FOREST SERVICE

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

United States Department of Agriculture

National Technology & Development Program • 7700—Transportation Mgmt • 1277 1814—SDTDC • October 2015

Storm Damage 

Risk Reduction Guide 

for Low-Volume Roads

Forest 

Service

Storm Damage Risk Reduction 

Guide for Low-Volume Roads

By:

Gordon Keller, P.E., Geotechnical Engineer

Forest Service, Pacific Southwest Region, 

Plumas National Forest (Retired)

Gary Ketcheson, Hydrologist

Forest Service, Pacific Northwest Region (Retired)

The Forest Service, an agency of the U.S. Department of Agriculture (USDA), has developed this 

information for the guidance of its employees, its contractors, and its cooperating Federal and State 

agencies. The Forest Service assumes no responsibility for the interpretation or use of this information by anyone except its own employees. The use of trade, firm, or corporation names is for the 

information and convenience of the reader. Such use does not constitute an official endorsement or 

approval of any product or service to the exclusion of others that may be suitable.

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status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or part of an individual’s income is derived from any public assistance 

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alternative means for communication of program information (Braille, large print, audiotape, etc.) 

should contact USDA’s TARGET Center at (202) 720-2600 (voice and TDD). To file a complaint 

of discrimination, write USDA, Director, Office of Civil Rights, 1400 Independence Avenue, S.W., 

Washington, D.C. 20250-9410, or call (800) 795-3272 (voice) or (202) 720-6382 (TDD). USDA is an 

equal opportunity provider and employer.

Table of Contents i

Table of Contents

Acknowledgements .......................................................................................................................... iv

1. Introduction ................................................................................................................................1-1

1.1 Storm Damage Risk Reduction Objectives ..................................................................1-4

1.2 Scope of the Problem...................................................................................................1-8

 Flood History...........................................................................................................1-8

 Emergency Relief for Federally Owned (ERFO) Roads History ...........................1-10

 Vulnerable Road Placement .................................................................................1-10

 Examples of Storm Damage.................................................................................1-14

Definition of Risk Reduction Treatments...............................................................1-16

 Open Roads..........................................................................................................1-16

Closed Roads (Roads Placed In Storage)............................................................1-17

 Relative Treatment Costs......................................................................................1-17

2. Strategic Risk Assessment.........................................................................................................2-1

2.1 Hazard Assessment .....................................................................................................2-1

 Landscape-Scale Considerations ...........................................................................2-1

Watershed-Scale Considerations ...........................................................................2-3

 Site-Scale Considerations.......................................................................................2-5

 Road Location.........................................................................................................2-6

 Road Standards......................................................................................................2-7

 Failure Potential......................................................................................................2-8

2.2 Consequence Assessment...........................................................................................2-8

 Consequences of Failure........................................................................................2-9

2.3 Overall Risk Assessment............................................................................................2-10

3. Selection of Appropriate Treatment............................................................................................3-1

4. Implementation...........................................................................................................................4-1

4.1 Planning and Preparation and Response.....................................................................4-1

4.2 Best Management Practices as Mitigation ...................................................................4-2

5. Common Storm Damage Risk Reduction Treatments ...............................................................5-1

5.1 Road Maintenance .......................................................................................................5-2

5.2 Road Surface Drainage Improvements ........................................................................5-4

 Adding Rolling Dips (Broad-Based Dips)................................................................5-7

 Cross-Drain Culverts (Ditch Relief Culverts).........................................................5-12
ii Storm Damage Risk Reduction Guide for Low-Volume Roads

Table of Contents

Waterbars..............................................................................................................5-15

 Ditches and Ditch Treatments...............................................................................5-17

 Ditch or Pipe Outlet Armoring ...............................................................................5-21

5.3 Stream Crossing Structure Protection and Improvements .........................................5-23

 Culvert and Channel Maintenance........................................................................5-23

Culvert Diversion Prevention and Overflow Protection.........................................5-23

5.4 Bridge Protection and Improvements .........................................................................5-29

 Channel Maintenance and Debris/Sediment Clearing..........................................5-29

5.5 Erosion Prevention and Erosion Control ....................................................................5-31

Erosion Prevention and Drainage Improvements .................................................5-31

 Physical Methods for Erosion Control...................................................................5-33

 Vegetative Methods for Erosion Control ...............................................................5-36

 Use of Deep-Rooted, Native Vegetation...............................................................5-38

 Soil Bioengineering and Biotechnical Erosion Control Methods...........................5-39

 Gullies and Gully Prevention.................................................................................5-42

5.6 Fillslope Stabilization Measures .................................................................................5-46

 Sidecast Fill Pullback for Slope Stabilization ........................................................5-46

6. Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments.................6-1

6.1 Less Used Road Surface Drainage Treatments...........................................................6-1

Converting Inslope Roads to Outslope ...................................................................6-1

Surface Water Diversion Structures........................................................................6-3

Culvert Cross-Drain Inlet Protection and Drop Inlets..............................................6-6

6.2 Stream Crossing Structure Protection and Improvements ...........................................6-8

Increasing Culvert Capacity..................................................................................6-11

 Trash Racks..........................................................................................................6-16

 Culvert Reinstallation and Realignment................................................................6-18

 Aquatic Organism Passage Considerations .........................................................6-21

Pipe Inlet/Outlet Protection ...................................................................................6-23

Fords or Low-Water Crossings versus Culverts....................................................6-25

 Local Structure, Streambank, and Channel-Stabilization Measures.....................6-30

 Local Scour Protection..........................................................................................6-31

 Beaver Plugging Prevention .................................................................................6-40
Table of Contents iii

Table of Contents

6.3. Bridge Protection and Improvements ........................................................................6-43

Increasing Freeboard............................................................................................6-43

 Major Bridge Scour Protection..............................................................................6-45

6.4 Road-Stream Encroachment......................................................................................6-48

Floating Log Weirs ................................................................................................6-50

 Other Channel Redirection and Bank Protection Measures (Spurs, Veins, 

 Drop Structures, and Log Jams)...........................................................................6-54

6.5 Erosion Control...........................................................................................................6-55

 Gully Stabilization .................................................................................................6-55

 Other Gully Control Considerations ......................................................................6-59

 Roadway Surfacing Materials ...............................................................................6-60

6.6 General Slope Stabilization Measures .......................................................................6-60

 Achieving Stable Cut and Fillslopes......................................................................6-60

Increasing Stability with Structures.......................................................................6-65

Drainage Improvements........................................................................................6-70

 Use of Deep-Rooted Vegetation or Soil Bioengineering/Biotechnology ...............6-72

 Strengthening Existing Structures.........................................................................6-72

 Deep Patch Shoulder Strengthening ....................................................................6-77

Changing Road Grade, Alignment, and Width......................................................6-79

6.7 Cold Regions Storm Issues........................................................................................6-80

 Thawed-to-Frozen Condition ................................................................................6-80

 Frozen-to-Thawed Condition ................................................................................6-82

 Frozen, Plugged Culverts and Ditches .................................................................6-82

River Ice Jams ......................................................................................................6-84

 Other Cold Regions Problems ..............................................................................6-84

6.8 Road Storage and Closure.........................................................................................6-84

7. Summary and Conclusions ........................................................................................................7-1

8. Appendixes

A1 Design and Construction Drawings/Notes ...................................................................8-1

A2 Road Options in Wet, Very Flat Terrain......................................................................8-10

A3 Outlet Protection/Velocity Dissipation Devices ..........................................................8-11

A4 Road Storm Damage Risk Reduction Assessment Field Evaluation/

 Road Condition Form............................................................................................8-12

A5 Glossary.....................................................................................................................8-16

9. References ................................................................................................................................9-1
1-iv Storm Damage Risk Reduction Guide for Low-Volume Roads

Acknowledgements

The original version of this document was developed under the guidance of Brian Staab, regional 

hydrologist, and Tom Erkert, transportation group leader, in the Pacific Northwest Region for use in 

the Pacific Northwest and West. It has since been expanded to deal with many of the storm damage 

issues encountered nationwide. The authors are thankful for the original support from the Pacific 

Southwest and Pacific Northwest Regions.

Thanks to the many individuals who have been involved with storm damage repairs, helped forests 

recover from the damage and prevent future damage, offered suggestions on the development of 

this guide, sent information, or helped in the review of this document. William Weaver, principal 

hydrologist for Pacific Watershed Associates, and Karen Bennett, Pacific Northwest regional soil 

scientist, provided very detailed reviews with comments on the entire document. Roger Nichols 

contributed much of the section on Floating Log Weirs, while Robert Douglas and Maureen Kestler 

contributed the information on Cold Regions Storm Issues. Special thanks to Paul Karr, retired 

Forest Service engineer, who drew most of the graphics (figures) in this document and to Susan 

Clements for her editing skills and Janie Ybarra for the publication layout.

Substantial review comments were received from:

Robert Douglas, geotechnical engineer, Golder Associates, Toronto, Canada.

Bill Shelmerdine, geotechnical engineer, Pacific Northwest Region, Olympic National Forest. 

Robert Metzger, engineering, Pacific Northwest Region, Olympic National Forest. 

Steve Coupal, operations and maintenance engineer, Rocky Mountain Region (retired).

Ron Copstead, civil engineer, Pacific Northwest Research Station, Information Resources, 

Seattle, WA.

Pete Bolander, geotechnical engineer, Pacific Northwest Region, Willamette National Forest.

Roger Nichols, engineering, Pacific Northwest Region, Mt Baker-Snoqualmie National Forest 

(retired).

Traci Sylte, soils and water manager, Northern Region, Lolo National Forest.

Tom Erkert, transportation group leader, Pacific Northwest Region.

Tom Black, research hydrologist, Rocky Mountain Research Station.

Robert Gubernick, watershed geologist, Eastern Region.

Maureen Kestler, civil engineer, San Dimas Technology and Development Center.

Paul Anderson, transportation planner, Washington Office Engineering (retired).

iv
Acknowledgements v

Thanks also to the following individuals for their comments and other contributions to this document:

Vincent Barandino, civil engineer, San Dimas Technology and Development Center.

Brian Staab, regional hydrologist, Pacific Northwest Region.

John Booth, assistant regional engineer, Pacific Southwest Region.

Odell Sanders, Technology and Development Program Manager, WO Engineering.

Penny Luehring, Technology and Development Program Manager, WO Engineering.

Robin Stoddard, forest hydrologist, Pacific Northwest Region, Olympic National Forest.

Bruce Sims, regional hydrologist, Northern Region.

Lonnie Ruchert, forest road manager, Pacific Northwest Region, Umatilla National Forest.

Caty Clifton, forest hydrologist, Pacific Northwest Region, Umatilla National Forest.

Virginia Gilmore, transportation manager, Pacific Northwest Region, Colville, National Forest.

James Barry Jones, engineering, Southern Region, National Forests of North Carolina.

Ben Lindley, assistant forest engineer, Pacific Northwest Region, Malheur National Forest.

Sandra Wilson-Musser, regional geotechnical engineer, Pacific Northwest Region.

Fred Bower, transportation planner, Northern Region.

William Goodman, regional hydrologist, Intermountain Region.

Matthew Murphy, transportation engineer, Eastern Region.

Terry Benoit, hydrologist, Plumas County (CA) Coordinated Resource Management Group.

Charlie Luce, research hydrologist, Rocky Mountain Research Station, Boise, ID.

Drew Coe, hydrologist, California Central Valley Regional Water Quality Control Board.

Justin Humble, operations engineer, Intermountain Region.

Mark Truebe, geotechnical engineer, Pacific Northwest Region, Willamette National Forest (retired).

Mark Leverton, geotechnical engineer, Pacific Northwest Region, Willamette National Forest.

Steve Romero, geotechnical engineer, Pacific Southwest Region.

Juan De La Fuente, zone geologist, Pacific Northwest Region, Klamath National Forest.

Rene Renteria, materials engineer, Pacific Northwest Region.

Don Elder, geologist, ACT2 Enterprise Team, Forest Service.

Michael Furniss, hydrologist, Pacific Southwest Research Station, Redwood Sciences 

Laboratory.

John Fehr, director, National Technology and Development Program (retired).

Leslie Boak, assistant national road system O&M engineer, Washington Office Engineering

Michael Eberle, surface water program leader, Washington Office Watershed, Fish, Wildlife, Air & 

Rare Plants, Washington, DC.

Liz Berger, assistant water program leader, Washington Office Watershed, Fish, Wildlife, Air & 

Rare Plants, Washington, DC.
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CHAPTER ONE 1INTRODUCTION
Introduction 1-1

1. Introduction

The Forest Service, an agency of the U.S. 

Department of Agriculture, manages more 

than 375,000 miles of road throughout the 

Nation. Although designed to handle severe 

storm events, land use changes, infrastructure 

degrades, and changes to climatic patterns 

may put those roads at risk of failure. Millions 

of dollars are spent on road storm damage 

repair each year, and whole road systems 

may be inaccessible for long periods of time 

while appropriate repairs are determined 

and implemented. Stream crossing and road 

drainage failures have caused extensive 

resource damage. Modification of stream 

crossings and control of road drainage, 

whether generated on the road surface or 

intercepted by the road cut and ditch, are 

the most important issues for preventing 

storm damage. Addressing streamside road 

locations, slope stability issues, and adequate 

vegetation erosion control also is important 

to reduce the risk of storm damage. In order 

to meet the Clean Water Act requirements, 

one must be aware of impending impacts and 

implement practices capable of reducing risks 

associated with large storm events. This guide 

provides a framework to assess the potential 

risks to a low volume road system and the 

selection and implementation of appropriate 

treatments to reduce those risks.

The purpose for storm damage risk reduction 

is watershed protection; additional benefits 

are reduced road repair costs, reduced road 

maintenance, and uninterrupted access. Roadrelated watershed damage during storm events 

can vary from inconvenient to catastrophic. 

When deciding on priorities and methods for 

storm damage risk reduction, strive to meet 

the intent of the Clean Water Act. Section 101 

of the Clean Water Act states “Protect and 

restore the physical, biological and chemical 

integrity of the Nations waters.” If there is 

a choice between protecting the road and 

the watershed, choose watershed. This may 

result in moving the road or finding alternate 

access. Many road decisions in the past 

have contributed to damage to the roads and 

watershed today.

Storm damage risk reduction (SDRR), as used 

in this guide, refers to nonrecurring treatments 

on existing roads that reduce the potential for 

resource impacts and damage or failure of 

a road feature or road system resulting from 

storm events. SDRR treatments are needed to: 

❏ Reduce the potential for future damage 

(risk reduction).

❏ Reduce the magnitude of failures and 

resource damage that occurs when major 

storms do occur.

❏ Add redundant systems to protect roads 

receiving less frequent maintenance. 

❏ Improve hydraulic efficiency and resilience 

of existing road drainage features. 

These treatments relate to open and 

stored roads as defined in Forest Service 

Handbook (FSH) 7709.59 - Chapter 60, Road 

Maintenance (U.S. Department of Agriculture, 

Forest Service 2009). Stored roads do not 

have active traffic, but may still have many of 

the same erosion prevention, drainage, and 

maintenance needs as an open road. However, 

the selection and implementation of treatments 

must take into account the differences in road 

maintenance levels for open and closed roads 

in relation to the risk of damage over the life 

of the treatments. Stored roads do not have 

active traffic! They will receive less frequent 

maintenance than open roads, but as long 

as elements of the road drainage system are 

retained and expected to function (i.e., culverts 

and ditches), some periodic evaluation or field 

checking is needed to assure the long-term 

stability and functionality of the road and to 

minimize environmental damage. 
1-2 Storm Damage Risk Reduction Guide for Low-Volume Roads

In selecting maintenance frequencies, it is important to understand how geomorphic and climatic 

conditions may affect the road, i.e., some roads may be revegetated within 5 years and may be 

inaccessible. Anticipate plugged culverts in areas where sediment supply is high. Maintenance 

and/or treatments necessary to protect drainage facilities and runoff patterns remain as critical 

management functions, even on stored roads. Stored roads must not pose any more risk to 

resources than open roads. In future years, maintenance—particularly on closed roads—will be 

minimal, so the most problematic areas must be prioritized to receive needed attention. 

Forest road management must be comprehensive and multifaceted. As part of, or prior to, the 

selection of SDRR treatments, basic questions need to be answered for roads and road systems 

(roads analysis). Is the road needed? Should the road be decommissioned? Based on site 

conditions and stability concerns, can the road be stabilized in its current location, or should 

relocation be considered? SDRR assessments and treatment selection should not substitute for 

making clear choices regarding road management. Road decommissioning (permanent closure), 

while a very valuable tool for addressing critical road issues, is outside the scope of this guide. 

However, decommissioning may be the best choice for a road. Many of the treatments described 

in this guide have applicability to road decommissioning because the physical processes that affect 

roads also must be accounted for when prescribing treatments to decommission a road.

SDRR treatments are not in the same category as capital improvements and are not specifically 

road maintenance. However, there is considerable overlap across the spectrum when it comes 

to some of the treatments. SDRR treatments are measures applied to existing roads to reduce 

or eliminate impacts from storm-related effects on site productivity (loss of soil and vegetation on 

hillslopes) and on downstream aquatic habitat and water quality, as well as minimize road damage. 

Storm-related affects include:

Outside of the Road Prism Within the Road Prism

Hillslope failure (mass wasting). Cutslope failure or erosion.

Stream channel migration. Road surface erosion (gully and rill).

Stream sediment and debris loads. Road surface drain plugging.

Flooding. Fillslope erosion or mass wasting (sidecast failures).

Formation of gullies downslope of road. Culvert scour or plugging. 

Drainage feature damage. Ditch scour or blocking.

Upslope surface erosion. Bridge and ford scour or failure.

Stream capture. Drainage diversion and cascading failures.

Annual or other frequent measures taken to maintain drainage features and drivability (including 

safety measures) and minimize water concentration may include measures found in this guide. 

Road maintenance is defined in FSH 7709.59, and the types and frequency of maintenance 
Introduction 1-3

are defined by the road maintenance level. 

SDRR treatments are not prescribed and 

implemented on an ongoing basis; however, 

the treatments may be items that require some 

maintenance over time to continue to function 

properly. On the other end of the spectrum, 

SDRR treatments are not specifically capital 

improvement projects, although the separation 

can be indistinct in some cases. Capital 

improvement projects are implemented as 

funding allows to improve the safety, drivability, 

or long-term use of the road. They may be 

projects to move a road from one maintenance 

class to another. Practitioners should 

understand what treatments accomplish storm 

damage reduction objectives and use funding 

as available to achieve SDRR objectives. 

Some of the SDRR measures include:

❏ Keep needed road maintenance up to date.

❏ Have adequate road surface drainage 

measures to move water off the 

road surface rapidly and prevent the 

accumulation of water.

❏ Have culverts in good condition, free of 

debris, and with diversion prevention 

measures installed.

❏ Keep bridge channels cleared and free of 

excessive debris.

❏ Have slopes well covered with deeprooted vegetation or other erosion control 

methods.

❏ Pull back marginally unstable or failing 

sliver fillslopes.

SDRR treatments may be necessary to protect 

the investment in a road or protect critical 

natural resources while road decisions are 

being made, or while funding is being sought for 

capital improvement work. In these instances, 

SDRR treatments are interim rather than 

final treatments. In other instances, SDRR 

treatments will constitute the final treatments 

for a road and will contribute to reducing future 

maintenance costs. The treatments become 

a permanent feature of the road and will still 

require some regular maintenance to function 

properly over time. On lower use roads, the 

selection of proper treatments must take into 

account the need to function with infrequent 

maintenance. All roads probably can benefit 

from SDRR treatments. In the context of this 

guide, SDRR treatments are measures applied 

to existing roads to correct situations that 

contribute to a higher risk to resources and 

of road damage or failure. However, many 

measures that are considered SDRR should 

be part of good road design or as road best 

management practices since they offer ongoing 

protection for the road, from construction 

through maintenance and decommissioning. 

Many SDRR measures cannot practically be 

added into the road on an emergency response 

basis, such as when a large storm is forecast 

(time and resources are not that flexible or 

necessarily available), so the measures are 

installed as an upgrading practice through an 

ongoing SDRR program.

Given the recent unpredictable and often 

extreme weather events we are experiencing, 

much emphasis is being given to the issue 

of global climate change and its impacts on 

infrastructure, including roads. Most Federal 

agencies, including the Forest Service; U.S. 

Department of Transportation, Federal Highway 

Administration (FHWA); U.S. Department of 

Commerce, National Oceanic and Atmospheric 

Administration; and U. S. Department of the 

Interior, U.S. Geologic Survey; as well as the 

National Academy of Sciences, are working to 

understand, model, and ideally minimize climate 

change impacts. Also, many international 

concerns exist and efforts are being undertaken 

by groups, such as the Organization of 

American States, the World Road Association, 

World Bank, the United Nations, and others, to 

deal with the issue.
1-4 Storm Damage Risk Reduction Guide for Low-Volume Roads

An increase in the size and intensity of forest 

fires also appears to be the result of global 

climate change. Post-fire storm events 

typically cause increased runoff and erosion 

from burned-over watersheds. Watershed 

managers need to prescribe treatments that 

account for this increase in runoff in channels 

and ditchlines, and debris movement that may 

block stream channels and culverts. Many 

SDRR treatments are effective for preventing or 

mitigating damage from post-fire storms.

1.1 SDRR Objectives

A Storm Damage Risk Reduction Guide 

is needed because there are more forest 

roads. Through the 1980s, the forest road 

system expanded rapidly, received regular 

maintenance, and was used extensively. 

Roads were constructed for efficiency of use 

and, typically, used 25-year drainage design 

standards. Construction standards were less 

stringent, and construction often included 

leaving stumps and other organic debris in 

fills, extensive sidecasting of fill material, and 

fewer and smaller drainage culverts. Large 

storms caused considerable road damage and 

cumulative resource impacts. Repair of stormdamaged facilities replaced failed structures 

with similar structures.

Over the last two decades, road construction on 

National Forest System lands has almost ended 

and road maintenance has been deferred. 

Except where forest management projects 

are being implemented and where roads 

access popular recreation sites (campgrounds, 

trailheads, ski areas, and so forth). 

Road drainage designs typically now use 

a 50- to 100-year storm event for design 

purposes. The Northwest Forest Plan Aquatic 

Conservation Strategy requires new culverts, 

bridges, and other stream-crossing structures 

to accommodate at least a 100-year flood, 

including associated bedload and debris. 

The lack of maintenance and old, undersized 

road drainage features leaves roads at high 

risk of failure or damage during climatic 

events. Aquatic resources that are already 

impacted also are at elevated risk to impacts 

from increased sediment delivery from road 

failures. Repair costs are escalating, and storm 

frequency and intensity may be increasing 

due to climate change. Many road features 

have met or exceeded their design life. Certain 

locations are expected to be more susceptible 

than others or have a higher response potential 

to change under climate-change scenarios; 

for example rain-on-snow zones and firesusceptible forests.

This guide is intended to provide a framework 

for the selection of appropriate SDRR 

treatments. There are very important tasks and 

processes that are necessary to make informed 

treatment selections that are not covered 

in detail in this guide, but should be part of 

comprehensive road management programs. 

They include road condition inventories, hazard 

assessments, and strategic plans for treating 

high-hazard sites.

Treatments outlined in this guide are 

intended to minimize road damage and the 

accompanying environmental harm associated 

with how roads respond to storm events. There 

are multiple benefits from reducing storm 

damage, including the following: 

❏ Maintaining soil on hillslopes.

❏ Reducing rapid delivery of storm runoff 

and debris to streams by decoupling road 

drainage runoff from the stream network.

❏ Reducing catastrophic sediment and debris 

delivery to streams.

❏ Maintaining stream channel structure that 

absorbs stormflow without damage to 

aquatic habitat.

❏ Reducing the cost, effort, and 

inconvenience of having to repair damaged 

roads.
Introduction 1-5

The treatments themselves may actually 

increase the number of initiation or source 

points in the short term (i.e., adding culverts or 

waterbars). Care is required to construct these 

treatments in locations and at a frequency 

using techniques that do not increase erosion 

potential. With proper implementation, SDRR 

treatments should substantially reduce 

consequences during large storms. 

Successful implementation of SDRR treatments 

has additional benefits. Many treatments 

will reduce chronic sediment delivery that 

results from road runoff during all storms. The 

reduction of sediment delivery (chronic and 

catastrophic) to streams will reduce the risk of 

failure for in-stream projects implemented to 

improve stream function and fish habitat. Where 

appropriate, consider fish and other aquatic 

organism passage needs when implementing 

SDRR treatments. By nature of the interrelated 

processes, the reduction of sediment delivery 

involves changing how water is intercepted and 

runs off of the road network. A secondary effect 

of SDRR treatments may be to alter how roads 

change the magnitude and timing of streamflow 

peaks; and, alternately reduce impacts from 

storm runoff in the channels downstream. Road 

maintenance and storm damage repair costs 

are expected to decrease after implementation 

of SDRR treatments. 

Risk reduction is a variable target. The goal 

is to reduce to what level of risk? Different 

individuals, various managers, and scientists 

have differing tolerances for risk. Different sites 

have characteristics that affect the tolerance of 

risk. It is not possible for this guide to prescribe 

treatments that eliminate all risk. It will be up to 

the implementation team, in consultation with 

their public, to evaluate the risks, the values at 

risk, and the amount of time and resources to 

apply to reduce the risk to a desirable level. 

This guide is intended for use in the Forest 

Service regions nationwide, including Alaska. 

The treatments have been developed and used 

primarily for the climate and geomorphology 

of western landscapes, but most of the 

SDRR treatments apply in all regions. Some 

storm damage issues specific to cold regions 

are briefly discussed in section 6.7, but a 

thorough discussion of this topic is beyond 

the scope of this guide. In any geographic 

location, the implementation team should 

perform assessments to assure that the 

treatments discussed herein are appropriate 

for the situation. Include the assessment in 

the determination of the values at risk, site 

conditions, failure mechanisms, and climatic 

conditions that contribute to failure.

A summary of most key SDRR—or 

stormproofing—principles applicable to forest 

and low-volume roads are the following:

1. Identify areas of historic or potential 

vulnerability. Certain high-risk sites are 

well known; others may be more subtle. 

Chronically undersized culverts will have 

a history of failure. Geologically unstable 

materials or slopes, roads on steep slopes 

with sidecast fills, or roads that cross 

steep channels subject to debris flows, wet 

slopes, areas subject to flooding, or areas 

of high soil erosion near streams (inner 

gorges) all have increased vulnerability.

2. Avoid local problematic and high-risk 

areas. Consider road closure or relocation 

to avoid problematic areas and poor 

road locations. Common problematic 

areas include steep slopes (over 60 to 70 

percent), deep-seated rotational landslides, 

and areas prone to shallow rapid landslides 

and debris torrents, avalanche chutes, 

rock-fall areas, wet areas, saturated soils, 

highly erodible soils, and so forth. 
1-6 Storm Damage Risk Reduction Guide for Low-Volume Roads

3. Use appropriate minimum design 

standards. Road standards, particularly 

road width, should be minimized, while still 

considering traffic safety and road user 

needs. Because SDRR treatments involve 

existing roads, road standards are already 

in place. Use SDRR treatments, however, 

to lower the standard as appropriate and 

result in less earthwork, lower cuts and 

fills, and less concentration of runoff, all 

of which reduce risk of damage or failure 

during storms. 

4. Employ self-maintaining concepts 

into the selection and implementation 

of treatments. Resources for road 

maintenance often are limited and the 

road systems are extensive. Implementing 

those treatments that reduce the amount 

of road miles that need frequent and costly 

maintenance will allow limited resources 

to be applied to more of the road system 

where it is needed. Examples might include 

outsloping (on appropriate soil types), 

additional cross drains, and redundant 

(back up) or larger drainage structures.

5. Incorporate relevant, cost-effective 

technology. Apply current appropriate 

technology to improve identification of 

priorities and for planning, design, and 

reconstruction practices. This includes the 

use of geographic information systems, 

and global positioning systems technology; 

geosynthetics for filters, separation, and 

reinforcement; mechanically stabilized 

earth retaining structures; current riprap 

sizing criteria for bank stabilization; 

bioengineered and biotechnical slope 

stabilization and erosion control measures, 

and so forth.

6. Perform scheduled maintenance. 

Perform scheduled maintenance at a 

planned frequency to be prepared for 

storms. Ensure that culverts have their 

maximum capacity, ditches drain well, 

and channels are free of excessive debris 

and brush that can plug structures.1

 Keep 

the roadway surface shaped to disperse 

water rapidly and avoid areas of water 

concentration. There may be insufficient 

time to do the routine work as a storm is 

approaching.

7. Use simple, positive, frequent 

roadway surface drainage measures 

and use restrictions. Provide good 

roadway surface drainage so that water 

is dispersed off the road frequently and 

water concentration is minimized. Where 

soil properties are insufficient to support 

traffic when wet (i.e., volcanic ash), restrict 

use during wet seasons to prevent rutting 

and gullying. Outslope roads whenever 

appropriate and practical and use rolling 

dip cross drains for surface drainage rather 

than a system of ditches and culverts that 

require more maintenance and can plug 

easily during major storm events. Frequent 

cross drains, insloping and outsloping, and 

rolling road grades all need to be in good 

working order. Failed cross-drain culverts 

are very common after major storm events.

8. Properly size, install, and maintain 

culverts. Improperly installed, undersized, 

and plugged pipes are common reasons 

for culvert failure during storms. Improper 

alignment or grade relative to channels and 

1 Limit clearing debris from stream channels to smaller 

mobile pieces that pose an immediate risk to a structure. 

Larger pieces that are integrated into the channel bed or 

banks or large enough to resist movement during most 

flows are important for stream function and aquatic habitat. A fisheries biologist or hydrologist should be consulted 

prior to any extensive debris removal.
Introduction 1-7

ditchlines, excessive woody debris in the 

channel, excessive channel constriction 

and headwater elevation, excessively 

wide inlet areas, and inadequate capacity 

all contribute to pipe plugging and 

subsequent failure. Concrete or masonry 

headwalls greatly improve the resistance 

of culverts to failure during overtopping. 

Another cause of culvert failure is a lack 

of proper maintenance. Maintaining inlet 

configurations and removing debris that 

may plug the pipe are essential for proper 

function during storms. 

9. Use simple fords or vented low-water 

crossings. Use simple fords or vented 

low-water crossings (vented fords) as often 

as appropriate for small or low-flow stream 

crossings on low-volume roads, instead of 

culvert pipes that are more susceptible to 

plugging and failure. Protect the entire (100 

year) wetted perimeter of the structure, 

and the downstream edge of the structure 

against scour; provide for aquatic organism 

passage as needed.

10.Stabilize cut and fillslopes. Remove 

or treat unstable fillslopes as necessary 

to improve stability. Cut and fillslopes 

should be well covered (stabilized) with 

vegetation, to minimize surface instability 

problems as well as minimize surface 

erosion. Uncompacted sliver fills and 

settling or cracking fills are a high priority 

for stabilization or removal. Fillslopes also 

may be undercut and oversteepened by 

a stream or channel. Failing, over-steep 

slopes from road construction where 

material enters a stream can cause 

downstream problems to the watershed 

and contribute to plugging of structures.

11.Use deep-rooted vegetation to 

“anchor” soils. Promote slope stability 

by using deep-rooted vegetation for soil 

bioengineering and biotechnical treatments. 

Combine deep-rooted plants with a mixture 

of shallow-rooted grasses for good ground 

cover and erosion control on slopes, 

preferably using native species. 

12.Design high-risk bridges and culverts 

with armored overflows. High-risk 

bridges and culvert structures often can be 

designed with armored overflow areas near 

the structure in case of overtopping, or they 

can have a controlled failure point that is 

easy to repair and minimizes environmental 

damage. Alternatively, oversizing the 

structure and allowing for extra freeboard 

on bridges will maximize capacity and 

minimize risk of plugging. Do not constrict 

the natural channel. Consider culverts with 

a span that is at least bank-full channel 

width and bridges that span the flood plain.

13.Eliminate diversion potential. Design 

and construct (or upgrade) all stream 

crossings, especially culvert crossings, to 

have no diversion potential. Design and 

construct (or upgrade) stream crossings in 

steep stream channels that are subject to 

debris flows to withstand such debris flows 

without being washed out or resulting in 

subsequent streamflow diversion. Structure 

damage from a plugged culvert may be 

minimal, but road damage from a stream 

diverted down the road can be extensive.

14.Use scour prevention measures for 

structures on questionable foundation 

materials. Install bridges, retaining 

structures, and structural foundations 

into bedrock or on firm, in-place material 

with good bearing capacity to minimize 
1-8 Storm Damage Risk Reduction Guide for Low-Volume Roads

foundation failures. Apply foundation 

strengthening and scour prevention 

measures when foundation conditions are 

known to be marginal or a bridge is scour 

susceptible. 

15.Be aware of channel morphology and 

stream channel changes near a bridge, 

culvert, ford, or road along a creek. 

Significant changes in stream gradient, 

from a steeper reach to a flatter area, can 

cause channel aggradation (deposition) 

and subsequent plugging of structures or a 

stream jumping out of its original channel. 

This is particularly problematic on alluvial 

fans where avulsion can damage roads and 

structures. Also, tight bends in the channel 

promote concentration of flow to the outside 

edge, often leading to scour. Woody debris 

tends to accumulate at bends in a channel. 

Road work or improvements also might cut 

off a stream’s natural access to its flood 

plain.

1.2 Scope of the Problem

Flood History

The Western United States is subjected to 

severe weather that moves on shore from 

the Pacific Ocean. The severity and nature of 

the storms is dependent on the season and 

whether individual storm cells originate in the 

northern Pacific and Gulf of Alaska, or from 

the southern Pacific and the Hawaiian Islands. 

Flooding in the coastal mountains and western 

Cascades is most common during the fall 

and early winter months (November through 

February) when deep low pressure pulls large 

quantities of moisture from the southern Pacific 

Ocean. These storms may produce 24-hour 

rain totals of 5 to 10 inches or more. Flooding 

may be exacerbated if low-elevation snow 

is present when these storms arrive. These 

rain-on-snow storms typically cause the largest 

floods on record in the Coast Ranges, Olympic 

Peninsula, and western Cascade watersheds 

(U.S. Department of Agriculture, Forest Service 

1992) and Northern Sierras. 

Rain-on-snow events are less frequent on the 

east side of the Cascades and Sierra, and in 

the interior of the continent. They do, however, 

account for a significant number of the peak 

flows. Rapid spring snowmelt is the more typical 

flood-generating process to the east of the 

Cascade mountains and in the Inter-mountain 

west. Unlike the rain-on-snow floods of the west 

side that only last for a couple days, snowmelt 

flooding may last for a week or more. Ice-jam 

flooding also may occur during very cold winter 

weather followed by a rapid warmup. In this 

case, blocks of river ice may form large rafts 

and dams across rivers, causing widespread 

flooding before the dams breach. When the ice 

dams breach, the resulting surge flows may 

cause additional downstream flooding. 

Extreme weather variations and uncommonly 

intense and frequent storms in the Northeast 

have caused local to widespread heavy 

precipitation, power loss, and damage.

Hurricanes have increasingly impacted the 

Southern Region and along the East Coast. 

Infamous events, such as Hurricane Hugo in 

1989, Katrina and Rita in 2005, Irene in 2011, 

and Sandy in 2012, did widespread damage in 

those regions. Increasing droughts and violent 

thunderstorms in the Southern Rockies and 

tornadoes in the Midwest and South also have 

had huge impacts on lives and infrastructure.

Some of the largest magnitude floods recorded 

in the Northwest occurred in the late 1890s 

and early 1900s during a wetter, colder climate 

period at the end of the Little Ice Age. These 

floods preceded the onset of extensive forest 

management. Much of the early and middle 

1900s were marked by comparably modest 

floods with an occasional very large flood. 
Introduction 1-9

The Christmas storm of 1964 was large in 

magnitude and geographic extent and reset the 

thinking of road construction techniques and 

culvert design in much of the Northwest. Floods 

across the region in the late 1900s and early 

2000s have led to another look at roads and 

the practice of replacing road features in place 

and in-kind after floods. Road relocation is now 

considered a viable option for consideration in 

planning flood repair. 

Toward the end of the 20th century, stream 

gauge records have shown a marked 

increase in the frequency of large floods (U.S. 

Department of Agriculture, Forest Service 

1992; U.S. Department of Agriculture, Forest 

Service 2010). During the last 35 years, more 

than 1,500 bridges have collapsed nationwide, 

at least half of which are attributed to flooding 

and scour. Whether the increase in flood 

frequency is related to land use, climate 

change, or a combination, is a subject of 

continued discussion, but there is mounting 

evidence of both mechanisms having the 

potential to increase the magnitude and 

frequency of severe floods (Casola et al. 2005; 

U.S. Environmental Protection Agency 2010; 

National Cooperative Highway Research 

Program 2014). 

“Global Climate Change Impacts in the United 

States” (Karl et al. 2009) report and “Adopting 

Infrastructure and Civil Engineering Practice 

to a Changing Climate (American Society of 

Civil Engineers 2015) each discuss many 

of the storm and weather changes that are 

predicted and that affect land management and 

roads, including increasing frequency of heavy 

precipitation events; increasing streamflows in 

some regions, particularly the East; and earlier 

snowmelt in the West and Northeast. Other 

descriptions of weather related to global climate 

change have included more radical or extreme 

events, more variability, warmer and moister 

air, more intense storms and more droughts, 

unseasonable temperatures, etc. Expect more 

intense storms, but where and when are difficult 

to predict. Ultimately the problem of increased 

risk of damage from storms, both to roads and 

to the watersheds, comes from a combination of 

global climate change, land use changes, and 

an aging infrastructure.

Given the unpredictable and often extreme 

weather events we are experiencing today, 

a great amount of emphasis is being given 

to global climate change and its impacts on 

infrastructure, including roads. Most Federal 

agencies, including the U.S. Department of 

Agriculture Forest Service, U.S. Department of 

Transportation Federal Highway Administration, 

U.S. Department of Commerce National 

Oceanic and Atmospheric Administration, 

and the U.S. Department of the Interior U.S. 

Geologic Survey, are working to understand, 

model, and ideally minimize its impacts. The 

Intergovernmental Panel on Climate Change 

(IPCC) has produced a great deal of information 

on causes and impacts of climate change, 

including their Working Group publications 

“Climate Change 2014: Mitigation of Climate 

Change”, Climate Change 2014: Impacts, 

Adaptation, and Vulnerability”, and “Climate 

Change 2013: The Physical Science Basis”. 

IPCC publications are available through <http://

www.ipcc.ch>.

The U.S. Department of Transportation, FHWA, 

has been actively involved in climate change 

issues and measures to lessen the impacts 

of more frequent and severe storms, such as 

disaster planning, management and evacuation 

issues during disasters, and emergency and 

permanent repairs, as well as emergency relief. 

Useful Web sites for information are available 

at <http://ops.fhwa.dot.gov/publications/

publications.htm#eto>

and <http://www.fhwa.dot.gov/programadmin/

erelief.cfm>. 
1-10 Storm Damage Risk Reduction Guide for Low-Volume Roads

More intense storm events following fires 

further contribute to local flooding, debris 

slides, and significant sediment movement in 

channels; resulting in ditch erosion, plugged 

culverts, and other damage to infrastructure. 

The Burn Area Emergency Response (BAER) 

program includes the assessment of risk and a 

prescription of treatments to minimize post-fire 

damage from increased runoff and peak flows. 

Many of the SDRR treatments discussed in this 

publication are typically prescribed by BAER 

assessments. Some of the treatments are 

discussed in the BAER catalogue (BEARCAT, 

Napper 2006).The Forest Service, Rocky 

Mountain Research Station publication “Climate 

Change, Forests, Fire, Water, and Fish: Building 

Resilient Landscapes, Streams, and Managers” 

(Luce et al. 2012) further discusses the 

problems with fires and watershed responses 

from climate change.

Emergency Relief of Federally Owned 

Roads (ERFO) History

The ERFO program (see Forest Service Manual 

7700, Section 7732.26-Repairs Performed with 

Emergency Relief Federally Owned Funds, 

U.S. Department of Agriculture, Forest Service 

2014b) administered by the U.S. Department 

of Transportation, FHWA, provides funding 

to repair flood-damaged roads on Federal 

lands. Currently Maintenance Level 3, 4, and 

5 roads may qualify, as justified by applicable 

road management objectives for those roads. 

Historically, when a culvert was washed out, 

ERFO would fund a replacement-in-kind, 

replacing the existing structure with the same 

structure. No consideration was made for the 

probability of recurring failure, so sites often 

became repeat repair projects. 

From the 1950s through the 1980s, the 

road system on national forests expanded 

exponentially and accessed higher risk 

terrain in response to a growing domestic 

and international demand for timber. With 

the increase in roads came an increase in 

vulnerability to damage. 

The ERFO program covers all Federal land in 

the United States and all causes of damage. 

From 1971 to 1990, ERFO paid out more than 

$1.1 billion on road and bridge repairs2

 across 

the United States. It is a rare year when there 

are no ERFO funding requests. On average, 

there are 14 qualifying events each year with 

nearly 35 flood-related bridge projects initiated. 

In 2004, three major storms hit North Carolina 

in a row, causing widespread devastation and 

$50 million in damage. Hurricane Irene in the 

summer of 2011 caused widespread damage 

in the East and New England States with over 

a 100 year precipitation event in many areas. 

The overall cost of this damage is estimated 

at $20 billion. Hurricane Sandy was a massive 

storm that caused widespread damage and 

flooding throughout the Northeast in 2012, with 

estimated damage amounting to $50 billion. 

Whether the cause is climate change, an everexpanding road system on high-risk terrain, 

or just an aging road system with deferred 

maintenance, the need for ERFO funding 

and the escalating ERFO repair costs are 

staggering.

The major storm patterns from the Pacific 

Ocean shift up and down the coast. On rare 

occasions, the entire area from northern 

California to the Canadian border is affected by 

major storms triggering ERFO response. For 

one forest in the northwest Cascades, average 

annual ERFO road repair funding went from 

$190,000 per event for the 1970s decade to 

$411,000 for the 1990s decade (actual costs) 

(Doyle and Ketcheson 2004). The Forest 

2

 Jennifer Rhodes and Roy Trent; An Evaluation of 

Highway Flood Damage Statistics. Undated report. 

Federal Highway Administration, Offices of Research 

and Development, 6300 Georgetown Pike, McLean, VA 

22101.
Introduction 1-11

Service Pacific Northwest Region averaged 

more than $6 million a year in ERFO repair 

between 2002 and 2010. The Pacific Southwest 

Region spent more than $40 million on the 1997 

storm damage program alone. 

Useful information on the ERFO program, 

funding, eligibility, and types of storm damage, 

the use of damage survey reports, and 

regulations are found in the FHWA publication 

“Emergency Relief for Federally Owned Roads: 

Disaster Assistance Manual“ (FHWA 2015). It is 

available at <http://flh.fhwa.dot.gov/programs/

erfo/documents/erfo-2015.pdf>.

One of the challenges of working with the 

FHWA and obtaining adequate ERFO funding 

is that ERFO guidelines still are targeted at 

“replace in kind.” Forest Service standards, 

however, have changed and the road that failed 

will no longer meet the new standards. The 

forests often need to acquire additional funds, 

over the ERFO allocation, to fix storm-damaged 

facilities to current standards. Obtaining the 

additional funds is challenging and may not 

occur in a timeframe that complies with ERFO 

funding time limits.

It is the intention of the Forest Service, through 

the use of this SDRR Guide and the travel 

analysis process, to reduce the reliance of the 

forests on emergency funding by implementing 

an effective program of storm damage risk 

reduction, combined with proactive road 

maintenance and a vital capital improvement 

program that will result in minimal storm 

damage to roads. Where forests decide to 

decommission rather than repair damaged 

roads, emergency funds may be used for 

decommissioning. However keep in mind that 

currently ERFO funds can only be used on 

Level III or higher roads.

Vulnerable Road Placement

Forest road systems are often damaged 

by floods when drainage features are 

overwhelmed by water and debris. Roads 

located in riparian corridors and on flood plains 

also face the erosive power of streamflow at 

streamside locations and may be completely 

removed by the river.

Roads crossing alluvial fans or located at the 

transition from a steeper slope to flatter terrain, 

are at risk since drainage channels in this area 

can shift over time. Intense storms leading to 

debris slides or debris torrents may fill existing 

channels with debris, causing the channel to 

shift to a new location.

The southwest regions and parts of the West 

have been impacted lately by severe droughts, 

fires, and then debris flows following heavy 

rains. Roads crossing ephemeral channels and 

arroyos are at risk of damage from extreme 

events and the associated debris torrents. 

Road-stream crossings in these areas should 

either involve very large structures, such as 

bridges, or minimum investment structures, 

such as simple fords. Culvert structures have 

a high risk of plugging. In the northern and 

northeast regions, more extreme events or ice 

and snow can lead to more ice dams, blocked 

structures, and high snow loads. Inconsistent 

cold and warm weather can lead to more 

subgrade damage and freeze-thaw damage on 

roads, as discussed in section 6.7. 

Road design and repair must take into account 

the probability of flooding and the potential 

effects on the road. Older roads often were 

designed for small storm events and may not be 

adequate to handle more severe floods. Pipes 

and other drainage features on older roads also 

are aging and failing. For road-stream crossings 

placed on unstable terrain or on dynamic 

landscapes, stream channels may shift during 

storm events, severely damaging the road and 

bypassing existing drainage structures. 
1-12 Storm Damage Risk Reduction Guide for Low-Volume Roads

Hillside road locations are subject to a number 

of threats:

❏ Hillslope failure that blocks ditch drainage 

and forces the flow down or across the 

road, resulting in rilling or gullying of the 

road surface and often mass failure of the 

fillslope, or a cascading series of failures 

down slope at culvert crossings.

❏ Hillslope failure that buries and plugs ditchrelief or cross-drain culverts causing water 

to cross the road prism and erode or wash 

out the fillslope.

❏ Cutslope failures and landslides from 

unstable slopes either burying the road or 

removing the road prism.

❏ Rockfall that damages the road or creates 

a safety problem.

❏ Debris slides or debris avalanches that 

race down small drainages and overwhelm 

culverts or bridges, resulting in washed out 

drainage crossings or plugging and stream 

diversion.

❏ Excessive ditch erosion due to the 

accumulation of water from long distances 

between ditch-relief culverts or due to the 

bypassing of runoff past a plugged relief 

culvert.

❏ Ditch plugging or loss of ditch capacity due 

to dry ravel of cutslope materials, forcing 

runoff out of the ditch and down or across 

the road.

❏ Saturation of the road fill from subsurface 

moisture either from upslope or from water 

ponding in flat ditch grades or sags that 

causes mass failure of the road fill. 

❏ Settling of road-fill materials forming open 

cracks at the surface that collect surface 

runoff, resulting in mass failure of the road 

fill.

❏ Excessive erosion and possible rilling of the 

road tread on steep grades, resulting from 

inadequate surfacing, channeling of water 

in rutted or compacted tire tracks (loss of 

inslope or road crown) or down tire ruts in 

mud or snowpack.

❏ Reduced ditch capacity over time due to 

vegetation encroachment into ditch and 

roadside zones, forcing water onto the 

road.

Road systems that place several roads on 

a single hillside (road stacking) or a set of 

switchbacks have an increased risk of loss 

during storms because of the domino, or 

cascading failure, effect of a road failure high 

on the slope. Water from upper road segments 

often concentrates and accelerates as it flows 

down the road or downslope and the water, 

eroded soil, and debris can damage roads in 

succession lower on the slope, causing multiple 

failure areas down the road. 

Roads on valley floors or flood plains are 

subject to some similar and some different 

threats that merit special considerations 

depending on their specific locations:

❏ Roads on alluvial fans are subject to debris 

avalanche deposits plugging drainage 

culverts, shifting drainage channels, and/or 

burying the road prism in debris.

❏ Stream channels on alluvial fans migrate 

and can cause erosion of the road prism 

and breaching of the road.

❏ Rivers migrate across their flood plains 

forming a channel migration zone (CMZ) 3

. 

Roads located within the CMZ of the river 

are subject to washing away during floods.

❏ Roads on high terraces or upslope from 

flood plains may fail as streams undercut 

the toe of the old terrace or slope.

3

 Channel migration zone is defined as an area adjacent 

to an unconfined stream where channel migration is likely 

during high-flow events. The presence of side channels or 

oxbows, stream-associated wetlands, and low terraces are 

indicators of these zones. (Southwest Oregon State Forests 

Management Plan 2001. Glossary. Other definitions and 

mapping guidance in A Framework for Delineating Channel 

Migration Zones. Rapp and Abbe 2003. Washington State 

Department of Ecology Pub. 03-06-027).
Introduction 1-13

❏ Road crossings, bridges and culverts, and 

associated approaches may be lost due to 

migrating river channels.

❏ Roads within flood plains are subject to 

inundation during floods, which may result 

in the compromising of road surfacing and 

drainage systems and erosion of road 

prisms. Roads in narrow canyons are at 

risk of inundation or being washed away by 

high water or bank scour.

❏ River corridors, flood plains, and CMZs 

are often characterized by a history of 

diking and riprapping in an effort to protect 

roads from river migration and erosion. 

Nonriprapped sections of road are at 

greater risk because dikes and riprap 

merely translate stream energy to banks 

downstream.

❏ Roads adjacent to aggrading stream 

channels resulting from land-use induced 

sediment delivery are threatened by 

increased channel instability and migration.

Figure 1 shows the inundation of the Feather 

River Canyon in northern California in the 

floods of 1986 and 1997, each roughly a 100-

year event (statistically). The highway is located 

in a narrow river canyon that is vulnerable to 

flooding. Each flood resulted in road closure 

for months and millions of dollars in damage 

repairs.

Figure 1—The Feather River Canyon, Plumas 

County, CA, where floods have resulted in major 

road damage and long periods of road closure (a). 

Signs (b) show water levels reached in February 

1986 (bottom sign) and January 1997 (top sign).

a

b
1-14 Storm Damage Risk Reduction Guide for Low-Volume Roads

Examples of Storm Damage

Figures 2 through 10 show examples of forest 

road storm damage.

Figure 2—Severe gully erosion damaging forest 

roads during storm events.

a

b

Figure 3—Bridge scour occurring at the abutment of 

bridges during major storms in northern California 

because of channel constriction (a) and natural 

stream meander (b).

Figure 4—A bridge abutment washed away by a 

debris torrent and channel widening during a storm 

event in the Pacific Northwest. (Courtesy of Mark 

Leverton.)
Introduction 1-15

Figure 5—This road washed out because of 

placement on a flood plain and channel widening 

during a storm event.

Figure 6—Damage to a low-water ford caused by 

scour of the bedding material under concrete planks.

Figure 7—Culvert plugging (a) and a total culvert 

washout (b) during major storm events.

Figure 8—Examples of road closure caused by 

cutslope and fill failures during storms.

a

b
1-16 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 9—Streambank erosion during a major flood 

and loss of riprap streambank stabilization material. 

Surrounding land management contributes to the 

susceptibility of a road to damage. In this instance, 

grazing practices and poor riparian conditions 

(compacted soils devoid of deep-rooted vegetation) 

contributed to severe streambank erosion. Poor 

installation and undersized riprap contributed to the 

problem.

Figure 10—Road-stream encroachment and flooding 

along the South Fork, Salmon River, Idaho, due to a 

rain-on-snow event. (Courtesy of Tom Black.)

Definition of Risk Reduction 

Treatments

As discussed above, storm damage risk 

reduction is neither road maintenance nor 

capital improvement. Prescribed SDRR 

treatments fall into a category between the 

two; but may involve some combination of 

both types of treatments. The primary purpose 

of these treatments is to reduce the risk of 

damage to the road and to the environment 

from the stresses experienced during storm 

events. Understanding the hazards and 

controlling water is paramount. Erosion and 

sediment delivery from roads to streams and 

the consequence of the sediment and related 

debris in the streams are primary concerns. 

Therefore, these treatments are ones that 

control surface runoff within the road prism, 

increase mass-stability factors for road cuts and 

fills, and make the road features more resilient 

to forces from outside the road prism, such 

as upslope mass wasting and debris, rain-onsnow runoff, and other severe climatic events. 

Treatments often will involve increasing the 

size and decreasing the spacing of drainage 

features, fortifying stream crossings, and 

stabilizing slopes.

Open Roads

Maintenance level 3-5 roads are open and 

maintained for travel by prudent drivers for 

standard passenger cars. Maintenance level 

2 roads are open for use by high-clearance 

vehicles. The selection of treatments for open 

roads is constrained by the need to provide 

for the passage of vehicles. Depending 

on the maintenance level, this may mean 

consideration for passenger cars, vehicles 

pulling trailers (horse, camp, utility, snowmobile, 

and so forth), log trucks, or equipment on 

lowboys. Treatments that would restrict or 

prevent the passage of these types of vehicles 

would not be selected for maintenance level 3-5 

roads. 
Introduction 1-17

Rolling dips (broad-based dips) may require 

varying design criteria depending on the 

steepness of the road and whether the 

expected use is passenger cars and light trucks 

versus logging trucks and lowboys. Where 

high-clearance vehicles are the intended 

use (maintenance level 2), more aggressive 

treatments that affect the travel surface may be 

selected. Drivable waterbars and rolling dips 

may be more pronounced on maintenance level 

2 roads compared with maintenance level 3-5 

roads.

Closed Roads (Roads Placed in 

Storage)

When traffic is not a factor in selecting 

appropriate SDRR treatments, the types of 

treatments available expand significantly. 

However, the expectation of a lower 

maintenance frequency must be factored into 

the selection and design of treatments. A closed 

road, or road in storage, is a road that may be 

closed to use for a long period of time, say 5 

to 20 years, but with the expectation that it will 

be used again. Therefore, implement SDRR 

treatment prescriptions for closed roads with 

that understanding. 

On a closed road, road width, surface 

roughness, and drainage structures all can 

be viewed differently. The road does not have 

to be wide enough to carry traffic so unstable 

fills can be removed. Drainage structures can 

be removed completely, and large gaps in the 

road surface are acceptable, even desirable. 

The amount of road prism left intact on a closed 

road depends on the amount of time until the 

next expected use, the physical setting and 

failure risk posed by the local climate, the age 

of the road and associated drainage structures, 

the return interval flow for which the existing 

drainage structures were designed, and the 

resource values at risk. In sensitive landscapes 

and severe climates, a closed road may appear 

similar to a decommissioned road (permanently 

closed) due to the level of treatment. The 

anticipated length of closure, and expected 

maintenance, may affect the degree of 

treatment.

Closed roads remain as part of the forest road 

system and should receive periodic inspection 

and maintenance to protect the road and forest 

resources. However, the reality is that there is 

limited funding or other resources available to 

inspect and perform maintenance on closed 

roads. For closed roads, SDRR and closure 

treatments must be prescribed with the caveat 

that maintenance will be very limited or not 

occur at all until the road is reopened. 

Treatment selection and design criteria must 

result in road features that will stand up to years 

of storms without an impairment of function. 

As with all SDRR treatments, the design is not 

expected to be for the largest storm events. 

Designing for the 100-year storm event (plus 

debris) necessitates some allowance for 

increased capacity and redundancy in order 

to absorb some effects by the more common 

storms without losing capacity or stability.

Relative Treatment Costs 

The SDRR treatments described or 

referenced in this guide span a wide range 

of implementation costs. Costs for the 

same treatment vary widely over time and 

geography, due to specific site conditions and 

the availability and price of materials. Various 

economic factors will determine actual costs 

once proposals go to bid. Cost is an important 

factor in the selection of the appropriate 

treatment, but cost-effectiveness over time 

should carry more weight in the decision. 
1-18 Storm Damage Risk Reduction Guide for Low-Volume Roads

Some treatments may be very expensive, 

such as bridge-scour countermeasures, 

but can be cost-effective if it prevents a 

bridge failure. If a higher cost treatment 

is warranted, a lower cost, less effective 

treatment should not be substituted unless 

it is viewed as an interim treatment to lower 

an imminent risk. Because of the variables 

influencing costs, this guide does not 

include cost items for individual treatments, 

but rather categorizes treatments as to most 

common and lower cost, to less common 

and higher cost. 

Table 2 with a summary of the SDRR measures 

discussed in this guide is found in section 3.

The appendix A5-Glossary presents the 

definition of a few terms commonly associated 

with storm damage risk reduction and road 

management.

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CHAPTER TWO 2STRATEGIC RISK ASSESSMENT
Strategic Risk Assessment 2-1

2. Strategic Risk Assessment

There are essentially two risk components 

in the storm damage risk reduction (SDRR) 

evaluation for selection of appropriate 

treatments. The two factors of a hazard risk 

assessment are:

1. Probability of occurrence and failure 

(how big and frequent the storm runoff 

occurrence is and the probability that it will 

wash out a bridge, plug a culvert, or cause 

a landslide).

2. Expected consequences (loss of life, 

safety, cost of damage to infrastructure, 

environmental costs, etc.).

The probability of a failure is related to the 

factors that contribute to the potential for a 

failure to occur. The consequence, should 

a failure occur, relates to the infrastructure 

or natural resource impacts resulting from a 

failure. The values of natural resource(s) or 

the road component that would incur damage 

are important in determining the amount of 

resources to allocate towards preventing a 

failure and the level of risk that is acceptable.

The selection and application of the most 

cost-effective SDRR treatment depends on a 

thorough assessment of the factors that place 

a particular road or group of roads at risk, 

and an understanding of the consequences 

(relative to health and safety, environment, and 

infrastructure) should a road failure or damage 

occur. There are a number of physical and 

climatic factors that need to be considered. This 

assessment should involve an interdisciplinary 

team and should be completed early in the 

storm response process, or it can be completed 

on a whole road system in a programmatic 

way. Having the risk assessment completed 

expedites response to storms and is invaluable 

for preparing annual road maintenance plans. 

This document does not fully address risk 

assessment. However, it is intended to offer a 

framework for the concept and needs of a risk 

assessment, considering the hazards involved 

and the consequences of failure, along with 

some tools and references to aid in doing one.

2.1 Hazard Assessment

It is important to understand and recognize 

the origin of different factors that affect storm 

intensities and types, and the responses of a 

landscape to storms. A complex set of these 

factors determines the risk of storm damage 

for a given road. Some of these factors are 

associated with regional- or landscape-scale 

phenomena, such as climate patterns; some 

are expressed at a watershed or subwatershed 

scale; and others are site-specific factors with 

very local effects.

Landscape-Scale Considerations

The risk of damage from severe storms, and 

the severity and frequency of those storms, 

relates to the regional climate and weather 

patterns. Understanding how a particular area 

is affected by storms throughout the year is 

important for road design and the selection and 

implementation of SDRR treatments.

Climate

Climate patterns control the types of storms that 

typically occur within a given watershed. The 

climate overlay of the Western United States 

is influenced heavily by the Pacific Ocean and 

the movement of storm cells onshore. In the 

Northwest, from northern California north to 

Canada, deep low-pressure systems arriving 

from the Pacific Ocean can carry large amounts 

of moisture that push up over the coastal or 

inland mountain ranges where much of the 

moisture is released. Much larger quantities 
2-2 Storm Damage Risk Reduction Guide for Low-Volume Roads

of moisture are released on the west and 

southwest faces of these mountain ranges than 

east of the crest due to the prevailing storm 

tracks. The air mass off the ocean in winter is 

relatively warmer than air masses that travel 

across the cold land masses. This condition 

results in more frequent rain-on-snow storms 

on the west side of mountains as the jet stream 

alternates from the Gulf of Alaska to warmer 

waters of the Hawaiian Islands. Storms from 

Alaska bring cold air and lower elevation snow; 

storms originating more southerly from the 

Pacific bring warm rain and wind that melts 

snow rapidly. 

The air mass dries out and cools as it crosses 

the mountains and, most of the time during 

winter, encounters a colder air mass on the east 

side, resulting in mountain and valley snow. The 

snow accumulates during the winter and melts 

out in the spring.

The opposite occurs in the summer, when 

the air mass arriving from the ocean is cooler 

than the air mass over land. The air mass 

warms over the land and holds more moisture 

that may carry across to the east side of the 

mountains where additional heating causes 

thunderclouds to form. Unstable atmospheric 

conditions may lead to large amounts of energy 

being generated in these clouds that then 

release large quantities of rain during short, 

high-intensity downpours. These intense storms 

can cause flash flooding. The most damaging 

storms typically are isolated thundershowers 

or cells that stall over one particular area or 

watershed for a long period of time. These 

localized storms are the least predictable. This 

phenomenon is more prevalent on the east side 

of the Cascade and Sierra mountains and the 

hot desert basins of eastern Washington and 

Oregon. 

In the Interior West of the United States, 

“Chinook Winds” can blow down from Canada 

and send atypically warm winds into the region 

from Oregon to Montana, and particularly in the 

Rocky Mountain Region. These warm winds 

occurring in the winter can drastically change 

the temperature and have been known to melt a 

foot of snow in a day, resulting in local flooding.

Other phenomenon such as an ”Alberta Clipper” 

can rapidly move cold air from Alberta into the 

northern Plains and eventually move to the East 

along the mid-Atlantic Coast. These storms can 

bring atypically heavy snowfall over the regions. 

Other events, such as “derechos,” can move 

very strong winds and severe thunderstorms 

along a relatively narrow but long path, from the 

mid-West to New England, or along the East 

coast. 

The “El Niño” and “La Niña” weather pattern 

extremes have had dramatic effects on U.S. 

weather patterns as well as the weather 

worldwide. As ocean temperatures either tend 

to be warmer than usual (El Niño) or colder 

than usual (La Niña) in the central and eastern 

Pacific, resulting weather across the United 

States becomes quite variable. During El Niño, 

the west coast may expect a stormy winter 

while the North may be warmer and the South 

wetter. A La Niña winter may produce a cold 

northern winter and drought in the Southern 

states. A La Niña year may also result in fewer 

Pacific hurricanes but more Atlantic hurricanes. 

El Niño patterns have become more frequent 

in recent decades rather than the historic more 

even fluctuations between the two extremes.
Strategic Risk Assessment 2-3

Topographic Influences

In many areas along the west coast, 

precipitation patterns and intensities vary 

dramatically due to the physical features of 

the landscape. Western mountain ranges 

oriented north to south lift air masses moving 

west to east and cause higher intensities of 

precipitation on the western slopes and drier 

rain shadows on the east side. The west coast 

mountain ranges are characterized by large 

volcanic peaks that extend to greater heights 

than the surrounding mountains. These peaks 

experience heavy precipitation on the slopes 

facing the oncoming storms but often cause a 

split in the airflow around the peak. This results 

in a localized rain shadow on the lee (east) side 

of the mountain range; but, if these split-storm 

cells converge beyond the mountain, a zone 

of higher precipitation may result (Mass 2008) 

from the convection caused by the collision. 

It is important to understand these influences 

of topography, combined with regional storm 

patterns, when planning for road drainage 

needs.

Ecoregions

The ecological hierarchy described in 

“Description of Ecological Subregions: 

Sections of the Conterminous United States” 

report (McNab et al. 2005) and the U.S. 

Environmental Protection Agency (EPA) Level 

III and IV information characterize climatic 

differences (precipitation amounts and patterns 

and temperature), vegetation, and large-scale 

terrain features throughout the United States. 

Characteristics of these various terrains can 

guide managers into areas where there may 

be concerns for road stability issues. This 

hierarchy is refined and available locally with 

land-type association and land type or soil 

maps. Interpretations resulting from these 

mapping efforts are invaluable in determining 

risks associated with roads.

The EPA information on ecoregions is available 

at <http://www.epa.gov/wed/pages/ecoregions/

level_iii_iv.htm#Level IV>. 

Watershed-Scale Considerations

The geomorphology and condition of a 

watershed influences the runoff response to 

storm events. Geomorphology partially dictates 

how a watershed responds to climatic inputs 

(Leopold et al. 1995) and the condition of the 

watershed affects how the watershed absorbs 

those inputs (Swanson 1980). The combination 

of geomorphology and watershed condition 

determines the response of a watershed to 

storm events and the level of hazard to be 

addressed. 

Shape and Orientation

Watersheds in all climate regions may display 

an asymmetry of hillslope profiles on opposite 

sides of the valley (Leopold et al. 1995). In 

the middle latitudes of the United States, 

this expression may be dramatic. Southfacing slopes tend to be drier (less snow and 

more rapid snowmelt and more intense sun 

exposure) and have greater diurnal temperature 

swings; whereas, north-facing slopes are 

wetter (hold snow longer and have less intense 

sun) with more uniform moisture. Drier slopes 

generally have shallower soils and are longer 

with fewer stream channel crenulations.4

Wetter slopes may be steeper and shorter 

with a higher density of stream crenulations 

and deeper soils. While these characteristics 

are generalized and may be affected by 

storm patterns and how the underlying 

geologic structure controls subsurface water, 

understanding what these landform expressions 

mean for water movement is important to 

managing road drainage and stability.

4

 A crenulation is an indentation in a contour line on a 

topographic map that represents a course for flowing 

water.
2-4 Storm Damage Risk Reduction Guide for Low-Volume Roads

At river-basin scales, the orientation of the 

watershed can influence the response of the 

watershed to storms. Watersheds that are 

aligned with the storm track progressing from 

the mouth to the headwaters will experience 

higher storm intensities across the higher 

slopes in the watershed. Watersheds that 

are perpendicular to the typical storm track 

may experience greater storm intensities on 

slopes facing the oncoming storm and lower 

intensities on the lee slopes. This can alter the 

runoff timing from one side of the watershed 

to the other and reduce the peak flows in 

the lower watershed. Roads located on the 

slopes receiving higher intensity precipitation 

may require additional drainage facilities. This 

leeward effect on precipitation intensity also can 

result if the watershed is oriented such that a 

storm travels from headwaters to the mouth. 

At the watershed scale, storm duration and 

intensity may most influence the size of runoff 

from storms. Localized variability in intensity 

may be similar to that discussed for larger 

basins, but overall, it is how rapidly the storm 

passes over the watershed and the relative 

precipitation intensity that determines the 

amount of water falling on the watershed. 

Rainfall intensity, more than total rainfall, 

dictates the design of smaller drainage 

structures and road surface drainage 

measures.

Geomorphology

The geomorphology of a watershed controls 

how that watershed responds to severe storms 

and what stresses a road may encounter during 

storm events. This includes the orientation of 

the watershed and the hillslope characteristics, 

which result from the underlying geology 

and structure, as well as the climate history. 

Particularly valuable information can be 

obtained from geology, land-type association 

(LTA), and soil maps.

Geologic mapping identifies the underlying rock 

structure that influences slope stability (the 

strike and dip of the bedrock in relation to that 

of the hillslopes) and the specific rock types 

and their distribution across the watershed. Soil 

and LTA maps show the distribution of distinct 

combinations of soil types, slope characteristics 

(slope steepness, orientation, shape), and 

vegetation. From this information, the stability of 

slopes and the type of runoff expected from the 

watershed can be determined (i.e., flashy runoff 

from shallow, unconsolidated materials). Soils 

with finer soil texture and more clay-forming 

minerals may be most prone to instability. 

Understanding if the topography is convergent, 

which concentrates surface and groundwater, or 

divergent, which distributes water, is important 

for planning on the volume of stormwater a road 

may need to handle.

Stream Pattern

The climate and geology of a particular 

area determine the development of stream 

channels draining a watershed. Similarly, the 

stream network has a dramatic effect on the 

topography within a watershed. Mountainous 

terrain, in wetter climates, tends to have a 

higher density of stream channels that are 

deeply incised into the hillsides. Roads across 

these hillslopes must deal with a higher number 

of stream channels, steeper slopes, and deeper 

stream crossings. Drier climates, in general, 

have fewer stream channels and less deeply 

incised stream channels. Roads in these 

watersheds encounter fewer stream channels 

and typically have shallower crossings.

The geology has a broad effect on the pattern 

of the main stream channel in a watershed. 

Geologic structure may dictate the direction 

of channel development, such as where a 

channel may follow a straight line along a fault, 

or a dominate joint set, or a more erodible rock 
Strategic Risk Assessment 2-5

layer. The location of these channels may be 

more set than channels that are not controlled 

by geologic structure. Stream channels in 

watersheds with recent past or current glacial 

activity may move dramatically across the 

glaciated valley floor. Large amounts of glacially 

derived sediment tax the stream’s ability to 

transport the sediment, resulting in active and 

continual channel shifts. Roads that cross or 

are located near these streams are at a higher 

risk of damage from the shifting channel. 

Where available, LTA, topographic, and 

geologic maps provide invaluable information 

regarding stream patterns that are useful for 

evaluating channel influences on roads and 

appropriate SDRR treatments. 

Site-Scale Considerations

Roads can cross a large variety of conditions 

within a watershed and even across a single 

hillside. Even where the majority of the road 

crosses a stable landscape, missing the proper 

interpretation and road drainage needs for one 

critical site can result in a failure and significant 

environmental damage. Other factors that 

determine the risks faced by a road system 

are related to watershed condition expressed 

at a given road location. The capacity of a 

watershed to process runoff and sediment 

generated during storm events is affected by 

past and current land use and fire history. If 

a road location is affected by a recent timber 

sale or a recent fire, this will modify (and likely 

increase) the risk of damage. 

Geomorphology

The selection of SDRR treatments relies 

heavily on the expression of hillslope and 

stream characteristics at the site level. A clear 

interpretation of the different conditions along 

a road route is important for selecting and 

implementing the appropriate treatment.

Slope characteristics are the expression of 

complex interactions of climate, geology, and 

soils. Storm runoff patterns and intensity are a 

reflection of these interactions and the influence 

of land use on the natural patterns. In general, 

long, uniform hillslopes with shallow soils tend 

to shed runoff faster than complex slopes with 

deep soils; steeper slopes will move water 

faster than gentler slopes. Deeper soils provide 

a larger reservoir to store rainfall and snowmelt 

and will release water slower. Concave hill 

slope depressions tend to accumulate soil and 

water, thus having a relatively high incidence of 

failures or debris slides.

A history of glaciers within a watershed often 

produces oversteepened slopes and deposits of 

glacial material that affect water movement and 

slope stability.

Roads on steep slopes have larger or higher 

cutslopes; and, depending on the groundwater 

conditions, will likely intercept more subsurface 

water than roads on gentle slopes. Thus, 

roads on steep slopes will need more drainage 

capacity to handle the intercepted water and 

lower the risk of failure during storms. Also 

disconnect the road from the stream system.

Soils and Stability

Local soil characteristics play a major role in 

determining the stability of a road location. 

Water movement in the slope above, at, 

and below the road is influenced by the soil 

and bedrock features. Shallow bedrock or 

consolidated glacial till represents a vulnerable 

failure plane where water cannot move deeper 

into the hillside and therefore moves laterally 

across the bedrock interface causing increased 

pore water pressure in the soil and reducing 

slope stability. Fine textured soil layers, such 

as clay, may act in a similar manner, impeding 

vertical water movement. 
2-6 Storm Damage Risk Reduction Guide for Low-Volume Roads

Bedrock underlying hillslopes rarely is uniform; 

it is the result of fracturing, weathering, and 

erosion over time. Small hollows or depressions 

in bedrock may have been filled in during the 

passage of glaciers. These in-filled hollows 

collect water during storms and may remain 

relatively stable when undisturbed. But if a 

road cuts through a hollow, the in-filled material 

and the road may fail due to the changes in 

stability from the road cut. Road runoff may also 

increase the amount of water entering a hollow, 

resulting in failure. 

Soil characteristics affect how water moves 

through the soil and therefore slope stability 

of cut and fillslopes. Soil texture and cohesion 

downslope of road drainage features affect the 

susceptibility of slope to erosion or mass failure. 

Soil depth, combined with slope steepness, 

affects the amount of bedrock into which a road 

is cut. While a full-benched road may be stable 

because it is located completely on bedrock, 

runoff from road drainage features may cause 

downslope instability due to soil and slope 

characteristics.

The presence of deep-seated hillslope slumps 

and earthflows play a major role in roadlocation stability. Failure planes can concentrate 

subsurface moisture that can then be captured 

by roads or cause slope failure. Ground 

movement can sever roadways and pose safety 

and access issues. A road cut may destabilize 

or cut into the toe of an old slide furthering 

instability.

The soil types across which a road passes 

affect whether the road needs additional 

surfacing to support traffic and minimize rutting 

and roadway erosion. Volcanic ash soils, in 

particular, have very low shear strength when 

wet and do not support vehicles, which leads 

to rutting and gullying. Soils with a high clay 

content will become slick when wet and, unless 

otherwise surfaced, will become safety hazards.

Road Location

Historical road location took advantage of 

easy construction along gently sloping valleys 

and on flat flood plains, which placed roads in 

proximity to meandering channels. These roads 

continue to constitute the primary connecting 

routes for forest road systems. As roads were 

used to access resources further upslope, 

roads were constructed on steeper slopes 

and crossed more unstable lands. In many 

instances, stability problems were recognized 

and design features addressed those issues; 

however, not all stability issues were recognized 

or appropriately addressed. Some stability 

problems come to light only after miles of 

road are constructed in a watershed. Stability 

concerns that were dealt with in the original 

road design may again become issues due to a 

lack of maintenance.

Road location considerations include:

❏ Landform–hillslope, canyon wall, active 

flood plain, stream terrace, slump scarp or 

bench, and so forth.

❏ Slope position–upper, middle, and lower 

part of the slope.

❏ Slope aspect–orientation as it relates to 

storm tracks, water accumulation, and 

snow melt.

❏ Slope shape–concave, convex, plane.

❏ Slope angle or steepness.

❏ Road stacking–are there other roads 

upslope, including “ghost” roads and skid 

trails.

❏ Presence of unstable or erodible soils/

slopes.

❏ Site specific morphology at stream 

crossings.

❏ Size and efficiency of upslope watershed 

area draining to a particular channel 

crossing.

❏ Land use and watershed condition above 

road and any stream crossings.
Strategic Risk Assessment 2-7

Road Standards

Road standards also dictate varying 

considerations for SDRR treatment selection 

(see FSH 7709.56, Road Preconstruction 

Handbook, Chapter 40, Design) (U.S. 

Department of Agriculture, Forest Service 

2014c). As noted in the introduction, SDRR 

applies to open and closed (stored) roads, but 

certain treatments also may have applicability 

to decommissioned roads.

Road standards relate to the drivability and 

type of vehicles appropriate to the road and 

have a bearing on the type and frequency of 

maintenance needed. Knowledge of the road’s 

maintenance history is important since many 

roads do not receive prescribed maintenance 

over time due to many reasons. In many 

instances, maintenance is more dependent on 

funding than the road management objective 

or road maintenance level. As important as the 

maintenance history is, the expected future 

level of maintenance also has a bearing on the 

selection of treatment. 

Lower standard roads typically have less risk 

to storm damage than higher standard roads. 

Factors that contribute to the reduced risk are 

that lower standard roads:

❏ May be single lane, disturbing considerably 

less area than a wider two-lane road.

❏ Have lower design speeds allowing the 

road to have rolling grades and smaller 

cuts and fills (less earthwork).

❏ May use fords and dips rather than culverts 

and cross-drain pipes, thus having a lower 

risk of plugging and failure during a storm. 

Nevertheless, lower standard roads may still 

have high-risk sites for diversion potential and 

be located on highly unstable slopes or cross 

steep drainages subject to debris slides. Lowstandard roads with low-use levels may have 

less design requirements, poor construction 

quality control, and receive less inspection and 

maintenance than higher standard, high-use 

roads. Figure 11 shows some of the differences 

in a low- and relatively high-standard road.

Figure 11—Varying risk of a high-standard road with 

pipes and larger cuts and fills (a) versus a lowstandard road with small cuts and fills and a ford (b).

A specific road may have been originally built to 

a certain standard. A subsequent administrative 

decision could change that standard, either 

higher or lower, yet little physical change may 

have occurred to the road. Any SDRR treatment 

will need to be consistent with the current road 

standard. The full history of the road (when 

available) and expected future management 

are important when considering risk and proper 

treatment. Knowing the original standard to 

which the road was built, if different from the 

current standard, has a bearing on how the 

road may respond to storm stresses. 

a

b
2-8 Storm Damage Risk Reduction Guide for Low-Volume Roads

Construction Practices/Methods

How the road was built often affects long-term 

performance and susceptibility to damage 

during storms. This information may be 

unknown. Often, the age or year of construction 

is used as an index to identify the most 

common construction practices at the time. 

Examples of practices that increase hazard 

(or reduce long-term stability/performance) 

include sidecast construction/loose sliver fills 

and the use of organic retaining walls (burying 

logs behind stumps to retain loose fill) on 

slopes that are too steep for the fillslope to 

catch. Relatively small culverts were typically 

installed on old roads compared to those used 

today. When identification of these practices 

can be integrated into risk assessment or 

prioritization at a watershed, subwatershed, or 

site scale, it can help target treatment locations 

and solutions. When left untreated, these 

sites will fail and often necessitate expensive 

solutions, such as retaining walls or difficult 

road realignment. The failures can cause 

considerable damage to site productivity and 

streams downslope. 

Failure Potential

The hazard discussion above demonstrates 

the complex nature of factors that influence 

the potential for damage or failure during storm 

events. An assessment of these factors is 

critical to selecting the proper treatment to lower 

the risk. A number of procedures have been 

developed by forests to rank the failure risks of 

roads. Many potential risk factors exist, but only 

a short list is presented here. 

Factors contributing to failure risks include:

Watershed Factors:

❏ Rain-on-snow.

❏ Stream density.

❏ Debris (wood and sediment) loading on 

slopes and in drainages.

❏ Various land management activities that 

affect vegetation, soil properties, and 

runoff. 

❏ Fires and lack of ground cover.

❏ Best management practices 

implementation.

Slope Factors:

❏ Slope angle.

❏ Presence of unstable soils; historic mass 

wasting.

❏ Soil texture and stratigraphy: shallow and 

dry versus deep and wet, presence of fault 

traces and restrictive layers. 

❏ Upslope vegetation type and age.

❏ Presence of bioperturbation (riverine and 

mountain beaver).

Road Factors:

❏ Road location and grade.

❏ Road stacking.

❏ Age of road/type of construction (i.e., 

sidecast).

❏ Ability to maintain ditch integrity.

❏ Cutslope stability and ravel.

❏ Diversion potential: debris plugging 

culverts, filled ditches, uniform downslope 

road grade.

❏ Road maintenance history.

2.2 Consequence Assessment

The consequence assessment looks at the 

values at risk—the infrastructure or natural 

resource impacts that would result should a 

failure occur. 

There are three general categories considered 

within the SDRR umbrella. First, consider the 

potential for effects on life and safety. SDRR 
Strategic Risk Assessment 2-9

treatments can contribute to alleviating some 

of the risk to health and safety by reducing the 

potential for failure. Road failures pose a threat 

to persons travelling forest roads. Due to road 

or bridge washouts, users may be stranded 

without proper supplies to endure an extended 

stay. Limited sight distances on low-volume 

roads may obscure washouts and debris across 

the road. Upslope road and slope failure may 

threaten private property downslope.

Second, there is the value of the infrastructure 

itself. The loss of or damage to road 

components, campgrounds and other buildings, 

roads, culverts, and bridges downslope may 

impair or prevent use until repaired. These 

require funding and other resources to repair or 

replace.

Third, there are environmental values at risk. 

Impairment of site productivity results when 

hillslopes fail or when upslope debris and soil 

bury downslope forest areas. The delivery of 

sediment and road debris to stream channels 

poses significant risks to fish and can disrupt 

normal channel processes for extended 

periods of time, extending over miles of river 

channel. Some salmonid populations are at 

risk of extinction due to habitat degradation 

from sediment impacts. Some stream reaches 

are designated as water-quality impaired and 

included on a State’s 303d list5

 due to direct 

or indirect sediment impacts. Riparian plant 

communities are impacted by streams migrating 

around debris jams and sediment deposits. 

Downslope wetlands and other sensitive 

habitats also may be severely impacted by 

sediment and debris deposition from upslope 

failures. 

Consequences of Failure

If a road fails, consider the following safety 

concerns. 

❏ Are there structures downslope; are they 

inhabited? 

❏ What is the use level of the road and can 

access be blocked readily? 

❏ What would be the expected type and 

amount of damage to the road and what 

would be the cost to repair the road? 

❏ What would be the disposition of the failure 

material (sediment and debris)? 

❏ What is the likelihood of a debris jam 

forming and creating a dam-break flood in 

the stream? That could extend the potential 

damage farther downstream.

If the material travels a short distance 

downslope and comes to rest in the forest or 

on a river terrace, the harm would be less than 

if that debris entered a critical environmental 

site, such as a wetland or a salmon spawning 

stream reach. An evaluation of the probable fate 

of the failure material (i.e., sediment and debris 

delivery potential) is important to determine risk 

tolerance for the site. For delivery to aquatic 

sites, knowing the volume and grain size of 

delivered material is important to assess the 

potential harm.

Evaluate downslope factors in order to predict 

the disposition of the failure material:

❏ Topography–presence of broken slopes 

with terraces or benches to catch material 

versus steep, straight slopes that deliver 

directly to water or wetlands.

❏ Distance downslope to valuable resources–

how much material would travel the 

distance?

❏ The resource affected–how resilient is it to 

5 States are required by the Clean Water Act to maintain impacts?

a list of “impaired” waterbodies. The 303(d) list identifies 

waterbodies where reliable data show water quality is impaired (does not meet State water quality standards) and 

what pollutants are responsible for the impairment.
2-10 Storm Damage Risk Reduction Guide for Low-Volume Roads

❏ Additional infrastructure downslope–will 

other road segments, culverts, or other 

improvements be involved?

❏ Potential for road failure to initiate a mass 

failure, debris torrent and/or dam-break 

flood.

Understanding both the hazard (potential for 

failure) and the consequences relating to a 

failure is paramount to selecting treatments 

and appropriately setting treatment priorities. 

Not all hazard sites will be a high priority for 

treatment, particularly if the consequence is 

low. All high-consequence sites should be high 

priority for treatment, but also may be ranked 

based on the details of the consequences and 

an assessment of the level of acceptable loss.

2.3 Overall Risk Assessment

Risk assessment is the combination of 

the potential hazard assessment and the 

consequence to values at risk. This guide does 

not provide a detailed process for conducting 

a risk assessment; however, an accurate 

and comprehensive risk assessment must 

be incorporated into all SDRR programs. 

Table 1, adapted from Exhibit 02 in Forest 

Service Manual FSM 2500-Watershed and Air 

Management, Chapter 2520-Water Protection 

and Management (2520-2014-1 Interim 

Directive) (U.S. Department of Agriculture, 

Forest Service 2014a) demonstrates a possible 

scenario.The first consideration may use a 

numerical or relative degree rating for the 

magnitude of the consequence. The second 

factor considers probability of damage or loss. 

The factors result in a combined ranking of risk 

from very high to very low. This risk rating would 

influence the priority of the work. 
Strategic Risk Assessment 2-11

Table 1—Risk assessment matrix

Probability of Magnitude of Consequences

Damage or Loss RISK

Major Moderate Minor

Very likely Very high Very high Low

Likely Very high High Low

Possible High Intermediate Low

Unlikely Intermediate Low Very low

. 

Probability of Damage or Loss:

The following descriptions provide a framework to estimate the relative probability that damage or 

loss would occur (to reduce the subjectivity of these ratings, develop criteria to express these more 

quantitatively).

Very likely: Nearly certain occurrence (greater than 90 percent).

Likely: Likely occurrence (greater than 50 percent to less than 90 percent).

Possible: Possible occurrence (greater than 10 percent to less than 50 percent).

Unlikely: Unlikely occurrence (less than 10 percent).

Magnitude of Consequences:

Major: Loss of life or injury to humans, major road damage, irreversible damage to critical natural 

or cultural resources.

Moderate: Possible injury to humans, likely long term, but temporary road closure and lost use 

of major road or road system, degradation of critical natural or cultural resources resulting in 

considerable or long-term effects.

Minor: Road damage minor, little effect on natural or cultural resources resulting in minimal, 

recoverable or localized effects.

Risk and Priority:

A. Very high and High risk: Highest priority for SDRR treatments. 

B. Intermediate risk: SDRR treatments needed; may be incorporated into annual maintenance.

C. Low and Very low risk: SDRR treatments may not be necessary. 
2-12 Storm Damage Risk Reduction Guide for Low-Volume Roads

Proper risk assessment is not only imperative 

for setting meaningful priorities for SDRR 

treatments, but useful for making all road 

management decisions. There are a number of 

formal and informal risk assessment procedures 

to help as a guide. The document “Upslope 

Erosion Inventory and Sediment Control 

Guidance, Part X of California Salmonid Stream 

Habitat Restoration Manual” (Weaver et al. 

2006) provides considerable information about 

site assessment. 

Depending on the resource values and the 

complexity of the site, a site evaluation or area 

assessment may require a data-intensive site 

review. One such tool is the “Geomorphic Road 

Analysis and Inventory Package” (GRAIP) 

(Black et al. 2012) (Cissel et al. 2012b), along 

with its publications and software. GRAIP is 

a tool developed by the Forest Service, Utah 

State University, and the U.S. Environmental 

Protection Agency to help predict the impacts of 

roads or a road segment, and prioritize where 

reconstruction work is needed most. It uses 

resource data and field road inventory data in 

an Arc-GIS (geographic information system) 

platform to predict and quantify sediment 

production, diversion potential, or slope stability 

risks. It also is useful to assess hydrologic 

connectivity of roads to streams. This 

information can be used to identify, prioritize, 

and help select SDRR work. A comprehensive 

field-based inventory of existing and potential 

road-related sediment sources is inexpensive 

compared to the widespread application of 

SDRRs. Information on the GRAIP analysis 

method is available at <http://www.fs.fed.us/

GRAIP/index.shtml>.

Another tool to use in assessing road treatment 

needs and priorities is NetMap <http://www.

netmaptools.org/>.

The publication “Soil and Water Road-Condition 

Index-Field Guide” (Napper 2008) is still 

another tool that can help assess the condition 

of a road and identify problem areas, which may 

be exacerbated during a major storm event. 

Those sites often would be good candidates for 

SDRR treatments. This document is available at 

<http://www.fs.fed.us/eng/pubs/pdf/08191815.

pdf>.

The following section discusses various factors 

and processes that should be addressed 

using a two-tiered assessment of hazards and 

consequences.

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CHAPTER THREE

SELECTION OF APPROPRIATE 3TREATMENT
Selection of Appropriate Treatment 3-1

3. Selection of Appropriate Treatment 

This guide assumes that an inventory of all potential hazard areas and road conditions is complete 

or will be completed as part of the process. Selection of the appropriate treatment will depend on 

the assessment of priorities, the hazards and values at risk, and funds available. This should be 

an interdisciplinary process. The development of this guide relied on the experience of long-term 

road managers and other specialists. Each evaluation team should seek out and rely on those with 

the greatest experience with road management and the resources at risk. However, keeping the 

process and selection as simple as possible is desirable. Treatment selection also depends to a 

certain extent on whether the treatment is a repair of existing damage, or preventative, to reduce 

the risk of future failure. Once a failure has occurred, repair and treatment of the road to reduce 

future risk may be quite different from a preventative treatment that would reduce the risk of a 

failure.

Table 2 presents a summary of storm damage risk reduction measures used. The most commonly 

used treatments are listed first (and discussed in section 5), followed by the less common 

treatments that typically are more costly or more difficult to implement (discussed in section 6). 

Note that separate sections do create some redundancy in the document, but it was done to best 

highlight the most common and cost-effective treatments used by practitioners.

Table 2—Summary of storm damage risk reduction measures

 

MOST COMMON TREATMENTS

 Effectiveness Cost effectiveness

 Short Long Low High

 Term Term

 Easy, Low Cost, or Most Cost Effective

Road Maintenance

Normal grading, cleaning, and shaping

maintenance items. ❋ ❋

Road Surface Drainage

Add rolling dips. ❋ ❋

Add ditch-relief culverts (cross drains). ❋ ❋

Add waterbars. ❋ ❋

Ditch treatments/armoring/check structures. ❋ ❋

Add leadoff ditches. ❋ ❋

Cross-drain pipe/dip/ditch-outlet

protection/armoring. ❋ ❋ ❋
3-2 Storm Damage Risk Reduction Guide for Low-Volume Roads

Table 2—Summary of storm damage risk reduction measures (continued)

MOST COMMON TREATMENTS

Effectiveness Cost effectiveness

Short 

Term

Long 

Term

 Low High

Easy, Low Cost, or Most Cost Effective

Stream Crossing Structures

Culvert maintenance. ❋ ❋

Minor channel debris removal and clearing. ❋ ❋

Culvert diversion prevention/armored overflow 

protection.

❋ ❋

Bridge Protection and Improvement

Channel maintenance and debris/sediment 

clearing around footings.

❋ ❋

Erosion Protection

Physical erosion control measures. ❋ ❋

Vegetating barren areas/deep-rooted native 

plants.

❋ ❋

Gully prevention (limiting water 

concentration).

❋ ❋

Slope Stability Measures

Sidecast fill; pull-back/sliver-fill failure 

prevention.

❋ ❋
Selection of Appropriate Treatment 3-3

Table 2—Summary of storm damage risk reduction measures (continued)

MOST COMMON TREATMENTS

Effectiveness Cost effectiveness

Short 

Term

Long 

Term

 Low High

Medium-Cost Treatments

Surface Drainage

Converting inslope to outslope road (only on 

appropriate soil types).

❋ ❋

Other surface water diversion structures. ❋ ❋

Cross-drain pipe inlet protection or drop inlets. ❋ ❋ 

Stream Crossing Structures

Increase culvert capacity. ❋ ❋

Use of trash racks. ❋ ❋ ❋

Culvert reinstallation/realignment. ❋ ❋ ❋

Culvert pipe inlet/outlet protection, armoring, 

adding headwalls.

 ❋ ❋ ❋

Convert problem culverts to low-water 

crossings.

❋ ❋

Local stream channel stabilization measures. ❋ ❋

Local scour protection. ❋ ❋

Beaver protection. ❋ ❋

Road-Stream Encroachment

Add floating log weirs. ❋ ❋

Relocate road away from stream. ❋ ❋

Erosion Protection

Soil bioengineering and biotechnical methods. ❋ ❋

Gully stabilization. ❋ ❋

Add roadway surfacing materials. ❋ ❋

Slope Stabilization Measures

Modifications to cut-and-fillslopes. ❋ ❋

Slope-drainage improvements. ❋ ❋

Use deep-rooted vegetation. ❋ ❋

Strengthen existing structures (launched soil 

nails).

❋ ❋

Deep patch shoulder repair. ❋ ❋

Road Storage and Closure Issues

Road closure/storage. ❋ ❋ ❋

LESS
3-4 Storm Damage Risk Reduction Guide for Low-Volume Roads

MOST COMMON TREATMENTS

Effectiveness Cost effectiveness

Short 

Term

Long 

Term

 Low High

High-Cost Treatments

Bridge Protection and Improvement

Increase freeboard on bridges. ❋ ❋

Major bridge-scour protection. ❋ ❋

Slope Stability Measures

Add retaining structures. ❋ ❋

Change road grade/alignment/width. ❋ ❋

Table 2—Summary of storm damage risk reduction measures (continued)

LESS

Simple, inexpensive, and cost-effective 

preventative measures should always be the 

first priority and are most commonly used. 

Mitigation measures should be a second priority. 

More expensive treatments reduce the number 

of miles of road that can be treated using limited 

resources. The simpler preventative treatments 

often are the less expensive ones, but cost 

effectiveness should be more important than 

cost alone. Less costly treatments might include 

work such as:

❏ Maintaining or improving roadway surface 

drainage.

❏ Adding cross drains and leadoff ditches.

❏ Armoring ditches and drain outlets.

❏ Vegetating barren slopes and exposed 

areas.

❏ Preventing gully formation with improved 

drainage.

❏ Adding armored overflow dips, secondary 

culverts, or diversion prevention dips/drains 

on high-risk culverts.

❏ Clearing minor channel debris at culvert 

inlets and bridges.

❏ Excavating (pulling back) unstable fill 

material. 

Other treatments that involve much more work, 

major land disturbance, higher cost, or more 

detailed site evaluation and design detail may 

be important but more difficult to implement. 

These treatments likely will be implemented 

less often. Items with long-term effectiveness in 

this category include:

❏ Reconforming the road surface (from 

inslope to outslope).

❏ Replacing or increasing culvert capacity.

❏ Raising a bridge deck for added freeboard.

❏ Moving the road away from a stream.

❏ Reshaping a slope to a flatter angle.

❏ Strengthening retaining structures or 

adding a deep patch fill repair.

❏ Realigning a road section. 

❏ Stabilizing of unstable cut or fill slopes.

❏ Adding biotechnical slope stabilization and 

erosion control measures.

Short-term or less effective treatments may 

include:

❏ Adding trash racks to culverts.

❏ Adding streambank stabilization measures.
Selection of Appropriate Treatment 3-5

Evaluation and prioritization of the many factors 

involved in treatment selection can be difficult. 

It is important to assess the risk and the valuesat-risk accurately and then tailor the treatment 

to reduce the risk to an acceptable (agreed 

upon) level using the most cost-effective 

treatment. At times, these decisions may be 

controversial and agreement may be difficult to 

obtain. Good interdisciplinary communications 

and thorough study of the situation are critical 

to this process. Agreeing on effective solutions 

for high-use roads that also are at high risk 

of failure can be difficult and controversial 

when the treatment options are limited by site 

conditions and funding. 

Climate change may add to the uncertainty as 

to what level of risk is tolerable for some sites. 

Channel margin areas and rain-on-snow zones 

may be riskier in the future. Certain species 

may be more vulnerable to impact due to the 

effects of climate change on their habitat. 

Conversely, in some areas climate change 

might basically be a nonissue.
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CHAPTER FOUR 4IMPLEMENTATION
Implementation 4-1

4. Implementation

Implementation of storm damage risk reduction 

(SDRR) measures will vary by forest, depending 

on funds available to accomplish the work. A lack 

of specific funding to implement SDRR treatments 

does not necessarily preclude taking actions to 

increase a road’s resistance to damage. The 

routine application of best management practices 

and many actions inherent in preventative 

maintenance on the road system can go a long 

way toward reducing storm damage. Preventative 

maintenance practices are discussed in section 

5.1.

Our goal is to reduce road damage as well as 

the overall resource impacts resulting from 

a major storm. The objective is to implement 

SDRR measures as resources and funding allow. 

However, planning and preparation for storm 

events before they come is desirable too. This 

work is valuable considering that predicting storms 

is difficult. Storms are unique, and some damage 

will always happen from the largest ones, no 

matter how much SDRR work is done. 

4.1 Planning, Preparation, and 

Response

Storm events will happen. The question is not 

if, but rather when, where, and how often or 

how big. Thus, to minimize the damage and 

impacts of natural disasters, one must develop 

a storm damage risk reduction mindset. This 

mindset should permeate all aspects of road 

management, from inventory and design 

through storm response, maintenance, and 

repair.

Planning before a disaster occurs includes 

issues, such as having current information 

about the transportation system and identifying 

alternative routes to use in case of closures, 

knowing the weakest links in the system, having 

local contractors available to do needed work, 

preparing for temporary repairs, and having a 

reliable system of communication.

Each forest should develop a process that 

maximizes the ability to prevent storm damage, 

minimizes resource impacts and the disruption 

of the transportation system. Some key 

elements include:

❏ A transportation inventory that includes:

n All drainage features, ranked as to 

vulnerability. 

n Culvert and culvert-condition 

inventory.

n Road segments or sites of greatest 

risk or vulnerability.

❏ List of priority sites for prestorm inspection 

and storm patrols (based on historic 

damage and projected vulnerability), 

including maps to facilitate planning.

❏ Prioritized work lists of:

n Systematic inspection schedule.

n Prioritized sites for SDRR treatment.

❏ Storm contingency plan:

n Identification of alternative 

transportation routes.

n Available maintenance equipment.

n Temporary bridges on hand.

n An organization, such as the incident 

command system (ICS), in place to 

deal with disaster coordination.

n Available maps and a reliable system 

of communication in place that 

functions during a disaster. 

n Storm damage patrol organization 

(ICS) or other identified structure.

n Preparations for storm damage 

assistance, such as cooperation with 

other agencies.

n Emergency procedures coordinated 

with regulatory agencies to deal 

with Clean Water Act and National 

Environmental Protection Act issues.
4-2 Storm Damage Risk Reduction Guide for Low-Volume Roads

A note on storm patrol organizations. The 

organization should be identified and ready to 

respond as needed. Map and communication 

resources should be readily available and 

operational. Cooperative agreements with other 

agencies should be in place and communication 

protocols established. 

Storm patrols can be successful in preventing 

severe damage to sites by relieving blockages 

at culverts and bridges, but storm patrols also 

can be dangerous and costly should personnel 

and equipment get stranded during storms. 

Knowledge of the magnitude of precipitation 

that triggers potentially damaging events 

in different areas can be used to mobilize 

storm patrols and limit patrols in dangerous 

conditions. During storms, limited patrols may 

be used on easily accessible main roads. To 

remain safe and effective, patrols should occur 

prior to the major runoff season or in the fall, 

and after the peak of major storms has passed. 

Damage from the next storm may be prevented 

by clearing the latest storm debris and repairing 

drainage features.

The “Simplified Guide to the Incident 

Command System for Transportation 

Professionals” contains useful information on 

the ICS system for overall coordination during 

disaster conditions, particularly with regard to 

transportation facilities. It is available at <http://

www.ops.fhwa.dot.gov/publications/ics_guide/>.

4.2 Best Management Practices as 

Mitigation

The regular application of roads best 

management practices (BMPs) is a powerful 

tool for SDRR implementation and should be 

viewed as part of a storm damage risk reduction 

strategy. BMPs result in the following: 

❏ Less storm damage. 

❏ Less environmental damage and water 

quality degradation. 

❏ Less impacts from road delays, closures, 

and repair costs. 

Roads need to be built, protected, inspected, 

and maintained as well as possible with 

appropriate design standards and mitigation 

measures. The application of BMPs helps to 

achieve these goals. BMPs also help guarantee 

a reasonable level of design quality and 

environmental protection, which is useful when 

a major storm hits. 

An environmentally friendly road is a 

road that is well located, constructed with 

minimum ground disturbance, is hydrologically 

disconnected from the watershed drainage 

network, is well drained, and is appropriately 

surfaced to control surface erosion and 

minimize the loss of surfacing material. Roaddrainage structures are suited to the site and 

have adequate flow capacity, cut and fillslopes 

are stable, and erosion control measures are 

effective and minimize sediment delivery to 

hillslopes and stream channels. 
Implementation 4-3

The “Low-Volume Roads Engineering Best 

Management Practices Field Guide” (Keller 

and Sherar 2003) presents concepts and lists 

of best management practices for each key 

area of low-volume road design (i.e., surface 

drainage, culverts, slope stabilization, erosion 

control, etc.). These points complement the 

BMPs documented in the new approved 

“National Best Management Practices for Water 

Quality Management on National Forest System 

Lands” (USDA FS 2012), particularly in the 

section on Road Management Activities. Note 

that the above-referenced “Low-Volume Road 

Engineering BMP Guide” is cited in the “List of 

Resources for Road Management Activities” 

(U.S. Department of Agriculture, Forest Service 

2012).

The Forest Service publication “National Best 

Management Practices for Water Quality 

Management on National Forest System 

Lands”, Volume 1 (U.S. Department of 

Agriculture, Forest Service 2012), provides 

a general set of best practices for most 

aspects of forest management, including 

roads. This guidance should be used in new 

planning efforts, National Environmental Policy 

Act analysis, and evaluation of proposed 

activities, particularly if those projects affect 

water resources. The national program will 

contain the National Core BMPs, standardized 

monitoring protocols, national directives, and 

the data management structure. The document 

is available at <http://www.fs.fed.us/biology/

resources/pubs/watershed/FS_National_Core_

BMPs_April2012.pdf>. 
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CHAPTER FIVE

COMMON STORM DAMAGE RISK- 5REDUCTION TREATMENTS
Common Storm Damage Risk-Reduction Treatments 5-1

5. Common Storm Damage RiskReduction Treatments

A wide variety of storm damage risk reduction 

(SDRR) measures exist that can be cost 

effective and reasonably implemented or 

incorporated into forest roads to reduce their 

likelihood of failure and minimize their adverse 

environmental impacts. Their use depends 

first on an assessment of the risks involved 

and then on priorities and funds available to 

accomplish the work. Treatments are grouped 

into the following categories:

❏ Road maintenance. 

❏ Road surface drainage improvements.

❏ Stream crossing structure protection and 

improvements.

❏ Bridge protection and improvements.

❏ Road-stream encroachment.

❏ Erosion prevention and erosion control. 

❏ Slope stabilization measures.

❏ Cold region storm issues.

❏ Road storage and closure. 

In each category there are preventive measures 

that are relatively simple to implement and are 

cost effective. The most common, useful, and 

cost-effective treatments or measures used 

are consolidated and discussed in this section. 

Other less common measures that involve 

structure replacement, road relocation, major 

rehabilitation, structural improvements, or are 

more expensive to implement are discussed in 

section 6. 

While these are important treatments for 

consideration in the right application, some may 

be less practical, have only a short-term benefit, 

require excessive maintenance, possibly be 

quite expensive, or be less cost-effective and 

therefore implemented less often. Justify their 

use by analyzing risk, potential damage, and 

cost effectiveness. 

The analysis team, with knowledge about 

the entire forest road system, must weigh 

the tradeoffs of high-cost treatments at a few 

sites and the treatment of a larger number 

of high-risk sites with lower cost treatments. 

Leaving more sites untreated in order to treat 

a few sites with high-cost solutions may leave 

more resources at risk than treating more 

sites and cutting back on the higher cost 

treatments. It cannot be stressed enough that 

cost effectiveness both by site and across a 

landscape needs to be carefully considered.

Many SDRR treatments are site-specific so 

field analysis and good judgment are needed 

for prescribing and designing on-the-ground 

treatments. This guide is not intended to be an 

exhaustive list of potential treatments; there 

may be other treatments of equal or greater 

value that are not covered here.

PIARC, the World Roads Association, has 

presented a general discussion of all natural 

hazard impacts and reduction options in its 

publications “Risks Associated with Natural 

Disasters, Climate Change, Man-made 

Disasters, and Security Threats” (PIARC, 2013) 

and “Natural Disaster Reduction for Roads” 

(PIARC 1999). Additionally, the Forest Service 

publication “Burn Area Emergency Response 

Treatments Catalog (BAERCAT)” (Napper 

2006) describes a number of useful drainage, 

channel, and erosion-control treatments to 

minimize damage from storms after a forest fire 

and treatments that also apply to general storm 

damage risk reduction. It is available at <http://

www.fs.fed.us/eng/pubs/pdf/BAERCAT/lo_res/

lo_res.shtml>.

Road drainage deficiencies typically are the 

number one priority for SDRR treatments. 

This includes road surface drainage measures 

and drainage crossing structure and channel 
5-2 Storm Damage Risk Reduction Guide for Low-Volume Roads

problems that can lead to structural failures. 

Relatively inexpensive road surface drainage 

improvement measures are particularly costeffective and can prevent significant road 

damage, hillslope erosion, and sediment 

delivery to streams. 

Drainage crossing structures, such as culverts, 

bridges, and fords, typically are expensive; 

failure may result in major resource damage, 

sediment production, and loss of road use for 

a significant period of time. Risk of failure may 

be caused by poorly designed structures, lack 

of flow capacity or freeboard, poor location, 

foundation scour, or problems with the channel, 

such as a confined reach or excessively 

sharp bend. Roads that remain closed while 

repair decisions are made will not receive 

maintenance beyond the damage site, perhaps 

increasing their potential for further damage. 

Thus, these structures can be a high priority for 

SDRR measures.

One of the most fundamental actions that one 

can take is staying current with needed road 

inspections and road maintenance. This work 

should be routine or periodic and not just a 

function of the road crew. All Forest Service 

employees should actively engage in observing 

and reporting road drainage problems. Blocked 

ditches, rutted roads, loss of road crown, 

damaged culverts, and so forth all contribute to 

additional damage during major storm events.

Erosion protection measures around structures, 

in exposed areas, along road shoulders and 

fillslopes, in quarries, and so forth are critical to 

prevent erosion and subsequent sedimentation 

and water-quality degradation. Erosion control 

measures typically are relatively inexpensive. 

While road erosion presents a widespread, 

chronic source of pollution and may reduce 

land productivity, a landslide can be an acute 

catastrophic source of sediment. Landslides 

can close roads for significant periods of time 

and can be costly to cleanup or stabilize. The 

resulting resource damage may be difficult 

and expensive to cleanup or mitigate. Thus, 

landslide stabilization or slide prevention can 

be a specific goal in storm damage reduction 

on some specific cut slopes and fill slopes. 

Road closure and storage, decommissioning, 

or obliteration are other ways to eliminate 

problematic or potentially damaging roads.

5.1 Road Maintenance 

Road inspection and maintenance is a 

fundamental part of road-system management. 

Therefore, it should be done both routinely 

for many items and periodically for recurring 

issues. This is important for roads to function 

properly any time of year, and particularly when 

the large storms occur. Figure 12 shows a 

road badly in need of road maintenance that 

is very vulnerable to additional damage from 

any storm. When a large storm hits, needed 

inspection and maintenance becomes critical, 

particularly properly functioning road-surface 

drainage and stream-crossing culverts. This 

applies to roads of all maintenance levels. 

Also, talk to local personnel about priorities and 

problem areas. 

Knowledgeable local individuals often have 

useful historic information on problem sites. 

Document the information, and develop 

a list of priority areas, based upon local 

experience that should receive maintenance 

first. Keep in mind, however, that road 

surface and ditch maintenance will lead to 

increased sediment production following 

the maintenance (Luce and Black 2001). 

Thus, road maintenance should only occur 

when really needed, particularly when roadsurface drainage defects need correction.
Common Storm Damage Risk-Reduction Treatments 5-3

Road maintenance items that are particularly 

important to implement in advance of any large 

storm (keeping in mind that there may not be 

much time to react just prior to a storm) are 

listed below, roughly in order of priority:

❏ Remove logs and debris from around the 

inlet area of culverts.

❏ Remove debris from the inlet area of 

ditch-relief cross-drain culverts to prevent 

plugging. 

❏ Clean out debris from trash racks upstream 

of culverts.

❏ Clean ditches to avoid blockage and 

ponding of water that can saturate the road 

subgrade and/or fill material.

❏ Replace missing riprap armor around the 

inlet/outlet of culverts.

❏ Reshape surface drainage features, such 

as rolling dips, waterbars, and so forth.

❏ Remove unwanted berms that have formed 

along the outside edge of the road.

❏ Grade and shape the roadway surface to 

maintain a distinct inslope, outslope, or 

crown shape to move water rapidly off the 

road surface, keep the roadbed dry, and 

prevent water concentration.

❏ Remove ruts in the road surface that trap 

and concentrate water.

❏ Patch potholes and seal cracks in an 

asphalt surface to prevent water intrusion 

and accelerated damage. 

❏ Compact the graded roadway surface to 

keep a hard driving surface and prevent the 

loss of fines.

Figure 12—Ruts and erosion in the road 

surface because of lack of surface drainage 

and maintenance. This road in this condition is 

particularly susceptible to damage from any storm. 

(Photo courtesy of Vincent Barandino.)

A note on ditch cleaning: During the 

1970s and 1980s concerns over sediment 

production from roads led to changes 

in road maintenance practices in many 

areas. Ditch cleaning was identified as a 

procedure that exposes erodible soils in 

the ditch and reinitiates cutslope erosion 

processes, generating chronic sources of 

road sediment. As a result, less intrusive 

ditch cleaning became common. Over 

time, the result of less ditch cleaning 

was vegetation encroachment and lower 

hydraulic efficiency within the ditch. 

Brushing activities cut the stems in the ditch 

but left stubs that trapped debris, this further 

reducing ditch efficiency and resulting in 

plugged ditches. Today a balance should 

be met between maintaining hydraulic 

efficiency and keeping a rough or armored 

surface or vegetative cover to reduce ditch 

erosion.
5-4 Storm Damage Risk Reduction Guide for Low-Volume Roads

Ditches that are hydrologically connected to 

the stream system are the greatest concern. 

To maintain hydraulic efficiency of the ditches 

and also allow for ditch stabilization and 

stabilization of adjacent cutslopes, ditches 

need to be oversized so that adequate capacity 

is maintained in the absence of regular 

cleaning. Any ditch enlargement needs to be 

done in such a way that cutslopes are not 

undercut, creating a new source of erosion and 

instability. Some sites may not be appropriate 

for oversized ditches and will require another 

technique, such as outsloping and/or insloping 

without a ditch and rolling the grade to manage 

water.

Additional road maintenance or improvement 

items that are useful for long-term prevention of 

damage are: 

❏ Armor ditches in areas of particularly 

erodible soils or steep grades.

❏ Add frequent ditch relief cross drains or 

rolling dips. 

❏ Convert an inslope section of road to an 

outslope. 

❏ Install diversion prevention dips at or 

downslope of stream-crossing culverts 

that have the potential to divert the stream 

down the road.

❏ Add riprap armor or soil bioengineering 

protection around the inlet of undersized 

culverts or bridges.

❏ Plant deep-rooted vegetation on oversteep 

cut and fillslopes or slopes with a history of 

movement. 

❏ Install a deep patch slope stabilization 

repair on chronically settling fills.

Despite the need for maintenance, particularly 

on open roads, current funding suggests that 

most roads will not receive the maintenance 

they need. Closed roads will receive little to 

no maintenance. Unneeded roads should 

be decommissioned. For most roads that 

cannot be closed and will likely receive little 

maintenance, stormproofing with good drainage 

measures (as discussed in the following 

sections) will be important. Roads need to be 

put into a condition to be as self-maintaining as 

possible. 

5.2 Road Surface Drainage 

Improvements

A number of commonly used roadway surface 

drainage improvements that prevent or 

minimize storm damage are summarized below. 

With traffic and time, ruts will form down most 

roads, thus maintenance is needed periodically. 

In the absence of maintenance, the best road 

drainage measures to prevent ruts are rolling 

grades, rolling dips or waterbars, or an inslope 

or outslope road. The following measures are 

used to construct and improve roads to prevent 

the concentration of water, move water rapidly 

off the road, and facilitate control of water:

❏ Close or relocate/reconstruct road 

segments with excessively steep road 

grades. The steeper the road grade, the 

more difficult it is to achieve road surface 

drainage. With steep grades, even roads 

with moderate cross slopes keep water on 

the road surface for a long distance. Road 

grades less than 10 percent are easiest for 

control of surface flow. Grades less than 

6 percent are helpful on roads receiving 

infrequent maintenance.

❏ Maintain positive surface drainage with 

an outslope, inslope, or crown roadway 

section, as seen in figure 13. A cross slope 

of 4 to 6 percent is typical on an unsealed 

road. Six percent may be ideal for drainage 

and requires the least maintenance. Two 

to three percent may be adequate for 

a hardened surface on flatter grades, 

particularly with log haul.

❏ Roll grades or undulate the road profile 

frequently to provide locations to disperse 

water off the road (figure 14).
Common Storm Damage Risk-Reduction Treatments 5-5

❏ Use frequently spaced leadoff ditches to 

prevent the accumulation of excessive 

water in the roadway ditches.

❏ Use roadway cross-drain structures, such 

as rolling dips (or pipe cross-drain culverts, 

open-top culvert flumes, or deflectors), to 

move water across and off the road surface 

and to move water from the inside ditch 

across and off the road surface. Space the 

cross-drain structures frequently enough to 

remove all surface water without excessive 

water accumulation. Since these features 

are intended to cutoff and discharge 

accumulated ditch water, they must be cut 

to the bottom of the ditch, which may be 

relatively deep, and dam the ditch on the 

downgrade side.

❏ Protect cross-drain outlets with rock 

(riprap), brush, or logging slash to dissipate 

energy and prevent erosion, or locate the 

outlet of cross drains on stable, nonerodible 

soils, rock, or in well vegetated areas. Also 

use downspouts or downdrains to move 

water down a fillslope to a stable outlet 

area. A stable outlet location is important. 

The ideal spacing listed in tables needs to 

be adjusted to meet field conditions. Closer 

cross-drain spacing may eliminate the need 

for outlet armoring and minimize water 

concentration.

❏ Construct waterbars on infrequently used 

roads or closed roads to control surface 

runoff and remove water from the road 

before it accumulates and causes erosion. 

Waterbars are road surface features 

that may not intercept the ditch and may 

redirect road surface flow into inboard 

ditches on strongly insloped templates. 

❏ Repair entrenched roads that are dificult to 

drain. The road may effectively become a 

canal. Ideally, raise the road grade above 

the level of the adjacent terrain with fill 

material to be able to drain the road surface 

(see appendix A2 for a drawing of road 

options in wet, very flat terrain).

❏ Use catchwater ditches (intercept ditches) 

across the natural ground above a cutslope 

in areas with high-intensity rainfall and 

overland flow. These ditches will capture 

overland sheet flow before it pours over the 

cutslope and erodes or destabilizes the cut. 

However, be aware that catchwater ditches 

that are not properly maintained can 

become counterproductive pools of water 

above slopes, increasing the probability of 

slope failure or gully erosion.

Note that many of the measures listed 

above may not be necessary if you can 

disperse the flow adequately and/or reduce 

the concentration or amount of water. If you 

are getting ditch erosion or need armor at 

the outlet of a pipe, ditch, or dip, it is a sign 

of too much concentrated flow. First try 

to add additional cross drainage or more 

leadoff ditches to reduce the flow in the 

problem area.

For all road surface water drainage 

applications, it is extremely important to know 

the soil and hillslope conditions where water 

is discharged from the road onto the slope. 

Most mass slope failures below roads, as 

well as many gullies, result from excessive 

concentration of surface water running off the 

roads and saturation of marginally stable or 

unstable hillslopes. Information regarding road 

surface drainage, road damage prevention, 

and water quality protection is available in the 

U.S. Department of Agriculture, Forest Service, 

National Technology and Development Program 

publication series “The Water/Road Interaction 

Technology (WRIT) Series” (U.S. Department of 

Agriculture, Forest Service 2000). For additional 

technical information, see the Web site <http://

www.stream.fs.fed.us/water-road/>.

Additional information on general road drainage 

can be found in “Roadway and Roadside 

Drainage” (Orr 2003).
5-6 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 13—Basic road surface drainage options of an inslope, outslope, or a crown section. The photo shows 

a crown road with an inside ditch.
Common Storm Damage Risk-Reduction Treatments 5-7

Figure 14—Examples of a rolling grade to minimize 

concentration of water on the road.

Adding Rolling Dips (Broad-Based 

Dips)

Rolling dips, or broad-based dips as they are 

called occasionally, are designed to divert 

and remove water off the road surface and 

the roadway ditch while safely allowing for the 

passage of relatively low-speed traffic. They 

do not involve culverts that can plug during 

storm events. Figure 15 shows the concept of a 

rolling dip on a road while figure 16 shows two 

different rolling dips. Rolling dips are a cross 

between a waterbar and a grade break. They 

have a reverse grade to direct water off the 

road rather than down the road. Like waterbars, 

they rely on a mound or high point at the 

downhill side to reverse or change direction of 

the waterflow. 

Rolling dips are an alternative to conventional 

ditch relief cross-drain culverts with the added 

advantage that they drain the roadway surface 

as well as the ditch. They can eliminate 

problems with long, sustained grades and 

ditches. Rolling dips also can be carved into 

existing roadbeds. Rolling dips usually cost 

less, require less maintenance, and are less 

likely to plug and fail during a storm than culvert 

cross-drain pipes. The outlet area is often 

armored with rock to prevent erosion or gully 

formation, as seen in figure 17. 

Rolling dips are appropriate on low-volume, 

low-to-moderate speed roads (15 to 30 miles 

per hour (25 to 50 kilometers per hour)). They 

are difficult to drive and are dangerous on 

high-speed roads. Rolling dips typically are not 

constructed on road grades over about 8 to 10 

percent, particularly for truck traffic. Rolling dips 

occasionally are constructed on considerably 

steeper road grades but the excavation 

becomes significant with construction of a 

canyon in the road to adequately turn the water 

off the roadway, making sight distance poor. 

For that reason they are not commonly used 

on steep road grades. Use and design depend 

greatly on the road-design vehicle.
5-8 Storm Damage Risk Reduction Guide for Low-Volume Roads

Construct rolling dips nearly perpendicular 

to the road, or ideally at a slight skew (of 25 

degrees maximum), to minimize damage 

to truck frames driving through them yet 

effectively change the direction of waterflow. 

For commercial log-haul roads, install rolling 

dips perpendicular to the road. For light traffic 

and infrequently maintained roads or roads in 

storage, a 20- to 25-degree skew is ideal. The 

bottom of the dip should have a 2- to 5-percent 

outslope to ensure positive drainage. The entire 

structure should be long enough, typically 50 

to 200 feet (15 to 60 meters), to comfortably 

pass vehicles and equipment. If the road has 

an aggregate surfacing, salvage the existing 

aggregate before a dip is constructed and 

respread it once the dip is completed. 

Additional armoring material may be needed 

in the bottom of the dip, where it intercepts the 

road subgrade, to prevent rutting in soft soils 

and at the dip outlet. If erosion is occurring at 

the dip outlet, however, a better solution may 

be to add more dips, with a closer spacing. 

Figure 18 shows the basic form of a rolling dip. 

The rolling dip needs to be moderately deep to 

function properly, particularly on the outlet side; 

have a distinct reverse slope out to properly 

drain water off the road; and be constructed 

using a hand level, rod, and tape (or other 

simple survey instruments) to ensure that 

the proper grades are established. Armor the 

mound and dip with gravel or rock, particularly 

in soft soils, to maintain the shape of the rolling 

dip during traffic use. Maintenance of dips with 

a grader is a learned skill, so operators need 

to understand the form and function of a dip. 

Otherwise, a dip can be damaged or destroyed 

during grader maintenance.

In terrain that may receive snow combined 

with traffic, construct the dips relatively deeply, 

accentuating the depth of the trough, so that 

water will exit the road even when tire ruts are 

formed in the snow. Otherwise, the ruts in the 

snow can act as canals that channel the water 

on down the road, through the dip, rather than 

allowing the water to be diverted off the road.

Figure 15—Sketch of a typical rolling dip on a logging road. (Adapted from California Division of Forestry 

Roads Handbook 1968).
Common Storm Damage Risk-Reduction Treatments 5-9

Since different vehicles have different clearance requirements, figure 19 shows dimensions for 

rolling dips and drivable waterbars (discussed later) designed for passage of a logging truck, a low 

boy, and a high-clearance vehicle. Additional examples of rolling dips, drawings, and information on 

their design and use are found in appendix A1. The table is included to aid in the design of rolling 

dips for a variety of traffic (a design spreadsheet is available electronically for those with access to 

the Forest Service internal Web site at <http://fsweb.sdtdc.wo.fs.fed.us/programs/eng/drain_dip_

spreadsheet>).

Figure 16—The form of rolling dip cross drains on various roads.

Figure 17—An armored and grid-rolled rolling dip on the Umatilla National Forest. Note the armored dip outlet 

using small riprap. (Coutesy of Caty Clifton, Umatilla National Forest.)
5-10 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 18—A rolling dip layout and shape, where the dip is used to move surface water off the road, drain any 

inside ditch, and prevent water concentration.
Common Storm Damage Risk-Reduction Treatments 5-11

Figure 19—A typical drawing of dips and their dimensions for different design vehicles. (Note: Lengths and 

depths are shown in feet.)
5-12 Storm Damage Risk Reduction Guide for Low-Volume Roads

The recommended range of spacing of rolling 

dips varies widely. Spacing for maximum 

distance between rolling dip cross-drain 

construction on forest roads must be site 

specific and should be adapted to the local 

climate and existing soil and slope conditions. 

As shown in figure 20, many studies have 

been conducted and recommendations made 

for locating surface cross drains with a fairly 

wide range of spacing values. The spacing 

distances presented in table 3, adapted 

from Packer and Christensen (1964), are 

relatively conservative values commonly used 

in a range of erodible to nonerodible soils to 

minimize rilling in the road surface. Thus, use 

local experience and judgment in selecting 

appropriate spacing values based upon field 

performance, topographic location on the slope, 

soil type, road surfacing material, rainfall, traffic, 

approach grade, and other local conditions. 

In sensitive areas, such as near riparian 

zones, spacing might be much closer than in 

other upland areas. The specific spacing and 

discharge location should be field adjusted to 

find locations that will not erode or form a gully. 

Ideal dip or cross-drain exit locations are in 

brushy areas, rocky areas, or natural drainage 

features or ravines.

Cross-Drain Culverts (Ditch-Relief 

Culverts)

Culvert cross drains (relief culverts) are 

buried beneath the road surface to discharge 

ditchwater from the toe of the cut to the outside 

edge of the road. They are crucial on most 

inslope and crown roads to prevent excess 

concentration of water in the ditch. They are 

the most common type of road drainage ditch 

relief and are appropriate for high-standard 

roads where a smooth road surface profile 

is desired. They also are very common on 

low-standard roads anywhere a ditch is 

constructed. However, cross-drain pipes are 

another expense and the relatively small culvert 

pipes used for cross drains are susceptible to 

plugging during storms. Consider rolling dips as 

an alternative.

Figure 20—Range of recommended cross-drain spacing (Copstead et al. 1998). 

Note: Conversion 1 meter = 3.28 feet.
Common Storm Damage Risk-Reduction Treatments 5-13

Construct relief culverts with circular or arch 

pipes or rectangular concrete or wooden boxes. 

An 18-inch (450-millimeter [mm]) minimumdiameter round culvert is used most often for 

ditch relief to help prevent failure from debris 

blockage. Smaller pipes plug easily. If the pipe 

has plugged, then install a larger pipe, such as 

a 24-inch (600-mm) culvert. Some State forest 

practices rules require 18 inches (450 mm) 

as the minimum size for ditch-relief culverts 

(Washington State Forest Practices Act). Also, 

consider additional cross-drain pipes, reducing 

the spacing between the pipes to reduce the 

water volume. Calculate pipe size and spacing 

by using the rational formula with the small road 

watershed and local rainfall intensity-duration 

data. Note that pipes installed on a skew slightly 

affect the effective pipe capacity. However, pipe 

size and spacing are based more commonly on 

local experience or on a recommended spacing 

from tables. 

Table 3 lists criteria for spacing of ditch-relief 

cross drains (and rolling dips) as a function 

of road grade and soil erodibility (Unified Soil 

Classification System). Actual spacing can 

depend on field conditions and on ditch capacity 

to prevent overflow or limit the water volume to 

prevent erosion or gully formation at the outlet, 

or to minimize hydrologic connectivity near a 

stream crossing. Also, spacing criteria, which 

was developed in the Central United States, 

may need to be adjusted for areas with higher 

rainfall intensity. Downslope stability issues are 

extremely important when determining spacing 

and outlet location of a ditch-relief structure.

Table 3—Guidelines for maximum recommended distance between ditch-relief cross drains and rolling-dip 

cross drains, with spacing in feet, based on soil type. (Adapted from Packer and Christensen 1964, and 

Copstead, Johansen, and Moll 1998.)

Maximum Recommended Distance Between Ditch-Relief and 

Rolling-Dip Cross Drains (feet)

Low to Moderate to Very

 Non-Erodible Soils Erodible Soils

Road Aggregate, Silty and Silty SW, SP, ML

Grade Gravels Clay-Rich Sand (SM), Sand, Well-to-

(percent) (GW/GP) Gravels Clayey Sand Poorly Graded,

 (GM/GC) (SC) Silt

2 400 300 170 95

4 340 275 150 85

6 300 230 130 75

8 250 200 110 65

10 200 160 90 55

12+ 150 130 75 50

Conversion: 3.28 feet = 1 meter

Note: Soil types are listed by ASTM Unified Soil Classification System. Plastic clay rich soil (CH, CL, MH) types fall 

between the above range of values for low-to-moderately erodible soils. 

Note that rolling dips are very difficult to install on grades over 8 to 10 percent.
5-14 Storm Damage Risk Reduction Guide for Low-Volume Roads

The guidelines should be adjusted, according to 

Packer and Christensen (1964):

1. Reduce the spacing by 15 feet (5 m) if the 

road is located in the middle one-third of a 

slope.

2. Reduce the spacing by 35 feet (11 m) if 

the road is located in the bottom one-third 

of a slope.

3. Reduce the spacing by 10 feet (3 m) if the 

road is on an east or west exposure.

4. Reduce the spacing by 20 feet (6 m) if the 

road is on a south slope.

5. If the resulting spacing after items 1 

through 4 falls below 55 feet (17 m), 

use relief culverts at 55 feet (17 m) 

spacing and apply aggregate surfacing 

and erosion protection measures, 

such as vegetative seeding to ditches, 

road surface, fills, shoulders, and 

embankments.

Culvert cross-drain-pipe installation details are 

shown in figure 21. Install culvert cross-drain 

pipes with an ideal angle of 15 to 30 degrees 

skew to the centerline of the road, using a 

minimum outslope of at least 2 to 3 percent. 

Both are important to move water efficiently into 

the pipe and to prevent plugging. Additionally, 

the outslope should be at least 2 percent 

steeper than the ditch grade it is draining to 

reduce deposition at the inlet and prevent 

debris from plugging the culvert. Usually a berm 

or ditch-block structure is needed in the ditch 

immediately beyond the cross drain to ensure 

that water turns and enters into the pipe. This 

ditch block should completely fill and span the 

ditch. Commonly, an excavated inlet basin is 

used as well. The pipe should exit at ground 

level to prevent a waterfall and erosion. On 

very steep ground, the pipe outlet area may 

need specific reinforcement, such as live stakes 

Figure 21—A typical culvert cross-drain installation.
Common Storm Damage Risk-Reduction Treatments 5-15

and riprap. Again, if much outlet erosion is 

occurring, additional cross drains may be the 

best solution to reduce the quantity of water. 

In some cases, place rock armor at the outlet 

for energy dissipation and erosion control, as 

shown in figure 22. 

Figure 22—A cross-drain culvert with an armored 

outlet and leadoff ditch on fairly flat ground.

Waterbars

Use waterbars to prevent concentrated 

waterflow from accelerating down a sloping 

road and to divert water off a road. Waterbars 

typically are used on roads that are closed 

or are limited-use roads and trails. It is an 

excellent method of closing or decommissioning 

roads and trails where surface water running 

down the road may cause erosion of exposed 

mineral soil. Spacing may be close, depending 

on road grade and soil type, such that erosion 

does not occur between or within waterbars. 

Improperly installed or spaced waterbars can 

allow water to end-run (pass the waterbar) or 

cause too much water concentration, resulting 

in additional erosion. Waterbars are installed 

on grades up to 30 percent or more. On open 

roads, waterbars are designed as drivable.

Drivable waterbars, as shown in figure 23, 

have the same function as normal waterbars 

(to impede waterflow down a road) but are 

constructed in a manner such that highclearance vehicles or 4-by-4 vehicles can 

reasonably drive over them. Drivable waterbars 

occasionally are used on inactive roads, and 

4-wheel-drive roads that receive little use yet 

occasionally need to pass vehicles. Table 4 

shows that the spacing of waterbars is much 

closer than the spacing of rolling dips or cross 

drains. Drivable waterbar information also is 

found in appendix A1.

Nondrivable waterbars are waterbars or 

mounds that are constructed so radically that 

they cannot reasonably be driven over by 

vehicles, including 4-wheel-drive vehicles. 

They are intended to prevent use (by motorized 

vehicles) and stop water from accumulating or 

running down the road. They typically are built 

into roads in storage, decommissioned roads, 

or skid trails that are closed. Spacing criteria is 

the same as used for drivable waterbars.
5-16 Storm Damage Risk Reduction Guide for Low-Volume Roads

Table 4—Recommended waterbar spacing (feet)

Recommended Waterbar Spacing (feet)

Road/Trail Grade (percent) Low-to-Nonerodible Soils (1) Erodible Soils (2)

0-5 250 130

6-10 200 100

11-15 150 65

16-20 115 50

21-30 100 40

30+ 50 30

Note:

(1) Low-erosion soils=Coarse rocky soils, gravel and some clay.

(2) High-erosion soils=Fine, friable soils, silt, fine sands, volcanic ash.

Conversion: 3.28 feet=1 meter Adapted from Packer and Christensen (1964) and Copstead, Johansen, and Moll (1998)

Figure 23—Drivable waterbar construction. (Adapted from Wisconsin Department of Natural Resources 

Publication FR093 1995.)
Common Storm Damage Risk-Reduction Treatments 5-17

Ditches and Ditch Treatments

Ditches are a key tool to collect, move, and 

discharge water from the roadway. They 

should be large enough to carry the anticipated 

accumulation of water, and possibly be 

somewhat oversized to function during major 

storm events, even when no one has cleaned 

or maintained the ditches. They also can be 

a source of erosion if too much water is in 

the ditch or its velocity is too high. Water is 

removed from the ditch with ditch-relief cross 

drains, rolling dips, and leadoff ditches. If water 

cannot be adequately dispersed or removed 

from the ditch, then armor the ditch with rock 

or vegetation or reduce the velocity with 

small check-dam structures. Check dams are 

problematic and need attention to installation 

detail. If a ditch is armored or lined, it may have 

to be oversized initially to accommodate the 

armor and still have the needed flow capacity, 

particularly if maintenance is infrequent. With 

armoring or check dams, ditch maintenance can 

be difficult.

Leadoff Ditches

Leadoff ditches (or turnout ditches) are 

another way to discharge water and prevent 

accumulation of excess water in the roadway 

ditches (figure 24). They are an inexpensive 

alternative to culvert cross drains and used at 

every opportunity where the terrain is suitable. 

They usually do not use pipes that might plug in 

a storm.

They are used in flat terrain where there is no 

cutbank at approaching drainage crossings and 

at fill areas across a swale or ravine. In very 

flat terrain, a leadoff ditch may have to be quite 

long to daylight out into the forest. They work 

best with an elevated roadway. They also are 

used at switchbacks where the road quickly 

changes direction across the slope to divide the 

waterflow. As with rolling dips or culvert cross 

drains, they should discharge in nonerosive 

areas or protected outlets to prevent erosion. 

Alternatively, if terrain or circumstances do 

not allow for the use of a leadoff ditch, it may 

be possible to discharge the ditchwater into a 

sediment catchment basin. To disconnect the 

road drainage from the stream, discharge the 

water into the forest or a vegetated area before 

the ditch reaches a stream channel, as seen 

in figures 25 and 26. Figure 26 also shows a 

number of measures used to prevent sediment 

from entering streams at a road-stream 

crossing.

Figure 24—Ditch layout and leadoff before a 

stream. (Adapted from Wisconsin’s Forestry Best 

Management Practices for Water Quality 1995.)

Figure 25—A leadoff ditch discharging into the forest 

before reaching a live stream at the bottom of the 

grade (near the parked vehicles).
5-18 Storm Damage Risk Reduction Guide for Low-Volume Roads

Ditch Armoring

Water that runs in the ditch can erode and 

move large quantities of soil and debris. 

Avoiding the need for a ditch is ideal, but when 

a ditch is necessary, frequent ditch-relief cross 

drainage is ideal to prevent water accumulation 

and reduce or prevent ditch erosion. However, 

this is not always possible where the ditch is 

deep or the road template is strongly insloped. 

Alternatively, when cross drains are not possible 

to construct, an eroding ditch can be armored 

with graded rock to decrease the velocity of 

water, prevent erosion and downcutting, and 

to allow the deposition of sediment (figure 27). 

Small rock riprap typically is used as a lining 

material. A graded 3- to 6-inch (75- to 150-mm) 

rock size is ideal. A geotextile may be placed 

under the rock as a filter to separate the rock 

from the soil and keep soil from eroding under 

the rock. For relatively slow velocities, a ditch 

armored with grass may be adequate.

By decreasing the velocity, silt and debris are 

deposited in the ditch instead of additional bed 

material being eroded away. Increasing the 

roughness of the ditch decreases the velocity 

of water. Rock-armor ditches are common 

but need periodic maintenance to remove 

sediments. They also are labor intensive to 

construct and can be difficult to clean and 

maintain. 

Figure 27—A rock armored ditch to control the 

waterflow and prevent downcutting of the ditch.

Check dams (also called ditch dikes) in 

ditches can be an alternative to solid-lined 

ditches. They can be made with many types 

Figure 26—A sketch of a number of sediment protection measures used at a road-stream crossing, including 

discharging leadoff ditches into a vegetated area before reaching the stream.

Armor or stabilize the stream crossing structure and add surface armoring to the roadbed and approach. Drain water off the road 

surface before reaching the crossing. Road surface armor should be a minimum of 150 feet (50 m) and should extend to the nearest 

cross-drain structure. Actual distance depends on road grade, soil type, and rainfall, etc. For fords, set stream channel armoring at the 

elevation of the natural stream bottom. Armor outlets and fills as needed.
Common Storm Damage Risk-Reduction Treatments 5-19

of materials, including sand bags, loose 

rocks, masonry or concrete, branches, straw 

bales, logs, gabions, or live vegetation. Of all 

these materials, loose rock is one of the most 

commonly used and most effective designs, 

as seen in figures 28 and 29. Note, however, 

that ditch check-dam structures typically are 

not recommended for stormproofing roads 

that may be closed or receive infrequent 

maintenance. If not properly constructed or 

maintained, they can force flow around the 

structure and erode or undermine the road 

shoulder and cutbank.

The purpose of check dams is to increase 

roughness and decrease the velocity of water 

moving down the ditch with grade control. A 

series of check dams along a ditch reduces 

the effective gradient of the ditch, transforming 

a relatively steep gradient to a stair-stepped 

channel; they also trap sediment. Water 

successively flows on gentle slopes between 

structures and then cascades over the 

stabilized structure. A settling pond or sediment 

catchment structure also might be added as a 

ditch approaches a stream crossing if the water 

cannot be discharged into the forest.

The “California Division of Forestry Roads 

Handbook” (1968) presents some criteria for 

spacing of check structures in roadside ditches. 

Assuming that check structures are about 12 

inches (300 mm) high, spacing is every 12 to 

100 feet (4-30 m) along the ditch, depending on 

ditch slope and soil erosion potential, as seen in 

table 5.

These spacing values are approximate and can 

be proportionally greater in a deep ditch with 

18-inch-high (450 mm) dike structures. Adjust 

spacing for local soil and rainfall conditions, 

particularly based upon field performance of the 

structures and maintenance frequency.

Table 5—Recommended maximum distance 

between ditch check dams (Adapted from “California 

Division of Forestry Roads Handbook”1968.)

Recommended Maximum Distance Between 

Ditch Check Dams, in feet (meters)

Road Low-to- Erodible

Grade Nonerodible Soils

(percent) Soils

0-4 100 (30) 50 (15)

4-7 50 (15) 25 (8)

7-10 25 (12) 12 (4)

10-12+ 12 (4) <12 (4)

Rock check structures need maintenance to 

retain their function and to remove excess 

sediments; they also are labor intensive to 

build, but very effective in reducing flow velocity 

and trapping sediment. Depending on spacing 

and location, they can be difficult to maintain. 

They are a dam-like structure, so they have 

design details that are needed to make them 

function properly. Key design elements include 

the following:

❏ A V- or U-shaped top to keep the flow in the 

middle of the ditch and prevent an end run 

around the structure that can cut into the 

road. The bottom of the V-notch must be 

below the elevation of the road surface.

❏ The spacing is critical to achieve a 

nonerodible effective gradient between 

structures.

❏ The structures need to be placed firmly 

into the bottom of the ditch and into 

the ditchbank and designed to prevent 

undercutting. Since water cascades over 

the structures, there is the potential for 

erosion at the base of the structures that 

can undermine the structures and render 

them useless.
5-20 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 28—A typical rock-lined ditch and ditch check dams.

Figure 29—Check structures of rock or bagged gravel installed in the ditch to reduce flow velocity and trap 

sediment.
Common Storm Damage Risk-Reduction Treatments 5-21

❏ The structure should be lower than the 

elevation of the road shoulder to prevent 

water from flowing onto the road (see figure 

29).

More detailed information on check-dam design 

and installation is presented in section 6.5.

Vegetation-lined ditches, typically using 

noninvasive grasses, offer a natural alternative 

to other ditch armoring or check structures on 

gentle slopes or nearly flat ground. Grass-lined 

ditches often are suitable for ditch slopes up to 

5 to 10 percent, depending on soil, grass type, 

and climate. Vegetated ditches are inexpensive 

and present an aesthetic natural look, as seen 

in figure 30. They need periodic maintenance 

to remove sediments, and this maintenance 

can be difficult. On flat slopes with a lack of 

maintenance, the vegetation (as well as other 

debris) can block the flow and pond water, thus 

saturating the adjacent road. Ideally, select 

grasses for good growth properties, hardiness, 

dense ground cover, deep roots to stabilize 

the ditch, and some tolerance to periodic 

inundation. If necessary, reseed the ditch if 

maintenance damages the grass. 

On steeper slopes, grasses may be inadequate 

and more durable ditch protection is needed, 

such as turf reinforcing mats, rock riprap, 

masonry liner, and so forth. Since rock armoring 

or ditch structures partially block the ditch, a 

ditch may need to be initially oversized during 

construction or reconstruction to accommodate 

the armoring and the anticipated flow, plus 

some freeboard. If widening or deepening the 

ditch, do not undercut the adjacent cutslope.

Figure 30—A ditch lined with vegetation (grasses). 

Note that as the ditch gradient increases the grasses 

alone may not be enough to prevent higher water 

velocities from eroding the ditch.

Ditch or Pipe Outlet Armoring

The accelerated velocity of water leaving a 

roadway ditch or culvert pipe can cause severe 

erosion or gullying if discharged directly onto 

erodible soils. Always try to align and discharge 

culverts and cross-drain dip-drainage structures 

at the natural ground level away from a live 

stream and on a stable, nonerodible soil area 

in a stable area, such as an existing natural 

(dry) swale or channel, rocky areas, or wellvegetated areas. When necessary, stabilize the 

pipe, dip, or drain-outlet area, and the energy 

of the water dissipated by discharging the water 

onto a few cubic yards (1-2 cubic meters) of 

a graded rock riprap, as shown in figure 31. 

Extend the riprap several feet (1-2 meters) 

beyond the end of the pipe, and ideally, fan 

out the rock to spread out the flow and prevent 

erosion (appendix A2). Outlet erosion also is a 

sign that closer cross-drain spacing is needed. 
5-22 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 31—Photos of pipe riprap outlet protection (a) and flow into a sediment catchment basin (b), and a 

drawing detail of culvert-outlet protection (c).

a

b

c
Common Storm Damage Risk-Reduction Treatments 5-23

Other energy dissipation measures include 

stilling basins or settling ponds, reinforced 

splash aprons, gabion baskets, dense 

vegetation, slash and limbs, logs, boulders, or 

bedrock. When using slash, press the material 

into good contact with the ground or mix with 

varying sizes of debris to provide a ground 

surface protection layer. 

A pipe should discharge beyond the toe of any 

fillslope. Extend the pipe 1.5 to 3 feet (0.5 to 

1.0 m) beyond the toe of the fillslope to prevent 

erosion of the fill material. In high fills, you 

might need to add a downdrain pipe or armored 

channel to safely convey the water to the toe 

of the fill. To minimize the outlet erosion, install 

a “T” fitting at the bottom on a long, steep 

downspout to divide the flow. Downdrain pipes 

are subject to high shear stresses and may be 

unreliable in areas of heavy snow accumulation. 

Freezing temperatures, particularly in shade 

areas or on north-facing slopes, also can cause 

small pipes to plug with ice.

When ditch-relief discharge cannot avoid 

unstable slopes, adjust the spacing so that no 

structure discharges an amount of water that 

will increase gully formation or the risk of mass 

wasting. These problems typically are avoided 

by field observation during maintenance 

activities or by local experience. In some cases, 

you may need to perform a hillslope stability 

analyses to determine the added risk. 

5.3 Stream Crossing Structure 

Protection and Improvements

Culvert and Channel Maintenance

Culvert maintenance and periodic cleaning of 

the pipe and channel near the inlet to the pipe 

are critical to the proper pipe function. Lack 

of maintenance contributes to many culvert 

failures. Ideally, pipes will receive maintenance 

before any major storm, though that can 

really only be guaranteed by performing 

maintenance routinely and after the last major 

storm. Common maintenance items include the 

following:

❏ Keep the inlet clear of wood, sediment 

buildup, rocks, and vegetation.

❏ Ensure that headwalls are in good 

condition. 

❏ Reline worn culvert barrels or replace the 

pipe. 

❏ Replace damaged or missing splash 

aprons or riprap.

❏ Bend back damaged metal blocking the 

entrance.

Culvert Diversion Prevention and 

Overflow Protection

Diversion prevention is one of the most costeffective and common SDRR treatments for 

culverts at risk of overtopping and washing 

down the road. The physical consequences 

of exceeding the capacity of a streamcrossing structure usually depends on the 

degree of exceedence, crossing fill volume, fill 

characteristics, soil characteristics, and the flow 

path of the overflowing stream discharge. When 

the structure capacity is exceeded, or if the 

culvert pipe plugs with debris, the stream backs 

up behind the fill. If the low point of the crossing 

is the ditchline, the water will divert down the 

road rather than flow directly over the road fill 

and back into the natural channel. At some 

point, the water will leave the road and erode 

the fill and the hillside all the way to a channel, 

as shown in figures 32b and c. 

If the low point is over the pipe, or an armored 

dip is constructed in the roadway near the 

structure, water will flow across the road and 

return quickly to the channel (figures 32a and 

33). Armoring the dip or low point is important 

to prevent erosion, downcutting, and additional 

damage to the road and fill. Figure 34 shows 

photos of a stream diversion (b) and the 

subsequent damage to the road (b), where 
5-24 Storm Damage Risk Reduction Guide for Low-Volume Roads

a

b

c

Figure 32—Existing undersized culvert fitted with an armored overflow dip to pass water without stream 

diversion or washing out the fill (a). Figures (b) and (c) show a stream diversion where a plugged culvert 

crossing sends water down the road rather than staying in its natural channel, causing considerable off-site 

damage. (Adapted from Furniss et al. 1997.)
Common Storm Damage Risk-Reduction Treatments 5-25

Figure 33—A stream diversion dip built into a stream crossing, showing an ideal location of the dip at the 

transition from the fill to the native ground. (Adapted from Weaver and Hagans 1994.)
5-26 Storm Damage Risk Reduction Guide for Low-Volume Roads

the road was basically washed out for several 

hundred feet. Diversion potential exists on 

roads that have a continuous climbing grade 

across the stream crossing or where the road 

slopes downward away from a stream crossing 

in at least one direction. 

Figure 34—A plugged pipe that “floated” out of the 

fill (a), resulting in a stream diversion down the road 

and past a culvert cross drain with metal drop inlet, 

(b), causing extensive damage to the road and 

significant environmental impacts, Plumas National 

Forest.

Crossings with diversion potential typically pose 

much greater overall risks than those without 

diversion potential because the resulting road 

and hillslope erosion delivers large volumes 

of sediment to the stream network and can 

destroy a section of the road, requiring 

extensive repairs. Figure 35 shows a plugged 

culvert location, allowing water to run down 

the road (top photo). On down the road, the 

creek eventually left the road, causing a major 

roadway and fill washout (bottom photo). In 

almost all cases, diversion will create more 

damage than stream flows that breach the 

fill but remain in the channel. Research in 

Redwood Creek, California, showed an average 

10 times increase in sediment delivery due 

to gullying and debris slides triggered by 

stream diversion as compared to a washout 

of the stream crossing fill (Weaver, personal 

communication).

Figure 35—Stream diversion caused by a plugged 

48-inch corrugated metal pipe that forced water 

down the road (a) and caused the roadway fill 

washout (b) when it exited the road, White Mountain 

National Forest. (Courtesy of Bob Gubernick.)

b

a

b

a

Plugged 

48-inch

culvert
Common Storm Damage Risk-Reduction Treatments 5-27

Figure 36 shows another stream diversion 

site where a culvert pipe plugged and sent the 

water down the ditchline, destroying the road. 

Here a cascading failure was created where 

excessive water in the ditchline caused a series 

of cross drains and culverts to plug and fail 

on down the road, compounding the problem 

at each new crossing. Stream diversion 

occasionally can be caused by accumulations 

of snow and ice in a channel or on the road that 

directs overflow out of the channel (Swanson, 

personal communication). Snow removal 

operations need to consider this potential effect 

and configure removed snow such that stream 

diversion will not occur (Furniss et al. 1997). 

Stream diversion also can be caused where a 

channel has severe aggradation, particularly on 

a fan deposit, where the channel periodically 

aggrades and shifts to a new location. Figure 

37 shows a stream diversion caused by stream 

aggradation at the location of a road-stream 

crossing. The site was aggravated additionally 

by the blocking of the small bridge with 

sediment and debris. 

Typically, the solution to prevent stream 

diversion is to add a dip near the culvert pipe, 

as seen in figures 32a and 33, and in figures 38 

and 39. The rolling dip can be placed directly 

over the pipe, but preferably it should be placed 

just downgrade of the drainage crossing at the 

transition point between the fill and the native 

ground to prevent stream diversion in the event 

that the drainage crossing culvert plugs (figure 

33). This minimizes erosion delivered to the 

stream in the event of overtopping and is easier 

to reconstruct. 

On relatively high-standard roads that have 

a risk of stream diversion yet travel speeds 

make a diversion dip or rolling dip undesirable, 

a designed failure point can be built into the 

road. This is a soft spot that will wash out if flow 

goes over it and is located where a dip would 

otherwise be located. This relatively soft failure 

point in the road subgrade can be constructed 

with fine gravel or sand rather than compacted 

soil (Gubernick, personal communication). 

With the dip or a designed failure point, water 

can be diverted back into the natural drainage 

before flowing down the road and causing 

road and additional resource damage. The 

cost of an overflow dip or designed failure 

point is relatively small compared to the cost 

of replacing the entire fill, or repairing major 

damage to the road.

Figure 36—Stream diversion caused by plugging 

of a 48-inch culvert (a) and subsequent cascading 

failure and road damage on down the road (b), 

region 6, Washington. (Courtesy of Robin Stoddard.)

a

b
5-28 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 37—Stream diversion down a road caused 

by stream aggradation that filled the channel and 

plugged a bridge. (Courtesy of Bob Gubernick.)

Figure 38—A concrete headwall (a) and an overflow 

dip placed just down the road (as seen at the low 

point where the person is standing down the road 

(b)) to prevent future stream diversion. This is the 

culvert repair and diversion-prevention dip installed 

at the damage site seen in figure 34.

a

Figure 39—Another storm damage diversion site 

with road damage down the road at the curve (a) 

and the diversion-prevention dip added up the road 

above the damaged area (b) just below the culvert 

crossing (not seen in photo) to prevent future stream 

diversion, Plumas National Forest.

For technical information on diversion potential, 

consult the “Water/Road Interaction Technology 

(WRIT) Series” publication “Diversion Potential 

at Road-Stream Crossings” (Furniss et al. 

1997). The publication discusses the physical 

effects of diversion potential and provides 

design considerations for remediation of 

existing crossings that have diversion potential. 

Link to the document <http://www.fs.fed.us/eng/

pubs/pdf/w-r/97771814.pdf>.

a

b

Low point 

in dip

b

New dip

Damage area 

down the road
Common Storm Damage Risk-Reduction Treatments 5-29

5.4 Bridge Protection and 

Improvements

Many changes can occur at a bridge site and 

in the watershed upstream of the bridge over 

time. Climate change and changes in the 

watershed can affect the peak flows and the 

amount of sediment transported through the 

stream or river system. More glacial melt may 

be expected in some regions. Thus, evaluate 

bridges carefully for increases in storm damage 

risks that were not anticipated at the time of 

construction. Understanding the processes 

at work in the watershed upstream and 

downstream of a bridge site is very important 

for determining the most cost-effective 

treatment.

Bridges are a large investment and, therefore, 

a large liability if they fail. Build good bridge 

design and bridge scour protection measures 

into the initial design, understanding that 

additional channel maintenance and cleaning 

often is required to maintain the bridge flow 

capacity and reduce the chance of blockage 

during a storm. 

Channel Maintenance and Debris/

Sediment Clearing

Channel debris and vegetation may cause 

scour or plugging problems for bridges. 

Common countermeasures for an existing 

bridge with debris problems include:

❏ Monitor debris buildup for prompt removal.

❏ Clear undesirable upstream debris and 

vegetation.

❏ Install debris catchers or deflectors (which 

require maintenance).

❏ Remove sediment or areas of aggradation 

that decrease channel capacity.

Channel clearing and maintenance are 

important to maintain bridge-flow capacity. 

The key thing to understand about this type 

of treatment is that the symptom is likely the 

result of natural stream and riparian processes, 

or altered processes due to the presence of 

the structure itself or an encroaching roadway 

or land use upstream. Most treatments are 

temporary and will require occasional clearing 

of debris and vegetation. For the best long-term 

solution, consider the replacement of the bridge 

with a longer, higher structure, or relocation of 

the structure to a less susceptible site.

Remove undesirable vegetation, brush, 

trees, and debris from the channel near the 

structure, particularly for the bankfull width of 

the channel. This work may be controversial 

since channel disturbance should be minimized, 

yet the structure flow capacity should be 

maximized and risk of blocking a bridge 

should be minimized. A balance is needed 

to remove counterproductive vegetation 

yet leave vegetation needed for channel 

stability and ecological benefits. Clearing and 

removing channel material can be damaging 

and difficult. Thus, decisions regarding the 

extent of channel clearing should be an 

interdisciplinary process. Figure 40 shows 

a channel encroached on by vegetation while 

figure 41 shows a channel relatively clear of 

vegetation and debris. Vegetation established in 

a stream-channel bottom is not normal unless 

there has been a change in the runoff pattern 

or volume. Try to understand the watershed 

condition, what has caused the change, 

and what ramifications the change has on 

streamflow at the structure site.
5-30 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 40—Stream channel under a bridge that 

needs clearing and tree removal if flows are being 

blocked, diverted, or if there is risk of plugging the 

bridge. 

Figure 41—A bridge opening and channel relatively 

free of vegetation and debris.

Aggradation in the channel also can reduce 

the cross-sectional area of a bridge and 

reduce flow capacity, as seen in figure 42. 

This tends to occur on a river bend where a 

point bar develops on the inside of the bend. 

Some channel deposits need to be removed 

periodically to maintain bridge flow capacity. 

This material typically is sandy gravel, so it may 

be useful as a fill material or surfacing material 

elsewhere. Note that some gravel and boulders 

should be left in the channel for fish spawning 

bed material. The timing of removal must 

consider the presence of fish in the channel. 

Coordinate and work with a fisheries biologist.

Figure 42—A bridge with significant channel 

aggradation on left from a growing point bar on 

the inside of a river bend that now needs removal 

to maintain bridge flow capacity. The point bar will 

rebuild with time, creating a continual maintenance 

problem at this bridge site. Bends are often poor 

locations for bridge sites. 

Figure 43 shows a damaged bridge that nearly 

failed due to minimum freeboard and its location 

on an alluvial fan. During a storm, tall trees fell 

into the drainage and formed a log jam at the 

bridge. This subsequently caused aggradation 

in the channel at the bridge (seen in the 

photo), thus losing hydraulic capacity through 

the bridge, and this forced a large amount 

of stormflow around the left abutment of the 

structure. This damaged the bridge approach 

and washed out a section of road.

“Steepland Forests: A Historical Perspective of 

Protection Forestry in New Zealand” (McKelver 

1995) offers some insights into stream channel 

aggradation problems, particularly when moving 

from a steep upland topography to a flatter 

alluvial plain.
Common Storm Damage Risk-Reduction Treatments 5-31

Figure 43—A bridge with minimum freeboard and 

located on an alluvial fan that nearly failed due to a 

log jam and subsequent aggradation in the channel. 

This forced some flow around the left abutment of 

the structure. (Courtesy of Bob Gubernick.)

5.5 Erosion Prevention and Erosion 

Control

Erosion Prevention and Drainage 

Improvements

Erosion prevention on roads, the entire road 

prism, and on disturbed areas is fundamental 

for the protection of water quality, particularly 

during storms. Two causes of erosion are the 

concentration of flowing water and lack of 

ground cover over the soil. Vegetative ground 

cover and good water infiltration are the primary 

long-term defenses against erosion. 

Implement erosion control measures 

immediately following construction and every 

time an area is disturbed. Implement the 

measures before the first winter period following 

construction or ground disturbance and before 

any major storm event. In areas of construction, 

ground cover is difficult to achieve, so sediment 

typically is trapped around the site. The area of 

disturbance also can be limited and areas can 

be rehabilitated progressively. 

Prevent waterflow concentration or armor/

stabilize eroding channels. Trap any sediment 

before it enters natural drainage channels, but 

give priority to treatments at the source of the 

erosion. Cover bare ground with some form of 

matting or mulch to reduce initial erosion and to 

promote growth of grass seed or other types of 

appropriate vegetation (ideally native) for longterm erosion control. Use rapid-growing annual 

grasses to provide quick ground cover; then 

replace with native vegetation over time.

As discussed in “Forest Roads: A Synthesis of 

Scientific Information” (Gucinski et al. 2001), 

surface erosion from road surfaces, shoulders, 

cuts, and fills is significant. Movement of 

sediment can occur during and after road 

construction, after road maintenance, during 

logging or mining activities, as the road is being 

used, if a road is closed but not stabilized, or 

from poor land management practices near the 

road. For instance, roughly half of the erosion 

from a logging operation comes from the 

associated roads and skid trails. Mass erosion 

rates from roads typically are one to several 

orders of magnitude higher than from other land 

uses. Much of that erosion occurs during storm 

events.

Erosion control is a two-step process; step one 

is to prevent short-term erosion from bare or 

exposed soil. Step two is control of long-term 

erosion through establishment of vegetation. 

Sometimes in steep or severe conditions, a 

structural solution, such as a retaining wall, 

ground armoring with rock or a gully plug, is 

required. The ideal erosion control solution 

promotes the germination and growth of plants 

and encourages the natural recruitment of 

the surrounding native plant community while 

it protects the soil from short-term erosion. 

Conserving native topsoil and respreading it 

over an area helps promote native plant growth. 

There are numerous treatments, combinations 

of treatments, and emerging products that may 

be suitable for a site. 

An erosion control plan is a very useful tool for 

any project to help describe the local conditions 

and problems at the site, evaluate possible 
5-32 Storm Damage Risk Reduction Guide for Low-Volume Roads

solutions, and determine the cost of the erosion 

and sediment control measures. Use this plan 

for construction projects, SDRR measures, and 

site repairs. Make an evaluation of the most 

effective physical, vegetative, or biotechnical 

treatments, or use of some combination of 

them. Plan short-term and long-term measures, 

as well as an evaluation of needs, such as 

fertilizers, irrigation, protection measures, and 

so forth. 

The “Erosion Control Treatment Selection 

Guide” by Todd Rivas (2006) describes 

erosion control principles, erosion types, and 

soil types. The guide also details erosion 

control treatments and proper treatment 

selection for use by engineers, soil scientists, 

and other resource specialists. Link to the 

document <http://www.fs.fed.us/eng/pubs/pdf/

hi_res/06771203hi.pdf>.

Another useful reference combining many 

erosion and sediment control practices in a 

forest road setting, including drainage control, 

vegetation, use of mulches, biotechnical 

measures, and various physical structures 

is presented in the publication “Erosion and 

Sediment Control Practices for Forest Roads 

and Stream Crossings” by Clayton Gillies with 

FP Innovations-FERIC Division in Canada 

(2007).

Where Erosion Occurs Along Roads

Most disturbed road areas, including the road 

surface, road fills, some road cuts, shoulders, 

and drainage ditches, are exposed to erosion 

at some time. Other associated areas, such as 

landings, skid roads, construction staging and 

storage areas, borrow pits and quarries, and 

other working areas can erode and produce 

sediment. Figure 44 shows the common types 

Figure 44—Common types of erosion along a road.
Common Storm Damage Risk-Reduction Treatments 5-33

of road erosion, including sheet and rill erosion 

on the cutbank (and fillslopes), ditch and roadsurface erosion, and gully erosion when water 

becomes concentrated. Figure 45 shows severe 

erosion in a road cut where a road passes 

through very erodible soil.

Figure 45—Severe erosion in a road cutslope in 

erodible soil. (Courtesy of Vincent Barandino.)

Most erosion control prevention practices 

fall into three basic types. They are physical 

methods, vegetative methods, and soil 

bioengineering or biotechnical methods. 

Common erosion control practices include: 

❏ Install drainage and sediment control 

structures.

❏ Disperse water/surface runoff.

❏ Armor surface and ground cover with 

netting, mulch, slash, rock, and so forth. 

❏ Prepare bed, mulching, and grass seeding.

❏ Establish vegetation for ground cover.

❏ Use simple soil bioengineering methods.

Figures 46 through 52 show a variety of these 

erosion and sediment control methods. Many 

erosion control treatments exist, along with an 

entire profession and industry to help achieve 

effective erosion control. Effective erosion 

control requires assessment of the situation, 

attention to detail, inspection and quality 

control during installation, and post-project 

maintenance. 

Physical Methods for Erosion Control

Drainage control is one of two key elements 

for physical erosion control (the other is 

ground cover). Prevent water concentration 

by dispersing the flow or controlling the water. 

Concentrated, fast flowing water has a large 

amount of energy and therefore a great ability 

to scour, erode, and form gullies. There are 

many physical erosion control measures to help 

disperse or control the flow of water, such as 

armored ditches, berms, check dams, or turfreinforcing mats. Section 5.2 discusses many 

other measures used to disperse and control 

surface drainage, such as rolling dips, road 

shaping, and use of ditch relief culverts and 

leadoffs.

Physical methods also include the wide range 

of materials used to provide protective ground 

cover, such as mats and netting, silt fences, 

turf-reinforcing mats, slash and mulches, 

bonded fiber matrix, rock and concrete, and 

so forth. Often these products are used in 

conjunction with drainage and vegetation.

Physical methods include a range of 

alternatives:

❏ Berms to control and direct waterflow or 

berm removal to disperse runoff.

❏ Walls, barriers, and sinks or sediment 

basins to trap sediment.

❏ Mulch and soft ground cover with straw, 

wood chips, slash, leaves, bark, shredded 

paper, mats, bonded-fiber matrices, and 

so forth to temporarily protect the ground 

surface against erosion.

❏ Hard armor/ground cover with rocks, riprap, 

articulated concrete blocks, geocells, 

gabions, and so forth for permanent ground 

cover.
5-34 Storm Damage Risk Reduction Guide for Low-Volume Roads

❏ Rock or aggregate on the roadway surface, 

particularly on steep road grades, erodible 

soils, areas of concentrated water flow, 

and hydrologically connected roads at 

approaches to stream crossings.

❏ Rolled erosion control products (erosion 

control and revegetation blankets) and 

mats to provide ground cover and promote 

vegetation. Mats and blankets need to 

have good contact with the soil and be 

pinned down in accordance with the 

manufacturers’ recommendations.

❏ Turf reinforcement mats to armor high-flow 

channels.

❏ Silt or sediment fences to trap sediment, 

particularly around work sites.

❏ Stabilizers and tackifiers to modify the 

soil surface to make it more resistant to 

erosion.

❏ Hydromulch and hydroseed.

❏ Modified soil surfaces (terracing, 

roughening, etc.) to control runoff and aid 

revegetation.

❏ Waterbars, rolling dips, and other crossdrain structures to disperse and divert 

water from the road surface and ditches.

❏ Check dams and rock armor used in 

ditches to reduce velocity and prevent 

downcutting and erosion.

Each method has advantages or 

disadvantages, installation details and 

requirements, and performance characteristics. 

Most should have good contact with the soil, 

and mats need to be closely pinned down; 

hard products need a compact soil surface 

and often a filter layer under them; silt fences 

need support, the appropriate geosynthetic, 

and some soil embedment, and so forth. Some 

treatments require more maintenance than 

others. Treatment selection should consider 

whether it will be maintained. The “Erosion 

Control Treatment Selection Guide” (Rivas 

2006) helps describe these methods and select 

the most appropriate or cost-effective solution. 

Figures 46 and 47 show some of these physical 

measures.

Figure 46—Common physical erosion control 

measures, including straw (a), netting, wood chips 

and straw wattles (b), to provide ground cover until 

vegetation can grow.

a

b
Common Storm Damage Risk-Reduction Treatments 5-35

Figure 47—Common physical erosion control measures.

c. Silt and Brush Fences

 to Trap Sediment

Bales keyed (buried)

 4 inches (100 mm)

 deep into soil.

Flow

Filter fabric 

silt fence

Note: Problems can develop from

water running between and under 

straw bales. Install them according to

 contractor’s or manufacturer’s specifi cations. 

Figure 13.2 PHYSICAL EROSION CONTROL MEASURES

Ground protected with 

mulch and brush

Ditch armored with 

rock, concrete, masonry 

or grass

30-50 cm 

Deep

1 foot (300mm) 

 wide 

 a. Control Water with Ditches and/or Berms. 

e. Brush Barrier (set brush into the soil).

d. Straw Bales

 

Compacted soil 

berm protected with brush

 and rock cover.

Leave no gaps 

between bales.

Staked and

entrenched

straw bale. (Use

two stakes

 per bale.)

b. Protect the Ground Surface with

 Mulch and Cover

Aggregate and rock armoring, leaves, 

straw, wood chips, or scarifi cation, seed and mulch for ground cover on 

barren soil or closed road surfaces.

Overland

Flow

(Runoff)

Tamped 

soil

1 to 2 feet

 (300-600 mm)

 deep
5-36 Storm Damage Risk Reduction Guide for Low-Volume Roads

Vegetative Methods for Erosion Control

Ground cover is the most critical site factor influencing erosion. Vegetation is the most desirable 

type of long-term ground cover in forest and range settings and provides these major benefits. 

Vegetation:

❏ Reduces raindrop impact via top growth and leaf litter.

❏ Reduces runoff velocity via increased roughness from growing plants and leaf litter.

❏ Provides structural integrity (reinforcement) of the soil via the root system.

❏ Filters chemical pollutants and sediments from runoff.

❏ Increases water infiltration into the soil.

❏ Increases percolation through the soil (the lateral movement of water in the soil).

❏ Increases evapotranspiration (the vertical movement of water to the air through plant tissues).

Vegetative methods use grasses, brush, and trees to offer ground cover, root strength, and soil 

protection with inexpensive and aesthetic natural vegetation. Live vegetative hedgerows on contour 

help trap sediment on a slope. Figures 48 and 49 show some of the considerations and types of 

vegetative erosion control used in conjunction with physical methods.

Figure 48—Common types of vegetative erosion control measures including grasses on a closed road (a) and 

deep-rooted shrubs and trees (as well as grass) on a cutslope (b).

Good soil preparation is key to the long-term success of vegetative treatments. The quality and 

fertility of the soil directly affects its productivity and ability to grow vegetation. Loosen compacted 

soils with scarification or subsoiling and add organic material. Sterile soils may need amendments to 

promote growth. Other chemicals or minerals in soil may retard growth and need mitigation. Native 

species do not typically require fertilizer since they are adapted to the nutrients available in the 

local soil.The erosion control plan should consider the soil condition and often include a chemical 

analysis of the soil.

a b
Common Storm Damage Risk-Reduction Treatments 5-37

Design and install vegetative treatments to 

provide immediate, short-term, and long-term 

protection. Often physical methods are used to 

protect seeds initially and promote long-term 

revegetation. Thus, a phased and/or mixed 

application of vegetation is needed, typically 

with a variety of plants. Ideally, select vegetation 

for good growth properties, hardiness, dense 

ground cover, and deep roots for slope 

stabilization. Local native species having these 

properties are preferable. Consider them first. 

When natives are not practical, select nonnative 

plants with noninvasive characteristics. Some 

shrubs, such as willows (Salix family), have 

been used extensively in the Western United 

States, particularly in wet sites, because of 

their strong, deep roots; adaptability; and ability 

to resprout. Refer to local forest botanists 

and native plant guides and policies before 

prescribing vegetative treatments.

Figure 49—Vegetative erosion control measures along a road (often mixed with some physical measures).
5-38 Storm Damage Risk Reduction Guide for Low-Volume Roads

Vegetation typically is the best, least expensive, 

and most aesthetic long-term treatment. 

Vegetation typically has the following 

characteristics:

❏ Plants need to be compatible with local 

climate.

❏ Vegetation needs water and sunlight.

❏ Local native species will likely do best.

❏ Vegetation is live and needs care; 

follow appropriate handling and planting 

procedures, and keep the materials moist 

during collection, transport, storage, and 

installation.

There are a number of considerations for the 

most effective use of seeds, cuttings, or 

potted transplants:

❏ Select seed for quality, resistance, and 

germination properties. Rates of seed 

application and specific mix for the location 

and time of year should be determined.

❏ Prepare, loosen, amend, and improve 

the seedbed, planting site, or individual 

planting holes as needed to promote 

growth. Add mulch and organic material to 

a site as necessary to retain soil moisture 

and improve the soil and microclimate. 

❏ Handle plants with care and do not 

allow them to dry out during storage, 

transportation, and planting.

❏ Prune broken branches and reduce the 

size of woody shrubs by one-third prior to 

planting. Prune trees but do not prune the 

leader (center main growing trunk).

❏ Protect plants from animals and disease; 

fertilize, if needed; and maintain conditions 

for good growth. 

Use native species whenever possible for 

the best growth and adaptation to the site. 

Nonnative species of annual grasses may 

be needed for the first few years to protect 

disturbed areas against surface erosion. For 

difficult sites, such as arid environments, test 

plots should be set up to determine what 

species and methods achieve the best results. 

Consider setting up onsite nurseries to harden 

and adapt plants to the local project area. In 

some cases, use completed projects as sources 

for live cut stock on a new project. Try to select 

native vegetation that does not require watering 

or fertilizers.

The U.S. Department of Transportation, 

FHWA publication “Roadside Revegetation: 

An Integrated Approach to Establishing Native 

Plants” (Steinferld et al. 2007) provides a 

thorough discussion of benefits and issues 

dealing with native vegetation, including 

project initiation, planning, implementation, and 

monitoring for roadside revegetation. Link to the 

Web site <http://www.wfl.fhwa.dot.gov/td/>.

Use of Deep-Rooted, Native 

Vegetation

Choose the type and source of vegetation 

carefully to accomplish the specific purpose. 

Project planning (the erosion control plan) 

should assess the problem and determine 

the effective solution. Information, such as 

location, aspect, climate and microclimate, soil 

type, fertility, time of planting, and subsequent 

land use, are critical factors in making the final 

design determination. Refer to the local forest 

botanist, soil scientist, or native plant specialists 

and policies before prescribing vegetative 

treatments. Check with local agricultural 

extension offices, the U.S. Department of 

Agriculture, Natural Resources Conservation 

Service, and State or local road departments. 
Common Storm Damage Risk-Reduction Treatments 5-39

The advantage of vegetation with deep root 

systems is that they are more resistant to 

drought conditions and the deep roots provide 

slope stabilization as well as ground cover. 

Many grasses provide excellent dense ground 

cover but have shallow roots that do little 

to deter shallow mass failures on saturated 

slopes.

Figure 50 shows two applications where 

vegetation with deep root systems help provide 

slope stability as well as ground cover. 

Figure 50—Deep root systems from pine trees (a) 

and willows (b) that provide stability to slopes and 

streambanks. (Photos courtesy of Donald Gray (a) 

and Robbin Sotir (b).)

Soil Bioengineering and Biotechnical 

Erosion Control Methods

Soil bioengineering and biotechnical treatments 

can be applied to any number of instances 

where erosion protection is needed. Depending 

on the site conditions and the values at risk, 

the application of these techniques can be 

relatively inexpensive or expensive. Section 6.2 

discusses soil bioengineering and biotechnical 

applications for streambank instability, 

and section 6.6 discusses uses for slope 

stabilization.

Soil bioengineering is a technology that uses 

integrated ecological principles to assess, 

design, construct, and maintain living vegetative 

systems to repair damage caused by erosion 

and slope failures (Sotir 2001). Biotechnical 

treatments combine the use of vegetation 

with other physical structures, such as 

vegetated gabions or vegetated reinforced soil 

slopes (Gray and Leiser 1982). Biotechnical 

stabilization is a specialized field, and 

consultation with experts and other guides is 

highly recommended. 

Some more common soil bioengineering and 

biotechnical treatments include:

❏ Live stakes (willow and others) embedded 

in the face of the slope. 

❏ Fascines/bundles/wattles; bundles of 

branches that are laid in a trench along 

contour lines that sprout and grow. 

❏ Brush layering laid onto terraces in 

the slope, covered with moist soil and 

compacted.

❏ Vegetated reinforced soil slopes (that 

is, geosynthetic reinforced soil slopes 

with brush placed on each lift with the 

reinforcement).

❏ Vegetated gabions and walls, such as 

retaining structures interplanted with live 

vegetation cuttings.

a

b
5-40 Storm Damage Risk Reduction Guide for Low-Volume Roads

❏ Live cribwalls, wooden cribwalls using 

material that will sprout or constructed with 

vegetation layers.

Figure 51 shows examples of biotechnical 

stabilization measures with live cribwalls and 

retaining structures. Figure 52 shows examples 

of common soil bioengineering and biotechnical 

measures used to control erosion and to 

stabilize cuts and fills and/or adjacent slopes. 

The U.S. Department of Agriculture, Natural 

Resources Conservation Service publication 

EFH 18, “Soil Bioengineering for Upland 

Slope Protection and Erosion Reduction” 

(NRCS 1992) describes many of the common 

bioengineering techniques. It is available at

<ftp://ftp-nhq.sc.egov.usda.gov/NHQ/pub/

outgoing/jbernard/CED-Directives/efh/EFHCh18.pdf>.

Also “Soil Bioengineering–An Alternative for 

Roadside Management: A Practical Guide” 

(Lewis 2000) provides field personnel with the 

basic merits of soil bioengineering concepts and 

gives examples of several techniques especially 

effective in stabilizing and revegetating upland 

roadsides. Link to the document <http://www.

fs.fed.us/eng/pubs/pdf/00771801.pdf>.

Figure 51—Biotechnical slope stabilization and 

erosion control methods, using a live crib wall (a) 

and gabions with live stakes (b). (Lower photo 

courtesy of John McCullah.)

a

b
Common Storm Damage Risk-Reduction Treatments 5-41

Figure 52—Examples of some common soil bioengineering and biotechnical erosion control measures.
5-42 Storm Damage Risk Reduction Guide for Low-Volume Roads

Gullies and Gully Prevention

Gullies are a severe form of surface soil 

erosion. Typically, they are caused by 

concentrated waterflow on erodible soils. 

Concentrated waterflow may begin as a minor 

sheetflow (micro-rills) that produce rills, and 

eventually results in major gully formation. 

Gullies can have major impacts on an area by 

taking land out of production and by lowering 

the ground-water table, as well as being a 

major source of sediment. They can be caused 

by concentrated water flowing off roads or 

they can impact roads by creating another 

drainage crossing or need for more frequent 

maintenance. If a gully can be prevented by 

diverting or dividing a concentrated flow of 

water, damage typically can be prevented. 

Gullies are particularly problematic and 

common in dry, arid climates and areas with 

deep, fine soil deposits. Rainfall is minimal, 

so vegetative cover is often sparse. When it 

rains, storms are often brief, high-intensity 

thundershowers that overwhelm the soilinfiltration capacity. Thus, gullies form and 

enlarge with each high-intensity event.

Once formed, gullies grow with time and will 

continue downcutting until resistant material 

is reached or the contributing flow is reduced 

or eliminated. They also expand laterally as 

they deepen. Gullies often form at the outlet of 

culverts or cross drains due to excessive and 

concentrated flows and relatively fast water 

velocities, as shown in figure 53. Occasionally 

gullies formed below cross drains have 

translated into debris slides that have caused 

extensive offsite damage. Also, gullies can 

form upslope of culvert pipes, especially in 

meadows, if the pipe is set below the meadow 

elevation. This causes a drop in the meadow or 

channel elevation and subsequent headward 

migration of the gully through the meadow. 

Gully prevention is important, but gully 

stabilization measures and structures are 

infrequently used for storm damage risk 

reduction. However, stabilization measures 

are very useful to correct ongoing gully 

erosion problems and prevent their growth and 

headward migration. Section 6.5 discusses 

commonly used gully stabilization structures.

Figure 53—Gully formation below a road where excessive water was discharged from a ditch relief crossdrain culvert.
Common Storm Damage Risk-Reduction Treatments 5-43

Gully Risk and Prevention

The ideal way to deal with gullies is to prevent 

them from happening. Investigate the source 

of the water and, if possible, remove the water 

source that is causing the gully. Gully formation 

at the outlet of cross-drain structures or road 

drainage points is a common problem and 

the subject of considerable research. Gully 

formation, or gully initiation along a road, is a 

function of the contributing area (road length) 

and the ground slope over which the water 

runs (hillslope gradient). These functions 

relate to the volume of water accumulated and 

the energy that water has to cause erosion. 

Resistance to gully formation also is a function 

of soil and vegetation characteristics where the 

water exits onto the slope; drain exit location is 

important. Thus, the spacing of road drainage 

features is particularly important in preventing 

gully formation. 

On steep slopes, there is potential for gully 

initiation after a couple hundred feet (50 

meters) if the pipe exits on an unprotected, 

compact hillslope. An outlet discharging on 

forest litter takes a longer distance to initiate a 

gully, and with an energy dissipator below the 

outlet, the initiation distance may be 1,000 to 

2,000 feet (several hundred meters) long. Also, 

gully initiation is likely on shorter segments of 

road that have an inslope versus an outslope 

because of the more concentrated flow at 

the outlets. Figure 54 presents curves for the 

critical road length and ground slope factors 

observed to be critical for gully initiation, 

either with an outlet energy dissipater, or with 

discharge into forest litter or compacted soil. 

These curves represent a 50-percent probability 

of gully initiation if conditions are exceeded. 

To prevent gully formation below cross-drain 

culverts, controlling the road length draining to 

the culvert effectively controls the contributing 

area and the amount and velocity of water; 

therefore, preventing gully initiation. This 

should be the basis for many culvert-spacing 

recommendations.

Figure 55 shows somewhat similar results for 

gully initiation as a function of contributing road 

length and hillslope for three different soil types 

with a low, medium, and high erodibility. The 

low soil-erodibility data was collected in volcanic 

soils in central Oregon, the medium erodibility 

data from granitic soils in the Idaho batholith, 

and the high erodibility data from young basalt 

and glacial deposits on the Olympic Peninsula, 

Washington. The critical Erosion Sensitivity 

Index is the parameter used to establish the 

three curves shown in figure 55 and define the 

limit between high incidence and low incidence 

(no formation) of gullies (Cissel et al. 2012a.) 

Experiences in the Pacific Northwest, California, 

and Australia have shown variability in the 

data for gully initiation. Thus, values shown 

in figures 54 and 55 may be conservative 

but are recommended for use, particularly if 

the drainage outlet is in a critical or sensitive 

area, such as above a steep slope or near a 

stream channel. Updated research and ongoing 

monitoring data can be found at the following 

Geomorphic Road Assessment and Inventory 

Package (GRAIP) Web site <http://www.fs.fed.

us/GRAIP/case_studies.shtml>.
5-44 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 54—Gully initiation risk as a function of road length, hillslope gradient, and outlet area conditions. 

(Adapted from Drew Coe 2006).
Common Storm Damage Risk-Reduction Treatments 5-45

Figure 55—Gully initiation data from the contributing road length as a function of hillslope for various soil 

erodibility categories. (Adapted from Cissel et al. 2012a.)
5-46 Storm Damage Risk Reduction Guide for Low-Volume Roads

5.6 Fillslope Stabilization Measures

A wide variety of slope stabilization measures 

are available to land managers, engineers, and 

resource specialists. Measures, such as use 

of vegetation and drainage, can be simple and 

relatively inexpensive, while measures, such 

as retaining structures, can be effective but 

expensive. One slope modification technique, 

removing loose, high-risk fill material, is a 

common storm damage risk reduction (SDRR) 

measure and is discussed below. Section 6.6 

discusses the many other less commonly used 

but effective SDRR techniques, such as walls, 

reinforced fills, slope modification, soil nails, 

deep patch reinforcement, and so forth.

A summary of many cost-effective and 

sustainable slope stabilization techniques is 

presented in the National Cooperative Highway 

Research Program synthesis 430 “CostEffective and Sustainable Slope Stabilization 

and Erosion Control Practices” (Fay et al. 

2012).

Sidecast Fill Pullback for Slope 

Stabilization

Sidecast fill material, or loose material placed 

on top of native soil on a slope, can absorb 

water and fail on the plane at the base of the 

sidecast material. Old logs or stumps used to 

support the fill will decay with time, leading 

to fillslope failures. These old sliver fills are 

common failures on older sidecast road fills 

on steep slopes where the fill material was not 

compacted. Newer roads do not use as much 

sidecast construction; excess material is end 

hauled to another flatter area or to a disposal 

site. However, there may still be fillslopes 

that exhibit instability and warrant treatment 

because of an excessively steep slope, rotting 

logs in the fill, or local groundwater conditions, 

as seen in figure 56. Also the toe of a fillslope 

can be undercut by a stream if the road is 

located too close to the stream.

Figure 56—Road fill failure in a sliver fill that was 

partially supported by old logs. The failure triggered 

a downslope debris slide, Olympic National Forest. 

(Courtesy of Bill Shelmerdine.)

In many cases, if road surface water is being 

properly managed and drained, sidecast fills 

may not pose a stability problem. If there 

is subsurface water that moves below the 

roadway and saturates fill material, the risk of fill 

failure remains high. Also, sidecast fill material 

in concave swales and drainages may fail under 

saturating rainfall conditions, independent 

of roadway drainage. The terrain tends to 

concentrate groundwater into this landform.

On steep, natural slopes (typically steeper than 

50 to 60 percent), a road fill failure can trigger 

a debris slide downslope of the road fill. These 

debris slides can travel for long distances, 

increase the volume of material involved 

in the slide, and can damage the hillslope 

itself, particularly if there is infrastructure or 

watershed values (such as a stream or wetland) 

at the bottom of the slope. Debris flow hazards 

have presented problems worldwide and have 

been common on forest roads, particularly in 

the West and Northwest (figures 57 and 58). 

Many of the fill failures that cause such an 

event are both identifiable and preventable. 

Small scarps and curved cracks in the road 

surface, particularly in the outside half of the 
Common Storm Damage Risk-Reduction Treatments 5-47

road, are indicators of fill settlement or incipient 

failure. One can map the potential flow path and 

assess the risks. 

Figure 57—Debris slides, often triggered during 

a storm event from failing fills, that started at the 

upper road and ultimately impacted the lower road. 

(Courtesy of Jim Doyle.)

Treatments may include periodic maintenance 

of the site to seal the cracks against water 

intrusion; ditch the road or use measures to 

direct water away from the fillslope; repair the 

site using a deep patch (or other appropriate 

measure for the site); or remove the failing fill 

material. Collins (2007) discusses detection 

and mitigation measures in “Debris Flows 

Caused by Failure of Fillslopes: Early Detection, 

Warning, and Loss Prevention.” The U.S. 

Geological Survey publication “The Landslide 

Handbook” (Highland and Babrowsky 2008) 

also describes many useful debris slide 

mitigation measures.

Sidecast fill pullback or removal of high-risk 

fill material is one positive method to reduce 

the risk of failure. However, just pulling back 

sidecast fill (and end hauling it to a stable 

disposal site) has the drawback of narrowing 

the roadway. In some cases a roadway ditch 

can be eliminated to gain road width. If the road 

standard is changing from a maintenance level 

3 to maintenance level 2, or a maintenance 

level 2 to maintenance level 1, narrowing the 

road may not be a problem. 

For higher standard roads, and heavily used 

roads, road width may need to be retained. In 

those cases additional cutting into the hillside 

and/or changing the road grade or alignment 

may be required. On severe or very steep 

slopes, especially with subsurface water, 

retaining structures with drains may be needed 

to provide long-term stability.

Figure 58—Debris slides that subsequently formed 

gullies below the road in Colorado (a) and on the 

Blue Ridge Parkway, Pisgah National Forest, North 

Carolina (b). (Lower photo courtesy of Tom Collins.)

a

b
5-48 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 59 shows the process of pulling back loose fill material to stabilize a road shoulder. Either the 

road surface will be narrowed or a cutslope excavation will be needed to compensate for the loss of 

road width.

Figure 59—A fill failure repaired by pulling back or removing loose, sidecast shoulder and fill material and 

shifting into the hillside, or possibly narrowing the road.

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CHAPTER SIX 

LESS COMMON OR HIGHER COST STORM

DAMAGE RISK REDUCTION (SDRR) 6TREATMENTS
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-1

6. Less Common or Higher 

Cost Storm Damage Risk 

Reduction (SDRR) Treatments

6.1 Less Used Road Surface Drainage 

Treatments

Converting Inslope Roads to Outslope

Conversion of roads from an inslope template, 

typically with an inside ditch, to an outslope 

road has many long-term advantages, 

particularly if a road will receive minimal 

maintenance. It is a treatment that is discussed 

often to reduce the risk of storm damage. 

However, it also is an expensive treatment and 

one that involves a considerable amount of 

labor and earthwork. Thus, the reality is that 

despite its desirability it is not commonly used. 

Advantages of an outslope road are that the 

roadway template is as narrow as possible (not 

requiring a ditch); there is a relatively small cut 

and fill; slumping in the cutslope is less of a 

problem, construction is least expensive; there 

is less maintenance without a ditch and crossdrain system; and the dispersed flow avoids 

water concentration and quickly gets the water 

off the road. Since ruts and berms concentrate 

water on any road surface, an outslope road 

functions best when built in conjunction with 

rolling dips. Thus, an outslope road can be the 

most desirable roadway template to use if it 

suits the local conditions. On most roads some 

combination of inslope and outslope exists to 

accommodate the terrain, drainage needs, and 

traffic safety. To function properly, ensure that 

maintenance on an outslope road does not 

build up a berm along the outside edge of the 

road.

Conditions that are not conducive to outsloping 

include where interception of water from the 

cutslope occurs, or where a slippery, icy, or 

erodible road surface and fillslope is a concern. 

Clay-rich soils, some silts and volcanic ash 

soils, or polished limestone rock can be quite 

slippery when wet. Intercepted water in the 

cutbank may cause erosion or instability 

downslope. Drivers can feel unsafe and fear 

that they will slide off the mountain, particularly 

on outsloped curves and in steep terrain. In 

steep terrain and on steep grades where the 

road surface may be slippery or have snow and 

ice, it is safer to use an inslope road template. 

Better to slide into the ditch than off the hillside. 

In some cases, use an inside ditch along with 

an outslope road template where there are 

seeps or springs in the cutbank.

Advantages of an inslope road are that water 

is better controlled; it moves into a ditch, and 

then the ditch can discharge onto a stable 

nonerodible location. Also, it can be safer and 

prevent a vehicle from sliding off the road. 

The disadvantages of an inslope road and a 

ditch are the need for additional road width, 

more excavation with a higher cut and more 

fill material, concentrated flow in the ditch, 

and the need for ditch relief cross drains or 

leadoff ditches. Cross-drain culverts commonly 

plug during large storms, leading to significant 

damage to the road. 

Figure 60 shows the form and relative 

dimensions of an inslope versus an outslope 

road. Conversion of an inslope road to outslope 

requires removing some of the fill and road 

shoulder material, and typically filling the ditch, 

as seen in the lower figure. New material 

may need to be imported to fill and raise the 

roadbed at the inside ditchline. This is desirable 

from a drainage standpoint, but requires a 

considerable amount of work on the road. Some 

drainage slope conversion work has been 

as expensive as the initial road construction. 

Figure 61 shows an outslope road accentuated 

on an outside bend in the road.

The road cross slope with an inslope or 

outslope road should be accentuated or 

constructed to its maximum recommended 

slope for a road that will be closed or that will be 

maintained infrequently. The road cross slope 
6-2 Storm Damage Risk Reduction Guide for Low-Volume Roads

is typically 2 to 6 percent, and a 4- to 6-percent 

range is recommended where maintenance 

may be infrequent. Under slippery conditions 

and log haul traffic, a 2- to 3-percent maximum 

outslope may be desirable. Traffic and time tend 

to wear down or smooth out a cross slope and 

flatten the road.

The importance of regular and proper 

maintenance cannot be stressed enough for 

the proper function of road surface drainage 

features, especially on native material and 

gravel roads. Traffic will eventually create ruts 

or surface depressions on the road surface. 

These ruts or gravel berms compromise the 

inslope and outslope of the surface and may 

concentrate water for long distances down 

(grade) the road. Eventually the concentrated 

water breaks through and often causes 

additional erosion problems where it leaves the 

roadway. Frequent use of rolling grade dips can 

mitigate some of the problems with rutting and 

infrequent maintenance. Over the full length of 

a road, insloping and outsloping combined with 

rolling grade dips may be the best drainage 

solution. More and more roads are being built 

this way or retrofitted with these drainage 

features.

A ditch and additional 

excavation are needed 

with the inslope road

Figure 60—Examples of an inslope versus outslope road template and the work involved in converting an 

inslope road to an outslope configuration (lower figure adapted from Mendocino County, CA).
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-3

Figure 61—An outslope road template on a curve 

in the road. This photo also shows the tendency 

of a center and outside berm to form on gravel 

roads. The presence of these berms disrupts the 

dispersion of water and can cause significant 

water concentration if not eliminated by routine 

maintenance or dispersed with use of rolling dips.

Surface Water Diversion Structures

A variety of structures have been used to 

remove water off the road surface. Each can 

work in particular circumstances, and most 

are maintenance intensive because of their 

limited capacity for water and to accommodate 

sediment. Also, use of these other structures 

typically does make routine maintenance of the 

road more difficult. Rolling dips appear to be 

the most effective surface drainage measure for 

low-volume roads. 

However, some successful use has been made 

of the following measures, particularly if the 

road has a smooth profile and a rolling dip is 

not wanted or appropriate. Other alternatives 

include:

q Open-top culverts.

q Rubber deflectors. 

q W-beam guardrail channels.

q Small concrete canals.

Publications “Introduction to Surface Cross 

Drains” (Copstead et al. 1998) and “Cross 

Drain Update” (Gonzales 1998) document 

techniques and materials used to divert 

water off the road surface. They are available 

respectively at <http://www.fs.fed.us/eng/

pubs/pdf/w-r/98771806.pdf> or <http://www.

fs.fed.us/eng/pubs/pdf/w-r/98771804.pdf>. The 

National Technology and Development Center 

publication "Drain Dips, Waterbars, Diverters, 

and Open-top Culverts—A Guide for Surface 

Water Drainage of Forest Roads" (Russell et 

al., in preparation) provides a summary of water 

diversion structure options, their design, and 

construction/maintenance details.

Open-Top Culverts. Open-top culverts are the 

most common water diversion structure after 

rolling dips. They may be made from wood, 

logs, metal, small culverts, or concrete. The 

open top is wide enough to have surface water 

drop into the culvert and run off the road, yet 

narrow enough to allow vehicle tires to roll 

over it. Figure 62 shows a drawing of common 

wooden open top culverts. Opening width 

commonly is 3 to 4 inches (75 to 100 mm). 

Alternatively, some relatively wide open-top 

culverts have metal grates at the road surface 

to allow tires to drive over them smoothly, as 

seen in figure 63. Typically, they are placed 

at a skew, ranging from 0 to 30 degrees 

perpendicular to the road direction to divert 

water off the road surface, and are placed on 

road segments with a slight to moderate road 

grade (2 to 12 percent). 

The greatest disadvantage of open-top culverts 

is that often they can fill with sediment and 

not work, particularly on native soil or gravel 

surfaced roads, as seen in figure 64 (a). 

They do function well on hard-surfaced roads 

and under circumstances where they can be 

cleaned frequently.
6-4 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 62—Drawing of typical open-top wooden box culverts using lumber or small logs. 
Figure 64—A 

narrow wooden 

open-top culvert 

that is plugged

with sediment (a), and an open-top culvert that has 

been recently cleaned on a gravel road (b).

Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-5

Figure 63—A relatively wide concrete open-top 

culvert with a metal grate. 

Rubber Deflectors. Deflectors, typically made 

of rubber strips or old conveyor belt materials, 

are buried in the roadway surface and stick 

up 2 to 4 inches (50 to 100 mm) above the 

road surface to deflect flowing water off the 

surface, as seen in figure 65. Tires from traffic 

bend down the deflector when driven over. 

Typically, they are installed at a skew to the 

road. They have been used effectively on 

relatively steep road grades (greater than 12 

percent) where rolling dips are inappropriate, 

and on unsurfaced and paved roads. The main 

disadvantage of the deflectors is that they have 

a relatively short lifespan on roads with traffic. 

Figure 65—Rubber deflectors used on a rocky native 

soil road (a) and a paved road surface (b).

W-Beam Guardrail and Small Concrete 

Channels. Place small channels, or canals, 

into the road surface to capture and deflect or 

direct water off the road surface. Use W-beam 

materials typically for guard rails. Other 

a

a

b

b
6-6 Storm Damage Risk Reduction Guide for Low-Volume Roads

channels can be made of concrete or masonry, 

as seen in figure 66. Typically, each is set into 

the road surface at road grade and on a skew 

to deliver the water off the road. 

The challenge with open-channel deflectors is 

that they have a very limited capacity to move 

much water. A large channel with moderate 

capacity becomes difficult to drive over. Like 

open-top culverts, these channels can fill with 

sediment and add challenges to maintenance. 

Figure 66—Small concrete channels placed across a 

native surfaced road for surface drainage. (Courtesy 

of Simon Done, TRL.)

Culvert Cross-Drain Inlet Protection 

and Drop Inlets

Culvert inlet control structures (or drop inlets) 

occasionally are placed in the inside ditchline at 

the location of a culvert cross drain. Construct 

drop inlets with concrete or masonry boxes 

or from round metal or concrete pipe. They 

need to be consistent with the size of the ditch 

and pass the accumulated flow in the ditch. 

They are an alternative to a typical excavated 

culvert inlet basin (catch basin) and typically are 

used where the ditch carries large amounts of 

sediment or is eroding and downcutting. Figure 

67 shows several drop inlet types. Sometimes 

a window is cut in the riser pipe and set at the 

desired ditch bottom elevation, as shown in 

figures 67 and 68. Thus, flow has to go over this 

edge at this elevation, preventing further ditch 

downcutting. 

Figure 67—A variety of culvert cross-drain drop inlet 

structure types used on forest roads.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-7

Make culvert inlet structures large enough 

to prevent debris accumulation and easy to 

clean and maintain. Inlet structures are useful 

to change the direction of water flowing in 

the ditch into a cross-drain pipe, particularly 

on steep grades. Also inlet structures are 

particularly useful to stabilize the ditch elevation 

at the level of the inlet window before entering 

the culvert. They also can be useful when the 

cutbank is steep, unstable, or has a high rate of 

ravel to buttress and help stabilize the cutslope, 

thus preventing sediment from entering the 

culvert inlet. Additionally, concrete and masonry 

box structures often have a bottom set below 

the cross-drain pipe elevation so that this area 

or reservoir serves as a trap for sediment (sand 

trap). Clean out trapped sediment periodically, 

particularly if there is a basket arrangement to 

facilitate sediment removal.

Figure 68—Drop inlet design and installation details for concrete box or corrugated metal riser pipe with builtin sand traps.

Energy 

Dissipation 

with Riprap, 

Concrete 

Apron or 

Splash pool 

with water

Design and Installation Detail

Roadbed surface

Use drop inlet structure to control the level of water, 

turn water into the pipe, and prevent downcutting and erosion of the ditch.

General Information

Grass for 

Erosion 

Control

Slope

Culvert Pipe 12-24 inch 

(300-600 mm) diameter

Sand trap

 2-4 feet

(0.6-1.2 m)

Roadbed Surface

Drop inlet 

structure

Bottom of ditch

 3 ft.

 (1 m)

Masonry Metal

Window

 18 in. 

(450 mm)

 12 in.

(300 mm)

 36 inch 

(900 mm)

 12 in.

(300 mm)

 Slope

 3 ft.

 (1 m)

 18 in. 

(450 mm)
6-8 Storm Damage Risk Reduction Guide for Low-Volume Roads

6.2 Stream Crossing Structure 

Protection and Improvements

Natural drainage crossing structures, including 

culverts, fords, or low-water crossings and 

bridges require hydrologic and hydraulic 

design to determine the proper type and size 

of structure. Also, it is important to identify and 

understand hillslope processes and landform 

features since many structures have failed 

during storm events from landslides and debris 

slides that initiate outside the stream channel 

and overwhelm the structure with sediment and 

debris. Fluvial geomorphology processes also 

are important to appreciate the dynamic nature 

of stream and river systems, and their ability 

to shift location with time. It is important to 

recognize the characteristics of the watershed 

and thus the type of material that may be 

moving through the river system. Evaluate 

structures considering their setting, changes 

to the watershed over time, and the types of 

material expected in the channel. 

Culverts most often fail due to plugging 

with debris and sediment, and occasionally 

fail because of lack of capacity. Bridges often 

fail because of scour under foundations or 

the abutments. Low-water crossings have 

failed due to scour, channel erosion, or end 

runs around the structure. Impacts from 

failures include degraded water quality, bank 

erosion, channel scour, and impacts to aquatic 

organisms. Traffic delays and costly repairs 

often are associated with these failures. 

The likelihood of a culvert failure can be 

minimized in several ways. A culvert should 

be properly sized consistent with the width 

of the natural stream channel (ideally with a 

culvert width matching at least the channel 

bankfull cross-section width), aligned with the 

upstream channel, and have an efficient inlet 

to have the least chance of being plugged with 

debris. Debris comes in two types: organic and 

inorganic. It is important to know the dominant 

type for the site. Some sites are subject to high 

levels of both organic and inorganic debris.

Organic woody debris is the most common type 

in forested environments, resulting from tree 

breakage, blowdown, and logging. Even small 

limbs and branches can lodge across a culvert 

inlet and trap other organic and inorganic 

material, reducing culvert capacity or causing 

complete blocking of the culvert entrance.

Streams with highly mobile beds or steep 

channels subject to debris flows may deliver 

huge volumes of gravel, cobbles, and even 

very large boulders to a stream crossing, 

burying the culvert inlet (figure 7a). The best 

solution for these sites may be a structure that 

also will pass the debris over the road rather 

than through the culvert. Most culverts that 

are properly designed will work fine. However, 

debris flows and torrents can plug even 

very large pipes, so in these circumstances, 

some overflow protection is recommended 

or a vented ford may be a better design. The 

structure must include provisions to prevent 

stream capture or stream diversion. 

Culvert failure risk might be managed with 

appropriate flow and debris capacity for the 

culvert size, proper installation and alignment, 

lowering the height of the fill over the pipe, 

providing a spillway for overtopping, the 

addition of trash racks, or additional flood 

capacity through other structures in the 

floodway. Figure 69 summarizes a number of 

problematic installation details for a culvert and 

solutions to minimize the likelihood of plugging. 

Plugging commonly occurs because of the 

following factors:

q Figure 69a. A high headwater-to-pipe 

diameter (Hw/D) ratio that promotes 

accumulation of debris above the culvert 

inlet.

q Figure 69b. An atypically wide culvert 

inlet excavation that promotes debris 

accumulation.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-9

q Figure 69c. A flattened culvert gradient that promotes accumulation of sediment.

q Figure 69d. A sharp change in alignment that allows floating debris to lodge across the pipe 

inlet.

q Figure 69e. Large rocks lodging against the inlet lip of a culvert. A flared end section can help 

minimize this problem.

Figure 69—Common plugging hazard problems found with culverts (Furniss et al. 1998).
6-10 Storm Damage Risk Reduction Guide for Low-Volume Roads

Flow velocity typically accelerates in a culvert 

pipe, so the pipe outlet area commonly is 

subject to scour and may need armoring or 

other scour protection. Armoring, such as riprap 

is common, cutoff walls may be used, or a 

stable energy-dissipation pool can be designed 

at the pipe outlet. 

Two publications that provide insight into culvert 

failures during storms and provide information 

to reduce the likelihood of culvert failures are 

“Response of Road-Stream Crossings to Large 

Flood Events in Washington, Oregon, and 

Northern California” (Furniss et al. 1998), and 

“Water/Road Interaction: Examples of Three 

Flood Assessment Sites in Western Oregon” 

(Copstead and Johansen 1998). They are 

available at <http://www.fs.fed.us/eng/pubs/

pdf/w-r/98771807.pdf> or <http://www.fs.fed.us/

eng/pubs/pdf/w-r/98771805.pdf>.

A California Department of Forestry and Fire 

Protection document that discusses a number 

of treatments to help design and prevent pipe 

failures during major storms is “Designing 

Watercourse Crossings for Passage of 100-

Year Flood Flows, Wood, and Sediment” 

(Cafferata et al. 2004). It is available at

<http://www.fire.ca.gov/resourcemanagement/

PDF/100yr32links.pdf>

One large pipe is almost always better than 

multiple small pipes. Not only is a larger pipe 

more efficient hydraulically, a single, relatively 

large pipe is much less susceptible to plugging 

compared to multiple smaller pipes. The fill area 

between multiple pipes acts as a trash rack 

to catch debris and plug the pipes. However, 

multiple pipes often are used to minimize the 

height of the structure (for vertical alignment 

considerations), to drain a flood plain, and in 

emergency situations, small pipes often are 

readily available. Avoid multiple pipes if aquatic 

organism passage is an issue because they can 

create a barrier for passage.

Pipes often are designed with an Hw/D 

(headwater depth to pipe diameter) ratio of 

1.0 to 1.5, allowing for some buildup of water 

in front of the pipe to maximize pipe capacity. 

However, in a mountainous environment, such 

a practice contributes to plugging potential with 

debris that can accumulate at the mouth of the 

culvert. To minimize plugging potential, an Hw/D 

ratio of around 0.8 often is used. In watersheds 

with considerable woody debris that must pass 

through culverts, Hw/D ratios in the range of 0.5 

to 0.67 are recommended (Furniss et al. 1998).

Improve pipe capacity somewhat with the 

addition of a concrete headwall, smooth liners, 

or ensuring that the inlet is not damaged. If a 

pipe might plug and there is diversion potential, 

construct an armored dip over or near (just 

down gradient from) the culvert to accommodate 

overtopping without washing out the structure or 

diverting water down the road, thus preventing 

additional damage (section 5.3).

If a stream channel contains considerable debris 

and there is a history of or potential for culvert 

plugging, add a trash rack in the channel or onto 

the culvert to trap debris before it plugs the pipe. 

However, trash racks are one more item that 

requires periodic cleaning and maintenance. A 

discussion of trash racks follows. 

To function properly with less risk of failure, 

install culverts properly and align with the 

drainage. If the inlet is not aligned with the 

channel, a directional change of flow must 

occur. This may result in deposition on one 

side and erosion from an eddy on the other 

side. Debris is less likely to be directed into the 

structure, leading to plugging. Misaligned pipes 

will erode the hillslope on the downstream end 

and possibly destabilize the slope. Culverts can 

create a deep plunge pool at the outlet after the 

channel has adjusted to the different alignment. 

Remove and reinstall poorly installed pipes if 

possible. 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-11

Accelerated flows out of culverts often cause 

embankment and stream channel scour. To 

prevent this type of damage, add armoring 

with riprap, gabions, biotechnical measures, or 

other solutions around pipes as a preventative 

measure or for outlet stabilization. Other energy 

dissipation measures also may be used. Some 

culverts may need beaver protection to prevent 

beaver dams from plugging the structure.

Where flow fluctuates dramatically or where a 

channel is subject to debris slides and torrents, 

culverts may have a history of plugging. 

In these instances, a lifetime cost analysis 

may indicate that fords or vented low-water 

crossings are a better alternative than culverts. 

This conversion requires a moderate amount of 

work, but low-water crossings have advantages 

over culverts in certain settings and are less 

likely to fail during major storms.

For all natural stream crossings, the need for 

fish and other aquatic organism passage must 

be determined. If passage is needed, there 

are tools to assess the structural requirements 

necessary to retrofit or replace the existing 

structure. In most cases, replacing the existing 

structure, if it is a barrier, is more desirable. 

For techniques for determining the barrier 

effect of a structure and designing aquatic 

organism-friendly stream crossing structures, 

see the section on aquatic organism passage 

considerations and the “Stream Simulation: An 

Ecological Approach to Providing Passage for 

Aquatic Organisms at Road-Stream Crossings” 

publication (Forest Service Stream Simulation 

Working Group 2008).

Increasing Culvert Capacity

Regardless of the culvert type used, check the 

high-flow capacity of the culvert. This ensures 

the survival of the culvert and road fill during 

extreme storm flow events. Most new culverts 

are designed to pass at least a 50- to 100-year 

storm event (Q50-Q100). However, many older 

pipes were designed to pass only 20- to 25-year 

events, or less. Also, design flows may increase 

due to changes in the watershed, a recent 

fire, global climate change with increased 

snowmelt, and so forth. Road-fill stability, 

road overtopping, allowable headwater depth, 

the likelihood of debris plugging the culvert, 

backwater effects, or a combination of these 

factors may determine the culvert high-flow 

capacity. In some instances, culvert capacity is 

dictated by forest plans or other Federal, State, 

or local regulations or documents. The “Record 

of Decision for Amendments to Forest Service 

and Bureau of Land Management Planning 

Documents within the Range of the Northern 

Spotted Owl” (U.S. Department of Agriculture, 

Forest Service; U.S. Department of the Interior, 

Bureau of Land Management 1994) amended 

many forest plans with a standard of culverts 

passing the 100-year stormflow plus debris.

Whether a pipe or structure is being designed 

for a 25-, 50-, or 100-year storm event (Q25, 

Q50, Q100), you must determine that design flow. 

Fortunately, many methods exist to determine 

streamflows, and in particular peak-design 

stormflows, using methods such as the U.S. 

Department of the Interior or U.S. Geological 

Survey (USGS) regression equations, various 

agency or USGS hydrology programs (Ries 

2007), local stream gauging data, or the 

Rational Method for small watersheds. 

The US Geological Survey has published state 

reports with regression equations for every 

state in the United States. See the fact sheet 

describing the National Streamflow Statistics 

(NSS) program “The National Streamflow 

Information Program: Estimating High and Low 

Streamflow Statistics for Ungaged Sites: U.S. 

Geological Survey Fact Sheet 2007-3010” 

(Turnipseed and Ries 2007). Updated State 

information is available at the USGS Web 

site <http://water.usgs.gov/software/NSS/>. 

Additionally the US Geological Survey Program 
6-12 Storm Damage Risk Reduction Guide for Low-Volume Roads

“StreamStats” is a Web-based Geographic 

Information System (GIS) that can be very 

useful since it allows the user to choose a 

location in most States of the United States 

and determine hydrologic data for that site. 

StreamStats data is available at <http://water.

usgs.gov/osw/streamstats/>.

A summary of many of the useful hydrologic 

methods can be found in “Highway Hydrology,” 

FHWA HDS-2 (McCuen et al. 2002), the 

“Highway Drainage Guidelines” (AASHTO 

2007a), or the California Department of Forestry 

publication “Designing Watercourse Crossings 

for Passage of 100-Year Flood Flows, Wood, 

and Sediment” (Cafferata et al. 2004).

For culverts in small watersheds and for 

ditch relief cross-drain culverts, use of the 

Rational Method may be most appropriate. 

The Rational Method relies on rainfall intensity 

data (as well as the watershed area and runoff 

characteristics of the terrain). It can be a very 

useful method to estimate current watershed 

discharge when the characteristics of the 

watershed have changed since the original 

design of a culvert, such as after a fire or for 

some development in the watershed. These 

watershed changes affect the value of the 

Runoff Coefficient. Rainfall intensity can be 

quite localized, but information on rainfall 

intensities is generally available in all areas 

of the United States. One of the best sources 

of Rainfall Intensity-Duration-Frequency data 

nationwide is from NOAA, the National Oceanic 

and Atmospheric Administration <http://hdsc.

nws.noaa.gov/hdsc/pfds/>.

Compare different hydrologic methods since 

each method has advantages and limitations, 

such as period of record, consideration of 

climate change, and changes in watershed 

characteristics. Note that most hydrologic 

methods do not consider debris loading in a 

channel, so considerable judgment and local 

experience is needed to properly assess 

plugging potential from vegetation and sediment 

(fine sediment, boulders, and rocky debris).

Determine the design flow and check the 

existing pipe capacity, using methods, such as 

the U.S. Department of Transportation, FHWA 

pipe capacity nomographs found in “Hydraulic 

Design of Highway Culverts” (Normann et 

al. 2005). If pipe capacity is found to be 

inadequate, then assess the degree of risk of a 

failure. 

The most obvious way to reduce the risk is 

to increase culvert capacity by increasing the 

size of the pipe or adding additional pipes in 

some circumstances. Where additional pipes 

are added, maintain the primary pipe at bed 

level for proper function (and aquatic organism 

passage), while additional pipes can be set 

higher at a flood-plain level to accommodate 

floodflows. 

However, since adding pipes is expensive and 

typically requires excavation, alternatives exist 

that slightly increase the capacity of existing 

pipes. Increase culvert capacity somewhat by:

q Improving the pipe inlet efficiency with 

concrete headwalls, beveled inlets, or 

metal end-sections rather than a projecting 

pipe.

q Reducing the pipe friction by installing a 

smooth liner, such as epoxy or a plastic 

liner (that may be detrimental to aquatic 

organism passage).

q Removing any obstacles or damaged 

portion of the pipe inlet.

q Increasing the headwater elevation at the 

pipe entrance (Note that this does increase 

capacity, but is not recommended because 

it also promotes the pipe plugging with 

debris).
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-13

Headwalls and metal end sections (figure 70) 

added to pipes have the SDRR benefits of 

slightly increasing the capacity and improving 

resistance to plugging or overtopping failure. 

Figure 70 (top) shows a culvert with a concrete 

headwall that was overtopped but did not fail.

Figure 70—Culverts with a concrete headwall (a) 

and a metal end section (b) that offer improved 

resistance to plugging and damage from 

overtopping.

Increasing the headwater depth (the possible 

water height above the culvert before 

overtopping the road) at the upstream entrance 

to the pipe can increase pipe capacity. This 

could require raising a road fill. However, 

it should be understood that this treatment 

increases the potential damage should the 

culvert crossing fail (due to a higher fill) and 

also increases the likelihood of the pipe 

plugging from debris that can accumulate 

around the pipe inlet. Some increased capacity 

might be gained by the addition of a riser 

pipe, or snorkel (with trash rack), as seen in 

figure 71. The snorkel can increase culvert 

flow substantially in a long pipe, but its primary 

purpose is to allow some flow if the culvert inlet 

has a history of becoming plugged. 

Figure 71—Culvert with a riser pipe and trash rack 

to prevent plugging in a channel that carries a lot of 

debris. Note that the roadway barely is seen in the 

background above the riser-pipe elevation.

Another treatment for organic debris at culvert 

crossings where the fill depth is sufficient is to 

add additional (usually smaller) culverts higher 

in the fill, adjacent to the existing pipe, or at 

other locations across a broad flood plain, as 

seen in figure 72. Use these where the site and 

the additional culverts are accessible during 

storm events. The smaller or additional culverts 

are at a greater risk of plugging than the larger 

main culvert and may need to be cleaned 

during a storm event. The additional culverts 

serve as an emergency overflow and may 

prevent a complete crossing failure by adding 

more capacity and providing extra time before 

reaching capacity during a flood.

Repair old and damaged pipes to extend 

their life by grouting the damaged areas. This 

involves placing a concrete or epoxy lining 

in worn out sections of the pipe, such as the 

bottom, that receives the most constant flow 

and abrasion from sediment. Also, installing 

a new slip lining of plastic or rubber inside an 

old pipe, as seen in figure 73, can preserve 

a

b
6-14 Storm Damage Risk Reduction Guide for Low-Volume Roads

the life of the culvert, prevent a failure, and 

possibly increase the capacity of the pipe by 

using a liner with a relatively low Manning’s 

(n) roughness coefficient. However, smooth 

liners may create additional problems for fish 

or other aquatic organism passage. Many new 

lining and installation options are available 

today within trenchless technology for culvert 

rehabilitation or replacement. For additional 

technical information about the trenchless 

technology, consult “Summary of Trenchless 

Technology for Use with USDA Forest Service 

Culverts” (Piehl 2005). Techniques for replacing 

or rehabilitating corrugated metal pipe culverts 

18 inches (450 mm) or greater in diameter are 

emphasized because they commonly are used 

for culverts. Link to the document <http://www.

fs.fed.us/eng/pubs/pdf/05771201.pdf>.

Figure 72—Increase culvert capacity using multiple 

pipes or stacking pipes on top of the original pipe. 

Typically, stacking pipes is only an emergency 

solution. Also note that multiple pipes increase the 

risk of plugging.

Figure 73—Culvert rehabilitation with a cement grout 

or a new rubber slipliner. (Bottom photo courtesy of 

Clackamas County, Oregon.)

For damaged culvert inlets repair, replace, 

or add an end section to the pipe to maintain 

its hydraulic capacity as well as to minimize 

plugging potential (figure 74). A relatively small 

reduction in inlet area causes a significant 

decrease in pipe capacity, as seen in figure 75.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-15

Figure 74—Damaged culvert inlets that need repairing and cleaning. Damaged culvert inlets can greatly 

reduce the culvert flow capacity.

Figure 75—Culvert capacity versus reduction in inlet area (Flanagan and Furniss 1997).

Information regarding culverts, design, installation, problems and solutions, and so forth is found in 

the U.S. Department of Transportation, FHWA publication HDS No. 5, “Hydraulic Design of Highway 

Culverts” (Normann et al. 2001) (Revised May 2005). This comprehensive culvert design publication 

is available from FHWA at the following Web site <http://www.fhwa.dot.gov/engineering/hydraulics/

culverthyd/index.cfm>.
6-16 Storm Damage Risk Reduction Guide for Low-Volume Roads

Trash Racks 

In forest environments, culverts fail far more 

often due to plugging with organic debris, 

sediment, and rock than due to lack of flow 

capacity. Trash racks can be effective in 

channels with significant amounts of organic 

debris to prevent pipe plugging. Place the trash 

racks upstream of a pipe or, in some cases, 

place immediately at the inlet of the culvert, as 

seen in figure 76. Large debris racks also are 

placed occasionally in channels upstream of 

bridges. If a trash rack is placed upstream of a 

pipe or bridge, ensure that there is access to the 

location for periodic cleaning of the trash rack.

Limit trash racks to stream crossings where 

culverts are undersized and/or prone to plugging. 

They are one more item that requires 

maintenance. Generally, a better solution or 

new construction is the installation of a larger 

pipe or an overflow pipe or dip in case of 

plugging. However, many types of trash racks 

have been used effectively to minimize the 

chance of culvert plugging on an existing culvert.

Before adding a trash rack to a crossing, 

carefully evaluate the site and assess the 

conditions that cause debris accumulation. 

These conditions can potentially cause more 

bank scour as water tries to flow around the 

structure and can cause channel diversion. 

Trash racks also may create a barrier to fish 

passage. Trash racks that are not maintained 

may become a severe liability. Certainly before 

any storm event, trash racks need to be clean. 

Figure 77 shows large trash racks placed in 

large channels to trap heavy sediment. 

Figures 77 through 79 show a wide variety of 

materials and configurations of trash racks. The 

spacing of the bars on a trash rack should be 

about the same as the diameter of the culvert, 

or it can depend on the size of material moving 

in the channel. A riser pipe with trash rack is 

commonly placed on small pipes where there is 

a lot of debris or sediment in the drainage, such 

as after a wildfire. The slanted trash rack over 

the culvert inlet is more self-maintaining since 

organic debris will slide up the rack, keeping the 

entrance to the culvert free. However, the inlet 

capacity of the culvert is diminished due to the 

change in entrance hydraulics. Consider the 

altered inlet hydraulics in sizing the pipe. Figure 

79 shows a trash rack both before and after a 

storm event. It shows the need for maintenance 

and cleaning of the structure after any storm 

(and before a storm hits).

Figure 76—Options for trash racks used with culverts.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-17

Figure 77—Use of trash racks to prevent plugging 

of a culvert or bridge from upstream large boulders 

and debris. (Bottom photo courtesy of Vincent 

Barandino.)

Another form of trash rack that has been 

effective for small-to-moderate streams is a 

single-post trash rack placed about one culvert 

diameter upstream of the pipe inlet. This post 

turns floating debris so that it will go through 

the pipe. If the debris is wider than the channel, 

then the post stops the debris and backs it 

up against the post and streambank before it 

reaches the pipe entrance. Flared inlet sections 

on pipes and their wing walls also serve to turn 

debris and funnel it through the pipe. 

The order of desirability to prevent culvert 

failure is (1) a vented ford (discussed 

in Fords versus Culverts below), (2) an 

oversized pipe, and (3) an overflow dip. A 

trash rack may be the least desirable way 

to prevent long-term pipe plugging, but it 

also is a relatively simple and inexpensive 

solution on many existing pipes.

Evaluate each site to determine the most costeffective and best solution to protect a pipe.

Figure 78—Examples of trash racks added to small 

culvert pipes to prevent plugging. Note that the 

riser with trash rack (a) is downstream of a fire area 

where considerable sediment is anticipated that 

often plugs small pipes. Debris can float over the 

culvert inlet at high flows (b). The single post (c) 

upstream of the pipe is used to help orient debris to 

go through the pipe rather than plugging across the 

inlet.

a

b

c
6-18 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 79—A trash rack upstream of a culvert, both 

before a storm ((a) looking downstream) and full 

of debris after a major storm event ((b), looking 

upstream). Note that trash racks must be cleaned 

and maintained!

Culvert Reinstallation and 

Realignment

Culverts may need to be replaced, reinstalled, 

or realigned because of poor alignment, which 

causes scour on an adjacent streambank; poor 

performance, such as piping under the old 

culvert; plugging; or deteriorating due to age. 

Figure 80 provides examples of a problematic, 

undermined culvert and an old, deteriorating 

culvert that has a high risk of failure during 

major storms.

Figure 80—Problematic culverts due to piping under 

the structure (a) or deterioration from a worn-out 

bottom (b). These culverts are at risk of failure, 

particularly during a major storm event.

Some regions are inappropriate for small culvert 

pipes because the culverts tend to plug with 

sediment and debris. This is particularly true 

in desert-to-semiarid regions where channels 

typically are dry but are subject to infrequent 

but periodic flashfloods or debris torrents. This 

is a condition observed in many areas of the 

Southwest, drier areas of the Rocky Mountains, 

and in parts of southern California. It also can 

a

b

a

b
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-19

be a problem in any steep, rocky mountainous 

terrain. Channels that transport large amounts 

of coarse sediments frequently plug culvert 

pipes, particularly if the pipes are small. In 

these areas, use either relatively large box 

culverts or small bridges. Alternatively, simple 

unvented fords may be most appropriate; they 

can pass a large amount of debris right over the 

top of the structure. Figure 81 shows examples 

of two culvert pipe installations that have 

plugged from coarse channel debris. Figure 82 

shows the same problem with relatively large, 

multiple culvert pipe vents used in vented lowwater crossings.

Figure 81—Culverts that have plugged due to their 

location in sediment and debris laden channels in a 

semiarid region.

Figure 82—Culverts in vented low-water crossings 

that have totally or partially plugged with coarse 

channel debris. 

Other important culvert reinstallation and 

alignment considerations and factors are 

shown in figures 83 and 84. These important 

installation details for culverts include: 

q Minimizing channel modifications. 

q Avoiding constriction of the bankfull flow 

channel width.

q Maintaining the natural channel grade and 

alignment. 

q Using quality, well compacted bedding and 

backfill material. 

q Using inlet, outlet, and streambank 

protection measures.
6-20 Storm Damage Risk Reduction Guide for Low-Volume Roads

Align culverts as close to the specific reach 

of the channel where it is being placed as 

possible. Avoid location on channel bends, but 

if necessary, the culvert needs to fit into the 

bend as much as possible, keeping in mind 

plugging and scour possibilities at the inlet and 

bank scour at the outlet. Set the pipe right at 

the elevation of the natural stream channel 

bottom, as seen in figure 83. Consider the 

average channel thalweg elevation through that 

reach of the stream. A culvert set too low can 

fill with sediment, lose capacity, and possibly 

cause headward (upstream) channel erosion. A 

culvert set too high can create a waterfall at the 

outlet, causing downstream channel scour and 

a possible fish and aquatic organism passage 

barrier. Culverts installed as described here 

also may constitute a fish barrier simply by 

increasing the water velocity through the pipe. 

Culverts specifically designed for fish passage 

may include oversizing the pipe and burying the 

bottom in stream substrate to simulate natural 

channel characteristics. See the next section, 

Aquatic Organism Passage Considerations, for 

additional culvert installation considerations.

Bedding and backfill material for culverts 

commonly is specified as select granular 

material or select mineral soil. Most soils are 

satisfactory if they are free of excess moisture, 

muck, lumps of frozen soil or highly plastic clay, 

roots, or rock larger than 3 inches (75 mm). 

Bedding material beneath the pipe should not 

have rocks larger than 11⁄2 inches (38 mm). 

Clay soil can be used if it is carefully compacted 

at a uniform, near-optimum moisture content. 

Ideal backfill material is a moist, well-graded 

granular or sandy gravel soil with up to 10 

percent fines and rock free. The material should 

be well compacted, to at least the density of the 

adjacent ground, and preferably to at least 90 to 

95 percent of the American Association of State 

Highway Transportation Officials T-99 maximum 

density, and placed in 6- to 8-inch- (150-200 

NO—TOO DEEP

YES

NO—TOO HIGH

Figure 83—Installation of a culvert right at the natural channel elevation.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-21

mm) thick layers (lifts), brought up uniformly on 

both sides of the pipe in lifts, as seen in figure 

84. A dense, uniform backfill is important to 

structurally support the lateral pressure from the 

pipe. Compaction is particularly important in the 

haunch area of the pipe and for aluminum and 

plastic pipe that easily deform. 

Installation details should follow the 

manufacturers’ information and 

recommendations, and manufacturers’ literature 

is available for all types of pipes, including 

plastic pipe, concrete pipe, and metal pipe. 

Concrete and metal pipes (usually corregated 

metal pipe) have been the most traditionally 

used products. They have performed well if they 

were properly designed and installed. Today, 

plastic pipe is used frequently, particularly 

because of its lower cost, ease of installation, 

and its suitability in corrosive environments. 

However, plastic pipe can burn, particularly in a 

forest environment, so consider the potential for 

fire when contemplating its use; add concrete 

or masonry headwalls to reduce this risk. 

Manufacturers of each type of culvert material 

have their own handbooks or manuals for 

design and installation information. For use of 

corrugated metal pipe, the “Handbook of Steel 

Drainage and Highway Construction Products” 

(Corrugated Steel Pipe Institute 2007) is an 

excellent reference. It is available at <http://

www.cspi.ca/node/158>.

Uniform fine sand and silt soils can be 

problematic when used for culvert bedding or 

backfill material. These fine, noncohesive soils 

are very susceptible to scour and piping from 

moving water (figure 80a). Thus, their use is 

discouraged. If used, they should be very well 

compacted against the pipe. Ideally, place 

antiseepage collars, made of metal, concrete, 

geotextile, or a zone of compacted clay around 

the culvert pipe to force any water channel 

to flow in a longer path through the soil and 

prevent a piping failure. Concrete headwalls 

also deter piping.

Aquatic Organism Passage 

Considerations

Most stream crossing structures built or 

replaced today need to ensure adequate 

aquatic organism passage. A goal is often to 

replace or retrofit a culvert to improve aquatic 

organism passage, as seen in figures 85 and 

86. Implementation of SDRR measures only 

need to avoid work that would degrade fish 

passage, and hopefully work would improve fish 

and other aquatic organism passage.

Figure 84—Ideal bedding and backfill compaction around a culvert pipe. (Adapted from Montana Department 

of State Lands 1992.)
6-22 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 85—An improved or stream simulation culvert installation compared to many traditional culvert 

installations.

Figure 86—A stream simulation design through a culvert.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-23

Properly designed stream simulation projects 

for aquatic organism passage should function 

well during major storms, particularly where 

the structure width is equal to or greater than 

the channel bankfull width. Several stream 

simulation culvert projects with flows exceeding 

the Q100 design flood were evaluated on the 

Green Mountain National Forest, Vermont, 

in September 2011, after the storm impacts 

from Hurricane Irene. Each structure survived 

well with minimal problems and performed 

flawlessly, maintaining aquatic organism 

passage at each site (Gillespie et al. 2014). 

Only some movement of the bed material was 

observed (Gubernick 2012a and b). Many other 

conventional culverts and bridges in the region 

were damaged or destroyed. 

For a comprehensive discussion on aquatic 

organism passage, consult the U.S. Department 

of Agriculture, Forest Service publication 

“Stream Simulation: An Ecological Approach 

to Providing Passage for Aquatic Organisms 

at Road-Stream Crossings” (Forest Service 

Stream Simulation Working Group 2008). The 

document is available at <http://fsweb.sdtdc.

wo.fs.fed.us/pubs/pdf/StreamSimulation/index.

shtml>. Forest Service policy is to use stream 

simulation for aquatic organism passage.

The U.S. Department of Transportation, FHWA, 

also has produced a comprehensive summary 

of fish passage options in its publication titled 

“Culvert Design for Aquatic Organism Passage,” 

Hydraulic Engineering Circular (HEC) 26 

(Kilgore et al. 2010). Available at <http://

www.fhwa.dot.gov/engineering/hydraulics/

pubs/11008/index.cfm>. This circular is not 

based on stream simulation.

Pipe Inlet/Outlet Protection

There are several ways to minimize the 

likelihood of a culvert failure. A culvert should 

be properly sized consistent with the design 

flow and width of the natural stream channel, 

aligned with the upstream channel, and have 

an efficient inlet for the least chance of being 

plugged with debris. You also might manage 

risk with appropriate flood and debris capacity 

and culvert height, a critical dip and spillway 

to prevent stream diversion and for minimizing 

overtopping damage, and/or increased flood 

capacity through additional structures in a 

floodway. Flow velocity typically accelerates in 

an undersized or narrow culvert pipe, so the 

pipe outlet area commonly is subject to scour 

and may need armoring or scour protection 

(figure 87). 

Figure 87—Common scour problems with a culvert due to confining the flow at the inlet and accelerated flows, 

producing scour at the pipe inlet and outlet. A properly sized crossing structure would span the full natural 

bankfull stream channel.
6-24 Storm Damage Risk Reduction Guide for Low-Volume Roads

Many old pipes are undersized and installed 

in fills that partially block or constrict the 

natural stream channel. For culvert pipe and 

fill protection, armoring the fill with materials, 

such as riprap, or using concrete headwalls is 

common (figure 88). For 2:1 or flatter fillslopes, 

riprap armoring may only extend slightly above 

the top of the culvert. For steeper fillslopes, 

armor the entire fillslope. Extent of armoring 

also depends on the design capacity of the 

pipe and expected headwater elevation. Use 

biotechnical measures in place of rock riprap. 

An advantage of a concrete or masonry 

headwall is that it can reduce the needed length 

of culvert pipe exiting the fill. Thus, weigh the 

cost of concrete and labor to build the headwall 

against the cost of additional pipe. 

Design downstream cutoff walls, roughened 

energy dissipation slabs, splash aprons, or 

stable energy dissipation pools (plunge pools) 

at the pipe outlet to reduce erosion and scour, 

as seen in figure 89. These measures reduce 

the likelihood of scouring and undermining a 

pipe at its outlet, but evaluate these measures 

carefully as they can create problems for fish 

passage.

The size of a splash apron or riprap basin 

depends on the diameter of the culvert and 

the flow exit velocity and volume. For small 

drainage culverts and culvert cross drains, 

place a few cubic yards (cubic meters) of riprap 

at the pipe outlet. Some recommended outlet 

protection details are shown in appendix A3. 

a b

c

Figure 88—Armoring and protection of a culvert pipe fill using riprap (a) (b) or concrete headwalls and 

wingwalls (c).
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-25

For major culverts, the apron length typically 

is 4 to 8 times the pipe diameter. Riprap size 

and thickness are functions of pipe size and 

discharge. Specific design equations are 

found in HEC14 and other FHWA references. 

Useful information specific to the design of 

structures to protect against outlet scour is 

found in the U.S. Department of Transportation, 

FHWA publication “Hydraulic Design of Energy 

Dissipators for Culverts and Channels,” HEC 14 

(Thompson and Kilgore 2006). It is available at 

<http://www.fhwa.dot.gov/bridge/hec14SI.pdf>

Fords or Low-Water Crossings versus 

Culverts

Low-water crossings, or fords, can provide a 

desirable alternative to culverts and bridges 

for stream crossing repairs or replacement on 

low-volume roads where road use, streamflow 

conditions, and topographic setting are 

Figure 89—Outlet protection options to prevent scour at the pipe outlet. 
6-26 Storm Damage Risk Reduction Guide for Low-Volume Roads

appropriate. Like other hydraulic structures 

for stream crossings, their construction or 

repair requires specific site considerations and 

geomorphic, soil, hydrologic, hydraulic, and 

biotic analyses. Conversion or selection of a 

ford is useful, particularly if a culvert pipe has 

had a history of plugging from channel debris. 

Construct fords, and particularly vented lowwater crossings, to pass large flows and large 

amounts of debris, and provide suitable aquatic 

organism passage (figure 90). In this example, 

a large waterway open area is provided, and 

the natural substrate material is maintained 

through the structure.

Figure 90—A large vented low-water crossing 

with predominantly natural stream channel bottom 

material that is ideal for fish passage.

For good foundation conditions, locate lowwater crossings at a relatively narrow, shallow 

stream location in an area of bedrock or coarse 

soil. An armored ford can be narrow or broad, 

but do not use it in deeply incised drainages 

that require a high fill or excessively steep road 

approaches. These settings require either a 

culvert or an armored fill. An improved ford 

with a hardened driving surface is a desirable 

structure on some very low traffic roads to 

minimize turbidity and potential pollution 

problems. However, some State agencies 

discourage fords because of negative impacts 

to the stream and fish when driving through the 

water and for traffic safety concerns. A vented 

low-water crossing can resolve this problem.

Design a ford to not create a low-flow depth 

barrier to fish passage. This can happen if a 

ford is wider than the natural channel. Ensure 

that neither scour nor perching develops along 

the downstream edge of the ford, which may 

turn into a fish passage barrier.

Fords and low-water crossings may have 

a simple, rock reinforced (armored) driving 

surface, as shown in figure 91, or an improved, 

hardened surface, such as gabions or a 

concrete slab (figure 90). Vented low-water 

crossings combine culvert pipes, box culverts, 

or open bottoms (figure 90) to pass low flows 

and a reinforced driving surface over the 

culverts to support traffic and keep traffic out 

of the water most of the time. The reinforced 

driving surface over the pipes also resists 

erosion during overtopping at high waterflows. 

Protect the entire wetted perimeter of the 

structure to a level above the anticipated design 

high-water elevation. Because basic designs 

require tailoring to individual site requirements 

and locally available materials, many variations 

of each of the basic types of low-water crossing 

structures have developed over time.

Figure 91—A simple rock-armored ford in an 

ephemeral channel. Coarse rock is placed across 

the channel bottom and a fine aggregate is placed 

on the road surface for driver comfort.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-27

Key factors to consider for the location, design, 

or repair of a low-water crossing include: 

q Low- and high-water levels. 

q Foundation conditions.

q Scour potential.

q Allowable traffic delays.

q Approach grades of the road. 

q Channel cross-section shape and 

confinement. 

q Protection of the downstream edge of the 

structure against local scour. 

q Stream channel and bank stability. 

q Material transported in the stream, 

particularly from debris slides or debris 

torrents.

q Locally available construction materials.

q Ability to dewater the site or work during 

the dry season. 

q Hydraulics for fish passage.

For fish or aquatic species passage, maintain a 

natural or rough stream channel bottom through 

the ford, and do not accelerate water velocities, 

similar to requirements through a culvert. Ideal 

structures are vented fords with box culverts 

and a natural stream bottom or simple on-grade 

fords with a reinforced, rough driving surface. 

Some fords can be designed as low-water 

bridge structures. They must be designed to be 

overtopped periodically and have an erosionresistant deck and approaches. This structure is 

ideal for fish passage.

Vented fords have a driving surface elevated 

some distance above the streambed with 

culverts (vents) that enable low flows to pass 

beneath the roadbed (figure 93). The vents 

can be one or more pipes, box culverts, or 

open-bottom arches. In streams carrying large 

amounts of debris, the driving surface over 

the vent may be removable, such as a cattle 

guard, permitting debris to be cleared after 

a large flow event. Figure 92 shows a large 

pipe that had plugged several times in large 

storm events. Eventually the pipe was replaced 

with a concrete-vented ford designed with a 

metal cattle guard top that can be removed for 

cleaning. Note that a fish barrier exists at the 

downstream edge of the concrete ford, and 

the entire downstream wetted perimeter of the 

structure needs to be armored. 

Figure 92—A large, 10-foot-diameter culvert that had 

plugged several times with debris (a)(b) and was 

replaced eventually with a vented low-water crossing (c).

a

b

c
6-28 Storm Damage Risk Reduction Guide for Low-Volume Roads

Vented fords fall into two categories—low 

vent-area ratio (VAR) and high VAR—each 

affects stream channels differently. Vented 

fords with culverts that are small relative to 

the bankfull channel area have a low VAR. 

The Forest Service and other agencies have 

built many low-VAR fords. A vent opening 

that approximates or exceeds the size of the 

bankfull channel has a high VAR, as seen in 

figure 93. Low-VAR structures plug with debris 

easily; act as a dam and cause deposition of 

sediment upstream of the structure; and may 

accelerate flows downstream, creating a barrier 

to fish passage through the pipes, as well as 

channel scour. A high-VAR structure is much 

better for aquatic organism passage and to 

maintain the natural function of the stream.

Figure 94 summarizes many of the key design 

issues necessary to make a ford or low-water 

crossing function properly and avoid damage. 

Many existing fords found throughout the United 

States and used by the Forest Service have 

been damaged at some time during a flood and 

have required repairs or have been replaced. To 

avoid damage to the structures as well as to the 

environment, consider the following key design 

issues:

q Provide an armored surface through the 

high-water wetted perimeter of the entire 

structure, plus some freeboard.

q Keep the driving surface as low and 

as close to the natural stream channel 

elevation as possible.

q Provide scour protection below the 

downstream edge of the structure.

q Maintain a natural stream channel bottom 

through vented fords.

q Stabilize and properly drain the road 

surface on both approaches to the crossing 

using aggregate, concrete, or paving.

Figure 93—Examples of fords with a low and a high vent-area ratio.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-29

q Use appropriate vertical and horizontal 

alignment through the crossing to 

accommodate the anticipated traffic safety 

and use. 

q Use delineators, signs, and depth markers 

as needed to make the crossing safe.

For additional technical information about 

fords and low-water crossings, consult “LowWater Crossings: Geomorphic, Biological and 

Engineering Design Considerations” (Clarkin 

el al. 2006). The publication reviews both the 

advantages and disadvantages of a variety of 

low-water crossing structures in various 

Figure 94—Key design components for an unvented, improved low-water ford.
6-30 Storm Damage Risk Reduction Guide for Low-Volume Roads

stream environments and illustrates situations 

in which low-water crossings may be the 

optimal choice of crossing structure. Link to the 

Web site <http://www.fs.fed.us/eng/pubs/pdf/

LowWaterCrossings/index.shtml>.

Local Structure, Streambank, and 

Channel-Stabilization Measures

If local or average velocities exceed the 

permissible velocities of the materials for 

movement (shear stresses), erosion and scour 

will result. These problems often occur where 

a structure has confined or redirected the flow 

of the channel, where flows have increased 

in the channel, or where natural protection, 

such as vegetation, has been removed from 

the channel or around a structure. Therefore, 

either take measures to reduce the velocities, 

redirect the flow, dissipate the energy of the flow, 

provide stability below the likely depth of scour, 

or armor the areas with materials that can resist 

the flow’s forces. Reducing stream velocity is 

generally more desirable and cost-effective than 

streambank armor.

A variety of streambank-stabilization treatments 

are available to minimize the susceptibility of 

structures or streambanks to disturbance-caused 

erosion processes. They may be engineered 

grade-stabilization structures or vegetationoriented remedies ranging from conventional 

plantings to a combination of biological and 

engineering elements, such as soil bioengineering. 

Measures include the use of rock riprap, gabions, 

concrete slabs, cable concrete, vegetation, and 

various biotechnical treatments. Use lighter 

treatments, such as turf-reinforcing mats, in small 

channel banks in some circumstances to help 

promote the growth of vegetation. 

Historically, many organizations involved in water 

resource management have given preference 

to engineered structures; they are viable 

options. However, in a growing effort to restore 

sustainability and ensure ecological diversity, 

give preference to those methods that restore 

the ecological functions and values of stream 

systems as well as protect the structure. The 

value of vegetation in civil engineering and the 

role woody vegetation plays in the stabilization 

of streambanks has gained considerable 

recognition in recent years.

Once a stream-crossing structure is placed in a 

channel, it may change the dynamics of the site 

or the structure itself may need protection. The 

structure may need armoring, the stream channel 

may need protection, streambanks may need 

stabilization, or it may be desirable to control 

the flow in the channel with some river training 

measures to protect the structure and/or the 

banks. "River engineering" is an evolving field, 

and measures should not be undertaken without 

proper consultation with qualified engineers, 

hydrologists, and geomorphologists. Improper 

design and implementation of river engineering 

techniques can create additional localized and 

systemic problems.

Different structures have different scour 

risks that commonly need protection. Some 

structures accelerate flows through pipes 

or vents, some confine channel flow, some 

accelerate flow across the driving surface, and 

some create water drop off the downstream 

edge. Many structures create more than one of 

these effects. Depending on the velocity of flow 

and erosion potential, scour protection and/or 

bank stabilization may be needed. The following 

options are available:

q Vegetation, erosion control mats, or small 

riprap for low velocities.

q Soft armor systems, such as biotechnical 

treatments, vegetated turf-reinforcing mats, 

rootwads, logs, and boulders for moderate 

velocities.

q Hard armor systems, such as articulated 

concrete blocks, gabions, large riprap, 

grouted riprap, or concrete for high channel 

velocities or high shear stress areas where 

flows are turbulent or impinging upon the 

streambank. 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-31

Figure 95 provides general guidelines for 

selecting channel and bank stabilization 

measures as a function of mean channel 

velocity and the duration of flow (i.e., how long 

the area is subject to inundation). Note that 

often a transition zone is needed between a 

hard armor zone and the native streambed 

material. Ideally, this work is tied into a stable 

channel feature, such as bedrock or large 

boulders. “Environmentally Sensitive Channel 

and Bank Protection Measures” (McCullah and 

Gray 2005) presents an excellent summary of 

the many river training, channel modification, 

and bank stabilization/ armoring options 

available today in this National Cooperative 

Highway Research Program Report 544. Link 

to the document <http://onlinepubs.trb.org/

onlinepubs/nchrp/nchrp_rpt_544.pdf>.

Local Scour Protection

Scour protection is needed in many areas for 

stream-crossing structures (culverts, bridges, 

and fords). Scour is one of the principal 

causes of failure or damage to structures 

during major storms. Thus, scour protection 

around structures is a key mitigation to reduce 

the risk from storm damage. Many scour 

protection measures are similar to streambank 

stabilization measures. Figure 96 shows some 

of the areas where local scour affects stream 

(ECVM)

TRM/ECVM

Large riprap

Vegetated gabions

Live stakes

Willow cuttings

Small riprap and vegetation

Brush layers

Root wads

Well vegetated

Poor cover

Concrete blocks

1. Hard armor—

2. Soft armor—

3.

Figure 95—Allowable velocities and flow duration for various erosion and bank protection measures. (Adapted 

from Fischenich 2001 with information from Theisen 1992 and McCullah and Gray 2005.)
6-32 Storm Damage Risk Reduction Guide for Low-Volume Roads

channels and structures the most, including the 

following:

q Vertical streambanks, particularly on the 

outside bend of a channel where flow 

velocities are the fastest.

q Areas where there is a sharp bend in the 

channel, leading to scour on the outside 

edge of that bend.

q Over waterfalls and around midchannel 

structures where flow is accelerated and 

may be turbulent.

q Where flow is constricted, thereby 

accelerating stream channel velocity, such 

as through narrow bridges, box culverts, or 

in culvert pipes.

Section 6.3 discusses some of the scour 

protection measures, such as riprap, logs, and 

soil bioengineering treatments. The following 

sections present additional scour information.

Use of Rock Riprap

Rock riprap is one of the most commonly used, 

and misused, erosion and scour protection 

measures because of its resistance to high 

stream velocities when properly installed, 

availability, relatively low cost, durability, 

adaptability to many sites, revegetation 

opportunities, and self-healing aspects of 

loose rock. As an SDRR treatment, riprap is 

useful for structure and erosion protection, 

such as around the inlet and outlet of culverts, 

for culvert or ford fill armoring, around bridge 

abutments to protect against scour, at the 

outlet of cross-drain culverts, leadoff ditches, 

and rolling dips, or to protect and maintain the 

shape of a rolling dip. However, avoid armoring 

lengths of streambanks if possible. Road 

relocation should be thoroughly investigated in 

cases of road encroachment next to a stream, 

especially fish-bearing streams (section 6.4). 

Also rootwads, boulders, vegetation, and other 

materials can be used as alternatives to rock 

riprap. Because riprap is a loose-rock structure, 

it can move to some degree, deform, and 

conform to scour areas and still offer erosion or 

scour protection. It can armor an entire channel 

cross section (above water and underwater), 

armor streambanks to the expected high-water 

level, and armor a plunge pool or stilling basin. 

Place riprap at the outlet of pipes, along the 

downstream edge of a structure, in a scour 

hole, or around and along channel protrusions, 

such as bridge piers. 

The two most common reasons for riprap 

failure are improper sizing and poor installation. 

Many agencies have developed riprap-sizing 

criteria. The most rigorous criteria are based 

upon shear stresses or tractive forces exerted 

by flowing water and sediment moving along 

the rock surface. The U.S. Department of 

Transportation, FHWA publication HEC-11, 

“Design of Riprap Revetments” (Brown and 

Clyde 1989) provides a comprehensive design 

process for riprap sizing, using permissible 

tractive forces and velocity, along with design 

examples. HEC 23, “Bridge Scour and Stream 

Instability Countermeasures” (Lagasse et 

al. 2009) also discusses riprap design. Both 

publications are available at <http://www.fhwa.

dot.gov/engineering/hydraulics/>. 

Criteria based upon permissible velocity often 

are used because velocity information may 

be available from Manning’s Equation, direct 

measurements, or other sources. Install rock 

large enough that the forces of flowing water 

do not displace it. Figure 97 shows relatively 

simple riprap-sizing criteria based upon average 

channel velocity determined using Manning’s 

Equation. Depending on the slope of the bank, 

the riprap diameter (median size) is chosen 

based on the average streamflow velocity (Vave). 

Use smaller rock on areas of straight, parallel 

flow to the bank (Vp

), while larger riprap is 

needed in curves where the flow is impinging 

(Vi

) upon the rock. The impinging velocity (Vi) is 

estimated at 4/3, or 133 percent of the average 

channel velocity (Vave). Use well-graded riprap 

and ensure that half of the stones are at least 

as large as the median size.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-33

Figure 96—Common scour problem areas found along stream channels and around structures.
6-34 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 97—Sizing criteria for riprap based upon average channel velocity (Note that 1 foot/second=0.305 

meters/second and 1 pound=0.454 kilograms).
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-35

Common modes of riprap failure, in addition 

to undersized rock, are failures at the end of a 

section of rock armoring or undermining of the 

rock by scour at the toe of a slope. The armored 

reach of the stream or structure should begin 

and end ideally at a strong or hard point in the 

channel, such as a bedrock outcrop, at large 

boulders, or a heavily vegetated bank area. 

Also, on steep streambanks, the slope can 

fail under the riprap, particularly if the toe area 

scours and is not well buttressed. Riprap also 

can fail by sliding if it is placed upon a smooth 

bedrock surface. If riprap is placed upon inplace smooth boulders or bedrock, pin down the 

riprap with rebar or bolts anchored into the rock. 

The maximum rock size used in remote areas 

is often dictated by the rock size available. If 

large rock is not available, then grout smaller 

rock with concrete or use gabions. Otherwise, 

risk of failure becomes higher, the investment 

is wasted, and the structure may become a 

liability. Relatively large riprap is needed around 

a bend in a stream where the streambanks are 

subject to the force of impinging flows. Use 

smaller riprap in areas of parallel flow along the 

streambank (figure 97).

Figure 98 shows a stream channel with 

light riprap armoring but little channel and 

toe protection. Additionally, the riprap has 

encroached upon the channel, reducing its 

capacity. Thus, this site is likely to fail in a future 

storm. Place large riprap in the bottom of the 

channel and bury in the toe of the fill to at least 

the depth of expected scour along the channel, 

as shown in figure 99. Bury large footer rocks 

below the level of scour, then build upon these 

footer rocks. Scour depth will depend on the 

flow, velocity, and materials, as discussed 

previously and in section 6.3, but is frequently 

at least several feet (a couple meters) deep. 

Also, the channel is relatively smooth, with 

little roughness to help slow the velocity of 

flow and dissipate the channel energy. An 

armored, hard, smooth section of stream can 

accelerate the flow and move or create a 

problem downstream where the stream enters 

a less resistant channel material. Use energydissipation structures, such as rock vanes, 

weirs, large boulders, log jams, and so forth, 

in conjunction with riprap wherever possible. 

Dissipate the energy or else it just moves away 

from the hardened area (Gubernick, personal 

communication). This is discussed further in 

section 6.4.

Figure 98—A stream with light riprap armoring 

on the roadway fill, but the channel bottom is not 

armored, nor is the toe of the riprap slope keyed into 

the channel below the depth of potential scour. This 

site is likely to fail in a future storm. Also the channel 

is constricted. Road relocation should be considered 

here. (Photo courtesy of Bob Gubernick.)

Several other design and installation details are 

important when using riprap, as seen in figure 

99: 

q Use well-graded riprap to provide a dense, 

armoring layer. The riprap layer should be 

at least as thick as the maximum rock size, 

and preferably 1.5 times the maximum.

q Use hard, durable, and angular rock. 
6-36 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 99—Typical riprap installation details for streambank protection. The upper drawing shows the use 

of riprap while the lower drawing shows a detail using gabions, with both showing measures for toe scour 

protection.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-37

q Place riprap upon a filter layer of either 

gravel or geotextile. The filter allows water 

to drain from the soil while preventing soil 

particle movement behind or beneath the 

riprap. In critical applications, a multiple 

filter layer may be desirable. A sand 

cushion over a geotextile can be desirable 

to prevent damage to the geotextile. 

q Key in riprap around the layer’s perimeter, 

particularly along the toe of an armored 

slope and at the upstream and downstream 

ends of the rock layer such that scour will 

not undermine the whole structure, nor 

make an end run behind the structure. 

Extend the protection through a curve or 

beyond the area where fast or turbulent 

flow is expected. Excavate the toe key to 

the depth of expected scour or to at least 

several feet deep. 

q Place riprap with an excavator or by hand 

to help achieve interlocking of the individual 

pieces. Dumped riprap results in an uneven 

thickness and an unstable structure overall. 

q Add extra rock or an extra length of 

gabions at the toe of the protected area to 

help prevent scour and undermining of the 

structure.

Other Solutions for Streambank Instability

The solutions for streambank instability often 

involve a combination of physical and soil 

bioengineering techniques. Streambank 

stabilization measures often are needed at 

road-stream crossings where a road fill may 

encroach on the stream, a culvert fill is placed 

across the stream, or where a flow constriction 

accelerates the natural stream channel velocity, 

leading to local scour.

The two basic categories of protection 

measures for structures and streambanks are: 

1. Those that armor the soil and increase the 

local resistance to erosion. 

2. Those that reduce the force of water 

against the structure or streambank through 

flow redirection or energy dissipation. 

Examples of ways to increase local resistance 

to erosion include:

q Conventional vegetation. 

q Soil bioengineering measures, such as live 

stakes, joint planting, brush mattresses, 

and live fascines.

q Conventional engineering measures, 

such as rock riprap, gabions, concrete, 

structural biotechnical measures, erosion 

control blankets, turf reinforcement mats, 

root wads and boulder revetments, 

articulated concrete blocks, and so forth. 

Examples of ways to reduce the force of water 

include many river training structures, such as 

spur dikes, groins, jetties, barbs, weirs, drop 

structures, in channel logs (large woody debris) 

and boulders, increased channel sinuosity, 

vegetated floodways, and so forth. Section 6.4 

provides more information on some of these 

treatments. 

Often a combination of methods is used. Soft 

treatments, such as wood and vegetation, 

typically are the most desirable to emulate 

natural energy dissipation features and to help 

create the best fish habitat. Figure 100 shows 

a style of streambank stabilization using logs, 

rootwads, and boulders; figure 101 shows 

logs and rootwads and brush layering for 

streambank stabilization where channel-flow 

velocities are low. Log, rootwad, and boulder 

revetments have the advantage of creating 

channel roughness along with streambank 

protection; they create fish habitat, too. 
6-38 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 100—Log, rootwad, and boulder streambank stabilization measures. (Note: 1 foot = .305 meters)

Some considerations for the type of stabilization 

method to use include:

q Selecting self-sustaining, permanent 

solutions that, in the case of soil 

bioengineering measures, have the ability 

to grow stronger with age and require 

minimum future maintenance.

q Protecting or restoring the natural functions 

and values of the stream as much as 

possible.

q Using materials that will add channel 

roughness and help dissipate stream 

energy.

q Using native, natural living plants and 

locally available inert materials. 

q Protecting or improving water quality 

by reducing water temperatures and 

sedimentation problems.

q Selecting measures that are strong or 

durable enough to resist the erosive forces 

of the stream during a major storm event.

One key reference on this topic is the U.S. 

Department of Agriculture, Natural Resources 

Conservation Service, Engineering Field 

Handbook, Chapter 16, “Streambank and 

Shoreline Protection” (NRCS 1996). This 

classic reference describes and emphasizes 

use of vegetation, soil bioengineering, and 

biotechnical methods, as well as traditional 

structural streambank stabilization methods. 

Link to document <http://directives.

sc.egov.usda.gov/OpenNonWebContent.

aspx?content=17553.wba>.

Another reference for streambank stabilization 

is the Washington State Department of 

Fish and Wildlife publication “Integrated 

Streambank Protection Guidelines” (Cramer et 

al. 2003). It is available at <http://wdfw.wa.gov/

publications/00046/wdfw00046.pdf>.

LOG, ROOT WAD, AND BOULDER LOG, ROOT WAD, AND BOULDER

 STREAMBANK REVETMENT STREAMBANK REVETMENT

PLAN VIEW PROFILE VIEW

Adapted from Rosgen (1996); NRCS Engineering Field Guide Handbook, Chapter 16; and USDA Soil Bioengineering Guide (2002).
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-39

Figure 101—Rootwads, logs, and boulders (a) and 

brush layering (b) used for streambank stabilization 

(Upper photo courtesy of John McCullah). 

Soil Bioengineering Solutions for 

Streambank Instability

Soil bioengineering is a technology that uses 

sound engineering practices in conjunction 

with integrated ecological principles. It takes 

advantage of the benefits of vegetation 

systems, arranged in specific ways, to prevent 

or repair damage caused by erosion and stream 

scour. Soil bioengineering for streambank 

stability is instrumental in road rehabilitation and 

stabilization for use along the road, adjacent to 

an eroding streambank, or at structures, such 

as a bridge. Adapted types of woody vegetation 

(shrubs and trees) are installed initially in 

specified configurations that offer immediate 

soil protection and reinforcement.

Additionally, soil bioengineering systems create 

resistance to sliding or shear displacement in 

streambanks as they develop roots or fibrous 

inclusions. Environmental benefits derived 

from woody vegetation include diverse and 

productive riparian habitats, shade, organic 

additions to the stream, cover for fish, 

temperature reduction, and improvements in 

aesthetic value and water quality. 

Under certain conditions, soil bioengineering 

installations work well in conjunction with 

structures to provide more permanent 

protection and function, enhance aesthetics, 

and create a more environmentally acceptable 

product. This combination is called biotechnical 

stabilization. Soil bioengineering systems 

normally use plant parts in the form of live, cut 

branches and/or rooted plants. For protective 

measures for streambanks, live stakes, live 

fascines, joint planting through rock (vegetated 

riprap), vegetated geogrids and gabions, 

live cribwalls, branch packing, and live brush 

mattresses are used in various configurations 

as appropriate for a specific location. Figure 

102a shows a joint planting system with 

live stakes inserted through riprap, and an 

installation after several years of growth.

Figure 102—Biotechnical streambank stabilization 

using rock riprap and live willow stakes. (Photo (a)

courtesy of Robbin Sotir.)

a

b

a

b
6-40 Storm Damage Risk Reduction Guide for Low-Volume Roads

Useful information on streambank 

stabilization is found in the publication “A 

Soil Bioengineering Guide for Streambank 

and Lakeshore Stabilization” (Eubanks and 

Meadows 2002). Link to this document <http://

www.fs.fed.us/publications/soil-bio-guide/>.

Beaver Plugging Prevention

Beavers often build dams or structures that 

block culvert inlets. They may plug culverts 

to create ponds upstream of the pipes. 

Consequently, the pipes may not function or 

they may have reduced capacities, increasing 

the chances of culvert failure during a major 

storm event. Keeping beaver dams cleaned out 

from culverts also increases the cost of culvert 

maintenance. Removal of the beaver structures 

is a common and sometimes chronic treatment.

Frequent removal of beavers and their 

structures from an area is not desirable or costeffective. It also is difficult to keep beavers from 

colonizing a suitable site, and efforts often have 

been unsuccessful. Removing food sources is 

difficult; trapping and relocating is expensive; 

and no environmentally safe chemicals or toxins 

are available for specific use against beavers. 

Additionally, beavers may be an important part 

of the local natural environment and be native 

to the area. Thus, removal of the beavers may 

be undesirable, but the culverts need to be kept 

functioning. 

A recommended practice is to keep an 

inventory of sites where beaver problems are 

found. Then, these sites can be a high priority 

for cleaning in advance of storms, and also for 

storm patrol inspections. Figure 103 shows a 

typical beaver structure decreasing the capacity 

of a large culvert.

Figure 103—A beaver dam partially plugging a large 

culvert, Routt National Forest. (Courtesy of Steve 

Coupal.)

Beavers seem to be able to plug culverts faster 

than staff can clean them, and beavers are 

able to repair a dam or lodge by the next day. 

To alleviate this problem, staff have developed 

measures to thwart the efforts of beavers 

to plug culverts and to discourage beaver 

dam construction at culverts. These include 

exclusion fences, secondary flow bypasses, 

water control devices, perforated pipes through 

culverts, or ways to regulate the water and 

pool level in a pond behind the culvert. Place 

coarse screens and meshes on the inlet of 

pipes to keep beavers out. However, they 

make a good location for beaver activity at the 

pipe inlet (outside the pipe). Also,the screens 

and meshes restrict the flow of water and can 

easily plug with debris. Water control devices 

also may create a problem for fish passage, so 

evaluate the implications of various treatments 

carefully.

Figures 104 and 105 show retrofits that have 

been added to large culverts to prevent them 

from being totally plugged by beavers. Figure 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-41

104 shows drop-inlet pipes connected to 

the culverts (a pond is behind the pipes in a 

meadow) with additional larger rings to allow 

waterflow into the pipes even if the top is 

covered with debris. Figure 105 shows small 

perforated pipes running through the large 

culvert pipe to ensure some waterflow (and 

to frustrate the beaver). Figure 106 shows 

a screened-in cage in front of the culvert 

inlet, known as a beaver deceiver, to prevent 

plugging.

Figure 104—Large riser pipes connected to culverts 

with entrance rings to allow waterflow into the pipe, 

yet thwart beaver activity, Plumas National Forest, 

California.

Figure 105—Perforated pipes flowing through or 

next to larger main pipes to maintain some flow 

through the pipe even with beaver dam activity.

Figure 106—Beaver deceiver protection by 

screening off the entrance to the culvert in Wyoming. 

(Courtesy of Steve Coupal.)
6-42 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 107—A Clemson beaver pond leveler to control water level and/or provide bypass flow to plugged 

culverts and beaver dams. (Wood et al. 1984) (Note: Metric units were not available on this drawing.)
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-43

The Clemson beaver pond leveler (figure 107) 

is a relatively low-cost, low-maintenance device 

to pass water through a beaver dam or plug 

(Wood et al. 1994). It can control the level of 

water in a pond behind a beaver dam or ensure 

that some flow goes through a plugged pipe 

or past a beaver dam. Staff in the southern 

and northern regions of the country have used 

this successfully since 1987. A Forest Service 

publication discussing methods used to mitigate 

beaver problems is “How to Keep Beavers from 

Plugging Culverts” (Nolte et al. 2005).

6.3 Bridge Protection and 

Improvements

Bridges are a major investment for rural 

infrastructure and need to be protected to 

prevent failures. Incorporate good bridge design 

and bridge scour protection measures into 

the initial design. However, changes over time 

can affect the hydrology of the watershed and 

increase stormflows, putting existing bridges 

at risk. Watershed characteristics may change 

due to new land uses, logging, or forest fires. 

Global climate change may be contributing to 

more intense storms, longer duration storms, 

or more glacial melt-water runoff. Thus, old 

bridges may have marginal hydraulic capacity 

for storm runoff. Some bridge protection may 

be needed over time and erosion and scour 

protection added to prevent undermining of 

foundation piers or abutments. Since bridges 

are costly, scour prevention measures can be a 

high-priority item. This may involve the following 

work:

q Foundation repairs and grouting.

q Riprap placement around piers or 

abutments.

q Channel lining with riprap or gabions.

q Redirecting flows with barbs or rock jetties, 

and so forth. 

Work should involve an interdisciplinary process 

including fisheries biologists, hydrologists, 

geomorphologists, earth scientists, and 

engineers. 

For some structures with a known inadequate 

flow capacity, raise the superstructure to 

increase freeboard. In some circumstances, 

an overflow dip can be built into approach fills, 

particularly fills that block a historic flood plain, 

to provide a controlled failure point that can be 

repaired easily rather than losing the bridge 

structure.

Channel maintenance, as discussed earlier, 

also can increase the bridge flow capacity and 

reduce the chance of plugging or blocking 

during a storm. 

The publication “Stream Instability, Bridge 

Scour, and Countermeasures: A Field Guide 

for Bridge Inspectors” (U.S. Department of 

Transportation, FHWA 2009b) has many 

insights into bridge instability problems, what to 

look for, and appropriate countermeasures.

Increasing Freeboard

Lack of capacity and freeboard can lead to 

bridge failure. Lack of freeboard may be a lack 

of capacity for the design flow or, as shown 

in figure 108, it may cause a bridge failure 

by catching floating debris that can push 

the superstructure off its abutments. Stream 

channels in areas of aggradation, on alluvial 

fans, and channels carrying significant amounts 

of large woody debris, all have the potential 

of plugging a bridge. In these areas, evaluate 

bridges for their freeboard and consideration 

given for raising the structure if possible. 

Alternatively, some channel clearing may be 

warranted (after consideration of other adverse 

environmental impacts of channel clearing) as 

discussed in section 5.4. 
6-44 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 108—A failed bridge due to lack of freeboard and accumulation of debris behind the bridge deck.

Solutions to lack of freeboard are not simple. Either replace the structure with a longer span to 

increase the waterway open area of the structure or raise the superstructure (girders and deck). The 

success of these measures depends on the topography around the bridge. Raising the bridge deck 

in flat terrain may accomplish nothing if the floodwaters just spread out across a flood plain. Here 

a longer structure would be more useful. Figure 109 shows a Bailey Bridge (a portable, temporary 

bridge) superstructure with marginal freeboard and debris stuck in the superstructure after a flood. 

The problem was solved by adding height to the abutments and piers (right photo), thus raising the 

deck a couple feet, and placing a new concrete superstructure across the span.

Figure 109—A bridge with marginal freeboard (a) where the abutments and piers were raised (b) to add 

freeboard and flow capacity, Plumas County, California.

a b
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-45

Major Bridge Scour Protection

Bridge foundation scour is one of the most 

common causes of bridge damage or failure 

during storm events. The most common forms 

of scour at bridge sites are contraction scour, 

general channel scour, and local scour around 

piers and abutments, as shown in figure 

110. Section 6.2 presents some additional 

discussion of scour.

Determining the depth of scour and amount 

of potential bedload movement are critical 

factors that need to be evaluated at bridge 

sites, particularly if the site has had historic 

scour problems or appears undersized. In 

coarse, rocky channels with boulders, scour 

depth may be minimal or only a couple feet (a 

meter); in gravel channels with cobbles, scour 

may be as deep as several feet (a few meters); 

and in fine sandy river channels, scour depth 

has been observed to over 50 feet (15 meters 

plus). Determining scour depth requires study 

and careful evaluation of the channel and bedmaterial characteristics. The references listed 

here provides information on determining scour 

depth. The U.S. Army Corps of Engineers HECRAS programs for analysis of river systems 

have methodologies for determining scour 

depth. It is available at <http://www.hec.usace.

army.mil/software/hec-ras/>.

Key areas needing scour protection include:

q Along the downstream edge of an inchannel grade-control structure, where 

accelerated velocities or water dropping 

off a structure produces a waterfall with 

erosive energy.

q Around or beneath mid-channel piers, 

posts, piles, or box walls that create 

turbulence or accelerated flows (figure 111).

q Along the edges and beneath abutments 

and footings, where locally accelerated 

flows and scour occur.

q Along or around the approaches to 

structures (outflanking) or wingwalls, 

where the high-water level may exceed the 

elevation of armoring or a shifting channel 

scours behind the approach armoring. 

Figure 110—Types of scour and common scour problem areas around a bridge. (Adapted and reproduced 

with the express written authority of the Transportation Association of Canada, from Neill 1973.)
6-46 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 111—Common scour seen around a midchannel pier or abutment. Thus mid-channel piers 

and abutments are problematic, undesirable, and 

should be avoided if possible.

Common mitigation measures used for 

protecting bridges against scour include the 

following:

q Move a structure to a location where the 

local materials are not scour susceptible, 

such as areas of coarse rock and bedrock.

q Widen a structure to avoid constricting 

the flow channel, thus avoiding flow 

acceleration.

q Armor the entire channel with scourresistant materials (grouted gabions, riprap, 

concrete, and so forth).

q Protect the channel, streambanks, and 

abutments against local scour, using 

vegetation, rootwads and logs, riprap, 

sack concrete, articulated concrete blocks, 

vegetated turf reinforcing mats, gabions, 

and so forth, as seen in figure 112.

q Redirect stream-channel flow with barbs, 

spur dikes, weirs, cross vanes, and so forth 

(section 6.4).

q Install deep foundations, placed below the 

anticipated scour level, such as relatively 

deep spread footings, or drilled or driven 

piles and piers.

q Add shallow scour cutoff walls, gabions or 

concrete splash aprons, plunge pools, or 

riprap layers along the downstream edge of 

an in-channel structure. 

q Install deep cutoff walls or deep sheet piles 

installed to a depth below the depth of 

scour, or to scour-resistant material, such 

as bedrock. 

For additional technical information about scour, 

consult U.S. Department of Transportation, 

FHWA, HEC-18 “Evaluating Scour at Bridges” 

(Richardson and Davis 2001). The manual is 

part of a set of Hydraulic Engineering Circulars 

(HECs) issued by FHWA to provide guidance 

for bridge scour and stream stability analyses. 

A companion document is HEC-23, “Bridge 

Scour and Stream Instability Countermeasures” 

(Lagasse et al. 2009). Links to HEC 18 <http://

isddc.dot.gov/OLPFiles/FHWA/010590.

pdf> and HEC 23 <http://www.fhwa.dot.gov/

engineering/hydraulics/library_arc.cfm?pub_

number=23&id=142>.

Also, technical information about bridge 

scour evaluation is provided in “Bridge 

Scour Evaluation: Screening, Analysis, and 

Countermeasures” (Kattell and Eriksson 1998). 

Link to the document <http://www.fs.fed.us/eng/

pubs/pdf/98771207.pdf>.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-47

Figure 112—Bridges with rock riprap (a) and gabion 

(b) scour protection around the abutments since the 

bridges are located on bends in the stream (river 

bends are problematic locations).

Stream channels are dynamic and tend to 

meander or shift laterally, longitudinally, and 

vertically over time, particularly in low-gradient 

environments. They can change flow direction 

during flood events, move out of an existing 

channel, and may reoccupy some historic 

flood plain deposit. Figure 113 shows a bridge 

site where stream meander or change in 

flow direction eroded a bridge abutment and 

approach fill. The repair involved extending the 

concrete wingwall and providing upstream bank 

protection with riprap. Evaluate the location of 

bridge abutments for long-term channel stability 

and possible stream channel adjustments. 

Figure 114 shows a damaged bridge approach 

due to channel widening from a debris torrent 

in the watershed. Figure 115 shows a bridge 

damaged due to channel constriction and scour 

on the outside of a river bend.

Figure 113—Abutment scour caused by channel 

meander (a) and the repair (b), extending bank 

protection upstream from the bridge. Note that the 

bridge also constricts the original stream channel, 

Plumas National Forest.

Figure 114—Channel widening and scour around a 

bridge abutment, removing part of the approach fill, 

caused by a debris torrent and shifting river channel. 

A temporary Bailey Bridge is being used to cross the 

gap, Mount Hood National Forest. (Courtesy of Mark 

Leverton.) 

a

b

a

b
6-48 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 115—Bridge failure during Hurricane Irene 

due to a bridge span too short for the channel 

width, with the abutments constricting the channel 

and accelerating the streamflow, plus the bridge is 

located on a river bend, all causing abutment scour, 

White Mountain National Forest. (Courtesy of Bob 

Gubernick.)

6.4 Road-Stream Encroachment

Historically, roads were located following the 

path of least resistance; in essence, where 

construction required the least amount of 

excavation. This resulted in most of the 

initial roads into a watershed located on river 

terraces adjacent to the river and or in channel 

flood plains. These areas, now referred to as 

channel migration zones, are where rivers 

shift in response to storm flows, sediment, and 

obstructions to shape and reshape their flood 

plain. Areas where there are significant changes 

in stream gradient, or alluvial fan areas, are 

particularly problematic since over time the 

stream channel may fill with sediment and shift. 

Road work, such as raised embankments or 

levees, may in fact block off a stream’s access 

to its historic flood plain or an overflow channel, 

thus concentrating more flow in the channel 

that can lead to erosion and scour problems, or 

create a problem elsewhere downstream. 

The constant shifting by rivers frequently 

undermines or removes road sections, which 

are costly to repair and disruptive to travel 

(figure 116). At times, the road may capture the 

entire river, resulting in a complete relocation 

of the channel to the road alignment. A road 

located in or near the channel migration zone 

is an encroachment on the river and its natural 

function. Figure 117 shows road damage 

and a nearly total road washout caused by 

constriction of the natural stream channel by the 

roadway and fill. Note that the road is located in 

a relatively steep, narrow canyon.

 

Figure 116—Examples of roads located in flood 

plains (a) and on the edge of a channel migration 

zone (b) where flood damage may be frequent and 

costly. Road access may be lost for long periods of 

time, Pacific Northwest. (Courtesy of Roger Nichols.)

a

b
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-49

Figure 117—Road-stream encroachment where 

a road is located too near the stream in a narrow 

river canyon, confining the streamflow during a 

major storm event, Green Mountain National Forest. 

(Courtesy of Bob Gubernick.)

The usual response following road damage is 

to move the road or restrain the river with rock 

armor placed on the road fill to protect the road 

from the erosive power of the river. The roadbed 

elevation may be raised above expected flood 

levels and the fill armored to maintain the 

alignment. The river reacts to this displacement 

by shifting the erosion downstream or by totally 

removing an obstruction, such as an undersized 

bridge or culvert crossing. 

Section 5.3 covers some common treatments 

for stream encroachment and protection at 

stream crossings. Use the treatments included 

in this section where roads parallel streams and 

at stream crossings when appropriate.

In more recent road construction practices, 

road locations along streams, within flood 

plains, and within or adjacent to channel 

migration zones are viewed as high-risk sites 

that should be avoided. Close high-risk roads 

or relocate. Move roads away from the 

channel or upslope on a hillside, outside 

any potential channel migration zone. As 

storm damage occurs to roads located in these 

sites, relocation should be the first option. While 

relocation may be costly and administratively 

and physically difficult, relocating roads away 

from streams, flood plains, and channel 

migration zones will eliminate future costly 

channel encroachment repairs and loss of 

road function for extended periods. In addition, 

channel functions will be better managed if road 

features do not interfere with natural stream 

processes.

If road relocation is eliminated from possible 

solutions, a detailed review of stream processes 

at the site and geomorphic processes in the 

watershed should be used to determine the 

hydraulic forces and channel dynamics that 

need to be included in treatment selection and 

design.

The most common stream encroachment site is 

when a road encounters the outside boundary 

of a channel meander. Select treatments that 

treat the site where the road and meander are 

in contact; however, meanders migrate (laterally 

and in a downstream direction) and remember 

that the outside of a meander bend is where 

considerable stream energy is expended. 

Also aggradation of the streambed or channel 

widening due to loss of riparian vegetation can 

contribute to channel migration. A relatively 

smooth, but resistant treatment (including 

riprap) will only pass the stream energy to 

the next downstream unprotected bank, and 

likely exacerbate the problem. The following 

techniques attempt to work with stream energy 

and alter stream dynamics around the site. 

The Pacific Northwest and northern California 

have used these techniques successfully. It is 

a holistic approach, but the techniques are still 

experimental and the practices are evolving. 
6-50 Storm Damage Risk Reduction Guide for Low-Volume Roads

They appear best suited for wide, degrading 

river systems that move significant bedload. 

They also are applicable to moderate gradient 

mountain streams with low-to-moderate 

sinuosity and where whole-tree jams are 

common (Benoit, personal communication).

Floating Log Weirs

Many streambank protection treatments involve 

a rigid, massive structure designed to deflect 

flow away from the treatment site, such as 

riprap armored banks, gabions, and rock or 

log deflectors. Those structures can withstand 

the maximum expected drag forces and scour 

at the site and are built to a height equal to 

the highest design flow, plus some freeboard. 

Often these structures increase the scour at the 

structure due to how the structure interacts with 

the flow. A structure with a rigid uniform surface, 

including a riprap streambank, reduces channel 

roughness and increases water velocities. The 

result is an increase in local and downstream 

scour, which can contribute to the failure of the 

structure itself. 

Natural wood jams form on key features or logs 

and capture floating debris on the falling limb 

of the hydrograph. During a runoff event, the 

collected buoyant debris expands as it floats 

up in the water column. As the flow recedes, 

the debris settles and compacts. This process 

is repeated many times on varying sized storm 

events resulting in the debris jam increasing in 

size until the channel shifts or the river reacts 

by cutting a new channel. Secondary processes 

that accompany this dynamic jam flexing are 

continual renewal by debris collecting and 

disruption of the water column, which results in 

localized eddy formation. The backwaters and 

eddies trigger sediment deposition. Constructed 

structures that utilize these traits of natural 

wood jams can be utilized to slow the water 

velocities along the eroding bank and slowly 

increase the bed elevation through sediment 

deposition downstream. The eventual result is a 

channel shift away from the eroding bank. 

Ballasted log structures are a flexible design, 

which attempt to mimic the natural flexing of a 

wood debris jam with similar local scour and 

deposition characteristics. The technique has 

been employed successfully at a number of 

sites and minimizes the impact on fisheries 

resources. Monitoring has shown improvement 

in aquatic habitat over the project life. 

Ballasted long-line log weirs

For long-line cabled log weirs, logs are placed 

perpendicular to the streambank and cabled 

to boulders (figures 118 and 119) that help 

maintain the log positions. The cables are slack 

so that the logs float up as the flow increases 

and interact with the upper velocity vectors 

in the water column. This does not increase 

the bed scour around the structure, but rather 

interrupts flow velocities and creates lower 

velocity eddies within the structure and nearbank area. At the same time, the floating logs 

have the potential to capture other logs floating 

downstream on the surface of the flow. The 

result is the deposition of sediment within the 

structure and a shift in the stream thalweg away 

from the bank being protected (figure 120).

Figure 118—Floating log weirs just after installation. 

These weirs are designed to move the river thalweg 

away from the near bank, Warnick Bluff, Nooksack 

River, Washington. (Courtesy of Roger Nichols.) 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-51

Figure 119—Floating log-weir design and anchor details.
6-52 Storm Damage Risk Reduction Guide for Low-Volume Roads

Logs are held in place by two large boulders 

cabled to the ends of the log. Size these 

boulders to withstand the shear, buoyant, and 

drag forces expected at the site. Since the logs 

are on long cables, the shear force experienced 

by the boulders is reduced since the logs float 

up and do not exert a buoyant force directly on 

the boulders. The buoyant force is exerted at an 

angle instead of directly upward. 

A discussion of this treatment can be found in 

the “Use of Long-Line Cabled Logs for Stream 

Bank Rehabilitation” (Nichols and Sprague 

2003).

Figure 120—Warnick Bluff floating log weirs at high 

flow. Note the turbulent flow zone located outside the 

weir installation and slack water within the structure. 

(Courtesy of Roger Nichols.)

Four Log-Ballasted Long-Line Jams

Another long-line technique has been used to 

protect roads from damage. In this case, log 

jams are constructed within the river reach 

where the road-stream encroachment has 

become a problem. The log jams are designed 

and positioned to encourage the river to migrate 

or split flow energy around lateral or midchannel 

bars.

By combining three ballasted base logs and 

a rear top log (figure 121), an interactive 

log jam is formed, which amplifies debris 

capturing characteristics and additional local 

sedimentation and scour. By placing this type 

of structure in series of three, the top, middle, 

and bottom of lateral or medial gravel bars, 

it is possible to split flows and reduce river 

power significantly. This splitting of flows 

greatly reduces effective stream power and 

encourages localized deposition.

Figure 122 shows the application of the 

ballasted floating log weirs being used to slow 

the water velocity along the bank and protect 

the streambank along a highway, both after 

installation at high flow and after 7 years in 

service during a low-flow period. Riprap in 

this location had previously been inadequate 

and had allowed streambank erosion. These 

ballasted floating logs can be useful for 

streambank protection, as well as to encourage 

streambank revegetation.

Another option involves using whole trees 

cabled to the upper bank. The long-term 

objective is to move the center of flow away 

from the eroding bank while live vegetation 

takes over. Boulder vanes also can help redirect 

the flow away from the eroding bank, followed 

by sloping the eroding bank (usually to a 1:1 

slope) and vegetating the area. Live vegetation 

is the key to long-term success. 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-53

Figure 121—Design details for four log-ballasted long-line structure.
6-54 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 122—Ballested logs slowing the water along 

the bank during high flow in 2004 along State Route 

542, Washington (a). Same location at low flow 

in 2011 (b). Note the streambank erosion in the 

riprap (in 2004) that took place before installing the 

ballested logs. (Courtesy of Roger Nichols.)

Other Channel Redirection and Bank 

Protection Measures (Spurs, Veins, 

Drop Structures, and Log Jams)

Floating log weirs were discussed in the 

previous section and some bank armor and 

hard and soft protection measures were 

discussed in section 6.2. In addition to these 

measures, other treatments have been 

developed to help protect structures and are 

used as river-training structures to confine, 

direct, or focus waterflow, as well as provide 

some armoring and energy dissipation. These 

include use of spurs, barbs, dikes, J-hooks, 

drop structures, guidebanks, vanes, and so 

forth. These treatments are useful when flow 

needs to be deflected or directed away from 

the streambank, a structure, or the road. Again, 

moving the road or structure may be the best 

long-term solution. 

Flow redirection techniques include using spurs, 

dikes, jetties, vanes, groins, barbs, engineered 

log jams and large woody debris, boulder drop 

structures, and porous weirs. They are most 

commonly constructed with rock or boulders, 

and frequently used in conjunction with 

vegetation, but they also may be constructed 

using gabions, wood posts, fencing, concrete, 

or other materials. Rock cross veins, boulder 

drop structures, and stone weirs are used to 

focus the waterflow, prevent channel erosion 

and headcutting, as well as provide pool 

habitat. Examples are shown in figure 123. 

Figure 123—Flow redirection or training measures 

used to protect streambanks and structures, such 

as spur dikes or rock vanes (a), and rock drop 

structures or weirs (b). (Photos courtesy of John 

McCullah.)

a

b

a

b
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-55

A textbook that addresses many considerations 

in river dynamics, channel morphology, 

assessment of stream condition, and effects of 

structures on the channel (and vice versa) is 

“Applied River Morphology” by David Rosgen 

(1996). 

Four other references (previously mentioned) 

that contain information on streambank 

stabilization, channel control, and flow direction 

measures are: 

q “Environmentally Sensitive Channel and 

Bank Protection Measures” (McCullah and 

Gray 2005).

q HEC 23 “Bridge Scour and Stream 

Instability Countermeasures” (Lagasse et 

al. 2009). 

q “Streambank and Shoreline Protection, 

Chapter 16” (NRCS 1996). 

q “Integrated Streambank Protection 

Guidelines” (Cramer et al. 2003).

6.5 Erosion Control

Erosion control on roads and disturbed areas 

is fundamental for the protection of water 

quality, particularly during major storms. The 

two main causes of erosion are concentration 

of flowing water and lack of ground cover over 

the soil. Many erosion control treatments are 

low cost, effective, and commonly applied. 

Vegetative ground cover is the primary longterm defense against surface erosion. Section 

5.5 discussed a number of basic erosion control 

and protection measures. 

Some erosion situations and treatments are 

more complicated and costly to implement, but 

have great long-term benefits in environmental 

protection. Some soil bioengineering and 

biotechnical slope stabilization measures can 

offer excellent and aesthetic slope treatments, 

but most are labor-intensive to install. Gullies, 

as mentioned previously, are a particularly 

severe form of erosion, and efforts should be 

made to prevent their formation, particularly 

those caused by concentrated runoff from 

roads. Section 5.5 discussed prevention criteria. 

However, once a gully has formed, various 

measures exist to help stabilize the area or 

prevent it from getting larger. 

Gully Stabilization

Stabilization of gullies typically begins with 

removing or reducing the source of water 

flowing through the gully. The following 

information also applies to gully formation 

caused by road drainage features or poor 

land-use practices. Typically, gully stabilization 

structures are not desirable on natural stream 

channels. Only use these structures in natural 

drainages to correct some severe problems, 

and then only after careful study, examining 

a range of alternative solutions. For gullying 

of ditches or gully formation below the road, 

the best solution is prevention by adding more 

frequent drainage features (ditch-relief culverts) 

to reduce the concentrated water volume (as 

discussed in section 5.5).

If a gully already exists, stabilization measures 

help, but the first step is to remove as much of 

the source of the water as possible. With time 

and the source of water removed, a gully may 

tend to stabilize itself, particularly if the area 

can be revegetated. However, as a common 

practice, the gully can be stabilized with 

vegetation or refilled with dikes, or small dams, 

built at intervals along the gully. Reshaping and 

stabilizing the over-steepened gully sides also 

may be needed, as well as some treatment on 

the headcut area. Gully stabilization can require 

a lot of work and expense. 

Typical gully stabilization check-dam structures 

are constructed from materials including rock, 

gabions, logs, wood stakes with wire or brush, 

rock and brush, bamboo, vegetative barriers, 

and so forth. Each has its advantages and 

disadvantages. Loose rock structures are 
6-56 Storm Damage Risk Reduction Guide for Low-Volume Roads

somewhat forgiving and can deform a bit and 

still function. The rock should be well graded, 

but it will still be porous so it detains water 

temporarily and then attenuates the runoff. 

Gabions commonly are used but can be 

problematic and need extra care in installation. 

They are subject to settlement, piping through 

the rock, and flow diversion around or under the 

structure. Place a geosynthetic filter material 

behind and under the structure to minimize 

piping. Examples of log and loose-rock 

structures are shown in figure 124. Biotechnical 

methods offer a combination of physical 

structure and vegetative measures for physical 

protection, as well as additional long-term root 

support and aesthetics. Typically, a headcut 

structure is needed to stabilize the upslope, 

or top-most portion of the gully and prevent 

additional headward movement. 

Figure 124—Gully stabilization structures made of 

logs (a) and of loose rock (b).

Important design details for gully stabilization 

structures are shown on figure 125. They 

include:

q Have a weir, notched, or U-shaped 

spillway that is constructed for the design 

peak flow capacity to keep the water flow 

concentrated in the middle of the channel.

q Key the structure into the adjacent banks 

tightly and far enough to prevent erosion 

around the ends of the structures.

q Bury the structures deep enough in the 

channel to prevent flow under the structure.

q Spill the water over the structure onto a 

splash apron, protective layer of rock, or 

into a pool of water to prevent scour and 

undermining of the structure. 

q Space the structures close enough so 

that the flow over the structure spills into 

backwater caused by the next structure 

downstream (figure 126).

q Use of well-graded rock is important if rock 

check dams are used so that the structure 

is relatively impermeable. Use of vegetation 

with the rock will add integrity to the 

structure.

The recommended spacing for structures 

depends on the slope of the terrain or gully 

channel, and the height of each structure, 

as presented in figure 126a. This physical 

relationship between structure height and 

spacing in a sloped channel is so that water 

and material stored behind the lower structure 

is level with the toe of the upper structure. This 

allows water from the upper structure to spill 

into the pool above the lower structure, thus 

minimizing toe scour at the upper structure 

(figures 125 and 126b). Figure 126c shows 

the arrangement of a series of rock check-dam 

structures in a channel (as well as installation 

details). 

a

b
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-57

Another consideration is height of the 

structures. Occasionally, one large dam is built 

in an area to fill and stabilize an entire gully 

system. Also, large structures typically are 

built as debris-retention dams where massive 

quantities of sediment and debris are moving 

in a canyon, such as some steep Southwest 

arroyos. Generally, a series of small, well-built 

structures is more desirable than one or a few 

large structures, particularly if grade control 

is the objective to keep a gully from eroding 

deeper and to reduce the effective gradient 

of the channel. Smaller structures are more 

versatile, easier to build using local materials, 

and have less fall at the outlet that can cause 

erosion. Also, there is less risk of a catastrophic 

failure of just one structure. Structures typically 

are less than 5 feet (1.6 m) high.

In gully prevention proper erosion control 

measures result in less sediment and erosion 

caused damage. Take action to prevent 

Figure 125—Check dams used to control or stabilize a gully. Note the details for keying the structure into the 

gully, having a U-shaped weir, and protecting the toe area against scour and with other downslope structures. 
6-58 Storm Damage Risk Reduction Guide for Low-Volume Roads

a

b

c

Figure 126—Ideal gully control check-dam spacing criteria (a) and installation details for loose rock-check 

dams (b and c). Note that water from the upper structure should spill into the pool above the lower structure 

(adapted from Gray and Leiser 1982). Note: 1 meter = 3.28 feet. 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-59

the formation of gullies, remove or divide the 

source of water, and stabilize existing gullies 

before they grow larger. Once large, gully 

stabilization measures can be very expensive. 

Other Gully Control Considerations

In addition to removing the source of water, and 

building gully control structures (check dams), 

several other concepts are important and may 

be useful for controlling gullies. 

Zeedyk et al. (2006) discusses some healing 

principles for gullies including: 

q Disperse upland flow to prevent water 

concentration and increase infiltration and 

percolation.

q Reduce channel slope to reduce runoff 

velocity.

q Widen the channel bottom to lessen 

erosion forces.

q Increase channel roughness.

q Retain soil moisture to improve conditions 

for plant growth.

Gully repair or stabilization projects should 

begin with an assessment of the scope of 

the problem, the severity of the conditions, 

and whether water control and revegetation 

treatments will be adequate or if structures 

are needed. Global climate change, wildfires, 

and intense rainstorms all contribute to more 

potential for severe gully problems. Also, clarify 

the objective concerning grade control or 

sediment retention check dams, or both. 

Methods to minimize gully problems include 

reforestation in the watershed above a gully 

and planting vegetation to reduce the amount 

of runoff, as well as other erosion control 

measures. Treatments may include large 

gully plugs that provide both grade control 

and sediment storage. Adopt a pond-and-plug 

design to fill in parts of the gully and establish a 

new waterway. A variety of check-dam designs 

and materials have been used, as discussed 

above. Reshape and design the gully area as 

a waterway if the water cannot be diverted. 

Alternatively, reshape the entire gully area 

to eliminate the gully, but the area needs to 

include diversion ditches or waterways to 

accommodate any flows. 

Treat headcut areas at the top of the gully 

specifically to prevent the gully from migrating 

further headward. Typically, the area must be 

reshaped and smoothed, and backfilled with 

rock to form a rock-filled basin that will resist 

erosion. Usually, there is a drop, or change 

in elevation from the natural ground level to 

the bottom of the gully. The armoring must 

accommodate this drop yet remain stable. 

Other designs have used logs, biotechnical 

meansures, or materials that form steps and 

break up or divide the fall of the water. 

References on gully control and stabilization 

are: “Biotechnical Slope Protection and 

Erosion Control” (Gray and Leiser 1982) and 

“Gullies and Their Control” (U.S. Department of 

Agriculture, Natural Resources Conservation 

Service 2007). The Natural Resources 

Conservation Service document is available 

at <http://www.nae.usace.army.mil/reg/nrrbs/

TECHNICALSUPPLEMENTS/TS14P.pdf>.

“Gully Development and Control: The Status 

of our Knowledge” (Heede 1976) is a classic 

document and basic primer on gully formation, 

gully stabilization techniques, and types of 

control structures. 

“An Introduction to Erosion Control” (Zeedyk 

et al. 2006) offers an overall perspective and 

useful suggestions for erosion and gully control, 

particularly for semiarid conditions. Available 

at <http://www.comanchecreek.org/images/

links/115-Erosion%2520Control%2520Field%25

20Guide.pdf>.
6-60 Storm Damage Risk Reduction Guide for Low-Volume Roads

Roadway Surfacing Materials

Roadway surfacing materials, such as gravel, 

crushed aggregate, or even a rocky soil, can 

be useful to reduce road surface erosion. An 

aggregate surfacing also can make the road 

more resistant to washing and damage during 

major storm events. Roads that have had a 

dense surfacing material have withstood severe 

storm events much better than unsurfaced 

roads or roads with a loose, poorly graded 

material (Copstead and Johansen 1998). 

Over time, roadway surfacing is displaced or 

worn out and is ineffective at protecting the 

road surface from erosion and traffic damage. 

Therefore, road surface material replacement 

often becomes an important SDRR treatment. 

Fortunately many main collector roads in 

the Forest Service currently are surfaced 

with aggregate, particularly in the Pacific 

Northwest. However, many roads need surface 

replacement and many lower standard roads 

would benefit from armoring with a surfacing 

aggregate.

Key characteristics of the aggregate to be most 

resistant to flowing water are to have a dense 

gradation, be well compacted, have angular 

rock particles, and have adequate plasticity 

to resist ravel, particularly in arid to semiarid 

regions of the West. In high rainfall areas, such 

as the East, plasticity may not be desirable 

since it can be a source of sediment. Thus, a 

dense gradation with adequate fines and good 

compaction are all the more important.

A dense graded 1- to 2-inch (25 to 50 mm) 

maximum size aggregate is ideal. Coarser 

aggregate may seem better than a small 

aggregate to resist erosion, but the coarser 

aggregate also tends to ravel more during 

normal road use, likely leaving the road surface 

loose if a storm hits. Figure 127 shows a dense, 

well-graded aggregate on a forest road, with 

minimal ravel.

Information on use and properties of roadway 

materials, both for structural support and 

erosion control, is found in “Stabilization and 

Rehabilitation Measures for Low-Volume Forest 

Roads” (Keller et al. 2011). This document is 

available by chapters at <http://www.fs.fed.us/

eng/php/library_card.php?p_num=1177%20

1801P>. Also see the FHWA/LTAP publication 

“Gravel Roads Maintenance and Design 

Manual” (Skorseth and Selim 2000). It is 

available at <http://water.epa.gov/polwaste/

nps/gravelroads_index.cfm> and <http://

www.mnltap.umn.edu/publications/videos/

gravelroadmaintenance/documents/manual.

pdf>.

Figure 127—A dense, compact, well-graded 

roadway aggregate that is making the road surface 

more resistant to erosion and washing.

6.6 General Slope Stabilization 

Measures 

Achieving Stable Cut and Fillslopes

Cut and fillslopes are routinely constructed in 

new construction or road reconstruction and 

repair projects. Over-steep cut and fillslopes are 

a hazard during storm events and risk failure. 

Failed slopes typically need to be repaired or 

removed. They usually do not involve analysis, 

but rather are constructed at slope angles 

thought to be stable based on local experience 

and general guidelines. Guidelines are meant to 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-61

produce stable slopes in most soils most of the 

time. If a specific problematic, unstable, or wet 

area is encountered, the road can be realigned 

around this area or the slope can be flattened, 

drained, or a retaining structure or a buttress 

built. Designs typically are site specific and may 

require input from geotechnical engineers and 

engineering geologists. Figure 128 shows the 

typical range of stable cut and fillslopes. Slopes 

steeper than those presented in the figure 

have a high risk of failure during a major storm 

event. Table 6 also presents common stable 

slope ratios for cuts and fills in a variety of soil 

and rock types. Existing slopes steeper than 

these values again have a risk of failure during 

storms.

Add simple rock buttresses, gabions, or 

geosynthetic reinforced soil or mechanically 

stabilized earth walls to improve site stability. 

Drainage improvements might include ditches, 

cutoff trenches, or horizontal drains. The use 

of deep-rooted vegetation is always desirable 

on marginally stable slopes, and it is fairly 

inexpensive. Live stakes, brush layers, and 

various soil bioengineering or biotechnical 

treatments with vegetation are common to 

improve slopes and the stability of structures.

Figure 128—Typical conditions and treatments for stable cut and fillslopes. Figure is adapted from “Stream 

Corridor Restoration,” by the U.S. Department of Transportation, Federal Interagency Stream Restoration 

Working Group (1998).
6-62 Storm Damage Risk Reduction Guide for Low-Volume Roads

Table 6—Common stable slope ratios for varying soil/rock conditions

 Soil/Rock Condition Slope Ratio (Horizontal:Vertical)

Most rock 1/4:1 to 1/2:1

Very well cemented soils 1/4:1 to 1/2:1

Most in-place soils 3/4:1 to 1:1

Very fractured rock 1:1 to 1 1/2:1

Loose coarse granular soils 1 1/2:1

Heavy clay soils, volcanic ash 2:1 to 3:1

Soft clay rich zones or wet seepage areas 2:1 to 3:1

Fills of most soils 1 1/2:1 to 2:1 

Fills of hard, angular rock 1 1/3:1

Low cuts and fills (<7-10 ft) (<2-3 m.) 2:1 or flatter (for revegetation)

Note: All slope references are shown as Horizontal:Vertical (H:V). However, current Federal Highway Administration-FP03 

Specifications use a designation of Vertical:Horizontal (V:H). FILL CUTS

Two general references on slope stability 

assessment, slide recognition, investigation, risk 

analysis, and slope stabilization treatments are: 

q Transportation Research Board Special 

Report No. 247, “Landslides-Investigation 

and Mitigation” (Turner and Schuster 1996).

q The three-volume Forest Service 

publication “Slope Stability Reference Guide 

for National Forests in the United States” 

(Prellwitz and Steward 1994). 

Links to the Forest Service documents: <http://

www.fs.fed.us/rm/pubs_other/wo_em7170_13/

wo_em7170_13_vol1.pdf>.

<http://www.fs.fed.us/rm/pubs_other/wo_

em7170_13/wo_em7170_13_vol2.pdf>.

<http://www.fs.fed.us/rm/pubs_other/wo_

em7170_13/wo_em7170_13_vol3.pdf>.

The National Cooperative Highway Research 

Program Synthesis 430, “Cost-Effective and 

Sustainable Road Slope Stabilization and 

Erosion Control” (Fay et al. 2012), addresses a 

variety of relatively low-cost solutions for slope 

stabilization, including vegetative, biotechnical, 

and structural solutions. 

Cutslopes

For most cutslopes, typical slope angles are 

prescribed based upon the general soil or rock 

type found in that area and field observations. 

For most rocky, silty-to-sandy soils in the 

Western United States, cutslopes of 1:1 or 

3/4:1 (H:V) are used. In rock cuts and rocky or 

cemented soils, use near vertical cutslopes, or 

use a 1/4:1 slope, as shown in figure 129. In 

clay rich, fine-grain soils or zones of saturation, 

flatter slopes such as 2:1 or 3:1 may be 

required for stability. 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-63

A balanced cut-and-fill design is used in 

gentle terrain with slopes less steep than 50 

percent where material excavated from the 

cutslope is placed into the adjacent roadway 

fill embankment. On slopes in the 50- to 

65-percent range, the suitability and stability 

of balancing the cut-and-fill material should 

be evaluated carefully, depending on local soil 

conditions. On steeper slopes, a steep fullbench cutslope is used with no fillslope. Note 

that stable cutslope angles are very particular to 

local soil, weather, and groundwater conditions. 

Local experience is invaluable. Values 

presented in this publication should be adapted 

for your local conditions and based upon local 

observations.

Fillslopes

A slope of 1 1/2:1 to 2:1 is recommended 

for fillslopes constructed with the majority of 

common soils. Rock fills can be stable on 

slopes as steep as 1 1/3:1 or even 1:1 with 

angular rock and careful placement. To achieve 

good vegetative stabilization on a constructed 

fillslope, the slope should be 2:1 or flatter, 

especially for low fills. 

Figure 130 shows the construction of typical fills 

under a variety of conditions and natural ground 

slopes. Routine fills or through fills placed upon 

relatively flat ground, with a slope less than 30 

percent, are commonly built with a 

Figure 129—Commonly used stable cutslope angles. 
6-64 Storm Damage Risk Reduction Guide for Low-Volume Roads

1 1/2:1 slope. If the ground is relatively flat 

and a fillslope will easily catch and not be 

excessively long, a 2:1 or flatter fillslope is used 

to help promote the growth of vegetation. On 

ground slopes steeper than 30 to 40 percent, 

place the base of the fill upon a terraced 

surface to key the fill into the slope and prevent 

a failure along the plane of contact between 

the fill and natural ground. The foundation is 

critical to the fill’s stability. On slopes over 50 

to 65 percent, construct a full-bench road and 

no fill. Alternatively, design and construct a 

reinforced soil fill, using layers of geosynthetic 

reinforcement. End-haul excavated cutslope 

material to a designated stable disposal area or 

fillslope area on flatter ground.

Figure 130—Common fillslope design requirements and slope angles.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-65

Thin sliver fills are a common problem on 

old roads where construction was done by 

sidecasting the fill material on steep slopes. 

The material may only be 2 to 4 feet (0.6 to 

1.2 meters) wide at the road surface elevation. 

These slopes commonly have failures or 

problems with settlement, particularly during 

major storms. Repair of these sites by 

pulling back the fill material was described 

in section 5.6. Also, using relatively flat 

fillslopes or avoiding fillslopes and using fullbench construction might be considered at 

the approach to stream channel crossings, 

particularly if the stream channel or sideslopes 

are deeply incised or steep. This will minimize 

sidecast material getting into the stream 

channel.

Increasing Stability with Structures

Retaining structures are used in many 

applications with roads. Their primary use is 

to resolve a space constraint in steep ground, 

where a wall is needed to support the roadway 

in a tight location and to avoid a large cut or 

fill. They also are used to rebuild the roadway 

where fills fail to avoid cutting into a hillside in 

a slide area, to support a roadway across a 

steep, narrow saddle, to buttress a marginally 

stable slope, and to provide vertical, lowprofile abutments for bridges. The Forest 

Service publication “Retaining Wall Design 

Guide” (Mohney 1994) offers a comprehensive 

coverage of basic retaining walls, their use, 

selection, design details for a variety of wall 

types, and sample calculations. It is available 

on the Association of Environmental and 

Engineering Geologists, Geoscience Library 

Web site under section 5, Transportation 

Geology, Low-Volume Roads Collection, Slope 

Stability Issues. (To access this site, you must 

register initially with GeoSci Library at <http://

www.geoscilibrary.org>.)

Retaining walls are relatively expensive 

structures so they are not routinely used 

for SDRR work. They should not be used 

without looking at other options, such as road 

relocation, cutting into the hillside to place the 

road prism on a full bench, using a reinforced or 

rock fill, and so forth. However, when needed, 

walls offer a positive solution to support 

the roadway. Their use can avoid creating 

additional slope stability problems, avoid long 

fillslopes that may be erodible or unstable, 

and keep the toe of fills out of drainages (all 

of which can have adverse environmental 

impacts). Walls constructed into cutslopes 

need to be designed and constructed to allow 

ditch cleaning without undermining the wall or 

damaging its facing.

Design and construct walls considering the 

existing or potential failure plane of any slide 

and the depth of failure. Determine the size and 

height of the wall based upon slope stability 

analysis to ensure that the structure will have 

enough mass to resist the driving forces of the 

slide or slope. A wall needs to be deep (tall) 

enough to have its base placed upon firm, inplace material (ideally bedrock) below the depth 

of slide movement or a slide plane. Build walls 

with a subsurface drain behind the structure. In 

some cases a lightweight structure, constructed 

with sawdust, shredded tires, or geofoam, can 

be designed to minimize the driving forces if 

the wall is placed on a slide or has a marginal 

foundation material.

Several basic types of retaining structures exist, 

with a variety of wall options within each type. 

The fundamental types are:

q Gravity retaining structures where the 

mass of the structure resists sliding and 

overturning, such as gabions, cantilever 

concrete, concrete blocks, or cribwalls.

q Earth reinforced systems where the 

backfill material is actually reinforced with 

reinforcing layers of material, such as 

welded wire, geogrids, or geotextiles, to 

form a composite unit, which becomes the 

wall.
6-66 Storm Damage Risk Reduction Guide for Low-Volume Roads

q Special types, such as cantilever H-piles 

or tieback walls, which are used in special 

applications, such as high walls on very 

steep slopes or bedrock areas to avoid 

excavation. 

q Simple rock buttresses also are used in 

many small slump repairs. 

Figure 131 shows two simple retaining 

structures, a rock buttress (a) and a gabion 

retaining wall (b).

Alternatively, less expensive soil bioengineering 

or biotechnical measures, such as live crib 

structures, vegetated rock walls or vegetated 

gabions, which rely both on the engineered 

structure and the anchoring effects of roots from 

vegetation, may be appropriate for small slopes. 

Consult a geotechnical or geological engineer 

when selecting and designing retaining 

structures.

Figure 131—Commonly used retaining structures, 

such as a rock buttress (a) and a gabion wall (b).

a

b
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-67

Gravity Structures. The most common 

gravity structures are those made of reinforced 

concrete, cellular bins, gabions, masonry, 

dry rock walls, or large rocks. The size of 

the structure depends on the height of the 

wall needed to fit the site and provide the 

desired roadway width and elevation, loading 

conditions on the wall, and allowable foundation 

conditions. Common heights for gravity 

structures are a few feet (a meter) to 25 feet (8 

meters). Above this height, gravity structures 

become relatively difficult and expensive to 

build. For simple gravity structures, the base 

width typically is about 0.6 to 0.7 times the 

height to achieve a stable design for simple 

loading conditions. With traffic loading, the 

base-to-height ratio ranges from 0.6 to 0.8. 

For a hillslope immediately above the wall, 

the base-to-height ratio ranges from 0.7 to 

1.0. A wider base may be needed for unusual 

conditions, such as a soft foundation, high 

lateral loads, or seismic loads. Set any structure 

onto firm, in-place materials.

Also, use gabions in stream channels to 

buttress the toe of a fillslope and prevent 

scour of the fill, or for streambank protection, 

particularly on the outside bend of a stream 

near a structure. They are an alternative to 

loose rock riprap or other bank stabilization 

measures. However, with time, the wire baskets 

will corrode or wear through from abrasion. 

After 20 to 30 years, many gabion structures 

in a stream environment begin to fail. Their 

life can be maximized by use of galvanized 

or plastic-coated wire. Gabions also are 

susceptible to piping of soil under or behind the 

basket, so install them on a filter blanket, such 

as a geotextile.

Earth Reinforced Systems. Today, use of 

earth reinforced systems, reinforced soils, 

geosynthetic confined soils, or mechanically 

stabilized earth (MSE) walls, as they are 

commonly called, offers an economical and 

effective alternative to traditional gravity 

type structures for most wall heights and 

applications. Actually, the use of reinforcement 

fibers to strengthen soil has been a concept 

used since biblical times. For walls over 25 

feet (8 m) high, MSE walls offer significant 

cost advantages over gravity structures. In the 

case of rural or forest low-volume roads, where 

the access may be very difficult and when 

the budget is limited, the use of prefabricated 

or lightweight materials, combined with local 

or onsite soils, as used in MSE technology, 

generally is recommended.

Soil reinforced gabion designs also have been 

developed where typical gabion baskets form 

the face and gabion wire mats are used to 

reinforce the backfill. Reinforcing spacing is 

typically 3 feet (1 m) (the height of a basket). 

The length of reinforcement is a function of 

the wall height and loading conditions, similar 

to other MSE designs. Advantages of this 

design are the comfort people have using 

traditional gabion baskets, combined with 

reinforced soil technology. Fewer baskets go 

into any moderately high wall compared to a 

conventional gravity structure and use of a 

reinforced soil backfill reduces cost. Figure 132 

depicts the construction process of a reinforced 

soil wall with a gabion face.
6-68 Storm Damage Risk Reduction Guide for Low-Volume Roads

Reinforced soil or mechanically stabilized earth 

retaining structures use strength properties of 

wire or geosynthetics and soil reinforcement 

concepts. MSE structures include welded wire 

walls, geotextile reinforced walls, modular 

block walls, lightweight wood or sawdust walls 

reinforced with geosynthetics, and so forth. 

MSE walls use a variety of facing materials, 

including tires, wood beams, straw bales, 

modular concrete blocks, gabions, concrete 

panels, geotextile or turf reinforcing mats, and 

other facings. Soil reinforcement commonly 

is achieved using geotextile and geogrid 

reinforcing layers, though welded wire, chain 

link fencing, metal bars, and metal strips 

have been used. MSE structures are most 

commonly constructed today because of their 

flexibility, minimal foundation pressure, ease 

of construction, and relatively low cost. Figure 

133 shows two earth reinforced (MSE) types of 

retaining structures commonly used on forest 

roads. Note that the geotextile reinforcing and 

facing shown on the photo on the right will 

degrade with time, often a few years, when 

exposed to the sun. Thus, at a minimum, 

add some ultraviolet protection, such as an 

application of asphalt emulsion, and ideally a 

durable permanent facing, such as shotcrete or 

concrete.

Figure 134 shows a typical drawing for a 

relatively simple MSE wall using geotextile for 

reinforcement and a geotextile-wrapped face. 

Note that the face does need to be protected 

in some way against damage from ultraviolet 

light (the sun) and possibly vandalism. Note 

again that layers of vegetation could easily be 

introduced between the layers of geosynthetic 

reinforcement to improve both the long-term 

stability and the aesthetics of the structure. 

Figure 132—A soil reinforced gabion design. (Terramesh, courtesy of Maccaferri Gabions Inc.) Note that this 

design could easily be improved by incorporating layers of vegetation between the baskets.
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-69

a b

Figure 133—A welded wire MSE wall (a) and a geotextile wall (b), both used because of their relatively low 

cost and ease of construction.

Figure 134—Typical drawing for a simple geotextile faced retaining wall.
6-70 Storm Damage Risk Reduction Guide for Low-Volume Roads

Comprehensive information on MSE wall 

design and construction is found in the FHWA 

publication titled “Design and Construction 

of Mechanically Stabilized Earth Walls and 

Reinforced Soil Slopes,” Volume 1 and 2 

(Berg et al. 2009). Additionally comprehensive 

information on general use of geosynthetics is 

found in the FHWA publication “Geosynthetic 

Design and Construction Guidelines – 

Reference Manual” FHWA-NHI-07-092 (Holtz et 

al. 2008).

Drainage Improvements

Localized wet areas, clay-rich or deeply 

weathered soil pockets, and shear or fault 

zones require relatively flat cutslopes to reduce 

the risk of failure. Seeps, springs, or wet areas, 

often recognized by water-loving vegetation, 

almost always require special consideration and 

drainage. In any excavation, the water table 

should be below the exposed surface (where 

practical) to prevent instability. If an excavation 

opens a wet area, or a fill is placed on a wet 

area, take extra measures to drain the slope, 

flatten the slope more than normal, or buttress 

the toe of the slope. A stable wet slope angle 

may be roughly half the angle of the same 

stable dry slope. Drain slopes using surface 

ditches, cutoff trenches, collection galleries, 

horizontal drains, and so forth as discussed 

below.

Use drainage measures, including cutoff 

trenches or underdrains (figure 135), toe 

drains, drainage blankets or filter blankets 

(figure 136), or horizontal drains to remove the 

water and lower pore-water pressures within 

the slope. Any reduction in the water table or 

pore pressures in the slope will improve slope 

stability. Place underdrains, as seen in figure 

Figure 135—Typical road underdrain or trench drain, enveloped in a geotextile, used to remove subsurface 

water. 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-71

135, along roads in wet cutslope and seepage 

areas. They typically are constructed with a 

filter aggregate and perforated pipe wrapped 

in a geotextile, or with a geocomposite drain 

comprised of a core material and geotextile 

wrapped around the core to serve as a filter. 

Drainage measures typically are less expensive 

than walls, buttresses, slide removal, and so 

forth, and can greatly improve the slope’s factor 

of safety. However, drainage measures often 

are difficult to predict in terms of effectiveness 

and reliability. Install piezometers (water level 

monitoring or observation wells) if necessary 

to measure the groundwater level and the 

effectiveness of drainage measures.

Install deep internal drains, such as the 

horizontal drains being drilled in figure 137, to 

lower the groundwater table and to intercept 

groundwater before it reaches the face of the 

slope. A road cut may intercept groundwater 

that creates high pore pressures within the 

cutslope. This increases the risk of a cutslope 

failure. Horizontal drains change the direction 

of the groundwater flow, remove water, and 

reduce the water pressure before water reaches 

the face of the slope. The Washington State 

Department of Transportation has a publication 

that offers a comprehensive treatment of 

groundwater issues and horizontal drain 

designs, "Design Guidelines for Horizontal 

Drains Used for Slope Stabilization." The 

publication is available at <http://www.wsdot.

wa.gov/research/reports/fullreports/787.1.pdf>

Figure 137—Horizontal drains being drilled to drain 

groundwater and stabilize a large landslide.

Figure 136—A filter blanket drain used under a fill to provide subsurface drainage for the fill placed upon a wet 

or spring area. 
6-72 Storm Damage Risk Reduction Guide for Low-Volume Roads

Use of Deep-Rooted Vegetation or Soil 

Bioengineering/Biotechnology

Vegetative slope stabilization is achieved using 

soil bioengineering methods or biotechnical 

methods, particularly for shallow failures. 

Vegetation use is encouraged since it is 

relatively inexpensive, though labor-intensive, 

and it improves slope stability. Advantages 

of soil bioengineering are low initial cost; a 

visually pleasing result using natural, biological 

systems; and minimum long-term maintenance.

Vegetative stabilization works well on most 

projects (as mentioned in section 5.5). 

Vegetation as a slope treatment and the 

benefits of root strength are shown in figure 

138. Vegetative and soil bioengineering 

measures are appropriate for surface erosion 

control and shallow slope failures, such as 

debris slides and small cutslope failures. Do 

not use vegetative stabilization by itself for 

stabilizing large and deep-seated slides. Use 

deep-rooted shrub and tree species rather 

than shallow-rooted grasses for most slope 

stabilization applications. Conservation of 

topsoil is encouraged for later placement 

on slopes to aid in the process of slope 

revegetation. Also, vegetation and slash placed 

at the toe of any slope or fill will help control 

erosion and trap sediment coming off that 

slope. Figure 139 shows photos of cutslopes 

stabilized with vegetation.

Live stakes for slope stabilization are shown 

in figure 140 as a drawing and a photo. Brush 

layering (figure 141) also is a very effective 

vegetative soil bioengineering technique for 

slope stabilization. Figure 142 shows an entire 

hillslope being stabilized using brush layering. 

Ground cover with grasses, ideally mixed with 

deep-rooted shrubs and trees, also is effective 

for erosion control and preventing surface 

instability.

As also discussed in section 5.5, biotechnical 

stabilization is a combination of vegetation 

and conventional structures used for slope 

stabilization. Some of the more common types 

of biotechnical slope stabilization measures 

include live cribwalls, vegetated rock walls and 

gabions, and vegetated reinforced soil slopes. 

These treatments depend on the strength and 

design of the traditional structure combined 

with the supplemental benefits of root strength 

and the long-term durability and aesthetics 

of vegetation. Figure 143 shows photos of 

two vegetated live cribwall designs, as well 

as sketches of a vegetated rock wall and a 

vegetated reinforced soil slope. In vegetated 

reinforced soil structures, the roots of the woody 

vegetation have a very real and important 

stabilizing role as they knit the system together. 

Root penetration through the geosynthetic 

is anticipated to provide a composite root/

geosynthetic structure with a net gain in 

reinforcement strength.

Strengthening Existing Structures

Soil Nails

Launched soil nails are a rapid, economical 

alternative to recurring maintenance or 

other reconstruction solutions, particularly 

for road-shoulder failures. Often workers 

can fix several small fill failures in one day 

without any excavation. They can move the 

launcher easily between trees and shrubs 

with little or no vegetation removal and little 

need for environmental or visual mitigation. 

The launcher, which is mounted on a tracked 

excavator, uses suddenly released highpressure air to project steel nails up to 1.5 

inches (37 mm) in diameter and up to 20 feet 

(7 m) in length into the soil to depths ranging 

from 5 to 20 feet (1.5 to 7 m). Fiberglass bars 

are now available that can be launched into 

corrosive soils. Hollow bars with drilled holes 

also can be launched that serve as
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-73

Figure 138—Use of deep-rooted vegetation to stabilize slopes (A) and the benefits of root strength (B) to help 

stabilize a slope.
6-74 Storm Damage Risk Reduction Guide for Low-Volume Roads

a

Figure 139—A variety of vegetation used to stabilize cutslopes. Selection of vegetation type depends a great 

deal on the local climate and environment, but deep-rooted species are critical to achieve slope stabilization. 

(Right photo courtesy of the Pennsylvania State University Center for Dirt and Gravel Road Studies.)

Figure 140—Use of live stakes for slope stabilization. (Sketch (a) courtesy of Robbin B. Sotir & Associates, 

Inc.)
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-75

Figure 141—Brush layering used for slope stabilization and erosion control on large or small slopes. Brush 

layers are labor intensive but very effective, particularly for shallow slope stability and erosion control. 

(Adapted from NRCS Engineering Field Handbook, Chapter 18, 1992.) 
6-76 Storm Damage Risk Reduction Guide for Low-Volume Roads

a b

a

b

Figure 142—Before (a) and after one season (b) photos of brush layering used to stabilize an entire hillside 

after storm damage in Pakistan. (Photos courtesy of Asif Faiz, World Bank.)

Figure 143—Biotechnical treatments using vegetation to strengthen timber cribwalls (upper photos), a rock 

retaining wall (a), and a geosynthetic reinforced fillslope (b). (Right photo courtesy of Neha Vyas, India.) 

(Lower sketches courtesy of Robbin B. Sotir & Associates, Inc.)
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-77

tensile inclusions and horizontal or vertical 

drains. Threaded bars can be used as well 

for tie downs and micropiles. The number, 

depth, and spacing of the nails depend on 

the slide geometry and loading conditions. 

Site investigation and analysis are needed to 

develop a soil-nail design.

The launcher, because it is mounted on a 

tracked excavator, can reach very remote 

locations to install nails and drains. One of its 

most useful applications is to stabilize roadway 

shoulder fill failures and shallow slides rapidly 

and without needing excavation for a wall that 

can result in long traffic delays. Figure 144 

shows a sketch of a roadway shoulder failure 

stabilized with launched soil nails. Also use soil 

nails to stabilize a toe zone for the foundation 

of a retaining wall or launch them through some 

failing walls.

For additional technical information about the 

launched soil nails, consult U.S. Department of 

Agriculture, Forest Service (1994 a, b).

Link to volume 1 <http://www.fs.fed.us/

eng/pubs/pdf/em7170_12a.pdf>. Link to 

volume 2 <http://www.fs.fed.us/eng/pubs/pdf/

em7170_12b.pdf>.

Currently launched soil nails is a proprietary 

method used by Soil Nail Launcher, Inc. Link 

to its Web site at <http://soilnaillauncher.com/

dnn/>.

Deep Patch Shoulder Strengthening

Uncompacted sliver fills (as discussed in the 

previous section) often settle progressively, 

are a maintenance problem, and are at risk of 

failure. Certain maintenance approaches are 

considered inexpensive methods of dealing with 

settlement because road maintenance crews 

can do the work as part of their normal routine. 

These methods usually consist of grading over 

the areas of settlement and cracks (aggregatesurfaced roadway) or filling cracks and adding 

asphalt (paved roadway) to level the road 

surface. While these approaches temporarily 

restore the road’s driving surface, the cause 

of the cracking and continual settlement in the 

road remains untreated. Grading does not stop 

Figure 144—A roadway shoulder failure stabilized with launched soil nails. (Courtesy of Bob Barrett.)
6-78 Storm Damage Risk Reduction Guide for Low-Volume Roads

the settlement either, but begins a long-term 

commitment to continual roadway repair. Deep 

patch, as a maintenance technique, reduces or 

stops the continual settlement. Deep patches 

have slowed, but not stopped slope surface 

movement on sections of roads crossing 

areas of large-scale slope movement. Road 

settlement and road maintenance costs have 

been reduced using deep patch repair.

The deep patch design is a shallow roadfillslope repair where the upper 3 to 6 feet (1 to 

2 meters) of the subsiding section of roadway 

is excavated; the fill material is replaced with 

compacted select backfill, and several layers 

of geogrid or other reinforcing material are 

installed, as shown in figure 145. Geogrid has 

been the most used type of reinforcement. 

However, multiple layers of closely spaced 

geotextile (every 6 to 8 inches (150 to 200 

mm)) might offer additional cost savings to this 

technique for road shoulder fill stabilization. 

Figure 146 shows a typical cross section of a 

deep patch design.

The cost of repairing a road embankment failure 

with the deep patch method depends on backfill 

material (type and source), type and number of 

reinforcement layers, and drainage (if needed). 

However, when compared to other methods, 

such as road realignment or reconstruction, or 

retaining structures, the deep patch generally is 

the least expensive option. 

For additional technical information about 

deep patch, consult the “Deep Patch Road 

Embankment Repair Application Guide” 

(Wilson-Musser and Denning 2005). The 

guide describes the background, performance, 

design, and construction details of the deep 

patch technique. Link to the document <http://

www.fs.fed.us/eng/pubs/pdf/05771204.pdf>. 

Additionally the Federal Highway Administration 

and Western Transportation Institute have 

published “Deep Patch Repair, Phase 1: 

Analysis and Design” (Cuelho et al. 2012) that 

updates the design methodology.

Figure 145—Over-steep settling fillslope before 

deep patch repair (a) and a geosynthetic reinforcing 

material (a geogrid) being placed in a deep patch 

shoulder repair (b).

a

b
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-79

Changing Road Grade, Alignment, and 

Width

The first question to be asked in cases of 

roads adjacent to streams is: “Can the road be 

relocated?” If the answer is no, or only at too 

great a cost, then employ onsite measures. 

Road realignment or narrowing a section of the 

road may be ways to deal with slope instability 

and erosion at the toe of a slope, especially if 

that slope toe ends at a stream. Shifting and 

widening into the cutbank gains space in a 

narrow canyon or steep area, which is often 

caused by a stream channel eroding and 

over-steepening the roadway fill. Figure 147 

demonstrates this situation where a roadway 

fill toe was originally in the stream. After 

reconstruction, the road level is raised and the 

template shifted into the cutbank, away from 

the creek. Also the toe of the fill is armored with 

riprap along the creek. Additional discussion of 

this topic is found in section 6.4.

Raising or lowering the road grade may be 

effective for stabilizing unstable fill or cutslopes. 

Where a shallow fill failure surface is present 

at the shoulder of the road section, lowering 

the road grade below the failure zone can 

eliminate the problem (figure 148, top). Also 

if the cutslope is steeper than the fillslope 

(typically it is), lowering the grade will gain 

some road width. If a low cutslope chronically is 

unstable, raise the road bed using compacted 

backfill such that the added backfill buttresses 

the cutslope and reduces the failure potential 

(figure 148, bottom). If the cutslope is weeping 

subsurface water or surface water is present, 

provide drainage, such as a trench drain 

(underdrain) behind the new compacted backfill, 

or a surface ditch drain (or both).

Figure 146—Cross section of typical deep patch road embankment repair.
6-80 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 147—Road centerline is shifted away from 

the river after reconstruction, with a flatter, stable 

fillslope and riprap armor along the river. (Courtesy 

of Mike Balen.)

Narrowing the road can add stability to the 

roadway prism either by removing part of the 

fill, particularly the uppermost material (as 

discussed in section 5.6), or by placing material 

at the toe of the cut, thus buttressing the cut. 

In either case, the slopes can gain some 

improvement in stability, but part of the roadway 

is removed or filled, narrowing the road. In 

some cases, an inside ditch can be eliminated 

by outsloping the road and some road width can 

be gained back. Depending on the road use, 

closure status, or maintenance level, this can 

be an acceptable, relatively expensive SDRR 

treatment and a viable alternative to improve 

marginally stable slopes and reduce the road 

surface area subject to erosion. 

6.7 Cold Regions Storm Issues

Whether attributed to latitude or altitude, more 

than half the road network maintained by the 

Forest Service lies within what is categorized 

as a “seasonal frost area.” Problems caused by 

this severe environment present a set of unique 

challenges for road managers. While preventive 

and mitigating measures from noncold regions 

all still apply, it also is recommended that they 

be supplemented by additional mitigation 

measures designed to deal with cold regions' 

storm issues. A full treatment of this subject is 

beyond the scope of this document. However, 

some of the key issues are mentioned below, 

along with some references on the issues. 

Since problems in cold regions span the 

spectrum from solely temperature-induced to 

solely winter-storm induced, problems/damages 

of both types occur. Problems, such as ice 

weighing down and breaking trees and limbs 

are almost exclusively winter-storm induced. 

On the other end of the spectrum, problems 

with the pavement structure itself, such as frost 

heaving or thaw weakening, are caused almost 

exclusively by cold temperatures (although 

damage does not occur from cold temperatures 

alone; freezing temperatures must be 

accompanied by the presence of water, which 

can originate from storms/rain, groundwater, 

or meltwater, and frost-susceptible soils). 

Although this temperature-induced category 

falls outside the scope of this publication, frost 

heave and thaw weakening are major problems 

for roads. Most cold-regions’ problems are 

some combination of storm impacts and 

cold temperature. Problems, such as iceblocked culverts, are caused primarily by cold 

temperatures, but they can be exacerbated by 

winter storms. Another category of problems 

that are indirect results of winter storms include 

roads that are treated with salt in response 

to snowstorms; however, salt, itself, is the 

cause of other problems, such as tenting of 

pavements and browning of roadside trees. 

Problems occuring with low-volume roads in 

cold climates are often caused by changes 

in temperature from thawed to freezing, or 

freezing to thawed conditions. Storm events 

with heavy precipitation add to the problems.

Thawed-to-Frozen Condition

Sudden drops in temperature can lead to 

the freezing of water flowing overland, or 

underground. Bigger problems occur if the 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-81

Figure 148—Raising or lowering road grade to improve slope stability. (Adapted from “Landslide Remedial 

Measures” Royster 1982.) 
6-82 Storm Damage Risk Reduction Guide for Low-Volume Roads

drop in temperature is followed by rain while 

the ground is still frozen. Freezing can cause a 

buildup of ice on streams, which subsequently 

breaks up and flows downstream. The ice floes 

plug culverts and jam up at bridges. Large floes 

also may scour the streambed. If a particular 

site, such as a culvert is prone to repeated 

jams, install ramped trash racks upstream 

to protect the culvert. Construct in-stream 

bridge piers with ramped cutwaters, which are 

protected with steel leading edges. When a floe 

encounters the cutwater, it is lifted up. Being 

weak in tension, the lifted, bending ice floe breaks 

and passes in halves either side of the pier.

The freezing of flowing groundwater may lead 

to icings—thick sheets of ice developing over 

the road surface. Construction details may 

be responsible for the problem. For example, 

groundwater, flowing laterally, encounters the 

relatively impermeable consolidated soils under 

a road embankment. With enough pressure, 

the groundwater is driven upwards and over 

the road surface. The cold temperatures freeze 

the water at the surface, and a thick slab of ice 

grows across the road, with the accretion of 

more and more water, freezing in place, just as 

it does on an icicle. Solutions include finding 

a better location for the road, away from the 

site of the groundwater flow, and directing the 

groundwater flow away from the road.

Frozen-to-Thawed Condition

A sharp rise in temperature can lead to rapid 

thawing of frozen soil or permafrost, causing a 

loss of bearing capacity and large settlements. 

Thawing permafrost can release water, which 

starts a progressive degradation cycle with 

the released water melting more permafrost, 

releasing more water, which melts more 

permafrost, cutting deeply into the permafrost 

and moving substantial amounts of soil.

One solution is to be careful of drainage, 

preventing water from contacting frozen soil 

or permafrost. Another is to insulate the soil or 

permafrost, so it does not melt in the first place. 

Place soil or aggregate blankets over the frozen 

soil or permafrost, to a sufficient depth that it 

never melts. Place rigid foam insulation on the 

frozen soil or permafrost, and cover to protect 

the foam. In extreme cases, install thermopiles, 

keeping the soil or permafrost frozen.

Imposing seasonal load restrictions, i.e., 

either reducing allowable load or completely 

prohibiting truck traffic during the damagesusceptible period, is the most common, and 

the most widely accepted road-usage technique 

to reduce damage in seasonal frost areas. 

Technical information on simple diagnostic 

techniques for determining when to place and 

remove seasonal load restrictions is provided 

in Kestler et al. (2003, 2011), and number of 

models are reviewed in Miller et al. (2015). 

Also using trucks with reduced tire pressure 

is not widely practiced, but serves as a viable 

alternative road-usage technique during the 

spring thaw period.

Frozen, Plugged Culverts and Ditches

Culverts that are plugged with ice can be a 

significant problem for road managers (figure 

149). There are several techniques to address 

this problem. Some are after-the-fact (after the 

ice blockage has already occurred), and others 

are more preventive, but may require action or 

a site visit to be effective. 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-83

Figure 149—Iced culvert at milepost 2.5 on Mt. 

Washington Auto Road, NH. (Photo courtesy of the 

Mt. Washington Auto Road.)

Mitigating measures include: 

1. Push a steam point through the culvert 

after the blockage has occurred. This works 

well, but uses an expensive steam boiler 

mounted on a truck. It also requires special 

training.

2. Use steam pipes permanently mounted in 

the culvert. There is some variation and 

debate on whether the steam pipe should 

be on the top or bottom of the culvert. 

Outlets typically are drilled at intervals 

along the pipe. 

3. Use electrical thaw wires, but they must 

meet electrical code. (Note, a person was 

killed when a system failed and the water 

was electrified.) The system is efficient, but 

can only be used where there is power.

4. Use solar-powered thawing systems (which 

have had limited success). These systems 

circulate hot water through a pipe. The 

power to run the pumps uses solar panels. 

The water is also heated by the sun. These 

systems require a bit of maintenance and 

are subject to vandalism.

5. Dislodge ice with jets of water; then 

vacuum the ice and debris with a vacuum 

truck. 

6. Ensure a smooth flow into the inlet. This is 

perhaps the most natural way to eliminate 

the likelihood of freezing, as ideally the 

flow will continue and never freeze solid. 

However, this is generally outside the road 

manager’s control. 

7. Preinstall electric heat tapes.

Ice-blocked ditches also can cause significant 

problems, as seen in figure 150. Water 

flowing over the road typically is most 

problematic in the spring. Water in ditches and 

culverts typically freezes due to cold winter 

temperatures. In the spring, ice in the culvers 

melts more slowly than snow on the ground 

surface. Meltwater running down ditches cannot 

pass through frozen culverts, and overflows the 

road. Preventive and mitigation methods are 

similar to those of frozen culverts.

Figure 150—Culvert clogged with ice causing water 

to flow across the road. (Courtesy of Dale Higgins, 

Chequamegon-Nicolet National Forest.)
6-84 Storm Damage Risk Reduction Guide for Low-Volume Roads

River Ice Jams

The Army Corps of Engineers, Engineering 

Research and Development Center, Cold 

Regions Research and Engineering Laboratory 

(CRREL) maintains a comprehensive Web 

site and database of reported ice jams. 

Also, it provides a wealth of publications and 

presentations on case studies and preventive 

measures (figure 151). 

Figure 151—Photos of ramifications of ice jams from 

presentation on the Cold Regions Research and 

Engineering Laboratory Web site. 

Consultation with the Web site and publications 

is recommended for details. Some mitigations 

include the following suggestions: 

1. Winter flow control can reduce ice jam 

flooding. (But it should also be noted, 

winter reservoir operation influences the 

natural ice regime on rivers, and affects 

downstream ice cover, which may, in turn, 

affect fish survival.) 

2. Dark materials can be distributed on river 

ice, since they absorb heat faster, to 

accelerate melting. Leaves were shown 

to perform in a similar manner to the 

traditionally used dusting materials, such as 

fly ash and coal slag. However, unlike the 

latter, leaves are not believed to adversely 

affect aquatic organisms when applied in 

small quantities. 

3. Tuthill (1995) reviews ice control structures. 

The purpose of ice control structures are 

either for sheet ice retention, breakup ice 

control, or ice diversion.

4. Haehnel (1998) discusses several 

additional nonstructural methods to reduce 

ice jams and subsequent damage. These 

include a variety of mechanical, thermal, 

chemical, and physical methods. 

Other Cold Regions Problems

Other cold region storm and temperature 

impacts on roads include:

q Culvert differential heaving and washout.

q Longitudinal cracking in pavements.

q Localized puddling and ice buildup on 

roads.

q Snow drifts across roads.

q Impacts to roadside vegetation. 

q Road distress from frost heaving or thaw 

weakening, and associated road cracking 

and damage. 

6.8 Road Storage and Closure 

The post-World War II era brought a high 

demand for wood products for home 

construction and rebuilding war damaged 

areas. The baby boom and increased prosperity 

fueled an expanding economy with an appetite 

for wood and places for recreation. Road 

systems on private and public lands expanded 

rapidly to access forests and keep pace with 

demand. The environmental movement of 

the 1970s clashed with the expansion of 

industrial forestry into undeveloped forests. 

By the 1980s, the pace of construction slowed 

as more public lands were set aside and 

environmental impacts from development and 

habitat alteration were identified. The result is a 

large transportation system on public lands that 

is no longer used as intensively. Federal road 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-85

maintenance budgets were never large because 

maintenance was performed by the activity 

using the road, mostly timber management. 

Inadequate maintenance can result in 

environmental and access problems and storm 

damage risks increase.

A critical need now exists to bring road 

maintenance costs in line with budgets. One of 

the benefits to SDRR treatments is that general 

maintenance and storm damage repair costs 

will decrease. But the reduced costs will not be 

sufficient to close the gap between maintenance 

need and budget levels. So, as mentioned 

throughout this guide, when a road needs 

repair, the first question that should be asked is 

“Is this road needed?”

National forests have been directed to define 

a minimum road system (36 CFR (Code 

of Federal Regulations) 212.5(b) through 

a science-based process to identify the 

necessary transportation system and those 

roads no longer needed. With this mandate, 

managers are looking hard at road closure and 

decommissioning.

In the introduction to this guide, we stated 

that the road decommissioning is outside the 

scope of the document, but it may be the best 

management decision for a road. In the context 

of this guide, decommissioning is in a separate 

category specific to roads that will not be used 

again. Storage applies to roads that will be 

used in the future. Road decommissioning 

applies to roads that are no longer needed 

and results in the removal of the road from the 

maintained transportation system. Selection 

of appropriate decommissioning treatments is 

similar to that of SDRR, but is not discussed in 

detail in this document. Decommissioned roads 

are permanently blocked to traffic, drainage 

structures are removed, and a series of earth 

and rock berms or waterbars may be added that 

will revegetate and obscure the existence of 

the road. Structural barriers also may be used. 

In some sensitive areas, full road obliteration 

may be undertaken, where the entire roadway 

template is removed and the terrain is returned 

to its natural shape.

Road closure is discussed here because 

closed roads are expected to be used again 

in the future. Risks from storm damage and 

impacts to resources need to be considered 

and managed just as on open roads. Closed 

roads, or maintenance level 1 roads (see Forest 

Service Handbook 7709.59, U.S. Department of 

Agriculture, Forest Service 2009), are closed to 

vehicular traffic or stored until access needs are 

required again. 

Road closure may be for one or more purposes 

and for varying periods of time. A road may 

be closed if it will not be used for some period 

of time, or if the road is causing unacceptably 

high maintenance costs or environmental 

damage. Closure may be annual, may be 

for a short period of time, or may last a few 

decades. A clear understanding of the purpose 

and expected duration of closure is key to 

determining the appropriate SDRR treatment. 

Use short-term road closures (less than a year) 

to prevent traffic use that may damage surface 

drainage structures, to prevent use during wet 

periods or the rainy season, to protect them 

from erosion, or when use must be prevented 

for safety concerns. In most cases, short-term 

closure does not change the selection of SDRR 

treatments because the road is used at least 

part of the year. The same evaluation and 

decisionmaking process for open roads should 

be used in these instances. Treatments should 

be those that are effective at preventing erosion 

and other failure hazards that may occur during 

the time of closure. Road closure typically is a 

barricade or berm.
6-86 Storm Damage Risk Reduction Guide for Low-Volume Roads

Consider long-term road closure as a way to 

reduce road maintenance costs. Although the 

long-term cost savings are less than for road 

decommissioning, substantial savings and 

reduced damage often results from proper road 

storage. However, the implementation costs for 

storage may be significant, especially for roads 

with high storm damage risk, and involves a 

more complex evaluation to select appropriate 

SDRR treatments. Road closure often involves 

input from the public and other affected road 

users, and may be controversial. The main 

reality of long-term closures is that road 

maintenance will be limited or lacking during the 

period of closure. Thus, confidence is needed in 

any SDRR treatments that are used.

Figure 152 shows the range of options 

commonly considered in road closure and 

decommissioning. The figure includes road 

decommissioning as a closure treatment. 

Road obliteration is the most extensive 

decommissioning treatment where all road 

features are removed. In other, less extensive 

decommissioning treatments, only high-risk 

features are removed. Figure 153 shows 

various road closure treatments. Block closed 

roads with a barricade or berm to keep traffic off 

the road.

Consider a number of factors when selecting 

treatments for long-term storage:

q Term of closure (5, 10, 30 years).

q On and offsite risk factors.

q Road age.

The term of storage and the current age of 

the road act in combination to suggest the 

susceptibility of the road to storm damage 

during closure. The longer the road is stored the 

more deterioration of road features will occur. 

Deterioration includes loss of ditch capacity 

(infilling by cutslope ravel, erosion, slope failure, 

or vegetation growth), settling or cracking of 

fill material, loss of culvert capacity (partial or 

complete plugging by sediment and/or debris), 

and degradation of surface drainage shape 

and features. If road drainage structures, such 

as culverts, are already nearing their useful 

life, they are at high risk of failure during a long 

storage period. It is prudent to remove those 

features since they will need to be replaced 

when the road is reopened and they represent 

a real risk of failure and environmental impact 

during the time of storage. 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-87

a. Gate Closure (Temporary or Seasonal) b. Earth Mound or Berm Closure (Storage)

c. Decommissioning - Permanent Road Closure with Surface Ripping or Subsoiling and

 Seeding for Revegetation, but Keeping Most of Road Template (Shape).

d. Road Obliteration

Old, ripped, revegetated

roadway surface

(1) Road template before 

 obliteration.

(2) During obliteration, old 

 road is scarifi ed and refi lled

 by pulling back the fi llslope or

 importing fi ll material.

(3) Final obliteration, with 

fi lling and recontouring to 

 the original natural 

 topography, followed by 

 revegetation.

1

2

3

a b

c

d

Figure 152—Road closure options, including temporary or seasonal closure, storage, 

decommissioning (permanent closure), and obliteration. In the context of this guide, decommissioning 

is in a separate category specific to roads that will not be used again. Closure and storage apply 

to roads that will be used in the future. Road obliteration is the most extensive decommissioning 

treatment where all road features are permanently removed.
6-88 Storm Damage Risk Reduction Guide for Low-Volume Roads

Figure 153—Examples of road closure treatments 

including a decommissioned road (a), an obliterated 

road and recontoured area (b), and closure of a 

road-stream crossing (c). (Lower photo courtesy of 

James Jones.)

Roads stored for long periods of time may 

require extensive reconstruction to reopen, 

especially in wet climates with lush vegetation. 

Organic litter fall will cover road surfacing and 

retain moisture that will seep into the subgrade, 

a

b

c

b

a

requiring substantial removal and resurfacing. 

Figure 154 shows examples of roads that have 

been placed into storage, using waterbars or 

berms and logs to close the road.

Figure 154—Examples of roads that have been 

placed into storage. Photo (a) shows a road that 

has been closed using a waterbar. (Courtesy of Ken 

McCall/Karen Bennett.) Photo (b) shows the road 

has been closed with a log and berm, plus some 

vegetation. (Courtesy of Greg Napper.)

Although the probability of a given sized storm 

is the same for a given year, the probability of a 

road suffering storm damage by a large storm is 

greater the longer the road is in storage. Roads 

that are to be stored for long periods should 

receive treatments that build in a factor of 

safety to allow for the degradation of drainage 

features. Schedule periodic inspections to 

assure all features are functioning, but realize 

this may not happen. Thus, a factor of safety 
Less Common or Higher Cost Storm Damage Risk Reduction (SDRR) Treatments 6-89

can be achieved through sizing ditches and 

other features larger than would be done for 

an open road receiving regular maintenance, 

or using redundant or backup features like 

waterbars and dips in case of individual failures. 

The more factors that contribute to higher risk, 

the more extensive treatment will be required 

to assure the stored road does not contribute to 

environmental degradation during storage.

Roads with a number of high-risk factors and 

older roads that will be stored for many years 

will need treatments that will appear similar 

to those of road decommissioning. Drainage 

structures (culverts) will be removed; ideally, 

the ditch removed and the road bed outsloped; 

high-risk fills pulled up and the material stored 

in stable locations; and frequent waterbars 

added for backup drainage or to replace 

removed ditch-relief culverts.

Roads that will be stored for just a few years 

may only need to be waterbarred. Consider 

unstable fills for removal. Remove and replace 

high-risk stream-crossing structures when use 

is resumed; consider temporary (portable) 

bridges that do not require the replacement of 

large volumes of fill.

There are just as many considerations, if not 

more, for closed roads as open roads as they 

relate to SDRR. Do not skimp on the analysis 

for a closed road since impacts may be as 

important as for an open road. The analysis 

must look more long term to factor in the 

uncertainty of how long the road will stay in 

storage and not receive maintenance. Should 

all culverts be pulled or just stream crossings? 

That may depend on the age of the culverts as 

much as the risk factors. If a portable bridge will 

be used in the future, how should the crossing 

be left upon closure? If some structures are 

left that may need maintenance, can they be 

accessed? It is important to understand the 

landscape and watershed above a crossing, 

and how the terrain will perform over time.

Economics will enter into the decision on 

SDRR treatments for stored roads as well. 

The initial direct costs of storage may be high, 

but if the road will be stored for a long time, 

annual maintenance costs will be saved. 

Over a number of years, the savings may 

be substantial. Roads stored long term will 

need considerable work to reopen, including 

replacement of culverts that were removed or 

that have rusted, reconstruction of settled or 

sagging fills, reconstruction of the subgrade, 

reshaping the surface, and likely addition of 

new surfacing materials. 
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CHAPTER SEVEN 7SUMMARY AND CONCLUSIONS
Summary and Conclusions 7-1

7. Summary and Conclusions

Closure or damage to forest and rural roads 

can present a significant hardship to rural 

populations and economies, particularly during 

a time of disaster. Considerable experience 

has been gained in the assessment of storm 

damage to low-volume roads, subsequent 

repairs, and implementing measures to reduce 

the risk of the road system to future events and 

associated damage and environmental impacts 

caused by road failures during storms. These 

measures are an instrumental part of storm 

damage risk reduction (SDRR).

SDRR assessment for both assessing 

vulnerability and determining needed 

preventative measures or repairs should involve 

a process of working with local land managers, 

engineers, specialists, and local road agencies 

to identify their highest priority sites. Work 

should include an objective inventory of the 

transportation system, an evaluation of on-theground conditions and risks, an understanding 

of landscape and channel geomorphic 

processes, and identification of SDRR options. 

Specify needed work on work lists and with sitespecific designs. Appendix A4 is an example 

of a road condition form that can be useful to 

identify, assess, and document needed SDRR 

work.

Many planning, location, design, and 

maintenance measures exist that can greatly 

reduce the risk and vulnerability of low-volume 

roads to major storm events. Most measures 

involve avoiding problematic areas, having 

adequate designs, or controlling drainage in 

a positive manner. A key factor is the value of 

practical, cost-effective, and preferably simple 

measures to reduce the risk of storm damage. 

Table 2 presents a summary of most of the 

SDRR measures used to protect roads. The 

most commonly used treatments are identified, 

as well as a list of less common or more 

expansive, yet useful treatments used at times.

These measures include minimum road widths, 

frequent and well disbursed road surface 

drainage, use of rolling dips and fords, use of 

relatively flat cutslopes that will not fail, bridge 

and culvert scour protection, having adequately 

sized culverts with diversion protection, and well 

vegetated areas for both slope stabilization and 

erosion control.

Proper selection and implementation of SDRR 

treatments will reduce annual maintenance 

costs and catastrophic storm repairs. SDRR 

treatments can be approached on a systematic 

basis, following an assessment of risk and 

treating the highest risk sites first. Watersheds 

where SDRR treatments are complete are 

expected to show improvements in watershed 

condition and less damage suffered during 

storm events.
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CHAPTER EIGHT

8APPENDIXES
Appendixes 8-1

Appendix—A1. DESIGN AND Construction Drawings/Notes

A1a—Dip design information and details for common vehicles (adapted from 

Russell, et al., in preparation)

Dimensions of a drain dip for design (Note: vertical scale is exaggerated).

Schematic of vehicle clearance dimensions on vertical curves.
8-2 Storm Damage Risk Reduction Guide for Low-Volume Roads

Table A1a—Common drain dip dimensions for various design vehicles

Vehicle: Passenger Cars, wheelbase = 10 feet, center clearance on flat ground = 6 inches

Road 

Grade (G) 

%

Length (feet) Total Dip 

Length 

(TDL)

Depth, H (feet) Approach 

Grade %

Dip 

(reverse) 

Grade %

A B Outside 

Edge

Inside 

Edge

0–4 14 46 120 1.1 0.7 7 6

5–8 11 75 172 1.0 0.6 10 8

9–12 8 110 236 0.9 0.5 14 10

Vehicle: RV with low trailer hitch, wheelbase = 16 feet, center clearance on flat ground = 

8 inches; rear overhang = 6 feet, rear clearance = 5 inches

Road 

Grade (G) 

%

Length (feet) Total Dip 

Length 

(TDL)

Depth, H (feet) Approach 

Grade %

Dip 

(reverse) 

Grade %

A B Outside 

Edge

Inside 

Edge

0–4 16 65 162 0.8 0.4 7 4

5–8 10 90 200 0.6 0.2 11 3

9–12 4 112 232 0.3 0.1 13 2

Vehicle: Fire Truck/4-by-4 Truck, wheelbase = 18 feet, center clearance on flat ground = 

18 inches

Road 

Grade (G) 

%

Length (feet) Total Dip 

Length 

(TDL)

Depth, H (feet) Approach 

Grade %

Dip 

(reverse) 

Grade %

A B Outside 

Edge

Inside 

Edge

0–4 12 48 120 0.8 0.4 8 4

5–8 10 66 152 0.7 0.3 12 5

9–12 5 75 160 0.6 0.2 14 6

Vehicle: Log Truck, wheelbase = 50 feet, center clearance on flat ground = 12 inches

Road 

Grade (G) 

%

Length (feet) Total Dip 

Length 

(TDL)

Depth, H (feet) Approach 

Grade %

Dip

 (reverse) 

Grade %

A B Outside 

Edge

Inside 

Edge

0–4 14 30 88 1.0 0.6 11 4

5–8 14 45 118 1.0 0.5 15 5

9–12 13 58 142 0.8 0.4 20 3

Vehicle: Lowboy, wheelbase = 38 feet, center clearance on flat ground = 5 inches

Road 

Grade (G) 

%

Length (feet) Total Dip 

Length 

(TDL)

Depth, H (feet) Approach 

Grade %

Dip 

(reverse) 

Grade %

A B Outside 

Edge

Inside 

Edge

0–4 18 54 144 0.8 0.5 8 4

5–8 Not recommended without special analysis

9–12

Notes: 

1. Values for H are based on 3-percent drain-line pitch and 14 feet lateral distance between inside and outside edges of 

road.

2. The values in this table are based on calculations and some field experience assuming generic vehicle dimensions. 

Local conditions and vehicle types may vary.
Appendixes 8-3

The values in table A1a are based on the dip dimensions and the physical geometric clearance 

between the vehicle and the road surface or the vehicle and its load (i.e., bottoming out, tail 

dragging, front scraping, stinger-log contact, and so forth) (the figures above). These values do 

not account for dynamic issues, such as vehicle speed and acceleration, vehicle jounce or dive, or 

driver comfort. See Hafterson (1973) for an evaluation of vehicle acceleration through drain dips.

Complete discussions and calculations for geometric design of drain dips for vehicle passage are 

given by Hafterson (1973), French et al. (2002), and Ohmstede (1976). Detailed spreadsheets for 

geometric drain-dip design (Cummings 2002 and Russell and Messerlie 2013a) are available on the 

Forest Service Intranet, which is available only to Forest Service and Bureau of Land Management 

employees at: <http://fsweb.sdtdc.wo.fs.fed.us/programs/eng/Applications/dips.xls> and <http://

fsweb.sdtdc.wo.fs.fed.us/programs/eng/drain_dip_spreadsheet/Drain%20Dip%20Vehicle%20Pass_

v8-30-13.xlsx>

A similar, but less detailed, online calculator for drain dips may be found at: <http://fsweb.sdtdc.

wo.fs.fed.us/programs/eng/ENG_1/sample-project/sample-project/index.html> on the right sidebar 

of the Web page.
8-4 Storm Damage Risk Reduction Guide for Low-Volume Roads

A1b—Drain Dip Typical Examples and Information

Drain Dip Typical—Flat and Steep Grades Umatilla National Forest
Appendixes 8-5
8-6 Storm Damage Risk Reduction Guide for Low-Volume Roads
Appendixes 8-7

Horizontal Alignment. Dip construction shall include provisions (e.g. elevating ditch dam, shifting berm or backslope alignment, etc.) to satisfactorily guide driver away fromany blind approach into a ditch or other unsafe conditionwithin the dip.Constructed Slopes. All excavated back slopes shall be 1:1 orflatter, unless otherwise SHOWN ON DRAWINGS. All embanked fill slopes and berms shall be 1;3 or flatter.Utilize all excavated material and side casting shall not beallowed.Compaction Unless otherwise SHOWN ON DRAWING, compact each part of the dip by operating spreading andhauling equipment uniformly over the full width at a moisturecontent suitable for compaction.Traveled Way Width. The finished traveled way widththrough the dip shall be no narrower than 900 mm less thanthe adjoining travel width, but not less than 3.5 m, unless otherwise SHOWN ON DRAWINGS.Drain Line. The drain line of the dip shall be shaped smoothand free draining. Skew of drain line shall be 80-100 degrees.Lead-in Berm. Outside edge of roadbed shall be retained asberm leading into the outlet ditch. Riprap shall beincorporated into 300 mm of the lower end of each berm.

Outlet Ditch Elevation. The elevation of the outlet ditchthrough the berm, or upper edge of the riprap, shall closely match that of the finished drain line of the dip where they join.Outlet ditch and dip drainline shall be constructed at similar'grade.Outlet Riprap. Place riprap at drainage outlet when SHOWNON DRAWINGS. Excavated oversize rocky material whichmeets gradation requirements may be used as riprap belowdrainage outlet. Alignment of riprap placement and openingthrough the berms shall be centered to the low point of thedip.Aggregate Surfacing. Salvage and reuse existing aggregatesurfacing when SHOWN ON DRAWINGS. Import crushedaggregate surfacing and place to designated depth whenSHOWN ON DRAWINGS. Aggregate from commercial source shall be Forest Service certified as free of POC disease.Material shall meet requirements of Specification 304, typebase, grading E, compaction A. Savaged or importedsurfacing shall be spread against the inside bank, or nativeshoulder material at same elevation, in a manner thatfacilitates water drainage away fro inside edge of thetraveled way.

ROLLING DIP GENERAL NOTES

Rev. 5-03 ACAD rolldip-3(notes)

SHEET TOTAL

 NUMBER SHEETS
8-8 Storm Damage Risk Reduction Guide for Low-Volume Roads

A1c—Drivable and Non-Drivable Waterbars
Appendixes 8-9
8-10 Storm Damage Risk Reduction Guide for Low-Volume Roads

Appendix—A2. ROAD OPTIONS IN WET, VERY FLAT TERRAIN
Appendixes 8-11

Appendix—A3. OUTLET PROTECTION/VELOCITY DISSIPATION DEVICES
8-12 Storm Damage Risk Reduction Guide for Low-Volume Roads

Appendix—A4. ROAD STORM DAMAGE RISK REDUCTION ASSESSMENT—

FIELD EVALUATION/ROAD CONDITION FORM

Adapted from Karen Bennett, regional soil scientist, Pacific Northwest Region

Road Name _______________________________ Date _________________________________

Surveyor(s) Names ______________________________________________________________

Road Segment Information

Mile Post: From ______ To_____ GPS: From ___________________To____________________

Total Segment Length: ______________________

Road Width:_______________________________ Road Gradient:_______________________

Topographic position: Ridgetop _____ Midslope ______ Valley Bottom _____ Vertical ______

Traffic Use Intensity: Low _____ Moderate _____ High _____

Reason for Segment Break: ________________________________________________________

Road Surface Condition Assessment

Road Surface Material: Paved ______ Gravel ______ Native Soil ______ 

Road Prism Cross Section: Insloped ________ with ditch ________ without ditch ________

Crowned ________ with ditch ________ without ditch ________

Outsloped ________ with ditch ________ without ditch ________

Flat ___________ Entrenched ___________

Surface Condition: Good _________ Corrugated _________ Potholes _________

Ruts __________ Gully ______________ Rough___________

Condition Severity: (H,M,L) _________ _________ _________ _________ _________

RECOMMENDED TREATMENT_____________________________________________________

_______________________________________________________________________________
Appendixes 8-13

Slope Stability

Cutbank Height __________ Cutbank angle __________ Stable: Yes______ No______ 

Fillslope Length ___________ Fillslope Angle _________ Stable: Yes______ No______ 

Evidence of landslides? (movement?) Yes______ No______ Erosion: Yes______ No______

Seeps and springs in cutbank? Yes______ No______ 

Retaining Structures: Functioning _________ Needing Repairs _________

Add problem description?: __________________________________________________________

RECOMMENDED TREATMENT_____________________________________________________

_______________________________________________________________________________

Road Surface Drainage

Drainage type: Cross-Drain Culvert _________ Rolling Dip __________ Water Bar_________

Number of cross drains ____________ _____________ ______________

Average Spacing ____________ _____________ ______________

Length to drain more important that spacing (needs to consider divides, etc.)

Need road grade paired w/ assessment of spacing and # to adequately assess.

Plugged Cross drain with location ____________________________________________________

Erosion at cross drain outlet with location ______________________________________________

Ditch Condition: scoured: Y/N Scour length: Depth: (max & ave?)

RECOMMENDED TREATMENT_____________________________________________________

_______________________________________________________________________________

Sediment Transport Assessment

Ditch delivers sediment to stream: Yes______ No______ Ditch length_____ Ditch width ___

Road delivers sediment to stream: Yes______ No______ 

Distance from road to bankfull channel _________________

Other sources of sediment __________________________

RECOMMENDED TREATMENT_____________________________________________________

_______________________________________________________________________________
8-14 Storm Damage Risk Reduction Guide for Low-Volume Roads

Stream / Wetland Crossing Assessment

Number of Stream/Wetland Crossings__________

Is flow or drainage being backed up? Yes______ No______ 

Stream Crossing type: Open channel ____ Ford ____ Vented Ford ____ Culvert ____ Bridge ____

Dimensions: ___________________________________________________________________

RECOMMENDED TREATMENT_____________________________________________________

_______________________________________________________________________________

Culvert Assessment

Shape/Size: Circular _________ Eliptical ________ Recrangular ________

 Squash _________ Arch __________

Single or Multiple Pipes: _______________ Number ______________

Are all pipes functioning?: Yes______ No______ Explain: ____________________________

_______________________________________________________________________________

Stream Gradient: ____________ Bankfull Width ___________ or active channel width

Culvert Grade: _______________ Culvert Length ___________

Inlet Blocked: Yes______ No______ Depth of sediment _______________

Outlet Eroded: _________ Height of drop__________ Length of erosion __________

Piping along culvert: Yes______ No______ ; through bottom of pipe (holes) Y/N 

RECOMMENDED TREATMENT_____________________________________________________

_______________________________________________________________________________

Consider adding: Evidence of recent flood or disturbance: Y/N (would provide context or validate 

width rating if condition = good after experiencing recent flood)

Description: (or damage/effects of disturbance) 
Appendixes 8-15

Bridge/Ford Assessment

Type__________________________________ Span__________________________________

Structure Concerns: Yes______ No______

Channel Constriction_____________________ Debris_________________________________

Scour Problems__________________________________________________________________

RECOMMENDED TREATMENT_____________________________________________________

_______________________________________________________________________________

 Photo Point # Description
8-16 Storm Damage Risk Reduction Guide for Low-Volume Roads

APPENDIX—A5. GLOSSARY

Road decommissioning—A permanent 

road closure treatment (including obliteration) 

applied to roads that are no longer needed 

or are undesirable. This removes the road 

from the maintained transportation system. 

Treatments range from simply blocking the 

road and allowing it to brush in, to removing all 

drainage structures and unstable sidecast fills. 

Decommissioning removes those elements 

of a road that reroute or impede hillslope 

drainage and present slope stability hazards. 

Decommissioning removes the road from the 

road system. 

Road obliteration—A permanent road 

closure with full physical site restoration that 

attempts to re-contour slopes with the intent of 

completely removing the footprint of the road 

from the landscape. Natural drainage patterns 

are re-established as much as possible. The 

term is more frequently used by Washington 

Department of Natural Resources for private 

and State road treatment.

Storage—This treatment involves closing and 

stabilizing those roads that are not currently 

needed, but have been identified as having a 

future access purpose. The road may be closed 

for a year or many years, such as until the next 

forest management cycle, but it will be used 

again in the future. Treatment can involve wide 

ranges of options on a case-by-case basis. 

These roads, built using sidecast excavation 

methods and undersized drainage structures, 

require removal of drainage structures and 

sidecast fill material but leaves the road prism 

intact. The road becomes maintenance level 1.

Open roads—Roads that are to remain open 

for necessary access, either all year or for 

a portion of the year (i.e., seasonal wildlife 

restriction, snow, etc.). Depending on the 

technique of the original construction of the 

road and other factors such as number of 

streams crossed , need for fish passage and 

steepness of side slope involved, results in 

varying amounts of drainage correction work 

required. Correcting these drainage deficiencies 

is sometime referred to as an upgrade. Open 

roads are maintenance at maintenance level 2 

to 5. 

Forest Service nonsystem routes—These 

are road segments that are not system roads. 

These roads were frequently built under timber 

sales and never became part of the Forest 

Service maintained road transportation system 

due to their short-term need. 

Road maintenance levels—one of five levels 

assigned based on the maintenance required to 

provide the desired type of access:

Level 1—These are roads that have been 

placed in storage between intermittent 

uses. The period of storage must exceed 

1 year. Basic custodial maintenance is 

performed to prevent damage to adjacent 

resources and to perpetuate the road 

for future resource management needs. 

Emphasis is normally given to maintaining 

drainage facilities and runoff patterns. 

Planned road deterioration may occur at 

this level. 

Roads receiving Level 1 maintenance 

may be of any type, class, or construction 

standard and may be managed at any 

other maintenance level during the time 

they are open for traffic.
Appendixes 8-17

Level 2—Assigned to roads open for use 

by high-clearance vehicles. Passenger car 

traffic, user comfort, and user convenience 

are not considerations. Warning signs and 

traffic control devices are not provided with 

the exception that some signing, such as 

W-18-1 “No Traffic Signs” may be posted 

at intersections. Motorists should have no 

expectations of being alerted to potential 

hazards while driving these roads. Traffic is 

normally minor, usually consisting of one or 

a combination of administrative, permitted, 

dispersed recreation, or other specialized 

uses. Log haul may occur at this level.

Level 3—Assigned to roads open and 

maintained for travel by a prudent driver in 

a standard passenger car. User comfort and 

convenience are not considered priorities. 

The “Manual on Uniform Traffic Control 

Devices” (FHWA 2009a) is applicable. 

Warning signs and traffic control devices 

are provided to alert motorists of situations 

that may violate expectations.

Roads in this maintenance level are 

typically low speed with single lanes and 

turnouts. Appropriate traffic management 

strategies are either "encourage" or 

"accept.” "Discourage" or "prohibit" 

strategies may be employed for certain 

classes of vehicles or users.

Level 4—Assigned to roads that provide 

a moderate degree of user comfort and 

convenience at moderate travel speeds. 

Most roads are double lane and aggregate 

surfaced. However, some roads may be 

single lane. Some roads may be paved and/

or dust abated. “Manual on Uniform Traffic 

Control Devices” is applicable. The most 

appropriate traffic management strategy 

is "encourage." However, the "prohibit" 

strategy may apply to specific classes of 

vehicles or users at certain times.

Level 5—Assigned to roads that provide 

a high degree of user comfort and 

convenience. These roads are normally 

double lane, paved facilities. Some may 

be aggregate surfaced and dust abated. 

“Manual on Uniform Traffic Control 

Devices” is applicable. The appropriate 

traffic management strategy is "encourage."

Diversion potential—The possibility that 

streamflow will leave its established channel 

and flow down a road or roadway ditch that 

slopes away from a road-stream crossing. 

Stream diversion occurs when a culvert pipe 

capacity is exceeded or a culvert plugs with 

debris, causing the stream to overtop the 

culvert and follow the road rather than stay in its 

own stream channel. 

Drivable waterbars—Low berms or barriers, 

typically of soil, that are constructed across 

roads or skid trails with the purpose of blocking 

the flow of water or directing it off the road 

surface. Drivable waterbars are constructed 

such that they can be driven over with highclearance vehicles.

Hydrologically connected (hydraulic 

connectivity)—Hydraulic connectivity, or items 

that are hydraulically connected, refers to a 

circumstance, such as a roadway ditch or other 

drainage structure, that is directly connected 

to a watercourse, such that water, and any 

associated sediment it is carrying, is delivered 

directly to that watercourse or a natural channel 

network.
8-18 Storm Damage Risk Reduction Guide for Low-Volume Roads

River engineering—A coined term, 

similar to “Natural River Rehabilitation,” or 

“Geomorphologic Restoration” referring to 

work in channels to restore the natural or 

proper function of a stream. Work is commonly 

associated with repairs around structures 

such as bridges, or bank protection where a 

roadway encroaches upon a natural channel. 

Traditional work may have involved use of hard 

structures such as riprap and concrete, while 

current practice places emphasis on use of 

natural products such as logs and vegetation, 

as well as a better understanding of channel 

morphology and dynamics.

Rolling dip (drivable dip, broad-based 

dip)—A roadway surface drainage structure, 

with a constructed break in the road grade, 

specifically designed to drain water from an 

inside ditch as well as the roadway surface off 

the road. The dip, mound, and ditch outlet area 

are often reinforced or armored. Vehicle travel 

speed is typically somewhat reduced, and dips 

are used on low-volume, relatively low-speed 

roads. 

.

Stormproofing—Measures taken to reduce 

the risk or amount of damage to roads from 

major storms. Storm Damage Risk Reduction

pertains to similar storm proofing measures, 

but considers the fact that risk or probability 

of damage can be reduced, as well as actual 

amount of damage, but that low-volume roads 

can never afford to be built to prevent all 

damage. 

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CHAPTER NINE 9REFERENCES
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