Hazard 

Mitigation 

Field Book

Roadways

FEMA B-797 / May 2010

FEMA

 HAZARD MITIGATION FIELD BOOK 

Foreword

The Federal Emergency Management Agency (FEMA) continually strives to improve the delivery of disaster assistance to states and 

local governments. This Hazard Mitigation Field Book (HMFB) for Roadways assists entities directly affected by catastrophic events 

and disasters by suggesting mitigation measures. These mitigation measures are intended to help in identifying mitigation options 

and solutions for local jurisdictions and can be used at any time, and not just after a disaster.

As disasters have grown in frequency and severity, the costs of response and recovery have escalated to unsustainable levels. 

Obligations through the Federal Disaster Relief Fund ballooned from $2.8 billion in 1992 to $34.4 billion in 2005 due to damages 

associated with the 2004 and 2005 hurricane seasons. The most effective way to reduce these excessive losses is through disaster preparedness and mitigation. To best achieve this goal, we need to pursue two objectives:

Break the disaster-rebuild-disaster cycle. Merely repairing substandard infrastructure and elements to their pre-disaster 

condition does not protect the community from future disaster damages or reduce long-term costs. Mitigation improvements 

should always be considered in the rebuilding process, utilizing a multi-hazard approach whenever possible.

Ensure that communities address natural hazards. Comprehensive plans should acknowledge all hazards that pose a risk 

and identify steps to avoid these hazards altogether or incrementally reduce a community’s exposure to its hazards.

The outcome of achieving these objectives will be more resilient and economically sustainable communities. Although following and 

implementing the solutions in the HMFB does not guarantee FEMA funding, an analysis by the National Institute of Building Sciences’ 

Multihazard Mitigation Council established that every dollar spent in damage prevention saves four dollars in future repairs.

 http://maps.grida.no/go/graphic/trends-in-natural-disasters

Mitigating Misery: Land Use and Protection of Property Rights before the Next Big Flood by Edward A. Thomas & Sam Riley Medlock (2008). http://www.floods/org/PDF/ASFPM_

Thomas&Medlock.pdf

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 HAZARD MITIGATION FIELD BOOK ii

Table of Contents

Foreword .......................................................................................................i

Purpose........................................................................................................1

Methodology .................................................................................................1

Step One − Project Identification Diagram ....................................................1

Step Two − Selection Matrix ........................................................................4

Explanation of Selection Matrix − Factors and Weights...............................5

Case Studies ...............................................................................................18

County Roads Endure After Mitigation Solution is Implemented....................18

Highway Repairs are Designed to Avoid Damage .........................................22

Saving Our Roads: Larger Culverts Help .....................................................25

Pond Culvert Upgrade...............................................................................30

Mitigating County Road 55........................................................................34

General Design Guidance .............................................................................38

General Design Guidance − Examples........................................................38

General Design Guidance − References .....................................................41

Contributors List..........................................................................................42

Evaluation...................................................................................................43

Notes .........................................................................................................44

Safety

You are responsible for your personal safety at all times. 

Teams or individuals performing inspections may be exposed to hazards, especially following a disaster. Work 

areas may be isolated, extreme weather conditions may 

still exist, and heavy equipment may already be operating 

at the work site. In addition, state and local jurisdictions 

may be focusing on saving lives and response issues; 

other issues related to building science or recovery might 

not be immediately addressed.

Be aware of new safety risks created by an event. These 

may include washed out roads, downed power lines, nonfunctioning traffic signals, eroded road surfaces, washed 

out culverts and roads, high flood levels, etc.

At all times be careful of other motorists on the road.

Ensure that you have taken necessary precautions. Wear 

appropriate personal protective equipment such as highvisibility protective vests, eye protection, hard hats, leather 

boots with slip resistant soles, hearing protection, insect 

repellent, and gloves. Make sure you have a map and a 

way to call for help if needed.

Remember: Nothing is more important than your personal safety and your ability to safely carry out your task!
 HAZARD MITIGATION FIELD BOOK

Purpose

The FEMA Hazard Mitigation Field Book (HMFB) for Roadways helps local government entities choose the best hazard mitigation (HM) solution(s) given their 

operational constraints and design considerations. By offering the user a quick 

selection tool, based on broad characteristics, the HMFB reduces a wide array of technical solutions to a few practical options. Although there are many 

causes of damage to roadways, this Field Book focuses primarily on flood-related causes of damage.

Methodology

The HMFB uses a two-step selection process that includes 1.) a Project Identification Diagram that quickly outlines the specific hazard and presents the 

relevant mitigation solutions to be considered and 2.) a Selection Matrix that 

introduces HM solutions with their principal considerations and respective 

weights. 

Step One – Project Identification Diagram

The Project Identification Diagram is a linear decision tool that screens possible HM options for practical solutions based on clearly identified criteria and 

a qualitative weighing process. 

The HMFB does not discuss bridges. The discussion is limited to culverts, embankments, and road surfaces 

and shoulders.

Hazard Mitigation Steering Committee

On December 4, 2009, FEMA convened a Steering 

Committee of experts in roadway hazard mitigation. Practitioners from several state Departments of 

Transportation, local Departments of Public Works, FEMA, 

Federal Highway Administration (FHWA), organizations 

such as the American Association of State Highway 

and Transportation Officials (AASHTO), Association of 

State Floodplain Managers (ASFPM), and private-sector subject-matter experts were represented. See the 

Contributors List on page 42. 

The Steering Committee further identified the extensive 

source of existing technical information already available 

that need not be duplicated in a FEMA manual. Instead the 

Steering Group recommended that 1.) A decision process 

be identified for roads and drainage structures subject to 

disaster damages; 2.) Sample case studies be created 

using this decision process and; 3.) Additional sources of 

technical information be provided. This document is a direct response to those recommendations.
 HAZARD MITIGATION FIELD BOOK

Project Identification Diagram

Decision to Consider HM Solutions – What has 

happened, or may occur, that will cause unacceptable 

impacts or damages to a community.

Cause of Damage – Damage to roadways is related 

to three general water-related causes of damage to 

roadways.

Erosion and Scour – Damage occurs when moving 

floodwaters shift or remove sand along a coastline 

or shoreline (erosion) and/or undermine foundations 

(scour) that support road infrastructure. In some 

areas, the effects of erosion and scour can result in 

severe damage or collapse of infrastructure.

Inundation – Damage occurs when floodwaters 

surround and infiltrate infrastructure. This damage 

may be associated with high-velocity floodwaters. 

Debris Blockage – Damage occurs when various 

natural materials and man-made objects are carried 

by moving floodwaters and either collide with or clog up drainage structures. In some instances, debris can collect on 

other infrastructure, such as bridges, and adversely affect flow, resulting in damage to the infrastructure leading up to or 

adjacent to a bridge or other structure.

Damaged Infrastructure – Subdivides all non-bridge road infrastructure into three main categories: culverts, embankments, 

and road surfaces and shoulders. 

Culverts – Drain crossing under a road.

Embankments – Raised structure to hold back water or carry a roadway.

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Embankment 

Erosion/Scour

see page 14

Insufficient Culvert 

Capacity

see page 8

Misalignment

see page 12

Road Surface and 

Shoulder Damage 

see page 16

Obstructions That 

Reduce Culvert 

Capacity 

see page 12

Damage 

Description

Embankments

Culverts

Road Surfaces and 

Shoulders

Damaged 

Infrastructure

Erosion and Scour

Inundation

Cause of Damage

Event

Decision to 

Consider HM 

Solutions

Debris Blockage Culverts
 HAZARD MITIGATION FIELD BOOK

Road Surfaces and Shoulders – Portion of pavement designed to carry traffic. This includes the lateral support of pavement 

layers, and the edge of the roadway designed for accommodation of stopped vehicles, emergency use, and recovery area 

for errant vehicles.

Damage Description – Identifies types of damage that occur when a cause from the “Cause of Damage” column interfaces 

with a structure from the “Damaged Infrastructure” column. 

Misalignment – Damage to a culvert caused by its horizontal and/or vertical misalignment within the stream channel and 

subsequent erosion of the embankment. 

Insufficient Culvert Capacity – Damage or failure of a culvert resulting from overtopping and/or erosion of embankments 

due to insufficient culvert capacity and/or inefficient end sections. The inadequate capacity may be a result of inappropriate 

hydrologic analysis of flood peaks and volumes, and/or application of inappropriate culvert design criteria.

Embankment Erosion – Damage or failure of a culvert caused by erosion of the embankment at its entrance and/or outlet, 

or around the outside of the culvert. The embankment erosion and subsequent culvert damage or failure may result from 

inadequate culvert end sections. 

Road Surface and Shoulder Damage – Damage caused by water flowing over the top of the roadway, due to low roadway 

elevation or inadequate drainage structure capacity.

Obstructions That Reduce Culvert Capacity – Damage or failure of a culvert caused by overtopping and erosion of the 

embankment due to plugging of the culvert with debris caught or wedged in the culvert, restricting water flow. A culvert can 

then be washed out or damaged due to increased water surface elevations upstream. 

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 HAZARD MITIGATION FIELD BOOK

Step Two – Selection Matrix

Based on the final Project Identification Diagram selection, the user matches the appropriate “Damage Description” grouping to 

the Selection Matrix and begins a step-by-step process to evaluate each factor (consideration). The list of possible HM solutions 

includes a qualitative weight in each column for the user to determine if a solution is a viable option. With each selection, the user 

has a reduced set of possible solutions. Once the user has stepped completely through the process, a small number of possible 

mitigation solutions remain. At this point, the final selection from the remaining qualifying solutions is based on the discretion of the 

user as to which solution is the most appropriate. Additional sources of technical information are provided that can be used for the 

final solution decision/recommendation. 

Design Components of the Selection Matrix

Columns – The column headings are the factors that influence HM selection. They are grouped into four primary categories: 

Time to Implementation, Feasibility Considerations, Design Considerations, and Environmental Considerations. The cells 

below the column headings carry the ranking of the factor (column heading) to the corresponding HM solution (row). 

Ordering of the Columns – The ordering of the columns from left to right creates a built-in assumption that columns 

(factors) to the left are generally more critical than the columns (factors) to the right. Therefore any column (factor) moving 

from left to right must be considered simultaneously or before any other column (factor) to the right is considered. 

Rows – The rows contain commonly implemented roadway HM solutions. 

Key Assumptions of the Selection Matrix

The user is familiar with road construction, maintenance, and repair. 

The Selection Matrix is designed to provide details about specific solutions. It is possible that multiple measures can be 

combined into a single project (e.g., a trash rack can be included as part of a modified culvert).

Qualitative weights are assigned to different solutions to ease use of this manual. The user may determine different 

weights that are more appropriate based on region, local practices, and availability of materials. 

Five sample case studies are provided to illustrate how the user may proceed through the Selection Process under realistic scenar-

ios. Please refer to the cases beginning on page 18.

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 HAZARD MITIGATION FIELD BOOK

Explanation of Selection Matrix – Factors and Weights

Time to Implementation – This factor weighs the total time to receive 

permitting, make designs, and finish construction of an HM solution. 

Roads critical to emergency traffic, evacuation, or that function as the 

sole outlet for residents must be reopened as quickly as possible. 

Moreover, available funds, seasonal working conditions, acquiring 

permits, and other local considerations may influence the allowable 

time for repairs. The user must consider these variables and determine 

the maximum time available to restore services. Only then can they 

consider the HM solutions that can be implemented within that time. 

The HM solutions are weighted as shown to the right:

Feasibility Considerations – This factor weighs the relative cost 

feasibility of implementing various HM solutions. Recognizing that prices 

may vary widely among localities, relative costs between HM solutions 

should generally remain constant. Factors that influence costs may 

include labor (force account vs. contract), overhead, cost of materials, 

availability, etc. Cost weights current as of the writing of this Field Book 

are shown to the right:

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H – High < 30 days to implement

M – Medium 30 – 90 days to implement

L – Low > 90 days to implement

$ – Low $ < 10,000

$$ – Medium $10,000 – $25,000

$$$ – High >$25,000 
 HAZARD MITIGATION FIELD BOOK 

Design Considerations – Certain mitigation solutions may inherently 

involve a greater degree of design complexity than others. The user 

must decide whether these considerations present an obstacle to 

selecting certain HM solutions. 

Engineer Required? – The user must evaluate whether they have 

the staff with the skill-set(s) necessary to complete the job, or if 

they will have to seek outside assistance. HM solutions range from 

prescriptive engineering solutions, to requiring engineers to develop 

specific designs, to a combination of the two. 

Right-of-Way Constraints? – Damage in a high-density traffic area may 

require traffic control to manage Right-of-Way (ROW) constraints and 

detours, which can substantially complicate reconstruction, impacting 

cost and time to implement. 

Permit Required? – HM solutions may require an inspection and/or 

permit, which can impact the implementation timeframe. 

Environmental Considerations – Each HM solution may affect the 

environment differently. It is crucial that the user research the HM 

solutions’ impacts on the local watershed and comply with federal/local 

laws/codes. 

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Engineered Solution Required?

S – Standard Design

O – Original Design

C – Combination

Right-of-Way Constraints?

Y – Yes, ROW will be affected

N – No, ROW will be uneffected

Permit Required?

Y – Permit Required

N – No Permit Required

Environmental Impact

Y – Yes, Determination is required

N – No, Determination is not required

Floodplain/Wetland Impact D – Depends on the situation

Comprehensive or Master Plan 

Requirement

Structural Aesthetics Impact

ESA – Endangered Species Act

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CWA – Clean Water Act

NEPA – National 

Environmental Policy Act

CBRA – Coastal Barrier Resources Act

NHPA – National Historic Preservation Act

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 HAZARD MITIGATION FIELD BOOK 

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 HAZARD MITIGATION FIELD BOOK

Selection Matrix for Insufficient Culvert Capacity

Time to 

Implementation

Feasibility 

Considerations Design Considerations Environmental Considerations

Mitigation 

Solutions

Speed to 

Implementation 

(source of funding, 

funding cycle, and 

seasonal effects)

Cost 

(material type, 

labor, overhead, and 

availability)

Engineer 

Required? 

(standard design, 

original, or 

combination)

Right-of-Way 

Constraints?

Permit 

Required?

Floodplain/

Wetland 

Impact

Comprehensive 

or Master Plan 

Requirement

Structural 

Aesthetics 

Impact 

ESA CWA NEPA CBRA NHPA

Insufficient Culvert Capacity

Improve Culvert 

Entrance 

Efficiency

H $ S N Y N N N N N N N D

Install Emergency 

Spillway/HighWater Overflow 

Crossing

M $$$ C Y Y N Y N N D D N D

Install Low-Water 

Crossing L $$$ O Y Y Y Y Y D D D N D

Increase Culvert 

Size L $$$ C Y Y Y Y Y D D D N D

Install Diversion 

Channel to 

Detention Pond

L $$$ O Y Y Y Y Y D D D N D

Increase 

Floodplain 

Storage Capacity 

with Setback 

Levees

L $$$ O Y Y Y Y Y D D D N N
 HAZARD MITIGATION FIELD BOOK

Explanation of the Mitigation Solutions

Insufficient Culvert Capacity Solutions

Improve Culvert Entrance Efficiency − Properly designed entrance 

structures may improve the hydraulic performance of the culvert. A 

well-rounded entrance is more efficient than a sharp-edged entrance. 

Entrances can be made less sharp edged by installing flared aprons or 

wingwalls. To be most effective, the flared aprons or wingwalls should 

be oriented perpendicular to the approaching stream, not the culvert. 

If there is an abrupt change in flow direction at the culvert entrance, 

an “L” shaped endwall is the more efficient choice.

Install Emergency Spillway/High-Water Overflow Crossing − If alteration of an existing culvert/embankment system is not feasible, an 

alternative is to construct an emergency spillway, or high-water crossing 

to accommodate flow in excess of culvert capacity. The entrance to the 

emergency spillway should be designed to carry flood flows in excess 

of existing culvert capacity. Generally the bottom of the emergency 

spillway will be lower than the existing roadway. Lowering the roadway 

to cross the emergency spillway creates a low-water crossing to 

accommodate high-water overflows.

New culverts can be used to carry emergency spillway flow beneath 

the existing roadway elevation. This effectively increases the 

number of culverts, but requires additional work in the existing 

embankment.

Install Low-Water Crossing − In relatively flat areas where stream 

flow is infrequent and brief, eliminate culverts and install the roadway 

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Flood Damage at Culvert Locations

A requirement to provide access to homes, businesses, 

and recreational facilities occasionally requires roadway 

crossings of low-lying areas or small streams in floodprone or Special Flood Hazard Areas (SFHAs). Those 

roadways are frequently constructed on earthen fill embankments with culverts through the embankments to 

accommodate normal and/or flood flow. Flows exceeding 

the Design Flood may exceed the capacity of the culverts 

and result in overtopping the roadway embankment and/

or increased flood depths upstream of the embankment. 

For convenience, the mitigation solutions are arranged in 

three general categories:

1. Insufficient culvert capacity

2. Obstructions that reduce culvert capacity

3. Misalignments that reduce culvert capacity

This guide suggests and offers mitigation solutions to reduce the impact of flooding at roadway crossings.
 HAZARD MITIGATION FIELD BOOK 10

directly on the stream bed. The upstream edge of the roadway should be even with the stream bed to avoid scour. The 

downstream edge should be protected with rock riprap or a cutoff wall for the same purpose. Roadway flood gauges, warning 

signs, and/or barriers are required to prevent vehicles from driving into the low-water crossing during flood events.

Increase Culvert Size: The most direct way to reduce damages caused by insufficient culvert capacity is to replace the 

existing culvert with a larger one. For small to mid-size culverts, this can be accomplished by installing a new, wider pipe. If 

increasing the pipe diameter is not feasible because of a lack of clearance between the top of the embankment and the new 

pipe, alternate culvert shapes may be used (e.g., arch culverts, which allow widening the culvert while keeping the height 

low to meet site specific requirements). In extreme cases, sufficiently increasing the culvert size may lead to construction 

of a box culvert.

Install Diversion Channel to Detention Pond: If sufficient land is available, it may be feasible to divert a portion of the 

stream flow into a stormwater detention basin, releasing it back into the stream as permitted by culvert capacity when flood 

flows attenuate.

Increase Floodplain Storage Capacity with Setback Levees: An alternative to a diversion channel and detention pond is to 

increase stormwater storage capacity near the culvert entrance by constructing relatively low levees along the stream bank. 

If properly graded, the levees can also divert stormwater upstream of the culvert embankment, thereby increasing travel 

distance and time to entry into the culvert. 

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 HAZARD MITIGATION FIELD BOOK 11

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 HAZARD MITIGATION FIELD BOOK 12

Selection Matrix for Obstructions That Reduce Culvert Capacity and Misalignment 

Time to 

Implementation

Feasibility 

Considerations Design Considerations Environmental Considerations

Mitigation 

Solutions

Speed to 

Implementation 

(source of funding, 

funding cycle, and 

seasonal effects)

Cost 

(material type, 

labor, overhead, and 

availability)

Engineer 

Required? 

(standard design, 

original, or 

combination)

Right-of-Way 

Constraints?

Permit 

Required?

Floodplain/

Wetland 

Impact

Comprehensive 

or Master Plan 

Requirement

Structural 

Aesthetics 

Impact 

ESA CWA NEPA CBRA NHPA

Obstructions That Reduce Culvert 

Capacity

Clear Source of 

Flood Debris/ 

Increase 

Maintenance

H $ N/A N N N N N N N N N N

Install Trash Rack 

or Debris Barrier H $ S N Y Y N Y N N N N D

Install Debris 

Barrier Riser H $ S N Y N N Y N N N N D

Install a Relief 

Culvert M $$ C N Y Y Y Y D D D N D

Misalignment

Install Flow 

Diverters H $ S N Y Y N N N N N N N

Realign Culvert M $$ C Y Y N Y N N N N N D

Install Additional 

Culverts L $$$ O Y Y Y Y Y D D D N N

Realign the 

Stream Channel L $$$ O Y Y Y Y Y D Y Y N N 
 HAZARD MITIGATION FIELD BOOK 13

Explanation of the Mitigation Solutions 

Obstructions That Reduce Culvert Capacity Solutions

Clear Source of Flood Debris/Increase Maintenance − Remove debris accumulation in a culvert that led to blockage. 

Install Trash Rack or Debris Barrier − Install an entrance debris barrier to prevent blockage of the culvert, or debris 

fins, designed to orient the floating debris for easy passage through the culvert. Although effective in areas that have 

significant debris loading in the upstream drainage, there must be adequate stream channel storage available for debris 

accumulation. 

Install Debris Barrier Riser − Allows debris to float up with the rising floodwaters without blocking flow into the culvert. Area 

upstream should be suitable for storing floodwaters. 

Install a Relief Culvert − Located at the crossing site and in the embankment above the flow line of the primary culvert, 

providing an alternate route for the flow if the main culvert gets plugged, and prevents sedimentation through the high-flow 

scouring action.

Misalignment Solutions

Install Flow Diverters − Design barbs to redirect the flow away from the embankment and into the culvert.

Realign Culvert − Align centerline of the culvert (either vertically or horizontally) to the centerline of the stream to eliminate 

erosion along the embankment and subsequent damage to the culvert. Alignment may also require relocating a culvert. 

Install Additional Culverts − Locate additional culverts at previous and/or new stream alignments at road crossing site to 

increase drainage. 

Realign the Stream Channel − Channel flow should be directed into and at same angle as the culvert and away from the 

embankment to reduce erosion along the embankment and subsequent damage to the culvert. 

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 HAZARD MITIGATION FIELD BOOK 14

Selection Matrix for Embankment Erosion/Scour

Time to 

Implementation

Feasibility 

Considerations Design Considerations Environmental Considerations

Mitigation 

Solutions

Speed to 

Implementation 

(source of funding, 

funding cycle, and 

seasonal effects)

Cost 

(material type, 

labor, overhead, and 

availability)

Engineer 

Required? 

(standard design, 

original, or 

combination)

Right-of-Way 

Constraints?

Permit 

Required?

Floodplain/

Wetland 

Impact

Comprehensive 

or Master Plan 

Requirement

Structural 

Aesthetics 

Impact 

ESA CWA NEPA CBRA NHPA

Embankment Erosion/Scour

Extend Culvert 

Discharge H $ S N D N N N D D D N D

Riprap Slope 

Protection H $$ C Y Y Y Y Y D D D N D

Construct a 

Wingwall and 

Endwall

H $$$ S N Y Y N Y D D D N D

Install Energy 

Dissipation 

Measures

M $$ C N Y Y N Y D D D N N

Enlarge Stream 

Channel M $$ S Y Y Y Y Y D D D N N

Toe Stabilization 

Using Gabions M $$$ C Y Y Y Y Y D D D N D

Install Check 

Dams L $$ C N Y Y Y Y D D D N N

Bio-Engineered 

Slope Protection L $$ C Y Y Y Y Y D D D N N
 HAZARD MITIGATION FIELD BOOK 15

Explanation of the Mitigation Solutions

Embankment Erosion/Scour

Extend Culvert Discharge − For embankments that are only slightly susceptible to erosion or scour, and where the design 

discharge velocity of the culvert is relatively low (e.g., less than 2 feet per second), extending the discharge end of the 

culvert beyond the toe of the embankment may be enough protection.

Riprap Slope Protection − Stream bank erosion can be reduced by protecting the embankment with a layer of riprap – large 

stones, rocks, or manufactured materials that are heavy enough to resist erosion and provide a dispersion of hydraulic 

energy when impacted by the stream flow. The effectiveness of this method can be enhanced by installing the riprap over a 

layer of geotextile erosion fabric.

Construct a Wingwall and Endwall − Where discharge velocity is relatively low and erosion concern is limited to undermining 

the culvert, a straight or U-shaped endwall may be enough. As flow velocity increases, scour of the embankment may result, 

particularly if the culvert outlet is much narrower than the outlet channel. In those cases, flared wingwalls at the outlet will 

be more effective in mitigating embankment erosion and scour.

Install Energy Dissipation Measures − Erosion and scour at the discharge of culverts can be mitigated by the energy 

dissipation. Three general types of energy dissipation measures are:

Aprons at culvert discharges to reduce turbulent flow that can scour the toe of the embankment or undermine the culvert. 

Aprons can be made of non-corrosive metal or reinforced concrete.

Baffle structures can be used to relocate the zone of high velocity discharge downstream to a location that does not pose 

a risk to the embankment.

Increase tailwater depth by excavating a discharge pool or stilling basin to control turbulent flow at the culvert discharge.

Enlarge Stream Channel − The capacity of a stream is a function of the cross-sectional area and flow velocity. Erosion 

and scour are a function of stream velocity. Thus, enlarging the stream channel will decrease the flow velocity and reduce 

erosion and scour.

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 HAZARD MITIGATION FIELD BOOK 16

Toe Stabilization Using Gabions − Gabions are wire mesh baskets filled with stone and placed along the toe of stream 

embankments to resist toe erosion, which can cause slope failure. Gabions typically increase slope stability of the stream 

embankment by increasing the weight of materials resisting slope failure.

Install Check Dams − In small streams and man-made drainage channels, check dams can be used to retard flow velocity, 

trap a portion of the bed load, and allow settling of a portion of the suspended load. Each of these impacts can reduce 

potential downstream erosion.

Bio-Engineered Slope Protection − Carefully selected grasses, shrubs, and other ground cover can be effective in reducing 

stream bank erosion. Selection and design of bio-engineered embankment protection should consider steepness of 

embankment, expected flow rates, and growing season of the vegetation selected.

Selection Matrix for Roadway Surface and Shoulder Drainage

Time to 

Implementation

Feasibility 

Considerations Design Considerations Environmental Considerations

Mitigation 

Solutions

Speed to 

Implementation 

(source of funding, 

funding cycle, and 

seasonal effects)

Cost 

(material type, 

labor, overhead, and 

availability)

Engineer 

Required? 

(standard design, 

original, or 

combination)

Right-of-Way 

Constraints?

Permit 

Required?

Floodplain/

Wetland 

Impact

Comprehensive 

or Master Plan 

Requirement

Structural 

Aesthetics 

Impact 

ESA CWA NEPA CBRA NHPA

Roadway Surface and Shoulder Damage

Construct 

Shoulder 

Protection

H $ S Y Y N N Y N D N N N

Improve Shoulder 

Drainage H $ C Y Y N N Y N D N N N

Improve Subgrade 

Using Geotextile 

Drainage Systems

M $$ O Y Y Y N N D D D Y N

Increase Ditch 

Capacity M $$ S Y Y Y Y Y D D D N N

Increase Roadway 

Elevation L $$$ O Y Y N Y Y N D N N N

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 HAZARD MITIGATION FIELD BOOK 17

Explanation of the Mitigation Solutions

Roadway Surface and Shoulder Damage

Construct Shoulder Protection − Protection of the structural integrity of 

roadway shoulders can reduce the amount of water that can enter the 

roadway subgrade, thereby reducing damage to the roadway. Structural 

protection can be added to dirt shoulders by adding a layer of clean 

gravel or crushed stone. A layer of macadam can be added to reduce 

permeability of the shoulder and to reduce material loss due to traffic 

on the shoulder.

Improve Shoulder Drainage − Install shoulder subsurface drains. 

Typical shoulder subsurface drains consist of a perforated drain pipe 

placed near the bottom of a gravel-filled, geotextile-lined ditch. The 

pipes are directed to catch basins at designed collection and discharge 

locations.

Improve Subgrade Using Geotextile Drainage Systems − Roadway 

strength and durability can be significantly improved using geotextile 

drainage blankets between the pavement section and subbase. The 

geotextile drainage blankets can be used with free draining base course 

material or natural subgrade soils. The drainage blankets are especially 

effective in removing water from the pavement section before it substantially weakens the subgrade support.

Increase Ditch Capacity − Increasing the capacity of shoulder ditches will increase drainage capacity and reduce roadway 

flooding. An annual maintenance program will reduce future damages.

Increase Roadway Elevation − In relatively small areas of localized flooding, it may be feasible to elevate frequently flooded 

sections of roadway above the base flood elevation. In most cases, elevating the roadway will require removal of the existing 

pavement, compaction, and potentially improving the exposed subgrade, as well as constructing a new pavement section.

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Surface Damage to Roadways and Road 

Shoulders

Roadways experience millions of dollars of damage annually due to flooding. Damages include pavement and 

shoulder failure caused by loss of strength and durability in the roadway structure, including the subgrade, and 

erosion along roadway shoulders. Mitigation solutions to 

reduce roadway damages caused by flooding generally 

fall into the following groupings:

1. Elevating roadway

2. Improving drainage of the roadway structure

3. Improving shoulder drainage
 HAZARD MITIGATION FIELD BOOK 18

Case Studies

Introduction

Five sample case studies have been included to illustrate how the HMFB can be applied by practitioners to a variety of realistic situations. By reviewing each case, the user can analyze the considerations, the assumptions, and the line of reasoning employed by the 

decision-maker in each narrative. In addition, each case uses the HMFB methodology to evaluate a different damaged infrastructure, 

with different solution type(s), relating to different considerations. Selected solutions in both the Project Identification Diagram and 

Selection Matrix are highlighted in green. Please review each case study and consider how the HMFB methodology can be broadly 

employed. 

1. County Roads Endure After Mitigation Solution is Implemented

Background

A recent hurricane dropped nearly 20 inches of rain on the Florida Panhandle, making many of the sandy clay, unpaved county roads 

impassable because of very soft, slippery road surfaces and major washouts, especially in low-lying areas. Although the roadways 

lack drainage ditches, they are elevated on fill to be about 12 inches above surrounding grade. Although the roadways are crowned 

in the center to provide surface drainage to both sides, in heavy rains in even moderately windy conditions, run-off forms streams 

across the roads in the direction of the wind, creating small erosion channels. In severe storms, the erosion channels increase to 

become significant washouts in some areas. Further, as the erosion channels increase in size, the moisture content of the subgrade 

soils increases, resulting in a loss of shear strength and an inability to support traffic.

The area and depth of many of the washouts prevented access to neighborhoods requesting emergency assistance during and for 

several days after the storm. County officials recognized the need to not only reconstruct the roads to restore access to isolated 

neighborhoods, but also to include mitigation techniques that would make the roads more hazard resilient to future storm events.
 HAZARD MITIGATION FIELD BOOK 19

Selection

County officials decided they preferred a mitigation solution 

that increased the roadways’ damage resistance to local-

ized flooding by reinforcing the subgrade and by improving 

subgrade drainage. The County Department of Roads Super-

intendent consulted the FEMA HMFB to review and select 

feasible alternative solutions. Beginning with the Project 

Identification Diagram, the County quickly identified the problem as “Roadway Surface and Shoulder Damage” since the 

roadway washout was not the result of stream flooding, but 

inundation (see 1.1). As time was of the essence to restore 

the roadways to full service, the selection process started 

with reviewing the Selection Matrix (see 1.2) for relative project completion times for the listed alternatives. Although 

construction of shoulder protection was shown as the quickest alternative, less than 30 days to complete, it was decided 

that shoulder protection would not provide the level of mitigation needed. The “mid-range” project time of 30 to 90 days 

for “Improve Subgrade Using Geotextile Drainage Systems” 

was deemed to be acceptable and to provide the mitigation 

benefits needed. The relatively long project time (over 90 days) and high project cost made increasing the roadway elevation 

unacceptable. 

The County Department of Roads Superintendent consulted several sources, including Florida Department of Transportation design 

standards, and Design Manual for Roadway Geocomposite Underdrain Systems (http://www.tenaxus.com/roads/designinformation/

designmanual/DesignManual-Roadway-%20Drain.pdf). The design consisted of targeting and repairing those specific road sections 

highly prone to washout by reconstructing a reinforced patch with improved subgrade drainage. The patch design consisted of grading 

the washout area to a depth of 2 feet below the original roadway, compacting the exposed soils, and installing a geotextile drainage 

Damage 

Description

Damaged 

Infrastructure

Cause of 

Damage

Decision to 

Consider HM 

Solutions

Embankment 

Erosion/Scour

Insufficient Culvert 

Capacity

Misalignment

Embankments

Culverts

Erosion and Scour

Inundation

Culverts

Event

Debris Blockage

Road Surface and 

Shoulder Damage

Road Surfaces and 

Shoulders

Obstructions That 

Reduce Culvert 

Capacity

1.1
 HAZARD MITIGATION FIELD BOOK 20

blanket. The drainage blanket was covered by 12 inches of crushed stone coarse aggregate, then by a biaxial geomembrane reinforc-

ing grid, and finally by 8 inches of well-compacted, low plasticity sandy clay. The sandy clay was covered with 4 inches of base course 

aggregate compacted into the clay to add strength to the roadway surface (see 1.3).

Although design and construction of the selected mitigation alternative was expected to be acceptable, the County needed to 

complete the review to determine if other variables presented impediments to the project. Information in the “Environmental Considerations” of the HMFB gave the County a reliable source for identifying specific environmental issues related to the chosen 

alternative. They noted that, because the roadway subgrade drainage system might have a small impact on the local floodplain, the 

mitigation plan should be reviewed by the County Floodplain Manager before implementation.

Time to 

Implementation

Feasibility 

Considerations Design Considerations Environmental Considerations

Mitigation 

Solutions

Speed to 

Implementation 

(source of funding, 

funding cycle, and 

seasonal effects)

Cost 

(material type, 

labor, overhead, and 

availability)

Engineer 

Required? 

(standard design, 

original, or 

combination)

Right-of-Way 

Constraints?

Permit 

Required?

Floodplain/

Wetland 

Impact

Comprehensive 

or Master Plan 

Requirement

Structural 

Aesthetics 

Impact 

ESA CWA NEPA CBRA NHPA

Roadway Surface and Shoulder Damage

Construct 

Shoulder 

Protection

H $ S Y Y N N Y N D N N N

Improve Shoulder 

Drainage H $ C Y Y N N Y N D N N N

Improve Subgrade 

Using Geotextile 

Drainage Systems

M $$ O Y Y Y N N D D D Y N

Increase Ditch 

Capacity M $$ S Y Y Y Y Y D D D N N

Increase Roadway 

Elevation L $$$ O Y Y N Y Y N D N N N

1.2
 HAZARD MITIGATION FIELD BOOK 21

Implemented Mitigation Solution

The County successfully used reconstruction of the roadways using force account labor from its roads department. The project took 75 days and used an 

original mitigation design for improved subgrade drainage and a stabilized road 

surface (see 1.4). Since completion of the mitigated roadways, sections have 

experienced several hurricanes and tropical storms with only very minor damage, none of which closed the roads to residents or emergency services.

1.4

1.3

Geosynthetic Systems
 HAZARD MITIGATION FIELD BOOK 22

2. Highway Repairs are Designed to Avoid Damage

Background

Anytown, IA – Severe summer storms with record-breaking heavy rains caused flooding that washed out roads, eroded shoulders, and 

compromised ditches and culverts. One segment of County Road 16 received damage due to culvert blockage. Despite routine debris clearing from the Public Works Department, floodwaters lodged vegetative debris at the culvert inlet. The impeded water flowed 

over the embankment, causing water to run over the road and severely scour the road surface. Prior hydrologic and hydraulic stud-

ies indicated that the culvert size should have been sufficient to convey the flood flow. Had there been no blockage, the water would 

have flowed freely, causing only minor erosion and scour around the inlet and surrounding embankments. 

The Director of Public Works wanted the repair project to go 

beyond restoration to reduce or avoid future damages. He 

thought, “A little extra money spent now may save untold expenditure of funds later.” With this perspective in mind, the 

Public Works Director wanted to know his options. 

Selection

Using the HMFB, he referred to the Project Identification Diagram to diagnose the damage (see 2.1). Beginning at “Event,” 

the Director determined that vegetative debris lodging in the 

culvert would fall under the category “Debris Blockage” as 

a cause of damage. Debris blockage in the culvert had led 

to plugging – debris deposition across the culvert entrance 

that also led to increased water surface elevations upstream. 

With the root problem now identified, the Director cross-referenced “Obstructions That Reduce Culvert Capacity” with the 

Selection Matrix to find possible mitigation solutions. 

Damage 

Description

Damaged 

Infrastructure

Cause of 

Damage

Decision to 

Consider HM 

Solutions

Embankment 

Erosion/Scour

Insufficient Culvert 

Capacity

Misalignment

Embankments

Culverts

Erosion and Scour

Inundation

Culverts

Event

Debris Blockage

Road Surface and 

Shoulder Damage

Road Surfaces and 

Shoulders

Obstructions That 

Reduce Culvert 

Capacity

2.1
 HAZARD MITIGATION FIELD BOOK 23

County Road 16, albeit small, serves as a route of commerce and commuting for the area. It is essential that the road be reopened 

as quickly as possible following a flood event. Additionally, the Director knew that Anytown had limited resources, both in money and 

personnel. Along with repairs to the roadway, any mitigation enhancements to the culvert would have to be economical. 

Referencing the Selection Matrix (see 2.2), the Director began by evaluating his choices under the “Speed to Implementation” column, the most important factor due to how vital the road is to the local community. He then considered the mitigation solutions with 

a “High” weighting under the “Speed to Implementation” column. The “High” ranking signifies the most rapid completion of a given 

mitigation solution for “Obstructions That Reduce Culvert Capacity.”

Time to 

Implementation

Feasibility 

Considerations Design Considerations Environmental Considerations

Mitigation 

Solutions

Speed to 

Implementation 

(source of funding, 

funding cycle, and 

seasonal effects)

Cost 

(material type, 

labor, overhead, and 

availability)

Engineer 

Required? 

(standard design, 

original, or 

combination)

Right-of-Way 

Constraints?

Permit 

Required?

Floodplain/

Wetland 

Impact

Comprehensive 

or Master Plan 

Requirement

Structural 

Aesthetics 

Impact 

ESA CWA NEPA CBRA NHPA

Obstructions That Reduce Culvert 

Capacity

Clear Source of 

Flood Debris/ 

Increase 

Maintenance

H $ N/A N N N N N N N N N N

Install Trash Rack 

or Debris Barrier H $ S N Y Y N Y N N N N N

Install Debris 

Barrier Riser H $ S N Y N N Y N N N N N

Install a Relief 

Culvert M $$ C N Y Y Y Y D D D N N

2.2
 HAZARD MITIGATION FIELD BOOK 24

By selecting the “High” weighting, he reduced his choices for mitigation solutions for “Obstructions that Reduce Culvert Capacity” from four choices to 

three. Those choices with “High” speed to implementation ranking are “Clear 

Source of Flood Debris/Increase Maintenance,” “Install Trash Rack or Debris 

Barrier,” and “Install Debris Barrier Riser.” Given the remaining solutions were 

all rated “$”, or economical, he then considered the design requirements and 

environmental constraints of each and chose “Install Trash Rack or Debris Barrier” as the best possible solution. Given Anytown’s steady maintenance of 

debris clearing at the culvert site and around the area upstream, “Clear Source 

of Flood Debris/Increase Maintenance” was not considered since it was already being implemented – leaving only “Install Trash Rack or Debris Barrier” 

and “Install Debris Barrier Riser” as the remaining solutions.

Implemented Mitigation Solution

After the Director selected the “Install Trash Rack or Debris Barrier” as a mitigation measure for County Road 16, he referred the project to the town’s road 

and bridge department, who had prior experience implementing trash racks on 

other culverts around town. Using prior designs (referenced from the Iowa Department of Transportation – Office of Design’s Design Manual Chapter 8 Safety 

Design 8B-4, pg. 3 [http://www.iowadot.gov/design/dmanual/08b-04.pdf]), 

the road crews began to construct the trash rack (see 2.3 and 2.4). 

2.3

Iowa Department of Transportation Haala Industries, Inc.

2.4
 HAZARD MITIGATION FIELD BOOK 25

3. Saving Our Roads: Larger Culverts Help

Background

Central Arkansas – In March 2008, levees along rivers in northern and central 

Arkansas were straining to hold back floodwaters that were cresting at levels 

not seen in more than a quarter-century. Subsequent flooding particularly damaged County Road 357, a low-trafficked road used only by an estimated 50 to 

70 vehicles daily to transport farm equipment. Flooding at two sites resulted in 

the washouts of a 36-inch diameter x 32-foot long corrugated metal pipe (CMP) 

culvert and another 48-inch diameter x 45-foot long CMP culvert. The primary 

cause of damage was erosion to the culvert entrances, the 

culvert outlets, and the roadway embankment. In addition, 

culvert misalignment occurred (see 3.1). 

Selection

County officials wanted to compare the relative characteristics 

of standard mitigation solutions and decided to use the HMFB. 

They referred to the Project Identification Diagram to confirm 

the damage. They determined that the erosion to the existing 

culverts would fall under the categories “Insufficient Culvert 

Capacity” as the cause of damage to the culvert inlet and outlets, and “Embankment Erosion/Scour” for the embankment 

erosion (see 3.2). Consequently, the project needed two mitigation solutions to address the chronic culvert washout problem. 

With the sources of the problem now confirmed, officials reviewed the mitigation solutions in the Selection Matrix listed 

under each category to address both hazards (see 3.3). 

Damage 

Description

Damaged 

Infrastructure

Cause of 

Damage

Decision to 

Consider HM 

Solutions

Embankment 

Erosion/Scour

Insufficient Culvert 

Capacity

Misalignment

Embankments

Culverts

Erosion and Scour

Inundation

Culverts

Event

Debris Blockage

Road Surface and 

Shoulder Damage

Road Surfaces and 

Shoulders

Obstructions That 

Reduce Culvert 

Capacity

3.1

3.2

Butler County Engineer’s Office, Butler Co., OH
 HAZARD MITIGATION FIELD BOOK 26

Beginning with solutions relating to “Insufficient Culvert Capacity,” the officials evaluated the choices under the “Speed to Implementation” column. Due to the low volume of traffic of the road, the County officials considered mitigation solutions with a “Low” 

weighting under the “Speed to Implementation” column. By selecting the “Low” weight, all solutions in that category would be considered. Moving to the “Feasibility Considerations” column, the County officials determined that County funds would be able to 

finance solutions within the “$$$” range. With ample time to implement the solution, ready funds available, and near identical design 

and environmental considerations among the remaining solutions, the County was only able to focus on which solution would be the 

most effective. Therefore, they performed a final analysis of the six mitigation solutions listed under “Insufficient Culvert Capacity.” 

The analysis was based primarily on determining a long-term answer for each solution because of the continuing frequency of flood 

events. The final choice was “Increase Culvert Size” at the respective damage sites.

However, when the officials evaluated solutions for the “Embankment Erosion/Scour,” they allowed for a “Med” time to Implement. 

They assumed that an embankment erosion solution would be implemented after a new larger culvert had been put in place. Given 

the time to install the new culvert, the County would have less time available to improve the embankments. The available funding 

for the “Embankment Erosion/Scour” solution also fell in the “$$$” range, affording them the broadest range of solutions. Lastly, 

they reviewed the “Design” and “Environmental” considerations and, like the “Insufficient Culvert Capacity” solutions, found that 

they were very similar. “Toe Stabilization Using Gabions” was selected to prevent erosion by increasing slope stability of the stream 

embankment and to provide a dispersion of hydraulic energy when impacted by the stream flow (see 3.4). 
 HAZARD MITIGATION FIELD BOOK 27

Time to 

Implementation

Feasibility 

Considerations Design Considerations Environmental Considerations

Mitigation 

Solutions

Speed to 

Implementation 

(source of funding, 

funding cycle, and 

seasonal effects)

Cost 

(material type, 

labor, overhead, and 

availability)

Engineer 

Required? 

(standard design, 

original, or 

combination)

Right-of-Way 

Constraints?

Permit 

Required?

Floodplain/

Wetland 

Impact

Comprehensive 

or Master Plan 

Requirement

Structural 

Aesthetics 

Impact 

ESA CWA NEPA CBRA NHPA

Insufficient Culvert Capacity

Improve Culvert 

Entrance 

Efficiency

H $ S N Y N N N N N N N D

Install Emergency 

Spillway/HighWater Overflow 

Crossing

M $$$ C Y Y N Y N N D D N D

Install Low-Water 

Crossing L $$$ O Y Y Y Y Y D D D N D

Increase Culvert 

Size L $$$ C Y Y Y Y Y D D D N D

Install Diversion 

Channel to 

Detention Pond

L $$$ O Y Y Y Y Y D D D N D

Increase 

Floodplain 

Storage Capacity 

with Setback 

Levees

L $$$ O Y Y Y Y Y D D D N N

3.3
 HAZARD MITIGATION FIELD BOOK 28

Time to 

Implementation

Feasibility 

Considerations Design Considerations Environmental Considerations

Mitigation 

Solutions

Speed to 

Implementation 

(source of funding, 

funding cycle, and 

seasonal effects)

Cost 

(material type, 

labor, overhead, and 

availability)

Engineer 

Required? 

(standard design, 

original, or 

combination)

Right-of-Way 

Constraints?

Permit 

Required?

Floodplain/

Wetland 

Impact

Comprehensive 

or Master Plan 

Requirement

Structural 

Aesthetics 

Impact 

ESA CWA NEPA CBRA NHPA

Embankment Erosion/Scour

Extend Culvert 

Discharge H $ S N D N N N D D D N N

Riprap Slope 

Protection H $$ C Y Y Y Y Y D D D N N

Construct a 

Wingwall and 

Endwall

H $$$ S N Y Y N Y D D D N N

Install Energy 

Dissipation 

Measures

M $$ C N Y Y N Y D D D N N

Enlarge Stream 

Channel M $$ S Y Y Y Y Y D D D N N

Toe Stabilization 

Using Gabions M $$$ C Y Y Y Y Y D D D N N

Install Check 

Dams L $$ C N Y Y Y Y D D D N N

Bio-Engineered 

Slope Protection L $$ C Y Y Y Y Y D D D N N

3.3 Continued
 HAZARD MITIGATION FIELD BOOK 29

Implemented Mitigation Solution

The County undertook the project using an emergency management assistance 

contract with a neighboring county. The contract provides mutual aid between 

governmental jurisdictions following a disaster. Since the damaged county road 

and culverts were within the city boundaries, a cooperative project was carried 

out that used County force account labor and equipment with city design and 

engineering supervision services. Culvert design and placement, as well as 

gabion design and placement, followed guidelines and specifications set forth 

in the Arkansas State Highway and Transportation Department’s (ASHTD’s) 

Drainage Manual (http://www.arkansashighways.com/foi_list.aspx). ASHTD’s 

Roadway Design Plan Development Guidelines (http://www.arkansashighways.

com/foi_list.aspx) were also used to implement the project. 

3.4
 HAZARD MITIGATION FIELD BOOK 30

4. Pond Culvert Upgrade

Background

Anytown, ME – During periodic rain events, West Street, a major road in Anytown, is often closed due to flooding. The area 

of West Street, near the pond outflow culvert, overtops during 

these events, causing washout and structural damage to the 

road bed. In addition to the loss of road access, the washout 

creates a 7-mile detour for public safety vehicles and the potential for hazards to the health and safety of the residents. 

Although overtopping can often be attributed to a culvert with 

insufficient capacity, the stream leading up to the culvert has 

sharp curves, which contributes to scour at the upstream culvert opening. A further site inspection performed by the City’s 

Public Works Department revealed that the location of the 

culvert, in relation to the stream’s geometry, increased the 

likelihood of undercutting and scour. 

Selection

After inspecting the damage to West Street from the outflow culvert and considering the disruption in road access, 

Anytown’s Director of Public Works decided that the City should investigate repair alternatives to help mitigate further damages. 

Referencing the HMFB, the Director used the Project Identification Diagram to quickly diagnose the problem (see 4.1). Selecting “Culverts” as the damaged infrastructure, and “Misalignment” as the underlying cause of damage (see 4.2), the Director then viewed 

the Selection Matrix (see 4.3) to determine the appropriate alternatives.

Damage 

Description

Damaged 

Infrastructure

Cause of 

Damage

Decision to 

Consider HM 

Solutions

Embankment 

Erosion/Scour

Insufficient Culvert 

Capacity

Misalignment

Embankments

Culverts

Erosion and Scour

Inundation

Culverts

Event

Debris Blockage

Road Surface and 

Shoulder Damage

Road Surfaces and 

Shoulders

Obstructions That 

Reduce Culvert 

Capacity

4.1
 HAZARD MITIGATION FIELD BOOK 31

Although there was pressure from the public to reopen traffic on West Street, 

there was also a clear mandate to find a solution that would prevent future damage. So when the Director evaluated “Speed to Implementation,” he was able 

to select the “mid-range” choice, which estimates 30 to 90 days timeframe 

for implementation. Mitigation solutions that could be completed within that 

timeframe included “Realign Culvert” and “Install Flow Diverters.” Although 

“Installing Flow Diverters” would be more cost-effective, the Director knew that 

Anytown was willing to spend a little more to get a better solution (thereby 

choosing the “$$” Cost Selection). 

In addition, the Director wanted to consider other mitigation solutions to fortify 

against embankment erosion after the culvert was reset. Referring back to the 

HMFB under “Embankment Erosion/Scour,” the Director reviewed the alternatives (see 4.1). Given the reduced timeframe (after the culvert realignment 

was complete), the Director sought out solutions that could be implemented 

quickly and were moderately priced. In the end, the City decided that during the 

pond culvert upgrade, they would have the contractor reset the culvert angle to 

permit direct outflow downstream and install riprap on the stream banks and 

around the culvert opening to reduce erosion (see 4.4). 

4.2 
 HAZARD MITIGATION FIELD BOOK 32

Time to 

Implementation

Feasibility 

Considerations Design Considerations Environmental Considerations

Mitigation 

Solutions

Speed to 

Implementation 

(source of funding, 

funding cycle, and 

seasonal effects)

Cost 

(material type, 

labor, overhead, and 

availability)

Engineer 

Required? 

(standard design, 

original, or 

combination)

Right-of-Way 

Constraints?

Permit 

Required?

Floodplain/

Wetland 

Impact

Comprehensive 

or Master Plan 

Requirement

Structural 

Aesthetics 

Impact 

ESA CWA NEPA CBRA NHPA

Misalignment

Install Flow 

Diverters H $ S N Y Y N N N N N N N

Realign Culvert M $$ C Y Y N Y N N N N N D

Install Additional 

Culverts L $$$ O Y Y Y Y Y D D D N N

Realign the 

Stream Channel L $$$ O Y Y Y Y Y D Y Y N N 

Embankment Erosion/Scour

Extend Culvert 

Discharge H $ S N D N N N D D D N D

Riprap Slope 

Protection H $$ C Y Y Y Y Y D D D N D

Construct a 

Wingwall and 

Endwall

H $$$ S N Y Y N Y D D D N D

Install Energy 

Dissipation 

Measures

M $$ C N Y Y N Y D D D N N

Enlarge Stream 

Channel M $$ S Y Y Y Y Y D D D N N

Toe Stabilization 

Using Gabions M $$$ C Y Y Y Y Y D D D N D

Install Check 

Dams L $$ C N Y Y Y Y D D D N N

Bio-Engineered 

Slope Protection L $$ C Y Y Y Y Y D D D N N

4.3
 HAZARD MITIGATION FIELD BOOK 33

Implemented Mitigation Solution

To implement the project, the City’s Public Works Department hired a local 

contractor to provide labor and equipment and had the City Engineer’s staff supervise the project. To design the project, department engineers used culvert 

placement information from Maine Erosion and Sedimentation Control BMPs

(http://www.state.me.us/dep/blwg/docstand/escbmps/index.htm). 

 BMPs = Best Management Practices

4.4
 HAZARD MITIGATION FIELD BOOK 34

5. Mitigating County Road 55

Background

Central Arkansas – Continued heavy rainfalls often caused washouts on County Road 55 in the community. Although County Road 55 has ditches and is 

crowned 10 inches to provide drainage, soils underlying the road are extremely susceptible to erosion and collapse when they become saturated. Severe 

spring flooding caused a 32-foot washout on County Road 55. The ends on 

two 48-inch diameter x 30-foot long corrugated metal culverts were bent and 

undermined at one location. A hydraulic study conducted by the County Floodplain Manager suggested that larger culverts were needed to convey the high 

volume of water from the floods. Mitigation measures were developed that recommended replacement of the two 48-inch diameter x 30-foot long culverts with two 10-foot wide x 10-foot high x 40-foot long box 

culverts (see 5.1). In addition, the County Director of Public Works decided to consider enforcing the culvert embankments as an 

added mitigation solution to prevent culvert misalignment and embankment erosion.

Selection

Although two mitigation solutions were recommended as part of the hydraulic study, County officials decided to verify those suggested solutions by using the HMFB. Using information from the hydraulic study, they determined that the existing culvert would fall 

under the category “Insufficient Culvert Capacity” as the cause of damage (see 5.2). With the root problem now confirmed, officials 

reviewed the mitigation solutions in the Selection Matrix under that category (see 5.3). 

They began by evaluating the choices under the “Speed to Implementation” column. Although one of the key concerns was the 

criticality of the road to the local community, the officials were willing to accept a longer implementation time if it meant choosing 

the best option. They started to consider the mitigation solutions with a “High” or “Medium” weighting under the “Speed to Implementation” column, signifying the most rapid completion of a given mitigation solution for “Insufficient Culvert Capacity.” However, 

the only mitigation solutions with those rankings, “Improve Culvert Entrance Efficiency” and “Install Emergency Spillway/High-Water 

5.1
 HAZARD MITIGATION FIELD BOOK 35

Overflow Crossing,” were deemed as insufficient. The first solution was insubstantial given the high volume of floodwaters 

and the second solution did not maintain roadway access. So 

officials decided that a slower implementation time was acceptable if it broadened their options to include more robust 

solutions. By accepting a “Low” “Speed to Implementation,” 

all mitigation solutions remained available.

Continuing on, County officials considered the “Cost” weight-

ing, of which all five of the remaining mitigation solutions had 

a “$$$” ranking. Accepting the high price tag to increase 

the culvert capacity, the County officials then considered the 

design requirements and environmental constraints for each 

mitigation solution and determined that each one was relatively equal in that they would all require an engineer (either 

as an original design, or as a combination of an original design with a standard design). 

They performed a final analysis of the remaining solutions 

based on the appropriateness and long-term answer for each solution. They chose the solution that most closely mirrored the two 

solutions suggested in the hydrologic and hydraulic study. The final choice was “Increase Culvert Capacity.” 

In addition, the County officials decided to strengthen the embankment from erosion and scour. Seeking a solution that could be 

implemented quickly (“H”) and fairly economically (“$”, “$$”), the officials selected “Riprap Slope Protection.” 

Implemented Mitigation Solution

County officials conferred with two engineers from the Arkansas State Highway and Department of Transportation (ASHDT) to determine design specifications for the project. Once a “modified standard” design was established, the County provided force account 

labor to complete the project while an ASHDT engineer supervised (see 5.4). 

Damage 

Description

Damaged 

Infrastructure

Cause of 

Damage

Decision to 

Consider HM 

Solutions

Embankment 

Erosion/Scour

Insufficient Culvert 

Capacity

Misalignment

Embankments

Culverts

Erosion and Scour

Inundation

Culverts

Event

Debris Blockage

Road Surface and 

Shoulder Damage

Road Surfaces and 

Shoulders

Obstructions That 

Reduce Culvert 

Capacity

5.2
 HAZARD MITIGATION FIELD BOOK 36

Time to 

Implementation

Feasibility 

Considerations Design Considerations Environmental Considerations

Mitigation 

Solutions

Speed to 

Implementation 

(source of funding, 

funding cycle, and 

seasonal effects)

Cost 

(material type, 

labor, overhead, and 

availability)

Engineer 

Required? 

(standard design, 

original, or 

combination)

Right-of-Way 

Constraints?

Permit 

Required?

Floodplain/

Wetland 

Impact

Comprehensive 

or Master Plan 

Requirement

Structural 

Aesthetics 

Impact 

ESA CWA NEPA CBRA NHPA

Insufficient Culvert Capacity

Improve Culvert 

Entrance 

Efficiency

H $ S N Y N N N N N N N D

Install Emergency 

Spillway/HighWater Overflow 

Crossing

M $$$ C Y Y N Y N N D D N D

Install Low-Water 

Crossing L $$$ O Y Y Y Y Y D D D N D

Increase Culvert 

Size L $$$ C Y Y Y Y Y D D D N D

Install Diversion 

Channel to 

Detention Pond

L $$$ O Y Y Y Y Y D D D N D

Increase 

Floodplain 

Storage Capacity 

with Setback 

Levees

L $$$ O Y Y Y Y Y D D D N N

5.3
 HAZARD MITIGATION FIELD BOOK 37

Time to 

Implementation

Feasibility 

Considerations Design Considerations Environmental Considerations

Mitigation 

Solutions

Speed to 

Implementation 

(source of funding, 

funding cycle, and 

seasonal effects)

Cost 

(material type, 

labor, overhead, and 

availability)

Engineer 

Required? 

(standard design, 

original, or 

combination)

Right-of-Way 

Constraints?

Permit 

Required?

Floodplain/

Wetland 

Impact

Comprehensive 

or Master Plan 

Requirement

Structural 

Aesthetics 

Impact 

ESA CWA NEPA CBRA NHPA

Embankment Erosion/Scour

Extend Culvert 

Discharge H $ S N D N N N D D D N D

Riprap Slope 

Protection H $$ C Y Y Y Y Y D D D N D

Construct a 

Wingwall and 

Endwall

H $$$ S N Y Y N Y D D D N D

Install Energy 

Dissipation 

Measures

M $$ C N Y Y N Y D D D N N

Enlarge Stream 

Channel M $$ S Y Y Y Y Y D D D N N

Toe Stabilization 

Using Gabions M $$$ C Y Y Y Y Y D D D N D

Install Check 

Dams L $$ C N Y Y Y Y D D D N N

Bio-Engineered 

Slope Protection L $$ C Y Y Y Y Y D D D N N

5.3 Continued

5.4

USDA Natural Resources Conservation Service
 HAZARD MITIGATION FIELD BOOK 38

General Design Guidance

There are a number of regulatory authorities and industries dealing with public infrastructure impacted by flooding and erosion, but it 

is the transportation industry that generally experiences the widest range of impacts from those hazards. Over the years, the transportation industry has been a leader in research and development of procedures and design standards for surface and subsurface 

drainage systems, erosion and scour mitigation solutions, and embankment stabilization methods. The Federal Highway Administration (FHWA) and the American Association of State Highway and Transportation Officials (AASHTO) are leaders in conducting and 

sponsoring research in the sciences and engineering fields that develop methods and materials to mitigate the adverse effects of 

water on national transportation infrastructure.

Although federal funding supports much of the road construction in the U.S., the roads are owned and managed by states and localities. In many areas, state and local roads are not part of the national highway system and are constructed and maintained without 

benefit of federal funds. As a result, establishing roadway design standards, including drainage, erosion, and scour mitigation, and 

embankment protection is the responsibility of state and local governments. That responsibility is frequently fulfilled by adapting national research findings to local conditions and by using new and improved design approaches and standards that are appropriate 

for local conditions.

General Design Guidance − Examples

Examples of federal (FHWA), State Departments of Transportation, and industry organizations (AASHTO) manuals are provided 

below. They include general design standards, drawings, and methodologies for developing custom applications for culverts and embankments. These links are not intended to be prescriptive of the designs endorsed by the HMFB. Instead they serve as example 

resources on where to locate design/detail information. 
 HAZARD MITIGATION FIELD BOOK 39

Mitigation Solutions Designs/Details Source, Title, Website Links

Insufficient Culvert Capacity

Improve Culvert Entrance 

Efficiency

Federal Highway Administration, Hydraulic Design of Highway Culverts, Hydraulic Design Series No. 5, FHWA-NHI-01-020, Washington, DC, 2001, revised 2005 

(http://isddc.dot.gov/OLPFiles/FHWA/012545.pdf)

Install Emergency Spillway/

High-Water Overflow Crossing

Federal Highway Administration, Hydraulic Design of Highway Culverts, Hydraulic Design Series No. 5, FHWA-NHI-01-020, Washington, DC, 2001, revised 2005 

(http://isddc.dot.gov/OLPFiles/FHWA/012545.pdf)

Install Low-Water Crossing U.S. Department of Agriculture, Forestry Service, Low-Water Crossings: Geomorphic, Biological and Engineering Design Considerations

(http://www.fs.fed.us/eng/php/library_card.php?p_num=0625%201808P ) 

Increase Culvert Size Iowa Department of Transportation (http://www.iowadot.gov/design/stdplne_rf.htm)

Install Diversion Channel to 

Detention Pond Requires detailed hydrologic and hydraulic analyses

Increase Floodplain Storage 

Capacity with Setback Levees Requires detailed hydrologic and hydraulic analyses

Obstructions That Reduce Culvert Capacity

Clear Source of Flood Debris/

Increase Maintenance

Federal Highway Administration, Debris-Control Structures, Hydraulic Engineering Circular No. 9, FHWA-EPD-86-106, Washington, DC, 1971 

(http://www.fhwa.dot.gov/engineering/hydraulics/library_listing.cfm?archived=true)

Install Trash Rack or Debris 

Barrier

Federal Highway Administration, Debris-Control Structures, Hydraulic Engineering Circular No. 9, FHWA-EPD-86-106, Washington, DC, 1971 

(http://www.fhwa.dot.gov/engineering/hydraulics/library_listing.cfm?archived=true)

Install Debris Barrier Riser Federal Highway Administration, Debris-Control Structures, Hydraulic Engineering Circular No. 9, FHWA-EPD-86-106, Washington, DC, 1971 

(http://www.fhwa.dot.gov/engineering/hydraulics/library_listing.cfm?archived=true)

Install a Relief Culvert Iowa Department of Transportation (http://www.iowadot.gov/design/stdplne_rf.htm)

Misalignment

Install Flow Diverters Mississippi State University (http://www.abe.msstate.edu/csd/NRCS-BMPs/pdf/streams/bank/streambarbs.pdf)

Realign Culvert Federal Highway Administration, Hydraulic Design of Highway Culverts, Hydraulic Design Series No. 5, FHWA-NHI-01-020, Washington, DC, 2001, revised 2005 

(http://isddc.dot.gov/OLPFiles/FHWA/012545.pdf)

Install Additional Culverts Requires detailed hydrologic and hydraulic analyses

Realign the Stream Channel Requires detailed hydrologic and hydraulic analyses
 HAZARD MITIGATION FIELD BOOK 40

Mitigation Solutions Designs/Details Source, Title, Website Links

Embankment Erosion/Scour

Extend Culvert Discharge Iowa Department of Transportation, Standard Road Plans – RF Series (http://www.iowadot.gov/design/stdplne_rf.htm)

Riprap Slope Protection

Federal Highway Administration, Hydraulic Design of Highway Culverts, Hydraulic Design Series No. 5, FHWA-NHI-01-020, Washington, DC, 2001, revised 2005 

(http://isddc.dot.gov/OLPFiles/FHWA/012545.pdf)

Federal Highway Administration, Geosynthetic Design and Construction Guidelines, FHWA-HI-95-038, 1995, revised 1998 

(http://www.fhwa.dot.gov/engineering/geotech/library_sub.cfm?keyword=020)

South Carolina Department of Transportation, Standard Drawings (http://www.scdot.org/doing/sd_book.shtml)

Construct a Wingwall and 

Endwall

Nevada Department of Transportation, 2010 Standard Plans Index (http://www.nevadadot.com/business/contractor/standards/index/) 

Tennessee Department of Transportation (http://www.tdot.state.tn.us/Chief_Engineer/engr_library/structures/stdboxculvertdrawings.htm) 

Install Energy Dissipation 

Measures

Federal Highway Administration (http://www.fhwa.dot.gov/engineering/hydraulics/pubs/06086/hec14ch01.cfm)

South Carolina Department of Transportation (http://www.scdot.org/doing/sd_book.shtml)

Enlarge Stream Channel Requires detailed hydrologic and hydraulic analyses

Toe Stabilization Using Gabions

Federal Highway Administration (http://www.fhwa.dot.gov/engineering/hydraulics/pubs/05114/hec1507.cfm)

Nevada Department of Transportation, 2010 Standard Plans Index (http://www.nevadadot.com/business/contractor/standards/index/) 

Install Check Dams

Federal Highway Administration (http://www.fhwa.dot.gov/environment/fish5.htm)

State of Washington Department of Transportation (http://www.wsdot.wa.gov/publications/fulltext/Standards/english/PDF/i50.10-00_e.pdf)

Bio-Engineered Slope Protection Iowa Department of Transportation (http://www.iowadot.gov/design/stdplne_ec.htm)

Roadway Surface and 

Shoulder Damage

Construct Shoulder Protection Iowa Department of Transportation (http://www.iowadot.gov/design/tnt/PDFsandWebFiles/IndividualPDFs/e7153.PDF)

Improve Shoulder Drainage South Carolina Department of Transportation (http://www.scdot.org/doing/sd_book.shtml)

Improve Subgrade Using 

Geotextile Drainage Systems Iowa Department of Transportation (http://www.iowadot.gov/operationsresearch/reports/reports_pdf/hr_and_tr/reports/TR-525%20Final%20Report.pdf)

Increase Ditch Capacity Iowa Department of Transportation (http://www.iowadot.gov/design/SRP/IndividualStandards/erf19c.pdf)

Increase Roadway Elevation Iowa Department of Transportation (http://www.iowadot.gov/design/stdplne_rl.htm)
 HAZARD MITIGATION FIELD BOOK 41

General Design Guidance − References

Examples of state and local websites containing schematic and/or construction drawings for the mitigation measures outlined in 

this document. 

Federal Highways Department of Transportation Master Site to All State Roadway Design Manuals 

(http://www.fhwa.dot.gov/programadmin/statemanuals.cfm), (http://flh.fhwa.dot.gov/resources/pse/standard/state.htm)

State of New Jersey Department of Transportation Design Manual, Standard Roadway Construction Details, Aug. 2006

(http://www.nj.us/transportation/eng/docsuments/drainage/drainage.shtm)

South Carolina Department of Transportation (http://www.scdot.org/doing/sd_book.shtml)

Nevada Department of Transportation (http://www.nevadadot.com/business/contractor/standards/index/)

Iowa Department of Transportation (http://www.iowadot.gov/bridge/v8eculstd.htm)

Idaho Department of Transportation (http://www.itd.idaho.gov/design/StandardDrawings.htm)

Hamilton County, Ohio Department of Public Works (http://www.hamiltoncountyohio.gov/pubworks/std_drawings.asp)

Metropolitan Planning Council for the Twin Cities Metropolitan Area, Minnesota 

(http://www.metrocouncil.org/environment/water/BMP/manual.htm)

Colorado Department of Transportation (http://www.dot.state.co.us/DesignSupport/MStandards/

2006%20M%20Standards/2006%20Index/2006%20M%20Standards%20Index.htm)

Mississippi State University (http://www.abe.msstate.edu/csd/NRCS-BMPs/pdf/streams/bank/streambarbs.pdf)

State of Washington Department of Transportation (http://www.wsdot.wa.gov/Environment/WaterQuality/ErosionControl.

htm)

Five Counties Salmonid Conservation Program 

(http://www.5counties.org/Projects/FinalGeneralProjectPages/RoadsManual800.htm)

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 HAZARD MITIGATION FIELD BOOK 42

Contributors List

FEMA 

Paul Tertell

Preston Wilson

Jack Anderson

Steering Committee 

Mark Bush − AASHTO

Perry Cogburn − VDOT

Peter Drenan − Dewberry

Bill Gadberry − FEMA

Alexander Greenberg − FEMA

Rey de Guzman − Prince George’s County, Maryland

Sergio Mayorga − FHWA

Michael Powell − ASFPM

Rebecca Quinn − RCQuinn Consulting, Inc. 

Steve Suprata − Montgomery County, Maryland

Patrice White − DC Hazard Mitigation and PC II Manager

Robert Wright − FHWA

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 HAZARD MITIGATION FIELD BOOK 43

Evaluation

This is the first edition of the Hazard Mitigation Field Book. Please email any feedback or suggestions you may have regarding 

the content, format, or methodology of the document to the FEMA Office of Building Design at FEMA-Buildingsciencehelp@dhs.gov

or call our office hotline at (866) 927-2104.

Comments are encouraged and will be considered in the development of future editions. 

Thank you. 
 HAZARD MITIGATION FIELD BOOK 44

Notes

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797 EMA B- F

9-2 11 01 Catalog No.