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Table of Contents

60.2 - Objectives......................................................................................................................... 3

60.3 - Policy................................................................................................................................ 3

60.6 - References ........................................................................................................................ 3

60.61 - Open-Channel Flow.................................................................................................... 3

60.62 - Bridge Hydraulics....................................................................................................... 4

60.63 - Fords, Vented Fords, and Low-water Bridges............................................................ 5

60.64 - Geomorphic Design, Culvert Hydraulics, and Fish Passage ...................................... 5

60.65 - Websites...................................................................................................................... 7

60.66 - Software...................................................................................................................... 7

61 - CHANNEL DESIGN................................................................................................. 8

61.1 - Open-Channel Hydraulics................................................................................................ 8

61.11 - Flow States ................................................................................................................. 8

61.12 - Open-Channel Flow Equations................................................................................... 8

61.2 - Roadside and Trailside Design Features .......................................................................... 9

61.3 - Channel Modification..................................................................................................... 10

61.4 - Channel Stabilization...................................................................................................... 10

61.41 - Bank Stabilization Structures ................................................................................... 11

61.42 - Grade Control Structures.......................................................................................... 11

62 - BRIDGE DESIGN .................................................................................................. 11

62.1 - Bridge Location .............................................................................................................. 11

62.2 - Hydraulic Analysis......................................................................................................... 11

62.3 - Scour............................................................................................................................... 12

62.4 - Pier Design ..................................................................................................................... 13

62.5 - Abutment Design............................................................................................................ 13

62.6 - Relief Structures............................................................................................................. 14

63 - FORD AND LOW WATER CROSSING SELECTION AND DESIGN.................... 14

63.1 - Fords............................................................................................................................... 14

63.2 - Vented Fords .................................................................................................................. 15

63.3 Low-Water Bridges........................................................................................................... 16

64 - CULVERT DESIGN ............................................................................................... 16

64.1 - Location and Installation ................................................................................................ 16

64.2 - Site Surveys.................................................................................................................... 17

64.3 - Culvert Type................................................................................................................... 17

64.31 - Shape ........................................................................................................................ 17

64.32 - Materials................................................................................................................... 17

64.33 - Number of Barrels .................................................................................................... 18

64.4 - End Treatments............................................................................................................... 18

64.41 - Projecting Culverts................................................................................................... 18

64.42 - Mitered or Beveled Culverts..................................................................................... 18

64.43 - Headwalls and Wingwalls ........................................................................................ 19

64.5 - Culvert Hydraulics.......................................................................................................... 19
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65 - AQUATIC ORGANISM PASSAGE DESIGN......................................................... 19

65.1 - Design Priority: .............................................................................................................. 19

65.2 - Aquatic Organism Passage Design Methods.................................................................. 20

65.3 - Design Requirements for Aquatic Passage..................................................................... 20
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60.2 - Objectives

To maintain stream channel integrity and function, maximize structure life, minimize structure 

maintenance, and optimize user safety while providing for appropriate species passage. 

Address watershed characteristics that currently influence, or have potential to influence, stream 

crossings. 

60.3 - Policy

To the extent possible, design bridges, culverts, and low-water crossings to accommodate the 

natural passage of ice, woody debris, water, sediment, and aquatic species; accommodate 

passage of terrestrial species where such passage is necessary; provide for ecological 

connectivity; and meet the intent of applicable laws and regulations. 

Bridges, culverts, and low-water crossings should minimize constriction, relocation, 

encroachment, and modification to stream channels, adjacent riparian vegetation, and 

floodplains. Where there are feasible opportunities, use channel and stream crossing designs to 

promote favorable stream channels and improve watershed conditions. 

Use an interdisciplinary process to develop site specific design criteria including, but not limited 

to, project limits, structure type, structure material type, width, elevations, and skew angle. 

Identify upstream and downstream watershed influences, the current and most probable 

geomorphic stream setting, and species passage needs. Identify risks to the watershed and local 

stream conditions created by the structure and mitigation measures to reduce those risks. 

60.6 - References

60.61 - Open-Channel Flow

1. American Association of State Highway and Transportation Officials (AASHTO). 

Highway Drainage Guidelines. Current edition. AASHTO, 444 North Capitol Street, 

N.W., Suite 249, Washington, D.C. 20001.

2. AASHTO. Model Drainage Manual. Current edition. AASHTO, 444 North Capitol 

Street, N.W., Suite 249, Washington, D.C. 20001.

3. Chow, Ben Te. Open-Channel Hydraulics. Current edition. McGraw Hill, Inc., 1221 

Avenue of the Americas, New York, NY 10020.
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4. U.S. Department of Transportation, Federal Highway Administration (DOT-FHWA). 

Design Charts for Open-Channel Flow, Hydraulic Design Series No. 3 (HDS-3), FHWAEPD-86-102. 1961. 

Website: http://www.fhwa.dot.gov/engineering/hydraulics/index.cfm.

5. U.S. Department of Transportation, Federal Highway Administration (DOT-FHWA). 

Debris Control Structures Evaluation and Countermeasures, Hydraulic Engineering 

Circular No. 9 (HEC-9), FHWA-IF-04-01. 2005. 

Website: http://www.fhwa.dot.gov/engineering/hydraulics/index.cfm.

6. U.S. Department of Transportation, Federal Highway Administration (DOT-FHWA). 

Highways in the River Environment, Hydraulic Engineering Circular No. 16 (HEC-16), 

FHWA-HI-90-016. 1990. 

Website: http://www.fhwa.dot.gov/engineering/hydraulics/index.cfm.

7. U.S. Department of Transportation, Federal Highway Administration (DOT-FHWA). 

Bridge Scour and Stability Countermeasures Experience, selection, and Design Guidance, 

third edition, Hydraulic Engineering Circular No. 23 (HEC-23). FHWA-NHI-09-111. 

2009. 

Website: http://www.fhwa.dot.gov/engineering/hydraulics/index.cfm.

8. U.S. Department of Agriculture, Forest Service. (USDA – FS) "National Best 

Management Practices for Water Quality Management on National Forest Lands”. 

Volume 1: National Core BMP Technical Guide. The web address to order this 

publication is: http://www.fs.fed.us/biology/resources/pubs/watershed/index.html

60.62 - Bridge Hydraulics

1. AASHTO. Highway Drainage Guidelines. Current Edition. AASHTO, 444 North

Capitol Street, N.W., Suite 249, Washington, D.C. 20001.

2. U.S. Department of Agriculture, Forest Service. (USDA - FS). Bridge Scour 

Evaluation: Screening, Analysis, and Countermeasures. 7700 - Transportation Systems, 

September 1998, 9877 1207-SDTDC. http://www.fs.fed.us/eng/techdev/sdtdc.htm.

3. U.S. Department of Transportation, FHWA. Hydraulics of Bridge Waterways, 

Hydraulic Design Series No. 1 (HDS-1), FHWA-EDP-86-101. 1978. 

Website: http://www.fhwa.dot.gov/engineering/hydraulics/index.cfm.
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4. U.S. Department of Transportation, FHWA. Evaluating Scour at Bridges, Hydraulic 

Engineering Circular No. 18 (HEC-18). FHWA-NHI-01-001. 2001. 

Website: http://www.fhwa.dot.gov/engineering/hydraulics/index.cfm.

5. U.S. Department of Transportation, FHWA. Stream Stability at Highway Structures, 

3rd Edition, Hydraulic Engineering Circular No. 20 (HEC-20). FHWA-NHI-01-002. 

2001. Website: http://www.fhwa.dot.gov/engineering/hydraulics/index.cfm.

60.63 - Fords, Vented Fords, and Low-water Bridges

1. U.S. Department of Agriculture, Forest Service. (USDA - FS). Use of Precast 

Concrete Ramp Planks For Low-Water Crossings, Engineering Field Notes, Volume 15, 

April-June 1983. 4. USDA-FS, Engineering Staff, P.O. Box 96090, Washington, DC 

20090-6090.

2. U.S. Department of Agriculture, Forest Service. (USDA - FS). Low-Water 

Crossings: Geomorphic, Biological, and Engineering Design Considerations. Forest 

Service National Technology and Development Program, 2006. 

Website: http://www.fs.fed.us/eng/pubs/pdf/LowWaterCrossings

3. U.S. Department of Transportation, FHWA. Design and Construction of Low-Water 

Stream Crossings, Executive Summary, FHWA-RD-83-015. September 1983. The web 

address to order this publication is: http://www.fhwa.dot.gov/bridge/elibrary.htm. 

60.64 - Geomorphic Design, Culvert Hydraulics, and Fish Passage

1. AASHTO. Highway Drainage Guidelines. Current Edition. AASHTO, 444 North 

Capitol Street, N.W., Suite 249, Washington, D.C. 20001.

2. AASHTO. Model Drainage Manual. Current edition. AASHTO, 444 North Capitol 

Street N.W., Suite 249, Washington, DC 20001.

3. American Iron and Steel Institute (AISI). Handbook of Steel Drainage and Highway 

Construction Products. Current edition. Published by AISI, 1000 16th Street N.W., 

Washington, DC 20036.

4. Federal Interagency Stream Restoration Working Group. 1988 Stream Corridor 

Restoration: Principles, Processes, and Practices. ISBN-0-934213-59-3.

5. Watts, F. J. Design of Culvert Fishways. Current edition. Available through the 

Water Resources Research Institute, University of Idaho, Moscow, Idaho 83843.
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6. Knighton, D. Fluvial Forms and Processes. New York: John Wiley and Sons. 383p. 

7. Montgomery, D. R., & Buffington, J. M. 1997. Channel-Reach Morphology in 

Mountain Drainage Basins. Geological Society of America Bulletin. 109p. 596-611.

8. Portland Cement Association (PCA). Handbook of Concrete Culvert Pipe Hydraulics. 

Current edition. PCA, 33 West Grand Avenue, Chicago, IL 60610. 

9. Rosgen, D. 1996. Applied River Morphology. Pagosa Springs, CO. Wildland 8. 

Hydrology.

10. Somon, A. 1989. A Model of Channel Response in Disturbed Alluvial Channels. 

Earth Surface Process and Landforms. 14p. 11-26.

11. Rosgen, D. 2001. The Cross-Vane, W-Weir and J-Hook Structures: Their 

Description, Design and Application for Stream Stabilization and River Restoration.

12. U.S. Department of Agriculture, Forest Service. (USDA - FS). The Water/Road 

Interaction Series. San Dimas Technology Development Center. 1997. 

http://fsweb.mtdc.wo.fs.fed.us/search

13. U.S. Department of Agriculture, Forest Service. (USDA - FS). Stream Simulation: 

An Ecological Approach to Providing Passage for Aquatic Organisms at Road-Stream 

Crossings, Forest Service Stream-Simulation Working group, Forest Service National 

Technology and Development Program, 2008. 

http://www.fs.fed.us/eng/pubs/pdf/StreamSimulation/

14. U.S. Department of Agriculture, Forest Service. (USDA – FS). Keeping Beavers 

From Plugging Culvert Inlets, Engineering Field Notes, Volume 18. January-February 

1986. USDA-FS, Engineering Staff, P.O. Box 96090, Washington, DC 20090-6090.

15. U.S. Department of Transportation, Federal Highway Administration (DOT-FHWA). 

Hydraulic Design of Highway Culverts, Hydraulic Design Series No. 5 (HDS-5), FHWANHI-01-020. 2005. http://www.fhwa.dot.gov/engineering/hydraulics/index.cfm.

16. U.S. Department of Transportation, Federal Highway Administration (DOT-FHWA). 

Culvert Design for Aquatic Organism Passage, Hydraulic Engineering Circular No. 26 

(HEC-26). FHWA-HIF-11-008. 2010. 

http://www.fhwa.dot.gov/engineering/hydraulics/index.cfm
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60.65 - Websites

1. FHWA - Hydraulic Engineering Website: 

http://www.fhwa.dot.gov/engineering/hydraulics/index.cfm.

2. FHWA - Western Federal Lands Website: 

http://www.wfl.fhwa.dot.gov/resources/hydraulics/fishpassage/bibliography.htm.

3. NRCS - Hydraulics and Hydrology Website: 

http://www.wsi.nrcs.usda.gov/products/W2Q/H&H/H&H_home.html.

4. U.S. Army Corps of Engineers–Hydrologic Engineering Center (HEC) Website: 

http://www.hec.usace.army.mil/.

5. U.S. Department of Agriculture Forest Service, Stream Systems Technology Center 

Website: http://www.stream.fs.fed.us/.

6. U.S. Department of Agriculture Forest Service, National T&D Publications Website: 

http://fsweb.mtdc.wo.fs.fed.us/search/.

7. U.S. Geological Survey, Office of Surface Water Website: 

http://water.usgs.gov/osw/techniques/indirect.html.

8. U.S. Geological Survey, Water Resources Website: http://water.usgs.gov/.

60.66 - Software

1. U.S. Army Corps of Engineers. Hydrologic Engineering Centers River Analysis 

System (HEC-RAS). http://www.hec.usace.army.mil/software/hec-ras/.

2. U.S.D.A-FS. Stream Systems Technology Center, WinXSPRO: A Channel CrossSection Analyzer. http://www.stream.fs.fed.us/publications/winxspro.html.

3. DOT-FHWA. HY-7, Bridge Waterways Analysis Model, Micro Computer Software, 

FHWA-WSPRO. Current version. 

http://www.fhwa.dot.gov/engineering/hydraulics/software/softwaredetail.cfm#hy7.

4. DOT-FHWA. HY-8, FHWA Culvert Analysis, Micro Computer Software, FHWAHY-8. Current version. http://www.fhwa.dot.gov/engineering/hydraulics/software/hy8/.

5. U.S. Geological Survey. National Streamflow Statistics Program. 

http://water.usgs.gov/software/NSS/.
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61 - CHANNEL DESIGN

Channels should be designed to use a combination of geomorphic and hydraulic principles and 

approaches. Use the same hydraulic principles to analyze both natural stream and manmade 

channels through structures or channel section to be restored relative to water surface profiles, 

flow and velocity distributions, lateral and vertical stability of the channel, sediment 

mobility/stability, flood risk, and the potential reaction of the stream to changes in alignment, 

shape, or slope. 

61.1 - Open-Channel Hydraulics

61.11 - Flow States

Determine appropriate flow states using criteria and equations outlined in "Open-Channel 

Hydraulics" (Chow) (sec. 60.6). Open-channel flow is usually classified as one or a combination 

of:

1. Steady or unsteady flow, or

2. Uniform or non-uniform flow, or

3. Subcritical or supercritical flow.

See the zero code of this handbook for definitions of flow states. Steady and uniform flow 

seldom occurs in nature. However, Engineers generally use the simplifying assumption of 

steady and uniform flow to simplify hydraulic calculations. This assumption would not be valid 

for flow during a dam failure or breach where steady and uniform flow obviously does not occur. 

Steady, but non-uniform flow generally occurs at restrictions in the channel, such as at some 

bridge locations. Use engineering judgment to ensure that the use of simplifying assumptions 

will not result in inaccurate calculations.

Stable natural stream channels should always exist in subcritical flow conditions. Supercritical 

flow is only experienced with isolated hydraulic situations such as hydraulic jumps, channel 

obstructions, and constrictions. Design channels should mimic these subcritical flow conditions. 

61.12 - Open-Channel Flow Equations

As appropriate, use the following basic equations from "Open-Channel Hydraulics" (Chow) and 

"Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains" 

(FHWA TS-84-204) (sec. 60.6) to determine flows for the hydraulic design of open-channels:

1. Manning's Equation. Use Manning's Equation to make uniform flow and gradually 

varied flow calculations and to calculate normal depth in an open channel. (Normal 

depth is the depth of flow for uniform flow for a given cross-section, roughness, and 
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slope.) The most difficult part of using Manning's Equation is selecting the appropriate 

channel roughness coefficient, "n". Always validate numeric outputs for realistic values. 

Average stable channel conditions are always in a subcritical flow scenario, and most 

stream channels will have bankfull velocities ranging from 3-6 fps. If realistic values are 

not generated, the roughness coefficient or perhaps other variables should be adjusted. 

Use actual discharge measurements to back calculate Mannings to calibrate flow models 

and perform a Froude number check to validate subcritical flow regime. 

2. Energy Equation. Use the Energy Equation, or results derived from it, for calculating 

gradually varied flow in channels, for determining backwater elevations, and for 

calculating headwater for culverts in outlet control.

3. Momentum Equation. Use the Momentum Equation, or results derived from it, for 

calculating rapidly varied flow in the design of energy dissipaters.

4. Continuity Equation. Use the Continuity Equation, or results derived from it, for 

calculating all types of flow conditions in channels and for determining backwater 

elevations and upstream effects due to storage. Use commercially available charts and 

computer programs (available from the Federal Highway Administration, State highway 

departments, and private vendors) to simplify and speed calculations using these 

equations. For additional procedures see "Design Charts for Open-Channel Flow" 

(FHWA HDS-3) (sec. 60.6).6.11 - Stage Discharge Curves. Use a stage discharge curve 

to display the elevations of the water surface at a particular cross-section for different 

discharges. Develop a stage discharge curve to find channel freeboard, to predict 

flooding, or to determine culvert tailwater elevations.

61.2 - Roadside and Trailside Design Features

Ditches along the side of the traveled way either prevent water from reaching the traveled way or 

drain water that would otherwise collect on the traveled way. Design ditches to carry flows 

commensurate with the Road Management Objectives (RMOs, FSM 7714) and Trail 

Management Objectives (TMOs). Ensure that side features are stable for the selected flows. 

Ditches and associated structures should be designed and maintained to be disconnected from the 

natural stream system with adequate buffer distances and/or devices to eliminate or reduce 

sediment delivery to streams. This is best achieved through frequent cross drains that discharge 

onto stable areas of the forest floor allowing water to infiltrate into the ground. Design cross 

drain alignment to maintain sediment transport to the extent possible by skewing and sloping. 

Sediment buffering devices, such as low-profile slash filter windrows, coir rolls, straw bales, and 

so forth, should be used at the outlets of cross-drains to trap sediment when sediment has the 

potential to be delivered to a stream or wetland.

For further information on traveled way drainage, see FSH 7709.56, section 44.
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61.3 - Channel Modification

Streams are dynamic and extremely powerful; attempts at stream modification are expensive, 

frequently create unintended consequences, and are often only temporary. Every attempt should 

be made to move the route and structure to a stable location rather than manipulate the stream. 

Channel modifications should only be attempted with an interdisciplinary team as a last resort 

and should not be manipulated beyond natural geomorphic ranges of profile, pattern, and 

dimension. When manipulated beyond natural geomorphic ranges, streams that have been stable 

for years frequently can respond by several outcomes such as head-cutting, straightening, 

widening, meandering, down cutting, or aggrading. Perform a geomorphic assessment to

identify the critical design information required for channel modifications.

61.4 - Channel Stabilization

Use interdisciplinary expertise to characterize and properly identify stabilization issues and 

remedies. Assure that both causal and symptomatic mechanisms of the instability are understood 

and addressed by performing a geomorphic assessment. Avoid placing roads, trails, or structures 

across or immediately adjacent to unstable or braided channels, across debris flow or avalanche 

paths or on alluvial fans. Channel stabilization designs should be developed by an experienced 

interdisciplinary team. Check upstream and downstream stability at any proposed stream 

crossing by identifying potential bank sloughing, head cutting, aggradation, or down cutting. 

Where stream stabilization may be necessary, all design components should be designed to 

address identified failure mechanisms such as, but not limited to, flanking, channel down cutting, 

local scour, or systemic down cutting. Consider the stabilization method relative to efficacy of 

both local and systemic dynamics. Localized spot stabilization may fail or be expensive to 

maintain if they are located within long reaches of unstable channels.

Understand the dynamics of discontinuous grades above and below the stream crossing. When 

replacing a culvert with a bottomless structure, assure that adequate grade control is used to 

avoid head cutting through the structure.

Select stabilization measures based on an evaluation of the slope, velocity, shear stress on the 

channel and bank, projected scour, grain size of the bed load, bed and bank material, design 

flood water surface elevations, volume and type of debris, whether the channel is horizontally or 

vertically unstable, and experience with stabilization efforts in the area.

Every attempt should be made to adjust the structure to the natural or most probable stable long 

profile elevation of the stream channel. Attempts to change a streams elevation beyond natural 

geomorphic ranges for the valley and local setting controls are expensive and frequently 

temporary. 
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61.41 - Bank Stabilization Structures

Many types of stabilization structures exist. Consult with appropriate resource specialists to 

decide which structures best accommodate the channel relative to the remedy and the risk. 

Harden or protect banks only to a level of protection necessary to address the desired risk and 

public safety. Incorporate vegetation whenever possible. Avoid activities that remove or retard 

natural bank vegetation and deep rooting strengths. If stream banks are unstable and horizontal 

movement and erosion must be retained, utilize natural materials whenever possible and 

minimize the extent of hardened structures. When hardened structures are used, use natural 

channel design techniques that work with natural stream processes and cross-section hydraulics 

whenever possible. 

61.42 - Grade Control Structures

Use grade control structures as needed to avoid or control vertical instability, head cutting, and 

undermining of structure components. Understand and design for the causal mechanism of 

vertical instability in both local and systemic contexts. Utilize structures that provide for natural 

flow and sediment transport hydraulics. Natural channel design techniques will in most cases be 

the desired approach. Design structures in context to reduce the risk of identified potential 

failure mechanisms such as flanking, local scour, and down cutting. If techniques other than 

natural channel design approaches are used, assure the grade control structure(s) do not interfere 

natural flow hydraulics or with passage requirements for aquatic organisms.

62 - BRIDGE DESIGN

Hydraulic design of bridges is site specific and varies by geographic and geomorphic area. Use 

the following general requirements, modified as necessary by Regions and Forest guidance. 

62.1 - Bridge Location

Direction on bridge location is found in chapter 20 of this Handbook. 

62.2 - Hydraulic Analysis

Perform hydraulic analyses of the design discharges for road and trail bridges and Aquatic 

Organism Passage structures to determine the:

1. Headwater elevation just upstream from the bridge.

2. Water surface profile upstream from the bridge, with and without the proposed bridge.

3. Freeboard clearance.
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4. Depth of overtopping of the road or trail and the associated design discharge.

5. Velocity of flow transitioning upstream, through the bridge opening, and downstream.

Bridge openings should not restrict the bankfull stream width. Consider sloping abutment fills to 

create spill through openings to improve flow efficiency and reduce scour potential. 

The bridge opening should also account for freeboard above the maximum design flood event’s 

upstream water surface elevation. A minimum freeboard of three feet is recommended for most 

structures. If large floating debris is possible, half a typical root wad, or half a typical log should 

be added to the minimum three foot freeboard to provide clearances. A minimum freeboard to 

accommodate boat, canoe, and floater recreation use should be five feet. 

Use the HEC-RAS, “Hydrologic Engineering Center – River Analysis System” or a similar 

program that may be available to model the water surface profile through the bridge opening. 

Bridge openings should be sized to pass the 100-year flood event with an appropriate freeboard 

and considering debris and sediment passage and blocking if applicable. 

The hydraulic model should consider debris blockage in the analysis. If site specific debris data 

is not available, a 30-50 percent blockage of the bridge opening using ineffective flow areas can 

account for debris blockages. Debris blockage should be considered if there is history or 

probability of transported debris or high intensity wildfire. 

Modeling contraction and expansion coefficients should be 0.1 and 0.3 for most channels and 0.3 

and 0.5 for the cross sections nearest the bridge. The 2-year flood flow event, bankfull or the 

flow corresponding to the clear Ordinary High Water (OHW) geomorphic indicators should be 

used to determine the OHW surface elevation. This design elevation should be confirmed with 

existing OHW or bankfull at the site as validation of the design discharges. If elevations of 

predicted and existing OHW or bankfull are not similar, then further refinement of the design 

discharges is necessary. 

Temporary construction detour drainage structures should be sized to pass the 2-year flood flow, 

bankfull flow or the peak flow anticipated during the time of year construction will occur based 

on a hydrologic analysis of the site and nearby stream gages, unless provisions are in place to 

remove the structure during seasonal high-water.

62.3 - Scour

For details on estimating and reducing scour, see the following:

1. “Bridge Scour Evaluation: Screening, Analysis, and Countermeasures” (USDA-FS) 

(sec. 60.6). 

2. "Evaluating Scour at Bridges" (FHWA HEC-18) (sec. 60.6).
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3. "Highways in the River Environment" (FHWA HEC-16) (sec. 60.6).

4. "Stream Stability at Highway Structures" (FHWA HEC-20) (sec. 60.6).

Validate predicted scour with existing channel information. Evaluate the longitudinal profile and 

sediment size and mobility in the reference channel reaches to provide a check of scour depth 

that occurs naturally in the stream. Investigate local maximum scour depths in the existing 

channel, realizing that substrate deposition during recession of the peak flood commonly occurs, 

which reduces the measured maximum scour. Placement of structure elements in dynamic 

stream channels commonly has greater scour potential at flood flows than the natural stream 

because of greater flow constriction and higher shear stresses. 

62.4 - Pier Design

Eliminate piers when possible. Consider the following when piers must be constructed:

1. Pier Width and Length. Minimize the area a pier presents to the flow to decrease 

backwater and local scour.

2. Pier Shape. Streamline the pier so that it produces less backwater and scour potential 

and so that it catches less debris. 

3. Alignment. Align piers with the maximum design flood flow to produce less 

backwater and scour and to catch less debris. Use geomorphology, aerial photographs 

and other available site specific data to determine appropriate pier alignment. Avoid 

using multiple pier bents that are not aligned to the stream flow since they catch debris, 

which in turn increases local scour and backwater.

4. Location. Pier location affects the debris catching characteristics of a bridge. Avoid 

placing piers in the center of streams that carry a large amount of debris where velocity 

and debris concentration is highest. Consider alternatives, such as using a three-span 

bridge with two short spans and one long central span or using two unequal spans to 

avoid placing a pier in the center of the stream.

62.5 - Abutment Design

Choose either a vertical or spill-through abutment depending on economic and structural 

considerations and in accordance with the following conditions:

1. Spill through abutments is preferred and may minimize scour impacts.

2. Vertical abutments should have wing walls to improve scour resistance.

3. Align abutments with the flow to reduce backwater and to avoid excessive scour.
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4. Locate spill-through abutments so that the toe of the embankment is equal to or 

greater than bank full width and does not constrict the main channel. 

5. Design riprap or other appropriate countermeasures based on maximum design flood 

flow velocities and shear with debris blockages that may accelerate flows. If the channel 

and banks are stable, riprap or other appropriate countermeasures should be installed 

within areas of channel and bank disturbance. Riprap mat thickness must be a minimum 

of twice the average rock size of the riprap class/gradation or D50. A layer of geotextile 

may be placed under the riprap layer. 

62.6 - Relief Structures

Consider using relief structures on wide floodplains with large amounts of overbank flow. Relief 

structures allow floodplains to drain without using lateral channels along the upstream 

embankment slope. Relief structures also distribute the flow across the floodplain, reducing 

constriction losses and increasing the efficiency of the structures. Relief structures reduce 

abutment scour described in section 62.3.

63 - FORD AND LOW WATER CROSSING SELECTION AND DESIGN

Use these structures only when their installation is consistent with the RMOs and TMOs. 

Consider the need for depth markers to warn of dangerous crossing conditions. Install fords or 

low-water structures if traffic can be interrupted during floods and the structure can be built to 

prevent frequent severe damage.

Designs must meet aquatic organism passage requirements including low flow depth needs and 

minimum jump limits. Consult U.S. Department of Agriculture, Forest Service. Low-Water 

Crossings: Geomorphic, Biological, and Engineering Design Considerations. Forest Service 

National Technology and Development Program, 2006. 

63.1 - Fords

Use paved or natural fords under the following conditions:

1. Traffic can be interrupted during periods of mild to severe flooding.

2. Fisheries and water quality requirements allow vehicles to enter the stream.

3. The normal daily flow is less than 6 inches deep.

4. When there is extreme variation in normal flow to flood flow levels.
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If a stream has a flat slope and a rock or gravel bottom, construct a natural ford by merely 

dropping the grade to the stream bottom. On streams with steep slopes, or with rough, rocky, or 

soft sandy bottoms, level the bottom with coarse gravel. Except in very low gradients, a grade 

step will occur at the ford site. To dissipate flow energy and provide for aquatic organism 

passage when it is necessary, construct one or more grade control features downstream to evenly 

transition the downstream grade with the ford.

Use paved fords for sites similar to those of natural fords, but where a more permanent structure 

requiring less maintenance is desirable. Extend the paved ford horizontally to a point above the 

high water for a specified design flood frequency. Base slab thickness on the design wheel load 

and the foundation strength.

Use vertical cutoff walls on the downstream side of paved fords to prevent undermining of the 

slab. The depth of the cutoff wall below the streambed depends on the streambed material. 

Large gravel and cobble streams normally require only 1-foot depth, while a sandy streambed 

may require a deeper cutoff wall.

Build fords at the same elevation as the streambed.

63.2 - Vented Fords 

Use a vented ford under the following conditions:

1. Traffic can be interrupted during flooding.

2. Fishery and water quality requirements do not allow vehicles to enter the stream. 

3. Fish passage is not required. 

4. Normal daily low flow can pass through one or more small culverts.

5. The type and volume of bed load in the stream is unlikely to clog the culverts.

Vented fords obstruct the stream channel and are subjected to large drag and lift forces. 

Reinforce and tie all parts of the structure together so the weight of the entire structure will resist 

the drag and lift forces.

Use culverts as large as possible within the height restrictions for the vented ford; because large 

culverts are less likely to clog than small ones. Use as many culverts as are needed to pass the 

low flow in the stream.
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63.3 Low-Water Bridges

Use a low-water bridge under the following conditions:

1. Traffic can be interrupted during periods of moderate to severe flooding.

2. Normal daily flow cannot pass economically or effectively through a vented ford.

3. It is necessary to provide aquatic organism passage and associated natural bottoms.

Design low-water bridges similar to conventional bridges, but allow for overtopping. When 

inundated, these bridges may be subjected to significant scour. Design low-water bridges to 

resist scour and lateral stream flow forces. 

64 - CULVERT DESIGN

When designing culverts, use the direction outlined in this section in conjunction with direction 

in FSH 7709.56, section 44 along with guides and design charts available from culvert 

manufacturers. 

64.1 - Location and Installation 

Consult chapter 20 for direction in locating bridges and culverts. See U.S. Department of 

Agriculture, Forest Service. Stream Simulation: An Ecological Approach to Providing Passage 

for Aquatic Organisms at Road-Stream Crossings , "Handbook of Steel Drainage and Highway 

Construction Products" (AISI) and "Highway Drainage Guidelines" (AASHTO) (see sec. 60.6) 

for guidance in locating and installing all types of culverts. In addition, consider the following:

In flat country where the stream leaves its banks during high water and spreads over a wide 

floodplain, flow must be accommodated in the active main channel, as well as flood swales, side 

channels and sometimes along the floodplain for continuous, long embankments. In low 

gradient streams and where sediment and debris transport are an extremely low occurrence or 

probability, placing multiple adjacent culverts can be considered. Avoid concentrating water in 

one place due to the danger of scour and upstream aggradation. 

Identify and provide for proper functions of all side channels adjacent to the primary channel. 

For wetland drainage or drainage of the floodplain it is extremely important to ensure that the 

culvert invert is not placed lower than the primary surface elevation of the wetland or floodplain 

as head cutting and lowering of the water table can result.

See U.S. Department of Agriculture, Forest Service. Stream Simulation: An Ecological 

Approach to Providing Passage for Aquatic Organisms at Road-Stream Crossings for 

methodology to determine vertical, horizontal alignment of structure with the stream and cross 

sectional and streambed attribute necessary for design.
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64.2 - Site Surveys

Direction on site surveys is found in chapter 30 of this Handbook.

64.3 - Culvert Type 

Consider the shape, material, and number of barrels when selecting culvert types and base 

selection on accommodating the desired stream width, total annual cost, local availability, flow 

capacity and desired headwater depths, flow capacity relative to counter-sinking depth, service 

life, and fill height. 

64.31 - Shape

Use one of the following culvert shapes:

1. Circular. Circular culverts are the most common. They cost less because of their 

efficient use of material and ease of construction. To gain maximum desired width of the 

channel, they may require relatively deep counter-sinking depths, which may limit 

desired flow capacity.

2. Pipe Arch. Use pipe arch culverts when there are fill height limitations. Pipe arch 

culverts are not as structurally strong as circular culverts, require more intensive 

construction inspection for the backfill, and are usually more expensive for equal 

hydraulic capacity. However, for stream simulation design, they are commonly preferred 

to maximize stream width accommodation relative to culvert rise and flow capacity.

3. Rectangular. Rectangular box-shaped culverts are the most efficient in reducing 

headwater elevations for a given culvert cross sectional area, but they are poor for fish 

passage unless they are counter-sunk. They are generally more expensive than circular or 

arch pipes.

4. Arch (Without Bottom). Avoid restricting bankfull flow when using open bottom arch 

pipes. Locate footings on rock or below the estimated scour depth. Scour depth is 

difficult to estimate inside an arch because the flow is non-uniform and highly turbulent.

64.32 - Materials

Culverts are usually made of reinforced concrete, corrugated steel, corrugated aluminum, and 

polyethylene. Evaluate the following variables when choosing culvert material:

1. Abrasion. When evaluating abrasion potential, consider the water velocity and the 

size, shape, and quantity of the streambed material that can be transported. Provide 

additional protective measures for metal pipe to avoid loss of culvert material when the 

average flow velocity in the culvert exceeds 7 feet per second and the bed load consists of 

sharp, angular rock.
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2. Corrosion. Evaluating corrosion is complex and involves appraising abrasion and 

factors that cause chemicals to erode the culvert surface. Use the performance of existing 

structures in the same channel as the primary design criterion. 

The "Handbook of Steel Drainage and Highway Construction Products" (AISI) (see sec. 60.6) 

contains values for corrosion indicators for different types of soils. Use these values as 

guidelines only. 

64.33 - Number of Barrels

Avoid using multiple barrels in stream channels with the exception being where vertical 

alignment cannot be raised due to right-of-way, paving, grade control points, or environmental 

issues. Keep in mind that multiple culvert openings are more susceptible to debris and ice 

clogging than a larger single opening.

64.4 - End Treatments

Consult "Hydraulic Design of Highway Culverts" (FHWA HDS-5) (see sec. 60.6) for 

information on culvert end treatments. The following are four common types of end treatments: 

projecting culverts, mitered or beveled culverts, and headwalls and wingwalls. 

64.41 - Projecting Culverts

Use this type for the smaller diameter culverts and where projecting inlets and outlets will not be 

visually objectionable. This type of treatment will have the least first cost. 

64.42 - Mitered or Beveled Culverts

A mitered culvert is formed when the culvert is cut to conform with the plane of the embankment 

slope. Use this treatment with large metal culverts to improve the appearance of the culvert 

ends.

Cutting corrugated metal pipe significantly reduces its strength and rigidity, making the inlet end 

susceptible to damage by hydraulic forces during large flows. When stream flows depths will 

not exceed one half the height of the miter, hydraulic pressures can generally be ignored. 

Horizontal earth pressures on the mitered section of the pipe should be considered in the design. 

Sharp skews on a structure can result in uneven loading from the structure fill, so contours at the 

inlet and outlet may need to be adjusted to balance the earth pressures. In general, miters not 

exceeding 6 feet in length (along the axis of the pipe) are not in jeopardy of earth pressure 

failure. Mitered, or bevels exceeding 6 feet in horizontal length should be structurally evaluated.
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Mitered ends may be on a continuous slope from the top to the bottom of the culvert, or with a 

vertical step on the bottom, or with vertical steps on both the top and bottom. The ends with 

steps are referred to as step mitered or step beveled.

Stabilize fill slopes, paying particular attention to the inlet end, with riprap or other erosion 

control methods. Riprap placement should be monitored to prevent damage to the pipe.

64.43 - Headwalls and Wingwalls

Headwalls and wingwalls are usually vertical walls that are commonly reinforced concrete. 

Headwalls and wingwalls are typically constructed on the ends of culvert barrels to retain the fill 

material, reduce erosion of the embankment, and improve hydraulic efficiency. 

Although headwalls are sometimes skewed to the culvert barrel to fit the embankment slope, 

aligning them with the direction of flow provides a more hydraulically efficient opening. Where 

feasible, warp the fill slope to accommodate this more favorable orientation. 

64.5 - Culvert Hydraulics

Hydraulic capacity design of culverts must accommodate aquatic organism passage and natural 

substrate within the culvert. See the references in section 60.6 for detailed information. In most 

instances, headwater depths should not exceed the height of the structure in order to avoid back 

water conditions and assure proper transport of flow, sediment, and debris. For the desired 

design discharge, designs should accommodate headwater to depth ratios of 0.8 for maximum 

hydraulic efficiency.

65 - AQUATIC ORGANISM PASSAGE DESIGN

This guidance sets expectations for aquatic organism passage (AOP) design for new structures 

and replacement structures at road or trail stream crossings. Priority setting, design, and 

monitoring must be accomplished as part of an interdisciplinary process.

65.1 - Design Priority:

Aquatic organism passage and ecological connectivity is the goal and is second only to user 

safety in design priority for crossing streams that provide habitat for aquatic life. The other 

design considerations include, in order of importance:

1. Minimizing the consequences of plugging and overtopping, including the ability to 

prevent stream diversion;

2. Hydraulic capacity, including the requirement that headwater depth does not cause 

pressurized flow at the maximum flood; and

3. Maximize benefits while minimizing life cycle cost.
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65.2 - Aquatic Organism Passage Design Methods

Consider route realignment to avoid crossing the stream where possible. Temporary structures 

that span the stream may be desirable on roads with intermittent use. If a permanent crossing is 

necessary, the following alternatives for stream crossings should be considered in order of 

preference:

1. Stream simulation strategies. Bridges, arches or embedded culverts provide stream 

simulation. The USDA Forest Service stream simulation guidance should be used when 

possible.

2. Geomorphic-based Channel Design. Reconnects the upstream and downstream 

channel while meeting most fish and other aquatic organism movement and habitat 

needs.

3. Hydraulic design. Designs based primarily on hydraulic capacity should be limited to 

low stream gradients, where the culvert is constantly partially submerged. Baffled 

culverts or structures designed with a fishway are discouraged and should be used as a 

last resort, especially if they also hinder terrestrial organism passage. Note, in some 

limited circumstances, a fishway hydraulic design for a high gradient stream may be 

required to meet limited fish passage only objectives.

The design method must respond to the site specific conditions found at the crossing, a 

combination of design methods may be needed. New and replacement structures will use the 

most preferred design method possible or as much of the preferred design methods as possible. 

Document the design rationale especially when variance from this guidance is planned. 

Document the specific nature of any variance, along with biological and/or hydrologic rational to 

support the alternative approach. 

65.3 - Design Requirements for Aquatic Passage

Aquatic habitat stream crossings, regardless of the design option used, must be designed to 

accommodate the peak flood flow (100 year in most cases) plus debris and bed load without 

structural damage to the crossing or diversion of the stream. Hydraulic capacity must 

compensate for expected deposition in the culvert bottom.

Structure opening width, as a minimum, should not constrict the stream or accelerate velocity at 

bankfull flow (bankfull width). 

Stream form and substrate within the structure should conform to the reference channel crosssection, provide a thalweg for low flow conditions, and avoid continuous stream flow along the 

structure wall. The natural stream gradient and substrate material should be simulated through 

the structure and should be restored whenever possible. Consider the need for terrestrial 

organism passage in the design of materials placed in and around structures.
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Conduct geomorphic and hydrologic assessments to determine design parameters, feasibility, and 

watershed risk.

All structures should be evaluated for appropriate regrading and grade control measures to avoid 

head cutting through and above the structure.

Hydraulic analysis designs should be done in a manner to ensure bed and bed form stability. 

They should also evaluate ecological process (large woody debris transport) and associated 

failure risk (flood history). 

Baffles, weirs, and other mechanical devices inside culverts and artificial instream structures 

below culverts should only be employed when stream simulation or use of natural stream 

features is not physically possible or practical. Before using these devices, seek advice from the 

Regional Bridge Engineer.