerations and 

Recommendations

5.5 Reference Reach: The Pattern for StreamSimulation Design
Stream Simulation

Sketch a planview map

Topographic survey 

l Site and road topography. 

l Channel longitudinal profile.

l Channel and flood-plain cross sections. 

Measure size and observe arrangement of bed materials

l Pebble count or bulk sample.

l Bed mobility and armoring.

l Bed structure type and stability (steps, bars, key features).

Describe bank characteristics and stability

Conduct preliminary geotechnical investigation

l Bedrock.

l Soils. 

l Engineering properties.

l Mass wasting.

l Ground water.

Analyze and interpret site data

l Bed material size and mobility.

l Cross section analysis.

Flood-plain conveyance.

Bank stability.

Lateral adjustment potential.

l Longitudinal profile analysis.

Vertical adjustment potential.

l General channel stability.

Document key design considerations and recommendations.

 RESULTS

Geomorphic characterization of reach.

Engineering site plan map for design.

Understanding of site risk factors and potential channel 

changes over structure lifetime.

Detailed project objectives, including extent and objectives 

of any channel restoration.

Design template for simulated streambed (reference reach).

Figure 5.1—Steps and considerations in site assessment.

Steps and Considerations in Site Assessment
Chapter 5—Site Assessment

After verifying that the site is suitable for a crossing and will probably 

be suitable for stream simulation, the next step is to conduct a thorough 

site assessment. In this phase, you will collect the topographic and other 

data necessary for designing both the stream-simulation channel and the 

crossing structure and road approaches. Crossing-removal projects require 

virtually the same set of data and observations. 

Interpret the additional information gathered here to predict how the 

structure and stream will interact, and to design a stable structure that 

avoids or minimizes adverse effects to the stream over the long term. 

Document your key considerations, findings, and recommendations. This 

work requires close communication among team members who are skilled 

in biology, geomorphology, hydrology, and engineering. A thorough 

understanding of channel form and fluvial processes—the basics of 

which are in appendix A—is essential for interpreting the site assessment 

information.

5.1 Collecting site data

Data collection for site assessment consists of surveying channel, valley 

and road topography, and tying the survey to observations of geomorphic 

and other features, including subsurface materials. Much of the assessment 

is aimed at understanding the site and the stream processes that will 

have to be accounted for in design of the new crossing. You need this 

understanding to predict channel changes expected over the structure’s 

lifetime and design for them. Again, the level of effort and detail should 

correspond to the complexity of the site and the risks associated with 

placing a structure there.

The second goal of site assessment is obtaining a model for design of the 

simulated channel—that is, characterizing the reference reach. However, 

the reference reach must have a slope very similar to the slope of the 

simulated channel, and that slope will not be known for sure until the 

project profile design is complete (section 6.1.2). The actual reference 

reach cannot be identified with certainty until after that first design step. 

There are two ways to handle this logistically: 

1. Enough data can be collected during the site assessment to 

characterize several potential reference reaches at different slopes. 

This avoids the need to revisit the site and collect additional data 

once the reference reach is selected during design (chapter 6).
5—2

Stream Simulation

2. After analyzing the project area survey and determining one or 

more potential slopes for the simulated streambed, identify one or 

more applicable reference reach(es) from the longitudinal profile, 

and return to the site to characterize their cross-section dimensions, 

entrenchment, bed material, etc.

Section 5.5 goes into detail on selecting the reference reach. Channel 

morphologic data needed for the reference reach are summarized there. 

Good documentation of the field observations is essential for interpreting 

the survey data, and a complete sketch map is a key complement to the 

narrative field notes. 

5.1.1 Sketch Map

Often the site sketch map will have been started during the initial site 

reconnaissance (section 4.4). More information should be added as 

site assessment progresses. The sketch map helps in evaluating road 

and channel alignments, and interpreting survey results. Draw the map 

approximately to scale, and illustrate the spatial relationship of the channel 

and flood plain features and their relation to the road-stream crossing. As 

you walk through the reach drawing the map, take the opportunity to flag 

key features, cross sections, bankfull elevations, flood-prone zone limits, 

etc., to ensure their inclusion in the topographic survey.

The sketch is a plan view of the project reach, showing: 

l Channel pattern (straight, meandering, or braided). On existing roads, 

attempt to estimate the location and pattern of the natural channel 

before the road was built.

l Channel and road alignments relative to each other.

l Channel width and variations in width.

l Channel units (pools, riffles, steps, etc.).

l Valley and flood plain features, such as side channels, width of the 

flood-prone zone, evidence of past flood elevations, terraces, valley 

slopes, abandoned channels, etc. It is sometimes possible to use 

abandoned channel segments to visualize the natural channel location 

and planform through existing crossings. 

l Valley features that might influence construction, such as wetlands, 

old roads, utilities, and property boundaries.
5—3

Chapter 5—Site Assessment

l Important stream features such as large boulders or bedrock, 

large woody debris structures, gravel bars, submerged vegetation, 

vegetation changes, eroding banks, on-bank trees, bank irregularities, 

bankfull elevation markers. 

l Location of detailed measurements, such as cross sections, pebble 

counts, and photo points.

l Survey instrument setup locations, benchmarks.

l Possible reference reaches (see section 5.5).

(For additional information and explanation regarding constructing a site 

sketch map, see Harrelson et al. 1994.) 

Newbury Creek Site Assessment—Sketch Map

The sketch map in figure 5.2 shows a crossing on Newbury Creek 

on the Olympic National Forest that we will follow through the site 

assessment process (figures 5.8 - 5.11 and 5.17). 

The dotted lines bordering the stream channel on the sketch indicate 

the edges of the valley bottom, where the flatter valley surface meets 

the steeper side slopes. Note that the stream is closely bounded by 

a high terrace (GLFL) upstream of the culvert, and there are several 

places where bedrock is exposed in the channel. Downstream of the 

culvert, the valley broadens and a low terrace and flood plain (FPLT) 

border the channel. The crossing is located at a transition where the 

bedrock-controlled channel changes to an alluvial one that is less 

confined. 

Upstream of the culvert, plane-bed segments are mixed in with poolriffle segments (see appendix A for descriptions of these channel 

types). Downstream of the culvert, the channel type is pool-riffle, 

with riffles dominating. Gravel bars on the inside of bends are narrow 

(that is, little sediment is stored in the channel), and woody debris is 

not present in large amounts. Log weirs installed in the mid 1980s 

and early 1990s to increase pool habitat are both upstream and 

downstream of the crossing.

The road crosses the stream at a slight bend in the channel. Upstream 

of the road, a riprap blanket on the left bank (facing downstream) 

indicates there have been some erosion problems. 

Later we will see how all of these observations enter into the site 

assessment recommendations for design. 
5—4

Stream Simulation

MAP LEGEND

GLFL

FPLT

PB

bedrock (basalt)

bankfull margins (QBF)

valley margins

eroded bank

Point bar

flow direction

drilling locations

pool 

Riffle

plane bed

glacio fluvial terrace

flood plain terrace

Figure 5.2—Example project site sketch map with valley cross sections. Newbury Creek, Olympic NF, WA. 

Redrawn from original by Dan Cenderelli.
5—5

Chapter 5—Site Assessment

5.1.2 Topographic Survey

The topographic survey has two overlapping objectives. It needs to 

include: 

1. The detailed topographic data the project engineer needs to prepare 

the site plan, structural design, and the construction contract. 

2. Geomorphic information required for designing the simulated 

streambed and tying it into the adjacent stream sections. Generally, 

this will involve a longer length of channel than traditional 

engineering site surveys at road-stream crossings.

Sometimes these two objectives are considered distinct from each other 

and two surveys are done separately. However, there are good reasons 

for doing a single integrated survey. First, any surveys must use the same 

elevation controls and benchmarks. Second, different team members have 

the expertise to observe different types of features and conditions. Working 

together on the survey is an excellent opportunity to exchange information 

and arrive at a common interpretation of site conditions and limitations.

This topographic survey can be seen as a standard engineering site survey 

expanded to include a longer reach of stream that may not be surveyed to 

the same level of detail. The engineering site survey is typically a radial 

survey in which points are not necessarily taken along straight transects. 

The product is a contour map. This part of the survey must extend far 

enough upstream and downstream from the road to support planning for 

alignment changes, channel restoration, and temporary road or stream 

diversions during construction. On the other hand, channel longitudinal 

profiles and cross sections, which are used for simulated channel design, 

are displayed as linear plots. If the radial survey covers the entire area in 

sufficient detail, the profile and cross sections can be generated from the 

digital elevation models. As the survey moves away from the worksite 

itself, however, it is more common to survey only those points needed for 

the longitudinal profile and cross sections. In either case, good notes and 

sketch map annotations are essential for identifying what each point is; 

without them the linear plots can be extremely difficult to interpret.
5—6

Stream Simulation

Topographic Survey Methods

The standard engineering site survey collects an array of three 

dimensional points that, when plotted, is detailed enough to create 

a contour map that accurately represents the landform and site 

features. The key is look at the terrain and visualize the locations 

of the points that will accurately depict the shape of the terrain, 

both horizontally and vertically, and then survey those points so 

that the topographic map accurately represents the actual terrain 

in the field. Be sure to include not only the obvious slope breaks 

in the channel, etc., but also include points that define swales 

and high areas in the general landform.

There are several methods accurate enough for site topographic 

maps:

Traverse and cross sections. One method is to survey numerous 

cross-sections of the channel and valley at selected locations 

along a traverse. Be careful using this method—a cross section 

must be taken at every horizontal or vertical change along the 

stream to accurately draw a terrain model from the cross sections. 

The cross section method is not as accurate as the radial survey; 

it works best when the landform is fairly regular.

Radial survey. The recommended method is to survey key points 

that are not necessarily along straight transects; instead, each 

three dimensional point is defined by azimuth, distance, and elevation from a control point or set of control points. The array of 

points defines the topography and features on the map. This type 

of survey usually is done with a total station, which combines a 

theodolite, electronic distance meter, and data storage device in the 

same instrument. Data collected with a total station is electronically 

transferred to a computer and the contour map is quickly generated 

using software.

Combining the radial survey with the cross-section method can be 

efficient when the channel survey extends beyond the area where a 

contour map accurate enough for site layout purposes is needed. In 

this case, use the radial survey close to the crossing where accuracy is more important, and survey linear cross sections further out. 
5—7

Chapter 5—Site Assessment

During the site survey, keep good notes and annotate the sketch 

map. The survey includes the following work items: 

l Establish two horizontal reference points for each control point. 

(A control point is where the survey instrument is set up.) Two reference points per control point allow the set-up location to be relocated later. Often it is convenient to locate reference points at each end 

of the roadway outside of the construction work area. 

l Establish vertical controls using temporary benchmarks. Benchmarks should be reoccupiable during and after construction. 

l Clear vegetation, but limit vegetation removal to only what is 

necessary for facilitating safe travel and seeing the survey target. 

(Avoid destabilizing banks and removing large amounts of stream 

cover.) 

l Survey all topographic break points. 

l Collect enough topographic points to accurately detail the site 

(both road and stream), including locations of hazard trees or 

trees to retain, probe/boreholes, utilities, and property lines. 

l Survey channel and valley features (thalweg, water’s edge, top 

and bottom of banks, foot of valley slope or terraces, key grade 

control features, steps, gravel bars, bedrock exposure, etc.) in 

accordance with guidance in sections 5.1.3 and 5.1.4. Take more 

points around bends than in tangent sections, and take points at 

the top and bottom of banks vertically very close together if you 

plan to use HEC-RAS or another step-backwater model.

l Ensure enough ground and stream coverage to allow for 

potential road or stream realignment.

After completing the field survey, most surveyors and designers 

use a digital terrain or contour modeling program—such as 

AutoCad Land Development Desktop, Terramodel, Surfer, or 

Eagle Point—to create a topographic map for the site. As these 

software packages use break lines to control the interpolation 

between points, topographic break points (top and base of bank, 

toe of roadfill, etc.) must be accurately identified and surveyed. Be 

sure to plot the surveyed points on the map so that the accuracy 

of the contour lines that the program generates can be checked. 

If the design engineer does not conduct the survey, (s)he should 

ground-proof the contour map before starting final design.
5—8

Stream Simulation

This guide does not go into further depth on standard engineering 

surveying procedures that are well documented elsewhere (see appendix 

A in USACE 2006). Instead, this guide focuses on the survey data and 

observations needed for designing the simulated streambed. These 

measurements and observations include: 

l Channel longitudinal profile, key grade controls, scour depths. 

l Cross-section channel geometry: top of bank, bottom of bank, etc. 

l Width and elevation of valley surfaces; flood plain inundation 

frequency and depth. 

l Streambed and bank materials.

l Channel and bank stability, sediment and debris processes.

5.1.3 Longitudinal Profile

The longitudinal profile is perhaps the single most valuable tool in the 

stream-simulation design process. It shows the natural channel gradient, 

the local gradient variability, the features controlling channel gradient, the 

depth and variability of scour, the length and spacing of channel units, 

such as pools, riffles, and steps, the length and depth of any accumulated 

sediment upstream from the culvert (channel aggradation), and the 

length and depth of channel scour downstream from the culvert (channel 

degradation). The longitudinal profile is necessary for determining the 

appropriate channel elevation and design gradient through the crossing, 

identifying a reference reach with a similar gradient, and determining the 

range of potential vertical streambed adjustment (vertical adjustment 

potential).

5.1.3.1 What and where to survey

Use survey equipment capable of 0.01-foot precision to survey the 

longitudinal profile. This kind of precision is required for surveying 

benchmarks and water surface slope. Take ground shots to tenths of a 

foot. Include the inlet and outlet invert of the existing structure, road fill 

boundaries, and the center point of the road.

Most longitudinal profiles have highly variable local slopes reflecting 

different channel units, such as pools, riffles, steps, and cascades (figure 

5-3). The survey should include enough points to clearly delineate these 
5—9

Chapter 5—Site Assessment

units and the streambed structures (steps, pool tail crests, etc) that 

control their elevations. As described in appendix A.5.5, these channel 

units typically occur in repeating sequences, with regular spacing 

between them. Delineating units on the longitudinal profile enables us 

to mimic their dimensions and spacing if channel units, such as steps, 

are constructed inside the culvert, and it permits us to tie the constructed 

streambed into the adjacent channel units. Table 5-1 lists channel points 

and features to survey and describe in the survey notes.

Given the importance of selecting the survey points and making accurate 

observations about them, the person who will be primarily responsible 

for interpreting the survey and designing the simulated channel should 

run the rod. For each survey point, identify the local channel feature (e.g., 

pool, riffle crest, base of step, etc.), and note other relevant characteristics, 

such as size, packing, shape, and stability of the particles. These notes are 

critical for interpreting the longitudinal profile survey later.

Generally, points for the longitudinal profile should be along the thalweg—

the deepest part of the channel and the main thread of flow. However, 

in some channels the thalweg is substantially longer than the channel 

centerline. In a meandering channel, for example, the thalweg swings to 

the pool near the outside of each bend, and thalweg slope can be much less 

than slope calculated from centerline length. In such cases, survey both 

thalweg and centerline points, distinguishing them with separate codes. 

Channel slope calculations will use the centerline points. 

Steep channels often have randomly distributed scour holes that are not 

in the main center of flow. On these channels, represent the thalweg 

by selecting points along the general trend of deepest flow rather than 

zigzagging across the channel from hole to hole. Also survey centerline 

points, especially at grade controls like step crests, and use the centerline 

distances to calculate channel slope between grade controls.
5—10

Stream Simulation

Figure 5.3—Typical measuring points needed to define the longitudinal profile

for a pool-riffle channel (a), step-pool channel (b), and cascade channel (c). The

plan view sketches show the approximate location of the main thread of water to 

survey. In the cascade channel, one would occasionally take a point on top of a 

rock to indicate the general height of the bed material. 

x

x

x

/

x = CENTERLINE POINTS

= THALWEG POINTS
5—11

Chapter 5—Site Assessment

Profile points 

(include all major slope breaks)

Beginning (riffle head or pool tail crest) and

end of riffles (figure 5.3a).Top and bottom of rock or wood steps (figure5.3b).Beginning, end (pool tail crest), and

maximum depth of pools (figure 5.3a and b)

Other Features

Features controlling grade and/or retaining sediment: large woody debris, small embedded wood, large rocks, beaver dams, etc.Changes in bed material sizeTributary junctionsZones of bank instabilityCross-section locationsBankfull or ordinary high water elevations at various points along the profileElevation and extent of sediment accumulation above undersized culvertDepth and extent of incision downstream of culvert

Notes

Note riffle head (pool tail crest) material, size and mobility.Note step material, size, embedment, stability.Note what is causing the pool: a channel bend, bedrock outcrop, woody debris jam, a step formed by boulders, cobbles, woody debris, etc.Note size, durability of material, and mobility.May be associated with steps, embedded wood, mid-channel bars, bedrock outcrops, local aggradation above undersized culvert, etc. Characteristics like lower gradient and general lack of bed diversity distinguish aggraded areas above culverts from the rest of the channel.Note tributary width, any sediment accumulation at junction.It is usually not necessary to survey these; just note their presence, height, materials, riparian vegetation, as needed.Select cross sections along the longitudinal profile to characterize channel variationsin width, depth, slope, bed material size, etc.).See box “Identifying Bankful Elevation” (section 5.1.4.2). Some jurisdictions requireshowing ordinary high water elevations on the site plan map.Field evidence might include gravel or sand bars. Bed may appear featureless or simplified as compared to the rest of the channel (due to burial of features).Field evidence might include coarser bed material than upstream of culvert; banks higher than upstream and possibly unstable; wider channel than upstream.

Table 5.1—Longitudinal profile points and observations.
5—12

Stream Simulation

5.1.3.2 Length of the longitudinal profile 

The profile should be long enough to display on paper the general profile 

of the reach, including any grade breaks. If the profile extends beyond the 

detailed survey area (section 5.1.2), ensure the surveys are tied together 

with a common datum. 

At most sites, the channel longitudinal profile extends 20-30 channel 

widths in each direction from the culvert. Generally, this ensures the 

profile meets the following criteria:

l Extends well beyond the influence of the existing crossing structure. 

l Includes several sequences of repeating bedforms, for example pools 

and riffles, to get a good representation of their length, spacing, 

and slope. Including the range of variability in channel slopes, 

scour depths, and bedforms gives you a good chance of including a 

segment that can be used as a reference reach.

l Extends beyond the length of stream expected to adjust (usually to 

downcut) when the existing structure is replaced. Crossings with 

large elevation drops might require longer surveys because a longer 

reach of stream might adjust to the crossing replacement. Wherever 

possible, end the survey at stable points that will limit vertical 

adjustment, such as bedrock outcrops or other stable features

The reference reach is discussed in detail in section 5.5. The reference 

reach has characteristics (most importantly slope) similar to those of 

the crossing segment if the road were not there. Generally, the reference 

reach is upstream and outside the influence of the existing crossing, and is 

included in the longitudinal profile. In fact the longitudinal profile is often 

long enough to include several options for the reference reach. In some 

cases, however, you may need to look for a better reference reach at some 

distance from the crossing. The actual reference reach will be selected later 

based largely on the design slope through the crossing. 
5—13

Chapter 5—Site Assessment

5.1.3.3 Grade controls

Grade controls are key structural features that control channel elevation 

and grade, dissipate flow energy, and store sediment. On different 

channels, these grade controls might include steps, pool-tail crests (riffle 

crests), bedrock outcrops, large woody debris structures, beaver dams, 

or debris flow or landslide deposits. In stream-simulation design, it is 

important to know how mobile or immobile the key grade controls are 

relative to the life of the crossing structure, and evaluating their stability 

is an important part of the survey. If grade controls are highly unlikely 

to move over the life of the crossing structure, even during large floods, 

the design can rely on a stable longitudinal profile. If the grade controls 

move relatively frequently, the design will need to accommodate vertical 

adjustment in the channel. In this context, mobile bed structures do not 

necessarily imply an unstable channel. For example, a stable fine gravel 

bed stream is likely to be highly mobile and to adjust under even moderate 

flows; on average, though, it retains its equilibrium dimensions and slope 

(see appendix A, section A.3). Evaluating bed mobility is discussed 

further in section 5.1.5.

Stability of these grade-control structures depends on material strength 

and durability, size and orientation of the particles or wood pieces, and the 

feature’s relationship to nearby structures (table 5.2). As the survey moves 

along the channel, the person holding the rod should document bedform 

length and width, as well as particle size, packing and embedment. They 

should also qualitatively evaluate the stability of the bed structures relative 

to the lifespan of the crossing. Manmade structures like diversion dams 

may play the same roles as natural structures, and the possibility that such 

structures might be removed will also need to be considered in design. 

Table 5.3 lists specific types of channel-bed structures and describes 

characteristics for each type that lead to a qualitative rating as high, 

moderate, or low stability. The table offers an example of a rating system 

for key feature stability—a system that has proved useful in Alaska. 

Modify the table as needed to fit your area.

In low-gradient, fine-grained channels with highly mobile streambeds, 

there may be no persistent grade-control structures. Any combination of 

channel bends, submerged and embedded wood, bank irregularities or 

other bank roughness features, for example, overhanging or submerged 

vegetation, may control slope and roughness. 
5—14

Stream Simulation Table 5.2—Factors contributing to channel-bed structure stability. Material strength

• Durable vs. nondurable rock.• Shape of the substrate

– degree of angularity or roundedness.

• Condition of wood (sound or decayed

– degree of decay).

• Diameter of wood (longevity).Orientation and size of particles and pieces• Key pieces (boulder, small cobble, wood,

a combination of wood and rock) are large. Logs are well

anchored in the bank, so that the stream cannot cut around the end.

• Particles are imbricated and/or embedded rather than loose and readily available for transport.• Wood has roots attached.• Length of the wood in relation to stream width (logs are longer than stream width).Relationship to other bedform structures• Structures are/are not subjected to undermining if adjacent structure is lost.• Immobile structures in the reach govern the extent of vertical and lateral adjustments.
5—15

Chapter 5—Site Assessment

Table 5.3—A qualitative method for determining channel-bed structure stability.

Structure composition

BedrockBoulder-cobble stepsCobble-boulder or cobblegravel pool tail crests or rifflecrestsLogComposite log and rockBoulder-cobble steps, cobblegravel steps Cobble-boulder or cobblegravel pool tail crests or rifflecrestsLogComposite log and rock, beaver damsCobble-gravel steps or pool tail crestsLogComposite log and rock, beaver dams

Stability Rating

HighHighHighHighHigh

ModerateModerateModerateModerate

LowLowLow

Structure Characteristics

Bedrock ledges or falls span entire stream width Boulder-cobble steps span entire width of stream. Rocks are tightly keyed in place, and keyed-in material extends below base of scour pool below step. Cobble-boulder or cobble-gravel pool tail crests or riffle crests span the entire width of stream.Particles are tightly packed, embedded into the channel bed, and coarser than the remainder ofthe channel bed.Wood is sound and well anchored, spanning entire stream width.Wood is sound and well anchored, may or may not span entire stream width. Rock pieces are well keyed in place and bridge gaps so that composite structure controls width from bank to bank.Steps do not span entire width of stream or are loosely keyed in place. Keyed-in rocks may notextend below base of scour pool below step. Alternatively, step key pieces are not in contact with each other. Pool tail crests span entire width of stream, but the largest particles are similar in size to thoseelsewhere observed along the channel bed. Alternatively, particles are moderately packed and/or moderately embedded into the channel bed. Wood is rotten and punky. It may span entire stream width, but anchoring is susceptible to bank scour and movement during high flood events.Wood is rotten, punky, not anchored well, or does not span the entire stream width. Rock pieces are not well keyed in place and subject to movement at higher flood events.Rock bridges gaps so that structure extends from bank to bank, but there are indicators of lateral scour. Beaver dam is well constructed with a good distribution of large logs, small sticks and mud (but consider the possibility that even a stable dam could break during the life of the crossingstructure).Steps do not span entire width of stream, and/or are composed of loosely packed materials. Pooltail crests are constructed of material no coarser than rest of stream bed.Wood is very rotten and punky, may or may not span entire stream width, and anchoring is poor and susceptible to bank scour and movement during bankful flood events. Indications ofmovement are visible where pieces are anchored into the bank.Wood is very rotten and punky, or structure is made of loosely packed pieces that are poorly anchored. Structure does not span entire stream width. Rock is small in size and subject to movement at bankfull flood events. Beaver dams are poorly constructed or old and inactive.Large key logs are not present.
5—16

Stream Simulation

Where wood is present, describe its size, condition, mobility, and function. 

See section 5.1.6.3 for details on describing wood in the project area and 

reference reach. 

5.1.4 Cross Sections

Cross sections represent the channel and flood-prone area as they vary 

with local slope, entrenchment, materials, etc. When viewed together, 

the plan view map, the longitudinal profile and cross sections provide 

a three-dimensional perspective of valley and channel topography. 

Relating the cross sections to the longitudinal profile and to bed-material 

observations helps one understand how the channel works in terms of 

erosion, deposition, and sediment transport. The goal is to understand 

the extent and causes of variability in channel width, depth, and particle 

sizes throughout the reach. Data from one or more cross sections in the 

reference reach will be used to design the simulated streambed. 

Cross sections also provide information on the height and stability of 

banks. The question of whether to allow upstream incision at crossings 

where the downstream channel has incised should always take these 

variables into consideration. 

As with the longitudinal profile, survey channel cross sections to at 

least 0.1 foot. Either ensure the topographic survey is detailed enough 

to generate accurate cross sections from the digital elevation model, or 

survey the cross sections individually. If cross sections are taken outside 

the topographic survey area, ensure the surveys use a common datum. 

. 

5.1.4.1 Location and number of cross sections

At existing crossings, survey cross sections immediately upstream 

and downstream from the culvert to show the geomorphic effects of 

the existing crossing on channel conditions, channel and flood plain 

relationships, and construction accessibility. These cross sections will be 

important for designing smooth transitions at the inlet and outlet of the 

new crossing structure. 
5—17

Chapter 5—Site Assessment

Base the number of cross sections for the project area as a whole 

on the variability in channel characteristics and on risks at the site. 

Understanding the variability in channel dimensions like width and 

depth is very important in properly sizing the simulated channel as well 

as the new structure. Channel dimensions vary depending on many 

factors, such as entrenchment, composition of the bed and banks, large 

woody debris, valley form, channel planform, channel gradient, and 

flood history. On relatively uniform channels, surveying two or three 

cross sections upstream and downstream from the crossing may be 

sufficient to adequately characterize the channel and its variability. On 

complex channels, to properly characterize the site, understand the risks, 

and provide a design template additional cross sections upstream and 

downstream from the crossing will be needed. Consider measuring cross 

sections on a representative range of channel units, such as riffles, pools, 

steps, runs, etc., and widths. Those measurements will provide various 

options for a reference reach and will help you understand the variability 

within the reference reach. 

Be sure to cover the entire reach that may be part of the final project, 

including locations where you might install grade control structures 

or restore the channel. In some cases where the entrenchment ratio 

and apparent flood-plain conveyance are high, the designer may use 

a hydraulic step-backwater model such as HEC-RAS for quantifying 

flood-plain conveyance at different flood stages. If so, the designer should 

evaluate the terrain in the field, and locate the number of cross sections 

needed to accurately represent reach and flood-plain geometry in HECRAS. 

5.1.4.2 Typical cross-section measuring points

Each cross section should include all major topographic slope breaks. 

Survey and describe all features (see table 5.4 and figure 5.4) that pertain 

to the cross section. Of these features, bankfull elevation is one of the most 

important.
5—18

Stream Simulation

Table 5.4—Cross-section survey points and observations.

Survey and observation 

points: channel (include 

all major slope breaks)

Top and bottom of banks

Channel thalweg

Left and right bankfull 

elevations

Left and right edges of active 

streambed

Changes in bed and bank 

materials

Undercut banks

Left and right edges of water 

at time of survey 

Survey and observation 

points: flood plain and 

valley bottom

Edges of flood-plain

channels and terrace(s) if

applicable

Side channels, flood swales,

vegetation type transitions

Flood high water marks 

Notes

Sediment characteristics, vegetation type

Ensure cross-section location is shown on longitudinal profile.

Defines bankfull width; allows surveyors to estimate elevation of the

floodprone zone. (See textbox “Identifying Bankfull Elevation” page 5—20)

Width of channel devoid of vegetation.

Bedrock, gravel bars, colluvium, etc. Note bank stability, bank vegetation 

type, rooting density, and depth.

Measure dimensions of the undercut bank (depth and height). Small streams

with dense vegetation can have 2 to 3 of their area in undercuts, enough 

to affect discharge and sediment entrainment estimates and the simulatedchannel width.

If you measure flow at time of survey, these measurements permit calibrating

hydraulic models for the cross section.

Terrace edges, toe of valley slope, top and bottom of flood-plain channel

banks, etc. To ensure good coverage of floodable areas, include the entire

floodprone zone: extend the cross sections to an elevation that is double the

maximum bankfull depth measured from the channel thalweg (see figure 5.5).

Note evidence of flood-plain conveyance: scour, vegetation washed

away, large woody debris accumulations on the flood plain. Describe the

roughness elements on the flooded area: vegetation type and density,

ground debris, topographic irregularity, etc.

Fine sediment on top of vegetation; debris caught in or wrapped around 

shrubs or trees; flood water line on trees or other flood-plain features.
5—19

Chapter 5—Site Assessment

An example of the features to include in a cross-section survey appears in 

figure 5.4. 

Figure 5.4—Schematic channel cross section showing recommended survey 

points.

Include the flood-prone area in the surveyed cross sections by extending 

the cross sections to an elevation that is double the maximum bankfull 

depth measured vertically from the channel thalweg (Rosgen 1996). 

This will encompass the frequently inundated flood plain (if one exists) 

and permit calculation of the entrenchment ratio (see appendix A section 

A.3.4). If the channel is confined or entrenched, the cross-section 

endpoints may be on the valley slope or a terrace. 

Identify surveyed points in the survey notes, with descriptive comments. 

Since the cross section represents the channel segment, descriptions 

need not be limited to the cross-section line. The notes should describe 

the general character of the channel segment upstream and downstream 

of the cross section. They should also describe flood-plain features and 

characteristics, and flood-plain features should be included on the site 

sketch.

Understanding the interaction between the main channel and the adjacent 

valley surfaces is crucial in designing a crossing that obstructs flood-plain 

functions as little as possible. Where a flood plain is present, identify side 

channels, flood swales, and wetlands that should be considered during 

design. Make note of any indicators of recent flood elevations you find. 

There also may be evidence of beaver activity, rapid bank erosion, and 

lateral channel shift across the flood plain. Look for relict channels that 

Edge of flood 

prone zone 

Bankfull 

Elevation Edge of 

flood plain

Edge of low 

terrace 

Debris jam with 6-inch 

logs on low terrace 

Scoured side channel 

on low terrace 

Coarse-gravel flood plain is 

vegetated with willows and grasses

Arrows show rod locations for cross-section survey 

Note: low terrace is densely vegetated with conifers, cottonwood, and shrubs. 

Slope break 

Top of bank 

Bottom of bank Bottom of bank 

Thalweg 
5—20

Stream Simulation

Identifying Bankfull Elevation

Bankfull elevation is the point where water fills the channel just before 

beginning to spill onto the flood plain. Bankfull discharge is the flow in 

the channel (cubic feet per second) when the water surface is at bankfull 

elevation. Bankfull discharge typically occurs every 1 to 2 years (Leopold et 

al. 1964), but its frequency of occurrence can vary depending on channel 

type, hydrologic regime, and watershed conditions. Bankfull is recognized 

as a surrogate for the range of flows that maintain channel shape and size 

(Emmett 2004). It is often referred to as the effective discharge of a stream: 

the flow responsible for moving the most sediment (Dunne and Leopold 

1978) and maintaining channel form. This is why bankfull flow width is the 

minimum structure width required for simulating and maintaining channel 

form and functions through a crossing. 

Strictly speaking, bankfull applies only to alluvial streams with flood plains. 

In alluvial stream types, use some or all of the following indicators for 

recognizing bankfull elevation, depending on the situation (Harrelson et al. 

1994):

l Elevation of the edge of an active flood plain (flood plain may be 

present as discontinuous patches).

l Elevation associated with the top of the highest depositional features 

such as point- and mid-channel bars.

l Changes in slope on the banks [figure 5.5(a)].

l Changes in particle size of bank materials (from coarser to finer).

l Changes in vegetation types (from moss to lichens, from grass to 

alder, etc.).

l Stain lines on rock and scour lines in moss and lichens. 

Be careful when using vegetation as a geomorphic indicator as vegetation in 

some channels is inundated by bankfull flows. Depositional features should 

be the primary geomorphic indicator for identifying bankfull flow in alluvial 

channels.

Not all indicators will be present at each cross section. They vary with 

channel type, and false or confusing indicators have to be sorted out at each 

site. Flagging and surveying many bankfull elevations along a substantial 

length of channel helps to eliminate misleading indicators and is essential for 

accurate identification. The ideal method for consistently identifying bankfull 

elevations is to plot the bankfull longitudinal profile using points where 

bankfull was confidently identified. Then—where the profile crosses any 

cross section—that is the bankfull elevation at that cross section (Emmett 

2004). 
5—21

Chapter 5—Site Assessment

In entrenched and nonadjustable channels (bedrock or strongly cohesive 

materials), ordinary high water (OHW) level is used instead of bankfull 

for stream-simulation design purposes. OHW marks are characteristic of 

frequent high flows that are sustained long enough that the vegetation or bank 

material is distinctly different from the adjoining higher ground. OHW marks 

in nonadjustable channels include many of the same features in the list for 

alluvial channels: stain lines on rocks, high points of depositional features, 

and vegetation changes. In figure 5.5(b), OHW is taken as the elevation of 

the boundary between the moss (which survives long submergence) and 

woody vegetation. 

Figure 5.5—(a) Bankfull elevation on an unentrenched alluvial channel. (b)

Ordinary high water elevation in an entrenched coarse-grained channel 

without depositional features.

There are numerous guides to using channel physical features for identifying 

bankfull elevations (e.g., Leopold et al. 1964; Williams 1978, Dunne and 

Leopold 1978; Harrelson et al. 1994; Rosgen 1994; Knighton 1998). The 

Forest Service has produced several multimedia presentations describing 

the techniques and procedures for identifying bankfull flow for different 

channel types in different parts of the country (USDA Forest Service 2003; 

USDA Forest Service 2005).

(a)

(b)
5—22

Stream Simulation

may be blocked by the road fill, and consider whether they can and should 

be reconnected. Also note the smoothness or roughness of the flood-plain 

surface, because these characteristics influence the velocity of overbank 

flows. Together with entrenchment ratio and slope, roughness controls the 

volume of water conveyed on the flood plain (flood-plain conveyance). 

Figure 5.5(a) is an example of a rough flood plain, where grasses, shrubs, 

and trees slow overbank flows.

Field evidence of high flood-plain conveyance following a flood might 

include: 

l Scoured flood-plain swales and side-channels. 

l Scoured flood-plain surface.

l Impact scars high in trees or logs suspended above banks. 

l Accumulations of large woody debris and/or sediment on flood plain.

Recognizing if and how an existing crossing has altered the natural 

channel’s location and length can be important for correctly interpreting 

channel response, and designing the layout for the replacement. Often the 

aerial photo or the sketch map suggests the predisturbance planform. In 

the field, look for old abandoned channel segments, berms, or any other 

evidence that the channel was moved or that the culvert replaced a bend. 

Cross sections also can help distinguish reaches where channel incision 

has occurred downstream of a crossing. In this case, the crossing structure 

is acting as a grade control protecting upstream reaches from headcutting, 

and the downstream reach may be quite different in cross section than the 

upstream reach. Compared to the channel upstream from the crossing, an 

incised channel downstream from the crossing may have: 

l A lower width-depth ratio.

l Higher banks, with older vegetation higher on the bank. 

l Over-steepened, failing banks. 

l Cut into weathered bedrock, clay, or other nonalluvial material below 

the valley alluvium. 

l A flat bed in cross section.

l No buried debris within the bed. 

l Less gravel accumulation. 

l Coarser bed material or a more armored bed.
5—23

Chapter 5—Site Assessment

5.1.5 Channel Types and Bed Mobility

Channel-type classification is a fundamental step toward understanding 

both current conditions and future channel changes. Classifying the 

channel—using both the Montgomery and Buffington and the Rosgen 

systems (see appendix A, section A.6)—can provide insights on the 

dominant geomorphic processes associated with the reach, and on the type 

and intensity of future channel response to a new or replacement structure, 

or to structure removal. For example, bedrock, cascade, and step-pool 

channels are transport channels that convey most of the sediment 

supplied to them and undergo minimal channel changes in response to all 

but very large disturbances (Montgomery and Buffington 1993, 1997). 

In contrast, plane bed, pool-riffle, and dune-ripple channels are response 

channels that may undergo substantial changes in response to disturbances 

(appendix A, table A.1).

In transport channels, the larger bed-forming rocks or logs are generally 

quite stable. They do not move in frequent floods, although finer bed 

material does move over or around them during bankfull and larger events. 

Because these bed structures—essential for energy dissipation—do not 

self-form in frequent floods, they need to be designed and constructed in 

the simulated streambed. 

Response channel beds mobilize at flows from slightly above bankfull to 

much smaller flows, depending on the bed particle size and structure. For 

highly mobile channels, such as dune-ripple and fine-grained pool-riffle 

types, bed features are usually not constructed in the simulated channel, 

because they are expected to self-form during the first high flows after 

construction. 

For intermediate channels, such as coarser pool-riffle and plane bed types, 

the frequency of bed mobility depends on such things as armoring and

imbrication. Evaluate the mobility of these channels in the field and 

determine whether bed structures should be constructed in the simulated 

channel. The decision will depend not just on bed mobility, but also 

on risk. In a high-risk channel—say, where the watershed has recently 

burned—the team might lean toward constructing bed structures to be sure 

energy dissipation functions are in full operation immediately. 
5—24

Stream Simulation

Design of stream-simulation channel-bed material varies depending on 

bed mobility in the natural channel, and the bed material sampling method 

also depends on it. Bed mobility is a distinguishing characteristic of 

Montgomery-Buffington channel types and they are used in the following 

discussion for that reason. Appendix A describes them more fully. For 

channel types with intermediate mobility (the coarser pool-riffle and 

plane-bed types), the team should judge mobility before sampling the bed 

material, and select the sampling method accordingly. 

5.1.6 Channel-bed and Bank-material Characteristics 

Characterizing bed and bank material and structure helps the team predict 

how the channel might respond to disturbances in the future, or how it 

might recover from past disturbances.

Two other specific objectives for characterizing bed and bank composition 

and structure are:

1. To design bed material sizes and arrangement for the simulated 

streambed. 

The bed-material size distribution in the reference reach is the basis 

for the stream-simulation bed material mix. Likewise, the size of 

rocks or wood making up key energy dissipation and grade control 

features in the reference reach is the basis for sizing any stabilizing 

features in the stream simulation bed. 

2. To understand bed material sizes and mobility in the reach upstream 

of the crossing. 

 As bed material is eroded from the simulated channel during high 

flows, the upstream reach must be able to resupply similar particle 

sizes at similar flows. If not, the simulation will not retain its intended 

bed characteristics. The bed may coarsen or be washed out. 

Often, both these objectives can be achieved by sampling the streambed 

and describing banks in the reach upstream of the crossing and outside 

the crossing’s area of influence. However, even when the reference reach 

is not upstream, the team will still need to assess bed material sizes, 

channel roughness, and bed mobility upstream of the crossing to assure 

they approximate those of the reference reach. The assessment need not be 

quantitative, but the team should satisfy itself that the upstream reach will 

indeed resupply the simulated streambed. 
5—25

Chapter 5—Site Assessment

The bed and bank characteristics that are of primary interest in streamsimulation design are those of the reference reach. One strategy for data 

collection is to wait until the reference reach is selected before collecting 

detailed data. The alternative is to take enough data, while you are already 

onsite studying the reach, to support several possible reference reach 

selections.

5.1.6.1 Sampling strategies and methods

Sediment sizes vary longitudinally, laterally, and vertically across the 

channel bed, reflecting the spatial variability of channel units (for 

example, channel margin, thalweg, pools, riffles), small-scale bedforms 

(for example, particle clusters, transverse bars, longitudinal bars), and 

bed layers (for example, armor, subarmor). 

For the purpose of designing the simulation bed material, the sample 

should represent the entire reference reach. Be aware of the variability 

in particle size distribution between different channel areas along the 

reference reach, and sample those areas proportionally to their coverage 

(Harrelson et al. 1994; Rosgen 1996; Bunte and Abt 2001). 

For detailed flow modeling, bed-material sampling may need to be 

stratified by channel units, such as pools, riffles or steps. It may take 

several samples to represent the range of variability present (Reid et al. 

1997; Wohl 2000; Bunte and Abt 2001). Data specific to a channel-unit 

might be needed, for example, if a designer wants to estimate the flow 

that mobilizes specific grade control structures in the natural channel (see 

section 6.4). The number of samples needed depends on the complexity 

of the channel and the objective. The designer/analyst should specify the 

amount and type of data that is required. 

This section relies heavily on information from Bunte and Abt (2001) 

“Sampling Surface and Subsurface Particle Size Distributions in 

Wadeable Gravel- and Cobble-Bed Streams for Analyses in Sediment 

Transport, Hydraulics and Streambed Monitoring,” published by the 

Forest Service, Rocky Mountain Research Station. The book is readily 

available from Rocky Mountain Research Station, Fort Collins, CO. We 

strongly recommend reading the many pertinent sections, especially 

those on sampling methods, rock size measurement techniques, 

sample sizes, and armoring. 
5—26

Stream Simulation

Except as described below for different channel types, bed surface material 

is normally characterized by measuring particles in place using the pebblecount method, and sampling in a grid pattern. Measure 100 to 400 particles 

selected either systematically along a measuring tape or from the toe of the 

boot in a heel-to-toe walk (Wolman 1954; Bunte and Abt 2001). For wellsorted (poorly graded) streambeds, 100 particles are sufficient; for poorlysorted (well graded) streambeds, up to 400 particles are necessary. 

The grid is formed by spanning the channel with a measuring tape 

repeatedly along the channel at close intervals. The sampling interval 

along the transect is one to two times the diameter of the largest particle. 

The grid method is the preferred sampling technique for pebble counts in 

cobble and boulder materials as well as gravel, because it reduces the bias 

against sampling the very small and very large particles. For purposes of 

stream-simulation bed design, pebble counts should include the channel 

bed between the base of each bank, and exclude the banks themselves. 

Review Bunte and Abt (2001) for details about selecting and measuring 

particles and laying out the sampling scheme. If the pebble count 

represents an entire reach, ensure the tape placements adequately cover the 

range of variability present in the reach.

Pebble count results are reported as a cumulative frequency distribution 

of particle sizes. In conventional notation, D50 (reported in millimeters) 

represents the median particle size; fifty percent of all particles are finer. 

Likewise, 84 percent of all particles are finer than D84. The pebble count 

parameters most commonly used in stream simulation bed design are D95

(representing the largest mobile particles), D84, and D50. Where immobile 

particles function as key energy dissipation and grade control features, 

their sizes also are used in design.

Distinguishing alluvial particles (those moved by the current river) from 

rocks that are not mobile is important. Immobile rocks may have fallen 

or slid into the stream during a landslide or debris torrent, or they may 

have been transported by ice-rafting. These rocks are generally much 

larger than the largest alluvial rocks, commonly two to three particle size 

classes larger. If they are mistaken for the largest mobile particle size, 

the simulated bed may end up with much coarser bed material than the 

reference reach. Nonalluvial material can be recognized by its limited 

distribution along the channel, and by its larger size. Rocks derived from 

the adjacent hillslopes (by landslides, rockfalls, etc.) are usually angular to 

subrounded, rather than round, and may therefore look out of place in the 

stream. Section 5.1.6.2 describes data collection for nonalluvial material 

and other key features. 

Table 5.5 summarizes the recommended methods for characterizing bed 

sediment in different channel types for stream-simulation design purposes. 

The channel types are described in more detail in appendix A, section A.6.1.
5—27

Chapter 5—Site Assessment

REFERENCE 

CHANNEL 

TYPE

Dune-ripple: 

high mobility

Pool-riffle

and planebed: mobile

Pool-riffle

and planebed: 

intermediate

mobility

Bed 

material

Sand to medium gravel Gravel, may be slightly armoredGravel to cobble, usually armored

Dominant roughness 

& structural element

Sinuosity, bedforms, banks. Small debris may provide structure.Bars, pools, grains, sinuosity, banksGrains, banks

Slope1

Low

<1.5%

<4% 

Entrenchment

SlightSlight 

Slight to 

entrenched

Streambed mobility

Termed “live bed”; significant sedimenttransport at most flowsNear bankfullFeatures controlling streambed elevations (riffle crests) mobilizeat flows greater thanbankfull

RECOMMENDED SEDIMENT 

SAMPLING METHOD FOR 

BED DESIGN

•	 Visual estimate representing

whole reference reach or

•	 Bulk sample/sieve•	 Note role of wood in stabilizing

bed

•	 If fine gravels (up to 16 mm),

same as dune/ripple

•	 Grid pebble count2

•	 If armored, note armor

thickness and packing. Visuallyestimate percent fines insubsurface layer

•	 Grid pebble count•	 If armored, note armor

thickness and packing. Visuallyestimate percent fines insubsurface layer

•	 Measure dimensions of 10-

25 of the largest rocks on upstream portion of riffle crests.

* Note imbrication, 

embeddedness, consolidation, particle shape

TYPICAL CONDITIONS

Table 5.5—Bed sediment sampling and observations for different Montgomery and Buffington (1997) channel types.

1 Slope is not diagnostic of Montgomery and Buffington channel types. Ranges given here include extremes. Typically, pool-riffle channels are <1.5%; plane

bed channels are 1.5% to 3%, and step-pool channels are 3% to 6.5%. Forced channels can have steeper slopes.

2 For the grid method, measurement points are spaced at one to two times the diameter of the largest particle. Transects are also located at that spacing.
5—28

Stream Simulation REFERENCE CHANNEL TYPE Step-pool

Cascade

Bedrock

Channels in 

cohesive 

materials 

Bed 

material

Cobble to boulder BoulderRock with sediment of various sizes in transport over rock surfaceSilt to clay

Dominant roughness 

& structural element

Steps, pools, banks. Debris may be an important component of streambed structure.Grains, banksBed and banks. Sinuosity, banks, bed irregularities

Slope1

3–8%

>3.5% 

(usually

>6.5%)

AnyAny

Entrenchment

Moderately 

entrenched 

to entrenched

Entrenched

AnyAny

Streambed mobility

Fine material moves over larger grains at frequent flows. Bedforming rocks move at higher flows dependingon size; often >Q30.

Smaller bed material moves at moderate frequencies (floodshigher than bankfull).Larger rocks are immobile in flowsless than ~Q50. 

Bedload moves over bedrock at various flowsdepending on its size. May be thin layer of alluvium over bedrock. Wood can strongly affect sediment mobility. Fine sediment moves over immobile bed at moderate flows dependingon its size. May be thin layer of alluvium over immobile bed. 

RECOMMENDED SEDIMENT 

SAMPLING METHOD FOR 

BED DESIGN

•	 Grid pebble count.•	 Measure 10-25 step-forming

rocks individually. Sample enough to get a range representative of all the steps in the reference reach.

•	 If steps are on wood, measure

wood diameter and step height.

•	 Grid pebble count.•	 Visually characterize alluvial

material patches or use pebble count. Note features controlling sediment accumulation (e.g.,wood).

•	 No estimate needed—bed

cannot be constructed of cohesive materials. 

TYPICAL CONDITIONS

1 Slope is not diagnostic of Montgomery and Buffington channel types. Ranges given here include extremes. Typically, pool-riffle channels are <1.5%; plane

bed channels are 1.5% to 3%, and step-pool channels are 3% to 6.5%. Forced channels can have steeper slopes.

Table 5.5—Bed sediment sampling and observations for different Montgomery and Buffington (1997) channel types (continued).
5—29

Chapter 5—Site Assessment

Dune-ripple and fine-grained pool-riffle channel types: high mobility.

Streambeds composed primarily of medium gravel and finer materials (less 

than 16 millimeters) are generally dune-ripple or pool-riffle channel types. 

Visual estimates of dominant particle size classes are normally sufficient 

on these channels. Estimate maximum particle size, and percentages of 

the bed covered by different size classes, such as coarse gravel, medium 

gravel, fine and very fine gravels, sand, and silt/clay. (Particle-size classes 

are defined in appendix A, table A.1.) Platts et al. (1983) recommended 

doing this along transects, visually estimating the particle-size class 

that comprises the largest part of each 1-foot section. Visual estimation 

is adequate in these fine-grained channels because it is generally not 

necessary to design the simulated bed material as carefully as in less 

mobile streambeds. The fine particles move at very frequent flows (below 

bankfull), and the simulated streambed reshapes itself rapidly as new 

material is transported into it from upstream. 

If more certainty about the particle size distribution is needed, then use 

bulk-sampling and standard laboratory sieve analysis to characterize the 

entire particle-size distribution for medium-gravel and finer channels. 

See Bunte and Abt (2001), section 4.2.2, for recommended sampling 

procedures. 

Pool-riffle and plane-bed channel types: mobile. Streambeds in these 

channels mobilize at flows near bankfull, and bed features are expected to 

form naturally in the simulated channel within a short period of time after 

construction. In these mobile channels, the bed-material sample should 

represent the whole reference reach. Use the grid pebble-count method, 

tailoring the number of individual particles measured to the variability in 

bed material sizes. 

Not all pool-riffle and plane-bed channels are mobile, so evaluate as many 

mobility indicators as possible. Besides small particle sizes, indicators 

of relatively frequent mobilization include the absence of algal stains or 

moss on particles, steep faces and a lack of vegetation on bars, and loose 

bed material. Be careful if doing this evaluation shortly after a large flood; 

particle packing is looser after the bed mobilizes during such a flood. If it 

has been some time since a high flow, lesser flows will have reworked the 

streambed particles so that they are more tightly packed (Reid et al. 1985).

The degree of armoring also influences streambed mobility. Gravelbed streams frequently have surface layers that are coarser than the 
5—30

Stream Simulation

subsurface (appendix A, figure A.6). In such armored channels, the size 

and packing of the armor layer strongly influences streambed mobility, 

while the subsurface fines limit flow infiltration and control subsurface 

flow. Inspect the material underneath the surface and compare it to the 

surface to determine whether a streambed is armored (appendix A, figure 

A.6). If it is armored, the subsurface has a much higher content of fines 

(particles less than 2 millimeters in diameter including silt and clay). The 

armor layer median particle size (D50) is usually 1.5- to 3-times larger than 

the subsurface material, and can be up to 4-times larger (Reid et al. 1998; 

Bunte and Abt 2001). Characterizing both armor and subarmor layers is 

important for designing realistic bed material for the simulation. Figure 

5.6 illustrates the difference between surface and subsurface material in a 

gravel-cobble stream in Colorado. 

Figure 5.6—Surface armor and subsurface particle size distribution curves for the 

South Fork Cache la Poudre River (data from Bunte 2004). The surface armor

was characterized by a pebble count. The subsurface was bulk sampled and 

sieved. Although the subsurface has a higher content of fines, it also includes the

full range of coarser sizes found in the surface armor.

Visually estimating the subsurface fines content is usually adequate for 

stream-simulation design purposes. Sometimes you may be able to find 

an exposed scour pool, where you can clear off the exposed surface from 

a bank and estimate the content of fines. Otherwise, remove the coarse 

armor layer (usually one to two particles thick) from a bed area 1.5 to 

2.0 square meters (16 to 22 square feet). If the area is submerged, use a 

Surface and Subsurface Material Particle Size Distributions--South 

Fork Cache la Poudre River 

Field data provided by K. Bunte

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100 1000

particle diameter (mm)

cumulative percent finer (%) 

subsurface sieve analysis

surface pebble count

silt/clay sand gravel cobble boulder
5—31

Chapter 5—Site Assessment

plywood shield to protect it from flowing water (Bunte and Abt 2001, 

p. 209). Estimate the percent area covered by fines, including fine gravels, 

sands, silts, and clay. These estimates will determine the content of sand 

(less than 2 millimeters) and silt/clay (less than 0.063 millimeters) in the 

simulation bed mix. Note any unusual situations, such as a layer of cobbles 

overlying very fine sediments.

Pool-riffle and plane-bed channel types: intermediate mobility. In 

steeper, coarser pool-riffle channels, where particle sizes increase to very 

coarse gravel and cobble, streambed mobility is likely to decrease. This 

is especially true for imbricated, embedded, consolidated or heavily 

armored streambeds (see section 4.4, table 4-2). Particle shape and 

angularity also affect mobility: mobilizing angular particles requires higher 

shear stresses than mobilizing spherical particles of similar size (Reid 

and Frostick 1996). Flat, disc-shaped particles are usually well imbricated, 

making them more resistant to entrainment (Carling 1992). 

As in the mobile channels, measure bed material using a pebble count 

method that samples the different channel units proportionally to their 

areas within the reach. In these coarser channels, tightly packed or 

embedded rocks making up the heads of riffles (or pool-tail crests) may be 

stable up to flows much larger than bankfull. It’s important to distinguish 

these less mobile grade controls where they exist. The whole-channel 

pebble count includes the riffle crests where the grid crosses them, but 

a separate assessment of the larger particle sizes comprising the upper 

segment of the riffle crests is also needed, so that these key features can 

be constructed in the simulated channel. Measuring 10 to 25 of the largest 

rocks on the riffle crests is probably sufficient. (Figure 5.8 shows an 

example of bed material evaluation in an intermediate-mobility pool-riffle 

channel.) Also note any other characteristics that influence mobility. If the 

rocks are tightly imbricated, embedded, or packed, particle size alone may 

not be an adequate index of stability of the grade controls. Where rocks 

are highly asymmetrical, it may be necessary to measure the long, short, 

and intermediate axes to describe their relative dimensions and create 

appropriate specifications later.

Step-pool and cascade-channel types: low mobility. Assess particle 

sizes on these channel types using grid-based pebble counts covering the 

entire streambed. For step-pool channels, measure on the order of 10-25 

step-forming rocks, separately if necessary. Again, for highly asymetrical 

particles, measuring dimensions of all three axes may be necessary to 

write a good specification.Where steps are formed by wood, measure log 
5—32

Stream Simulation

diameters. Ensure a good representation of the range of sizes of the stepforming rocks or wood in the reach as a whole. Measurements of the stepforming features comprise the first estimate of rock size for the steps in the 

stream simulation design bed. Design the overall bed mix from the wholechannel pebble count that includes the step-forming rocks only where the 

grid crosses them. 

5.1.6.2 Key features

In stream-simulation practice, the term key feature means any element 

on the streambed or banks that is large and immobile enough to control 

channel slope and dimensions, affect water velocity and flow direction, 

and/or retain sediment over a fairly long period of time. Key features 

often play crucial roles in maintaining the stability and diversity of 

the streambed and stream banks. Key features are either permanently 

immobile or, as in the case of the pool-tail crests and steps mentioned 

above, they are low-to-intermediate mobility grade controls that cannot be 

expected to form naturally within a culvert in a reasonable period of time. 

In addition to bedforms like steps, they include large wood, rock outcrops, 

large living tree roots, large boulders, etc. 

Key features are characterized separately from the alluvial material so 

their functions can be replaced in the stream-simulation channel. They 

should be shown on the site sketch map and surveyed and noted during the 

topographic survey. Where water drops over a feature, include the height 

of the drop in the surveyed longitudinal profile. It will probably be used 

directly in the simulated channel design. Field notes should cover type, 

condition, size, function, and stability of each key feature (see section 

5.1.3.3). Possible functions include providing grade control, hydraulic 

roughness, and bank stability. In some cases, key features may prevent the 

channel from shifting laterally or widening. 

Table 5.6 is an example of a form that can be used to summarize the field 

notes describing wood and other key features. 

5.1.6.3 Wood

Note: all wood is included in table 5.6 even when it may not be a longlasting key feature. The table classifies the wood by size, and describes each 

category in terms of diameter, length, condition (rotten or sound), amount 

or spacing, and function. This is simply a handy way to summarize the field 

observations for later reference during design. Where logs or trees are true 

key features, their size and stability should be noted individually and they 

should be located on the site sketch.
5—33

Chapter 5—Site Assessment

Table 5.6—Example key-feature summary table.

KEY FEATURE SUMMARY

 Key feature Size Function Spacing Plunge height Condition 

 (bed elevation change) & mobility/

 stability

Wood debris and 6”-15” G @ 15’ 0.4’-0.7’ Rotten—

live trees low stability 

 10”-15” C continuous Live tree

 tree diameter on left bank root systems

 36” R,C,B @ 20’ Live tree

 tree diameter both banks root systems

 

Large boulders 40” x 23” x 15” R,C,B irregular immobile

 37” x 18” x 18” R,C,B irregular immobile

Bedrock None

 

Bedforms (steps, 	 Steps are	

clusters, pool tail formed by See above

crests, etc.) 	 wood (6”-15”)

 

 

Function key: Grade control, Roughness, Bank stability, lateral Confinement
5—34

Stream Simulation

Wood need not be a long-lasting key feature to exert strong effects on 

channel morphology. For example, woody debris, living trees and roots, 

and other roughness elements can reduce bed-surface particle size by 

dissipating some of the boundary shear stress that would otherwise be 

exerted on the bed. Because the shear stress on the bed is less, the bed 

responds by becoming finer than it would otherwise be (Buffington and 

Montgomery 1999a).

Even small pieces of wood can affect the channel. In sand and fine-gravelbed streams, buried fine woody debris can stabilize the bed at a steeper 

slope than it would otherwise sustain. Make sure to note the frequency and 

size of the fine debris if it is present and playing this role. If the streamsimulation bed design does not include the stabilizing effects of the small 

wood, the bed material may scour.

5.1.6.4 Bank materials and morphology 

Streambanks can be relatively straight and uniform, or irregular with 

localized sections projecting into the channel. Woody vegetation and 

rock projecting from banks into the channel can have a substantial 

effect on channel form and processes by increasing flow resistance, 

obstructing or deflecting flow, stabilizing banks, and influencing erosional 

and depositional processes on the streambed (Poff et al. 1998). Bank 

irregularities also influence channel margin habitat for aquatic species by 

creating lateral scour pools and depositional zones. These habitats can 

be critical for passage of weak-swimming species that need slow and/or 

shallow water along the channel margin. 

Mimicking the diversity, roughness, and shape of the channel margins and 

banks is important for simulating the degree of hydraulic roughness in the 

reference reach and for satisfying aquatic organism passage objectives. 

Where bank irregularities are important for edge habitat, bank stability, or 

channel roughness (figure 5.7), measure their spacing and length, that is, 

the distance they extend out into the channel. Note the type (large woody 

debris, standing trees, rock) and size of material that forms the bank 

protrusions. These features can be simulated with rock. 
5—35

Chapter 5—Site Assessment

Figure 5.7—Channel margin diversity in Ore Creek, Oregon.

Understanding bank stability is also important when considering the 

effects of potential downcutting after a culvert replacement. Because there 

is often an elevation differential across older culverts, some adjustment 

of the longitudinal profile is likely during or after replacement with a 

stream-simulation culvert. If the replacement structure causes the upstream 

channel to degrade, the stability of the banks becomes an issue. Their 

stability may affect the decision about whether or how to control any 

headcutting that may occur (see section 5.3.3).

Qualitatively evaluate bank stability by observing: 

l Bank materials and their layering. 

l Rooting depth, density, and root sizes.

l Large, stable woody debris on banks. 

l Live trees and shrubs that may overhang the banks.

l Evidence of active bank erosion such as vegetated chunks lying near 

the edge of the streambed. 
5—36

Stream Simulation

5.1.7 Preliminary Geotechnical Investigation 

The initial assessment phase (chapter 4) included collecting existing 

information on site geology from geological reports, watershed analyses, 

or past projects in the area. Usually these reports provide only general 

geological information. Complete a field geotechnical investigation to 

evaluate if a more detailed study of subsurface material properties is 

needed, and to help determine the cost and feasibility of the proposed 

project. The geotechnical site investigation assesses the spatial variability 

and physical characteristics of soil and bedrock, and the presence of 

ground water. 

The list that follows summarizes the geological and geotechnical 

observations that may be needed. These observations apply to any site, 

whether steam-simulation design is used or not. Techniques are not 

discussed in detail because they are standard engineering practice. Ensure 

the geotechnical data are tied to the common datum of the topographic site 

survey. 

Bedrock.

l Location, elevation.

l Type, durability, dip, strike, orientation, thickness (these 

characteristics become important at bridge or open-bottom arch sites).

l Structural features (fracture and joint patterns, width, depth, 

orientation, continuous or discontinuous, extent, shear, and fault 

zones). 

l Weathering (distribution and extent). 

Soil.

l Type (Unified Soil Classification System).

l Physical characteristics (thickness, cementation, occurrence).

l Engineering properties of the materials at the site. 

l Durability. 

l Plasticity. 

l Load-bearing capacity (friction angle, cohesion, unit weight).

l Permeability. 

Mass-wasting risk at the site (Benda and Cundy 1990).

l Debris flow. 

l Slides and rock falls. 

Ground water. 

l Occurrence and distribution.

l Relationship to topography.
5—37

Chapter 5—Site Assessment

At most sites, sufficient subsurface data can be collected using simple 

hand methods (probing, hand augering, drop hammer, shallow excavations, 

etc.) Probing is a simple method of estimating some subsurface conditions, 

such as relative density of subsurface material, depth to bedrock, depth 

to probe refusal, and type of subsurface material (Williamson 1987). It 

is appropriate on most low-volume forest roads where no pavement is 

planned and the design structure is a culvert.

The probe is 2-inch galvanized steel pipe (actual dimensions are 

approximately w-inch outside diameter) and uses an 11-pound slide 

hammer for driving the probe into the soil. Stouter probes—such as 

stainless steel—may be needed in coarse-bed channels where rock is 

likely to be encountered. Probe immediately upstream and downstream of 

the existing structure and laterally across the stream (at least to bankfull 

width), including the area that the structure will cover. If bedrock is 

encountered during excavation, probe beyond bankfull width to develop 

more accurate estimates of excavation quantities. To assess localized 

changes in subsurface material and bank composition, extend the probing 

to the banks away from the fillslopes. Probe in scour holes to obtain 

information deeper in the subsurface. If riprap precludes probing near the 

culvert outlet, probe farther downstream and in the bank areas near the 

outlet. Include probe site locations on the site sketch, and flag them for the 

topographic survey. Using the surveyed surface elevations of each probe 

hole, calculate the elevation of the probed depth. For a more in-depth 

discussion of probing, see Williamson (1989).

During low-flow conditions, the plunge pool immediately downstream 

from an undersized culvert often has well-exposed scoured banks. 

Descriptions of sediments in the banks may provide insights into the 

material beneath the existing culvert. The vertical stratigraphy of the 

plunge pool sediments can highlight geotechnical concerns, such as the 

load-bearing capacity of the underlying sediments (how much weight the 

material can support), dewatering (how much ground water is expected 

and whether flow diversion is feasible), and susceptibility of the sediment 

to scour. Bank seepage can indicate potential problems with ground water 

during construction. 

The results from the preliminary investigation may indicate the need for a 

more intensive, detailed geotechnical investigation involving core drilling, 

seismic surveying, and/or ground penetrating radar to fully characterize 

the geology at the road-stream crossing. Such an investigation may be 

desirable anyway if the site has high associated risks and costs. For 

example, if the replacement structure might be a bridge or an open-bottom 
5—38

Stream Simulation

arch, and the preliminary geotechnical investigation shows that there 

is soft material at the site, a detailed geotechnical investigation will be 

required.

5.1.8 Road Travel-way and Construction Considerations 

Logistical constraints affect what you can do at any site. During the site 

assessment and preliminary design, identify all the limitations that could 

constrain design. A list of common constraints follows:

l Vertical constraints: Maximum road grade, and fixed or required 

elevations influence structure type and clearance and impact the site 

layout.

l Horizontal constraints: Issues of site visibility and maximum or 

minimum curve radius can affect site layout.

l Right of way and property boundaries: These affect the length of 

stream segment that can be regraded, along with the type and length 

of structure that can be installed.

l Utilities and property developments: These can affect the ability to 

reconfigure the site.

l Material constraints: Unavailability of materials may require a 

compromise on material used or an alternative design solution to 

stream simulation. 

l Site access: Access issues may affect the type of equipment you 

can use, as well as the feasibility of regrading the channel profile. 

The availability of space for storing materials can also affect the 

construction schedule.

l Road closure and detour feasibility: The importance of a road 

for public travel and access during construction may constrain 

construction activities.

l Time constraints: Regulatory limitations to protect threatened or 

endangered species may limit the ‘work window’ to a few weeks out 

of the year. This can preclude some construction techniques, such as 

building cast-in-place concrete footings.

These logistical constraints may limit the extent of regrading or the type 

of structure, forcing a less-than-ideal solution for the site. For instance, a 

narrow right-of-way may force a steeper-than-ideal project profile to limit 

the footprint of the work.
5—39

Chapter 5—Site Assessment

The site assessment should answer other construction-related questions as 

well: 

l Are the existing crossing embankment materials suitable for backfill? 

(See section 7.3.4.) 

l What onsite materials (trees, downed logs, riparian vegetation, 

topsoil, large rocks) are suitable for possible inclusion in the streamsimulation design or stabilization plan? 

l Are there nearby areas that might be suitable for treating dirty water 

by filtration through soil and vegetation? (See section 7.8.4.)

l What is the diversion potential at the site? Where would diverted 

water go? 

l Where might topsoil and construction materials be stockpiled? 

l Will streambank stabilization measures be necessary upstream or 

downstream? If so, what kinds of measures are needed? 

5.2 Analyzing and Interpreting Site Data

5.2.1 Interpreting Sediment Processes and Mobility 

Site assessment documentation for bed mobility should include:

l Channel types upstream and downstream of the crossing.

l Apparent bed mobility in upstream reach, and mobility indicators: 

degree of armoring, imbrication, bed structures, dominant particle 

sizes.

l Evaluation of whether grade controls need to be constructed in the 

stream simulation design bed.

Information for the reference reach should include:

l For gravel and coarser channels, particle size distribution curve(s) 

including particle sizes of grade controls if necessary.

l A visual estimate of subsurface fines.

l A qualitative description of the degree of armoring and the apparent 

stability of the armor layer (determined by packing, particle shape, 

etc.). 

l For highly mobile streambeds, qualitative evaluation of particle sizes: 

maximum mobile particle size, dominant class, range of sizes present.

l Key feature type, size, function. 
5—40

Stream Simulation

In all cases, describe any effects of the existing crossing structure on 

bed material sizes to help in predicting channel response to removal or 

replacement. 

The composition and characteristics of bed and bank material can 

provide insight on the frequency of sediment transport, channel stability, 

and sediment supply. These insights are important during design when 

decisions must be made about regrading the project profile, realigning 

the crossing structure or the adjacent reaches, and designing streambed 

structures that move at similar flows to the reference reach. 

5.2.2 Analyzing the Longitudinal Profile

Plot the surveyed longitudinal profile and cross sections, and annotate 

them from the survey notes to help interpret the relationships between 

channel characteristics and stream processes. Locate the cross sections 

and bed material site(s) on the longitudinal profile, as well as the grade 

controls and other features that were identified in the field (table 5-1). 

Channel slope typically varies considerably along the longitudinal profile, 

directly reflecting the influences of large woody debris, slope and bank 

failures, bedrock, bedforms, and spatial variability of bed-material sizes. 

Integrating all of this information allows assessment of how streambed 

elevations and the longitudinal profile may change over the life of the 

project. 

Usually, plotting the profile and cross sections with a vertical exaggeration 

(VE) between 2 and 10 makes them easier to interpret, as it makes 

segments with different slopes stand out from each other. Beware of 

using large VE’s, however, especially on streams with steep (greater than 

6 percent) slopes and high steps. Too much VE can give the misleading 

impression of many short channel segments. 

On the cross-section plots, show bankfull width and floodprone width, 

and identify key geomorphic features. Plotting all the cross sections at the 

same scale makes it easier to visualize changes in cross section dimensions 

along the channel. 
5—41

Chapter 5—Site Assessment

Newbury Creek Site Assessment—Bed Material

Figure 5.8 shows pebble-count data from a riffle in a potential reference reach downstream of the existing culvert at the Newbury Creek site [see figure 5.2 and 5.10(b)]. The 

bed is well-armored, tightly packed, and imbricated. Well-established moss can be seen 

on the largest particles, suggesting that riffle particles do not move very frequently. The 

channel type is pool-riffle with intermediate mobility. Riffle-crest particles were measured 

separately. A sample of 10 of the largest rocks on the riffle crest averaged 244 millimeters 

in diameter, which is in the large cobble range. The surface layer has less than 1-percent 

sand and finer material, but a visual estimate of subsurface fines is about 20 percent. 

Field notes indicate that gravel bars on the insides of bends are narrow, woody debris is 

not present in large amounts, and little sediment is stored in the channel. From the initial 

assessment, there is a low-gradient meadow a short distance upstream of the crossing 

reach. The meadow reach traps most sediment moving down Newbury Creek, and the 

supply of sediment to the crossing reach is fairly low. Aggradation is unlikely to be a major issue at this site.

Figure 5.8—Particle-size distribution curve from a potential reference reach in the vicinity of

cross sections 8 and 9. Cross sections are located on figure 5.9.
5—42

Stream Simulation

The following steps are a systematic way of analyzing the longitudinal 

profile. Having the annotated cross sections handy will help with the 

analysis and interpretation. 

1. Visually identify pools and grade controls. Identify geomorphic 

controls on pool formation (e.g., log, boulder weir, channel bend, 

culvert outlet plunge pool, etc.). Document the type and stability of 

the grade controls. 

2. Delineate slope segments by drawing straight lines connecting 

successive grade controls. End a segment when the next grade control 

does not fall on the straight line. Calculate segment gradients, and 

combine adjacent segments when their slopes do not differ by more 

than 20 to 25 percent. For each of the final segments, determine (a) 

segment length, (b) the number and distance between grade controls, 

and (c) maximum pool scour depth. 

3. Identify the length and depth of aggradation and degradation 

associated with the existing crossing. Identifying these areas of 

local aggradation and degradation helps in assessing the response 

of the channel to the existing structure, and predicting the channel’s 

response to a new structure.

4. Identify the shape of the longitudinal profile to interpret the dominant 

geomorphic processes occurring at the crossing, and predict channel 

adjustments after the replacement structure is installed. Section 

5.2.2.1 describes profile shapes and their implications for streamsimulation design.

5. Determine upper and lower vertical adjustment potential lines for the 

streambed through the crossing as if no crossing structure was present 

(section 5.2.2.2).

5.2.2.1 Identify longitudinal profile shape

Bedforms, woody debris, bedrock, etc, are not the only possible controls 

on channel slope. Slope also may vary where the crossing is located at 

a geomorphic transition, where the downstream channel has incised, or 

where the crossing itself has modified channel slope by causing sediment 

deposition upstream. 

Many forest roads are located at geomorphic transitions—natural terrain 

breaks such as the edge of a valley at the base of the hillslope, or on a 

natural bench. These terrain breaks [figure 5.12 (c), (d), and (e)] can create 

an abrupt change in stream slope, influencing the shape of the profile and 

affecting sediment transport along the channel. Project teams need to 

identify these transitions and understand their potential effects on sediment 

transport and channel stability to accommodate them in the design. 
5—43

Chapter 5—Site Assessment

Newbury Creek Site Assessment 

Longitudinal Profile Analysis Steps 1 Through 3

Figure 5.9 is the annotated longitudinal profile for Newbury Creek (sketch map 

is shown in figure 5.2). The longitudinal profile plot identifies the surveyed cross 

sections, and it shows channel features such as log weirs (installed in the 1980s and 

early 1990s to improve aquatic habitat), bankfull and flood plain surface elevations, 

and exposed bedrock. Bedrock occurs at the base of pools associated with log weirs 

upstream from the crossing. 

Figure 5.10 shows two typical cross sections upstream and downstream of the 

crossing (locations are on the longitudinal profile). The upstream cross section (a) is 

substantially more entrenched, bounded by the adjacent slope on one side and a high 

glaciofluvial terrace on the other. The downstream channel (b) is less entrenched; the 

adjacent surface is a low terrace only slightly higher than bankfull elevation. 

Step 1. Pools are identified on figure 5.9, as are grade controls, which include 

bedrock steps, moderate-to-high stability pool-tail crests (the heads of riffles) 

and low-to-moderate stability log weirs. Pool-tail crests are designated high 

stability when composed of tightly packed and embedded boulders and 

cobbles. Pool-tail crests of more loosely packed cobbles and gravels are 

considered moderate stability (see table 5.3).

Upstream of the road, the channel is relatively straight (figure 5.2), and the 

primary controls on pool formation are obstructions created by bedrock steps 

and log weirs. Downstream of the road, the channel is more sinuous and 

the primary controls on pool formation are channel bends and obstructions 

created by log weirs that have partially failed (compare figures 5.2 and 

5.9).

Step 2. The channel upstream of the road has five segments ranging from 0.5 to 

6.7 percent slope. Downstream of the road, three segments were initially 

identified. Segments F and G were combined because their slopes differ by 

only 21 percent and the segments are the same channel type. Grade control 

spacing and maximum residual pool depth for each segment are summarized 

in the table in figure 5.9.
5—44

Stream Simulation

Figure 5.9—Longitudinal profile of Newbury Creek, Olympic National Forest, showing various channel features.

H-tsH- c

tp

M-wl

M-wl

M-wl

M-ctp

L-wl

M-ctp

L-wl

0 50 100 150 200 250 300

distance downstream (m)

999897969594939291

103102101100

)m(noitaveleevitaler

distance downstream (ft)

)tf(noitaveleevitaler

vertical exaggeration = 10

2

3 4

5 6

7

8

9

10

A

E

H

G

B

C

D

F

culvert

rock

weir

Existing culvert

diameter 3.1 m

length 22 m

bedrock

channel

bedrock

plunge

pool bedrock along right bank bedrock, left bank of plunge pool

tributary

H-ctp

M-ctpH

-ctp

H-ctp

H-ctp

M-ctpsediment

wedg

{

e

channel-bed profilepooltop of bank/floodplain

cross section location

grade controlpool tail crest, high stability

pool tail crest, moderate stability

step, high stability

log weir, moderate stability

log weir, low stabilityslope segments (A,B,C,D,E,F,G,H)

lw-M

2

ptc-H

ptc-M

st-H

lw-L

164.1 328.1 656.2 492.2 820.3 984.3 0

298.6

301.9

305.1

308.4

311.7

315.0

318.3

321.5

324.8

328.1

331.4

334.7

337.9

segment

elevation 

change 

(ft)

segment 

length 

(ft) gradient

% gradient 

difference 

between 

successive 

segments

maximum 

residual 

pool depth 

(ft)

number 

of grade 

controls

distance between 

grade controls 

(ft)

A 0.94 53.17 0.0178 n/a 1.54 2 53.2B 0.41 81.53 0.0050 -71.9 0.34 2 81.7

C 2.68 185.21 0.0145 190.4 1.07 4 62.0, 101.7, 21.6

D 1.78 92.69 0.0193 32.9 2.29 3 57.7, 34.8

E 0.73 11.01 0.0665 245.2 0.99 2 11.2

culvert 0.27 72.09 0.0037 -94.4 4.38 2 72.2

F 5.70 234.28 0.0243 551.5 0.82 3 145.7, 88.6

G 1.27 66.25 0.0192 -21.0 1.70 2 66.3

F,G 6.97 300.53 0.0232 -4.6a 1.70 4 145.7, 88.6, 66.3

H 4.69 151.32 0.0309 60.8b,c 0.68 3 92.2, 59.1

c. When compared to combined slope segments of F and G, the percent gradient difference is 33.3% a. Percent gradient difference when compared to slope segment F.

b. Percent gradient difference when compared to slope segment G. 

channel-bed profile

pool

top of bank/floodpla ni

cross section location

grade control

2

segment

elevation 

change 

(ft)

segment 

length 

(ft) gradient

% gradient 

difference 

between 

successive 

segments

maximum 

residual 

pool depth 

(ft)

number 

of grade 

controls

distance between 

grade controls 

(ft)

A 0.94 53.17 0.0178 n/a 1.54 2 53.2

B 0.41 81.53 0.0050 -71.9 0.34 2 81.7
5—45

Chapter 5—Site Assessment

Figure 5.10—Newbury Creek cross-section profiles and photos. (Cross-sections are plotted looking

downstream and their locations are shown in figure 5.9.) (a) Looking downstream toward cross

sections 3 and 4 (photo taken between cross section 2 and cross section 3). Two log weirs are

visible. In the background a bedrock outcrop is exposed on the right bank of the channel. (b) Looking

downstream at cross section 9.

(a)

(b)

0 5 10 15 20 25 30

distance from lb to rb (m)

94

100

relative

elevation (m)

0 20 40 60 80 100

distance from lb to rb (ft)

310

325

relative

elevation (ft)

XS9 bankfull water surface

lower terrace

lower terrace terrace terrace

0 5 10 15 20 25 30

distance from lb to rb (m)

98

104

relative

elevation (m)

0 20 40 60 80 100

distance from lb to rb (ft)

323

338

relative

elevation (ft)

XS3

terrace/abandoned

logging road terrace

bankfull water surface

floodplain

floodprone width

water surface

floodprone width water surface

0 5 10 15 20 25 30

distance from lb to rb (m)

94

100

relative

elevation (m)

0 20 40 60 80 100

distance from lb to rb (ft)

310

325

relative

elevation (ft)

XS9 bankfull water surface

lower terrace

lower terrace terrace terrace

0 5 10 15 20 25 30

distance from lb to rb (m)

98

104

relative

elevation (m)

0 20 40 60 80 100

distance from lb to rb (ft)

323

338

relative

elevation (ft)

XS3

terrace/abandoned

logging road terrace

bankfull water surface

floodplain

floodprone width

water surface

floodprone width water surface
5—46

Stream Simulation

Step 3. Like most undersized culverts, the one at Newbury Creek has an area of 

sediment deposition immediately upstream of the inlet. Low sediment 

loads (due to the upstream meadow) and the steep, confined channel keep 

the sediment wedge small. Nonetheless, because the culvert is nearly flat 

(0.4 percent), some minor deposition has occurred in the culvert. 

The plunge pool downstream of the culvert outlet is much deeper than other 

pools. Residual pool depth is 4.4 feet, about twice the residual depth of pools 

that form naturally elsewhere in the channel. The plunge-pool tail crest is a 

constructed rock weir of angular rocks (riprap) much larger than the native bed 

material (600 to 750 millimeters) (figure 5-11). 

Steps 4 and 5 of the Newbury Creek longitudinal profile analysis are in 

sections 5.2.2.1 and 5.2.2.2, respectively.

Figure 5.11—Outlet of existing culvert on Newbury Creek, Olympic National Forest, 

Washington. 
5—47

Chapter 5—Site Assessment

Uniform A uniform profile has no slope transition, making this the ideal crossing 

situation [figure 5.12 (a)]. Even where the profile is uniform, though, 

aggradation upstream of an undersized culvert [figure 5.12 (b)] can 

reduce the local slope. Such a profile can be mistaken for convex [figure 

5.12 (d)] if the surveyed longitudinal profile does not extend beyond the 

aggradation, or if the aggradation is not recognized. Field evidence of 

aggradation upstream of an undersized culvert can include a relatively high 

gravel deposit in the center of the channel above the existing structure, 

a widened and/or divided channel, bank erosion, or a bar deposit just 

upstream from the culvert with finer sediment than at other locations. An 

aggraded reach may also appear simpler and more homogenous because 

structural features such as steps may be buried by sediment. Backwater

aggradation is not limited to uniform profiles, of course. It can occur 

upstream of any undersized culvert. 

Concave A concave transition is an abrupt slope transition from steep to flatter 

[figure 5.6 (c)], such as on a flat valley bottom near the toe of a hillslope. 

Such an area is a natural depositional zone, where sediment accumulation 

through the crossing structure can reduce the structure’s hydraulic capacity 

(see figures 4.7 and 4.8). Occasionally, sediment deposition can also 

plug the channel, and cause the stream to cut a new channel in a different 

location. If the excavation for a replacement structure cuts into the bed 

of the steeper reach and no upstream grade control exists, upstream 

headcutting and additional sediment deposition may result. 

Convex A convex transition is a slope transition from a mild slope to a steeper 

one [figure 5.12 (d)]. Depending on how close the crossing is to the 

grade break, flow acceleration resulting from either the structure or a 

disturbance during construction can destabilize bed structures that control 

the downstream grade. Destabilization, in turn, could create a headcut that 

might migrate upstream through the structure and undermine it. 

Complex A complex transition is a profile with both a convex and concave shape 

[figure 5.12 (e)]. This type of transition has both the upstream problems of 

the concave type and the downstream problems of the convex type. 
5—48

Stream Simulation

Figure 5.12—Longitudinal profile shapes: (a) uniform; (b) uniform reach affected by local scour and aggradation

due to undersized culvert; (c) concave transition; (d) convex transition; (e) complex transition; (f) incised channel;

(g) road-impounded wetland.
5—49

Chapter 5—Site Assessment

A road crossing placed at a convex or concave site may exacerbate 

the natural tendency toward aggradation or degradation if the crossing 

constricts the stream, or construction disrupts key grade controls. This 

can lead to a perpetual need for maintenance and the chronic channel 

disturbance associated with it. Consider road relocation away from 

concave or convex sites. Even though relocation may appear expensive, 

it may sometimes be cheaper than long-term costs associated with 

maintaining a poorly located crossing. 

Local scour versus 

regional incision Longitudinal profiles at culverts often show that the culvert is perched, but 

the elevation differential can have several causes: the downstream channel 

may have incised since the culvert was installed [regional incision, figure 

5.12 (f)]; high velocity flow from the culvert outlet may have scoured a 

local plunge pool [figure 5.12 (b)]; or the culvert may have been placed too 

high during construction [figure 5.12 (g)]. Distinguishing local scour from 

regional incision is important, because the scale of the design solutions 

will be very different (see also section 6.1.2.1). 

The vertical offset between the upstream and downstream channel bed 

profiles is a primary tool for determining whether degradation at the 

culvert is a local effect or the result of larger-scale channel incision 

(review appendix A). In figures 5.12 (b) and (g) and 5.13 (a), channel 

scour is local. When the downstream profile is extended upstream beyond 

the influence of the culvert, the profile aligns vertically with the upstream 

channel. The culvert is perched, but the perch is caused by local scour. 

In contrast, in figure 5.12 (f) and 5.13 (b), when the downstream profile 

is extended, it is approximately parallel to the upstream channel but at 

a lower elevation. A longitudinal profile with this channel-bed offset 

identifies an incised channel where the existing culvert is functioning as a 

grade control. 
5—50

Stream Simulation

Figure 5.13—Distinguishing (a) downstream local scour from (b) channel incision.

Channel-bed offsets on either end of a culvert can also occur from other 

causes. For example, a natural slope transition can sometimes appear 

as an offset (figure 5.12 (c), (d), or (e)). Abrupt changes in streambed 

elevations also occur in steep streams where bedrock or large logs control 

steps. If the existing culvert was placed on top of an earlier failed culvert, 

the upstream channel could have massively aggraded, and both road 

and streambed profiles are higher than otherwise. Or, the culvert could 

have been constructed on a bedrock ledge. In all these cases, it is less 

likely that the upstream and downstream profiles would be parallel. Field 

observations and historical information about the crossing will help define 

which of several possible causes is responsible for the change in streambed 

elevation.
5—51

Chapter 5—Site Assessment

Cross sections are an excellent way to verify whether a downstream reach 

is incised (see appendix A, section A.7.2). An incised channel downstream 

from a crossing structure where the crossing is functioning as a grade 

control will have different cross-section characteristics from the unincised 

upstream channel (see end of section 5.1.4.2). Bed material also is likely to 

be different—possibly coarser—with less accumulation of gravel or fines. 

Newbury Creek Site Assessment

Longitudinal profile analysis step 4, identify profile shape

At first glance, the Newbury Creek longitudinal profile (figure 5.9) leads 

one to suspect that the downstream reach may have incised. When 

a straight line along the downstream grade controls (the longitudinal 

profile) is extended upstream of the culvert, it is substantially lower 

than the upstream streambed. However, several pieces of evidence 

suggest this is not a case of channel incision. For one, the cross 

sections (figure 5.10) indicate that the banks downstream are not 

higher than those upstream; in fact, the downstream reach is less—

not more—entrenched. There is no evidence of bank instability and 

no indication that either bed or banks have adjusted to a lowering of 

the channel bed. 

The evidence confirms what the site sketch (figure 5.2) suggested—

that the crossing is located at a geomorphic transition. The valley is 

narrow and controlled by bedrock upstream from the crossing, and the 

valley is wider and alluvial downstream from the crossing. The road 

crosses the stream at the head of the alluvial valley. The longitudinal 

profile shape is complex due to local steepening immediately upstream 

from the crossing, where bedrock outcrops in segment D constrict the 

channel for a short distance.

See figure 5.17 for step 5 of the longitudinal profile analysis for the 

Newbury Creek site. 
5—52

Stream Simulation

For incised channels, verify the cause, scale, extent and stage of incision if 

at all possible. For design of any crossing, it is important to know whether 

incision is actively progressing, stabilizing, or recovering. If the cause is 

an upstream-migrating headcut, comparing the downstream reach to the 

channel evolution model (appendix A, section A.7.2) can help determine 

the stage of evolution. If the cause is a local influence, such as removal of 

woody debris or loss of a local grade control, then, with time, the bed may 

aggrade naturally back to its original profile. To accelerate the recovery 

process, the crossing project could include restoring the incised section to 

grade.

Road-impounded

wetlands Some road crossings with culverts that are undersized or that were 

installed too high cause ponding upstream (figure 5.12(g)). The ponding 

causes sediment deposition, which reduces the supply of sediment to the 

downstream channel. At these sites, the longitudinal profile usually shows 

an aggradation wedge, bed material is likely to be distinctly finer upstream 

than downstream, and vegetation may be different. The team will need to 

choose whether to preserve the wetland area, remove it, or allow it adjust 

naturally to a stream simulation replacement culvert. Because of a general 

loss of wetland habitat in some basins, resource managers are often 

motivated to preserve these wetland areas. 

To preserve the wet area and provide some measure of aquatic organism 

passage, a design method other than stream simulation is usually needed. 

Stream simulation may not be possible in these cases because simulating 

the natural channel slope, form, and processes through the crossing 

would cause incision in the upstream wet area when some or all of the 

accumulated sediment is remobilized. On the other hand, if you design an 

over-steepened channel to preserve the wetland, the channel would not 

be self-sustaining because the sediment sizes necessary for sustaining the 

steeper slope could not be transported through the wetland to the channel. 

(Refer to appendix B for design methods other than stream simulation. 

Use these methods where the channel through the crossing must be 

substantially steeper than the natural channel, and achieving stream 

simulation objectives is unlikely.) 

5.2.2.2 Determining vertical adjustment potential

One of the first steps in stream-simulation design involves selecting the 

gradient and elevation for the streambed that will be constructed—that is, 

the project profile. (See section 6.1.2.2 for detailed discussion of project 

profiles. It might be a good idea to review that section now, to get an idea 
5—53

Chapter 5—Site Assessment

of how the design uses the interpretations discussed here.) Before selecting 

the project profile, however, the team needs to predict the elevations 

between which the stream bed might vary over the service life of the 

structure: the vertical adjustment potential (VAP). The upper and lower 

VAP lines represent respectively the highest and lowest likely elevations 

of any point on the streambed surface in the absence of any crossing 

structure. This section describes the considerations that go into forecasting 

the VAP lines for the structure’s lifetime. There is no cookbook approach 

to selecting the upper and lower VAP lines; they are based on the team’s 

interpretation of conditions and processes in the stream that might affect 

the elevation of the channel in the future.

Depending on channel type and condition, processes that can change the 

streambed elevation, whether permanently or temporarily, include: 

l Channel incision caused by downstream base-level change. 

l Increased flows or sediment inputs resulting from land management 

changes or climatic events in the watershed. 

l Aggradation or degradation at a slope transition.

l Erosion and deposition of key features like boulders, steps, and large 

woody debris. 

l Channel scour and fill during floods and debris flows. 

l Headcutting upstream of a larger replacement culvert, as aggraded 

sediment is mobilized. 

l Pool formation.

Try to predict what types of changes might occur and estimate how the 

channel might respond to those changes. Consider first the potential for 

large-scale, long-term channel change, such as deposition due to debris 

flow, or regional channel incision due to base-level changes downstream. 

Then consider local changes, such as movement of one of more key 

features or formation of a debris jam. Predicting how such changes may 

affect bed elevations is necessarily subjective; use every available piece 

of field and historical evidence available. Be conservative where the 

probability of vertical adjustment is high, such as where large amounts 

of wood are in the channel, or where channel incision is expected. If 

you are uncertain how the channel might change in the future, design 

conservatively and consider getting additional expertise to help predict 

future conditions.
5—54

Stream Simulation

In channels where large wood or rock steps control bed elevation, if these 

key features do not move, they will control the lower limit of vertical 

adjustment for the lifetime of the replacement structure. On the other hand, 

loss or outflanking of one or more of these key features could cause a large 

change in bed elevation over some length of stream as the channel adjusts 

toward a new equilibrium. The length of stream affected depends on the 

stability of the adjacent grade controls and on the depth of channel bed 

lowering. Usually, the material from the failed step moves only a short 

distance downstream, filling in the downstream pool and reorganizing the 

bed to form a new grade control. See the Fire Cove Road VAP analysis, 

figures 5.14 and 5.15.

If the key features are less stable, project how bed elevations are likely to 

change when they move. In intermediate and low-mobility channels, some 

amount of channel-bed fluctuation will always occur as wood pieces or 

rock grade controls enter or move through the channel, or as bedforms and 

bend locations change. Debris jams or buried small debris can temporarily 

retain sediment upstream, and they may form a scour pool downstream. If 

the debris moves, how will the stream adjust? Generally, the height of the 

grade controls, (log or rock steps, pool-tail crests, debris accumulations) 

indicates the scale of bed adjustment expected after one or a series of 

grade controls moves.

In stable channels where the bed surface as a whole is not expected to 

change (e.g., due to base level lowering or changes in flow), the depth of 

ordinary pools is a reasonable estimate of the lowest likely bed elevation 

in any slope segment. Unusually deep pools formed by large key features 

would not be considered in this analysis since they would not form inside a 

culvert. The depth of surveyed pools, however, represents only a snapshotin-time of a dynamic channel that undergoes scour and fill during high 

flows. Limited research has shown that, in armored gravel-cobble bed 

streams, flood scour depths are on the order of twice the thickness of the 

armor layer, or about twice D90 (Bigelow 2005; Haschenburger 1999). It 

makes sense in these cases to expect that—temporarily at least—the bed 

may be that much lower than the bottoms of pools. If the level of risk 

warrants, the lower VAP line can be lowered to account for that. 

Channel incision that affects long stream reaches can occur due to a 

variety of causes. Downstream influences include in-stream gravel mining 

or channel straightening that cause a headcut to begin moving upstream; 

upstream causes might be an upstream dam that reduces sediment loads, or 

any land management activity that reduces infiltration and increases peak 

runoff rates. Predicting the lower VAP line under these conditions requires 

estimating how much of this large-scale incision may occur at the crossing 

site, and then adding the depth of pool scour to that estimate. 
5—55

Bank 

seepage 

Riffle with 

2”- 4” rock 

steps 

Boulder/cobble tailwater control 

Cross-section 

Sediment deposit 

Rd 8060200 

Tributary 

(BF width = 3-4’) 

P 

P 

P 

P 

P 

P 

Rootwad HS 

100’ 

Debris/boulder cascade HS (see photo b)

2.5’ log step 3’ high. HS (see photo c)

Stump embedded 

in bank 

Undercut bank 1’-2’ 

Wood/boulder steps 4”- 6” 

2’- 3’ spacing 

20’ 

10’ 

50’ 

100’ 

P 

P 

Abundant logs (>12”) over & 

in channel 

18” log step HS 

60’ 

40’ 

Channel width 5’- 6’

rock and wood

steps, 1’ spacing

P = Pool

BF = Bankfull

HS = High stability

N

Chapter 5—Site Assessment

Figure 5.14—Fire Cove Road crossing, Tongass National Forest, Alaska: (a) site sketch; (b) looking upstream

from the crossing at a high-stability step/cascade; (c) downstream of the crossing looking upstream at a highstability log step (road is in light background area).

(b)

(c)

(a)
5—56

Stream Simulation

Figure 5.15—Longitudinal profile with two possible lower VAP lines for the Fire Cove Road crossing. Either of the 

two lower VAP lines could be used depending on how stable the downstream control is judged to be. In chapter 6, 

we will see how this crossing was actually designed (figure 6.7).

The Fire Cove Road crossing on the Tongass National Forest is a good example 

of predicting vertical adjustment potential (VAP) in a steep stream with large log 

steps (figure 5.14). In this example, no regional channel incision or aggradation 

is expected. The solid, well-embedded 2.5-foot diameter log about 50 feet 

downstream of the culvert [figure 5.14(c)] is a key feature controlling the grade. Just 

upstream of the culvert is a high-stability feature: a debris-and-boulder cascade 

where bed elevation is unlikely to change. Figure 5.15 shows two alternatives for 

the lower VAP line at this site. VAP line 1 assumes the stable downstream log 

does not move over the lifetime of the project. VAP line 2 indicates how deeply 

new pools in the project reach could scour if the log does move. Headcutting 

would end at the high-stability cascade section even if the downstream grade 

control is lost.
5—57

Chapter 5—Site Assessment

Also think about any features or processes upstream that may cause the

channel to rise. Some examples are:

l	 Headcuts, bank failures, landslides, or debris flows occurring

upstream may create a potential for large amounts of sediment

deposition in the structure. Debris released by the headcut can

exacerbate the deposition problem. (See Benda and Cundy 1990, for

a method of predicting the risk of debris flow deposition).

l	 Formation of a debris jam and sediment accumulation behind it can

easily cause local bed elevations to rise.

l	 Evidence of recent aggradation or heavy bedload movement may

indicate the channel is aggrading, or it may be recovering from

aggradation.

l	 If the channel is unnaturally lacking in debris, consider whether trees

falling into the stream in the future might retain sediment and raise

the channel-bed elevation.

l	 Crossings located on tributaries near their junctions with a larger

river may experience aggradation if they are backwatered by high

flows in the river.

Using all the information, draw at least two lines on the longitudinal

profile to show the range of possible future bed elevations at the site

(figure 5.16). Delineate the lines for channel segments outside the

influence of the existing structure, and then connect them through the

project reach as though no structure were there. Draw them approximately

parallel to the average grade of each slope segment unless bedrock or other

immobile controls dictate a different slope.

The scenarios represented in figure 5.16 illustrate how the VAP lines were

delineated in three different hypothetical cases. Figure 5.16 (a) shows

the longitudinal profile of a 10-foot-wide stream crossing a road in a

4-foot culvert. The channel profile shape is uniform, and the stream is in

dynamic equilibrium. Watershed conditions are stable; there is no reason

to expect regional channel incision due either to headcut migration from

downstream or to changes in flow or sediment loads. The channel is an

armored gravel-cobble pool-riffle channel with some woody debris. Pools

not associated with large key features or the existing undersized culvert

are a maximum of 2 feet deep. The lower VAP line is at 2.8 feet below the

existing profile, 0.8 foot being added as a safety factor for potential scour

during floods. The depth of potential scour is estimated as twice the D90

size of 0.4 foot.
5—58

Stream Simulation

The upper VAP line in figure 5.16 (a) is at the top of the 2.5-foot-high bank 

because debris accumulations in this vicinity can extend that high. The top 

of the bank is the maximum elevation to which sediment could aggrade 

behind such an accumulation.

Figure 5.16 (b) shows the same channel after a 2.5-foot headcut moved 

up from downstream and was stopped by the existing culvert. The incised 

channel profile is 2.5 feet lower than the undisturbed (upstream) channel 

profile projected downstream. Here, if the culvert were not in place, the 

headcut could continue to move upstream causing incision up to 2.5 feet. 

Thus, the lower VAP line is 2.8 feet below the incised-channel longitudinal 

profile. Downstream of the road, a 3-foot-high debris jam of small trees 

that were undermined by bank erosion constitutes one piece of evidence 

for locating the upper VAP line at 3 feet above the incised channel profile 

(below the top of the bank). Again, if the culvert were not in place, the 

headcut would continue migrating upstream, and upstream VAP conditions 

would be essentially the same as those downstream. 

Figure 5.16(c) is a very different scenario, a concave profile. The road is 

located where a steep (8 percent) step-pool channel meets the valley floor 

of a larger river. Downstream of the transition zone, the stream meanders 

across the valley on a 2-percent grade to join the river. The steeper channel 

currently appears stable, but the height and composition of the banks at 

the valley edge show that the channel has deposited substantial sediment 

and debris there during past floods. Private property makes road relocation 

impossible here. 

The upper VAP line in this example is drawn at the top of the 2-foot-high 

banks in the valley section, and at the top of the higher banks in the slope 

transition section. We are presuming that at least short reaches of channel 

can fill to the top of the bank behind debris accumulations. The lower VAP 

lines in each channel segment are below the bottoms of the pools by a 

depth of two times D90. 

As shown in figure 5.16(c), where a channel has distinct gradebreaks, 

VAP lines can be drawn in segments. The high- and low-potential profiles 

might not be parallel where some feature will limit the possible channel 

elevation from going higher (e.g., flood-plain elevation) or lower (e.g., 

bedrock). Drawing several possible profiles—to show the range that might 

be expected at the site, given the existing grade controls and how they 

might change—is helpful. Where substantial uncertainty in the degree of 

potential vertical adjustment exists (e.g., in a channel with a highly mobile 

bed and good potential for debris jam formation), you might increase the 

range of potential vertical adjustment to offset the risk of error. Note your 

assumptions and relevant observations on the profile.
5—59

Chapter 5—Site Assessment

Figure 5.16—Range of vertical adjustment potential for three longitudinal profile types: (a) uniform profile, (b)

incised channel profile, (c) concave slope transition. The “channel profile” lines are the “slope segment” lines

drawn in step 2 of the longitudinal profile analysis (section 5.2.2).
5—60

Stream Simulation

Newbury Creek Site Assessment

Longitudinal profile analysis step 5, determine vertical adjustment potential

Figure 5.17—Newbury Creek longitudinal profile showing vertical adjustment

potential.

As noted in the previous Newbury Creek sidebar (section 5.2.2.1), the channel 

downstream of the crossing has not incised, and there is no reason to expect 

incision in the future. Therefore, the lower VAP line includes only the maximum 

residual depth of pools (1.6 feet) for each slope segment, plus the anticipated 

flood scour depth (1.3 feet, twice the D90 of 0.65 foot). The lower VAP line is 

therefore 2.9 feet below and parallel to the slope segment lines except where 

bedrock forces the projected lower VAP line higher (segments A and D, and 

cross section 7).

The upper limit of vertical adjustment potential is taken as the top of the bank, 

and again the line approximately parallels the slope segment lines. Near the 

culvert inlet, the line is lower than the upper bank because backwater from 

the undersized culvert has caused the streambed to aggrade there. When 

the culvert is removed, the aggraded material is expected to erode and the 

streambed should stabilize at its natural, lower elevation.
5—61

Chapter 5—Site Assessment

5.3 Project Site Risk Assessment

Continuing to build on the initial assessment and the longitudinal profile 

analysis, assess all risks at the site. Use all available data and observations 

to interpret current project site conditions, predict potential channel 

changes, and identify significant risks that the design will have to deal 

with. Review the site suitability determination in light of your more indepth understanding of the site.

Sometimes, design issues are associated with specific channel types (see 

table 5.7). For example, slightly entrenched channels have wide flood 

plains which can convey high flows during floods. Such road-stream 

crossings have risks associated with flood-plain constriction and lateral 

channel migration. Other risks can pertain to any channel type, depending 

on watershed and reach conditions. 

5.3.1 High Flood-plain Conveyance

When it occurs, high flood-plain conveyance (i.e., a high flow on the flood 

plain during floods) is an important factor affecting design. When floodplain conveyance is high and overbank flow occurs frequently, it may be 

necessary to install other flood-plain drainage structures under or across 

the road. The objective is to avoid funneling overbank flows through the 

main crossing structure, which would destabilize the simulated streambed 

in the culvert. Alternatively, a bridge or viaduct could be considered as a 

replacement structure.

To determine whether high flood-plain conveyance is an important issue 

at the site, estimate the depths and velocity of recent overbank flows. Use 

observations of past flood elevations and flood-plain scour and deposition 

features (section 5.1.4.2), together with historical flood data. Flood-plain 

vegetation and erosional and depositional features observed during the 

cross-section surveys may indicate recent overbank flow depths and should 

give a qualitative indication of the frequency and intensity of overbank 

flows. The presence of flood swales or side channels, for example, 

indicates enough overbank flow to cause significant scour. These channels, 

which can convey large amounts of flow, also may be important refuge 

or juvenile habitat for aquatic species. Identify them as key locations for 

flood conveyance and, where appropriate, aquatic organism passage. Be 

sure to evaluate whether evidence of overflow on the flood plain upstream 
5—62

Stream Simulation Table 5.7—Crossing design issues associated with specific channel characteristics. CHANNEL CHARACTERISTIC High flood-plain conveyance, frequent overbank flow

Channel migrating 

laterally across 

valley floor

Large elevation 

drop across existing 

structure caused 

by downstream 

incision

COMMON MAJOR ISSUES 

FOR CROSSING DESIGN

Crossing may constrict flood-plain 

flow, increasing velocities through crossing structure.

Road approach may block flow 

in flood plain, causing backwater ponding.

Alignment may change over time 

so that stream approaches inlet at a greater angle. Bank and roadfill erosion is progressive as alignment worsens.

Preventing upstream incision 

requires over-steepening the structure’s profile and/or adjacent reaches. 

Lowering the crossing to the 

level of the downstream channel may initiate a headcut moving upstream.

If channel is still incising, 

dimensions, elevation and grade may change over the structure lifetime. 

POSSIBLE DESIGN SOLUTIONS 

(SEE ALSO TABLE 6.7)

• Add overbank flow surfaces inside culvert.• Widen simulated channel by adding flood 

plain inside culvert. Consider permeable roadfill (6.5.1.1), bridge, or viaduct.

• Add flood-plain culverts and/or road dips 

for floodrelief and flow distribution. 

• Add flood-plain culverts or road dips.• Consider permeable roadfill (6.5.1.1).• Relocate crossing. 

• Widen crossing structure.• Build inlet transition (6.1.1.4). 

• Build structure that can be moved in future.• Reconsider designed project profile 

(6.1.2.3).

• Design steepened profile using appropriate 

permanent grade controls (Appendix B.2).

• Restore incised channel to original grade.• Extend length of project to permit 

controlled incision upstream. Consider moderate-stability grade control structures upstream to control rate of headcut migration.

• Restore downstream channel to original 

grade.

• Mitigate specific risks. 

• Wait to replace crossing structure until 

incision has stopped.

• Size culvert (or foundation) to 

accommodate possible range of profiles (see section 5.2.2.2). 

• Add downstream grade controls to control 

incision (6.1.2.3). 

• Restore channel to original grade.

SPECIFIC RISKS

• Simulated bed may not be sustainable.• Depositional/erosional processes that maintain 

flood-plain habitats can be interrupted. 

• Increased probability of sediment/debris 

blockage at inlet.

• Culvert capacity may be reduced due to 

aggradation or inlet energy losses.

• Bed scour can occur in stream simulation 

culvert inlet.

• Some aquatic species may not pass 

steepened crossing. 

• Simulated streambed may not be sustainable 

if upstream reach does not provide amount or size of sediment required.

• Habitat loss or degradation upstream.• Upstream channel incision can isolate stream 

from flood plain. 

• Upstream channel incision can destabilize 

banks. 

• Crossing structure and/or grade control 

structures may become perched or fail.
5—63

Chapter 5—Site Assessment

Table 5.7—Crossing design issues associated with specific channel characteristics (continued).

CHANNEL 

CHARACTERISTIC

Channels 

transporting high 

volumes of woody 

debris or sediment, 

or subject to debris 

flows

Concave transitions

(6.1.2.2)

including 

braided streams 

and alluvial fans 

Convex transition

(6.1.2.2)

COMMON MAJOR ISSUES 

FOR CROSSING DESIGN

Crossing structure may plug 

and fail. 

Channel may aggrade and/or 

shift location.

Downstream bed may be 

destabilized during construction or during floods.

POSSIBLE DESIGN SOLUTIONS 

(SEE ALSO TABLE 6.7)

• Consider structures with removable

tops for clean-out.

• Harden approaches and fill for 

overtopping.

• Prevent stream diversion if crossing is 

overtopped.

• Relocate crossing away from transition 

or alluvial fan.

• Build structure that can be moved in 

future.

• Use wider, higher crossing structure 

(e.g., bridge).

• Adjust project profile to intermediate 

grade (section 6.2.2.2).

• Avoid destabilizing steeper upstream 

reach during construction.

• Size structure to accommodate vertical 

adjustment potential .

• Reevaluate vertical adjustment 

potential; consider lowering VAP line to accomodate risk of headcutting.

• Construct new grade controls or 

reinforce natural controls.

• Widen structure to avoid concentrated 

outflow.

SPECIFIC RISKS

• Road and aquatic habitat damage.• Frequent maintenance may be needed.• Traffic interruptions may be long.• Sediment deposition may reduce culvert 

capacity. 

• Debris may block inlet.• Stream may move across fan or valley 

away from culvert.

• Headcut moves upstream through 

culvert.

• Habitat loss or degradation upstream.• Upstream channel incision can isolate 

stream from flood plain.

• Upstream channel incision can 

destabilize banks.

• Crossing structure and/or grade control 

structures may become perched or fail.
5—64

Stream Simulation Table 5.7—Crossing design issues associated with specific channel characteristics (continued). CHANNEL CHARACTERISTIC Channels with large key features (large wood, boulders) controlling slope and roughness (6.1.2.2)

Bedrock 

Cohesive material 

(silt/clay)

COMMON MAJOR ISSUES 

FOR CROSSING DESIGN

Key features must be simulated 

inside the structure.

Key features (wood) may 

stabilize alluvial veneer over bedrock in reference reach. 

Bed construction inside pipe is 

not practical because silt/clay material cannot be moved and stabilized.

POSSIBLE DESIGN SOLUTIONS 

(SEE ALSO TABLE 6.7)

• Select materials and construct key 

features carefully.

• Use shorter, wider crossing structure.• Span channel without disturbing 

streambed and banks.

• Construct fixed key features to retain 

alluvium. 

• Span channel without disturbing 

streambed and banks.

SPECIFIC RISKS

• Simulated key features are of different 

material (i.e., rock simulates logs) and may not function the same (eg., diversifying water velocity, retaining sediment).

• Simulated features may trap sediment 

and debris to form a dam.

• Alluvial veneer may not be 

sustainable in culvert without stable key features. 

• Constructed bed (or no bed) may not 

simulate natural channel and aquatic species may not be able to move through.
5—65

Chapter 5—Site Assessment

of the road crossing might simply be the result of flow constriction at an 

existing undersized crossing. If so, a larger structure may be all that is 

needed to solve the problem. 

Flood-plain observations will also help in selecting a roughness factor 

for flood-plain flow estimation, if you intend to use a model such as 

WinXSPRO or HEC-RAS. 

5.3.2 Lateral Adjustment Potential and Alignment

On streams with a high potential for lateral channel migration, the 

channel’s angle of approach to the crossing structure may become more 

acute over time. As described in appendix A, a poor alignment is an 

especially important risk factor in streams transporting woody debris. 

Evidence of past channel shifting (e.g., an acute angle of approach to the 

culvert inlet, bank erosion on one bank) can help in evaluating the risk 

to the replacement structure. Also consider factors, such as current bank 

stability (section 5.1.6.4), land use and vegetative condition, and probable 

future land use changes. 

Understanding the natural channel’s (pre-disturbance) pattern is essential 

for proper layout of a stream-simulation installation. Culverts shorten 

and steepen channels when they replace a bend. In the case of a streamsimulation culvert, such an increase in channel slope could put the 

simulated streambed at risk. Using the sketch map and field observations, 

try to detect the natural channel location and pattern. This would be the 

starting point for designing the replacement crossing alignment. 

It is especially important to consider natural channel pattern where a 

crossing must be located on a meandering stream. Several options are 

described in section 6.1.1 for minimizing risk by keeping the crossing 

short, aligning it with the stream, and providing efficient transitions. 

Preview that section and consider the various alignment options (figure 

6.4) while still in the field. Observations of bed and bank stability are 

vital in selecting the least damaging option. If a skewed culvert-tochannel alignment is being considered, bank materials and stability will 

determine whether bank stabilization measures are needed near the inlet or 

outlet. Where channel straightening cannot be avoided, the channel may 

respond by eroding either its banks or its bed. Try to predict likely channel 

responses to such changes by considering the relative resistance of bed and 

bank materials. 
5—66

Stream Simulation

5.3.3 Headcutting Potential 

Even in a uniform longitudinal profile, simply replacing an undersized 

culvert with a larger one set lower in elevation can cause the adjacent 

stream reaches to adjust. Sediment accumulated above the old culvert 

remobilizes, although usually the adjustment is not large enough to create 

a problem. Where the downstream reach has incised, however, headcutting 

upstream of the replacement structure (section 5.2.2.2) can be substantial 

enough to affect buried infrastructure, destabilize streambanks, modify 

aquatic habitats, etc. Decide whether to control such a headcut or allow it 

to progress upstream, considering the trade-offs between the extent and 

duration of impacts, versus the benefit of allowing the channel to evolve to 

a natural self-sustaining condition. 

Deciding how to handle any expected headcutting requires answers to 

questions such as the following:

l How much headcutting is likely if no controls are implemented? How 

far upstream might it go?

l What effects will the expected headcut have on streambed and banks? 

How long will they last?

l Should headcutting be prevented? 

l Should headcutting be allowed to occur at an uncontrolled rate?

l Should the rate of headcutting be slowed by temporary grade 

controls?

Before making these decisions, be aware of the types of effects headcuts 

can have. Bates (2003) identified the following physical, biological, and 

infrastructure issues for teams to consider when determining whether to 

control a headcut or allow it to occur. 

Extent of headcut The upstream distance that a headcut can travel depends on the stream 

slope, bed composition, sediment supply to the reach, and the presence of 

stable debris and/or large rock in the channel. The extent of headcutting is 

usually less in coarse-grained or debris-laden channels than in finer-bedded 

streams, because the headcut is more likely to encounter a stable grade 

control that prevents it from moving further upstream. A channel with a 

high supply of mobile bed material will reach equilibrium more rapidly 

than a channel with a low rate of sediment supply. 
5—67

Chapter 5—Site Assessment

Condition of

upstream channel

and banks Where a reach has aggraded above an undersized culvert, the channel can 

stabilize and return to its natural condition after some headcutting occurs 

through the aggraded area. If the upstream banks are already marginally 

stable, however, the degrading channel can undermine and destabilize 

them. 

Habitat impacts of

upstream channel

incision Allowing a large headcut to travel freely upstream can damage aquatic 

habitats. For example, a newly incised channel may be narrow and 

confined, with habitat diversity and stability reduced because the channel 

cannot access its flood plain during high flows. Although the channel 

may evolve back into its initial configuration (appendix A, figure A.28), 

substantial bank erosion and habitat instability may persist for a long 

time, up to a century in some cases (figure 5.18). Where bedrock is 

shallow, a headcut may expose it; and, if no debris or sediment structure 

is left, the stream will have difficulty trapping new sediments to recover 

habitat diversity and stability. Some bedrock (such as siltstone) is easily 

erodible once exposed. A headcut can also cause enough incision to 

leave side channels perched, inaccessible, or dry. Avoid headcuts in such 

areas. Restoring incised stream channels may require substantial channel 

reconstruction with wood and/or rock structures. 

Figure 5.18—Major channel instability occurring on the Homochitto River, MS. 

Bank erosion and widening follow channel incision on this fine-grained channel.
5—68

Stream Simulation

Wetlands have formed upstream of many undersized or perched culverts. 

Although artificial, these wetlands may perform important functions 

for the riparian ecosystem. Carefully consider their fate when replacing 

culverts. 

Presence of fish or

other organisms A headcut can pose a short-term risk of loss of organisms in the bed or 

pools just upstream of a culvert. The bed may scour at a lower flow than 

normal in a headcutting situation. Eggs and fry in the gravels may be lost. 

Habitat impacts to

downstream channel

from sediment release The risk to downstream aquatic habitats depends on the volume and rate 

of sediment released by a headcut, as well as the transport capacity in 

downstream reaches. Downstream of large headcuts, not only will the 

total volume of sediment in transport increase, but sediment will move 

at lower flows until the upstream channel and banks have stabilized. 

Sediment deposition may occur in streambed areas not normally subject to 

deposition. Small headcuts may not pose much risk at all to downstream 

reaches in many steep mountain streams. 

Decrease in culvert

and channel capacity

from initial slug of

bed material Where bed material is mobile, allowing an uncontrolled headcut upstream 

of a culvert may result in mobilizing a slug of material during a single 

flow event. As this material moves through the culvert and the downstream 

channel, it can reduce the capacity of both. A loss of capacity can result in 

additional deposition and, in extreme cases, can fill the entire channel and 

plug the culvert. 

Allow less headcutting where the culvert and/or channel have even a shortterm risk of plugging by sediment and debris. Consider similar limitations 

where structures further downstream are at risk from a loss of channel 

capacity or where banks are at risk of erosion. 

Proximity of upstream

utilities and structures If a headcut is allowed to continue upstream, it can jeopardize structures in 

or beneath the channel or on the banks. Asking the utility company to visit 

the site and locate any lines is common practice. Be aware of the potential 

effects of increased bank erosion on structures near the channel. 
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Chapter 5—Site Assessment

Potential for new fish

passage barriers

within the degraded

channel Consider the potential for channel incision to create barriers to passage 

of fish or other aquatic species. Buried logs, nonerodible materials, and 

infrastructure, such as buried pipelines, are commonly exposed by channel 

headcuts. As the channel headcuts to such a feature, the feature itself may 

become a new fish passage barrier. Adding to the difficulty, these problems 

may occur where they are not visible from the project site, where access 

is more difficult, or across a property boundary. In addition, upstream 

culverts could become perched, or, if they are embedded, their beds may 

wash out. 

Readers may also want to consult Castro’s 2003 discussion of headcutting 

considerations for the planning phase of a culvert replacement or removal 

project.

5.3.4 Debris 

To determine whether woody debris poses a potential hazard to the 

crossing structure, evaluate the stability, size, and accumulation potential 

of wood in the project reach, especially upstream of the road crossing. 

Look for debris accumulations, and dead or undermined trees that could 

fall into the stream. Review the debris risk assessment in section 4.3, the 

key-feature summary in table 5.6, and include historical information. Ask 

the following questions:

l Is the crossing in a land type where floods transport large wood?

l Has the existing structure ever had problems with woody debris 

plugging? 

l Are other nearby structures subject to plugging? 

l How large is the wood in transport? 

l What is the condition of wood in the reach? Is it durable, or fragile 

enough to break apart in transport?

To project future debris availability and stability, consider the long-term 

management plan in the watershed upstream of the crossing. Are debris 

inputs likely to change? 

Where wood is an important structural component of the channel, also 

consider whether downstream channel conditions and stability depend on 
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Stream Simulation

upstream woody debris inputs. If so, wood transport through the crossing 

structure may be critical to the long-term stability of the whole reach.

In general, stream-simulation culverts with good alignments tend to be 

large enough that debris passes freely through. However, difficulties might 

occur with large wood and rootwads in low-profile structures or where 

structures are poorly aligned with the stream. 

5.3.5 Unstable Channels

If the channel is unstable (rapidly incising, aggrading, shifting laterally, 

etc. See channel stability in glossary), the design will have to deal with 

changing conditions as the stream evolves toward a new equilibrium. Any 

work performed in these situations must factor in both reach-scale and 

watershed-scale processes:

l What is the cause of the channel instability? Is it caused by 

local land-use activities? Higher peak flows, due to watershed 

development? Downstream channel incision? Sudden, large lateral 

movements? Extensive bank failures?

l What is the proximity and extent of channel instability in relation to 

the crossing? 

l Are any restoration activities already planned for improving channel 

stability?

l What are the anticipated dimensions and configuration of the 

recovered channel? What is the time frame for recovery?

Where a channel has been recently disturbed by mass wasting events 

or extreme floods, consider leaving the road closed to allow time for the 

channel to adjust to the new conditions. If the road must be reopened, 

consider whether a channel restoration project is feasible, given watershed 

conditions and trends. If restoration is not feasible, the stream-simulation 

design approach may not work, and you might need to use an alternative 

design style (see appendix B).

If stream simulation is chosen, then it is important to estimate not only 

the vertical adjustment potential but also future channel dimensions 

and pattern. The uncertainty about channel change, as well as the 

unpredictability of future disturbances, can make this kind of prediction a 

very uncertain. Only a qualified and experienced team should perform the 

site assessment and replacement structure design on an unstable channel—

and, even then, the team should plan for maintenance. 
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Chapter 5—Site Assessment

 5.4 Document Key Design Considerations and

 Recommendations

At this point, document the results of the site assessment by summarizing 

the project site characteristics listed below. See also the assessment 

checklist in appendix C.

Project reach characteristics and risks:

l Longitudinal profile; what key features control channel slope? How 

mobile are they?

l Downstream channel incision; is the crossing acting as grade control? 

l Vertical adjustment potential. 

l Bed material size and mobility.

l Bank materials, height, and stability. 

l Variability in channel bankfull width; what controls differences in 

width?

l Potential for lateral channel shift and bank erosion. 

l Estimate of bankfull and 100-year flows. 

l Flood-plain conveyance; sites for flood-plain drainage structures. 

l Flood-plain constriction potential.

l Geotechnical concerns: soft soils, bedrock, ground water. 

l Key grade controls that anchor the longitudinal profile and that 

should not be disturbed in construction. 

l Habitats requiring special protection in design and during 

construction. 

l Site logistical constraints (property boundaries, infrastructure, etc.). 

l Construction and maintenance access.

l Sensitive areas (to avoid during construction) in vicinity of crossing. 

l Potential locations for construction equipment and materials storage.

l Construction recommendations: topsoil and vegetation salvage 

needs and opportunities, potential areas for dispersing and filtering 

sediment-laden water pumped from the excavation, etc. 
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Stream Simulation

Also, document interpretations of important geomorphic processes that 

may affect the site, the new structure, and the feasibility of a streamsimulation design. How will the channel respond to the replacement or 

removal of the crossing? How should the channel and/or road be modified 

to accommodate a new structure? With this detailed understanding of the 

site, revisit the project objectives defined earlier, and develop them into 

specific design objectives. If stream simulation appears to be infeasible, 

consider other design methods (see appendix B). Site-specific design 

objectives might deal with some of the following topics:

l Need for alignment control.

l Need for grade controls outside the crossing. 

l Need for channel restoration or habitat protection.

l Special sediment control or stabilization measures needed at road 

crossing or in stream.

l Characteristics needed for aquatic species passage. 

l Characteristics needed for passage of semi-aquatic and terrestrial 

species. 

A key task is to agree on the channel characteristics needed to achieve 

the desired degree of passage. For example, if weak-swimming species, 

amphibians, and small mammals that depend on channel margins for 

movement need to pass through the structure, the structure will need to 

be wide enough to maintain banklines or dry margins at low to moderate 

flows. 

5.5 Reference Reach: The Pattern for StreamSimulation Design

The reference reach will not be finally selected until the project profile 

design is complete (see section 6.1). However, geomorphic data on one 

or more potential reference reaches are generally collected during the site 

assessment. For that reason, criteria for selecting a reference reach are 

discussed here, along with the additional data requirements.

The ideal reference reach represents the physical, hydrologic, and 

hydraulic characteristics of the channel that would be at the culvert site if 

the road did not exist. This ideal will not always be achieved because the 

reference reach depends on the project profile—the longitudinal profile 

of the stream simulation channel to be constructed. The project profile 

may have to differ from the natural channel slope for a number of reasons 

(section 6.1). Although the reference reach may not represent historical 
5—73

Chapter 5—Site Assessment

or average conditions of the project reach, it must be within the range of 

variation found in the vicinity. Looking at the range of variability in slope, 

width, etc. in the project area can provide an idea of how far a stream 

segment can depart from average and still be stable in the system. 

Slope is a primary criterion for selecting a reference reach because it 

drives sediment erosion, transport, and deposition. These processes, in 

turn, control sediment characteristics at a given location in the channel. 

Thus, the reference reach slope must be similar to the design slope through 

the crossing. However, keep in mind that the reference reach is simulated 

in its entirety; width, slope, length, channel shape, bed characteristics, 

and roughness are all included in the simulation. The reference reach also 

should be similar in cross-section dimensions and entrenchment to the 

reaches upstream and downstream of the crossing. It represents the channel 

that will reconnect those reaches without creating flow discontinuities. 

The reference reach is a stable reach upstream or downstream from 

the crossing but always outside the influence of the existing structure. 

The factors that control channel dimensions (water discharge, sediment 

supply) in the reference reach must be similar to those that will control the 

simulation. At most sites, a reference reach can be identified close to the 

crossing, and the site data collected during the site assessment typically 

include a reach suitable for use as a reference. Occasionally, the most 

suitable reference reach may be some distance from the crossing site. 

There is no problem with this, so long as flow and sediment regimes are 

very similar. The reference reach should not be separated from the crossing 

by a major tributary junction, sediment source, or sediment sink. 

The following considerations go into selecting a reference reach: 

l The reference reach should be out of the area of influence of the 

existing crossing. Generally, it is upstream of the crossing to avoid 

any downstream channel changes the crossing may have caused. 

However, it can also be downstream if crossing effects are localized, 

and channel dimensions and slope are more appropriate to simulate at 

the crossing.

l The reference reach channel slope should be similar to the project 

profile slope through the road-stream crossing. Before selecting a 

final reference reach, determine the alignment and profile for the 

crossing project (section 6.1).

l Cross-section dimensions in the reference reach should be similar to 

the reaches near crossing. Entrenchment also should be similar. 
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Stream Simulation

l Flow and sediment regimes at the reference reach should be similar 

to those at the crossing. No major tributary junctions or sediment 

sources should be between the reference reach and the crossing. The 

reference-reach bed material must be similar in size and mobility to 

the reach upstream of the crossing that will supply sediment to the 

stream-simulation channel. 

l The length of the reference reach should be at least as long as the 

road-stream crossing structure.

l Determine the stability of both the reference reach and project reach. 

The reference-reach approach for channel design applies only to 

relatively stable channels.

l Where possible, avoid selecting a highly sinuous reference reach. 

A good method for testing the feasibility of using a particular reach 

as a reference reach is to visualize it enclosed in a culvert. Consider 

the characteristics that cannot be simulated, and whether they might 

compromise the simulation. 

l Consider the distribution of channel units upstream and downstream 

from the road-stream crossing. For example, pool locations and 

spacing may dictate that the simulated channel include a run or pool. 

The reference reach should include those channel units.

At new crossings, the undisturbed natural channel at the site is the 

reference reach. Ideally, you would build the crossing over the stream 

without disturbing it.

Where the site has a concave- or convex-profile shape, it may be necessary 

to measure possible reference reaches upstream and downstream of the 

crossing. Near grade breaks, a common method of reconnecting the two 

different slope segments is by constructing an intermediate-gradient 

transition inside the pipe. Elements of both upstream and downstream 

reaches may be incorporated in the design (see for example figure 6.8). 

Theoretically, a similar transition reach on another nearby stream could be 

used as the reference reach, but it is relatively uncommon to find streams 

and watersheds that are that comparable.

If a long reach outside the new structure will be regraded, conduct the 

reference-reach survey more carefully than in simpler cases. In this case, 

the data will have to support design of not only the simulated streambed 

inside the crossing structure, but also a channel-reconstruction project. 
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Chapter 5—Site Assessment

In the reconstructed reach outside of the road-stream crossing, features 

typically not built inside a structure (such as soil banks, planform 

characteristics, and large-wood grade controls) will be constructed and 

stabilized.

If the stream channel in the crossing vicinity has been recently disturbed, 

it is likely to be in a state of flux, evolving toward an equilibrium shape 

and grade. If the road can remain closed for an extended period, wait to 

construct the crossing until the stream reestablishes some measure of 

stability. Otherwise, you may be able to find a reference reach upstream of 

the disturbance.

For streams undergoing regional channel incision, if the headcut will be 

allowed to progress upstream through the crossing site, use downstream 

reaches that have already stabilized as the reference reach. Accomodate 

changes expected as the channel evolves (see appendix A, section A.7.2). 

If the crossing will be retained as a grade control, select a reference reach 

that has a gradient similar to the simulated-streambed design gradient. 

The incised channel is one possible situation where the channel through 

the crossing may have a steeper grade than the adjacent reaches. Project 

objectives (e.g., avoid channel incision upstream, preserve wetland habitat 

above crossing) or constraints (e.g., rights-of-way, property boundaries) 

may dictate the steeper grade. In cases like these, achieving stream 

simulation may or may not be possible, depending on whether reference 

reaches at the necessary grade exist. Until better information becomes 

available about how much of a difference is sustainable, a reasonable 

guideline is to keep the simulated channel within 25 percent of the slope of 

the reference reach. 

If the immediate area clearly cannot provide a reference reach, be sure you 

understand why not. If the reason is that the channel is highly unstable 

or the reach has characteristics like tortuous meanders that cannot be 

simulated inside a crossing structure, reconsider whether the crossing 

location is a good one. If the crossing cannot be moved, stream simulation 

may not be an appropriate design strategy.
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Stream Simulation

Where no appropriate reference reach exists close to the road-stream 

crossing, it is occasionally possible to find a reach with similar discharge, 

slope, streambed materials, and channel type elsewhere in the same 

watershed or a nearby watershed. Use great care here. Species inhabiting 

the project reach must be able to negotiate the transposed channel. Also, 

this kind of transfer may not result in a sustainable simulation because 

of the differences in particle size or amount of sediment input from the 

upstream reach. In cases when data is transferred from a reach with 

a different drainage area from the project site, a regional relationship 

between drainage area and bankfull width and depth may allow you to 

size the simulated channel correctly. Refer to Rosgen (1994, 1996) for 

procedures on scaling channel dimensions from regional relationships 

between channel dimensions and drainage area. However, again, be aware 

that, in this situation, sediment availability could be quite different, and 

the reach upstream of the simulated channel may not be able to supply 

the size and amount of sediment that the steeper reach needs for longterm sustainability. If long-term streambed sustainability appears unlikely, 

stream simulation may not be feasible, and you may have to settle for a 

hybrid or other design strategy (see appendix B). 

5.5.1 Reference Reach Data Required for Stream- Simulation 

Design

Assuming that the reference reach is included in the longitudinal profile 

already surveyed, most or all of the data needed for design may already 

be in hand. Additional data collection and analysis of the longitudinal 

profile, cross sections, and other survey data may be needed to define the 

following reference-reach characteristics. 

l Residual pool depth (figure 5.19). Average residual pool depth is used 

in stream simulation design to determine how deeply to embed a 

full-bottom culvert, and it is considered in decisions about how deep 

to construct foundations for an open-bottom structure. Pools formed 

by unusual controls that would not be simulated in a culvert (debris 

jams, large logs, large boulders) should not be included here.

l Size, spacing, height, and mobility of grade controls and other key 

features (figure 5.19).

l Bed material size distribution, degree of armoring (see section 5.2.1).

l Bankfull channel dimensions: depth, width, and width variability. 

l Bank or channel margin structure and diversity.
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Chapter 5—Site Assessment

Figure 5.19—Some reference reach longitudinal profile measurements.

x

x

x

x = CENTERLINE POINTS

= THALWEG POINTS

6Stream-Simulation 

Design

6.1 Project Alignment and Profile

6.2 Design of the Stream-Simulation Channel Bed

6.3 Crossing Structure Dimensions and Elevation

6.4 Bed-Mobility and Stability Analysis

6.5 Managing Risk Factors

6.6 Design Documentation
Stream Simulation

Determine project alignment and profile

l Crossing alignment relative to road and channel.

l Lateral channel adjustment potential.

l Vertical adjustment potential. 

l Upstream and downstream project profile control points.

Verify reference reach and stream simulation feasibility 

l Reference reach slope similar to project profile.

l Reference reach length similar to crossing structure.

l Reference reach bed characteristics, and water and sediment inputs similar 

 to crossing site.

Design bed material size and arrangement

l Bed mix particle size gradation.

l Bank rock size and placement.

l Key feature rock sizes and placement (clusters, bars, steps, etc.).

Select structure size and elevation

l Channel bankfull width including margins.

l Range of possible streambed profiles (vertical adjustment potential)

l Flood and woody debris capacity. 

l Largest rock sizes in bed.

l Results of bed mobility analysis.

Verify stability of simulated streambed inside structure

l Bed mobility similar to reference reach and upstream reach.

l Key features stable during high bed design flow.

Document design decisions and assumptions

RESULTS

Sketches or descriptions of project elements

l Simulated streambed longitudinal profile, cross section dimensions. 

Grade controls, bank stabilization measures, etc. in upstream 

and downstream channel segments

Stream-simulation bed material gradation 

Bed material placement including banks, edges, overbank flow 

surface

Flood-plain drainage structures

Crossing structure dimensions and invert elevation

Figure 6.1—Steps and considerations in the stream-simulation design.

Steps and Considerations in the Stream-simulation Design
Chapter 6—Stream-Simulation Design

In this phase of the project, the team integrates the information from the 

watershed and site assessments and designs the streambed through the 

crossing—the stream-simulation channel. The crossing structure is then 

designed to fit around the stream-simulation channel. The design process 

is not linear: as design decisions are made, previous steps may have to 

be repeated to include or compensate for changes that affect their results. 

Whoever takes the lead in this phase should ensure that all team members 

continue to be involved as needed. Issues relevant to all fields (biology, 

hydrology, geomorphology, engineering, construction) may arise in this 

phase of the project.

Match the level of care in design to the risks at the site. If the site is prone 

to channel change or if the consequences of failure would be severe, 

recheck assumptions, use multiple methods to estimate stability, be more 

careful with stabilization outside the crossing structure, get help from 

experienced designers, etc. 

6.1 Project Alignment and Profile

The first step in stream-simulation design—as with any crossing design 

project—is to establish the project layout in three dimensions, including: 

l The two-dimensional plan view that connects the upstream and 

downstream channels through the crossing.

l The streambed longitudinal profile that connects stable points 

upstream and downstream of the crossing.

The longitudinal profile and the plan view must be considered together 

because they are interdependent. When a culvert straightens the natural 

channel, as most culverts do, it also shortens and steepens the channel, 

increasing the velocity and energy of flow through the culvert. Figure 6.2 

shows how straightening a channel reduces its length and increases its 

gradient. 

The first step in designing the project layout is to understand the natural 

channel location and pattern through the crossing area. There may be 

various types of evidence: sometimes the natural pattern is obvious from 

a plan map; sometimes the site survey produces clues about a previous 

channel location, such as an abandoned channel segment. A relocated 

or realigned channel may have eroded one bank near the existing culvert 

inlet as it tried to reestablish its natural pattern, or it may have incised in 

response to straightening. Understanding the natural channel pattern helps 

explain how the existing culvert affected both stream length and slope. Try 

to formulate different layout options that approximate the natural pattern so 

that the replacement culvert conforms better to the natural channel.
6—4

Stream Simulation

Figure 6.2—Cutting off a bend results in channel length and slope changes.

Ideally, the project layout approximates the natural channel pattern 

and slope at the site. The simplest situations occur where the crossing 

is a new installation and/or the road crosses perpendicular to a stable, 

uniform stream channel. In such cases, the existing channel defines the 

project layout and profile. For more complex sites, evaluate the tradeoffs 

associated with the issues discussed in sections 6.1.1 and 6.1.2. It may be 

worthwhile to compare the pros and cons of a number of different profiles 

and alignments to find the best combination. 

6.1.1. Alignment 

`Culvert alignment is the orientation of the culvert structure relative to 

both the road and the stream channel. If the road crosses a straight uniform 

channel at right angles, the upstream and downstream channel reaches can 

be easily connected through a straight crossing. Alignments, however, are 

often not this simple. 
6—5

Chapter 6—Stream-Simulation Design

A crossing that best maintains ecological connectivity over the long 

term has a channel cross-section area, slope, and streambed similar 

to that of the upstream channel, and does not disrupt the natural 

channel pattern.

Poor structure alignment with respect to the stream (skew) is a perennial 

source of problems. Over 90 percent of culvert failures studied after the 

1995–96 floods in the Pacific Northwest resulted from debris plugging and 

sediment accumulations attributable in part to poor alignment (Furniss et 

al. 1998). Pieces of wood may rotate as they approach a skewed culvert, 

increasing their likelihood of lodging at the inlet. Energy losses due to 

the channel bend at a skewed inlet mean that backwatering and sediment 

deposition frequently occur upstream (even if the inlet is not plugged). 

Local bed scour inside the culvert inlet is a common problem caused by 

the inlet contraction or because flow is focused to one side. A skewed 

inlet or outlet can also cause severe bank erosion outside the culvert 

by directing the flow at erodible banks. Because all of these risks are 

associated with high flows, visualize the flow patterns at high flows when 

considering alignment. 

The relationship between the radius of curvature (Rc) of the upstream 

bend and bankfull width is an indicator of the level of risk posed by a 

skewed alignment (refer to figure 6.6). When Rc is greater than 5 times 

bankfull width, sediment and debris transport are essentially the same as 

on a straight channel. As Rc decreases, the risk of affecting sediment and 

debris transport increases and when Rc is less than twice bankfull width, 

the risk of impeding sediment and debris transport is substantial. More 

flow is forced to the outside of the bend, and large eddies form on the 

inside of the bend, impeding flow and reducing the effective width of the 

channel (Bagnold 1960; Leopold et al. 1964). Figure 6.6 shows a skewed 

culvert where the radius of curvature is well within the danger zone.

Aligning a properly sized structure parallel to the upstream channel 

minimizes the risk of backwatering, sediment deposition, debris blockage, 

and capacity exceedence for that structure. However, aligning the crossing 

structure with the channel often results in a skewed alignment relative to 

the road, which can require a longer structure and/or the installation of 

headwalls. 
6—6

Stream Simulation

6.1.1.1. Risks of longer culverts

Longer culverts are less forgiving of erroneous design assumptions or 

construction inadequacies. The longer the structure, the higher the risk that 

hydraulic energy is not adequately dissipated within the culvert. The length 

of the crossing structure should not be longer than the reference reach

(section 5.5). When a culvert would exceed the length of the reference 

reach, consider alternative structures, such as bridges.

One hazard of longer culverts in meandering streams is that they are 

more likely to cutoff channel bends and steepen the channel (figure 6.2), 

increasing the risk of streambed instability inside the culvert. 

In steep channels, which are usually straighter than flatter ones, channel 

straightening is less of a risk. However, steep channels often have jutting 

banks, debris jams, large exposed rootwads, and abrupt bends, all of which 

add roughness and dissipate energy. Take care, when designing long 

culverts on steep streams, to ensure that energy is adequately dissipated. 

Otherwise, the streambed may wash out of the culvert. 

Always consider minimizing structure length to manage risk. In some 

locations, shifting the road location to avoid a bend can be a solution. You 

can also shorten structures by: 

l Adding retaining walls and/or wingwalls: in some cases, this adds 

cost to the project.

l Lowering the road elevation to reduce the width of the roadfill.

l Steepening the embankment: on high volume roads, required 

additional safety measures may increase cost.

Increasing structure width can partially mitigate the risks associated 

with long culverts. A wider culvert permits more lateral variability in the 

channel and provides space for overbank flows inside the structure. Space 

will also be available inside the wider culvert for replicating reference 

channel roughness by placing large rocks as roughness elements.

There is no universal rule about which is better: a longer culvert with a 

good alignment relative to the stream, or a shorter crossing with a poor 

alignment. Do not reduce culvert length by realigning the channel to be 

normal to the road without first evaluating the tradeoffs associated with 

the poorer alignment relative to the stream. One of the tradeoffs is a higher 

risk of debris-plugging; however, stream simulation culverts are less 

subject to debris-plugging because they are as wide as the natural stream 

channel. If a site has easy access for maintenance, the benefit of a shorter 

skewed culvert may outweigh that of the better-aligned but longer one. 

These decisions are highly site specific. 
6—7

Chapter 6—Stream-Simulation Design

6.1.1.2. Channels skewed to the road

One common alignment challenge is shown in figure 6.3, where the road 

is aligned at an acute angle to the stream. Three alignment options for this 

situation are: 

(a) Matching culvert alignment to stream alignment. 

(b) Realigning the stream to minimize culvert length. 

(c) Widening and/or shortening the culvert. 

A project can combine elements of all three options. Other possible 

approaches include relocating the road to a better stream alignment or 

building a bridge with a wider span.

Of the options above, (b) entails the greatest risk. The risks listed in table 

6.1 should be evaluated and compared for projects where the road crosses 

the stream on a strongly skewed alignment. Minor skews are not likely 

to have important effects on the stream. The effects and impacts listed in 

table 6.1 are general, and may not apply to all situations. 

Figure 6.3—Three alignment options for a culvert where the road crosses the stream at an acute angle (high 

road-to-channel skew).
6—8

Stream Simulation Table 6.1—Comparison of alignment options, attributes, and associated effects for road crossings acutely skewed relative to the stream channel Alignment Option a. Crossing on stream alignment 

b. Realign channel 

c. Widen and/or 

shorten culvert 

Associated Effects and Comparison of Options

l Risk of debris and/or sediment blockage is low.l Permanent direct loss of aquatic habitat is highest. l Risk of bedform failure in the simulated channel and loss of aquatic organism 

passage is higher than in shorter culverts 

l Special design and construction methods may be required. l Probability of blockage by debris and sediment is greatest.l Passage of aquatic organisms may be blocked at times. l Risk of culvert failure is greatest. l Riparian area is removed, and habitat impacted. l Newly constructed and/or oversteepened banks are less stable and risks of 

bank failure or erosion are higher.

l Realignment may extend beyond right-of-way. l Risk of upstream aggradation is increased. l Need for maintenance to remove sediment is increased. l Risk of bank erosion downstream is greatest. l Risk of debris and/or sediment blockage or plugging is low. l Culvert capacity is greatest; lowest risk of culvert failure. l Risk of failure due to debris blockage or plugging is lowest. l Opportunities for passage of aquatic and terrestrial organisms are greatest.l Risk of construction activity detrimentally affecting wildlife is greatest. l Road closure is required for longer time.l Project may be most expensive. l Permanent direct habitat loss is least. 

Attributes

Inlet and outlet match channel alignment.Culvert is long. Culvert is skewed to road.Inlet is skewed to channel.Channel, riparian area and banks are disturbed.Channel grade is flattened due to added length.Outlet may be skewed to channel.Inlet and outlet match channelalignment. Open area is large. Construction duration may be long. Channel area covered by project is small. 
6—9

Chapter 6—Stream-Simulation Design

6.1.1.3. Culvert on a bend 

Another common alignment problem arises where the crossing is located 

at a bend in the channel (figure 6.4). Where road relocation is not feasible, 

the same three options pertain: matching channel alignment, realigning the 

stream, and widening and/or shortening the culvert.

None of these options necessarily stands alone. The best solution might 

be optimizing a combination of skew, culvert length, and culvert width 

changes. Table 6.2 lists attributes and effects of each channel-bend option.

Consider how far the channel is likely to migrate laterally during the life of 

the project (sections 4.4 and 5.3.2). Options for accommodating expected 

changes include the following:

l Widen the culvert and offset it in the direction of meander movement.

l Control meander shift at the inlet with appropriate bank stabilization 

measures or training structures, such as rock weirs or J-hook vanes.

If banklines are constructed within the culvert, the rocks on the outside 

bank (the bank in the direction of channel shift) will be exposed to higher 

shear stresses and might therefore need to be bigger than bank rocks in 

other locations (see section 6.4.2).

Figure 6.4—Three alignment options for a culvert on a channel bend.
6—10

Stream Simulation Table 6. 2—Comparison of alignment options, attributes, and associated effects for a road crossing on a channel bend Alignment Option a. Crossing on stream alignment

b. Realign channel 

c. Widen and/or 

shorten culvert 

Attribute

Bend location results in skewed outlet. Bend location results in skewed inlet. Channel bends in culvert. Disturbance to channel, banks, and riparian area. Inlet and outlet match channel alignment.Channel is shortened and steepened.Culvert length is short, open area is large. Construction duration may be long.Channel area covered by project is low. 

Associated Effects and Comparison of Options 

l Risk of bank erosion downstream is higher.l Risk of upstream sediment deposition and debris blockage increased over 

straight alignment.

l Likelihood of bank erosion upstream increased. l Natural bend characteristics (increased shear on outside of bend, pool, point 

bar) may not be feasible in a culvert. 

l Riparian area is removed and habitat impacted. l Channel realignment is excavated through high ground leaving bank slopes 

vulnerable to erosion or failure. 

l Risk of debris and/or sediment blockage is low. l Risk of bedform failure in the structure is higher.l Risk of upstream headcutting is higher than other options. l Realignment may extend project beyond right-of-way. l Hydraulic capacity is greatest. l Risk of culvert failure is least. l Risk of passage obstruction and culvert failure due to debris blockage or 

plugging is least. 

l Opportunities for passage of aquatic and terrestrial organisms are greatest. l Risk of detrimental effects due to construction is greatest. l Road closure is required for longer time. l Project may be most expensive. l Permanent direct habitat loss is least. 
6—11

Chapter 6—Stream-Simulation Design

For long pipes on bends, a curved pipe offers an alternative solution. 

A curved pipe is a series of culvert sections formed into a bend that 

preserves the inlet and outlet channel alignments, as well as channel length 

and slope (figure 6.5). Curved pipes might be useful, for example, in 

incised channels where alignment cannot be changed, or where property 

boundaries limit alignment options. They require special culvert design, 

special product, and careful construction. The simulated streambed 

should have the characteristics associated with a bend of similar radius of 

curvature. For example, the design might anticipate the formation of a pool 

at the apex of the bend and include a higher bank there.

Figure 6.5—Curved concrete pipe installation at Arrington Development, Durham, 

North Carolina, June 2001. (Pipe is 142 feet long, with a 24-foot span and a 

7-foot rise.)Courtesy of CON/SPAN Bridge Systems.

Many projects require comparing the relative merits of a longer versus a 

steeper culvert, or a poor channel-to-culvert alignment versus a channel 

realignment. See section 6.1.4 for an example from the Tongass National 

Forest where all these alternatives were considered. 

6.1.1.4. Transitions 

Transitions into and out of the culvert are important, especially if the 

alignment is not ideal. A good transition can smooth an abrupt change 

of flow direction. It can also eliminate poor inlet conditions caused by a 

previous pipe; for example, the wedge of sediment deposited upstream 

of an undersized culvert might be removed, and the widened channel 

might be restored to its normal width. Design the transition by contouring 

the banklines smoothly, beginning at the natural streambank upstream, 
6—12

Stream Simulation

continuing through the section to be modified by the project, and into the 

crossing (figure 6.6). 

Figure 6.6—Channel bend upstream of existing culvert has a radius of curvature 

less than two times bankfull width (Rc/w = 1.3), with serious potential to obstruct 

sediment and woody debris. New culvert is realigned, and banklines are 

excavated and reinforced to create smooth transitions at inlet and outlet.

If the stream must make a turn into the inlet, the bend should be no sharper 

than bends in the natural channel, so that debris that moves in the channel 

will also move through the structure. Visualize the bend during high flow 

when most debris will be moving. 

A poor transition will exacerbate all of the alignment risks that the 

previous section described. For example, where a channel widens 
6—13

Chapter 6—Stream-Simulation Design

immediately upstream of the culvert inlet (as in basins excavated during 

road maintenance), the wider basin causes pieces of floating wood to 

swing perpendicular to the channel and plug the culvert inlet. The wider 

cross section also reduces the shear stress exerted by flow, thereby 

reducing sediment-transport capacity per unit of channel width. As a 

consequence, both woody debris and sediment tend to accumulate (Furniss 

et al. 1998).

On the other hand, a replacement culvert that is much wider than the 

existing one may direct water against streambanks that have encroached 

into the stream channel below the previous narrow culvert. Consider the 

possible effects of bank erosion, and transition the culvert bed and/or 

banks into the natural streambanks to minimize erosion risk. Banklines 

built within a stream-simulation culvert should be continuous with the 

upstream- and downstream-channel banklines. Rebuilding eroded banks 

around an outlet scour pool, such as in figure 6.6, usually requires filling 

the pool. 

A good way to evaluate transitions is to compare the cross section of the 

simulated channel with the natural channel upstream and downstream 

from the crossing. The geometry and dimensions of the adjacent cross 

sections should be similar to one another. 

6.1.2. Designing the Project Longitudinal Profile 

The project profile represents the surface of the streambed that will 

be constructed through the project reach to connect the upstream and 

downstream channel profiles. It corresponds to the slope segments 

discussed in section 5.2.2, which connect the grade controls in the 

natural channel. At new culvert installations where the road alignment is 

perpendicular to the stream, the existing channel longitudinal profile is

the project profile. The project-profile analysis is one of the most critical 

elements in a stream-simulation design, whether the project is a new 

crossing, a replacement, or a crossing removal. A good project-profile 

analysis ensures that the new structure will accommodate expected future 

vertical streambed adjustment.

The scale of any channel adjustment problem caused by the previous 

culvert determines the scale of the solution. The project profile can be 

short if no large scale vertical adjustment is anticipated, such as where 

nearby stable steps or bedrock outcrops anchor the ends of the profile. The 

project profile will be longer where upstream aggradation and downstream 
6—14

Stream Simulation

incision at an undersized culvert create a large elevation drop. The 

profile will be longer still if large-scale downstream channel incision has 

occurred. In this case, connecting the upstream and downstream channels 

requires dealing with potential upstream headcutting (and/or downstream 

channel rehabilitation) over a longer stream reach.

Designing the project profile involves the following steps.

1. Identify stable endpoints for the project profile. 

Select stable grade control features upstream and downstream of 

the crossing that will anchor each end of the project profile. They 

should be stable enough that they will not be affected by removal of 

the existing crossing structure. Profile endpoints might be bedrock 

outcrops or highly stable steps, riffle crests, debris accumulations 

(e.g., large, well-embedded logs), etc. Several features may be good 

candidates for stable endpoints, and you might evaluate various 

project profiles using different combinations of endpoints. In this 

context, ‘stable’ means the bedform will last as long as the structure 

lifetime. It does not necessarily have to be permanently immobile. 

The cobbles on a high-stability riffle crest (table 5.3), for example, 

may mobilize in the 10- or 25-year flood, but the riffle crest itself will 

remain at or very near its current location and elevation if the channel 

is stable.

If the downstream channel is incised, the lower VAP line (section 

5.2.2.2) indicates the length and depth of potential channel incision 

upstream. Most alluvial bedforms higher than the lower VAP line 

would not be expected to constitute stable endpoints in this case. If 

you decide to allow a headcut to progress through the crossing, the 

upstream project profile endpoint would need to be upstream of the 

projected extent of incision. Alternatively, if you decide to maintain 

the crossing as a grade control, you may need to construct permanent 

grade control structures as the project profile endpoints (see section 

6.1.3). 

2. Delineate possible project profiles.

Draw one or more tentative project profiles between sets of control 

points to connect the upstream and downstream segments across the 

crossing. The project profile should extend at least as far upstream and 

downstream as the new culvert installation could directly affect the 

channel. The profile does not show bed topography, only the elevation 

and slope of the streambed that will be constructed (see figure 6.7 for 

an example). Calculate slope and length of the profile options. 
6—15

Chapter 6—Stream-Simulation Design

The best project profile is a uniform one beginning and ending on 

stable bedforms. However, some project profiles may have two 

segments with different grades. Sites with convex or concave profiles, 

for example, might have more than one segment. In these cases, 

we recommend the slope break be outside the culvert. The incised 

channel solution in figure 6.10 (c) is an example of a project profile 

in two segments. The same type of segmented project profile, with 

the steeper section constructed outside the culvert, could be used at 

any site where the elevation change exceeds the slope of available 

reference reaches and where the adjacent natural channel is stable 

enough to sustain the transition.

3. Verify the reference reach.

After identifying one or more good project-profile options, recheck 

the reference reach tentatively identified during the site assessment 

(section 5.5). Determine whether it adequately represents the 

preferred slope. The reference reach should be straight, and as long 

as the crossing structure. Ideally the reference reach should also 

be as long as the project profile, but this is not always feasible on 

meandering streams or where wood is a frequent bed feature. If the 

tentative reference reach does not match the desired project profile, 

evaluate other slope segments in the site survey (section 5.2.2) as a 

possible reference reach. 

If the site assessment survey did not include a reach as long as the 

project profile and within 25 percent of its slope, revisit the site to see 

if the natural channel includes reaches closer to your needs. If not, 

consider controlling the project profile to more closely fit an available 

reference reach (section 6.1.2.5). This need commonly arises when 

(1) there has been a large amount of aggradation upstream and 

deep local scour downstream of an undersized crossing or, (2) the 

downstream channel has incised and the existing culvert is acting 

as a grade control to prevent upstream headcut migration, or (3) the 

natural channel profile is concave, convex, or complex.

If profile modification will not work, the remaining options for 

crossing design are to: 

l Use a hydraulic or hybrid design method to achieve partial passage 

(see appendix B) or,

l Locate a reference reach on a different channel that has similar 

landscape characteristics: valley type, streambed materials, 

watershed size, hydrologic regime, etc. This option has strong 

limitations (see section 5.5).
6—16

Stream Simulation

4. Adjust VAP lines if necessary.

Where the project profile will be controlled by permanent grade 

control structures, the VAP lines may require adjustment to 

correspond with the project profile and reference reach. Examples 

are shown in figures 6.10b and 6.10c, which show an incised-channel 

site where the project profile will be controlled to avoid headcutting 

upstream of the replacement culvert. The lower VAP line in and 

upstream of the culvert is adjusted upward since the constructed 

grade controls will stop the progress of incision. 

5. Locate key bed features.

Based on the reference reach, determine the spacing, height, and 

location of any bedforms that need to be constructed. Bedforms are 

generally spaced based on average spacing in the reference reach. 

Tying them into the endpoint bedforms, however, sometimes requires 

varying bedform spacing. Meander bends, which control pool 

locations, must also be considered when locating the bedforms in the 

project reach. The average spacing may need to be varied to locate 

the pool appropriately in relation to the bend. Limit the variability in 

spacing to the range found in the reference reach. 

The following sections describe project profile delineation on various 

channel profile types. 

 6.1.2.1 Uniform channels with local scour and fill around an under

 sized culvert

In uncomplicated channels with uniform profiles (not incised), the 

project profile simply connects profile control points in the upstream 

and downstream channels at the same slope as the channel profile. The 

design slope is the same as the upstream and downstream channels. In 

figure 5-16a, for example, the project profile is the existing channel profile 

extended through the crossing. The replacement project entails nothing 

more than installing an appropriately sized and embedded culvert and 

filling the scour pool. Since the volume of sediment accumulated above the 

culvert inlet is not large, the sediment can be allowed to regrade naturally 

if desired. The project footprint will be quite limited. 

In some cases, the amount and extent of aggraded sediment upstream of an 

undersized culvert are so large that allowing the sediment to flush through 

the system all at once would be undesirable. In such cases, the team may 

elect to place control structures in the aggraded reach to meter sediment 

movement more gradually. This will extend the project’s footprint.
6—17

Chapter 6—Stream-Simulation Design

6.1.2.2 Steep channels with large key features

On streams controlled by large key features (bedrock outcrops, large 

woody debris, stable debris jams, boulder steps, manmade structures), the 

project profile reflects the team’s assessment of the probability that key 

features might move. In the Fire Cove Road example (site sketch and VAP 

analysis shown in section 5.2.3), several project profiles were evaluated 

under different assumptions about potential movement of the upstream and 

downstream key features. 

Recall that the Fire Cove Road crosses a wood-forced step-pool Rosgen 

A channel, where a 2.5-foot-diameter log about 50 feet downstream of the 

culvert (figure 5.14c) controls channel slope across the crossing. A debrisand-boulder cascade over 20 percent slope is about 30 feet upstream of the 

culvert. The existing culvert slope is 5-percent, flatter than the adjacent 

channel, where slopes range between 6 and 22 percent. In spite of the 

complex profile shape, this steep transport channel has had no problems 

with aggradation at the culvert inlet.

Figure 6.7 displays possible project profiles at the Fire Cove Road 

crossing. The steepest profile assumes that the downstream log control 

moves or will be removed, and that a boulder step in the middle of the 

cascade also may move. For solid anchor points, this profile uses the 

highly stable boulder-log structure at the top of the cascade upstream 

of the crossing, and a log-boulder complex further downstream of the 

crossing. The intermediate slope profile also assumes the downstream 

control moves. Both of these steeper profiles would entail constructing a 

very steep simulated streambed with a design gradient of over 6 percent. 

These options would not only avoid any potential aggradation problems 

but also would result in a channel where stability does not depend on the 

downstream log.
6—18

Stream Simulation

Figure 6.7—Project-profile options on a channel with large key features, and the

selected project profile: Fire Cove Road, Tongass National Forest, Alaska.

The flattest profile in figure 6.7 has a 4.6-percent slope, and assumes that 

no existing grade controls move. This design project profile was used 

because the probability is very low that either of the nearest grade controls 

will move over the lifetime of the new culvert. The existing culvert, at a 

5-percent grade, had no problems with aggradation. This option preserves 

the valuable pool habitat in the vicinity of the culvert, and requires the 

least channel regrading. A reference reach with a similar slope exists 

downstream of the crossing. 

(ELEVATION CONTROL POINT)

(ELEVATION CONTROL 

 POINT)
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Chapter 6—Stream-Simulation Design

6.1.2.3 Concave slope transitions

The concave transition (see section 5.2.1.4) is common, because many 

roads are located at the outer edge of valleys, where the steeper sideslope 

meets the valley floor. Shear stress decreases abruptly with the change in 

channel slope, and these areas are natural sediment depositional zones. 

A crossing that constricts the stream will exacerbate the natural tendency 

toward sediment deposition. Even where no constriction exists, natural 

aggradation can reduce a structure’s hydraulic capacity. 

If a culvert has to remain at or near a concave grade break where it could 

be affected by aggradation, the project profile should include the grade 

break. Figure 6-8a shows an undersized culvert at a concave-channel 

transition, along with the upper and lower VAP lines. No regional channel 

incision is anticipated here, so the lower VAP line is drawn below the 

typical depth of pools in each segment. The upper VAP line here is at 

the top of the streambank. The channel has downcut through a sloping 

bench (an old depositional surface) where the hillside meets the valley 

bottom. Upstream of the crossing on the hillside is an entrenched step-pool 

channel; downstream is a less well-entrenched pool-riffle channel. 

Replacement option 1 would be the desirable project profile if a reference 

reach can be found at an intermediate grade. Such a reference reach might 

be a steep, riffle-dominated reach with transverse bars, like the project 

profile shown in figure 6.8b. This alternative reduces risk by moving the 

probable locus of aggradation away from the culvert, where maintenance 

can access the channel if necessary. Note that the lower VAP line has 

been adjusted upward in this scenario, because the project profile is 

raised and its elevation is controlled by constructed riffle crests. Option 

2 (figure 6.8c) involves oversizing the structure so as to accommodate 

any aggradation that may occur. The project profile is a smooth transition 

between the profiles of both adjacent channel segments. This alternative 

is less than ideal because of the difficulty of predicting future aggradation 

(see section 4.5, Brewster Creek example). 

Table 6.3 lists and compares common options for design solutions at 

concave transitions. Note that the vertical curve of the roadway influences 

design options, because it controls how much the road surface can 

be raised to allow more room for sediment deposition in the crossing 

structure. 
6—20

Stream Simulation

Figure 6.8—Hypothetical determination of VAP lines and project profile at a concave transition: (a) undersized

culvert before replacement showing upper and lower VAP lines; (b and c) two options for possible project profiles

inside replacement culverts (see text). Steps or constructed riffle crests could be designed for these installations,

based on bedform spacings in the respective reference reaches (see sections 6.2.2.4 and 6.2.2.2). 

CONSTRUCTED RIFFLE CREST LOCATIONS

BASED ON SPACING IN REFERENCE REACH

CONSTRUCTED RIFFLE CREST LOCATIONS

BASED ON SPACING IN DOWNSTREAM

CHANNEL

ELEVATION CONTROL

POINT
6—21

Chapter 6—Stream-Simulation Design

Table 6.3—Design options for concave transitions

Profile Option

Relocate crossing away from grade break.Adjust channel profile to ensure sediment is transported through the crossing.Oversize culvert to accommodate sediment accumulation (to upper VAP line).Design for long term maintenance.

Associated Effects and Comparison of Options

l Most reliable solution.l Reduces maintenance requirements.l Depending on road alignment, can have undesired trade-offs, e.g., 

changes in sight distance, safe driving speeds, etc.

l Only possible when downstream channel is steeper than ~1% and 

somewhat entrenched (so flow does not spread out and deposit sediment immediately downstream of culvert).

l Carries risk that downstream deposition may progress upstream 

toward crossing (important on low gradient alluvial fans).

l Depositional area may be moved to channel reach not adjusted to 

it, or on another property.

l Road grade may be raised to accommodate steeper culvert; 

diversion potential can increase.

l Road grade may need to be raised to accommodate larger culvert.l Only desirable where geometric road requirements can be 

met without causing a potential for stream diversion (i.e., road approaches should slope down to crossing).

l Estimate of high potential profile (and therefore culvert size) is 

subject to considerable uncertainty.

l Useful where roadway constraints or rapid sedimentation rates 

make other options infeasible.

l Requires a maintenance commitment.l Upstream excavation carries a risk of destabilizing the steeper 

channel upstream and possibly causing a headccut and/or loss of aquatic habitat.

Attributes

Places crossing upstream or downstream of the depositional zone.Steepens profile inside pipe and moves grade transition away from culvert.Allows aggradation without sacrificing structure performance.High potential profile is estimated based on site history, sediment sources, amount of debris moving in system, etc. Alternatives include:

l Embedded concrete boxes with 

removable lids.

l Bridge.l Excavated sediment pond 

accessible to maintenance equipment upstream of crossing. 
6—22

Stream Simulation

6.1.2.4 Convex slope transitions

Where the channel gradient steepens downstream of a crossing, there is 

an inherent risk of headcutting unless permanent grade controls exist or 

are constructed. Traditional culverts at these locations control streambed 

elevations, but stream-simulation culverts do not function that way. 

Local headcutting might occur due to disturbance during construction or 

movement of local grade controls (steps, short cascades) during floods. 

The risk depends on the stability of the grade controls. Unless grade 

controls are highly stable, protecting the simulated streambed in the 

replacement culvert may require constructing additional grade control 

structures. 

Figure 6.9—Road crossing near convex slope transition. (a) Existing crossing 

with bed topography, channel profile, and VAP lines. (b) Two possible project

profiles.
6—23

Chapter 6—Stream-Simulation Design

Figure 6.9a shows a crossing near a convex slope transition, where a 

pool-riffle channel breaks to a steeper step-pool channel. If one or more 

of the downstream steps is destabilized by construction or a flood, the 

downstream channel could incise to approximately the height of the grade 

controls. In this example, we are not anticipating regional channel incision 

such as might occur with a base level change somewhere downstream. 

We are only designing for local bed elevation changes that could occur if 

one or two log or boulder grade controls move during a flood. If regional 

channel incision were anticipated, the lower VAP line in figure 6.9a would 

need to be lowered to account for that, or permanent steps would need to 

be constructed downstream.

Two possible project profiles are delineated in figure 6.9(b). Both start at 

the same upstream elevation control point—a stable riffle crest. Profile 1 

has a slope intermediate between the two adjacent channel segments. It 

could be selected if a reference reach with a similar slope exists nearby, 

and if the elevation control points are stable enough to sustain the steeper 

slope. Both the outlet pool-tail crest (the downstream profile control 

point) and the upstream riffle would need to be highly stable structures 

to make this a viable option. Profile 2 extends the channel profile of the 

upstream reach through the new crossing, and would require constructing 

an immobile grade-control structure downstream of the new culvert to 

maintain the slope. The reference reach for profile 2 would be the reach 

immediately upstream of the culvert. 

6.1.2.5 Incised channels

Where a culvert is protecting the upstream channel from incision, but the 

amount of prospective incision is acceptable, you may decide to simply 

lower the culvert and allow the upstream channel to regrade naturally. 

Once again, see section 5.3.3 for a checklist of things to consider when 

deciding whether to allow incision to progress. Either ensure incision 

downstream of the crossing is not ongoing, limit it by constructing 

permanent grade controls, or provide adequate depth to accommodate it.

One way of mitigating some of the effects of expected channel incision

is to limit the rate of upstream headcut migration using temporary gradecontrol structures, such as scattered, buried, or other rock structures, which 

are expected to fail over time. Although you can place woody debris for 

the same purpose, be aware of the potential impact on the culvert, should 

that debris move.
6—24

Stream Simulation

Where the projected VAP is not tolerable, several options exist for 

adjusting and controlling the project profile. Most of these situations are 

where the downstream channel has incised, and the depth or extent of 

possible upstream incision is unacceptable. Again, the first step in dealing 

with these situations is to identify stable grade controls (or control points 

that can be stabilized) upstream and downstream of the crossing, and 

connect those points to delineate a tentative project profile. Determine the 

slope of the profile and verify that a reference reach exists at that slope. 

If the project profile exceeds the slope of potential reference reaches, 

adjusting the profile may be possible using one or more of the following 

strategies.

l Reconstruct the incised channel to pre-incision conditions. 

l Steepen the culvert. 

l Lower the culvert and steepen the adjacent reach(es); control grade 

with key features like boulder weirs or logs, or constructed gradecontrol structures.

Figure 6.10 illustrates these options and table 6.4 describes and compares 

them. Many projects include a combination of two or all of these options.

Projects dealing with large-scale channel incision are often much longer 

than those dealing only with local scour because they require restoring 

or controlling streambed elevations on the adjacent channel segments. 

The objective is to smooth the transition between the unincised channel 

upstream and the incised channel downstream so as to avoid impeding 

aquatic organism passage. Right-of-way limits, property boundaries, and 

other infrastructure can sometimes constrain the length of the project. 

However, do not automatically assume that they do. Instead, consider 

options that cross or move these features if those options have advantages. 

Reconstruct the 

channel Channel reconstruction [figure 6.10(a)] should be considered as an option 

in any project associated with an incised channel. Channel reconstruction 

is the reestablishment of equilibrium channel dimensions, structure, and 

grade, with the goal of achieving a self-sustaining channel that can remain 

in dynamic equilibrium over the long term. It is a more elegant, durable 

way of correcting a large elevation drop resulting from channel incision, 

as opposed to forcing the culvert into an artificially oversteepened profile. 

Reconstruction might involve realigning a straightened channel to restore 

meander pattern and length at its original elevation. Oversteepened banks 

could be laid back and the excess material used to build the incised bed 

back up to an elevation that provides access to the culvert. 
6—25

Chapter 6—Stream-Simulation Design

Figure 6.10—Several project-profile options for an incised channel (reference figure 5.16b).(a) Reconstruct

channel; (b) steepen stream-simulation channel; (c) steepen adjacent channel segments The lower VAP lines 

represent the lowest channel elevations expected over the life of the replacement structure given the profile

controls constructed in each case. Incision is judged to have ended.
6—26

Stream Simulation Table 6.4—Comparison of project-profile design options for incised channels Profile Option a. Reconstruct channel. 

b. Steepen culvert.

c. Lower culvert and steepen 

upstream and/or downstream 

reaches.

d. Lower culvert and allow 

upstream headcutting.

Potential Effects and Comparison of Options

l Greatest habitat gain of the three options.l Most self-sustaining of the three options.l Risk that downstream channel may continue to incise if the cause 

of instability is not resolved.

l Initial disturbance may be more extensive and construction cost 

may be higher. Potential issues with property boundaries or rightsof-way, short-term wildlife habitat impacts.

l Creates or enhances in-channel, flood plain and/or riparian 

habitats.

l Improves flood-plain water storage and other flood-plain functions.l Stream-simulation crossing is more sustainable and less 

vulnerable to headcutting or sediment deposition.

l Risk that simulation will be unsustainable if upstream reach does 

not resupply the same caliber of sediment eroded from culvert.

l More likely an impediment to aquatic species passage than other 

options (minimize this risk by staying within 25% of reference reach slope for stream simulation).

l Risk of headcutting upstream and sediment deposition 

downstream.

l Grade controls may be required upstream and/or downstream.l Less risk of simulated channel instability compared to (b).l Less risk of impeding passage compared to (b) because the 

variety of pathways is greater. 

l Potential effects of allowing upstream headcutting are outlined in 

section 5.3.3.

Attributes

May restore downstream channel to natural length and grade.Project scope includes longer reach than other options. Usually includes habitat improvements. Often improves channel floodplain connectivity. Avoids abrupt slope changes along profile.Higher streampower and coarser rock in simulation than in adjacent reach.Slope transitions at upstream and/or downstream ends of culvert.Natural banklines and roughness elements (especially wood) in open channel dissipate energy and help stabilize the steepened reachesMaintains channel connectivity through crossing but permits free channel incision.
6—27

Chapter 6—Stream-Simulation Design

Before deciding to reconstruct a channel, it is critical to understand the 

cause of channel incision. Channel incision can sometimes result from 

long-term watershed changes (for example, in land use and the amount, 

timing, and distribution of runoff). In that case restoring the channel to 

historic, predisturbance conditions may not be possible and the channel 

should be designed for current and future flow regimes. Understanding 

the stage of incision is also crucial. If incision is still on-going, it could 

destabilize the reconstructed channel. Channel reconstruction may not 

be feasible for many reasons, and you should evaluate feasibility before 

deciding to implement this option. See the Federal Interagency Stream 

Restoration Working Group (1998) for an introduction to the channelreconstruction planning process.

The reconstructed channel must tie into a stable downstream base-level 

control so that incision does not recur. The downstream control in figure 

6.10(a)—a stable debris jam—would probably not be considered an 

adequate elevation control point in real life. Most channel-reconstruction 

projects would involve reconstruction of a longer reach, with either a more 

solid downstream control, such as bedrock, or a more gradual tie into 

the incised channel. The downstream channel might be reconstructed at 

a slightly steeper gradient to tie gradually back into the natural channel. 

Designing the steeper reconstructed channel would require finding a 

reference reach at that steeper gradient.

A project that includes reconstruction of an incised channel can extend a 

considerable distance downstream. It may have habitat-restoration values 

that go far beyond passage of aquatic organisms. For example, such a 

project can restore in-stream, riparian, and flood-plain habitats and channel 

flood-plain interactions; reconnect side channels previously blocked by 

the roadfill; and stabilize eroding banks. Channel reconstruction may be 

the most expensive option, but such a project is likely to be more selfsustaining and lower in maintenance costs than others. 

Steepen the streamsimulation channel A more local solution to the incised-channel problem is to steepen the 

simulated channel [figure 6.10(b)]. Look at the site longitudinal profile 

and consider the variability of reach slopes. You may find short punctuated 

steps that are steeper than the average gradient; these could serve as 

a reference reach if they are long enough. If necessary, go back and 

investigate beyond the surveyed longitudinal profile.
6—28

Stream Simulation

How much steeper than the reference reach can the stream-simulation 

channel be? The increase should not be great, because at some point, the 

bed material in the simulated channel must be so much larger than in the 

upstream reach that the upstream reach cannot replenish it if it erodes. 

In other words, the simulation will not be self-sustainable. Keep in mind 

that the premise of stream simulation is that the simulated channel is 

close enough to the natural one that organisms will move through it 

equally easily. If the difference between the slopes is great—especially 

if the steeper slope requires a different channel shape or bed material for 

stability—aquatic organisms may not be able to move through at the same 

flows as in the natural channel. Stream simulation may not be feasible in 

that case.

Bates et al. (2003) suggest a slope increase of no more than 25 percent of 

the natural or reference reach. The suggestion is a conservative guideline, 

as we have no data thus far to support a specific criterion. We use a 

maximum percent change of slope, because a flatter channel is much more 

sensitive to a given absolute change than a steeper one. For example, 

increasing a 1-percent slope channel by 1, to create a 2-percent channel, 

is a substantial change, whereas increasing a 10-percent slope channel 

by the same amount, to create an 11-percent channel, is reasonable. We 

recommend doing a bed-mobility analysis (section 6.4) for any slope 

greater than the reference reach, even if the slope of the simulation channel 

is within the 125-percent guideline. 

Steepen adjacent 

reaches The reaches upstream and/or downstream of the culvert can be steepened, 

either as an alternative to or in addition to the steepened crossing [figure 

6.10(c)]. Steepening channels outside of a culvert is less risky for the 

following reasons:

l If necessary, the channel can be widened.

l The culvert wall does not constrict high flows.

l Natural banklines and channel margins provide the added benefit of 

vegetation for roughness and root strength. 

l It is easier to repair grade-control structures outside culverts.

Reference-reach features are the basis for designing the dimensions and 

spacing of grade controls such as those shown in figures 6.10(b) and 

6.10(c) (see sections 6.2.2.2 and 6.2.2.4). Such structures should not 

be placed near the culvert inlet to avoid exposing them to unusual flow 

patterns near the inlet at flows higher than bankfull. 
6—29

Chapter 6—Stream-Simulation Design

Appendix F briefly describes some common grade-control structures 

used to steepen reaches upstream and downstream of crossings. Where 

channel incision has occurred and control structures are the sole means of 

maintaining elevation and grade downstream of a culvert, these structures 

should be long-lasting and stable enough to maintain the designed 

elevation. The designer must assess the possibility that further incision 

downstream of the project could create a passage barrier at the lowest bed 

control and/or jeopardize the controls and the project.

6.1.3 Project Alignment and Profile Design: Two Examples

Newbury Creek

Crossing Project

Profile and

Reference Reach In chapter 5, we used the Newbury Creek crossing on the Olympic 

National Forest to demonstrate the site-assessment process, including 

analysis of the longitudinal profile and VAP. Here we examine how the 

alignment and project-profile issues were handled at the Newbury Creek 

site, which channel segment was selected as the reference reach and how 

bedforms were spaced in the design channel. Newbury Creek illustrates a 

case where the VAP was acceptable, and the project profile did not require 

modification to control vertical adjustment.

Figure 6.11 shows that the original culvert straightened a slight bend on 

Newbury Creek, which explains the need for riprap on the east bank just 

above the inlet. The degree of straightening is slight, and the replacement 

culvert requires no alignment adjustments. 

Figure 6.12 shows the longitudinal profile with two possible project 

profiles drawn between stable grade-control features upstream and 

downstream of the crossing. The downstream elevation control point for 

both profiles is the riprap rock weir at the outlet pool tail crest (photo in 

figure 5.11). The flatter profile uses bedrock as the upstream elevation 

control point, assuming that the sediment wedge above the existing culvert 

will erode. Erosion of the sediment wedge is expected to destabilize the 

log weirs and other grade controls in the steeper reach above the crossing, 

allowing for some channel downcutting there. Slope of this profile is 2.26 

percent, less than 3 percent steeper than combined segments F/G, which 

are downstream of the crossing and can function as a reference reach.
6—30

Stream Simulation

Figure 6.11—Newbury Creek site plan map showing interpretation of natural channel alignment. 
6—31

Chapter 6—Stream-Simulation DesignFigure 6.12—Newbury Creek: two possible project profiles.

0 50 100 150 200 250 300

distance downstream (m)

9291

vertical exaggeration = 10

p

g

p

298.6

301.9

refere

n

c

e

re

a

c

h

H- ct p

M- ct p

H- ct p

H- ct p

H- ct p

M- ct p

H- ct p

H-t s

M- wl

M- wl

M- wl

M- ct p

L- wl

L- wl

M- ct p

0 50 100 150 200 250 300

distance downstream (m)

999897969594939291

103102101100

) m( noit avel e evit al er

distance downstream (ft)

)tf( noit avel e evit al er

vertical exaggeration = 10

2

3 4

5 6

7

8

9

10

A

E

H

G

B

C

D

F

culvert rock

weir

Existing culvert

diameter 3.1 m

length 22 m

bedrock

channel

s

e

dim

ent

w

e

d

g

{ e

bedrock

plung

e

p

o

ol bedrock along

right bank bedrock, left bank

of plunge pool

tributary

d

o

wnstre

a

m

d

e

sign ele

v

ation

c

ontrol p

oint

up

stre

a

m

d

e

sign ele

v

ation

c

ontrol p

oint 2

up

stre

a

m

d

e

sign ele

v

ation

c

ontrol p

oint 1

channel-bed profilepooltop of bank/floodplain

cross section location

grade controlpool tail crest, high stability

pool tail crest, moderate stability

step, high stability

log weir, moderate stability

log weir, low stabilityslope segments (A,B,C,D,E,F,G,H)

upper vertical adjustment potential line

lower vertical adjustment potential line

design channel-bed profile 1 (S=0.0226)

design channel-bed profile 2 (S=0.0319)

lw-M

2

ptc-H

ptc-M

st-H

lw-L

164.1 328.1 656.2 492.2 820.3 984.3 0

298.6

301.9305.1308.4311.7315.0318.3321.5324.8328.1331.4334.7337.9

segment

elevation 

change 

(ft)

segment 

length 

(ft) gradient

% gradient 

difference 

between 

successive 

segments

maximum 

residual 

pool depth 

(ft)

number 

of grade 

controls

distance between 

grade controls 

(ft)

A 0.94 53.17 0.0178 n/a 1.54 2 53.2

B 0.41 81.53 0.0050 -71.9 0.34 2 81.7

C 2.68 185.21 0.0145 190.4 1.07 4 62.0, 101.7, 21.6

D 1.78 92.69 0.0193 32.9 2.29 3 57.7, 34.8

E 0.73 11.01 0.0665 245.2 0.99 2 11.2

culvert 0.27 72.09 0.0037 -94.4 4.38 2 72.2

F 5.70 234.28 0.0243 551.5 0.82 3 145.7, 88.6

G 1.27 66.25 0.0192 -21.0 1.70 2 66.3

F,G 6.97 300.53 0.0232 -4.6a 1.70 4 145.7, 88.6, 66.3

H 4.69 151.32 0.0309 60.8b,c 0.68 3 92.2, 59.1

c. When compared to combined slope segments of F and G, the percent gradient difference is 33.3% a. Percent gradient difference when compared to slope segment F.b. Percent gradient difference when compared to slope segment G. 
6—32

Stream Simulation

Figure 6.13—Newbury Creek channel profile anticipated after channel response to culvert replacement.

refere

n

c

e

re

a

c

h

H- ct p

M- ct p

H- ct p

H- ct p

H- ct p

M- ct p

H- ct p

H-t s

M- wl

M- wl

M- wl

M- ct p

L- wl

L- wl

M- ct p

0 50 100 150 200 250 300

distance downstream (m)

999897969594939291

103102101100

) m( noit avel e evit al er

)tf( noit avel e evit al er

distance downstream (ft)

vertical exaggeration = 10

2

3 4

5 6

7

8

9

10

A

E

H

G

B

C

D

F

culvert rock

weir

Existing culvert

diameter 3.1 m

length 22 m

bedrock

channel

sediment

wed

g

{ e

bedrock

plunge

p

o

ol bedrock along

right bank bedrock, left bank

of plunge pool

tributary

d

o

wnstrea

m

design elev

ation

c

ontrol p

oint

upstrea

m

design elev

ation

c

ontrol p

oint 1

164.1 328.1 656.2 492.2 820.3 984.3 0

298.6

301.9

305.1

308.4

311.7

315.0

318.3

321.5

324.8

328.1

331.4

334.7

337.9

segment

elevation 

change 

(ft)

segment 

length 

(ft) gradient

% gradient 

difference 

between 

successive 

segments

maximum 

residual 

pool depth 

(ft)

number 

of grade 

controls

distance between 

grade controls 

(ft)

A 0.94 53.17 0.0178 n/a 1.54 2 53.2

B 0.41 81.53 0.0050 -71.9 0.34 2 81.7

C 2.68 185.21 0.0145 190.4 1.07 4 62.0, 101.7, 21.6D 1.78 92.69 0.0193 32.9 2.29 3 57.7, 34.8

E 0.73 11.01 0.0665 245.2 0.99 2 11.2

culvert 0.27 72.09 0.0037 -94.4 4.38 2 72.2

F 5.70 234.28 0.0243 551.5 0.82 3 145.7, 88.6

G 1.27 66.25 0.0192 -21.0 1.70 2 66.3

F,G 6.97 300.53 0.0232 -4.6a 1.70 4 145.7, 88.6, 66.3

H 4.69 151.32 0.0309 60.8b,c 0.68 3 92.2, 59.1

c. When compared to combined slope segments of F and G, the percent gradient difference is 33.3% a. Percent gradient difference when compared to slope segment F.b. Percent gradient difference when compared to slope segment G. 

channel-bed profilepooltop of bank/floodplain

cross section location

grade controlpool tail crest, high stability

pool tail crest, moderate stability

step, high stability

log weir, moderate stability

log weir, low stability

slope segments (A,B,C,D,E,F,G,H)

upper vertical adjustment potential line

lower vertical adjustment potential line

design channel-bed profile 1 (S=0.0226)

design bedform locations (head of riffles)

fill plunge pool with channel-bed material

base of footing of replacement structure (on bedrock)

predicted long-term bed surface (headcut, erosion)

lw-M

2

ptc-H

ptc-M

st-H

lw-L
6—33

Chapter 6—Stream-Simulation Design

For the steeper alternative profile, the upstream elevation control point is 

a medium-stability pool tail crest composed of gravel and small cobbles. 

Slope of this project profile is 3.19 percent, and if it were selected the 

steepest section of this complex profile would extend through the culvert. 

Because the grade is about 38-percent steeper than segments F/G, some 

downstream aggradation might be expected with this alternative; however, 

as seen earlier, the probability of aggradation is low in this stream. 

Segment H is steep enough to constitute a viable reference reach for this 

alternative. Both potential profiles are well within the VAP lines. 

The lower gradient project profile was chosen because of the lower risk 

associated with the lower gradient, and the lack of confidence that the 

medium-stability pool tail crest (elevation control point 2) would remain 

stable at the steeper grade. The steeper alternative might have required 

construction of more grade controls, extending the project’s footprint 

further upstream than the selected alternative. 

Figure 6.13 shows the expected final channel profile after culvert 

replacement, in-channel construction, and projected future channel 

adjustments. The riffle crests (“head of riffles”) are similar in spacing to 

the pool tail crests in the reference reach. Minor local downcutting may 

occur upstream of elevation control point 1 as the log weirs deteriorate 

and fail. Shallow bedrock will limit downcutting, and trees falling into 

the channel may offset it. The projected final profile in figure 6.13 is an 

estimate based on all those considerations. 

Tongass National

Forest, Mitkof Island,

Road 6245

The 6245 road crossing is a situation where culvert replacement could 

have caused unacceptable channel incision. Avoiding incision in this 

case required modifying the crossing alignment. The example does not 

showcase an ideal solution; however, it does demonstrate the trade-offs 

between channel alignment and slope that are sometimes needed. At this 

site, no ideal solution existed and the final alignment required substantial 

engineering control. 

Existing condition The unnamed stream at this crossing is a 6- to 10-foot-wide step-pool 

channel (Rosgen A3) with steps formed of cobbles, boulders, and wood. 

Average channel slope is 6.4 percent, with short steep segments up to 20 

percent. The gravel layer on the streambed is thin, and bedrock outcrops 

frequently. The existing 36-inch pipe has a slope of 3.5 percent, and was 

probably constructed with a perch. Currently, the outlet invert is perched 

2.7 feet above the outlet tail crest of the outlet plunge pool (figure 6.14). 
6—34

Stream Simulation

Natural grade controls upstream and downstream of the road indicate that 

the segment now covered by the crossing was at least 7 percent. Flatter 

segments where log jams control grade (one is just downstream of the 

crossing) provide good spawning gravels, which are in short supply in this 

watershed.

Figure 6.14—Looking upstream at outlet of existing pipe, road 6245. Photo:

Chinook Engineering.

The log jam controlling the flat reach immediately downstream of the 

crossing is only moderately stable, and is likely to readjust or fail over the 

life of the replacement. The lower VAP line in figure 6.15 (longitudinal 

profile) accounts for the probability that the log jam may move, and that 

incision could progress upstream, as sequential steps readjust to the steeper 

local slope. There is little or no risk of larger-scale (regional) channel 

incision here.

The existing culvert approximates the natural channel alignment, and it 

lines up well with the upstream reach. The sharp bend downstream of 

the outlet is a natural bend, but erosion caused by the crossing has made 

the bend more acute. The pipe is skewed relative to both the road and the 

downstream channel (figure 6.15 planview). Issues with both alignment 

and vertical adjustment potential complicate stream-simulation design at 

this site.

t
6—35

Chapter 6—Stream-Simulation Design

Figure 6.15—Existing condition: planview and longitudinal profile.
6—36

Stream Simulation

Options for streamsimulation replacement

Option 1 Aligning culvert with channel. To improve the culvert’s hydraulic 

alignment, option 1 would increase skew relative to the road and lengthen 

the pipe (figure 6.16). Both inlet and outlet would be aligned with the 

stream, but the simulation would be steep—8.5 percent (figure 6.17). 

This slope is within the range of variability in the natural channel, but 

segments this steep are shorter than the culvert, and could not function 

as reference reaches. This alternative is also steeper than the upstream 

channel segment, and the streambed material for the simulation would 

need to be larger to achieve stability. In addition, the log jam, which is the 

downstream grade control, is only 30 feet downstream of the outlet pool in 

this option. When the log jam moves, incision through the simulation will 

be a real possibility unless additional grade controls are constructed.

Option 2 Using the existing culvert alignment. This option has similar drawbacks 

to option 1: it is steep (7.9 percent) and only slightly further upstream 

from the questionably stable debris jam (figure 6.16). In addition, the bend 

at the outlet would require bank-stabilization measures. 

Figure 6.16—Alternative alignmentsfor replacement culvert.
6—37

Chapter 6—Stream-Simulation Design

Option 3 Realigning the channel and shortening the culvert. Option 3 accepts a 

poorer culvert-to-channel alignment at the outlet for the sake of a shorter 

and flatter pipe, and better control of VAP. The channel downstream of the 

crossing would be lengthened to meet the outlet of the pipe, which here 

is placed perpendicular to the road. The added channel length raises the 

outlet elevation so that culvert slope is only 6.25 percent, near the average 

channel slope for the entire reach, and only slightly steeper than the 

upstream reach. 

Selected design option Option 3 was selected largely because no valid reference reach exists in 

the surveyed longitudinal profile (figure 6.15) for either option 1 or 2. 

In addition, the steeper culverts in options 1 and 2 would require larger 

streambed material for stability, creating a risk of loss of surface low flows 

due to infiltration into the streambed. The simulated channel would also be 

less self-maintaining because the flatter upstream reach may not resupply 

the larger bed material as it moves out of the culvert during floods. 

In option 3, the simulated channel slope is similar to the slope of the 

upstream channel, and the simulated streambed is more likely to be 

self-maintaining; that is, sediment washed out of the simulation will be 

replaced by incoming sediment of similar size from the upstream reach. 

The upstream reach will serve as a reference reach. Option 3 constructs 

21 feet of new channel at a moderate grade between the culvert outlet and 

the log jam [figure 6.18(a)]. When the jam does break up and the channel 

downcuts locally, two rock weirs constructed in the new channel segment 

will mitigate any risk to the stream-simulation channel in the culvert. A 

secondary benefit of the new channel segment is that it adds spawning 

habitat to the reach. 

Because of the abrupt bend at the outlet, the culvert-channel transition 

is very important in this design, to avoid bank erosion and excessive 

sediment deposition. The design overwidens the bend at the outlet to leave 

space for a gravel bar that is expected to form at the inside of the bend 

[figure 6.18(a) and (c)]. Riprap is placed on the outer bank. The two rock 

weirs below the bend not only stabilize grade, but also bring the thalweg

to the center of the channel. They are designed to be immobile during the 

100-year flood. 
6—38

Stream Simulation

Figure 6.17—Channel and existing ground profiles associated with the alignment options. Project profiles are

drawn between stable grade controls.
6—39

Chapter 6—Stream-Simulation Design

Figure 6.18—( a) Design site plan.( b) Design longitudinal profile. (c) Realigned channel at outlet. (d) Looking

downstream through the finished stream simulation culvert. (Design by Robert Gubernick, Tongass National

Forest, and Chinook Engineering.)

(c) (d)

(a)

(b)
6—40

Stream Simulation

6.2 Design of the Stream-Simulation Channel Bed

After determining the best site layout (i.e., horizontal alignment and 

vertical slope profile), design the stream-simulation channel using the 

characteristics and dimensions of the reference reach. 

This section describes design of the following streambed elements:

l Particle-size distribution of the bed material. 

l Channel width and cross-section shape. 

l Banklines, margins, and key features. 

l Bedforms: pool-riffle, step-pool, or other sequences. 

These elements control channel gradient and provide enough flow 

resistance (roughness) to maintain the diverse range of water depths 

and velocities needed for fish and other aquatic species passage. The 

reference reach is the template for all these elements. Flood conveyance 

considerations and other project objectives, such as terrestrial animal 

movement, will determine the amount of bank space allowed inside the 

structure.

One of the keys to stream-simulation design is creating roughness 

conditions that are similar to the reference reach. Total roughness depends 

on a number of features (see appendix A), including: 

l Bed material particle-size distribution. 

l Channel shape.

l Bedforms (fixed or mobile).

l Key features that constrict the channel and are major roughness 

elements.

l Vegetation.

l Bank irregularities.

l Channel bends. 

Not all these features can be replicated inside the crossing structure, but 

the design still needs to approximate total reference-reach roughness. 

The following sections describe how to simulate those elements that can 

be simulated. Clearly, since channel bends cannot be simulated (except 

in very unusual circumstances—see section 6.1.1.3), a straight, uniform 

reference reach is ideal.
6—41

Chapter 6—Stream-Simulation Design

Section 6.2.1 describes basic procedures for designing a simulated stream 

bed using reference reach characteristics. Section 6.2.2 covers special 

considerations for specific channel types. The key is to mimic those 

features in the reference reach that influence channel gradient, energy 

dissipation, bed stability, and physical and hydraulic diversity. 

6.2.1. General Procedures for Simulated Streambed Design 

6.2.1.1. Bed material size and gradation in armored channels

Stream-simulation bed material is designed based on the reference reach 

particle-size distribution (see section 5.1.6.1). It should be well graded 

(consisting of a wide range of particle sizes), and it must include enough 

sand, silt, and clay (particles less than 2 millimeters in diameter) to fill 

voids between larger particles and reduce infiltration into the channel 

bed. The procedure described here produces a particle size distribution 

curve that approximates the reference reach. Later in the design process, 

particle sizes may need to be modified to deal with various risk factors; 

for example, you might increase particle sizes somewhat if the simulation 

needs to be slightly steeper than the reference reach (see section 6.5.1). 

Section 7.4.3 shows how to work the particle-size distribution curve into a 

contract specification.

If particle size results from a depth-integrated bulk sample of the 

reference reach are available, the simulation can have the same grainsize distribution as the bulk sample. However, bulk sampling is unusual 

in coarse-bedded streams because representative samples must be very 

large(section 5.1.6.1). Usually, stream-simulation bed-material gradation 

is based on the reference reach pebble count, which represents only the 

bed surface. In unarmored or weakly armored channels, the surface pebble 

count characterizes the entire streambed, and the simulation bed mix 

will have the same gradation as the pebble count. In armored channels, 

however, the surface pebble count underrepresents the smaller sizes in the 

subsurface, and therefore the smaller particle size classes must be either 

estimated or calculated. The D95, D84, and D50 percentile particle sizes 

of the reference reach bed become the corresponding grain sizes of the 

stream-simulation gradation in both armored and unarmored channels. 

The smaller grain sizes in the streambed are extremely important for bed 

permeability and stability. A porous bed can allow substantial infiltration 

and loss of surface flow. The simulation bed mix must therefore have 

enough fine materials (2 millimeters and finer) to fill the voids between 
6—42

Stream Simulation

the larger particles. Do not assume that the stream will transport sufficient 

fines to seal an open-graded bed surface, because a natural filling-in of the 

voids could take years. Cases exist where the entire summer streamflow 

infiltrated into the subsurface and flowed through the porous culvert bed 

for at least a decade after construction. The problem of loss of surface 

flow is especially critical in steep channels, where bed particles and voids 

between them are larger, and the steeper hydraulic slope can drive the flow 

into the subsurface. 

Since pebble counts of armored bed surfaces underrepresent the finer 

material in the subsurface, grain sizes smaller than D50 must be determined 

another way. One method is the equation developed by Fuller and 

Thompson (1907), which defines dense sediment mixtures commonly 

used by the aggregate industry. This equation has not yet been widely 

field-tested for this application, so apply good professional judgment when 

using it. 

The Fuller-Thompson equation is: 

Equation 6.1

P/100 = [

 d ]

n

 Dmax

where d is any particle size of interest, P is the percentage of the mixture 

smaller than d, Dmax is the largest size material in the mix, and n is a 

parameter that determines how fine or coarse the resulting mix will be. An 

n value of 0.5 produces a maximum density mix when particles are round. 

The Fuller-Thompson equation can be rearranged to base the particle 

size determination on D50 rather than Dmax. Basing the calculation on 

D50 avoids a discontinuity in the particle size distribution curve, which 

otherwise occurs when the actual D50 is different from the value calculated 

from Dmax. The equations for D30, D10 and D5 are: 

Equation 6.2 D30 = 0.61/n D50

Equation 6.3 D10 = 0.21/n D50

Equation 6.4 D5 = 0.11/n D50
6—43

Chapter 6—Stream-Simulation Design

To develop the particle-size distribution curve for the for the finer portion 

of the simulation bed mix, use n values between 0.45 and 0.70, a standard 

range for high-density mixes. The goal is a dense, well-graded bed mix 

with a percentage and type of fine material (sand, silt, clay) similar to 

the percentage and type in the reference reach subsurface. The fines are 

essential to limit infiltration into the bed and to help lock the larger pieces 

together. Type and percentage of fines vary with geology and stream slope, 

but generally the bed mix should contain at least 5-percent fines. If the D5

resulting from the Fuller-Thompson equation is larger than 2 millimeters 

(for n = 0.45, this occurs when D50 is larger than 330 millimeters or 13 

inches), adjust the mixture so that fines comprise at least 5 percent. If 

your field estimates of fines (section 5.1.6.1) differ substantially from this, 

adjust the mixture to approximate the field composition. 

Figure 6.19 shows how the results of the Fuller-Thompson method 

compared to field data for the South Fork Cache la Poudre River (figure 

5.6). Field data for the surface armor are from a pebble count. The 

subsurface particle size distribution curve is from a sieved bulk sample.

Figure 6.19—Bed material particle size distribution designed using the FullerThompson method, compared to field data for the South Fork Cache la Poudre

River. Field data provided by K. Bunte, 2004.

Surface and Subsurface Material Particle Size 

Distributions--South Fork Poudre River with 

Fuller-Thompson distributions 

Field data provided by K. Bunte

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100 1000

particle diameter (mm)

cumulative percent finer (%) 

subsurface sieve analysis

surface pebble count

design bed mix, F-T n=0.45

design bed mix, F-T n=0.7

size class

silt/clay sand gravel cobble boulder

cumulative percent finer (%)
6—44

Stream Simulation

The surface (“pebble count”) curve in figure 6.19 was used directly to 

define the larger particles of the design gradation. The lower half of 

the particle-size distribution curve can be anywhere between the two 

Fuller-Thompson distributions (labeled “F–T”) with n values of 0.45 and 

0.70. In this case, selecting an n value of 0.45 produces a gradation with 

approximately 10-percent finer than 2 millimeters, a percentage close to 

the actual fines content in the subsurface. 

Using the Fuller-Thompson method does not reproduce the natural 

subsurface particle size distribution in the reference reach subsurface, 

but it does result in a dense, well-graded distribution. Similar results may 

be obtained by smoothly redrawing the lower half of the particle size 

distribution curve by hand, such that the tail has an appropriate percentage 

of fines smaller than 2 millimeters. 

Note that these design procedures result in a bed mix that is coarser overall 

than the reference reach subsurface gradation. This constitutes a safety 

factor for the simulated bed; if the bed scours, there will be additional 

armor material below the surface, and the resulting bed surface will 

become coarser and rougher. 

The method of deriving a design gradation from the pebble count is not 

critical. What is critical is that the design gradation have the following key 

characteristics: 

l Large particles (D95, D84, and D50) that provide bed structure and 

buttress finer material should be accurately sized based on the 

reference reach. In channels where wood controls or influences the 

channel form, structures composed of angular rock can substitute 

for wood to simulate channel features in the crossing structure (see 

section 6.2.1.5).

l The entire bed mix should be well graded (poorly sorted). A dense, 

stable bed requires all particle sizes, so no gap in sizes should exist 

between any classes of material in the design bed mix. Ideally, each 

class of bed material that makes up the mix will be well graded, so 

that all sizes within the category are represented. This representation 

is especially important for the smaller-size fractions in a mixture that 

includes large particle sizes. 

l The percentage of sand, silt, and clay should approximate the 

reference reach channel bed subsurface (visually estimated, see table 

5.5), and should be adequate to limit bed permeability by filling 

voids between the larger particles. Including sand, silt, and clay 

in the simulation bed material commonly arouses concerns about 
6—45

Chapter 6—Stream-Simulation Design

water quality and habitat impacts, because some fine sediment in a 

freshly constructed bed will move during low flows, and could affect 

downstream fish habitats. Any such effects can be limited during 

construction by using water to wash the fine material down into voids 

between the larger particles in the bed (sections 7.5.2.3 and 8.2.11.2). 

l Bed material rock should be durable, and it should be at least as 

angular as in the reference channel. If it is less angular, it may be 

significantly more mobile than intended. It makes sense to try to 

find local material, as it will more likely resemble the natural bed 

material.

6.2.1.2 Channel cross section

The width of the simulated channel is typically the bankfull width of the 

reference reach or greater. This is not necessarily equal to the culvert width 

(see section 6.3 for selecting culvert dimensions). Bank features and/or 

overbank flow surfaces may require additional culvert width.

In channels with mobile beds (dune-ripple, fine-grained pool-riffle), 

complex channel shapes like those that develop over time in a natural 

channel need not be constructed. However, some bank features should be 

constructed to set the stage for channel margins to develop (figure 6.20). 

Without constructed features, the bed initially tends to flatten into an 

unnatural flat surface. Then, the main thread of flow often migrates to the 

culvert wall and progressively erodes a trench along the wall.

In mobile channels, in addition to banks and any other key features, a 

roughly V-shaped low-flow channel can be constructed to help keep flow 

from hugging the culvert wall until a natural bed structure develops. 

The V-shape is not intended to persist; when high flows occur, they will 

redistribute the bed material and construct a diverse channel with a natural 

thalweg. The precise shape of the V-shaped initial low-flow channel is 

not critical; the channel in figure 6.20 has a 5h:1v lateral slope which is a 

reasonable starting point. 

Stream simulations in less mobile channels are often constructed with 

some initial bed structure such as steps. Specifics for each channel type are 

described in section 6.2.2. 
6—46

Stream Simulation

Figure 6.20—Channel cross-section shape in a stream-simulation channel with 

rock banks.

6.2.1.3 Bank and channel margin features

In natural channels, the diversity, roughness, and shape of channel 

margins and banklines are critical for movement of some species. For 

example, terrestrial animals may need dry passage; weak swimmers and 

crawling species may need margins of slow, shallow water with eddies in 

which they can rest. At flows between low-flow and bankfull, channeledge diversity is necessary for accommodating the different movement 

capabilities of all aquatic species. Banks must continue through the inlet 

and outlet transitions. 

Bars may form in a crossing structure—perhaps on just one side or 

through part of its length—and they may provide some of the benefits of 

a bankline. However, without root structure, cohesive soils, or the ability 

to scour into parent bed material, true banklines will not form naturally 

inside the structure. Therefore, specific channel-margin features should be 

designed into the project when they are needed for hydraulic roughness, 

habitat diversity, or for preventing channel trenching along culvert walls 

and protecting footings from scour. In designing the bankline/margin, 

use the reference reach bank height and bankline diversity (including 

frequency and size of wood or rock protrusions) as a guide. Where wood is 

an important feature on the channel banks, use permanent rock to simulate 

its functions. 
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Chapter 6—Stream-Simulation Design

Because the intent is to create permanent bankline features, use material 

large enough to be stable during the high bed design flow. In the absence 

of vegetation, bank stability inside the structure will depend primarily on 

rock size, packing, clustering, and embedment. Base an initial estimate of 

rock size on the reference channel. As a starting point, bank material might 

be up to twice the size of D95 in the reference reach. If D95 is 3 inches 

or less, you can use 6-inch-minus quarry spalls or other rock. The size 

of rocks that appear to be immobile in the reference reach may also be a 

clue to sizing bankline rocks. Later in the design process (section 6.4.2), 

a stability analysis will verify that the bank rock and other key pieces are 

large enough to be immobile. 

The simplest bankline is an irregular line of large rock placed along 

each wall (figure 6.20). Most natural banks are rougher and more diverse 

than that, and a discontinuous line of rocks or rock clusters may better 

simulate the reference reach (see figure 6.21). Clusters of rock obstruct 

any tendency to scour along the culvert wall, and help create the bed 

diversity that exists in natural channels where water deflects off bankline 

irregularities like woody debris or root-wads. Fill the spaces between 

individual bank rocks and between the rocks and the culvert wall with 

‘filler’ material (section 7.5.3), so that the finer material helps to stabilize 

the larger rocks.

Overbank flow surfaces, or flood-plain benches, are sometimes constructed 

inside culverts (see 6.5.1.1). Construct them the same way as bank clusters 

or banklines, with the entire surface being stable rock infilled with filler 

material. The flood-plain bench should start at bankfull elevation on the 

margin of the bankfull channel, and slope up and out at about 10h:1v 

(figure 6.22). 

One way of simulating a bankline in an open-bottom arch might be to 

roughen the concrete stem wall using embedded rocks or shaped concrete 

elements built into the wall. To our knowledge, no one has tested this 

method.
6—48

Stream Simulation

Figure 6.21—Open-bottom arch stream-simulation design showing bankline diversity. Design by Kim 

Johansen, Ore Creek, Siuslaw National Forest.
6—49

Chapter 6—Stream-Simulation Design

Figure 6.22—Stream-simulation channel with overbank flow surface.

6.2.1.4. Key features

Many forest streams have highly stable features, such as large wood, embedded or jammed wood, 

and large boulders, which may have fallen or slid into the stream or are 

remnants of glacial action. Other woody debris in the reference reach 

might take the form of small jams, buried wood that buttresses the bed 

and/or forms steps, or wood protruding from a bank. These ‘key features,’ 

often partially buried in the bed, may block part of the channel cross 

section and are long-lasting grade control and/or energy dissipation 

structures in the channel. Key features also include stable steps and 

imbricated or well-packed riffle crests that move only in infrequent high 

flows. 
6—50

Stream Simulation

Functions of these key features that need to be replaced in the simulated 

channel include buttressing the bed material and controlling grade, 

providing diverse hydraulic conditions that aquatic species can use for 

cover and resting areas, and providing hydraulic roughness. 

In current practice, key-feature roughness is simulated directly by 

imitating the size and distribution of individual elements using rock. 

Intermediate-mobility riffle crests and steps are constructed using rock 

sized like the rock forming those features in the reference reach (section 

6.2.2.2). In the site assessment, separate measurements of 10 to 25 rocks 

from key features like riffle crests and steps were taken (section 5.1.6.1) 

as the basis for specifying rock sizes for these features. Slightly larger 

rock sizes, or more angular rock, may be needed to simulate the stabilizing 

effects of imbrication and particle packing in the natural channel that 

cannot be replicated in the simulation. Later in the design process (section 

6.4), a bed mobility analysis will be conducted to check that these rocks 

are sized properly and will be as stable in the simulated channel as in 

the reference reach. In chapters 7 and 8—dealing with construction and 

contracting—these key-feature rocks are referred to as “channel rocks.” 

Various size classes of channel rock may be specified to simulate different 

channel-bed features. 

Key features, such as embedded logs, often span the entire channel, and 

you should simulate them that way, constructing them like a step (see 

section 6.2.2.4), and simulating the height of the features in the reference 

reach. A cluster of rocks jutting out from the culvert wall can simulate a 

bank log in a natural stream, providing some edge diversity and helping 

prevent a low-flow trench next to the culvert wall. If space permits, 

simulate the roughness and functions of scattered or clustered boulders in 

the reference channel by placing the same general size and pattern of rocks 

in the stream simulation.

An alternative method of simulating roughness created by individual 

roughness elements in the reference reach would be to measure the 

total frontal area of all roughness elements in the reference channel and 

use boulders to reproduce it in the simulation. Ferro (1999) describes a 

method of quantifying the roughness created by various arrangements 

and concentrations of boulders placed on a gravel streambed. To our 

knowledge, this method has not yet been applied to stream-simulation 

design.

Streambed mobility is discussed in 

sections 5.1.6.1 and 

5.1.6.2
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Chapter 6—Stream-Simulation Design

For immobile key features, mimic the size of immobile rocks in the 

reference channel, and/or do a stability analysis (section 6.4). Rocks 

locked together in clusters are more stable than individual rocks and can be 

somewhat smaller. Angular rock is more stable than round rock. For these 

permanent key features, you can over-design rock sizes to reduce the risk 

of failure. 

To protect the culvert floor, place large rocks carefully by embedding them 

into the bed mix rather than allowing them to drop directly on the culvert 

floor. Careful construction is essential, especially in steeper (greater 

than 6 percent) channels where less experience with stream-simulation 

construction exists. Energy dissipation by key features is critical for the 

stability of steep channel designs; if possible, consult with experienced 

stream-simulation practitioners about steep simulated-channel designs. 

6.2.2. Bed Design Considerations for Specific Channel Types

The general procedures described in section 6.2.1 apply to all channel 

types. This section describes additional bed design considerations that apply 

to specific channel types. Table 6.5 summarizes important channel bed 

characteristics and channel design strategies for each type. 

6.2.2.1. Dune-ripple channels 

Although dune-ripple channels are usually sand-bed streams (table 6.5), 

for design purposes we include channels with fine- and medium-gravel 

beds (Dmax is medium gravel, 16 millimeters or smaller). Creating custom 

bed material gradations (section 6.2.1.1) with these materials is impractical 

because D95, D84, D50, etc., are close in absolute size. In addition, custom 

bed material designs usually are unnecessary for these fine materials. The 

bed typically mobilizes during moderate flows, and bed material turnover 

occurs frequently. Bank features may need to be constructed to avoid culvert 

wall trenching by providing edge diversity and the roughness present in the 

reference reach.

You might choose to allow the culvert to fill naturally with bed material 

if sediment loads are high and/or the culvert is backwatered by the 

downstream channel. This technique, however, has the potential to create 

a headcut in the upstream channel. Native bed material approximating 

This discussion uses the channel 

classification system developed 

by Montgomery and Buffington 

(1997).
6—52

Stream Simulation REFERENCE CHANNEL TYPE Dune-ripple 

Pool-riffle 

(mobile)

Pool-riffle 

(intermediate 

mobility)

Plane-bed 

Step-pool

Bed 

material

Sand to medium gravel Fine to coarse gravel Coarse gravel to cobbleGravel tocobble, usually armoredCobble to boulder 

Dominant roughness 

and structural 

elements2

Sinuosity, bedforms, banks. Small debris may provide 

structure.

Bars, pools, grains, sinuosity, banksBars, pools, grains, sinuosity, banksGrains, banksSteps, pools, banks. Debris may add significant structure.

Streambed mobility

Termed “live bed”; significant sediment transport at most flows. Bed is often armored; usually mobilizes near bankfull. Finer sediment moves over immobile armor layer at flows near bankfull. Armor layer mobilizes at higher flows.May be either ‘mobile’ or ‘intermediate mobility’ (see poolriffle).Fine material moves over larger grains at frequent flows.Bed-forming rocks move at higher flowsdepending on size;often >Q30

STREAM-SIMULATION 

DESIGN 

STRATEGIES

l Simulated bed can be native bed material or imported 

dense mix based on reference reach.

l Rock clusters (key features) added to simulate diversity 

from wood and bank shape.

l Key features and banks designed to be immobile.l

D95, D84, D50 based on reference reach.

l Material smaller than D50 is dense mix based on D50.

(section 6.2.1.1)

l Key features (rocks or rock clusters in bed and/or banks) 

added for diversity.

l Key features and banklines designed to be immobile.l

D95, D84, D50 based on reference reach.

l Material smaller than D50 is dense mix based on D50.

l Riffle crests constructed with material sized based on riffle 

crests in reference reach.

l Key features, banklines designed to be immobile.l

D95, D84, D50 based on reference reach.

l Material smaller than D50 is dense mix based on D50.

l Key features, banklines designed to be immobile.l Steps are spaced similar to reference reachl	Step-forming rocks are sized to be immobile.l	Smaller material size distribution is dense mix based on

D50

of material other than steps in reference reach 

l Banklines designed to be immobile.

TYPICAL CONDITIONS1

Table 6.5—Bed material design considerations for various channel types

1 Based on Montgomery and Buffington (1997) with some modifications.2 Woody debris can be an important structural feature in any channel type.
6—53

Chapter 6—Stream-Simulation Design

Table 6.5—Bed material design for various channel types (continued)

1 Based on Montgomery and Buffington (1997) with some modifications.2 Woody debris can be an important structural feature in any channel type.

REFERENCE 

CHANNEL 

TYPE

Cascade

Bedrock

Channels 

in cohesive 

materials 

Bed 

material

BoulderRock with sediment of various sizesin transport over rock surfaceSilt to clay

Dominant roughness 

and structural 

elements2

Grains, banksBed and banks. Sinuosity, banks, bed irregularities

Streambed mobility

Smaller bed material moves at moderate frequencies (floods higher than bankfull). Larger rocks are immobile in flows smaller than ~Q50. 

Bedload moves over bedrock at various flows depending onits size. May be thinlayer of alluvium over bedrock. Wood can strongly affect sediment mobility. Fine sediment moves over immobile bed at moderate flowsdepending on its size. May be thinlayer of alluvium over immobile bed. 

STREAM-SIMULATION 

DESIGN 

STRATEGIES

l

D95, D84, D50 based on reference reach.

l Smaller material size distribution is dense mix based on D50.

l Key features, banklines designed to be immobile.l Stream simulation bed is bedrock.l	Banklines and roughness elements are important but difficult

to design as stable.

l Condition, extent, and shape of bedrock are important.l Bottomless structure reduces rock removal compared to full 

pipe and can be anchored and shaped to rock.

l Stable cohesive bed and banks cannot be constructed in 

culvert. 

l Culvert walls may simulate smooth natural clay banks.l Bottomless structure might leave clay bed undisturbed.

TYPICAL CONDITIONS1
6—54

Stream Simulation

the reference reach size distribution may be available from the crossing 

excavation. If so, use it to fill the culvert. Since dune-ripple channels are not 

armored, mixing and replacing excavated bed material carries no great risk. 

Either use the bed material by itself, or supplement with imported material to 

make up the required channel-fill volume. 

To achieve more or less the same initial mobility, use material that is similar 

to (and not larger than) the reference reach bed material. Rounded river rock 

is not always available as fill material, and quarried angular rock can be 

substituted. However, recognize that sediment mobility for angular rock may 

initially be somewhat lower than in the adjacent reaches. Including fines for 

sealing the bed is not necessary in these mobile channels.

Bed structures form readily in fine-grained channels, so building structure 

into the simulated channel generally is unnecessary. Nonetheless, consider 

the roughness characteristics of the reference reach. Small pieces of debris 

scattered and partially buried in the bed stabilize some fine-grained channels 

at slopes steeper than they would otherwise be. Although the small wood 

will be transported into the simulated channel over time, you may need 

to place it during construction if it is critical for maintaining initial slope. 

Again, bank features also may be needed to simulate the reference reach. 

Dune-ripple streams are usually—though not always—unentrenched, 

and overbank flood-plain flows may occur frequently. The design issues 

associated with the road fill obstructing flood-plain flows (see section 

6.5.1.1) can be very important. To accommodate some flood-plain flow 

and to avoid excessive bed and outlet scour at the culvert during floods, 

banklines and a flood-plain surface may be important components of a duneripple channel simulation. Off-channel flood-plain drainage structures also 

may be important.

6.2.2.2 Pool-riffle channels 

The basic design process described in section 6.2.1.1 applies directly 

to mobile pool-riffle channels. For pool-riffle channels of intermediate

mobility, riffle crests may need to be constructed as key features 

(6.2.1.4). Place constructed riffle crests or bars at locations on the project 

longitudinal profile where a riffle crest would naturally fit. Locate the riffle 

crests based on average riffle crest spacing in the reference reach unless 

the crossing site includes a channel bend where a pool is expected to form. 
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Chapter 6—Stream-Simulation Design

In that case, locate the riffle crests where they would naturally fit upstream 

and downstream of the pool. Then, place any other constructed riffle 

crests at spacings within the range found in the reference reach (see riffle 

spacing, figure 5.19).

A constructed riffle crest is a structure that spans the channel as illustrated 

in figure 6.23. Rocks of similar size to reference reach riffle crest particles 

compose the structure (see section 5.1.6.1 for assessment procedure), 

and it is constructed with a low point in the center of the channel to help 

form the low-water channel. The objective is to establish grade control 

and energy dissipation structures with spacing and mobility similar to the 

reference reach.

Figure 6.23—Placement of constructed riffle crests to simulate natural riffle crests

of intermediate mobility.

Where imbrication, embeddedness, or any mode of particle packing 

increases the stability of riffle crests in the reference reach, constructed 

riffle crests will not achieve the same stability unless they are carefully 

constructed to replicate the particle packing. Figure 6.24 shows an 

example design, done by the Olympic National Forest project team for 

the simulated channel at Newbury Creek, which required constructing 

riffle crests by embedding rocks in an imbricated pattern. Recall that this 

is a cobble-bed pool-riffle channel, where riffle-crest materials are not 

mobilized until flows substantially exceed bankfull. Because the team did 

not expect bed structures to form rapidly, it decided to construct them. 

The transverse bars in figure 6.24 are analogous to riffle-crests in the 

reference reach. Rock spurs jutting out from the culvert wall mimic the 

indented and debris-strewn bankline in the reference reach. (see photo, 

figure 5.10). 

Refer to the Newbury 

Creek site assessment 

text boxes throughout 

chapter 5. They begin in section 5.1.1.
6—56

Stream Simulation

Figure 6.24—Newbury Creek stream-simulation design, Olympic National Forest. See chapter 5 for site assessment information.
6—57

Chapter 6—Stream-Simulation Design

6.2.2.3 Plane-bed channels

Plane-bed channels do not have regularly spaced bedforms. In these 

channels, although the bed is relatively featureless, large rocks protrude 

from the water surface at most times of the year. The basic design process 

(6.2.1.1) applies to plane-bed channels. Rock clusters along the culvert 

walls are recommended for helping to keep the thalweg from trenching 

along the wall and for fostering sediment deposition on channel margins. 

Bank features are important, because the bed itself has less hydraulic 

diversity than most channel types. 

6.2.2.4 Step-pool channels

In this channel type, both steps and pools are important for energy 

dissipation and channel stability. Steps form when the largest particles in 

the bed congregate and support each other, creating a bedform that is more 

resistant to movement than the individual pieces. Usually boulders or logs 

form the step framework, which supports smaller cobbles and gravels. In 

nature, steps can take several decades to form (Madej 2001), depending on 

when channel-organizing flows occur and what key features are present. 

Bed-organizing flows are generally higher than bankfull; depending on the 

size and embedment of the step-forming materials, steps may not form at 

flows less than the 30-year flow or higher (Grant et al. 1990). Because steps 

are critical for energy dissipation and channel stability but are unlikely to 

form naturally in a short period of time, they should be constructed and 

monitored carefully after high flows.

Base step height and length on the reference reach, and keep step spacing 

within the range of variability observed in the reference reach (see figure 

5.19). This ensures that step spacing is similar to the reference reach, while 

still allowing enough flexibility to tie the step-pool sequence into the stable 

profile endpoints. It also permits you to accommodate a channel bend that 

forces a pool in a specific location on the project plan and profile. Spacing is 

important because pools large enough for adequate energy dissipation must 

have room to develop, and each step affects the stability of the adjacent one. 

Steps in natural channels are typically spaced one- to four-channel widths 

apart and are closer in steeper channels.

To construct the steps, use rocks at least as large as the step-forming rocks in 

the reference reach. The rocks should have similar roundness or angularity. 

Embed or layer the rocks to below the expected depth of pool scour in such 
6—58

Stream Simulation

a way that the lower rocks support those above them. Construct steps with 

the expectation that individual rocks will adjust their position during high 

flows and lock together. Until the larger particles adjust and support each 

other, they are vulnerable to being scoured out of the culvert. Therefore, a 

conservative objective—designing the steps to be immobile for the life of 

the project—is a wise approach (section 6.4). 

In a step-pool channel, even with a bankfull-width culvert, bed-organizing 

flows may be more confined than in the natural open channel, and shear 

stress in the culvert may be higher. Steps may not reform inside the culvert if 

the constructed ones wash out. For this reason and because of the potentially 

long time before new boulders might be recruited during subsequent high 

flows, designing steps for immobility is common practice. You can increase 

rock angularity and/or size (as compared to the reference reach) to increase 

stability. 

Figure 6.25—Schematic of step-pool stream-simulation design.

Aside from the steps themselves, design the step-pool bed mix based on the 

reference channel pebble count (see basic design process, section 6.2.1.1). 

Frequent high flows scour and replenish the finer material between steps as 

bedload moves through the system.
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Chapter 6—Stream-Simulation Design

Once the steps are in place, pools will form naturally during moderate flows 

and generally do not need to be constructed. Pools are typically wider than 

steps, and the simulated channel should be as wide as maximum pool width 

in the reference reach. This will allow pools to form that are large enough to 

achieve the same degree of energy dissipation as in the reference reach. 

As a safety factor in steep (greater than 6 percent) simulations, bed retention 

sills are sometimes used for buttressing the steps and preventing material 

from sliding or washing out of the culvert. These sills may consist of metal, 

wood, or logs fastened in place. Unlike baffles, bed retention sills are not 

intended to control water velocity, and they do not extend above the surface 

of the streambed. Neither are they intended for placement in a bare pipe 

to trap bedload in transport. Place the tops of the sills below the lowest 

potential bed profile (lower VAP line), so that they will not be exposed above 

the streambed surface over the project lifetime (figure 6.26). 

Figure 6.26—Profile view of steel bed-retention sill stabilizing a boulder step

(cutaway view).

6.2.2.5 Cascade channels

Cascade channels are steep (greater than 8 percent), and their largest 

bed particles are large relative to normal flow depths (Montgomery and 

Buffington 1993). Energy is dissipated by water flowing over or around 

individual rocks. Smaller sediments move over or around the larger rocks 

at flows somewhat larger than bankfull. Rocks that are key to bed structure 

and stability, however, are immobile up to very high flows (greater than 

50-year). Again, at these flows, shear stresses inside a pipe may be higher 
6—60

Stream Simulation

than in an open channel. Bed stability is critical in a simulation because, 

if the bed fails, the bare culvert is unlikely to recover naturally. On a 

simulation this steep, after sizing the bed material based on the reference 

reach, conduct a hydraulic stability analysis (section 6.4) to ensure that the 

largest bed-forming particles (e.g., D84 and D95) are stable in the design 

flood. Also consider using bed-retention sills as in step-pool channels. 

6.2.2.6 Bedrock channels

If a culvert is being replaced and the adjacent channel is primarily 

bedrock, investigate the channel and likely footing locations to determine 

bedrock location, elevation, and suitability for a foundation. If the road is 

located on a concave transition, be aware that the steeper channel upstream 

may be bedrock while the flatter culvert site is on erodible fluvial material. 

If the bed at the site of a new crossing is sound bedrock continuous 

throughout the site, stream simulation may consist merely of placing an 

open-bottom arch culvert over the bedrock. Depending on the shape of 

the rock surface, you might anchor the entire footing to it, with a stem 

wall extending up to the bottom of the prefabricated culvert. The height 

of the footing and stem wall accommodate any variation in the bedrock 

surface. Where exposed bedrock is tilted, a deep, smooth channel may 

form along one wall of the culvert at low flow. In such situations, consider 

adding boulders for roughness and to deflect flow toward the center of the 

structure. You may need to use special construction procedures, such as 

embedding, anchoring, or clustering, to keep large boulders from rolling or 

sliding out of a bedrock channel. 

Frequently, bedrock is exposed in the channel bottom while the 

streambanks are composed of alluvial or colluvial material. The banks 

may have large roughness elements, such as wood and single or clustered 

boulders. These may be important key features for retaining sediment and 

debris that provide diverse habitats and migration pathways in bedrock 

channels (McBain and Trush 2004). Channel margins and/or banklines 

therefore may be important to the objective of the project. 

Bedrock channels sometimes exist where a bed of alluvial material has 

scoured, leaving the bedrock exposed. This exposure often occurs where 

woody debris has been removed or where a debris flow has scoured the 

channel to bedrock. Bedrock that does not show typical erosional features, 

such as fluting, longitudinal grooves, or potholes, could indicate that an 

alluvial veneer has recently washed away. In these cases, consider placing 
6—61

Chapter 6—Stream-Simulation Design

debris and/or immobile key feature rocks to help develop a natural alluvial 

bed and/or to stabilize a constructed bed. Exposed bedrock with no 

evidence of fluvial erosion also may result from channel incision caused 

by channel realignment and straightening during placement of the previous 

culvert. This can be a signal to seriously consider correcting the alignment 

during the replacement project. 

6.2.2.7 Channels with cohesive bed material

A channel with cohesive bed or banks cannot be constructed inside a pipe. 

For new installations in cohesive bed channels, avoid disturbing the bed. 

The best stream-simulation alternative is probably to span such a channel 

completely, using a bridge or arch. Cohesive-bed channels often pose 

foundation challenges, and require a good geotechnical investigation. 

6.3 Crossing Structure Dimensions and Elevation

Now, for the first time in the design process, we consider the crossing 

structure itself. Up to this point, we have used geomorphic design methods 

to define both the probable range of stream profiles at the site and the size, 

shape, materials, and arrangement of the stream-simulation channel bed. 

Now we size the structure by fitting it around the designed channel. This 

discussion is primarily about culvert design, but similar width and height 

considerations also apply to bridges.

Culvert elevation and dimensions are determined at this point because 

they affect the bed mobility calculations in the next design step. It may 

take several iterations to select the final dimensions, because the bed 

mobility calculations (section 6.4) may indicate the need to change 

culvert dimensions. Only the dimensions and elevation of the culvert are 

determined in this step; many other considerations enter into the final 

choice of structure type and materials. Section 7.2 discusses them at 

length. 

One of the goals in stream simulation is that the simulated channel be 

self-sustaining. That means it must simulate the hydraulics of the natural 

channel at sediment-transporting flows, especially the flows that create and 

rearrange major bed structures. To achieve these objectives, the simulated 

channel must be free to adjust to changes in incoming flow and sediment 

loads, and the culvert must be large and embedded deeply enough to 

accommodate both vertical and lateral adjustments.
6—62

Stream Simulation

Several factors go into determining culvert size and elevation. These 

include: 

l The bankfull width of the channel. 

l The width of any banklines and overbank surfaces. 

l The range of possible bed profiles (VAP).

l The maximum sizes of alluvial and immobile rocks. 

l The results from the bed stability and flow capacity analyses (6.4 and 

6.5.2.1).

The structure must satisfy all these conditions at the same time. 

6.3.1 Culvert Width

A variety of factors determine the structure width needed to achieve 

project objectives and to accommodate site conditions (see table 6.6). 

Table 6.6—Considerations affecting choice of stream-simulation culvert width

Based on project objectives: 

l Width of bankfull channel. 

l Stability of the simulated streambed.

l Hydraulic capacity of the culvert.

l Risk of blockage by floating debris or beaver activity.

l Construction, repair, and maintenance needs. 

l Passage of nonaquatic species.

l Meandering channel pattern. 

l Protection of flood-plain habitats.

Based on site characteristics: 

l High flood-plain conveyance and potential to concentrate overbank 

flows in culvert.

l Channel migrating laterally. 

l Wider channel expected in future.

l Channel skewed to road crossing or crossing on channel bend.

l Ice plugging in cold climates.

l Large bed material relative to culvert width.

Extra structure width is necessary for creating a stable bankline without 

constricting the bankfull channel. In entrenched and moderately 

entrenched channels, the first estimate of culvert width is simply the 

width needed to span the simulated bankfull channel plus the size of 

the rocks used to construct the banks (figure 6.27). This initial estimate, 
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Chapter 6—Stream-Simulation Design

of course, is subject to change depending on the results of the stability 

analysis of the bankline rocks. As noted in section 6.2.1.3, where the 

reference reach has a rough, irregular bankline, the simulated banks may 

be laterally deeper and may require more structure width.

Figure 6.27—For a stream-simulation design with banks, minimum culvert width 

is bankfull width plus twice the maximum diameter of rocks used to construct the 

banks.

In an unentrenched channel with an active flood plain, the road fill 

could block overbank flood flows and force them through the culvert. 

Section 6.5.1.1 discusses at some length the risks associated with flow 

concentration in active flood plains and their possible solutions. Placing 

additional culverts or dips that permit flood-plain flow through or across 

the road fill may reduce the risk to acceptable levels. If not, you may also 

need additional culvert width to allow for an overbank-flow surface within 

the culvert (figure 6.22). 

In choosing culvert width, also consider how the largest key-feature rocks 

(or rock clusters) in the simulated bed will interact with rock and wood 

pieces moving during high flows. A natural channel can usually scour 

around a large boulder or debris accumulation. In a culvert, however, a 

large individual boulder can create a constriction or form a bridge with 

other large particles, creating a culvert-wide drop structure or debris jam, 

and possibly limiting aquatic species passage, culvert capacity, and/or bed 

stability. A good guideline is that bankfull bed width inside the culvert 

should be at least four times the intermediate diameter of the largest 

immobile particles in the simulated bed.
6—64

Stream Simulation

Early in their development, incising channels may look narrow, but they 

will widen with time because the banks become unstable and fail in 

response to bed lowering (Schumm et al. 1984). Size a stream-simulation 

culvert to anticipate the expected widening of the natural channel near 

the crossing. On the other hand, if a channel is unnaturally wide from 

disturbance, and you expect it to narrow in the future, size the culvert for 

the current channel, with the expectation that recovery will occur inside 

the culvert as in the adjacent reaches.

As noted in section 6.1.1, you may need to increase culvert width if the 

culvert is skewed to the road alignment or if natural lateral migration of 

the channel will likely create a skewed-inlet condition. 

6.3.2. Culvert Elevation and Height 

Points on the stream channel bed may at some time be at any elevation 

within the range of potential vertical adjustment (see section 6.1.2.2). The 

culvert invert elevation and culvert height must allow for these vertical 

bed elevation adjustments over time. The stream simulation bed should 

be thick enough (and the invert deep enough) to avoid exposing the bare 

culvert floor during floods, and to allow large particles to be supported by 

the finer bed matrix, even at the bottom of a pool at the lowest potential 

bed elevation (figure 6.28).To achieve this, set the elevation of the bottom 

of the culvert or footing below the lower VAP line, adjusted to include the 

estimated depth of streambed scour during floods (2 times D90, see section 

5.2.2.2). For bottomless culverts, structural design of the footing and any 

engineering scour analysis that may be conducted may dictate a lower 

elevation (see section 7.3.2). Placement of bank rocks to protect footings 

may also affect their depth.

Once the culvert invert elevation is set, determine the culvert height 

needed to maintain flood and debris capacity when the bed is at its highest 

possible elevation. Setting the widest point of a round culvert at or above 

the highest potential bed elevation is an efficient design technique because 

it uses the full width of the culvert. Generally it also ensures headroom for 

floodwater and debris, although very large floating debris may not clear the 

inlet of the pipe during very high flows.

Recheck both culvert height and width after selecting the high bed-design flow. 

The bed-design flow is the highest flow that immobile particles are designed 

to sustain without moving. They are unlikely to remain in place if the culvert 

inlet becomes submerged and pressurized during a flood. For stability, we 
6—65

Chapter 6—Stream-Simulation Design

Figure 6.28—Embedment for full-pipe and bottomless culverts. 

recommend that the inlet not exceed 80-percent submergence during the 

high bed-design flow, 67 percent where woody debris is a significant 

concern. Ensure that the actual free space is large enough to accommodate 

the size of debris moving in the channel. Naturally, this does not apply to 

submergence caused by backwatering when water levels are similar on 

both sides of the crossing. 

The culvert also must be able to convey the structural-design flow. Flows 

that exceed the structural-design flow may destroy the crossing or cause 

the stream to divert down the road. Select the structural- and bed-design 

flows based on tolerable probabilities of exceedence (section 6.5.2.1) 

and the consequences of each type of failure. The two flows may be 

different, because the consequences of each failure type are different. 

For example, you may be able to accept the bed’s washing away in a 

lower flow than one that could destroy the entire structure—because the 

bed material is replaceable. Where bed-load transport is high enough, 

sediment will be replenished, and the bed may reconstruct itself as the 

flood recedes. Provide a safety factor for invert depth and/or culvert height 

commensurate with the level of uncertainty and the risk of failure. Where 

the consequences of failure are large, use a larger culvert or a deeper 

footing. 

LOWER VAP LINE ELEVATION

SHOULD INCLUDE EXPECTED

DEPTH OF SCOUR DURING 

FLOODS (2 TIMES D90)
6—66

Stream Simulation

Hypothetical small-stream culvert sizing example

Some small streams can have a relatively large range of vertical adjustment potential. A good 

example is a 4-foot-wide meadow stream with densely vegetated 1-foot-high banks and pools as 

deep as 1 foot. Bed material D90 and D95 are 2 inches and 2.5 inches, respectively. After delineating 

the VAP lines on the longitudinal profile, we find that at the cross section shown in figure 6.29 the 

upper VAP line is at the top of the banks, and the lower VAP line is 1.33 feet below the project 

profile. The lower VAP line here includes 1 foot for reference reach pool depth, and 0.33 foot to 

allow for the depth of scour during floods (2 times D90). 

The culvert should be wide enough to allow for placement of bank rocks, which are needed for 

simulating reference reach bank roughness. An initial estimate of the size of stable bank rocks is 

2 times D95 or 0.4 foot (see section 6.2.1.4). The first estimate of culvert width is 4.8 feet: 4 feet 

to accommodate bankfull width, and 0.8 foot to allow for stable rocks on both banks. The culvert 

also should be embedded deeply enough that the channel bed never scours to bare metal. In this 

example, we could conveniently use a 5-foot round culvert. Then, if the invert is 0.25 foot below 

the lower VAP line (2.6 feet below the upper VAP line at bankfull elevation), the project profile 

(bed elevation to be constructed) will be at 32 percent of culvert height, and the upper VAP line will 

be at 52 percent. 

Next, check culvert capacity for the eventuality that the streambed aggrades to the upper VAP line. 

Ideally, the culvert will be large enough to allow passage of the bed design flow with at least 1 foot 

of headroom (80-percent submergence). Assuming headroom is adequate, embedding the culvert 

slightly below the lower VAP line is simply a small additional safety factor; it may or may not be 

necessary depending on site risk factors affecting scour potential. 

After you analyze bed stability (section 6.4), you may need to reevaluate culvert size.

Figure 6.29—Hypothetical example of using reference reach channel size and bed material (described

in text) to size a small culvert. VAP lines shown in the figure were determined from a longitudinal profile

analysis. This drawing does not show the designed simulated channel bed; rather, it shows how the 

culvert would fit around the reference reach.
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Chapter 6—Stream-Simulation Design

6.3.3 Culvert Shape and Material

Aside from the size, elevation, and alignment issues already discussed, 

most of the considerations for culvert shape and material involve site 

conditions, designer preference, and cost. These considerations include: 

l Commercial availability. 

l Structure longevity.

l Road elevation and fill height.

l Streambed and culvert constructability.

l Construction time, sequencing, and allowable ‘in-water’ work period.

l Soil-bearing capacity.

l Site access.

l Flood capacity. 

Guidance for selecting culvert shape and material is in section 7.2.

6.4 Bed-Mobility and Stability Analysis

The purpose of the bed-mobility and stability analyses is to answer the 

following questions: 

Do the bed materials in the simulation move at the same flows as those

in the reference reach? (mobility analysis)

Do key particles that control channel form and hydraulics stay in place 

during the high bed-design flow (see section 6.4.5)? (stability analysis)

The bed-mobility analysis is useful where the simulated channel design 

differs from the reference reach with respect to slope or entrenchment. At 

other sites, the analysis may or may not be needed, depending on channel 

type, risks associated with the site, etc.

The bed-mobility analysis compares critical flow for entrainment (the 

flow at which a particle just begins to move) in the reference reach to 

critical flow in the culvert for the particle size of interest. Except for 

the least mobile channel types (coarse step-pool and cascade channels), 

stream simulations should be designed such that bed particles of similar 

size become mobile at similar flows in the reference reach and the streamsimulation reach: 

Equation 6.5 Qc culvert g Qc reference reach 
6—68

Stream Simulation

where Qc is the critical entrainment flow (or the critical shear stress) 

for the particle size of interest. When this goal is achieved, the amount 

and size of incoming and outgoing sediment balance, maintaining the bed 

structure and bed forms that are necessary for aquatic organism passage. In 

the mobile channel types, only banklines and other large key features are 

routinely designed to be permanently stable.

If the simulated channel closely mimics the reference reach, bed-mobility 

analysis will show that similar particle sizes move at similar flows in both 

channels. If the simulation is steeper than the reference reach, the designer 

can use the analysis results to adjust the simulation design for similar bed 

mobility. Adjustments can be made to one or a combination of design 

parameters, such as bed-material size, channel width, and flood-plain 

capacity within the culvert. Adding flood-plain relief dips or pipes and 

changing the project profile are other ways to adjust the design. 

To ensure that the simulation achieves its objectives, keep it within the 

range of natural variability in the reference reach. As a rule of thumb, 

increase slope, bed-material sizes, and/or active or bankfull channel width 

no more than about 25 percent unless you have a clear understanding of 

the implications of a greater change. 

6.4.1 When is a Bed-mobility Analysis Necessary?

Mobility analysis usually is not conducted on low-gradient, fine-grained 

response channels where the bed is fully mobile during frequent high 

flows. After a flood, such channels reestablish preflood-channel form more 

quickly than coarser-grained channels. In straightforward projects (e.g., 

a stable, moderately entrenched, moderate-gradient, gravel pool-riffle 

channel where the culvert bed closely replicates the reference reach) you 

can assume similar bed mobility. Again, bed-mobility analysis usually is 

not necessary.

Intermediate-mobility channels (coarse pool-riffle, plane-bed, and perhaps 

some cobble step-pool channels) do require a bed-mobility analysis. 

They may be fully mobile at flows that are fairly frequent (5- to 10-year 

recurrence interval), yet infrequent enough that a partial bed failure may 

not recover to its preflood channel form within a reasonable time. In these 

channels, risks may justify evaluating whether the same sizes are entrained 

in both the structure and the reference reach over a range of flows from 

bankfull to the high design flow (see the example illustration of such an 

analysis in appendix E). 
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Chapter 6—Stream-Simulation Design

It is most important to analyze bed mobility when the slope or 

entrenchment of the simulated channel differs somewhat from the 

reference reach. Sections 6.5.1.1 and 6.5.1.2 discuss the analysis in greater 

detail.

6.4.2 What Particle Sizes Are Analyzed? 

Generally bed-mobility analysis is done on the portion of the bed material 

that provides structure, stability, and roughness, that is, the larger sizes. 

D84 is the recommended grain size to analyze in most cases because when 

D84 is mobile, most of the smaller bed sediments are mobile as well. 

D95 also can be used as a more conservative indicator of ‘bed mobility.’ 

Where riffle crests or bars are designed and built, as in the Newbury 

Creek example (figure 6.23), the particle-size class used to construct 

those features would be analyzed. The aim of the mobility analysis is 

not to make the channel stable; the goal is to create a channel bed in the 

simulation reach that has similar sediment transport characteristics to the 

reference reach. 

6.4.3 What Flows Are Analyzed?

This is a comparative analysis, which does not require working with a 

flow of any predetermined return interval. First, find the flow that entrains 

D84 or D95 in the reference reach. Then, determine whether the same flow 

entrains D84 in the simulated channel. To verify that the calculated critical 

flow is valid, estimate its recurrence interval and compare it to the bed 

mobilization flow ranges listed in table 6.5 for the channel type.

If the critical flow in the simulation is different from that in the reference 

reach, various design parameters can be adjusted until the same flow 

moves D84 in the simulation. See section 6.5 and appendix E for examples 

and more explanation. 

6.4.4 Bed Mobility Analysis Equations

Bed mobility is evaluated using equations that estimate the critical flow 

for entrainment of specified particle sizes (the flow at which a particle 

just begins to move). Because these equations do not apply equally to all 

stream types, and because a great many variables are involved, this guide 

devotes appendix E to presenting and discussing them. 
6—70

Stream Simulation

Briefly, the most useful equations for stream-simulation applications are:

l The critical unit discharge equation (Bathurst 1987).

This equation estimates the critical unit discharge (flow per unit 

channel width) at which a particle of a certain size will begin to 

move in a steep, rough channel. The equation applies to steep (3.6 to 

5.2 percent), gravel-cobble channels where water depth is shallow 

compared to the size of the bed material. 

l The modified critical shear stress equation. 

This equation can be used to assess particle stability in channels with 

gradients less than 4 to 5 percent and D84 particles ranging between 

10 and 250 millimeters.

Like all hydraulic and hydrologic models, these equations approximate 

and simplify the real world. The Bathurst and modified critical shear 

stress equations apply best in alluvial settings; they do not account for the 

stabilizing effects of key features, such as embedded debris or colluvium. 

All the equations are based on empirical field and laboratory studies with 

data sets of limited size and variability, and they should be applied within 

those limits. In some cases, where it is not evident which equation is most 

appropriate, use more than one and compare the results. Understanding 

why the results differ can be important in developing a good design. 

Appendix E describes in some depth the background and limitations of the 

equations. It also provides examples of applying the equations to streamsimulation problems. 

Do not allow the equations to drive the design. Instead, use them as tools 

to validate the design and check the results against your understanding of 

how the channel will function in real life. Visualize how the channel will 

look and function as it adjusts over time, and use the equations to help 

predict bed mobility in different channel/structure configurations. The 

equations allow you to test the sensitivity of the bed to changes in different 

design parameters (e.g., slope, width, bed-material size). Test sensitivity by 

varying design values in the equations to see if the changes greatly affect 

the results. The risk of error is less when changes to the results are small.

If increasing bed-material size or channel width by 25 percent is not 

sufficient to match bed mobility in the simulated channel with bed 

mobility in the reference reach, review section 6.5 on managing risk in 

various situations. You may need to consider selecting a new project 

profile. Alternatively, you may decide that stream simulation is not feasible 

at the site.
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Chapter 6—Stream-Simulation Design

6.4.5 Stability Analysis for Immobile Key Feature Rocks 

The stability of key features that are intended to be permanent is crucial to 

a stream-simulation installation’s long-term sustainability. Because of the 

closed boundary in a culvert, a large flood may exert higher shear stresses 

and cause more turbulence than an open channel. Particles of a given size 

may move at lower flows than in the reference reach, and large rocks may 

not be replaced as the flood recedes. Loss of bed structure—possibly of 

the whole bed—could be essentially permanent. Therefore, design these 

structural pieces to be immobile at the high bed design flow:

Equation 6.6. Qc key feature in culvert $ Q bed design 

where Qc is the critical entrainment flow (or critical shear stress) for the 

rock size of interest.

This analysis consists of verifying that the bed-design flow will not 

mobilize the rocks that comprise the key features. The bed-mobility 

equations described in 6.4.4 and appendix E can be used for this analysis. 

Their results should be compared to results from equations developed 

to size boulder clusters and riprap blankets (appendix E.4). Accurately 

estimating entrainment flow for rocks that are embedded in much 

finer material is difficult, so it is wise to compare the results of several 

equations. The best validation is the size of material that appears to be 

immobile in the reference reach. 

6.5 Managing Risk Factors

This section recaps the risks associated with stream-simulation culverts 

as well as other culvert types, and outlines approaches to mitigating them. 

Section 6.5.1 focuses on risks specific to stream-simulation installations, 

while section 6.5.2 looks at risks that apply to all culverts.

In any situation, there are two ways to “manage” risk:

First, reduce the probability of failure by identifying the processes or 

conditions that could lead to failure, and by mitigating them in design 

or construction. “Failure” in this context means not only structural 

failure (culvert washes out, flow diverts down road, etc.), but also 

failure to achieve stream-simulation objectives. Simply having bed 
6—72

Stream Simulation

material inside a culvert does not constitute stream simulation. For 

the project lifetime the simulated streambed should maintain a suite 

of characteristics similar to those found in the natural channel near 

the culvert (bed material type and structure, channel dimensions, flow 

velocities and depths). Any of the risk factors listed in table 6.7 could 

lead to failure. 

Second, recognize that any crossing can fail in an extreme event, and 

design to reduce the consequences of failure. Methods for reducing 

failure consequences include preventing diversions down the road or 

ditch if water overtops the road fill, armoring road fill overflow dips, 

and ensuring that the culvert is accessible and large enough to permit 

future access for maintenance and repair. Chapter 7 discusses these 

strategies in more detail.

6.5.1 Potential Culvert Failure Risks—Stream-simulation 

Culverts

An installation can have multiple failure risks; evaluate and mitigate 

each risk in the context of all the others. For example, a straight culvert 

and road fill placed over a sinuous stream in a wide active flood plain 

constrict the flood plain and shorten the channel. In addition to adding 

flood-plain relief dips or pipes and increasing culvert width to mitigate 

these risks, you could also increase the size of the bed material. However, 

increasing bed-material size to mitigate for flood-plain constriction, and 

then again to mitigate for channel straightening, could defeat the purpose 

of stream simulation. A bed-mobility analysis integrates the risk factors, 

and is frequently the key to determining the magnitude of the risk and 

finding appropriate ways to mitigate it. In table 6.7, asterisks denote design 

strategies that involve bed-mobility analysis.

If bed-mobility analysis indicates that the simulated streambed materials 

will move at lower flows than in the reference reach, revisit the site to 

see if you can find a more appropriate reference reach. For example, if 

you have selected a project profile that is steeper than the reference reach, 

see if a natural-channel reach exists at the higher slope—one that may 

be appropriate as an reference reach. Be sure the new reach meets all the 

requirements, such as similar length, flow regime, sediment loading, and if 

possible entrenchment (see section 5.4). Other design solutions may have 

to be considered also, such as modifying the project profile or enlarging 

the culvert. 
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Chapter 6—Stream-Simulation Design

Table 6.7—Potential risk conditions and design strategies

RISK FACTOR

Flood-plain constriction.Rapid lateral channel migration.Steepened channel: culvert steeper than 

reference reach. 

Note: if a slope steeper than 125% of 

the reference reach is required, consider alternative designs or design methods (appendix B).

Downstream channel instability. Pressurized inlet. (Inlet is submerged; 

outlet is not submerged)

Submerged inlet.

OUTCOME OF DESIGN STRATEGY 

l Permits high flows to occupy wider ‘flood plain’ inside culvert.1

l Increases bed stability.1l Avoids flow concentration.l Reduces risk of complete loss of all embedment. l Reduces risk of upstream headcutting if simulation fails.l Requires larger culvert to allow for combined depth of rock layer and 

fully vertically adjustable streambed.

l Slows development of channel-to-culvert skew caused by channel shift.l Stream-simulation channel may function normally for a longer period of 

time before being constrained by culvert.

l Prevents channel movement. l May move channel alignment problems to reaches further from culvert.l Simulation is more sustainable over long term. l Increases bed stability.1l Reduces shear stress inside culvert.1l Reduces risk of loss of structural rocks.l Allows for natural variation in streambed elevation as long as actual 

degradation is within projected limits.

l Ensures simulated bed is protected from downstream headcut.l Grade controls themselves may become passage barriers.l Deeper foundation reduces probability of structural failure by 

undermining.

l Reduces adjustment potential of simulation.l If simulated bed is eroded, the bed is more likely to reconstruct itself on 

rough rock surface than on bare metal.

l Reduces incidence of very high water velocity in culvert.l Roadway vertical curve can be problem with round culverts. l Lower water elevation upstream of crossing. l Lowers inlet energy loss and increases culvert capacity.

DESIGN / CONSTRUCTION STRATEGY 

Widen culvert*Increase bed material size. *Add flood plain relief culverts, road overflow dips. *Place layer of large rock under simulated bed. Widen culvert and offset it in the direction of expected 

channel shift.

Provide best possible culvert alignment; stabilize 

banks; provide flow control structures such as rock weirs or J-hooks.

Minimize slope increase; modify downstream and/or 

upstream channel. 

Increase bed material size. *Increase width of stream-simulation channel, widen 

culvert. * 

If simulation is step-pool type, install bed retention sills. Verify vertical adjustment potential, and ensure 

simulated bed is deep enough and culvert is large enough to accommodate range of potential profiles. 

Provide adequate downstream grade controls. Use full-bottom pipe or deepen foundation of openbottom structure; place layer of large rock under 

simulated bed.

Increase culvert size to limit headwater depth during 

high bed design flow to 80% of culvert height above bed.

Add flood-plain relief culverts and/or road overflow 

dips. *

Optimize inlet alignment and transition; bevel pipe inlet.

Note: “*” denotes strategies that can be designed using bed-mobility analysis. Mobility analysis may indicate a need for bed material larger than reference reach, a wider culvert,

flood-plain culverts or dips, bed sills, etc.

1 These strategies are effective within limits. See sections 6.5.1.1 and 6.5.1.3 for their limitations.
6—74

Stream Simulation Table 6.7—Potential risk conditions and design strategies (continued) RISK FACTOR Long culvert. Initial lack of bed consolidation.

Excessive infiltration into streambed.Debris blockage, debris flows. Stream diversion.

OUTCOME OF DESIGN STRATEGY 

l Allows use of shorter culvert .l Compensates for compounding design flaws.l Increases initial bed stability. l Increases initial bed stability. l Increases initial bed stability.l Increases construction cost.l Allows for initial streambed erosion.l Smaller particles fill voids between larger particles.l Minimize large void spaces in new streambed.l Provides space for debris to float through culvert.l Facilitates debris and sediment passage.l Structure and road survive overflow and debris blockages.l Reduce stream bank erosion caused by backwater eddies during very 

large flood events.

l Allows removal of debris jam in culvert or at inlet.l Reduces probability of exceding culvert capacity or blocking with debrisl Contains overtopping flow at crossing.l Minimizes flood damage to soils and habitats.l Prevent a stream diversion into a roadside ditch downgrade from the 

crossing. 

l Reduce erosion caused by overtopping flows.

DESIGN / CONSTRUCTION STRATEGY

Minimize length of culvert using headwalls, lower road 

profile, etc. 

Add safety factor to stability analysis (e.g., increase 

bed material size or culvert width). *

Compact bed layers during construction. Wash fines in between and around larger material to 

embed and stabilize it .

Hand-place key bed features for stability.Construct thicker streambed (to elevation higher than 

design project profile).

Design and use well-graded bed material mix (section 

6.2.1.1) with adequate content of sand, silt and clay.

Construct densely packed streambed by compacting 

bed in layers and/or jetting fines into bed layers. 

Increase culvert size: Limit headwater depth during 

high bed design flow to 80% culvert height above bed; ensure open area is large enough for debris being transported. 

Ensure efficient transition from upstream channel 

(match alignment and width); bevel pipe inlet.

Harden fill; design for overtopping and cleanout; plan 

for possible streambed maintenance after overtopping.

Provide inlet riprap or other protection. Provide access for maintenance. Increase culvert size. Provide roadway dip over culvert. Sag vertical curve to 

avoid diversion during floods and minimize fill height; armor fill.

Provide ditch dams; redesign road ditches to direct 

flood and overtopping water to erosion resistant areas.

Note: “*” denotes strategies that can be designed using bed-mobility analysis. Mobility analysis may indicate a need for bed material larger than reference reach, a wider culvert,

flood plain culverts or dips, bed sills, etc.

1 These strategies are effective within limits. See sections 6.5.1.1 and 6.5.1.3 for their limitations.
6—75

Chapter 6—Stream-Simulation Design

6.5.1.1 Flood-plain constriction

A wide active flood plain is often considered a highly valuable hydrologic 

and biological resource. Overbank flows and sediment moving down a 

flood plain build and maintain many of the unique flood-plain habitats 

that can be critical for some aquatic and terrestrial species (Naiman et al. 

1992). Project objectives will usually include protecting and/or restoring 

flood-plain processes and habitats. 

The major challenge in constructing a sustainable stream-simulation 

culvert on a high-conveyance flood plain is the potential for the road fill 

to block overbank flood flows and force them to concentrate through the 

culvert. In such installations, bed scour inside the culvert occurs at lower 

flows than in the natural channel upstream. Material eroded out of the 

culvert may not be replenished, and the culvert is at risk of bed failure 

during floods. The inlet area is more susceptible to scour than other areas 

of the culvert under these conditions, because water-surface elevation 

drops abruptly as the water moves from the backwatered flood plain into 

the culvert inlet. The inlet may scour even when hydraulic conditions in 

the rest of the culvert are similar to the reference reach.

Depending on the site, you may want to use a combination of some or all 

of the following design strategies to mitigate the risk. 

Minimize flow 

concentration In valleys with very high flood-plain resource values, such as important 

aquatic and riparian habitats, consider building a viaduct or bridge that 

spans as much of the active flood plain as possible. For stable multichannel 

systems (anastomosing channels), consider providing for stream 

simulation on each channel.

Another strategy is to keep the road fill as low across the flood plain as 

is feasible given traffic needs. If the road can be closed during floods, 

designing it for overtopping can avoid the need for many flood-plain 

culverts. Combining some flood-plain culverts with a low road fill 

designed for overtopping allows smaller floods to drain under the road 

without forcing a road closure. Larger floods overtop the road so that 

the road fill does not work like a dam funneling water through the main 

crossing structure.
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Stream Simulation

Provide flood-plain culverts and/or dips at swales, side-channels, and 

other locations as needed (figure 6.30). Add enough drainage structures 

to avoid unduly concentrating flow in any one area. Maximize the cross 

sectional area of dips, and armor them to sustain expected flow depths and 

velocities as well as the drop over the downstream edge. Providing well 

distributed flood-plain culverts and dips minimizes the risk that floodplain flows concentrated in a single side channel might divert and capture 

the main channel. Nonetheless, side channels may carry more flow than 

normal because of the backwater caused by the road fill, and the potential 

for them to scour should be examined during the design process. In some 

cases buried rock may need to be installed just downstream of a flood 

plain or side-channel culvert to prevent incision. Be aware of the potential 

for woody debris to plug flood-plain culverts, and provide enough dips to 

handle flood-plain flow if needed. 

Side-channels are sometimes important fish habitat requiring aquatic 

organism passage. Culverts at these sites should simulate the size and 

character of the side channel, while providing protection against scour that 

flow concentration may cause. 

Figure 6.30—Stream simulation on an unentrenched channel may include a 

flood-plain surface inside the culvert and flood-plain relief culverts and dips.

Permeable roadfills can replace flood-plain culverts in some situations. 

Permeable fills are constructed with coarse granular fill, such as 2- to 

6-inch rock, sandwiched between layers of geotextile. On the downstream 

side, the base of the fill has a small toe drain of geotextile and rock 

to let water exit the fill safely without scouring (Pekuri, personal 

Culvert width including

flood plain widthvv

Reference channel 

bankfull width 

Flood plain culvert

in flood swale

Armored 

road dip 

Road fill 

Floodprone width 

Flood relief 

culvert in swale 

10 

1 
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Chapter 6—Stream-Simulation Design

communication). Although a permeable roadfill can allow a more natural 

and uniform movement of water and maintain some flood-plain function, 

it does not allow movement of most aquatic species or debris. For more 

information on permeable fills, see USDA Forest Service 1996. 

Conduct a backwater

and bed mobility/

stability analysis These analyses should be done at any site where significant overbank 

flow is expected on the flood plain. We particularly recommend it where 

the entrenchment ratio (flood-prone width: bankfull width) is around 6 or 

higher. This recommendation is based on model results for several forested 

flood plains in western Washington. This entrenchment ratio threshold will 

be lower for smoother, unforested flood plains with high conveyance. 

Compare the critical unit discharge or critical shear stress in the streamsimulation channel to the reference reach during a range of flows that will 

be constricted by the road. The choice of which flows to analyze depends 

on risks at the site and on flow conveyance. A 10-year recurrence interval 

flood seems a reasonable minimum flow to use for this analysis in mobile 

channels with considerable movement of bed material. In intermediatemobility channels, the flood that moves D84 in the reference reach might be 

a good choice for a minimum flow for this analysis. 

The reference reach critical shear stress or critical unit discharge for this 

analysis is not the average of the entire floodway. Instead, the analysis 

considers only the flow within the bankfull or active channel width, 

because that is the flow condition that entrains sediment on the reference 

reach bed. Use a step-backwater model like HEC-RAS to predict 

backwatering behind the road fill, accounting for the effects of multiple 

flood-plain culverts and/or road dips planned for the site. Compare the 

reference reach shear stress or unit discharge to the stream-simulation 

channel, factoring in the additional flood-plain flow that will be forced 

through the culvert.

If you have already added flood-plain relief dips and pipes to the design, 

and shear stresses are still higher in the main channel culvert than in 

the reference channel, the following two strategies provide options 

for offsetting the difference. These two strategies should normally be 

combined.
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Stream Simulation

Increase culvert

width Widen the pipe and construct a flood-plain surface inside. The width 

of the simulated bankfull channel should remain the same to avoid 

aggradation during moderate flows, and possible loss of low-flow passage. 

The constructed flood plain will relieve some of the excess shear stress 

by accommodating some of the overbank flow. All surfaces above the 

bankfull channel should slope toward the bankfull channel at a slope of 

about 10h:1v (see figures 6.22 and 6.30). 

Widening the culvert is not a panacea. Channel adjustments inside 

the pipe are likely to change the installation over time. For example, 

unless the culvert flood-plain surface is wide enough that water depth 

and velocity in the simulated active channel are similar to the reference 

reach, the simulated channel may incise. After that, flood flows will not 

access the overbank surface as easily, water depth and velocity at flows 

above bankfull will increase, and the original problem will not have been 

solved. For this reason, widening the culvert is generally combined with 

increasing bed material particle sizes. 

Increase bed material

particle size As mentioned previously, particle size can be changed only to a moderate 

degree if the simulated bed is expected to be self-sustainable. We 

recommend not increasing D84 more than 25-percent over the reference 

reach. 

If you increase bed-material sizes, increase each size class D50 and higher 

by the same percentage, and recalculate the finer particle sizes to maintain 

the dense-bed mixture (review section 6.2.1.1 bed design). Consider how 

the new particle-size distribution will fit into the channel context and 

whether that distribution is likely to achieve stream-simulation objectives. 

If an unacceptable risk of bed failure still exists after all the mitigation 

measures above have been applied, place individual large rocks in the bed 

to buttress the bed and provide additional roughness. Another option is to 

bury a layer of riprap deeply below the simulated streambed. The riprap 

should be deep enough that under normal conditions the simulated bed can 

scour and fill on top of it without being affected by it. Thus, the depth of 

the stream-simulation bed on top of the rock layer should be the same as 

if it were on top of the culvert floor (section 6.3). Base the thickness of the 

riprap sublayer on a riprap design protocol such as the U.S. Army Corps 

of Engineers method referenced in appendix E. That method requires a 

thickness not less than the Dmax stone, or 1.5 times D50, whichever is larger.
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Chapter 6—Stream-Simulation Design

6.5.1.2 Rapid lateral channel migration

Where a channel is experiencing rapid lateral shift, culvert-to-channel 

skew will intensify over time. Section 6.1.1.1 described the problems 

associated with skew, and ways to mitigate them. If a channel is shifting 

very rapidly, the most effective solutions might be relocating the road to a 

more stable site, or placing a temporary structure that can be moved. 

Table 6.7 lists possible solutions for channels where lateral shift is 

less extreme. They include widening the culvert and offsetting it in the 

direction of expected shift. Adjust the size of bankline rocks if needed 

to accommodate a deeper pool that can form as the bend becomes more 

acute. Bank-stabilization and flow-training structures such as rock weirs or 

J-hook vanes can be built above the crossing to slow down or minimize 

channel shift. 

6.5.1.3 Steepened channel

Section 6.1.2.3 described conceptual design options for sites where the 

downstream channel is incised. As emphasized there, downstream-channel 

rehabilitation may be the solution with the highest probability of long-term 

success, as opposed to maintaining a culvert as grade control. 

Steepening the simulated channel relative to the reference reach increases 

bed slope and shear stress (compared to the reference reach) and creates 

a higher potential for bed failure. Increases of up to 25 percent in particle 

size and/or channel width are likely to be within the range of variance 

of most natural channels and constitute a reasonable design limit. 

Nevertheless, conduct a bed mobility analysis whenever the streamsimulation channel is steeper than the reference reach. 

The analysis may suggest that an increase in bed-material size or channel 

width is necessary to offset the increase in slope. An increase in channel 

width reduces the calculated average shear stress to resemble a flatter 

reference reach. Do not accept such a solution without thinking through 

how it will work in the real simulation. For example, in a natural channel, 

short, steep reaches are normally narrower than average rather than wider, 

with larger bed material and/or key pieces. If the thalweg in the steeper 

simulation incises so that flow width narrows, the calculated increase 

in stability due to increased channel width may not persist. In such a 

situation, burying a layer of large-size rock below the simulated streambed 
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Stream Simulation

to prevent excess scour might be a useful added safety factor. An added 

benefit of the extra channel width is that it provides capacity for large 

floods, making failure less likely. 

Where the reference reach is steeper than the channel immediately 

upstream, analyze the mobility of the larger particle sizes in the simulated 

channel compared to the same sizes in the upstream reach that will be 

supplying sediment. Those sizes should be mobile at similar flows in both 

reaches in order for the simulated channel to be self-sustaining.

Avoid steepening a channel past a geomorphic threshold (see table 6.5 

and appendix A, figure A.25) that would—in nature—make the channel 

a different type. Staying within the 25-percent guideline will usually 

prevent the design from exceeding a channel-type threshold; however, if a 

threshold would be exceeded, first verify that a more appropriate reference 

reach does not exist. For example if the reference reach is a 4-percent 

plane-bed channel but the required crossing slope is 5 percent, investigate 

whether step-pool reaches exist nearby. If no more appropriate reference 

reaches exist, consider building the appropriate channel type as a hybrid 

design. In this example, the hybrid installation would be a step-pool 

channel. Steps would be designed for immobility during the high beddesign flow, because if the step-forming rocks wash away, they may not be 

replenished from upstream. If either a step-pool channel or one with other 

key features (such as wood) is steepened, consider decreasing the spacing 

of steps or key features to increase roughness. (See appendix B for more 

on hybrid design.)

6.5.1.4 Downstream channel instability

If the elevation of the channel bed downstream of the crossing degrades 

beyond the range to which the project can adjust, the simulated streambed 

could fail to function. If a risk of continued channel degradation 

downstream could jeopardize the structure, reevaluate your plans to 

control VAP (section 6.1.2.3). Consider restoring the downstream channel 

and/or adding grade control structures to support the project profile. 

Design conservatively. Take extra care in projecting VAP and, if possible, 

ensure that the culvert can accommodate it. One safety measure is to use 

a full-bottom pipe with a layer of large rock placed below the simulated 

bed. Even if the simulated bed partially or entirely washes away, the 

opportunity to reconstruct it will still exist. The layer of large rock will 
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Chapter 6—Stream-Simulation Design

protect the upstream reach from channel incision. In a bottomless pipe, 

increase the depth of footings. Consider placing a layer of immobile rock 

below the streambed elevation and constructing the simulated bed on top 

of it, giving the bed enough depth to make normal vertical adjustments 

(such as scour pools). 

6.5.1.5 Inlet control with submerged inlet

A stream-simulation bed will likely fail if the culvert is in inlet control, 

especially if the inlet is submerged and a high head differential exists 

between inlet and outlet. These conditions produce a strong flow 

contraction in the pipe near the inlet. In culverts flowing in inlet control, 

supercritical flow—a very high velocity flow extremely rare in alluvial 

channels—occurs in at least part of the pipe. 

Conduct a culvert analysis and verify that supercritical flow does not occur 

at the high bed-design flow. FishXing and HEC-RAS with the lid function 

are good tools to use for this because they analyze flow inside the barrel 

of an embedded or open-bottom pipe. Be conservative, because high-flow 

hydrology, effects of debris, and culvert inlet losses are all uncertain. 

If supercritical flow is likely to occur, or if the inlet may be submerged, 

one obvious solution is to increase the pipe’s size. We recommend that 

headwater depth at the high bed-design flow not exceed 80 percent of the 

culvert opening above the bed (67 percent where debris is a significant 

hazard). Improving the culvert’s alignment with the upstream channel and/

or designing an efficient culvert inlet configuration, such as a wingwall, 

may lower the headwater and reduce the flow contraction near the inlet. 

Again, if the site has an active flood plain, adding flood-plain culverts and/

or road dips will reduce flow concentration through the culvert.

6.5.1.6 Long culvert 

Review section 6.1.1.1 on risks associated with long culverts. We can 

presume that a culvert can safely be as long as a straight reference 

reach at the same slope. If a culvert is longer than straight segments of 

the reference reach, it is likely that channel bends were straightened 

to construct the culvert; therefore, the simulated bed is not as rough as 

the natural channel. The excess culvert length exacerbates the risks of 

any design uncertainties, invalid assumptions, flaws, or construction 

inadequacies. Unfortunately, there is no specific hydraulic method for 

quantifying the risks of bed failure due to culvert length. 
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Stream Simulation

As section 6.1.1.1 notes, the risk can be minimized by locating the 

crossing so that it avoids channel bends and minimizes culvert length. 

Adding headwalls and/or lowering the road fill may permit shortening the 

pipe. Other possible measures include adding a safety factor to the size 

and/or embedment of the culvert or the size of the bed material. Larger 

bed material or key roughness pieces will add roughness, thereby helping 

to dissipate energy in long culverts. However, be aware that the additional 

turbulence caused by the larger material may affect opportunities for 

aquatic species passage. 

6.5.1.7 Initial lack of bed consolidation 

In natural channels, hydraulic forces sort and structure bed materials 

so that they are in relatively stable positions and orientations. In newly 

constructed streambeds, the risk of bed failure during a flood is somewhat 

higher until moderate flows sort, structure, and consolidate the new bed. 

Characteristics like armoring and imbrication cannot be constructed, and 

must be allowed to develop naturally.

Although we cannot quantify this lower initial stability, there are several 

ways of managing the risk: 

l Add extra material initially to allow for some bed erosion and 

consolidation. 

Barnard (personal communication 2003) monitored steep streamsimulation channels after construction. He found that the constructed 

beds had lowered by about 20 percent of their depth in the first few 

years after construction, likely from a combination of consolidation

and erosion of fine material. These were steep channels, and the 

material had not been consolidated or compacted during construction. 

l For beds composed of grain sizes up to cobbles, compact the bed 

during installation. 

Compaction can be done mechanically, by washing fines into the bed, or 

both. As bed material size increases, mechanical compaction becomes 

more difficult and more likely to damage the culvert. Bed structures 

such as steps and key features therefore become more important. 

These bed structures will support the alluvial part of the bed until it 

is consolidated. Ensure step and key-feature stability by specifying 

that individual rocks be placed so that they are in direct contact and 

support one another (see sections 6.2.2.4 and 7.5.2.3). 

l Increase the size of the bed material slightly. 

l Monitor the effects of highflows until bed structure develops, and be 

prepared to repair any bed failures.
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Chapter 6—Stream-Simulation Design

6.5.1.8 Excessive infiltration into the streambed

The lack of natural sorting and bed consolidation also results in a potential 

for excessive streambed permeability and the risk of losing surface flow 

during low flows. A well-graded bed mix with at least 5-percent sand, silt, 

and clay content (section 6.2.1.1) is designed to avoid large empty spaces 

in the new, loose bed. Construction practices, such as ensuring the bed 

material is not segregated during handling, compacting the bed in layers 

and washing the fines into each layer help to reduce initial infiltration 

rates. 

6.5.2 Potential Culvert Failure Risks—All Culverts 

6.5.2.1 Flow exceeds culvert capacity 

Like all crossings, stream-simulation designs must be checked to ensure 

that the culvert will convey floods up to the high structural design flow 

(the flow that, if exceeded, could cause culvert failure). Even when floodcapacity calculations indicate the culvert has adequate capacity, however, 

the potential for structural failure exists. The 50- to 100-year recurrenceinterval flow is commonly used as the high structural-design flow, with 

the notion that this reduces the risk to an appropriate level. However, in 

reality the probability of a 50- to 100-year flow occurring over the lifetime 

of a culvert is not low. Suppose, for example, that the designer expects a 

culvert to last for 50 years and wants to design it so that a structural failure 

does not have more than a 5-percent probability of occurring. According to 

the following equation, the design analysis would have to be based on the 

1,000-year flood! 

This equation calculates the probability (Pn) that a flow with a given 

recurrence interval (Tr

) will occur at least once during a given timespan 

(n): 

Equation 6.7 Pn = 1 - Tr

-1 n

For n = 50-year project life, and P = .05, Tr

 = 1,000-year recurrenceinterval flood. That is, there is a 5-percent probability that a flood with 

a recurrence interval of 1,000 years will occur during any 50-year span. 

For any 50-year period, there is a 40-percent probability that the 100-year 

flood will occur, and a 64-percent chance that the 50-year flood will occur. 

Tr
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Stream Simulation

Thus, there is a significant risk that the design flow—and even higher 

flows—will occur during a culvert’s lifetime. Although safety factors built 

into the design can offset errors and uncertainty in the flow estimates and 

other analyses, structures should be designed to be overtopped or to fail 

with minimal destructive consequences.

If a likelihood of debris and/or sediment plugging exists, the culvert 

hydraulic analysis should also factor in partial debris blockage of the 

culvert inlet. 

6.5.2.2 Debris or sediment blockage

In forested environments with large amounts of woody debris, hydraulic 

calculations may not accurately predict culvert failures. Furniss et al. 

(1998) state: 

“The loading of sediment and woody debris is difficult to predict and 

subject to the stochastic nature of landsliding, streambank erosion, treefall, 

and other processes that contribute these materials. We might be able 

to anticipate which crossings are more likely to fail based on upslope/

upstream geomorphology, crossing inlet configuration, and hydraulic 

models, but we expect that actual failures will remain difficult to predict.”

Mitigate the risk of debris and sediment plugging the culvert by matching 

culvert width and alignment to the upstream bankfull channel. Furniss et 

al. (1998) suggest limiting headwater depths at the maximum design flow 

to 50 to 67 percent of the culvert opening, to account for sediment and 

debris. Correct an over-widened basin upstream of the culvert, since it 

allows wood to rotate perpendicularly to the culvert, exacerbating plugging 

potential. If an undersized culvert has widened the upstream channel, 

restore the channel dimensions to those of the reference reach. Avoid 

damaging the banks further upstream and possibly increasing their erosion 

potential during construction.

Consider designing the entire crossing to sustain plugging and overtopping 

by hardening fillslopes and approaches, and preventing stream diversion 

down the road or ditch. Be sure to factor maintenance into the design. 
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Chapter 6—Stream-Simulation Design

6.5.2.3 Stream diversion potential

For every culvert design, when the preliminary design is complete, ask “if 

the culvert plugs, where will the flow go?” If a plugged culvert backwaters 

flood flow so that it enters a ditch sloping away from the crossing, water 

will flow along the ditch until it either crosses the road (see figure 6.31) 

or drains into another stream channel. If diverted water outlets onto the 

roadfill or a slope, it can jeopardize slope stability. In some cases, entire 

stream channels have been diverted out of their normal alignment onto 

steep slopes with no capacity to carry flow, and large gullies have formed, 

causing slope failures. Diverted flows that enter another stream channel 

can cause channel erosion there. 

To mitigate the risk of stream diversion at a crossing where plugging is 

possible, first mitigate the potential for debris or sediment blockage as 

described above. Then, design the road surface to keep overflow localized 

at the crossing. For example, you might build a sag vertical curve into 

the road alignment over the crossing or place a diversion dip in the road 

surface (section 7.7.2.1). 

Figure 6.31—Stream diversion at plugged pipe. (photo: Stream diverted down road,

Plumas National Forest 1997)
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Stream Simulation

6.6 Design Documentation

Summarizing key site data, design assumptions, and decisions is important 

for others to understand the basis of the design. Good documentation is 

important for the final design phase (chapter 7) and during monitoring, 

when questions may arise about the intent of the stream-simulation design. 

Such documentation will also help reviewers and managers understand the 

project and design process well enough for permitting, prioritizing, and 

funding. 

This completes the simulated-channel design. In the next step (chapter 7), 

the design engineer completes the design details for the installation and 

prepares the contract. 

 
7Final Design 

and Contract 

Preparation

7.1 Phase Overview

7.2 Crossing Structure Selection

7.3 Structural Design

7.4 Handling Traffic During Construction

7.5 Developing Specifications

7.6 Designing for Flood and Debris Failure Prevention

7.7 Planning for Erosion and Pollution Control

7.8 Dewatering, Bypass, and Water Treatment During 

Construction

7.9 Special Contract Requirements
Stream Simulation

Figure 7.1—Steps and considerations in final design.

Select structure type

l Project objectives and stream-simulation sustainability 

l Fill height

l Construction issues

l Costs

Design the crossing installation

l Foundations or bedding

l Structure

l Mitigate failure potential

Specify streambed materials and placement

l Gradation

l Key features, bedforms, banks, grade controls

l Bed elevation

l Auxiliary grade-control structures up- and/or downstream of crossing 

 structure

Specify dewatering and water quality protection 

requirements

l Diversion system

l Animal protection and removal

l Sediment treatment system

l Rewatering

Provide for short-term pollution control

Provide for long-term stabilization (revegetation)

RESULTS

Contract solicitation package

Steps and Considerations in Final Design
Chapter 7—Final Design and Contract Preparation

7.1 Phase Overview

The previous chapters presented the tools needed for designing the 

stream-simulation channel, including size and orientation, streambed 

characteristics, and restoration needs outside the culvert. The next task is to 

finalize the design for the installation as a whole: to verify the engineering 

plans for both the crossing structure and the roadway, and to prepare the 

documents necessary for soliciting bids for construction. 

At this point in the project the focus shifts to completing important design 

details, and project responsibility passes from the project team to the 

design engineer. The design details discussed in this chapter are either 

unique to stream-simulation projects or require more emphasis because the 

projects are generally bigger and take longer to construct than traditional 

culverts.

This phase of project design can be accomplished either with in-house 

resources or by contracting (or a combination of the two methods.) The 

assumption that Architectural and Engineering contractors require only 

minimal oversight can lead to poor results. As a minimum, the agency 

must have a staff with a level of technical expertise that allows them to 

recognize poor or inaccurate work, as well as enough skilled people to 

provide prompt and proper technical oversight for the contracted work. The 

design engineer is responsible for recognizing and correcting situations 

where expertise is not represented adequately within the team. Whether the 

final design is done in-house or by contract, the final product must be the 

same quality. 

Develop construction drawings from the site plan produced during the site 

assessment (see section 5.1.2). Along with the original topography, the 

new plan includes profile and cross-section drawings of the new structure 

and its related channel features, details of the roadway, and other project 

details. This development process may take a few days to several weeks 

(depending on the complexity of the site,) and is often the most timeconsuming part of design and contract development. 

As you develop the detailed contract drawings of the stream-simulation 

design, numerous questions may arise that require consultation with 

the project team. This need for consultation, along with possibly 

short deadlines, will always add pressure and confusion to a project. 

Nevertheless, you should be proactive, communicating regularly with other 

members of the project team to solve design issues. Both the inspector 
7—4

Stream Simulation

and the contracting officer’s representative (COR) can offer valuable 

information and assistance, particularly about construction techniques for 

difficult sites. Integrate these experts into your design team as the design 

progresses and include them in all pertinent communications. Definitely 

involve the COR in decisions about what aspects of the dewatering and 

erosion and pollution control plans must be performed inhouse. 

Finally, assemble all elements of the project into a package that includes 

drawings, specifications, supplemental specifications, special contract 

requirements, and the contract boilerplate. The contracting officer then 

offers the contract package to the public for construction bids. The 

specifications and special contract requirements cover elements of the 

design that the detailed drawings cannot adequately describe. When 

the standard specifications do not adequately describe the work, write 

supplemental specifications to modify them. The Forest Service uses 

Standard Specifications for Construction of Roads and Bridges on Federal 

Highway Projects (FP-03: FHA, 2003b) for standard specifications. See 

appendix H for sample supplemental specifications. Special contract 

requirements (Federal Acquisition Regulations Section H—part of the 

contract boilerplate) cover other aspects of the project, such as water 

quality and environmental protection. Appendix H also includes examples 

of special contract requirements.
7—5

Chapter 7—Final Design and Contract Preparation

Construction BMP Checklist

While completing the final design, consider the following list of BMPs that will help minimize 

sediment in the stream. These BMPs should be in the back of your mind as you make decisions 

on the project. Even as early in the final design as structure selection, BMPs can influence 

your decisions. Different types of structures involve different levels of site disturbance and 

different lengths of time for construction. All of the items on the BMP list are discussed in detail 

in either this chapter, chapter 8, or appendix G. Where ever appropriate, include these items in 

the contract to provide proper control during construction. To include them, place them in the 

specifications, the special contract requirements, or on the drawings.

BMPs are usually required in construction permits

Federal, State, and county permits often include required BMPs and performance standards (e.g., 

turbidity requirements). Apply for permits early, because these requirements must be in the special 

contract requirements, the erosion control plan, and may need notes and details in the drawings. 

Stormwater Management, Erosion, and Sediment Control

l Minimize bare ground.

	 l Minimize impact to riparian vegetation.

l Prevent excavated material from running into water bodies and other sensitive 

areas.

	 l Use appropriate erosion barriers (silt fence, hay bales, mats, coir logs).

l Dewater before excavation.

l Manage sediment-laden water encountered during excavation. 

 s	 Sediment basins.

 s Fabric, biobag, or hay-bale corrals.

 s Sand filter.

 s Geotextile filter bags.

As a quick check (not to replace required monitoring,) be sure that the turbidity of water 100 to 200 

feet downstream of the site is not visibly greater than turbidity upstream of the project site.
7—6

Stream Simulation

Dewatering

l Minimize the extent and duration of the hydrological disruption.

l Consider using bypass channels for maintaining some river and stream continuity during 

construction.

l Develop a storm management plan.

l Use dams to prevent backwatering of construction areas.

l Gradually dewater and rewater river and stream segments to avoid abrupt changes in 

streamflow and water temperature.

l If fish are present, prevent them from entering the construction site by placing block nets at 

the upstream and downstream ends of the dewatered section.

l Salvage aquatic organisms (fish, salamanders, crayfish, mussels) stranded during dewatering.

l Segregate clean bypass water from sediment-laden runoff or seepage water.

l Use antiseep collars.

l Use upstream sumps to collect ground water and prevent it from entering the construction site.

l Collect construction drainage from ground water, storms, and leaks, and treat it to remove 

sediment.

l Use a downstream sediment control sump to collect water seeping out of the construction 

area.

l Use fish screens around the bypass pipe intake.

l Use appropriate energy dissipators and erosion control at the outlet.

l Make sure to have adequate pumping capacity for handling storm flows.

Pollution Control

l Wash equipment to remove leaked petroleum products and avoid introduction of invasive 

species.

l	 Repair equipment before construction to minimize leaks.

l Be prepared to use petroleum-absorbing “diapers” if necessary.

l Locate refueling areas and hazardous material containment areas away from streams and 

other sensitive areas.

l Establish appropriate areas for washing concrete mixers, and prevent concrete wash water 

from entering rivers and streams.
7—7

Chapter 7—Final Design and Contract Preparation

l Take steps to prevent leakage of stockpiled materials into streams or other sensitive areas 

(i.e., locate the stockpiles away from water bodies and other sensitive areas, use sediment 

traps, cover during heavy rains).

Streambed and Banks Within Structures

l Check construction surveys to ensure appropriate slopes and elevations. 

l Use appropriately graded material that has been properly mixed before placing it inside the 

structure.

l Avoid segregation of bed materials.

l Compact the bed material.

l Wash in fines to ensure that fine materials fill gaps and voids.

l Construct an appropriate low-flow channel and thalweg.

l Carefully construct any designed bed forms to ensure functionality and stability.

l Where included in the design, construct well-graded banks for roughness, passage by small 

wildlife, and instream bank-edge habitat.

l Tie constructed banks into upstream and downstream banks.

Soil Stabilization and Revegetation

l Ensure soil surface is rough enough to collect seeds and moisture.

l Implement seeding and planting plan for both short-term stabilization and long-term 

restoration of riparian vegetation.

l Water the vegetation to ensure adequate survival.

l Use seed, mulch and/or erosion control fabrics on steep slopes and other vulnerable areas.

l Avoid jute netting (which has been known to trap and kill fish and wildlife) near streams or 

rivers.

l Avoid placing gabions in contact with the stream (for the same reason as above.)

Timing of Construction

l Generally, time construction for periods of low flow, observing any required work windows.

l Ensure all lifestages of resident aquatic species are protected adequately during construction.

l Consider whether construction should be limited during periods of high flows.
7—8

Stream Simulation

7.2 Crossing Structure Selection

Search for specific products that will meet the stream, roadway, traffic, 

and construction needs according to earlier design decisions (see chapter 

6.) A wide variety of structures may fit the site criteria, such as circular 

pipes, pipe arches, concrete or metal boxes, open-bottom concrete or metal 

arches, and many bridge types. All have their specific advantages and 

disadvantages. Use the structure type that best fits the specific needs and 

objectives of each crossing.

Developing a pool of local knowledge by gaining experience with various 

stream types and roadways is important. Study and compare options, and 

monitor projects objectively after construction. The goal is to learn which 

structures best meet project objectives by comparing their total costs 

(for example, planning, design, administration, contract, maintenance, 

replacement, and salvage) to the benefits they offer (for example, 

aquatic species passage, and long-term maintenance of channel form and 

function). 

Stream-simulation sustainability influences structure type selection 

because the structure must accommodate the potential variation in channel 

alignment and bed elevation (section 6.1.) over its lifetime. Structure 

width and embedment depth were determined in chapter 6 and usually by 

now the project team has identified a tentative structure type. However, 

as you draw the structure and fit it into the site, better ways to meet 

project objectives may become evident. Construction objectives, such as 

the duration of construction, also may be important. With input from the 

project team, develop structure alternatives and identify costs, risks, site 

impacts, and effectiveness in meeting site objectives. The project team 

should review the alternatives and make a final decision on the structure 

choice before you proceed to the remaining design details. 

One-piece embedded metal pipes are usually used on small streams 

because of their low cost and generally simple installation. Actual width 

is limited to what can be legally hauled to the site. Larger road-stream 

crossings may be constructed with a wide variety of structure types (see 

figures 7.2 through 7.6).

While the design of a stream-simulation structure is based primarily on 

accommodating natural stream function, the roadway also influences the 

selection of the structure type, height, and length. Road-design (as opposed 

to stream-simulation design) features that will influence structure selection 

include:
7—9

Chapter 7—Final Design and Contract Preparation

Figure 7.2—1-piece corrugated metal pipe 

(embedded).

Figure 7.3—1-piece corrugated metal pipe arch 

(embedded).

Figure 7.4—1-piece open-bottom arch. Figure 7.5—Multiplate open-bottom pipe arch.

Figure 7.6—Multiplate open-bottom box. 
7—10

Stream Simulation

l Rights-of-way limits.

l Road and site geometry.

l Traffic handling during construction.

l Initial and lifecycle costs.

l Lifespan.

l Risk.

l Environmental impacts caused by the construction.

Where more than one alternative satisfies the design criteria, consider 

designing several alternative crossings for the contract and advertise them 

as separate alternative bid items, so that the final design structure is based 

on cost. You can also define specific design criteria and request that a 

design firm analyze possible alternatives. Using more than one alternative 

is particularly useful when analysis of the alternatives requires design 

skills that are not readily available. 

7.2.1 Site Geometry

Nearly all parameters of the site geometry influence structure design and 

selection. To ensure that all traffic can pass safely over the site, base the 

road width, horizontal and vertical alignment, and curve widening on 

standard geometric design methods. The following checklist indicates 

important roadway factors that affect the position, length, and shape of the 

structure:

l Horizontal and vertical alignment. 

l Skew of structure to road centerline.

l Adequate curve widening. 

l Adequate sight distance.

l Road intersections. 

l Adequate fill cover over the crossing structure for the life of the 

structure.

l Vertical curves and road surface. 

l Type and thickness of roadway surface, shoulders, and slough 

widening. 

l Widening for curbs and guardrail, where required. 

l Proximity to existing utilities, both buried and overhead.
7—11

Chapter 7—Final Design and Contract Preparation

7.2.1.1. Dipping the road profile to prevent stream diversion 

Where a risk of debris plugging and embankment overtopping exists, the 

stream-simulation design will call for a dip over the crossing structure or 

adjacent to it, down grade. This dip will prevent the stream from running 

down the road if the culvert overtops. Check the remaining fill height to 

see which structures will fit under the road grade with sufficient cover. 

On relatively low fills, a dip may mean that a low-profile structure is 

needed (see table 7.1). Consider how normal erosion and road grading 

will affect cover over the structure during its life. To maintain adequate 

cover to protect the structure, it may be necessary to add measures such as 

informative signs for maintenance crews or paving/hardening the dip. 

7.2.1.2. Low embankment options

When the height of the road embankment is low compared to stream 

width, consider using a low-profile structure. Each culvert has a unique 

range of cover heights—that is, where the culvert will support the design 

load without failure. For circular pipe, pipe arch, and open-bottom arch 

structures, cover height becomes an issue when the fill height is less than 

about one-half the structure width plus the required cover. Cover height is 

important for metal culverts because they require the structural backfill to 

help support the load. Check the manufacturer’s literature for the allowable 

cover height range for the highest expected loads during the structure’s 

lifetime. Increasing pipe thickness may reduce the required cover. 

Although the cost will be higher, the structure’s lifespan will increase. 

Alternatively, investigate the feasibility of raising the road profile to gain 

proper cover over the structure. If neither of these alternatives is feasible, 

various structure types are available in low-profile shapes. Low-profile 

shapes tend to be more expensive than standard shapes.

Concrete boxes, vaults with lids, and precast bridges are often used at lowclearance crossings. The lid or roof can be structurally designed to act as 

the driving surface.

Table 7.1 displays the variety of shapes available and height-to-width ratio 

(i.e., how “short” they are). Use this table to help choose a structure to fit 

beneath a low embankment.
7—12

Stream Simulation

Table 7.1—Structures suitable for low-embankment sites, with approximate 

height-to-width values (will vary with manufacturer and material type)

EMBEDDED PIPE TYPE Height-to-width ratio

Pipe arch— 76-86% 

single piece and multiplate (subtract embedded depth)

Low-profile horizontal 75% 

ellipse—multiplate (subtract embedded depth)

Low-profile metal arch— 

steel or aluminum 32-50%

Low-profile concrete box culvert 3’—varies

BOTTOMLESS PIPE TYPE Height-to-width ratio

Low-profile concrete arches 

(BEBO E-series) 30-36%

Bottomless box culvert, 

5.5” x 15” corrugation—steel 22-42%

Bottomless box culvert, 

2” x 6” corrugations— 18-50%

steel or aluminum

BRIDGE TYPE Minimum clearance 

Various bridge options ~3’—varies
7—13

Chapter 7—Final Design and Contract Preparation

7.2.2. Construction Considerations 

Refer to section 6.5.1 and table 6.7. The table lists potential risks to longterm sustainability of the stream-simulation channel, along with design 

features that can reduce these risks. Several of the design strategies listed 

in table 6.7 affect the choice of structure size and type. 

Table 7.2 highlights some of the construction issues that may affect 

structure selection and dimensions. 

Table 7.2—Construction issues that may affect choice of structure

CONSTRUCTION RELATED 

PROBLEMS

Pipe too small to construct streamsimulation bed.

Lengthy dewatering time (1-10 days) 

(Structures with poured concrete 

footings may take 1-4 weeks).

Excessive construction noise.

Lengthy construction time.

Near-surface bedrock

Limited in-channel access

Poor foundation material

SOLUTIONS

l Provide a minimum pipe height (diameter) 

of 6’ to allow most workers to stand upright 

while constructing the streambed. Pipes as 

small as 5’ have been used successfully. 

Smaller diameters can be used if they 

are constructed in half diameter sections, 

but smaller pipes may not have enough 

embedment depth to accomodate natural 

fluctuations in streambed elevation.

l Top-load an open-bottom or lidded culvert.

l Use one-piece embedded pipe.

l Use precast or metal footings for openbottom arch.

l Use a bridge with precast spread-footings.

l Avoid blasting, use nonexplosive methods. 

l Avoid pile driving.

l Use simple designs: CMPs, or 

prefabricated box culverts, or bridges 

where possible instead of complex, labor 

intensive structures.

l Use open-bottom culvert with concrete 

stemwalls formed to bedrock.

l Use open-bottom or top-loaded culvert

l Use full-bottom pipe.

l Lower the road if possible to reduce total 

dead load on the foundation soils.

l Use a geotechnically designed foundation 

(geotextile, geogrids, etc.)
7—14

Stream Simulation

7.2.3. Cost Considerations

Cost considerations related to the design, material and labor, expected 

life, and ultimate replacement of the structure often influence structure 

selection (table 7.3). Changes in the structure’s size may have an influence 

on the project cost but not proportionally; for example, a structure twice as 

large does not cost twice as much (see sample cost estimates in appendix 

G.3). Manufacturers will often help find the most economical structure 

shape for the design criteria. Structure types and sizes also influence 

maintenance and replacement costs; for instance, large structures, while 

initially more costly, also are less prone to flood damage and debris 

plugging.

Table 7.3 lists factors that affect total project costs (e.g., initial costs and 

projected lifetime and replacement costs).

Table 7.3— Cost factors that affect choice of structure

COST FACTOR

Initial costs

Durability and 

replacement cost

Maintenance costs

CONTROLLING FACTORS

l Structure type (one piece is less expensive than 

multiplate).

l Structure type (one piece embedded is less expensive 

than open-bottom arch in small sizes).

l Special shapes (squashed, low-profile, box).

l Special features (collars, thrust beams, special backfill, 

headwalls).

l Delivery.

l Shape control engineering (super-span culverts).

l Construction duration.

l Resistance to corrosion and abrasion (see table 7.4).

l Ability to salvage existing foundations and streambed 

(open-bottom arches and bridges) when replacing 

structure in the future.

l Vulnerability to flood damage.

l Debris removal. Structure type and size will influence 

debris-removal costs.

l Repairing flood-related damage to eroded streambanks, 

stream-simulation bed, grade-control structures.
7—15

Chapter 7—Final Design and Contract Preparation

Table 7.4 lists the durability of different structure material types from the 

most durable to the least. To help weigh cost and durability, use tables 7.4 

and 7.5 in conjunction with each other.

Table 7.4—Durability factors that affect choice of structure

Durability factor

Corrosion or deterioration 

rate.

Soil pH and conductivity 

influence corrosion and 

deterioration rate in 

metal culverts. Increasing 

metal thickness, concrete 

strength, or adding special 

coatings will enhance 

longevity.

See table 7.5.

Abrasion rate.

Size, shape, and flow rate 

of sediments influence 

abrasion rate.

See Ault and Ellor 2000.

Structure material

(listed in order of longest to shortest design life)

l Prestressed concrete. 

l Reinforced concrete bridges and culverts.

l Steel bridges — weathering steel or if 

maintained with paint. 

l Aluminum culverts.

l Aluminized steel culverts.

l Galvanized steel culverts.

l Treated timber bridges (durability varies with 

treatment and climate).

l Untreated timber bridges.

l Concrete.

l Aluminum culverts (more vulnerable to 

abrasion in sandy sediment).

l Aluminized steel culverts (more vulnerable to 

abrasion in cobble sediment). 

l Galvanized steel culverts (more vulnerable to 

abrasion in cobble sediment).
7—16

Stream Simulation

This table, from the Oregon Department of Transportation’s Hydraulics Manual (ODOT 2005), is an 

example of the type of information that may be available and helpful in choosing a structure material 

appropriate for the site.

Table 7.5—Pipe material service life for Oregon (ODOT 2005) PIPE MATERIAL SERVICE LIFE: Average 

Years to Maintenance, Repair or Replacement Due to Corrosion (includes effects of scour as well)

Bituminous-coated (AASHTO M190) metal pipe adds 10 years to the service life in all locations. Apply the 

factors from the previous two items to the total service life. (Many regions do not permit bitumous-coated pipes 

because of water quality issues.)

Soil resistivity or pH readings outside the indicated limits will require special design considerations.

Material

Galvanized Steel

Aluminum

Aluminized Steel

Concrete

Polyethylene

Location 

East or West 

of Cascades

East

East

East

West

West

West

East or West

East

West

All Locations

All Locations

Water 

& 

Soil pH

4.5 – 6.0

>6 – 7

>7 – 10

4.5 – 6.0

>6 – 7

>7 – 10

4.5 – 10

5 – 9

5 – 9

4.5 – 10

4.5 – 10

Soil 

Resistivity 

(ohm-cm)

1,500 – 2,000

1,500 – 2,000

1,500 – 2,000

1,500 – 2,000

1,500 – 2,000

1,500 – 2,000

>1,500

>1,500

>1,500

>1,500

>1,500

Service 

Life 

(Years)

30

35

40

15

20

25

75

65

50

75+

75

For galvanized steel, the service life 

increases for soil resistivity as follows:

Resistivity (ohm-cm) Factor

2,000 – < 3,000 1.2

3,000 – < 4,000 1.4

4,000 – < 5,000 1.6

5,000 – < 7,000 1.8

> 7,000 2.0

The service life indicated is for 16-gauge 

metal pipes. Multiply the service life by 

the appropriate factor for different thickness:

Gauge 14 12 10 8

Factor 1.3 1.7 2.2 2.9
7—17

Chapter 7—Final Design and Contract Preparation

7.2.4 Tips for Choosing Structures

The following tips may be helpful when choosing between different 

structure types: 

l Embedded pipes are most economical of all the structures and quick 

to construct, at least up to sizes where they become multiplate 

structures (12 to 15 feet, depending on the manufacturer); however, 

except for box culverts, these structures require large excavations.

l When fill heights are relatively low (one-half to two-thirds of 

design width), round and pipe-arch culverts may not fit under the 

embankment with sufficient cover. Consider using low profile 

and box structures, raising the fill height, or using a bridge. Fill is 

relatively inexpensive if raising the grade over the structure does not 

affect the road grade or alignment for a long distance. However, if 

the grade is raised over a long distance to accommodate a large pipe, 

fill costs may become excessive and there may be significant wetland 

impacts with large increases in the embankment height.

l In bottomless structures, and box culverts with lids, the streambed 

can be constructed from the top, reducing the need for equipment to 

operate in the channel.

l Embedded pipes more than 25 feet in diameter may have to be buried 

over 10 feet deep for filling to design width. These pipes therefore 

may not be practical if dewatering is either difficult or impossible, or 

if bedrock is too close to the surface.

l Compared to culverts, channel-spanning bridges tend to have lower 

risks and higher longevity, and provide better passage for aquatic, 

semiaquatic, and terrestrial animals. When they are close in cost to 

other structures, they are generally preferable. 

l Bridges are worth considering for active flood-plain locations 

and debris-flow or landslide-prone areas where high clearance is 

necessary.

7.3 Structural Design

Design elements of the crossing structure include: 

l Crossing structure.

l Foundation.

l Structural backfill.
7—18

Stream Simulation

7.3.1 The Crossing Structure 

Pipe, pipe arch, and bottomless structures are constructed of either 

corrugated metal or reinforced concrete. Structural design is not necessary, 

because manufacturers supply this information in brochures and for 

individual projects to ensure correct use of their products. Culvert 

brochures usually have tables giving design solutions for various culvert 

dimensions, corrugation types, thickness, traffic loads, and range of fill 

heights. You can get this information directly from the manufacturer for 

specific designs. To do so, have the following minimum site information 

available before contacting them:

l Maximum traffic load.

l Fill height range.

l Soil weight.

l Soil type.

l Foundation bearing capacity.

l Structure dimensions.

Bridges are constructed of a variety of modular and individually 

engineered materials with steel, concrete, and wood as the common 

building materials. Structural bridge design or review is beyond the scope 

of this document. Whenever a bridge may be a suitable option, a bridge 

engineer should be part of the design team. 

Standards for designing bridges, culverts, foundations, and backfill are 

in Standard Specifications for Highway Bridges, 17th edition (AASHTO 

2002). Another good resource for all pipes is the installation manual for 

corrugated steel pipe, pipe arches, structural plate (NCSPA undated).

7.3.2 Footing Design

You must be able to recognize foundation situations that are risky or 

complex enough to require expert assistance for design of an openbottom structure—or even to preclude such a structure. The geotechnical 

investigation conducted during the site assessment (section 5.1.7) should 

yield enough information for you to determine the degree of complexity 

and risk. Unsuitable soils or foundation conditions that will require further 

expert analysis include:
7—19

Chapter 7—Final Design and Contract Preparation

l Silts and clays.

l Soils with high organic content.

l Unconsolidated soils.

l Bed rock.

If these materials are present, particularly if the site is geologically 

complex, a detailed site investigation is needed. 

Footing design requires the following analyses: 

l Structural analysis: quantifying and analyzing stresses on the footing, 

and adjusting footing dimensions until the load distributes evenly on 

the footing. 

l Bearing capacity analysis: analyzing the soil bearing capacity for 

various footing depths and widths.

l Scour scenario analysis: ensuring that the worst-case scour condition 

leaves enough embedment depth to develop sufficient bearing 

capacity to support the foundation loads. 

l Foundation design: designing the footing details, including 

reinforcement, culvert attachment, shape, and constructability 

aspects. 

l Settlement estimation: estimating the amount of settlement expected 

to occur. 

The above analyses are within the skills of most bridge, structural, 

foundation, geotechnical, and geological engineers. Ensure that the 

required expertise is available if you do not have all the skills necessary 

for designing bottomless arch or box-culvert footings. For more detailed 

discussion regarding footing design and foundations, see appendix G.4.2

The following example illustrates inadequate footing design methods. One 

type of open-bottom arch—a half-round corrugated metal pipe (CMP) with 

flat lengths of corrugated sheet metal welded on each edge of the arch to 

function as a footing (figure 7.7)—has been used in a number of locations 

to provide continuity in small streams. Some of these structures have failed 

because they were not adequately embedded and scour occurred under the 

corrugated sheet metal footings. Therefore, when considering using these 

less-expensive structures, use the same design procedures as you would 

use on larger more complex open-bottom arches. Ignoring proper design 

procedure makes failure likely.
7—20

Stream Simulation

Figure 7.7—Open-bottom pipe arch with metal footings.

7.3.3 Structure Backfill 

Backfill material in the special backfill zone (figure 7.8) interacts with the 

structure to provide more strength than either material could provide by 

itself. Backfill requirements vary for different types and sizes of structures 

and are usually specified by the manufacturer. Backfill and compaction 

specifications for culverts are covered in FP-03, Section 209 under: 

l Backfill material (for general backfilling of culverts).

l Lean concrete (for both bedding and partial backfill material). 

l Bedding material (for placing beneath pipe structures as a leveling 

and piping prevention layer (figure 7.9).

l Foundation fill (for replacing unsuitable material and for long-span 

structures). 

Choose foundation fill gradation A-1-a from FP-03, Section 705 for 

long-span (greater than 25 feet) structures, because you can easily place 

it and compact it to high strength without overstressing or distorting 

corrugated steel structures. Consult the structure manufacturer for specific 

recommendations.
7—21

Chapter 7—Final Design and Contract Preparation

Figure 7.8—Special backfill zone for an open-bottom arch.

Figure 7.9—Shaping culvert bedding.
7—22

Stream Simulation

7.3.4 Existing Site Materials

The crossing design may be able to use several types of materials available 

on site; for example: 

l Large boulders.

l Large woody debris.

l Bedding material from the old culvert.

l Streambed materials in areas that will be disturbed.

l Clearing debris.

These materials may be suitable for constructing streambed features such 

as steps, banks, or other key features. The old bedding (figure 7.10) may 

be useful in the stream-simulation bed material recipe (section 7.5.2.2), 

and clearing debris can be used for erosion control (figure 7.11). 

Figure 7.10—Old culvert bedding may be used in the stream-simulation bed mix.
7—23

Chapter 7—Final Design and Contract Preparation

Figure 7.11—Clearing debris scattered for erosion control.

Also evaluate the existing embankment to determine if the soil meets 

structural and general backfill requirements. Estimate whether additional 

backfill will be required or if a surplus exists. Old embankments 

sometimes have large trees and other surprises buried in them. These 

“surprises” are normally handled during construction under the changes 

clause. Trees and other native materials may be suitable for placement 

as instream structures upstream or downstream of the structure. The site 

assessment documentation should contain recommendations on how to use 

these materials on the project. You may place them in disturbed areas to 

control erosion, in riparian zones for habitat, or in the stream for additional 

aquatic habitat or grade control. Depending on long-term goals, trees and 

other native material may or may not be anchored to the bank; consult with 

the project team. 
7—24

Stream Simulation

7.4 Handling Traffic During Construction

Four options are generally available for accommodating or controlling 

traffic during the project. 

(1) Redirecting traffic to alternate routes. 

(2) Closing the road briefly (3 days to 1 week). 

(3) Providing an adjacent temporary road-stream crossing (often 

over the dewatering dam). Either ensure that the roadway has 

sufficient width, slope, traction, and geometric alignment to allow all 

expected traffic to use the bypass, or provide signs indicating vehicle 

limitations. Keep in mind that this option affects the dewatering 

system, clearing limits, excavation volumes, and traffic management 

efforts. Figure 7.12 illustrates this option but does not use the 

dewatering dam. 

(4) Passing traffic over the construction site while constructing the 

structure in two stages. 

(a) Allow enough road-surface width for building more than half the 

new structure at one time. Sometimes, you can achieve the needed 

width by lowering the road surface temporarily. 

(b) Construct a stable roadway to support traffic safely (according 

to Occupational Safety and Health Administration standards.) To 

support the excavation side of the embankment, you may need some 

form of retaining wall. (Because of the need to construct the road fill 

in two stages, this option may require a longer structure.)

Traffic bypasses can account for anywhere from 10 percent to as much as 

50 percent of the total project cost, depending on the size of the project 

and the complexity of the bypass. The total cost of a traffic bypass includes 

the combined increased costs of slowing the construction work and adding 

traffic control personnel, signs, traffic control lights, and other project 

details. Figures H.4 and H.5 show examples of a sign plan and a gate plan.
7—25

Chapter 7—Final Design and Contract Preparation

Figure 7.12—Typical construction site traffic bypass. 

7.5 Developing Specifications

Chapter 6 covered design of particle-size gradations and other features 

of the simulated streambed using data from the reference reach. This 

section develops contract specifications based on the stream-simulation 

design. Stream-simulation construction contracts require modifying 

standard specifications to describe their specialized construction. The 

Forest Service uses Standard Specifications for Construction of Roads and 

Bridges on Federal Highway Projects (Federal Highways publication FP03) for standard specifications. Use Specifications 151-Erosion Control, 

251-Riprap, and 705-Materials for the parent specifications to describe 

dewatering, streambed construction, and streambed materials in streamsimulation projects. Appendix H provides examples of supplemental 

specifications. 

All construction specifications that describe work to be done—

specifications in FP-03 Divisions 200 through 600—consist of three parts: 

l Description: This part describes the scope of work covered in the 

specification.

l Materials: This part nearly always refers to a materials specification. 

In the case of stream simulation, Supplemental Specification 705 

covers rock and filler material.

l Construction methods: This part describes all features and how to 

construct them. Often, to clarify features difficult to describe in 

words, the specification refers to drawings.
7—26

Stream Simulation

Some aspects and requirements of stream-simulation construction will 

be unfamiliar to contractors, even those with instream experience. Wellwritten notes and specifications for aspects outside the normal practice 

will allow bidding that is more accurate and minimizes expensive change 

orders. 

7.5.1 Submittals

You may often use specifications to require the contractor to design and 

submit a plan for portions of the project work for approval. When using 

this method, expected results should normally be specified—not methods 

for performing the work. For some work, contractor design is more 

appropriate, allowing the contractor to perform the work in a manner 

that best fits his or her work methods and, most important, making the 

contractor responsible for the end result. Allow reasonable time for a 

submittals process, i.e., adequate time for the contractor to design and 

submit the proposal for the specified work and adequate time for a 

thorough but timely agency review of the proposal. Work items often 

specified in the contract and designed or performed by the contractor 

through a submittals process are:

l Quality control.

l Construction surveying.

l Temporary erosion and pollution control.

l Dewatering and water treatment.

l Storm management plan.

l Structural backfill materials.

l Concrete mix designs.

l Stream-simulation bed mixture.

l Revegetation.

7.5.2 Supplemental Specification 251: Streambed Construction

7.5.2.1 Description 

The description is an introduction to the specification. Briefly describe 

the features— especially unique features—that you want to construct 

under this specification. (See appendix H for an example of Supplemental 

Specification 251.) 
7—27

Chapter 7—Final Design and Contract Preparation

7.5.2.2 Materials

The Materials section of Supplemental Specification 251 should refer to 

material specification Supplemental Specification 705 (section 7.5.3). 

Supplemental Specification 251 includes a description of work required to 

achieve the gradations specified in Supplemental Specification 705. 

The streambed may contain material that you can salvage from the 

excavation and use for at least a portion of the stream-simulation bed mix. 

Excavated material that appears too dirty to use may simply be the natural 

subsurface layer, which is often much richer in fines that the surface 

of an armored streambed. At some culvert-replacement sites, natural 

streambed materials may be covered by the old culvert bedding material 

(figure 7.10). Bedding depths can vary, depending on the roughness of the 

underlying channel surface or whether the channel is incised or not. 

Consider making provisions in the contract for using the native streambed 

material if it meets gradation requirements. Alternatively, native material 

can be part of the recipe for the streambed-simulation bed mix. If the 

material cannot be used for the streambed-simulation bed, it can be used 

elsewhere on the project as common excavation for other backfill. Provide 

locations for stockpiling, mixing, and disposing of the material depending 

on the final determination for the use of the onsite materials.

The drawback to using onsite materials in the bed mix recipe is that you 

will not know the mix proportions when the project is advertised. It may 

be far more expedient and economical for the project not to depend on 

onsite materials. If, during construction, you determine the onsite materials 

are useable, the government can take a deduction for using the onsite 

material in lieu of purchased or hauled material through a change order.

If you are going to include onsite materials in your bed mix, you must 

sample the onsite materials and determine their gradation. The best time 

to sample is during excavation of the existing structure. Two sampling 

methods can be used: the pebble count method (section 5.1.6.1), or bulk 

sampling. Keep in mind that representative samples of material for bulk 

sampling where the largest particles are over 4 to 5 inches must be several 

hundred pounds (reference American Society for Testing and Materials 

standard C136-06). If sampling and gradation testing of onsite materials is 

performed after the contract is awarded, contract administrators will use a 

change order to incorporate the onsite materials.
7—28

Stream Simulation

Once the gradations of all the materials (both onsite and commercial) 

have been determined, determine the proportions of each material that 

will be needed to produce the stream-simulation bed mix (the gradation 

specified in Section 705, see figure 7.18). The process of developing a 

stream-simulation bed mix recipe is identical to developing a mix design 

for Portland cement or asphalt concrete from several differently graded 

stockpiles. 

Sampling can be done in-house or by the contractor. Specify either option 

in the materials section of Supplemental Specification 251.

Sampling by the

contractor Specify a submittal for the bed-mix recipe (the proportions of the different 

aggregate stockpiles to be used in the bed mix) based on the gradations 

determined during the stream-simulation design (see section 6.2.1.1.) 

The contractor will develop the mix recipe as a submittal using materials 

recovered from the site excavation, from commercially available materials, 

or from a mix of both.

Sampling by contract

administrators Specify in the contract that the engineer will perform sampling and testing 

during structure excavation and that the bed-mix recipe will be designed 

“in-house.” Be sure to include a provision that (a) states that the contractor 

cannot proceed with any streambed construction until the analysis and 

streambed-simulation recipe are complete and, (b) provides a reasonable 

length of time for the sampling, testing, and analysis. 

7.5.2.3 Construction methods

To develop the Construction Methods section of Supplemental 

Specification 251, use or modify the example in appendix H to describe 

features such as: 

l Stream-simulation bed cross section and profile.

l Low-water thalweg. 

l Steps, constructed riffle crests.

l Banks, edge features.

l Rock clusters.

l Grade-control structures. 
7—29

Chapter 7—Final Design and Contract Preparation

l Handling of known or discovered natural key features (for example, 

bedrock, natural rock steps that are part of the stream-simulation 

design).

Describe the streambed features designed in chapter 6 in detail in the 

contract and show them on the contract drawings. (See figure H.9 and 

H.14, and section 6.2.) Determine which onsite materials, if any, can be 

used for constructing these features, and incorporate those materials and 

features into the specification. If possible, use detail drawings and refer to 

them with the specification. Include language in the specification or special 

contract requirements that provides protection for the structure against 

damage while streambed materials are placed.

Constructing streambeds and other features inside very small culverts 

usually involves hand labor (figure 7.13). Hand labor will be required 

to help seal streambeds and for compaction close to the structure where 

compaction by equipment is impossible. (See also figure 8.16.)

Figure 7.13—Hand labor walk-behind equipment.
7—30

Stream Simulation

Stream-Simulation

Bed Details Supplemental Specification 251 (appendix H) covers placing streambed 

material. It specifies the size, depth, surface profile, and compaction of the 

bed material, as well as layer placement when needed. 

You may need fine-grained filler material (referred to as “select borrow” in 

the sample specification) to fill in voids between larger rocks and against 

the sides of the culvert. As discussed in chapter 6, the filler material is 

washed into the voids in the streambed (figure 7.14), reducing streambed 

permeability and helping to keep the streamflow on the surface during 

low-flow periods. This practice also reduces the loss of fines and thus 

decreases turbidity during the initial rewatering.

Figure 7.14—Washing filler material into the voids in the stream-simulation bed.
7—31

Chapter 7—Final Design and Contract Preparation

When using footings in high-risk scour areas, specify placing a layer of 

larger more stable streambed material against the footings to prevent scour 

of the footings (figure 7.15). Provide for protecting the stemwalls and the 

structure during construction. 

Figure 7.15—Footing armor.

Channel Margins Continuous channel banklines or other margin features, such as rock 

clusters, are part of the stream-simulation design (section 6.2.1.3). The 

margins may be a single row of rocks, or they may be wide enough 

to simulate a flood plain in the culvert (figure 6.22). Banks should be 

constructed carefully to limit void space between the large rocks. Voids 

should be filled by jetting or flooding in filler material.

Figure 7.16—Newly constructed (2006) stream-simulation channel and banks, 

Surveyor Creek, Lolo National Forest, ID. The top of the bank is at bankfull 

elevation, indicated by the painted line. Note the transition between natural banks 

outside and constructed banks inside the culvert.
7—32

Stream Simulation

Key Features Key features are grade-control or diversity-enhancing structures consisting 

of rock or wood, placed to mimic natural conditions where they are called 

for in stream-simulation design plans. Ensure rock is carefully placed to 

produce the desired degree of stability. Individual rocks and rock clusters 

should be embedded a minimum of one-third of their size.

The stream-simulation plans may call also call for steps, bands of rifflesized rock, and rock clusters (figures 6.23, and 6.25). In steep step-pool 

channels where steps must be as stable as natural steps, the rocks must 

be carefully placed, bearing against—and interlocked with—other step 

rocks (section 6.2.2.4). Steps generally have two tiers, an upper tier 

of rocks immediately upstream and a lower tier of footer rocks below 

and immediately downstream of the upper tier, to prevent scour and 

undermining (figure H.9).

In pool-riffle channels, the stream-simulation design may call for 

constructed riffle crests to simulate intermediate mobility key features like 

pool tailouts, and promote natural development of diverse bed structures 

over time. Construct these by placing streambed material to full depth for a 

distance along the length of the culvert, then switching to coarser material 

for the width of the band, alternating this pattern through the length of the 

culvert (section 6.2.2.2). Both bands and the rest of the channel are shaped 

with a low-flow thalweg, so that the cross section dips in the middle and 

rises toward the walls of the structure (figure H.15).

Where bank stability and/or habitat requires placing wood outside the 

structure, place it with about two-thirds of the tree’s length on the bank, 

with the remainder lying in or over the water and pointing upstream at a 

sharp angle. The wood must be well buried, anchored, or large enough 

to remain immobile. To ensure these features will be stable for the life of 

the structure, work with an experienced biologist or hydrologist. Where 

possible, develop site-specific designs to use available local materials. 
7—33

Chapter 7—Final Design and Contract Preparation

7.5.3 Developing SPS 705: Specifying Rock Sizes

Section 705 specifies characteristics of aggregates, including the gradation 

of the materials used for various purposes. To modify Section 705 for 

stream simulation, we need to specify the gradations of all the materials 

needed for the features discussed in the Supplemental Specification 251, 

Construction Methods. The project team has already developed a gradation 

curve for the bed mix (section 6.2.1.1), with units of millimeters, the most 

common units used for pebble counts. The bed-gradation specification 

must be in a format that material suppliers understand. Generally, this 

format is a table of sieve sizes, with percent-finer values (the percentage 

of aggregate by weight passing the particular sieve) accompanied by a 

percentage range of tolerances (for example, 50-percent passing through 

the sieve, plus or minus 5 percent, expressed “45% - 55%”).

If using bulk sampling, simply insert the values determined from the 

laboratory analysis of the sample into table 705-7 (figure 7.17), and use the 

table in Supplemental Specification 705. 

If using the particle-size distribution curve from chapter 6, do the 

following:

l Determine the closest sieve sizes (the next largest) to the D95, D84, 

D50, D30, and D10 values (or other key values) on the particle-size 

distribution curve, and insert those values in table 705-7 (figure 7.17). 

l Verify that the sieve size is no more than 5-percent greater than the 

desired particle size. If the size is greater, choose another point on the 

distribution curve, close to the desired size, that better coincides with 

a standard sieve.

l Using the particle-size distribution curve, find for each sieve size 

the percent-finer value on the vertical axis (figure 7.18). Insert those 

values in table 705-7. (These are the values for the stream-simulation 

bed gradation, expressed as “percent finer values.”) 

l To provide flexibility, use a tolerance range of 10 percent (plus or 

minus 5 percent) for each sieve size. Generally, no less than 5-percent 

fines (finer than number 8 sieve) are allowed in the manufactured 

streambed-simulation rock. The stream-simulation bed mix design 

(6.2.1.1) may specify a different fines content based on the reference 

reach. Similarly, 90 to 100 percent of the material should pass the D95

size.

l For the filler material, use 1-inch minus or D16, whichever is smaller. 

(A minimum of 50 percent of the filler material should pass the sieve 

representing the D5 value of the streambed-simulation bed.)
7—34

Stream Simulation

Using the values determined from the curve in figure 7.18, fill in the values 

in the table in figure 7.17.

Figure 7.17—Example of table 705-7, Project Requirements for StreamSimulation Bed Material.

Channel Rocks For the purpose of definition in the construction contract, “channel rocks” are 

rock materials needed for constructing key features, such as steps, constructed 

riffle crests, banks, and clusters. Specify them separately from the streamsimulation material, using sizes already determined for key features in section 

6.2.1.3 and 6.2.1.4. Not only diameter but also shape characteristics are 

important. For example, elongated rocks interlock better and can form a more 

stable feature in the simulated streambed. 

(1) U.S. Standard Sieve size 

closest to D100, D84, 

D50, D30, D10, are:

12”, 6”, 3”, 2”, #4

(2) Filling in the corresponding % finer 

values allowing +/- 5% of the value 

from the distribution curve:

12” = 99% +/- 5% = 94-104

(use 90-100)

6” = 84% +/- 5% = 79-89

3” = 50% +/- 5% = 45-55

2” = 34% +/- 5% = 29-39

#4 = 9% +/- 5% = 4-14

(3) Finally, filling in the values for filler 

material: Sieve sizes closest to D16

and D5 are 3⁄4” and #40.

Standard

sieve

Stream simulation

bed material

(percent finer)

Filler 

material

(percent finer)

12” 90-100

6” 79-89

3” 45-55

2” 29-39

#4 4-14

3⁄4” 100

#40 > 50
7—35

Chapter 7—Final Design and Contract PreparationFigure 7.18—Developing a gradation table from a particle-size distribution curve.
7—36

Stream Simulation

Table 705-4 (figure 7.19) defines the channel rock size classes and lists approximate weights and 

acceptable range of rock diameters for each class. Size classes are shown on the drawings for each key 

feature in the design.

Table 705-4 Size Requirement for Channel Rocks

Channel Rock Approximate Median Axis

Class Weight Dimension &

(diameter, inches) (pounds) Variation in inches

Rock-4 3 4 +/- 1

Rock-6 10 6 +/- 1

Rock-9 33 9 +/- 2

Rock-12 80 12 +/- 2

Rock-16 185 16 +/- 2

Rock-20 365 20 +/- 2

Rock-24 630 24 +/- 3

Rock-30 1,230 30 +/- 3

Rock-36 2,120 36 +/- 4

Rock-42 3,370 42 +/- 4

Rock-48 5,030 48 +/- 5

Rock-54 7,160 54 +/- 5

Rock-60 9,820 60 +/- 6

Figure 7.19—Table 705-4 defines channel rock-size classes.

An example of Supplemental Specification 705 for stream simulation is in appendix H. Tables 705-4 (size 

requirement for channel rocks) and 705-7 (gradation requirements for stream simulation bed material) 

are added to the standard specification. In the example in appendix H, channel rocks are required to have 

a long axis at least 33-percent longer than the median axis. The 133-percent elongation should be field 

verified for each site. In places where you are constructing permanent features from the channel rocks, you 

may wish to specify that the rocks are to be fractured and angular. 
7—37

Chapter 7—Final Design and Contract Preparation

7.6 Designing for Flood and Debris Failure Prevention

See table 6.7 and section 6.5.2 for discussion of risks caused by high flows, 

woody debris, and sediment, along with methods of minimizing those 

risks. Additional information is available in Furniss et al. 1997.

7.7 Planning for Erosion and Pollution Control

An erosion and sedimentation-control plan details the suite of methods and 

tools that will be used to minimize sediment delivery to the stream channel 

during and after construction. The plan contains actions and practices 

that occur before, during, and after construction, including long-term 

stabilization elements, such as the revegetation plan. Depending on the site 

and conditions, the plan may include the following elements:

Before-construction actions

l Planning for water quality monitoring during and after construction.

l Salvaging and storing topsoil. 

l Salvaging plants or cuttings. 

During-construction actions

l Construction timing and sequencing.

l Site dewatering and rewatering.

l Treating water. 

l Providing short-term erosion control on disturbed areas and storage 

piles.

l Preventing and controlling pollution from equipment and facilities.

l Methods of stabilizing disturbed areas, such as placing rocks and logs 

for long-term bank stabilization. 

l Special treatment of imported or excavated streambed material, such 

as segregating stockpiles to prevent contamination or covering them 

to prevent loss. 

Post-construction actions

l Removing temporary erosion- and sediment-control measures.

l Revegetating the site.

l Maintaining the site.
7—38

Stream Simulation

Federal, State, and county permits often include BMPs and performance 

standards (for example, turbidity requirements) that apply directly to the 

erosion-control plan. Be sure to include these requirements in the special 

contract requirements and the erosion-control plan as well as any notes 

and detail drawings that you may need. You may need to create detailed 

drawings, applying the BMPs to specific site features and paying for them 

directly via pay items in the contract. 

Including the major features of erosion control in the design gives the 

project team maximum input into long- and short-term erosion control. 

Including major features of the dewatering system, long-term revegetation, 

and site-stabilization plans in the design will also provide greater overall 

project efficiency. For example, you can clean and retain sedimentretention basins (constructed to control storm flows in the contributing 

road ditches during construction) as long-term ditch sediment-control 

measures. 

7.7.1 General Erosion Control During Construction

The most important rule for erosion control is to minimize site disturbance 

within the limits of project goals. First, mark clearing and disturbance 

limits, and reduce the disturbed area as much as possible. Second, control 

potential erosion by covering disturbed surfaces (for example, storage 

piles), or by routing water away from them (for example, using stormwater 

controls). Third, capture and treat sediment-laden water before releasing 

it to the stream. Fourth, provide for long-term stabilization of the site 

through revegetation and other permanent measures.

Standard specifications and contract clauses allow you to (a) specify 

erosion-control measures, (b) specify outcomes and require the contractor 

to submit an erosion-control plan to meet them, or (c) combine the two 

methods. Risk to the owner (the government in this case) is greater when 

methods and measures are specified, because the responsibility for any 

failure then remains with the owner. Performance-based specifications are 

generally encouraged for this reason. 

Erosion control can be paid directly as a separate pay item, or made 

incidental to other work such as installation of the culvert and paid under 

that pay item. A successful result with either method depends primarily on 

diligent and consistent enforcement of the requirements. Be sure to include 

contract language requiring the contractor to maintain all erosion control 

and prevention features.
7—39

Chapter 7—Final Design and Contract Preparation

Consider the following items for the temporary erosion prevention, 

control, and treatment plan: 

l Construction site layout with clearing limits.

l Work schedule, including timing of erosion-control items.

l Dewatering and sediment treatment plan (see section 7.8).

l Storm management plan.

l Sediment-trapping silt fences or straw bales.

l Drainage-control plans directing water away from disturbed areas.

l Ditches and check dams.

l Road drainage details.

l Ditch relief culvert details.

You may need to include the following in your special contract 

requirements to cover temporary erosion and sediment control: 

l Cover aggregate stockpiles to prevent wind and rainfall erosion. 

l Cover excavated slopes to reduce surface erosion.

l Sweep and clean off road surfaces.

l Submit a storm management plan, including the following as a 

minimum:

s List of contacts including contract administration and contractor 

personnel.

s Site specific list of action items, for example:

n Maintain erosion control measures including ditches, 

barriers, silt fences, etc.

n Maintain the construction bypass system and any 

components, such as trash screens.

n Have extra pumping capacity onsite ready to use in 

emergency.

	 n Block traffic or provide traffic control if necessary.
7—40

Stream Simulation

l If the project is longer than one construction season:

s Be prepared for an early winter storm and construct over-winter 

erosion-control measures early.

s Provide for periodic maintenance checks during winter and during 

spring runoff.

s Inspect and maintain all erosion-control measures before spring 

restart of construction. 

s Remove and dispose of temporary erosion-control measures and 

accumulated sediment after construction and after the site has 

stabilized.

For projects that could extend over more than one construction season, see 

appendix G.4.3.7.

7.7.2 Permanent Erosion Control Measures

Develop necessary drawing details and special-project specifications for 

permanent erosion control on roads, road embankments, streambanks, and 

other disturbed areas. 

Many long-term stabilization measures, such as in-channel wood, 

streambank rocks, and engineered slope-stabilization measures, are design 

features included in Supplemental Specification 251. Where vegetation 

may be difficult to establish in a mat thick enough to provide erosion 

control, combine vegetation with other measures such as riprap, root wads 

or logs, or erosion-control matting. 

Typical components of a long-term stabilization plan include:

l Seeding, mulching, and planting of exposed soils.

l Scattering construction slash on exposed soil areas for erosion 

control.

l Ditches, relief culverts, and dips that drain to natural sedimentfiltering vegetation and stable landforms where runoff can infiltrate, 

rather than running directly into the stream.

l Erosion protection for road cut-and-fill embankments. 

Specifications that 

have an end result 

are much easier to 

administer than 

process-oriented 

specifications.
7—41

Chapter 7—Final Design and Contract Preparation

l Integrated streambank protection: 

s Although riprap is generally very successful and stable, it is 

sometimes not aesthetically desirable on some visually sensitive 

sites and may not be desirable due to habitat loss.

s For vegetation, use native plant species such as willows, 

groundcovers, and other native species.

s Other bioengineering methods (WDFW 2003).

For detailed discussion on revegetation, see appendix G.4.3.

7.7.2.1 Diversion-prevention dips

In many cases, a diversion-prevention dip will be an essential part of the 

permanent erosion control system (section 6.5.2.3). Diversion-prevention 

dips provide a drainage pathway across the road to avoid stream diversion 

down the road (figure 7.20). Design the dip without severe grade changes 

that exceed the design standard for the road and could pose a traffic 

hazard. Make sure the dip will capture all the overtopping water and carry 

it in a controlled way to the intended relief drainage pathway. Plan to plug 

any continuous road ditches on the downgrade side of the stream crossing 

to prevent them from diverting ponded water down the road. 

Figure 7.20—Diversion-prevention dip on the Plumas National Forest, California. 

The diversion dip is located just down the road from the stream crossing because 

the crossing is on a tight curve.
7—42

Stream Simulation

When a culvert plugs and sends water over the road through the relief dip, 

the water tends to pool relatively gently on the upstream side. However, 

once through the relief dip and over the road, the water rushes down the 

much steeper embankment slope and can cause considerable erosion. 

Make sure the downstream slope of the relief dip is well protected with 

vegetation and or riprap.

A relief dip also may be used to provide stormflow relief by means of a 

controlled failure. In such a scenario, the dip is protected from erosion in 

the same way as other fillslope areas. If the stream-simulation structure 

plugs, the stormflow causes failure at the relief dip location, preventing 

the stormwater from running down the road and thereby limiting overall 

damage.

A good diversion-prevention dip has the following characteristics:

l Accommodates the critical vehicle at the design speed.

l Cross section is adequate to contain the design stormflow volume.

l Outsloped at less than 5 percent.

l Incorporates embankment erosion-control measures.

l Associated ditches are plugged to prevent floodwater escape down 

the ditch.

7.8 Dewatering, Bypass, and Water Treatment During 

Construction 

Live streams require dewatering to prevent mixing soil with streamwater 

during construction. Unless subsurface water exists, a dry streambed may 

not require dewatering. However, if water quality is an issue, create and 

implement a reliable bypass plan for handling stormflows. Summer storm 

events may be the most intense storms during the year in some areas, and 

unusual events can happen at any time. 

Often, engineers do not take dewatering seriously enough. Although 

the dewatering system does not have to be elaborate, it needs to work 

effectively. The bypass dam is the first line of defense on the project, and 

the downstream sediment collection point—whether an excavated pool, 

an existing scour pool, or a dammed pool—is the last. These components 

of the dewatering system must work well and reliably. The failure of a 

dewatering system can cause serious damage to the stream habitat, delay 

the project, and result in cost overruns. 
7—43

Chapter 7—Final Design and Contract Preparation

Only a gross estimate of the amount of surface and subsurface water and 

sediment that need capturing and treating can be made until the site is 

actually excavated. We recommend that the engineer and a hydrologist 

work together on the dewatering-system design, and take into account 

historical flows during the construction season. Be sure to require that 

the contractor provide adequate pumping ability, regardless of project 

conditions, and to have a backup pump always available for handling 

stormflows and taking over if the primary pump malfunctions. 

A successful dewatering and bypass system does all of the following:

l Captures streamflow and successfully diverts it around the project.

l Handles stormflows without failure, with backup pumps readily 

available onsite. 

l Captures water that seeps around the bypass before it reaches the 

excavation, and reroutes and treats it (if necessary) before releasing it 

back to the stream.

l Captures and removes sediments from water that seeps into the 

excavation from its edges or from springs, mixes with soil and 

becomes turbid.

l Does not backwater the site. 

l Captures water that seeps into the excavation from downstream and 

either treats it or—if it is kept clean—releases it back into the stream.

l Protects fish and other species of concern by providing suitable 

screens on all pump intakes in areas containing aquatic organisms.

l Accomplishes dewatering in a controlled manner, slowly and in 

stages, allowing capture and transport of aquatic organisms out of the 

construction area.

l Accomplishes rewatering by releasing any large pools of water 

dammed during construction in a slow, controlled manner avoiding 

downstream water heating during rewatering. 

l Provides for fish passage around the construction site where 

necessary.

tSupplemental Specification 157 (example in appendix H) requires the 

contractor to take the measures necessary for dewatering and treating 

sediment to meet turbidity requirements. Figure 7.21 shows a generic 

dewatering plan demonstrating key components of a complete plan, 

including a stop-work requirement to permit relocating aquatic species 
7—44

Stream Simulation

before the dewatering takes place. An actual dewatering plan, however, is 

site-specific; details, configuration, and components of the plan will vary 

by site. Appendix G.4.1 includes more detailed information on elements of 

the bypass and dewatering system.

The length of time the bypass and dewatering system must be in place 

varies with each project. Small embedded pipes or precast structures 

may only require a site to be dewatered for a few days or less. Projects 

with cast-in-place concrete usually need at least 2 weeks. Sites requiring 

a bypass road may require continuous dewatering until the bypass road 

is removed. Complex projects may require more than one construction 

season, along with bypasses capable of handling high-flow events 

throughout the year. 

7.8.1 Bypass Dams

As long as the existing culvert is still in place, you can direct water

through it and use it for the bypass. Once the culvert is removed, however, 

you will need a bypass dam or convenient natural pool to gather water, 

direct it into a transport structure, and divert it around the project site. 

This bypass dam or pond location is important. By locating it close 

to the excavation, you create the best chance of capturing most of the 

water entering the construction site. Using a natural pool, when one is 

conveniently available, will reduce the height of the bypass dam. When 

doing extensive upstream channel work, use more than one bypass dam 

to capture the flow from springs and side drainages. Do not locate bypass 

dams on any stream features that control the channel gradient (e.g., steps, 

or pool tail-outs). Those features tend to allow more seepage beneath 

a dam built on top of them than other more well-graded and smoother 

channel areas. If constructing the dam in those locations is the only option, 

preserve stream stability by reconstructing those features as close as 

possible to the original features. 

Three different methods for diverting water are in common use:

l Pumping and transport hoses: A gas, diesel, or electric pump 

pumps from a stream pool or an excavated sump during the entire 

dewatering period, diverting the water around the site and back into 

the stream. Float switches control the pumps as water levels fluctuate 

to save energy and keep the pumps from running dry. Screens must 

be used to protect organisms (figure G.5) and must be maintained—if 

screens plug, pumps lose efficiency or can run dry. See the biologist 

on the project team for help in sizing this screen. 
7—45

Chapter 7—Final Design and Contract Preparation Figure 7.21—Dewatering system details—generic drawing.
7—46

Stream Simulation

Pumping systems that will reliably convey the bypass design flow 

can be complicated to design where water must be pumped up, or 

far away. You may want to contact the pump manufacturer to verify 

system design is adequate.

l Bypass dam and pipe: This method uses a single dam and bypass 

pipe to dewater the site. Construct the bypass dam from an 

impermeable membrane and a support structure. The dam can be 

made of excavated streambed materials, small or very large sandbags, 

waterbags, or other materials (section G.4.1.1). Since the bypass dam 

impounds water, it must be stable (e.g., if using streambed materials, 

you need minimum slopes of 1:1 upstream and 1:1.5 downstream). 

Place a membrane upstream of the dam, embedded 2 to 4 feet 

into the stream bottom and sides, to intercept subsurface flow and 

prevent seepage through bank materials when the dam pools water. 

If possible, construct the dam adjacent to a pool or excavation, where 

the membrane can line the entire dam and pool edge to the bottom 

to maximize capture of subsurface flow. Weigh down the membrane 

to keep it from floating. Cut a hole in the membrane smaller than 

the bypass pipe, stretching it around the pipe and binding it to the 

pipe to make an impermeable seal. The trench for the bypass pipe 

often collects some of the leakage from the bypass dam. If the water 

is clean, you can pump it upstream to eventually flow through the 

bypass pipe. If it is not clean, you can allow it to flow downstream 

to the sumps or to flow in an erosion-protected ditch alongside the 

bypass pipe, where it can be captured and treated. Leaves and woody 

debris can plug the diversion inlet and quickly cause overtopping 

of the diversion dam; consider placing a coarse mesh screen or 

fence upstream of the pipe inlet a few feet and tying it back into the 

diversion dam to catch debris before it can plug the inlet.

l Feeder dam, bypass dam, and pipe: This method uses an additional 

dam to pool and divert water with pumps during the construction 

of the main bypass dam. This method allows easier construction 

of the main dam and is more suitable in larger streambeds where 

dewatering is difficult due to subsurface flows and permeable bank 

materials. Any water that seeps by the feeder dam collects between 

the two dams and enters the annular area created by placing the 

smaller bypass pipe in the feeder dam into the larger bypass-dam 

pipe. In practice, the two-dam system will make the bypass much 

more efficient and reduce the amount of seepage that reaches the 

excavation (see figure 7.21). However, this system is more costly and 

is only necessary when subsurface flows make construction of the 

bypass dam difficult.
7—47

Chapter 7—Final Design and Contract Preparation

Creating a good seal of the bypass dam can be difficult. Expect about 

95-percent capture in a good system. If the amount of seepage is a 

problem, consider deepening or lengthening the membrane to decrease 

seepage.

7.8.2 Bypass Design 

Size the bypass pipe to carry the highest flow reasonably expected to 

occur during construction, including surface and subsurface flows. The 

project team should determine the design flow for the bypass system after 

assessing risks and consequences of exceeding the design flow. Note that 

some State permits set a minimum return frequency for the design storm 

for bypass systems. 

We recommend that a hydrologist estimate surface flow rates, and 

that either a hydrologist or a geologist help estimate subsurface flow 

volumes. (See appendix D for a brief discussion of methods for estimating 

streamflow.) Once you have estimated the design-flow volume for the 

bypass, design the pipe to carry the flow at an inlet depth of one pipe 

diameter or less. You can examine various pipe sizes and inlet-flow depths 

to find a pipe size and dam height capable of carrying the peak flow 

without overtopping the bypass dam or plugging the pipe with leaves or 

woody debris. To determine flow depth at the inlet and water velocity at 

the pipe gradient, use culvert-design charts or software such as FishXing 

or HY-8. (You can find FishXing and HY-8, as well as other useful 

hydraulic software downloads, at the Federal Highway Administration’s 

Hydraulic Engineering Web site: http://www.fhwa.dot.gov/engineering/

hydraulics/software.cfm.) Be sure that the bypass dam is at least as high 

as the calculated backwater at the pipe inlet, preferably higher by at least 

6 inches to 2 feet, depending on the stream size, slope, and risk. Costs 

for the pipe and bypass dam are significant. Evaluate various scenarios to 

determine the least expensive reliable combination.

The bypass pipe requires protection from the considerable thrust that 

occurs at elbows and bends (both horizontal and vertical.) Weigh down 

or bury bypass pipes at elbows, bends, and vertical curves to prevent the 

pipes from moving or coming apart at the couplings.

To prevent seepage into the excavation, the pipe should have sealed joints. 

Given specifications, manufacturers can provide a pipe with a reliable 

seal. The pipe usually goes in a trench adjacent to the excavation. Use 

the calculated pipe velocity to design appropriate outlet erosion-control 
7—48

Stream Simulation

measures or a suitable pool to dissipate energy and reduce damage to 

organisms that may be transported downstream through the bypass pipe 

(for gravity bypass systems).

On some relatively flat sites, you can divert water into a natural or 

constructed channel around the project. The channel can be a lined ditch, 

raised sandbag, or other type of channel structure. Design the channel to 

carry the high flow expected either during the construction season, or, for 

multiseason projects, the expected annual high flow.

Other bypass options that you can design or allow in the contract include:

(1) A constructed erosion-resistant transport ditch lined with rock or a 

membrane.

(2) An existing flood-plain channel.

(3) Isolated footing areas, with sandbags maintaining streamflow 

through the center of the project.

(4) Pumping or siphoning the water through hoses 100 percent of the 

dewatering time.

Of these four, either you or your hydrologist can design the first three or 

check them for capacity. For pumping and siphoning systems, because of 

the difficulty in estimating flows, your best bet is to estimate the needed 

capacity, then plan on adjusting the capacity in the field. 

7.8.3 Sump Design

Use sumps to collect ground water or seepage that escapes capture by 

the bypass dam (figure 7.21). Locate one or more at low points at the 

upstream and downstream ends of the excavation area. The upstream sump 

captures any ground water or seepage that gets past the bypass dam. If 

this water contains sediment, collect the water for further treatment before 

it reenters the stream channel (see figures 8.5 and 8.6). The downstream 

sump collects any sediment and drainage seeping though the area from any 

source and is the final insurance against sediment entering the stream. If a 

scour pool already exists at the culvert outlet, the downstream sump may 

not need to be excavated. If no scour pool exists, construct a waterproof 

downstream dam to create a sump below the excavation. 

To help determine the correct pump size for the estimated seepage 

into the sump, pump manufacturers provide pump-performance curves 

(volume versus head). Depending on the application, pumps range from 

relatively small electric sump pumps to large gasoline- or diesel-powered 
7—49

Chapter 7—Final Design and Contract Preparation

pumps. Automatic float switches for controlling the pumps are available 

(see figure 8.7). Electric sump pumps are lower in capacity than enginepowered trash pumps (see appendix G.4.1.2). 

One way to estimate seepage rates to determine pump capacity needed is 

to do a pump test near the channel. The pump test is normally done during 

a geotechnical investigation. It consists of determining how long it takes 

for seepage to refill a pit of known volume that has been pumped dry.

Estimate the sump collection areas and draw them on the site plan. 

Because seepage volumes and pumping requirements are only estimates, 

the design should be conservative. The sump must be large enough to 

capture all seepage and deep enough so the pump always has enough head 

to work properly. The contract can also state a requirement that “all sump 

water must be captured and treated before being released back into the live 

stream.”

The upstream sump may contain clean water that can be pumped directly 

back into the stream. If the water does not need treatment, pumping it 

either into the live-stream channel above the bypass dam or directly into 

the bypass system to avoid unnecessary treatment is often a convenient 

tactic. The downstream sump is the main collection point for sedimentladen water from excavation and other site disturbances, and it will always 

require treatment. 

7.8.4. Sediment Treatment Methods

Using soil information and/or onsite drilling records, you can predict the 

type of sediment likely to be trapped in the sump. Due to the presence of 

suspended silt and clay, all projects will generate some turbidity. While 

sand-sized sediments settle quickly, silt and clay take much longer to 

settle; this water must be treated before being released into the stream 

channel.

A common and often suitable method of treating sediment-laden water 

is by natural filtration through soil and vegetation adjacent to the stream. 

Forest soils with thick layers of organic material, dense ground covers, 

and soils with at least moderate permeabilities at least 100 feet from a 

streambed can provide good filtering media for sediments (figure 8.8). 

You can use a perforated-pipe drainfield, or even irrigation sprinklers to 

disperse water over a broad area. Be aware that highly permeable riparian 

areas close to the stream may be ineffective for filtration. 
7—50

Stream Simulation

The project team may have located suitable filtration areas during the site 

assessment. If none are in the immediate vicinity, you can transport water 

further away in roadside ditches, swales, excavated ditches, or piping 

systems to more suitable treatment areas. 

A variety of alternative sediment-treatment methods exist (also see 

appendix G.4.1.3):

l Use a subsurface drain in low-permeability material. Construct it 

by excavating a hole and filling it with drain rock to increase the 

absorption area and head.

l Pump sediment into small constructed pools to remove coarse 

sediment before treating for silt and clay. The ground disturbance 

associated with large settling ponds may be excessive on most sites.

l In treatment pools, ponds, or containers, include chemical polymers 

or natural-based flocculants such as: 

s Polyacrylamide (PAM), such as Chemco 9107GD and 9836A 

(Tobiason et al. 2001). 

s Chitosan-based water clarifier, such as Storm-Klear Liqui-Floc 

(For more information on polymer use for water treatment, see 

“Conclusions” in the following article: http://www.forester.net/

ec_0101_polymer.html.)

l Filter sump water, using sediment-filter bags similar to those 

from JMD Company (see http://www.jmdcompany.com/EnviroProtection_bag.cfm ).

Figure 7.22—Typical silt-fence installation.
7—51

Chapter 7—Final Design and Contract Preparation

Silt fences are typically capable of trapping only small quantities of liquid, 

sand, and coarse silts, down to about 125 microns. They effectively can 

control overland sediment transport, but are not useful in deeper water, 

which overtops the silt fence as it becomes plugged with sediment. Include 

requirements to maintain silt fences when they are used; once the silt fence 

is filled, it is useless until maintained.

7.8.5 Backwatered Sites

Where the stream is not entrenched and is relatively flat, the excavation 

may be backwatered easily. Any excavation done in a backwatered site 

will produce a large volume of dirty water that may require extensive, 

high-volume treatment methods. Study the long profile to determine 

the backwatering potential and need for a downstream dam (in addition 

to the upstream bypass dam). Backwater dams are similar to bypass 

dams and use the same construction methods. If the backwater is deep, 

hydrostatic forces on the dam can be substantial, and the dam may require 

an engineering design. If little water is present, straw bales and plastic 

sheeting may be all you need for a backwater dam. Another possible 

solution when there is sufficient grade is lengthening the bypass pipe and 

outletting water further from the excavation.

Some backwatered sites, especially those adjacent to pools or reservoirs, 

cannot be dewatered effectively. In those cases, consider different structure 

types and construction methods that will reduce water quality impacts. 

For instance, a precast structure may be better suited to this kind of site 

than a cast-in-place structure. Bridges with driven-pile foundations or 

spread-footings near the ground surface will cause little impact to the 

site. Embedded pipes that can be placed quickly may also be suitable, 

especially if they do not require significant excavation because they are 

located in a backwatered “pool” location. 

7.8.6 Deep Fills

At crossings with deep fills, carefully consider where to locate the bypass 

pipe to minimize the amount of excavation required for its placement. An 

open-bottom arch may be more desirable at this kind of site, because the 

existing pipe can be left in place to act as the dewatering pipe while the 

arch is constructed around it. Using an open-bottom arch may require a 

wider structure than selected in chapter 6. You will need to use sandbags or 

other damming materials to direct the water into the culvert while keeping 

it out of the excavation. When the existing pipe must finally be removed, 

you will need to either pump the water or route it through a bypass pipe

while the streambed is prepared. 
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Stream Simulation

If constructing an embedded pipe, consider construction methods that 

require the least time, because you will have to divert the stream during 

the entire construction. To avoid future leaks in the fill, remove the bypass 

pipe as the embankment is constructed.

7.8.7 Large Streams 

Large streams may require the full suite of dewatering techniques 

described so far. The key to determining when to cut back or increase 

dewatering details is to evaluate the risks of failure. For example, 

when stream sediments contain large quantities of fines, more stringent 

measures to recover the fine material may be required to meet turbidity 

requirements. Although collecting all the water on a project before 

it reaches the excavation is often difficult, providing a conservative 

sediment-control system is better than causing stream turbidity problems, 

especially in sensitive habitat.

7.8.8 Small Streams 

Although the dewatering system does not have to be elaborate, it does 

need to work effectively. The failure of a dewatering system on a small 

stream can sometimes cause just as much damage as a failure on a larger 

project. 

7.8.9 Bedrock Channels 

Sediment control is relatively easy in bedrock channels. The key is 

to create a well-sealed dewatering dam at the upstream end. Once the 

bedrock is cleaned off and dried, little sediment will be generated. 

Nonetheless, expect seepage from banks and through the dewatering dam. 

Because the water that has seeped in will almost never be clean, especially 

during excavation, construct a downstream sediment trap.

7.8.10 Field Modifications 

Because streamflow and seepage volumes are hard to predict and can 

be highly variable, expect some modification of the dewatering plan in 

the field by contract administrators working in conjunction with you, 

the project team, and the contractor. Some modifications may also be 

necessary for optimizing the system for site conditions that become 

evident only during excavation. 
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Chapter 7—Final Design and Contract Preparation

7.8.11 Pollution Control 

Use special contract requirements, Federal Acquisition Regulations (FAR) 

Section H, to include pollution controls on a project. (See section 7.9.) 

Typically, pollution controls include:

l Equipment washing—to prevent bringing in invasive plant species or 

petroleum-product pollution.

l Equipment repair—to prevent hydraulic leaks before beginning work.

l Petroleum-absorbing “diapers”—to be on hand and close by.

l Specially constructed fueling areas to contain spills.

l Limitations on camping and control of garbage and litter. 

l Onsite toilets.

For jobs involving placing concrete in forms, locate suitable waste areas 

for dumping bad concrete and for washing mixers before concrete work 

begins. Never allow concrete washwater and fresh concrete to enter live 

streams, because the cement in the concrete is deleterious (due to the lye 

content) to all aquatic species.

Controlling invasive species and disease is a very important part of 

pollution control. Invasive plants may be accidentally imported into the 

project area from remote sources of soil, rock, plant, and seed materials. 

Ensure that the erosion- and pollution-control plan includes provisions 

against contaminating the project with invasive species (either plants or 

animals). Provide for washing equipment before bringing it to the project 

and when using vehicles to haul materials to or from contaminated areas. 

In addition, to ensure that soil and aggregate sources do not contain 

invasive plant species, provide for surveying the aggregate sources before 

using them. Do not use any aggregate source that has invasive plants.

7.9 Special Contract Requirements

Special contract requirements or “H-clauses” modify the main contract 

clauses or FAR. Following is a summary of the content of H-clauses 

typically used with aquatic organism passage contracts (see appendix 

H). These clauses often cover items also specified on the drawings, 

specifications, and supplemental specifications. Note: In this section, 

clauses are numbered as a typical contract for reference between chapter 7, 

chapter 8, and appendix H. Some of these clauses may or may not apply to 

your contract and thus your numbering may be different.
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Stream Simulation

Clauses related to species protection

l H.1—Seasonal Restrictions: H.1 specifies the overall dates for the 

work period, site disturbance, and in-water work. If extensions for 

site disturbance and in-water work periods are necessary, contact the 

project team biologist. 

l H.13—Protection of Habitat of Endangered, Threatened, and 

Sensitive Species: H.13 specifies measures to protect plants 

or animals listed as threatened or endangered. If measures are 

inadequate or new species are found, the Government may 

unilaterally modify or cancel the contract. Discovery of threatened, 

endangered, or sensitive species requires notifying the contracting 

officer. Site dewatering methods fall under this clause.

Clauses related to water quality

l H.3—Landscape Preservation: H.3 replaces FAR clause 52.236, 

Control of Erosion, Sedimentation, and Pollution, and specifies 

requirements for:

s Protecting vegetation outside clearing limits.

s Preventing fuel and oil pollution.

s Preventing or removing objectionable materials deposited in water 

bodies.

s Specifying erosion- and pollution-control measures that must be 

available onsite.

s Specifying turbidity limits and monitoring frequency.

s Submitting contractor’s plans and obtaining approval—before 

construction—for the following work items (all which have the 

potential for causing sedimentation and pollution of the stream 

and work area):

n Clearing and grubbing.

n Removing existing pipe.

n Dewatering and water treatment.

n Erosion control. 

n Excavating.

n Placing channel rock, streambed simulation rock, and select 

borrow.

n Placing structural concrete.
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Chapter 7—Final Design and Contract Preparation

l H.4—Moisture Sensitive Soils: H.4 requires the contractor to design 

bypass and temporary roads to support highway-legal loads during 

construction. It also requires the contractor to repair any damage 

associated with unsuitable material (such as saturated backfill), that 

would result in silt deposits in streams.

l H.16—Final Cleanup: H.16 requires removing trash and unused 

material, and requires sweeping and washing the road surface to 

remove sediment.

Clauses related to pollution control:

l H.14—Sanitation and Servicing Requirements: H.14 requires 

approval for camping, as well as the placing of oil-absorbing mats 

under stationary landing equipment and during equipment servicing.

Clauses related to structure or material changes:

l H.5—Value Engineering (VE): H.5 requires that the project team 

review VE proposals and it limits the use of VE proposals that change 

the functional service of a facility. (Typically, a change in structure 

type will not be suitable unless it is an upgrade, such as a sufficiently 

wide and durable bridge for a culvert structure.)

l H.6—Product Substitution: H.6 requires that the substitution meet the 

“or equal” clause in all respects, along with written documentation 

and testing information verifying that the substituted material meets 

specification requirements. The contractor is responsible for any 

other modification that the substitution causes. The project team must 

review any substitution of materials.

l H.10—Control of Material: H.10 specifies the type of excavation 

expected on the project, along with earthwork tolerances. It requires 

testing and written documentation of onsite materials to meet 

project specifications. (Although stream-simulation material is not 

earthwork, that material still must be placed accurately.) H.10 also 

specifies requirements for treating borrow, storage, stockpile, and 

disposal areas.
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Stream Simulation

Clauses related to traffic:

l H.7—Road Use and Maintenance: H.7 specifies requirements for 

road closures, traffic controls, and permits. Traffic-control plans are 

often subject to change after contract award. Contact the project team 

if a proposed changed would affect either the project timeline or any 

physical site detail.

l H.9—Prosecution of Work: H.9 specifies requirements for providing 

for public safety throughout the construction (including traffic 

controls), and notifying the public when the construction work, e.g., 

road closures or blasting, will affect the public.

l H.11—State Permits: H.11 requires the contractor to obtain and 

follow State permits. 

l H.17—Protection of Improvements: H.17 requires the contractor to 

protect improvements at the site throughout the construction. The 

contractor must replace signs, and other site features disturbed by 

construction, unless the contract specifically says otherwise.

Clauses related to safety:

l H.15—Potential Safety Hazards: H.15 requires the contractor to 

provide safe working conditions. Occupational Safety and Health 

Administration (OSHA) regulations apply for working in excavations 

and for working in confined areas. (For example, using power 

equipment to place stream materials inside a culvert is covered by 

OSHA clauses covering working in trenches, working in the vicinity 

of operating equipment, and working in the vicinity of excavated 

slopes.)

Miscellaneous clauses:

l H.2—Physical Data (FAR 52.236-4): H.2 states that physical 

conditions indicated on the drawings and in the specifications are 

the result of site investigations by the Government and that the 

Government is not responsible for the contractor’s use of the site. H.2 

also describes the normal fire season. (Many forests and regions have 

a fire plan describing the contractor’s fire-related responsibilities, 

including types of equipment that must be kept onsite, hours that may 

be worked during high fire danger, people to contact in case of fire, 

preventive measures, and fire weather updates.)

l H.8—Construction Stakes, Lines, and Grades: H.8 specifies 

requirements for contractor surveys and for protecting survey control 

points.

l H.12—Protection of Cultural Resources: H.12 requires protecting 

and reporting any cultural resources discovered during the project 

(stream settings are often cultural-resource sites). The Government 

may unilaterally modify or cancel the contract under this clause.
8Stream-Simulation 

Construction

8.1 Brief Introduction to Stream-Simulation 

Construction

8.2 Stream-Simulation Construction Topics

8.3 Post Constuction
2

Stream Simulation Stream Simulation

Let contract

Inspect and control all aspects of crossing construction

Document the installation as-built

Figure 8.1—Major steps in the stream-simulation construction.

RESULTS

The completed stream simulation crossing

Baseline for monitoring

Major Steps in the Stream-simulation Construction
Chapter 8—Stream-Simulation Construction

This chapter focuses on contract administration, addressing itself primarily 

to the contracting officer (CO), the contracting officer’s representative 

(COR), and the inspectors. The chapter is not intended to stand alone. It 

builds on material presented earlier in this guide, because it is essential 

that people involved in construction have a good understanding of project 

design elements and objectives. Ideally, the COR who takes primary 

responsibility at this stage was also involved in the design phase, at least in 

a consulting role, and is already familiar with the design of the project. 

Figure 8.2—Open-bottom arch stream-simulation culvert on Wilson Creek, Boise 

National Forest, Idaho.

This chapter describes how to administer construction of the complete 

project, paying particular attention to unique and special emphasis 

elements that make stream-simulation projects different. Although 

many aspects of these projects are identical to other road-construction 

and stream-crossing projects, stream-simulation projects are often large 

structures, and they require streambed construction (figure 8.2). These 

considerations, along with more rigid survey and construction tolerances, 

add significantly to the complexity of construction. Proper attention 

to detail, continual indepth assessment of site details as the project 

progresses, good communication, and careful, informed decisionmaking 

are equally important for the construction phase.
8—2

Stream Simulation

This chapter emphasizes factors critical to the performance of streamsimulation structures, including:

l Construction survey.

l Structure grade control.

l Structure alignment.

l Structure foundation and backfilling.

l Stream-simulation bed construction. 

8.1 Brief Introduction to Stream-simulation 

Construction

The following subsections highlight areas where contract administration 

for stream-simulation projects differs somewhat from traditional streamcrossing projects. None of this is intended to replace policy and direction 

in the Forest Service Manual and Handbook, it merely emphasizes topics 

that are either unique to stream simulation, or that sometimes cause 

problems in construction. 

For policy and direction for contract administration on Forest Service 

contracts, go to Forest Service Handbook (FSH) 6309.11 Contract 

Administration. The Road Construction Handbook is FSH 7709.57, 

and FSH 7709.56b contains guidance for Transportation Structures 

including major and minor culverts. For policy and direction for the 

Forest Service Engineering Construction Certification Program, go to FSH 

7109.17 Engineering Certification. For the entire self-study Engineering 

Construction Certification program, go to the Engineering Manual 7115 

series of manuals. You can download these documents from the Forest 

Service internal Web site. (Other public lands agencies may have similar 

programs and policy.)

8.1.1 Roles

For a stream-simulation project, the project team includes individuals not 

always found on construction projects. The experience of hydrologists, 

geomorphologists, and fisheries biologists is essential to the success of 

aquatic organism passage projects. In some cases, these specialists may be 

participating in the engineering project development process for the first 

time. The key personnel working on most stream-simulation projects are 

listed below. Depending on the complexity of the project and the availability 

of personnel, some of those listed may or may not be members of a specific 

project team.
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Chapter 8—Stream-Simulation Construction

l Land manager (district ranger or forest supervisor).

l Contracting officer (CO).

l Contracting officer’s representative (COR).

l Inspector(s).

l Project team— subject matter specialists.

s Designer.

s Geotechnical or structural engineer.

s Hydrologist/geomorphologist.

s Fisheries biologist.

l Contractor.

8.1.2 Communications

Because good communication is critical for construction projects, we 

reemphasize it here. All communications must be complete, accurate, 

and honest. During the final design process, the design engineer must 

determine the needs of the land manager, the project team, and the 

permitting agencies, and convert those requirements into accurate 

drawings, specifications, and other contract requirements. Otherwise, 

intelligent communication with the contractor becomes impossible. The 

contractor, in turn, must construct the project precisely to the contract 

drawings and specifications. Many of the concepts involved in streamsimulation projects may be new to many contractors; for example, the tight 

tolerance for elevation control is not widely known in the low-volume road 

construction industry, and in many cases, the work requires hand labor. 

The entire process, therefore, requires a great deal of time, commitment, 

and communication among all the members of the project team including 

contractors and contract administrators. 

Contract administrators may not completely understand all the 

performance details of the structure or its stream-simulation design 

features. Therefore, when unexpected problems arise on a project, contract 

administrators should immediately contact the designer for input leading to 

solutions that preserve design performance.

Similarly, the project team and design engineer should involve contract 

administrators early in the planning process. The separation in many 

organizations between planning/design and contract administration 

imposes artificial barriers and decreases the level of communication. 
8—4

Stream Simulation

Both the COR and the inspector can provide good information on project 

constructability, and valuable advice on the types of hazards that exist 

at particular construction sites. Their knowledge of the capabilities and 

limitations of various types of equipment is extremely valuable to project 

teams choosing site location or considering construction access. CORs and 

inspectors may be able to solve problems that are difficult to solve without 

firsthand field experience; they may be able to offer solutions that are 

simpler and less costly, while providing the same level of effectiveness. 

Therefore, contract administration personnel should be involved early in 

the project-development process.

8.1.3 Contact Administration Meetings

This section discusses meetings that are particularly important to streamsimulation projects. For successful stream-simulation projects, formal 

contract administration meetings include the following:

8.1.3.1 Prebid tour

This meeting is particularly important for stream-simulation projects, 

which may be unfamiliar to some potential bidders. The prebid tour is 

an onsite meeting during solicitation, allowing the offering agency and 

prospective bidders to view the project together to clarify the project 

drawings, specifications, and contract requirements before bidding. 

Generally attending are the CO, designers, COR, inspectors, and 

prospective bidders. Having the project team at this meeting is often 

useful, as they can explain the rationale behind any special construction 

features in the design. If stream simulation is relatively new in an area, 

it may be desirable to begin with an office slide show illustrating the 

different aspects of an installation.

Often, during the prebid tour and other reviews, questions will arise 

regarding the project that cannot be answered through the solicitation. 

In this case, the CO will issue an amendment to the solicitation. The 

amendment provides the necessary clarification and provides identical 

information to all bidders to ensure fair and equal competition.
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Chapter 8—Stream-Simulation Construction

8.1.3.2 Prework meeting

This is the first meeting after the contract has been awarded. It is generally 

an office meeting for reviewing the contract, including contract clauses, 

special contract requirements, drawings, specifications, and any final 

clarifications with the successful bidder. This meeting gives everyone 

a chance to discuss and reinforce any special or unusual contract 

requirements, such as permit requirements or special construction 

requirements.

Generally attending are the CO, COR, inspector(s), a district or program 

representative, the design engineer, and the contractor. Having the project 

team present is useful, as they can explain why special construction 

requirements for stream-simulation construction are important to the 

success of the project. The CO or the COR should brief the team members 

beforehand on contract authority, to avoid potentially embarrassing 

breaches during the meeting (project team members do not ordinarily have 

contract authority). The Notice to Proceed is usually issued at this meeting.

8.1.3.3 Prework field meeting

This is the first field meeting between the contract administration 

personnel and the contractor. Additional attendees might include a district 

representative and possibly a representative from the permitting agency. 

On complex projects, the designer should be present, to provide any 

necessary clarification of the drawings and specifications. Again, members 

of the project team should be present to explain the importance of special 

construction requirements unique to stream-simulation construction. The 

meeting will cover the overall project, with an emphasis on such initial 

items as surveying, clearing, dewatering, traffic bypass (if appropriate), 

project limits, temporary erosion control, storage and stockpile areas, 

camping, and general land use, as well as any permit requirements. At this 

meeting, contract administration personnel should establish day-to-day 

working relationships, communications channels, and ground rules.
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Stream Simulation

8.1.3.4 Final inspection/post-construction meeting

The final formal onsite construction meeting reveals to all team members 

the results of their collective efforts in designing and constructing a 

stream-simulation project. At this point, work on the project is essentially 

complete, and the project is ready for use, except for a few small “clean 

up” items. The project is inspected in its entirety, and the final punch list 

of items needing completion is finalized. Ideally, all project team members 

and contract administrators should participate in this meeting, and all 

parties should express, objectively, what went right or wrong with the 

project. Lessons learned should be well documented with suggestions for 

future projects. When the punch-list items are completed, the project is 

formally accepted, and final payment is processed.

8.1.4 Construction and Inspection 

In many respects, stream-simulation projects are no different than other 

stream-crossing projects. However, some features make stream-simulation 

projects different and more complex. The way the streambed is treated 

inside the structure as well as up and downstream of the crossing generally 

differs from past construction of stream crossings. The structure infill 

is the most unique feature of any stream-simulation project; its proper 

construction is vital to its performance. Upstream and downstream controls 

also play critical roles in the way that the infill of the structure performs. 

All of these features need your extra attention, because of the strict 

tolerances required for proper performance and because of the relative 

newness of these features to the construction industry. In addition, much of 

this work must be performed by hand, particularly in small structures.

8.1.5 Construction BMPs

BMPs for construction are in section 7.1, in the Construction BMP

checklist. All of the items in the BMP list are discussed in detail in either 

this chapter, chapter 7, or appendix G. The list provides an excellent 

“watch list” for protecting the construction site, the stream and aquatic 

organisms during construction, as well as for proper construction of the 

stream-simulation channel inside the culvert.
8—7

Chapter 8—Stream-Simulation Construction

8.1.6 Construction Survey and Tolerances

Surveying for a stream-simulation project requires more time and attention 

than normal (section 8.2.2). For stream-simulation projects, both the 

construction survey and the original site survey require great accuracy and 

attention to detail. Construction tolerances on stream-simulation projects 

are likewise critical. Small changes in gradient, location, or bed material 

can profoundly affect the structure’s performance. 

For example, if the gradient in the structure is steeper than designed, the 

resulting increase in stream velocity can cause the infill to wash out of the 

structure. If the gradient is shallower than designed, the resulting decrease 

in stream velocity can cause the stream to deposit material in the structure. 

In either case, the structure will not match the stream long profile and 

may cause an aquatic passage barrier to form at the inlet or outlet. If the 

structure is placed with a change in alignment, similar consequences could 

occur, with poor inlet or outlet performance or unanticipated bank erosion. 

If the bed material does not include enough fine material, the infill may 

be permeable enough to allow the stream to travel below the surface. If 

this happens, low flow may not be deep enough to provide for aquatic 

organism passage, or the channel may become completely dry.

8.1.7 Permits and Permit Requirements

Generally, stream-simulation projects are constructed under permit from 

State and Federal agencies. Permits may include strict requirements 

on protection of aquatic species, levels of suspended sediment, and 

construction pollutants. Often a seasonal restriction on the construction 

timeframe (often called the “construction window”) defines when 

construction can actually occur on the site during a normal year. Along 

with these restrictions, permits will include requirements for site closure, 

including seasonal closures (if the construction will take more than one 

season to complete).

8.1.8 Contract Modifications/Design Changes

Given so many variables, projects seldom flow from beginning to end 

without a contract modification (i.e., a change order). With such complex 

projects, anticipating every site problem during the final design is difficult, 

if not impossible. In addition, once onsite, the contractor, inspector, or 

COR may find a simpler, more effective, or more economical way of 

accomplishing the intent of the design and may initiate a proposal for a 

modification. (In this document, the terms “change order” and “contract 

modification” are interchangeable.)
8—8

Stream Simulation

Whatever the need is for the modification, the process for solving the 

problem is the same. Whoever identifies a problem proposes a solution 

(along with team help, if necessary), and the COR estimates the cost. 

Depending on the complexity of the problem, all members of the project 

team may need to review the proposed solution. The contractor’s input 

to the solution of any design change can be invaluable, particularly in 

the area of construction methods and constructability. The designer will 

provide the engineering solution, with input from contract administration 

personnel and the remainder of the project team and the contractor. 

8.1.9 As-built Drawings and Final Construction Report

As-built drawings begin with the contract drawings for the project. All 

change orders, including minor deviations, are clearly marked and the 

drawings modified, so that they accurately depict the structure as it was 

finally constructed. While time-consuming, this process is key to the 

success of future projects. Being able to study the current generation 

of projects—through the “as-built” drawings and the final construction 

report—gives future designers a better understanding of similar projects.

For Forest Service projects, FSM 7721.36 requires a final construction 

report. Specific requirements for the document are included in FSH 

7709.57, Chapter 7. Contract-administration personnel prepare the final 

construction report. Its purpose is to provide background for future similar 

projects, and it should thoroughly and objectively document “lessons 

learned” (both good and bad). 

8.2 Stream-Simulation Construction Topics

The remainder of this chapter emphasizes areas of work that require 

special attention in stream simulation. It provides lists of items to be 

routinely checked and lists of common problems, with possible solutions 

and helpful hints. This section will follow the work progress of a typical 

project, beginning with Section H “Special Contract Requirements” and 

ending with the final cleanup and post-construction monitoring.

The first item on the construction project is planning. The contractor is 

required to submit a project schedule. The purpose of the schedule is not 

only to track the contractor’s work progress, but also to give you—the 

COR—a useful tool for work planning. When the contractor submits the 
8—9

Chapter 8—Stream-Simulation Construction

schedule, be sure that it provides realistic information that will benefit the 

contract-administration process. Obviously, the contractor must update 

the schedule periodically to reflect current progress of the project (see 

figure G-1 for a sample project schedule.) A project schedule can take 

many forms, ranging from showing proposed work progress on a calendar 

to using detailed Gantt or Critical Path Charts. At the very least, the 

schedule should identify project start and completion dates and proposed 

timeframes for important work items such as:

l Construction survey.

l Mobilization.

l Stream diversion and dewatering.

l Aquatic organism capture and transport (timeframe for others to 

perform work).

l Existing structure removal when applicable.

l Clearing.

l Structure excavation.

l Structure installation.

l Structure backfill.

l Road reconstruction.

l Site cleanup and demobilization.

l Seasonal site closure for projects spanning more than one 

construction season.

8.2.1 Safety 

Inspect the contractor’s operations to ensure that all work is accomplished 

safely. (An inspector who witnesses unsafe acts that lead to an accident 

and does not intervene can be held personally liable.) Most safety 

issues are standard ones for a variety of construction projects, including 

operations for excavation, confined spaces, concrete placement, heavy 

lifting, underground utilities, power equipment, potential fire hazards, and 

machinery. Be familiar with FSH 7709.57 section 2.5 and Occupational 

Safety and Health Administration (OSHA) regulations that apply to 

the particular project. Safety is a personal responsibility, as well as the 

contractor’s responsibility. (For OSHA regulations, go to http://www.osha.

gov/dts/osta/otm/otm_v/otm_v_2.html.)
8—10

Stream Simulation

Follow OSHA regulations when working with power equipment in a 

confined space. When installing streambed material inside the culvert with 

power equipment, or working in the narrow spaces between footings and 

excavated slopes, consider:

l Replacing some power equipment work with hand labor.

l Providing large fans to exhaust air from confined spaces.

l Other placement methods.

In addition to OSHA regulations governing construction, you should 

emphasize safety for both contract administration personnel and anyone 

visiting the job site. (See Forest Service Health and Safety Code 

Handbook, FSH 6709.11.) 

Perform a job hazard analysis. Identify job tasks, their known hazards, and 

abatement actions for each hazard. For each project, fill out a FS-6700-7 

job hazard analysis (JHA), and file it in the project folder. Review the JHA 

before going to project sites. 

8.2.2 Construction Survey 

This phase of the project requires more attention to detail than ordinary 

stream-crossing projects. The COR, the inspector, the project team, and the 

contractor should visit the site together to create a thorough understanding 

of the site, design objectives, and details. Visually examine the site to 

make sure that it looks like the drawings. Spot check elevations to find 

obvious discrepancies in the survey or design. Because streams change 

occasionally between site surveys and construction, design changes may 

be necessary. Always contact the project team with any questions about 

the location of project features when the drawings differ from actual site 

conditions. 

Protect control points that were established during original topographic 

survey. They are important references that are necessary to establish 

construction stakes for the project, for monitoring the construction, 

for developing as-built drawings, and for monitoring the project in the 

future. Control points may consist of a reference point on any permanent 

structure. One simple control point is a 24- to 48-inch reinforcing bar, 

driven into soil by hand a safe distance away from the maintained roadway 

and stream in a stable location. Locate offset reference stakes in the area of 

the culvert inlet and/or outlet in locations where they will not be disturbed 

by construction or potential stormflows but where they can be easily 

checked during the project.
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Chapter 8—Stream-Simulation Construction

An error in placing the foundation or bedding can badly affect the outcome 

of the entire project (figure 8.3), and may result in the eventual failure of 

the stream-simulation bed or the culvert structure. 

Figure 8.3—Results of a survey error. Footings were constructed 2 feet higher 

than the designed location because of a construction survey error. Always 

doublecheck surveys!

In figure 8.3, the footings were constructed approximately 2 feet higher 

than designed, due to a contractor survey error that the inspector did not 

catch. The stream-simulation bed still had to be constructed to match 

the stream profile, so that the footings were not embedded as deeply as 

designed. 

This error resulted in:

l An increased risk of the foundation being undermined by scour.

l Less fill (on the inside of the footings) for resisting the overturning 

forces (on the outside) thereby reducing the safety factor for 

overturning and bearing capacity.

l Insufficient cover height over the pipe, which was designed for 

minimum cover. To compensate, the contractor had to raise road 

grade 2 feet, creating an obvious hump in the road profile and 

limiting some truck traffic.
8—12

Stream Simulation

Inspectors should verify contractor surveys by checking—and 

rechecking—the work. Mistakes in the survey and construction staking 

will affect the entire project. Your job is to insist on accuracy at all stages.

Survey Inspection Checklist

Take care of the following items before beginning any work on the 

site: 

4 Verify control points, which are typically established during the 

original topographic survey (section 5.1.2).

4 Reestablish any missing control points and, if necessary, 

establish additional control points to aid construction of the 

stream channel and any channel restoration work.

4 Resolve discrepancies with the surveyor and design engineer 

before construction begins.

4 Clearly mark all clearing and construction limits, especially near 

the stream channel, with stakes and flagging.

4 Clearly mark stockpile-storage areas, waste areas, and borrowsource areas with stakes or flagging.

4 Review and establish any proposed construction access with the 

contractor. 

4 Document agreements on a work order.

4 Ensure accurate placement of slope stakes and references 

for the road travel way, embankment limits, and all excavation 

slopes. 

4 Check that erosion and sediment control and dewatering/

sediment removal features are properly located.

4 Check construction stakes to ensure accurate location of the 

structure, especially the elevation, position, grade, alignment, 

excavation slopes, and width.
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Chapter 8—Stream-Simulation Construction

8.2.3 Special Contract Requirements (H Clauses)

Review the contract for any special contract requirements in section H or 

in other sections of the contract. Review Section H requirements, Standard 

Specifications and Supplemental Specifications, not only for special 

requirements but also for any potential conflicts between them. If conflicts 

exist, notify the CO and design engineer to resolve them. See appendix H 

for sample H clauses, and section 7.9 for a summary of the clauses.

8.2.4 Signs and Traffic Control Plans

The contract may require a traffic-control plan. Options usually include: a 

traffic detour onto other roads, a traffic bypass over the site or adjacent to 

it, or a traffic barrier.

If a change in public traffic access is necessary, be aware that it can 

affect other aspects of the project. For example, a change may require 

constructing a temporary road, lengthening the culvert, or moving the 

dewatering dam and bypass pipe. It may cause additional resource damage, 

and in some cases, may even increase the cost enough to make a bridge 

more practical and economical. 

8.2.5 Erosion, Sediment, and Pollution Control

Maintaining water quality throughout the project (by preventing erosion, 

and preventing sediment and pollutants from entering streams) should 

be a major focus for contract administration. Most sediment is generated 

during embankment and foundation excavation, and by erosion of freshly 

disturbed slopes, constructed embankments, stockpiles, and road surfaces. 

(See section 8.2.6 for dewatering. Check section H, Special Contract 

Requirements, for turbidity requirements.) 

Erosion control means that the soil remains in place, either undisturbed 

or protected with a protective covering such as mulch, rock, or a 

membrane. 

Sediment control means that soil already eroded is captured and 

prevented from harming the stream or sensitive riparian areas. 

Sediment control includes dewatering the site to prevent sediment 

transport downstream and capturing the sediment with sedimenttrapping mats, dams, or silt fences. See appendix G figures G.8 and 

9 for example drawings of temporary erosion and sediment control 

features.
8—14

Stream Simulation

8.2.5.1 Reviewing erosion- and sediment-control plans

If the guidelines in chapter 7 were followed, the contract does not specify 

specific methods of erosion and sediment control. Instead, it makes the 

contractor responsible for the end result of meeting the requirements 

spelled out in the supplemental specifications and section H. For ease 

of contract administration, if any changes in the contractor’s plan are 

required, write them in terms of “end results” or performance rather than 

specifying specific methods. 

The contract may have specific sections relating to erosion and pollution 

control, such as a Supplemental Specification 157, or a special contract 

requirement similar to H3 (appendix H). If not, make sure that the 

specifications and drawings adequately cover any erosion and sediment 

control requirements in the National Environmental Policy Act document 

(including BMPs) and in the water quality permit. 

Normally, the contractor is required to submit a plan for erosion, 

sediment, and pollution control (H.3, appendix H). The erosion-control 

plan can consist of a variety of erosion-prevention and sediment-trapping 

methods. These methods and details can be obtained from a variety of 

sources, including instream work permit requirements (BMPs), standard 

engineering practices, and standard OSHA requirements for excavations 

and preventing slope failures.

Review the erosion and sediment plan with the project team and ensure it 

protects the site as required in the contract. Much more detail on erosion 

and sediment control is in sections 7.7 and 7.8, but some common methods 

are: 

l Erosion control

s Minimize cleared and disturbed area.

s Dewater the construction area (section 8.2.7).

s Use slope treatments, such as seed, mulch, erosion-control fabrics, 

geotextile fabrics, and membrane, during and after construction.

s Maintain temporary erosion and sediment control measures.

s Provide armor and/or ditch dam energy dissipators for newly 

constructed or maintained road drainage ditches.
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Chapter 8—Stream-Simulation Construction

s Control erosion at lead-out ditches and pipe outlets.

s Prevent road surface runoff from entering the excavation by using 

surface dips, berms, or outsloping the road.

s Maintain road surfaces and drainage systems during the contract.

s For disturbed areas, stockpile sites, and waste areas, scatter 

clearing slash and debris, seed and mulch, and slope to drain. 

(Long slopes over about 50 feet are broken by water collection 

ditches or berms to dissipate energy and control runoff.) 

s Cover temporary stockpiles with an impermeable membrane to 

prevent erosion and control moisture.

l Sediment control 

s Place silt fences, straw bales, or other sediment trapping systems 

at the bottom of excavated slopes, and temporary drainages.

s Use sumps for collecting sediment-laden water upstream and 

downstream of construction, and in bypass pipe ditches.

s Locate the area for the water treatment/sediment removal system 

and verify the system will function as intended at that location.

s Use pumps (or gravity when possible) for transporting water to 

treatment areas.

s Monitor and maintain pumping equipment during dewatering 

operations.

s Install erosion controls in the treated water release area if not 

being released directly into the stream.

s Ensure prescribed method for removing sediment functions 

as intended. Identify alternative methods if more treatment is 

necessary.

s Wash paved road surfaces at the end of the project to prevent 

sediment from entering the stream, and to restore safe traffic 

conditions.

The Storm Action Plan should require monitoring and maintaining erosion 

and sediment control measures and immediate repair or replacement in the 

case of damage.
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Stream Simulation

Temporary Erosion and Sedimentation Control Inspection Checklist

For maximum effectiveness, make certain that appropriate erosion-control measures are in place 

at the beginning of site-disturbance activities. (Refer to sections 7.7 and 7.8.) Tips on handling 

common problems that arise with erosion and sediment controls are in appendix G.4.3.6. 

Inspect and monitor the following activities during each site visit throughout construction, and 

make changes as necessary to control sediment production. Verify that:

4 The dewatering system is installed according to the approved dewatering plan and is 

functioning properly.

4 Sediment collection systems are installed according to the approved erosion-control plan and 

are functioning properly.

4 Excavation and waste stockpiles are protected from rainfall and are located where they will 

not fail or erode directly into the stream. In locations where protection is not practical, be sure 

sediment-control measures are in effect.

4 Drainage from open excavations, fresh cut banks, and embankments is captured and treated.

4 Water treated and discharged back to the stream meets contract requirements. (Ground water 

intrusion will increase sediment production. Despite the best dewatering efforts, ground water 

seepage occurs at many sites. Drainage may come from many different places and increase 

substantially during storm events.)

4 The contractor takes measures to reduce sediment production, such as by minimizing bucket 

spill into construction area drainage and by avoiding slope failures in over-steepened excavations. 

(Because all water discharged back to the stream requires treatment to remove sediments, remind 

the contractor that it may be more cost effective to reduce sediment production during excavation 

in order to reduce the amount of water treatment necessary—especially in silt-clay rich soils.) 

4 No excavated slopes remain vulnerable to erosion or failure. (If left for long periods (days) these 

slopes may benefit from using a membrane cover. Assess the slope condition frequently. While 

membrane covers can be helpful, covering slopes will also slow drying the soil in embankments 

and in turn cause sloughing. Keep in mind that excavated slope erosion is often the largest source 

of sediments on the project.)

4 The contractor is careful when loading and hauling wet materials, especially near the site. Be 

sure that the contractor either avoids spilling excess soil onto the road during haul or provides a 

means of preventing this sediment from entering the stream. (Sediments around loading areas 

can be trapped with berms or sand bags. Existing roadside vegetation may provide sufficient 

trapping elsewhere.)

4 All sediment- and erosion-control methods function as intended. (Silt fences filling with soil 

and/or water will fail, either by structure failure or by overflowing, unless the soil is removed before 

the sediment traps fill.)

4 For multiple-year projects, the contract should contain adequate site protection provisions 

covering conditions peculiar to the site at the end of each work season. (If additional protection 

is necessary because of expected storm events, snowmelt, frost heave, ravel, wind, etc., the 

inspector and the project team should initiate a change order to encompass the necessary work. 

If the area has a history of vandalism, for example, vehicle barriers may be necessary to close the 

site to traffic, especially during hunting season or other high-use in recreation periods.)
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Chapter 8—Stream-Simulation Construction

8.2.5.2 Pollution control and prevention plans 

The primary sources of pollution on a project are vehicle fuels, hydraulic 

fluids, lubricants, invasive plants and animals carried in on equipment, and 

human waste. Special contract requirements specify methods, procedures, 

and rules to follow to reduce the risk of pollution. Depending on the 

contract, the contractor may be required to submit a pollution-control plan. 

Be proactive in ensuring that the contract meets water-quality and soilprotection goals. Items such as hydraulic oil or other fluid leaks can cause 

serious damage in a very short time. To meet project and environmental 

requirements, deal with spills immediately and firmly. Review plans and 

operations to ensure that construction activities comply at all times with 

specified pollution control objectives. Make sure that protections are in 

place or are ready to deploy immediately when necessary.

Special contract requirements generally include all or some of the 

following. See also sections 7.8.11 and 7.9, and H-clause 3 in appendix H. 

l Landscape Preservation and Hazardous Materials 

s Written approval required for operating equipment in live streams.

s Service equipment only in approved areas.

s Transport waste offsite. 

s Treatment for general construction debris. (Usually, the contract 

will require that construction debris be removed to an off-forest 

site according to local, State, and Federal regulations.)

l Hazardous Materials

sSpill plan submittal. 

n Specifies hazardous material cleanup kit.

n Specifies which materials must be on hand to contain spills. 

n Specifies that required spill containment devices, pads, and 

booms are onsite and ready to deploy immediately. 

s Review the spill plan and require modifications as necessary to 

meet project objectives and goals.

l Industrial Camps

s Self-contained toilet facilities onsite.
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Stream Simulation

l Equipment Cleaning

s Pressure-wash equipment to remove foreign terrestrial, aquatic 

weed, and animal species.

s Repair fuel, lubricant, and hydraulic leaks.

s Inspect daily (depending on activity).

8.2.6 Dewatering and Sediment Removal 

A dewatering system bypasses the streamflow around the site and removes 

most of the water from the excavation area of the project (figure 8.4). 

(See appendix H for a sample dewatering supplemental specification, and 

figures H.6, 7, 8, and 13 for sample dewatering system drawings.) Water 

that escapes the bypass system—by flowing around or beneath the dam 

or seeping into the excavation—must be captured and treated to remove 

the sediments before returning it to the stream. In areas prone to seasonal 

storms during the construction season, even channels with dry stream beds 

may require dewatering plans. A simpler system than that shown in figure 

8.4 may be suitable in such areas. 

Figure 8.4. Diversion dam and gravity pipe bypass system. Bypass road is visible 

in the background, and excavation in main road is just beyond it.

Excavation activities produce most of the sediments on a project. As 

part of the dewatering, encourage the contractor to avoid inadvertently 

mixing excavated soil and water. Not mixing soil and water will reduce 

the amount of water needing treatment and reduce the risk of exceeding 

turbidity levels in the stream. The contractor will also find this practice 

advantageous because it reduces water-treatment costs.
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Chapter 8—Stream-Simulation Construction

8.2.6.1 Protection of aquatic organisms when dewatering

To maximize protection of aquatic organisms, review the dewatering plan 

and the species-removal plan with the biologist and contractor in the field 

before beginning construction. When endangered and threatened species 

are present, dewatering may take on critical importance, and regulatory 

agency personnel may review it in the field. See appendix G.4.4 for case 

examples of dewatering and species protection for a stream crossing and a 

stream-restoration project.

Dewatering often traps aquatic animals, and construction activities in the 

dewatered stream channel frequently kill organisms that have retreated into 

moist gravels. To avoid stranding, stressing, or killing aquatic organisms: 

dewater gradually, capture the organisms, and transport them to the best 

available stream habitat above or below the construction site. 

No standard method exists for capturing and handling aquatic organisms. 

The biological opinion from the regulatory agency should cover the 

methods for endangered species act-listed species. State fish and game 

agencies are a good source for guidelines for handling captured aquatic 

organisms. Generally, placing captured fish in a bucket of water kept at 

ambient stream temperature is best. The exact methods for capture and 

transport will depend on channel features such as dimensions, shape, 

substrate size, and location of hiding places. Choose trap and transport 

techniques that reduce stress on individuals selected for protection.

To determine a practical and reliable way to dewater, collect and transport 

aquatic organisms, and rewater the site in a controlled and staged manner, 

the contractor, inspector, and project biologist should coordinate their 

timing and work together. The construction contractor generally does 

not perform aquatic organism removal, but the contract should provide 

a stop-work requirement that allows time for that work (section 7.8). 

It is important to ensure that when dewatering begins enough qualified 

personnel are present to collect and move species safely and efficiently. 

The contractor and inspector may also be able to help with species 

transport under the guidance of the fish biologist. For example, if aquatic 

organisms will be transported over rough ground to an upstream or 

downstream habitat, the contractor’s assistance in clearing a pathway for 

this effort may be extremely helpful.
8—20

Stream Simulation

Protecting aquatic organisms also includes:

l Minimizing damage to aquatic habitat by limiting turbidity and 

sedimentation of the streambed. 

l Minimizing water temperature increases by avoiding vegetation 

removal and retaining shade. Before implementing any proposed 

changes to clearing limits, review them with the project biologist. 

l Preventing the spread of invasive species such as zebra mussels, 

snail species, and weeds. Be sure that all tools and equipment are 

thoroughly cleaned before they are brought to the construction site. 

l Avoiding chemical contamination and rapid changes in water 

temperature. 

l Adhering to the instream construction window, which is usually 

determined by the permit. The timing is set to avoid critical aquatic 

organism life cycle periods such as migration and spawning.

8.2.6.2 Dewatering plan review

If the contract does not contain a dewatering and water-treatment plan 

but instead requires the contractor to submit one, review the contractor’s 

submittal carefully. Be sure it includes the elements described in section 

7.8. Review SupplementalSpecification 157 and the sample dewatering 

plan drawings in appendix H for features that may be used on the project. 

A successful dewatering plan includes the following:

l A diversion dam to direct water into a bypass pipe or pump system to 

capture the majority of stream flow. About 90- to 95-percent capture 

is considered successful. The dam must have sufficient height and 

width for stability.

l A method for temporarily pumping or diverting water around the 

bypass dam area during its construction. 

l Screens on pump intakes to prevent leaves and other debris and 

aquatic organisms from entering the pumps.

l A hydraulically designed water bypass system (leak-proof pipe, linedditch, or pump system) capable of handling construction-season flow 

conditions, including possible storm flows.
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Chapter 8—Stream-Simulation Construction

l A sump above the excavation to collect water seeping past the 

diversion dam; and a pump, bypass ditch, or other means for 

transporting the water to the treatment system (figure 8.5). (If the 

water is clean, it can be pumped back upstream to the bypass system 

to reduce the load on the water treatment system.)

l A sump immediately downstream from the excavation to collect 

sediment-laden seepage water, and a means to transport it to a 

treatment system (figure 8.6).

l Sump pumps that are capable of handling expected flows. 

l A specific sediment-removal method, including backup if the 

preferred method fails.

l A storm action plan that includes both Government and contractor 

contacts along with specific action items for avoiding a catastrophic 

failure.

Figure 8.5—Upper sump collects water that bypasses or seeps through the 

diversion dam.
8—22

Stream Simulation

Figure 8.6—Lower sump collects water bypassing the construction. The hose on 

the right is discharge from the upper sump. The water is pumped from the sump 

to a treatment area where the water is dispersed in the vegetation.

The design engineer, COR, and inspector should all review the dewatering 

and sediment-removal plan carefully to ensure that the objectives are 

understood, and that the plan is effective at collecting and treating dirty 

water, meets hydraulic needs, and allows for project limitations such as 

rights-of-way and instream work permit requirements.

8.2.6.3 Dewatering inspection recommendations

Because the amount of ground water can vary significantly and is difficult 

to predict before construction begins, the dewatering plan should allow 

for necessary adjustments. If the dewatering plan was written as an endresult or performance-based specification, the contractor must be prepared 

to make appropriate changes to the dewatering system to eliminate 

sedimentation. Either the COR or the inspector must be quick to recognize 

any potential failure of the dewatering system and require any necessary 

changes. 
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Chapter 8—Stream-Simulation Construction

Above all, all sites must have a method of capturing and treating sediment and drainage downstream of 

the project (sections 7.8.3 and 7.8.4). The method usually consists of a sump and pump system that feeds 

the dirty water to a treatment system. This system is the project’s final opportunity to keep sediment out 

the stream.

Follow the approved diversion plan exactly, adjusting it only to fit the site. Contact the design engineer 

if changes are necessary. If something is not working correctly, have the contractor repair it immediately, 

because the integrity of the individual design elements is key to preventing leaks from reaching the 

stream. 

Dewatering System Inspection Checklist

Check the following items to ensure that the dewatering system protects aquatic species and 

functions properly: 

Bypass dams and downstream backwater dams.

4 Dam materials are tightly packed. 

4 Dam elevation is correct (provides for flood capacity). 

4 Dam length and width is correct. (Even though the dam is small, it is still a dam and 

must be stable and safe.)

4 Membrane is properly installed, embedded into banks and stream bottom to the 

dimensions specified (or as necessary to intercept both surface and subsurface water).

4 The dam has a reasonably good seal against the streambed, bank, and bypass pipe. 

(Expect some seepage to get past the dam; collecting 100 percent of the water is 

nearly impossible.)

Stream bypass by pumping

4 Screens are on all pump intakes.

4 Pump intake is placed deep enough that the entire screen is submerged, providing 

enough head for the pump to operate efficiently.

4 Pump capacity is sufficient to pump entire stream around construction area. Multiple or 

unusually large pumps are required in larger stream flows. Pumps and stream flows are 

measured differently. One cubic foot per second (typical streamflow unit) equals 448 gallons 

per minute (typical pump measurement).

4 During excavation activities, check all pumps until the excavation water clears. (Pumps may 

have to run constantly, and you may need one or more backup pumps.)
8—24

Stream Simulation

Stream bypass by gravity through pipe.

4 Pipe capacity is adequate to pass required streamflow for the construction season and any 

anticipated extensions. Pipe capacity can be determined with either pipe-flow nomographs 

or software such as HY-8.

4 The pipe is effectively sealed to the dam according to specification (section 7.8.1).

4 Pipe diameter, material type, joint type, and sealer or gasket must be as specified.

4 Pipe joints are straight, unstressed, and leakproof. 

4 Pipe is placed at the designed grade and elevation.

4 The pipe outlet area is scour resistant. If downstream fish passage is allowed, outlet is 

placed in a pool or other suitable release area. 

4 Any seepage from the diversion dam that flows into the bypass pipe ditch is collected 

in a sump before it enters the stream. (If it is very clean [test it to be sure] it can be 

pumped back into the stream).

Stream bypass by gravity into existing side or constructed channel.

4 The diversion channel slope, width, and bank slopes are constructed as designed.

These characteristics ensure stability and keep the channel scour-resistant. 

Supplemental erosion control measures may consist of: rock lining, a membrane, 

straw, check dams, geotextiles, etc.

4 The area between the channel and the construction area must be sufficiently stable 

to prevent an accidental diversion into the construction area, and to prevent channel 

seepage from reaching the excavation area. (Reinforce the channel, if necessary.) 

Stream bypass and water treatment system—general.

4 Check the pipe diversion inlet daily. Make sure that any debris such as leaves and 

twigs are removed periodically. If screens are required on the pipe or on pumps and 

hose inlets, cleaning may be required daily, especially in areas of significant leaf fall 

during construction. 

4 Enough fuel is available to keep pumps running up to 24 hours a day, when required, 

to prevent stream sedimentation during storm events.

4 System includes automatic pump control floats to conserve fuel and maintain pool 

elevation within an acceptable range (figure 8.7)

4 Backup pumps and extra pump capacity (including fuel) are onsite to accommodate 

increased stream flows during storm events.

4 Where storms, vandalism, or other events could cause a system malfunction, check 

the system daily—including weekends.
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Chapter 8—Stream-Simulation Construction

Figure 8.7—Pump controls: float switches are attached to stake in sump. 

Dewatering and aquatic-organism removal.

4 The project team biologist is kept informed of the contract schedule, so that he or 

she can be present for species removal during dewatering.

4 A staged (slow, controlled) dewatering procedure is clearly understood and agreed 

to by the contractor.

4 Necessary equipment such as buckets, nets, shock equipment, and dip nets are 

onsite before dewatering begins.

4 Equipment necessary for special-capture methods is on hand if needed for listed 

species.

4 All equipment is cleaned before it is brought to the site as well as when it leaves the 

site, to protect against introducing invasive species.

4 The release area is located and a safe pathway from the dewatering area to the 

release area has been cleared. 

4 Before dewatering begins, enough people for the capture/removal job are in place.

4 Dewatering begins gradually so that aquatic organisms are not stranded in the dry 

streambed.
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Stream Simulation

8.2.6.4 Tips for collecting and treating sediment-laden water

Upstream sump l Maintain sump pool volume with enough capacity to prevent flooding 

the excavation.

l Capture the seepage and pump it to the treatment system. The water 

may eventually clear enough for pumping back into the stream.

l For both convenience and reliability, run all sumps from a central 

electric generator.

Water collection

within the

excavation area l Collect as much water as possible within the excavation area. Then 

concentrate and divert the water away from loose soil to reduce sedimentation.

l Maintain the drainage area between the foundations and excavation 

edge by removing accumulated soil deposits and debris.

Downstream sump l This sump is important, as it offers the last chance for collecting and 

diverting dirty water for treatment (figure 8.6).

l Use the downstream sump at all times to capture and divert dirty water 

to the treatment system.

l Be sure that the sump-pool capacity is sufficient for stormflows, 

especially if sedimentation rate is high.

l Have enough pumps to handle seepage during storm events, and keep 

pumps in good working order. 

l Keep transport hoses for sediment removal stable and leak-free to 

avoid causing erosion or allowing sediment to enter live streams.

l Use a downstream backup instream filter in case the downstream sump 

capacity is exceeded. The filter may be geotextile wrapped straw bale 

or other sediment filter.

l Ensure that the backup sediment filter is constructed and positioned 

properly, and sealed against the streambed. 

l Monitor the filter to ensure that it remains effective but do not expect it 

to completely eliminate turbidity in the stream.
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Chapter 8—Stream-Simulation Construction

Downstream dam l On flat sites or in deep water, you may need a downstream dam to 

prevent backwatering of the excavation and sump.

l Construct the dam the same way as the upstream bypass dam and size 

it according to the downstream need.

Water treatment

methods l Follow contract drawings and specifications. If the system is not 

working, follow normal contract protocol and modify the contract. 

(Again, performance-based specifications will make the specified end 

result the contractor’s responsibility.) Be sure that water released into 

the forest for natural filtering (figure 8.8) does not “short circuit” back 

to the stream untreated. (See various filtration and water treatment 

methods in sections 7.8.4 and appendix G.4.1.3.)

l Untreated or inadequately treated water will yield high turbidity 

levels in the stream below the construction site. To enforce turbidity 

requirements in the contract, know how to check stream turbidity 

levels during construction, or know who can. If necessary, ask the 

project team for help with this item.

Figure 8.8—Natural filtration on the forest floor isolated from the stream channel 

is one means of treating sediment-laden water.
8—28

Stream Simulation

Dewatering and water treatment—what can go wrong?

l Unanticipated floods and thunderstorms (see figure 8.9) exceeding normal conditions 

can overwhelm dewatering systems sending flow through excavation areas, causing soil 

erosion, structure damage, and stream turbidity and sedimentation.

l Hydraulic failure of dewatering dams or pipes can occur.

l Poorly designed or constructed systems may fail during normal summer storms, creating 

damage similar to that of an unanticipated large flood event. (This scenario is preventable.)

l Stockpiles located close to the stream may not be covered in time to prevent saturation or 

erosion. The material may become unusable long enough to cause a delay in the project. 

l Sediment capture and treatment systems may not provide adequate treatment, allowing 

dirty water to reenter the stream and calling for additional methods to prevent harm to 

aquatic organisms from sediment, turbidity levels, or toxicity.

l Equipment such as pumps and generators can break down.

l Equipment breakdowns can leak petroleum products into the project site, requiring an 

expensive hazmat-treatment action.

l Generators or pumps can run out of fuel when no one is onsite.

l Sumps and pumps can be too small to keep water from exceeding sump capacity.

l Bypass pipes or channels can leak drainage into excavation or embankment areas.

l Early freezing, wet weather, fires, or other unanticipated emergencies can set back the 

entire project.

Figure 8.9 shows what happened when a flood—300 percent of normal high summer flow 

volume—exceeded the capacity of the bypass dam. The bypass structure was overtopped, and 

water diverted through the construction area, causing erosion, high turbidity in the stream, and 

partial infilling of concrete forms with fine sediments.

Figure 8.9—Unusual summer storm overwhelms bypass system. (a) Looking downstream at bypass 

pipe, now in mid-channel. (b) Looking downstream through the open excavation.

(a) (b)
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Chapter 8—Stream-Simulation Construction

8.2.7 Excavation

In this document, excavation refers to all excavation (including removal 

of the road embankment surrounding an existing pipe structure) except 

structural excavation. Section 8.2.10 covers structural excavation which is 

defined as the part of the excavation necessary to install the new structure, 

including its footings.

Check the contract for specific requirements and encourage the contractor 

to minimize the length of dewatering time. If erosion protection measures, 

such as silt fences at the base of embankments, are in place and working 

well, delaying dewatering until excavation is close to stream channel level 

may be suitable. If possible use the existing culvert while excavating the 

existing road embankment to place the bypass. Otherwise, temporary 

dewatering with pumps will be necessary before the bypass can be 

constructed.

8.2.7.1 OSHA and excavation safety

Maintain safe, stable slopes during excavation, and be aware of conditions 

that indicate slope instability. The slope ratio either is stated on the 

contract drawings or is governed by OSHA regulations. OSHA guidelines 

regulate maximum slopes and configurations for trenches and excavations 

up to 20 feet deep. A registered professional engineer must design 

excavations deeper than 20 feet. OSHA recognizes that excavating is one 

of the most hazardous construction operations. Therefore, it revised OSHA 

Part 1926, Subpart P, Excavations, of 29 CFR 1926.650, .651, and .652 to 

make the standard easier to understand, to permit the use of performance 

criteria where possible, and to provide construction employers with 

options when classifying soil and selecting employee protection methods. 

Contract administrators must thoroughly understand this document!

The common hazards for embankment excavation (table 8.1) may be 

encountered at stream crossings. A number of stresses and deformations 

can occur in an open cut or trench. For example, increases or decreases 

in moisture content can adversely affect the stability of a trench or 

excavation. OSHA classifies soil into five categories and recommends 

maximum excavation slope angles for various benching and trenching 

options. 
8—30

Stream Simulation

TENSION CRACKS. Tension cracks usually form at a horizontal 

distance of 0.5 to 0.75 times the depth of the trench, measured 

from the top of the vertical face of the trench.

SLIDING. Sliding or sluffing may occur as a result of tension 

cracks. 

TOPPLING. In addition to sliding, tension cracks can cause 

toppling. Toppling occurs when the trench’s vertical face shears 

along the tension crack line and topples into the excavation. 

SUBSIDENCE AND BULGING. An unsupported excavation can 

create an unbalanced stress in the soil, which, in turn, causes 

subsidence at the surface and bulging of the vertical face of the 

trench. If uncorrected, this condition can cause face failure and 

entrapment of workers in the trench. 

 

HEAVING OR SQUEEZING. Bottom heaving or squeezing is 

caused by the downward pressure created by the weight of 

adjoining soil. This pressure causes a bulge in the bottom of the 

cut, as illustrated in the drawing above. Heaving and squeezing 

can occur even when shoring or shielding has been properly 

installed. 

BOILING. Boiling is evidenced by an upward water flow into 

the bottom of the cut. A high water table is one of the causes 

of boiling. Boiling produces a “quick” condition in the bottom of 

the cut, and can occur even when shoring or trench boxes are 

used. 

TENSION CRACK

SLIDING

TOPPLING

HEAVING OR SQUEEZING

BOILING

SUBSIDENCE & BULGING

Table 8.1—Slope failure mechanisms (OSHA 1999)
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Chapter 8—Stream-Simulation Construction

8.2.7.2 Excavation—what can go wrong?

l Inaccurate construction survey (double-check the stakes for 

location and elevation before beginning excavation).

l Excavation slope failure.

l Intense rainstorms, erosion. 

l Dewatering system failure during flood event. 

l Springs within excavation area causing slope instability or 

generating large amounts of sediment. 

l Very weak subsurface. (Contact the design engineer to determine 

a suitable solution, such as subgrade reinforcement or a 

foundations design change.) 

l Bedrock is unexpectedly encountered, making redesign of 

footings or structure necessary.

8.2.8 Structural Excavation 

The final embankment excavation, structural excavation, and removal of 

existing structures create the most sediment and turbidity. To capture and 

treat the construction water as this work proceeds, the dewatering system 

must be functioning properly. 

To verify that the final depth and location of the excavation are correct, 

make an accurate survey check as the bottom of the excavation approaches 

the design depth. You can use a rebar or pipe probe to locate the bedrock 

depth.

As the excavation approaches the final depth, the design engineer should 

review the foundation materials to verify soil-bearing capacity, verify 

that conditions match design assumptions, and approve the foundation 

conditions. Be prepared to have the contractor reinforce soft areas with 

subgrade reinforcement material, such as free draining crushed rock. A 

geotextile may be useful as a filter and reinforcement. 
8—32

Stream Simulation

If appropriate under the contract (section 7.5.2.2), examine the material 

beneath the culvert and determine if any material can be salvaged. Such 

material may include:

l Gravel bedding for the existing pipe: This bedding may be usable for 

part of new culvert bedding.

l Streambed materials: The project team may decide to either keep 

them in place or remove them for processing with other streambed 

materials. Streambed materials must meet specification requirements.

s The material may appear “dirtier” than other streambed surface 

materials. (This “dirt” is often only natural subsurface fines.)

s If streambed material is removed during construction, it is often 

too wet and its moisture content may need to be reduced before 

replacement into the structure. Place it in a separate stockpile and 

protect from contamination from other materials.

l Soil embankment and backfill: These may meet backfill requirements 

for the new structure.

l All materials: Must meet contract specifications and must be tested 

for gradation and other engineering properties.

Keep the foundation area relatively dry. To facilitate subsurface drainage 

away from the work area and avoid pooling, you may find it helpful to 

start the excavation downstream of the structure and extend it upstream. 

This approach also improves the work area and reduces erosion and 

turbidity. Concentrate seepage into one or more flow paths; ensure that the 

downstream sump is deep enough and that the pump has enough capacity 

to keep water levels below the bottom of the excavation. 

8.2.8.1 Bedrock and blasting

If the excavation encounters bedrock (or other unsuitable material) that 

was not detected earlier and that interferes with the foundation, contact 

the design engineer to identify an appropriate solution. Changes made in 

culvert alignment or elevation without consultation can seriously affect 

stream-simulation stability. 

Blasting may be necessary when bedrock is encountered. Contact the 

project biologist before proceeding for help with developing a blasting 

plan. Blasting may be prohibited during certain time periods to protect 

species, such as nesting birds or listed fish. Such time periods and concerns 

should be listed in project National Environmental Policy Act documents. 
8—33

Chapter 8—Stream-Simulation Construction

Fish near blast sites may suffer swim bladder rupture, tissue and organ 

damage, or internal bleeding. The damage to fish depends on the size of 

the charge, distance to the fish, depth of water, substrate type, and the size 

and species of fish (Keevin 1997). 

If the design engineer anticipated blasting on the project, the contract will 

include FP03 section 205. If blasting is added to the contract because of a 

design change, include FP03 section 205 and require an approved blasting 

plan. The contractor must submit the plan and obtain approval in writing 

before doing any preparatory work (i.e., drilling) for blasting. Obtain the 

help of a blasting expert to review the blasting plan. 

8.2.8.2 Settlement beneath foundations and pipes

Settlement should be limited to avoid adversely affecting alignment, grade, 

and structural shape. The amount of settlement depends on soil properties 

and compaction. Contact the design engineer to verify that soil conditions 

meet design values. If very soft materials (such as wet clay, silt, or other 

soft plastic fine-grained soils) are unexpectedly present, foundation 

soils may need reinforcement or replacement or the footings may need 

redesigning. Differential settlement—caused by the footing’s settling more 

over the soil region than the bedrock region of the foundation—will create 

undue stress on the structure. 

Recommendations for Structural Excavation Inspection 

 Verify that :

4 Foundation soils have been tested for shear strength and results 

meet design assumptions.

4 Foundation excavation elevation and location are correct.

4 Either the design engineer or the geotechnical engineer approved 

the foundation conditions.

4 The contractor has completed the required compaction testing 

and compaction meets contract requirements.

4 There is plenty of operating space for equipment for structure 

assembly and placement of footings, forms, steel, and concrete.
8—34

Stream Simulation

8.2.9. Constructed Concrete Features

Figure 8.10—Concrete forms and reinforcing during concrete pour.

8.2.9.1 Concrete form inspection

When inspecting concrete forms, verify that:

l Shop drawings and contract drawings agree with each other. Resolve 

any discrepancies. Send a copy of the drawings to—and discuss any 

potential changes with—the design engineer. 
8—35

Chapter 8—Stream-Simulation Construction

s Note that a change in arch shape may result in a change in the 

foundation design shape or position.

l Forms match design drawings and that dimensions are correct.

l Forms are stable and well braced to withstand the hydraulic stresses 

of the fresh concrete. Forms for deep footings and stem walls may 

require design by a licensed engineer.

l Embedded bolts and fixtures are installed in the formwork.

s Anchor positions and angles are correct for open bottom arch 

location. 

s For multiplate pipes, hardware for collars and haunch beams is 

installed according to manufacturer’s instructions. (For large 

complex structures, manufacturers often provide their own 

inspector for forms and hardware.)

s Channels are installed correctly per the manufacturer’s 

instructions. 

l Reinforcing bars match design drawings for bar sizes, spacing, and 

position.

l Bars are tied down to prevent movement during concrete pour.

l Bottoms of footings are flush with excavation or plugged, to prevent 

concrete leaks into stream water.

l Water is diverted away from forms, if possible, to prevent diluting 

concrete.

s Seal concrete is used if water is present in forms.

l The work site is safe and workers have a safe place to stand during 

concrete pour.

8.2.9.2 Pouring concrete 

Discuss common problems and contingency plans with the contractor in 

advance; for example, pump trucks can plug, concrete can spill, forms 

can be damaged, concrete can arrive early or late, and mistakes or poor 

planning may cause cold joints. Concrete should be tested for quality to 

ensure it meets specifications. 

Do not spill concrete, cement, or concrete additives into the stream. 

Concrete and related products that mix with water and enter live streams 

can kill fish very quickly because high pH levels (cement contains lye) are 

corrosive to fish gills.
8—36

Stream Simulation

Special concrete placement methods may be necessary when standing or 

running water is present in or adjacent to forms. To avoid mixing with 

standing or flowing water, seal concrete (normal concrete with an extra 

sack of cement) may provide a more stable mass. The pump hose should 

touch existing wet concrete when the hose is near any water to prevent 

mixing the concrete with water. Ensure that water does not pond on the 

excavation side of forms, where it can be forced through concrete joints 

and wash away concrete and expose reinforcing steel.

When standing water cannot be pumped from the form, it can be displaced 

with the fresh concrete. Placing the concrete involves putting the concrete 

pump hose on the footing bottom in the lowest elevation of the ponded 

water. Keep the pump hose at the foundation bottom. As concrete is 

pumped in, the water is displaced. Avoid movement that would agitate the 

concrete mass and mix the cement with water, such mixing would dilute 

and weaken the concrete, as well as risk getting cement in the stream 

water.

When running water is present, place the pump hose at the source of entry 

to quickly form a plug. The plug seals the leak during the concrete pour 

and leaves the hose submerged in the concrete. As you encounter other 

leaks, plug them in the same manner. 

8.2.9.3 Inspection recommendations for concrete placement

Before the pour, verify that:

4 The concrete design mix was approved including admixtures. 

4 The foundation forms have been approved.

4 Test equipment and test cylinders are onsite before the first concrete 

truck arrives.

4 The contractor’s equipment is checked and operable (pump if 

applicable, vibrator and a spare).

4 There is adequate access for equipment and personnel before concrete 

is ordered.

4 A safe and appropriate place to dump excess or reject concrete has 

been designated.

4 A safe and appropriate place to wash out the interior of the concrete 

mixer has been designated.
8—37

Chapter 8—Stream-Simulation Construction

During the pour, check that:

4 Concrete is not outside the allowed time, and slump or percent air is 

correct. (Otherwise, reject the concrete.)

4 Forms do not shift and fail. If this occurs, stop the concrete pour and 

repair forms. Concrete delivery may have to be delayed and concrete 

already onsite may have to be dumped. 

Most things that go wrong on a concrete pour—if not solved 

immediately—lead to a cold joint. If necessary, prepare the joint before 

the concrete hardens with a rough unfinished surface to develop effective 

friction. When lateral forces are high, form keyways to increase the shear 

strength of the joint. 

Contact the design engineer for advice on correcting mistakes on any 

concrete pour.

8.2.10 Culvert Installation 

8.2.10.1 Closed-bottom culvert bedding

Closed-bottom culverts require bedding material to be placed and shaped 

to match the bottom of the structure. The top of the bedding should 

conform to the design elevation, slope, and alignment of the pipe. Have the 

contractor place the soil tight against the structure to prevent subsurface 

stream flow and to develop soil-structure stress interaction.

Verify the following for the culvert bedding:

4 Bedding material is the correct gradation and thickness.

4 Bedding elevation and alignment match the design elevation for the 

structure inverts.

4 Bedding shape reasonably matches pipe shape in approximately 

the center third of the pipe before placing pipe. (See figure 7.9.) A 

plywood form may be useful for shaping the bedding. 

4 Bedding is sufficiently compacted to prevent culvert distortion during 

bedding and backfill operations. The following two methods are 

typically used to avoid leaving voids in the culvert bedding:
8—38

Stream Simulation

s Voids beneath the culvert are filled by pushing and tamping bedding 

material into place.

s Voids are filled with low-strength, high-slump (flowable) concrete 

called “controlled low strength material.” This material is used in 

place of shaped and tamped bedding.

With controlled low-strength material (CLSM), ensure that the contractor 

secures the pipe by placing some of the stream-simulation bed material 

inside, or by weighting the top of the pipe with soil (to keep it from 

floating as the heavy CLSM flows under the pipe). To avoid distortion, 

check that the pipe is fully assembled before contractor places backfill 

material or CLSM.

8.2.10.2 Open-bottom culvert attachment

Open-bottom structures have footings usually made of concrete, steel, or 

aluminum. Open-bottom culverts come in a variety of shapes, including 

metal half-circle arches, low-profile arches, and boxes; high-profile arches, 

pear shapes, and ellipses, as well as concrete boxes. These structures 

attach to footings with bolted connections or grouted slots. Be aware of 

the manufacturer’s requirements, including any certifications required to 

install their products.

For attachments, verify that embedded bolts and fixtures, and grouted slottype culvert attachments, meet contract requirements and, if applicable, 

shop drawings. See figure H.11. 

When metal footings are used for open-bottom arches, placing some 

stream-simulation material between the footings before full pipe assembly 

is possible, as long as backfill on the outside of the footing is brought up 

in equal lifts. This method is risky, however, because the footings can 

shift, making the remaining arch assembly difficult or impossible without 

resetting the footings.

8.2.10.3 Pipe assembly

All pipe segments require careful handling and alignment to maintain 

their shape, so they can be properly joined to form a watertight seal. 

Pipe transporting and handling must be according to manufacturer’s 

recommendations to prevent damage and assure proper fit. Concrete box 

culverts are rigid and easily aligned for a tight fit. Metal pipes are flexible, 
8—39

Chapter 8—Stream-Simulation Construction

depending on their material, thickness, corrugation depth, and dimensions. 

To slide into proper position, they often need small adjustments during 

placement.

Leaks can lead to loss of water from the stream-simulation bed or piping 

of backfill outside the culvert, leading to eventual structure failure. Metal 

pipe joints are very susceptible to differential movement and shape change 

during the backfill operation. Deflection and variations in shape of the 

pipe on either side of the joint can result in joint leaks. Careful assembly 

is critical to making a leak-proof joint. The joints are either grouted 

or sealed with waterproof joint wrap, according to the manufacturer’s 

recommendations. For information on pipe couplings for metal pipes, see 

National Corrugated Steel Pipe Association Installation Manual.

Pipe Assembly Inspection Checklist

4 Check for careful handling of pipes upon delivery, transport to 

location, and assembly to ensure that they are within shape 

tolerance and that watertight joints are constructible. Reject 

materials that do not meet specification (reference AASHTO 

Standard Specification M36-01 for corrugated steel pipe 

manufacturing tolerances, zinc coating, etc.)

4 Verify that culvert materials match those specified in the 

contract.

4 Review coupling materials and installation instructions to ensure 

that the couplings are properly fitted around the entire pipe joint.

4 Reject leaking joints. (The manufacturers and the design 

engineer can help solve joint problems.)

8.2.10.4 Multiplate pipes

Multiplate pipes are manufactured in a wide variety of shapes. They 

consist of multiple corrugated metal plates assembled with bolts and 

tightened to a specific torque. Because the weakest places in a multiplate 

pipe are the joints, make sure that all required bolts are installed and 

tightened to specifications.

Small multiplate pipes may be assembled offsite and placed in one piece. 

They are not designed to be coupled together in segments like one-piece 
8—40

Stream Simulation

pipes. Instead, they can be partially assembled, placed, and the remaining 

plates can be bolted on later one at a time. Large multiplate pipes are 

usually assembled in place. Wide-span multiplate structures sometimes 

have special assembly requirements. The manufacturer may either send 

its own inspector or provide guidelines for monitoring shape during 

assembly and backfill operations. Obtain details covering structural plate 

installations from the pipe manufacturer, particularly on large structures. 

8.2.10.5 Backfill and embankments

For embedded pipes, begin backfilling as soon as bedding is completed 

and the structure is in place. For open-bottom arches with footings poured 

in place, backfilling can begin as soon as the concrete has cured long 

enough (i.e., usually 80 percent of full specified strength) to withstand 

backfill forces up to the top of the footings. After the arch or box is in 

place, place the remaining backfill. Leave the dewatering system in place 

until the backfill is high enough to prevent the stream from flowing along 

and outside the structure in the event of a sudden storm. The inspector 

should monitor the shape of metal culverts during backfill and compaction. 

Many structures require a specific width of special structural backfill on 

either side of the structure.

 Backfilling Tips

l Place backfill at the proper moisture content in thin lifts, 

according to the specifications on both sides of the footing or 

culvert.

l Ensure that all backfill material meets the project specifications, 

especially within special backfill zone areas next to the structure. 

(Controlled compaction is required on most culvert installations. 

Check the specifications; the contractor is usually required to 

provide quality control testing.)

l To prevent damage to the structure, use hand-operated 

compaction equipment near the structure (e.g., hand-operated in 

confined areas and machine-compacted in broad areas).

l To prevent damage to the structure, provide a fully compacted, 

minimum cover height above the pipe before allowing 

construction equipment or other traffic to cross the structure. 

Construction equipment may require more cover than design 

traffic; check fill height tables for the structure or check with the 

manufacturer.

See National Corrugated Steel Pipe Association Installation Manual 

for additional information on installing culverts.
8—41

Chapter 8—Stream-Simulation Construction

Figure 8.11—Example of uneven backfill and compaction. The right side was 

backfilled before the left side instead of even lifts on both sides, causing the 

distortion visible in this photo. The distortion induces eccentric loadings on the 

footings, as well as bending stresses at the anchor bolts on both footings.

8.2.11 Stream-simulation Bed Material Placement

The most important detail of a stream-simulation project is the streambed 

which, when constructed properly, will enable a variety of aquatic 

organisms in the stream to travel up and down through the structure at 

will. Large flood events are likely to occur during the life of the structure. 

Therefore, the quality of construction is critical to developing the design 

channel form with the energy-dissipating structures (inside the streamcrossing structure) that ensure sustainability over time.

Contract drawings will show, at a minimum, a shaped streambed in the 

structure, usually sloping downward toward the center to form a low-water 

channel. Other features common to many sites include cross-channel steps 

formed with large rocks, raised stream banks along the interior culvert 

edges, and fields or clusters of large rocks (figure 8.12). Sills (see section 

6.2.2.4) are occasionally attached to the culvert before bed placement, 

to help support rock steps in the culvert. Their design is unique to each 

project.
8—42

Stream Simulation

Figure 8.12—Newly constructed (2007) step-pool channel inside culvert on 

Eustache Creek, Lolo National Forest, Idaho. Two visible rock weirs (steps) have 

their lowest points offset to provide some sinuosity at low flow. The top of the 

sloped banks is bankfull elevation.

Review the stream-simulation design and the natural streambed with the 

design engineer, and examine local features and discuss placement details 

with the contractor. If the streambed design needs modifying for any 

reason, contact the design engineer immediately to fix the problem before 

making any changes. Failure to understand all of the stream-simulation 

parameters can easily lead to error during construction and result in failure 

of the stream-simulation bed.

8.2.11.1 Size of streambed materials

The gradations, depth, configuration, and extent of the streambed materials 

are provided in the contract (section 7.5.3, figure 7.18). Review the 

contract specifications and drawing details, especially the streambed rock 

gradation, to determine the size and gradation of the mix. To ensure that 

the correct materials are obtained and that they are correctly placed inside 

the structure, discuss these details with the design engineer and contractor 

in advance. The gradation of the streambed materials is critical to the 

performance of the streambed simulation. 
8—43

Chapter 8—Stream-Simulation Construction

Table 8.2—Example of a stream-simulation bed material recipe

The contract should specify streambed material sources. Sources may 

include onsite material, borrow sites, quarry sites, road maintenance rock 

fall debris piles, and commercial quarries. Material produced in a quarry 

can be crushed and screened to the proper gradation. Very large boulders 

may be hard to obtain from some quarries, but are sometimes available 

on site in the form of old riprap or colluvium. The design engineer should 

have identified such material in the contract. 

If the material comes from different sources, gradations and quantities of 

the individual materials will have to be determined to establish a recipe for 

the mixture. The mix recipe states the proportions of each material type 

in the mix, and the mixed material requires testing to ensure the proper 

gradation is being supplied. When reviewing stockpiles of materials for 

suitability, familiarity with and use of the pebble-count method—for 

estimating the volumetric or weight-based material gradation—is useful. 

Channel rocks are generally large and easy to measure. Samples of smaller 

bed material (see section 7.5.2.2) can be taken to a laboratory for sieve 

analysis. Use standard submittal procedures for the stream-simulation 

bed mix recipe (table 8.2) if the contractor is responsible for it. Have the 

design engineer review the mix design (recipe) before allowing delivery of 

the materials to the site. 

% Sieve % Material Source 

Passing Size Volume Volume Identified

100 12 in 16% of volume is 1cy Boulders borrowed

 6 to 12 in from: glacial 

 outwash borrow 

 source

84 6 in 34% of volume is 2cy Unwashed river

 1.5 to 6 in dredging 

 from 

 McKenzie River

50 1.5 in 34% of volume is 2cy same

 0.5 to1.5 in

16 0.5 in 16% of volume is 1cy same

1/8 to 0.5 in

 

5 0.125 in 5% of volume is 0.3cy Excess excavation

 smaller than 0.125 in from coarse 

 sand-silt road

 embankment soil
8—44

Stream Simulation

Regardless of the source(s), make sure that the materials from different 

sources are well mixed. If materials are mixed onsite, be sure the mixing 

area is large enough to accommodate all the stockpiles and equipment. 

If the mixing is done elsewhere, be aware that transport can cause mixed 

materials to segregate somewhat. Do not attempt to mix the material inside 

the structure.

8.2.11.2 Constructing the simulated streambed

This is a critical step in the project construction. The inspector and a 

project team member should be on site to review the constructed bed and 

ensure that the final shape, slope and details are correct before rerouting 

water through the structure.

Final bed height should be measured at the center of the surface pieces, not 

at the top of the largest rocks.

If necessary, wash the surface to force fines deeper into the bed to reduce 

the permeability of the bed (figure 7.14). The sump pump may be used for 

washing the bed material in with water from the sump. The downstream 

sump will capture sediment generated during the washing.

General Inspection Checklist 

for Placing Streambed Materials 

(all structures)

 Verify that: 

l Gradation of stream-simulation bed materials meets 

specifications.

l Specified compaction methods are followed for every lift.

l Material is carefully tamped around large rock features, to 

provide good interlocking and low permeability.

l Voids in stream-simulation bed material are filled after each lift 

by washing filler material into the voids and tamping by hand. 

l Elevations are correct during placement to ensure the bed 

surface is shaped correctly.

l Fill elevation line is painted accurately on the inside of the pipe 

for the contractor and for monitoring after construction.

l Grade stakes for the bed are accurately placed, especially 

where the bed extends outside the structure.
8—45

Chapter 8—Stream-Simulation Construction

l Large channel rocks are incorporated into the first or second 

upper lifts and placed as shown in the drawings (this step is 

important as the channel rocks are embedded in the streamsimulation bed material—not placed on top of it).

l Proper equipment is used for more delicate work.

l The stream-simulation bed is the proper shape according to the 

drawings.

8.2.11.2.1 Recommendations for placing material in open-bottom arches

Constructing a streambed in an open-bottom arch is relatively easy, 

because the material can be placed before the arch is set in place (figure 

8.13). Headroom does not limit the size of the equipment as in closed 

structures. An excavator is usually used for placing bed materials, and 

tracked or rubber-tired equipment can be used for compacting the bed.

l Place the bed, compacted in layers, at the same time that the backfill 

outside of the footing is being compacted (figure 8.14). This should 

prevent the footing from moving, and avoids excessive horizontal 

loading on the stem walls.

l Use proper compaction equipment to ensure there is no damage to the 

structure as well as to provide proper compaction.

Figure 8.13—Machine placing stream-simulation material. It took 4 days to place 

600-cubic yards.
8—46

Stream Simulation

Figure 8.14—Newly constructed stream-simulation bed for a bottomless arch. 

The stream-simulation bed material is manufactured rock from a commercial 

source. Footing backfill and stream-simulation bed material were backfilled and 

compacted at the same time. Placing the stream-simulation bed is much easier 

before the structure is in place.

8.2.11.2.2 Recommendations for placing material in embedded pipes

Since embedded pipes are closed structures filled nearly half-full with 

streambed material (figure 8.15), installing the streambed usually requires 

hand labor or small equipment (e.g., rubber-tired loaders, garden-sized 

tractors and trailers, small dozers that can run on a cushion of previously 

placed bed material).

l Take precautions to avoid damage to the galvanized coating in steel 

pipes during bed placement.

s Do not push material along the culvert bottom; pushing removes 

galvanizing, speeding up corrosion of the culvert and shortening 

the life of the structure.

s End-dump streambed materials. 
8—47

Chapter 8—Stream-Simulation Construction

l Use rubber-tired equipment whenever possible, to avoid accidental 

steel-track damage to the structure.

s If the contractor must use a tracked vehicle, place a cushion 

of fine streambed material in front of the vehicle, deep enough 

to spread the load and protect the culvert invert from the track 

grousers.

l Avoid overfilling buckets, and remove spilled material from the travel 

path.

l The contractor may use hand labor to place streambed materials. With 

smaller structures, hand placement is often the only option (figure 

8.16).

Once material is dumped, it can be spread by hand (or by machine, if 

clearance allows). A good way to place the bed material is working up 

from the downstream end, and placing lifts on an angle sloping down in 

the upstream direction. This technique allows dumping, spreading, and 

compacting subsequent loads on this slope while maintaining maximum 

headroom for the remaining length of the culvert for transporting material. 

It is also a good way of meeting placement and compaction requirements, 

and it facilitates placing larger channel rocks in the culvert. In small 

culverts, it is easiest to place lifts full length in the culvert.

Figure 8.15—Bobcat placing bed material to marked elevation.
8—48

Stream Simulation

Figure 8.16—Assuring proper cross-section shape, rock-band spacing and 

placement of channel rocks and footers often require hand work.

8.2.11.2.3 Placing channel rocks

Placing channel rocks is a somewhat subjective aspect of construction, 

calling for careful observation of the existing channel. Depending on the 

project, some features will need well-interlocked rocks in some places and 

individual or clusters of rocks elsewhere. Because the placement process 

is very difficult to specify precisely, it is a difficult procedure to enforce. 

To make the job more difficult, since the channel rocks are embedded, they 

must be placed during the construction of the stream-simulation bed and 

the bed material must be compacted around the channel rocks. 

l Check the contract for sizes and any shape requirements of channel 

rocks. 

l Check the contract for details for the locations and placement of 

channel rocks (figure 8.17).

l When constructing steps with footers, footers should extend deeper 

than the projected depth of pools that will form in the future.

l Because machinery is necessary for moving large rocks, use care to 

avoid damaging the structure. 
8—49

Chapter 8—Stream-Simulation Construction

l Positioning large rocks often requires fitting them together so that 

they interlock for stability. This often requires several attempts to 

obtain an acceptable fit.

l Pack with finer material by hand, or place by machine and wash fines 

in for a tight, stable fit. 

Figure 8.18 illustrates the difference between loosely and tightly packed 

rock structures—both scenarios are important in stream simulation. With 

few exceptions the rocks in figure 8.18(a) are not well interlocked. In most 

cases, individual rocks can move independently. The step in figure 8.18(b) 

is tightly packed and the rocks are well interlocked. Moving any piece 

requires moving adjacent pieces as well. Well-interlocked rock is much 

more stable in a stream channel.

8.2.12 Permanent Erosion Control Measures

Permanent erosion control measures may include conserving and replacing 

topsoil; planting erosion-control grasses, ground covers, and larger plants; 

and placing individual rocks, riprap, logs, reinforced slopes, or retaining 

walls for bank stabilization. Although most of these are outside the scope 

of this document, they may be included in the contract. 

The project should have provisions for stabilizing banks disturbed by 

construction, and may include provisions to mitigate some anticipated 

post-construction stream-channel changes. These provisions may include 

large wood, root wads, biotechnical plantings, plant cuttings, large rock 

placements, or special stream structures. As these features or structures 

may be unfamiliar to the contractor and the inspector, contact the design 

engineer for assistance for placing these structures. Pay special attention to 

the quality of work on these structures. To withstand multiple flood events 

during the life of the structure, they will need solid construction. 

8.2.12.1 Revegetation

The planned vegetation may have specific planting requirements, and it 

may require water or mulch during dry spells. Some projects may require 

an irrigation system to adequately water the plants. Erosion control 

fabrics or mulch may be placed to stabilize soil while vegetation becomes 

established. For a discussion of common problems with revegetation and 

erosion control, and their solutions, see appendix G.4.3.
8—50

Stream Simulation

Figure 8.17—Stream-simulation bed details.

12'

1.42

42"

56.00'

12'

Infill pattern for

streambed simulation

material

60" 36"

18"

24"

12"

25-30'

4.5%

NEW PIPE 6%

8' x 12' pipe arch

12'

step-pool step 

boulder details

x-section AA'

44"

12"

60"

18"

24"

36"

A' step pool grade 

controls

8.5%

20'

COOK CREEK STREAM CROSSING

STEP POOL STEP DETAILS

8' 20' 8'

A

COOK CREEK STREAM CROSSING

STEP-POOL STEP DETAILS

Infill pattern for

streambed-simulation

material.

Step-pool step

boulder details

x-section AA’.

step-pool grade

controls
8—51

Chapter 8—Stream-Simulation Construction

Figure 8.18—Streambed features such as steps must be well interlocked to withstand large flood events. 

(a) Poorly interlocked rock; (b) a tightly interlocked rock step.

(b)

(a)
8—52

Stream Simulation

8.2.12.2 Riprap inspection

Riprap commonly is used for stabilizing streambanks at culvert entrances, 

channel edges, embankments, and steep slopes. Over-steepened slopes can 

be stabilized with a thin layer (rockery) or thick layer (buttress).

Riprap is designed to be a well-graded but free-draining material because 

it lacks fine aggregates. Riprap is much more stable when well graded; a 

cluster of large rocks does not have the same interlocking forces as well 

graded riprap material. To be stable in a stream setting when covering a 

streambank, either the riprap must sit on a stable surface such as bedrock, 

or a foundation of riprap must be constructed below the possible scour 

zone to support the mass. 

Riprap design generally includes:

l Enough thickness (twice Dmax) to provide good interlocking with 

other riprap materials.

l A maximum slope limit of 1:1 (12:1 is preferable).

l Geotextile behind and beneath to prevent piping of soil through the 

riprap (piping can eventually undermine the riprap’s stability).

Inspectors should check:

4 Riprap gradation by measuring individual pieces. Adjust riprap 

gradation as necessary to obtain specified gradation.

4 The staking for the riprap limits.

4 The specified placement method.

4 The final product for thickness, interlocking, slope, height, and width.

4 Riprap is placed flush with the existing channel edge to avoid 

restricting the stream channel (requires excavating the streambank).
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Chapter 8—Stream-Simulation Construction

8.2.13 General Road Construction

l Ensure that road construction does not damage the structure. 

l Ensure loose soil is not spilt where it will erode directly into the 

stream without being treated. 

l Make sure that survey-control points are protected. If control points 

are disturbed or destroyed by construction, the contractor may have 

to provide additional surveying or even roadway design. 

l Check the embankment slope, width, and height during construction. 

Correcting an error in the embankment after construction is complete 

is both difficult and costly. 

8.2.13.1 Roadway drainage structures

l Make sure that road surface runoff drains into areas that can filter the 

water before it enters the stream directly. 

l Configure road dips or surface-shape changes to divert road-surface 

runoff before the stream crossing. Make sure that outsloped roadways 

drain onto stable landforms. 

l Ensure that culverts draining onto disturbed areas have downpipes or 

other means to carry water beyond vulnerable areas.

l In erosion-prone areas, spread slash, straw, or other erosion-control 

materials to stabilize bare soil.

For diversion prevention dips (see 7.7.2.1)

l Make sure that longer tapers and gentler rate of grade change are 

provided where lowboys must be accommodated (figure 8.19).

l Verify that downslope erosion protections are in place.

l Plug any downgrade ditches or other escape routes to prevent 

downgrade flooding of the roadway.

l Ensure that aggregate surfacing is spread evenly throughout the dip. 

A very common mistake is to grade the aggregate surfacing thin at 

the high part of the dip and thick at the low part of the dip, thereby 

seriously reducing the capacity of the dip.
8—54

Stream Simulation

Figure 8.19—The grade change at this dip is too severe to accommodate a 

lowboy, which has high-centered on top of the dip.

8.2.14 Demobilization/Cleanup

Check the contract and specifications for any special requirements. Always 

ensure that the following items have been taken care of:

4 All construction debris cleaned up, hauled off, and disposed of 

according to specifications, special contract requirements, and local 

regulations.

4 Campsites cleaned up according to specifications and special contract 

requirements.

4 Hazmat items removed and cleaned up according to specifications, 

special contract requirements, and local regulations.

4 Sediments cleaned or washed from roadways according to 

specifications and special contract requirements to prevent washing 

directly into the stream.

4 Stockpile areas, waste areas, and aggregate pits treated and cleaned 

up according to specifications and special contract requirements. 

4 Temporary erosion and sediment controls removed and cleaned up. 

Trapped sediments removed and properly disposed of.

4 Drainage and final erosion control measures in place and functional.
8—55

Chapter 8—Stream-Simulation Construction

8.3 Post Construction

8.3.1 Post-construction Project Review

After construction is complete, an extremely important step—and the final 

item on the project timeline—is the post-construction project review. Even 

though this review process is time-consuming, review the project from 

beginning to end—and beyond. Your review observations on this project 

will contribute invaluable insight for the success of future projects. Include 

answers to the following questions:

l Overall thoughts and impressions

s Was the project a success?

s What went well with the construction?

s What problems were encountered?

s How were these problems solved?

l Project development and design process

s How well did communications work?

s Did you have the proper mix of project team members?

s How would you assess the available skills? 

s Is the proper skill mix available within the organization?

s Should the agency solicit more specialized help for the next 

project?

l Advertisement and solicitation for bids/proposals

s Was the prebid meeting helpful? Could it be improved? Did it 

answer all questions? If not:

s Did the amendment process answer all questions?

l Construction

s Was the contract adequate?

s Were specifications clear and sufficient?

s Did you have special contract requirements?

s Were the drawings accurate and clear?

s Were the supplemental specifications clear?

s Were design and project team personnel available for promptly 

solving unforeseen issues?

s How would you rate the contractor’s performance (necessary for 

best value contracting)?
8—56

Stream Simulation

Reviewing and discussing the development of the project—and the time 

and people involved—is extremely beneficial to all team members and 

the organization as a whole. The discussion should lead to the following 

questions (and probably more): 

l How would you go about project planning and development 

for future projects to improve the overall process efficiency, 

communications, design quality, and project overhead costs? 

l What worked well on this project? What did not work well? How 

would you avoid those issues on the next project?

8.3.2 Post-construction Monitoring

Post-construction monitoring provides invaluable information, not only 

about the design and construction of the particular project, but also about 

how to improve aquatic organism passage design in general. Detailed 

review and analysis of project features, such as the structure, stream 

simulation bed, any stream restoration, and erosion control features, 

will provide very useful information. These lessons learned add to the 

knowledge base for developing informed decisions on future projects.

8.3.2.1 Physical monitoring of structure performance

A convenient method for physically monitoring the structure is the bridge 

inventory. The method uses a standard form listing physical features and 

conditions of bridges and major culverts, and it adds physical information 

into the INFRA database and the Federal Highways National Bridge 

Inventory. Major culverts—which many stream-simulation projects are—

are included in the inventory. 

Monitoring to date shows that most stream-simulation structural failures 

are due to poor bedding material placement, poor backfill compaction, 

unstable footing design, and flood damage where pipes or bed material 

were undersized.

8.3.2.2 Physical monitoring of streambed performance

The Forest Service culvert-assessment procedure (Clarkin et al. 2005) 

includes observations of streambed depth and continuity through the 

structure, as well as basic culvert details. Many recently inventoried 

culverts have this information and it should be kept up to date as culverts 

are replaced. The assessment procedure has been modified and expanded 
8—57

Chapter 8—Stream-Simulation Construction

for use as a monitoring tool on some forests. Forms modified for 

monitoring include observations and measurements of general channel 

changes both in the stream and in the structure.

Here are some simple observations for monitoring the success of a streamsimulation project:

l Overall examination: Is there a continuous channel through the 

culvert, without excessive jumps or velocity barriers? 

l Photos taken from the same points year after year, with the same 

camera or lens.

s Inlet upstream and downstream.

s Outlet upstream and downstream.

s Bed details.

s From the road upstream, downstream, and along road both 

directions.

s Other strategic points: Specific stream features or areas expected 

to undergo changes such as scour and deposition.

l Presence of bed material in the pipe: What has changed? 

l Stability of constructed bed forms: What has changed?

l Bed material depth measurement from the top of the culvert, and 

references painted on the culvert wall.

l Long-profile survey for comparing channel adjustment over time.

l Either channel cross sections near the culvert and in strategic spots, 

or a complete site plan (which is sometimes quicker and more 

informative) for monitoring channel alignment changes.

The amount of data you collect will depend on your monitoring objectives, 

and your ability to analyze and keep track of the data. Commonly cited 

problems in both embedded pipes and open-bottom arches involve bed 

material scour.

9Glossary

Glossary

Abandoned channel—An inactive channel. One example is an oxbow isolated by a meander 

cutoff. Abandoned channels also occur where streambed aggradation causes the stream to 

overflow and shift to a new location. 

Active channel—A portion of the channel that is somewhat lower than bankfull, as in the following 

definition: “the portion of the channel commonly wetted during and above winter base flows... identified 

by a break in rooted vegetation or moss growth on rocks along stream margins” (Taylor and Love 2003). 

The ordinary high water mark is sometimes given as the elevation defining the active channel. 

Adjustable channel—A channel where dimensions, slope, planform, etc. change relatively readily in 

response to changes in inputs of water, sediment, and debris. See response reach.

Aggradation—The accumulation of sediment on a streambed, causing streambed elevation to rise. 

Alluvium—Sediment deposited by flowing water, as for example on streambeds, flood plains, and 

alluvial fans. Alluvium does not refer to subaqueous deposits in lakes and seas. 

Anastomosing—A type of channel with multiple channels that separate, meander, braid, or remain 

relatively straight and then rejoin. (A.3.3) Well-vegetated islands separate individual channels. Generally 

have low bed load and stable banks. 

Anadromous—Fish that are born and rear in freshwater, travel to the ocean, then return to spawn in 

their natal stream.

 

Armored streambed—In gravel- and cobble-bed streams, the bed is often segregated into a coarser 

surface layer (the armor layer) over a finer subsurface. This is due to winnowing of the finer particles 

from the surface that is exposed to the force of flow. (A.3.1 and figure A.5)

Arroyo—Flat-floored gullies of ephemeral or intermittent streams in arid or semiarid areas. Arroyos are 

often formed in unconsolidated alluvium and have steep walls. They are dry much of the time, but flash 

floods can transform them into dangerous torrents. 

Backwater—An area where water-surface elevation is controlled by some downstream 

obstruction, such as a constricting bridge, dam, or prevailing countercurrent. 

Bankfull—Describes the volume of flow, and the flow width or depth associated with the 

bankfull elevation: that point where water fills the channel just before beginning to spill onto the flood 

plain. See also active channel. For more discussion of bankfull, see section 5.1.4.2.

Bankline—The sloping ground bordering a stream that confines flow in its natural channel for a range 

of flows below bankfull. 

A
10—4

Stream Simulation

Barrier—A natural or manmade structure that impedes or prevents movement. 

A partial barrier prevents movement of (1) some individuals, or some species some or all of the time, 

or (2) all species and individuals some of the time. The meaning of this term varies with context, and 

should be defined whenever used.

A complete barrier prevents movement of all individuals of the species being discussed all the time. 

Base level and base-level control—The level below which a stream cannot erode its bed. The ultimate 

base level is sea level, although there can be local base levels, such as a resistant formation or lake. A 

base-level control is any structure or feature that prevents downcutting below its elevation.

Bed permeability—The ability of the channel bed materials to transmit fluid. Permeability depends on 

the size voids between particles, how well the voids are connected, and the tortuosity of the path that the 

water travels. Surface flow over a permeable bed may infiltrate so that, at low flows, most or all of the 

water flows below the surface. 

Bedforms—Accumulations of bed material in an alluvial channel formed by stream flow over the 

channel bed. Ripples, dunes, and antidunes are bedforms found mainly in sand-bed channels. Pebble 

clusters and transverse bars are bedforms found in gravel-bed streams. Steps form in cobble- and 

boulder-bed streams.

Bedload transport rates—The volume rate of sediment moving on the bed by rolling, sliding, or 

saltation. Bedload transport rates depend on the transport capacity of the flow, and on the surface 

characteristics of the bed (packing, armoring). 

BMPs—Best management practices are guidelines and procedures for the protection of water quality 

and beneficial uses during land management activities. 

Channel adjustment—Changes in channel cross-sectional form, bed configuration, 

planimetric geometry, and channel bed slope in response to changes in flow, sediment type, and 

sediment and debris loads. Channels can adjust very slowly (e.g., downcutting over decades or 

centuries) or rapidly (e.g., sudden channel shift during floods).

Channel avulsion—Sudden switching of the main flow into a new channel. Avulsion can happen, for 

example, when aggradation or debris jams cause backwatering, overflow, channel incision in a new 

location, and stream capture (Hooke 1997).

Channel incision—The process of channel bed lowering (also downcutting or degradation). ‘Regional’ 

channel incision is downcutting over a long length of channel, sometimes on a watershed scale. Many 

causal factors can enter into regional channel incision, including base level lowering, increases in peak 

or total flows, and decreases in sediment load. 

Channel migration—Change in channel location, commonly caused by bank erosion, point bar 

formation, and/or channel avulsion.
10—5

Glossary

Channel pattern—Describes the channel in planview; most common terms are straight, meandering, 

anastomosing, and braided. (A.3.3)

Channel rocks—A term used in some construction contracts for large rocks used to simulate various 

types of channel-bed structures and key features, such as banks, steps, boulder clusters, bars, etc. 

Gradation is specified separately from the stream-simulation bed mix. Placement in the streamsimulation channel often requires special methods (see 7.5.3).

Channel stability—Stable channels adjust to a wide range of flows and sediment loads by eroding 

and depositing sediments; however, their dimensions and slope fluctuate around averages that remain 

approximately constant over periods of decades or longer. This means that, on average, the amount of 

sediment coming into a stable reach is the same as the amount leaving it. Unstable channels are those 

experiencing large rapid changes in dimensions or slope. (See section A.4)

Channel unit—Section of stream with characteristic bed topography, water-surface slope, depth, and 

velocity; for example, pool, riffle, step, etc. 

Channel-forming flow—Flow that represents a range of flows which determines channel parameters, 

such as cross-sectional geometry and meander wavelength. It also can be thought of as the flow which 

performs the most work by transporting the most sediment. It is sometimes called dominant discharge, 

and is often equated with bankfull discharge. (A.4.1)

Channelized stream—A stream that has been altered by straightening and (usually) deepening (see 

channelization). Streams are sometimes channelized along roads to drain marshy acreage for farming, or 

to control flooding. 

Complete barrier—See barrier.

Consolidation—Describes a sedimentary unit in terms of bulk density or how closely packed the grains 

are. After deposition, the unit becomes more compact as particles adjust under the weight of overlying 

materials. 

Cohesive materials—Silt and clay-rich materials that are bound together by attractive forces. Compared 

to sands, cohesive materials are strong when dry, are more resistant to surface erosion, and have lower 

permeability. Cohesive streambanks tend to fail due to toppling or caving when banks are undercut or 

saturated. 

Colluvium—Soil and rock that has been moved by gravity processes, such as in landslides, debris flows, 

avalanches, or rock falls. 

Confined channel—A channel that is unable to shift laterally because it is bounded by valley walls, or 

other topographic or manmade boundary. 
10—6

Stream Simulation

Contracting officer’s representative (COR)—In the Forest Service, the contracting officer (CO) is 

responsible for all contract actions. The COR represents the CO in the field, evaluating the progress of 

the contract and recommending contract actions when necessary.

Cost-risk analysis—A way to assess the desirability of a long-range project by weighing the probability 

that a costly event will occur in the future against the current cost of a project. For example, in urban 

flood control planning, engineers might weigh the cost of building higher levees against the probability 

and consequences of a large flood.

Cutoff channel—Forms when flow takes a shortcut between two points along the stream, straightening 

the channel and increasing the slope. Neck cutoffs develop across meander bends; chute cutoffs develop 

over point bars. Cutoffs occur when streams can no longer transport the sediment load at the current 

gradient. 

Debris—Material transported by the stream, such as wood or sediment. Commonly used to 

refer to woody debris.

Debris torrent—A rapid channelized flow of water mixed with rock, soil, and mud. Debris 

torrents are generally caused by saturation of the land surface or snowpack by heavy rains. 

Dewatered stream—A stream affected by water withdrawals or upstream storage. Flow is lower than 

it would be naturally, and even if there is some water flowing, water temperature, depth, and continuity 

may be problems for aquatic organisms.

Dip of rock—The angle, measured perpendicular to strike, that a tilted bed or fault forms with the 

horizontal. 

Discontinuous flood plains—Flood plains that are patchy or are obstructed in the longitudinal direction 

along a channel. Discontinuous flood plains occur where the valley is narrow or constricted by natural or 

manmade features.

Diversion potential—The possibility for streamflow to leave its established channel and flow down a 

road or ditch that slopes away from a road-stream crossing (Moll 1997).

Dynamic equilibrium—A stream channel is considered to be in dynamic equilibrium when channel 

dimensions, slope, and planform do not change radically even though they constantly adjust to changing 

inputs of water, sediment, and debris. See channel stability.

Earthflow—A landform formed by (usually) slow movement of a mass of soil and rock 

downhill. They usually occur in fine-grained materials and may move slowly over a period of 

years. Earthflows most often terminate in a lobelike form. 

Embeddedness—Describes the degree to which the voids between larger sedimentary particles are 

filled with finer grains. Embedded gravel streambeds may be less mobile than unembedded ones because 

fines filling the gaps between the larger rocks reduce the surface area exposed to the pressure of water. 
10—7

Glossary

Entrainment (sediment)—The initiation of motion of a sediment particle by flowing water. 

Entrainment flow—The lowest flow at which a particle first begins to move.

Entrenched channel/entrenchment—A channel that does not have a wide flood-prone zone (Rosgen 

1994). May be gullied or confined. (Section A.3.4 discusses entrenchment)

Ephemeral stream—A stream that flows briefly only in direct response to precipitation or snowmelt 

(Wilson and Moore 1998). 

Fines—Streambed particles smaller than 2 millimeters in diameter: sand, silt, and clay.

Flashy—Describes a flow regime characterized by large floods with short peaks. Arid-climate 

rivers are often flashy. 

Flood frequency analysis—The process of analyzing a multiyear record of peak flows (usually 

a gauging station record) to determine the probability that a flood equaling or exceeding a given 

magnitude will occur in any year or during a period of years. (D.2) 

Flood plain—The flat-lying area adjacent to a channel that is flooded on a fairly frequent basis and is 

being constructed by the stream. (See section A.5.4)

Flood-plain conveyance—Refers to the volume and rate of flow carried on the flood plain during 

floods. In this guide, the term is used qualitatively (high or low). Rougher flood plains with forest or 

dense shrubby vegetation would be considered lower-conveyance flood plains, whereas smoother grassy 

surfaces are higher-conveyance flood plains. (5.3.1)

Flood-plain function—Flood-plain functions include the temporary storage of sediment and floodwater. 

Flood plains may also provide diverse habitat for both aquatic and terrestrial wildlife, support riparian 

vegetation that shades and supplies nutrients and debris to the stream, and they may be movement 

corridors for large mammals. (See also valley flat.)

Flood-prone zone—The valley bottom area up to an elevation of twice maximum bankfull depth, 

measured vertically above the thalweg, at any channel/valley cross section. Flood-prone width is the 

width of that zone at a given cross section.

Flow boundary—The wetted perimeter, or contact zone between stream flow and the channel bed and 

banks.

Flow regime—Describes how flow in a stream is distributed throughout the year or across years, both in 

terms of discharge volume and timing. 

Flow resistance—Drag force exerted on flowing water by its boundary. Flow resistance is the force that 

opposes the downslope component of the weight of water (the driving force) and controls water velocity. 

(A.3.6)
10—8

Stream Simulation

Fluvial—Of or pertaining to rivers or streams. 

Forced channel—Montgomery and Buffington (1993 and 1997) define forced channels as those in 

which flow obstructions (such as wood) “force” a channel morphology that is different from what would 

exist if the obstructions were not present. For example, a wood-forced step-pool channel has log steps. 

Fords—A stream crossing where the road surface is elevated only slightly or not at all above the 

channel bed. 

Free-surface resistance—A component of flow resistance associated with the boundary between the 

water surface and the atmosphere. Caused by the distortion of the water surface by waves and hydraulic 

jumps. 

Freshet—A flood of any size resulting from rainfall or snowmelt. 

Gaining streams—An effluent stream, or a stream with a channel below the water table so that 

base flow is provided from the zone of saturation. Antonym: losing stream. 

Genetic drift—A primary mechanism of evolution, by which gene frequency in a population 

changes from one generation to the next, due to chance processes rather than natural selection, 

mutations, or immigration. Genetic drift is especially important in small populations. (from Wikipedia 

and Primate Info Net [Online]. 2008, January)

Geomorphologist—A scientist who studies landforms and land surface processes. Geomorphology is 

the study of the classification, description, nature, and origin of landforms.

Grade control—Anything that controls channel elevation and therefore local channel slope. Grade 

controls can be natural streambed structures or manmade dams, sills, culverts, etc. As used in this guide, 

the term most commonly refers to natural structures, such as logs, riffle crests, boulder steps, etc. See 

also key features and base level. See section 5.1.3.3.

Granular materials—Sediment made up of small rock fragments, or grains. 

Headcut—An abrupt change in channel bed elevation resulting in local steepening. Headcuts 

may be nearly vertical, or more gradual, depending on grain size and consolidation of channel 

materials. 

Headwater streams—Streams at the upstream end of a drainage network. Headwater streams are most 

often classified as first- and second-order streams (see zero-, first-, and second-order streams). 

HEC-RAS—Hydrologic Engineering Center, River Analysis System: a step-backwater model for 

estimating streamflow velocity and other flow characteristics in a river reach.
10—9

Glossary

High bed-design flow—A high flow, which when exceeded may mobilize rocks designed to be 

permanently immobile and possibly cause the simulated streambed to wash out of the culvert (See 

section 6.3.2).

Hydraulic jump—An abrupt change in water depth at a transition from supercritical to subcritical flow, 

forming a stationary wave. Hydraulic jumps occur in a number of situations: over significant obstacles in 

the bed, below a vertical drop, or where an expansion or contraction of the flow occurs. 

Hydraulic radius—The ratio of a streams cross-sectional area to its wetted perimeter. In wide, 

rectangular channels the depth often is used to approximate hydraulic radius. (Figure E-2)

Hydro-physiographic province—A region with characteristic climate, geology, landforms, and 

vegetation. The relief features and landforms of a hydro-physiographic province differ significantly from 

surrounding regions, e.g., Valley and Ridge and Coastal Plain in Eastern United States; Basin and Range 

and Great Plains in Western United States. 

Imbricate/imbrication—Overlapping arrangement of rocks in a stream, similar to overlapping of 

shingles on a roof. Imbricated rocks are more resistant to entrainment than loosely-packed rocks.

Inbreeding depression—A reduction in overall health and vigor of individuals in a population 

as a result of breeding with close relatives over multiple generations (Primate Info Net [Online]. 2008, 

January)

Incised channel—See channel incision.

Intermittent stream—A stream that flows part of the year. Intermittent streams generally flow 

continuously for a month or several months during and after the rainy or snowmelt season—the time of 

year when the ground water table is high enough to supply surface flow. 

Invert—The bottom of a full-bottom culvert.

J-hook vane—A streambank stabilization structure designed to reduce near-bank shear stress 

on the outside bank at channel bends. The structure causes scour in the center of the channel, 

maintaining sediment transport capacity (Rosgen 2006). 

Karst—A type of topography formed on limestone, and characterized by dissolution features such as 

sinkholes, caverns, and underground streams. 

Key features—Anything in the stream channel that the current stream either cannot move or that moves 

only in infrequent floods, and that plays an important role in channel morphology and stability. Key 

features may control grade, provide roughness, retain bed material, and stabilize banks, among other 

functions. They can be rocks, logs, living trees, roots, etc. (5.1.6.2)
10—10

Stream Simulation

Lateral accretion –Sediment deposition along one bank as a river migrates across its flood plain and 

erodes the opposite bank. Point bars are a common lateral accretion feature. 

Lateral migration—see channel migration.

Longitudinal rib or bar—A ridge of gravel, cobbles, or pebbles extending along the channel parallel to flow. 

Like transverse ribs, these ridges are microtopographic features found in gravel-bed streams. 

Losing streams—Streams that lose surface flow as water percolates into their beds to the water table.

Mass wasting—The downslope movement of soil and rock material under the direct influence of 

gravity. Examples include rockslides, landslides, earthflows, and soil creep.

Maximum bankfull depth—In any channel cross section, the distance from the streambed’s lowest 

point (thalweg) vertically up to bankfull elevation.

Meander belt—The zone along the valley floor across which a meandering stream shifts its channel from time 

to time. It may be 15 to 18 times the stream width. (Wilson and Moore 1998)

Mobile-bed channels—Channels where streambed materials move frequently, even at relatively low flows 

below bankfull. 

Nickpoint or Knickpoint—An abrupt drop, or point of inflection, in the longitudinal profile of a 

stream. Usually associated with a lowering of base level, nickpoints migrate upstream and can cause 

rapid channel incision upstream. See also headcut.

Noncohesive—Lacking in attractive forces that cause particles to stick together, so that resistance 

to erosion is based on intergranular friction. Granular materials (e.g., sands, gravels) are generally 

noncohesive. 

Openness—The opposite of confinement. Some species are reluctant to enter a structure if it 

appears to be too confining, possibly due to lack of light or security (predator ambush potential 

or lack of escape routes). Openness is often expressed as the ratio of the cross-section area of a 

structure’s opening (m2 ) divided by structure length (m). 

Ordinary high water—Water surface elevation below bankfull, defined variously by different entities. 

It is defined as follows in Bates (2003): “The Ordinary High Water mark can usually be identified by 

physical scarring along the bank or shore, or by other distinctive signs. This scarring is the mark along 

the bank where the action of water is so common as to leave a natural line impressed on the bank. That 

line may be indicated by erosion, shelving, changes in soil characteristics, destruction of terrestrial 

vegetation, the presence of litter or debris, or other distinctive physical characteristics.”

Outsloping—Cross-sectional road-surface profile that is angled slightly away from the cutbank. This 

method of road construction is used to disperse water from the road surface rather than allowing it to 

flow directly into a stream.
10—11

Glossary

Overbank flow—Water flowing outside the channel boundary over the adjacent land surface. Overbank 

flow generally occurs during floods or when in-channel flow is constricted. 

Palustrine—Generally relates to vegetated, non-tidal wetlands.

Particle size class—Named categories with standard ranges of sediment particle sizes (e.g., 

sand, silt, gravel, cobble, etc.). See table A-1.

Pebble clusters—Microtopographic bed features, on the scale of 10 to100 centimeters in length, 

oriented along the local streamline in gravel-bed streams. Generally, an obstacle protrudes above 

neighboring grains allowing both upstream and downstream accumulation of smaller grains. Pebble 

clusters are known to delay both entrainment and transport of constituent class, and to reduce bedload 

transport rates by increasing flow resistance. 

Perennial stream—A stream that flows year round. 

Permeable roadfill—Fill material (soil and rock) with a relatively high capacity for transmitting water 

used to construct road embankments that permit through-flow of water. Objective is often permitting 

overbank flood flows to filter through the embankment rather than forming a solid dam across the flood 

plain. 

Pivot angle—The angle of repose for noncohesive sediment. The angle that a particle, with a particular 

diameter, has to overcome when rolling over a particle, with a different diameter, that is partly 

underneath and partly in front of it. 

Planform—The channel pattern, or the appearance of a stream from above. The most common 

categories are straight, meandering, and braided. (A.3.3)

Plasticity—The ability to be molded into a different shape without breaking, and to retain that shape 

when the deformating force is removed. 

Pool spacing—The distance between two successive pools, measured from pool tail to pool tail or pool 

head to pool head.

Profile control structure—A structure placed to control grade and elevation of the simulated channel, 

such as a log or boulder weir. Profile controls can be inside or outside the culvert. 

Project profile—The streambed longitudinal profile designed for construction in and around the new 

crossing structure.

Project reach—The stream segment that will be affected by the project, including segments not directly 

constructed, but expected to adjust to the changes made by the project. 
10—12

Stream Simulation

Radius of curvature—Describes the tightness of a meander bend (figure A-11).

Reach—A stream segment. Usually refers to a stream section that is somewhat homogenous, 

and can be characterized based on discharge, depth, area, or slope. A reach can also be the 

length of stream between two gauging stations, between two tributary junctions, or between any two 

points. 

Recruitment—Introduction of woody debris or other elements into a stream from banks, valley 

walls, and from upstream. Wood can be recruited by trees falling into the channel, by landslides, or by 

transport from upstream. Recruitment can also mean the addition of individual animals to a population.

Recurrence interval—The average time interval, in years, between occurrences of a hydrologic event 

of a given or greater magnitude. The probability remains the same from year to year for any event of any 

recurrence interval. A 20-year event will not necessarily occur once every 20 years, but the probability 

that the event will occur in any given year is 0.05. 

Reference reach—A natural stable channel reach used as the design template for stream simulation. (3.2, 5.5) 

Regulated river—River whose flow is controlled by artificial means such as a dam. 

Residual pod depth—Maximum depth of a pool, measured from pool bottom to the pool tail crest (or 

riffle head). See figure 5.19.

Residual soil—Soil formed from the underlying rock. 

Response reach—A stream reach that adjusts to changes in flow and sediment loads by changing its 

morphology. Changes can include widening or narrowing, straightening or increasing sinuosity, incising, 

aggrading, etc. Generally, response reaches have erodible bed and bank material, and they tend to be 

flatter than transport reaches. When upstream sediment inputs increase, sediment tends to deposit in 

response reaches. 

Riparian vegetation—Vegetative community located near a body of water such as a stream. Riparian 

vegetation significantly influences and is significantly influenced by its adjoining body of water. 

River dynamics—Processes and mechanisms of channel change; water, sediment and debris transport; 

and interactions between the channel and surrounding area; 

Road management objective—A statement of the intended purpose of a road, as well as standards for 

its design, management, and maintenance.

Roughness (and relative roughness)—Channel characteristic that causes a drag on flow, limiting 

velocity and increasing diversity. Roughness elements include grains, bedforms, woody debris, 

manmade structures, and bank irregularities. Relative roughness is the ratio of hydraulic radius to grain 

size. As depth increases with discharge at a cross section, relative roughness decreases and the effects of 

grain roughness are drowned out. 
10—13

Glossary

Sediment load—The amount or volume of sediment that is being transported by a stream, 

including both bed load (sediment that rolls or bounces along the streambed) and suspended 

load (finer sediment that travels suspended in the water column). 

Sediment regime—Describes how sediment transport in a stream is distributed throughout the year or 

across years, in terms of particle size, amount, and timing.

Sediment transport capacity—The maximum amount of sediment that a given flow can move in a 

stream channel. 

Shear stress—a measure of the hydrodynamic (erosive) force exerted by flowing water on channel bed 

and banks (see sections A.5.1 and E.1).

Side channel—A secondary channel that carries a small volume of the total flow. Many processes for 

side channel development exist; for example, a side channel can be an abandoned meander bend, or it 

may have formed by scour during overbank flood flows.

Simulated channel—The stream simulation channel bed contained in the crossing installation, 

generally inside the culvert. 

Slump—Type of mass wasting event in which a mass of rock or unconsolidated material slides along a 

concave slide plane. 

Sorting—A process that occurs during sediment transport events by which sedimentary particles are 

segregated by size, shape or weight. A well-sorted streambed is composed of a narrower range of 

sediment sizes than a poorly-sorted streambed. 

Stage—The elevation of the water surface in a stream channel. A flood stage is the elevation of the water 

surface during the flood.

Stream connectivity—Describes the transfer of matter, energy, and organisms by water within and 

between all components of the stream ecosystem including the channel, flood plain, and alluvial aquifer. 

Stream corridor—The stream channel and associated riparian area, including the flood plain. 

Streambed continuity—Describes how well connected (or how fragmented) the streambed is along 

its length. Weirs, baffles, bare culverts, etc. disrupt streambed continuity and may limit movement of 

benthic organisms and aquatic and riparian-dependent species that require dry or shallowly-submerged 

surfaces for movement. 

Streambed structure—The geomorphic forms comprising a streambed: channel units such as pools and 

riffles, steps, etc.; grade controls; bank configuration and composition. 

Strike of rock—The geographic direction and angle between true north and a horizontal line of any 

planar geologic feature: bedding, faults, or dikes. 
10—14

Stream Simulation

Structural design flow—A high flow which, when exceeded may cause the crossing structure to fail. 

(Contrasts to high bed design flow.) 

Terrace—A relatively level bench or flat steplike surface above the flood plain. An alluvial 

terrace is the relict of a former flood plain before regional uplift or increase in discharge caused 

erosion and incision into the former flood plain. Other types of terraces include marine terraces 

and structural terraces. 

Thalweg—The longitudinal profile line, or line connecting the lowest points along a streambed. (5.1.3)

Tidal rivers—Coastal rivers influenced by tidal fluctuations. 

Transport reach—A stream reach that resists changes in morphology when flows and sediment loads 

change. Transport reaches are generally steeper and coarser-grained than response reaches. When 

sediment input increases, the added load is simply transported through the reach. 

Transverse bars—Relatively broad flat surfaces with a crest oriented perpendicular to flow. Transverse 

bars are at least several particles in length (from upstream to downstream) and can extend completely 

or partially across the channel. They typically form downstream of pools where flow begins to diverge 

as the channel widens, and are located at the pool-tail crest (or riffle head). Coarse-grained bars are 

typically well armored with particles that are tightly packed and well imbricated. They may be immobile 

up to high discharges, and usually function as reach-scale hydraulic grade controls. 

Valley flat—The area adjacent to the channel that is relatively flat and is bordered by hillslopes. 

The valley flat may include the flood plain and one or more terraces. Also valley floor or valley 

bottom. 

Vertical accretion—Process of accumulation of sediment on flood plains during overbank flows. 

Vertical adjustment potential—the vertical range of possible streambed elevations over the life of the 

structure (5.2.2.2).

Well-graded—Refers to coarse-grained sediments that have a continuous distribution of 

particle sizes, such that smaller grains fill the spaces between the larger grains (AGI 1962). 

Synonym: poorly sorted. 

Width-depth ratio—Bankfull width divided by mean bankfull depth (average across the cross section). 

(A.3.4).

Woody debris—Logs, limbs, and rootwads found in streams. Woody debris plays important roles in 

stream ecosystems by increasing boundary roughness and flow resistance; providing storage areas for 

sediment and organic material; providing cover for fish; controlling grade and increasing profile and 

substrate diversity. 
10—15

Glossary

Zero-, first-, and second-order streams—Stream ordering is a system of classifying stream 

segments based on location in the drainage network. A zero-order stream is an unchannelized 

valley or swale. First-order streams are segments with no tributaries. Second-order streams are 

formed by the junction of two first-order streams.

10 References
Stream Simulation
References

Abbe, T. B.; Montgomery, D. R. 1996. Large woody debris jams, channel hydraulics, and 

habitat formation in large rivers. Regulated Rivers: Research and Management. 12, 201-

221.

Abt, S. R.; Wittler, R. J.; Ruff, J. F.; LaGrone, D. L.; Khattak, M. S.; Nelson, J. D.; Hinkle, N. E.; Lee, 

D. W. 1988. Development of riprap design criteria by riprap testing in flumes: phase II; 

Follow-up Investigations Volume 2, NUREG/CR-4651, ORNL/TM-10100/V2, prepared 

for U.S. Nuclear Regulatory Commission, Washington D.C.

American Association of State Highway and Transportation Officials. 2002. Standard specifications for 

highway bridges, 17th edition (HB-17). Washington DC: American Association of State 

Highway and Transportation Officials. 1,028 p.

Andrews, E. D. 1979. Scour and fill in a stream channel, East Fork River, western Wyoming. U.S. 

Geological Survey Professional Paper 1117. 49 p.

Andrews, E. D. 1983. Entrainment of gravel from naturally sorted riverbed material: Geological Society 

of America Bulletin. 94, 1,184-1,192.

Andrews, E. D.; Parker G. 1987. Formation of a coarse surface layer as the response to gravel mobility. 

In: Sediment transport in gravel-bed rivers, Thorne, C. R.; Bathurst, J. C.; Hey, R. D. 

(eds). New York: John Wiley and Sons. 269-300.

Arcement, G. J., Jr.; Schneider, V. R. 1989. Guide for selecting Manning’s roughness coefficient for natural 

channels and flood plains. U.S. Geological Survey Water-Supply Paper 2339. 38 p.

Ault, P; Ellor, J. A. 2000. Durability analysis of aluminized type 2 corrugated metal pipe. FHWARD-97-140. McLean VA: Federal Highway Administration. 106 p.

Bagnold, R. A. 1960. Some aspects of the shape of river meanders. U.S. Geological 

Survey Professional Paper 282E. 135-144.

Barnard, R. 2003. [Personal communication]. June 11. Olympia, WA: Washington 

Department of Fish and Wildlife.

Barnes H. H., Jr. 1967. Roughness characteristics of natural channels. U.S. Geological 

Survey Water-Supply Paper 1849. 213 p.

A

B
11—4

Stream Simulation

Bates, K. K. 2003. Design of road culverts for fish passage. Washington Department of Fish and 

Wildlife. http://wdfw.wa.gov/hab/engineer/cm/, accessed 1/20/06.

Bathurst, J. C. 1985. Flow resistance estimation in mountain rivers. American Society of Civil 

Engineers. Journal of Hydraulic Engineering. 111:625-641.

Bathurst, J. C. 1987. Critical conditions for bed material movement in steep, boulder-bed streams. 

In: Erosion and Sedimentation in the Pacific Rim. Wallingford, U.K.: Institute of 

Hydrology, IAHS Publication. 133, 91-6.

Bathurst, J. C. 1997. Environmental river flow hydraulics. In: Thorne, C. R.; Hey, R. D.; Newson, M. 

D., eds. Applied Fluvial Geomorphology for River Engineering and Management. 

Chichester, U.K.: Wiley and Sons. 69-93.

Bathurst, J. C.; Graf, W. H.; Cao, H. H. 1987. Bed load discharge equations for steep mountain rivers. 

In: Thorne, C. R.; Bathurst, J. C.; Hey, R. D., eds. Sediment Transport in Gravel-bed 

Rivers. Chichester, U.K.: Wiley and Sons. 453-77.

Beamish, F. W. H. 1978. Swimming capacity. In: Hoar, W. S.; Randall, D. J., eds. Fish Physiology. 

Volume VII. New York: Academic Press. 101-187.

Behlke, C. E.; Kane, D. L.; McLean, R. F.; Travis, M. D. 1991. Fundamentals of culvert design for 

passage of weak-swimming fish. FHWA-AK-RD-90-10. 

Benda, L. E.; Cundy, T. W. 1990. Predicting deposition of debris flows in mountain channels. Canadian 

Geotechnical Journal. 27(4):409-417.

Bigelow, P. E. 2005. Testing and improving predictions of scour and fill depth in a northern California 

coastal stream. River Research and Applications. 21: 909-923.

Brayshaw, A. C.; Frostick, L. F.; Reid, I. 1983. The hydrodynamics of particle clusters and sediment 

entrainment on coarse alluvial channels. Sedimentology. 30, 127-43.

Brown, S. A.; Clyde, E. S. 1989. Design of riprap revetment. Hydraulic Engineering Circular 11. U.S. 

Federal Highway Administration, McLean VA. NTIS # PB89-218424. 169 p.

Browning, M. C. 1990. Oregon culvert fish passage survey. Vancouver, WA: U.S. Department of 

Transportation, Federal Highway Administration, Western Federal Lands Division.
11—5

References

Brudin, C. O. 2003. Wildlife use of existing culverts and bridges in north central Pennsylvania. In: 2003 

Proceedings of the International Conference on Ecology and Transportation. Irwin, C. 

L.; Garrett, P.; McDermott, K. P., eds. Raleigh, NC: North Carolina State University, 

Center for Transportation and the Environment. 344-352.

Bryant M. D.; Zymonas, N. D.; Wright, B. E. 2004. Salmonids on the fringe: abundance, species 

composition, and habitat use of salmonids in high-gradient headwater streams, southeast 

Alaska. Transactions of the American Fisheries Society. 133:1,529-1,538.

Buffington, J. M.; Montgomery, D. R. 1997. A systematic analysis of eight decades of incipient motion 

studies, with special reference to gravel-bedded rivers. Water Resources Research. 33, 

1,993-2,029.

Buffington, J. M.; Montgomery, D. R. 1999a. Effects of hydraulic roughness on surface textures of 

gravel-bed rivers. Water Resources Research. 35(11): 3,507-3,521.

Buffington, J. M.; Montgomery, D. R. 1999b. Effects of sediment supply on surface textures of gravelbed rivers. Water Resources Research. 35(11): 3,523-3,530.

Bunte, K. 2004. [Personal communication]. Fort Collins, CO: Engineering Research Center, Colorado 

State University.

Bunte, K.; Abt, S. R. 2001. Sampling surface and subsurface particle-size distributions in wadable 

gravel-and cobble-bed streams for analyses in sediment transport, hydraulics, and 

streambed monitoring. Gen. Tech. Rep. RMRS-GTR-74. Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research Station. 428 p.

Carling, P. A. 1992. In-stream hydraulics and sediment transport. In: Calow, P.; Petts, G. 

E., eds. The Rivers Handbook. Oxford, U.K.: Blackwell Publishing. 101-125.

Castro, J. 2003. Geomorphic impacts of culvert replacement and removal: avoiding channel incision. 

Portland, OR: U.S. Fish and Wildlife Service, Oregon Fish and Wildlife Office. 

http://library.fws.gov/Pubs1/culvert-guidelines03.pdf

Castro, J. M.; Jackson, P. L. 2001. Bankfull discharge recurrence interval and regional hydraulic 

geometry relationships. Patterns in the Pacific Northwest, USA. Journal of American 

Water Resources Association. 37(5):1,249-1,262

Chin, A. 1989. Step pools in stream channels. Progress in Physical Geography. 13, 391-407.

C
11—6

Stream Simulation

Chow, V. T. 1959. Open-channel hydraulics. New York: McGraw-Hill Book Company. 679 p.

Chow, V. T.; Maidment, D. R.; Mays, L.W. 1988. Applied hydrology. New York: McGraw-Hill Book 

Company. 572 p.

Church, M. 1978. Paleohydrological reconstructions from a holocene valley fill. In: Miall, A. D. ed. 

Fluvial Sedimentology. Calgary: Canadian Society of Petroleum Geologists, Memoir 5. 

743-772.

Clarkin, K.; Connor, A.; Furniss, M.; Gubernick, B.; Love, M.; Moynan, K.; WilsonMusser, S. 2003. 

National inventory and assessment procedure for identifying barrriers to aquatic 

organism passage at road-stream crossings. San Dimas, CA: U.S. Department of 

Agriculture, Forest Service, National Technology and Development Program, San 

Dimas Technology and Development Center. 

http://fsweb.sdtdc.wo.fs.fed.us/programs/eng/aopxing/

Clarkin, K. L.; Keller, G.; Warhol, T.; Hixson, S. 2006. Low-water crossings: geomorphic, biological, 

and engineering design considerations. 0625-1808—SDTDC. San Dimas, CA: U.S. 

Department of Agriculture, Forest Service, National Technology and Development 

Program, San Dimas Technology and Development Center. 360 p.

Coffman, J. S. 2005. Evaluation of a predictive model for upstream fish passage through culverts. 

Harrisonburg, VA: James Madison University. 104 p. Thesis.

Cooper, A. B.; Mangel, M. 1999. The dangers of ignoring metapopulation structure for the conservation 

of salmonids. Fisheries Bulletin. 97(2): 213-226.

Cooper, R. M. 2006. Estimation of peak discharges for rural, unregulated streams in western Oregon. 

Oregon Water Resources Department. Open File Report SW 06-001. 86 p.

Copeland, R. R.; McComas, D. N.; Thorne, C. R.; Soar, P. J.; Jonas, M. M.; Fripp, J. B. 2001. Hydraulic 

design of stream restoration projects. ERDC/CHL TR-01-28. Vicksburg, MS: U.S. 

Army Corps of Engineers, Coastal and Hydraulics Laboratory, Engineer Research and 

Development Center, Waterways Experiment Station. 109 p.

Dunham, J. B.; Rieman, B. E. 1999. Metapopulation structure of bull trout: influences 

of physical, biotic, and geometrical landscape characteristics. Ecological Applications. 

9(2): 642-655.

D
11—7

References

Dunham, J. B.; Vinyard, G. L.; Rieman, B. E. 1997. Habitat fragmentation and extinction risk of 

Lahontan cutthroat trout. North American Journal of Fisheries Management. 17: 1,126-

1,133.

Dunne, T.; Leopold, L. B. 1978. Water in environmental planning. New York: W.H. Freeman and Co.

Emmett, W. W. 2004. A historical perspective on regional channel geometry curves. 

Stream Notes January 2004. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station, Stream Systems Technology Center. 

Available at: http://www.stream.fs.fed.us

Erman, D. C.; Hawthorne, V. M. 1976. The quantitative importance of an intermittent stream in the 

spawning of rainbow trout. Transactions of the American Fisheries Society. 105: 675-

681.

Fahrig L.; Merriam, G. 1985. Habitat patch connectivity and population survival. 

Ecology. 66(6): 1,762-1,768.

Fausch, K. D.; Rieman, B. E.; Young, M. K.; Dunham, J. B. 2006. Strategies for conserving native 

salmonid populations at risk from nonnative fish invasions: tradeoffs in using barriers 

to upstream movement. Gen. Tech. Rep. RMRS-GTR-174. Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research Station. 44 p.

 

Federal Emergency Management Agency. 1995. Flood insurance study guidelines and specifications for 

study contractors: Federal Emergency Management Report, FEMA 37. 200 p.

Federal Highway Administration. 2003. Manual on uniform traffic control devices. [Online]. Available: 

http://mutcd.fhwa.dot.gov [7/18/2007].

Federal Highway Administration. 2003b. Standard specifications for construction of roads and bridges 

on federal highway projects. FP-03. [Online]. Available: 

http://www.wfl.fha.dot.gov/design/specs/fp03.htm [7/9/07].

Federal Interagency Stream Restoration Working Group. 1998. Stream Corridor Restoration: Principles, 

Practices, and Processes. PB 98158348. Available from National Technical Information 

Service at. http://www.ntis.gov 

Ferro, W. 1999. Evaluating friction factor for gravel-bed channel with high boulder concentration. 

Journal of Hydraulic Engineering. 125(7): 771-778.

E

F
11—8

Stream Simulation

Fetherston, K. L.; Naiman, R. J.; Bilby, R. E. 1995. Large woody debris, physical process, and 

riparian forest development in montane river networks of the Pacific Northwest. 

Geomorphology, 13, 133-144. 

Findlay, C. S.; Bourdages, J. 2000. Response time of wetland biodiversity to road construction on 

adjacent lands. Cons. Biol. 14(1): 86-94.

Findlay, C. S.; Houlahan, J. 1997. Anthropogenic correlates of species richness in Southeastern Ontario 

wetlands. Conservation Biology. 11: 1,000-1,009.

Fischenich, C.; Seal, R. 2000. Boulder clusters. Technical Note ERDC TN-EMRRP-SR-11. Vicksburg, 

MS: U.S. Army Engineer Research and Development Center. Ecosystem Management 

Restoration Research Program EMRRP Technical Notes Collection. http://el.erdc.

usace.army.mil/elpubs/pdf/sr11.pdf accessed 6/11/2007.

Fleming, D. F. 1989. Effects of spawning run delay on spawning migration of artic grayling. Fairbanks, 

AK: University of Alaska, Alaska Cooperative Fisheries Research Unit, Unit 

Contribution #29.

Food and Agriculture Organization of the United Nations. 2002. Fish passes - Design, dimensions and 

monitoring. Originally published in German by the German Association for Water 

Resources and Land Improvement (DVWK) in 1996. Translated and published in 

English by Food and Agriculture Organization of the United Nations.

Forman, R. T. T.; Sperling, D.; Bissonette, J. A.;[and others]. 2003. Road Ecology. Washington: Island 

Press. 481 p.

Franklin, I. R. 1980. Evolutionary change in small populations. In: Soulé, M. E.; Wilcox, B.A. (eds). 

Conservation Biology: An Evolutionary-Ecological Perspective. Sunderland, MA: 

Sinauer Associates. 135-149.

Fuller, W. B.; Thompson, S. E. 1907. The laws of proportioning concrete. Journal of the Transportation 

Division, American Society of Civil Engineers. Volume 59. Paper No. 1053. 67-143.

Furniss, M. J.; Ledwith, T. S.; Love, M. A.; McFadin, B. C.; Flanagan, S. A. 1998. Response of roadstream crossings to large flood events in Washington, Oregon, and Northern California. 

9877-1806—SDTDC. In: Water-Roads Interaction Technology Series. San Dimas, CA: 

U.S. Department of Agriculture, Forest Service, National Technology and Development 

Program, San Dimas Technology and Development Center. 14 p.
11—9

References

Furniss, M. J.; Love, M.; Flanagan, S. A. 1997. Diversion potential at road-stream crossings. 

9777-1814—SDTDC. In: Water-Roads Interaction Technology Series. San Dimas, CA: 

U.S. Department of Agriculture, Forest Service, National Technology and Development 

Program, San Dimas Technology and Development Center. 12 p.

Gehrals, G. E.; Berg, H. C. 1992. Geologic map of southeastern Alaska. U.S. Geological 

Survey Miscellaneous Investigation Series MAP 1-1867. 24 p.

Gowan, C.; Young, M. K.; Fausch, K. D.; Riley, S. C. 1994. Restricted movement in resident stream 

salmonids: a paradigm lost? Canadian Journal Fisheries and Aquatic Sciences. 51: 

2,626-2,637.

Grant, G. 1988. The RAPID technique: a new method for evaluation of downstream effects of forest 

practices on riparian zones. Gen. Tech. Rep. PNW-220. Portland, OR: U.S. Department 

of Agriculture, Forest Service, Pacific Northwest Forest Research Station. 40 p.

Grant, G. E.; Swanson, F. J.; Wolman, M. G. 1990. Pattern and origin of stepped-bed morphology 

in high-gradient streams, western Cascades, Oregon. Geological Society of America 

Bulletin. 102, 340-352.

Hall, C. A. 1972. Migration and metabolism in a temperate stream ecosystem. Ecology. 

53(4): 585-604.

Hanski, I.; Gilpin, M. E. 1991. Metapopulation dynamics: brief history and conceptual domain. 

Biological Journal of the Linnean Society. 42: 3-16.

Hardison, C. H. 1971. Prediction error of regression estimates of streamflow characteristics at ungauged 

sites. U.S. Geological Survey Professional Paper 750-C. 228-236.

Hardy, T.; Palavi, P.; Mathias, D. 2005. WinXSPRO, A channel cross section analyzer, user’s manual, 

Version 3.0. Gen. Tech. Rep. RMRS-GTR-147. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Research Station. 95 p.

Harig, A. L.; Fausch, K. D. 2002. Minimum habitat requirements for establishing translocated cutthroat 

trout populations. Ecological Applications. 12(2): 535-551.

Harrelson, C. C.; Rawlins, C. L.; Potyondy, J. P. 1994. Stream channel reference sites: an illustrated 

guide to field technique. Gen. Tech. Rep. RM-245. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 61 p.

G

H
11—10

Stream Simulation

Haschenburger, J. K. 1999. A probability model of scour and fill depths in gravel-bed channels. Water 

Resources Research. 35, 2,857-2,869.

Hazlett, B.; Rittschof, D.; Rubenstein, D. 1974. Behavioral biology of the crayfish Orconectes virilis I. 

Home Range. American Midland Naturalist. 92(2): 301-319.

Hey, R. D. 1997. Stable river morphology. In: Thorne, C. R.; Hey, R. D.; Newson, M. D. eds. Applied 

Fluvial Geomorphology for River Engineering and Management. Chichester, U.K.: 

Wiley and Sons. 223-236.

Hickin, E. J. 1984. Vegetation and river channel dynamics. Canadian Geography. 28, 111-126.

Hicks, D. M.; Mason, P. D. 1998. Roughness characteristics of New Zealand rivers. National Institute of 

Water and Atmospheric Research, ltd. Water Resources Publications. Englewood, CO. 

329 p.

Hooke, J. M. 1997. Styles of channel change. In: Thorne, C. R.; Hey, R. D.; Newson, M. D. eds. Applied 

Fluvial Geomorphology for River Engineering and Management: Chichester, U.K.: 

Wiley and Sons. 237-268.

Interagency Advisory Committee on Water Data. 1982. Guidelines for determining flood 

flow frequency. Water Resources Council Bulletin 17B. 28 p.

Jackson, W. L.; Beschta, R. L. 1982. A model of two-phase bedload transport in an 

Oregon coast range stream. Earth Surface Processes and Landforms. 7, 517-27.

Jarrett, R. D. 1985. Determination of roughness coefficients for streams in Colorado. Water-Resources 

Investigations Report 85-4004. Denver, CO: U.S. Geological Survey. 54 p. 

Johnson, P. A.; Gleason, G. L.; Hey, R. D. 1999. Rapid assessment of channel stability in vicinity of road 

crossing. Journal of Hydraulic Engineering. 25(6): 645-651.

Julien, P. Y. 1995. Erosion and sedimentation. New York: Cambridge University Press. 280 p.

Katapodis, C. 2005. Developing a toolkit for fish passage, ecological flow management, 

and fish habitat works. Journal of Hydraulic Research. 43(5): 451-467.

I

J

K
11—11

References

Keevin, T. M., Ph.D.; Hempen, G. L., Ph.D., P.E., R.G. 1997. The environmental effects of underwater 

explosions with methods to mitigate impacts. U.S. Army Corps of Engineers, August 97. 

(https://www.denix.osd.mil/denix/Public/ES-Programs/Conservation/WaterX/water1.

html#toc). 

Knighton, D. 1998. Fluvial forms and processes: A new perspective. London: Arnold. 383 pp.

Komar, P. D. 1987. Selective grain entrainment and the empirical evaluation of flow competence. 

Sedimentology. 34, 1,165-1,176.

Komar, P. D.; Carling, P. A. 1991. Grain sorting in gravel-bed streams and the choice of particle sizes for 

flow-competence evaluations. Sedimentology. 38, 489-502.

Komar, P. D.; Li, Z. 1986. Pivoting analyses of the selective entrainment of sediments by shape and size 

with application to grain threshold. Sedimentology. 33, 425-36.

Koster, E. H. 1978. Transverse ribs; their characteristics, origin, and paleohydraulic significance. In: 

Miall, A. D. ed. Fluvial Sedimentology. Canadian Society of Petroleum Geologists. 

Memoir 5, Calgary. 161-86.

Lang, M.; Love, M.; Trush, W. 2004. Improving stream crossings for fish passage. 

Report prepared for U.S. Department of Commerce, National Marine Fisheries Service, 

Southwest Region. Contract 50ABNF800082 by Humboldt State University Foundation. 

128 p. 

Leigh, E. G., Jr. 1981. The average lifetime of a population in a varying environment. Journal of 

Theoretical Biology. 90: 213-239.

Lemkuhl, J. F. 1984. Determining size and dispersion of minimum viable population for land 

management planning and species conservation. Environmental Management. 8: 167-

176.

Leopold, L. B.; Wolman, M. G.; Miller, J. P. 1964. Fluvial processes in geomorphology. New York: 

Dover Publications, Inc.

Letcher, B. H.; Nislow, K. H.; Coombs, J. A.; O’Donnell, M. J.; Dubreuil, T. L. 2007. Population 

response to habitat fragmentation in a brook trout population. Public Library of ScienceBiology.

L
11—12

Stream Simulation

Levy, S. 1997. Pacific salmon bring it all back home. BioScience. 47(10): 657-660.

Lewin, J.; Macklin, M. G.; Newson, M. D. 1988. Regime theory and environmental change—

irreconcilable concepts? In: White, W. R. ed. International conference on river regime. 

Chichester, U.K.: Wiley and Sons. 431-445.

Limerinos, J. T. Determination of the Manning coefficient from measured bed roughness in natural 

channels: U.S. Geological Survey Water-Supply Paper 1898-B. 47 p.

Linsley, R. K.; Kohler, M. A.; Paulhus, J. L. 1982. Hydrology for engineers. Third edition. New York: 

McGraw-Hill Book Company. 508 p. 

Lowe, W. H.; Bolger, D. T. 2002. Local and landscape-scale predictors of salamander abundance in New 

Hampshire headwater streams. Conservation Biology. 16(1):183-193.

Macklin, M. G.; Lewin, J. 1997. Channel, floodplain and drainage basin response to 

environmental change. In: Thorne, C. R.; Hey, R. D.; Newson, M. D. eds. Applied 

Fluvial Geomorphology for River Engineering and Management. Chichester, U.K.: 

Wiley and Sons. 15-45.

Madej, M. A. 2001. Development of channel organization and roughness following sediment pulses in 

single-thread, gravel bed rivers. Water Resources Research. 37(8): 2,259-2,272.

McBain, S.; Trush, W. 2004. Attributes of Bedrock Sierra Nevada River Ecosystems. Stream Notes, January 

2004. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Research Station, Stream Systems Technology Center. http://

www.stream.fs.fed.us

McCuen, R. H. 2003. Modeling hydrologic change: statistical methods. Washington D.C.: Lewis 

Publishers. 433 p.

McIntosh, B. A.; Sedell, J. R.; Smith, J. E.; Wissmar, R. C.; Clarke, S. E.; Reeves, G. H.; Brown, L. A. 

1994. Historical changes in fish habitat for select river basins of Eastern Oregon and 

Washington. Northwest Science. 68 Special Issue.

Meyer J. L.; Strayer, D. L.; Wallace, J. B.; Eggert, S. L.; Helfman, G. S.; Leonard, N.E. 2007. The 

contribution of headwater streams to biodiversity in river networks. Journal of the 

American Water Resources Association. 43(1): 86-103.

M
11—13

References

vMeyer-Peter, E.; Muller, R. 1948. Formulas for bed-load transport: Stockholm: International 

Association of Hydraulic Engineering and Research. Proceedings 2nd Meeting. 39-64.

Meyers et al. 1992 as cited in Fausch, K. D.; Torgersen, C. E.; Baxter, C. V.; Li, H.W. 2002. Landscapes 

to Riverscapes: bridging the gap between research and conservation of stream fishes. 

BioScience. 52(6): 483-498.

Moll, J. 1997. Glossary of water-road interaction terminology. In: Water/road interaction technology 

series. 9777-1806—SDTDC. San Dimas, CA: U.S. Department of Agriculture, Forest 

Service, National Technology and Development Program, San Dimas Technology and 

Development Center. 16 p.

Montgomery, D. R.; Buffington, J. M. 1993. Channel classification, prediction of channel response, and 

assessment of channel condition. Report TFW-SH10-93-002 prepared for the SHAMW 

committee of the Washington State Timber, Fish, and Wildlife Agreement. 84 p.

Montgomery, D. R.; Buffington, J. M.; 1997. Channel classification, prediction of channel response, and 

assessment of channel condition. GSA Bulletin. 109(5): 596-611.

Morita, K.; Yamamoto, S. 2002. Effects of habitat fragmentation by damming on the persistence of 

stream-dwelling charr populations. Conservation Biology. 16(5): 1,318-1,323.

Naiman, R. J.; Beechie, T. J.; Benda, L. E.; Berg, D. T.; Bisson, P. A.; MacDonald, L. H.; 

O’Connor, M. D.; Olson, P. L.; Steel, E. A. 1992. Fundamental elements of ecologically 

healthy watersheds in the Pacific Northwest Coastal Ecoregion. In: Watershed 

Management: Balancing Sustainability and Environmental Change, Naiman, R. J. ed. 

New York: Springer-Verlag, Inc.

Nanson, G. C.; Croke, J. C. 1992. A genetic classification of floodplains. Geomorphology. 4, 459-486.

National Corrugated Steel Pipe Association (NCSPA). No date. Installation manual for corrugated steel 

pipe, pipe arches, structural plate. [Online]. Available: http://

www.ncspa.org/images/InstallManual_00.pdf [12 July 2007].

Nelson, J. M.; Shreve, R. L.; McLean, S. R.; Drake, T. G. 1995. Role of near-bed turbulence structure in 

bed transport and bed form mechanics. Water Resources Research. 31, 2,071-2,086.

Neville, H. M.; Dunham, J. B.; Peacock, M. M. 2006. Landscape attributes and life history variability 

shape genetic structure of trout populations in a stream network. Landscape Ecology. 21: 

901-916.

N
11—14

Stream Simulation

Newbury, R.; Gaboury, M. N. 1993. Stream analysis and fish habitat design: a field manual. Gibsons, BC 

Canada: Newbury Hydraulics Ltd. [Online]. Available: 

http://www.newbury-hydraulics.com. [7/6/07]

Northcote, T. G. 1997. Potamodromy in Salmonidae – living and moving in the fast lane. North 

American Journal of Fisheries Management. 17: 1029-1,045.

Occupational Health and Safety Administration (OSHA). 1999. Excavations: hazard 

recognition in trenching and shoring. OSHA technical manual section V chapter 2. 

Washington D.C.: Office of Science and Technology Assessment. [Online] Available: 

http://www.osha.gov/dts/osta/otm/otm_v/otm_v_2.html [2007, July 27]

Omang, R .J. 1992. Analysis of the mangnitude and frequency of floods and the peak flow gaging 

network in Montana. U.S. Geological Survey Water Resources Investigations Report 92-

4048. Helena, MT.

Oregon Department of Transportation, Highway Department. 2005. Hydraulics Manual. [Online]. Available: 

http://www.oregon.gov/ODOT/HWY/GEOENVIRONMENTAL/hyd_manual_info.shtml. 

[16 July 2007]..

Peake, S. 2004. An evaluation of the use of critical swimming speed for determination 

of culvert water velocity criteria for smallmouth bass. Transactions of the American 

Fisheries Society. 133: 1,472-1,479.

Pennsylvania Department of Environmental Protection (DEP). 2000. Erosion and sediment pollution 

control program manual. [On-line]. Available: 

http://www.filtrexx.com/downloads/Penn_DEP_Erosion_and_Sediment_Control_Manual.pdf 

[2007, August 3].

Pfankuch, D. J. 1978. Stream reach inventory and channel stability evaluation: a watershed management 

procedure. Unpublished report. Missoula, MT: U.S. Department of Agriculture, Forest 

Service, Northern Region. 25 p.

Platts, W. S.; Megahan, W. F.; Minshall, G.W. 1983. Methods for evaluating stream, riparian, and biotic 

conditions. Gen. Tech. Rep. INT-138. Ogden, UT: U.S. Department of Agriculture, 

Forest Service, Intermountain Forest and Range Experiment Station. 70 p.

O

P
11—15

References

Plummer, M. V.; Mills, N. E.; Allen, S. L. 1997. Activity, habitat, and movement patterns of softshell 

turtles (Trionyx spiniferus) in a small stream. Chelonian Conservation and Biology. 2(4): 

514-520.

Reed, D.F. 1981. Mule deer behavior at a highway underpass exit. Journal of Wildlife 

Management. 45(2): 542-543.

Reid, I.; Bathurst, J. C.; Carling, P. A.; Walling, D. E.; Webb, B. W. 1997. Sediment Erosion, Transport, 

and Deposition. In: Thorne, C. R.; Hey, R. D.; Newson, M. D. eds. Applied Fluvial 

Geomorphology for River Engineering and Management. Chichester, U.K.: Wiley and 

Sons. 96-133.

Reid, I.; Frostick, L. E. 1994. Fluvial sediment transport and deposition. In: Pye, K. (ed), Sediment 

transport and depositional processes. Oxford: Blackwell Scientific Publications. 89-156.

Reid, I.; Frostick, L. E.; Latmn, J. T. 1985. The incidence and nature of bedload transport during flood 

flows in coarse-grained alluvial channels. Earth Surface Processes and Landforms. 10, 

33-44.

Richardson, E. V.; Simons, D. B.; Julien, P.Y. 1990. Highways in the River Environment. FHWAHI-90-016. Design and training manual for U.S. Department of Transportation, Federal 

Highway Administration. 683 p.

Rieman, B. E.; Allendorf, F. W. 2001. Effective population size and genetic conservation criteria for bull 

trout. North American Journal of Fisheries Management. 21: 756-764.

Rieman, B. E.; McIntyre, J. D. 1993. Demographic and habitat requirements for the conservation of bull 

trout Salvelinus confluentus. Gen. Tech. Rep. INT-302. U.S. Department of Agriculture, 

Forest Service, Intermountain Research Station.

Riggs, H. C. 1973. Regional analysis of streamflow characteristics. U.S. Geological Survey Techniques 

of Water Resources Investigations, Book 4, Chapter B3. 15 p.

Rosgen, D. L. 1996. Applied river morphology. Pagosa Springs, CO: Wildland Hydrology. 

Rosgen, D. L. 1994. A classification of natural rivers. Catena. 22, 169-199.

R
11—16

Stream Simulation

Rosgen, D. L. 2006. The cross-vane, w-weir and J-hook vane structures: their description, design and 

application for stream stabilization and river restoration. Web-published by Wildland 

Hydrology, Inc. Available: www.wildlandhydrology.com [June 2007].

Saldi-Caromile, K.; Bates, K.; Skidmore, P.; Barenti, J.; Pineo, D. 2004. Stream habitat 

restoration guidelines: final draft. Co-published by the Washington Departments of 

Fish and Wildlife and Ecology and the U.S. Fish and Wildlife Service. Olympia, WA. 

Available at http://wdfw.wa.gov/hab/ahg/shrg/

Schumm, S. A. 1977. The fluvial system. New York: Wiley and Sons.

Schumm, S. A.; Harvey, M. D.; Watson, C. C. 1984. Incised channels. Littleton, CO: Water Resources 

Publications. 220 p.

Shaffer, M. L. 1981. Minimum population sizes for species conservation. BioScience. 31(2): 131-134.

Shaffer, M. L.; Samson, F. B. 1985. Population size and extinction: a note on determining critical 

population sizes. American Naturalist. 125: 144-152. 

Shvidchenko, A. B.; Pender, G. 2000. Flume study of the effect of relative depth on the incipient motion 

of coarse uniform sediments. Water Resources Research. 36(2) 619-628.

Simon, A.; Downs, P. W. 1995. An interdisciplinary approach to evaluation of potential instability in 

alluvial channels. Geomorphology 12. 215-232.

Simons, Li and Associates. 1982. Engineering analysis of fluvial systems. Fort Collins, CO. Simons, Li 

and Associates. 1,128 p.

Soulé, M. E. 1980. Thresholds for survival: maintaining fitness and evolutionary potential. In: Soulé, M. 

E.; Wilcox, B. A. eds. Conservation Biology: An Evolutionary-Ecological Perspective. 

Sunderland, MA: Sinauer Associates. 151-167.

Stillwater Sciences. 2005. Santa Clara River Parkway Floodplain Restoration Feasibility Study: 

Assessment of Geomorphic Processes (DRAFT REPORT). Prepared for the California 

State Coastal Conservancy. Berkeley CA. 

http://www.santaclarariverparkway.org/newsfolder/wkb/projects/scrfeasibility accessed 

8/9/2006

S
11—17

References

Stuckey, M. H.; Reed, L. A. 2000. Techniques for estimating magnitude and frequency of peak flows for 

Pennsylvania streams. U.S. Geological Survey Water resources Investigations Report 

00-4189. 22 p.

Sumioka, D. L.; Kresch, D. L.; Kasnick, K. D. 1997. Magnitude and frequency of floods in Washington. 

U. S. Geological Survey, Water Resources Investigations Report 97-4277. 91 p.

Taylor, R. N.; Love, M. 2003. Fish passage evaluation at stream crossings. Part IX, 

California Salmonid Stream Habitat Restoration Manual, 1998 3rd edition. California 

Department of Fish and Game. http://www.dfg.ca.gov/nafwb/manual.html accessed 

1/6/06.

Thomas, B. E.; Hjalmerson, H. W.; Waltemeyer, S. D. 1993. Methods for estimating magnitude and 

frequency of floods in the southwestern United States. U. S. Geological Survey Open 

File Report 93-419. 91 p.

Thomas, D. M.; Benson, M. A. 1969. Generalization of streamflow characteristics from drainage 

watershed characteristics. U.S. Geological Survey Open-File Report. 45 p.

Thomas, D. B.; Abt, S. R.; Mussetter, R. A.; Harvey, M. D. 2000. A design procedure for sizing steppool structures. Fort Collins CO: Mussetter Engineering, Inc. [Online] Available: 

http://www.crwcd.org/page_5. [7/6/07] 

Thorne, C. R. 1990. Effects of vegetation on riverbank erosion and stability. In: Thornes, J. B. ed. 

Vegetation and Erosion. New York: Wiley and Sons. 125-144.

Thorne, C. R. 1997. Channel types and morphological classification, in Thorne, C. R.; Hey, R. D.; 

Newson, M. D. eds. Applied Fluvial Geomorphology for River Engineering and 

Management. Chichester, U.K.: Wiley and Sons. 175-222.

Thorne, C. R.; Allen, R. G.; Simon, A. 1996. Geomorphological river channel reconnaissance for 

river analysis, engineering and management. Transactions of the Institute of British 

Geographers 21. 469-483.

Thurow, R. F.; Lee, D. C.; Rieman, B. E. 1997. Distribution and status of seven native salmonids in 

the interior Columbia River basin and portions of the Klamath River and Great Basins. 

North American Journal of Fisheries Management. 17, 1,094-1,110.

T
11—18

Stream Simulation

Tinkler, K. J.; Wohl, E. E. 1998. A primer on bedrock channels. In Tinkler, K. J.; Wohl, E. E. eds. Rivers 

over Rock: Fluvial Processes in Bedrock Channels. American Geophysical Union 

Geophysical Monograph 107. 1-18.

Tobiason, S.; Jenkins, D.; Molash, E.; Rush, S. 2000. Polymer use and testing for erosion and 

sediment control on construction sites: Recent experience in the Pacific Northwest. 

In: Proceedings Conference 31, 21-25 February 2000. International Erosion Control 

Association, Steamboat Springs, CO.

U. S. Army Corps of Engineers. 1994. Engineering and Design - Hydraulic Design of 

Flood Control Channels. EM-1110-2-1601. 

http://www.usace.army.mil/publications/eng-manuals/em1110-2-1601 accessed 1/20/06

U. S. Army Corps of Engineers. 1999. Ice Engineering. EM-1110-2-1612. 

U. S. Army Corps of Engineers. 2006. Control and topographic surveying. EM-1110-1-1005. Final draft 

at http://crunch.tec.army.mil/Information/Publications/em1110-1-1005draft/toc.htm. 

accessed 3/5/2007.

U.S. Department of Agriculture, Forest Service. 1996. Managing roads for wet meadow ecosystem 

recovery. FHWA-FLP-96-016. U.S. Department of Agriculture, Forest Service, 

Southwestern Region.

U.S. Department of Agriculture, Forest Service. 2003. Identifying bankfull stages in the eastern and 

western United States. DVD. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station, Stream Systems Technology Center.

U.S. Department of Agriculture, Forest Service. 2005. Guide to identification of bankfull stage in the 

northeastern United States. 4 CDs. Gen. Tech. Rep. RMRS-133-CD. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, 

Stream Systems Technology Center.

U.S. Department of Agriculture, Natural Resources Conservation Service. 2001. Design of rock weirs. 

Engineering Technical Note 13. Boise, ID: U.S. Department of Agriculture, Natural 

Resources Conservation Service.

. 

Vaughan, M. D. 2002. Potential impact of road-stream crossings (culverts) on the 

upstream passage of aquatic macroinvertebrates. Report prepared for the USDA Forest 

Service. [Online]. Available at: http://www.xerces.org/pubs_merch/ [30 July 2007].

U

V
11—19

References

Wilson, W. E.; Moore, J. E. eds. 1998. Glossary of hydrology. Alexandria, VA: American 

Geological Institute. 248 p.

Washington Department of Fish and Wildlife. 2003. Integrated streambank protection guidelines. 

Washington State Aquatic Habitat Guidelines Program. 

http://wdfw.wa.gov/hab/ahg/ispgdoc.htm

Whitaker, J. G.; Jaeggi, M. N. R. 1982. Sediment availability and the predication of storm-period 

sediment yields. In: Walling, D. E. ed. Recent Developments in the Explanation and 

Prediction of Erosion and Sediment Yield. International Association of Hydrological 

Sciences Publication 133. 177-194.

Whitaker, J. G. 1987. Sediment transport in step-pool streams. In Thorne, C. R.; Bathurst, J. C.; Hey, R. 

D. eds. Sediment transport in gravel-bed rivers. Chichester, U.K.: Wiley and Sons. 545-

570.

Wiberg, P. L.; Smith, J. D. 1987. Calculations of the critical shear stress for motion of uniform and 

heterogeneous sediments. Water Resources Research. 23(8): 1,471-1,480.

Wiley, J. B.; Atkins, J. T., Jr.; Tasker, G. D. 2000. Estimating magnitude and frequency of peak 

discharges for rural, unregulated streams in West Virginia. U.S. Geological Survey Water 

Resources Investigations Report 00-4080. 93 p.

Williams, G. P. 1978. Bankfull discharge of rivers. Water Resources Research 14. 1,141-1,158.

Williamson, D. 1989. Geotechnical field methods. Engineering Field Notes. Washington DC: U.S. 

Department of Agriculture, Forest Service, Engineering. 

Wipfli, J. S.; Hudson, J. P.; Chaloner, D. T.; Caouette, J. P. 1999. Influence of salmon spawner densities 

on stream productivity in Southeast Alaska. Canadian Journal of Fisheries and Aquatic 

Science. 56, 1,600-1,611.

Wipfli, J. S.; Richardson, J. S.; Naiman, R. J. 2007. Ecological linkages between headwaters and 

downstream ecosystems: transport of organic matter, invertebrates, and wood down 

headwater channels. Journal of the American Water Resources Association. 43(1): 72-

85. 

W
11—20

Stream Simulation

Wipfli, M. S.; Gregovich, D. P. 2002. Export of invertebrates and detritus from fishless headwater 

streams in southeastern Alaska: implications for downstream salmonid production. 

Freshwater Biology. 47, 957-969.

Wissmar, R. C.; Smith, J. E.; McIntosh, B. A.; Li, H. W.; Reeves, G. H.; Sedell, J. R. 1994. A history 

of resource use and disturbance in riverine basins of Eastern Oregon and Washington. 

Northwest Science. 68 Special Issue.

Wohl, E. E.; Cenderelli, D. A. 2000. Sediment deposition and transport patterns following a reservoir 

sediment release. Water Resources Research. 36, 319-333. 

Wohl, E. E. 2000. Mountain Rivers: Washington, D.C.: American Geophysical Union. Water Resources 

Monograph. 14, 320.

Wolman, M. G. 1954. A method of sampling coarse river-bed gravel. Transactions of American 

Geophysical Union. 35, 951-56. 

Wolman, M. G.; Miller, J. P. 1960. Magnitude and frequency of forces in geomorphic processes. Journal 

of Geology. 68, 54-74.
AGeomorphic 

Principles Applied 

in Stream 

Simulation

A.1 Why Consider Fluvial Processes in Crossing 

Design?

A.2 The Watershed Context

A.3 Channel Characteristics

A.4 Channel Stability and Equilibrium

A.5 Fluvial Processes

A.6 Channel Classification Systems

A.7 Unstable Channels
Stream Simulation
Appendix A—Geomorphic Principles Applied in Stream Simulation

This appendix very briefly reviews fluvial processes (i.e., processes

pertaining to river or stream action) and channel characteristics that project

teams consider when evaluating site conditions at road-stream crossings

and designing stream-simulation structures. Chapters 4, 5, and 6 describe

how teams apply these concepts in stream-simulation site assessment and

design.

Training and experience in geomorphology are essential for assessing

channel conditions, interpreting channel responses and fluvial

processes, and designing a simulated streambed. Most hydrologists,

geomorphologists, geotechnical engineers, and hydraulic engineers already

will be familiar with many of the concepts we are presenting here. If you

are a reader for whom the material is new, the information in this appendix

is not adequate for developing journey-level geomorphology skills. You

may want to review the references cited here and attend training courses

to expand your knowledge. Project team members are responsible for

recognizing when additional expertise must be brought in—especially when

channel conditions are complex and difficult to interpret (see sidebar in

section 3.3).

A.1 Why Consider Fluvial Processes In Crossing Design?

Streams are dynamic systems that can readily change in response to human

or natural disturbances. Streams continually erode sediment and wood

from their boundaries and redeposit that material at other locations in the

channel. Many streams also shift location laterally across the valley bottom.

Streambed elevations change as the stream transports, deposits, and stores

woody debris and sediment. During floods, streams overflow the floodplain surface, eroding and depositing sediment and debris, and constructing

riparian habitats.

Road-stream crossings are rigid structures that lock the stream in place

and elevation, preventing these normal dynamic processes. In the

past, crossings have typically been narrower than the stream, causing

backwatering and sediment deposition at the inlet [figure A.1(a)].

Narrow culverts also increase water velocity causing channel scour in or

downstream of the crossing [figure A.1(b)].
A—2

Stream Simulation

Figure A.1—(a) Aggraded (filled) channel upstream of narrow culvert; (b) incised

(scoured) channel downstream of culvert, Save Creek, Olympic National Forest,

Washington.

As chapter 1 explains, such channel responses to culverts can ultimately

inhibit or prevent aquatic species passage. These responses also can cause

massive problems—both for the road and the stream—during large floods.

Plugging with debris and sediment is common at culverts. Fill failure or

stream diversion can follow, as the water overtops the road or runs along

the road until it pours off onto a hillslope or into another drainage (figure

1.17). Scouring at narrow bridges or open-bottom arches can also cause

these structures to fail.

Stream-simulation design provides for both aquatic species passage

and long-term stability of the structure and the constructed streambed.

Within the limits of a necessarily rigid structure, stream simulation aims

to provide enough space for the stream channel to adjust to changing

flows and sediment loads, just as the natural channel does. To achieve

this objective, the project team must understand how fluvial processes

(a)

(b)
A—3

Appendix A—Geomorphic Principles Applied in Stream Simulation

shape the current channel at a site. The team must be able to predict future

channel responses to changes in watershed and climatic conditions, and

they must also be able to predict how the channel will respond to the new

crossing structure.

A.2 The Watershed Context

The site’s location in the watershed is important. Depending in part on

their position in the watershed, channel reaches (stream segments with

relatively homogenous characteristics) can be divided into three general

types (Montgomery and Buffington 1993, 1997):

(1)	 Source reaches are headwater channels with few if any fluvial

characteristics. Hill-slope processes such as surface erosion and soil

creep deliver sediment to these channels, which store it until large

flow events or debris flows scour it out.

(2)	 Transport reaches are typically steep streams that tend to resist

erosion, because they have persistent bed and bank structures

dominated by large particle sizes (boulders, cobbles, gravels, and

wood). Although these reaches store some sediment (e.g., behind

pieces of woody debris), in general they have high transport

capacities. When sediment supply increases, they tend to pass the

increase quickly to lower-gradient reaches. Channel morphology does

not change very much in response to changes in water or sediment

inputs.

(3)	 Response reaches are lower-gradient reaches where sediment

transport is limited by relatively low transport capacity. That is, when

sediment supply from upstream increases, it is likely to deposit in a

response reach. The reach will often respond to changes in sediment

supply or discharge by making large adjustments in channel size,

shape, slope, or pattern. As Montgomery and Buffington (1993) point

out, the first response reach downstream of a series of transport reach

is likely to be an extremely sensitive site when water or sediment 

regimes change in the upstream watershed.

This appendix refers to these reach types throughout. They are helpful as

shorthand descriptors of likely channel responsiveness to environmental

change. Understanding the differences between streams in their

responsiveness to environmental changes is very important in streamsimulation design.
A—4

Stream Simulation

While some watersheds have a more or less regular sequence of source,

transport, and response reaches from headwaters to mouth (figure A.2),

reach types are often distributed in a more complex way. Local geologic

controls can create meandering mountain meadow streams (response

reaches) near the headwaters, and very steep transport reaches may be near

the downstream end of tributaries on river breaks.

Figure A.2—Idealized distribution of reach types in a watershed. Drawn by

L’Tanga Watson.

As integral parts of the watershed ecosystem, streams reflect the effects

of climate, geology, soils, vegetation, basin shape, and land use in the

watershed. These factors control water and sediment inputs to the stream.

In turn, water and sediment, interacting with riparian vegetation and 

channel boundary materials, control fluvial processes and determine

channel characteristics.

Much can happen to change these controlling factors over the lifetime

of a crossing structure. Land use is changing rapidly in many areas,

particularly near national forest boundaries where people can build homes

and interface directly with “nature.” Road building is continuing in some

locations, and roads are being improved for recreation access. Off-road

vehicle use can affect the hydrologic system, as can grazing and fire. In

many locations, streams are experiencing or recovering from large-scale

mining, logging, and removing of woody debris. All these changes can

have large individual and cumulative effects on the hydrologic regime.

Even a single unusual flood can create large, long-lasting changes in a

stream system, requiring decades for recovery.

Response reaches

Mostly transport reaches

Smallest headwater

channels are source

reaches.

Larger streams are 

transport reaches. 
A—5

Appendix A—Geomorphic Principles Applied in Stream Simulation

	Obviously, what happens upstream in a watershed affects downstream

channel reaches. However, downstream-land use or river changes also can

affect upstream areas if they induce channel incision (i.e., downcutting).

For example, channelization for urban or agricultural development

speeds up water flow, increases its erosive power and causes channels

to incise. Removal of woody debris from a channel, e.g., to reduce the

risk of flooding can have the same effect. Gravel-mining operations that

dig in-channel pits can lower the base level for all upstream reaches.

These actions often produce headcutting, in which an oversteepened

nickpoint migrates upstream (figure A.3), causing the bed to incise until

it equilibrates at a lower, less erodible slope. Many existing culverts are

functioning as grade controls, protecting upstream reaches from channel

incision caused by migrating headcuts.

Figure A.3—Active nickpoint migrating upstream, Meadow Creek, Nez Perce

National Forest, Idaho. (a) Looking downstream across nickpoint; (b) looking

upstream at nickpoint. Bright streambed indicates recently mobilized material.

(a)

(b)
A—6

Stream Simulation

Cause and effect can be difficult to determine, not only because unseen

offsite changes may be affecting a site but also because a significant lag

time may exist between cause and effect. For example, headcuts related to

channel straightening in the 1960s were still actively migrating upstream

in northern Mississippi in the 1980s (Harvey et al. 1983). There can also

be cascading effects. If bank vegetation is removed (e.g., by agriculture,

logging, grazing, or construction) from a particularly sensitive reach,

the channel may respond dramatically. Bank erosion could cause the

affected reach to widen significantly, releasing large volumes of sediment.

That sediment may be deposited in a downstream reach, potentially

destabilizing streambanks there.

Existing channel conditions may depend on factors or events far removed

spatially and temporally from the site. To understand the past and predict

future channel responses, analyze the temporal sequence and spatial

distribution of watershed activities. This information is critical to making

informed and accurate interpretations of channel conditions at the roadstream crossing. This analysis is part of phase 1 of a stream-simulation

project—the initial watershed review (see chapter 4).

A.3 Channel Characteristics

A.3.1 Streambed Material

A channel reach can be described as bedrock, colluvial, or alluvial

according to the composition of its bed and banks (Montgomery and

Buffington 1997; Knighton 1998). Bedrock channels have considerable

segments of resistant bedrock (in excess of 50 percent) exposed along

the flow boundary or the bedrock may be overlaid by a thin veneer of

alluvium, i.e., material transported by the stream (Tinkler and Wohl 1998)

(figure A.4). Bedrock channels tend to be quite stable. Many are situated

in narrow valleys and lack flood plains. The lack of sediment in bedrock

channels indicates that sediment is efficiently transported through the

reach (Montgomery and Buffington 1997). Even in these transport reaches,

however, there are usually localized, transient sediment accumulations

behind woody debris or other channel features, and these accumulations

may form very important habitats for aquatic species (McBain and Trush

2004). 

Channels composed of material deposited by gravity-driven processes

such as creep, surface erosion, debris flows, landslides, and rockfalls

are referred to as colluvial channels (a type of source reach, figure A.2).

Typically, they are located in the steep headwater areas of the watershed,
A—7

Appendix A—Geomorphic Principles Applied in Stream Simulation

where mass wasting is the dominant geomorphic process (Montgomery

and Buffington 1993, 1997). Colluvial channels are composed of angular

boulders, cobbles, gravels, and sands. Normal (shallow) streamflow is

insufficient for mobilizing most of the material; intermittent debris flows

are the primary process for mobilizing and delivering the coarse colluvial

material downstream (Montgomery and Buffington 1997).

Figure A.4—Bedrock channels are transport reaches.

Alluvial channels are composed of alluvium; that is, their bank and bed

materials were transported and deposited by the stream. They are able

to adjust their form by eroding and depositing sediment in response to

changes in flow and sediment transport conditions. The frequency and

degree of channel adjustment is strongly related to particle size; channels

composed of gravel and small cobbles (figure A.5) are more responsive to

flow and sediment supply changes, whereas channels composed of large

cobbles and boulders are relatively stable at most flows and may only
A—8

Stream Simulation

change form during infrequent, exceptional floods with large sediment

inputs. Sand-bed channels are highly responsive, and their beds are usually

continuously in motion at most flows.

Figure A.5—Alluvial response reach.

Channels in cohesive materials (with significant clay content) may or

may not be alluvial. Many are incised into residual soils. Although their

characteristics vary greatly depending on slope, in general they do not

transport very much bed load. Most sediment is transported in suspension. 

In channels composed of gravels, cobbles, and boulders, bed material is

often segregated into two layers (figure A.6). The bed surface consists of a

one- or two-grain-thick layer of coarser particles overlying smaller gravels

or sands beneath the surface. This overlying coarse layer is referred to

as the armor layer. The median particle size of the armor layer is usually

1.5- to 3.0-times coarser than the median particle size of the subarmor

layer (Reid et al. 1998; Bunte and Abt 2001), although ratios as high as

6 and 7 have been reported (e.g., Andrews and Parker 1987; King et al.

2004; Barry et al. 2004). The presence of an armor layer indicates that

the channel can transport more sediment than is available from upstream

areas, whereas the lack of an armor layer indicates a balance between

sediment supply and transport capacity (Montgomery and Buffington

1997). The armor layer increases the streambed’s resistance to erosion.

Once the armor breaches, however, the whole streambed can mobilize, and

general scour occurs. In general, unarmored streambeds are more mobile

than armored ones; that is, bed sediment moves at lower flows and more

frequently in an unarmored streambed than it would in an armored one.
A—9

Appendix A—Geomorphic Principles Applied in Stream Simulation

Note: The particle size terminology we use in this document is from the

Wentworth classification system, in which particle diameter doubles

for each successive category (table A.1).

Table A.1—Definitions of particle size categories used in this guide: Wentworth

classification system

Particle Description mm inches

Bedrock	 >2,048	 80

Large – very large boulders	 1,024 – 2,048	 40 – 80

Medium boulders	 512 – 1,024	 20 – 40

Small boulders	 256 – 512	 10 – 20

Large cobbles	 128 – 256	 5 – 10

Small cobbles	 64 – 128	 2.5 – 5

Very coarse gravels	 32 – 64	 1.26 – 2.5

Coarse gravels	 16 – 32	 0.63 – 1.26

Medium gravels	 8 – 16	 0.31 – 0.63

Fine gravels	 4 – 8	 0.16 – 0.31

Very fine gravels	 2 – 4	 0.08 – 0.16

Very coarse sands	 1.0 – 2.0	 0.04 – 0.08

Coarse sands	 0.50 – 1.0	 0.02 – 0.04

Medium sands	 0.25 – 0.50	 0.01 – 0.02

Fine sands	 0.125 – 0.25	 0.005 – 0.01

Very fine sands	 0.062 – 0.125	 0.002 – 0.005

Silts/clays	 < 0.062	 < 0.002

Figure A.6—The armor layer can be seen on this eroded gravel bar, Flathead

River, Montana.
A—10

Stream Simulation

A.3.2 Channel Slope

Slope is an important variable determining the overall energy of the stream

for transporting water and sediment. Slope is also one of the channel

characteristics most frequently altered by crossing structures that are

undersized or installed at slopes different from that of the natural channel.

As a general rule, channel slope decreases going downstream in the

watershed from the headwaters to the lower sediment deposition zone

(figure A.2). Locally, the channel slope may steepen or flatten because of

factors such as bedrock, coarser material, tectonic activity, and base-level

changes (Knighton 1998). The general decrease in channel slope across the

watershed corresponds to an increase in flood-plain width, channel sinuosity

(see A.3.3), and average flow depth; a decrease in bed material size; and

a decrease in the interactions between valley slopes and the stream. Steep

channels usually have coarser sediments, discontinuous narrow flood plains

or no flood plains, narrow valley bottoms, and relatively straight planforms

when compared to low-gradient channels.

A base-level control is any structure that fixes the lowest elevation to which

a stream reach can downcut. Common examples of base-level controls

are very stable debris jams or concrete weirs. For a tributary, the ultimate

base level is the elevation of the master stream at a tributary junction.

When a base-level control is removed or altered, upstream channel slope

changes concomitantly. Base-level control is an important concept in stream

simulation. If the base-level control changes over the life of the structure, the

altered slope may destabilize the simulated streambed.

At the reach scale, channel slope can be measured as the slope of the channel

bed or as the slope of the water surface. It also can be measured along

the thalweg (representing low flow) or along the midpoint of the channel

(representing high flow). In stream-simulation design, the channel bed along

both the thalweg and the bankfull water surface slope can be important (see

section 5.2.2.2 bankfull sidebar).
A—11

Appendix A—Geomorphic Principles Applied in Stream Simulation

The thalweg is a line running along the channel bed (i.e., longitudinally), 

connecting the lowest points. In figure A.7, the thalweg meanders 

along the bottom of the otherwise straight channel. The thalweg in 

figure A.7 is longer than the channel as a whole, because the thalweg 

bends back and forth along the channel bottom. The thalweg’s longer 

length makes its slope lower than the average channel slope. As 

the water surface rises in this channel during a high-flow event, flow 

straightens out and slope increases.

Figure A.7—This straight reach of the San Pedro River, Arizona, has a

meandering thalweg. 
A—12

Stream Simulation

Local channel slopes vary, reflecting the presence of multiple bedforms

such as steps, riffles, pools, and obstructions (figure A.8). At higher flows,

water surface slope evens out somewhat because bedforms are submerged.

Figure A.8—Pool-riffle and step-pool channel profiles showing variable local

slopes. From Knighton (1998). permission to use requested.
A—13

Appendix A—Geomorphic Principles Applied in Stream Simulation

A.3.3 Channel Pattern

Channel patterns—also referred to as planform characteristics—are

usually classified as straight, meandering, braided, or anastomosing (figure

A.9). Pattern is determined by factors like slope, confinement, sediment

supply, channel and valley materials, and riparian vegetation (Knighton

1998).

Straight alluvial channels are relatively rare in nature. Most streams tend

to meander, unless they are tightly confined in a narrow valley or gully.

Channel sinuosity—the ratio of stream length to valley length—describes

the degree of meandering (see figure A.10). Meandering streams are

inherently more dynamic, and their tendency to shift location across the

valley bottom increases with sinuosity, bed load, and slope. The more

erodible the banks, the more changeable the stream.

Figure A.9—Channel patterns. From Thorne et al. (1997), reproduced with

permission from John Wiley and Sons, Ltd.

Meander wavelength (L), amplitude (A), and radius of curvature (Rc)

describe the geometry of individual meanders (figure A.11). The radius of

curvature is of particular interest in stream-simulation design, because it

affects the distribution of water velocities across the channel. At a bend,

water velocity is higher near the outside bank than near the inside bank.

This cross-sectional difference in velocity causes erosion on the outer bank

and deposition on the inside bank, often resulting in meander shift. At

road-stream crossings, radius of curvature can affect the risk of alignment

changes over the life of the crossing (see section 6.1.1).
A—14

Stream Simulation

Figure A.10—Channel sinuosity is channel length divided by valley length.
A—15

Appendix A—Geomorphic Principles Applied in Stream Simulation

Figure A.11—Common meander geometry measurements.

Braided channels consist of multiple wide and shallow channels separated

by poorly vegetated bar deposits. Individual channels and bars frequently

shift position [figure A.12(b)]. A braided pattern indicates that sediment

supply is high and that the channel bed and banks are readily eroded.

Despite the fact that channels and bars continually shift, the size and slope

of the channel within the limits of the braided area may remain the same.

A braided channel like this is in dynamic equilibrium with existing

geomorphic conditions (Knighton 1998).

Anastomosing channels are also multithreaded. However, the individual

channels are separated by highly stable vegetated bars or islands [figure

A.12(c)]. Anastomosing channels typically form in environments where

the valley bottom is wide, flooding is highly variable, flood plains

are frequently inundated, and banks are relatively resistant to erosion

(Knighton 1998).
A—16

Stream Simulation

(a)

(c)

(b)

Figure A.12—Stream patterns (a) meandering reach on the Dosewallips River, Olympic National Forest,

Washington; (b) braided river in the Arctic National Wildlife Refuge. (USFWS Alaska photo gallery); 	 	

(c) anastomosing reach on Medicine Bow National Forest, Wyoming.
A—17

Appendix A—Geomorphic Principles Applied in Stream Simulation

A.3.4 Channel Dimensions, Confinement, and Entrenchment 

Width-to-depth ratios are often used to characterize channel dimensions

(usually bankfull channel dimensions—see section A.4.1). Low width-todepth ratios indicate the channel is narrow and deep, whereas high widthto-depth ratios indicate that the channel is wide and shallow. Width-depth

ratios, however, do not describe a cross-section’s symmetry. Both symmetry

and width-to-depth relations vary longitudinally along a given channel, and,

in meandering channels, they are strongly influenced by the cross-section’s

location relative to bends. Cross sections located at channel bends typically

have asymmetric shapes reflecting the pool and point bar (channel type

C, figure A.13), whereas cross sections in straight channel segments have

symmetrical, more rectangular shapes (channel type B, figure A.13).

Vegetation strongly influences channel shape. Banks densely vegetated with

deep-rooted species have narrower and deeper channels than those with

thinly vegetated, grassy banks (Hey and Thorne 1986). The cohesiveness

of the bank material also influences channel shape. Channels with cohesive

banks (silts and clays) have narrower and deeper channels than channels

with noncohesive (sand, gravel) banks (Knighton 1998).

The term “channel entrenchment” describes the degree to which flow is

vertically contained (figure A.13). That is, as discharge increases, flow in

an entrenched stream is confined either by the valley walls or by steep,

high streambanks. This guide uses Rosgen’s (1994) definition of channel

entrenchment: the ratio between flood-prone width and channel bankfull

width. Flood-prone width is the width of the flood plain or valley bottom at

an elevation two times the maximum bankfull depth. Generally, the floodprone width is considered to correspond with floods having recurrence 

intervals of less than 50 years (Rosgen 1994).

Channels with entrenchment ratio values less than 1.4 are “entrenched,”

indicating either that the valley bottom is narrow or that the adjacent

valley surface is not frequently flooded (e.g., it is a terrace). Channels

with entrenchment-ratio values greater than 2.2 are “slightly entrenched,”

indicating that the flood-prone valley bottom surface is wide relative to the

channel. Channels with entrenchment ratio values between 1.4 and 2.2 are

considered moderately entrenched.
A—18

Stream Simulation

Figure A.13—Channel entrenchment (from Rosgen 1994).

In stream simulation we use the entrenchment ratio as an indicator of

potential site risks associated with future alignment changes; that is, slightly

entrenched channels tend to undergo alignment changes as they shift across

the flood plain. Slightly entrenched channels also are more likely to have

roadfills that obstruct flood plains. Flood-plain obstruction can cause

problems for a crossing structure by concentrating flood flows through it.

A.3.5 Channel Bedforms 

Natural stream channels have a variety of bed structures known as

bedforms, which reflect local variations in hydraulics, particle size, and

sediment transport. In coarse-grained channels, structures such as pebble 

clusters, transverse ribs, and cobble-boulder steps cause complex flow

patterns of convergence and divergence. These patterns in turn influence

bedload transport rates and patterns (Brayshaw et al. 1983; Koster

1978; Whitaker and Jaeggi 1982). In sand-bed channels (figure A.14), the

channel bed is easily mobilized into different bedforms (ripples, dunes,

antidunes) that correspond to variations in flow intensity (Knighton 1998).
A—19

Appendix A—Geomorphic Principles Applied in Stream Simulation

Figure A.14—Depending on flow intensity, bed structures such as ripples, dunes,

and antidunes can form in sand bed channels, dramatically changing channel

roughness. Redrawn after Simons, Li & Associates 1982.

In gravel-bed channels, the dominant form of bed topography tends to be

alternating pools and riffles in low-gradient channels, and pools and steps

in high-gradient channels. In pool-riffle channels, pools are scoured along

the outer margins of channel bends and downstream from obstructions

such as bedrock outcrops or large woody debris structures that locally

constrict the channel. Pools and point bars are located at bends, and riffles

are located in straight channel segments between successive meanders. At

low flows, flow is deep and slow in pools, whereas flow in the adjacent,

steeper riffles is shallow and fast (figure A.15). The average spacing

between pools in a pool-riffle channel is generally between 5- to 7-channel

widths, but spacing is variable along a given channel and can range from

1.5- to 23.3-channel widths (Keller and Melhorn 1978). The spacing

of pool-riffle sequences can be influenced by large woody debris, large

obstructions, or bedrock outcrops (Lisle 1986; Montgomery et al. 1995).

Figure A.15—A pool-riffle reach on the Flathead River, Montana.
A—20

Stream Simulation

Step-pool sequences are common bedforms in high-gradient, coarse-bed

alluvial channels. Steps are composed of cobbles, boulders, bedrock, and/

or large woody debris that extend across the entire channel perpendicular

or oblique to flow (figure A.16). Plunge pools form at the base of each

step and often contain finer material. In step-pool channels, the spacing

between steps ranges between 1- and 4-channel widths and is primarily a

function of gradient, with less distance between steps as gradient increases

(Whitaker 1987; Chin 1989; Montgomery and Buffington 1997). The

height and length of steps are also a function of gradient, with step heights

increasing and step lengths decreasing as gradient increases (Whitaker

1987; Grant et al. 1990).

Figure A.16—Step-pool channel in northern Idaho.

A.3.6 Flow Resistance or Channel Roughness 

Water velocity in a stream depends on channel resistance (roughness),

as well as water depth and channel slope. A stream simulation mimics

natural-channel roughness to keep velocities similar and to recreate the

velocity diversity that allows for a wide variety of species to pass the

crossing.
A—21

Appendix A—Geomorphic Principles Applied in Stream Simulation

Total flow resistance is influenced by the combined interactions of

channel-bed material, bedforms, water-surface and bed-surface slope

variability, channel alignment, bank irregularities, and vegetation. Total

flow resistance can be divided into the following three categories (Bathurst

1997; Knighton 1998):

l Free-surface resistance represents energy losses associated with

surface waves and hydraulic jumps (e.g., flow plunging over a step).

l Channel resistance represents energy losses caused by water-surface

and bed-surface slope variability (e.g., slope variability associated

with pool-riffle and step-pool sequences), bank irregularities (e.g.,

bedrock outcrops, large woody debris complexes), and variability in

channel alignment (e.g., channel bends).

l Boundary resistance represents energy losses caused by a number of

factors, including grain roughness, form roughness, and vegetation

roughness.

Channel resistance can be very significant in channels with many pieces

of debris, rock outcrops or large boulders, and/or sharp bends. However,

boundary resistance is the primary factor influencing total flow resistance

of most channels (Limerinos 1970; Hey 1979; Bathurst 1985; Jarrett

1985). Boundary resistance includes the following components:

l	Grain roughness represents energy losses caused by the size of the

particles and the height to which they project into the flow: Larger

particles have greater flow resistance than small particles.

l	Form roughness represents energy losses caused by bedforms.

l	Vegetation roughness represents energy losses associated with type

and density of vegetation along channel banks. Taller, more rigid, and

more densely packed stems increase vegetation resistance to flow and

reduce shear stresses on bank and flood-plain surfaces (Arcement and

Schneider 1989).

Boundary resistance varies with discharge, because the depth of water

influences the degree to which the channel-bed sediments, bedforms,

and bank vegetation interact with the flowing water. As water depth

increases, the influence of grain and form roughness decreases while

vegetation roughness increases, because more water is in contact with

the bank vegetation. Boundary resistance on the flood plain, caused by

microtopography, vegetation, etc., also controls the amount of water

flowing over the flood plain (i.e., flood-plain conveyance).
A—22

Stream Simulation

In gravel- and cobble-bed channels, grain roughness is the primary

component of boundary resistance. In boulder-bed channels with step

topography, the combination of individual particles (grain roughness)

and steps (form roughness) determines boundary resistance. In sand-bed

channels, form roughness is more important than grain roughness, because

continual bedform changes (ripples, dunes, antidunes) cause variations in

boundary resistance (figure A.14).

A.4 Channel Stability and Equilibrium

Stable channels are channels that are not experiencing rapid, lasting

change in dimensions or slope. While stable channels adjust to a wide

range of flows and sediment inputs, their average dimensions remain the

same over long periods (decades to centuries).

In the short term, a stable channel reach may adjust width, depth, and/

or slope in response to a flow or sediment input event such as a flood

or landslide. However, with time, channel dimensions return to the

equilibrium state. On average, a stable reach is neither aggrading nor

incising, neither widening nor narrowing, and the amount of sediment

coming in is the same as the amount leaving it. Recognizing that such

channels are stable but not static, we describe them as being in quasiequilibrium (figure A.17).

Figure A.17—In quasi-equilibrium channels, width and depth vary around longterm average values. After Schumm (1977).
A—23

Appendix A—Geomorphic Principles Applied in Stream Simulation

For a channel to be in quasi-equilibrium, environmental conditions, such

as the amount and timing of runoff and sediment input, also must be

approximately constant (or changing very slowly) over the decade-tocentury time scale. Base level also must remain the same. If these controls

change enough to cross a “response threshold,” the destabilized channel

can change dramatically and rapidly, going through a series of adjustments

before reaching a new quasi-equilibrium state (Schumm 1977).

As we gain more understanding of climatic variability, and as human 

uses of land and rivers intensify, geomorphologists are increasingly 

skeptical about whether modern streams actually achieve quasiequilibrium over “engineering time” (Macklin and Lewin 1997). El Niño 

and the North Atlantic Oscillation cause changes in rainfall regimes 

large enough to cause river adjustments (Lewin et al. 1988) on decade 

and longer time scales. In many forested environments, changing 

land management may be expected to progressively alter runoff and 

sediment-load regimes. Crossing designers should recognize the 

possibility that the conditions controlling stream morphology may not 

be stable over a structure’s lifetime. Watershed-scale investigations 

that deal with past, present, and future conditions, such as those 

outlined in chapter 4, are critical for providing the context needed for 

prudent design. 

Most channels immediately adjacent to a narrow road-stream crossing

structure adjust their form to establish a “new” quasi-equilibrium with

the conditions imposed by the undersized structure (culvert). Typical

responses include aggradation and channel widening immediately

upstream from the culvert inlet, and channel widening and incision

immediately downstream from the culvert outlet. These adjustments

make the channel more efficient in transporting sediment and dissipating

flow energy, and create a more stable channel form. However, these same

adjustments may prevent aquatic organisms from migrating freely along

the stream corridor. A stream-simulation structure will restore stream

and ecological connectivity at the road-stream crossing. During and after

construction of the stream-simulation structure, the channel will adjust its

form to establish a new quasi-equilibrium. 
A—24

Stream Simulation

A.4.1 Equilibrium and Bankfull Flow

Observable channel characteristics are the result of both a range of past

discharges and the temporal sequence of floods. Nonetheless, a single

discharge value is commonly used to represent the “channel-forming 

flow” (Knighton 1998). Bankfull discharge—the maximum discharge the

channel can contain before water overtops its banks onto the flood plain—

is generally taken to represent the channel-forming discharge in response

channels and moderate-gradient transport channels. In many environments,

bankfull is a peak that is equaled or exceeded frequently—about every 12

to 2 years. Because this peak is frequent and because it usually transports

a significant amount of sediment, it is generally found to transport more

sediment cumulatively than any other flow over a long period of time (Hey

1997).

Since water and sediment inputs continually fluctuate, the channel

continually adjusts. However, unless it is truly unstable, its dimensions

will vary around equilibrium values that can often be consistently related

to bankfull discharge (Emmett and Wolman 2000) (see figure A.18). Based

on these relationships, bankfull discharge is often used as the reference

discharge for designing channels (Hey 1997). We use bankfull in stream

simulation for the same reason.

Figure A.18—Relationship of bankfull channel dimensions (determined in the

field using geomorphic indicators) to bankfull discharge (determined from gauge

records at observed bankfull elevation). Data from Castro and Jackson (2001).

0

1

10

100

1,000

1 10 100 1,000 10,000 100,000

bankfull discharge (cfs)

bankfull width and depth (ft)

width

depth

data from Castro and Jackson 2001 
A—25

Appendix A—Geomorphic Principles Applied in Stream Simulation

Bankfull is not the channel-forming flow in all streams. In steep transport

streams with large bed material, the flow that moves the large, structural

bedforms can be much higher (i.e., less frequent) than in low-gradient

alluvial channels. The channel-forming flow may be the 25-year flow or

higher in a boulder-bed channel, depending on sediment inputs from the

watershed (Montgomery and Buffington 1996; Grant et al. 1990).

A.5 Fluvial Processes

This section describes key processes that both are created and affected by

channel morphologic characteristics such as pattern, channel shape, slope,

and bed structure. Understanding these processes is central to designing a

stream-simulation structure that can sustain itself in the changing stream

environment over the long term.

A.5.1 Sediment Dynamics

The morphology of a channel reflects the interaction between

hydrodynamic forces acting on the channel bed and the resisting forces of

the materials that make up the channel bed. When the hydrodynamic (lift

and drag) forces exceed the resisting forces (particle weight and friction),

sediment is entrained (mobilized), transported, and later deposited, causing

the channel to change its form or grain-size distribution.

Generally, sediment is entrained and transported as water rises and peaks

in a runoff event, and it is deposited again as high flow recedes. Stability

of a constructed streambed—like all streambeds—depends on the balance

between entrainment and transport of bed material and resupply by

deposition of material transported from upstream.

Entrainment of noncohesive sediments by flowing water depends on:

l	Sediment properties: size, shape, density, pivot angle.

s	 Larger, heavier particles require faster deeper flow to move.

Angular rocks tend to lock together better than rounded rocks,

and they resist rolling. Elongated rocks tend to ‘shingle’ or

imbricate (overlap) along the direction of flow, and they can

form very resistant bed surfaces.
A—26

Stream Simulation

l	Channel-bed composition: particle packing and orientation, sorting,

distribution of bedforms, and degree of particle exposure to flow.

s	 In poorly sorted channel beds, the stability of a particle is

influenced by the particles adjacent to it (Andrews 1983; Wiberg

and Smith 1987; Komar 1987) (figure E.1). Smaller particles

are shielded behind larger particles in poorly sorted beds, and

stronger flows are necessary for entraining them than in a wellsorted bed. Larger particles, in contrast, are entrained at weaker

flows than in a well-sorted bed, because they project into the

flow. Particles that project higher are more exposed to the

force of the water, and this increased exposure enhances their

entrainment despite their greater weight. 

l	Flow hydraulics: velocity, slope, water depth, and turbulence.

Shear stress is a measure of the hydrodynamic force exerted by flow 

on the channel bed and banks. Critical shear stress for a particle is 

the force that entrains it, that is, that initiates its motion by lifting it off 

or dragging it along the bed. 

Water velocity and shear stress vary with local changes in channel slope

controlled by such things as woody debris, rock weirs, steps, or gravel

bars. These bed structures flatten local slope so that the upstream bed

retains smaller particles than a bed of uniform slope. Even small embedded

pieces of wood can control slope. In stream simulation, average slope is

an important parameter, but the team must also pay attention to the bed

structures that control slope and create both ‘sediment storage sites’ and

diverse pathways for animal movement.

Understanding the relative mobility of different bed materials and

structures is also critical. For example, sand-bed channels are highly

mobile, and their beds are continuously in motion at most flows. In

some gravel- and cobble-bed channels, the surface of coarse gravels and

cobbles is relatively stable during frequent, moderate floods, although

large quantities of sands and gravels move over the coarse surface layer

(Jackson and Beschta 1982). Many gravel-bed streams are armored, and

their tightly packed surface layers have been winnowed of finer materials.

These intermediate-mobility streams may transport very little sediment

until flow is able to breach the armor layer. Cobble- and boulder-bed

channels are quite resistant to erosion, and these large rocks move only
A—27

Appendix A—Geomorphic Principles Applied in Stream Simulation

during infrequent, exceptional floods (Montgomery and Buffington

1993; Knighton 1998). During frequent, moderate floods, however, large

quantities of sand and gravel can be transported over and around the

relatively immobile cobble and boulder structures.

A.5.2 Vertical Channel Adjustment

As high flow entrains sediment, parts of the streambed may lower or rise

by inches or even feet. Then, as flow recedes and sediment transport 

capacity drops, the scoured or filled sections may return to their preflood

elevation (Andrews 1979). After the event, that scour and fill occurred may

not be at all evident, because the streambed often equilibrates at the same

elevation as before. Stream-simulation culverts need enough headroom

and bed depth to permit these processes to occur. High flow scour and fill

is less important in streambeds that are resistant to erosion (e.g., where bed

material is large, well-packed, or imbricated).

Longer-lasting vertical changes occur when sediment or water regimes

change, or when channels are straightened or cleared of debris. Channels

aggrade (fill) when sediment supplied from upstream exceeds the local

transport capacity, and they degrade or incise (cut) when the reverse is

true. Aggradation is the vertical rise in the bed elevation, a rise resulting

from sediment deposition, which can occur upstream of a backwater

structure such as a beaver dam or an undersized culvert. Aggradation is a

common risk at concave slope transitions (figure 5.12). It also can occur if

flow is removed from a channel by diversion or if sediment loads increase

as a result of land use changes.

Channel incision (or degradation) is a lowering of channel elevation that

occurs when local erosion exceeds deposition of sediment transported

from upstream. Following are some familiar locations where channel

incision commonly occurs:

l	Stream reaches below dams, which cut off the supply of sediment and

alter the flow regime.

l	Forest streams where wood that controlled grade has been removed.

l	Watersheds where the frequency or magnitude of peak flows has

increased due to land cover or climatic changes.

Channel incision can create a self-reinforcing feedback loop. As the

channel deepens, larger and larger floods are contained within its banks.

The stream bed experiences increasing shear stress, and continues to incise

until it encounters erosion-resistant material.
A—28

Stream Simulation

All these processes can severely affect simulated streambeds. Project

teams should understand the direction and magnitude of probable vertical

channel change over the lifetime of the planned structure, and they should

design the structure to accommodate those changes.

A.5.3 Lateral Channel Adjustment

Many styles of lateral channel adjustment exist, and some of them occur

in response to vertical adjustments. Aggrading channels tend to widen

because, as the channel fills, flows apply more erosive pressure to the

banks (figure 4-3). On the other hand, sediment deposition also can result

in channel narrowing if vegetation is able to colonize new bar deposits

along the banks. Although incising channels are initially narrow, they tend

to widen as their banks become taller and more prone to sloughing (figures

4.6 and A.28).

Another fluvial process important in stream-simulation design is lateralchannel migration. As described in chapter 1, lateral shifting can change

the stream’s alignment to a crossing, and affect the crossing’s ability

to pass water, sediment, and debris. A crossing located on a channel

bend may need to be positioned asymmetrically over the channel to

accommodate future channel shifting. If the bend is sharp or the rate of

channel migration is high, alternative solutions such as a bridge spanning

the zone of potential lateral migration may be necessary.

In narrow valleys where the valley walls are close to the channel, the

potential for lateral-channel migration is limited. However, streams in

wide alluvial valleys shift position laterally across the valley bottom, and

the process may be either gradual or rapid. Low-gradient sand and gravel

channels gradually shift by meander migration; during frequent, moderate

floods, the stream erodes the outer banks of bends and builds the point

bar on the inside bank. Sudden and pronounced lateral shifting can occur

during infrequent, large-magnitude floods or when water scours around

obstructions such as sediment or wood accumulations.

The rate of meander migration depends on:

l	Bend geometry (tighter bends tend to migrate faster).

l	The resistance of the outer bank to erosion (bank height, materials,

vegetation, moisture, etc.).

l	The magnitude and duration of the hydraulic forces acting on the

bank (Knighton 1998).
A—29

Appendix A—Geomorphic Principles Applied in Stream Simulation

Certain types of sinuous planform patterns indicate a systematic

downstream, down-valley meander migration, while others indicate a

process of periodic bend cut-offs (Thorne 1997; Knighton 1998) (figure

A.19).

Figure A.19—Types of lateral-channel adjustment. From Thorne (1997).

Reproduced with permission from John Wiley & Sons, Ltd.

Regardless of valley width, standing trees and large woody debris in and

along the stream can substantially affect channel processes by increasing

flow resistance, affecting bank erodibility, and providing obstructions to

flow (Hickin 1984; Thorne1990). Large woody debris deposited in and

along channel/flood-plain margins can alter channel patterns by diverting 

flow around the obstruction or creating low-velocity zones where sediment

and organic matter deposit (Fetherston et al. 1995; Abbe and Montgomery

1996). This deposition in turn provides fresh surfaces for the establishment

of new vegetation. Depending on the vegetation type, rooting strength can

stabilize those surfaces and influence the degree of later channel migration.

Bank vegetation has a strong influence on lateral adjustability. Deep-rooted

native species often provide very strong bank reinforcement. If native

species are replaced by shallower-rooted exotic plants, bank erosion can

accelerate, causing the channel to widen or increasing the rate of meander

migration.
A—30

Stream Simulation

A.5.4 Flood-plain Inundation and Dynamics

A flood plain is a valley surface being constructed as the current stream

deposits sediment. It is a temporary sediment storage area along the valley

bottom, composed of sediments deposited during overbank floods. In

meandering, low-gradient channels with relatively large, well-developed

flood plains, lateral accretion is the dominant flood-plain formation

process. In other words, the flood-plain surface is formed as the stream

builds point bars during meander migration (Nanson and Croke 1992).

In steep channels with narrow, discontinuous flood plains, vertical 

accretion (sediment deposition on top of the flood plain) is the dominant

flood-plain forming process, because coarse channel sediments inhibit

channel lateral migration (Nanson and Croke 1992).

Flow occurs frequently over a true flood plain (whenever bankfull

discharge is exceeded). Other, higher flat valley surfaces (terraces)

are flooded at less frequent intervals. Terrace surfaces are not being

constructed by the current stream, although it may be eroding them. Both

low terraces and flood plains can have erosion and deposition features, and

the “flood-prone zone” (figure A.13) may encompass both.

Flood plains are key elements affecting channel stability in many response

reaches. The stream’s ability to overflow the flood plain limits channel

erosion during high flows by limiting flow depth inside the main channel.

During a flood, flow in the main channel is fast and deep, while flow

over the flood-plain surface is slower and shallower. There is growing

recognition that riparian forests play a significant role in the development

of channel and flood-plain morphology. These forests stabilize flood plains

during high flows and contribute large woody debris in and along channels

that modifies flow hydraulics and sediment transport (e.g., Thorne 1990;

Abbe and Montgomery 1996).

The density and type of vegetation on the flood plain influence the

velocity and depth of flow over its surface, thereby influencing floodplain conveyance, which is the water discharge (volume per unit time)

across the flood plain or flood-prone zone. Flood-plain conveyance is a

very important variable at a stream-simulation crossing, because during a

flood the volume of flow on a high-conveyance flood plain may be so large

that it requires special handling to avoid concentrating flow through the

crossing.
A—31

Appendix A—Geomorphic Principles Applied in Stream Simulation

A.6 Channel Classification Systems

To provide a framework for assessing channel conditions, interpreting

fluvial processes, predicting channel responses, and making design

recommendations, this guide uses the channel-type classifications that

Montgomery and Buffington (1993, 1997) and Rosgen (1994, 1996)

developed. Both classifications are useful in stream simulation for

somewhat different purposes.

As the information in this appendix only summarizes these classifications

briefly, we strongly encourage you to read the original papers.

A.6.1 Montgomery and Buffington Channel Classification

The Montgomery and Buffington channel-classification system is based

primarily on streambed structure (bedforms). The classification, which

applies to mountain streams, identifies six distinct alluvial channel types

and two nonalluvial channel types (bedrock and colluvial, section A.3.1).

The classification of the alluvial types is based on bed structure and

the resulting channel roughness and energy dissipation characteristics.

Montgomery and Buffington (1993, 1997) also distinguish “forced

morphologies,” in which flow obstructions (such as wood) “force”

a channel morphology that is different from what would exist if the

obstructions were not present.
A—32

Stream Simulation

Cascade channels (figure A.20) generally occur on steep slopes (i.e., about

10- to 30-percent slope), and are frequently confined by valley walls. Their

beds are ‘disorganized,’ with cobbles and boulders scattered or clustered

throughout. Small pools that do not span the entire channel width—and

tumbling, turbulent flow over the individual rocks—characterize this

type. The large particles that form the bed mobilize only during very large

floods (50- to 100-year flows), and they may include hillslope-derived

materials (e.g., colluvium from debris flows, rock falls) as well as fluvially

placed sediments.

Step-pool reaches (figure A.21) have large rocks or pieces of wood

that form channel-spanning steps, usually spaced at about one to four

channel widths. Below each step is a pool containing finer sediment.

Because energy is efficiently dissipated as flow falls into the pools, this

bed structure is more stable than would be expected for a less organized

streambed. The steps mobilize and reform during large floods, but finer

sediment moves over the steps during moderate high flows. Typical

average channel slopes range from 3- to 10-percent slope.

Figure A.20—Cascade reach: (a) schematic planview and profile, and (b)

cascade reach on Selway River, Idaho.

(a)

(b)
A—33

Appendix A—Geomorphic Principles Applied in Stream Simulation

Figure A.21—Step-pool reach: (a) schematic planview and profile, (b) step-pool

reach on Boulder Creek, Colorado, and (c) forced step-pool channel, Mitkof

Island, Alaska.

(a)

(c)

(b)
A—34

Stream Simulation

Plane-bed reaches (figure A.22) “have long stretches of relatively

featureless bed” (Montgomery and Buffington 1993, 1997) without

organized bedforms. They are on “moderate to high slopes in relatively

straight channels,” usually with armored gravel-cobble beds. Bed

mobilization occurs at flows near bankfull. In Rosgen’s system, a planebed reach might be either a B- or G-channel type, and could have bed

material as fine as sand.

Figure A.22—Plane-bed reach: (a) schematic planview and profile, and (b) 

plane-bed reach on the Sitkum River, Washington.

Pool-riffle reaches (figure A.23) have longitudinally undulating beds, with

a repeating sequence of bars, pools, and riffles regularly spaced at about 5-

to 7-channel widths apart. Large woody debris can alter the spacing. These

(a)

(b)
A—35

Appendix A—Geomorphic Principles Applied in Stream Simulation

channels, which usually have flood plains, may be sand- to cobble-bedded

streams. Depending on their degree of armoring, bed mobilization may

occur at or below bankfull. These may be Rosgen C, E, or F streams (see

section A.6.2 for Rosgen classifications).

Figure A.23—Pool-riffle reach: (a) schematic planview and profile, (b) pool-riffle

reach on Libby Creek, Washington.

(a)

(b)
A—36

Stream Simulation

Dune-ripple reaches (figure A.24) have low gradients with sand and finegravel beds. These streambeds transport sediment at virtually all flows, and

the bedforms change depending on water depth and velocity (figure A.14).

If the channel is sinuous, these streams also can have point bars.

Figure A.24—Dune-ripple reach: (a) schematic planview and profile, and (b)

dune-ripple reach on Coal Creek, Washington. Photo: Kozmo Ken Bates.

(a)

(b)
A—37

Appendix A—Geomorphic Principles Applied in Stream Simulation

Because the Montgomery and Buffington channel types are based on

streambed morphology, they are highly useful for stream-simulation

design, where we mimic bed structure and channel roughness to create a

simulated channel that will adjust similarly to its surrounding reaches.

Each type is uniquely adjusted to the relative magnitudes of sediment

supply and transport capacity. This relationship determines how sensitive

the channel is to changes in water and sediment inputs.

Montgomery and Buffington (1997) were able to determine for each

channel type the typical frequency with which the streambed is mobilized

(table A.2). Knowing the typical frequency is important for stream

simulation, because the simulated bed should mobilize at the same

flows as the surrounding reaches. Transport reaches such as cascade and

step-pool channels, for example, are relatively stable. The coarse bed

material that controls channel form in these channel types mobilizes only

in infrequent floods, although finer sediments and debris are efficiently

conveyed over the large rocks during normal high flows. Response reaches

such as pool-riffle and dune-ripple channels can experience significant

and persistent changes in channel dimension, slope, and planform when

hydrologic conditions and sediment supply change. These channels offer

more challenge to crossing designers than do the more stable transport

reach types. Chapter 6 outlines design options for stream simulations in

various channel types. See Montgomery and Buffington (1993, 1997) for a

complete explanation of their classification system.
A—38

Stream Simulation Streambed mobility Termed “live bed”; significant sediment transport at most flows. Armored beds usually mobilize near bankfull. 

Near bankfull or at higher flows, 

depending on grain size.

Fine material moves over larger grains 

at frequent flows. Bed-forming rocks move at higher flows depending on size; often >Q30.

Smaller bed material moves at 

moderate frequencies (floods higher than bankfull). Larger rocks are immobile in flows smaller than ~Q50.

Bedload moves over bedrock at 

various flows depending on its size. May be thin layer of alluvium over bedrock. Wood can strongly affect sediment mobility. 

Fine sediment moves over immobile 

bed at moderate flows depending on its size. May be thin layer of alluviumover immobile bed.

Entrenchment

SlightSlight 

Slight to 

entrenched

Moderately 

entrenched to 

entrenched

Entrenched

AnyAny

Slope2

<0.1%

0.1-2%

1-3%3-6%

6-30%

AnyAny

Dominant roughness and structural 

elements1

Sinuosity, bedforms, 

banks. Small debris may provide structure.

Bars, pools, grains, 

sinuosity, banks.

Grains, banks.Steps, pools, banks. 

Debris may add significant structure.

Grains, banks.Bed and banks. Sinuosity, banks, bed 

irregularities.

Bed material

Sand to 

medium gravel

Gravel, often 

armored

Gravel to 

cobble, usually 

armored

Cobble to 

boulder 

Boulder

Rock with 

sediment of 

various sizes 

in transport 

over rock 

surface

Silt to clay

Dune-ripple 

Pool-riffle

Plane-bed 

Step-pool

Cascade

Bedrock

Channels 

in cohesive 

materials 

(si/cl)

 Alluvial reaches Nonalluvial

 reaches

 REACH TYPE TYPICAL CONDITIONS

 Response Transport reaches

reaches

Table A.2—Characteristics of channel types (adapted from Montgomery and Buffington, (1993, 1997)

1 Any channel type can be ‘forced.’ In forced channels, woody debris is an important structural element.2 Slope is not

a diagnostic criterion, and slope ranges overlap more than the ‘typical’ values in this table reflect. Slope ranges shown here are

from figures 16 and 19 in Montgomery and Buffington (1993). See also figure

6 and related text in Montgomery and Buffington (1997).
A—39

Appendix A—Geomorphic Principles Applied in Stream Simulation

A.6.2 Rosgen Channel Classification

Rosgen’s (1994) major channel types are based on the following channel

variables: entrenchment, width-depth ratio, pattern, and gradient.

Rosgen’s major channel type classes are particularly useful in stream

simulation because they reflect the degree of channel entrenchment—an

important variable for assessing risks associated with stream simulation.

Streams with high entrenchment ratios (unentrenched channels, Rosgen

types C, DA, and E) have relatively wide flood plains that may be flooded

frequently. To avoid concentrating overbank flood-plain flows through the

pipe, teams must incorporate special design features in stream-simulation

installations on these channel types. Streams with low-entrenchment

ratios (entrenched channels, Rosgen types A, B, and G) have fewer risks

associated with flood-plain inundation and lateral adjustment potential.

Each of Rosgen’s nine major channel types (see figure A.25) has typical

slope ranges that can be quite broad. Subgroups within each of the major

types are divided by bed material type and designated with numbers.

Rosgen’s system does not specifically consider channels where woody

debris is a dominant influence on morphology.

Rosgen (1994) developed interpretations of each channel type’s sensitivity

to a disturbance, its recovery potential, susceptibility to bank erosion, and

reliance on vegetation for form and stability. His interpretations about

channel responses to disturbance are very useful for predicting how the

channel might change when some change occurs in water or sediment

input, when local conditions (such as riparian vegetation) change, or

during and after channel incision (see also section A.7). Project teams need

to consider these potential changes when assessing site and watershed risks

and potential channel responses to the crossing (chapter 4). 
A—40

Stream Simulation

Figure A.25—Channel types defined by Rosgen (1994). Used by permission.
A—41

Appendix A—Geomorphic Principles Applied in Stream Simulation

A.7 Unstable Channels

A.7.1 Inherently Unstable Landforms and Channel Types 

Some channel types are inherently unstable; that is, they are naturally

subject to rapid changes in channel location, dimension, or slope. Certain

landforms also are naturally unstable, and the channels that drain them are

subject to episodic (and sometimes unpredictable) changes, which may

destabilize them for a period of time. Like streams affected by unusually

large floods or other events, recovery can take years or decades, depending

on channel resilience after disturbance.

Braided streams [figure A.12(b)] are difficult sites for road-crossing

structures, because they have high sediment loads that can plug structures

and because individual channels can change location during floods. These

streams are best avoided as crossing sites. (However, where the braided

channel as a whole is confined and unable to shift location, a team might

consider an open structure that crosses the entire channel.)

Active alluvial fans are located where a confined channel emerges into a

wider valley, spreads out, and deposits sediment (figure A.26). During high

debris-laden flows, so much sediment may be deposited that it blocks the

major channel; consequently, flow jumps to a new location and forms a

new channel. Several channels may be active at once. Crossing structures

can be isolated when the channel changes location, and structures can also

exacerbate the likelihood of channel shift if they plug frequently. Even

where a fan does not appear to be active, it still constitutes a risky location

for structures of any kind, because a rare flood/debris flow event can result

in catastrophic sediment deposition.

Figure A.26—Alluvial fan bordering the Noatak River, Alaska. Photo: USFWS

Alaska Image Library.
A—42

Stream Simulation

For all of these reasons, avoid placing new crossings on fans and braided

channels.

Arroyos are incised or incising channels, usually with ephemeral flow

regimes. They are found in semiarid and arid environments where high

flows are often extremely flashy. Little or no riparian vegetation may

border an arroyo channel, and the banks can be highly erodible. During

high flows, the channel may carry large amounts of sediment and debris,

and may be prone to shifting location. Some of these channels are braided,

and the problems they pose for crossings of any kind are the same as those

for braided streams.

On or near slopes prone to mass wasting, large erosional events can be

expected to cause significant changes in the downstream channel (figure

A.27). Even stable transport reaches, if they are immediately downstream

of a slope prone to landslides, earthflow, gullying, or severe bank erosion,

can be expected to undergo flow events where sediment loads are high

enough to cause a culvert to plug. In steep terrain, where many crossings

exist on a single channel, the domino effect of a single crossing failure can

cascade downstream and actually cause a debris flow. Unconsolidated finegrained glacial deposits are especially subject to rapid surface erosion and

slumping, and we can expect channels draining them to experience large

bed-elevation changes from both headcutting and episodic sediment inputs

from surrounding slopes. Sites located at the transition point between

a transport and response reach are particularly vulnerable to sediment

deposition during large erosional events.

Figure A.27—Stream eroding the toe of a slump is likely to transport large

volumes of sediment that may plug downstream culverts.
A—43

Appendix A—Geomorphic Principles Applied in Stream Simulation

Unconfined meandering streams on wide flood plains are prone to

channel shift by meander migration, as described earlier. Such streams

are nonetheless considered to be in equilibrium as long as they maintain

consistent channel dimensions and slope. In many cases, their rate of

meander migration may be slow relative to the life of the structure.

However, land development and management frequently accelerate this

natural process of channel migration, a consideration to bear in mind

before investing in a crossing structure. A shifting channel can move so

that it no longer approaches the crossing perpendicularly—and a sharp

angle of approach tends to increase sediment deposition above the inlet by

forcing the water to turn. Likewise, a sharp angle increases the potential

for debris blockage and therefore overtopping failure.

An additional effect of crossings on such channels is that their approaches

are often on roadfill raised above seasonally wet or inundated flood plains.

Blocking the flood plain obstructs to some degree the erosional and

depositional processes that construct and maintain flood plains and the

diverse habitats they offer. The roadfill may obstruct side channels that are 

essential habitats and migration corridors for fish. Forcing the overbank 

flows to concentrate in the structure can also cause scour through or

downstream of the crossing.

A.7.2 Channels Responding to Disturbances

Streams that have been destabilized by changes in vegetative cover,

base level control, climatic events, earthquake, etc., can undergo major

changes in elevation, channel width and depth, and/or other characteristics

before returning to a quasi-equilibrium state. The changes often occur in

a predictable sequence, represented conceptually as channel-evolution

models.

One classic channel-evolution model is especially important to understand

during work on stream crossings. This model (Schumm, Harvey, and

Watson 1984) describes channel incision that could be due either

to channelization (channel straightening and/or constriction), baselevel lowering, or increases in runoff. In this model (figure A.28), an

unentrenched stream downcuts, banks become unstable and erode, and the

channel widens until a new flood plain and/or unentrenched stream system

establishes at the lower elevation.
A—44

Stream Simulation

Figure A.28—Channel evolution model shows how a channel evolves from active

incision to stabilization (Castro 2003).

Channel incision progresses upstream unless the headcut is checked by

a natural- or engineered-grade control, such as a road-stream crossing

structure. Downstream reaches are at a later stage in the evolutionary

sequence than upstream ones, and can therefore be useful for predicting

the magnitude of changes to be expected upstream. This evolution can

take years, decades, or centuries, depending on the resistance of the

materials being eroded, and can affect entire drainage basins. Tributaries

far removed from the original cause of incision can be affected as headcuts

move up the main channel and lower the base level for tributaries. The

stages are more clearly distinguishable in streams with cohesive bed and

banks where actively eroding features (eroding banks, nickpoints) hold

steep slopes. In granular materials (figure A.3), the features are less

easily distinguished because they are less abrupt (Federal Interagency

Stream Restoration Working Group 1998). Where channel segments

upstream and downstream of a crossing have very different characteristics,

understanding whether those differences are due to channel evolution or

some other cause is critical to a stream-simulation design. 

If it is not possible to avoid an unstable channel by relocating the

crossing, predict the direction of future change, and design the structure

to accommodate it. Doing all of this well requires a background and

experience in fluvial geomorphology and river dynamics.
BOther Culvert 

Design Methods 

for Fish Passage

B.1 Hydraulic Design Method

B.2 Hybrid Design and Roughened-Channel Design

B.3 Velocity Simulation

B.4 “No-Slope” Design
Stream Simulation
Appendix B—Other Culvert Design Methods for Fish Passage

Several methods exist for designing culverts for fish passage. Different

methods produce different levels of passability for various aquatic species.

This appendix briefly describes common design methods other than stream

simulation. The method descriptions are only intended to put stream

simulation in the context of the other methods. They do not include enough

detail for design.

B.1 Hydraulic Design Method

Hydraulic design has been used for decades as the primary (if not the

only) design method for fish passage culverts at road crossings. It has been

included in design manuals and applied on roads in many countries, and

Bates (2003) provides a detailed description of it. Still used as a primary

design concept in many locations, hydraulic design also is used for

retrofitting impassable culverts to improve their passability.

The goal of hydraulic design is creating water depths and velocities suited

to the swimming ability of a target fish at the range of flows when the

fish moves in the natural channel. To accomplish this, the design process

simultaneously considers the hydraulic effects of culvert size, slope,

material, and length. The resulting culvert size is usually narrower than the

stream channel bankfull width.

Maximum average velocity and turbulence in the culvert cross-section

are basic design criteria in the hydraulic method. Increasing hydraulic

roughness—by adding baffles or by embedding the pipe—increases

resistance to flow and is one way to reduce water velocity. Theoretically,

increasing turbulence can always reduce the calculated velocity in a steep

channel to a level that is passable by specific species. However, if the flow

becomes too turbulent, the velocity barrier has simply been converted to a

turbulence barrier. Turbulence can be quantified as the energy dissipation

per unit volume of water, referred to as the energy dissipation factor

(EDF). Bates (2003) suggests limitations of EDF appropriate to the passage

of adult salmon. However, we have little data on the subject, and no EDF

limits have been suggested for other species or life stages.

Another problem with hydraulic design is the paucity of biological

information available for a prudent design. We know little about movement

timing and capabilities of many species of fish and other organisms that

migrate through the stream corridor. Species—and even different life

stages within species—move at different times of the year, during different

flow conditions. The variability in movement timing and swimming
B—2

Stream Simulation

capabilities means that designing culverts to meet specific depth, velocity,

and turbulence requirements for multiple species during all flow conditions

is impractical. (Refer to chapter 1, section 1.2.3 for a discussion of

problems related to using existing biological data for hydraulic design.)

The hydraulic design method targets distinct species of fish without

necessarily accounting for the requirements of nontarget species. Different

species use the variety of habitats in a stream channel for movement.

As chapter 1 notes, many weak-swimming or crawling species use the

slow water at bank edges and along the stream bottom itself. Specific

detailed information about the hydraulics in these boundary layers would

be necessary to account for those areas in a design. The hydraulic option

also does not deal with the ecological and habitat issues at road crossings

discussed in chapter 1, sections 1.1.3 and 1.3.1. 

B.2 Hybrid Design and Roughened-Channel Design

Hybrid design and roughened-channel design are styles of hydraulic

design that create a nonadjustable streambed inside of a culvert to pass

at least some aquatic species. The channel usually resembles the general

shape of a natural channel although it may be quite different from the

channel in which it is constructed. These design methods are useful when

stream simulation is not feasible. Their purpose may be to provide:

l	A crossing structure steeper than the natural channel slope (as in an

incised channel).

l	A streambed that will be stable in the absence of bed material supply

from upstream (as below a lake).

l	A stable streambed where no reference reach can be located (as in an

unstable channel).

A roughened channel is a well-graded mix of rock and sediment with

enough roughness to sustain the required gradient and enough hydraulic

diversity to provide passage for some fish. The design method is hydraulic,

combining channel dimensions, slope, and bed material to create the water

depths, velocities, and low-turbulence conditions that a target species can

negotiate. Ideally, a channel is roughened to the point where the potential

energy available at the upstream end of a reach is consistently dissipated in
B—3

Appendix B—Other Culvert Design Methods for Fish Passage

turbulence through the reach and no excess kinetic energy is present within

the reach or at the downstream end. The velocity-simulation method (B.3)

is similar, except that velocity simulation takes its velocity criteria from

the natural channel rather than from published swim-performance values

for a target species.

To improve fish passage, roughened channels can be designed to have

banklines, shallow water margins, and other diversity. Nonetheless, a

roughened channel is essentially a hydraulic design. The bed material

is not intended to evolve as a natural channel with bed material being

scoured and replenished; instead, it is a fixed, semirigid structure.

Although individual rocks are expected to adjust position, the larger grain

sizes are designed for permanence. Because culverts with roughened

channels often are steeper and more confined than the natural upstream

channel, recruitment of the larger rock in the bed from upstream is

not expected. In other words, if large material is scoured, it will not be

replaced, and the entire channel will therefore degrade.

If excess infiltration into the roughened channel bed and loss of low

surface flow are to be prevented, bed porosity must be controlled. Smaller

grains that control the porosity in the roughened channel may gradually be

washed out of the bed. If material transported from the natural channel is

too small to be trapped in the voids of the roughened channel bed, the bed

will become porous.

A hybrid is a roughened channel designed to be similar in shape and bed 

structure (but not bed mobility) to the channel type that would naturally

occur at the required culvert slope (see slope ranges for different channel

types in table A.2). For example, if a culvert has to maintain a slope of 6

percent in a 3-percent reach, the culvert streambed could be designed as

a step-pool channel even if the natural channel is a pool-riffle type. The

steps—the structural elements of the bed—would be designed to be stable

at all flows, because sediment from upstream is not expected to replenish

the larger bed particles if they are eroded away. The culvert streambed

will be enriched by smaller sediments moving across the top of the larger

material and depositing temporarily. Because a hybrid has hydraulic

microenvironments more similar to those of a natural channel, we expect

it to pass more species and life-stages than a roughened channel or other

hydraulic design.
B—4

Stream Simulation

For the hybrid design, bed structure might be based on a reference reach of

the appropriate channel type and slope, if one exists in the area. If not, the

streambed could be designed using stable channel design methods such as

those described in USDA-NRCS (2001). Examples of hybrid-type designs

in open channels are in Castro (2003), who describes artificial step-pool

and cascade reaches, and Newbury (1993). (For more examples and design

details of various hybrid channels currently used in Europe and elsewhere,

see FAO/DVWK 2002.)

B.3 Velocity Simulation

Browning (1990) described a hydraulic design method that uses the

natural channel for determining permissible velocities in the culvert.

In this method, velocities in the culvert are allowed to be 25-percent

greater than those calculated for the natural channel during a 2-year

flood event. Browning also recommends equalizing velocities for a range

of flows. There are no specific limitations on culvert slope, width, or

length, and depending on how the method is applied, there may or may

not be a limitation on the structures or features used within the culvert

for controlling the velocity. Similar to the hydraulic method, baffles or

permanent rock can be used for controlling velocity, with no consideration

of the effects of resulting turbulence on fish passage. Performance of the

method will likely vary depending on the capability of the culvert to hold

bed material, the occurrence of floods that may scour the bed out, and the

supply of bed material load in the stream.

B.4 “No-slope” Design

The Washington Department of Fish and Wildlife (Bates 2003) developed

“no-slope” design as a regulatory option to simplify fish passage design

and permitting for private landowners with short crossings under

driveways. The no-slope option requires few technical calculations and

results in reasonable culvert sizes.

According to the no-slope design method, the bed within the culvert

must be at least as wide as the channel bankfull width. Current thinking

is that width should be somewhat greater. The culvert is level, and the

downstream invert is countersunk below the channel bed by a minimum

of 20 percent of the culvert diameter or rise. The upstream invert is

countersunk by a maximum of 40 percent of the culvert diameter or rise.
B—5

Appendix B—Other Culvert Design Methods for Fish Passage

These countersinking requirements limit (a) the channel slope on which

you can install this kind of a culvert and/or (b) the length of any culvert

that may be designed with this concept. Variations of the method might

simply limit the slope and length, allowing the culvert to be sloped. A

culvert designed by this method must also be checked for adequate flood

capacity.

The published description of this method (Bates 2003) does not suggest

installing a bed, nor does it consider bed stability. However, before

countersinking a bare culvert into a channel, the designer should consider

the potential effects on the channel. Replacing a barrier culvert with a

larger embedded culvert can create a headcut. Without an understanding of

the effects of such a headcut, the replacement could pose a substantial risk

to channel stability (see section 5.3.3).

CSite Assessment 

Checklist
Stream Simulation
Appendix C—Site Assessment Checklist

ROAD

4 Long-term commitment and plans for the 

 road.

4 Road management objectives.

n Location of road and crossing.

4 Road and crossing maintenance history: 

chronic maintenance problems.

4 Vertical and horizontal constraints on road 

grade and location.

4 Rights-of-way.

n Associated infrastructure.

n Fillslopes: height, stability.

n Construction closure and detour 

options.

WATERSHED RISK FACTORS

4 Geologic or geomorphic hazards 

(landslides, avalanches, debris torrents, 

etc.).

4 History of flooding and geomorphic events.

4 Land management history and projected 

future change: expected changes in 

sediment and/or flow regimes.

4 Channel stability offsite (location/type/

potential to affect site.)

EXISTING STRUCTURE

n Dimensions, slope, fill, perch.

n Material, condition.

n Structure skew to stream and 

road.

4 Flood-plain constriction.

4 Site restrictions/sensitive areas.

4 Type of barrier (partial or complete).

4 Fish and other aquatic organisms affected 

by barrier. 

	 	 l Endangered species.

	 	 l Timing, swimming ability.

4 Terrestrial species affected.

4 Barriers upstream and downstream from 

structure. 

4 Structure priority for replacement.

RESOURCE VALUES

	 4 Aquatic- and riparian-dependent fish and 

wildlife populations.

	 4 Aquatic habitats requiring protection.

		 l Quality and extent upstream from 

structure.

		 l Critical habitats downstream.

		 l Flood-plain habitats.

		 l Work window timing.

	 4 Terrestrial animal migration routes/

specialized habitats.

	 4 Flood-plain habitats; wetlands.

	 4 Critical flood-plain water storage. 

	 4 Water supply.

	 4 Recreation.

PROJECT REACH 

n Annotated site sketch

	 4 Geomorphic features: channel and valley.

	 4 Road features.

	 4 Significant vegetation.

	 4 Land ownership.

	 4 Utilities.

	 4 Potential lateral adjustment.

	 4 Potential construction access.

	 4 Photo points.

	 4 Cross section, key feature locations.

n Channel morphology

	 4 Channel type.

	 4 Natural channel location.

 l Alignment.
C—4

Stream Simulation

	 4 Longitudinal profile. 

 l Stable endpoints.

 l Key features; mobility.

 l Residual pool depths/scour potential.

 l Natural channel elevation, gradient, 

and vertical adjustment potential.

	 4 Cross sections.

 l Bankfull width; variability in width.

 l Bank height.

 l Flood-prone zone width; flood-plain 

conveyance.

 l Flood-plain roughness.

 l Additional cross sections for backwater 

model.

	 4 Bed material.

 l Pebble count or other estimate of 

gradation.

 l Armoring. 

 l Key features; size and mobility.

	 4 Soils/foundation materials.

	 4 Ground water.

n Channel stability

	 4 Channel response to existing structure.

	 4 Vertical adjustment potential.

 l Bed mobility.

 l Perch.

	 4 Lateral adjustment potential.

 l Bank stability.

	 4 Flood-plain conveyance. 

	 4 Plugging potential.

 l Woody debris.

 l Ice. 

n Risk Assessment 

4 Site history (flood history, past/future land 

use, geologic/hydrologic setting, etc.).

4 Potential for change in sediment loading/

flow regime.

4 Vertical adjustment potential.

4 Headcut potential and effects.

4 Aggradation potential and projected effects.

4 Lateral adjustment potential and effects.

4 High flood-plain conveyance; constriction 

potential.

4 Habitats.

STATEMENT OF PROJECT 

OBJECTIVES (section 4.6)

	 4 Road.

 l Traffic level; interruptibility; safety.

 l Maintenance. 

 l Diversion potential.

	 4 Stream-simulation channel.

 l Desired design features.

 l Structure design flow.

REFERENCE REACH

	 4 Preliminary selection.

	 4 Longitudinal profile.

 l Gradient.

 l Key features: types, spacing, height. 

 l Channel roughness.

	 4 Cross section

 l Channel form/geometry.

 l Bankfull width.

 l Entrenchment. 

 l Channel margins and banklines.

	 4 Bed material. 

 l Bed material: gradation, armoring, 

angularity. 

 l Key features: particle sizes, packing/

consolidation.

 l Bed mobility.
DEstimating Design 

Stream Flows at RoadStream Crossings

D.1 Introduction

D.2 Design Flow Estimates

D.3 Verifying Flow Estimates at Ungauged Streams
Stream Simulation
D—1

Appendix D—Estimating Design Stream Flows at Road-Stream Crossings Appendix D—Estimating Design Stream Flows at Road-Stream Crossings

D.1 INTRODUCTION

Assessing the stability of any crossing structure requires estimating design 

peak flows for the site. This appendix provides guidance and resources 

for estimating peak flows at gauged and ungauged sites. It is intended as a 

desk reference rather than an introduction to hydrologic analysis.

Two types of design flows apply to stream-simulation design: 

l structural-design flows, for evaluating the structural integrity and 

stability of the culvert, bridge, etc., during flood events. 

l bed-design flows, for evaluating the stability of the particles intended 

to be permanent inside a drainage structure. 

Design flows are the flows that, if exceeded, may cause failure of 

the structure or the bed. The two design flows may be different if 

the consequences of bed failure are different from those of complete 

structural failure (see risk discussion in section 6.5.2.1). For example, 

if the acceptable risk of bed failure is 4 percent in any one year, the bed 

design flow would be the flow that is exceeded on average only every 25 

years—the 25-year flow. The acceptable risk of losing the structure might 

be lower, perhaps only 1-percent per year, in which case the 100-year flow 

would be the structural-design flow. These design flows are often taken 

to be the same in real applications, but it is important to understand the 

concept that design flows are determined based on acceptable risks and 

consequences.

All stream-simulation designs require estimating the structural and bed 

design flows. Some designs also require further hydraulic analysis—the 

bed-mobility analysis which compares key-piece entrainament flows in 

the reference reach to those in the project reach (appendix E). For the 

comparative analysis, we do not need to know the flow recurrence interval. 

However, determining the recurrence interval can be an independent check 

on the reasonableness of your estimate of entrainment flow. The frequency 

of the estimated flow can be compared to the bed-mobilization frequencies 

listed in table 6.5 for each channel type, or to field observations of actual 

floods of known recurrence interval. 

Estimating flood flows on small watersheds is particularly difficult, 

because relatively few stream gauges exist on small streams. The U.S. 

Geological Survey (USGS) maintains nearly 6,000 gauges, 26 percent of 
D—2

Stream Simulation

which are on or within 10 miles of national forest lands. However, only 

132 gauges are on streams with a contributing area of less than 5 square 

miles, and only 22 of those are on or within 10 miles of a national forest 

(figure D.1). Fewer still have a long-term flow record to allow for accurate 

estimates of extreme events. 

More inactive gauges exist than active ones and the historic records from 

inactive gauges are also very useful for estimating flood flows. There are 

407 inactive gauged sites with a contributing area of less than 5 square 

miles and within 10 miles of a national forest.

The lack of gauging stations in small, forested watersheds requires the 

analyst to use varied approaches, employing multiple flow-estimation 

methods to arrive at the best estimate of design flows. Methods can be 

grouped according to the project site’s proximity to a stream gauge: 

l Direct application of gauge data.

l Extrapolation of flow estimates from gauged sites. 

s To ungauged sites on the same stream.

s To ungauged sites on nearby streams.

l Predictions in ungauged basins with regional regression equations.

 

To meet project objectives, teams must invest appropriate time and effort 

in developing both structural- and bed-design flows. If structural-design 

flows are underestimated, then the risk of hydraulic failure is likewise 

underestimated. Conversely, if design flows are overestimated, both 

immobile bed particles and the structure itself may be oversized. 

D.2 Design Flow Estimates

Many sources of streamflow data exist. Stream gauges are most commonly 

operated by State or Federal agencies or by utilities. The best and most 

reliable data are generally those published by the USGS, which has welldefined protocols for data collection and quality control. Gauge data 

collected without defined protocols and documentation may be of lesser 

quality. At the National Water Information Systems (NWIS) Web site 

(http://water.usgs.gov/nwis/), you can download gauge station information, 

field measurements, summary statistics, and mean daily flow, peak flow, 

and partial peak flow data.
D—3

Appendix D—Estimating Design Stream Flows at Road/Stream Crossings

Figure D.1—U.S. Geological Survey stream gauges in and 

near national forests.
D—4

Stream Simulation

Professional papers and reports are a good source of historical flow data, 

regional regression equations, and other flow estimation tools. USGS 

publications for estimating flows describe the methods to be used for each 

State. Check the National Flood Frequency Program (NFF) Web site for 

the electronic documentation for the State in which your project lies: 

(http://water.usgs.gov/software/nff.html).

D.2.1 Design Flow Estimates at Gauged Sites

Although aquatic organism passage projects rarely occur at a gauged site, 

it may be necessary to analyze data from nearby gauges to determine flows 

of particular recurrence intervals (flood frequency) in the vicinity. 

Accuracy of flood frequency estimates at a gauged site depends on 

the length of record of the gauge. The longer the period of record, the 

better the estimate. As an example, figure D.2 shows the measured peak 

discharges for the period of record for the gauge on the North River in 

Alabama. Notice how the estimated magnitude of the 100-year flood (Q100) 

changes as different time periods are considered. In the development of 

flood frequency estimates, the general recommendation is that a gauge 

station have a minimum of 10 years of record. Gauges with fewer than 

10 years of data can be used to develop flow estimates for frequent floods 

(e.g., Q2), but they should not be used for higher recurrence interval flood 

flows.

For flood flow estimates for infrequent events (e.g., 25- to 100-year floods) 

at a gauged site, use the guidelines in Bulletin 17b (Interagency Advisory 

Committee on Water Data, 1982 http://www.floodmaps.fema.gov/pdfarchive/

dl_flow.pdf). The bulletin suggests using the Log-Pearson type III floodfrequency distribution. The required three parameters for this distribution 

are the mean, standard deviation, and skew of the logarithms of the annual 

series of peak streamflows. To determine the values of the parameters, 

follow the guidelines in the bulletin. In addition to Bulletin 17b, other 

useful references for flood-frequency analysis include McCuen (2003), 

Chow et al. (1988), and Linsley et al. (1982).

To facilitate flood-frequency analysis using the methods recommended in 

Bulletin 17b, you can download computer programs from private vendors 

or the USGS. The USGS program is called PEAKFQ; the most current 

version (4.1) is a DOS version, last updated in February 2002. You can 

download the program and user guide at http://water.usgs.gov/software/

surface_water.html.
D—5

Appendix D—Estimating Design Stream Flows at Road-Stream Crossings

Figure D.2—Variation in peak discharge estimates over difficult periods of record.

As part of the NFF Program, the USGS has completed flood-frequency 

analyses for most of their gauges with adequate data. The NFF Web site 

(http://water.usgs.gov/software/nff.html) has the information for each 

State, summarizing estimated discharges for a range of flood frequencies. 

Be aware of the date on which the summary was last updated; it may not 

include the most recent years of data.

D.2.1.1 Weighted flood frequency

Because a gauging station’s period of record is limited, computed floodfrequency values may contain some bias. The period of record for the 

station may or may not include years when large floods occurred (see 

figure D.2). Flood-frequency values calculated from a record that includes 

several large floods will be very different from one that happens to lack 

any large floods. To improve the reliability of the estimate—especially for 

gauges with short periods of record—you can weight the flow computed 

from streamflow data (QG) for a specific recurrence interval (RI) with the 

same RI flow computed from regression equations (QR). The weighting 

is based on the number of years of record for the gauging station and the 

equivalent period of record for the regression equation (see equation D.1). 

The equivalent period of record for the regression equation is the number 

of years of actual gauge record that would be required for producing the 

same accuracy as the equation. Hardison (1971) describes the calculations 

involved in estimating an equivalent number of years of record. For many 

Figure D-2. Variation in the estimates of 100-year peak discharge, when different 

subsets of annual peak flow data are used to make the estimate(1 m3/s = 35.3 ft3/s)

0

100

200

300

400

500

600

700

800

900

1000

1100

Feb-39Mar-44Feb-48

Dec-51

Apr-56

Mar-60

Apr-64

Dec-67

Jan-72

Oct-75

Mar-80

Dec-83

Feb-88Feb-92

Jan-96

Date

Annual Peak Discharge, ( m3/s ) -

North River, Alabama 1970-1984, Q100 = 1050 m3/s

1939-1999, Q100 = 631 m3/s

1939-1969, Q100 = 601 m3/s 1985-1999,

Q100 = 466 m3/s

1,000

1,100
D—6

Stream Simulation

States, equivalent years of record for regional regression equations are 

displayed on the NFF Web site. 

To obtain a weighted flood frequency for a gauging station, multiply the 

flow estimate for the station by the years of record at the station (N), and 

multiply the flow computed from the appropriate regional regression 

equation by the equivalent years of record (NE). Add the two values 

and divide by the sum of the years of record to obtain a weighted flood 

frequency for the stream at the gauging station (Stuckey and Reed 2000). 

Weighting of flood frequency records using the following equation is also 

discussed in detail by Cooper (2002) and Wiley et al. (2000).

Equation D.1 QW = (QG x N + QR x NE) / (N + NE)

where:

QW = Weighted discharge for a return interval of T-years.

QG = T-year discharge computed from measured streamflow data.

QR = T-year discharge computed from regional regression equations.

N = Number of years of record at the gauging station.

NE = Equivalent years of record for regional regression equations.

D.2.2 Design-flow Estimates Near Gauged Sites

Flood frequency estimates at gauged sites must be in hand before you can 

estimate flood flows at ungauged sites. 
D—7

Appendix D—Estimating Design Stream Flows at Road-Stream Crossings

D.2.2.1 Ungauged site on a gauged stream

If a project site is on the same stream as a gauge, you can calculate peak 

discharges at the ungauged site by weighting the gauge data by a ratio of 

drainage areas (e.g., Thomas et al. 1993; Sumioka 1997) as follows: 

Equation D.2 Q(ungauged) = Q(gauged) (Aungauged / Agauged )x

where:

Q = Discharge.

A = Basin area at gauge site and project site.

x = Slope exponent of the curve (power function) relating Q to A for 

 suitable gauges in the hydro-physiographic province.

The slope exponent (x) accounts for the difference between the ways in 

which larger basins and smaller basins react to precipitation. Larger basins 

usually have smaller peak discharges per unit area than smaller basins, 

because of differences in the amount of water storage (in ponds and soils), 

time of concentration, and spatial differences in precipitation during a 

storm. The exponent x is approximately the same value as the average 

exponent on basin area in the regional regression equation for that flood 

region. 

If necessary, you can directly determine the value of the exponent for 

any subset of gauges simply by plotting the flow estimates from floodfrequency analysis for a given recurrence interval (dependent variable) 

against drainage area (independent variable), and fitting a power function 

through the data (figure D.3). You can then average the drainage area 

exponents determined from this regression for several RI flows, to produce 

a single exponent for each flood region. However, you do not usually have 

to do this analysis, because the slope exponent is often reported by flood 

region in the USGS publications for individual States (see NFF Web site).
D—8

Stream Simulation

Figure D.3—Typical example of the relationship of drainage area to flow of a

specific recurrence interval (or exceedence probability), for determining the

exponent (x) in equation D.2.

Equation D.2 is valid as long as the drainage area of the ungauged site is 

between 0.5 and 1.5 times the area of the gauged site and the watersheds 

have similar characteristics. If the watersheds differ appreciably in 

topography, vegetative cover, geology, etc., make the peak discharge 

estimates using appropriate regional regression equations. 

Some investigators (e.g., Wiley et al. 2000; Stuckey and Reed 2000) 

recommend a different method of transferring gauged data to an ungauged 

site. This method uses a linear correction factor for the difference in 

drainage areas between the gauged and ungauged sites (equation D.3).

Equation D.3 Cu = Cg – [2(|Ag – Au|)/ Ag](Cg – 1)

where:

Cu = Correction factor for the ungauged site.

Cg = Weighted flow for the gauged site (Qw from equation 1) divided by

the regional regression estimate of the flow for the gauged site

(QR).

Ag = Drainage area at the streamflow gauge.

Au = Drainage area at the ungauged site.

Determination of Slope Exponent ( x)

Q100 = 81.5A0.68

Slope Exponent, x = 0.68

0

2,000

4,000

6,000

8,000

10,000

0 200 400 600 800 1,000

Drainage Area (sq. mi.)

Q

100

 (cfs) -

(x)
D—9

Appendix D—Estimating Design Stream Flows at Road-Stream Crossings

The flow estimate for the ungauged site is determined by multiplying the 

correction factor for the ungauged site (Cu) by the regional regression 

estimate for the ungauged site. Decide which transfer method (equation 

2 or 3) to use, given the recommendations in the applicable USGS flood 

frequency analysis documentation within the NFF program (FEMA 1995). 

D.2.2.2 Ungauged site near a gauged stream

If the project site is near a gauge—even if the project site is not in the 

same watershed—you can often use the gauge data to estimate design 

flows. The methodology is the same as that presented in section D.2.2.1. 

When extrapolating the data from a specific gauged site to a site in a 

nearby watershed, the two sites must have similar:

l Precipitation.

l Drainage area and shape.

l Orographic expression.

l Aspect.

l Vegetation.

l Lithology/geology.

Again, when you transfer gauge data to an ungauged site, the basin area 

should be within 0.5 to 1.5 times that of the gauged site. The accuracy and 

validity of the flow estimates are directly tied to the similarity of watershed 

characteristics between the gauged and ungauged sites. As the differences 

between the watersheds increase, be more cautious in using this technique. 

Be familiar with the gauged site and recognize the hydrologic influence of 

lakes, water diversions, regulated rivers, or dams. In addition, be cautious 

when using gauges on losing streams in arid areas, in karst terrain, or 

in areas with evident regional ground water contributions to streamflow. 

Extrapolation in these situations can produce invalid results. 

D.2.3 Flow Estimates on Ungauged Streams

You can also estimate peak stream flows for an ungauged watershed from 

equations that relate peak flows to climatologic and physical characteristics 

of the watershed (Thomas and Benson 1969; Riggs 1973). The equations 

are derived using multiple linear regression techniques. This generalization 

or regionalization of peak discharges from measured to unmeasured 

watersheds is known as “regional regression analysis.” Defining regions 

of relatively consistent geography, geology, and hydrology improves the 

accuracy of regional regression equations. States have different numbers 
D—10

Stream Simulation

of equations, depending on the number of flood regions they have 

defined. For example, the State of Colorado has a different set of regional 

regression equations for each of five different flood regions; New Mexico 

has eight flood regions, and Maine has one. 

For most States, the USGS has completed regional regression analyses 

and developed predictive equations. First, gauge-site peak discharges 

corresponding to a suite of recurrence intervals are computed using the 

flood-frequency analysis techniques discussed previously. These peak 

flows are then used as dependent variables in a multiple regression analysis 

against independent variables such as drainage area, mean basin elevation, 

mean maximum January air temperature, area of lakes and ponds, etc. 

Although the list of independent variables is extensive, only a small subset 

correlates well enough with peak flows to be used in predictive equations. 

Drainage area and some measure of precipitation are commonly the most 

important variables. The form of the regional regression equations takes 

the general form:

Equation D.4 Q = b0 + b1x1 + b2x2 + ..... +bmxm

Where Q represents the predicted streamflow for a selected recurrence 

interval and x1, x2, ..., xm represent the m watershed characteristics used 

as predictive variables. The regression coefficients b1, b2, ..., bm define the 

relationship among variables and are determined from the measured data 

in the flood region. 

Published regression equations typically provide some measure of their 

accuracy. The standard errors of prediction typically range between 30 

and 60 percent, although some exceed a standard error of 100 percent. An 

example of predictions using regional regression equations from Oregon 

(figure D.4) shows the range of potential error included within 95-percent 

prediction limits. 

All of the published regional regression equations have limitations. First, 

they should only be applied where basin characteristics are within the 

limits of those used for developing the equations. For example, if regional 

regression equations were developed from gauges with drainage areas 

between 10 and 100 square miles, the accuracy of those equations is 

suspect for a site with a 5-square-mile drainage area. 
D—11

Appendix D—Estimating Design Stream Flows at Road-Stream Crossings

Figure D.4—Example of prediction errors asssociated with regional regression

equations.

A second limitation is that the names of the independent variables within 

the regional regression equations do not necessarily convey the method 

by which they should be quantified. For example, watershed slope can 

be characterized in a variety of ways. Carefully read the supporting 

documentation for the equations to fully understand the methodology in 

which the predictor variables are determined. 

Third, regressions may not be applicable in areas with unique geohydrologic features affecting floods, such as seeps or springs that 

contribute large parts of streamflow or areas with extremely high soil 

permeability (Omang 1992). 

Fourth, be aware that urbanization, roads, timber harvest, streamflow 

diversions, or other land use changes will affect water yield and can thus 

have an influence on calculated design flows.

Peak Flow Predictions

Northern Oregon Coast

0

200

400

600

800

1000

1 10 100

Recurrence Interval 

Discharge ( cfs ) -

Predicted (Cooper 2002)

95 % Prediction Limits

1,000

95% Prediction Limits
D—12

Stream Simulation

D.3 Verifying Flow Estimates at Ungauged Streams

Given the potential errors in estimating flows at ungauged sites, using field 

data to check flow estimates can greatly enhance their credibility. Keep 

in mind that the accuracy-checking methods described here are likely to 

allow detection only of gross errors. 

One method of verifying flood-flow predictions at ungauged sites is 

to compare the predictions to real flows that were observed or that left 

evidence on the landscape from which to calculate flow. Information about 

flood frequency and water-surface elevations can come from querying 

long-time residents and/or identifying historic flood markers in the field. 

Knowing the flood elevation and the morphology of the stream reach, 

you can use a hydraulic model to route a calculated flood flow (e.g., Q100) 

through the stream reach. Compare the modeled water-surface elevation 

for the predicted flow to observed or field-identified flood levels to get an 

idea of whether your estimate is reasonable. Although you rarely know 

exactly what recurrence interval the historic flood was, news accounts or 

anecdotal information from residents often offer some indication of how 

unusual the flood was. You also can compare the modeled water-surface 

elevation to a geomorphic feature, such as a terrace, for which you can 

identify an approximate frequency of flooding. This check helps verify that 

the predictions are at the right order of magnitude so long as watershed 

changes (dam building, development of impervious areas, etc.) have not 

altered flow frequencies from those observed in the past. 

The routing analysis requires a measured cross section (or series of cross 

sections) and stream gradient at the site, along with some estimate of 

flow resistance in the channel and on the flood plain. You can perform 

hydraulic routing using Manning’s or other equations (see Hardy et al. 

2005) in uniform reaches or, for reaches where gradient or cross-sectional 

characteristics are changing, with backwater analysis programs such as 

HEC-RAS. 

Accurately determining flow resistance (typically the Manning’s 

roughness coefficient, n) is a key element in hydraulic modeling aimed 

at verifying predicted flows. A number of publications are available for 

estimating channel roughness from photos (Barnes 1967) or descriptive 

tables (Chow 1959). Equations exist for estimating n from physical 

channel characteristics such as slope and sediment size distribution (e.g., 
D—13

Appendix D—Estimating Design Stream Flows at Road-Stream Crossings

Bathurst 1985; Jarrett 1985). Other methods use a combination of photos 

and physical channel characteristics (Hicks and Mason 1998). A USGS 

publication also is available to estimate roughness characteristics of flood 

plains from photographs and site descriptions (Arcement and Schneider 

1989). Although these methods can give good estimates of channel 

roughness, the preferred method is to perform discharge measurements 

at high flows, then use Manning’s equation to back-calculate the actual 

roughness.

The most direct way of verifying the validity of calculated design flows 

is to check their reasonableness against measured values at nearby gauges 

(if they exist). Before doing so, check that the conditions affecting flow at 

the nearby gauge(s) (e.g., watershed characteristics and stream type) are 

similar to those at the project site. As a starting point, develop unit runoff 

relations at the local gauges: divide the flow for a given RI at the gauge by 

drainage area at the gauge to arrive at a discharge per unit area (i.e., cubic 

feet per second per square mile). These normalized values can provide a 

rough check on the magnitude of estimates made with regional regression 

equations at the project site.

Bankfull is the flow we can most confidently estimate from field 

observations alone, and when the field data confirms the bankfull 

discharge value estimated using regional regression equations or other 

indirect methods, we have more confidence in estimates of larger floods 

made using the same methods. (Depending on the region of the country, 

bankfull discharge at gauging stations often corresponds to a recurrence 

interval between 1 and 2 years.) At the project site, estimate bankfull 

discharge directly from a regional regression equation, if one is available 

for such a frequent flood. Alternatively, make a plot of discharge vs. RI 

(figure D.4), and extrapolate the curve to arrive at a flow estimate with the 

same recurrence interval as bankfull discharge. Then, route this estimate 

of bankfull flow through a representative cross section, using WinXSPRO 

(Hardy et al. 2005) or another hydraulic model, and check how well the 

calculated water surface matches the field-identified bankfull indicators. 

If the calculated discharge corresponds to a water-surface elevation 

significantly different from the bankfull indicators, adjustments in the 

calculated flood estimates may be necessary. On the other hand, if the 

calculated discharge is a reasonable representation of the bankfull stage, 

this suggests, although it doesn’t guarantee, that flood predictions of 

higher recurrence intervals are reasonable. We recommend confirming the 

accuracy of higher recurrence interval floods using other methods such as 

those described above.
D—14

Stream Simulation

Another way of checking on the reasonableness of an estimated flow is 

to check the Froude number (F) for that flow. The Froude number is a 

measure of whether flow is subcritical, critical, or supercritical. It is a ratio 

of inertial forces to gravitational forces and is calculated as follows:

Equation D.5 Fr = V/ gD 

where:

Fr = Froude Number

V = Average velocity = Q/A

g = Gravitational acceleration

D = Hydraulic mean depth = A/T

A = Cross-sectional area 

T = Top width of water

If:

Fr < 1 the flow regime is subcritical.

Fr = 1 the flow regime is critical. 

Fr > 1 the flow regime is supercritical.

Flows in natural channels are rarely critical or supercritical, so determining 

the Froude number for a calculated flood discharge at the project site may 

indicate whether problems with the estimate exist. For example, if the 

Froude number is greater than 1 for the calculated flood flow at the project 

site, this may indicate that the flow estimate is too high. Be sure to check 

the Froude number at local gauged sites. If the stream type is unusual (say, 

a bedrock channel), or the flow is extremely high, the high Froude number 

may be real.
E Methods for 

Streambed Mobility/

Stability Analysis

E.1 Flow Hydraulics: Shear Stress and Unit Discharge

E.2 Particle Entrainment in Natural Channels

E.3 Sediment Mobility/Stability Analysis Example: 

Schafer Creek Tributary

E.4 Sizing Immobility Key Pieces
Stream Simulation
Appendix E—Methods for Streambed Mobility/Stability Analysis

This appendix provides background on the use and limitations of several 

sediment entrainment equations that are the most practical for streamsimulation applications. These equations are used in stream-simulation 

design to verify whether the sediment sizes to be used in the simulation 

(sizes based on reference reach data) are as mobile or as stable as intended. 

Specifically, the purpose of these equations in stream-simulation design is 

to ensure that:

	 lSimilar particle sizes move at similar flows in both the referencereach channel and stream-simulation design channel.

	 lKey pieces (permanent features) in the stream-simulation design 

channel are stable for the high bed-design flow. 

The equations are one set of tools that help the designer modify the 

simulation-bed width, the bed-material size, and/or the design slope to 

compensate for a difference between the stream-simulation channel and 

the reference reach. The difference might be a flow constriction (as in a 

wide flood plain that is blocked by the road fill), or a steeper slope. Within 

limits, designers can use these equations to change the design parameters 

so that a given-size particle moves at the same flow as in the reference 

reach. 

The equations do not apply to all stream types and flow conditions. For 

example, they are not relevant to channels with cohesive soils making 

up their bed and banks. They do apply to alluvial channels composed 

of granular material where erosion occurs by entrainment of individual 

particles; however, each equation is applicable only in conditions similar 

to those for which it was developed. We strongly recommend that you 

understand the source, derivation, and limitations of these equations 

before you use them. It is always wise to compare results from more 

than one equation, and check those results for reasonableness in the field. 

Knowledgeable designers may elect to use other equations for specific 

applications, but the ones described here are a good starting point for many 

stream-simulation design situations.

E.1 Flow Hydraulics: Shear Stress and Unit Discharge

A particle on the streambed begins to move when drag and lift forces 

exerted by the flow on the particle exceed the forces resisting motion. 

Resisting forces include the submerged weight of the particle and 

intergranular friction between particles (figure E.1). The flow at which 

the particle just begins to move is called the critical flow or the critical 

entrainment flow. 
E—2

Stream Simulation

Figure E.1—Schematic diagram illustrating the interaction between drag and lift 

forces, the buoyant force, and resisting forces (submerged weight of the particle 

and intergranular friction between particles). Diagram is modified from Carling 

1992; Julian 1995; and Knighton 1998.

There are two common approaches to quantifying the driving forces acting 

on a particle during any specific flow: average boundary shear stress and 

unit discharge.

The average boundary shear stress exerted by flowing water on its 

boundary is:

Equation E.1 t = gRS

where:

	 t = average boundary shear stress (lb/ft2

)

	 g = specific weight of water (62.4 lb/ft3

)

R = hydraulic radius (ft)

S = energy slope or bed slope (ft/ft). 

Hydraulic radius is average flow depth, determined by dividing the crosssection flow area by the wetted perimeter. Because we are most interested 

in the mobility or stability of particles on the channel bed, boundary 

shear stress is calculated for flows within the active stream bed width or 
E—3

Appendix E—Methods for Streambed Mobility/Stability Analysis

bankfull width (figure E.2). Use active streambed width for streams with 

gently sloping or vegetated banks where that part of the cross section 

is subject to substantially lower shear stresses than the rest of the bed 

and there is less evidence of sediment transport. Where bankfull width 

is substantially the same as active streambed width, as in rectangular 

channels, either can represent active-channel width. Flows outside of 

those boundaries (i.e., flood-plain flow) should not be included in the 

calculations because they will underestimate the boundary shear stress 

being exerted on the channel bed.

Figure E.2—Active channel width and hydraulic radius.

For channels with gradients greater than 1 percent, and where the flow 

depth is shallow with respect to the channel-bed particle size (relative 

submergence, R/D50, values less than 10), Bathurst (1987) suggested 

using discharge-per-unit width instead of average boundary shear stress 

for determining particle mobility. The reason is that water depth in such 

channels can be highly variable and is more difficult to measure accurately 

than discharge (Bathurst 1987). The following equation defines unit 

discharge:

Equation E.2 q = Q/w

where:

q is the unit discharge (cfs/ft or ft2/s; cms or m2/s)

Q is discharge (cfs or cms)

w is the active channel width for bedload transport (ft or m) at a given 

cross section.

Active channel

width

Bankfull width 

Active channel 

wetted perimeter 

Active 

channel 

area 
E—4

Stream Simulation

Although there are no existing guidelines for defining the active-channel 

width for bedload transport, we suggest using the bed width between the 

lower banks to represent active-channel width because it is typically the 

zone of active bedload transport. The unit discharge should be determined 

for the portion of the total flow that occurs over the active channel bed 

(figure E.2). Calculating unit discharge using the total discharge instead 

of the portion of discharge occurring over the active-channel width 

would overestimate the flow actually exerting force on the active-channel 

bed. This overestimation of unit discharge would be magnified when 

floodwaters inundate a wide flood plain. 

E.1.1 Models for Calculating Flow Hydraulics 

The hydraulic parameters in both equations 1 and 2 are calculated for 

a range of discharges and require the use of hydraulic models such as 

a cross section analyzer (e.g., WinXSPRO, http://www.stream.fs.fed.

us/publications/winxspro.html) or step-backwater model (e.g., HECRAS, http://www.hec.usace.army.mil/software/hec-ras/). WinXSPRO 

uses a resistance-equation approach (e.g., Manning equation) and basic 

continuity to calculate channel geometry, flow hydraulics, and sediment 

transport potential at a single cross section (Hardy et al. 2005). Flow is 

assumed to be relatively uniform; that is, width, depth, and flow area are 

relatively constant along the channel, and the bed slope, water-surface 

slope, and energy slope are essentially parallel. WinXSPRO is also valid 

for gradually varied flow that is more typical of natural channels, so long 

as energy losses are primarily due to boundary friction (see section A.3.6). 

The program allows the user to subdivide the channel cross section so that 

overbank areas, mid-channel islands, and high-water overflow channels 

may be analyzed separately. The reliability of the WinXSPRO output data 

depends on the reliability of the cross section and bed slope data collected 

in the field for input into the program and the selection of channel 

boundary roughness or Manning’s roughness coefficient (n). Please refer 

to Hardy et al. (2005) for guidelines on collecting cross section and slope 

data, and the various methods available in the WinXSPRO program for 

determining channel roughness. WinXSPRO cannot model flow hydraulics 

through a culvert. We recommend modeling the stream-simulation design 

channel inside a culvert as an open channel, but with vertical walls having 

low roughness values as a surrogate for the culvert. Typically, streamsimulation culverts are of sufficient width and capacity that the culvert 

shape has a negligible effect on flow hydraulics. However, if overbank 

flood-plain flows are being funneled through the culvert or the crossing 

is prone to debris jams, culvert shape could potentially affect flow 

hydraulics.
E—5

Appendix E—Methods for Streambed Mobility/Stability Analysis

HEC-RAS utilizes the step-backwater method to calculate a onedimensional, energy-balanced, water-surface profile that is a function of 

discharge, channel/flood-plain boundary roughness, and channel geometry 

(usace 2006). For a specified discharge and assumed friction and form 

energy losses (channel/flood-plain boundary roughness, flow expansion/

contraction) the step-backwater method iteratively calculates an energybalanced, water-surface elevation between the surveyed cross-sections. 

When applying the step-backwater method to natural channels, the basic 

assumptions are that (1) flow is relatively steady or constant along the 

surveyed reach, (2) flow is gradually varied between successive cross 

sections, (3) flow is one dimensional, (4) slopes are less than 10 percent, 

and (5) the energy slope between successive cross sections is constant 

across the cross section. Based on the modeling results for a given 

discharge, various hydraulic parameters can be calculated at each cross 

section for both the total cross section and for sections of a subdivided 

cross section (e.g., channel, flood plain, channel banks, active bed 

width). The reliability of the HEC-RAS modeling results and subsequent 

hydraulic calculations depend on the accuracy with which surveyed 

channel/valley dimensions represent actual topography. The accuracy of 

estimating energy losses due to channel/flood-plain boundary roughness 

and to channel expansion/contraction also directly affects the reliability of 

model results. Please refer to USACE (2006) for guidelines on using the 

HEC-RAS step-backwater model. HEC-RAS can model flow through the 

stream-simulation design channel by using the “Lid” option. The crosssection data are entered as the bottom half of the structure and the “lid” 

data are entered as the top half of the culvert. Any culvert shape can be 

modeled, but the actual culvert shape the model uses will depend on the 

number of points the user inputs to define the pipe shape. Several cross 

sections with “lids” can be used to represent the length of the culvert. 

Stream-simulation culverts are almost always of sufficient width and 

capacity that flow is not pressurized; however, this can and should be 

verified in HEC-RAS at individual sites. Flow could become pressurized if 

a substantial volume of overbank flow is funneled through the culvert, or 

if a debris jam reduces the opening area and causes water to submerge the 

inlet.
E—6

Stream Simulation

E.1.2 What Flows to Analyze 

Your choice of analysis flows for sediment entrainment will depend on the 

question you are trying to answer. The most common questions in stream 

simulation are:

At what flow (e.g., bankfull, 10-year flood, 50-year flood, 100-year flood, 

etc.) are the D84 and/or D95 particle sizes of the channel bed mobilized? 

The same flow should mobilize the D84 and D95 particle sizes in the 

reference-reach channel and the stream-simulation channel. 

Are key pieces stable for the high bed-design flow (e.g., 10-year flood, 50-

year flood, 100-year flood, etc.)? Rocks used as permanent features such 

as banks or roughness elements should not be mobilized by the high beddesign flow. 

The following sections show how to answer these questions. 

E.2 Particle Entrainment in Natural Channels

Many readers will be familiar with the Shields equation, which predicts 

critical shear stress for particle entrainment based on particle size. The 

Shields equation is most applicable in well-sorted streambeds composed of 

particles of a narrow range of sizes. For these streambeds, the relationship 

of forces driving and resisting particle movement at the moment of 

entrainment (figure E.1) can be expressed as a dimensionless ratio known 

as the Shields parameter:

Equation E.3 t* = tc

 / (gs – g) D

where:

t* is the Shields parameter

tc

 is the critical average boundary shear stress at which the sediment 

particle begins to move (lb/ft2

)

gs

 is the specific weight of the sediment particle (lb/ft3)

g is the specific weight of the fluid (lb/ft3

)

D is the median size particle diameter of the channel bed (ft)
E—7

Appendix E—Methods for Streambed Mobility/Stability Analysis

The Shields parameter—the dimensionless ratio of hydrodynamic forces 

acting on the bed to the submerged weight of the particles—has been 

determined experimentally for a wide range of particle sizes (table E.1). 

The parameter increases nonlinearly as the particle size increases from 

medium size sands to very coarse gravels (ranging from 0.029 to 0.050). 

For cobbles and boulders, the Shields parameter approaches a constant 

value of 0.054. However, Shvidchenko and Pender (2000) demonstrated 

that channel slope and flow depth influence the Shields parameter; the 

Shields parameter increases as slope increases and as flow depth with 

respect to particle size decreases.

Table E.1—Shield’s parameter for different particle sizes. Modified from Julien 

1995

Particle size Particle size, Angle of Shield’s Critical

classification D (mm) repose, parameter, shear 

 f (degrees) t* stress,

 tc (lb/ft2)

very large boulders > 2,048 42 0.054 37.37

large boulders 1,024-2,048 42 0.054 18.68

medium boulders 512-1,024 42 0.054 9.34

small boulders 256-512 42 0.054 4.67

large cobbles 128-256 42 0.054 2.34

small cobbles 64-128 41 0.052 1.13

very coarse gravels 32-64 40 0.050 0.54

coarse gravels 16-32 38 0.047 0.25

medium gravels 8-16 36 0.044 0.12

fine gravels 4-8 35 0.042 0.057

very fine gravels 2-4 33 0.039 0.026

The equation used to determine the Shields parameter for gravels, cobbles, and 

boulders is t* = 0.06 tanf .

The Shield’s parameter and critical shear stress values are for the smallest 

number in the particle-size interval. 

Assuming gs= 165 lb/ft3 and g = 62.4 lb/ft3, equation 4 can be rearranged 

and simplified to calculate the critical shear stress to entrain a particle:
E—8

Stream Simulation

Equation E.4 tc = t* (102.6 D)

All equations can be used with either metric or English units, as long 

as the units are kept consistent. In metric units, equation 4 is tc

 = t* 

(16170 D). The metric unit of shear stress is newtons/square meter 

(N/m2

).

Most channels where bed mobility requires analysis for stream-simulation 

design are poorly sorted, that is they are made up of a wide variety of 

different particle sizes. In poorly sorted streambeds, the calculated values 

of t* and tc

 do not accurately predict sediment entrainment in the channel. 

To account for the variability of particle sizes in gravel- and cobble-bed 

channels, t* is often assigned a constant value of 0.045 in the unmodified 

Shields equation. However, Buffington and Montgomery (1997), in a 

thorough review of past entrainment studies, found that the assumption of 

a constant value of 0.045 is not always appropriate; reference based and 

visually based values of t* ranged from 0.052-0.086 and 0.030-0.073, 

respectively. 

Subsequently, the Shields equation was modified for poorly sorted 

channels to account for the influence of adjacent particles on the stability 

of a given particle (Andrews 1983; Wiberg and Smith 1987; Komar 1987; 

Bathurst 1987). Because larger particles shield smaller ones, stronger 

flows are needed in poorly sorted streambeds for entraining the small 

particles when compared to streambeds composed of uniformly sized 

particles. Similarly, in poorly sorted streambeds, larger particles are 

entrained at weaker flows, because the larger particles project into the flow. 

This increased exposure enhances their entrainment despite their greater 

weight (figure E.3). In addition, the larger particles surrounded by smaller 

particles have smaller pivoting angles, causing them to rotate more easily 

from their resting position on the bed (Komar and Li 1986; Wiberg and 

Smith 1987).
E—9

Appendix E—Methods for Streambed Mobility/Stability Analysis

Figure E.3—Critical shear stress for well-sorted sediments compared to critical 

shear stress for poorly sorted sediments. Higher critical shear stress is needed 

to entrain smaller particles in poorly-sorted sediments because they are shielded 

by the larger particles. Lower critical shear stress is needed to entrain larger 

particles because their protrusion into the flow causes them to experience 

greater hydrodynamic forces. The critical shear stress line for the poorly-sorted 

sediments crosses the critical shear stress line for well-sorted sediment at the 

reference particle size (D50).

In the sections that follow, we examine two approaches for evaluating the 

stability of particles in poorly sorted channel beds: (1) modified critical 

shear stress and (2) critical unit discharge.
E—10

Stream Simulation

E.2.1 Modified Critical Shear Stress Approach

The modified critical shear stress equation is based on the relationship 

between the particle size of interest (Di

) and D50, which is assumed to be 

unaffected by the shielding/exposure effect (Andrews 1983; Bathurst 1987; 

Komar 1987, 1996; Komar and Carling 1991). For stream simulation, the 

particle size of interest, Di

, is usually D84 and/or D95, because key grade 

controls are in this size range. When these particles begin to move, much 

of the streambed is in motion and the structure of the channel bed will 

change. 

The modified critical shear stress equation (Komar 1987, 1997; Komar and 

Carling 1991) is as follows:

Equation E.5 tci = tD50 (gs

 - g) Di

0.3 D500.7

where:

tci is the critical shear stress at which the sediment particle of interest 

begins to move (lb/ft2

 or N/m2).

tD50 is the dimensionless Shields parameter for D50 particle size (this value 

can either be obtained from table E.1, or the value 0.045 can be used for a 

poorly sorted channel bed).

D50 is the diameter (ft or m) of the median or 50th percentile particle size 

of the channel bed .

Di

 is the diameter (ft or m) of the particle size of interest. For stream 

simulation the particle size of interest is typically D84 and/or D95.

Assuming gs

 = 165 lb/ft3 and g = 62.4 lb/ft3, equation 5 can be simplified 

to:

Equation E.6 tci = 102.6 tD50 Di

0.3 D50

0.7

The modified critical shear stress equation is appropriate for assessing 

particle stability in riffles and plane-bed channels (i.e., where flow is 

relatively uniform or gradually varied between cross sections) with 

channel-bed gradients less than 0.05 (5 percent) and D84 particles ranging 

between 10 and 250 mm (2.5 to 10 inches). Because of the uncertainty 
E—11

Appendix E—Methods for Streambed Mobility/Stability Analysis

and variability of determining tD50 (see earlier discussion), multiple values 

should be used to assess how it influences the results. If the reference reach 

bed is tightly packed and/or imbricated, the Shields parameter and critical 

shear stress will be higher than in the newly constructed stream-simulation 

bed. The stream-simulation bed may need to have larger material to offset 

the difference in bed stability. Likewise, if angular material must be used 

for the stream-simulation bed, the Shields parameter and critical shear will 

be higher than for rounded river rock. 

When applying the critical shear stress equation, be sure that the diameter 

for the particle size of interest (e.g., D84 or D95) is not larger than 20 to 30 

times the D50 particle diameter. For Di

/D50 ratios greater than 30, equation 

E.6 is not accurate because a large particle will roll easily over surrounding 

smaller sediments (Komar 1987, 1996; Carling 1992). D84/D50 or D95/

D50 ratios are typically less than 5 in natural channels. However, where a 

design uses larger rock to create permanent, stable features such as banks 

or roughness elements, check that those rock diameters do not exceed 20 

to 30 times D50. 

To determine critical entrainment flow at a given cross section using the 

modified shear stress approach, use the following process: 

From equation E.6, find the critical shear stress (tci) for the particle size of 

interest (e.g., D84) at a given cross section. Assume tD50= 0.045 or use table 

E.1 to determine tD50 for the D50 particle size. 

Calculate the boundary shear stress (equation E.1) within the active 

channel for a range of discharges using a hydraulic model such as 

WinXSPRO or HEC-RAS.

To determine whether the particle will move, compare the active-channel 

boundary shear stress for a particular flow to the critical shear stress for the 

particle size of interest. If the critical shear stress (tci) of a given particle 

is less than the active-channel boundary shear stress (t) being exerted on 

the particle by the flow, the particle will be entrained. If the critical shear 

stress (tci) is greater than the active-channel boundary shear stress (t) 

being exerted on the particle by the flow, the particle will not be entrained. 

See the sidebar in section E.2.3 for an example calculation.
E—12

Stream Simulation

E.2.2 Critical Unit Discharge Approach

For channels steeper than 1 percent (S = 0.01) where the flow depth is 

shallow with respect to the channel bed particle sizes (R/D50 < 10), water 

depth can be quite variable because large rocks or wood pieces on or near 

the surface influence depth (Bathurst 1987). For such channels, Bathurst 

et al. (1987) used flume data to construct the following equation, which 

predicts the critical unit discharge for entraining the D50 particle size in 

well-sorted sediments: 

Equation E.7 qc-D50 = 0.15 g0.5 D50

1.5

 S1.12

where:

qc-D50 is the critical unit discharge to entrain the D50 particle size (cfs/ft or 

ft2

/s, cms/m or m2/s)

D50 is the median or 50th percentile particle size (ft or m)

g is gravitational acceleration (32.2 ft/s2

 or 9.8 m/s2)

S is bed slope (ft/ft or m/m)

In the flume studies, particle sizes ranged between 3 and 44 mm (0.1 

and 1.7 inches), the experimental bed materials were uniform (i.e., wellsorted), slopes ranged between 0.0025 and 0.20, and ratios of water depth 

to particle size approached 1 (Bathurst 1987).

Bathurst (1987) used equation E.7 to predict the entrainment of particles in 

poorly sorted channel beds, by comparing the particle size of interest (e.g., 

D84 or D95) to a reference particle size. The reference particle size is the 

D50 particle size, which is assumed to move at the same flow as in a wellsorted channel. The critical unit discharge for entraining a particle size of 

interest is determined by:
E—13

Appendix E—Methods for Streambed Mobility/Stability Analysis

Equation E.8 qci = qc-D50 (Di/D50)b

where:

qci is the critical unit discharge to entrain the particle size of interest (cfs/

ft or ft2

/s, cms/m or m2/s)

Di is the particle size of interest (mm)

D50 is the median or 50th percentile particle size (mm)

The exponent b is a measure of the range of particle sizes that make up 

the channel bed. It quantifies the effects on particle entrainment of smaller 

particles being hidden and of larger particles being exposed to flow. 

Calculate the exponent from:

Equation E.9 b = 1.5(D84/D16)-1

where:

D84 is the 84th percentile particle size (mm)

D16 is the 16th percentile particle size (mm)

Equations E.8 and E.9 were derived from limited data and are most 

appropriate for assessing particle stability in riffles and plane-bed channels 

(i.e., where flow is relatively uniform or gradually varied between cross 

sections) with slopes ranging between 0.0360 and 0.0523, widths ranging 

between 20 and 36 feet, D16 particle sizes between 32 and 58 millimeters 

(1.3 and 0.67 inches), D50 particle sizes between 72 and 140 millimeters 

(2.8 and 5.5 inches), and D84 particle sizes between 156 and 251 

millimeters (6 and 10 inches).

To determine the critical entrainment flow for a given particle size, use the 

following process (See the sidebar in section E.2.3 for an illustration):

1) Using equation E.7, calculate the critical unit discharge (qc-D50) 

needed to entrain the D50 particle size at any given cross section. 

2) Using equation E.9, calculate the exponent (b) based on the ratio 

between the D84 particle size and D16 particle size. 

3) Using equation E.8, calculate the critical unit discharge (qci) needed 

to entrain the particle size of interest at any given cross section (e.g., 

D84 or D95).
E—14

Stream Simulation

4) Using equation E.2, calculate the unit discharge within the active 

channel for a range of discharges using a hydraulic model such as 

WinXSPRO or HEC-RAS. 

To determine whether the particle will move at any given discharge, 

compare the unit discharge for that flow to the critical unit discharge for 

the particle size of interest. If the critical unit discharge (qci) of a given 

particle is less than the unit discharge (q) being exerted on the particle by 

the flow, the particle will be entrained. If the critical unit discharge (qci) is 

greater than the unit discharge (q) being exerted on the particle by the flow, 

the particle will not be entrained.

E.2.3 Uncertainty in Predicting Particle Entrainment

The modified critical shear stress equations (equations E.5 and E.6) and 

critical unit discharge equation (equation E.8) improved on the original 

critical shear stress equations (equations E.3 and E.4) by incorporating 

the effects of shielding and exposure on the entrainment of sediments. 

However, the modified critical shear stress and critical unit discharge 

equations do not account for other factors, such as:

Fluctuating flows Fluctuating flows can cause temporary increases in near-bed, instantaneous 

stresses that cause particles to be entrained at lower values than predicted 

by average shear stress values acting on the bed (Nelson et al. 1995; 

Knighton 1998). Depending on channel and flow conditions, instantaneous 

shear-stress values near the bed can be 2 to 3 times greater than the 

average boundary shear stress (Richardson et al. 1990; Knighton 1998). 
E—15

Appendix E—Methods for Streambed Mobility/Stability Analysis

Particle shape Angular particles require higher shear stresses to move than spherical 

particles of similar size, because of a greater pivot angle associated with 

angular particles (Reid and Frostick 1994). Flat, disc-shaped particles are 

usually well imbricated, making the particles more resistant to entrainment 

(Carling 1992). 

Channel-bed structure For channel beds composed of particles coarser than 8 millimeters (0.3 

inch), Church (1978) demonstrated that the Shields parameter can vary 

by a factor of two, depending on whether the channel bed is loosely 

consolidated or tightly packed. Recently deposited sediments can be 

poorly packed, making it easier for those particles to be entrained than if 

they were tightly packed or highly consolidated (Church 1978; Reid and 

Frostick 1994). With time after a large flood, the bed consolidates as low 

flows slightly rearrange particles so that they are more tightly packed and 

more difficult to entrain (Church 1978). Reid et al. (1985) demonstrated 

that the shear stress needed to entrain particles could be up to three times 

higher than the average when the flood occurred after an extended period 

of no bed disturbance.

Although table E.1 and equations E.4, E.6, and E.8 suggest a distinct 

threshold at which a particle is entrained, the previous discussion makes 

clear that the entrainment of a particle does not occur at a distinct critical 

shear stress value, but instead may occur over a range of critical shear 

stresses. Nevertheless, the equations provide insights on the relative 

mobility of channel-bed sediments for a range of flows. If the flows that 

mobilize sediment in the reference reach are different than those in the 

stream-simulation design channel, these equations can help you assess 

whether the difference is significant, and whether the stream-simulation 

design channel needs to be adjusted so that its particle mobility is similar 

to that of the reference reach. 
E—16

Stream Simulation

Using the equations to determine if D84 moves at bankfull flow in the reference reach

The reference reach selected for a stream-simulation culvert on Example Creek is a pool-riffle reach with a 

gravel-cobble bed (figure E.4). 

Bankfull flow, 106 cubic feet per second, was estimated using the HEC-RAS step-backwater model to 

generate a water-surface profile that matched bankfull elevations identified in the field using geomorphic 

indicators. The portion of flow over the active bed width was 102 cubic feet per second. Channel data for 

one of the cross sections are listed below:

Bankfull width (Wbf) = 18.7 feet

Active bed width (wa) = 15.3 feet 

Slope (s) = 0.0142 feet/feet

Hydraulic radius for the active channel during bankfull flow (Rbf) = 1 foot

D84 = 120 mm (0.39 feet) 

D50 = 52 mm (0.17 feet)

D16 = 27 mm (0.089 feet)

Determine whether the D84 particle moves at bankfull flow at this cross section.

Modified critical shear stress equation

Find critical shear stress for D84 using equation E.6:

tD50 = 0.050 (from table E.1) for 52 mm particles

tci = 102.6 tD50 Di

0.3 D500.7 (equation E.6)

tc-D84 = 102.6(0.050)(0.39 ft)0.3 (0.17 ft)0.7 = 1.12 lb/ft2

Find the average boundary shear stress in the reference reach at bankfull flow (tbf) using equation E.1:

t = gRS (equation E.1)

 tbf = (62.4 lb/ft2)(1 ft)(0.0142) = 0.90 lb/ft2

The D84 particle is stable at bankfull flow because because tc-D84 ( 1.12 lb/ft2

) is greater than tbf (0.90 lb/ft2)

The D84 particle size is stable at bankfull flow. 

Figure E.4—Downstream view of Example Creek, a 

pool-riffle channel composed primarily of gravels and 

cobbles and local inputs of wood.
E—17

Appendix E—Methods for Streambed Mobility/Stability Analysis

How well does the modified critical shear stress equation apply here?

l D84/D50 = 2.3, which is much less than 30.

l Slope < 5 percent.

l Channel unit is a riffle.

l D84 particle size of 120 millimeter is between the range of 10 and 250 millimeters.

Conclusion: The modified critical shear stress equation (equation E.6) is applicable to this stream.

Critical unit discharge equation

Find the critical unit discharge for D50 (qc-D50) using equation E.7:

qc-D50 = 0.15 g0.5 D50

1.5 (equation E.7) 

 S1.12

 qc-D50 = (0.15)(32.2 ft/s2)0.5 (0.17 ft)1.5 = 7.0 cfs/ft

 0.01421.12

Calculate b (which quantifies the range in particle sizes) using equation E.9:

b = 1.5(D84/D16)-1 (equation E.9)

b = 1.5(0.39 ft/0.089 ft)-1 = 0.34

 

Find critical unit discharge for D84 (qc-D84) using equation E.8: 

qci = qc-D50 (Di/D50)b (equation E.8)

qc-D84 = 7 cfs/ft(0.39 ft/0.17 ft)0.342 = 9.3 cfs/ft

Calculate unit discharge in the reference reach active channel at bankfull flow using equation 2:

q = Q/wa (equation E.2)

q = Qbf-active ch/wa

 = 102 cfs/15.3 ft = 6.7 cfs/ft 

The D84 particle is stable at bankfull flow because qc-D84 (9.3 cfs/ft) is greater than qbf-active ch (6.7 cfs/ft). The 

results for critical unit discharge agree with those of the modified critical shear stress equation.

Is the critical unit discharge equation (equation E.8) appropriate for this stream? 

l Slope > 1 percent.

l Channel unit is a riffle.

l D16, D50, and D84 are smaller than the particle sizes used to develop the critical 

 unit discharge equation for D84.

l Rbf/D50 = 5.9, which is < 10 (low relative submergence).

Figure E.4 shows there are some scattered large roughness elements (logs and rocks) that appear to be 

higher than 1-foot (Rbf) above the streambed. Conclusion: The critical unit discharge equation (equation 

E.8) should be used with caution since particle sizes are outside the range of particle sizes used to 

develop the equation.
E—18

Stream Simulation

E.3 Sediment Mobility/Stability Analysis Example: 

Schafer Creek Tributary 

This example applies the modified critical shear stress and critical unit 

discharge approaches to a stream-simulation design on a tributary of 

Schafer Creek in the Olympic National Forest, Washington. The purpose 

is to (1) evaluate whether the stream-simulation design bed would have 

similar mobility/stability as the reference-reach channel and (2) adjust the 

stream-simulation design so that it has similar mobility and stability as 

the reference-reach channel. At this site, sediment mobility was actually 

evaluated for several reaches within the project profile to determine the 

range of sediment mobility; however, this example limits the discussion 

to comparing sediment mobility/stability between the stream-simulation 

design bed and the reference-reach channel. 

E.3.1 Channel and Road-stream Crossing Background Information

The channel upstream and downstream from the road crossing has a planebed to pool-riffle morphology and is slightly to moderately confined with 

greater channel confinement downstream from the crossing. The channel 

upstream and downstream from the road-stream crossing has bankfull 

widths ranging between 5.5 and 7.6 meters (18 to 25 feet), pool residual 

depths ranging between 0.3 and 0.5 meters (1.0 to 1.6 feet), and channel 

gradients ranging between 1 and 2 percent (figure E.5). The channel-bed 

surface is composed primarily of gravel- and cobble-sized sediment. The 

channel bed is moderately to well armored; the subarmor layer consists 

of a poorly sorted mixture of cobbles, gravels, and sands. Channel bed 

structures in the riffles and plane-bed channel segments consist primarily 

of transverse bars or rock clusters composed of cobbles and small 

boulders. 

The existing culvert at the crossing is undersized, in a deteriorated 

condition, and is a partial barrier to anadromous fish at various life 

stages and flows (figure E.6). The culvert is a round corrugated pipe with 

a diameter of 1.52 meters (5 feet) diameter and a length of 30.5 meters 

(100 feet). There is a 0.4-meter (1.3-foot) drop at the culvert outlet and the 

associated plunge pool has a residual pool depth of 2.1 meters (6.9 feet) 

which is four times deeper than other pools along the channel (figure E.5). 

Sediment accumulation at the culvert inlet extends about 25-meters (82 

feet) upstream from the culvert (figure E.5). Based on evidence such as 

increased bank heights, undercut banks, and localized bank failures, the 

channel downstream from the road-stream crossing has incised about 0.5 

to 1.0 meter (1.6 to 3.3 feet) (figure E.5). 
E—19

Appendix E—Methods for Streambed Mobility/Stability AnalysisFigure E.5—A longitudinal profile and selected cross sections showing channel characteristics and dimensions along the Schafer Creek tributary.

vertical exaggeration = 10

Existing culvert:

diameter 1.52 m (5 ft)

length 30.8 m (101 ft)

slope 0.0201

downstream

design elevation

control point

H- ct p

M- ct p

H- d wl

M- ct p

M- ct p

L- d wl

M- ct p

2 3 4 5 6 7 8

9

10 11

8.5D sediment

wedge

upstream

design elevation

control point

plunge pool

0 25 50 75 100 125 150 175 200 225 250 275

distance downstream (m)

194192190188186184

) m( noit avel e evit al er

)tf( noit avel e evit al er

distance downstream (ft)

reference

reach

F

E

G

A

B C D

XS Explanation

fp

t

Q2, Q10, Q100 water surfaces

floodplain

terrace

upper vertical adjustment line

lower vertical adjustment line

M- ct p

L- dwl

M- ct p

M- ct p

M- ct p

H- dwl

L- ct p

L- ct p

636.5630.0623.4616.8610.3603.7

0 82.0 246.1 164.1 328.1 410.1 492.2 574.2 656.2 738.2 820.3 902.3

0 2 4 6 8

distance (m)

195194193192191190189188

evit al er

) m( noit avel e

evit al er

)tf( noit avel e

distance (ft)

XS8.5D

Qbf

Q10 Q100

VE = 1

639.8636.5633.2630.0626.7623.4620.1616.8

6.6 13.1 19.7 26.2 0

VE=1 XS4

15 20 25 30

distance (m)

194193192191

evit al er

) m( noit avel e

distance (ft)

evit al er

)tf( noit avel e

fp

Qb

Q

f 1

Q

0

100

636.5633.2630.0626.7

49.2 65.6 82.0 98.4

VE = 1

0 2 4 6 8

distance (m)

191190189188

evit al er

) m( noit avel e

evit al er

)tf( noit avel e

distance (ft)

XS10

t t

Qbf

Q10 Q100

626.7623.4620.1616.8

6.6 13.1 19.7 26.2 0

channel-bed profile

cross section location

top of bank/floodplain

pool

grade control

pool tail crest: high (H), moderate (M), or low (L) stability

large woody debris: high (H) or low (L) stability

slope segments (A,B,C,D,E,F,G)

upper vertical adjustment potential line

lower vertical adjustment potential line

2

ptc

lwd design channel-bed profile (S=0.02156)

design bedform locations (head of riffle)

base of footing, replacement structure

fill plunge pool with sediment

anticipated erosion of sediment wedge
E—20

Stream Simulation

Figure E.6—The culvert outlet and the upper segment of the reference-reach 

channel at the Schafer Creek tributary. Note the perch of the culvert outlet and its 

size relative to the channel. 

The Schafer Creek tributary site is an example of a common situation 

where an undersized culvert has created a moderate plunge at its outfall 

and the downstream channel has incised. In essence, the existing 

undersized culvert is acting as a nickpoint preventing channel incision

from continuing upstream. In situations such as this, reestablishing 

geomorphic continuity between the upstream and downstream channels 

requires either a stream-simulation channel that is slightly steeper than the 

reference reach or extensive restoration of the downstream channel to raise 

the channel bed. 

A downstream reference reach (XS10) was chosen because the stream 

is slightly steeper, more confined, and the bed is somewhat coarser than 

other segments of the channel; these characteristics are needed inside the 

replacement structure. The channel geometry and channel-bed sediment 

characteristics from XS10 were used to develop a preliminary design for 

the stream-simulation channel bed. The replacement structure for this 

crossing was a 6.1-meter (20-feet)-wide open-bottom arch; this width is 

similar to the bankfull width at XS10. The preliminary stream-simulation 

channel-bed design slope is 0.0216, which is 24-percent steeper than the 

reference-reach channel slope (figure E.5). The difference in slopes makes 

it important to check for similar mobility.
E—21

Appendix E—Methods for Streambed Mobility/Stability Analysis

Both the modified critical shear stress and critical unit discharge 

approaches are applicable at this site based on channel characteristics and 

both were used in the sediment mobility/stability analyses for comparison 

(tables E.2 and E.3). The step-backwater model HEC-RAS was used 

to model flow hydraulics for a range of discharges between just below 

bankfull to just above the Q100 discharge. Using geomorphic indicators for 

bankfull and the HEC-RAS model, bankfull discharge was estimated to 

be 3 cubic meters per second (m3/s). Regional regression equations were 

used to predict the discharges for the Q2, Q10, Q50, and Q100 floods. Selected 

hydraulic parameters from the model used in the sediment mobility/

stability analyses are summarized in tables E.2 and E.3.

E.3.2 Modified Critical Shear Stress Approach

Results from the modified critical shear stress analysis show that the D84

particle size of 160 millimeters in both the preliminary stream-simulation 

design channel and reference-reach channel has a critical shear stress 

(tc-D84) of 81 N/m2 (table E.2A-B). In the reference-reach channel the D84

particle size is mobilized at a lower discharge of about 5 m3/s when the 

active channel boundary shear stress is 82 N/m2 (table E.2A and figure 

E.7). In contrast, the D84 particle size in the design channel is mobilized 

at a discharge of about 4 m3/s when the active channel boundary shear 

stress is 85 N/m2 (table E.2B and figure E.7). The steeper slope in the 

preliminary design channel is the primary reason for the higher shear 

stresses when compared to the reference-reach channel for a given 

discharge. At this site, the project team decided not to reconfigure the 

channel to reduce the slope through the crossing. Instead they opted to 

increase the D50 and D84 particle sizes to achieve entrainment at the same 

discharge in both the design channel and reference-reach channel. The 

D50 and D84 particles were increased in size by 20 percent from 95 to 114 

millimeters and 160 to 192 millimeters, respectively (table E.2C and figure 

E.7). These sediment-size adjustments increased the critical shear stress for 

the D84 particle (tc-D84) in the stream-simulation design channel from 81 N/

m2 to 97 N/m2. The changes are within the acceptable range of 25-percent 

difference in slope and particle size between the stream simulation and 

reference channels. 
E—22 Table E.2—Summary of flow hydraulics and particle mobility/stability for the reference-reach channel and design channel using the modified critical shear stress approach

Critical

shear

Total Channel Shield’s stress to

Total Channel Total Channel boundary boundary entrain- entrain D84

flow flow hydraulic hydraulic shear shear Angle ment D84 particle Particle

Recur- Dis- Flood- Channel Channel Energy width width radius radius stress stress of for size mobile

rence charge, plain n n slope slope Wt Wc Rt Rc tt tc D50 D84 repose D50 tc-D84 (yes

Interval Q (m3/s) value value Sc Se (m) (m) (m) (m) (N/m2)a (N/m2)b (mm) (mm) f tD50 (N/m2) d /no)

 A. Reference reach cross section: XS10

2.00 0.150 0.050 0.0174 0.0174 5.19 5.19 0.30 0.30 51 51 95 160 42 0.045 81 no3.00 0.150 0.050 0.0174 0.0174 5.55 5.55 0.37 0.37 63 63 95 160 42 0.045 81 no4.00 0.150 0.050 0.0174 0.0174 5.81 5.81 0.43 0.43 73 74 95 160 42 0.045 81 no

2 4.20 0.150 0.050 0.0174 0.0174 5.86 5.86 0.44 0.44 75 75 95 160 42 0.045 81 no

5.00 0.150 0.050 0.0174 0.0174 6.01 6.01 0.47 0.48 80 82 95 160 42 0.045 81 yes6.00 0.150 0.050 0.0174 0.0174 6.10 6.01 0.52 0.53 89 91 95 160 42 0.045 81 yes

10 6.70 0.150 0.050 0.0174 0.0174 6.18 6.01 0.54 0.57 92 97 95 160 42 0.045 81 yes

7.00 0.150 0.050 0.0174 0.0174 6.22 6.01 0.55 0.58 94 99 95 160 42 0.045 81 yes

25 7.80 0.150 0.050 0.0174 0.0174 6.30 6.01 0.58 0.62 99 106 95 160 42 0.045 81 yes

8.00 0.150 0.050 0.0174 0.0174 6.32 6.01 0.59 0.63 101 108 95 160 42 0.045 81 yes

50 8.80 0.150 0.050 0.0174 0.0174 6.40 6.01 0.61 0.67 104 114 95 160 42 0.045 81 yes

9.00 0.150 0.050 0.0174 0.0174 6.42 6.01 0.62 0.67 106 114 95 160 42 0.045 81 yes

100 9.90 0.150 0.050 0.0174 0.0174 6.51 6.01 0.65 0.71 111 121 95 160 42 0.045 81 yes

10.00 0.150 0.050 0.0174 0.0174 6.52 6.01 0.65 0.72 111 123 95 160 42 0.045 81 yes

11.00 0.150 0.050 0.0174 0.0174 6.61 6.01 0.68 0.76 116 130 95 160 42 0.045 81 yes

a tt = g Rt Se (similar to equation E.1).b t

c

 = g Rc Se (similar to equation E.1).

c tD50 was assumed to be 0.045. Alternative values could be obtained from table E.1.d tc-D84 = 16170 (t*D50 ) ( D84)0.3 (D50)0.7 (similar to equation E.5).

Hydraulics Particle Mobility/Stability
E—23

Table E.2—Summary of flow hydraulics and particle mobility/stability for the reference-reach channel and design channel using the modified critical shear stress approach (continued)

Critical

shear

Total Channel Shield’s stress to

Total Channel Total Channel boundary boundary entrain- entrain D84

flow flow hydraulic hydraulic shear shear Angle ment D84 particle Particle

Recur- Dis- Flood- Channel Channel Energy width width radius radius stress stress of for size mobile

rence charge, plain n n slope slope Wt Wc Rt Rc tt tc D50 D84 repose D50 tc-D84 (yes

Interval Q (m3/s) value value Sc Se (m) (m) (m) (m) (N/m2)a (N/m2)b (mm) (mm) f tD50 (N/m2) d /no)

B. Preliminary stream simulation design channel: XS8.5D

2.00 0.028 0.050 0.0216 0.0212 4.67 4.67 0.31 0.31 66 64 95 160 42 0.045 81 no

3.00 0.028 0.050 0.0216 0.0211 5.49 5.47 0.36 0.36 76 75 95 160 42 0.045 81 no

4.00 0.028 0.050 0.0216 0.0212 6.02 5.90 0.40 0.41 85 85 95 160 42 0.045 81 yes

2 4.20 0.028 0.050 0.0216 0.0212 6.07 5.90 0.41 0.42 87 87 95 160 42 0.045 81 yes

5.00 0.028 0.050 0.0216 0.0211 6.09 5.90 0.45 0.47 95 97 95 160 42 0.045 81 yes

6.00 0.028 0.050 0.0216 0.0211 6.07 5.90 0.49 0.52 104 107 95 160 42 0.045 81 yes

10 6.70 0.028 0.050 0.0216 0.0210 6.06 5.90 0.52 0.56 110 115 95 160 42 0.045 81 yes

7.00 0.028 0.050 0.0216 0.0210 6.06 5.90 0.53 0.57 112 117 95 160 42 0.045 81 yes

25 7.80 0.028 0.050 0.0216 0.0209 6.05 5.90 0.56 0.61 119 125 95 160 42 0.045 81 yes

8.00 0.028 0.050 0.0216 0.0209 6.05 5.90 0.57 0.62 121 127 95 160 42 0.045 81 yes

50 8.80 0.028 0.050 0.0216 0.0209 6.04 5.90 0.59 0.66 125 135 95 160 42 0.045 81 yes

9.00 0.028 0.050 0.0216 0.0209 6.04 5.90 0.60 0.67 127 137 95 160 42 0.045 81 yes

100 9.90 0.028 0.050 0.0216 0.0208 6.02 5.90 0.63 0.71 133 145 95 160 42 0.045 81 yes

10.00 0.028 0.050 0.0216 0.0208 6.02 5.90 0.63 0.71 133 145 95 160 42 0.045 81 yes

11.00 0.028 0.050 0.0216 0.0208 6.01 5.90 0.66 0.75 140 153 95 160 42 0.045 81 yes

a tt = g Rt Se (similar to equation E.1).b t

c

 = g Rc Se (similar to equation E.1).

c tD50 was assumed to be 0.045. Alternative values could be obtained from table E.1.d tc-D84 = 16170 (t*D50 ) ( D84)0.3 (D50)0.7 (similar to equation E.5).

Hydraulics Particle Mobility/Stability
E—24 Table E.2—Summary of flow hydraulics and particle mobility/stability for the reference-reach channel and design channel using the modified critical shear stress approach (continued)

Critical

shear

Total Channel Shield’s stress to

Total Channel Total Channel boundary boundary entrain- entrain D84

flow flow hydraulic hydraulic shear shear Angle ment D84 particle Particle

Recur- Dis- Flood- Channel Channel Energy width width radius radius stress stress of for size mobile

rence charge, plain n n slope slope Wt Wc Rt Rc tt tc D50 D84 repose D50 tc-D84 (yes

Interval Q (m3/s) value value Sc Se (m) (m) (m) (m) (N/m2)a (N/m2)b (mm) (mm) f tD50 (N/m2) d /no)

C. Adjusted stream simulation design channel: XS8.5D

2.00 0.028 0.050 0.0216 0.0212 4.67 4.67 0.31 0.31 66 64 115 190 42 0.045 97 no

3.00 0.028 0.050 0.0216 0.0211 5.49 5.47 0.36 0.36 76 75 115 190 42 0.045 97 no

4.00 0.028 0.050 0.0216 0.0212 6.02 5.90 0.40 0.41 85 85 115 190 42 0.045 97 no

2 4.20 0.028 0.050 0.0216 0.0212 6.07 5.90 0.41 0.42 87 87 115 190 42 0.045 97 no

5.00 0.028 0.050 0.0216 0.0211 6.09 5.90 0.45 0.47 95 97 115 190 42 0.045 97 yes

6.00 0.028 0.050 0.0216 0.0211 6.07 5.90 0.49 0.52 104 107 115 190 42 0.045 97 yes

10 6.70 0.028 0.050 0.0216 0.0210 6.06 5.90 0.52 0.56 110 115 115 190 42 0.045 97 yes

7.00 0.028 0.050 0.0216 0.0210 6.06 5.90 0.53 0.57 112 117 115 190 42 0.045 97 yes

25 7.80 0.028 0.050 0.0216 0.0209 6.05 5.90 0.56 0.61 119 125 115 190 42 0.045 97 yes

8.00 0.028 0.050 0.0216 0.0209 6.05 5.90 0.57 0.62 121 127 115 190 42 0.045 97 yes

50 8.80 0.028 0.050 0.0216 0.0209 6.04 5.90 0.59 0.66 125 135 115 190 42 0.045 97 yes

9.00 0.028 0.050 0.0216 0.0209 6.04 5.90 0.60 0.67 127 137 115 190 42 0.045 97 yes

100 9.90 0.028 0.050 0.0216 0.0208 6.02 5.90 0.63 0.71 133 145 115 190 42 0.045 97 yes

10.00 0.028 0.050 0.0216 0.0208 6.02 5.90 0.63 0.71 133 145 115 190 42 0.045 97 yes

11.00 0.028 0.050 0.0216 0.0208 6.01 5.90 0.66 0.75 140 153 115 190 42 0.045 97 yes

a

. tt = g Rt Se (similar to equation E.1)

b

. t

c

 = g Rc Se (similar to equation E.1)

c

. tD50 was assumed to be 0.045. Alternative values could be obtained from table E.1.

d

. tc-D84 = 16170 (t*D50 ) ( D84)0.3 (D50)0.7 (similar to equation E.5).

Hydraulics Particle Mobility/Stability
E—25

Appendix E—Methods for Streambed Mobility/Stability Analysis

Figure E.7—Plot of shear stress versus discharge showing when the D84 particle 

is mobilized in the reference-reach channel and the preliminary stream-simulation 

design channel. To achieve similar D84 particle mobility in the stream-simulation 

design channel at the same flow as the reference-reach channel, the D50 and 

D84 particle sizes were increased in size by 20 percent to 114 mm and 192 mm, 

respectively. 

E.3.3 Critical Unit Discharge Approach

Results from the critical unit discharge analysis show that the D84 particle 

size of 160 millimeters has a critical unit discharge (qc-D84) of 1.59 m2

/s 

in the reference-reach channel, whereas it is 1.25 m2

/s in the preliminary 

stream-simulation design channel (table E.3A-B). The lower critical unit 

discharge in the design channel indicates the D84 particle size will mobilize 

at lower discharges when compared to the reference-reach channel because 

of the steeper slope (figure E.8). The D84 particle size is mobilized at a 

discharge of about 7.8 m3

/s in the reference-reach channel, whereas it is 

mobilized at a discharge of 5.6 m3

/s in the preliminary design channel. 

The unit discharges for the various discharges are similar between the 

reference-reach channel and the design-channel reach because they have 

similar active channel widths and entrenchment ratios (table E.3 and figure 

E.8). To achieve similar sediment mobility in the stream-simulation design 

channel at the same discharge as the reference-reach channel, the D16, 

D50, and D84 particle sizes were increased from 43 to 50 millimeters, 95 

to 112 millimeters, and 160 to 188 millimeters, respectively (table E.3C 

and figure E.8). These sediment size adjustments increased the critical 

unit discharge for the D84 particle (qc-D84) in the stream-simulation design 

channel from 1.25 m2

/s to 1.59 m2/s.

0

20

40

60

80

100

120

140

160

180

200

0 5 10 15

discharge (m 3/s)

shear stress (N/m

2 )

reference reach (XS10)

stream simulation design

channel (XS8.5D)

reference reach

τcD84=81 N/m2

D50=95 mm

D84=160 mm

Final bed design

τcD84=97 N/m2 

D50=114 mm

D84=192 mm
E—26

Stream Simulation Table E.3—Summary of flow hydraulics and particle mobility/stability for the reference-reach channel and design channel using the critical unit discharge approach Hydraulics Particle Mobility/Stability Critical unit Critical discharge Active Total Active Active Particle unit to entrain Channel flow channel Total channel size discharge D84 particle D84 Recur- Dis- width Flood width width unit unit Channel range for D50 size particle rence charge, discharge plain Channel Wt Wa discharge discharge slope D16 D50 D84 measure qc-D50 qc-D84 mobile Interval Q (m3/s) Qa(m3/s) n value n value (m) (m) q(mt 2/s)a qa(m2/s)b Sc (mm) (mm) (mm) bc (m2/s)d (m2/s)e (yes/no) A. Reference reach cross section: XS10

2.00 1.96 0.15 0.05 5.14 4.40 0.39 0.45 0.0174 43 95 160 0.40 1.29 1.59 no

3.00 2.89 0.15 0.05 5.46 4.40 0.55 0.66 0.0174 43 95 160 0.40 1.29 1.59 no

4.00 3.80 0.15 0.05 5.72 4.40 0.70 0.86 0.0174 43 95 160 0.40 1.29 1.59 no

2 4.20 3.98 0.15 0.05 5.76 4.40 0.73 0.90 0.0174 43 95 160 0.40 1.29 1.59 no

5.00 4.69 0.15 0.05 5.93 4.40 0.84 1.06 0.0174 43 95 160 0.40 1.29 1.59 no

6.00 5.54 0.15 0.05 6.04 4.40 0.99 1.26 0.0174 43 95 160 0.40 1.29 1.59 no

10 6.70 6.12 0.15 0.05 6.12 4.40 1.09 1.39 0.0174 43 95 160 0.40 1.29 1.59 no

7.00 6.37 0.15 0.05 6.15 4.40 1.14 1.45 0.0174 43 95 160 0.40 1.29 1.59 no

25 7.80 7.03 0.15 0.05 6.24 4.40 1.25 1.60 0.0174 43 95 160 0.40 1.29 1.59 yes

8.00 7.19 0.15 0.05 6.26 4.40 1.28 1.63 0.0174 43 95 160 0.40 1.29 1.59 yes

50 8.80 7.85 0.15 0.05 6.34 4.40 1.39 1.78 0.0174 43 95 160 0.40 1.29 1.59 yes

9.00 8.01 0.15 0.05 6.36 4.40 1.42 1.82 0.0174 43 95 160 0.40 1.29 1.59 yes

100 9.90 8.74 0.15 0.05 6.44 4.40 1.54 1.99 0.0174 43 95 160 0.40 1.29 1.59 yes

10.00 8.82 0.15 0.05 6.45 4.40 1.55 2.00 0.0174 43 95 160 0.40 1.29 1.59 yes

11.00 9.62 0.15 0.05 6.55 4.40 1.68 2.19 0.0174 43 95 160 0.40 1.29 1.59 yes

a qt = Q / Wt (similar to equation E.2).b qa = Qa / Wa (similar to equation E.2).c b = 1.5(D84/D16)-1 (equation E.9).d qc-D50 = 0.15 (g)0.5 (D50)1.5 (S)-1.12 (equation E.7).e qc-D84 = qc-D50 (D84/D50)b (similar to equation E.8).
E—27

Appendix E—Methods for Streambed Mobility/Stability Analysis Table E.3—Summary of flow hydraulics and particle mobility/stability for the reference-reach channel and design channel using the critical unit discharge approach (continued)Hydraulics Particle Mobility/Stability Critical unit Critical discharge Active Total Active Active Particle unit to entrain Channel flow channel Total channel size discharge

D84 particle

D84 Recur- Dis- width Flood width width unit unit Channel range for D50 size particle

rence charge, discharge plain Channel

Wt Wa discharge discharge slope

D16

D50

D84 measure

qc-D50

qc-D84 mobile

Interval Q (m3/s)

Qa(m3/s) n value n value (m) (m)

q(mt

2/s)a qa(m2/s)b Sc (mm) (mm) (mm)

bc (m2/s)d (m2/s)e (yes/no)

 B. Stream simulation preliminary design: XS8.5D

2.00 1.99 0.15 0.05 4.35 4.11 0.46 0.48 0.0216 43 95 160 0.40 1.01 1.25 no

3.00 2.96 0.15 0.05 4.90 4.27 0.61 0.69 0.0216 43 95 160 0.40 1.01 1.25 no

4.00 3.89 0.15 0.05 5.54 4.27 0.72 0.91 0.0216 43 95 160 0.40 1.01 1.25 no

2 4.20 4.08 0.15 0.05 5.62 4.27 0.75 0.95 0.0216 43 95 160 0.40 1.01 1.25 no

5.00 4.77 0.15 0.05 6.03 4.27 0.83 1.12 0.0216 43 95 160 0.40 1.01 1.25 no

6.00 5.61 0.15 0.05 6.09 4.27 0.99 1.31 0.0216 43 95 160 0.40 1.01 1.25 yes

10 6.70 6.18 0.15 0.05 6.08 4.27 1.10 1.45 0.0216 43 95 160 0.40 1.01 1.25 yes

7.00 6.43 0.15 0.05 6.07 4.27 1.15 1.51 0.0216 43 95 160 0.40 1.01 1.25 yes

25 7.80 7.09 0.15 0.05 6.06 4.27 1.29 1.66 0.0216 43 95 160 0.40 1.01 1.25 yes

8.00 7.25 0.15 0.05 6.06 4.27 1.32 1.70 0.0216 43 95 160 0.40 1.01 1.25 yes

50 8.80 7.89 0.15 0.05 6.05 4.27 1.45 1.85 0.0216 43 95 160 0.40 1.01 1.25 yes

9.00 8.06 0.15 0.05 6.05 4.27 1.49 1.89 0.0216 43 95 160 0.40 1.01 1.25 yes

100 9.90 8.78 0.15 0.05 6.04 4.27 1.64 2.06 0.0216 43 95 160 0.40 1.01 1.25 yes

10.00 8.86 0.15 0.05 6.04 4.27 1.66 2.07 0.0216 43 95 160 0.40 1.01 1.25 yes

11.00 9.65 0.15 0.05 6.03 4.27 1.82 2.26 0.0216 43 95 160 0.40 1.01 1.25 yes

a qt = Q / Wt (similar to equation E.2).b qa = Qa / Wa (similar to equation E.2).c b = 1.5(D84/D16)-1 (equation E.9).d qc-D50 = 0.15 (g)0.5 (D50)1.5 (S)-1.12 (equation E.7).e qc-D84 = qc-D50 (D84/D50)b (similar to equation E.8).
E—28

Stream Simulation Table E.3—Summary of flow hydraulics and particle mobility/stability for the reference-reach channel and design channel using the critical unit discharge approach (continued) Hydraulics Particle Mobility/Stability Critical unit Critical discharge Active Total Active Active Particle unit to entrain Channel flow channel Total channel size discharge D84 particle D84 Recur- Dis- width Flood width width unit unit Channel range for D50 size particle rence charge, discharge plain Channel Wt Wa discharge discharge slope D16 D50 D84 measure qc-D50 qc-D84 mobile

Interval Q (m3/s) Qa(m3/s) n value n value (m) (m) q(mt 2/s)a qa(m2/s)b Sc (mm) (mm) (mm) bc (m2/s)d (m2/s)e (yes/no)

 C. Adjusted stream simulation design channel: XS8.5D

2.00 1.99 0.15 0.05 4.35 4.11 4.11 0.48 0.0216 50 112 188 0.40 1.29 1.59 no

3.00 2.96 0.15 0.05 4.90 4.27 4.27 0.69 0.0216 50 112 188 0.40 1.29 1.59 no

4.00 3.89 0.15 0.05 5.54 4.27 4.27 0.91 0.0216 50 112 188 0.40 1.29 1.59 no

2 4.20 4.08 0.15 0.05 5.62 4.27 4.27 0.95 0.0216 50 112 188 0.40 1.29 1.59 no

5.00 4.77 0.15 0.05 6.03 4.27 4.27 1.12 0.0216 50 112 188 0.40 1.29 1.59 no

6.00 5.61 0.15 0.05 6.09 4.27 4.27 1.31 0.0216 50 112 188 0.40 1.29 1.59 no

10 6.70 6.18 0.15 0.05 6.08 4.27 4.27 1.45 0.0216 50 112 188 0.40 1.29 1.59 no

7.00 6.43 0.15 0.05 6.07 4.27 4.27 1.51 0.0216 50 112 188 0.40 1.29 1.59 no

25 7.80 7.09 0.15 0.05 6.06 4.27 4.27 1.66 0.0216 50 112 188 0.40 1.29 1.59 yes

8.00 7.25 0.15 0.05 6.06 4.27 4.27 1.70 0.0216 50 112 188 0.40 1.29 1.59 yes

50 8.80 7.89 0.15 0.05 6.05 4.27 4.27 1.85 0.0216 50 112 188 0.40 1.29 1.59 yes

9.00 8.06 0.15 0.05 6.05 4.27 4.27 1.89 0.0216 50 112 188 0.40 1.29 1.59 yes

100 9.90 8.78 0.15 0.05 6.04 4.27 4.27 2.06 0.0216 50 112 188 0.40 1.29 1.59 yes

10.00 8.86 0.15 0.05 6.04 4.27 4.27 2.07 0.0216 50 112 188 0.40 1.29 1.59 yes

11.00 9.65 0.15 0.05 6.03 4.27 4.27 2.26 0.0216 50 112 188 0.40 1.29 1.59 yes

a qt = Q / Wt (similar to equation E.2).b qa = Qa / Wa (similar to equation E.2).c b = 1.5(D84/D16)-1 (equation E.9).d qc-D50 = 0.15 (g)0.5 (D50)1.5 (S)-1.12 (equation E.7).e qc-D84 = qc-D50 (D84/D50)b (similar to equation E.8).
E—29

Appendix E—Methods for Streambed Mobility/Stability Analysis

Figure E.8—Plot of unit discharge versus discharge showing when the D84

particle is mobilized in the reference-reach channel and the preliminary streamsimulation design channel. To achieve similar D84 particle mobility in the stream 

simulation design channel at the same flow as the reference-reach channel, the 

D16, D50, and D84 particle sizes were increased in size by 17.5 percent to 50 mm, 

112 mm, and 188 mm, respectively. 

 

E.3.4 Summary

The modified critical-shear stress and the critical unit-discharge analyses 

resulted in similar increases in the D50 and D84 particle sizes in the design 

channel to achieve similar mobility to the reference-reach channel. 

Although the discharge at which sediment mobility occurs is different 

between the two approaches, the direct comparison between the design 

channel and reference reach channel minimizes any differences in the end 

result (sizing of sediments) because the assumptions that go into the flow 

models and sediment mobility/stability analyses are the same between 

the design channel and reference-reach channel. Thus any errors in our 

assumptions used in the analyses cancel each other out between the design 

channel and the reference channel. 

0.0

0.5

1.0

1.5

2.0

2.5

0 5 10 15

discharge (m 3/s)

effective unit discharge (m

2 /s)

reference reach (XS10)

preliminary stream

simulation design channel

(XS8.5D)

reference reach

qc-D84=1.59 m2

/s

D16=43 mm

D50=95 mm

D84=160 mm

preliminary bed design

qc-D84=1.25 m2

/s

D16=43 mm

D50=95 mm

D84=160 mm
E—30

Stream Simulation

E.4 Sizing Immobile Key Pieces

To size rocks intended to remain in place permanently (banklines and 

some key features), start from the size of rocks that appear to be immobile 

in the reference reach. Use all applicable equations to determine whether 

that size will move at the high bed-design flow. These rocks are often 

much larger than the rest of the stream-simulation material in which 

they are embedded, and accurately estimating critical entrainment flow 

is difficult because most equations do not account for such large size 

differences. Therefore, use several equations, compare their results, and 

size the key pieces accordingly. 

One analysis procedure has been developed specifically for determining 

when individual large rocks move (by sliding or rolling) (Fischenich and 

Seal 2000). The analysis applies to boulders on a flat bed or on a sloped 

bank, whether embedded or resting on the surface. 

Most equations for sizing large permanent rock material in streams are for 

designing riprap blankets, where rocks are embedded in a layer of other 

large rocks. These equations are not directly applicable to individual rocks 

or clusters, but they provide alternative estimates of stable rock sizes for 

comparison. One standard method is included in HEC-11 (Brown 1989). 

Rather than individual rock sizes, the method yields the median rock size 

for a stable riprap gradation in which Dmax is about 1.5 to 2 times D50. This 

method uses either shear stress or water velocity to represent the driving 

forces for entrainment. 

Two other riprap models developed by the U.S. Army Corps of Engineers 

(USACE) may be useful. They were developed from laboratory and 

analytical work for designing riprap bank protection and rock chutes such 

as spillways. For full descriptions of the two models, see EM 1110-2-1601 

(USACE 1994). The manual is available at http://www.usace.army.mil/

publications/eng-manuals/em1110-2-1601.

Both of the USACE riprap models are intended for the design of stable 

riprap banks and beds with angular rock. Angular rock locks and wedges 

together thereby resisting rolling and sliding. If using round rock, increase 

rock size to achieve the level of stability of an angular rock. Abt et al. 

(1988) studied the difference in stability of angular and rounded rock at 

slopes from 1 to 20 percent. Although the data set of Abt et al. (1998)

is not large, the data indicated the round rock was stable when D50 was 

40-percent greater than the angular rock.
E—31

Appendix E—Methods for Streambed Mobility/Stability Analysis

Both models also assume a riprap gradation that is relatively well sorted 

compared to a natural streambed. Most graded riprap mixtures have D84/

D16 ratios less than 3. Although it does not present supporting data, the 

USACE suggests that the equations do apply to mixtures with D84/D16

ratios up to 7. D84/D16 in natural cobble/gravel channels can be much 

higher than this, but until better models are developed, these are useful for 

estimating stable rock sizes. 

Unless supported or buttressed by other rocks of a similar size, individual 

large rocks may move when the smaller bed mixture around them is 

scoured. When that happens, the large rock can roll or sink into the bed. 

If you size rocks using these riprap equations, you will need similar-size 

rocks to support them. Be sure to buttress individual key pieces, including 

bank line rocks, with buried footer rocks.

FChannel Grade 

Control Structures

Boulder Weirs

Roughened Channels

Rigid Weirs
Stream Simulation
Appendix F—Channel Grade Control Structures

This appendix briefly describes permanent grade control structures

that are sometimes needed in the upstream and/or downstream reaches

adjacent to a stream-simulation culvert. Their purpose is to control channel

slope and elevation, and they are often used to raise the elevation of a

channel that has incised downstream of a culvert. The objective is to avoid

steepening the stream-simulation channel inside the culvert, and to allow

the design slope to match an available reference reach (section 5.5). In

some cases, grade-stabilization structures designed to adjust gradually

in response to high flows are also used upstream of stream-simulation

culverts to moderate the rate of erosion of a large wedge of sediment

accumulated above an undersized culvert. Some grade control structures

also can be shaped to improve bankfull flow alignment with the culvert

inlet.

How well these structures provide passage for all aquatic organisms

depends on how well they mimic structures found in the natural channel.

The more rigid a control structure, and the more uniform in cross section

and hydraulic characteristics, the less certain the provision of passage

for aquatic organisms. The key to providing as unrestricted a passage

as possible is to design the structure for maximum variety of passage

opportunities (water depth, velocity, substrate). Determining with certainty

how well specific aquatic organisms can pass grade-control structures may

necessitate biological monitoring.

The information in this appendix is general and not adequate for design

purposes. For details on design considerations, applications, and

limitations, consult other references, such as the U.S. Department of

Agriculture, Natural Resource Conservation Service (2001); Rosgen

(2006); and Salde-Caromile et al. (2004).

The three most common types of artificial grade control structures are

boulder weirs, roughened channels, and rigid weirs.

Boulder Weirs 	 Boulder and log weirs can be designed to imitate natural steps (figure

F.1). These weirs are appropriate grade controls in step-pool channels, and

channels with forcing features such as large rock and woody debris. In

other channels, the degree to which the weirs permit passage of aquatic

organisms will vary.
F—2

Stream Simulation

Figure F.1—Natural boulder step on a tributary to the Entiat River, Willamette 

National Forest, Washington.

Low weirs have been built for many years for backwatering perched

culverts and low dams, and for controlling grade. Though many of those

structures have deteriorated and disappeared over time, they can be

durable and effective if well designed and constructed. Their success

largely depends on the size and quality of material, the care and skill of

the equipment operator, the supervision of construction, and the equipment

used to place the rocks.

To create a long-lasting boulder weir, use durable rock shaped so that

individual rocks can be keyed together. As somewhat angular boulders are

much more stable than round ones, individually select specific rocks to fit

together. A common guideline is to use rock twice the size of the largest

mobile particles in the channel. The U. S. Department of Agriculture,

Natural Resources Conservation Service 2001 suggests that D50 of weir

rock be equal to what is calculated as stable riprap, and that Dmax be twice

that size. Keep in mind that scour depth is also a factor. For a 1-foot drop,

place rocks on footer rocks embedded from 2.5 feet to 3.5 feet in gravel

and sand beds, respectively. Boulders also can be sized conservatively

using a stability analysis procedure published by Fischenich and Seal

(2000) for individual boulders rolling or sliding on the streambed. Rosgen

(2006) and Thomas et al. (2000) provide empirical methods for sizing

rocks used for weirs and steps.
F—3

Appendix F—Channel Grade Control Structures

Figure F.2—Typical plan, cross section, and profile views of a rock weir, taken

from a stream-simulation contract in Alaska.

Drop structures concentrate energy as water plunges over the crest, and

turbulence and bed scour dissipate the energy. Bed scour can undermine

the structure, and bank erosion can cut around the ends. Outflanking is

one of the most common modes of failure. Designing the structures with a

V-shape in both plan view and cross-section view helps prevent these types

of failure.
F—4

Stream Simulation

This V-shaped configuration in both plan and elevation views concentrates

the spill towards the center of the channel as flow is directed perpendicular

to each leg of the structure. This hydraulic condition tends to concentrate

scour away from streambanks and can help maintain a natural and more

complex channel cross section. The scour hole will be longer and the

structures may therefore have to be placed further apart than if they are

straight and horizontal. This concept applies to any style of drop structure. 

Variations on this shape can help guide flow around channel bends.

In plan view, the weir is shaped like an arch or a convex-upstream V, so

that adjacent rocks support each other. Carefully place individual rocks

with equipment that allows the rock to be rotated for precise alignment

and fitting. Place footer rocks below the elevation of the final grade, to

support a second row of rocks. Then place the top row of rocks against the

footer boulders and slightly upstream of them, so that they are supported

with multiple points of contact. Footer rocks should be placed so that

they prevent the formation of a scour pool the top rock could roll into.

However, they should not be so far in front or so high as to act as a splash

pad that could prevent fish passage. When the arch is complete, each

boulder bears against its downstream neighbor and—ideally—against the

two footer rocks below it. The force of the streamflow and bedload will

then transfer through the weir to the footer rocks and banks. Fitting the

rocks together in this way often requires moving rocks several times.

In cross section, the weir crest should slope down toward the apex,

approximating the intended cross section of the channel. To avoid

outflanking, key boulders at the ends of the weir well back into the banks

to at least bankfull elevation and backfill them securely. Place well-graded

seal material with some fines on the upstream side of the control, to limit

permeability and leakage. Bed material that accumulates on the upstream

face of the weir provides much of the structural integrity and sealing of

boulder weirs. Without continued recruitment of sediment for maintaining

the weirs, they will become more porous, eventually leaking and becoming

vulnerable to failure.

Boulder weirs carry the risk of domino failure. If one weir in a series fails,

the risk of other failures increases, because the added head differential

increases the plunging flow, scour, and hydrostatic forces on the next weir

upstream. See Salde-Caromile (2004) for a fuller description of rock weirs

and other drop structures, and Rosgen (2006) for a discussion of boulder

weir styles and failure modes.
F—5

Appendix F—Channel Grade Control Structures

Roughened channels	 Oversteepened channel segments have traditionally been designed as

roughened channels, in which high bed-roughness limits water velocity

to allow the passage of a target fish, and bed material is sized to be

immobile at the design flow (see also appendix B). More recently, artificial

pool-riffle, step-pool, and cascade reaches have been constructed for

maintaining grades steeper than the reference reach or the project stream,

but in a more naturalistic manner (figure F.3). We refer to these as “hybrid”

designs because, while they are basically hydraulic designs, they also

incorporate elements of natural channel design. See appendix B for a

discussion of hybrid design.

Where a hybrid or roughened channel segment plays the role of grade

control downstream of a stream-simulation culvert, design both control

and culvert conservatively. If the control incises at all, the streamsimulation channel will also be at risk of incision. To minimize the risk,

countersink the culvert deeper than normal.

Figure F.3—Rock riffle used to backwater the outlet of a box culvert on Dickson

Brook, Bay of Fundy, New Brunswick, Canada. Photo courtesy of Dr. Robert

Newbury, Newbury Hydraulics, Okanagan Centre, British Columbia, Canada.
F—6

Stream Simulation

Rigid Weirs Rigid weirs are fixed, nondeformable structures used for precisely

controlling the channel profile. They can be built out of logs, sheet piling,

or concrete. One benefit of rigid weirs is that they can often be built at

a steeper grade than other structures, thus minimizing the footprint of a

project. Slopes steeper than about 5 percent require additional structure

(floors and walls) to protect bed and banks.

Log sills (log weirs) span the entire channel and create a series of small

drops, raising the downstream-water surface to the elevation of a culvert.

Log sills are a low-cost, durable means of fish passable grade control

for streams with moderate gradients and channel toe widths of less than

about 30 feet. Although they are typically used downstream of a culvert,

they may also be used upstream. Log sills include a variety of designs,

including single logs, multiple stacked logs, straight weirs, angled weirs,

and V-weirs. Any level sill should have a low-flow notch cut into it.

Simple, straight, double-log sills are the most secure. These require the

least overall channel length and are the least costly of the styles. Other

styles, such as sills that dip toward the middle of the channel or angle

downstream, tend to create more channel and hydraulic complexity.

Because of the recommended maximum slope for rigid weirs (5 percent),

steepening a channel with a natural slope already greater than about 3

percent is difficult. In small channels, planks can be used instead of logs.

Precast concrete or steel sheet-pile weirs are other options for rigid

controls. Their advantages are that they can be manufactured precisely,

resulting in a good seal, with a varied cross section similar to the natural

channel, and a crest shape specifically designed for fish passage. You can

custom-design their installation to fit the needs of the site; for example,

a single precast concrete unit could include a weir, a stilling basin, wing

walls, and a head wall. Steel-pile weirs can be solid sheet-piles or H-piles

with wood or precast concrete lagging between them. Concrete highway

median barriers and “ecology blocks” are not recommended for use in

grade-control structures; they commonly fail when used as weirs unless

they are anchored for stability, modified to provide a sharp crest and a deep

plunge pool, and sealed permanently to prevent leakage.

Straight, level, rigid weirs can have negative impacts on channels. They

tend to create channels that are trapezoidal and very uniform in cross

section. Even though full channel-spanning level structures may look like

natural embedded wood structures, they lack the variety of passageways

found in most complex natural structures. Poorly designed structures

commonly fail by scouring either under or around the end of the structure.

Rigid structures are more likely than adjustable rock structures to become

barriers to fish passage when downstream scour occurs.
GAdditional Tools 

and Tips

G.1 Contract Preparation Checklist

G.2 Sample Project Schedule

G.3 Estimated Project Costs: Comparing Different 

Structure Types and Sizes

G.4 Tips From Engineers and Biologists Experienced 

in Stream-Simulation Construction
Stream Simulation
Appendix G—Additional Tools and Tips

Sections G.1 through G.3 of this appendix provide additional information that may be useful in streamsimulation final design and contract development. Section G.4 summarizes results from a workshop 

where construction engineers and biologists shared their experience doing major in-stream work. 

Participants identified common problems and solutions they had used or developed. 

G.1 Contract Preparation Checklist

n Plan views

4 Plans drawn to scale with scale, north arrow, at 

least three reference points (more are preferred) 

outside of construction disturbance area.

4 Clearing limits: all work areas covered by 

special project requirements, notes.

4 Structure location: inlet and outlet inverts

located with XYZ coordinates, or equivalent 

(taken from long profile).

4 Extension of channel excavation and filling 

(taken from long profile).

4 Road locations, edges, centerline, geometric 

description of curvature, widths, and curve 

widening, p-line, or XYZ coordinates.

4 Channel work identified: bank erosion control 

features, grade control, channel linings.

n Dewatering system

4 Location, height, and width of diversion dam.

4 Bypass pipe size, length, location, coupling 

method. 

4 Sump locations, estimate of necessary flow and 

sump capacity. 

4 Backwater prevention method.

4 Sediment treatment plan with methods, release 

point, extent.

n Footings

4 Estimated bearing capacity parameters. 

4 Depth, width, and eccentricity from bearing 

capacity equations.

4 Dimension of footing or footing options with 

enough width for arch attachment.

4 Reinforced concrete details and calculate 

temperature steel. 

4 Quantities of concrete and reinforcing steel.

4 Estimated construction time.

n Streambed Details

4 Thalweg, slope, bank shape, material gradation(s).

4 Step, bank, rock-placement details: elevations, 

spacing, diameters, and locations.

4 Extent of streambed-simulation material, 

excavation, and infilling.

4 Quantities of materials.

4 Details for any retention structures (sills): 

construction, attachment, and backfilling.

n Structure Details

4 Structural section, gauge or thickness requirement 

for live load, dead load.

4 Minimum and maximum cover limits.

4 Structures (drawn to scale) on elevation view 

showing bed material location relative to 

structure, special backfill zones. 

4 Special concrete structure details for collar 

headwalls, inlets, thrust beams, footings.

4 Structure attachment to footing details.

4 Structure length, all details necessary for costing.

4 Structural excavation quantity, total excavation 

estimate.

n Details for creating a bypass road 

around project

4 Traffic control plan with signs.

4 Traffic bypass road design, including maximum 

grade, minimum width, surfacing, and curve 

radius.

4 Over-structure embankment details, estimate of 

delay time during construction.
G—2

Stream Simulation

G.2 Sample Project Schedule

Figure G.1, a Gantt chart, shows which tasks in this project depend on 

others and how many actual workdays (weekends are included with 

workdays on this chart) are necessary for completing a hypothetical 

project. To get total contract time, the designer should add any anticipated 

shutdown time for fire and weather, as well as holidays and “contingency” 

days. 

Most permits have a “construction window” of time when work is allowed 

to take place in the stream. This window protects spawning and migrating 

species and ensures ample time for “winterizing” a project if the project 

going to take more than one season. Using the project Gantt chart, you 

can fit the project into the construction window allowed in the permit and 

establish realistic start dates, completion dates, and a time estimate.

A thoughtful, well-constructed project schedule helps the contractor and 

inspector plan their work. It will help both to minimize unnecessary field 

trips and to plan for critical events such as aquatic organism capture and 

transport, compaction testing, and concrete pours (see figure G.2).
G—3

Appendix G—Additional Tools and TipsFigure G.1—Sample project schedule: Gantt chart.
G—4

Stream Simulation

Figure G.2—This project construction timeline is a reminder of the specifications that apply at various phases of construction. Work under some specifications continues throughout much of the project. 
G—5

Appendix G—Additional Tools and Tips

 G.3 Estimated Project Costs: Comparing Different Structure Types

and Sizes

G.3.1 12-foot and 18-foot Open-bottom Arch Multiplate Structures—20-foot Fill

Table G.1 compares estimated costs for two open-bottom arch multiplate structures on concrete footings, with step pools and stream-simulation bed, complete site dewatering and erosion control. Fill height is 20 feet; bankfull width is 12 feet.

Table G.1—Sample cost estimates for 12-foot and 18-foot open bottom multiplate arches with 20-foot fill

Item 

No.

152(02)152(03)201(03)203(02)157(19)157(20)208(02)208(07)

Description

Construction staking and surveying (road)Construction staking and surveying (structure)Clearing and grubbing, slash treatment methods for tops and limbs 12, logs 12, stumps 12, utilization of timber 2Removal of existing 6-ft. dia. multiplate pipe, disposal method ASoil erosion and pollution controlDewatering and sediment control Foundation fill (use mostly structural excavation, and bedding volume)Structural excavation

Method of 

Measurement

AQAQ

LSQ

AQ

LSQLSQ

DQDQ

Unit

Mile

Each

LSEaLSLSCyCy

Estimate 

Quantity 12’

0.2

1

All Req’d

1

All Req’dAll Req’d

80

1,212

Estimate 

Quantity 18’

0.2

1

All Req’d

1

All Req’dAll Req’d

120

1,434

Unit 

Price

$5,000$1,000$1,500$5,000$2,500

$15,000

$50$20

Total for 

12’

$1,000$1,000$1,500$5,000$2,500

$15,000

$4,000

$24,240

Total for 

18’

SameSameSameSameSameSame

Add $2,000Add $4,440
G—6

Stream Simulation Table G.1—Sample cost estimates for 12-foot and 18-foot open bottom multiplate arches with 20-foot fill (continued) Item No. 251(14) 251(15)a 251(16) 

301(10)552(03)554(03)601(01)607(03)617(06)625(02)

Description

Placed channel rock, rock-18, method D (2 steps constructed in pipe)Placed streambed simulation rock, bed class 12, method DPlaced select borrow Untreated aggregate course, type base, grading C, compaction BStructural concrete, class A (AE), for footingsReinforcing steelMobilizationGate temporary, type I barricade, size 4.9m wide by 800mm high (or other traffic-control features) Galvanized steel corrugated openbottom arch Seeding, hydraulic method (with mulch)

Method of 

Measurement

AQ

DQDQDQ

AQ

DQ

LSQ

AQAQ

DQ

Unit

EA

CyCyCyCy

Lb

LS

Each

Ft

acre

Estimate 

Quantity 12’

16

135

8

3745

2’wide by 4’ 

tall footings

880

All Req’d

2

12’ by 6’ by 

76’ @ $435/ft

0.25

Estimate 

Quantity 18’

24

202

123955

2.5’ wide by 

4’ tall footings

897

All Req’d

2

18’ by 9’ by 

74’ @ $650/ft

0.25

Unit 

Price

$80$65$50$40

$500

$2.50

$2,250$4,000

Total for 

12’

$1,280$8,775

$400

$1,480

$22,500

$2,200

$12,940

$4,450

$33,060

$1,000

$142,325

Total for 

18’

Add $480

Add $4,350

Add $200

Add $80

Add $5,000

Add $43

$16,118

same

Add $15,040

same

$177,301 Total estimate for 12’ and 18’ open bottom arches, fill height assumed at 20’

Cost difference = $34,976 (24.5%)
G—7

Appendix G—Additional Tools and Tips

 G.3.2 8-foot and 12-foot Embedded CMPs—20-foot FillTable G.2 compares estimated costs for two embedded pipe single-piece structures with step pools and stream-simulation bed, complete site dewatering, and erosion control. Fill height is 20 feet; bankfull width is 8 feet.

Item 

No.

171(02)171(03)201(03)202(02)157(19)157(20)206(02)206(07)251(14)

Description

Construction staking, precision C, method 1Staking structures, precision C, method 1Clearing and grubbing, slash treatment methods for tops and limbs 12, logs 12, stumps 12, utilization of timber 2Removal of existing 4-ft. dia. steel pipe, disposal method ASoil erosion and pollution controlDewatering and sediment control Foundation fill (use mostly structural excavation, and bedding volume)Structural excavationPlaced channel rock, rock-12, method D (2 steps constructed in pipe)

Method of 

Measurement

AQAQ

LSQ

AQ

LSQLSQ

DQDQ

AQ

Unit

Mile

Each

LSEaLSLSCyCy

EA

Estimate 

Quantity 8’

0.2

1

All Req’d

1

All Req’dAll Req’d

53

388

16

Estimate 

Quantity 12’

0.2

1

All Req’d

1

All Req’dAll Req’d

80

456

24

Unit 

Price

$5,000$1,000

$100

$2,500$1,000$8,000

$50$20$65

Total for 

8’

$1,000$1,000$1,000$2,500$1,000$8,000$2,650$7,760$1,280

Total for 

12’

SameSameSameSameSameSame

Add $1,350Add $1,360

Add $480

Table G.2—Sample cost estimates for 8-foot and 12-foot embedded CMPs with 20-foot fill
G—8

Stream Simulation Item No. 251(15)a 251(16)

304(10)601(01)607(03)617(06)625(02)

Description

Placed streambed simulation rock, bed class 6, method D, both pipes embedded 4’ add 10% for adjacent channel work Placed filler materialCrushed aggregate, type base, grading C, compaction BMobilizationGate temporary, type I barricade, size 4.9m wide by 800mm high (or other traffic control features) Galvanized steel 3” by 1” corrugated pipe (installed)Seeding, hydraulic method (with mulch)

Method of 

Measurement

DQDQDQ

LSQ

AQAQ

DQ

Unit

CyCyCyLS

Each

Ft

acre

Estimate 

Quantity 8’

78

8

15

All Req’d

2

8’ diam by 

76’ @ $200 /ft

0.25

Estimate 

Quantity 12’

981017

All Req’d

2

12’diam by 

74’ @ $300/ft

0.125

Unit 

Price

$65$50$40

$2,250

Always 

check for 

latest prices

$4,000

Total for 

8’

$5,070

$400$600

$5,240$4,450

$15,200

$500

$57,650

Total for 

12’

Add $1,300

Add $100

Add $80

$6,390

same

Add $7,000

same

$69,110

Table G.2—Sample cost estimates for 8-foot and 12-foot embedded CMPs with 20-foot fill (continued)

Total estimate for 8’ and 12’ embedded pipes, fill height assumed at 20’

Cost difference = $11,460 (20%) 
G—9

Appendix G—Additional Tools and Tips

G.3.3 8-foot and 12-foot Embedded CMPs—12-foot Fill

Table G.3 compares estimated costs for two embedded pipe single-piece structures with step pools and stream-simulation bed, complete site dewatering, and erosion control. Fill height is 12 feet; bank full width is 8 feet.

Item 

No.

171(02)171(03)201(03)202(02)157(19)157(20)206(02)206(07)251(14)

Description

Construction staking, precision C, method 1Staking structures, precision C, method 1Clearing and grubbing, slash treatment methods for tops and limbs 12, logs 12, stumps 12, utilization of timber 2Removal of existing 4-ft. dia. steel pipe, disposal method ASoil erosion and pollution controlDewatering and sediment control Foundation fill (use mostly structural excavation, ad bedding volume)Structural excavationPlaced channel rock, rock-12, method D (2 steps constructed in pipe)

Method of 

Measurement

AQAQ

LSQ

AQ

LSQLSQ

DQDQ

AQ

Unit

Mile

Each

LSEaLSLSCyCy

EA

Estimate 

Quantity 8’

0.2

1

All Req’d

1

All Req’dAll Req’d

53

1148

16

Estimate 

Quantity 12’

0.2

1

All Req’d

1

All Req’dAll Req’d

80

1296

24

Unit 

Price

$5,000$1,000$1,500$3,000$2,500

$10,000

$50$20$65

Total for 

8’

$1,000$1,000$1,500$3,000$2,500

$10,000

$2,650

$22,960

$1,280

Total for 

12’

SameSameSameSameSameSame

Add $1,350Add $2,960

Add $480

Table G.3—Sample and cost estimate for 8-foot and 12-foot embedded CMPs with 12-foot fill
G—10

Stream Simulation Item No. 251(15)a 251(16) 304(10)

601(01)607(03)617(06)

625((02)

Description

Placed streambed simulation rock, bed class 6, method D, both pipes embedded 4’ add 10% for adjacent channel workPlaced filler materialCrushed aggregate, type base, ggrading C, compaction BMobilizationGate temporary, type I barricade, size 4.9m wide by 800mm high (or other traffic control features) Galvanized steel 3” x 1”corrugated pipe (installed)Seeding, hydraulic method (with mulch)

Method of 

Measurement

DQDQDQ

LSQ

AQAQ

DQ

Unit

CyCyCyLS

Each

Ft

acre

Estimate 

Quantity 8’

78

8

37

All Req’d

2

8’ dia. by 76’ 

@ $200 /ft

0.25

Estimate 

Quantity 12’

981039

All Req’d

2

12’dia. by 74’ 

@ $300/ft

0.25

Unit 

Price

$65$50$40

$2,250

Always 

check for 

latest prices

$4,000

Total for 

8’

$5,070

$400

$1,480$7,350$4,450

$15,200

$1,000

$80,840

Total for 

12’

Add $1,300

Add $100

Add $80

$9,410

same

Add $7,000

same

$103.520 Total estimate for 8’ and 12’ embedded pipes, fill height assumed at 20’

Cost difference = $22,680 (28%)

Table G.3—Sample and cost estimate for 8-foot and 12-foot embedded CMPs with 12-foot fill (continued)
G—11

Appendix G—Additional Tools and Tips

G.4 Tips from Engineers and Biologists Experienced

in Stream-Simulation Construction 

This section includes details about tools, procedures, and problems 

common in stream-simulation projects. Several subsections summarize 

results of a workshop in February 2004 where experienced construction 

engineers and biologists discussed the common problems that can arise in 

placing embedded pipes, and their solutions. 

G.4.1 Diversion, Dewatering, and Water Treatment System 

 Components 

A dewatering plan contains the information in supplemental specification 

157 (appendix H, figures H.7, H.8, and H.13). 

Figures G.3 through G.6 from the State of Oregon Department of 

Transportation show various types of dewatering systems. 

G.4.1.1 Bypass and backwater dams

Different bypass and backwater dam structures are suitable for different 

conditions. Commercial cofferdams provide an ideal solution under some 

circumstances, especially in larger projects. 

Following are the advantages and disadvantages of different bypass and 

backwater dams:

Water-filled cofferdams: flexible bladders that conform to the ground to 

form a seal (figures 2.7 and G.4). (For additional information, see www.

waterstructures.com/, www.aquabarrier.com, www.portadam.com.)

	l Advantages.

 s Are flexible, conform to streambed surface.

 s Can be quickly installed.

 s Are reusable.

	l Disadvantages.

 sAre expensive.

 sMay not provide for inserting a pipe through the dam, although 

some do.
G—12

Figure G.3—Typical dewatering plan—culvert through work zone method—Oregon Department of Transportation.
G—13Figure G.4—Typical dewatering plan—water bladder isolation method—Oregon Department of Transportation.
G—14

Figure G.5—Typical dewatering details—fish screening and pump/generator containment—Oregon Department of Transportation.
G—15

Figure G.6—Typical dewatering plan—dam and pump isolation method—Oregon Department of Transportation.
G—16

Stream Simulation

 s May need to be placed on a smoothed surface for a good seal.

 s May require the addition of a impermeable membrane for 

intercepting additional subsurface flow. 

Pile cofferdams

l Advantages.

 sCan form a tight seal without a membrane.

 sAre sturdy.

 sCan be used to dewater deep water.

 sCan be used as retaining walls to hold back both water and deep 

excavations.

 sCan be fitted to conform to the site.

	l Disadvantages.

 sAre expensive (to mobilize pile driver).

 sCreate noise and vibration impacts.

 sSometimes difficult to remove. 

Sandbag dams

	l Advantages.

 sSimple and easy to construct.

 sCreate a good seal when used with an impermeable membrane 

liner.

 sRelatively inexpensive.

 sConform to the site easily.

	l Disadvantages.

 sLabor intensive.
G—17

Appendix G—Additional Tools and Tips

G.4.1.2 Pump types and characteristics

Table G.4—Characteristics of various pumps. Information was drawn from 

company Web sites.

 Brand/Model Pump Type Max GPM CFS Max 

 Solids Head

Little Giant 6E-CIA-RFSN Sump/effluent 1/2” 50 0.11 20’

Multiquip MQD306H Diaphragm 1.5” 85 0.19 25’

Tsurumi TE2-50HA Centrifugal 3/8” 137 0.31 115’

Tsurumi TE2-80HA Centrifugal 3/8” 264 0.59 105’

Tsurumi EPT2-50HA Trash 1” 190 0.42 90’

Multiquip QP2TH self prime Trash 1” 215 0.48 90’

Multiquip QP302TY diesel Trash 1.5” 416 0.93 90’

Multiquip QP301TI self prime Trash 1.5” 416 0.93 90’

Multiquip QP40TH self prime Trash 2” 611 1.36 90’

Multiquip41TDY diesel Trash 2” 611 1.36 90’

Multiquip MQ61TDH diesel Trash 2” 1,083 2.41 100’

Multiquip MQ600TD80 diesel Trash 3” 1,600 3.56 150’

6” Godwin CD150M Dri-Prime Trash 3” 1,750 3.90 160’

8” Godwin CD225M Dri-prime Trash 3 1/8” 3,250 7.24 180’

10” Godwin CD250M Trash 3 1/8” 3,600 8.02 180’

10” Godwin CD300M Trash 3 3/4” 3,601 8.02 180’

12” x 10” Godwin HL10M Trash 3” 4,500 10.03 390’

Pumping conditions—combinations of volume, pressure head, suction 

head, site access, available power, and water condition—determine what 

size and type of pump to use.

Sump Pumps Powered by electricity, these can handle small solids and low flows with 

moderate heads. Their best use is for removing minor amounts of clear 

seepage at the inlet and outlet dewatering sump ponds and for very small 

stream flows. You can use multiple pumps.

Diaphragm Pumps Engine-powered, these can handle shallow depths and slurry water, can 

handle air without losing their prime, and can handle water with a solid 

content greater than 25 percent by volume. These work where centrifugal 

pumps will lose their prime or plug. Nicknames include mud hogs, 

mud hen, and mud sucker. These pumps use a diaphragm rather than an 

impeller, and are more durable than other pumps. Their best use is for 

dewatering muddy sump ponds. 
G—18

Stream Simulation

Trash Centrifugal 

Pumps These are capable of handling large amounts of debris, with inlet diameters 

of 2 to 6 inches. They can handle solids, such as sticks, stones, and other 

debris. They can be quickly disassembled for service or inspection, 

and are available in diesel power. These are most suited for dewatering 

applications, such as diversion of flow during construction of dewatering 

dams, sump duty in large streams, and backup storm-water sump pumps.

High-Pressure 

Centrifugal Pumps These have high discharge and pressure. They are generally not capable 

of handling solids or even sandy water. Instead, they are used for washdown equipment, or for irrigation and emergency standby pumps in areas 

with a high risk of fire. They are not suited for dewatering applications 

(except in very clean water as a help or backup pump). They may have use 

as a project high-pressure pump for washing fines into bed material from 

a clean-water source and for cleaning rock and soil off asphalt haul road 

surfaces. 

G.4.1.3 Sediment removal methods

Following are some common methods for removing sediment from 

construction-area water, along with some of the strengths and weaknesses 

of each method.

Gravity-based 

settling systems—

large capacity 

Sediment basins A pump inlet is attached to a floating suction hose placed in a stable 

location in the sump basin. The sump pump operation may be controlled 

by floats to save fuel and to control pool elevation. The dirty water is 

pumped to a basin with sufficient volume to hold both water and sediment 

long enough to allow it to settle and for water to seep into the surrounding 

soil or evaporate.

Strengths: Sediment basins hold large volumes of sediment. Such heavy 

settling sediments as sands settle out very effectively.

Weaknesses: As sediments settle into the basin, the retention time 

decreases, along with efficiency. These basins require a relatively large 

surface area. Silt and clay particles may take days or weeks to settle out, 

requiring the construction of a very large settling pond.
G—19

Appendix G—Additional Tools and Tips

Tanks There are two basic tank types: a standard storage tank with a single 

storage chamber, and a weir tank with multiple chambers to help increase 

settling efficiency. Tanks with capacities of up to approximately 18,000 to 

21,000 gallons can be mobile.

Strengths: Mobile tanks eliminate the need for permanent dedication of 

space or construction of earthen dikes. The tanks can hold a large volume 

of solids before requiring clean out. Tanks require very little operational 

maintenance. The tanks can be operated in either batch or continuous 

operating modes. Properly designed tanks are easier to drain down than 

sediment basins, allowing a more rapid return to full storage capacity.

Weaknesses: Like sediment basins, tanks are rather ineffective in 

removing fine to medium-sized sediments. Mobile tanks have a limited 

storage capacity. A weir tank has a practical limit of 65 gallons per minute 

per tank for adequate sediment settling. (The flow capacity can be higher 

for larger sediments, such as large sands.) The tanks must be cleaned out 

when the project is completed. 

Passive Filtration 

Systems

Pressurized Sand Filters Sand filters have a high filtering rate, meaning that the area they occupy 

is very small compared to sediment basins and tanks. A 100 gallon per 

minute sand filter will typically be 3 feet wide by 8 feet long. Sand filters 

produce reliable results. A portable sand filter using very fine sand can 

remove sediment down to the 50 micron range.

Strengths: They need only a small area. The ability to backwash makes a 

sand filter a very cost-effective choice in situations with medium to heavy 

sediments. For multiple-year projects, their setting-up, self-cleaning, 

backwashing capability makes them effective in removing large amounts 

of sediments. An automatic backwash controller eliminates the need for 

constant operational supervision.

Weaknesses: Sand filters do not effectively remove fine silts or clays. A 

medium-head pump is required for pressurizing the system. The backwash 

generates a concentrated waste stream that must be disposed of or treated 

by other methods.
G—20

Stream Simulation

Bag Filters These filters are lightweight fabric bags capable of filtering particles as 

small as fine sand (figure G.7). They are available in various sizes to suit 

project requirements. When full, they must be removed, generally by 

cutting and disposing of sediments.

Strengths: They are effective in removing heavy sediments. They are best 

used in vegetated areas where the vegetation is used as additional filtration.

Weaknesses: They will not remove such fine sediments as silts and clays 

until a filter cake builds up. The length of time it takes for a filter cake to 

develop is unpredictable. When the filter cake is built up, the flow rate 

diminishes. Bag filters become heavy with sediment and are difficult to 

remove. They are not reusable.

Figure G.7—Typical bag filter system (Pennsylvania DEP 2000). 
G—21

Appendix G—Additional Tools and Tips

Pressurized Bag Filters These are fabric bags contained inside portable cylinders that allow the 

bags to be pressurized.

Strengths: They can be easily moved from one location to another. They 

are most effective in removing medium to heavy sediments.

Weaknesses: They do not efficiently remove fine sediments such as 

silts and clays. The smaller bag surface area and volume means that the 

sediment-holding capacity is much less than for gravity bag filters.

Wound Cartridge Filters These are tightly wound filaments that form a cartridge, which is used as 

the filter media inside pressurized cylinders.

Strengths: They are capable of removing silts and some clay not 

removed by sediment basins, sand filters, or bag filters. Wound cartridge 

systems provide the best sediment removal efficiency without the need 

for chemical treatment. They are highly portable and use a small area. 

Operational effectiveness is consistent.

Weaknesses: Wound cartridges will not remove colloidal clays. They have 

a low sediment-holding capacity.

Polymer Treatment 

Systems These systems work by adding a polymer to the untreated water, which 

creates a floc (a flocculent mass formed in a fluid through precipitation or 

aggregation of suspended particles). The flocs are either allowed to settle 

or are filtered out. 

Strengths: Water-based polymer treatment systems provide consistent 

removal of fine sediments. They are highly effective in removing colloidal 

clays. The settling tanks or ponds can be designed to hold large quantities 

of sediments. Ground application of the polymer enhances erosion and 

sediment control simultaneously, at a low relative cost. Small in-hose 

cartridges are available for projects with small flow rates and small 

sediment loads.

Weaknesses: Water-based polymer treatment systems are water treatment 

systems (as opposed to the simple passive filtration systems used to 

remove sediment.) As such, they are more complex and costly than other 

sediment removal measures. Depending on local regulations, these systems 

may require a permit for use, as well as licensed personnel to design and 

monitor the system during operation.
G—22

Stream Simulation

G.4.2. Foundation and Footing Design 

Considerations This section highlights several considerations that many designers miss 

during foundation design for open-bottom structures. Skilled designers 

should always review all preliminary and final designs.

G.4.2.1 Overturning forces

Actual foundation stresses are complex. 

l On the fill side of the footing is pressure from the overlying 

embankment soil. 

l On the streamside is a variable depth of saturated stream-simulation 

material that may change in depth from scour and aggradation during 

the project’s lifespan. 

l The footing receives the structure load at the angle of the structure to 

the footing. 

l The foundation rests on material that may be saturated or partially 

saturated. 

l The stress from the embankment weight and traffic can increase 

along the footing, ranging from none at the inlet and outlet to a 

variable amount at the embankment edges. This load can be averaged 

over the length of the footings. 

The resulting load on a footing is a combination of overturning forces that 

create an eccentric load beneath the structure’s leg. The footing is typically 

sufficiently offset to center the load and spread it evenly, thereby reducing 

peak soil stress. Short, wide footings (2 feet by 2 feet) experience a 

minimal eccentric load, which you may ignore under most circumstances. 

Taller footings develop increasing eccentricity with depth. To remove 

this eccentricity, move the center of footing to the center of the footing 

reaction. For most moderate or high strength soils, either a spread footing 

and stem wall or a rectangular or wedge-shaped footing will usually work. 

Concrete footings for arches tend to be massive, partly for constructability 

reasons. 

Design each footing’s reinforcement for the forces present, and then 

check to see if temperature steel is needed in addition. (Reinforcement 

for temperature is not normally added to structural steel.) Have a design 

engineer with expertise in reinforced concrete review the designs.
G—23

Appendix G—Additional Tools and Tips

G.4.2.2 Scour

Open-bottom arch footings should extend to 2.5 to 5 feet below the lower 

vertical adjustment potential line identified in sections 5.2.2.2 and 6.1.2. 

Although footings are normally 2 to 5 feet wide, larger sizes may be 

necessary when scour potential is large or soils are soft. 

Foundations are not inherently prone to scour, but material placed next to 

the foundations can scour if not sized properly for the hydraulic forces in 

the culvert during large flood events. Shaping the streambed and providing 

edge diversity can keep water from eroding a trench against the side of the 

culvert. Trench erosion tends to occur along footings because the smooth 

concrete surface provides less resistance to flow than a standard or deep 

corrugated culvert. The thickness of the stream-simulation bed against the 

structure edge is important for providing sufficient interlocking of large 

particles for stability. 

To increase footing roughness and help hold sediment against the 

smooth concrete, use deeply textured concrete forms. You can construct a 

roughened surface from deeply textured forms by simply attaching lumber 

(2 by 4 or larger) to the inside of the formwork.

The top of the footing normally reaches above the bankfull height 

measurement of the channel. If the footing is constructed above the 

bankfull line, the structure itself will receive less abrasion from mobile 

stream sediments and therefore last longer. 

G.4.2.3 Bedrock

When bedrock is located at shallow depths or only slightly deeper than 

necessary for the foundation support, you can pour the footings directly 

onto the bedrock. Although placing a footing on rigid bedrock is not 

standard engineering practice, the procedure is safe as long as the design 

(a) contains an adequate safety factor and (b) requires good quality, 

well-compacted backfill. Use structural backfill (A-1-a) for all bedrock 

foundations. Doweling to attach footings to bedrock is seldom necessary, 

because of the high friction developed between rock and concrete. That 

high friction prevents movement under all but the most extreme debrisslide impacts.

G.4.2.4 Soil strength

For sites with very soft soil, an embedded pipe or bridge with a driven 

pile footing may be a better option than a bottomless arch and footing. For 
G—24

Stream Simulation

bridges, you can use a driven or drilled pile foundation to support the load 

on a deep, firm surface below the softer surface soils. Alternatively, you 

can drive piles deeply into soft substrates to produce enough support. 

Settlement is a concern, especially with fine-grained streambeds. If the 

culvert is replacing an existing pipe and the embankment height is not 

changed, then the foundation may be consolidated and settle little under 

the new structure. If any special foundation treatment is required, it should 

be detailed on the drawings and covered with a supplemental specification 

based on the embankment compaction specification. For example, 

if settlement is projected to be excessive for the road standard, the 

foundation area can sometimes be excavated to a depth of stiffer material 

and backfilled with high-strength material. If doing so is not feasible, a 

bridge with pile foundations may be the only practical option to use. 

Settlement of embankment and backfill material can cause drag forces 

in the culvert if the culvert cannot settle at the same rate or slightly more 

than the embankment. Be sure to analyze both the foundation material and 

backfill material for settlement potential.

General principles for arch foundations

Chapter 12, article 12.1.6.3, in AASHTO Standard Specifications 

for Highway Bridges, Soil-Corrugated Metal Structure Interaction 

Systems (AASHTO 2002) is an excellent reference for footing design. 

The chapter lists the following design principles for arch foundations:

l Making the metal arch relatively unyielding or fixed, compared 

with the adjacent sidefill, is not a good idea.

l The footing design should provide uniform longitudinal settlement 

of acceptable magnitude, to reduce drag forces caused by 

consolidation of the adjacent roadfill.

l Footing reactions from the arch thrust should be calculated to 

act tangentially to the metal plate at its point of connection to the 

footing. For example:

 s Half-round arches have a 90-degree attachment angle.

 s Other arch shapes attach at an angle up to 20 degrees.

l Knowing the effect of the depth of the base of the footing and the 

direction of the footing reaction from the arch is important. The 

deeper the footing, the more eccentricity the arch-attachment 

angle will produce.
G—25

Appendix G—Additional Tools and Tips

G.4.3 Revegetation and Erosion Control

For a project to be successful, erosion control and revegetation must 

complement each other. Erosion control is a collection of tools working 

together; no single tool can be completely effective by itself. Success will 

come with a staged approach: first, minimize the amount of soil disturbed; 

second, stabilize whatever soil is disturbed; and, third, achieve long-term 

stabilization of all disturbed materials. Except in very arid environments, 

long-term site stabilization usually occurs only when the disturbed area is 

fully revegetated. 

Temporary erosion and sediment control measures should be designed 

with the long-term stabilization plan in mind, so that they do not conflict. 

Where possible, design long-term measures into the short-term erosion 

control plan (figure G.8). For example, a steep ditch on an approach road 

needs both short- and long-term erosion and sediment control measures. 

Short-term sediment control might consist of sandbag check dams to be 

removed after construction, along with a sediment basin that will detain 

ditch runoff water both during the project and during stabilization.

Supplemental specifications for revegetation and erosion control should 

be written as performance-based specifications (for example, “ % or 

# of plants must be alive in x years; downstream turbidity shall not 

exceed___” ). Do not specify methods: planting methods, watering 

intervals, filtration methods, silt fence locations, etc. Method-based 

specifications are difficult to administer, and failures are blamed on the 

method. With performance based specifications, contractors are free to 

choose the method that best suits their work style, and the responsibility 

for achieving the end product is theirs.

Environmental documents may require planting indigenous (native) 

species, providing for rapid site stabilization by seeding a pioneer species 

such as sterile wheat, or using bioengineering techniques. Note that the 

topic of revegetation includes several sub-categories, including:

l “Turf establishment” or “reseeding”—typically growing grasses, 

forbs, and sometimes shrubs from seed.

l “Plantings/cuttings” (involving the planting of container plants or the 

use of shrub cuttings).

l Bioengineered structures (willow wattles, live fascines, etc.). 
G—26

Figure G.8—Sample temporary erosion control drawing sheet 1 (Colorado Department of Transportation).
G—27

Appendix G—Additional Tools and Tips

Revegetation, particularly with native species, is an extensive topic that 

this guide presents only as an overview. (For bioengineered structures, see 

section G.4.3.9.)

Timing is critical in revegetation, particularly with native species. In some 

instances, plant materials must be grown from seed collected from the 

site. Doing so requires a project lead time of 1 to 2 years (and sometimes 

more). If you do not consider revegetation planning at the project’s start, 

your oversight can delay the entire project.

G.4.3.1 Salvaging and storing topsoil 

In the contract, define topsoil by depth and rock/wood content. Topsoil 

refers to the uppermost soil horizon, usually 2 to 8 inches deep. Topsoil 

may include live vegetation less than 3 feet in height, limbs less than 3 

inches in diameter, and organic duff.

Remove topsoil before doing anything else. To avoid compaction, do not 

drive on topsoil before, during removal, or after replacement.

If less topsoil is available for salvaging than you will need for 

revegetation:

	l Use commercial mulches in place of topsoil. The mixture must 

be free of weed seeds, harmful bacteria or disease spores, and 

substances toxic to plant growth.

	l Spread topsoil thin to maximize cover of the seed bank contained in 

topsoil. 

To maintain and store “living soil:”

	l Stipulate topsoil storage details in special contract requirements or 

supplemental specifications (depth of piles, length of storage time, 

number of moves, etc.).

 s After topsoil has been stored for extensive periods (i.e., 3 months 

for nonforest sites and 6 months for forest sites), spread it thin and 

allow it to revegetate. 

 s Do not store conserved wetland sod for more than 1 month.

l Avoid contaminating topsoil with unsuitable material. Do not mix 

topsoil with subsoils.
G—28

Stream Simulation

l Designate topsoil stockpile location.

l Store topsoil close to where you found and removed it, to protect it 

from loss and contamination. 

l Put up a sign indicating topsoil, to avoid mixing it with other 

excavated materials. Also designate the source location and return 

topsoil/seed bank to its original location.

l Avoid over-winter storage.

l Install sediment control measures around storage site.

Dealing with topsoil infested with noxious or invasive weeds:

l Specify that topsoil sources shall be free of weeds and invasive 

species.

		 sMap known areas of infestation and control them mechanically or 

chemically. (Note: Before using herbicides, consult national 

 herbicide BMPs.)

l Do not use infested topsoil. Enlist help from the project team for 

proper disposal or treatment.

l Make sure to pressure-wash or steam clean all incoming equipment. 

Replacing topsoil on steeper slopes: 

l You may need special equipment and manual methods for proper 

workmanship in difficult terrain. Design slopes with planting pockets 

and ledges that hold the soil in place to promote revegetation.

G.4.3.2 Collecting seeds and cuttings for native species 

 revegetation

Environmental documents sometimes require seedlings, which take 1 or 

2 seasons to grow from seed. Plan at least 2 to 3 years in advance to have 

native plant materials available (i.e., to find and collect seed and cuttings). 

l Understand safe storage requirements for the species you intend to 

use.
G—29

Appendix G—Additional Tools and Tips

G.4.3.3 Water quality monitoring

l Thoroughly understand water quality permit and NEPA document 

requirements.

 s Include requirements in special contract requirements or 

supplemental specifications to make sure they are part of the 

contract.

l Before construction, establish the baseline for the following factors 

(or others as required by the permit or NEPA document):

		 s Turbidity.

		 s Temperature.

		 s Dissolved oxygen. 

l During construction, ensure that proper water quality monitoring 

methods are employed, and that monitoring frequency is adequate to 

meet permit or environmental document requirements.

G.4.3.4 Training and quality control

	l Effectively communicate which resources the erosion control plan 

protects.

s Use well-written performance-based supplemental 

specifications. 

s Conduct an in-depth review of the erosion control plan, permit, 

and contract requirements with the contractor, the project team, 

and inspectors. 

s Use well-trained COR and inspectors for communicating and 

enforcing contract requirements.

	l Ensure sufficient quality control.

s Train designers and contract administrators in effective erosion 

control measures and temporary stabilization methods.

	l If the contractor requests—or unexpected site conditions demand—a 

change to a previously permitted erosion control plan, review the 

design/protection criteria from NEPA document (what you are 

protecting and why). Consult with the project team before making 

a change to ensure the change will meet the intent of the original 

erosion control plan. 
G—30

Stream Simulation

G.4.3.5 Temporary soil stabilization until vegetation is fully 

 established

Temporary methods for stabilizing disturbed soil (near streams) before 

permanent revegetation is fully established include:

l Temporary seeding (annual grass).

l Temporary cover such as plastic sheeting, mulch, netting.

l Rock blankets/riprap.

l Entrenched coir logs.

l Matting.

l Silt fence at ditch relief outlets.

l Chemical soil stabilizers.

‘In stream’ construction windows do not necessarily coincide with 

best streamside planting times. (These time differences often require 

two separate contracts—one for the crossing construction and one for 

revegetation.)

l Install and maintain temporary erosion control measures until 

revegetation criteria are met. Make sure that these measures 

are contract requirements. (If they are not in the contract before 

construction, add them through a contract modification.)

The agency must provide manpower and funding for several years, either 

to enforce revegetation and erosion control contract provisions or to 

maintain the erosion/revegetation plan after work on the contract has been 

accepted. The contractor is not responsible for erosion/revegetation work 

needed after the contract is closed.

G.4.3.6 Miscellaneous ‘things that can go wrong’ during 

 construction

“Maximum area disturbed” clause of contract is ignored.

l This clause is the first line of defense for sediment control, and 

is especially critical in rainy environments. Enforce contract 

requirements for maximum disturbance area.
G—31

Appendix G—Additional Tools and Tips

Occasionally, a contractor may deliberately violate water quality permit 

conditions. Actions that have been observed include shoveling sediment 

from a silt fence into the stream, and pumping muddy water from structure 

excavation into the stream.

l Check erosion control installations daily, especially after rain events.

Although silt fences must be maintained to remain effective, they are 

frequently placed in inaccessible locations.

l Use brush windrow wrapped with biodegradable fabric (may be left 

permanently). 

l Ensure that a difficult location is important to long-term erosion 

control objectives. If the location is negotiable, work with permitting 

agency in design phase. 

Temporary instream sediment traps (for example, filter cloth/lay-back trap) 

may trap too much sediment for hand removal. 

l Do not construct such structures unless they are accessible by 

equipment for maintenance.

l Rely on erosion controls first. Always think of sediment controls as a 

last resort.

Large storm events may exceed capacity of erosion control system.

l Check history of large events during construction window and any 

information available on what measures worked at that time of year.

l Design for controlled failure and minimize consequences of failure.

l Place the burden of performance on the contractor—use performancebased specifications.

G.4.3.7 Seasonal work shutdown and resumption of work

Design for temporary stabilization over winter. Plan longer-term sediment 

controls for multiseason projects or for shut-down in extreme weather 

conditions.

l Include provisions in supplemental specifications or special contract 

requirements for periodic maintenance.

l In the erosion control plan, define locations for sediment cleaned-out of 

silt fences, settling basins and other sediment control facilities. 
G—32

Stream Simulation

Temporary stabilization for winter is often underbid and frequently 

overrun. In late fall, the contractor may be behind schedule and view this 

work as a second priority.

l Enforce contract requirements for temporary stabilization. The COR 

must remind the contractor of the requirements well in advance of 

seasonal closure and ensure the site is protected according to the 

contract.

Final maintenance of erosion control measures before seasonal shut down: 

l Clean out silt fences.

l Water bar closed roads.

l Divert runoff to “safe” area with erosion control measures.

l Check periodically during winter for maintenance needs. (For 

example, rain on existing snow packs may cause greatly increased 

runoff and erosion.)

l Stabilize area left disturbed over winter.

Runoff-season drainage patterns may differ greatly from construction 

season.

l On south-facing cuts/slopes, frost will thaw more quickly. 

l Muddy runoff may enter stream running over snow while silt fences 

are still buried under snow.

l On disturbed soils, fluffy surfaces will thaw faster and can slide on 

top of frozen surfaces.

Before construction resumes in the spring, inspect, maintain, and enforce 

all erosion control measures (all erosion control contract provisions).

G.4.3.8 Common problems with revegetation

Revegetation is a critical element in the long-term stabilization of any 

construction project—and particularly vital to projects close to waterways 

requiring aquatic organism passage. The lack, or subsequent loss, of longterm stability can not only cause an otherwise successful project to fail but 

also damage the aquatic environment that the project originally intended to 

enhance.
G—33

Appendix G—Additional Tools and Tips

Common problems along with suggestions for avoiding or solving them 

follow. These problems have no quick or easy answers, so adequate 

research and planning are vital. Using performance-based specifications 

for revegetation, either as a part of the construction contract or as 

a separate contract, can avoid many of these problems by giving 

the revegetation contractor responsibility for successful vegetation 

establishment. 

Plant materials specified in contract are not available.

l Call local plant nurseries or local Natural Resource Conservation 

Service (NRCS) office to identify plants that are or are not 

commercially available. NRCS offices can also help you distinguish 

native plants from nonnative plants for a given ecological setting. 

(While this work should have been done during the design phase, 

you may need to do it during construction if plant substitutions are 

required.)

l Develop a list of acceptable grass, forb, and shrub plants in the local 

area. Then refine this list to identify species that are:

		 sNative or nonnative to that ecological area.

		 sCommercially available as seed or container plantings.

		 sAvailable to be collected (cuttings).

l Ensure you know the best time to plant each species, and which can 

substitute for others. 

l To be successful, plant substitutions should ideally mimic the 

ecologic role of the originally specified plant (assuming that the 

role was identified). If the original plant was intended to thrive in 

a riparian zone, is the substituted plant an upland species? Will the 

plant stabilize the soil with deep or widespread roots, or is it intended 

to serve as ground cover to protect riparian soils from rainfall impact? 

As these details will vary with each project, you need to factor them 

in for the intent of the revegetation to be successful.

l If the plants will be commercially ordered, know the delivery 

schedule and ensure it meets the desired planting window.
G—34

Stream Simulation

Plant materials are at risk of dying or do not grow as quickly as desired. 

For example, work might be complete in the fall, as planned, but a drought 

makes it unlikely that the plants will survive. 

l For projects encountering weather problems, either delay the 

revegetation work or modify the contract to do revegetation as a 

separate contract.

l If these options are not available, pay the contractor for watering 

the plants long enough for germination and establishment (typically 

several weeks, but possibly a month or two).

l Use “dry water,” a type of water-saturated gel that can be placed in 

the ground with container plants. (Guidelines based on successful 

experience are not widely available yet.)

l Native plants in many ecosystems may begin to establish themselves 

in the first year but may take 2 to 3 years to become fully established. 

Do not expect native plants to establish as quickly as some 

commercial turf grasses.

Plant materials do not grow at all. Improper selection, handling, or storage 

is a frequent cause. 

l Base the revegetation specifications on site-specific conditions. 

To be successful, the revegetation specifications must include 

site-appropriate species. Sampling and analysis may be needed to 

characterize the soil materials that will be revegetated; if so, they 

should be specified.

l Develop guidelines for the proper storage, transportation, and 

handling of the specified plant materials. Contact local nurseries and 

offices of the NRCS to learn what methods are necessary.

l Provide necessary training for CORs and Inspectors regarding 

transporting, handling, storing, and planting seed or plants.

Plant materials are damaged or killed after successful establishment.

l In areas where herbicides are part of the weed eradication program, 

use grass species rather than forb (broad-leaf) species in the 

specifications, since grass is often more resistant to herbicides.
G—35

Appendix G—Additional Tools and Tips

l In areas where wild grazing animals are common, select species that 

are less attractive to grazing animals to reduce chances of heavy 

grazing.

l In areas where the herding of domestic animals is common, seek 

cooperation with local animal owners: Ask that they delay herding 

or provide alternate herding routes during the times critical to 

germination and establishment of permanent vegetation.

Invasive or noxious plants are observed on site before or after 

construction.

l Determine which plants are invasive. For a list of invasive or noxious 

plant species, by state, see the USDA Animal and Plant Health 

Inspection Service Web site, listed under Resources.

l Before construction, physically remove weeds observed on-site 

or treat them with herbicide methods where legal. (See NEPA 

documentation before using herbicides.)

l During construction, take a proactive approach to avoid inadvertently 

transporting invasive species to the site. Include in the special 

contract requirements or supplemental specifications the provision 

that all construction vehicles be washed before entering the 

construction area. This provision will minimize the transport of 

noxious weed seeds from other areas.

l Require plant seed used in revegetation be tested for the presence of 

invasive or noxious weed seeds at appropriate state seed labs.

l Do all you can to foster a healthy revegetated area—it will be more 

resistant to invasive or noxious weed species than areas that are not 

successfully revegetated.

G.4.3.9 Resources for revegetation and erosion control

Lewis, Lisa. 2000. Soil bioengineering – an alternative to roadside 

management – a practical guide. 0077-1801—SDTDC. San Dimas, CA: 

U.S. Department of Agriculture, Forest Service, San Dimas Technology 

and Development Center. 44 p.

Luna, Tara; Dumroese, R. Kasten; Landis, Thomas D. 2006. Collecting 

dormant hardwood cuttings for western riparian restoration projects. 0624-

2334—MTDC. Missoula, MT: U.S. Department of Agriculture, Forest 

Service, Missoula Technology and Development Center. http://www.fs.fed.

us/t-d/pubs/htmlpubs/htm06242334/ (Username: t-d, Password: t-d)
G—36

Stream Simulation

Salix Applied Earthcare. 2002. Erosion Draw 4.0 , (http://www.

erosiondraw.com/, or SAE homepage: http://www.salixaec.com)

Salix Applied Earthcare. 2002. Bio Draw 2.0 , (http://www.biodraw.com/, 

or SAE homepage: http://www.salixaec.com)

U.S. Department of Agriculture. 2002. The PLANTS Database, Version 

3.5 (http://plants.usda.gov). U.S. Department of Agriculture, Natural 

Resources Conservation Service, National Plant Data Center, Baton 

Rouge, LA 70874-4490.

U.S. Department of Agriculture. Various links to Noxious Weed lists and 

Federal Seed Act information (http://www.aphis.usda.gov/ppq/weeds/

nwauthor.html)

G.4.4 Aquatic Organism Capture and Transport 

Following are recommended procedures for successfully capturing and 

transporting aquatic organisms during dewatering.

Block off site upstream and downstream. Set block nets to prevent 

organisms from entering the construction zone from upstream and 

downstream while the site is being dewatered.

Dewater all or part of the channel in stages. Dewatering slowly 

minimizes shock and harm to the organisms.

Trap and transport aquatic organisms. As the site is dewatered, trap 

organisms with dip nets and by hand. (Electroshocking may be 

necessary for some fish.)

Stage and control rewatering after the project is completed. 

Rewatering the site slowly prevents turbidity and temperature levels 

from rising suddenly, and minimizes harm and shock to aquatic 

organisms.

No standard method exists for capture and handling of aquatic organisms. 

The biological opinion from the regulatory agency should cover the 

methods for Endangered Species Act-listed species. State fish and game 

agencies are a good source for guidelines for handling captured aquatic 

organisms. As a general rule, place captured fish in a bucket of water kept 

at near stream temperature.
G—37

Appendix G—Additional Tools and Tips

Knowing which fish, mollusks, crustaceans, and amphibian species inhabit 

each site is important, because different species are more easily captured at 

different stages of dewatering. Capture begins immediately after you have 

isolated the construction area with block nets. Capture stages for aquatic 

species encountered in Oregon are as follows:

l Before actual dewatering begins, use nets and/or seines to efficiently 

capture juvenile salmon, adult and juvenile cutthroat trout, dace and 

red side shiners, other small minnow-like fish, and slow-moving 

amphibians. 

l After dewatering begins and as the water level is dropping, capture 

red-legged frogs, mollusks, and crustaceans.

l When the water level at the site is drawn down, capture streambottom oriented fish, sculpin, and three-spine stickleback. 

l After dewatering, capture stream-bottom species such as lamprey, 

because they come out of the substrate when the ground is disturbed.

Place traps and/or screens on the bypass system pumps and hoses. When 

using a passable pipe or channel bypass system, review the outlet, water 

depth, and velocity to ensure that aquatic organisms can pass through the 

system unharmed.

The following case examples demonstrate procedures for capturing and 

transporting aquatic organisms from construction sites. The principles 

are the same for stream restoration projects, as illustrated in the second 

example. 

G.4.4.1 South Fork Desolation Creek

Block nets were placed above and below the work site the day before 

construction started. Fish were removed from the construction site by 

electro-fishing (figure G.10). Captured fish were placed in 5-gallon buckets 

filled with water kept at stream temperature, and the buckets were carried 

downstream for release. Dewatering consisted of confining streamflow 

to the middle of the creek and dewatering the foundation trenches with 

pumps during placement of the precast concrete footings. As dewatering 

progressed in the foundation trenches, more aquatic organisms were 

captured and removed. Block nets were kept in place until construction 

was completed and the entire streambed within the open-bottom arch 

was rewatered. Figure G.11 shows block nets in place both upstream and 

downstream, with the stream diverted to the middle between the footings 

of the newly placed arch. 
G—38

Stream Simulation

Figure G.10—South Fork Desolation Creek looking upstream. The crew captured 

fish trapped within the construction area as the foundation trenches were 

dewatered.

Figure G.11—South Fork Desolation Creek construction site showing block nets.
G—39

Appendix G—Additional Tools and Tips

G.4.4.2 Karnowski Creek habitat restoration

Karnowski Creek was rerouted to the valley margin in the late 1800s 

when cross-valley drainage ditches were constructed to drain the valley 

bottom for pasture and hay land. During the habitat restoration project, 

the drainage ditches were plugged and Karnowski Creek was relocated 

in its historic midvalley location. Aquatic organisms were captured and 

relocated before, during, and after each ditch was dewatered, plugged, 

and backfilled. (See figures G.12 to G.13/9.) Figure G.14 shows the 

percentage of individuals of each species captured before plugging and 

after the ditches were fully plugged and dewatered. The majority of the 

salamanders, frogs, and lampreys were captured after plugging. 

Figure G.12—Trout and sculpin are seined from ditch prior to plugging, and 

captured with dip nets. 
G—40

Stream Simulation

Figure G.13—Capturing aquatic organisms by hand as the site is dewatered.
G—41

Appendix G—Additional Tools and Tips

Common Name Abbrev.

coho salmon...........................................coho

chinook salmon ......................................chnk

cutthroat trout (1+)....................................cutt

steelhead trout (1+)................................stlhd

trout (0+)................................................. trout

pacific lamprey .......................................... LA

sculpin...................................................... SC

red-sided shiner ....................................... SH

three-spine stickleback..............................ST

red-legged frog........................................ frog

dace .......................................................dace

crayfish..................................................... CR

newt........................................................newt

pacific giant salamander ........................PGS

mussel......................................................MU

coho mortalities ............................. coho mort

other mortalities............................. other mort

Figure G.14—(a) Percentages of total individuals of each species captured before and after dewatering. 

(b) Species identification table.

(b)

(a)

Mortalities

HSample Contract 

Provisions

Supplemental Specification 157—

Soil Erosion Control

Supplemental Specification 251—

Streambed Construction

Supplemental Specification 705—

Streambed-Simulation Materials

Special Contract Requirements 

(H-Clauses)
Stream Simulation
Appendix H—Sample Contract Provisions

This appendix includes sections from a contract developed on the Willamette National Forest in 

Oregon: the list of items, supplemental specifications, Section H - Special Contract Requirements, 

and drawings. The example contract documents can serve as “starting points” for your project, but 

they will need to be thoroughly modified for local conditions. 

Table H 1—Sample schedule of items (Bid Schedule)

Item 

No.

152(02)

171(03)

201(03)

202(02)

204(19)

204(20)

206(02)

206(07)

251(01)

251(14)

251(15)a

251(16)

301(10)

552(03)

Description

Construction surveying and staking 

(road)

Construction surveying and staking 

(structure)

Clearing and grubbing, slash 

treatment methods for tops and 

limbs 12, logs 12, stumps 12, 

utilization of timber 2

Removal of existing 13-ft.-diameter 

multiplate pipe, disposal method A

Soil erosion and pollution control

Dewatering and sediment control 

Foundation fill

Structural excavation

Placed riprap, class 6, method A

Placed channel rock, rock-36, 

method D

Placed streambed simulation rock, 

subarmor, method D

Filler material, placement method E

Untreated aggregate course, type 

base, grading C, compaction B

Structural concrete, class A (AE), for 

footings

Method of 

Measure

AQ

AQ

AQ

LSQ

AQ

LSQ

LSQ

DQ

LSQ

DQ

AQ

DQ

DQ

DQ

AQ

Unit

Sta

Each

LS

Ea

LS

LS

CY

LS

CY

EA

CY

CY

CY

CY

Estimate 

Quantity

3

1

All Req’d

1

All Req’d

All Req’d

1,565

All req’d

138

30

315

26

84

69

Unit 

Price

$_______ 

$_______ 

$_______ 

$_______ 

 $_______ 

 $_______ 

$_______ 

 $_______ 

 $_______ 

 $_______ 

 $_______ 

$_______ 

$_______ 

 $_______ 

Total

$_______ 

$_______ 

$_______ 

$_______ 

$_______ 

$_______ 

$_______ 

$_______ 

$_______ 

$_______ 

$_______ 

$_______ 

$_______ 

$_______ 

Reconstruction of Road , mp
H—2

Stream Simulation

Table H 1—Sample schedule of items (Bid Schedule)

Item 

No.

554(03)

601(01)

607(03)

617(06)

625(02)

Description

Reinforcing steel

Mobilization

Gate temporary, type I barricade, 

size 16’ wide x 2 8” high

Steel 5.75” x 15” corrugation longspan structure, plate zinc-coated, 

34’ span, 15’ 3” rise, .25” thickness

Seeding, hydraulic method (with 

mulch)

Method of 

Measure

AQ

LSQ

LSQ

AQ

AQ

DQ

Unit

LS

LS

Each

FT

Acre

Estimate 

Quantity

All Req’d

All Req’d

2

28

0.12

Unit 

Price

$_______ 

$_______ 

$_______ 

$_______ 

$_______ 

Total

$_______ 

$_______ 

$_______ 

$_______ 

$_______ 

Reconstruction of Road , mp
H—3

Appendix H—Sample Contract Provisions

SUPPLEMENTAL SPECIFICATION 157—SOIL EROSION CONTROL

Description

157.01 Add the following after the first sentence: 

The work shall also include stream bypass construction and 

dewatering.

Materials

157.02 Requirements Add the following:

Coarse Aggregate for Portland Cement Concrete.............703.02

Plastic Lining ....................................................................725.19

Bentonite ...........................................................................725.20

Erosion Control Culvert Pipe............................................713.15

Plastic Pipe........................................................................706.08

Aluminum-Alloy Corrugated Pipe....................................707.03

Metallic-Coated Corrugated Steel Pipe ............................707.02

Watertight Gaskets............................................................712.03

Construction 

Requirements

157.03 General Add the following after the first paragraph:

The contractor’s written plan shall include, as a minimum, the 

dewatering and sediment control requirements AS SHOWN ON THE 

DRAWINGS and in this specification. The contractor shall submit 

the complete plan at least 15 days prior to start of work and shall not 

commence work until approved in writing by the contracting officer. 

The plan shall be executed without modification unless authorized in 

writing by the contracting officer. The work shall be in conformance 

with applicable Federal, State, and local government regulations. 

157.04 Controls and 

Limitations on Work Add the following:

The contractor shall operate in a manner that will protect aquatic 

organisms.
H—4

Stream Simulation

Construct the dewatering and sediment control requirements AS 

SHOWN ON THE DRAWINGS, in accord with and according to the 

contractor’s approved plan.

Notify the contracting officer of the intention to dewater the stream 

at least 72 hours in advance. DO NOT REROUTE WATER until 

approved by the contracting officer. A fisheries biologist (approved 

by the contracting officer) and other Government personnel must be 

present and prepared to rescue aquatic organisms prior to rerouting 

of the stream. Work that would jeopardize fish shall not be permitted 

during the dewatering operation. Dewater the stream slowly and 

incrementally in order to facilitate the fish rescue. The rescue 

operation will generally take several hours.

The newly constructed simulated streambed must be approved by the 

contracting officer prior to releasing water through the project site. 

After approval, water shall be released slowly and incrementally over 

a period of at least 1 hour, or as approved by the contracting officer.

157.09 Diversions Add the following:

Stream Bypass Dam and Pipe. Construct a sandbag dam and bypass 

pipe to divert the stream water around the excavation. A channel lined 

with an impermeable membrane may be substituted for the bypass 

pipe when approved by the contracting officer.

Primary Bypass Dam. Construct the sandbag dam in a dry condition 

by first pumping the stream around the dam, placing a feeder dam, 

or placing temporary sandbag cofferdam(s). Place the sandbag dam 

AS SHOWN ON THE DRAWINGS or approved by the contracting 

officer. Remove rocks and other irregularities from the streambed to 

form a smooth bedding for the dam. Place the dam so that water does 

not seep from the downstream side of the dam; if seepage occurs, 

improve the dam by adding sandbags, improving or adding seals, or 

adding pumping or other means to eliminate seepage from the dam.

Bypass Pipe. Place bypass pipe AS SHOWN ON THE DRAWINGS 

or approved by the contracting officer and in accordance with Section 

603-Metal Pipe or 603B-Plastic Pipe. The upstream invert of the pipe 

shall be placed at the lowest point in the stream channel; remove rocks 

from the streambed, as needed. Install joints and elbows, as needed 

to accommodate the site layout. Use watertight seals, when SHOWN 

ON THE DRAWINGS. Lay of the pipe must be approved by the 

contracting officer prior to backfilling. 
H—5

Appendix H—Sample Contract Provisions

Compact the backfill according to method A. Allow water to pass 

through pipe only after a downstream splash apron has been prepared 

in a manner that will protect the stream from scour and turbidity. The 

installation shall be constructed in a manner that avoids injury to 

aquatic organisms, such as fish being dashed onto sharp rocks at the 

outfall of the pipe.

Feeder Dam and Pipe. Construct a sandbag dam and pipe upstream 

of the primary bypass dam/pipe AS SHOWN ON THE DRAWINGS 

or approved by the contracting officer, for the purpose of feeding the 

streamflow into the primary bypass pipe and improving the efficiency 

of the primary bypass dam.

Downstream Dam. When water flows into the work area from 

downstream, place a sandbag or geotextile/straw-bale dam AS 

SHOWN ON THE DRAWINGS or approved by the contracting 

officer to prevent water from entering the work area. 

Sandbags. Place sandbags AS SHOWN ON THE DRAWINGS or 

approved by the contracting officer. Prior to placing the lower rows 

of sandbags, remove the larger rocks from the streambed to form 

a smooth bed. Sandbags shall contain only clean sand or coarse 

concrete aggregate. The bags shall be loosely filled and tamped in 

place to minimize seepage between, under, and around the bags.

Primary Dam Impermeable Membrane. Place the membrane 

within the sandbag dam and entrenched in the streambed AS SHOWN 

ON THE DRAWINGS or approved by the contracting officer. The 

membrane shall have a minimum thickness of 10mil and be free of 

tears or punctures. Compact soil in the trench along bottom edge of 

the membrane to form a water seal; when approved by the contracting 

officer, a small amount of granular bentonite may be used along the 

bottom edge of the membrane to form a watertight seal between the 

membrane and the streambed. Cut a hole in the membrane to fit the 

bypass pipe and seal the membrane to the bypass pipe or the bypass 

pipe collar using such means such as adhesive strips to form a durable 

watertight seal.

Bypass Pipe Collar. Install and maintain a leak-proof pipe collar 

immediately downstream of the impermeable membrane AS SHOWN 

ON THE DRAWINGS or approved by the contracting officer. The 

collar shall be an Ethylene Propylene Diene Monomer (EPDM) liner 

having a thickness of 45mil. A smooth round hole shall be cut in the 

liner with diameter one-half that of the bypass pipe, and pulled over 

the end of the pipe into place. EPDM-seam tape and compression 
H—6

Stream Simulation

band(s) shall be used to form a durable watertight seal between the 

collar and the pipe. The liner shall extend to the sides and top by a 

distance of one pipe diameter. The lower edge of the collar shall be 

entrenched in the streambed along the downstream side of the dam’s 

impermeable membrane. When approved by the contracting officer, 

a small amount of granular bentonite may be used along the bottom 

edge of the collar to form a watertight seal between the collar and the 

streambed.

Pumps. Install pumps as required to reroute the stream around the 

construction site and dewater foundations. When failure of a pump 

would result in movement of sediment or turbidity beyond the work 

area, a back-up pump shall be readily available.

Bypass Pump. When SHOWN ON THE DRAWINGS, supply and 

operate a pump that has the pumping capacity greater than the flow 

in the stream, to be used for installing and removing the gravity 

bypass pipe(s) and dam(s), and at other times to facilitate construction 

operations (and used during storms to supplement the gravity 

bypass). The pump shall be equipped with approved fish screens, 

appropriate suction and discharge hoses, fittings, and flow regulation 

equipment needed to route the stream around the construction site to 

the discharge point SHOWN ON THE DRAWINGS or approved by 

the contracting officer. Pumps shall be clean and free of leaks. Oil 

lubricant in the pump seal systems shall consist of food-grade mineral 

oil.

Sump Pumps. Supply two pumps capable of dewatering the structure 

foundation AS SHOWN ON THE DRAWINGS or approved by the 

contracting officer. Pumps shall be clean and free of leaks. Sediment 

in the sump pump discharge shall be removed from the water prior to 

reentering the waterway.

Sump Water Discharge. Discharge sump water AS SHOWN ON 

THE DRAWINGS or as approved by the contracting officer. Apply 

one or more methods to remove sediment from sediment-laden 

water. Apply additional methods, as needed, to eliminate all visual 

evidence that sump water discharge is causing a downstream turbidity 

increase. Monitor operations to insure continuing compliance with 

water quality requirements. Note, in the following methods, where a 

manufacturer is shown, there may be other manufacturers who supply 

similar products or methods of treatment. Unless stated otherwise, it 

is not our intent to endorse a particular manufacturer in this document. 

The reader should further research similar products.
H—7

Appendix H—Sample Contract Provisions

(a) Natural Vegetation/Soil Dispersal and Filtration. Sump water 

may be discharged onto areas of ground most advantageous for 

dispersal and filtration of sediment, for example, flat heavily 

vegetated soil. When single point discharge does not function 

adequately, discharge sump water into a perforated pipe laid level so 

that the sump discharge will disperse over a wide area. 

(b) Silt Bag(s) Filtration. Discharge sump water into a silt bag. The 

bag shall be constructed of Mirafi 180N, or approved equal, with 

sewn seam strengths of 90-percent efficiency according to ASTM 

D4632. The bag shall be constructed to hold and filter sump water. 

Place silt bag(s) on level ground above a layer of straw 1-foot thick.

(c) Settling Basin(s). Discharge sump water into a basin or basins. The 

basins may be premanufactured tanks, folding tanks, geotextile, or 

membranes placed over a sandbag or weed-free straw berm, or other 

similar basins designed to separate sediment from the water. 

Suspended Sediment Coagulation Agent. When the above 

methods (a), (b), or (c) do not function adequately, add an approved 

coagulation agent to the water prior to discharging the water onto 

natural vegetation, silt bag(s), or settling basin(s) described in 

methods (a), (b), or (c). The flocculation agent shall be Chitosan-based 

Storm-Klear Gel-Floc, or an approved equal. Storm-Klear products 

are manufactured by Vanson HaloSource, Inc., and are distributed by 

Natural Site Solutions, Redmond, Washington. Use the suspended 

sediment coagulation agent according to the manufacturer’s 

recommendations.

After placement of the simulated stream materials AS SHOWN 

ON THE DRAWINGS, wash the fines into the surface of the new 

streambed. Treat the sump water discharge as before.

Sedimats. Place Sedimats across the streambed AS SHOWN ON 

THE DRAWINGS or approved by the contracting officer and 

as recommended by the product manufacturer. The Sedimat is a 

proprietary product manufactured by Indian Valley Industries, Inc. and 

distributed by Columbia Storage Inc., Vancouver, Washington. Use 

Sedimats according to the manufacturer’s recommendations.
H—8

Stream Simulation

157.13 Maintenance 

and Cleanup Add the following:

Maintain all elements of the operation in order to dewater the 

foundation, facilitate construction, prevent harm to aquatic organisms, 

and prevent sediment and turbidity from entering the stream. 

When removing the sandbag dam(s), sand must be removed from the 

waterway; if coarse concrete aggregate is used in the sandbags, the 

gravel may be distributed evenly across the waterway as directed by 

the contracting officer.

Geotextiles used in sediment control operations shall be removed 

from Government property after use.

Bare soil left from filtering or settling operations shall be shaped to 

drain, seed, and mulch with weed-free straw. 

Measurement

157.15 Add the following after the last item:

Measure dewatering and sediment control as a lump sum.
H—9

Appendix H—Sample Contract Provisions

SUPPLEMENTAL SPECIFICATION 251—STREAMBED CONSTRUCTION

Description

251.01 Add the following after the first sentence:

The work shall include streambed-simulation construction.

Materials

251.02 Requirements (Add the following materials)

Channel Rock 705.07

Streambed-Simulation Rock 705.08

Construction 

Requirements

251.04A Placed 

streambed-simulation 

rock and channel rocks Add the following:

Prior to the start of construction, submit a written plan for obtaining, 

mixing, placing, and shaping streambed-simulation rock, channel 

rocks, and select borrow. The plan must indicate how the material 

will be tested to verify that it meets all of the requirements of this 

specification. Do not substitute onsite materials for material sources 

specified in the contract, unless a revised plan is first submitted and 

approved in writing by the contracting officer.

Placed stream-simulation rock is rock placed on a prepared surface 

to form a well-graded, low-permeability mass, similar in appearance 

and texture to the adjacent natural streambed. No metal track or 

rubber-tired equipment shall be driven on or operated directly on 

metal or concrete structure surfaces. Onsite excavation materials will 

only be accepted as substituting for specified source material, if it 

can be shown by the contractor to meet all of the requirements of the 

specified material. Material not meeting the gradation or diameters 

specified will not be accepted, unless approved in writing by the 

contracting officer.

Method D, Machine Placed. Place streambed-simulation-rock in 

one or more layers, not to exceed 6 inches or 1.5 x* D84, whichever 

is larger. Fill voids within each layer with filler material according 

to 251.10A before placing the next layer. Do not place streambedsimulation rock by methods that cause segregation or damage to the 

prepared surface or culvert surface. Place or rearrange individual 
H—10

Stream Simulation

rocks by mechanical methods to obtain a compact, low-permeability 

mass matching the stream-simulation bed details SHOWN ON THE 

DRAWINGS. Place channel rocks in the configurations and locations 

SHOWN ON THE DRAWINGS.

Method E, End Dumped. Dump streambed-simulation rock in one or 

more layers not to exceed 6 inches or 1.5 x* D84 diameter, whichever 

is larger. Fill voids within each layer with filler material according 

to 251.10A before placing the next layer. Distribute larger rocks 

throughout the mass of stone. Obtain a uniformly dense, compact, 

low-permeability bed with a surface matching the stream-simulation 

bed details, as SHOWN ON THE DRAWINGS. Place filler material 

according to 251.10A. Place channel rocks in the configurations and 

locations as SHOWN ON THE DRAWINGS.

Method F, Hand Placed. Place stream-simulation rock by using 

hand labor. Material may be hand-carried, or carried in wheelbarrows 

and end-dumped to obtain its full thickness or in layers, if the depth 

exceeds 24 inches. Compact each load using hand-operated equipment 

to obtain a uniformly dense, compact, low-permeability bed with a 

surface matching the stream-simulation bed details as SHOWN ON 

THE DRAWINGS. Place filler material according to 251.10A before 

placing the next layer. Place channel rocks in the configurations and 

locations SHOWN ON THE DRAWINGS.

251.10A. Placed 

Filler Material Fill all voids between individual streambed-simulation rocks and all 

voids left during placement of channel rocks and streambed-simulation 

rock adjacent to footings, concrete structures, or corrugated pipes with 

select borrow as specified in Subsection 704.07. Use water pressure, 

metal tamping rods, and similar hand-operated equipment to force 

material into all surface and subsurface voids between the structure and 

rocks and between individual rocks. Fill shall extend to 100 percent of 

the rocks’ height between layers and 67 percent of their height on the bed 

surface or as SHOWN ON THE DRAWINGS.

Measurement Add the following:

Measure placed channel rocks by each. Measure streambed-simulation 

rock by the cubic yard in place. Measure filler material by the cubic 

yard in place.
H—11

Appendix H—Sample Contract Provisions

SUPPLEMENTAL SPECIFICATION 705—STREAMBED-SIMULATION 

MATERIALS

Add the following: 

705.07 Channel rocks – Channel rocks shall have a long axis 133 

percent or longer than the median axis.

Table 705-4—Size requirement for channel rocks

Channel Rock Approximate Median Axis

Class Weight Dimension &

(diameter, inches) (pounds) Variation in inches

Rock-4 3 4 +/- 1

Rock-6 10 6 +/- 1

Rock-9 33 9 +/- 2

Rock-12 80 12 +/- 2

Rock-16 185 16 +/- 2

Rock-20 365 20 +/- 2

Rock-24 630 24 +/- 3

Rock-30 1,230 30 +/- 3

Rock-36 2,120 36 +/- 4

Rock-42 3,370 42 +/- 4

Rock-48 5,030 48 +/- 5

Rock-54 7,160 54 +/- 5

Rock-60 9,820 60 +/- 6

 Note: Rock classes are shown on the drawings for all key features to be constructed.

Table 705-7– Project gradation requirements for streambed-simulation bed 

material, (inches)

 Note: Figure 7.18 shows how to fill out table 705-7.

Standard

sieve

Stream-simulation

bed material

(percent finer)

Filler 

material

(percent finer)
H—12

Stream Simulation

 SPECIAL CONTRACT REQUIREMENTS (H-CLAUSES)

H.1 SEASONAL 

RESTRICTIONS For protection of resources, time restrictions will apply. Work will be 

conducted only during the time frames listed below:

l All work shall be completed between ______ and ______.

l Site disturbance and all other general construction work may not 

begin until _____, unless wildlife restrictions are waived by the 

district wildlife biologist.

l All in-water work is restricted to _____ through _____, unless 

extended by the (local permitting agency, such as Oregon Department 

of Fish and Wildlife).

H.2 PHYSICAL DATA 

(FAR 52.236-4) 

(APR 1984) Data and information furnished or referred to below is for the contractor’s 

information. The Government shall not be responsible for any 

interpretation of, or conclusion drawn from, the data or information by the 

contractor.

The indications of physical conditions on the drawings and in the 

specifications are the result of site investigations by the _____ (Forest 

Service, FHWA, etc.). The investigational methods have included the site 

survey as shown on the drawings and visual observations of the ground 

surface.

Weather conditions typical for this area indicate the following normal fire 

season: ____________ to ___________.

H.3 LANDSCAPE 

PRESERVATION The contractor shall not remove, deface, injure, or destroy trees, shrubs, 

lawn, or natural features not designated for treatment. The contractor shall 

confine operations to within the clearing limits or other areas designated 

in the contract documents and prevent the depositing of rocks, excavated 

materials, stumps, or other debris outside of these limits. Material that falls 

outside of these limits shall be retrieved, disposed of, or incorporated in, 

the work as directed by the contracting officer.
H—13

Appendix H—Sample Contract Provisions

To prevent fuel and oil spills. The contractor shall maintain storage 

facilities for oil or oil products on site; appropriate preventive measures 

shall be taken to insure that any spill of such oil or oil products does 

not enter any stream or other waters of the United States. When pumps 

are used near a stream, a fuel containment pan shall be placed under the 

pump to prevent fuel and oil contacting the soil in the event of a spill 

from the pump. If a spill of a petroleum product should occur in water, the 

contractor shall immediately notify the engineer and the (local Emergency 

Response System, such as Oregon Emergency Response System).

Servicing of all equipment shall be done only in the areas approved 

by the contracting officer. If the total oil or oil products storage 

exceeds 1,320 gallons or if any single container exceeds a capacity 

of 660 gallons, the contractor shall prepare a spill prevention control 

and countermeasures plan. Such a plan shall meet applicable EPA 

requirements (40 CFR 112), including certification by a registered 

professional engineer.

No objectionable material shall be allowed to enter any stream, 

river, lake, or other body of water. Material which falls in these 

areas shall be retrieved and disposed of, or incorporated into the 

work, as directed by the contracting officer. Damage to vegetation or 

structures outside the project limits shall be repaired, as directed by 

the contracting officer.

The contractor shall not operate equipment or otherwise disturb the 

natural vegetation and soil beyond the areas flagged on the ground or 

beyond 2 feet from the top of cuts or toes of fills.

Prior to the start of construction, the contractor shall submit to 

the engineer for approval a schedule and plan for soil erosion and 

pollution control measures for the following phases of work:

l Item 157—Dewatering And Sediment Control.

l Item 201—Clearing and Grubbing.

l Item 203—Removal of Structures and Obstructions.

l Item 209 or 208—Structural Excavation. 

l Item 251—Channel Rock, Streambed Simulation Rock, 

Select Borrow.

l Item 552—Structural Concrete.
H—14

Stream Simulation

The soil erosion and pollution control measures shall be designed to 

prevent any visually perceptible difference in turbidity of the water 

flowing 100 feet downstream of the project (when compared to the water 

upstream of the project). The plan shall incorporate, as a minimum but 

not limited to, the measures AS SHOWN IN THE DRAWINGS. The 

following control measures and materials shall be available on the project 

site:

l Plastic sheets or other suitable covers for exposed soil during 

rainstorms.

l Weed-free straw bales, silt fences or other similar erosion barriers 

placed at the lower edges of soil slopes that prevent soils from 

eroding into adjacent streams.

l Covering of all exposed areas of soils with certified weed-free straw 

mulch upon final completion of the work.

l Sump discharge for dewatering the excavation shall use settling 

ponds or distribution systems (for example, perforated pipe laid on 

the ground away from streams) placed in a manner that will cause 

water infiltration into the surrounding soils.

l Temporary stream diversions, as shown in the drawing or as 

improved upon by the contractor and approved by the engineer.

l Other measures and materials proposed by the contractor and 

approved by the contracting officer.

If construction activities cause a visually perceptible increase of stream 

turbidity for a period in excess of 30 minutes, the contractor shall cease the 

operations that are causing the turbidity and modify the control measures, 

as needed to prevent further pollution. 

The contractor shall have a SPILL RESPONSE KIT on the project 

whenever equipment is operating. The spill kit shall be sufficient to absorb 

up to 34 gallons of oil and be designed to float on the surface, while 

absorbing oil and repelling water. The kit shall meet or exceed the physical 

properties of the “New Pig Products Spill Kit #408.”

Equipment shall be furnished on a fully operational basis of modern design 

and in good operating condition with no fuel or oil leaks. Repairs and 

move-in/move-out are the contractor’s responsibility. All equipment shall 

be power-washed to remove all foreign or noxious seeds/weeds prior to 

entering Forest Service land.

Straw shall be certified weed free.
H—15

Appendix H—Sample Contract Provisions

H.4 MOISTURE 

SENSITIVE SOILS Contractors are cautioned that the roadway structure must be designed so 

that the completed road will support highway-legal loads during a limiteduse season. Construction equipment often subjects the uncompleted 

roadway structure to loadings it was not designed to support. This is 

especially critical during periods of excessive moisture and will require 

careful selection and scheduling to permit efficient operation. The 

contractor at their expense shall correct any damage resulting from 

operations that render the material unsuitable for use or results in potential 

siltation of streams.

H.5 VALUE 

ENGINEERING Value engineering change proposals which change the service or function 

of a facility or produce irreconcilable conflicts with management 

objectives will not be considered.

The following work is excluded from consideration under the value 

engineering clause: NONE.

H.6 PRODUCT 

SUBSTITUTION Any modification of items, designs, materials, products, or equipment 

(including Government-furnished property), made necessary because 

of a substitution, shall be the responsibility of the contractor without 

adjustment in contract price or time. The contracting officer’s approval 

of any substitute shall not affect the contractor’s responsibility for such 

modification. Any and all substitutions shall be requested by the contractor 

after award of the contract has been made.

No approvals will be made prior to award. 

The contractor shall provide written documentation and all testing 

information to verify that the proposed substitution product meets all the 

of the specification requirements.

H.7 ROAD USE AND 

MAINTENANCE

H.7.1 Use of Roads See Special Project Specification 104.021 for use authorization and 

limitations.

H.7.2 Traffic Control

The contractor may close Road ______ as needed for construction for a 

period not to exceed _____ consecutive days. During the times of closure, 

the contractor shall provide and maintain “Road Closed Ahead” signs and 
H—16

Stream Simulation

other devices at locations leading to the project site, as prescribed in the 

traffic control plan. 

The contractor shall provide, erect, and maintain all necessary barricades, 

suitable and sufficient lights, danger signals, signs, and other traffic control 

devices; they shall take all necessary precautions for the protection of the 

work and safety of the public. Barricades and other obstructions shall be 

illuminated during the hours of darkness. Suitable warning signs shall be 

provided to control and direct traffic properly.

The contractor shall erect warning signs in advance to any place on the 

project, where operations may interfere with the use of the road or trail 

by traffic, and at all intermediate points, where the project crosses or 

coincides with an existing road or trail. 

H.8 CONSTRUCTION 

STAKES, LINES, AND 

GRADES The Forest Service has placed control points at the project site. The hubs 

and stakes constitute the field control from which the contractor shall 

execute the work, and shall be left in place until the engineer approves 

their removal.

The contractor shall do all further surveying, staking, and engineering to 

establish the horizontal and vertical control necessary for the finished work 

to comply with the lines and grades shown on the drawings or stated in the 

specifications. This work is incidental to the pay items.

If any construction control points have been destroyed or displaced by the 

contractor’s negligence or operation, the contractor shall promptly notify 

the engineer. If these points are destroyed or displaced due to contractor’s 

negligence or operation, the cost of replacing them will be charged to the 

contractor.

In the case of any construction changes, the contractor shall cooperate with 

the engineer and facilitate the prompt reestablishment of the field control 

for the readjusted work.
H—17

Appendix H—Sample Contract Provisions

H.9 PROSECUTION 

OF WORK The contractor shall conduct activities so that interference with the public 

shall be kept at a minimum. Any activities requiring any type of closure to 

the public will be scheduled with the COR at least ____ days in advance.

The contractor shall use measures and precautions necessary to warn 

and protect the public and Government personnel during work at the 

project site. Such actions include, but are not limited to, furnishing and 

maintaining barricades and signs around the work site and roping off the 

area.

For blasting precautions and methods, the contractor shall comply with 

State and Federal laws in regards to transportation, storage, and use of 

explosives. The contractor shall post a watchperson at a safe distance on 

all approaches to a blasting area on all approaching roads and trails. The 

contractor shall also notify all people in the vicinity prior to a detonation.

H.10 CONTROL OF 

MATERIAL

H.10.1 Rights in and 

use of materials The contractor may use on the project suitable stone, gravel, or sand 

encountered in the excavation that can be shown by testing and in written 

documentation that it meets the project specifications.

H.10.2 Excavation ________(specify type)

H.10.3 Material sources Borrow sources, if needed, must be approved in advance by the engineer. 

Such borrow sources shall be restored to a natural appearance. Rocks and 

mineral soil excavated within the normal excavation shall be conserved 

and used, as needed where they meet project specifications and are 

approved in writing by the contracting officer.

H.10.4 Storage and 

stockpiling of materials Materials shall be stored to assure the preservation of quality and fitness 

for the work. Stored materials shall be located to facilitate their prompt 

inspection. Sites on Forest Service-administered land, approved by the 

Forest Service, may be used for storage purposes and for the placing of 

the contractor’s plant equipment. All storage sites provided by the Forest 

Service shall be restored at the contractor’s expense. Contractor shall be 

responsible for making arrangements for storage on other than Forest 

Service-administered lands. 
H—18

Stream Simulation

H.10.5 Local 

disposal sites Designated disposal sites for this project are as shown on the drawings.

H.10.6 Earthwork 

tolerances Unless working tolerances are specified, all work performed and materials 

furnished shall be in reasonably close conformity with lines, grades, cross 

sections, dimensions, and material requirements shown on the drawings, 

indicated in the specifications, or designated on the ground. “Reasonably 

close conformity” shall be in compliance with what is reasonable and 

customary for manufacturing and construction tolerances.

H.11 STATE PERMITS Roads in the project work area necessary to complete the project 

are designated as “within the immediate construction project” for 

consideration under ORS 767.025 as to the nonapplicability of PUC 

requirements.

H.12 PROTECTION 

OF CULTURAL 

RESOURCES The location of known historic or prehistoric sites, buildings, objects, 

and properties related to American history, architecture, archeology, and 

culture (such as settler or Indian artifacts) protected by the American 

Antiquities Act of 1906 (16 U.S.C. 431-433), National Historic 

Preservation Act of 1966 (16 U.S.C. 470), and the Archeological 

Resources Protection Act of 1979 (PL 96-95 and 36 CFR 261.9(e)) shall 

be identified on the ground by the Forest Service. The Forest Service 

may unilaterally modify or cancel this contract to protect an area, object 

of antiquity, artifact, or similar object which is or may be entitled to 

protection under these acts regardless of when the area, object, or artifact 

was discovered or identified. Discovery of such areas or objects by either 

party shall be promptly reported to the other party.

The contractor shall protect all known and identified historic or prehistoric 

sites, buildings, objects, and properties related to American history, 

architecture, archeology, and culture against destruction, obliteration, 

removal, or damage during their operations. In accordance with 36 CFR 

296.14(c) the contractor shall bear the costs of restoration, provided that 

such payment shall not relieved the contractor from civil or criminal 

remedies otherwise provided by law.

Wheeled or track-laying equipment shall not be operated within such areas 

except on roads. Unless agreed otherwise, trees shall not be felled into 

such areas.
H—19

Appendix H—Sample Contract Provisions

H.13 PROTECTION 

OF HABITAT OF 

ENDANGERED, 

THREATENED, AND 

SENSITIVE SPECIES Location of areas needing special measures for protection of plants or 

animals listed as threatened or endangered under the Endangered Species 

Act of l973, as amended, or as determined to be sensitive by the regional 

forester under authority of FSM 2670, are shown on the drawings and 

identified on the ground. Measures needed to protect such areas have been 

included elsewhere in this contract or are as follows: None.

If protection measures prove inadequate, if other such areas are 

discovered, or if new species are listed as federally threatened or 

endangered or as sensitive by the regional forester, the Forest Service may 

either cancel or unilaterally modify this contract to provide additional 

protection regardless of when such facts become known. Discovery of 

such areas by either contractor or inspector shall be promptly reported to 

the other party.

H.14 SANITATION 

AND SERVICING 

REQUIREMENTS

Unless substitute measures or equipment are authorized in writing by the 

contracting officer, protection of air and water quality shall include the use 

of approved chemical toilets by all persons engaged in road construction 

or in removing timber under this contract while they are inside the forest 

boundary. Such facilities shall be furnished by contractor in quantities and 

at locations approved by the engineer. No habitation or overnight dwelling 

by employees of the contractor shall be permitted on national forest land 

without advance written approval from the contracting officer.

Oil-absorbing mats are required under all stationary landing equipment, 

or equipment being serviced within the forest boundary to prevent leaking 

or spilled petroleum-based products from contaminating soil and water 

resources. Such material will be furnished by the contactor and approved 

by the contracting officer. 

The contractor agrees that all persons engaged in work under this contract 

will have a certificate from a medical doctor certifying them to be free 

from all diseases communicable through drinking water.
H—20

Stream Simulation

H.15 POTENTIAL 

SAFETY HAZARDS Data and information furnished or referred to below is for the 

contractor’s information. The government shall not be responsible for any 

interpretation of or conclusion drawn from the data or information by the 

contractor. This list shall not be deemed to be all-inclusive. The contractor 

shall bear the sole responsibility for taking all appropriate actions 

necessary to prevent accidents and injuries to individuals at the worksite.

The following checked activities have been identified by the government 

as potential safety hazards.

[ ] Confined space entry.

[ ] Temporary excavation/deep trenching/slope stability.

[ ] Tree falling.

[ ] Fall hazard from work heights exceeding 6 feet.

[ ] Blasting.

[ ] Traffic control on high-volume and/or high-speed and/or limitedvisibility roads.

[ ] Heavy equipment operation.

[ ] Tree climbing and/or tower climbing.

[ ] Fire hazards.

[ ] Hazardous materials handling.

[ ] Electrical hazard.

[ ] Hydraulic and/or pneumatic and/or other high-pressure hazards.

[ ] Mechanical hazards such as pulleys, springs, etc.

[ ] Other _____________________________

H.16 FINAL CLEANUP Contractor shall remove and dispose all of their own trash and refuse from 

the contract area. Material to be removed includes, but is not limited to, 

camp refuse; for example, tin cans, aluminum foil, glass, paper, garbage, 

used engine oil, oil filters, oil cans, grease cartridges, etc. The contractor 

shall also remove and dispose of upon completion of the project, all stakes, 

old culverts, flagging, and similar debris within the project area. Roads 

shall be swept and washed to remove soil and rock materials. This cleanup 

is a subsidiary item for which no special payment will be made. All debris 

shall be removed from national forest land in accordance with State and 

local disposal requirements.
H—21

Appendix H—Sample Contract Provisions

H.17 PROTECTION 

OF IMPROVEMENTS Unless otherwise agreed to in writing, the contractor shall remove the 

existing signs within the work area and reinstall them to their approximate 

existing locations.

Figure(s) 

Title Sheet H.1

Vicinity Map H.2 

Estimate of Quantities H.3

Sign and Gate Plan H.4, 5

Site Plan (topographic map, structure and road 

location, storage area location) H.12

Dewatering Plan (including channel 

excavation work needing dewatering) H.6, 7, 13

Dewatering Details (specification, drawing, 

additional design details) H.8

Long Profile and Stream-simulation Details

(abbreviated) H.9, 14

Cross Sections and Stream-simulation Details H.10, 15

Road Template and P-line Location 

(coordinates)

Structure Design 

Structure Details

Concrete Details (footing, collar, etc.) H.11, 16

Drill Investigation Information

 [Note—Some of these drawings will take more than one sheet.]

SAMPLE LIST OF 

PROJECT DRAWINGS
H—22

Figure H.1—Humbug Sheet 1: Title Sheet.
H—23Figure H.2—Humbug Sheet 2: Vicinity Map.
H—24

Figure H.3—Humbug E-W Sheet 3: Estimate of Quantities.
H—25 Figure H.4—Humbug Sheet 4: Sign Plan.
H—26

Figure H.5—Humbug Sheet 5 - Gate Plan.
H—27 Figure H.6—Humbug West Sheet 6 - Site Plan with Dewatering System.
H—28

Figure H.7—Humbug West Sheet 7 - Profile of Dewatering Plan.
H—29Figure H.8—Humbug Sheet 8 - Dewatering Details.
H—30

Figure H.9—Humbug West Sheet 9 - Long Profile and Grade Control Details.
H—31Figure H.10—Humbug West Sheet 10 - Cross Section of Arch and Stream-Simulation Bed Details.
H—32

Figure H.11—Humbug West Sheet 12 - Foundation Details.
H—33Figure H.12—Humbug East Sheet 13 - Site Plan.
H—34

Figure H.13—Humbug East Sheet 14 - Site Plan with Dewatering System.
H—35Figure H.14—Humbug East Sheet 15 - Longitudinal Profile with Stream-Simulation Bed Details.
H—36

Figure H.15—Humbug East Sheet 16 - Cross Section of Arch and Stream-Simulation Bed.
H—37Figure H.16—Humbug Sheet 17 - Foundation Details.