Publication No. FHWA-IF-04-016 

October 2005 

U.S. Department Hydraulic Engineering Circular No. 9 

of Transportation 

Federal Highway 

Administration 

Debris Control Structures 

Evaluation and Countermeasures 

 Third Edition 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1. Report No. FHWA-IF-04-016 2. Government Accession No. 3. Recipient’s Catalog No. 

HEC-9 

4. Title and Subtitle 5. Report Date

Debris Control Structures – Evaluation and Countermeasures October 2005 Hydraulic Engineering Circular 9 

Third Edition 

6. Performing Organization Code 

 HIBT-20, HFL-15 

7. Author(s) 8. Performing Organization Report No. 

 J.B. Bradley, D.L. Richards, C.D. Bahner 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

WEST Consultants, Inc. 

2601 25th Street 

Suite 450 11. Contract or Grant No. Salem, Oregon 97302 

 DTFH61-01-Z-00012 

12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered 

Final Report 

 Office of Bridge Technology National Highway Institute 

 FHWA, Room 3203 4600 North Fairfax Dr., Suite 800 14. Sponsoring Agency Code 400 Seventh Street, SW Arlington, Virginia 22203 

 Washington, D.C. 20590 

15. Supplementary Notes 

 Project Manager: Brian Beucler, FHWA 

Technical Assistants: Joe Krolak, Abbi Ginsberg, Jorge E. Pagán-Ortiz, FHWA; Arthur Parola, University of Louisville 

16. This circular provides information on debris accumulation and the various debris control countermeasures available for 

culvert and bridge structures. It is a dual unit update of the first edition published in 1964. This circular presents various 

problems associated with debris accumulation at culvert and bridge structures, provides a procedure for estimating the potential 

of debris accumulating at a bridge structure, and provides general guidelines for analyzing and modeling debris accumulation on 

a bridge structure to determine the impacts that the debris would have on the water surface profile through the bridge structure 

and the hydraulic loading on the structure. Various types of debris countermeasures for culvert and bridge structures are 

discussed within this circular. General criteria for selection of these countermeasures and general design guidelines for some of 

the structural measures are also included in this manual. 

17. Key Words 18. Distribution Statement 

debris accumulation, debris, culverts, bridges, hydraulic This document is available to the public from 

loading, debris-control structures, debris impacts, debris The National Technical Information Service 

countermeasures, debris management, debris Springfield, VA 22151 (703) 487-4650

classification http://www.fedworld.gov/ntis/ 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

Unclassified Unclassified 179 

TABLE OF CONTENTS
 

TABLE OF CONTENTS ................................................................................................................ i
 

LIST OF FIGURES ....................................................................................................................... v
 

LIST OF TABLES ...................................................................................................................... viii
 

LIST OF SYMBOLS .................................................................................................................... ix
 

ACKNOWLEDGMENTS ............................................................................................................. xii
 

GLOSSARY ............................................................................................................................... xiii
 

CHAPTER 1 – INTRODUCTION ............................................................................................... 1.1
 

1.1 PURPOSE...................................................................................................................... 1.1
 

1.2 BACKGROUND ............................................................................................................. 1.1
 

1.3 DOCUMENT ORGANIZATION ...................................................................................... 1.1
 

1.4 DUAL SYSTEM OF UNITS ............................................................................................ 1.2
 

CHAPTER 2 – DEBRIS CHARACTERIZATION ....................................................................... 2.1
 

2.1 DEBRIS CLASSIFICATION ........................................................................................... 2.1
 

2.1.1 Types of Debris .................................................................................................. 2.1
 

2.2 FLOW BEHAVIOR OF FLOATING DEBRIS .................................................................. 2.3
 

2.2.1 Debris Transport and Transport Mechanisms .................................................... 2.3
 

2.2.2 Debris Jams and Debris Dams ........................................................................... 2.5
 

2.2.3 Debris Accumulation .......................................................................................... 2.6
 

2.3 PROBLEMS ASSOCIATED WITH DEBRIS................................................................... 2.8
 

CHAPTER 3 – ESTIMATING DEBRIS QUANTITIES ............................................................... 3.1
 

3.1 INTRODUCTION............................................................................................................ 3.1
 

3.2 DEBRIS VOLUMES ....................................................................................................... 3.2
 

3.3 POTENTIAL FOR DEBRIS DELIVERY ......................................................................... 3.3
 

3.3.1 Task A: Estimating Potential for Debris Delivery to Site ..................................... 3.3
 

3.3.2 Task B: Estimating the Largest Debris Size Delivered to Site............................ 3.5
 

3.3.3 Task C: Assigning Location Categories to All Parts of the Highway Crossing ... 3.7 

3.4 POTENTIAL FOR DEBRIS ACCUMULATION............................................................... 3.8
 

3.4.1 Task A: Assigning Bridge Characteristics to All Immersed Parts of the Bridge .. 3.8 

3.4.2 Task B: Determining Accumulation Potential for Each Bridge Component ........ 3.9
 

3.5 SIZE OF DEBRIS ACCUMULATION AT STRUCTURES ............................................ 3.10
 

3.6 FACTORS THAT AFFECT DEBRIS PRODUCTION ................................................... 3.12
 

i 
CHAPTER 4 – ANALYZING AND MODELING DEBRIS IMPACTS TO STRUCTURES ......... 4.1
 

4.1 INTRODUCTION............................................................................................................ 4.1
 

4.2 HYDRAULIC ANALYSES OF DEBRIS .......................................................................... 4.1
 

4.3 ONE-DIMENSIONAL DEBRIS ANALYSIS MODELING ................................................ 4.1
 

4.3.1 Data Requirements ............................................................................................ 4.1
 

4.3.2 Background of Modeling Methods and Approaches ........................................... 4.2
 

4.3.2.1 Low Flow Conditions............................................................................... 4.3
 

4.3.2.2 High Flow Conditions.............................................................................. 4.4
 

4.3.3 Scenarios for Hydraulic Modeling of Debris Accumulation ................................. 4.6
 

4.3.3.1 Bridge Debris Scenarios ......................................................................... 4.6
 

4.3.3.2 Culvert Debris Scenarios ...................................................................... 4.10
 

4.4 ADVANCED MODELING ............................................................................................. 4.11
 

4.5 LOCAL PIER SCOUR ASSOCIATED WITH DEBRIS ACCUMULATION.................... 4.11
 

4.6 HYDRAULIC LOADING ASSOCIATED WITH DEBRIS ACCUMULATION ................. 4.12
 

4.7 HYDRAULIC LOADING EXAMPLE PROBLEMS ........................................................ 4.15
 

4.7.1 Example 1 – Hydraulic Loading on a Single Pier (SI)....................................... 4.15
 

4.7.2 Example 2 – Hydraulic Loading on Two Adjacent Piers, Case 1 (SI)............... 4.19
 

4.7.3 Example 3 – Hydraulic Loading on a Superstructure (SI) ................................ 4.24
 

4.7.4 Example 4 – Hydraulic Loading on a Single Pier (CU) ..................................... 4.28
 

4.7.5 Example 5 – Hydraulic Loading on Two Adjacent Piers, Case 1 (CU) ............. 4.32
 

4.7.6 Example 6 – Hydraulic Loading on a Superstructure (CU)............................... 4.37
 

CHAPTER 5 – DEBRIS COUNTERMEASURES ...................................................................... 5.1
 

5.1 BACKGROUND ............................................................................................................. 5.1
 

5.2 COUNTERMEASURES FOR CULVERTS..................................................................... 5.1
 

5.2.1 Structural Measures ........................................................................................... 5.1
 

5.2.2 Non-structural Measures .................................................................................... 5.2
 

5.3 COUNTERMEASURES FOR BRIDGES ....................................................................... 5.2
 

5.3.1 Structural Measures ........................................................................................... 5.2
 

5.3.2 Non-Structural Measures ................................................................................... 5.3
 

5.4 COUNTERMEASURES FOR FIRE DAMAGED / DEFORESTED AREAS.................... 5.4
 

5.4.1 Fire Damaged Areas .......................................................................................... 5.4
 

5.4.2 Deforestation ...................................................................................................... 5.5
 

CHAPTER 6 – DESIGN PROCEDURES FOR DEBRIS COUNTERMEASURES .................... 6.1
 

6.1 GENERAL PROCEDURES AND CONSIDERATIONS.................................................. 6.1
 

6.1.1 Field Investigations............................................................................................. 6.1
 

6.1.2 Selecting the Type of Countermeasures ............................................................ 6.4
 

6.1.3 Design for Bridges versus Culverts .................................................................... 6.8
 

6.1.4 Existing Structures Versus Proposed Structures................................................ 6.9
 

6.1.5 Maintenance Accessibility of the Countermeasure........................................... 6.10
 

ii 
6.2 DESIGN GUIDELINES FOR CULVERTS .................................................................... 6.11
 

6.2.1 Debris Deflectors for Culverts........................................................................... 6.11
 

6.2.1.1 Debris Deflector Example (Culvert) ...................................................... 6.12
 

6.2.2 Debris Racks for Culverts................................................................................. 6.14
 

6.2.2.1 Debris Rack Example (Culvert) ............................................................ 6.15
 

6.2.3 Debris Risers for Culverts................................................................................. 6.17
 

6.2.4 Debris Cribs for Culverts .................................................................................. 6.18
 

6.2.5 Debris Fins for Culverts .................................................................................... 6.18
 

6.2.6 Debris Dams / Basins for Culverts.................................................................... 6.19
 

6.2.7 Combined Debris Controls for Culverts ............................................................ 6.21
 

6.3 DESIGN GUIDELINES FOR BRIDGES ....................................................................... 6.21
 

6.3.1 Debris Deflectors for Bridges ........................................................................... 6.21
 

6.3.2 Debris Fins for Bridges ..................................................................................... 6.22
 

6.3.3 Crib Structures for Bridges ............................................................................... 6.22
 

6.3.4 River Training Structures for Bridges ............................................................... 6.23
 

6.3.5 In-Channel Debris Basin for Bridges ................................................................ 6.23
 

6.3.6 Flood Relief Structures for Bridges................................................................... 6.23
 

6.3.7 Debris Sweepers for Bridges............................................................................ 6.25
 

6.3.8 Design Features for Bridges............................................................................. 6.25
 

6.4 NON-STRUCTURAL GUIDELINES (DEBRIS MANAGEMENT).................................. 6.27
 

CHAPTER 7 – MAINTENANCE ................................................................................................ 7.1
 

CHAPTER 8 – REFERENCES .................................................................................................. 8.1
 

APPENDIX A – METRIC SYSTEM, CONVERSIONS, AND WATER PROPERTIES .............. A.1
 

APPENDIX B – STATE SURVEY ............................................................................................ B.1
 

iii 
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iv
LIST OF FIGURES 

Figure 2.1. Hypothetical Patterns of secondary flow in straight and curved channels. .............. 2.5
 

Figure 2.2. Single pier debris accumulation (led to pier scour failure). ...................................... 2.7
 

Figure 2.3. Span blockage accumulation bridge failure. (Louisiana) ......................................... 2.8
 

Figure 2.4. Missouri Highway 113 bridge over Florida Creek near Skidmore, Missouri. ........... 2.9
 

Figure 2.5. Debris accumulation failure at bridge located in Oklahoma..................................... 2.9
 

Figure 2.6. Debris accumulation at a bridge structure.(17) ........................................................ 2.10
 

Figure 2.7. Debris induced bridge and bank failure structure. (Louisiana) .............................. 2.11
 

Figure 2.8. Effects of debris accumulation at a bridge structure. (New York) .......................... 2.12
 

Figure 3.1. Schematic Illustrating Estimate of Maximum Design Log Length ............................ 3.6
 

Figure 4.1. Sketch of the sluice gate type of pressure flow. ...................................................... 4.4
 

Figure 4.2. Sketch of fully submerged pressure flow. ................................................................ 4.5
 

Figure 4.3. Sketch of pressure and weir flow. ............................................................................ 4.5
 

Figure 4.4. Changing downstream expansion cross section location. ....................................... 4.7
 

Figure 4.5. Adding downstream ineffective flow locations. ........................................................ 4.7
 

Figure 4.6. Adding upstream ineffective flow locations. ............................................................. 4.8
 

Figure 4.7. “Dummy” bridge pier used to simulate an asymmetrical debris accumulation. ........ 4.9
 

Figure 4.8. Upstream face of the bridge for Example 1. .......................................................... 4.15
 

Figure 4.9. Water surface profile for Example 1. ..................................................................... 4.16
 

Figure 4.10. Upstream face of the bridge for Example 2. ........................................................ 4.19
 

Figure 4.11. Water surface profile for Example 2. ................................................................... 4.20
 

Figure 4.12. Upstream face of the bridge for Example 3. ........................................................ 4.24
 

Figure 4.13. Water surface profile for Example 3. ................................................................... 4.25
 

Figure 4.14. Upstream face of the bridge for Example 4. ........................................................ 4.28
 

Figure 4.15. Water surface profile for Example 4. ................................................................... 4.29
 

Figure 4.16. Upstream face of the bridge for Example 5. ........................................................ 4.32
 

Figure 4.17. Water surface profile for Example 5. ................................................................... 4.33
 

Figure 4.18. Upstream face of the bridge for Example 6. ........................................................ 4.37
 

Figure 4.19. Water surface profile for Example 6. ................................................................... 4.38
 

Figure 5.1. Steel rail debris deflector for large rock (looking upstream of culvert). .................... 5.5
 

Figure 5.2. Steel rail debris deflector (looking downstream). ..................................................... 5.6
 

Figure 5.3. Steel rail and cable debris deflector. In boulder areas, cable is more desirable for its 

flexibility than a rigid rail (looking towards entrance). ............................................ 5.6
 

Figure 5.4. Steel debris deflectors installed at entrances to a battery of culverts. ..................... 5.7
 

Figure 5.5. Steel rail debris deflector for battery of culverts (see Figure 5.6). ........................... 5.7
 

Figure 5.6. Installation of Figure 5.5 during flood; functions well under heavy debris flow. ....... 5.8
 

Figure 5.7. Steel rail debris deflector in area of heavy flowing debris (looking upstream). ........ 5.8
 

Figure 5.8. Timber pile debris deflector for boulders and large floating debris. ......................... 5.9
 

Figure 5.9. Timber pile debris deflector protected culvert during heavy floods. Nearby culverts 

without deflectors were plugged. ......................................................................... 5.10 

Figure 5.10. Rail debris rack over sloping inlet. Heavy debris and boulders ride over rack and 

leave flow to culvert unimpeded. ......................................................................... 5.11 

Figure 5.11. Post and rail debris rack, in place for 35 years, for light to medium floating debris 

installed 100 ft upstream of culvert...................................................................... 5.11
 

v 
Figure 5.12. Rail debris rack. ................................................................................................... 5.12 

Figure 5.13. Timber debris rack (note how suspended by cables). ......................................... 5.12 

Figure 5.14. Hinged steel debris rack in urban area. Due to nature of debris and possible entry 

by children, bar spacing is close. ........................................................................ 5.13 

Figure 5.15. Steel debris rack in urban area. ........................................................................... 5.13 

Figure 5.16. Debris rack used in State of Washington............................................................. 5.14 

Figure 5.17. Rail debris rack in arid region (see Figure 5.18). ................................................. 5.15 

Figure 5.18. Installation in Figure 5.17 after several years of fine silt deposition at entrance.. 5.15 

Figure 5.19. Steel rail debris rack. Note amount of debris accumulation in upstream channel. 

............................................................................................................................ 5.16 

Figure 5.20. Steel debris rack probably saved the culvert from plugging. ............................... 5.16 

Figure 5.21. Steel grill debris rack with provision for cleanout afforded by concrete paved area in 

foreground. .......................................................................................................... 5.17 

Figure 5.22. Steel grill debris rack on slope mitered culvert entrance. .................................... 5.17 

Figure 5.23. Metal pipe debris riser in basin (note anti-vortex device on top). ........................ 5.18 

Figure 5.24. Metal pipe debris riser placed during initial construction of culvert provides relief in 

case the culvert entrance becomes plugged (see Figure 5.25). ......................... 5.18 

Figure 5.25. Installation shown in Figure 5.24 after flood. Riser conveyed large flows during 

flood. Fence partially surrounding riser was of no value for debris control. ........ 5.19 

Figure 5.26. Debris crib of precast concrete sections and metal dowels. Height increased by 

extending dowels and adding more sections. ..................................................... 5.19 

Figure 5.27. Arid region debris crib of precast concrete sections and metal dowels. .............. 5.20 

Figure 5.28. Redwood debris crib with spacing to prevent passage of fine material. Basin had 

buildup of 30 feet. ................................................................................................ 5.20 

Figure 5.29. Concrete debris fins with sloping leading edge as extension of culvert walls...... 5.21 

Figure 5.30. Concrete debris fin with sloping leading edge as extension of center wall. ......... 5.21 

Figure 5.31. Concrete debris fin with rounded vertical leading edge as extension of culvert 

center wall. .......................................................................................................... 5.22 

Figure 5.32. Combined installation of concrete debris fin and metal pipe debris riser with single 

corrugated metal pipe culvert (looking downstream)........................................... 5.23 

Figure 5.33. Concrete debris fin for single culvert (Prefer more area between wingwalls and fin). 

............................................................................................................................ 5.23 

Figure 5.34. Debris fin and metal pipe debris riser in conjunction with single barrel culvert.... 5.24 

Figure 5.35. Timber debris fins with sloping leading edge. ...................................................... 5.24 

Figure 5.36. Metal bin type debris dam.................................................................................... 5.25 

Figure 5.37. Gabion debris dam. ............................................................................................. 5.25 

Figure 5.38. Debris dam of precast concrete sections fabricated to enable placement in 

interlocking fashion.............................................................................................. 5.26 

Figure 5.39. Debris dam of precast concrete sections fabricated to enable placement in 

interlocking fashion.............................................................................................. 5.26 

Figure 5.40. Debris dam and basin along with steel debris rack over culvert entrance in 

foreground. A metal pipe riser is visible over the spillway. .................................. 5.27 

Figure 5.41. Bendway wiers on outer bank of Hatchie River looking upstream (TDOT).......... 5.27 

Figure 5.42. Kellner jacks used for redirecting the flow patterns. ............................................ 5.28 

Figure 5.43. Debris deflectors installed at State Route 59 south crossing of the Eel River in 

central Indiana. .................................................................................................... 5.28
 

vi 
Figure 5.44. Debris sweeper being installed on a bridge over Staunton River in Altavista, 

Virginia. ............................................................................................................... 5.29 

Figure 5.45. Close up of a debris sweeper installed on the Cedar Creek in Washington. ....... 5.29 

Figure 5.46. Close up of a debris sweeper installed on the South Fork Obion River in 

Tennessee........................................................................................................... 5.30 

Figure 5.47. Close up of double-stacked installation debris sweeper on Interstate 24 over the 

Mississippi River.................................................................................................. 5.30
 

Figure 6.1. Debris deflector designed in example.................................................................... 6.14
 

vii 
LIST OF TABLES 

able 1.1. Commonly Used Engineering Terms in SI and CU Units. ........................................ 1.3

able 3.1. Major Phases and Tasks in Evaluating Debris Accumulation Potential at a Bridge.. 3.2

able 3.2. Determining Potential for Debris Accumulation across a Span or Vertical Gap...... 3.10

able 3.3. Determining Potential for Debris Accumulation on a Single Pier. ........................... 3.10

able 3.4. Maximum Extent of Debris Accumulation. .............................................................. 3.11

able 4.1. Drag Coefficient for Debris on Piers. ...................................................................... 4.12

able 4.2. Drag Coefficient for Debris on Superstructure. ....................................................... 4.12

able 4.3. Results of Hydraulic Calculations for Example 1. ................................................... 4.16

able 4.4. Results of Hydraulic Calculations for Example 2. ................................................... 4.20

able 4.5. Results of Hydraulic Calculations for Example 3. ................................................... 4.25

able 4.6. Results of Hydraulic Calculations for Example 4. ................................................... 4.29

able 4.7. Results of Hydraulic Calculations for Example 5. ................................................... 4.33

able 4.8. Results of Hydraulic Calculations for Example 6. ................................................... 4.38

able 6.1. Debris-Control Countermeasures Matrix. ................................................................. 6.5

able 6.2. Management Plan for the Large Woody Debris Formations. .................................. 6.28

viii 

T 
 

T 
 

T 
 

T 
 

T 
 

T 
 

T 
 

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T 
 

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LIST OF SYMBOLS 

A 

AB 

Abu 

Abd 

Ac 

= 

= 

= 

= 

= 

Cross sectional area of flow, m2

 (ft2) 

Net area of the bridge opening, m2

 (ft2) 

Net area of the bridge opening at the upstream face of the bridge, m2

 (ft2) 

Net area of the bridge opening at the downstream face of the bridge, m2

 (ft2) 

Unobstructed cross-sectional flow area in the contracted section, m2

 (ft2) 

Ad = Cross-sectional flow area blocked by debris in the contracted bridge 

 section, m2

 (ft2) 

AD = Area of wetted debris based on the upstream water surface elevation projected 

normal to the flow direction, m2

 (ft2) 

Ahd = Area of the vertically projected, submerged portion of the debris accumulation 

below the downstream water surface, m2

 (ft2) 

Ahu 

Ao, Ai 

Ao 

Arack 

= 

= 

= 

= 

Area of the vertically projected, submerged portion of the debris accumulation 

below the upstream water surface, m2

 (ft2) 

Outlet and inlet storm drain cross-sectional areas, m2

 (ft2) 

Orifice area, m2

 (ft2) 

Area of debris rack, m2

 (ft2) 

B = Blockage ratio 

C = Expansion and contraction loss coefficients 

Cg = Discharge coefficient for sluice gate type of pressure flow 

Cd = Discharge coefficient for fully submerged pressure flow 

CD = Drag coefficient 

Cw = Discharge coefficient for weir flow 

DBEL = Bottom elevation of the debris accumulation, m (ft) 

D = Culvert diameter, m (ft) 

DHW = Design high water elevation, m (ft) 

Do = Outlet pipe diameter, m (ft) 

D50 = Mean riprap size, m (ft) 

Et = Total energy, m (ft) 

F = Total segment force on the bridge structure, N (lbs) 

FD = Drag force, N (lbs) 

FDEL = Elevation of drag force on the bridge structure, m (ft) 

FDST = Station of drag force on the bridge structure, m (ft) 

FEL = Elevation of total segment force on the bridge structure, m (ft) 

Ff = External force due to friction, N (lbs) 

Fh = Total hydrostatic force on the bridge structure, N (lbs) 

Fhd = Hydrostatic force on downstream side of the bridge structure, N (lbs) 

FhEL = Elevation of hydrostatic force on the bridge structure, m (ft) 

Fhu = Hydrostatic force on upstream side of the bridge structure, N (lbs) 

FhST = Station of hydrostatic force on the bridge structure, m (ft) 

Fr = Froude number 

FST 

g 

= 

= 

Station of total force on the bridge structure, m (ft) 

Acceleration due to gravity, 9.81 m/s2

 (32.2 ft/s2) 

h = Vertical distance from water surface to center of gravity of flow area, m (ft) 

ix 
 

 

 

 

 

 

 

 

 

hf = Friction loss, m (ft) 

hv = Velocity head, m (ft) 

H = Increase in water surface elevation from the downstream side to the upstream 

side of the bridge, m (ft) 

H = The difference in the upstream energy gradient elevation and the downstream 

water surface elevation, m (ft) 

H = Height of debris-control structure, m (ft) 

hcu = Vertical distance from the upstream water surface to the centroid of 

 area Ahu, m (ft) 

hcd = Vertical distance from the downstream water surface to the centroid of 

 area Ahd, m (ft) 

HGLi = Hydraulic grade line elevation at the inflow pipe, m (ft) 

HGLo = Hydraulic grade line elevation relative to the outlet pipe invert, m (ft) 

hL = Energy head loss, m (ft) 

Hw = Difference between the upstream energy and the road crest, m (ft) 

INV 

K 

= 

= 

Inlet invert elevation, m (ft) 

Conveyance, m3

/s (ft3/s) 

K = Yarnell’s pier shape coefficient 

Kc = Units conversion factor or coefficient 

Ke = Expansion coefficient 

L = Horizontal length of curve, flow length, length of basin at base length of pipe, or 

length of culvert, m (ft) 

Lw = Effective length of the weir, m (ft) 

n = Manning's roughness coefficient 

P = Wetted perimeter, m (ft) 

P 

Q 

Q 

Qw 

= 

= 

= 

= 

Hydrostatic pressure force, N (lbs) 

Flow, m3

/s (ft3/s) 

Total discharge through the bridge opening, m3

/s (ft3/s) 

Total discharge over the roadway approaches and the bridge, m3

/s (ft3/s) 

R = Hydraulic radius (flow area divided by the wetted perimeter), m (ft) 

s = Spacing between the bars of a debris-control structure, m (ft) 

S = Surface slope, m/m (ft/ft) 

Sf = Friction slope, m/m (ft/ft) 

SL = Longitudinal slope, m/m (ft/ft) 

So = Energy grade line slope, m/m (ft/ft) 

SL = Main channel slope, m/km (ft/mi) 

t = Bar thickness, m (ft) 

T = Surface width of open channel flow, m (ft) 

V 

V 

= 

= 

Mean velocity, m/s (ft/s) 

Storage volume, m3

 (ft3) 

Vr = Reference velocity, m/s (ft/s) 

V2 = Mean velocity for the cross-section at the downstream side of the 

 bridge, m/s (ft/s) 

V3 = Average flow velocity at the cross section immediately upstream of the 

 bridge, m/s (ft/s) 

y = Flow depth, m (ft) 

x 
yr = Average flow depth corresponding with the reference velocity, m (ft) 

Y3 = Hydraulic depth at the cross section immediately upstream of the bridge, m (ft) 

w = Width of debris-control structure, m (ft) 

W = Force due to weight of water in the direction of flow, N (lbs) 

WD = Width of debris accumulation defined by design log length, m (ft) 

WSDS = Water surface elevation downstream of the bridge, m (ft) 

Wmin = Minimum width of debris rack, m (ft) 

WSUS = Water surface elevation upstream of the bridge, m (ft) 

Z = Elevation above a given datum, m (ft) 

z = Horizontal distance for side slope of trapezoidal channel, m (ft) 

γ = Specific weight of water, 9810 N/m3

 (62.4 lb/ft3) at 15.6 EC (60 EF) 

γs = Specific weight of sediment particle, N/m3

 (lb/ft3) 

τ

ρ

= 

= 

Average shear stress, Pa (lb/ft2

) 

Fluid density, kg/m3

 (slugs/ft3) 

ω = Ratio of velocity head to depth for the cross-section at the downstream side of 

the bridge 

α = Obstructed area of the piers divided by the total unobstructed area for the cross 

section at the downstream side of the bridge 

α = Apex angle for a culvert debris deflector, degrees 

xi 
ACKNOWLEDGMENTS
 

First Edition 

Mr. G. Reihsen, Highway Engineer with the Federal Highway Administration wrote the first 

edition of this Hydraulic Engineering Circular. The manual was dated February 1964. 

Second Edition 

Messrs. G. Reihsen and L.J. Harrison, FHWA revised the first edition in March 1971. In that 

second edition, the authors incorporated comments from practitioners and added additional 

remarks on safety. 

Third Edition 

This third edition substantially revises these earlier HEC-9 editions, adding new information, 

while keeping important portions of the original documents. The authors of this third edition wish 

to acknowledge the contributions made by the authors of earlier editions. 

The writers wish to also acknowledge the technical assistance of Dr. Arthur Parola, Professor at 

University of Louisville. Finally, the authors wish to thank everyone at FHWA who was involved 

in the preparation of this manual. 

xii 
 

GLOSSARY
 

abutment: The structural support at either end of a bridge usually classified as 

spill-through or vertical. 

aggradation: General and progressive buildup of the longitudinal profile of a channel 

bed due to sediment deposition. 

alluvium: Unconsolidated material deposited

floodplain, alluvial fan, or delta. 

by a stream in a channel, 

average velocity: Velocity at a given cross section determined by dividing discharge by 

cross-sectional area. 

backwater: The increase in water surface elevation relative to the elevation 

occurring under natural channel and floodplain conditions. It is induced 

by a bridge or other structure that obstructs or constricts the otherwise 

unobstructed flow of water in a channel. 

backwater area: The low-lying lands adjacent to a stream that may become flooded due 

to backwater. 

bank: The side slopes of a channel between which the flow is normally 

confined. 

bank, left (right): The side of a channel as viewed in a downstream direction. 

bankfull discharge: Discharge that, on the average, fills a channel to the point of overflow. 

bar: An elongated deposit of alluvium within a channel, not permanently 

vegetated. 

bed: The bottom of a channel bounded by banks. 

bed load: Sediment that is transported in a stream by rolling, sliding, or skipping 

along the bed or very close to it; considered to be within the bed layer. 

bed material: Material found on the bed of a stream (May be transported as bed load 

or in suspension). 

boulder: A rock whose diameter is greater than 250 mm. 

xiii 
bridge opening: The cross sectional area beneath a bridge 

conveyance of water. 

that is available for 

bridge waterway: The area of a bridge opening available for flow, as measured below a 

specified stage and normal to the principal direction of flow. 

channel: The bed and banks that confine the surface flow of a stream. 

channelization: Straightening or deepening of a natural channel by artificial cutoffs, 

grading, flow-control measures, or diversion of flow into a man-made 

channel. 

clay: A particle whose diameter is in the range of 0.00024 to 0.004 mm. 

cobble: A rock whose diameter is in the range of 64 to 250 mm. 

constriction: A natural or artificial control section, such as a bridge crossing, channel 

reach or dam, with limited flow capacity in which the upstream water 

surface elevation is related to discharge. 

contraction: The effect of a natural or man-made channel constriction on flow 

streamlines. 

countermeasure: A measure intended to prevent, delay or reduce the severity of stream 

or river problems. 

contraction scour: Contraction scour, in a natural channel or at a bridge crossing, involves 

the removal of material from the bed and banks across all or most of 

the channel width. This component of scour results from a contraction 

of the flow area at the bridge which causes an increase in velocity and 

shear stress on the bed at the bridge. The contraction can be caused 

by the bridge or from a natural narrowing of the stream channel. 

cross section: A section normal to the trend of a channel or flow. 

culvert: A drainage conduit that conveys flow from one side of an embankment 

to the other. 

dam jam: A type of debris jam that extends entirely across the channel as a 

result of the debris length being approximately equal to the channel 

width. 

xiv 
 

debris: Floating or submerged material, such as logs, vegetation, or trash, 

transported by a stream. 

debris accumulation: The collection of debris material on a fixed object. 

debris cribs: Open crib-type structures placed vertically over the culvert inlet in logcabin fashion to prevent inflow of coarse bedload and light floating 

debris. 

debris dams and 

basins: 

Structures placed across well-defined channels to form basins that 

impede the streamflow and provide storage space for deposits of 

detritus and debris. 

debris deflectors: Structures placed at the culvert inlet to deflect the major portion of the 

debris away from the culvert entrance. 

debris fins: Walls built in the stream channel upstream of a culvert or bridge. Their 

purpose is to align debris, such as logs, with the axis of the culvert or 

bridge so that the debris will move through the culvert or bridge 

opening. 

debris jam: Accumulation of floating or neutrally buoyant debris material formed 

around large, whole trees that may be anchored to the bed or banks at 

one or both ends, once in the stream system. 

debris racks: Structures placed across the stream channel to collect the debris 

before it reaches the culvert entrance. Debris racks are usually vertical 

and at right angles to the streamflow, but they may be skewed with the 

flow or inclined with the vertical. 

debris risers: A closed-type structure placed directly over the culvert inlet to cause 

deposition of lowing debris and fine detritus before it reaches the 

culvert inlet. 

deflector jam: A type of debris jam that redirects the flows to one or both of the banks. 

These types of jams usually occur when the channel width is slightly 

greater than the average tree height. 

degradation (bed): A general and progressive (long term) lowering of the channel bed due 

to erosion over a relatively long channel length. 

xv 
design log length:	 A length above which logs are insufficiently abundant and insufficiently 

strong throughout their full length to produce an accumulation equal to 

their length. This length does not represent the absolute maximum 

length of trees within the watershed upstream of the site. 

detritus: Non-debris sediment or bed load characterized as fine or course. Fine 

detritus is a fairly uniform bed load of silt, sand, gravel more or less 

devoid of floating debris, tending to deposit upon diminution of velocity. 

Coarse detritus consists of coarse gravel or rock fragments. 

dike:	 An impermeable or semi-permeable linear structure for the control or 

containment of overbank flow. A dike-trending parallel with a 

streambank differs from a levee in that it extends for a much shorter 

distance along the bank, and it may be surrounded by water during 

floods. 

dike (groin, spur, jetty): A structure extending from a bank into a channel that is designed to: 

(a) reduce the stream velocity as the current passes through the dike, 

thus encouraging sediment deposition along the bank (permeable 

dike); or (b) deflect erosive current away from the streambank 

(impermeable dike). 

drift: Alternative term for "debris" that is floating on or through a river. 

eddy current:	 A vortex-type motion of a fluid flowing contrary to the main current, 

such as the rotational water movement that occurs when the main flow 

becomes separated from the bank. 

effective length 

of debris: 

The length of the debris element that can support the load of the debris 

accumulation. 

ephemeral stream:	 A stream or reach of stream that does not flow for parts of the year. As 

used here, the term includes intermittent streams with flow less than 

perennial. 

erosion:	 Displacement of soil particles on the land surface due to water or wind 

action. 

floodplain:	 A nearly flat, alluvial lowland bordering a stream, that is subject to 

frequent inundation by floods. 

xvi
 

flow-control structure: A structure either within or outside a channel that acts as a 

countermeasure by controlling the direction, depth, or velocity of 

flowing water. 

Froude number: A dimensionless number that represents the ratio of inertial to 

gravitational fluid forces. High Froude numbers can be indicative of 

high flow velocity and the potential for scour. 

geomorphology 

/morphology: 

That science that deals with the form of the Earth, the general 

configuration of its surface, and the changes that take place due to 

erosion and deposition. 

gravel: A rock fragment with a diameter ranging from 2 to 64 mm. 

groin: A structure extending from the bank of a stream in a direction 

transverse to the current. Many names are given to this structure, the 

most common being "spur," "spur dike," "transverse dike," "jetty," etc. 

Groins may be permeable, semi-permeable, or impermeable. 

guide bank: An embankment extending from the approach embankment at either or 

both sides of the bridge opening to direct the flow through the opening. 

Some guide banks extend downstream from the bridge (also see spur 

dike). 

helical flow: Three-dimensional movement of water particles along a spiral path in 

the general direction of flow. These secondary-type currents are of 

most significance as flow passes through a bend; their net effect is to 

remove soil particles from the cut bank and deposit this material on the 

point bar. 

hydraulic problem: An effect of stream flow, tidal flow, or wave action such that the 

integrity of the highway facility is destroyed, damaged, or endangered. 

hydraulic radius: The ratio of a channel’s cross sectional area to its wetted perimeter. 

ice debris: Accumulation or transport of ice floes in the waterway. 

inlet: Entrance of the culvert at the upstream end. 

xvii 
island: A permanently vegetated area, emergent at normal stage that divides 

the flow of a stream. Islands originate by establishment of vegetation 

on a bar, by channel avulsion, or at the junction of a minor tributary 

with a larger stream. 

large floating debris: Type of debris consisting of trees, logs, and other organic matter with a 

length greater than 3.5 feet. Also referred to as Large Woody Debris 

(LWD) 

lateral erosion: Erosion in which the removal of material is progressing primarily in a 

lateral direction, as contrasted with degradation and scour that 

progress primarily in a vertical direction. 

light floating debris: Type of debris consisting of small limbs or sticks, orchard prunings, 

tules, and refuse. 

levee: An embankment, generally landward of top bank, that confines flow 

during high-water periods, thus preventing flooding into lowlands. 

local scour: Removal of material from around piers, abutments, spurs, and 

embankments caused by an acceleration of flow and resulting vortices 

induced by obstructions to the flow. 

medium floating 

debris: 

Type of debris consisting of tree limbs, and large sticks. 

mid-channel bar: A bar lacking permanent vegetal cover that divides the flow in a 

channel at normal stage. 

migration: Change in position of a channel by lateral erosion of one bank and 

simultaneous accretion of the opposite bank. 

outlet The downstream end of a culvert. 

overbank flow: Water movement that overtops the bank either due to stream stage or 

to overland surface water runoff. 

parallel jam: A type of debris jam that is oriented parallel to the flow. These types of 

jams usually occur when the channel width is significantly greater than 

the maximum debris length. 

xviii 
perennial stream: A stream or reach of a stream that flows continuously for all or most of 

the year. 

pressure flow scour: The increase in local scour at a pier subjected to pressure (or orifice) 

flow as a result of flow being directed downward towards the bed by 

the superstructure (vertical contraction of the flow) and by increasing 

the intensity of the horseshoe vortex. The vertical contraction of the 

flow can be the more significant cause of the increased scour depth. 

reach: A segment of stream length that is arbitrarily bounded for purposes of 

study. 

revetment: Rigid or flexible armor placed to inhibit scour and lateral erosion (see 

bank revetment). 

riprap: In the restricted sense, layer or facing of rock or concrete dumped or 

placed to protect a structure or embankment from erosion; also the 

broken rock or concrete suitable for such use. Riprap has also been 

applied to almost all kinds of armor, including wire-enclosed riprap, 

grouted riprap, sacked concrete, and concrete slabs. 

river training: Engineering works with or without the construction of embankment, 

built along a stream or reach of stream to direct or to lead the flow into 

a prescribed channel. Also, any structure configuration constructed in a 

stream or placed on, adjacent to, or in the vicinity of a streambank that 

is intended to deflect currents, induce sediment deposition, induce 

scour, or in some other way alter the flow and sediment regimes of the 

stream. 

roughness coefficient: Numerical measure of the frictional resistance to flow in a channel, as 

in the Manning’s or Chezy’s formulas. 

sand: A rock fragment whose diameter is in the range of 0.062 to 2.0 mm. 

scour: Erosion of streambed or bank material due to flowing water; often 

considered as being localized (see local scour, contraction scour, total 

scour). 

scoured depth: Total depth of the water from water surface to a scoured bed level 

(compare with "depth of scour"). 

sediment: Fragmental material transported, suspended, or deposited by water. 

xix 
sediment yield:	 The total sediment outflow from a watershed or a drainage area at a 

point of reference and in a specified time period. This outflow is equal 

to the sediment discharge from the drainage area. 

single pier Debris accumulation that occurs only on a single bridge pier as a result 

accumulation: of the maximum effective length of the floating debris being less than 

the effective opening between the bridge piers. 

slope (of channel or Fall per unit length along the channel centerline. 

stream): 

slope protection:	 Any measure such as riprap, paving, vegetation, revetment, brush or 

other material intended to protect a slope from erosion, slipping or 

caving, or to withstand external hydraulic pressure. 

span accumulation: Debris accumulation that accumulates across an entire span of a 

bridge structure as a result the length of floating debris exceeding the 

effective opening between piers. 

spill-through abutment: A bridge abutment having a fill slope on the streamward side. 

spur:	 A permeable or impermeable linear structure that projects into a 

channel from the bank to alter flow direction, induce deposition, or 

reduce flow velocity along the bank. 

spur dike:	 See guide bank. 

stable channel:	 A condition that exists when a stream has a bed slope and cross 

section which allows its channel to transport the water and sediment 

delivered from the upstream watershed without aggradation, 

degradation, or bank erosion (a graded stream). 

stage:	 Water surface elevation of a stream with respect to a reference 

elevation. 

stream:	 A body of water that may range in size from a large river to a small rill 

flowing in a channel. By extension, the term is sometimes applied to a 

natural channel or drainage course formed by flowing water whether it 

is occupied by water or not. 

xx 
streambank erosion:	 Removal of soil particles or a mass of particles from a bank surface 

due primarily to water flow in the channel. Other factors such as 

weathering, ice and debris abrasion, chemical reactions, and land use 

changes may also directly or indirectly lead to bank erosion. 

streambank failure:	 Sudden collapse of a bank due to an unstable condition such as due to 

removal of material at the toe of the bank by scour. 

streambank protection: Any technique used to prevent erosion or failure of a streambank. 

structural A structural component used to prevent, delay or reduce the severity of 

countermeasure: stream or river problems. 

substructure: The components of a bridge which includes all elements supporting the 

superstructure. Its purpose is to transfer the loads from the 

superstructure to the foundation soil or rock. 

superstructure: The entire portion of a bridge structure which primarily receives and 

supports traffic loads and in turn transfers these loads to the bridge 

substructure. 

subcritical, Open channel flow conditions with Froude Number less than and 

supercritical flow: greater than unity, respectively. 

thalweg:	 The line extending down a channel that follows the lowest elevation of 

the bed. 

toe of bank:	 That portion of a stream cross section where the lower bank terminates 

and the channel bottom or the opposite lower bank begins. 

toe protection:	 Loose stones laid or dumped at the toe of an embankment, groin, etc., 

or masonry or concrete wall built at the junction of the bank and the 

bed in channels or at extremities of hydraulic structures to counteract 

erosion. 

turbulence:	 Motion of fluids in which local velocities and pressures fluctuate 

irregularly in a random manner as opposed to laminar flow where all 

particles of the fluid move in distinct and separate lines. 

xxi 
underflow jam: A type of debris jam that exists near the bankfull level. These types of 

jams usually occur in small watersheds where the tree height is greater 

than the channel width. 

uniform flow: Flow of constant cross section and velocity through a reach of channel 

at a given instant. Both the energy slope and the water slope are equal 

to the bed slope under conditions of uniform flow. 

velocity: The rate of motion of a fluid in a stream or of the objects or particles 

transported therein, usually expressed in m/s (ft/s). The average 

velocity at a given cross section is determined by dividing discharge by 

cross-sectional area. 

Waterway opening 

width (area): 

Width (area) of bridge opening at (below) a specified stage, measured 

normal to the principal direction of flow. 

xxii 
CHAPTER 1 – INTRODUCTION 

1.1 PURPOSE 

The purpose of this Federal Highway Administration (FHWA) Hydraulic Engineering Circular 

(HEC) is to provide information on debris accumulation, guidelines for analyzing impacts 

associated with debris accumulation, and design guidelines for selecting debris control 

countermeasures. The design guidelines are based on countermeasures that have been 

implemented by federal, State, and local transportation agencies at culvert and bridge 

structures. 

1.2 BACKGROUND 

Debris accumulation at culvert and bridge structures openings is a widespread problem. The 

accumulation of debris at inlets of highway culverts and bridge structures is a frequent cause of 

unsatisfactory performance and malfunction. This accumulation may result in erosion at culvert 

entrances, overtopping and failure of roadway embankments and damage to adjacent 

properties, increased local scour at piers and/or abutments, and the formation of pressure flow 

scour. Consideration of debris accumulations and the need for debris-control structures should 

be an essential part of the design of all drainage structures. 

Structural and non-structural measures have been used effectively to prevent or reduce the size 

of debris accumulations at bridges and culverts. Structural measures can include features that: 

(a) intercept debris at or upstream of a structure inlet; (b) deflect debris near the inlet; or (c) 

orient the debris to facilitate passage of the debris through the structure. Non-structural 

measures include management of the upstream watershed and maintenance. This document 

provides measures for both culvert and bridge structures. The measures available for culverts 

are based on the information included in earlier editions of this manual. Selection of a certain 

debris countermeasure depends upon the size, quantity, and type of debris, the potential hazard 

to life and property, the costs involved, and the maintenance proposed. 

1.3 DOCUMENT ORGANIZATION 

This HEC is organized to provide the following: 

•	 Summarize the various types of debris and the problems associated with debris 

accumulation at culvert and bridge structures (CHAPTER 2) 

•	 Provide a general procedure for estimating the volume of floating debris upstream of a 

bridge site, the potential for the debris to accumulate on a bridge structure, and the 

potential maximum size of the debris accumulation (CHAPTER 3). 

1.1
 
•	 Provide general guidelines for analyzing and modeling debris accumulations on a bridge 

structure to determine the impacts the debris would have on the water surface profile 

through the bridge structure and the hydraulic loading on the structure (CHAPTER 4). 

•	 Summarize (describe) the various types of debris countermeasures available for culvert 

and bridge structures (CHAPTER 5). 

•	 Provide general criteria for selection of debris countermeasures for culvert and bridge 

structures, and provide design guidelines for structural countermeasures for which 

guidelines are available (CHAPTER 6). 

•	 Provide general information on maintenance practices (CHAPTER 7). 

•	 Provide references and source materials that provide additional, more comprehensive 

information on debris and debris issues. The references are grouped alphabetically, by 

author. 

•	 Provide a synopsis of a survey of State Department of Transportation and American 

Association of State Highway Transportation Officials (AASHTO) debris issues and 

mitigation practices. The survey was conducted as part of the effort to update this 

document. 

1.4 DUAL SYSTEM OF UNITS 

This edition of HEC-9 uses dual units, SI metric and English. The “English” system of units as 

used throughout this manual refers to U.S. Customary (CU) units. An explanation of the 

metric (SI) unit of measurement is provided in Appendix A. This appendix also contains 

conversion factors, physical properties of water in the SI and CU units, sediment particle 

size grade scale, and some common equivalent hydraulic units. 

This edition uses the meter (m) or foot (ft) for the unit of length; kilogram (kg) or slug for the unit 

of mass; Newton (N) or pound (lb) for the unit of weight/force; Pascal (Pa, N/m2

) or pounds per 

square foot (lb/ft2

) for the units of pressure; degree Centigrade (°C) or Fahrenheit (°F) for the 

unit of temperature. The unit of time is the same in both systems. The value of some of the 

common engineering terms used in this reference in SI units and their equivalent English Units 

are given in Table 1.1. 

1.2
 
Table 1.1. Commonly Used Engineering Terms in SI and CU Units. 

Term SI Units English Units 

Length 1 m 3.28 ft 

Volume 1 m3

 35.31 ft3 

Discharge 1m3/s 35.31 ft3/s 

Acceleration of Gravity 9.81 m/s2

 32.2 ft/s2 

Unit Weight of Water 9810 N/m3 62.4 lb/ft3 

Density of Water 1000 kg/m3 1.94 slugs/ft3 

Density of Quartz 2647 kg/m3 5.14 slugs/ft3 

Specific Gravity of Quartz 2.65 2.65 

Specific Gravity of Water 1 1 

Temperature  ̊C = 5/9 (°F – 32) °F 

1.3
 
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1.4
CHAPTER 2 – DEBRIS CHARACTERIZATION
 

2.1 DEBRIS CLASSIFICATION 

Flood flow reaching a culvert or bridge structure typically carries floating as well as submerged 

debris. As discussed in further detail in the next section of this manual, debris should be a 

concern to highway engineers because it can accumulate at and obstruct the waterway 

entrance of culverts or bridges, adversely affect the operation of the structure or cause failure of 

the structure.(14,17,40,47,49,50) A thorough study of the supply of debris originating in the drainage 

basin is essential for proper design of a drainage structure. 

2.1.1 Types of Debris 

The selection of a debris countermeasure depends on the type of debris transported to the site; 

therefore, the various types of debris should be defined and classified to assist in the selection 

of an effective debris countermeasure. This current edition retains, but slightly modifies, the 

classification system used in earlier editions. This classification is presented as follows: 

Very Small Buoyant Debris or No Debris. 

Small Floating Debris. Small floating debris includes small limbs or sticks, orchard prunings, 

tules, leaves, and refuse. This material can be easily transported by the stream and overland 

flow. Therefore, this type of debris can be introduced into the stream from the local runoff from a 

watershed, and then easily transported downstream by the stream flows. This type of debris 

also comes from tree and vegetation that are introduced into the stream due to bank erosion, 

landmass failures, wind action or collapsing due to biological factors such as decay and old age, 

or from the loss of foliage during the changing of seasons. There are usually no significant 

problems associated with this type of debris at bridge structures; however, it is an important 

component in the development of mature debris jams of large floating debris, and it could 

accumulate at and clog culvert structures. 

Medium Floating Debris. Medium floating debris consists of tree limbs or large sticks. The 

source of this material comes from trees introduced into the stream by bank erosion, mass 

wasting, windthrow, or collapsing of trees due to ice loading, beaver activities, or biological 

factors such as old age and diseases; or from erosion of emergent and riparian trees within the 

streams. Vegetation within the floodplain could also be a source of this type of debris. This type 

of debris could accumulate at both culvert and bridge structures. 

Large Floating Debris. Large floating debris consists of logs or trees (such large floating debris 

is also known as “drift”). The sources of this type of debris are the same sources discussed for 

“Medium Floating Debris”. Transport and storage of this material depends on discharge, 

channel characteristics, the size of the drift pieces relative to the channel dimensions, and the 

hydraulic characteristics (depth and slope) of the system.(17) In small and intermediate size 

2.1
 
streams, this material is not easily transported, and it is usually transported during larger floods 

or prolonged periods of high water.(17) Once introduced into the main channel of these small 

and intermediate size streams, this material can form into a jam, which is a collection of debris 

formed around large, whole trees that may be anchored to the bed or banks at one or both 

ends.(17,61) Larger streams and rivers do not store much of this material within the channel. 

During most flood events, a larger stream will transport nearly all large floating debris entering 

the reach.(17) The size of the jam depends on the type of vegetation existing within the 

watershed and the channel characteristics transporting the material. This type of debris causes 

a significant problem at bridge structures because of its size, shape, and facility for entrapment 

on bridge piers. 

Fine Detritus. Fine detritus consists of silt, sand, and fine gravel more or less devoid of floating 

debris. The size of this material ranges from 0.004 to 8 mm (0.00016 to 0.31 inches). This type 

of debris is transported along the bed and in the water column above the bed, i.e., as bed and 

suspended load. The source of this material is from sheet and rill erosion, gully erosion, 

landmass movement, and channel and bank erosion. Sediment yield rates for this material can 

be significantly influenced by the conditions of and changes within the watershed (e.g., 

urbanization, fire, etc.). Deposition of fine sediment could possibly block a culvert structure or 

significantly reduce the waterway opening through a bridge structure. 

Coarse Detritus. Coarse detritus consists of coarse gravel or rock ranging in size from 16 to 

256 mm (0.6 to 10 inches). The source of this material is from bed and/or bank erosion, gully 

erosion, or landmass movement. This material is usually transported as bed load, however it 

can be transported as both bed and suspended load within high gradient streams or gullies. 

Course detritus deposition can easily block a culvert entrance or significantly reduce a bridge 

waterway opening. 

Boulders. Material comprised of large rock ranging in size from 256 to 2048 mm (0.84 to 6.7 

feet). This type of material is usually associated with steep mountain streams or gullies, and it is 

transported as bed load. The source of the boulders is from bed and/or bank erosion or 

landmass movements. This material can easily block the entrance to a culvert and/or cause 

damage to the bridge piers from the impact forces. 

Flowing Debris. Flowing debris is a heterogeneous fluid mass of clay, silt, sand, gravel, rock, 

refuse, trees, and/or branches. In general, it is a combination of the different types of debris 

mentioned above. 

Ice Debris. Ice debris is accumulation or transport of ice floes in the waterway. This edition of 

the document does not describe or characterize this type of debris in any detail. In future 

editions, FHWA intends to add supplementary information based on results of on-going 

research efforts. 

2.2
 
2.2 FLOW BEHAVIOR OF FLOATING DEBRIS 

A brief discussion on the flow behavior of large floating debris is provided in the previous 

section. A more detailed discussion of the subject is provided in this section because it is an 

important concept to consider for evaluating the potential for debris accumulations at bridges 

and/or for developing watershed management plans for debris reduction. The potential for 

transport, mode of transport, formation of debris rafts, potential locations for debris 

accumulations, and general characteristics of debris accumulations at bridge structures are 

discussed in the remaining portions of this section. The information for these various topics was 

obtained from several different sources(14,17,18,43,59,63), but most of the information was obtained 

from the report prepared by Diehl.(17) 

2.2.1 Debris Transport and Transport Mechanisms 

The potential for transport depends on the discharge, channel characteristics, debris source 

availability, size of the floating debris relative to the channel dimensions, orientation of the 

debris relative to the channel alignment, and type of anchorage.(17,18,43) The potential for 

transport increases with increasing discharge due to the increase in the flow velocity, depth, and 

energy slope of the river.(17) Unfortunately at the present time, in most cases, there are no 

relationships available that define the minimum velocity and/or slope necessary to initiate 

transport of large floating debris. However, with respect to ice debris, the AASHTO LRFD Bridge 

Design Specifications describe several relationships regarding ice pressures and loads.(5) 

Ongoing research is investigating this and other ice debris issues. 

The width, depth, and slope of the channel are important channel characteristics that influence 

the potential for transport.(17) In general, this potential increases with increasing widths. 

Consequently the abundance of floating debris stored in the channel typically decreases with 

increasing widths. The length of the large floating debris transported increases with increasing 

channel widths.(17,63) Narrow streams rarely transport large floating debris, except for steep 

streams subject to debris torrents.(14,17) For narrow streams, trees and logs, (i.e., large floating 

debris) are usually longer than the width of the channel, so they typically become lodged across 

the channel, and rarely move without being broken into smaller pieces.(17) For most 

intermediate size streams, only some of the large floating debris is transported during large 

floods since most of it accumulates within the channel to form sizable debris jams.(17) 

Furthermore, for rivers and wide streams with adequate flow depth, nearly all of the large 

floating debris introduced into the main channel is transported by frequent flood events.(14,17) 

The depth of flow within the channel has to be deep enough to buoy up large floating debris. 

The depth sufficient to float logs and large trees is about the diameter of the tree butt plus the 

distance the roots extend below the butt.(17) The potential for transport is higher for highgradient streams than it would be for low-gradient streams with the same channel dimensions, 

since the forces of flowing water on stored debris in low-gradient streams are less.(17) Stored 

debris can be abundant in large, low-gradient channels.(17) 

2.3
 
The ratio of the effective length of the tree to the width of the channel is an important factor in a 

waterway’s capacity to transport a particular size of debris.(17) The relation of the length of the 

debris pieces to the channel width is a primary indicator in defining the transport rate and the 

type and amount of debris stored in the channel. The potential for debris transport increases as 

the ratio of the debris length over the channel width decreases. For example, more debris would 

be transported in an intermediate size stream if the debris length is one-half of the channel 

width, than would be transported if the debris length equals the width of the channel. The 

maximum size of the debris is limited by the channel width; however, the amount of debris is 

limited by the supply of debris and the capability of the channel to transport the debris. 

The potential for transport is influenced by the orientation of the debris relative to the channel 

alignment and type of anchorage.(18) Isolated pieces are more likely to be transported than 

pieces within debris jams or clumps. Also, trees with the root mass oriented upstream are more 

likely to be transported than trees with the root mass downstream or near the stream bank since 

it is easier for the piece of debris to be rotated by the flow.(18) Debris that is anchored to the side 

and perpendicular to the flow would most likely remain in place and not be transported 

downstream until the debris is dislodged from the bank due to bank erosion. 

After mobilization has occurred, large floating debris is transported either by floating along the 

water surface or dragging along the bed. Observations noted by Diehl indicate that debris is 

typically transported on the surface as individual pieces aligned with the flow and traveling at 

about the same velocity as the average water velocity at the surface.(17) The results of a 

physical model study performed by Ng(43) agrees with Diehl’s observation. Floating debris can 

occasionally be transported in short-lived clumps that eventually get broken apart due to 

turbulence.(14,17) The debris typically concentrates in a path occupying only a small fraction of 

the channel width. This path is defined by the zone of convergence that exists in some channels 

near the thalweg of the streams where the flow is the deepest and fastest under some flow 

conditions.(17,43) The zone of surface convergence for a straight and curved channel is 

illustrated in Figure 2.1.(17) The flow patterns reflected in this figure are hypothetical flow patterns 

at a particular location in a single bend during bankfull flow conditions, and the flow patterns 

would be entirely different for larger flood flows, different radii of curvature, or different channel 

conditions. 

Observations noted by Chang(14) indicate that floating debris within straight reaches tends to 

move inward to the thalweg at the rising stage of a flood and outward to the banks at the 

receding stage. The reasons are unclear; however, it could possibly be related to changes in the 

direction of the secondary flow patterns within the channel. As noted by Ng(43), the opposite 

pattern can occur, i.e., outward to the banks during the rising and inward to the thalweg during 

the receding, if water leaves the channel and flows into the floodplains. Diehl’s(17) observations 

in curved channel reaches indicate that floating debris may be transported on the outside of the 

curve during both rising and falling conditions. He also observed that debris typically travels 

between the center of the channel and the outside bank rather than in contact with the bank 

vegetation. 

2.4
 
Straight Reach 

Curved Reach 

Figure 2.1. Hypothetical Patterns of secondary flow in straight and curved channels. 

2.2.2 Debris Jams and Debris Dams 

Floating debris introduced into the channel can form into debris jams or dams.(17,63) Debris jams 

are usually formed when large, whole trees are introduced into the channel and are anchored to 

the bed or banks at one or both ends. The large trees act as a filter by trapping smaller floating 

debris and possibly sediment. The size and location of a jam depend on the size of the stream 

and the size of the trees. In small streams, a fallen tree may not be readily transported. 

However, this tree may trap and accumulate smaller debris from upstream and form a debris 

jam. Conversely, larger rivers can readily transport the debris downstream so it may not be able 

to accumulate into a large jam. Most of the accumulations in large rivers occur on the channel 

margins or outside the channel on islands, in floodplain forests, and in sloughs.(17,63) 

As noted by Wallerstein and Thorne(63), debris jams influence the geomorphology of rivers by 

influencing the overall channel form (i.e., they distort pool-riffle sequence and gravel bar 

formation); by changing the channel topography (i.e., they influence the erosional and 

depositional processes and widen the channel through bank erosion); and by increasing the 

channel apparent roughness through increased energy dissipation and eddy formation. (Note 

energy dissipation cannot be increased in a river, it can only be redistributed. Instead of the 

energy being dissipated along the channel, it is dissipated in intense drops over and around the 

debris. Upstream, the damming effect reduces the dissipation along the backwater-affected 

reach). 

2.5
 
Wallerstein and Thorne(63) classified debris jams according to what they called “engineeringgeomorphic impacts” as follows: 

Underflow Jams occur in small watersheds where the fallen trees span the channel at bankfull 

level (define bankfull level in glossary). The in-channel geomorphic impact associated with this 

type of debris jams is minimal; however, local bed scour could occur under the jam during high 

flows. 

Dam Jams usually occur when the tree height is approximately equal to the channel width. 

These types of jams can cause significant localized bank erosion and bed scour due to the 

constriction in flow, and backwater effects upstream that could cause sediment deposition 

upstream of the jam. 

Deflector Jams usually occur when the channel width is slightly greater than the average tree 

height. They usually redirect the flow to one or both of the banks causing bed and bank erosion 

that could result in more trees being introduced into the river. They can also create backwater 

sediment wedges and downstream bars depending on the level of dissipation caused by the 

jam. 

Parallel Jams exist when the channel width is significantly greater than the debris length, and 

the flow is capable of rotating the debris so that it is parallel with the flow. Bank erosion and bed 

scour associated with these jams are usually minimal. Parallel jams could actually stabilize the 

bank toe and protect it from erosion, and they may also initiate or accelerate the formation of 

mid-channel and lateral bars. 

2.2.3 Debris Accumulation 

Floating debris can accumulate at various locations and at obstructions within the river such as 

bridge piers and abutments, mid-channel bars, point bars, island heads, the streambed, or in 

pools along the base on the outside bank of bends. Debris accumulations typically grow in the 

upstream direction through the accretion of additional floating debris and fine and coarse 

sediment.(17) The rate of accumulation depends on the concentration, defined as number of 

debris per length of channel, of floating debris that is being transported and the magnitude of 

the flood.(14) In general, debris accumulations occur most frequently and in the largest sizes 

where the path of floating debris encounters obstructions.(17) The potential for trapping of debris 

at a bridge structure can be aggravated by the location and type of bridge piers.(14,17) Multiple 

columns can act as a sieve unless exactly aligned with flow; however, alignment of piers to flow 

at all flood levels for which debris is transported is unlikely. The gaps between columns are 

narrow relative to length of the floating debris, resulting in a high potential for accumulating 

debris. Floating debris can become entangled in a group of columns in ways that are not 

possible for a single-column pier. Floating debris accumulations at bridges generally fall into two 

classes: single-pier accumulations and span-blockage accumulations.(17) 

2.6
 
 

Single-pier accumulation occurs when the maximum effective length of the floating debris is 

less than the effective opening between the bridge piers. The effective length of debris is the 

length of the debris element that can support the load of the debris accumulation. The effective 

opening corresponds to the distance between the piers normal to the approaching flow. The 

width of the opening can be determined by extending lines parallel to the approaching flow 

upstream from the nose of each pier and measuring the perpendicular distance between the two 

lines. As noted by Diehl, single-pier accumulations typically contain one or more trees extending 

the full width of the accumulation perpendicular to the approaching flow.(17) The full-width tree 

can be either at the surface or submerged and concealed beneath smaller floating debris. Pier 

placement is extremely important for this type of accumulation. Even if the span length is 

significantly greater than the maximum length of the floating debris, a pier located within the 

path of floating debris (Figure 2.2) can result in a high potential for accumulation at the pier.(17) 

Figure 2.2. Single pier debris accumulation (led to pier scour failure). 

Span-blockage accumulations occur when the length of floating debris exceeds the effective 

opening between piers, resulting in the floating debris resting against two piers (Figure 2.3). 

This type of accumulation can also exist between a pier and an abutment. A similar type of 

accumulation can occur between a pier and a bank or other large fixed object, such as boulders 

and trees that can support one end of the floating debris. Like the potential for single-pier 

accumulations, the potential for span blockages is influenced by pier placement.(17) 

2.7
 
Figure 2.3. Span blockage accumulation bridge failure. (Louisiana) 

As noted by Diehl, most large debris accumulations are similar in shape. Floating debris is 

initially trapped on the pier perpendicular to the approaching flow, but as the accumulation 

increases in size, debris accumulates parallel to the upstream edge of the accumulation. This 

process results in an accumulation with a curved upstream edge, and with the upstream nose of 

the accumulation raft near the thalweg, where most of the debris is transported. The 

accumulation is typically deepest at the piers that support them, and widest at the surface. The 

potential to achieve a roughly rectangular cross section from the bed to the water surface 

depends on the abundance of debris, prolonged periods of high water, or multiple floods without 

removal of the accumulation.(17) 

Debris accumulations initially form at the water surface, grow toward and eventually become 

part of the streambed. As the water surface increases during a flood, floating debris already 

existing on the bridge usually remains in place as additional floating debris accumulates at the 

water surface. When the flood subsides, the new accumulated debris usually slides downward 

until it rests on the bed or on the previous debris accumulation to form a solid mass with 

irregular protrusions around the base of the pier. 

2.3 PROBLEMS ASSOCIATED WITH DEBRIS 

There are various potential problems associated with debris accumulations. In general, debris 

accumulations can adversely impact the conveyance through a culvert or bridge structure, 

exacerbate the contraction and local scour at a bridge structure, increase the hydraulic loading 

2.8
 
on a bridge structure, and cause upstream flooding. Several failures of highway bridges, 

roadway embankments and highway culverts have been attributed at least in part to debris 

accumulation. Figure 2.4 and Figure 2.5 shows two bridge failures attributed to debris 

accumulation during flood events.(49, 53) 

Figure 2.4. Missouri Highway 113 bridge over Florida Creek near Skidmore, Missouri. 

Figure 2.5. Debris accumulation failure at bridge located in Oklahoma. 

2.9
 
 

Debris accumulation can partially or totally block the waterway opening for a culvert or bridge 

structure.(14,17,40,47,49,50) A massive accumulation of woody debris at a bridge is shown in Figure 

2.6. Blockage of large portions of the waterway opening will increase backwater elevations 

upstream, increase flow velocity through the contracted opening under the structure, and modify 

flow patterns.(14,17,49,50) The increase in backwater upstream significantly increases the upstream 

inundation boundaries. High velocity contracted flow and large water surface elevation 

differences from the upstream to downstream side of the bridge can cause high drag and 

hydrostatic forces on the structure that can cause structural failure and collapse. Flows 

increased in velocity by the obstruction of the waterway and deflected away from the main 

channel can cause severe erosion near abutments or stream banks. Reduction in bridge 

waterway opening by debris can also cause a reduction in the flow rate required to overtop and 

potentially damage bridge approach roadways and embankments. Large accumulations could 

adversely affect the flow patterns near the structure by creating a strong lateral flow across the 

river towards the adjacent piers or embankment fill at unanticipated and potentially severe 

angles of attack, resulting in deep local scour at piers or abutment embankment fill. (17,50,51) 

Figure 2.6. Debris accumulation at a bridge structure.(17) 

2.10
 
Debris accumulations can also exacerbate the scour near the culvert or bridge structure. Figure 

2.7 depicts streambank failure associated with a span blockage (already shown in Figure 2.3). 

As stated above, the blockage of flow area from debris accumulations can cause a significant 

increase in the flow velocities through the bridge structure. This increase in flow velocities and 

boundary shear stresses may cause an increase in the contraction scour through the bridge if 

the entire bridge opening is affected. 

Bank Failure Abutment Scour 

Figure 2.7. Debris induced bridge and bank failure structure. (Louisiana) 

A laboratory study performed by Dongol(20) showed that debris accumulations (simulated using 

cylindrical shaped, PVC disks) cause larger and deeper scour holes to develop as a result of the 

significant increase in the downward velocity below the debris and the increase in both the 

horseshoe vortex size and the contact area of the vortex. The increase in both the contraction 

and local scour near the bridge structure could possibly damage or cause failure of the structure 

due to undermining of the pier footing or the abutment toe. Unfortunately, there has been only 

limited research conducted on local scour at piers with debris accumulation. Therefore, the 

scour associated with debris accumulation is extremely difficult to assess with any reliability. 

Damage and failure of several bridges has been related to the increase in the hydraulic loading 

on the structures caused by debris accumulations.(14,17,40,47,49,50) Highway bridges partially or 

fully submerged during a flood event are subjected to various types of forces. These forces 

include hydrodynamic drag and side forces, hydrostatic forces, buoyant forces, hydrodynamiclift forces, and impact forces.(49) 

2.11
 
Hydrodynamic drag forces result from the reaction of the water as it flows around an object, 

and it acts parallel to the direction of flow. 

Side forces are similar to drag forces, but act perpendicular to the flow direction. 

Hydrostatic forces on the bridge elements are related to the differential in water surface 

elevations at the upstream and downstream sides of the structure caused by the flow 

constriction through the bridge. 

Buoyant forces result from the displacement of water by the bridge or by the debris lodged 

under the bridge. 

Hydrodynamic-lift forces are related to the total dynamic pressure force acting in the vertical 

direction perpendicular to the flow direction and the side force. 

Impact forces are related to the moving debris colliding with the bridge structure. 

Debris accumulations cause an increase in these forces due to the increase in upstream water 

surface elevations, increase in the flow velocities through the bridge, and increase in projected 

area of these forces on the structure.(49) Increases of these forces may cause the bridge 

structure to collapse either by buckling of the bridge substructures, shearing of roadway deck 

supports, or overturning of the structure (see Figure 2.8).(49,50) 

Figure 2.8. Effects of debris accumulation at a bridge structure. (New York) 

2.12
 
Miscellaneous problems associated with debris accumulation include difficult and expensive 

maintenance programs required for debris removal, an increase in fire potential near the 

structure, and minor damage to the structure.(14) 

2.13
 
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2.14
CHAPTER 3 – ESTIMATING DEBRIS QUANTITIES 

3.1 INTRODUCTION 

The information presented in this Chapter focuses mainly on floating debris. This chapter does 

not address sediment yields and transport rates for fine and coarse sediment that is thoroughly 

documented in several references.(24,34,38,54,58,65) Most of the information presented in this 

chapter is based on a detailed study conducted by the U.S. Geological Survey (USGS), in 

cooperation with the FHWA. The study included an analysis of data from 2,577 reported debris 

accumulations and field investigations of 144 debris accumulations. Guidelines for the 

assessment of debris potential in the form of a detailed assessment method was proposed as a 

result of the study by Diehl.(17) The use of these guidelines requires familiarity with the specific 

regional characteristics of the local stream morphology and debris loading characteristics. As in 

all aspects of river problems, familiarity with the historical land-use activities, geologic and 

climactic conditions and the way in which these factors affect stream morphology and the debris 

loads in streams is imperative for making effective management decisions about debris 

production. 

The evaluation of debris accumulation on bridges has been separated into three major phases 

by Diehl:(17) 

1) Estimate the potential for debris delivery to the site, 

2) Estimate the debris accumulation potential on an individual bridge element, and 

3) Calculate hypothetical accumulations for the entire bridge. 

These phases can be further subdivided into eight tasks (Table 3.1). Each of these three 

phases is discussed in detail, even though the last phase is based on a qualitative methodology. 

It is presented because it is the most thorough information available on the subject matter. 

However, caution and a familiarity with the specific regional characteristics of the local 

stream morphology and debris loading characteristics should be used when applying 

this information. Simple examination of debris accumulations by state DOT’s during 

maintenance for important, but easily measured parameters outlined in the following procedure 

would provide specific local information necessary for improved future debris accumulation 

estimates. Although debris problems are widespread, conditions and parameters controlling 

debris production are specific to the local watershed conditions. 

As indicated in Table 3.1, estimating the volume of large debris within the watershed is not a 

major phase required in evaluating the potential for debris accumulations at a bridge. This 

information, however, is beneficial in developing a better understanding of the debris dynamics 

within the watershed, provide an indication of the overall debris conditions within the watershed, 

and provide an indication on the potential for debris being delivered to a structure within the 

watershed. Therefore, Diehl’s and Bryan’s(18) procedure for estimating the volume of large 

debris within the watershed is also presented within this chapter. 

3.1
 
Table 3.1. Major Phases and Tasks in Evaluating Debris Accumulation Potential at a Bridge. 

Major Phase Task 

1. Estimate potential for debris delivery 

a. 

b. 

c. 

Estimate potential for debris delivery to the 

site. 

Estimate size of largest debris delivered. 

Assign location categories to all parts of the 

highway crossing. 

2. Estimate debris potential on individual 

bridge elements (i.e., piers, 

abutments, etc.) 

a. 

b. 

Assign bridge characteristics to all immersed 

parts of the bridge 

Determine accumulation potential for each 

part of the bridge 

3. Calculate hypothetical debris 

accumulations for the entire bridge 

a. 

b. 

c. 

Calculate hypothetical accumulation of 

medium potential 

Calculate hypothetical accumulation of high 

potential 

Calculate hypothetical chronic accumulation 

Source: (17) 

3.2 DEBRIS VOLUMES 

The volume of large debris within a watershed can be determined using a procedure applied by 

Diehl and Bryan(18) for a basin of the West Harpeth River in Tennessee. The general procedure 

involves selecting several different reaches of the river that are representative of the conditions 

upstream and downstream of the selected reach. The representative reaches could be selected 

using aerial photographs and/or during the reconnaissance field investigation. Debris greater 

than a certain length is counted and measured within each of the reaches. Debris concentration 

is then calculated for each of the reaches as either: 

•	 the cubic meters of debris per kilometer of channel (e.g., 27 m3/km for a reach that is 3 

kilometers in length and contains 81 m3

 of debris), or 

•	 the number of pieces within a height range per kilometer of channel (e.g., 25 (10 meters 

long)/km for a reach that is 3 kilometers in length and contains 75 pieces of 10 meter long 

debris). 

The total volume of debris for each of the individual reaches is then estimated by multiplying the 

debris concentration by the total length that the selected reach represents. For example, a 

reach that has a debris concentration of 27 m3

/km (i.e., 3 kilometers in length containing 81 m3 

of debris) and a total representative length of 12 km would have a total volume of debris of 324 

m3

 (i.e., 27 m3/km times 12 km equals 324 m3). 

Finally, the volumes for each of the individual reaches are summed to determine the total 

volume of debris. For example, a watershed that has three reaches with individual debris 

3.2
 
volumes of 324 m3

, 500 m3, and 210 m3 would produce a total volume of debris of 1,034 m3 

(i.e., 324 m3

 plus 500 m3 plus 210 m3 equals 1,034 m3). 

During the counting and measuring of the debris, additional information about the debris should 

be noted and documented. The information should include: 

•	 The length of the piece, measured from root mass or spar butt to spar top; 

•	 The diameter at the butt and at the top of the piece; 

•	 If it has a straight or curved stem; 

•	 The abundance of branches, as in none, few, or many; 

•	 If the bark is present or not; 

•	 The condition of root mass, as in dirty, one-sided, symmetrical, worn, and/or gone; 

•	 The orientation within the channel, such as parallel, perpendicular, or diagonal to 

channel alignment; 

•	 The position in channel, such as on the bed, on the bank, or on the bed and bank; and 

•	 The type and extent of anchorage, such as on top of bed, entrenched in bed, tangled in 

vegetation, or part of a debris pile. 

3.3 POTENTIAL FOR DEBRIS DELIVERY 

The first phase in evaluating the potential for debris accumulation at a bridge is to estimate the 

potential for debris delivery to the bridge site. The tasks involved for this phase include 

estimating the potential for delivery of floating debris, estimating the largest size of floating 

debris delivered to the site, and assigning location categories to all parts of the highway 

crossing. 

3.3.1 Task A: Estimating Potential for Debris Delivery to Site 

The potential for debris delivery is evaluated based on the potential for the debris to be 

transported downstream to the bridge site and the potential for debris generation as defined by 

direct and indirect evidence. Observations of floating debris provide the most direct evidence for 

assessing the potential for debris delivery. These observations could be made of the channel 

system or of accumulations at bridges and/or at other sites upstream of the bridge structure or 

within a basin of similar characteristics. Even though present observations indicate that there is 

a low potential for debris delivery, infrequent catastrophic events or changes in the watershed 

3.3
 
could still result in abundant floating debris in the future. Therefore for such events, indirect 

evidence should be considered. 

Direct evidence for high delivery potential includes the following observations: 

•	 Multiple cases of floating debris accumulation at bridges. 

•	 Chronic floating debris accumulation at one or more sites. 

•	 Floating debris accumulation at sites where potential for accumulation would be low if 

floating debris were not abundant. 

•	 Abundant floating debris stored in the channel. 

• Past need for debris removal in the channel system or at bridges. 

Direct evidence of low potential for drift delivery may be indicated by the following observations: 

•	 Negligible floating debris delivered in major events, especially at sites with a high 

potential for trapping floating debris or at typical debris-accumulation sites. 

•	 All of the floating debris accumulates in forested channel upstream. 

•	 Floating debris in the channel is stationary during floods because of low flow velocity. 

The potential for debris delivery can also be assessed from indirect evidence of debris 

generation. As previously discussed, a major source of floating debris is from bank erosion. 

Therefore, evidence of existing or potential bank erosion can be considered as indirect evidence 

for high potential of debris generation. Observations of indirect evidence for abundant debris 

generation include: 

•	 Widespread bank erosion in the upstream channel system. 

•	 History of changes in the upstream channel system, including degradation, lateral 

migration, widening, channelization, in-stream gravel mining, widespread drainage, or 

dams. 

•	 Prospects of changes in the channel system. 

•	 Hydraulic and geomorphic factors indicating stream instability. 

•	 Widespread timber harvesting in the basin. 

•	 History or prospect of marked changes in basin land use. 

•	 In-stream gravel mining. 

3.4
 
Indirect evidence for low potential of debris generation includes the following observations: 

•	 The inability of woody vegetation to grow along the channel system and on steep slopes 

leading down to the stream channels. 

•	 The channel system is stable and is unlikely to experience any significant change. 

Where indirect evidence indicates that there is a high potential for existing or potential future 

debris generation, the ability of the channel system to transport the debris will control the 

potential of the debris delivery to the site. In general, most streams are capable of transporting 

some of the debris, and one should assume that the stream is capable of transporting the debris 

unless there is evidence to the contrary. Stable, densely forested streams transport little debris 

and can be assumed to have a low delivery potential as long as the forest will not be cleared in 

the future.(17) 

Applying the information above, a “High Delivery Potential” exists when there is an abundant 

amount of direct evidence of debris delivered to the site, or there is indirect evidence of existing 

or future debris generation within the watershed and the upstream channel is capable of 

transporting the floating debris to the site; and, a “Low Delivery Potential” exists when there is a 

sparse amount of direct evidence of debris delivered to the site and there is no existing indirect 

evidence of future debris generation within the watershed, or when the upstream channel is 

incapable of transporting the floating debris to the site. 

3.3.2 Task B: Estimating the Largest Debris Size Delivered to Site 

The second task of the first phase is to estimate the size of the largest debris delivered to the 

site (Maximum Design Log Length). This debris size influences the potential size of the debris 

accumulation. The largest debris delivered to the site is influenced by the channel dimensions 

upstream of the site, particularly the channel width. These dimensions may change over the 

project life of a bridge as a result of future stream instabilities, and these changes should be 

accounted for when defining the channel dimensions. 

As illustrated in Figure 3.1, the maximum design log length is estimated on the basis of the 

narrowest channel width immediately upstream from the site. This distance should be measured 

perpendicular to the banks or lines of permanent vegetation at the inflection points between 

bends. 

3.5
 
Figure 3.1. Schematic Illustrating Estimate of Maximum Design Log Length 

The minimum channel depth required to transport large trees is estimated to be about the 

diameter of the butt plus the distance the root mass extends below the butt, or roughly 3 to 5 

percent of the estimated tree length. 

The design log length represents a length above which logs are insufficiently abundant and 

insufficiently strong throughout their full length to produce an accumulation equal to their length, 

and it does not represent the absolute maximum length of trees within the watershed upstream 

of the site. Diehl(17) recommends estimating the design log length at a given site as the smallest 

of these three values: 

•	 Width of the channel upstream from the site. 

•	 Maximum length of sturdy logs. The height and diameter of mature trees on the banks 

determine the maximum length of the logs that can be delivered to the bridge as floating 

debris and capable of withstanding hydraulic forces when lodged against the piers. The 

maximum sturdy-log length seems to reach about 24 m (about 80 ft) in much of the 

United States; however, it may be as long as about 45 m (about 150 ft) in parts of 

northern California and the Pacific Northwest 

•	 9 m (30 ft) plus one quarter of the width of the channel upstream from the site, in much 

of the United States. As indicated by Diehl(17), this third constraint reflects the rarity of 

long logs and their breakage during transport, and it should not be considered for sites 

located in northern California or the Pacific Northwest. 

3.6
 
3.3.3 Task C: Assigning Location Categories to All Parts of the Highway Crossing 

The last task for the first phase involves assigning location categories to all parts of the highway 

crossing. As previously mentioned, debris is generally transported along a relatively small 

portion of the channel cross section. As a result, some areas of a site may be entirely free of 

debris transport, whereas other areas may receive a significant amount of debris. The various 

categories include: 

Sheltered Location. A sheltered location is defined for the section of the bridge that includes a 

forest area directly upstream of the bridge that traps the transported debris and prevents it from 

being delivered to the bridge. This category should only be applied when the gaps between 

trees are much narrower than the average tree height and the width of forest along the direction 

of flow is more than a double line of trees. Intuitively, this category should not be applied to the 

upstream forest area if it is potentially subject to clearing. 

Bank/Floodplain Location. This category includes the slope of the bank, top of the bank, and 

the floodplain since piers located on the slope of banks or at the top of the bank are just as likely 

to accumulate debris as piers located in the floodplain. The floodplain includes any area outside 

of the channel that is inundated in the design flood to a depth sufficient to transport drift, and it 

may be either clear of trees or a forested area that is subject to future clearing. If there is 

evidence that debris is transported within the slope of the channel banks, then the banks should 

not be assigned to this category. 

In the Channel Location. Debris can be transported anywhere in the channel. As expected, 

debris accumulations are more common for “in the channel” locations than for “bank/floodplain” 

locations, so the potential for debris accumulations for this category is higher than for the 

previous category. In humid regions, the “in the channel” location is typically defined by the base 

flow. In arid regions, where the base flow is relatively low, or for ephemeral streams, this 

location is typically defined between the toes of the banks. If there is evidence that debris is 

transported within the slope of the channel banks, then they should be assigned to this 

category. 

In the Path Location. This category is defined for the portion of the cross section in which the 

majority of the debris is transported. As previously mentioned, floating debris is generally 

transported in most streams along a relatively narrow path within the channel where the 

secondary circulation currents converge at the surface. In a straight reach, this convergence 

zone typically coincides with the thalweg of the channel where the flow is the deepest and 

fastest. In a curved reach, this zone generally exists between the thalweg and the outside bank 

of the bend. The best way to identify the debris path is to observe it during bank-full or high flow 

conditions. The observations do not need to be of large pieces of debris since all floating 

material responds similarly to the flow pattern. If observations indicate that the debris path 

includes part of the bank or part of the flood plain, then they should be assigned to this 

category. If high-flow observations are not available, observations during base flow can confirm 

the estimates based on channel characteristics. If direct observation is impossible for all flow 

conditions, then the location of the debris path can be estimated based on channel 

characteristics and assuming that the width of the debris path is about one-third the channel 

3.7
 
width. If the location of the debris path is indefinite, then several different locations of the debris 

path could be considered, i.e., the left third of the channel, the middle third, or the right third, or 

the entire channel could be assigned to this category, which would reflect the worst case 

scenario. 

3.4 POTENTIAL FOR DEBRIS ACCUMULATION 

The second phase in evaluating the potential for debris accumulation at a bridge is to estimate 

the debris potential on individual bridge elements. The tasks involved for this phase include 

assigning bridge characteristics to all immersed parts of the bridge and determining the 

accumulation potential for each of these parts. 

3.4.1 Task A: Assigning Bridge Characteristics to All Immersed Parts of the Bridge 

There are certain characteristics of a bridge structure that influence the potential for debris 

accumulation. So, the bridge structure should be divided into different components and the 

potential for accumulation should be evaluated separately for each of the components. The 

different components include piers, abutments, any gaps between fixed elements of the bridge 

opening, and the portion of the superstructure submerged during the design flood event. 

An effective width needs to be determined for both horizontal and vertical gaps in the bridge 

structure below the design water surface elevation. Horizontal gaps between adjacent piers, 

between each bank and the nearest pier in the channel, and between each abutment and the 

nearest pier in the channel are common locations for large accumulations. The potential for 

accumulation is high when the effective width of the horizontal opening is less than the length of 

the longest piece of debris delivered to the bridge. When this is the case, debris typically comes 

into contact with one of the bridge elements, and then rotates downstream until it becomes 

lodged against another of the bridge elements. The effective width of the horizontal gaps should 

be reduced to account for any skew in the bridge to the approaching flow. 

When the water surface elevation is at or above the bottom elevation of the superstructure ("low 

chord elevation"), debris can become trapped vertically between the superstructure and the 

streambed below it. When floating debris hits the superstructure, most of the pieces rotate to 

one side and remain at the water surface, resulting in an accumulation against the 

superstructure at the surface. However, some debris could be lodged between the streambed 

and the superstructure as a result of the upstream end of the debris rotating downward until it 

encounters the streambed after striking the superstructure roughly endwise. The height of the 

vertical gap between the low chord elevation of the bridge and the elevation of the streambed 

beneath should be based on the minimum height since the height most likely will vary along the 

bridge due to the changes in the elevations of the low chord and/or the streambed. 

3.8
 
Narrow openings of the structure elements of the bridge at the water surface elevation also 

determine whether debris would be deflected or trapped. Piers and superstructures with narrow 

openings that convey flow are significantly more likely to trap and accumulate debris. Examples 

of such structures include: 

• Multiple closely spaced pier or pile groups, 

• Closely spaced rows of piers, 

• Exposed pier footing piles, 

• A pile bent or a pier made of a single row of columns skewed to the approaching flow, 

• Open truss superstructures, 

• Superstructures with open parapets of pillars and rails, and 

• Various types of connections between the pier caps and the bridge deck. 

Also, any pier with existing accumulations should fall under this classification. Elimination of 

these narrow openings by using a single solid pier (wall, cylinder, or hammerhead), a 

superstructure with a solid parapet, and a solid beam that is connected directly to the pier would 

increase the likelihood of the debris being deflected and not trapped by the structure. 

3.4.2 Task B: Determining Accumulation Potential for Each Bridge Component 

The first step in determining the accumulation potential for each of the bridge components is to 

assign a location category described in the previous section to each of the components. The 

selected category for a horizontal gap should be based on the most debris-prone location 

category occupied by the fixed elements that define the gap. For example, a horizontal gap from 

a bank or abutment to a pier located in the debris path should be assigned to the “in the path” 

location category, or a gap that has one of the fixed elements sheltered while the other element 

in the floodplain should be assigned to the “floodplain” location category. 

After the categories have been assigned to each of the components, the potential for debris to 

span a horizontal or vertical opening between fixed elements of the bridge can be estimated 

using the information provided in Table 3.2, and the potential for accumulation on each of the 

piers can be estimated using the information provided in Table 3.3. 

3.9
 
 

 

Table 3.2. Determining Potential for Debris Accumulation across a Span or Vertical Gap. 

Accumulation 

Potential 

Gap Wider than 

Design Log Length Location Category Potential for 

Debris Delivery 

- Sheltered -

Low Yes - -

No Bank/Floodplain Low 

No In the Channel Low 

Medium No In the Path Low 

No Bank/Floodplain High 

High No In the Channel High 

High, Chronic No In the Path High 

Table 3.3. Determining Potential for Debris Accumulation on a Single Pier. 

Accumulation 

Potential Pier type Location Category Potential for 

Debris Delivery 

Low 

- Sheltered -

Solid Pier Bank/Floodplain -

Solid Pier In the Channel Low 

Piers w/ Openings Bank/Floodplain Low 

Medium 

Solid Pier In the Channel High 

Solid Pier In the Path Low 

Piers w/ Openings Bank/Floodplain High 

Piers w/ Openings In the Channel Low 

High 

Solid Pier In the Path High 

Piers w/ Openings In the Path Low 

Piers w/ Openings In the Channel High 

High, Chronic Piers w/ Openings In the Path High 

Both of these tables were generated from the information presented by Diehl.(17) As shown in 

these tables, the potential for debris accumulation is based on the estimated delivery potential, 

which is the same for the entire site, the location category, and the effective length of the span 

between fixed elements relative to design log length (gap wider or narrower) for span 

accumulations and the presence or absence of narrow openings that carry flow for single pier 

accumulations. 

3.5 SIZE OF DEBRIS ACCUMULATION AT STRUCTURES 

The last phase involves calculating the hypothetical accumulation over the entire length of the 

bridge with a medium, high, and high, chronic potential. The hypothetical accumulation with a 

medium and high potential should be used to evaluate the effects that the accumulation would 

have on the hydraulic characteristics through the bridge and on the hydraulic loading on the 

3.10
 
structure. The hypothetical accumulation with a high, chronic potential can be used to define the 

potential maintenance requirements for the bridge, i.e., the location and maximum extent of 

debris removal. 

The potential for debris accumulation estimated in the preceding section is related to the 

likelihood of occurrence relative to the various components of the bridge, and it does address 

the likely size of an accumulation. A pier with a “high potential” for accumulation indicates that 

there is a high potential for accumulation at the pier relative to the potential for accumulation at 

the other piers, and an accumulation will not necessarily form on such a pier. If an accumulation 

does form, it may be as wide as the design log length and extend vertically to the depth of flow, 

or it may be much smaller. The size of the accumulations depends mostly on the debris 

dimensions and delivery rates, the flow depth, and the number and proximity of gaps and piers 

affected. Accumulations in the channel can reach their maximum size during a single flood 

where delivery is high, but accumulations grow more slowly where the debris delivery is low or 

when the accumulation is outside of the channel. 

Diehl(17) proposed that accumulations on a single pier should have a width equal to the design 

log length over its full flow depth, accumulations across two or more piers should extend 

laterally half of the design log length beyond them, and accumulations on vertical and horizontal 

gaps should extend across the entire width and height of the gap. Diehl based this proposal 

upon conservative assumptions consistent with his largest observed debris accumulations.(17) 

Because of limited descriptions and observations available for debris accumulations on

superstructures, Diehl could not provide a means to estimate the maximum size of 

accumulations on superstructures.(17) Consequently he recommended following the suggestions 

provided by Wellwood and Fenwick(64), which define the vertical extent of the accumulation 

being 1.2 m (4 ft) above the top of the bridge parapet wall and below the low chord elevation. 

Based on the above information, the maximum extent of debris accumulations is summarized in 

Table 3.4. 

Table 3.4. Maximum Extent of Debris Accumulation. 

Accumulation Type Width Height 

Pier Design Log Length Flow Depth 

Superstructure Span Width Vertical Height of Superstructure plus 1.2 m 

Above and Below the Superstructure 

Horizontal Gap Width of Gap Smaller of Vertical Height of Gap or Flow 

Depth 

Vertical Gap Width of Gap Vertical Height of Gap 

The overall potential for debris accumulation at a bridge should be defined by the highest 

potential estimated for the different bridge components. Therefore, a bridge should be 

considered as a high potential for accumulation if any of its components have been determined 

to have a high potential for accumulation. 

3.11
 
Debris accumulations over the entire length of the bridge should be developed and evaluated 

for both medium and high potential conditions, with the second condition reflecting a condition 

that is more likely to occur than the first condition. As proposed by Diehl(17), an accumulation 

over the entire length of the bridge with a medium potential can be represented by assuming 

that all of the individual medium- and high-potential accumulations grow to their maximum size. 

Similarly, an accumulation with a high potential can be defined by assuming that all of the 

individual high-potential accumulations grow to their maximum size. 

As previously mentioned, debris accumulations can cause significant changes in the hydraulic 

characteristics through the bridge and the trapping characteristics at the bridge. These changes 

could cause an increase in the potential for accumulation at the bridge. For example, a highpotential blockage across the channel may cause skewed flow through the bents that were 

initially considered to be aligned with the flow, or the superstructure could become immersed as 

a result of the increased backwater caused by the debris accumulation on the structure. 

Therefore after the initial assessment, the bridge should be re-evaluated using the bridge 

comprised of the debris accumulation determined from the initial assessment and the 

corresponding hydraulics associated with the accumulation. 

Finally, the overall potential for debris accumulation at bridges depends on the probability or 

frequency of occurrence for the events that were used to define the potential for debris delivery 

and accumulation at the bridge. A high-potential assessment based on a large flood event and 

significant changes in the watershed and upstream channel would have different implications on 

the bridge design and maintenance compared to a high-potential assessment based on a 2-year 

flood event and existing channel conditions. 

3.6 FACTORS THAT AFFECT DEBRIS PRODUCTION 

There are several different factors that can influence debris production. These factors include 

floods, fires, urbanization, logging, land clearing (i.e., grazing and agriculture), conservation 

practices, and channel improvements. 

Flooding increases debris production as the associated discharges serve as a means to 

produce and deliver debris to a site. Higher discharges are more likely to cause erosive forces 

on bank and floodplains. The inundation of the flooding event affects more of the floodplain 

area; facilitating transport of debris into the main channel. 

Fires can decrease the amount of floating debris introduced into the stream system. However, 

fires increase the magnitude of runoff from the burned area, increase the erodibility of soils, and 

increase the probability of catastrophic events such as debris flows and landslides, resulting in a 

significant increase in sediment yield from the effected area. This increase could cause an 

increase in fine and coarse detritus to be transported to and deposited at a culvert or bridge 

structure. 

 

Urbanization over time causes an opposite effect in the yield of sediment from a watershed 

than that of fires. Initially, sediment yield can be significantly increased during the construction 

3.12
 
phase of development due to the removal of exiting vegetation and disturbance of the soil. 

However over time, the sediment yield decreases as the developed land becomes restabilized 

and land surface area exposed to the erosive effects of rainfall and runoff is reduced as a result 

of the increase in impervious area, such as roads, structures, and parking lots. Hydrological 

effects from urbanization include an increase in runoff volume, higher peak flows, and longer 

durations. These effects with the decrease in the sediment yield from the watershed could result 

in an increase in bank erosion and scour of the streambed, which could increase the generation 

and delivery of floating debris to a bridge site. 

Logging has been identified as a source of floating debris.(14,23,25) A study conducted by 

Froehlich(25) indicated that different logging practices cause substantial differences in the loads 

of floating debris. Practices that reduce the quantities of floating debris include directional felling 

uphill with a tree-pulling system and providing a buffer strip of undisturbed vegetation along the 

streams. 

Land Clearing associated with logging, grazing, or agriculture practices could cause the same 

effects associated with fires, however the magnitude of these effects would most likely not be as 

severe. Also, grazing allowed near a stream can result in a significant increase in bank erosion. 

Conservation practices have the opposite effects than the effects associated with clearing of 

the land. Implementation of a different conservation practice can reduce both the amount of 

erosion and runoff from the land. 

Channel improvements or modifications to the channel geometry and/or vegetation clearing 

from the channel can influence quantities of both floating debris and fine/coarse sediment. 

Improper design of such improvements can cause significant instabilities to develop within the 

system, including increased bank erosion, increased degradation and/or aggradation of the 

streambed, and/or significant changes in the planform, that could increase the generation and 

delivery of floating debris to a structure site. 

Growth of riparian forest buffer strips has been recommended and encouraged by the US 

Environmental Protection Agency (US EPA) for their water quality, ecological and bank erosion 

benefits especially in agricultural areas. These forested buffer strips adjacent to stream 

channels are now common with maturing trees especially in heavily agricultural areas. 

Extreme events, such as ice storms, debris flows, forest fires and insect infestations can 

drastically increase the debris load at some point in the life of the structure. 

3.13
 
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3.14
CHAPTER 4 – ANALYZING AND MODELING DEBRIS IMPACTS TO STRUCTURES 

4.1 INTRODUCTION 

After the location and extent of the debris accumulation on the bridge structure has been 

determined using the procedures discussed in the previous Chapter, a hydraulic analysis should 

be conducted to evaluate the affects the accumulation would have on the hydraulic 

characteristics through and upstream of the bridge structure, local scour at the piers, and 

hydraulic loading on the structure. General information for performing such an analysis is 

presented in this Chapter. This Chapter also includes information for estimating local pier scour 

and hydraulic loading on the bridge structure associated with debris accumulation. 

4.2 HYDRAULIC ANALYSES OF DEBRIS 

Hydraulic analyses of affects of debris upon a drainage structure are often conducted using a 

one-dimensional (1-D) water surface model. However, such analyses can also be performed 

using: 

• Hand calculations; 

• Two-dimensional (2-D) numerical (computer) models; 

• Three-dimensional (3-D) computer models; or 

• Physical (laboratory) modeling. 

The selection of any such analytical technique is based on the complexity of the hydraulics and 

debris, risk and importance of the drainage structure, and other project site characteristics. 

4.3 ONE-DIMENSIONAL DEBRIS ANALYSIS MODELING 

One-dimensional programs available for performing debris analyses include the U.S. Army 

Corps of Engineers (USACE) River Analysis System (HEC-RAS) and the FHWA Water Surface 

Profile (WSPRO) (among others). For culverts, the FHWA HY-8 program allows evaluation of 

complex hydraulic conditions. 

4.3.1 Data Requirements 

Data required for the hydraulic analyses include geometric and flow data. The geometric data 

consists of cross section data, reach length, energy loss coefficients, and hydraulic structure 

data. Flow data includes the discharges used in the analyses and the associated boundary 

4.1
 
conditions and regimes. A thorough discussion of this information, as well as modeling 

approaches is beyond the scope of this document. However, such information can be found in 

appropriate user manuals and model documentation. A brief description of each of these 

modeling data follows: 

Cross Section Geometry is the representation of the ground surface perpendicular to the 

direction of flow. Cross sections are located along a watercourse to define the conveyance 

capacity of the main channel and the adjacent floodplain. Cross sections are required at 

representative locations throughout the watercourse and at distinct locations where changes 

occur in discharge slope, shape, or roughness, or at location where hydraulic structures are 

located. In modeling debris, the cross section data can be modified to include or simulate 

ineffective flow areas, levees, and/or blocked obstructions. 

Reach Length is the measured distance between cross sections. Reach lengths are provided 

for the main channel, measured along the thalweg, and for the left and right overbanks, 

measured along the anticipated path of the center of mass of the overbank flow. In debris 

analyses, reach lengths serve to allow refined characterization of the extent of the debris field 

and the associated effects. 

Energy Loss Coefficients estimate losses caused by the resistance to flow from bed-surface 

and vegetative roughness (i.e., Manning’s n coefficient) (8,15,28,3,15), channel irregularities, channel 

alignment, obstructions, and by the contraction and expansion of the flow. Adjusting these 

coefficients allows simulation of the presence and extent of debris. These also provide a means 

to simulate the impacts and effects of debris. 

Hydraulic Structure Data is the geometric representation of structures that influence the water 

surface profile within a watercourse. The hydraulic structures can include bridges, culverts, 

spillways, diversion structures, weirs, etc. The information required to define the bridge structure 

are the dimensions of the bridge deck, piers, and bridge abutments. The geometry of the debris 

accumulation should also be accounted for when defining these features. Typically, the 

dimensions of these features have to be manually adjusted to account for the debris 

accumulation. 

Flow Data required for the model is the discharge in the watercourse and the flow conditions at 

the boundaries of the model. The discharge is based on the peak discharge for the design flood 

event of the bridge or for a specific flood event that is being used to estimate the hydraulic loads 

on the bridge structure. For this discharge, the flow depth is required at the downstream 

boundary for subcritical flow (Froude number less than 1) and at the upstream boundary for 

supercritical flow (Froude number greater than 1) to initiate the water surface profile 

computations. 

4.3.2 Background of Modeling Methods and Approaches 

There are several methods available for evaluating the hydraulics through a bridge structure for 

a one-dimensional flow analysis. The type of methods available depends on the flow conditions 

4.2
 
through the bridge. The flow through the bridge could be classified as either low or high flow 

conditions. 

4.3.2.1 Low Flow Conditions 

Low flow conditions exist when the flow through the bridge opening is open channel flow, i.e., 

the water surface is below the highest point on the low chord of the bridge opening. Three types 

of flow classes can exist for this condition: 

1. Class A exists when the water surface through the bridge is completely subcritical, i.e., 

above critical depth; 

2. Class B exists when the water surface profile passes through critical depth within the bridge 

structure, which can occur for either supercritical or subcritical flow; and 

3. Class C exists when the water surface profile through the bridge structure is completely 

supercritical, i.e., below critical depth. 

There are three methods commonly available for computing the hydraulics through the bridge 

for low flow conditions. These methods are: 

1. Energy Equation – This method uses the conservation of energy to determine water 

surface elevations, velocities, and losses in a waterway. This method is best used when the 

bridge piers are a small obstruction to the flow and the friction losses are the predominate 

consideration. This method can be used for both supercritical and subcritical flow (Class A, 

B, and C). 

2. Momentum Equation –The momentum equation uses the second law of thermodynamics 

to describe how change of momentum per unit of time in the body of water in a flowing 

channel is equal to the resultant of all the external forces that are acting on the body. Unlike 

the energy equation, this method does not account for non-uniform velocity distributions. 

The momentum equation method is best used when the bridge piers are the dominant 

contributor to energy losses or when the pier losses and friction losses are both 

predominant. As in the energy method, this method can be used for both supercritical and 

subcritical flow (Class A, B, and C). FHWA does not recommend this method for 1-D 

bridge hydraulics. 

3. Yarnell Equation – The Yarnell equation empirically predicts the change in water surface 

from just downstream of the bridge to just upstream of the bridge. The equation is based on 

about 2,600 lab experiments in which the researchers varied the shape of the piers, the 

width, the length, the angle, and the flow rate.(66) This method is most applicable when the 

piers are the dominant contributor to energy losses and the flow through the bridge remains 

subcritical (Class A). FHWA does not recommend this method for 1-D bridge 

hydraulics. 

4.3
 
The energy and momentum equation methods can be used for all of the classes for the low flow 

condition (Class A, B, and C), while the Yarnell equation method is intended only for Class A 

low flow. 

4.3.2.2 High Flow Conditions 

High flow conditions exist when the flow through the bridge opening comes in contact with the 

maximum low chord of the bridge deck. The type of flow conditions that can occur for high flow 

include pressure flow, a combination of pressure and weir flow, and a combination of weir flow 

and open channel flow through the bridge. Generally, three computational approaches exist to 

evaluate high flow conditions. 

1. Pressure Flow Condition – Pressure flow occurs when the flow comes into contact with the 

low chord of the bridge deck. The backwater upstream of the bridge associated with this 

type of flow condition causes the flow through the structure to behave as orifice flow. In 

general, there are two types of pressure flow that can exist (details of which are described in 

other FHWA documents).(10) Depending on the conditions, pressure flow can describe a 

bridge with full submergence of the low chord at the upstream side and open channel flow at 

the downstream side of the bridge (acting as a sluice gate – Figure 4.1). The second type of 

pressure flow exists when both the upstream and downstream sides of the bridge are 

submerged and uses the standard full flowing orifice equation (Figure 4.2). 

WS 

Bridge 

EG 

g2

V2 

2 

Y3 

Zbu 

Y2 

g2

V2 

3 

Figure 4.1. Sketch of the sluice gate type of pressure flow. 

4.4
 
V2
 

Figure 4.2. Sketch of fully submerged pressure flow. 

EG 

Bridge 

WS 

V2 

2 

2g

Y3 

Y2 

H 

3 

2g

2. Weir Flow Condition – Weir flow exists when water flows over the bridge structure and/or 

the roadway approaches for the bridge. Typically this condition is modeled using the 

standard weir equation (Figure 4.3). This illustration depicts pressure flow occurring through 

the bridge structure, which might not always be the case. Note that pressure flow may also 

occur through the bridge opening. Typically, some balancing of flow through the opening 

and over the “roadway” weir occurs before the model converges to a solution. 

Figure 4.3. Sketch of pressure and weir flow. 

WS 

Bridge 

EG g2 

V 2 

3 

Y3 

Y2 

Hw 

g2 

V 2 

2 

4.5
3. Energy Equation Approach – as with low-flow conditions, the energy method balances the 

total energy from the downstream side to the upstream side of the bridge structure. All of the 

computations are performed as though the flow is open channel flow, and the area 

obstructed by the bridge structure is subtracted from the flow area and the wetted perimeter 

is increased for the portions of the structure in contact with the water. This method should 

be used when the bridge is highly submerged and the flow over the road is not controlled by 

weir flow (low water bridges) or when the bridge deck is a small obstruction to the flow and 

the bridge opening is not acting like a pressurized orifice. This method is also best used for 

bridges that are perched above the floodplain. 

4.3.3 Scenarios for Hydraulic Modeling of Debris Accumulation 

These 1-D model data, methods, and approaches permit computation of water surface profiles 

at a hydraulic structure with debris accumulation. As previously mentioned, the general 

concepts discussed would apply to most 1-D models commonly used in the highway hydraulics 

community. 

4.3.3.1 Bridge Debris Scenarios 

Scenarios for analyzing debris accumulation at a bridge structure involve relocating cross 

sections, redefining the ineffective flow boundaries, modifying the cross section and bridge 

geometry, and changing the contraction and expansion coefficients. 

Scenario 1: Relocation of Downstream Wake. As depicted in Figure 4.4, relocating an 

“expansion” cross section further downstream would attempt to simulate an ineffective flow zone 

(downstream wake) created by the debris accumulation. The ineffective flow created by the 

debris accumulation should extend downstream from the upstream face. Assuming a 2:1 to 4:1 

expansion ratio for the reach downstream of the bridge, this distance would range from the 

width (2:1) to twice the width (4:1) of the debris accumulation. This scenario might be required 

where most of the flow downstream of the bridge is conveyed within the main channel, the 

overbank areas are not extremely wide, or the bridge structure and/or roadway do not 

significantly constrict the flow. 

4.6
 
Expansion Reach 

Contraction Reach 

1 

2 

Figure 4.4. Changing downstream expansion cross section location. 

Scenario 2: Creating additional downstream ineffective flow boundaries. As seen in Figure 

4.5, adding ineffective flow boundaries to downstream bridge face cross section would simulate 

the downstream wake created by the debris accumulation. Once again, assume a 2:1 to 4:1 

expansion ratio to create the locations of the ineffective flow. Adding additional downstream 

cross sections would assist in the transition. This scenario could be used alone or in conjunction 

with other scenarios. 

Contraction Reach 

2 

Ineffective Flow 

1 

Expansion Reach 

Figure 4.5. Adding downstream ineffective flow locations. 

4.7
 

Scenario 3: Creating upstream ineffective flow boundaries. An additional upstream cross 

section would simulate an ineffective flow zone created by the debris accumulation (Figure 4.6). 

Assuming a 1:1 to 2:1 contraction ratio the reach upstream of the bridge to the debris 

accumulation, the point where flow is not affected by the accumulation would be located 

upstream about one-half (1:1) to the entire (2:1) of the debris accumulation width. Depending on 

the accumulation width, adding additional upstream cross sections would assist in the transition. 

This scenario could be used alone or in conjunction with other scenarios. 

Expansion Reach 

Contraction Reach 

Ineffective Flow 

1 

1 

Figure 4.6. Adding upstream ineffective flow locations. 

Scenario 4: Modifying Bridge Geometry. Modification of the bridge geometry could be used 

to reflect debris accumulation. The modification could range from changing local structural 

elements, such as piers and abutments, to actual changes in the bridge opening area or low 

chord elevations. For piers, debris accumulation would change how pier width and height would 

be described in model input. The debris may collect either symmetrically or asymmetrically 

about the pier centerline. For an asymmetrical debris accumulation, the centerline of the pier 

would have to be moved to the centerline of the debris accumulation or an additional “dummy” 

bridge pier with an extremely narrow width would have to be defined at the centerline of the 

debris accumulation. This is illustrated in Figure 4.7. 

Scenario 5: Modifying Contraction and Expansion Losses. In some cases, increasing the 

contraction and expansion loss coefficients may be appropriate if the debris accumulation 

causes an abrupt contraction and expansion, respectively, of the flow. 

4.8
 
STA 11.3 STA 15

1.3 m 3.7 m 

Width = 10 m 

Debris Accumulation 

Water Surface 

Move Existing Pier or Add 

“Dummy” Pier to 

Centerline of Accumulation 

Figure 4.7. “Dummy” bridge pier used to simulate an asymmetrical debris accumulation. 

 

The results of the hydraulic analysis can be used to estimate the hydraulic loading on the bridge 

structure associated with the debris accumulation. Information required from a 1-D model to 

estimate the loads is briefly discussed in the following paragraphs. 

Upstream Water Surface Elevation. The upstream water surface elevation is used to compute 

the blockage ratio, B, the hydrostatic and hydrodynamic forces at the upstream side of bridge, 

and the location of the forces. This elevation should be selected at a location upstream of where 

the flow accelerates in response to the debris accumulation, causing flow-separation and a zone 

of ineffective flow. The upstream boundary of this zone is depicted in Figure 4.6. As shown in 

this figure, the boundary is based on the flow contraction ratio typically assumed near a bridge 

structure. Based on this assumption, the upstream water surface elevation should be obtained 

from a cross section located upstream of the debris accumulation at a minimum distance of at 

least one half of the total width of the debris accumulation. 

 

Downstream Water Surface Elevation. The downstream water surface elevation is used to 

compute the downstream hydrostatic forces on the bridge structure that is used to determine the 

net hydrostatic forces on the structure. As in the upstream water surface elevation, this 

elevation should be obtained sufficiently far downstream from the structure that the flow is not 

affected by the wake (ineffective flow zone) created by the debris accumulation (see Figure 4.5). 

Based on the assumptions discussed for the upstream water surface elevation, the downstream 

water surface elevation should be obtained from a cross section located downstream of the 

debris accumulation at a distance equal to the total width of the debris accumulation. The typical 

response within this reach is for the water surface elevation to recover from the drop in the 

water surface elevation (increased velocities) through the contracted section by increasing 

downstream of the bridge. However, there are some conditions (high rates of energy 

dissipation, large channel slopes, or large changes in channel geometry) where the water 

surface elevation downstream does not recover (rise in elevation) from the flow contraction. For 

4.9
 
these conditions, the water surface elevation downstream from the effects of the debris may be 

lower than the elevations in the contraction of the bridge section. For such cases, the water 

surface elevation in the contracted section should be used to compute the downstream 

hydrostatic forces. To summarize, the water surface elevation used to compute the downstream 

hydrostatic forces should be based on the higher water surface elevation at either the 

contracted section or the cross section immediately downstream of the wake created by the 

debris accumulation. 

Area within the Contracted Section. The flow area within the contracted section is used to 

compute the blockage ratio, B. The area should be based on the smallest area within the bridge 

section. 

Flow Velocity within the Contracted Section. The flow velocity within the contracted section 

is used to compute the dynamic forces on the structure. For accumulations on piers, the 

reference location for the velocity depends on the percentage of blockage of the debris and 

bridge structure. If the reduction in area is anticipated to be greater than 30 percent of the entire 

wetted cross-sectional flow area in the bridge opening, then the reference velocity is based on 

the maximum average velocity in the contracted section of the entire bridge opening. If the 

reduction is anticipated to be less than 30 percent, then the reference velocity is based on the 

maximum local average velocity near the pier and debris accumulation, i.e., maximum average 

flow velocity in the main channel for piers located in the main channel and maximum average 

flow velocity in the left overbank for piers located in the left overbank. For accumulations on 

superstructures, the reference velocity is the maximum contracted flow velocity in main channel 

under the superstructure for any degree of blockage. 

Average Flow Depth within the Contracted Section. The average flow depth within the 

contracted section is used to compute the Froude number that is utilized in selecting the drag 

coefficient. This depth should be based on the same area used to define the reference velocity, 

i.e., the average depth in the main channel should be used if the reference velocity is based on 

the average flow velocity in the main channel. 

4.3.3.2 Culvert Debris Scenarios 

Most of the common 1-D models have some culvert hydraulic capabilities incorporated within 

their algorithms. In several cases, these capabilities would allow application of the bridge-based 

scenarios to these culvert structures. However, these 1-D models do not simulate every barrel 

type or configuration. Additionally, for certain hydraulic and discharge conditions, these 1-D 

models may not replicate underlying assumptions of culvert hydraulics. 

Additionally, there is not a great deal of research available into the effects of debris upon 

hydraulic performance of culverts. Therefore, FHWA recommends use of specific culvert 

models, specifically HY-8, for both culvert hydraulic and debris analyses. In such use, debris 

analysis scenarios would modify barrel parameters to reflect changes in inlet efficiency (i.e., 

entrance loss coefficients), additional roughness, or reduced equivalent barrel area. 

4.10
 
4.4 ADVANCED MODELING 

Flow, geometric, boundary, topographic, or other conditions near the bridge structure may 

necessitate use of 2-D numeric, 3-D numeric, or physical models. Such models allow a more 

detailed prediction of the flow-separation regions and hydraulic pressure variations near the 

bridge structure and debris accumulation than what is assumed in the one-dimensional analysis. 

These models can also be very useful in defining the locations of the stream channel where 

high-debris transport would most likely occur. The numerical (i.e., 2-D and 3-D) models need 

approximately the same type of information as the 1-D model, however they use different 

means to represent data including: 

1. Using a finite element mesh of nodes and links to represent the topography, bathometry, 

and drainage structures; 

2. Requiring the resistance coefficient and turbulence parameter be defined at each node; and 

3. Defining the boundary conditions differently. 

The physical models require scaled design of the site to allow predictions of flow and debris 

characteristics and tendencies. 

Use of such models would be predicated on the relative importance of simulating the debris 

accumulation for a project. Several research projects are investigating the appropriate use of 

such advanced models in situations such as these. Additionally, increased power of computers 

and hydraulic software packages are making such analyses more cost effective for the 

transportation community. 

4.5 LOCAL PIER SCOUR ASSOCIATED WITH DEBRIS ACCUMULATION 

Debris accumulations on a bridge pier can increase local scour at the pier as a result of 

increased pier width and downward flow component upstream of the pier. When debris 

accumulates on a pier, the scour depth can be estimated by assuming that the pier width is 

larger than the actual width. A width equal to the design log length as defined by Diehl(17) and 

presented in the previous chapter of this manual can be assumed for estimating the scour at the 

pier. This assumption could be on the conservative side at large depths because the effect of 

the debris on scour depth diminishes. 

Only limited research exists on local scour at piers with debris accumulation. Melville and 

Dongol have conducted a limited quantitative study of the effect of debris accumulation on local 

pier scour and have made some recommendations which support the approach suggested 

above(39). An interim procedure for estimating the effect of debris accumulation on local scour at 

piers is presented in Appendix D of HEC-18(55). 

4.11
 
4.6 HYDRAULIC LOADING ASSOCIATED WITH DEBRIS ACCUMULATION 

There are three steps for computing the hydraulic loading on a bridge structure with debris 

accumulation. The first step is to define the geometry of the debris accumulation using the 

procedures and recommendations presented in Chapter 3 of this manual. The second step is to 

compute the flow hydraulics through the bridge structure using the procedures and 

recommendations presented in the previous sections of this chapter. The last step is to compute 

the hydrodynamic loads using the hydraulic characteristics associated with the presence of the 

debris accumulation and the following equations and general procedure developed by Parola(49). 

The hydrodynamic drag force is based on the general form of the drag equation and the drag 

coefficient relationship developed from a model study investigation by Parola at the University of 

Louisville(49). 

V 2 

F = r 

D C D γ A D (4.1) 2 g 

where: 

FD = Drag force, N (lbs) 

CD = Drag coefficient, see Tables 4.1 and 4.2 

γ = Specific weight of water, N/m3

 (lbs/ft3) 

AD = Area of wetted debris based on the upstream water surface elevation 

projected normal to the flow direction, m2

 (ft2) 

Vr = Reference velocity, see discussion in Subsection 4.3.3.1, m/s (ft/s) 

g = Acceleration of gravity, 9.81 m/s2

 (32.2 ft/s2) 

Drag coefficient for debris on piers is provided in Table 4.1 and for debris on superstructures in 

Table 4.2. 

Table 4.1. Drag Coefficient for Debris on Piers. 

Value of B Value of Fr CD 

B < 0.36 Fr < 0.4 1.8 

B < 0.36 0.4 < Fr < 0.8 2.6 – 2.0Fr 

0.36 < B < 0.77 Fr < 1 3.1 – 3.6B 

B > 0.77 Fr < 1 1.4 -1.4B 

Table 4.2. Drag Coefficient for Debris on Superstructure. 

Value of B Value of Fr CD 

B < 0.33 Fr < 0.4 1.9 

B < 0.33 0.4 < Fr < 0.8 2.8 – 2.25Fr 

0.33 < B < 0.77 Fr < 1 3.1 – 3.6B 

B > 0.77 Fr < 1 1.4 -1.4B 

4.12
 
The drag coefficient as provided in these tables is related to the blockage ratio and Froude 

number as defined below. 

Ad B = (4.2) Ad + A C 

where: 

B = Blockage ratio 

Ad = Cross-sectional flow area blocked by debris in the contracted bridge 

section, m2

 (ft2) 

Ac = Unobstructed cross-sectional flow in the contracted section, m2

 (ft2) 

V F r = r (4.3) 

gyr 

where: 

Fr = Froude number 

Vr = Reference velocity, see discussion in Subsection 4.3.3.1, m/s (ft/s) 

g = Acceleration of gravity, 9.81 m/s2

 (32.2 ft/s2) 

yr = Average flow depth corresponding with the reference velocity, m (ft) 

The total force on the structure that is caused by the hydrostatic pressure difference can be 

approximated as: 

F h = γ(h cu A hu − h cd A hd ) (4.4) 

where: 

Fh = Horizontal hydrostatic force on area Ah, N (lbs) 

γ = Specific weight of water, N/m3

 (lbs/ft3) 

hcu = Vertical distance from the upstream water surface to the centroid of area 

Ahu, m (ft) 

Ahu = Area of the vertically projected, submerged portion of the debris 

accumulation below the upstream water surface, m2

 (ft2) 

hcd = Vertical distance from the downstream water surface to the centroid of 

area Ahd, m (ft) 

Ahd = Area of the vertically projected, submerged portion of the debris 

accumulation below the downstream water surface, m2

 (ft2) 

4.13
 
The total resultant force is computed as the summation of the drag force (Equation 4.1) and the 

differential hydrostatic force (Equation 4.4). The loads computed using these equations 

corresponds to the pressure forces of the water on the debris accumulation. The transfer of the 

load from the debris to the structure depends on many factors, including the characteristics of 

the debris accumulation and the degree to which streambed and banks support the debris 

accumulation. Thus, a conservative approach of applying the resultant force as a point load is 

recommended in evaluating the forces on the structure. The vertical and horizontal location of 

the resultant hydrostatic and drag forces and that of the total force can be determined by adding 

the moments about convenient axes. A less conservative distribution of the load to the structure 

may be warranted where there is more information available on the debris configuration and 

structural susceptibility. 

Three scenarios should be evaluated when debris accumulation exists on two piers as a result 

of the opening between the piers being less than the length of the design log: 

1. Debris accumulation of maximum effective width (design log length) forms on Pile Bent 1, 

with a smaller effective accumulation on Pile Bent 2; 

2. Debris accumulation of maximum effective width forms on Pile Bent 2, with a smaller 

effective accumulation on Pile Bent 1; and 

3. A large log spans the opening and transfers or divides the load on the accumulation 

between the piers almost equally to each pier. Although the pressures on the debris 

accumulation are almost identical for each scenario, the distribution of the total force to each 

of the piers may be substantially different for each of the scenarios. 

Debris accumulations typically align themselves with the direction of the flow. There is a lot of 

uncertainty associated with debris accumulation geometry and the direction of the flood flows. 

Therefore, the resultant force should be applied using both consideration of the anticipated 

range of possible flow directions and the structure’s susceptibility to the resultant forces over the 

range of flow direction. For example, if the possible direction of flow is 20 degrees to the axis of 

the pier and the pier is most susceptible to a force applied at 15 degrees, then the force should 

be applied at 15 degrees to the axis of the pier. For superstructures and debris accumulations 

that span adjacent piers, the forces should be applied in at least two directions: (1) 

perpendicular to the face of the bridge and (2) in the direction of the flow with consideration to 

the structures susceptibility. 

4.14
 
4.7 HYDRAULIC LOADING EXAMPLE PROBLEMS 

4.7.1 Example 1 – Hydraulic Loading on a Single Pier (SI) 

Given: 

Design flow rate = 195 m3

/s 

Minimum upstream main channel width = 13.7 m; design log length of 13.7 m 

Depth of debris is full-flow depth 

Main channel width at the bridge = 60 m 

Debris accumulation only on Pile Bent 2 (see Figure 4.8) 

Superstructure is not submerged 

Ineffective flow areas from the debris defined by 1:1 contraction and 2:1 expansion 

Bottom elevation of Pile Bent 2 = 61.78 m 

Left station of debris = 160.33 m; Right station of debris = 174.03 

Hydraulic computation results are provided in Table 4.3 and shown in Figure 4.9 

Upstream water surface elevation, WSUS = 65.43 m (Table 4.3) 

Downstream water surface elevation, WSDS = 65.06 m (Table 4.3, see discussion 

Downstream Water Surface Elevation in Subsection 4.3.3.1) 

68 Le gend 

WS De sign Discharg e 

67 Ground 

Ba nk S ta 

66 Pi er De bri s 

10 0 15 0 20 0 25 0 

Ele vation ( m) 

65 

64 

63 

62 

61 

St ation (m) 

Figure 4.8. Upstream face of the bridge for Example 1. 

4.15
 
0 20 0 40 0 60 0 80 0 

60 

61 

62 

63 

64 

65 

66 

67 

Ma in C ha nne l Di stan ce (m) 

El eva tion (m) 

Le gen d 

WS D es i gn D isc har ge 

Gr oun d 

12 725 440 6.442 1.748 8.762 8.976 9.1 

Figure 4.9. Water surface profile for Example 1. 

Table 4.3. Results of Hydraulic Calculations for Example 1. 

River Station 

(m) 

Water 

Surface 

Elevation 

(m) 

Flow Area 

(m2

) 

Main 

Channel 

Velocity 

(m/s) 

Cross Section 

Average 

Velocity

 (m/s) 

Average 

Flow Depth1 

(m) 

0 64.27 230.98 2.48 0.84 2.34 

127 64.70 300.10 1.59 0.65 2.08 

254 64.89 307.65 1.03 0.63 1.55 

406.4 65.05 123.84 1.66 1.57 1.95 

421.7 BR D 65.06 110.19 1.77 1.77 1.94 

421.7 BR U 64.99 67.96 2.87 2.87 1.59 

436.9 65.43 149.06 1.39 1.31 2.32 

488.7 65.56 415.13 1.23 0.47 2.77 

628.9 65.63 439.48 1.29 0.44 2.73 

769.1 65.71 464.02 1.19 0.42 2.55 

Notes: 

1. For this example, the entire bridge opening was defined as the main channel. So, the average depth of the main channel is 

the same as the average depth of the entire cross section. 

4.16
 
Determine: 

Compute the hydrostatic and drag forces for a debris accumulation on a single pier. 

Solution: 

Hydrostatic Force on Single Pier Accumulation 

 Ahu = Area of the debris accumulation below the upstream water surface 

 Ahu = (WSUS – Debris bottom, DBEL)(Width of debris accumulation, WD) 

 A = (65.43 – 61.78)(13.7) = 50.01 m2 

hu

 Ahd = Area of the debris accumulation below the downstream water surface 

 Ahd = (WSDS – DBEL)( WD) 

 Ahd = (65.06 – 61.78)(13.7) = 44.93 m2 

 hcu = Vertical distance to centroid of Ahu = 0.5(WSUS – DBEL) 

 hcu = 0.5(65.43 – 61.78) = 1.83 m 

 hcd = Vertical distance to centroid of Ahd = 0.5(WSDS – DBEL) 

 hcd = 0.5(65.06 – 61.78) = 1.64 m 

 Fhu = Hydrostatic force upstream = γhcuAhu 

 Fhu = (9.81)(1.83)(50.01) = 898 kN 

 Fhd = Hydrostatic force downstream = γhcdAhd 

 Fhd = (9.81)(1.64)(44.93) = 723 ken 

 Fh = Total hydrostatic force on Pile Bent 2 = Fhu - Fhd 

 Fh = 898 – 723 = 175 kN 

Drag Force on Single Pier Accumulation 

A B = Blockage ratio = d
 

Ad + A c
 

B = = 0.42 

50.01 + 96 67.

B is greater than 0.3, therefore Vr should be based on the average velocity in the 

contracted section. 

 Vr = 2.87 m/s (Table 4.3)
 

Vr
 Fr = Froude number =
 g yr
 

 

50.01

 

4.17
 

2.87 Fr =	 = 0.73 

(9.81)(1.59) 

 CD = Drag coefficient = 3.1 – 3.6B (Table 4.1) 

 CD = 3.1 – (3.6)(0.42) = 1.59 

 

V2 

F r

D = Drag force on Pile Bent 2 = CD γAhu 2g 

 

(2.87)2 

FD = (1.59)(9810)(50.01) = 327 kN 

2(9.81) 

Total Force on Single Pier Accumulation 

 F = Total segment force = Fh + FD
 

 F = 175 + 327 = 502 kN 

Location of Forces on Single Pier Accumulation 

 

⎛ WS US − DB EL ⎞ ⎛ WS − DB F ⎜ ⎟ − F ⎜ DS EL ⎞ hu hd ⎟ ⎝ 3 ⎠ ⎝ 3 ⎠ FhEL 	 	 = Elevation of hydrostatic force = DB EL +

Fh 

 

⎛ 65.43 − 61.78 ⎞ ⎛ 65.06 − 61.78 ⎞ 898⎜ ⎟ − 723⎜ ⎟ ⎝ 3 ⎠ ⎝ 3 ⎠ FhEL = 61.78 +	 = 63.51m 

175
 

 FDEL = Elevation of drag force = 0.5(WSUS + DBEL) 

 FDEL = 0.5(65.43 + 61.78) = 63.61 m 

 

(FD )(F DEL ) + (Fh )(F hEL ) FEL	 	 = Elevation of total force = F 

 FEL =	 = 63.58 m 

502 

(327)(63.61) + (175)(63.51)

 FhST = Station of hydrostatic force = 0.5(Left station of debris + right station of debris) 

 FDST = Station of drag force = FhST 

 FST = Station of total force = FDST = FhST 

 FhST = FDST = FST = 0.5(160.33 + 174.03) = 167.18 m 

4.18
 
4.7.2 Example 2 – Hydraulic Loading on Two Adjacent Piers, Case 1 (SI) 

Given: 

Design flow rate = 85 m3

/s 

Minimum upstream main channel width = 13.7 m; design log length = 13.7 m 

Depth of debris is full-flow depth 

Main channel width at the bridge = 60 m 

Superstructure is not submerged 

Ineffective flow areas from debris defined by 1:1 contraction and 2:1 expansion 

Bottom elevation of Pile Bent 2 = 61.98 m; Pile Bent 3 = 61.92 m 

Total accumulation width = 25.4 m (defined by assuming that the accumulation extends 

laterally half the design log length beyond each pier). 

Accumulation width on Pile Bent 2 = 13.7 m for Case 1 and 11.7 for Case 2 

Accumulation width on Pile Bent 3 = 11.7 m for Case 1 and 13.7 for Case 2 

Pile Bent 2, left station of debris = 154.69 m; Right station of debris = 168.39 m 

Pile Bent 3, left station of debris = 168.39 m; Right station of debris = 180.09 m 

Hydraulic computation results are provided in Table 4.4 and shown in Figure 4.11 

Upstream water surface elevation, WSUS = 65.28 m (Table 4.4) 

Downstream water surface elevation, WSDS = 64.59 m (Table 4.4, see discussion of 

Downstream Water Surface Elevation in Subsection 4.3.3.1) 

68 Le gend 

WS De sign Di scharg e 

67 Ground 

Ba nk S ta 

66 Pi er De bris 

10 0 15 0 20 0 25 0 

Ele vation ( m) 

65 

64 

63 

62 

61 

St ation (m) 

Figure 4.10. Upstream face of the bridge for Example 2. 

4.19
 
67 

66 

65 

64 

63 

62 

61 

0 20 0 40 0 60 0 80 0 12 7 25 4 40 2.3 3 42 1.7 48 8.7 62 8.9 76 9.1 

Le gen d 

WS D es i gn D isc har ge
 

Gr oun d
 

El eva tion (m) 

60 

Ma in C ha nne l Di stan ce (m) 

Figure 4.11. Water surface profile for Example 2. 

Table 4.4. Results of Hydraulic Calculations for Example 2. 

River Station 

(m) 

Water 

Surface 

Elevation 

(m) 

Flow Area 

(m2

) 

Main 

Channel 

Velocity 

(m/s) 

Cross Section 

Average 

Velocity 

(m/s) 

Average 

Flow Depth1 

(m) 

0 63.22 28.81 3.52 2.95 1.38 

127 64.25 176.00 1.22 0.48 1.63 

254 64.42 176.31 0.82 0.48 1.08 

402.33 64.55 90.49 0.96 0.94 1.63 

421.7 BR D 64.56 83.47 1.02 1.02 1.63 

421.7 BR U 64.54 27.41 3.10 3.10 0.99 

440.46 65.30 142.17 0.64 0.60 2.20 

488.7 65.32 308.93 0.69 0.28 2.53 

628.9 65.35 308.60 0.79 0.28 2.44 

769.1 65.38 315.16 0.78 0.27 2.22 

Notes: 

1. For this example, the entire bridge opening was defined as the main channel. So, the average depth of the main channel is 

the same as the average depth of the entire cross section. 

4.20
 
Determine: 

Compute the hydrostatic and drag forces for a debris accumulation on two piers. The 

calculations are for only Case 1, which is based on the width of the debris accumulation 

on Pile Bent 2 being equal to the design log length and Pile Bent 3 having a smaller 

accumulation width. Case 2 is the reverse of Case 1, i.e., the width of the accumulation 

on Pile Bent 3 would be equal to the design log length and Pile Bent 2 would have a 

smaller accumulation width. Case 3 is based on the assumption that the design log 

length spans the opening approximately in the middle of the two piers and the resulting 

load on the accumulation is transferred equally to each pier. 

Solution: 

Hydrostatic Forces on Two Adjacent Piers (Case 1) 

 Ahu = Area of the debris accumulation below the upstream water surface 

 Ahu = (WSUS – Debris bottom, DBEL)(Width of debris accumulation, WD) 

 Ahu = (65.30 – 61.98)(13.70) = 45.48 m2 for Pier Bent 2 

 Ahu = (65.30 – 61.92)(11.70) = 39.55 m2 for Pier Bent 3 

 Ahd = Area of the debris accumulation below the downstream water surface 

 Ahd = (WSDS – DBEL)( WD) 

 A 2

hd = (64.55– 61.98)(13.70) = 35.21 m for Pier Bent 2 

A = (64.55– 61.92)(11.70) = 30.77 m2

hd for Pier Bent 3 

 hcu = Vertical distance to centroid of Ahu = 0.5(WSUS – DBEL) 

 hcu = 0.5(65.30 – 61.98) = 1.66 m for Pier Bent 2 

 hcu = 0.5(65.30 – 61.92) = 1.69 m for Pier Bent 3 

 hcd = Vertical distance to centroid of Ahd = 0.5(WSDS – DBEL) 

 hcd = 0.5(64.55 – 61.98) = 1.29 m for Pier Bent 2 

 hcd = 0.5(64.55 – 61.92) = 1.32 m for Pier Bent 3 

 Fhu = Hydrostatic force upstream = γhcuAhu
 

 Fhu = (9.81)(1.66)(45.48) = 741 kN for Pier Bent 2 

 Fhu = (9.81)(1.69)(39.55) = 656 kN for Pier Bent 3 

 Fhd = Hydrostatic force downstream = γhcdAhd
 

 Fhd = (9.81)(1.29)(35.21) = 446 kN for Pier Bent 2 

 Fhd = (9.81)(1.32)(30.77) = 399 kN for Pier Bent 3 

 Fh = Total hydrostatic force on Pile Bent = Fhu - Fhd
 

 Fh = 741 – 446 = 295 kN for Pier Bent 2 

 Fh = 656 – 399 = 257 kN for Pier Bent 3 

4.21
 
Drag Force on Two Adjacent Piers (Case 1) 

 

A d A + A

B = Blockage Ratio = = hu(pier 2) hu(pier 3)

A d + A c A hu(pier 2) + A hu(pier 3) + Ac 

 

45.48 + 39.55 B =	 = 0.76 

45.48 + 39.55 + 27.41 

 

B is greater than 0.3, therefore Vr should be based on the average velocity in the 

contracted section. 

 Vr = 3.10 m/s (Table 4.4)
 

Fr = Froude number =
 r

g yr
 

V
 

 

3.10 Fr =	 = 0.99 

(9.81)(0.99) 

 CD = Drag coefficient = 3.1 – 3.6B (Table 4.1) 

 CD = 3.1 – (3.6)(0.76) = 0.36 

 

V 2 

F = Drag force on Pile Bent = C γA r 

D	 D hu 2g 

 

(3.10)2 

FD = (0.36)(9810)(45.48) = 79 kN for Pile Bent 2 

2(9.81) 

 

(3.10)2 

FD = (0.36)(9810)(39.55) = 68 kN for Pile Bent 3

2(9.81) 

Total Force on Two Adjacent Piers (Case 1) 

 F = Total segment force = Fh + FD
 

 F = 295 + 79 = 374 kN for Pile Bent 2 

F = 257 + 68 = 325 kN for Pile Bent 3 

Location of Forces on Single Pier Accumulation 

⎛ WSUS − DBEL ⎞ ⎛ WS ⎜ ⎟ − ⎜ DS − DB ⎞ F hu	 F hd

EL ⎟ ⎝ 3 ⎠ ⎝ 3 ⎠ F	 = Elev hEL	 ation of hydrostatic force = DB EL +

Fh 

⎛ 65.30 − 61.98 ⎞ ⎛ 64.55 − 61.98 ⎞ 741⎜ ⎟ − 446⎜ ⎟
 ⎝ 3 ⎠ ⎝ 3 ⎠ FhEL = 61.98 +	 = 63.46 m for Pile Bent 2 

295 

4.22
⎛ 65.30 − 61.92 ⎞ ⎛ 64.55 − 61.92 ⎞ 656⎜ ⎟ − 399⎜ ⎟
 ⎝ 3 ⎠ ⎝ 3 ⎠ FhEL = 61.92 +	 = 63.43 m for Pile Bent 3 

257 

FDEL = Elevation of drag force = 0.5(WSUS + DBEL) 

FDEL = 0.5(65.30 + 61.98) = 63.64 m for Pile Bent 2 

FDEL = 0.5(65.30 + 61.72) = 63.51 m for Pile Bent 3 

(F D )(F DEL ) + (F h )(F hEL ) FEL = Elevation of total force = F
 

 

(79)(63.64) + (295)(63.46) FEL =	 = 63.50 m for Pile Bent 2 

374
 

 

(68)(63.51) + (257)(63.43) FEL =	 = 63.45 m for Pile Bent 3 

325 

FhST = Station of hydrostatic force = 0.5(Left station of debris + right station of debris) 

FDST = Station of drag force = FhST 

FST = Station of total force = FDST = FhST 

FhST = FDST = FST = 0.5(154.69 + 168.39) = 161.54 m for Pile Bent 2 

FhST = FDST = FST = 0.5(138.39 + 180.09) = 174.24 m for Pile Bent 3 

4.23
 
4.7.3 Example 3 – Hydraulic Loading on a Superstructure (SI) 

Given: 

Design flow rate = 220 m3

/s 

Low chord elevation of bridge = 65.5 m 

Depth of debris is 1.2 meters below the bridge low chord = 64.3 m 

Debris accumulation extends along the entire length of the structure (see Figure 4.12) 

Main channel width at the bridge = 60.0 m 

Left station of debris = 137.16 m; Right station of debris = 197.21 m 

Hydraulic computation results are provided in Table 4.5 and shown in Figure 4.13 

Upstream water surface elevation, WSUS = 65.71 m (Table 4.5) 

Downstream water surface elevation, WSDS = 65.13 m (Table 4.5, see discussion of 

Downstream Water Surface Elevation in Subsection 4.3.3.1) 

100 150 200 250 61 

62 

63 

64 

65 

66 

67 

68 

Station (m) 

Elevation (m) 

Legend 

WS Design Discharge 

Ground 

Bank Sta 

Debris 

Accumulation 

Figure 4.12. Upstream face of the bridge for Example 3. 

4.24
 
0 20 0 40 0 60 0 80 0 

60 

61 

62 

63 

64 

65 

66 

67 

Ma in C ha nne l Di stan ce (m) 

El eva tion (m) 

Le gen d 

WS D es i gn D isc har ge 

Gr oun d 

12 725 440 6.442 1.748 8.762 8.976 9.1 

Figure 4.13. Water surface profile for Example 3. 

Table 4.5. Results of Hydraulic Calculations for Example 3. 

River Station 

(m) 

Water 

Surface 

Elevation 

(m) 

Flow Area 

(m2

) 

Main 

Channel 

Velocity 

(m/s) 

Cross Section 

Average 

Velocity 

(m/s) 

Average 

Flow Depth1 

(m) 

0 64.36 255.76 2.54 0.86 2.43 

127 64.79 325.88 1.64 0.68 2.17 

254 64.98 333.72 1.07 0.66 1.64 

406.4 65.13 130.59 1.80 1.68 2.03 

421.7 BR D 64.30 75.34 2.92 2.92 1.45 

421.7 BR U 64.30 75.34 2.92 2.92 1.45 

436.9 65.71 171.53 1.39 1.28 2.61 

488.7 65.85 563.03 1.03 0.39 3.06 

628.9 65.89 573.58 1.10 0.38 2.99 

769.1 65.94 579.47 1.05 0.38 2.78 

Notes: 

1. For this example, the entire bridge opening was defined as the main channel. So, the average depth of the main channel is 

the same as the average depth of the entire cross section. 

4.25
 
Determine: 

Compute the hydrostatic and drag forces for a debris accumulation on a superstructure. 

Solution: 

Hydrostatic Force on Superstructure Accumulation 

 Ahu = Area of the debris accumulation below the upstream water surface 

 Ahu = (WSUS – Debris bottom, DBEL)(Width of debris accumulation, WD) 

 A 2 

hu = (65.71 – 64.30)(60.0) = 84.60 m

 Ahd = Area of the debris accumulation below the downstream water surface 

 Ahd = (WSDS – DBEL)(WD) 

 A 2 

hd = (65.13 – 64.30)(60.0) = 49.80 m

 hcu = Vertical distance to centroid of Ahu = 0.5(WSUS – DBEL) 

 hcu = 0.5(65.71 – 64.30) = 0.71 m 

 hcd = Vertical distance to centroid of Ahd = 0.5(WSDS – DBEL) 

 hcd = 0.5(65.13 – 64.30) = 0.42 m 

 Fhu = Hydrostatic force upstream = γhcuAhu 

 Fhu = (9.81)(0.71)(84.60) = 589 kN 

 Fhd = Hydrostatic force downstream = γhcdAhd 

 Fhd = (9.81)(0.42)(49.80) = 205 kN 

 Fh = Total hydrostatic force on Pile Bent 2 = Fhu - Fhd 

 Fh = 589 – 205 = 384 kN 

Drag Force on Superstructure Accumulation 

B = Blockage ratio = d 

A d + Ac 

A

84.60 B = = 0.53

84.60 + 75.34 

V
 Fr = Froude number = r

g yr 

B is greater than 0.3, therefore Vr should be based on the average velocity in the 

contracted section. 

 Vr = 2.92 m/s (Table 4.5)
 

 

4.26
2.92 Fr =	 = 0.77 

(9.81)(1.45) 

 CD = Drag coefficient = 3.1 – 3.6B (Table 4.1) 

 CD = 3.1 – (3.6)(0.53) = 1.19 

V 2 

F	 = Drag force on sup erstructure = C γA r 

D	 D hu 2g 

 

(2.92)2 

FD = (1.19)(9810)(84.60) = 429 kN

2(9.81) 

Total Force on Superstructure Accumulation 

 F = Total segment force = Fh + FD
 

 F = 384 + 429 = 813 kN 

Location of Forces on Superstructure Accumulation 

⎛ WS US − DB EL ⎞ ⎛ WS − DB ⎞ Fhu 	 ⎜ ⎟ − F hd ⎜ DS EL ⎟ ⎝ 3 ⎠ ⎝ 3 ⎠ FhEL = Elevation of hydrostatic force = DBEL +
 F h
 

 

⎛ 65.71− 64.30 ⎞ ⎛ 65.13 − 64.30 ⎞ 589⎜ ⎟ − 205⎜ ⎟ ⎝ 3 ⎠ ⎝ 3 ⎠ FhEL = 64.30 +	 = 64.87 m 

384 

 FDEL = Elevation of drag force = 0.5(WSUS + DBEL) 

 FDEL = 0.5(65.71 + 64.30) = 65.00 m 

(F D )(F DEL ) + (F h )(F hEL ) FEL = Elevation of total force =

 F
 

FEL =	 = 64.94 m 

 813 

(429)(65.00) + (384)(64.87)

 FhST = Station of hydrostatic force = 0.5(Left station of debris + right station of debris) 

 FDST = Station of drag force = FhST 

 FST = Station of total force = FDST = FhST 

 FhST = FDST = FST = 0.5(137.16 + 197.21) = 167.19 m 

4.27
 
4.7.4 Example 4 – Hydraulic Loading on a Single Pier (CU) 

Given: 

Design flow rate = 6,890 ft3

/s 

Minimum upstream main channel width = 45 ft; design log length of 45 ft 

Depth of debris is full-flow depth 

Main channel width at the bridge = 197 ft 

Debris accumulation only on Pile Bent 2 (see Figure 4.14) 

Superstructure is not submerged 

Ineffective flow areas from the debris defined by 1:1 contraction and 2:1 expansion 

Bottom elevation of Pile Bent 2 = 202.69 ft 

Left station of debris = 526.02 ft; Right station of debris = 570.97 ft 

Hydraulic computation results are provided in Table 4.6 and shown in Figure 4.15 

Upstream water surface elevation, WSUS = 214.66 ft (Table 4.6) 

Downstream water surface elevation, WSDS = 213.44 ft (Table 4.6, see discussion of 

Downstream Water Surface Elevation in Subsection 4.3.3.1) 

22 5 Le gend 

WS De sign Di scharg e 

Ground 

22 0 

Ba nk S ta 

Pi er De bris 

20 0 30 0 40 0 50 0 60 0 70 0 80 0 90 0 

Ele vation ( ft) 

21 5 

21 0 

20 5 

20 0 

St ation (ft) 

Figure 4.14. Upstream face of the bridge for Example 4. 

4.28
 
0 50 0 10 00 15 00 20 00 25 00 30 00 

19 8 

20 0 

20 2 

20 4 

20 6 

20 8 

21 0 

21 2 

21 4 

21 6 

21 8 

Ma in C ha nne l Di s tan ce (ft) 

El eva tion (ft) 

Le gen d 

WS D es i gn D isc har ge 

Gr oun d 

5.2 15.2 95.3 85.4 05.4 35.5 25.6 0 

Figure 4.15. Water surface profile for Example 4. 

Table 4.6. Results of Hydraulic Calculations for Example 4. 

River Station 

(miles) 

Water 

Surface 

Elevation 

(ft) 

Flow Area 

(ft2

) 

Main 

Channel 

Velocity 

(ft/s) 

Cross Section 

Average 

Velocity 

(ft/s) 

Average 

Flow Depth1 

(ft) 

5.13 210.86 2487.36 8.12 2.77 7.69 

5.21 212.27 3231.47 5.23 2.13 6.81 

5.29 212.90 3312.69 3.39 2.08 5.08 

5.38 213.41 1333.33 5.46 5.17 6.40 

5.39 BR D 213.44 1186.32 5.81 5.81 6.36 

5.39 BR U 213.21 731.66 9.42 9.42 5.21 

5.40 214.66 1605.03 4.56 4.29 7.62 

5.43 215.09 4472.12 4.04 1.54 9.08 

5.52 215.34 4733.89 4.22 1.46 8.95 

5.60 215.58 4997.62 3.91 1.38 8.36 

Notes: 

1. For this example, the entire bridge opening was defined as the main channel. So, the average depth of the main channel is 

the same as the average depth of the entire cross section. 

4.29
 
Determine: 

Compute the hydrostatic and drag forces for a debris accumulation on a single pier. 

Solution: 

Hydrostatic Force on Single Pier Accumulation 

 Ahu = Area of the debris accumulation below the upstream water surface 

 Ahu = (WSUS – Debris bottom, DBEL)(Width of debris accumulation, WD) 

 A 2 

hu = (214.66 – 202.69)(45) = 538.65 ft

 Ahd = Area of the debris accumulation below the downstream water surface 

 Ahd = (WSDS – DBEL)(WD) 

 Ahd = (213.44 – 202.69)(45) = 483.75 ft2 

 hcu = Vertical distance to centroid of Ahu = 0.5(WSUS – DBEL) 

 hcu = 0.5(214.66 – 202.69) = 5.98 ft 

 hcd = Vertical distance to centroid of Ahd = 0.5(WSDS – DBEL) 

 hcd = 0.5(213.44 – 202.69) = 5.38 ft 

 Fhu = Hydrostatic force upstream = γhcuAhu 

 Fhu = (62.4)(5.98)(538.65) = 200,998 lbs (100.5 tons) 

 Fhd = Hydrostatic force upstream = γhcdAhd 

 Fhd = (62.4)(5.38)(483.75) = 162,401 lbs (81.2 tons) 

 Fh = Total hydrostatic force on Pile Bent 2 = Fhu - Fhd 

 Fh = 200,998 – 162,401 = 38,597 lbs (19.3 tons) 

Drag Force on Single Pier Accumulation 

A B = Blockage ratio = d

Ad + A c 

538.65 B = = 0.42 

538.65 + 731.66 

 

B is greater than 0.3, therefore Vr should be based on the average velocity in the 

contracted section. 

 Vr = 9.42 ft/s (Table 4.6)
 

4.30
 
 

V Fr = Froude number =
 r

g yr
 

Fr = 9.42 = 0.73 

(32.2)(5.21) 

 CD = Drag coefficient = 3.1 – 3.6B (Table 4.1) 

 CD = 3.1 – (3.6)(0.42) = 1.59 

 

V2 

F r

D = Drag force on Pile Bent 2 = CD γAhu 2g 

 

(9.42)2 

FD = (1.59)(62.4)(538.65) = 73,638 lbs (36.8 tons) 2(32.2) 

Total Force on Single Pier Accumulation 

 F = Total segment force = Fh + FD
 

 F = 38,597 + 73,638 = 112,235 lbs (56.1 tons) 

Location of Forces on Single Pier Accumulation 

⎛ WS US − DBEL ⎞ ⎛ WS − DB ⎞ F hu ⎜ ⎟ − F hd ⎜ DS EL ⎟ ⎝ 3 ⎠ ⎝ 3 ⎠ FhEL = Elevation of hydrostatic force = DBEL +
 Fh
 

 

⎛ 214.66 − 202.69 ⎞ ⎛ 213.44 − 202.69 ⎞ 100.5⎜ ⎟ − 81.2⎜ ⎟ ⎝ 3 ⎠ ⎝ 3 ⎠ FhEL = 202.69 + = 208.39 ft 

19.3 

 FDEL = Elevation of drag force = 0.5(WSUS + DBEL) 

 FDEL = 0.5(214.66 + 202.69) = 208.68 ft 

 

(F D )(F DEL ) + (F h )(F hEL ) FEL = Elevation of total force = F
 

 

(36.8)(208.68) + (19.3)(208.39) FEL = = 208.58 ft 

56.1 

 

 FhST = Station of hydrostatic force = 0.5(Left station of debris + right station of debris) 

 FDST = Station of drag force = FhST 

 FST = Station of total force = FDST = FhST 

 FhST = FDST = FST = 0.5(526.02 + 570.97) = 548.50 ft 

4.31
 
4.7.5 Example 5 – Hydraulic Loading on Two Adjacent Piers, Case 1 (CU) 

Given: 

Design flow rate = 3,000 ft3

/s 

Minimum upstream main channel width = 45 ft; design log length = 45 ft 

Depth of debris is full-flow depth 

Main channel width at the bridge = 197 ft 

Superstructure is not submerged 

Ineffective flow areas from debris defined by 1:1 contraction and 2:1 expansion 

Bottom elevation of Pile Bent 2 = 203.35 ft; Pile Bent 3 = 203.15 ft 

Total accumulation width = 83.4 ft (defined by assuming that the accumulation extends 

laterally half the design log length beyond each pier). 

Accumulation width on Pile Bent 2 = 45 ft for Case 1 and 38.4 ft for Case 2 

Accumulation width on Pile Bent 3 = 38.4 ft for Case 1 and 45 ft for Case 2 

Pile Bent 2, left station of debris = 507.53 ft; Right station of debris = 552.48 ft 

Pile Bent 3, left station of debris = 552.48 ft; Right station of debris = 590.87 ft 

Hydraulic computation results are provided in Table 4.7 and shown in Figure 4.17 

Upstream water surface elevation, WSUS = 214.17 ft (Table 4.7) 

Downstream water surface elevation, WSDS = 211.89 ft (Table 4.7, see discussion of 

Downstream Water Surface Elevation in Subsection 4.3.3.1) 

22 5 Le gend 

WS De sign Di scharg e 

Ground 

22 0 

Ba nk S ta 

Pi er De bris 

20 0 30 0 40 0 50 0 60 0 70 0 80 0 90 0 

Ele vation ( ft) 

21 5 

21 0 

20 5 

20 0 

St ation (ft) 

Figure 4.16. Upstream face of the bridge for Example 5. 

4.32
 
0 50 0 10 00 15 00 20 00 25 00 30 00 

19 8 

20 0 

20 2 

20 4 

20 6 

20 8 

21 0 

21 2 

21 4 

21 6 

21 8 

Ma in C ha nne l Di s tan ce (ft) 

El eva tion (ft) 

Le gen d 

WS D es i gn D isc har ge 

Gr oun d 

5.2 15.2 95.3 85.3 95.4 35.5 25.6 0 

Figure 4.17. Water surface profile for Example 5. 

Table 4.7. Results of Hydraulic Calculations for Example 5. 

River Station 

(miles) 

Water 

Surface 

Elevation 

(ft) 

Flow Area 

(ft2

) 

Main 

Channel 

Velocity 

(ft/s) 

Cross Section 

Average 

Velocity 

(ft/s) 

Average 

Flow Depth1 

(ft) 

5.13 207.42 310.14 11.55 9.68 4.53

5.21 210.80 1894.63 4.00 1.57 5.35

5.29 211.36 1897.97 2.69 1.57 3.54

5.38 211.79 974.12 3.15 3.08 5.35

5.39 BR D 211.82 898.55 3.35 3.35 5.35 

5.39 BR U 211.76 295.07 10.17 10.17 3.25 

5.40 214.25 1530.45 2.10 1.97 7.22

5.43 214.31 3325.62 2.26 0.92 8.30

5.52 214.41 3322.07 2.59 0.92 8.01

5.60 214.51 3392.69 2.56 0.89 7.28

Notes: 

1. For this example, the entire bridge opening was defined as the main channel. So, the average depth of the main channel is 

the same as the average depth of the entire cross section. 

4.33
 
Determine: 

Compute the hydrostatic and drag forces for a debris accumulation on two piers. The 

calculations are for only Case 1, which is based on the width of the debris accumulation 

on Pile Bent 2 being equal to the design log length and Pile Bent 3 having a smaller 

accumulation width. Case 2 is the reverse of Case 1, i.e., the width of the accumulation 

on Pile Bent 3 would be equal to the design log length and Pile Bent 2 would have a 

smaller accumulation width. Case 3 is based on the assumption that the design log 

length spans the opening approximately in the middle of the two piers and the resulting 

load on the accumulation is transferred equally to each pier. 

Solution: 

Hydrostatic Forces on Two Adjacent Piers (Case 1) 

 Ahu = Area of the debris accumulation below the upstream water surface 

 Ahu = (WSUS – Debris bottom, DBEL)(Width of debris accumulation, WD) 

 A 2

hu = (214.25 – 203.35)(45) = 490.50 ft for Pile Bent 2 

 Ahu = (214.25 – 203.15)(38.4) = 426.24 ft2 for Pile Bent 3 

 Ahd = Area of the debris accumulation below the downstream water surface 

 Ahd = (WSDS – DBEL)(WD) 

 Ahd = (211.82 – 203.35)(45) = 381.15 ft2 for Pile Bent 2 

 A = (211.82 – 203.15)(38.4) = 332.93 ft2

hd for Pile Bent 3 

 hcu = Vertical distance to centroid of Ahu = 0.5(WSUS – DBEL) 

 hcu = 0.5(214.25 – 203.35) = 5.45 ft for Pile Bent 2 

 hcu = 0.5(214.25 – 203.15) = 5.55 ft for Pile Bent 3 

 hcd = Vertical distance to centroid of Ahd = 0.5(WSDS – DBEL) 

 hcd = 0.5(211.82 – 203.35) = 4.24 ft for Pile Bent 2 

 hcd = 0.5(211.82 – 203.15) = 4.34 ft for Pile Bent 3 

 Fhu = Hydrostatic force upstream = γhcuAhu 

 Fhu = (62.4)(5.45)(490.50) = 166,809 lbs (83.4 tons) for Pile Bent 2 

 Fhu = (62.4)(5.55)(426.24) = 147,615 lbs (73.8 tons) for Pile Bent 3 

 Fhd = Hydrostatic force upstream = γhcdAhd 

 Fhd = (62.4)(4.24)(381.15) = 100,843 lbs (50.4 tons) for Pile Bent 2 

 Fhd = (62.4)(4.34)(332.93) = 90,163 lbs (45.1 tons) for Pile Bent 3 

 Fh = Total hydrostatic force on Pile Bent 2 = Fhu - Fhd 

 Fh = 166,809 – 100,843 = 65,966 lbs (33.0 tons) for Pile Bent 2 

 Fh = 147,615 – 90,163 = 57,452 lbs (28.7 ton) for Pile Bent 3 

4.34
 
 

 

Drag Force on Two Adjacent Piers (Case 1) 

A d A

B hu pier 2 + A = Blockage Ratio = = hu pier 3

A d + A c A hu pier 2 + A hu pier 3 + Ac 

490.50 + 426.24 B = = 0.76 

490.50 + 426.24 + 295.07 

 

B is greater than 0.3, therefore Vr should be based on the average velocity in the 

contracted section. 

 Vr = 10.17 ft/s (Table 4.7)
 

Vr
 Fr = Froude number =
 g yr
 

 

10.17 Fr = = 0.99 (32.2)(3.25) 

 CD = Drag coefficient = 3.1 – 3.6B (Table 4.1) 

 CD = 3.1 – (3.6)(0.76) = 0.36 

 

V2 

F = Drag force on Pile Bent = C γA r

D D hu 2g 

 

(10.17)2 

F D = (0.36)(62.4)(490.50) = 19,589 lbs (9.8 tons) for Pile Bent 2 2(32.2) 

 

(10.17)2 

FD = (0.36)(62.4)(426.24) = 17,023 lbs (8.5 tons) for Pile Bent 3 2(32.2) 

Total Force on Two Adjacent Piers (Case 1) 

 F = Total segment force = Fh + FD
 

 F = 65,966 + 19,589 = 85,555 lbs (42.8 tons) for Pile Bent 2 

 F = 57,452 + 17,023 = 74,475 lbs (37.2 tons) for Pile Bent 3 

Location of Forces on Single Pier Accumulation 

⎛ WS US − DB EL ⎞ ⎛ WS DS − DB EL ⎞ F hu ⎜ ⎟ − F hd ⎜ ⎟ ⎝ 3 ⎠ ⎝ 3 ⎠ F hEL = Elevation of hydrostatic force = DB EL +

Fh 

⎛ 214.25 − 203.35 ⎞ ⎛ 211.82 − 203.35 ⎞ 83.40⎜ ⎟ − 50.4⎜ ⎟ ⎝ 3 ⎠ ⎝ 3 ⎠ FhEL = 203.35 + = 208.22 ft for Pile Bent 2 

33.0 

4.35
⎛ 214.25 − 203.15 ⎞ ⎛ 211.82 − 203.15 ⎞ 73.8⎜ ⎟ − 45.10⎜ ⎟ ⎝ 3 ⎠ ⎝ 3 ⎠ FhEL = 203.15 + = 208.12 ft for Pile Bent 3 

28.7 

 FDEL = Elevation of drag force = 0.5(WSELUS + DBEL) 

 FDEL = 0.5(214.25 + 203.35) = 208.80 ft for Pile Bent 2 

 FDEL = 0.5(214.25 + 203.15) = 208.70 ft for Pile Bent 3 

 

(F D )(F DEL ) + (F h )(F hEL ) FEL = Elevation of total force = F
 

 

(9.8)(208.80) + (33.0)(208.22) FEL = = 208.35 ft for Pile Bent 2 

42.8
 

 

(8.5)(208.70) + (28.7)(208.12) FEL = = 208.25 ft for Pile Bent 3 

37.2 

 FhST = Station of hydrostatic force = 0.5(Left station of debris + right station of debris) 

 FDST = Station of drag force = FhST 

 FST = Station of total force = FDST = FhST 

 FhST = FDST = FST = 0.5(507.53 + 552.48) = 530.00 ft for Pile Bent 2 

 FhST = FDST = FST = 0.5(552.48 + 590.87) = 571.67 ft for Pile Bent 3 

4.36
 
4.7.6 Example 6 – Hydraulic Loading on a Superstructure (CU) 

Given: 

Design flow rate = 7,770 ft3

/s 

Low Chord Elevation = 214.91 ft 

Depth of debris is 3.94 feet below the bridge low chord = 210.97 ft 

Debris accumulation extends along the entire length of the structure (see Figure 4.18) 

Main channel width at the bridge = 197 ft 

Left station of debris = 450.00 ft; Right station of debris = 647.01 ft 

Hydraulic computation results are provided in Table 4.8 and shown in Figure 4.19 

Upstream water surface elevation, WSUS = 215.59 ft (Table 4.8) 

Downstream water surface elevation, WSDS = 213.69 ft (Table 4.8, see discussion of 

Downstream Water Surface Elevation in Subsection 4.3.3.1) 

22 5 Le gend 

WS Su per Flows 

Ground 

22 0 

Ba nk S ta 

Debris 

Accumulation 

20 0 30 0 40 0 50 0 60 0 70 0 80 0 90 0 

Ele vation ( ft) 

21 5 

21 0 

20 5 

20 0 

St ation (ft) 

Figure 4.18. Upstream face of the bridge for Example 6. 

4.37
 
0 50 0 10 00 15 00 20 00 25 00 30 00 

19 8 

20 0 

20 2 

20 4 

20 6 

20 8 

21 0 

21 2 

21 4 

21 6 

21 8 

Ma in C ha nne l Di s tan ce (ft) 

El eva tion (ft) 

Le gend 

WS Supe r Fl ows 

Gr oun d 

5.2 15.2 95.3 85.4 05.4 35.5 25.6 0 

Figure 4.19. Water surface profile for Example 6. 

Table 4.8. Results of Hydraulic Calculations for Example 6. 

River Station 

(miles) 

Water 

Surface 

Elevation 

(ft) 

Flow Area 

(ft2

) 

Main 

Channel 

Velocity 

(ft/s) 

Cross Section 

Average 

Velocity 

(ft/s) 

Average 

Flow Depth 

Depth1 

 

(ft) 

5.13 211.17 2753.25 8.33 2.82 7.97 

5.21 212.58 3508.09 5.38 2.23 7.12 

5.29 213.20 3592.48 3.51 2.17 5.38 

5.38 213.69 1405.80 5.91 5.51 6.66 

5.39 BR D 210.97 811.03 9.58 9.58 4.76 

5.39 BR U 210.97 811.03 9.58 9.58 4.76 

5.40 215.59 1846.51 4.56 4.20 8.56 

5.43 216.05 6061.00 3.38 1.28 10.04 

5.52 216.19 6174.57 3.61 1.25 9.81 

5.60 216.35 6237.97 3.45 1.25 9.12 

Notes: 

1. For this example, the entire bridge opening was defined as the main channel. So, the average depth of the main channel is 

the same as the average depth of the entire cross section. 

4.38
 
Determine: 

Compute the hydrostatic and drag forces for a debris accumulation on a superstructure. 

Solution: 

Hydrostatic Force on Superstructure Accumulation 

 Ahu = Area of the debris accumulation below the upstream water surface 

 Ahu = (WSUS – Debris bottom, DBEL)(Width of debris accumulation, WD) 

A = (215.59 – 210.97)(197.0) = 910.14 ft2 

hu

 Ahd = Area of the debris accumulation below the downstream water surface 

 Ahd = (WSDS – DBEL)(WD) 

 A 2 

hd = (213.69 – 210.97)(197.0) = 535.84 ft

 

 hcu = Vertical distance to centroid of Ahu = 0.5(WSUS – DBEL) 

 hcu = 0.5(215.59 – 210.97) = 2.31 ft 

 hcd = Vertical distance to centroid of Ahd = 0.5(WSDS – DBEL) 

 hcd = 0.5(213.69 – 210.97) = 1.36 ft 

 Fhu = Hydrostatic force upstream = γhcuAhu 

 Fhu = (62.4)(2.31)(910.14) = 131,191 lbs (65.6 tons) 

 Fhd = Hydrostatic force upstream = γhcdAhd 

 Fhd = (62.4)(1.36)(535.84) = 45,474 lbs (22.7 tons) 

 Fh = Total hydrostatic force on Pile Bent 2 = Fhu - Fhd 

 Fh = 131,191 – 45,474 = 85,717 lbs (42.9 tons) 

Drag Force on Superstructure Accumulation 

A B = Blockage ratio = d

Ad + A c 

B = = 0.53

910.40 + 811.03 

 

910.40

B is greater than 0.3, therefore Vr should be based on the average velocity in the 

contracted section. 

 Vr = 9.58 ft/s (Table 4.8)
 

4.39
 
 

V Fr = Froude number =
 r

g yr
 

 

9.58 Fr = = 0.77 

(32.2)(4.76) 

 CD = Drag coefficient = 3.1 – 3.6B (Table 4.1) 

 CD = 3.1 – (3.6)(0.53) = 1.19 

V2 

F r

D = Drag force on sup erstructure = CD γAhu 2g 

 

 

 

(9.58)2 

FD = (1.19)(62.4)(910.14) = 96,313 lbs (48.1 tons) 2(32.2) 

Total Force on Superstructure Accumulation 

 F = Total segment force = Fh + FD
 

 F = 85,717 + 96,313 = 182,030 lbs (91.0 tons) 

Location of Forces on Superstructure Accumulation 

⎛ WS US − DBEL ⎞ ⎛ WS DS − DBEL ⎞ F hu ⎜ ⎟ − F hd ⎜ ⎟ ⎝ 3 ⎠ ⎝ 3 ⎠ FhEL = Elevation of hydrostatic force = DBEL +
 Fh
 

 

⎛ 215.59 − 210.97 ⎞ ⎛ 213.69 − 210.97 ⎞ 65.6⎜ ⎟ − 22.07⎜ ⎟ ⎝ 3 ⎠ ⎝ 3 ⎠ FhEL = 210.97 + = 212.85 ft 

42.9
 

FDEL = Elevation of drag force = 0.5(WSUS + DBEL) 

 FDEL = 0.5(215.59 + 210.97) = 213.28 ft 

 

(F D )(F DEL ) + (F h )(F hEL ) FEL = Elevation of total force = F
 

 

(48.1)(213.28) + (42.9)(212.85) FEL = = 213.08 ft 

91.0 

 FhST = Station of hydrostatic force = 0.5(Left station of debris + right station of debris) 

 FDST = Station of drag force = FhST 

 FST = Station of total force = FDST = FhST 

 FhST = FDST = FST = 0.5(450.00 + 647.01) = 548.51 ft 

4.40
 
CHAPTER 5 – DEBRIS COUNTERMEASURES 

5.1 BACKGROUND 

Countermeasures to mitigate and protect effects of debris depend on the type of structure. 

Typically, these countermeasures are grouped into structural and non-structural measures. The 

structural measures have many configurations and constructed from many materials. The nonstructural measures typically involve long-term approaches. 

This Chapter will describe debris countermeasures for culverts and bridges. As will be described 

in this and subsequent Chapters, while some countermeasures may have some applicability to 

both types of structures (as well as other not described herein), engineering judgment on use 

remains a key design consideration. 

5.2 COUNTERMEASURES FOR CULVERTS 

5.2.1 Structural Measures 

There are various types of structural measures available for culverts. These measures can have 

many shapes and can be constructed using various materials. The measures can generally be 

divided into the following types: 

Debris Deflectors are structures placed at the culvert inlet to deflect the major portion of the 

debris away from the culvert entrance. They are normally "V"-shaped in plan with the apex 

upstream. Examples of this type of structure measure are shown in Figure 5.1 through Figure 

5.9. 

Debris Racks are structures placed across the stream channel to collect the debris before it 

reaches the culvert entrance. Debris racks are usually vertical and at right angles to the 

streamflow, but they may be skewed with the flow or inclined with the vertical. Pictures of debris 

racks are shown in Figure 5.10 through Figure 5.22. 

Debris Risers are a closed-type structure placed directly over the culvert inlet to cause 

deposition of flowing debris and fine detritus before it reaches the culvert inlet. Risers are 

usually built of metal pipe. Examples of debris risers are shown in Figure 5.23 through Figure 

5.25. Risers can also be used as relief devices in the event the entrance becomes completely 

blocked with debris (Figure 5.25). 

Debris Cribs are open crib-type structures placed vertically over the culvert inlet in log-cabin 

fashion to prevent inflow of coarse bed load and light floating debris. Photos of this type of 

structure are provided in Figure 5.26 through Figure 5.28. 

Debris Fins are walls built in the stream channel upstream of the culvert. Their purpose is to 

align the debris with the culvert so that the debris would pass through the culvert without 

5.1
 
accumulating at the inlet. This type of measure can also be used at bridge. Examples of this 

type of structure measure for culverts are shown in Figure 5.29 through Figure 5.35. 

Debris Dams and Basins are structures placed across well-defined channels to form basins 

which impede the stream flow and provide storage space for deposits of detritus and floating 

debris. This type of structure is shown in Figure 5.36 through Figure 5.39. 

Combination Devices are a combination of two or more of the preceding debris-control 

structures at one site to handle more than one type of debris and to provide additional insurance 

against the culvert inlet from becoming clogged. Examples of combination devices are shown in 

Figure 5.40. 

5.2.2 Non-structural Measures 

The only type of non-structural measures available for culvert structures is to provide 

emergency and annual maintenance. Although not always feasible for remote culverts or 

culverts with small drainage areas, maintenance could be a viable option for larger culverts with 

fairly large drainage basins. Emergency maintenance could involve removing debris from the 

culvert entrance and/or an existing debris-control structure. Annual maintenance could involve 

removing debris from within the culvert, at the culvert entrance, and/or immediately upstream of 

the culvert, or repairing any existing structural measures. 

5.3 COUNTERMEASURES FOR BRIDGES 

5.3.1 Structural Measures 

Various types of structural measures are also available for bridge structures. Some of the 

measures discussed above for the culvert structures can also be utilized at bridges. The various 

types include: 

Debris Fins are walls built in the stream channel upstream of the bridge to align large floating 

trees so that their length is parallel to the flow, enabling them to pass under the bridge without 

incident. This type of measure is also referred to as a “pier nose extension”. Examples of debris 

fin deflectors are provided in Figure 5.35. 

In-channel Debris Basins are structures placed across well-defined channels to form basins 

which impede the streamflow and provide storage space for deposits of detritus and floating 

debris. These structures can be expensive to construct and maintain. This type of structure is 

shown in Figure 5.36 through Figure 5.40. 

River-Training Structures are structures placed in the river flow to create counter-rotating 

streamwise vortices in their wakes to modify the near-bed flow pattern to redistribute flow and 

sediment transport within the channel cross section. Examples of this type of structure include 

5.2
 
Iowa vanes, and impermeable and permeable spurs. This type of structure is shown in Figure 

5.41 and Figure 5.42. 

Crib Structures are walls built between open-pile bents to prevent debris lodging between the 

bents. The walls are typically constructed out of timber or metal material. 

Flood Relief Sections are overtopping or flow through structures that divert excess flow and 

floating debris away from the bridge structure and through the structure. 

Debris Deflectors are structures placed upstream of the bridge piers to deflect and guide 

debris through the bridge opening. They are normally "V"-shaped in plan with the apex 

upstream. An example of this type of structure is shown in Figure 5.43. A special type of debris 

deflector is a hydrofoil. Hydrofoils are submerged structures placed immediately upstream of 

bridge piers that create counter-rotating streamwise vortices in their wakes to deflect and divert 

floating debris around the piers and through the bridge opening. Unfortunately, no hydrofoils 

have been implemented within the field. They have only been tested within a physical model 

study.(56) 

Debris Sweeper is a polyethylene device that is attached to a vertical stainless steel cable or 

column affixed to the upstream side of the bridge pier. The polyethylene device travels vertically 

along the pier as the water surface rises and falls. It is also rotated by the flow, causing the 

debris to be deflected away from the pier and through the bridge opening. This type of device is 

shown in Figure 5.44 through Figure 5.47. 

Booms are logs or timbers that float on the water surface to collect floating drift. Drift booms 

require guides or stays to hold them in place laterally. Booms are very limited in use and their 

application is not covered within this manual. 

Design Features are structural features that can be implemented in the design of a proposed 

bridge structure. The first feature is freeboard, which is a safety precaution of providing 

additional space between the maximum water surface elevation and the low chord elevation of 

the bridge. The second feature is related to the type of piers and the location and spacing of the 

piers. Ideally, the piers should be a solid wall type pier that is aligned with the approaching flow. 

They should also be located and spaced such that the potential for debris accumulation is 

minimized. The third feature involves the use of special superstructure design, such as thin 

decks, to prevent or reduce the debris accumulation on the structure when the flood stage rises 

above the deck. The last feature involves providing adequate access to the structure for 

emergency and annual maintenance. 

5.3.2 Non-Structural Measures 

There are generally two types of non-structural measures available for bridge structures. The 

first type of non-structural measure is emergency and annual maintenance. Emergency 

maintenance could involve removing debris from the bridge piers and/or abutments; placing 

riprap near the piers, abutments, or where erosion is occurring due to flow impingement created 

5.3
 
by the debris accumulation; and/or dredging of the channel bottom. Annual maintenance could 

involve debris removal and repair to any existing structural measures. 

The second type of non-structural measure is management of the upstream watershed. The 

purpose of this measure is to reduce the amount of debris delivered to the structure by reducing 

the sources of debris, preventing the debris from being introduced into the streams, and clearing 

debris from the stream channels. The type of management system implemented varies 

depending on the type of debris. For organic floating debris, the management system could 

involve removing dead and decayed trees, and/or debris jams; providing buffer zones for areas 

where logging practices exist; implementing a cable-assisted felling of trees system; and 

stabilizing hillside slopes and stream banks. 

5.4 COUNTERMEASURES FOR FIRE DAMAGED / DEFORESTED AREAS 

5.4.1 Fire Damaged Areas 

Fires can decrease the amount of floating debris introduced into the stream system. However, 

fires increase the magnitude of runoff from the burned area, increase the erodibility of soils, and 

increase the probability of catastrophic events such as debris flows and landslides, resulting in a 

significant increase in sediment yield from the effected area. This increase could cause an 

increase in fine and coarse detritus to be transported to and deposited at a culvert or bridge 

structure. Countermeasures that can be implemented to reduce the amount of material 

transported to a drainage structure include: 

Surface Treatments are countermeasures that are placed directly on the burnt landscape to 

reduce the potential for erosion from the disturbed area. There are various types of surface 

treatments. One type of surface treatment is hydroseeding, which involves re-vegetation of the 

landscape by spraying grass or wildflower seeds. This method can be easily applied to large 

areas, and it is most effective when there is adequate time for the vegetation to develop. 

Another type of surface treatment consists of placing straw or wood fiber mulch on the 

landscape. A fabric mat can be used in lieu of mulch material to provide more resistance to 

erosive forces. 

Sediment barriers are temporary structures used to help retain the soil on the site and reduce 

the runoff velocity across areas below it. One type of sediment barrier is a silt fence, which is a 

temporary structures of wood or steel fend posts, weir mesh fencing, and a suitable permeable 

filter fabric. Another type of a sediment barrier structure is a straw bale dike, which are 

constructed out of straw bales. Both of these structures should be limited to small drainage 

areas that have a maximum slope of 2H on 1V and flow path length of around 100 feet. Another 

type of sediment barrier is straw wattles. Wattles are tubes of straw or coconut fiber. Wattles 

help stabilize the slope by shortening the slope length and by slowing, spreading and filtering 

overland water flow. They are placed in trenches on the slope at selected vertical spacing and 

held in-place by stakes. 

5.4
 
In-channel Debris Basins are structures placed across well-defined channels to form basins 

which impede the streamflow and provide storage space for deposits of detritus and floating 

debris. 

5.4.2 Deforestation 

Logging practices can cause a substantial increase in the volume of floating debris entering a 

channel system. Practices that reduce the quantities of floating debris include directional felling 

uphill with a tree-pulling system and providing a buffer strip of undisturbed vegetation along the 

streams. As in fires, logging can cause an increase in magnitude of runoff from the disturbed 

area, increase the erodibility of soils, and increase the probability of catastrophic events such as 

debris flows and landslides, resulting in an increase in fine and coarse detritus to be transported 

to and deposited at a drainage structure. The countermeasures that can be implemented to 

reduce the amount of material transported to a drainage structure include sediment barriers and 

in-channel debris basins as discussed above for fires. 

Figure 5.1. Steel rail debris deflector for large rock (looking upstream of culvert). 

5.5
 
Figure 5.2. Steel rail debris deflector (looking downstream). 

Figure 5.3. Steel rail and cable debris deflector. In boulder areas, cable is more desirable for its 

flexibility than a rigid rail (looking towards entrance). 

5.6
 
Figure 5.4. Steel debris deflectors installed at entrances to a battery of culverts. 

Figure 5.5. Steel rail debris deflector for battery of culverts (see Figure 5.6). 

5.7
 
Figure 5.6. Installation of Figure 5.5 during flood; functions well under heavy debris flow. 

Figure 5.7. Steel rail debris deflector in area of heavy flowing debris (looking upstream). 

5.8
 
Figure 5.8. Timber pile debris deflector for boulders and large floating debris. 

5.9
 
Figure 5.9. Timber pile debris deflector protected culvert during heavy floods. Nearby 

culverts without deflectors were plugged. 

5.10
 
Figure 5.10. Rail debris rack over sloping inlet. Heavy debris and boulders ride over rack and 

leave flow to culvert unimpeded. 

Figure 5.11. Post and rail debris rack, in place for 35 years, for light to medium floating 

debris installed 100 ft upstream of culvert. 

5.11
 
Figure 5.12. Rail debris rack. 

Figure 5.13. Timber debris rack (note how suspended by cables). 

5.12
 
Figure 5.14. Hinged steel debris rack in urban area. Due to nature of debris and possible entry 

by children, bar spacing is close. 

Figure 5.15. Steel debris rack in urban area. 

5.13
 
Figure 5.16. Debris rack used in State of Washington. 

5.14 
Figure 5.17. Rail debris rack in arid region (see Figure 5.18). 

Figure 5.18. Installation in Figure 5.17 after several years of fine silt deposition at entrance. 

5.15
 
Figure 5.19. Steel rail debris rack. Note amount of debris accumulation in upstream channel. 

Figure 5.20. Steel debris rack probably saved the culvert from plugging. 

5.16
 
Figure 5.21. Steel grill debris rack with provision for cleanout afforded by concrete paved area in 

foreground. 

Figure 5.22. Steel grill debris rack on slope mitered culvert entrance. 

5.17
 
Figure 5.23. Metal pipe debris riser in basin (note anti-vortex device on top). 

Figure 5.24. Metal pipe debris riser placed during initial construction of culvert provides relief in 

case the culvert entrance becomes plugged (see Figure 5.25). 

5.18
 
Figure 5.25. Installation shown in Figure 5.24 after flood. Riser conveyed large flows during 

flood. Fence partially surrounding riser was of no value for debris control. 

Figure 5.26. Debris crib of precast concrete sections and metal dowels. Height increased by 

extending dowels and adding more sections. 

5.19
 
Figure 5.27. Arid region debris crib of precast concrete sections and metal dowels. 

Figure 5.28. Redwood debris crib with spacing to prevent passage of fine material. Basin had 

buildup of 30 feet. 

5.20
 
Figure 5.29. Concrete debris fins with sloping leading edge as extension of culvert walls. 

Figure 5.30. Concrete debris fin with sloping leading edge as extension of center wall. 

5.21
 
Figure 5.31. Concrete debris fin with rounded vertical leading edge as extension of culvert 

center wall. 

5.22
 
Figure 5.32. Combined installation of concrete debris fin and metal pipe debris riser with single 

corrugated metal pipe culvert (looking downstream). 

Figure 5.33. Concrete debris fin for single culvert (Prefer more area between wingwalls and fin). 

5.23
 
Figure 5.34. Debris fin and metal pipe debris riser in conjunction with single barrel culvert. 

Figure 5.35. Timber debris fins with sloping leading edge. 

5.24
 
Figure 5.36. Metal bin type debris dam. 

Figure 5.37. Gabion debris dam. 

5.25
 
Figure 5.38. Debris dam of precast concrete sections fabricated to enable placement in 

interlocking fashion. 

Figure 5.39. Debris dam of precast concrete sections fabricated to enable placement in 

interlocking fashion. 

5.26
 
Figure 5.40. Debris dam and basin along with steel debris rack over culvert entrance in 

foreground. A metal pipe riser is visible over the spillway. 

Figure 5.41. Bendway wiers on outer bank of Hatchie River looking upstream (TDOT). 

5.27
 
Figure 5.42. Kellner jacks used for redirecting the flow patterns. 

Figure 5.43. Debris deflectors installed at State Route 59 south crossing of the Eel River in 

central Indiana. 

5.28
 
Figure 5.44. Debris sweeper being installed on a bridge over Staunton River in Altavista, 

Virginia. 

Figure 5.45. Close up of a debris sweeper installed on the Cedar Creek in Washington. 

5.29
 
Figure 5.46. Close up of a debris sweeper installed on the South Fork Obion River in 

Tennessee. 

Figure 5.47. Close up of double-stacked installation debris sweeper on Interstate 24 over the 

Mississippi River. 

5.30
 
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5.31
CHAPTER 6 – DESIGN PROCEDURES FOR DEBRIS COUNTERMEASURES 

6.1 GENERAL PROCEDURES AND CONSIDERATIONS 

6.1.1 Field Investigations 

Field investigations should be conducted prior to the design of a debris-control countermeasure 

or culvert/bridge structure. The purpose of the investigations is obtaining a general 

understanding of the debris problem at the site; acquiring data required for estimating the 

quantities of debris transported to the site; performing hydrologic, hydraulic, and sedimentation 

analyses; and attaining other miscellaneous design data. Several field investigations may be 

required for obtaining all of the data. 

The type of debris transported to the site will influence the selection of the debris-control 

countermeasure, and define the type of data and analyses required to estimate the quantity of 

debris transported to the site. The estimated quantity of debris is needed by the designer to 

provide adequate debris storage immediately upstream of the site or to evaluate the potential 

impacts associated with debris accumulating on the culvert/bridge structure. The most useful 

and desired source of information on the types and quantities of debris delivered to the site 

would be from past floods. Such information could be secured from maintenance personnel, 

from inhabitants of the immediate area, or by personal observation. Unfortunately, this type of 

information rarely exists. Therefore, information to assist the designer in estimating the types 

and quantities of debris transported to the site must be obtained from field investigations. This 

information may include soil, land use, and topographic mapping; additional survey data; stream 

and watershed characteristics upstream of the site; aerial photographs; observations of the flow 

characteristics near the site and any direct and indirect evidence of high delivery potential for 

floating debris upstream of the site; sediment and discharge data; and future changes in the 

watershed. 

Land use and soil maps are useful in estimating sediment yields of fine and coarse sediment. 

Land use maps can also provide an indication of future changes in the watershed that might 

influence the quantities of debris delivered to the site. There are many uses for topographic 

mapping and survey data (surveyed cross sections or digital terrain models, DTM). Some of 

these uses include developing a hydraulic model to evaluate the hydraulic characteristics 

upstream and downstream of the structure, defining the flow path of floating debris, defining the 

maximum allowable headwater elevation for a culvert structure, estimating the amount of debris 

storage available at the site, and defining potential access to the site. Sediment yield rates from 

gully, channel bank erosion, and mass wasting can be estimated by making a comparison 

between existing and historical topographic maps or survey data. 

Information on the stream and watershed characteristics would include the locations and 

approximate extent of any lateral channel instabilities, aggradation or degradation trends within 

the watershed, roughness coefficients of the main channel and floodplain, type of sediment in 

the streambed and banks, location of any hydraulic controls, high water marks, channel 

6.1
 
dimensions, locations of existing debris accumulations, vegetation characteristics, and any 

potential damage locations if the debris-control countermeasure and/or culvert/bridge structure 

becomes clogged. Most of this information could be obtained when applying the stream 

reconnaissance technique documented in Chapter 4 of HEC-20 (34). 

A well documented methodology for predicting bank erosion associated with stream meander 

migration using aerial photograph and maps has been developed by Lagasse, Spitz, 

Zevenbergen, Zachmann, and Thorne(36,37). The research for the development of this 

methodology was conducted as part of the National Cooperative Highway Research Program 

(NCHRP Project 24-16) for the Transportation Research Board, and the principal product of the 

research is a stand-alone Handbook(37). Information covered by the Handbook(37) includes: 

•	 Screening and classification of meander sites. A morphological classification scheme 

for alluvial rivers developed by Brice(11) was selected as the most appropriate system for 

the methodology developed. The original classification, however, was modified since not 

all of the original classes are commonly encountered. Based on the modified 

classification system, a screening system was established to define if the methodology 

would be applicable to a study reach. The methodology is not applicable for the 

classifications that possess either considerable stability or excessive instability. 

•	 Sources of mapping and aerial photographic data. The Handbook provides guidance 

on obtaining contemporary and historical aerial photographs and maps. 

•	 Basic principles and theory of aerial photograph comparison. The Handbook 

includes a brief discussion on the types of photogrammetry, photogrammetric products, 

and the application of photogrammetry to meander migration. 

•	 Manual overlay and computer assisted techniques. The procedures for the manual 

overlay techniques documented in the Handbook are briefly presented as follows: (1) 

obtain aerial photograph and maps; (2) convert aerials/maps to a common scale; (3) 

define common points for all aerial/maps; (4) trace banklines and registration points onto 

a transparent overlay for each set of aerials/maps; (5) define the average bankline arc, 

the radius of curvature of the bend, and the bend centroid position for best-fitted circles 

of the outer bank of each bend; and (6) define the future position of the bend by simple 

extrapolation based on the assumption that the bend will continue to move at the same 

rate and in approximately the same direction as it has in the past. The same general 

procedure for the manual overlay techniques can be accomplished more easily and 

efficiently using common computer software with drawing capabilities, such as most 

word processing and presentation applications or more powerful and versatile computer 

aided drawing (CAD) programs. 

•	 GIS-based measurement and extrapolation techniques. Another product of the 

research was the development of Geographic Information System (GIS) software based 

menu-driven extensions to assist in applying the developed methodology. Two 

extensions were developed: (1) Data Logger, and (2) Channel Migration Predictor. The 

Data Logger extension was developed to streamline the measurement and analysis of 

6.2
 
bend migration data and aid in predicting channel migration. This extension provides the 

user a quick and easy way to gather and archive river planform data. This extension 

records various river characteristics that are arranged by the river reach for each river 

bend and historical record. The data archived by this extension is then utilized by the 

Channel Migration Predictor extension to predict the probable magnitude and direction of 

the bend migration at some specified time in the future. Both of these extensions are 

provided on a CD in Appendix G of the Handbook. 

•	 Statistical analysis. A statistical analysis of an extensive data set (nearly 2,500 

measurements) was conducted to determine relationship for channel migration to the 

morphological classification scheme for alluvial rivers. This analysis showed that 

equiwidth meander reaches are fairly stable and will not change significantly over time, 

applying standard regression techniques to directly predict meander migration did not 

yield statistically significant relationships, and the results from the frequency approach 

should be used primarily as a supplement to the comparative analysis using aerial 

photography. 

•	 Sources of error and limitations. Information related to the sources of errors and 

limitations for using the developed methodology is provided in Chapter 6 of the 

Handbook. 

•	 Illustrated examples and application using manual overlay techniques. An 

illustrated example for applying the developed methodology is provided in Chapter 8 of 

the Handbook. 

Aerial photographs are useful in defining the roughness coefficients of the stream and 

floodplain, defining the representative reaches for estimating the volume of floating debris within 

a watershed as discussed in Chapter 3 of this manual, defining locations of bank erosion, and 

showing the present conditions of the watershed. Sequences of historical aerial photographs 

can be used to estimate channel bank or gully erosion by measuring the aerial differences 

between the sets of photos. The volume of bank erosion can be estimated for long term erosion 

and bank migration rates or for single hydrologic events, if photos are available before and after 

the event. 

Observations should be made of the flow conditions near the site during low and high flow 

conditions. These observations could be useful in defining the flow path of floating debris and 

the flow patterns near the site. For an existing structure, the observations could be useful in 

defining the region where the flow is affected by the structure, i.e., contraction and expansion 

lengths. Also, any direct and indirect evidence related to the delivery potential of floating debris 

as mentioned in Chapter 3 of this manual should be noted. For example, abundant floating 

debris stored in the channel is direct evidence for high delivery potential. There is considerable 

direct and indirect evidence of debris generation that can be collected and used to evaluate the 

potential for debris accumulation at a site. 

Measured sediment and discharge data are useful in estimating sediment yields of fine and 

coarse detritus. Unfortunately, this type of data is seldom available. Therefore, hydrological and 

6.3
 
sedimentation analyses are required to estimate the quantities of this type of debris. The only 

additional types of data needed for these analyses that have not already been mentioned are 

sediment gradation curves of the streambed and banks. 

Among the factors to be considered are possible future changes in the type and quantity of 

debris that might result from changes in land use within the drainage basin. Some of these 

potential changes include floods, fires, urbanization, logging, grazing, agriculture, channel 

improvements, and conservation practices. As an example, logging in a previously virgin area 

could increase the quantity of floating debris introduced into the stream and change the nature 

of the debris problem from one of “medium floating” to “large floating” debris. 

Lastly, any data needed for the debris structure should be obtained during the field 

investigations. An example would be the maximum allowable headwater and embankment 

height for culvert or debris-control countermeasure. This information might also be necessary in 

selecting the type of debris-control countermeasure best suited to the particular problem. 

In summary, the following information should be obtained during the field investigations: 

•	 Classification (as to the type) of the debris transported to the site. 

•	 Information for estimating the quantity of debris. 

•	 Land use and soil maps. 

•	 Existing and historical survey data. 

•	 Stream and watershed characteristics upstream of the site. 

•	 Aerial photographs. 

•	 Observations of the flow conditions near the site during low and high flow conditions. 

•	 Direct and indirect evidence related to the delivery potential of floating debris. 

•	 Information about future changes that could influence the quantity of debris. 

•	 Data required for design, i.e., maximum allowable headwater elevation for a culvert 

structure. 

6.1.2 Selecting the Type of Countermeasures 

As noted in the previous chapter of this manual, there are a wide variety of countermeasures 

available for debris-control. A debris-control countermeasures matrix, presented in Table 6.1, 

6.4
 
Table 6.1. Debris-Control Countermeasures Matrix. 

Countermeasure Characteristics 

DEBRIS CLASSIFICATION MAINTENANCE 

4

S

5 Estimated T

COUNTERMEASURE 

Floating Debris 

Flowing Debris 

Bed Material Allocation of 

Resources 

AESTHETIC

ENV. IMPAC 

Light Medium Large Fine Detritus Coarse Detritus Boulders H = High M = Moderate L = Low 

GROUP 1. STRUCTURAL COUNTERMEASURE 

GROUP 1.A. CULVERT STRUCTURES 

Deflectors x x x H A L C

Racks x x H A L C

Risers x x x L A L C

Cribs x x M A L C

Fin x M A L S

Dams and Basins x x x H A H W

GROUP 1.B. BRIDGE STRUCTURES 

Deflectors x x x H - M U L C

Fins x x M A L C

Crib Structure x x U L M

R.T.S. – Iowa Vanes x M A M IA 

R.T.S. – Permeable Spurs x x x M A M A

T

R.T.S. – Impermeable Spurs x x x H A H WIn-channel Debris Basins x x H A H W

Flood Relief Sections x x L A L WDebris Sweeper (Bridgeshark) x x L A L O

Booms x x K U M ID D.F. – Freeboard x x x L D L W

D.F. – Pier Type, Location, and Spacing x x L D L W

D.F. – Special Superstructure x x L D L T

GROUP 2. NON-STRUCTURAL COUNTERMEASURE 

Emergency and Annual Maintenance x x x H U M W

Debris Management Plan x x x H D L 

Notes: 1. “x” corresponds to a suitable device. 

2. R.T.S. corresponds to River Training Structures. 3. D.F. corresponds to Design Features. 4. Classification for Aesthetics is: (1) U for Undesirable, (2) A for Acceptable, and (3) D for Desirable. 5. Classification for Environmental Impact is: (1) L for Low, (2) M for Medium, and (3) H for High. 

6. Reference made above for the Design Guidance is related to the section indicated in this manual, i.e., information on deflectors for culverts is prov

6.5
 
has been developed to provide guidance in the selection of countermeasures suitable for 

various types of the debris. The matrix is organized to highlight the various groups of 

countermeasures and to identify their individual characteristics. The left side of the matrix lists 

the type of countermeasures available for the three general groups of structural 

countermeasures for culverts and bridges, and non-structural countermeasures for both. In each 

row of the matrix, the countermeasures suitable for the various types of debris are identified. 

The matrix also identifies the States that have used the countermeasure and the general level of 

maintenance resources required for the countermeasure. Finally, a resource for design 

guidelines is noted, where available. 

In addition to the information contained within the matrix, the selection of the countermeasure 

should be based on the construction and maintenance costs, risk of failure, risk of property 

damage, and environmental and aesthetic considerations. The safety of highway traffic should 

also be considered in the selection of the countermeasure. The culvert end and the 

countermeasure should be located beyond the usual recovery area for errant vehicles or the 

countermeasure should be designed to enhance the drivers' chance of recovery. At existing 

sites where modifications cannot be made to meet this objective, an appropriate vehicle 

restraining device or an impact attenuating device should be provided on the roadside. 

The countermeasure matrix (Table 6.1) was developed to identify distinctive characteristics for 

each type of countermeasure. Four categories of countermeasure characteristics were defined 

to aid in the selection and implementation of the countermeasures: 

•	 Debris Classification 

•	 Maintenance 

•	 Installation/Experience by State 

•	 Design Guideline References 

These categories were used to answer the following questions: 

•	 For what type of debris is the countermeasure applicable? 

•	 What level of resources will need to be allocated for maintenance of the 

countermeasure? 

•	 What States or regions in the U.S. have experience with this countermeasure? 

•	 Where do I obtain design guidance reference material? 

The Debris Classification Category describes the type of debris for which a given 

countermeasure is best suited or under which there would be a reasonable expectation of 

success. Conversely, this category could indicate the type of debris under which experience has 

shown that a countermeasure may not perform well or was not intended. 

6.6
 
The type of debris considered for this category is based on the debris classification provided in 

Chapter 2 of this manual: 

• Light Floating Debris 

• Medium Floating Debris 

• Large Floating Debris 

• Flowing Debris 

• Fine Detritus 

• Coarse Detritus 

• Boulders 

The suitable countermeasure for each debris classification is indicated by "x". When the debris 

is comprised of more than one type of debris, the information provided in this category can be 

used as guidance in selecting a combination of countermeasures to address the debris 

problems. 

The Maintenance Category identifies the estimated level of maintenance that may need to be 

allocated to service the countermeasure. The rating for this category is subjective, and it ranges 

from “Low” to “High.” The ratings represent the relative amount of resources required for 

maintenance with respect to other countermeasures provided within the matrix (Table 6.1). A 

low rating indicates that the countermeasure is relatively maintenance free; a moderate rating 

indicates that some maintenance is required; and a high rating indicates that the 

countermeasure requires more maintenance than most of the countermeasures in the matrix. 

The Aesthetics Category identifies the estimated level of appearance associated with the 

countermeasure with respect to other countermeasures provided within the matrix (Table 6.1). 

The rating for this category is subjective, and it ranges from “Undesirable” to “Desirable.” An 

undesirable rating indicates that the countermeasure is noticeably unpleasing to the sight; an 

acceptable rating indicates that majority of the structure is pleasing to the sight; and a desired 

rating indicates that the countermeasure is noticeably pleasing to the sight. 

The Environmental Impact Category identifies the estimated level of impact the countermeasure 

would have on the environment with respect to other countermeasures provided within the 

matrix (Table 6.1). The rating for this category is also subjective, and it ranges from “Low” to 

“High.” A low rating indicates that the countermeasure does not adversely impact the 

environment or the impacts are considered short term; a moderate rating indicates that some 

adverse impacts could occur with implementation of the countermeasure; and a high rating 

indicates that the countermeasure would adversely impact on the environment. 

The Installation/Experience by State category identifies DOTs that have used the 

countermeasure. This information was obtained from three sources: response of the DOTs to a 

debris-related questionnaire documented in “Debris Problems in the River Environment” 

(1979)(14); Brice and Blodgett, “Countermeasures for Hydraulic Problems at Bridges, Volumes 1 

and 2” (1978)(12, 13); and correspondence between FHWA and DOT staff. It is expected that 

additional information on state use will be obtained as this matrix is distributed and 

6.7
 
revised. Countermeasures that have been used by many States are given a listing of “Widely 

Used.” The listing reflects both successful and unsuccessful experiences. 

The countermeasures matrix (Table 6.1) is a convenient reference guide on a wide range of 

countermeasures applicable to addressing debris problems at culvert and bridge structures. A 

comprehensive plan of action would be to provide conceptual design and cost information on 

several alternative countermeasures, with a recommended alternative being selected based on 

a variety of engineering, environmental, and cost factors. The countermeasures matrix is a good 

way to begin identifying and prioritizing possible alternatives. The information provided in the 

matrix related to the suitable applications for the various types of debris and maintenance 

issues should facilitate preliminary selection of feasible alternatives prior to more detailed 

investigation. 

6.1.3 Design for Bridges versus Culverts 

The countermeasures provided in Table 6.1 have been divided into two groups: Structural 

countermeasures and Non-structural countermeasures. As seen in the table, the structural 

countermeasures have been further divided into structures available for culvert and bridge 

structures. Culverts, as distinguished from bridges, are usually covered with embankment and 

have structural material around the entire perimeter, although some are supported on spread 

footings with the streambed serving as the bottom of the culvert. Culverts are typically provided 

for small drainage basins and watercourses, and they are considered minor structures 

compared to bridges. Bridges, on the other hand, are usually used where the discharge of the 

watercourse is significant (larger drainage basins) or where the stream to be crossed is large in 

extent. 

Because of the significant difference in size and function of these structures, some of the 

countermeasures available for culvert structures cannot be used for bridge structures, and vice 

a versa. Some countermeasures available for culvert structures are also available for bridge 

structures even though the intended purpose is different for the two structures. The debris 

deflector is an example of such a countermeasure. Debris deflectors are used at culverts to 

prevent debris from going through the culvert by deflecting it to the side of the structure where it 

is stored (debris retention), whereas debris deflectors at bridges are used to deflect the debris 

away from the pier and through the bridge opening (debris passage). 

At many locations, either a culvert or bridge structure will satisfy both the structural and 

hydraulic requirements of the stream crossing. Structure choice at these locations should be 

based on construction and maintenance costs, risk of failure, risk of property damage, traffic 

safety, environmental, and aesthetic considerations. An additional deciding factor at these 

locations may be related to debris passage. Instead of providing a debris-control 

countermeasure at a proposed site, it may be desired to design the structure for debris 

passage. However, there are some obvious limitations in the case of culverts. There is no real 

assurance that doubling the size of a culvert will eliminate the threat of the culvert becoming 

plugged if debris poses a problem at the site. It is obvious that the probability of this occurring 

does decrease to some degree with increases in the size of the culvert. However, it is extremely 

6.8
 
difficult to demonstrate what level of protection would be obtained by such increases. Therefore, 

it may be necessary to use a bridge structure that is designed for debris passage with a higher 

degree of certainty in lieu of a culvert structure even though it can adequately convey the 

anticipated flows. 

Both types of non-structural countermeasures can be considered for culvert and bridge 

structures. However, a debris management plan would more likely be implemented at a bridge 

than a culvert due to the high cost and allocation of resources required to develop and execute 

such a plan making it infeasible for small drainage structures. 

6.1.4 Existing Structures Versus Proposed Structures 

The selection and design of the countermeasures presented in Table 6.1 could depend on if the 

countermeasure is for an existing or proposed structure. Constraints at an existing structure can 

prevent the use of certain countermeasures or influence the design of the countermeasure. The 

constraints could be related to the physical conditions at the site; the structure itself; monetary 

reasons; environmental or maintenance requirements; limited or no access to the culvert or 

bridge; or other reasons. Recent development adjacent to the watercourse upstream of an 

existing structure might prevent the use of an in-channel debris basin or dam because of 

potential flooding impacts or political pressure from the residents. The geometry of the existing 

structure could influence the configuration and dimensions of the proposed countermeasure. 

Certain countermeasures might require that part of the existing structure be demolished and 

significantly modified, making it too expensive to implement. It is also possible that 

environmental restrictions due to fish passage or vegetation removal could limit the type of 

countermeasure that could be selected and constructed at a particular site. 

All of the countermeasures presented in Table 6.1 could be used at proposed structures. 

However, this is not the case for existing structures. Unfortunately, the most common type of 

countermeasures used for bridge structures are usually infeasible to implement at existing 

bridge structures. These measures are identified in Table 6.1 as design features, i.e., “D.F.”, 

and they include adequate freeboard, the use of special superstructure, and considerations to 

the type, location, and spacing of piers for reducing the potential of accumulation. These 

countermeasures can easily be incorporated into the design of a proposed structure, whereas 

they are difficult to implement at an existing structure. The anticipated debris accumulation on a 

proposed bridge can be considered in the: (1) design of the hydraulic opening through the 

bridge to safely convey the design flood without overtopping the structure, (2) structural design 

of the bridge components to withstand the increase in lateral and overturning forces associated 

with the debris accumulations, and (3) design of the pier and abutment foundations to prevent 

undermining of the structure by the significant scour associated with debris accumulation. 

Proposed structures also have the benefit over existing structures in that access for 

maintenance can be included in the design of the structure. Access to a proposed structure can 

be incorporated into the design of the highway embankment where this might not be a viable 

option for an existing highway embankment. 

6.9
 
For existing structures, the problems associated with debris are usually more easily understood. 

There is generally sufficient information on the type of debris, quantity of debris transported to 

the site, and the associated problems with the debris available to select and design 

countermeasures to address the problem. In some instances where the investment is relatively 

small and there is little chance of interruption to current operations, it may be more desirable at 

a proposed site to select and design the countermeasure after the problems with debris have 

developed and are fully understood. 

6.1.5 Maintenance Accessibility of the Countermeasure 

Maintenance is an important factor to consider in selecting a debris-control countermeasure or 

designing a bridge structure. This should entail both regular and emergency maintenance 

activities. Considerations should be made as to the ease and cost of maintaining the 

countermeasure and accessibility to the countermeasure for performing the maintenance work. 

A countermeasure that is more expensive to construct may be more desirable if it is easier and 

less expensive to maintain. 

Provisions should be made for access to the countermeasure for maintenance purposes. 

Maintenance personnel should be consulted with when designing the access to the site. 

Unfortunately, access is often difficult to provide, and it may not be provided for 

countermeasures designed for secondary highways or lower class roads. If access roads to the 

countermeasure are impractical and the risk associated with flooding is high, it may be 

necessary to provide an area near the countermeasure where mechanical equipment, such as a 

crane, could perform maintenance activities, such as, debris and sediment removal. 

Maintenance accessibility for debris removal should be considered in the design of a new or 

replacement bridge. There are certain features that can be incorporated into the design to 

simplify debris removal. For instance, the use of solid wall piers that extend slightly upstream of 

the edge of the bridge deck. This type of pier configuration provides for easier removal of debris 

than other pier types. Debris not only accumulates more readily on multiple-column piers, but 

also may become entangled between the columns along the full width of the underside of the 

bridge, making it extremely difficult to remove the debris and/or causing access problems for the 

debris-removal crew. Debris trapped on trusses and piers with multiple columns can be 

entangled among multiple structural elements. The entanglement makes debris removal more 

difficult and increases the possibility that the bridge could be damaged during the removal 

operations. Hammerhead piers are an alternative to multiple columns. This type of pier 

eliminates the potential for entanglement. However, debris removal is still difficult since the pier 

nose is well beneath the bridge deck and it is extremely difficult to access and lift the debris 

from the bridge deck. Superstructures that allow access to the pier nose from directly above 

also ease debris removal. A wide deck with a simple parapet and adequate load-bearing 

capacity for heavy equipment at the upstream edge provides the best opportunity for debris 

removal from the bridge deck. 

Access should be provided to the substructure of bridges to ensure prompt and complete 

removal of debris that accumulates on the structure. Debris removal can usually be 

6.10
 
accomplished during low flows with tracked vehicles for bridges over small streams. However 

for bridges over large rivers, a barge might be required to remove the debris, so a launching site 

for the barge may be necessary at such a site. 

6.2 DESIGN GUIDELINES FOR CULVERTS 

6.2.1 Debris Deflectors for Culverts 

1. FUNCTION – The function of a debris deflector (Figure 5.1 through Figure 5.9) is to 

divert medium and large floating debris and large rocks from the culvert inlet for accumulation in 

a storage area where it can be removed after the flood subsides. Their structural stability and 

orientation with the flow make deflectors particularly suitable for large culverts, high velocity 

flow, and debris consisting of heavy logs, stumps, or large boulders. 

2. STORAGE AREA – The storage area provided must be adequate to retain the 

anticipated type and quantity of debris expected to accumulate during any one storm or 

between cleanouts. 

3. TYPE OF MATERIAL – Debris deflectors are usually built of heavy rail or steel 

sections (Figure 5.1 through Figure 5.9), although timber (Figure 5.8 and Figure 5.9) and steel 

pipe are sometimes used when the type of debris consists of light floating debris and/or fine 

detritus. The decision to use timber in lieu of steel could also be based on the availability of the 

material within the region and construction costs. Wire and post debris deflectors (Figure 5.3) 

have also been used for light floating debris. For economy, salvaged railroad rails may be used 

if available. Figure 5.3 shows a deflector that uses a cable as its lower longitudinal member. 

This modification has proved to be superior in locations where heavy boulders damage rigid 

members. 

4. LOCATION AND ORIENTATION – The deflector should be built at the culvert 

entrance and aligned with the stream rather than the culvert so that the accumulated debris will 

not block the channel. Individual deflectors can be built over each pipe (Figure 5.4) or a single 

deflector can be built over multiple pipe culverts (Figure 5.5). The deflector may be placed at the 

culvert entrance or a distance of 1 culvert dimension upstream. The apex of the deflector will 

“point” upstream. 

5. DIMENSIONS – The angle at the apex of the deflector should be between 15° and 

25°, and the total area of the two sides of the deflector should be at least 10 times the crosssectional area of the culvert. The base width and height of the deflector should be at least 1.1 

times the respective dimensions of the culvert. The upstream member is vertical on most 

installations. However, a sloping member at the apex (sloping downstream from bottom of 

member) would reduce the impact of large floating debris and boulders, and probably prevent 

debris from gathering at that point. Therefore, deflectors with a sloping member at the apex are 

recommended over a vertical upstream member. 

6.11
 
6. BAR SPACING – Spacing between vertical members should not be greater than the 

minimum culvert span dimension nor less than 1/2 the minimum dimension. A spacing of 2/3 the 

minimum dimension is commonly used. In addition to what is required for structural support, 

spacing of the horizontal bars along the sides of the deflector follow similar characteristics. 

Where headwater from the design flood is expected to be above the top elevation of the 

deflector and floating debris is anticipated, horizontal members should be placed across the top. 

The spacing of horizontal members on the top should be no greater than 1/2 the smallest 

dimension of the culvert opening. 

6.2.1.1 Debris Deflector Example (Culvert) 

Given: 

Circular Culvert, Diameter = 1.8 m 

Sediment material comprised of coarse detritus and medium floating debris 

Determine: 

Determine the dimensions of a triangular shaped debris deflector. 

Solution: 

Step 1: Determine the height and minimum width of the debris deflector 

Height, H = 1.1(Culvert diameter, D) = 1.1(1.8) = 1.98 m (use 2.0 m) 

Minimum width, Wmin = 1.1D = 1.1(1.8) = 1.98 m (use 2.0 m) 

Step 2: Select desired apex angle, bar spacing and thickness 

Apex angle, α, can range from 15° to 25°, use 20° 

Bar spacing, s, can range from 1/2(D) to D. 2/3(D) is common spacing, so use 1.2 m. 

Deflector will be constructed out of 76-millimeters-thick, t, steel rails. The thickness of the rail 

was selected taking into account the type of debris, availability of the material, cost, and 

structural stability. 

Step 3: Determine the side length of the debris deflector 

A trial and success procedure is required to determine side length of the debris deflector: 

a. determine the minimum side area of deflector, 

b. assume a length of the deflector, 

c. determine the number of vertical and horizontal bars, 

6.12
 
d. compute the gross area of the deflector, 

e. compute the total area of the steel rails, 

f. compute the net area of the deflector, and 

g. complete substeps “b” through “f” until the net area of the deflector is slightly greater 

than the minimum side area of the deflector. 

Implementing these substeps “a” through “g” 

Substep “a” 

Minimum side area of deflector on each side = 5(Area of culvert) 

Minimum side area of deflector on each side = 5(πD2

/4) = (5)((3.14)(1.8)2)/4) = 12.72 m2
 

As this is a trial and success solution, this example will only show the values in the first and final 

iteration. The “final” iteration will be a bold, italicized value provided within parenthesis. 
 

Substep “b” 

Assume a length, L, of 6.7 m (Final: 7.9 m) 

Substep “c” 

Number of vertical bars = (L + s)/(s + t) 

= (6.7 + 1.2)/(1.2 + 0.076) = 6.19 -> use 7 (Final: 8 bars) 

Number of horizontal bars = (H + s)/(s + t) 

= (2.0 + 1.2)/(1.2 + 0.076) = 2.51 -> use 3 (Final: 3 bars) 

Substep “d” 

Gross area per side = (L)(H) = (6.7)(2.0) = 13.4 m2

 

	 (Final: 15.80 m2

) 

Substep “e” 

Area of bars = (number of horizontal bars)(t)(L) + (number of vertical bars)(t)(H) 

Area of bars = (3)(0.076)(6.7) + (7)(0.076)(2.0) = 2.59 m2 

 (Final: 3.02 m2

)
 

Substep “f” 

Net Area = Gross area – Area of bars = 13.40 – 2.59 = 10.81 m2 

 (Final: 12.78 m2

) 

Substep “g” 

Compare Net Area and Side Area: 10.81 m2 less than 12.72 m2. Therefore increase length 

(substep “b”) and try another iteration. 

Step 4: Determine the distance to the apex of the deflector and the width of deflector 

Distance to the apex of the deflector = (L)(cos(0.5α)) = (7.9)(cos(10)) = 7.78 m 

Width of the deflector = (2)(L)(sin(0.5α)) = (2)(7.9)(sin(10)) = 2.74 m 

Figure 6.1 depicts the resulting culvert debris deflector developed in the example. The figure is 

for illustration purposes only and should not be used as a specification or detail. 

6.13
 
7.9 m 

1.8 m 

2.74 m 

20°

7.78 m 

Plan View 

°

7.78 m 

Side View 

1.2 m 1.8 m

2.0 m 

Figure 6.1. Debris deflector designed in example. 

6.2.2 Debris Racks for Culverts 

1. FUNCTION – The function of a debris rack (Figure 5.10 through Figure 5.22) is to 

essentially create a barrier across the stream channel to trap light and medium floating debris 

that is too large to pass through the culvert. 

2. STORAGE AREA – The storage area provided must be adequate to retain the 

anticipated type and quantity of debris expected to be accumulated during any one storm or 

between cleanouts. If a large debris storage area is provided upstream of the rack location, the 

frequency of maintenance can significantly be reduced and added safety is provided against 

overtopping of the installation during a single storm. 

3. TYPE OF MATERIAL – Debris racks are usually built of heavy rail or steel sections, 

although they can be constructed out of various types of material. Inclined racks and rubber 

tires have been used to help reduce the impact of heavy debris striking at high velocity. Chainlink fence has also been used to remove light floating debris from low velocity streams. This 

type of barrier is particularly advantageous in tidal areas where the function of flap or check 

6.14
 
gates can be hampered by light debris collecting on the gate seats and thereby blocking 

complete closure of the gates. Since vertical racks receive the full impact of floating debris and 

boulders, their structural design should incorporate brace members set in concrete. 

4. LOCATION AND ORIENTATION – Debris racks may be vertical or inclined and may 

be placed over the culvert inlet (Figure 5.10, Figure 5.14, Figure 5.15, Figure 5.21, and Figure 

5.22) or upstream from the culvert (Figure 5.11, Figure 5.12, Figure 5.13, Figure 5.16, Figure 

5.17, Figure 5.18, Figure 5.19, and Figure 5.20). Racks should not be placed in the plane of the 

culvert entrance, since they become easily plugged. Where a well-defined channel exists 

upstream of the culvert, the debris rack should be placed upstream from the culvert entrance a 

minimum distance of two times the culvert diameter. However, it should not be placed so far 

upstream that debris can enter the channel between the structure and the culvert entrance, or 

accessibility to and maintenance of the structure becomes difficult and/or costly. In addition, 

right-of-way constraints are important considerations in locating debris racks. Debris racks 

generally do not have top or horizontal members that extend from the rack to the culvert 

headwall, although there are exceptions. 

5. DIMENSIONS – The total straining area of a rack should be at least ten times the 

cross-sectional area of the culvert being protected. The overall dimensions of the rack should be 

a function of the amount of debris expected per storm, the frequency of storms, and the 

schedule of expected cleanouts. When a rack is installed at the upstream end of the wingwalls, 

it should be at least as high as the culvert. Also, the height of racks should allow some 

freeboard above the expected depth of flow in the upstream channel for the design flood. Racks 

10 to 20 feet high have been constructed. 

6. BAR SPACING – Spacing between vertical members should not be greater than 2/3 

the minimum culvert dimension nor less than 1/2 the minimum culvert dimension. This spacing 

permits the lighter debris to pass through the rack and the culvert. In urban areas, bar spacing 

of racks should be a maximum of 6 inches and tied to the culvert headwall by top bars to 

prevent children from entering the culvert. Unfortunately, the close spacing of the bars creates a 

debris trap and increases the maintenance required. To reduce the amount of debris becoming 

trapped, it is preferable to have the lowest edge of rack about six inches above the flow line of 

the ditch, permitting some debris to pass under the rack during low flows. 

6.2.2.1 Debris Rack Example (Culvert) 

Given: 

Culvert Diameter = 1.8 m 

Design Discharge = 9.9 m3

/s 

Design Headwater Depth = 2.7 m 

Upstream channel width = 7.3 m 

Flow carries light to medium debris 

Rack constructed out of 76-millimeter-thick steel rails 

6.15
 
Determine: 

Determine the dimensions of a vertical debris rack comprised of two horizontal 

members. 

Solution: 

Step 1: Determine the minimum area of the debris rack 

Minimum area of debris rack, Arack = 10(Area of culvert) 

Minimum area of debris rack, A πD2

/4) = 10(3.14)(1.8)2)/4 = 25.45 m2

rack = 10( 
 

Step 2: Determine the number of vertical bars and spacing 

Minimum spacing = 1/2(D) = smin = 1/2(1.8) = 0.9 m 

Maximum spacing = 2/3(D) = smax = 2/3(1.8) = 1.2 m 

Number of vertical bars for minimum spacing = (w - smin) / (smin + t) = 

 = (7.3 - 0.9) / (0.9 + 0.076) = 6.6 -> use 7 

Number of vertical bars for maximum spacing = (w - smax) / (smax + t) = 

 = (7.3 - 1.2) / (1.2 + 0.076) = 4.8 -> use 5 

Try 7 vertical bars for debris rack (assume this is the smallest material and fabrication cost). 

(w − t (number of vertical bars)) (7.3 −0.076 (5)) Spacing = ( = = 1.15 m

number of vertical bars + 1) (5 + 1)

The spacing of 1.15 m falls between the minimum (0.9 m) and maximum (1.2) spacing values. 

Therefore the spacing is adequate. 

Step 3: Determine the height of the debris rack 

A 25.45 Approximate height,H = rack 

( + ( )t (number )( ) of horizontal bars ) = ( ) nv + 1 s ( ) 6 ( ) + 0.076 2 = 3.84 m

1.15

(note: variable “n” is the number of vertical bars in the rack – in this example “5”). 

Use a height of 4 m to account for additional loss in area from the horizontal bars. No 

adjustment required to satisfy freeboard since the design headwater elevation is about 1.3 m 

below the top of the rack. 

6.16
 
Gross area = (w)(H) = (7.3)(4) = 29.2 m2 

 

Area of bars = (number of horizontal bars)(t)(w) + (number of vertical bars)(t)(H) 

Area of bars = (2)(0.076)(7.3) + (5)(0.076)(4) = 2.63 m2  

Net Area = Gross area – Area of bars = 29.2 – 2.63 = 26.57 m2 

 

Compare Net Area and Minimum Rack Area: 26.57 m2

 is greater than 25.45 m2 . Therefore, the 

design is adequate. 

6.2.3 Debris Risers for Culverts 

1. FUNCTION – Debris risers (Figure 5.23 through Figure 5.25, and Figure 5.32) 

generally consist of vertical culvert pipes that are commonly used as relief structures, either 

independent of the main culvert or in conjunction with it. Risers are used where considerable 

height of embankment is available and the type of debris consists of flowing masses of clay, silt, 

sand, sticks, or medium floating debris without boulders. They are seldom structurally stable 

under high-velocity flow conditions because of their vulnerability to damage by impact, and they 

are usually suitable for culvert installations of less than 54-inch diameter. 

2. STORAGE AREA – Storage area must be provided to adequately retain the 

anticipated type and quantity of debris expected to be accumulated during any one storm or 

between cleanouts. The use of risers induces deposition of the sediment material upstream of 

the riser as a result of the ponding created by the riser. 

3. TYPE OF MATERIAL – Debris risers are usually built out of corrugated metal pipe, 

although they can be constructed out of steel pipe. A corrugated metal pipe reducing elbow can 

be used to connect risers to an existing culvert inlet, although damage to the metal elbow from 

falling rocks may occur. Occasionally, concrete is placed inside the elbow to prevent the metal 

from wearing through by this abrasive action. A solution for extremely severe conditions is to 

connect riser and culvert by a concrete junction box having the inside shaped as an elbow. 

4. LOCATION AND ORIENTATION – Debris risers are typically vertical, however they 

have been built at an angle between vertical and the stream grade. This reduces the impact of 

debris at the elbow and assists in moving debris through the culvert. Risers can be either an 

independent structure or connected to an existing culvert. If connected to the existing culvert, 

the riser should be located a minimum of 3 feet or one half of the culvert diameter, whichever is 

greater, from the existing culvert headwall. 

5. DIMENSIONS – Good practice will build riser pipes at least 36 inches in diameter to 

provide an area large enough for maintenance access. To avoid vibration of the riser pipe and 

unstable flow conditions, the riser diameter should be about 1 foot larger than the culvert 

diameter. If the embankment is of sufficient height, provisions should be made to extend the 

riser vertically, if necessary. In the case of corrugated metal pipe risers, this can be 

accomplished by means of standard coupling bands. 

6.17
 
6. GRATE AND SLOT FEATURES – Debris risers should be covered by a grate or 

cage to prevent clogging of the culvert. The grate bars can be reinforcing steel or other such 

material with vertical spacing no greater than 1/2 the culvert diameter. Slots or holes are placed 

in the sides of the riser to carry low flow. It is preferable to have these holes punched before 

galvanizing to avoid deterioration by rust. The holes are considered to have no hydraulic 

capacity under peak flow conditions because of the likelihood of their becoming plugged by light 

floating debris and silt. It is also desirable to connect the grate bars to a coupling band, rather 

than directly to the riser pipe, so the grate can be removed should cleaning be required. 

6.2.4 Debris Cribs for Culverts 

1. FUNCTION – A debris crib (Figure 5.26 through Figure 5.28) is particularly adapted 

to small-size culverts where a sharp change in stream grade or constriction of the channel 

causes deposition of coarse detritus at the culvert inlet. Debris can almost envelop a crib 

without completely blocking the flow and plugging the culvert. Cribs are somewhat similar to 

risers, however cribs are more appropriate than risers where the culvert has little cover and the 

debris is comprised of coarse detritus. Due to the debris type and site conditions associated 

with debris cribs and also risers, these two types of countermeasures have shown to be the 

most consistently successful in producing an efficient, maintenance-free installation for culverts. 

2. TYPE OF MATERIAL – Debris cribs are usually constructed of precast concrete or 

wood, and precast concrete should be used when the debris consists of medium to large 

cobbles. 

3. LOCATION AND ORIENTATION – The crib is usually placed directly over the culvert 

inlet and is generally built in log-cabin fashion although other designs have been used. Debris 

cribs may be open (Figure 5.26 and Figure 5.27) or covered (Figure 5.28). 

4. DIMENSIONS – The spacing between the bars should be about 6 inches. This 

spacing also applies to the horizontal top members of a covered crib. The height of an open crib 

should be higher than the depth of debris deposited at the structure. When an open crib is used 

as a riser and an accumulation of detritus is expected to build up, provision should be made for 

the height of the structure to be increased as needed (Figure 5.26 and Figure 5.27). Cribs have 

been built as high as 50 feet above a pipe invert with little change in the efficiency of the facility. 

6.2.5 Debris Fins for Culverts 

1. FUNCTION – Debris fins are thin walls installed upstream of the culvert parallel with 

the flow (Figure 5.29 through Figure 5.35). They have been used successfully with large 

culverts or for multiple box culverts where the debris consists mostly of floating material that can 

pass through the culvert if oriented parallel with the culvert barrel. Debris that is not aligned by 

the fin to pass through the culvert is retained at the front of the fin for later removal by 

maintenance personnel. If the fin is sloped upward toward the culvert, the debris that does not 

6.18
 
pass through the culvert can float upward and prevent debris from blocking the culvert inlet. Fins 

are generally not used on culverts with a minimum dimension less than 4 feet. 

2. TYPE OF MATERIAL – Debris fins are usually concrete, although they have been 

constructed of steel and timber. 

3. LOCATION AND ORIENTATION – Debris fins can extend from the interior walls of 

culverts (Figure 5.29 through Figure 5.31) or located on the centerline of a single culvert (Figure 

5.32 through Figure 5.34). The upstream end of the fin should be rounded and sloped upward 

toward the culvert (Figure 5.29 and Figure 5.30) to reduce impact, turbulence, and the 

probability of gathering debris, rather than vertical as shown in Figure 5.31 through Figure 5.34. 

If the upstream end of the fin is vertical, rounding that edge would be preferable to a square 

edge (Figure 5.31). 

4. DIMENSIONS – A debris fin is usually constructed to the height of the culvert; thus, 

its effectiveness is limited after the inlet becomes submerged. Field experience indicates the fin 

length should be 1 1/2 to 2 times the culvert height. The leading edge would thus have a slope 

from 1-1/2:1 to 2:1 (from 33.7 to 26.6 degrees). The thickness of the fin should be the minimum 

needed to satisfy structural requirements in order to minimize disturbance to the flow. 

6.2.6 Debris Dams / Basins for Culverts 

1. FUNCTION – Debris dams are structures placed across a well-defined channel to 

form a barrier that impedes the stream flow. The dams also form a basin that provides storage 

for deposits of detritus and floating debris (Figure 5.36 through Figure 5.39). Debris dams and 

basins are used at sites that convey heavy debris loads where it is economically impracticable 

to provide a culvert large enough to convey the surges of debris. They are also used to trap 

heavy boulders or coarse gravel that would clog culverts, especially on low fills. In some 

locations, debris dams have been built to provide the added benefit of ground water recharge 

resulting from ponded water. 

2. TYPE OF MATERIAL – Debris dams are usually earth or rock filled structures. 

Debris dams, however, can be built out of metal (Figure 5.36), rock held in place by wire (Figure 

5.37), (i.e., gabions), or precast concrete beams placed in crisscross or log-cabin fashion with 

rock dumped between the members (Figure 5.38 and Figure 5.39). 

3. LOCATION AND ORIENTATION – Debris dams and basins are usually placed some 

distance upstream from a culvert. However at some locations, the highway embankment can 

serve as the embankment for the debris dam. 

4. DESIGN FEATURES –There are several features that must be considered in the 

design of the debris dam and basin. Some of the important features are the embankment, inlet 

protection, outlet structure, and emergency spillway structure. Information on the design of 

these features and sedimentation basins in general is provided in “Design of Sedimentation 

Basins”.(22) Prior to initiating the design of the debris dam, state agencies should be contacted 

6.19
 
to ensure that state regulations are met in the design of the structure. Also, hazards created by 

the failure of the debris dam need to be considered and evaluated during the design of the 

structure. 

Various items must be considered in the design of the embankment. One of the more important 

items is the height of the embankment. The top of the embankment should be set at an 

elevation sufficiently above the maximum ponding elevation associated with the design volume 

of runoff and debris to assure that the runoff and debris are contained with a high level of 

certainty, i.e., embankment has adequate freeboard. When defining this elevation, the debris 

storage volume should be based on the assumption that the deposition slope of the debris is 

horizontal and not one half of the natural valley slope that has commonly been used. This 

assumption eliminates the potential of the embankment being overtopped due to the momentum 

of the flow, which has occurred for some of the debris dams and basins in the Los Angeles area 

designed assuming a deposition slope equal to one half of the natural valley slope. Stability of 

the embankment is also a major concern. The embankment should be designed to withstand 

the total forces from soil and hydrostatic pressure, seepage uplift, and earthquake on the 

structure. Special considerations for slope stability should be made for earth-fill embankments. 

The upstream and downstream slopes of the earthen structure depends on the soil material 

used to construct the embankment; however, the slope typically ranges between 2.5 to 1 and 3 

to 1 for the upstream face and between 2 to 1 and 3 to 1 for the downstream face. Another 

important item for earthen structures is slope protection. Both the upstream and downstream 

face of the embankment should be protected with some type of slope protection measures, i.e., 

vegetation, riprap, matting, or mulch, to prevent erosion of the embankment. 

Occasionally, extensive excavation below the natural streambed is necessary to provide the 

required storage for the debris. For this type of basin design, the upper end of the basin should 

be protected with revetment to prevent any upstream erosion of the streambed due to 

headcutting. 

An outlet structure should be provided to drain the floodwater temporarily stored behind the 

structure. The structure could be either a closed conduit consisting of a culvert with a riser set 

above the expected level of the debris deposit or an open channel acting as a weir structure. 

The design of the structure will have an influence on the design volume of the basin and 

embankment height. In general, an outlet structure designed to convey more of the runoff 

volume will reduce the design volume of the basin and lower the embankment height, but the 

cost of the structure will increase. Therefore, several different types and sizes of the outlet 

structure should be considered in the design of the structure to optimize the total cost of the 

debris dam. Significant scour can develop downstream of the outlet structure due to the high 

velocity, turbulent flow leaving the structure and the significant reduction in the sediment load 

resulting from the upstream deposition. Therefore, protection measures must be provided at the 

downstream end of the structure to protect the structure and embankment from failure due to 

undermining. Access for maintenance and repair work should be provided to the upstream and 

downstream ends of the structure. 

The debris dam must have an emergency spillway to safely convey flows greater than the 

design event. The spillway should be located off to one side of the embankment and excavated 

6.20
 
into an adjoining hillside since this location is more stable against breaching than a spillway over 

a fill or over the embankment structure. Outlet structures designed as open channels can also 

be designed to serve as the emergency spillway. Protection measures must be provided at the 

downstream end of the spillway to protect the structure and embankment from failure caused by 

significant scour. 

6.2.7 Combined Debris Controls for Culverts 

Each drainage basin presents its own debris problem. Often more than one problem exists at a 

site and two or more types of debris-control countermeasures are required to adequately 

address the problems. Combined measures can also be used at locations where it may be 

preferable to remove the larger debris at a location upstream from the culvert and to remove the 

smaller material nearer the culvert inlet. Combined measures can also be used at locations 

where it may be advisable to install two types of devices so that one will function if the other 

fails. An example of this is shown in Figure 5.24 where a debris riser was installed over the 

entrance of a culvert to assure water is conveyed through the culvert in the event that the 

culvert entrance becomes plugged. A photograph of this installation after a flood event is shown 

in Figure 5.25. Other examples of combined countermeasures are shown in Figure 5.32, Figure 

5.34, and Figure 5.40. In these cases, Figure 5.32 and Figure 5.34 show a culvert protected by 

both a debris fin and a debris riser. Figure 5.40 shows an installation consisting of a debris dam 

and settling basin with a debris deflector at the inlet and a debris riser. 

6.3 DESIGN GUIDELINES FOR BRIDGES 

6.3.1 Debris Deflectors for Bridges 

1. FUNCTION – Debris deflectors are placed upstream of bridge piers to divert and 

guide debris through the bridge opening. Deflectors are used where the debris consists of 

medium to large floating debris. 

2. TYPE OF MATERIAL – Debris deflectors attached to the pier are usually constructed 

of steel rails, whereas steel piles filled with concrete are used for deflectors located some 

distance upstream. 

3. LOCATION AND ORIENTATION – Debris deflectors can be attached to the pier or 

located at some distance upstream from the pier. The effectiveness of deflectors is largely 

controlled by the direction of stream flow. Changes in flow direction can cause the deflector to 

be ineffective and in some cases actually worsen the situation. Therefore, deflectors can be 

greatly improved if the flow direction in the stream can be stabilized by auxiliary structures such 

as guide banks which confine and stabilize the flow in a certain direction. The flow patterns 

around the deflector are complex and cannot be easily predicted. The effectiveness of the 

structure is difficult to assess. Therefore, in the determination of proper location and 

configuration of the deflector, physical modeling is encouraged to assure proper functioning of 

deflector for various discharges. 

6.21
 
6.3.2 Debris Fins for Bridges 

1. FUNCTION – Debris fins are thin walls installed upstream of the bridge parallel with 

the flow (Figure 5.35). Debris fins have been successfully used to align debris with the waterway 

opening and to avoid the accumulation of debris on bridge piers. They are used when the debris 

consists mostly of floating material. Fins have also been successful in reducing ice clogging by 

displacing ice sheets upward along the sloping top surface. 

2. TYPE OF MATERIAL – Debris fins are usually concrete, although they have been 

constructed of steel and treated timber piling and bracing. 

3. LOCATION AND ORIENTATION – Debris fins are usually located on the centerline 

of the bridge piers, and they should be carefully aligned with the flow in order to avoid 

increasing the projected pier width and a corresponding greater depth of pier scour. The 

upstream end of the fin should be rounded and sloped upward toward the bridge to reduce 

impact, turbulence, and the probability of gathering debris. 

4. DIMENSIONS – The debris fin consists of a vertical and sloped section. The vertical 

section exists from the upstream face of the pier to 1.8 m (6 ft) upstream and has a minimum 

height of 0.76 m (2.5 ft) above the maximum water surface elevation for the design flood event. 

The sloped section has a height equal to the maximum water depth at the upstream end of the 

vertical section. The sloped section extends upstream a distance of twice the maximum water 

depth. The profile of the sloped section consists of a 3:1 sloped segment and a curved segment 

with a point of intersection located one-half of the maximum water depth above and downstream 

of the upstream end of the fin. The overall width of the debris fin transitions from the width of the 

bridge pier to a width of 0.3 m (1 ft) at the upstream end of the fin. The debris fin foundation 

must be sufficient to withstand the predicted scour depth. 

6.3.3 Crib Structures for Bridges 

1. FUNCTION – Debris cribs are used for open-pile bents to prevent debris from 

trapping and accumulating between the piles. The crib structure is constructed around the 

existing pier structures by doweling the sheathing members directly into the existing piers or to 

vertical columns that are tied into the foundation of the existing piers. 

2. TYPE OF MATERIAL – Debris cribs are usually built of timber or metal sheathing, 

although concrete sheathing has been used. 

3. SPACING – The effectiveness of debris cribs is largely dependent on the spacing 

between the sheathing members. Unfortunately, there are no guidelines available for defining 

the spacing of the crib structure. In general, large spacing should be avoided since it creates a 

favorable condition for entrapping and accumulating debris. 

6.22
 
4. FLOW DIRECTION – Special considerations should be made when the pile bents 

are skewed to the approaching flow. The narrow openings created by the structure increase the 

potential for debris trapping, and debris that would normally pass through the pile bents could 

accumulate on the structure. 

6.3.4 River Training Structures for Bridges 

River training structures are structures placed in the river flow to create counter-rotating 

streamwise vortices in its wakes to modify the near-bed flow pattern to redistribute flow and 

sediment transport within the channel cross section. Design guidelines for this type of structure 

are provided in HEC-23(35) as Design Guideline 9. The guidelines provided in HEC-23 cover the 

longitudinal extent of spur field, spur length, spur orientation, spur permeability, spur height and 

crest profile, bed and bank contact, spur spacing, shape and size of spurs, and rock sizes. 

6.3.5 In-Channel Debris Basin for Bridges 

In-channel debris basins are structures placed across well-defined channels to form basins 

which impede the streamflow and provide storage space for deposits of detritus and floating 

debris. Unfortunately, no design guidelines exist for these types of structures. The flow patterns 

around these structures are complex and cannot be easily predicted. Therefore, a physical 

model should be used to design and analyze the structure to assure that it functions properly for 

various discharges. 

In-channel debris basins for floating debris have been used in parts of Europe. Two such 

structures are the “Arzbach Treibholzfang” and the “Lainbach Treibholzfang”. Both of these 

structures were designed from physical model testing conducted at the Hydraulics Laboratory of 

the Technical University of Munich(61). The two structures have similar configurations, however 

there are some noticeable differences between them. The “Lainbach” structure was built with a 

double row of posts that was later found to be unnecessary, so the “Arzbach” structure was built 

only with a single row of posts. The posts within the two structures are the same. They have a 

diameter of 0.66 m (2.2 ft) and a height of 4 m (13.1 ft) above the channel bed. They are 

comprised of a steel sleeve with a concrete core with each post set into a concrete foundation 

that is supported on piles extending 4.4 m (14.4 ft) below the channel bed. Both of the 

structures have riprap revetment along the bed and side slopes of the channel upstream and 

downstream of the posts to protect against erosion. Another difference between the two 

structures is that a performed scour hole downstream of the post was incorporated into the 

design of the “Arzbach” structure. The maintenance requirements for these structures are high 

with debris having to be removed periodically and possibly on an annual basis. 

6.3.6 Flood Relief Structures for Bridges 

Flood relief structures are flow through or overtopping structures that divert excess flow and 

floating debris through the structure and away from the bridge structure. These structures can 

6.23
 
significantly reduce the risk of significant damage or failure of the structure by reducing the 

pressure of the flowing water on the increased width of a pier resulting from the lodged debris 

and the amount of debris conveyed to the bridge. These structures were determined to be very 

effective in preventing failure of several bridge structures with debris accumulations during 

severe flooding in Pennsylvania and New York from Hurricane Agnes(47). Therefore, a flood 

relief structure should be considered for bridges that have a high potential for debris 

accumulation and where there is space available and no physical constraints that would 

otherwise preclude their use. 

Flood relief structures should be located near the ends of the bridge. These structures can be 

incorporated into the design of a bridge, where the anticipated debris accumulation is included 

in the design of the structure, to function as an emergency structure for conveying flows greater 

than the design discharge. They can also be utilized at existing bridges where debris 

accumulated on the structure has significantly reduced the discharge conveyed through the 

bridge and has caused significant increases in the upstream water surface elevation. 

The discharge that a relief bridge would need to convey can be estimated using the following 

procedure: 

1. Compute the water surface profile through the bridge for the design discharge, assuming no 

debris accumulation on the structure. 

2. Estimate the location and extent of the debris accumulation using the procedures provided 

in Chapter 3 of this manual. 

3. Reflect the accumulated debris and re-compute the water surface profile through the bridge 

for the design discharge to determine the effect the debris accumulations has on the 

upstream water surface elevation. 

4. Compute a rating curve of discharge versus upstream water surface elevation for the bridge 

structure with debris accumulations. 

5. Determine the maximum allowable water surface elevation upstream of the bridge structure 

using topographic mapping, historical flood information, and information from the field 

investigation. This elevation could also be defined as the elevation associated with potential 

failure of the bridge caused by the increase in hydraulic loading on the structure due to the 

debris accumulation. 

6. Determine the flow through the bridge structure for the maximum allowable water surface 

elevation using the rating curve computed in the fourth step. 

7. Determine the design discharge for the relief structure by subtracting the discharge 

computed in the previous step from the design discharge. 

Relief structures should be protected with revetment where significant damage to the structure 

is undesired or when the anticipated difference between the upstream and downstream water 

6.24
 
surface elevations is large and there is a potential of catastrophic flooding downstream of the 

structure. Revetment should also be provided for the downstream slopes of highway 

embankments that are designed for overflow, or that are subject to overtopping, and the 

anticipated drop between the upstream and downstream water surface elevations is large. 

6.3.7 Debris Sweepers for Bridges 

A debris sweeper is a device, generally made of polyethylene, which is attached to a vertical 

stainless steel cable or column affixed to the upstream side of the bridge pier. The debris 

sweeper travels vertically along the cable or column as the water surface rises and falls. The 

devices are also rotated by the flow, causing floating debris to be deflected away from the pier 

and through the bridge opening. Two devices could be placed on the same track with one of the 

devices being completely submerged while the other device is near the water surface. The 

devices could be aligned with the pier or offset from the pier, and special considerations on the 

placement of the devices are required for skewed flow conditions. If access to the substructure 

from the bridge deck is a problem, then a column application can be utilized or the devices can 

be installed using a boat. Debris sweepers can be used for most types of floating debris with the 

larger, heavier debris requiring a stronger bracket design. Several States are still assessing the 

use of such sweepers. 

Maintenance and inspection of these devices is recommended after a high-water event. All 

cable and anchors for the bracketing system should be inspected for proper tension, and any 

debris near the device and/or bracket system should be removed immediately, so that the 

performance of the device is not compromised during subsequent events. 

An important design guideline appears to be carefully checking the suitability of the site to the 

sweeper application. For example, the device would not be an appropriate measure if the 

design log length is greater than the effective opening between the piers. 

6.3.8 Design Features for Bridges 

The most commonly used countermeasures for bridge structures are features incorporated into 

the design of the structure to reduce the potential for trapping and accumulating debris. 

Unfortunately, specific guidance or guidelines do not presently exist for these design features. 

However, general guidance is presented below. 

1. FREEBOARD. Freeboard is a safety precaution of providing additional space 

between the design water surface elevation and the low chord elevation of the bridge. 

Considerations to the delivery potential of floating debris should be made in defining the amount 

of freeboard for a proposed bridge structure. When the potential for floating debris is remote or 

relatively low, freeboard is less important, whereas a careful selection of the freeboard is 

required for a bridge over a stream with a high potential for floating debris. The minimum 

freeboard of a bridge structure should be 0.6 meters (2.0 feet) where there is a high potential for 

floating debris. The freeboard should be increased to 1 to 1.2 meters (3.3 to 3.9 ft) where debris 

6.25
 
is abundant and known debris problems exist. Unfortunately, freeboard alone cannot guarantee 

the complete elimination of damage because the degree of protection is limited by the everpresent chance that a flood will occur that exceeds the level of protection provided by the 

freeboard. 

Increasing freeboard will decrease the probability of debris hazards to a certain degree; 

however, the cost of construction may increase significantly depending on the geometry of the 

river crossing and the bridge. For such locations, a cost-risk analysis should be performed to 

establish the recommended freeboard at the site. 

2. PIER TYPE, LOCATION AND SPACING. As indicated in Chapters 2 and 3 of this 

manual, the potential for debris accumulation at a bridge structure is significantly influenced by 

the pier type, location and spacing. Therefore, these features should be evaluated during the 

design of proposed and replacement bridges where there is a high potential for debris delivery 

to the site. Piers located within the path of floating debris can have a high potential for 

accumulation even if the span length between the piers is significantly greater than the 

maximum length of the floating debris; or, piers that have adequate spacing and are out of the 

debris path can still have a high potential to accumulate debris if the piers have narrow 

openings that can easily trap debris. 

The type of pier can influence the potential for debris to become trapped rather than deflected. 

Piers with narrow openings that convey flow are significantly more likely to trap and accumulate 

debris than piers without openings. Therefore to minimize the potential for entrapment, the 

bridge piers should be solid, round-nosed piers that are aligned with the approaching flow. If 

multiple columns are used, then considerations should be made to reduce the potential 

entrapment of debris between the columns by providing a solid web wall between the columns. 

As previously stated, debris accumulations exist most frequently and in the greatest amount 

where the path of floating drift encounters fixed objects that divide the flow. Therefore, bridge 

piers should be placed outside of the debris path, which can be estimated using the information 

provided in Chapter 3 of this manual. In general, for a curved channel reach, piers should not be 

located near the bank toe on the outside bend, and in a straight reach, piers should not be 

located near the thalweg of the channel where the flow is the deepest and fastest. For critical 

locations, the piers should not be placed within the main channel, if this can be avoided. 

The span length can influence the type of debris accumulation occurring at the bridge and the 

overall width of the accumulation. If the span length is less than the design log length, debris 

could become lodged between two piers or between a pier and the adjacent abutment and 

potentially block the entire span opening, i.e., span-blockage accumulation. Debris for this type 

of accumulations can extend beyond the piers, so the total width of the accumulation could be 

greater than the design log length. On the other hand if the span length is greater than the 

design log length, debris only accumulates on the piers at a width approximately equal to the 

design log length, i.e., single-pier accumulations. As a minimum, the span length should be 

slightly greater than the design log length, which can be determined using the recommendations 

by Diehl(17) provided on page 3.6 of this manual. Pier spacing should be even greater for 

streams with a high potential for debris delivery to the site since longer spans are less prone to 

6.26
 
debris blockage. Since the total cost of the bridge generally rises with increasing pier spacing 

and span length, the total cost of the bridge in relation to the pier spacing should be carefully 

evaluated. 

3. SUPERSTRUCTURE. Where debris hazards persist and there is a high chance of 

the bridge being overtopped, the design of the superstructure should take into account the 

consequence of overtopping. The superstructure should be designed to withstand extreme 

floods even in a submerged condition. A thin deck and low railings could be incorporated into 

the design of the superstructure to minimize the lateral hydraulic forces on the structure. Also, 

the superstructure should be designed to minimize the potential for debris accumulation on the 

structure by eliminating any unnecessary narrow openings in the structure, i.e., solid parapet 

walls in lieu of open railings, and at the connection with the pier, i.e., a solid beam that is 

connected directly to the pier that would entrap and accumulate debris. 

6.4 NON-STRUCTURAL GUIDELINES (DEBRIS MANAGEMENT) 

The implementation of a debris management plan might be a cost effective method for 

structures on small watersheds. The purpose of this plan is to reduce excessive debris input into 

the stream network by clearing trash, debris jams and downed trees from the channel and 

floodplain of a stream and/or through multipurpose channel stabilization schemes. Large woody 

debris within a channel is a beneficial and vital geomorphologic and ecologic component of a 

river system (59,60,61) and the plan should recognize these benefits. Wallerstein and Thorne have 

developed such a plan(61) by taking into account the relationship between the large woody 

debris jam formations and channel processes discussed in Section 2.2 of this manual. This plan 

is summarized in Table 6.2. 

Based on information of a given reach, the management plan provides information on the type 

of debris jam formation most likely to be present within the reach, impacts on the channel 

morphology associated with the type of debris jam formation, and an appropriate management 

strategy for the reach. The information required for the given reach includes the vegetation type, 

average riparian tree height, average channel width, and the type of sediment within the reach. 

The vegetation type is defined as either forest, agricultural, or open water with forest being the 

only type where substantial jams can form. The ratio of tree height to channel width is used to 

define the type of debris jams most likely to be present within the reach. The precise limits used 

to define the type of debris jams were determined from empirical relationships developed from 

field studies. The type of sediment, either fine (sand) or coarse (gravel) detritus, is used to 

distinguish if backwater sediment wedges or downstream bars will occur at the jams. 

6.27
 
Table 6.2. Management Plan for the Large Woody Debris Formations. 

Vegetation 

Type 

Vegetation Height/ 

Channel Width Sediment (61) Management Strategy

Agricultural or 

Open Space n.a. n.a. 

Substantial debris jams are unlikely to form within the reach since the immediat

land or open water. Therefore, debris removal is unnecessary. Artificial debri

habitat enhancement, stabilization of sand bed channels through backwater sedbank velocities on the outside of meanders. 

Forest 1.3W ≤ H n.a. 

UNDERFLOW jams exist within the reach. Debris clearance is unnecessary sadverse geomorphic impacts associated with the jams (local scour may occur uand a significant quantity of heavy floating debris would unlikely be transpobridge and other structures in the reach should not be affected by persistent deb

Forest 0.95W < H < 1.3W Coarse Detritus 

DAM jams exist within the reach. Jams may cause significant local bed scour aconstriction. Backwater sediment wedges and bars may form upstream of tconsist of coarse detritus. The jams may also increase the duration of overbank floating debris may be transported downstream from the reach. Debris clearalocal bed and bank scour results in a significant increase of large woody debstream. 

Forest 0.95W < H < 1.3W Fine Detritus 

DAM jams exist within the reach. Jams may cause significant local bed scour aconstriction. Backwater sediment wedges and bars are unlikely to form upst

sediment consist of fine detritus. The jams may also increase the duration of amount of floating debris may be transported downstream from the reachnecessary if the local bed and bank scour results in a significant increase ointroduced into the stream. 

Forest 0.60W ≤ H ≤ 0.95W Coarse Detritus 

DEFLECTOR jams exist within the reach. Jams may cause significant bank e

that could result in a significant increase of large woody debris being introducesediment consist of coarse detritus, local bed scour induced by the jams will 

backwater sediment wedges may form upstream of the jams. Debris clearanclocalized bank erosion results in a significant increase of large woody debrstream. 

Forest 0.60W ≤ H ≤ 0.95W Fine Detritus 

DEFLECTOR jams exist within the reach. Jams may cause significant bank e

that could result in a significant increase of large woody debris being introducesediment consist of fine detritus, local bed scour induced by the jams might 

sediment wedges and bars would most likely not form upstream of the jams. Dprevent local bank erosion. 

Forest H < 0.60W n.a. 

FLOW PARALLEL jams exist within the reach. Large woody debris will be tranflows from this reach and deposited at bank base in meanders and at run

geomorphic impacts associated with the jams are minimal. Banks may be staband debris may also accelerate formation of mid-channel bars. Debris clearancif it is keyed into place at bank toes and bars. 

6.28
 
CHAPTER 7 – MAINTENANCE 

Maintenance is a vital component in how a debris-control countermeasure would function at a 

drainage structure. The lack of maintenance can cause improper functioning of the structure, 

resulting in possible damage to or failure of the structure. Although no specific guidelines for 

highway maintenance practices have been established, the general maintenance practice of 

these structures should involve regular inspections and cleaning, coupled with emergency 

removal of debris. Regular periodic inspections allow minor problems to be discovered and 

corrected before they become serious. The procedure and documentation of the inspection 

should be based on the guidelines provided in FHWA’s Culvert Inspection Manual(4) and 

FHWA’s Bridge Inspector’s Training Manual(27). Debris accumulated on these structures should 

be removed in a timely manner since the presence of debris can have several severe 

consequences as discussed in Chapter 2 of this manual. In addition to the adverse effects 

associated with debris accumulations, structures with an existing debris accumulation have a 

higher potential for trapping additional debris than if they were cleared of debris. Emergency 

debris removal during a flood event can be critical to the survival of a structure laden with debris 

and to the flooding impacts upstream of the structure. 

The frequency of maintenance must be considered in the design of these structures. Structures 

located on a primary highway may have a higher frequency of maintenance than those on a 

secondary highway. If a low standard of maintenance is to be provided at a debris-control 

countermeasure or structure, it may be desirable to use a debris-control countermeasure that 

requires less attention, a combination of debris-control countermeasures, and/or a larger 

structure. This consideration may also determine the choice of alternatives when two or more 

debris-control countermeasures are being considered for a site. 

As mentioned in Section 6.4 of this manual, provisions should be made for maintenance access 

to the debris-control countermeasure or culvert/bridge structure. Unfortunately, access is often 

difficult to provide for a debris-control countermeasure and structure, particularly when it is near 

an existing high embankment. Such installations, however, usually require less maintenance 

because of the additional storage available for the debris. If access roads are impractical and 

the risk associated with flooding is high, it may be necessary to provide an area near the 

structure that mechanical equipment, such as a crane, could be located for removing debris 

from the structure without disrupting highway traffic. Access should be provided to the 

substructure of bridges to ensure prompt and complete removal of debris accumulations on the 

structure. Tracked vehicles can usually be used to remove the debris during low flows at bridges 

over small streams. For bridges over large rivers, a barge might be required to remove the 

debris, so a launching site for the barge may be necessary at such a site. 

Debris removed from a culvert or bridge should be disposed in an acceptable fashion. It should 

not be disposed directly downstream of the structure, ignoring the consequences to any 

structure further downstream. Debris should not be placed where it may be carried away by the 

stream flow or where it may block the drainage of an area. Potential disposal options include 

burning, burial, using it as firewood, using it as chipped wood for horticultural purposes if low 

7.1
 
grade wood, or using it for structural purposes if high grade wood. The latter two options are 

preferable as they may create some financial return for the operator, while burial is costly and 

burning is environmentally undesirable, especially if the wood has become contaminated in the 

river by toxins, hydrocarbons, or heavy metals. In addition, this process seems to have limited 

success in reducing the amount of debris. 

Ideally, a maintenance plan that clearly defines the maintenance activities and the responsibility 

of the maintenance crew should be developed for a structure susceptible to debris problems. As 

a minimum, this plan should contain information about the various items discussed above, i.e., 

inspections, regular and emergency debris maintenance activities, frequency of maintenance, 

access, and disposal of the debris. In addition, the general location and maximum extent of 

debris removal as estimated using the procedures and recommendations provided in Chapter 3 

of this manual can be provided for bridges. 

Maintenance associated with debris removal should be considered in the design of a new or 

replacement bridge. As discussed in Section 6.4 of this manual, there are certain features that 

can be incorporated into the design to simplify debris removal. Solid wall piers that extend a 

short distance upstream of the bridge deck edge are easier to remove debris from and less 

likely to cause damage to the bridge structure during the removal operations than other pier 

types. Superstructures that allow access to the pier nose from directly above also ease debris 

removal, such as, a wide deck with a simple parapet and adequate load-bearing capacity for 

heavy equipment. 

7.2
 
CHAPTER 8 – REFERENCES 

1. Abt S.R., Brisbane, T.E., Frick, D.M. & McKnight, C.A., 1992, “Trash Rack Blockage in 

Supercritical Flow”, Journal of Hydraulic Engineering, ASCE, December pp 1692-1696. 

2. Apelt, C.J., 1986, “Flow Loads on Submerged Bridges: Institution of Engineers Australia 

(Queensland Division)”, Queensland Division Technical Papers, v. 27, no. 19, 

Publication No. QBG 1756, p. 17-23. 

3. Arcement, G.J., Jr., and Schneider, V.R., 1989, “Guide for Selecting Manning’s 

Roughness Coefficients for Natural Channels and Flood Plain, USGS Water-Supply 

Paper 2339”, Department of Interior, U.S. Geological Survey, Denver, CO. 

4. Arnoult, J.D., 1986, “Culvert Inspection Manual, Supplement to the Bridge Inspector’s 

Training Manual”, FHWA-IP-86-2, Research and Development, Turner-Fairbank 

Highway Research Center, McLean, Virginia: U.S. Department of Transportation, 

Federal Highway Administration. 

5. AASHTO, 2000, “LRFD Bridge Design Specifications”, 2nd edition; section 3.18 

6. Ballinger, C.A. and Drake, P.G., 1995, “Culvert Repair Practices Manual, Volume I”, 

FHWA-RD-94-096, Research and Development, Turner-Fairbank Highway Research 

Center, McLean, Virginia: U.S. Department of Transportation, Federal Highway 

Administration. 

7. Ballinger, C.A. and Drake, P.G., 1995, “Culvert Repair Practices Manual, Volume II”, 

FHWA-RD-95-089, Research and Development, Turner-Fairbank Highway Research 

Center, McLean, Virginia: U.S. Department of Transportation, Federal Highway 

Administration. 

8. Barnes, H.H., Jr., 1967, “Roughness Characteristics of Natural Channels,” U.S. 

Geological Survey, Water Supply Paper 1849, Washington D.C. 

9. Bates, D.W., Murphey, E.W., and Beam, M.G., 1971, “Traveling Screen for Removal of 

Debris from Rivers”, U.S. Department of Commerce, National Oceanic and Atmospheric 

Administration, National Marine Fisheries Service. 

10. Bradley, J.N., 1978, “Hydraulic of Bridge Waterways, Hydraulic Design Series No. 1”, 

Federal Highway Administration, U.S. Department of Transportation, Second Edition, 

revised March 1978, Washington D.C. 

11. Brice, J.C., 1975, “Air Photo Interpretation of the Form and Behavior of Alluvial Rivers”, 

Final report to the U.S. Army Research Office – Durham, Washington University, St. 

Louis. 

8.1
 
12. Brice, J.C., Blodgett, J.C., Carpenter, P.J., Cook, M.F., Craig, G.S., Jr., Eckhardt, D.A., 

Hines, M.S., Lindskov, K.L., Moore, D.O., Parker, R.S., Scott, A.G., and Wilson, K.V., 

1978, “Countermeasures for Hydraulic Problems at Bridge, Volume 1 – Analysis and 

Assessment”, FHWA-RD-78-162, U.S. Department of Transportation, Federal Highway 

Administration, Washington, D.C. 

13. Brice, J.C., Blodgett, J.C., Carpenter, P.J., Cook, M.F., Craig, G.S., Jr., Eckhardt, D.A., 

Hines, M.S., Lindskov, K.L., Moore, D.O., Parker, R.S., Scott, A.G., and Wilson, K.V., 

1978, “Countermeasures for Hydraulic Problems at Bridge, Volume 2 – Case Histories 

for Sites”, FHWA-RD-78-162, U.S. Department of Transportation, Federal Highway 

Administration, Washington, D.C. 

14. Chang, F.F.M., and Shen, H.W., 1979, “Debris Problems in the River Environment”, 

FHWA-RD-79-62, U.S. Department of Transportation, Federal Highway Administration, 

Washington, D.C. 

15. Chow, V.T., 1959, “Open Channel Hydraulics”, McGraw-Hill Book Company. 

16. Denson, K.H., 1982, “Steady-State Drag, Lift and Rolling Moment Coefficients for 

Inundated Inland Bridges”, Mississippi Water Resources Research Institute, Mississippi 

State University. 

17. Diehl, T.H., 1997, “Potential Drift Accumulation at Bridges”, FHWA-RD-97-28, U.S. 

Department of Transportation, Federal Highway Administration, Washington, D.C. 

18. Diehl, T.H., and Bryan, B.A., 1993, “Supply of Large Woody Debris in a Stream 

Channel”, in Shen, H.W., Su, S.T., and Wen, Feng, eds., Hydraulic Engineering 1993 

Conference, San Francisco, 1993, Proceedings: American Society of Civil Engineers, 

v.1, p. 1055-1060. 

19. Doheny E.J., 1993, “Relation of Channel Stability to Scour at Highway Bridges over 

Waterways in Maryland”, Hydraulic Engineering 1993, ASCE, pp 2243-2248. 

20. Dongol, M.S., 1989, “Effect of Debris Rafting on Local Scour at Bridge Piers”, Report No. 

473, School of Engineering, University of Auckland, Auckland, New Zealand. 

21. Fiebiger, G., 1993, “Structures of Debris Flow Countermeasures”, Debris-Flow Hazards 

Mitigation: Mechanics, Prediction, and Assessment, pp 596-605. 

22. Fox, S.V., Gilje, S.A., Johnson, F.L., Kober, W., McGuffey, V.C., Reed, L., Robinson, 

A.R., Spaine, L.F., and Zanganeh, S., 1980, “Design of Sedimentation Basins”, National 

Cooperative Highway Research Program Synthesis of Highway Practice 70, 

Transportation Research Board, Washington, D.C. 

8.2
 
23. Froehlich, H.A., 1973, “Natural and Man-Caused Slash in Headwater Streams”, Pacific 

Logging Congress, Loggers Handbook, Vol. 33, Oregon State University, School of 

Forestry. 

24. Genetti, A.J. Jr., 1995, “EM 1110-2-4000, Sedimentation Investigation of Rivers and 

Reservoirs,” U.S. Army Corps of Engineers, Washington D.C. 

25. Gilje, S.A., 1979, “Debris Problems in the River Environment”, Public Roads. 

26. Haile, H.H., 1982, “Criteria for Design of Debris Dams and Basins”, Los Angeles County 

Flood Control District. 

27. Hartle, R. A.; Amrhein, W. J.; Wilson, K. E.; Baughman, D. R.; and Tkacs, J. J., 1995, 

“Revised Bridge Inspector Training Manual”, FHWA-PD-91-015, Research and 

Development, Turner-Fairbank Highway Research Center, McLean, Virginia: U.S. 

Department of Transportation, Federal Highway Administration. 

28. Hicks, D.M. and Mason, P.D., 1991, “Roughness Characteristics of New Zealand 

Rivers”, Water Resources Survey, DSIR Marin and Freshwater, New Zealand. 

29. Hydrological Engineering Center, 1995, “Flow Transitions in Bridge Backwater Analysis, 

RD-42”, U.S. Army Corps of Engineers, Davis, CA. 

30. Hydrological Engineering Center, 2001, “HEC-RAS, River Analysis System, Hydraulic 

Reference Manual”, U.S. Army Corps of Engineers, Davis, CA. 

31. King, H.W. and Brater, E.F., 1963, “Handbook of Hydraulics”, Fifth Edition, McGraw Hill 

Book Company, New York. 

32. Johnson, P.A., and McCuen, R.H., 1989, “Silt Dam Design for Debris Flow Mitigation”, 

ASCE Journal of Hydraulic Engineering, Vol. 115, No. 9, pp. 1293 – 1296. 

33. Johnson, P.A., Hey, R.D., Horst, M.W., and Hess, A.J., 2001, “Aggradation at Bridges”, 

ASCE Journal of Hydraulic Engineering, Vol. 127, No. 2, pp. 154 – 157. 

34. Lagasse, P.F., Schall, J.D., and Richardson, E.V., 2001, “Stream Stability at Highway 

Structures”, Hydraulic Engineering Circular No. 20, Third Edition, FHWA NHI 01-002, 

Federal Highway Administration, Washington, D.C. 

35. Lagasse, P.F., Zevenbergen, L.W., Schall, J.D., and Clopper, P.E., 2001, “Bridge Scour 

and Stream Instability Countermeasures”, Hydraulic Engineering Circular No. 23, 

Second Edition, FHWA NHI 01-003, Federal Highway Administration, Washington, D.C. 

36. Lagasse, P.F., Zevenbergen, L.W., Spitz, W.J., and Thorne, C.R., 2003, “Methodology 

for Predicting Channel Migration”, NCHRP Project No. 24-16, Ayres Associations for 

8.3
 
National Research Council, Transportation Research Board, National Cooperative 

Highway Research Program, Washington, D.C. 

37. Lagasse, P.F., Spitz, W.J., Zevenbergen, L.W., and Zachmann, D.W., 2003, “Handbook 

for Predicting Stream Meander Migration using Aerial Photographs and Maps”, Ayres 

Associations for National Research Council, Transportation Research Board, National 

Cooperative Highway Research Program, Washington, D.C. 

38. MacArthur, R.C., Hamilton D., and Gee M.D., 1995, “Application of Methods and Models 

for Prediction of Land Surface Erosion and Yield”, U.S. Army Corps of Engineers, 

Hydrologic Engineering Center, Davis, CA. 

39. Melville, B.W., and Dongol, D.M., 1992, “Bridge Pier Scour with Debris Accumulation”, 

ASCE Journal of Hydraulic Engineering, Vol. 118, No. 9, pp. 1306 - 1310. 

40. McFadden, T., and Stallion, M., 1976, “Debris of the Chena River”, CRREL Report 76­

26, U.S. Army Corps of Engineers. 

41. Mueller, D.S., and Parola, A.C., 1998, “Detailed Scour Measurements Around a Debris 

Accumulation”, Water Resource Engineering 1998, ASCE, pp. 234 – 239. 

42. Naudascher, E. and Medlarz, H.J., 1983, “Hydrodynamic Loading and Backwater Effect 

of Partially Submerged Bridges”, Journal of Hydraulic Research, Vol. 21, No. 3, pp. 213 ­

231. 

43. Ng Y.L.A., and Richardson, J.R., 2001, “Transport Mechanics of Floating Woody 

Debris”, Thesis project presented to Faculty of the Graduate School, University of 

Missouri-Columbia. 

44. Odgaard, A.J. and Spoljaric A., 1986, “Sediment Control by Submerged Vanes”, ASCE 

Journal of Hydraulic Engineering, Vol. 112, No. 12, pp 1164-1181. 

45. Odgaard, J.A., and Wang, Y., 1990, “Sediment Control in Bridge Waterway”, IIHR 

Report No. 336, Iowa Institute of Hydraulic Research. 

46. Odgaard, A.J., and Wang, Y., 1991, “Sediment Management with Submerged Vanes II: 

Applications”, ASCE Journal of Hydraulic Engineering, Vol. 117, No. 3, pp 284-302. 

47. O’Donnell, C.L., 1973, “Observations on the Causes of Bridge Damage in Pennsylvania 

and New York due to Hurricane Agnes”, Highway Research Record, No. 479. 

48. Okubo, S., Ikeya, H., Ishikawa, Y., and Yamada, T., 1997, “Development of New 

Methods for Countermeasures against Debris Flow”, Recent developments on debris 

flow, Debris avalanches – Congresses, pp. 166 – 185. 

8.4
 
49. Parola, A.C., 2000, “Debris Forces on Highway Bridges”, NCHRP 445, National 

Cooperative Highway Research Program. 

50. Parola, A.C., Hagerty, D.J., and Kamojjala, S., 1998, “Highway Infrastructure Damage 

Caused by the 1993 Upper Mississippi River Basin Flooding”, NCHRP 12-39, National 

Cooperative Highway Research Program. 

51. Parola, A.C., and Kamojjala, S., Richardson, J., and Kirby, M., 1998, “Numerical 

Simulation of Flow Patterns at a Bridge with Debris”, Water Resource Engineering 1998, 

pp. 240 - 245. 

52. Perham, R.E, 1987, “Floating Debris Control: A Literature Review”, Technical Report No. 

REMR-HY-2, U.S. Army Corps of Engineers. 

53. Reihsen, G., and Harrison, L.J., 1971, “Debris-Control Structures”, Hydraulic 

Engineering Circular No. 9, U.S. Department of Transportation, Federal Highway 

Administration, Washington, D.C. 

54. Richardson, E.V. et at., 1975, “Highway in the River Environment, Hydraulics and 

Environmental Design Considerations, Training, and Design Manual”, U.S. Department 

of Transportation, Federal Highway Administration, Washington, D.C. 

55. Richardson, E.V., and Davis, S.R., 2001, “Evaluating Scour at Bridges”, Hydraulic 

Engineering Circular No. 18, Fourth Edition, FHWA NHI01-001, Federal Highway 

Administration, Washington, D.C. 

56. Saunders, S., and Oppenheimer, M.L., 1993, “A Method of Managing Floating Debris”, 

Hydraulic Engineering 1993, ASCE, pp 1373-1378. 

57. Savage, Jerry, 2003, Photograph of Oklahoma Bridge Debris failure, project 

correspondence 

58. Vanoni, V.A., 1977, “Sedimentation Engineering”, ASCE Manual and Reports on 

Engineering Practice – No. 54, ASCE. 

59. Wallerstein, N.,1997, “Impact of Large Woody Debris on Fluvial Processes and Channel 

Geomorphology in Unstable Sand Bed River”, Ph. D Thesis presented to the University 

of Nottingham. 

60. Wallerstein, N. and Thorne, C.R., 1995, “Impacts of Wood Debris on Fluvial Processes 

and Channel Morphology in Stable and Unstable Streams”, U.S. Army Research, 

Development and Standardization Group-UK. 

61. Wallerstein, N. and Thorne, C.R., 1996, “Debris Control at Hydraulic Structures – 

Management of Woody Debris in Natural Channels and at Hydraulic Structures”, U.S. 

Corps of Engineers, Waterways Experiment Station. 

8.5
 
62. Wallerstein, N. and Thorne, C.R., 1998, “Computer Model for Prediction of Scour at 

Bridges Affected by Large Woody Debris”, International Water Resources Engineering 

Conference. 

63. Wallerstein, N., Thorne, C.R., Abt S.R., 1997, “Debris Control at Hydraulic Structures in 

Selected Areas of the United States and Europe”, U.S. Army Engineer Waterways 

Experiment Station. 

64. Wellwood, N. and Fenwick, J., “A Flood Loading Methodology for Bridges,” Conference 

Proceedings, 15th ARRB Conference, part 3, 1989, pp. 315-341. 

65. Williams, J.R., and Berndt, H.D., 1972, “Sediment Yield Computed with Universal 

Equation’, ASCE Journal of Hydraulic Division, Vol. 98, No HY12, pp. 2087-2097. 

66. Yarnell, D.L., 1934, “Bridge Piers as Channel Obstructions,” Technical Bulletin 442, U.S. 

Department of Agriculture, Washington D.C. 

8.6
 
APPENDIX A – METRIC SYSTEM, CONVERSIONS, AND WATER PROPERTIES
 

A.1
 
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A.2
APPENDIX A 

Metric System, Conversion Factors, and Water Properties
 

The following information is summarized from the Federal Highway Administration, National 

Highway Institute (NHI) Course No. 12301, "Metric (SI) Training for Highway Agencies." For 

additional information, refer to the Participant Notebook for NHI Course No. 12301. 

In SI there are seven base units, many derived units and two supplemental units (Table A.1). 

Base units uniquely describe a property requiring measurement. One of the most common units 

in civil engineering is length, with a base unit of meters in SI. Decimal multiples of meter 

includes the kilometer (1000 m), the centimeter (1 m/100) and the millimeter (1 m/1000). The 

second base unit relevant to highway applications is the kilogram, a measure of mass which is 

the inertial of an object. There is a subtle difference between mass and weight. In SI, mass is a 

base unit, while weight is a derived quantity related to mass and the acceleration of gravity, 

sometimes referred to as the force of gravity. In SI the unit of mass is the kilogram and the unit 

of weight/force is the Newton. Table A.2 illustrates the relationship of mass and weight. The unit 

of time is the same in SI as in the English system (seconds). The measurement of temperature 

is Centigrade. The following equation converts Fahrenheit temperatures to Centigrade, °C = 5/9 

(°F - 32). 

Derived units are formed by combining base units to express other characteristics. Common 

derived units in highway drainage engineering include area, volume, velocity, and density. 

Some derived units have special names (Table A.3). 

Table A.4 provides useful conversion factors from English to SI units. The abbreviations 

presented in this table for metric units, including the use of upper and lower case (e.g., kilometer 

is "km" and a Newton is "N") are the standards that should be followed. Table A.5 provides the 

standard SI prefixes and their definitions. 

Tables A.6 and A.7 provide physical properties of water at atmospheric pressure in SI and 

English systems of units, respectively. Table A.8 gives the sediment grade scale and Table A.9 

gives some common equivalent hydraulic units. 

A.3
 
Table A.1. Overview of SI. 

Units Symbol 

Base units 

length 

mass 

time 

temperature* 

electrical current 

luminous intensity 

amount of material 

meter 

kilogram 

second 

kelvin 

ampere 

candela 

mole 

m 

kg 

s 

K 

A 

cd 

mol 

Derived units ** 

Supplementary units 

angles in the plane 

solid angles 

radian 

steradian 

rad 

sr 

* Use degrees Celsius (_C), which has a more common usage than kelvin. 

** Many derived units exist; several will be discussed in this session. 

Table A.2 Relationship of Mass and Weight. 

Mass 

Weight or 

Force of 

Gravity 

Force 

English slug 

pound-mass 

pound 

pound-force 

pound 

pound-force 

metric kilogram newton newton 

A.4
 
Table A.3. Derived Units With Special Names. 

Quantity Name Symbol Expression 

Frequency hertz Hz s-1 

Force newton N Kg · m/s2 

Pressure, stress pascal Pa N/m2 

Energy, work, quantity of heat joule J N · m 

Power, radiant flux watt W J/s 

Electric charge, quantity coulomb C A · s 

Electric potential volt V W/A 

Capacitance farad F C/V 

Electric resistance ohm Ω V/A 

Electric conductance siemens S A/V 

Magnetic flux weber Wb V · s 

Magnetic flux density tesla T Wb/m2 

Inductance henry H Wb/A 

Luminous flux lumen lm cd · sr 

Illuminance lux lx lm/m2 

A.5
 
Table A.4. Useful Conversion Factors. 

Quantity From English Units To Metric Units Multiplied by* 

Length Mile 

yard 

foot 

inch 

km 

m 

m 

mm 

1.609 

0.9144 

0.3048 

25.4 

Area square mile 

acre 

acre 

square yard 

square foot 

square inch 

km2 

m2 

hectare 

m2 

m2 

mm2 

2.590 

4047 

0.4047 

0.8361 

0.092 90 

645.2 

Volume acre foot 

cubic yard 

cubic foot 

cubic foot 

100 board feet 

gallon 

cubic inch 

m3 

m3 

m3 

L (1000 cm3

) 

m3 

L (1000 cm3

) 

cm3 

1 233 

0.7646 

0.028 32 

28.32 

0.2360 

3.785 

16.39 

Mass Lb 

kip (1000 lb) 

kg 

metric ton (1000 kg) 

0.4536 

0.4536 

Mass/unit length plf kg/m 1.488 

Mass/unit area psf kg/m2

 4.882 

Mass density pcf kg/m3

 16.02 

Force lb 

kip 

N 

kN 

4.448 

4.448 

Force/unit length plf 

klf 

N/m 

kN/m 

14.59 

14.59 

Pressure, stress, 

modulus of elasticity 

psf 

ksf 

psi 

ksi 

Pa 

kPa 

kPa 

MPa 

47.88 

47.88 

6.895 

6.895 

Bending moment, 

torque, moment of 

force 

ft-lb 

ft-kip 

N · m 

kN · m 

1.356 

1.356 

Moment of mass lb · ft kg · m 0.1383 

Moment of inertia lb · ft2 kg · m2

 0.042 14 

Second moment of 

area In4

 mm4 416 200 

Section modulus In3

 mm3 16 390 

Power ton (refrig) 

Btu/s 

hp (electric) 

Btu/h 

kW 

kW 

W 

W 

3.517 

1.054 

745.7 

0.2931 

A.6
 
Table A.4. Useful Conversion Factors (continued). 

Quantity From English Units To Metric Units Multiplied by* 

Volume rate of flow ft3

/s 

cfm 

cfm 

mgd 

m3/s 

m3

/s 

L/s 

m3

/s 

0.028 32 

0.000 471 9 

0.4719 

0.0438 

Velocity, speed ft/s m/s 0.3048 

Acceleration f/s2

 m/s2 0.3408 

Momentum lb · ft/sec kg · m/s 0.1383 

Angular momentum lb · ft2

/s kg · m2/s 0.042 14 

Plane angle degree rad 

mrad 

0.017 45 

17.45 

* 4 significant figures; underline denotes exact conversion 

Table A.5. Prefixes 

Submultiples Multiples 

deci 10-1 d deka 101

 da 

centi 10-2 c hector 102 h 

milli 10-3 m kilo 103 k 

micro 10-6 μ mega 106 M 

nano 10-9 n giga 109 G 

pica 10-12 p tera 1012 T 

femto 10-15 f peta 1015 P 

atto 10-18 a exa 1018 E 

zepto 10-21 z zeta 1021 Z 

yocto 10-24 y yotto 1024 Y 

A.7
 
Table A.6. Physical Properties of Water at Atmospheric Pressure in SI Units 

Temperature Density Specific Weight Dynamic Viscosity Kinematic Viscosity Vapor Pressure Surface Tension

Centigrade Fahrenheit kg/m3 N/m3 N · s/m2

 m

2/s N/m2 abs. N/m 

0° 32° 1,000 9,810 1.79 x 10-3 1.79 x 10-6 611 0.0756 

5° 41° 1,000 9,810 1.51 x 10-3 1.51 x 10-6 872 0.0749 

10° 50° 1,000 9,810 1.31 x 10-3 1.31 x 10-6 1,230 0.0742 

15° 59° 999 9,800 1.14 x 10-3 1.14 x 10-6 1,700 0.0735 20° 68° 998 9,790 1.00 x 10-3 1.00 x 10-6 2,340 0.0728 25° 77° 997 9,781 8.91 x 10-4 8.94 x 10-7 3,170, 0.0720 

30° 86° 996 9,771 7.97 x 10-4 8.00 x 10-7 4,250 0.0712 

35° 95° 994 9,751 7.20 x 10-4 7.24 x 10-7 5,630 0.0704 40° 104° 992 9,732 6.53 x 10-4 6.58 x 10-7 7,380 0.0696 50° 122° 988 9,693 5.47 x 10-4 5.53 x 10-7 12,300 0.0679 

60° 140° 983 9,643 4.66 x 10-4 4.74 x 10-7 20,000 0.0662 70° 158° 978 9,594 4.04 x 10-4 4.13 x 10-7 31,200 0.0644 80° 176° 972 9,535 3.54 x 10-4 3.64 x 10-7 47,400 0.0626 90° 194° 965 9,467 3.15 x 10-4 3.26 x 10-7 70,100 0.0607 

100° 212° 958 9,398 2.82 x 10-4 2.94 x 10-7 101,300 0.0589 

1Surface tension of water in contact with air 

A.8 
Table A.7. Physical Properties of Water at Atmospheric Pressure in SI Units 

Temperature Density Specific Weight Dynamic Viscosity Kinematic Viscosity Vapor Pressure Surface Tension

Fahrenheit Centigrade Slug/ft3 Weight lb/ft3 lb-sec/ft2 ft2/sec lb/in2 lb/ft 

32° 0° 1.940 62,416 0.374 x 10-4 1.93 x 10-5 0.09 0.00518 

39.2 4.0° 1.940 62,424 0 

40° 4.4° 1.940 62,423 0.323 1.67 0.12 0.00514 

50° 10.0° 1.940 62,408 0.273 1.41 0.18 0.0050

60° 15.6° 1.939 62,366 0.235 1.21 0.26 0.00504 70° 21.1° 1.936 62,300 0.205 1.06 0.36 0.004980° 26.7° 1.934 62,217 0.180 0.929 0.51 0.00492 90° 32.2° 1.931 62,118 0.160 0.828 0.70 0.0048

100° 37.8° 1.927 61,998 0.143 0.741 0.95 0.0047120° 48.9° 1.918 61,719 0.117 0.610 1.69 0.0466 140° 60° 1.908 61,386 0.0979 0.513 2.89 

160° 71.1° 1.896 61,006 0.0835 0.440. 4.74 

180° 82.2° 1.883 60,586 0.0726 0.385 7.51 

200° 93.3° 1.869 60,135 0.0637 0.341 11.52 212° 100° 1.847 59,843 0.0593 0.319 14.70 

1Surface tension of water in contact with air, weight of sea water approximately 63.93 lb/ft3 @ 15°C 

A.9 
Table A.8. Sediment Particles Grade Scale. 

Size Approximate Sieve Mesh 

Opening per Inch 

Millimeters Microns Inches Tyler U.S. Standard 

4000-2000 ---- ---- 160-80 ---- ---- Ve

2000-1000 ---- ---- 80-40 ---- ---- L

1000-500 ---- ---- 40-20 ---- ---- M

500-250 ---- ---- 20-10 ---- ---- S250-130 ---- ---- 10-5 ---- ---- 

130-64 ---- ---- 5-2.5 ---- ---- 

64-32 ---- ---- 2.5-1.3 ---- ---- Ve32-16 ---- ---- 1.3-0.6 ---- ---- 

16-8 ---- ---- 0.6-0.3 2 1/2 ---- M

8-4 ---- ---- 0.3-0.16 5 5 4-2 ---- ---- 0.16-0.08 9 10 V

2-1 2.00-1.00 2000-1000 ---- 16 18 Ve

1-1/2 1.00-0.50 1000-500 ---- 32 35 

1/2-1/4 0.50-0.25 500-250 ---- 60 60 

1/4-1/8 0.25-0.125 250-125 ---- 115 120 

1/8-1/16 0.125-0.062 125-62 ---- 250 230 V

1/16-1/32 0.062-0.031 62-31 ---- ---- 

1/32-1/64 0.031-0.016 31-16 ---- ---- ---- 

1/64-1/128 0.016-0.008 16-8 ---- ---- ---- 

1/128-1/256 0.008-0.004 8-4 ---- ---- ---- 

1/256-1/512 0.004-.0020 4-2 ---- ---- ---- 

1/512-1/1024 0.0020-0.0010 2-1 ---- ---- ---- 

1/1024-1/2048 0.0010-0.0005 1-0.5 ---- ---- ---- 1/2048-1/4096 0.0005-0.0002 0.5-0.24 ---- ---- ---- 

A.10 
Table A.9. Common Equivalent Hydraulic Units 

Volume 

Unit Equivalent 

cubic inch liter u.s. gallon cubic foot cubic yard cubic meter acre-fo

liter 61.02 1 0.264 2 0.035 31 0.001 308 0.001 810.6 E 

u.s. gallon 231.0 3.785 1 0.1337 0.004 951 0.003 785 3.068 E 

cubic foot 1728 28.32 7.481 1 0.037 04 0.028 32 22.96 E 

cubic yard 46,660 764.6 202.0 27 1 0.746 6 619.8 E 

meter3 61,020 1000 264.2 35.31 1.308 1 810.6 E 

acre-foot 75.27 E + 6 1,233,000 325,900 43 560 1.613 1 233 1 

sec-foot-day 149.3 E + 6 2,447,000 646,400 66 400 3 200 2 447 1.983 

Discharge (Flow Rate, Volume/Time) 

Unit Equivalent 

gallon/min liter/sec acre-foot/day foot3/sec million ga

gallon/minute 1 0.063 09 0.004 419 0.002 228 0.001 4

liter/second 15.85 1 0.070 05 0.035 31 0.022 8

acre-foot/day 226.3 14.28 1 0.504 2 325 9 

feet3/second 448.8 28.32 1.983 1 0.646 

meter3/second 15,850 1000 70.04 35.31 22.83 

A.11 
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APPENDIX B – STATE SURVEY 

B.1
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B.2
APPENDIX B – STATE SURVEY 

State Information 

Texas Provided a web site for their hydraulic manual. This manual generally flags the 

issue and provides some general guidelines (use a single box in lieu of a 

double box, multiple boxes at different elevation, increase freeboard, and 

increase span lengths). It is indicated in the response that problems 

associated with debris doesn’t appear to be a major problem for the state. 

Oklahoma Devices to control debris: debris sweeper, increase freeboard, and single or 

triple cell RCB in lieu of double RCB. No information available for debris 

sweeper since it is a new installation. 

Michigan No design procedures, practice, devices or strategies have been employed to 

control debris accumulation problem; other than, immediately remove any 

debris accumulated at the structures. 

Missouri No standard plans for debris control structures, and they are not routine used 

in their drainage designs. 

Connecticut Discourage the use of debris control structures on culverts. Use rounded nose 

for multiple culvert or for bridge piers. Debris racks have been used with 

varying degrees of success. However, these structures are discouraged 

except where absolutely necessary because they are invariably maintenance 

intensive. 

Virginia Very limited experience with debris control structures. A few structures were 

constructed about 25 to 30 years ago. Some or possibly all of these structures 

have been damaged in storms and have been removed. 

Montana Large culverts – They have tried H-piles placed upstream of the culvert on a 

limited basis. Bridges – location of piers, minimize number of piers, maintain 

adequate freeboard, and removal of any debris accumulations (maintenance). 

South Dakota Multiple barrel box culverts – extend the upstream end of the interior walls to 

the end of the apron with a height at the upstream end set at or above a 

computed water surface elevation (sloping nose). Bridges – use pier walls 

instead of series of columns. Safety bars at the upstream end of the culverts 

have services as debris racks and have reduced the culvert performance. 

Mississippi They have used drift deflectors and web walls for bridges. 

Kansas Bridges – no piers in the main channel, webwall for the width of the pier, align 

the piers to the streamflow, structure sized assuming a debris raft is present 

(drift potential is determined based on historical records, photos, and a site 

visit). Structures that span the main channel show limited amount of drift 

buildup. 

Florida District 3 will be installing a debris sweeper on one of their bridges, and they 

will be using this device on several other bridges depending on how it 

functions on the first bridge. Some of the local bridge owners have installed 

sacrificial 18” piling immediately upstream of the bridge pier to essentially shift 

the debris buildup and cleanup away from the structure. 

B.3
 
 

State Information 

Indiana Recommend 80’ to 90’ minimum span lengths for large stream crossings, and 

they have used debris deflectors. They are currently performing a research 

project to assess the success rate of the pile debris deflectors. 

South Carolina They have designed various structures on a case by case basis using the old 

version of HEC-9 as a guideline. They have no standard plans for these 

structures. There have been no significant complaints made by their field 

office. 

Arkansas They have not used any debris control structure at any culverts or bridges. 

Tennessee Have used debris fins at the inlet of culverts. They have also installed debris 

sweeper at two bridge sites. 

Kentucky They have not used any debris control structure at any culverts or bridges. 

B.4
 
Revisions and Errata 

November 16, 2005 – Joe Krolak – Final version of second edition & pdf 

October 31, 2007 – Joe Krolak – Corrected Debris Rack height and location (subsection 6.2.2) 

based on Eric Brown and USFS inquiry of January 2007. 

Revisions & Eratta.1