Pocket Safety Guide for 

Dams and Impoundments 

FEMA P-911 / October 2016 

FEMA
This document has been developed from the FEMA P-911 Pocket Safety Guide for Dams and 

Impoundments developed by the U.S. Forest Service (USFS) and 

Federal Emergency Management Agency (FEMA). 

It was revised in cooperation with 6 agencies of the Department of the Interior
 

(Bureau of Indian Affairs, Bureau of Land Management, Bureau of Reclamation, National Park Service, 

Office of Surface Mining, Reclamation and Enforcement and US Fish and Wildlife Service) as part of the 

Department of the Interior’s Enterprise Architecture Project “Reduce Dam Safety Risk”.
	

Further review was provided by the National Dam Safety Review Board and the 

Association of State Dam Safety Officials
	
Pocket Safety Guide for Dams and Impoundments 

October 2016 

The Federal Emergency Management Agency (FEMA) has developed 

this information for the guidance of its employees, its contractors, and 

its cooperating Federal and State agencies and is not responsible 

for the interpretation or use of this information by anyone except 

its own employees. The use of trade, firm, or corporation names in 

this document is for the information and convenience of the reader 

and does not constitute an endorsement by FEMA of any product or 

service to the exclusion of others that may be suitable. FEMA prohibits 

discrimination in all its programs and activities on the basis of race, 

color, national origin, age, disability, and where applicable, sex, marital 

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 program. 

(Not all prohibited bases apply to all programs.) FEMA is an equal 

opportunity provider and employer. 

Lily Lake Dam, Rocky Mountain National Park, Colorado. 

Photograph by Kevin Gagner. i 
Acknowledgements 

ii 

FEMA would like to thank the following people and organizations for 

assistance in developing and reviewing this guide: 

● Developing the original guide: 

○ James Scott Groenier, Project Leader for the USDA Forest Service 

Pocket Safety Guide for Dams and Impoundments, January 2012, 

assisted by the following individuals and organizations: 

○ Terri Anderson, U.S. Department of Agriculture, Forest Service, 

Bitterroot National Forest
	

○ Brad Campbell, U.S. Department of Agriculture, Forest Service, 

Boise National Forest
	

○ James Demby, U.S. Department of Homeland Security, Federal 

Emergency Management Agency, National Dam Safety Program
	

○ Cliff Denning, U.S. Department of Agriculture, Forest Service, 

Eastern Region
	

○ Brad Iarossi, U.S. Department of the Interior, U.S. Fish and 

Wildlife Service
	

○ Martin McCann, U.S. Department of Agriculture, Forest Service, 

Southern Region 

○ Sue Peterson, U.S. Department of Agriculture, Forest Service, 

Eastern Region 

○ Steve Romero, U.S. Department of Agriculture, Forest Service, 

Pacific Southwest Region 

○ Charles Showers, U.S. Department of Agriculture, Forest Service, 

Missoula Technology and Development Center 

○ Lori Spragens, Association of State Dam Safety Officials 

○ Nate Tatum, U.S. Department of the Interior, National Park Service 

○ Association of State Dam Safety Officials 

○ U.S. Department of Agriculture, U.S. Forest Service 

○ U.S. Department of Homeland Security, Federal Emergency 

Management Agency, National Dam Safety Program 

○ U.S. Department of the Interior, U.S. Fish and Wildlife Service 
● Revising the guide: 

●		Members of the 2013 – 2014 U.S. Department of the Interior 

Reduce Dam Safety Risk team: 

○ Mark Baker, National Park Service 

○ Jessica Bush, Bureau of Land Management 

○ Scott Goss, Bureau of Indian Affairs 

○ Dave Hibbs, U.S. Fish and Wildlife Service 

○ Stacy Johnson, Bureau of Reclamation 

○		Stefanie Self, Office of Surface Mining, 

Reclamation and Enforcement 

○ Jade Soddell, Bureau of Reclamation 

○ Jay Stateler, Bureau of Reclamation 

○ Mike Vail, Bureau of Indian Affairs 

● Figures 1, 2, 14, and 15: Ed Jenne, 

Missoula, Montana 

● Figures 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 

41, 43, 45, 47, 53, and 55 were adapted from the 

1986 FEMA publication “Dam Safety: An Owner’s 

Guidance Manual,” developed by the Colorado 

Division of Disaster Emergency Services 

iii 
iv 

Contents 

Acknowledgements..............................................................................................ii
 

Contents..............................................................................................................iv
 

Introduction...........................................................................................................1
 

Responsibilities and Liability.................................................................................2
	

Embankment Dam Diagram.................................................................................3
	

Noteworthy Observations at an Embankment Dam............................................4
	

Concrete Gravity Dam Diagram...........................................................................5
	

Concrete Arch Dam Diagram...............................................................................7
 

Concrete Buttress Dam Diagram.........................................................................9
 

Concrete Masonry Dam....................................................................................11
 

Noteworthy Observations at a Concrete Dam..................................................12
	

Outlet Structures................................................................................................13
	

 Gate Valve........................................................................................................13
	

 Drop Inlet Riser.................................................................................................14
	

 Spillway Overview.............................................................................................15
	

Embankment Dam Problems.............................................................................16

 Excessive Quantity and/or Muddy Seepage Water Exiting 

 From a Point.............................................................................................16
 

 Seepage Water Exiting From a Point Adjacent to the Outlet Pipe...................18

 Seepage Water Exiting as a Boil Downstream of the Dam.............................20 

 Seepage Water Exiting at the Abutment Contact.............................................22
	

 Sinkholes..........................................................................................................24
	

 Slide, Slump or Slip.........................................................................................26
	

 Transverse Cracking........................................................................................28
	

 Longitudinal Cracking.......................................................................................30
	

 Rodent Activity and Animal Impact...................................................................32
	

 Trees or Obscuring Vegetation.........................................................................34
	

 Surface Erosion................................................................................................36
	

 Deteriorated or Missing Riprap.........................................................................38

 Livestock and Cattle Traffic..............................................................................40

 Low Area in the Crest of a Dam.......................................................................42

Failure of Concrete or Rock Outfall (Energy Dissipation) Structures...............44
	

 Outlet Releases Eroding the Toe of the Dam .................................................46
	

Concrete or Masonry Dam Problems..................................................................48

 Cracking, Opening/Closing or Offsets at Joints, or 

 Apparent Deformations...................................................................................48
	

 Deterioration of Structural Materials..................................................................50
	

Spillway Problems...............................................................................................52

 Excessive Vegetation or Debris in the Spillway Channel or Around 

the Inlet...........................................................................................................52

 Erosion of Unlined Spillway Channels..............................................................54
	

Resources.......................................................................................................... 56
	

Web Sites........................................................................................................... 57



Useful Terms.......................................................................................................58
	
Introduction 

This “Pocket Safety Guide for Dams and 

Impoundments” was developed for dam owners, 

inspectors and other people as a quick reference 

when assessing dams and impoundments. For 

in-depth discussion, please refer to the technical 

manuals and guides available from FEMA at: 

http://www.fema.gov/technical-manuals-and-guides. 

Dam—Any artificial barrier of at least a minimum size, 

including appurtenant works, that impounds or diverts water 

or liquid-borne solids on a temporary or long-term basis (for a 

full definition see FEMA 93). The information contained in this 

document is also applicable to impoundments. 

Hazard potential classification—A system that 

categorizes dams according to the degree of adverse 

incremental consequences from their failure or misoperation 

that does not reflect in any way on their current condition (their 

safety, structural integrity, or flood routing capacity), and that 

includes the following categories: 

•		High hazard potential—loss of one or more human life 

is probable 

• Significant hazard potential—no probable loss of 

human life but possible economic loss, environmental 

damage, disruption of lifeline facilities, or other impacts 

•		Low hazard potential—no probable loss of human life 

and low economic and/or environmental losses (typically 

limited to the property of the dam owner). 

1 
2 

Various Federal and State agencies may have different 

definitions for the dams over which they have jurisdiction. 

Please refer to your agency or State dam regulatory 

requirements for direction. 

Responsibilities and Liability 

The owner is responsible for the operation and 

maintenance of a safe dam. Maintaining a safe dam is a 

key element in preventing failure and limiting the liability 

that an owner could face. The extent of an owner’s liability 

varies from State to State and depends on statutes and 

case law precedents. Federally owned and regulated dams 

are subject to the laws, regulations and guidelines of the 

owner agency. 

Owners can be fiscally and criminally liable for any failure 

of a dam and all damages resulting from its failure. Any 

uncontrolled release of the reservoir resulting from a dam 

failure or uncontrolled release can have a devastating effect 

on persons, property, and the environment. 

This guide is designed for a wide audience. The 

recommended action taken by individuals will be determined 

by their expertise with dam maintenance. Inexperienced 

individuals should photograph suspected problem(s) and 

report them promptly to the dam owner or operator. 

When you become aware of any unusual conditions that 

seem critical or dangerous to a dam, report them immediately 

to the appropriate Federal or State agency official. 
Embankment 

Dam Diagram
 

Figure 1 – Typical 

dam diagram 

showing common 

terms. 

3 
4 

Noteworthy 

Observations 

at an 

Embankment 

Dam 

Figure 2. 
Concrete 

Gravity 

Dam Diagram 

Figure 3 – Typical 

gravity dam views 

showing common 

terms 

(www.simscience.org). 

5 
6
 

Figure 4 – Gravity dam cross-section showing load 

transmission to foundation. Figure 5 – Grand Coulee Dam in Washington. 
Concrete Arch
 

Dam Diagram
 

Figure 6 – Typical arch dam 

views showing common terms 

(www.simscience.org). 

7 
8 

Concrete Arch Dams 

Figure 7 – Arch dam cross-section showing load 

transmission into foundation and abutment. Figure 8 – Theodore Roosevelt Dam in Arizona. 
Concrete 

Buttress Dam 

Diagram 

Figure 9 – Typical 

buttress dam 

diagram showing 

common terms 

(www.simscience.org). 

9 
10 

Concrete Buttress Dams 

Figure 10 – Arch dam cross-section showing load transmission 

into foundation and abutment. Figure 11 – Lake Tahoe Dam in California.
 
Concrete Masonry 

Dam 

Figure 12 – Masonry dam on 

the Cedar River southeast of 

Seattle, Washington.
	

11 
Noteworthy 

Observations at a 

Concrete Dam 

Figure 13. 

12 
Outlet 

Structures 

Gate Valve 

Figure 14 – 

Typical gate 

valve used 

with a spillway 

to control 

water level. 

13 
14 

Outlet 

Structures 

Drop Inlet 

Riser 

Figure 15 – 

Typical drop inlet 

riser serving as a 

principal or service 

spillway to control 

water level. 
Outlet 

Structures 

Spillway 

Overview 

Figure 16 – Spillway 

Overview with 

structures identified. 

15 
16 

Embankment Dam Problems 

Excessive Quantity and/or Muddy Seepage 

Water Exiting From a Point 

Probable Causes and Possible Consequences 

• Water has created an open pathway, channel, or pipe through

the dam. The water is eroding and carrying embankment

material.

• Rodents, frost action, or poor construction may allow

seepage water to create an open pathway or pipe through the

embankment.

• Continued flows can further erode embankment

and/or foundation materials and lead to dam failure.

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Estimate or measure outflow quantity. Determine whether seepage flows

are increasing with time, and whether material transport by the flow is

continuing to occur.

• Have a qualified engineer inspect the condition and recommend further

actions.

• Lower the water level in the reservoir.

• Search for an opening on the upstream side of the dam. Plug it if

possible.

• If possible, place filter sand at the seepage exit location to prevent

material from being transported along with the seepage. Gravel may

need to be placed initially so that the flow velocity is slowed, so the sand

is not washed away by fast-flowing water. Once the sand filter is in place,

cover it with gravel so it does not get washed away by the seepage flow.
Figure 17 – Excessive quantity and/or muddy seepage water exiting 

from a point. 

Figure 18 – Excessive seepage water exiting at the toe of 

the embankment. 

1 7
18 

Seepage Water Exiting From a Point Adjacent to the Outlet Pipe 

Probable Causes and Possible Consequences 

• A break or hole in the outlet pipe or poor compaction

around the pipe allows water to flow and creates a

pathway along the outside of the outlet pipe.

• Continued flows can lead to embankment and/or

foundation material erosion and dam failure.

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Determine if seepage is carrying soil particles (muddy

water).

• Determine quantity of flow, and if flow is increasing with

time.

• Have a qualified engineer inspect the condition and

recommend further actions.

• If flow increases or is carrying material, the reservoir level

should be lowered until seepage flow stops.

• Investigate embankment along alignment of pipe to see if

there are any signs of settlement or sinkholes.
Figure 19 – Seepage water exiting from a point adjacent to the 

outlet pipe. 

Figure 20 – Embankment erosion adjacent to an outlet pipe. 

19 
20 

Seepage Water Exiting as a Boil Downstream of the Dam 

Probable Causes and Possible 

Consequences 

• Some part of the foundation material is supplying a

path for reservoir seepage. This could be caused by a

sand or gravel layer in the foundation.

• Increased flows can lead to high upward gradients and

erosion of the foundation materials and dam failure.

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Examine the boil to see if foundation materials are being

transported (typically expressed as a ring of material

around the exit point).

• Have a qualified engineer inspect the condition and

recommend further actions.

• If soil particles are building up at exit points, use

sandbags or earth to create a dike around the boil. The

pressures created by the water level may halt the flow and

prevent further erosion.

• If the situation cannot be controlled using sandbags, and

soil materials are being carried by the flow, the reservoir

level should be lowered.
Figure 21 – Seepage water exiting as a boil downstream of the dam. Figure 22 – Sandbags surrounding a boil on the downstream side of

the dam (note the mound of material forming in the center of the sandbagged area. 21
22 

Seepage Water Exiting at the Abutment Contact 

Probable Causes and Possible 

Consequences 

• Water flowing through pathways in the abutment or

along embankment abutment contact can result in

internal erosion.

• Seepage can lead to erosion of the embankment

material and dam failure.

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Begin measuring outflow quantity. Determine whether seepage

flows are increasing with time, and whether material transport

by the flow is continuing to occur.

• Have a qualified engineer inspect the condition and recommend

further actions.

• Lower the water level in the reservoir.

• Search for an opening on the upstream side of the dam. Plug it

if possible.

• If possible, place filter sand at the seepage exit location to

prevent material from being transported along with the seepage.

Gravel may need to be placed initially so that the flow velocity is

slowed, so the sand is not washed away by fast-flowing water.

Once the sand filter is in place, cover it with gravel so it does

not get washed away by the seepage flow.
Figure 23 – Seepage exiting at the abutment contact (groin). 

Seepage 

Figure 24 – Seepage at the abutment contact (groin) can lead to dam 

failure. 

23 
24 

Sinkholes 

Probable Causes and Possible 

Consequences 

• Internal erosion (piping) of embankment materials or

the foundation can cause a sinkhole.

• An eroded cavern (cave-in) can result in a sinkhole.

• A small hole in the wall of an outlet pipe can develop

into a sinkhole.

• Seepage with sediment at the seepage exit indicates

potential internal erosion of the dam embankment or

foundation.

• Internal erosion can lead to dam failure as soil pipes

develop and erode through the dam embankment

and/or foundation.

Recommended Action 

Report the suspected problem to the appropriate Federal 

or State agency official and have the situation evaluated 

by a qualified engineer immediately. 

• Check seepage and leakage outflows for muddy

water.

• Inspect the dam and appurtenances for other

anomalies, such as new seepage areas, depressions,

cracks, etc.

• Have a qualified engineer inspect the conditions,

identify the exact cause of sinkholes, and recommend

further actions.
Figure 25 – Sinkholes. 

Figure 26 – Whirlpool action in the reservoir (unrelated to an outlet 

works intake) could be an indication of advanced piping. Dam failure

could be imminent. 

25 
26 

Slide, Slump or Slip 

Probable Causes and Possible 

Consequences 

• Foundation movement or a too steep slope can cause

earthfill to move along a slip plane which can lead to a

slump or slip of the embankment.

• Slide movements in the reservoir basin can lead to

inlet obstruction or dam failure (through instability in

the faces of the dam, or through wave action if the

slide is along the perimeter of the reservoir).

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Evaluate the extent of the slide.

• Inspect for new or changed seepage conditions

downstream of the slide area, and for longitudinal and

transverse cracks, scarps, etc. near the slide area.

• Monitor the slide.

• Draw down the reservoir level if the safety of the dam is

threatened.

• Have a qualified engineer inspect the conditions and

recommend further actions.
Figure 27 – Slide, slump or slip on either face. Figure 28 – Slumping on the downstream face of a dam. 

27 
28 

Transverse Cracking 

Probable Causes and Possible 

Consequences 

• Differential settlement between adjacent segments of

the embankment may cause transverse cracking.

• Deformation caused by differential settlement or slope

instability may provide a path for seepage through the

embankment cross section, which could initiate a breach

of the embankment.

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Inspect the crack and carefully record its location, length,

depth, width, and other pertinent physical features. Stake

out the crack limits.

• Have a qualified engineer determine the cause of cracking

and supervise all steps necessary to reduce danger to the

dam and correct the condition.

• Under the direction of a qualified engineer, excavate the

crest along the crack to a point below the bottom of the

crack. Backfill the excavation using suitable material and

correct construction techniques. This seals the cracks at the

crest surface to prevent surface water infiltration. Visually

monitor the crest routinely for evidence of future cracking.
Figure 29 – Transverse cracking. Figure 30 – Transverse cracks may provide open seepage paths through 

an embankment dam. 

29
	
30 

Longitudinal Cracking 

Probable Causes and Possible 

Consequences 

• Slope instability can result in longitudinal cracks.

• After an earthquake, longitudinal cracks could indicate

liquefaction of the foundation and/or embankment

materials may have occurred, which resulted in slope

movements.

• Differential settlement between zones or within the

embankment may cause longitudinal cracking.

• Cracks create a point for water to enter the embankment,

which could result in slope instability.

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Inspect the crack and carefully record the location,

length, depth, width, alignment and other pertinent

physical features.

• Have a qualified engineer determine the cause of

cracking and supervise the steps necessary to reduce

danger to the dam and correct the condition.

• Effectively seal the cracks to prevent surface water

infiltration.

• Monitor the crest routinely for evidence of further

cracking.
Figure 31 – Longitudinal crack. Figure 32 – Longitudinal cracks can be an indication of dam instability. 

31 
32 

Rodent Activity and Animal Impact 

Probable Causes and Possible 

Consequences 

• Cattail-filled areas and areas where trees are close to

the reservoir provide ideal habitat and foraging areas

for animals.

• An overabundance of rodents increases the chance of

animal burrowing, which creates holes, tunnels, and

caverns within an embankment dam.

• Tunnels may significantly reduce the length of the

seepage path through the dam embankment which

could cause initiation of an internal erosion problem.

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Start a rodent control program to reduce the population

and prevent future damage to the dam.

• Backfill existing rodent holes with suitable wellcompacted material to repair damages.

• For large or deep holes, include a zone of filter material

when backfilling.
Figure 33 – Rodent activity. Figure 34 – Rodent holes in the dam face can cause seepage issues 

that could lead to dam failure. 

33 
34 

Trees or Obscuring Vegetation 

Probable Causes and Possible Consequences 

• Natural vegetation (bushes) obscures visual inspection

and harbors animals.

• Large tree roots can create seepage paths.

• Large trees can blow over during a storm and damage

the dam, which may cause a breach.

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Control vegetation that obscures visual inspection of the

embankment.

• Under the direction of a qualified engineer, remove all

large, deep-rooted trees and shrubs on or near the

embankment with care to minimize disturbance to the

embankment.

• Backfill voids promptly.

• Remove trees at the toe and groins of the dam to provide a

25- to 50-foot buffer.

• Monitor area after removal of trees, since deterioration

of remaining roots from large trees may lead to voids in

the embankment and potentially cause sinkholes and/or

seepage paths.
Figure 35 – Trees or obscuring vegetation. Figure 36 – Trees growing on the crest and the faces of a dam. 

35 
36 

Surface Erosion 

Probable Causes and Possible Consequences 

• Water from intense rainstorms or snowmelt carries surface

materials down the slope and results in continuous

troughs, rills, gullies, etc.

• Erosion can lead to severe deterioration of the

downstream slope that can be expensive to repair. It is

preferable to catch the problem early, when resolving the

issue is much less costly.

• Surface erosion can also occur upstream if the face is not

protected with riprap.

Recommended Action 

Report the suspected problem to the appropriate 

Federal or State agency official and have the situation 

evaluated by a qualified engineer immediately. 

• If erosion is detected early, add protective grasses that

may resolve the problem.

• Restore and protect eroded areas; add rock or riprap,

where appropriate.
Figure 37 – Erosion. Figure 38 – Erosion on the downstream face of a dam. 

37 
38 

Deteriorated or Missing Riprap 

Probable Causes and Possible 

Consequences 

• Poor quality riprap deteriorates and does not protect

the slope.

• Wave or ice action can displace riprap allowing

erosion or over-steepening of the bank.

• Improperly sized riprap may displace and expose the

slope.

• Wave action against these unprotected areas

decreases the embankment width.

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Reestablish the normal slope.

• Place bedding material and properly sized riprap to protect

against wave action.
Figure 39 – Broken down or missing riprap. Figure 40 – Wave erosion on the unprotected face of a dam. 

39 
40 

Livestock and Cattle Traffic 

Probable Causes and Possible 

Consequences 

• Livestock paths and activities on the downstream dam

face can damage slopes, especially when wet.

• Livestock activities may reduce erosion protection

leading to erosion issues.

Recommended Action 

• Fence the embankment area to keep out livestock.

• Repair erosion protection by reestablishing grasses or

by placing bedding material and riprap, as applicable.
Figure 41 – Livestock and cattle traffic. Figure 42 – Livestock paths on dam faces can lead to dam erosion. 

41 
42 

Low Area in the Crest of a Dam 

Probable Causes and Possible Consequences 

• Excessive settlement of the embankment or internal erosion

of embankment material may cause a depression on the

crest of a dam.

• Foundation spreading upstream and/or downstream,

prolonged wind erosion, or improper final grading following

construction may cause a low area in the crest of a dam.

• Low area may have been intentionally included during

original construction.

• Low areas can reduce the freeboard available and result in

the embankment dam overtopping sooner.

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Establish survey monuments or survey along the length of the

crest from abutment to abutment to determine the exact amount,

location, and extent of low spots on the crest.

• Have a qualified engineer determine the cause of the low area

and supervise all necessary steps to reduce any threat to the dam

and correct the condition.

• Use proper construction techniques to fill in the low area and

reestablish the uniform elevation over the length of the crest.

• Routinely survey established monuments to routinely survey along

the dam crest to detect any unusual settlement along the crest of

the dam.

• Note: Low areas may be desirable to direct flow over areas of dam

less susceptible to damage or having lower consequences from

downstream failure.
Figure 43 – Low area in the crest of a dam. Figure 44 – Low areas in the crest of a dam reduce the freeboard

(therefore allowing overtopping at a lower frequency flood). 

43
	
44 

Failure of Concrete or Rock Outfall (Energy Dissipation) Structures 

Probable Causes and Possible 

Consequences 

• Excessive side pressures on a non-reinforced

concrete structure or poor concrete quality can cause

failure of the outfall structure.

• The embankment may be exposed to erosion by outlet

releases because of loss of an outfall structure.

• Too steep of a slope can cause rocks to roll down the

hill and partially block the outlet, which reduces the

flows that can occur through the pipe.

Recommended Action 

• Have a qualified engineer inspect the situation and

recommend corrective actions. The corrective actions

may involve improving drainage and water pressure

relief at the structure, as well as structure repair or

replacement.

• Repair the slopes and place riprap to stabilize them,

where applicable.

• Keep outlet free of blockages.
Figure 45 – Failure of a concrete outfall structure. Figure 46 – Rock blocking discharge of an outlet 

45 
46 

Outlet Releases Eroding the Toe of the Dam 

Probable Causes and Possible 

Consequences 

• The outlet pipe may be too short and result in scour at

discharge end.

• No energy-dissipating pool or structure at the

downstream end of the conduit can result in scour

damage.

• Erosion of the toe of the dam makes the downstream

slope too steep and may cause progressive sloughing.

Recommended Action 

• Extend the outlet pipe beyond the toe (use the

same size of pipe and material). Form a watertight

connection to the existing conduit.

• Stabilize the slope.

• Use riprap over suitable bedding to protect the

embankment.

• Construct a stilling basin or energy-dissipating pool.

• Involve a qualified engineer if the remedy is not

obvious.
Figure 47 – Outlet releases erode the toe of the dam. Figure 48 – A scour hole at an outlet resulting in erosion of the toe of 

the slope.

47
	
48 

Concrete or Masonry Dam Problems 

Cracking, Opening/Closing or Offsets at Joints, or Apparent Deformations 

Probable Causes and Possible 

Consequences 

• Structural cracking (longer cracks, not pattern cracks),

broken masonry, opening/closing or offsets at joints, and

apparent deformation in structures can be indications of

foundation problems.

• Foundation problems are the leading cause of concrete and

masonry dam failures.

• Failures of concrete and masonry dams typically are sudden

failures that can have catastrophic consequences due to

their suddenness.

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Have a qualified engineer assess the situation and supervise

the steps necessary to reduce the danger to the dam.

• Draw down the reservoir to reduce the risks of dam failure, as

warranted.
Figure 49 – Concrete deterioration, leakage and staining. 

Figure 50 – Open joints. 

49 
50 

Deterioration of Structural Materials

Probable Causes and Possible Consequences 

• Deteriorated concrete or masonry materials may have

less strength, and therefore less ability to carry the

reservoir loads imposed on the dam.

• Failures of concrete and masonry dams typically

are sudden failures that can have catastrophic

consequences due to their suddenness.

Recommended Action 

Report the suspected problem to the appropriate Federal or 

State agency official and have the situation evaluated by a 

qualified engineer immediately. 

• Have a qualified engineer assess the situation and

supervise the steps necessary to reduce danger to the

dam.

• Draw down the reservoir to reduce the risks of dam

failure as warranted.
Figure 51 – Concrete deterioration on the downstream face of a dam. Figure 52 – Deterioration of a concrete pier. 

51 
52 

Spillway Problems 

Excessive Vegetation or Debris in the Spillway Channel or Around the Inlet 

Probable Causes and Possible Consequences 

• An accumulation of slide materials, dead trees, debris,

excessive vegetative growth, etc., in the spillway

channel can reduce waterway capacity.

• Reduced discharge capacity may cause the dam to

overtop. Prolonged overtopping can cause dam failure.

Recommended Action 

• Clean out debris regularly as necessary to maintain an

open waterway and control vegetative growth in the

spillway channel.

• Install a log boom in front of the spillway entrance to

intercept debris.
Figure 53 – Excessive vegetation or debris in a spillway channel. Figure 54 – A blocked spillway channel may cause overtopping of the 

dam due to the loss of the spillway discharge capability. 

53 
54 

Erosion of Unlined Spillway Channels 

Probable Causes and Possible Consequences 

• Surface runoff from intense rainstorms or flow through

the spillway erodes spillway flow surfaces.

• Livestock traffic creates gullies where runoff flow can

concentrate and cause erosion.

• Irregularities in concrete structure flow surfaces can

lead to damage and loss of structure due to cavitation,

stagnation pressures, etc.

• Unabated erosion can result in headcutting, erosional

spillway failure, and unintended, uncontrolled, large

releases at the spillway location.

• Spillway damage can result in reduced spillway

capacity. Inadequate spillway capacity can lead to

embankment overtopping and result in dam failure.

Recommended Action 

• Have a qualified engineer inspect the conditions and

recommend remedial actions.

• Photograph the condition.

• Replace eroded material with compacted fill to repair

damaged areas, if appropriate.

• Revegetate the areas if appropriate.

• Install suitable rock riprap to protect against future

erosion., if appropriate.

• Closely monitor spillway performance during flood

events.
Figure 55 – Erosion channels. 

Figure 56 – Erosion of the spillway channel may lead to slumps or 

slides of the spillway sides, or large, uncontrolled, unintended releases 

in the event erosional headcutting in the spillway channel breaches the 

5 5 spillway crest.
56 

Resources 

Dam Operation, Maintenance, and Inspection Manual, 1985, revised 

2007. North Carolina Department of Environmental and Natural 

Resources, Division of Land Resources, Land Quality Section. 

Dam Safety: An Owner’s Guidance Manual, FEMA 145, July 1987. 

Federal Emergency Management Agency. 

Dam Safety Manual, 2002. John Schurer, Eric Wilkinson, 

James Norfleet, Jon Van Sciver, Clint Huntington, Chin Lee, and 

Allan Rogers. State of Colorado, State Engineer’s Office. 

Dam Safety Workshop PowerPoint Presentations. 

Association of State Dam Safety Officials (ASDSO). 

Glossary of Terms for Dam Safety, FEMA 148, February 1988. 

Federal Emergency Management Agency. 

Guidelines for Operation and Maintenance of Dams in Texas, 

November 2006. Texas Commission on Environmental Quality. 

Levee Owner’s Manual for Non-Federal Flood Control Works, 

March 2006. U.S. Army Corps of Engineers, The Rehabilitation and 

Inspection Program, Public Law 84-99.
	

Low Hazard Dams—Standing Operating Procedures (SOP), 

October 2008. U.S. Department of the Interior, U.S. Fish and Wildlife 

Service. RDSR Phase 1 Report: Glossary Terms and Standard Units, 

July 2010. U.S. Department of the Interior.
	

Safety Evaluation of Existing Dams: A Manual for the Safety 

Evaluation of Embankment and Concrete Dams,1992. 

U.S. Department of the Interior, Bureau of Reclamation (BOR), 

Denver, CO. 

Technical Manual for Dam Owners, Impacts of Plants on Earthen 

Dams, FEMA 534, September 2005. Federal Emergency 

Management Agency.
 

Training Aids for Dam Safety, Module: Evaluation of Seepage 

Conditions, 1990. David E. Wright, John M. Cyganiewicz, 

Charles H. McElroy, and Robert L. James. U.S. Department of the 

Interior, Bureau of Reclamation, Denver, CO. 
Web Sites 

Association of State Dam Safety Officials 

www.damsafety.org/ 

Bureau of Reclamation Training 

www.usbr.gov/ssle/damsafety/training.html 

Federal Emergency Management Agency 

www.fema.gov/dam-safety-publications-resources 

FEMA Training Aids for Dam Safety (TADS): A Self-Instructional Study Course in Dam Safety Practices 

www.fema.gov/library/viewRecord.do?id=3308 

57 
58 

Useful Terms 

Abutment—That part of the valley side against which the dam is 

constructed. An artificial abutment is sometimes constructed, as a 

concrete gravity section, to take the thrust of an arch dam where 

there is no suitable natural abutment. The left and right abutments 

of dams are defined with the observer viewing the dam looking in 

the downstream direction, unless otherwise indicated. 

Appurtenant structure—Ancillary features of a dam, such as 

outlets, spillways, powerplants, tunnels, etc. 

Arch dam—A dam (typically concrete or masonry) which is curved 

upstream in plan so as to transmit the major part of the water load 

to the abutments and to keep the dam in compression. 

Axis of dam (concrete)—A vertical reference surface coincident 

with the upstream face at the top of the dam. 

Berm— A nearly horizontal step in the sloping profile of an 

embankment to break the continuity of the slope; usually 

constructed to reduce erosion or to increase the thickness of the 

embankment. A shelf that breaks the continuity of a natural rock or 

earth cut or artificial ridge of earth. A ledge or shoulder, as along the 

edge of a road or canal. 

Breach—An opening through a dam that allows the uncontrolled 

draining of a reservoir. A controlled breach is a constructed opening. 

An uncontrolled breach is an unintentional opening. A breach is 

generally associated with the partial or total failure of the dam. 

Buttress dam— A dam consisting of a watertight upstream part 

(such as a concrete sloping slab) supported at intervals on the 

downstream side by a series of buttresses (walls normal to the axis 

of the dam). Buttress dams can take many forms. 

Channel—Natural or artificial watercourse with a definite bed and 

banks to confine and conduct continuously or periodically flowing 

water. 

Concrete Dam—See arch dam, buttress dam, or gravity dam. 
Conduit—A concrete lined or concrete/steel lined closed channel 

to convey water through, around, or under a dam or to house other 

conduits or pipes. 

Core wall—A wall of substantial thickness built of impervious 

material, usually of concrete or asphaltic concrete, in the body of an 

embankment dam to prevent leakage. 

Crest gate (spillway gate)—A gate on the crest of a spillway to 

control the discharge or reservoir water level. 

Crest length—The length of the dam measured along the axis or 

centerline crest of the dam, at the top level of the main body of the 

dam or of the roadway surface on the crest. 

Crest of dam—The top surface of the dam. 

Cross section—An elevation view of an object formed by passing 

a plane that is perpendicular to the object’s longest axis. An 

elevation view of a dam formed by passing a plane perpendicular to 

the dam axis; often perpendicular to the section view. 

Cutoff wall—A wall of impervious material (e.g., concrete, asphaltic 

concrete, timber, steel sheet piling, or impervious grout curtain) 

located in the foundation beneath a dam and which forms a water 

barrier and reduces seepage beneath a dam or spillway. 

Dam—A barrier built across a watercourse to impound or divert 

water or liquid-borne solids. A barrier that obstructs, directs, 

retards, or stores the flow of water. Usually built across a stream. 

A structure built to hold back a flow of water. 

Dam failure—Catastrophic type of failure characterized by the 

sudden, rapid, and uncontrolled release of impounded water or 

the likelihood of such an uncontrolled release. It is recognized 

that there are lesser degrees of failure and that any malfunction or 

abnormality outside the design assumptions and parameters that 

adversely affect a dam’s primary function of impounding water is 

properly considered a failure. These lesser degrees of failure can 

progressively lead to or heighten the risk of a catastrophic failure. 

They are, however, normally amenable to corrective action. 

59 
60 

Drain, toe—A system of pipe and/or pervious material along 

the downstream toe of a dam used to collect seepage from the 

foundation and embankment and convey it to a free outlet. 

Drainage area—The area that drains to a particular point on a 

river or stream. The drainage area of a stream at a specified 

location is that area, measured in a horizontal plane, enclosed 

by a topographic divide from which direct surface runoff from 

precipitation normally drains by gravity into the stream above the 

specified point. 

Drop Inlet—An uncontrolled location for flow to enter a spillway, 

typically by flowing over a wall or opening that serves as the sill of 

the spillway. 

Embankment dam—Any dam typically constructed of excavated 

natural materials. 

Face—Exposed surface of dam materials (earth, rockfill, or 

concrete), upstream and downstream. The external surface which 

limits the structure. The more or less vertical surface of rock 

exposed by blasting or excavating. 

Filter (filter zone)—One or more layers of granular material 

incorporated in an embankment dam and graded (either 

naturally or by selection) to allow seepage through or within the 

layers while preventing the migration of material from adjacent 

zones. 

Flashboards—Temporary barriers of timber, concrete or steel, 

anchored to the crest of a spillway as a means of increasing the 

reservoir storage. Flashboards can be removed, lowered, or 

carried away at the time of flooding either by a tripping device or 

by deliberate failure of the flashboards or their supports. 

Flood—A temporary rise in water surface elevation resulting in 

inundation of areas not normally covered by water. Hypothetical 

floods may be expressed in terms of average probability of 

exceedance per year, such as one-percent-chance-flood, or 
expressed as a fraction of the probable maximum flood or other 

reference flood. 

Foundation—Soil or rock materials at the damsite that underlie 

and support the dam structure. Foundation materials that are 

consolidated into rock or rock-like material may be referred to as 

bedrock, while unconsolidated materials may be referred to as 

surficial materials. 

Freeboard—The difference in elevation between the maximum 

water surface in the reservoir and the dam crest. The vertical 

distance between a stated water level and the top of a dam, 

without camber. 

Gate—A device that controls the flow in a conduit, pipe, or tunnel 

without obstructing any portion of the waterway when in the fully 

open position. Structure or device for controlling the rate of flow 

into or from a canal or ditch. A moveable watertight barrier for the 

control of water in a waterway. 

Gravity dam—A dam constructed of concrete and/or masonry 

which relies on its weight and internal strength for stability. Gravity 

dams are generally used where the foundation is rock, and earthfill 

in proper quality and quantity is not available. 

Groin—The line of contact between, or area along, the upstream 

or downstream face of a dam and the abutments. 

Heel of dam—The junction of the upstream face of a concrete 

gravity dam or arch dam with the ground surface. 

Hydraulic Height—The vertical difference between the lowest 

point in the original streambed at the dam axis (or the centerline 

crest of the dam) and the maximum controllable water surface 

(which often is the crest of an uncontrolled overflow spillway). 

Intake—Placed at the beginning of an outlet-works waterway 

(power conduit, water supply conduit), the intake establishes 

the ultimate drawdown level of the reservoir by the position of 

61 
62 

its opening(s) to the outlet works. The intake may be vertical 

or inclined towers; drop inlets; or submerged, box-shaped 

structures. Intake elevations are determined by the head 

needed for discharge capacity, storage reservation to allow for 

siltation, the required amount and rate of withdrawal, and the 

desired extreme drawdown level. 

Landslide—The movement of a mass, debris, or earth down 

a slope. In some instances, a landslide can involve large 

volumes of material, and landslide movements can damage 

or destroy embankments and/or appurtenant structures. 

Landslides occurring rapidly in a reservoir can lead to waves or 

displaced water that can overtop a dam. 

Leakage—Uncontrolled loss of water by flow through a hole or 

crack. 

Length of dam—The length along the top of the dam between 

contact abutments. This also includes the spillway, power 

plants, navigation lock, fish pass, etc., where these form part 

of the length of the dam. If detached from the dam, these 

structures should not be included. 

Longitudinal—Pertaining to or extending along the long axis, or 

length, of a structure. Lengthwise. 

Masonry Dam—Any dam constructed mainly of stone, brick or 

concrete blocks jointed with mortar. A dam having only a masonry 

facing should not be referred to as a masonry dam. Masonry 

dams differ from rockfill dams in that the stone is hand-placed with 

mortar resulting in the entire dam being impermeable. 

Outlet—An opening through which water can be freely 

discharged from a reservoir to the river for a particular purpose. 

Outlet gate—A gate controlling the flow of water through a 

reservoir outlet. 

Phreatic surface—The free surface of water seeping at 

atmospheric pressure through soil or rock. The location of the 

phreatic surface typically varies in response to changing reservoir 

and tailwater conditions. 
Piping—The progressive development of internal erosion by 

seepage. Classical piping occurs when soil erosion begins at a 

seepage exit point, and erodes backwards, supporting a pipe or 

roof along the way. 

Reservoir—A natural or artificial body of water impounded by a 

dam and in which water can be managed (stored, regulated, or 

controlled). 

Reservoir surface area—The area covered by a reservoir when 

filled to a specified level. 

Riprap—A layer of stones, broken rock, boulders, precast blocks, 

bags of cement, or other suitable material generally placed 

in random fashion on the upstream and downstream faces of 

embankment dams, stream banks, on a reservoir shore, on the 

sides of a channel, or other land surfaces to protect them from 

erosion or scour caused by current, wind, wave, and/or ice action. 

Very large riprap is sometimes referred to as “armoring”. 

Sand Boil—Seepage characterized by a boiling action at the 

surface surrounded by a cone of material from deposition of 

foundation and/or embankment material carried by the seepage. A 

swirling upheaval of sand or soil on the surface or downstream from 

an embankment caused by water leaking through the embankment. 

The ejection of sand and water resulting from piping. 

Seepage—The slow movement or percolation of water through small 

cracks, pores, interstices, etc., of soil or rock without formation of 

definite channels. The movement of water into and through the soil 

of embankment, abutment, foundation, unlined canals, ditches, or 

water storage facilities. 

Slope—Inclination from the horizontal. Sometimes referred to as 

batter when measured from vertical. 

Slope protection—The protection of a slope against wave action or 

erosion. See “Riprap.” 

Spillway—A structure over or through which flow is discharged from 

a reservoir. If the rate of flow is controlled by mechanical means, 

such as gates, it is considered a controlled spillway. If the geometry 

63 
64 

of the spillway is the only control, it is considered an uncontrolled 

spillway. Where more than one spillway is present at a dam, the 

service spillway begins flowing first, followed by the auxiliary 

spillway, and finally the emergency spillway. 

Spillway channel—An open channel or closed conduit conveying 

water from the spillway inlet downstream. 

Spillway chute—A steeply sloping spillway channel that conveys 

discharges at super-critical velocities. 

Spillway crest—The lowest level at which water can flow over or 

through the spillway. 

Storage—The retention of water or delay of runoff either by 

planned operation, as in a reservoir, or by temporary filling of 

overflow areas, as in the progression of a flood wave through a 

natural stream channel. 

Structural Height—The vertical distance between the top of the 

dam and lowest point of the excavated foundation area, excluding 

narrow fault zones. The structural height of an embankment dam 

is the vertical distance between the top of the embankment and the 

lowest point in the excavated foundation area, including the main 

cutoff trench, if any, but excluding small trenches or narrow backfilled 

areas. 

Toe of the dam—The junction of the slope or face of a dam with the 

ground surface; the upstream toe of a concrete dam is also called 

the heel. 

Transverse—Pertaining to or extending along the short axis, or 

width, of a structure. Perpendicular to or across the long axis, or 

length, of a structure. 

Trashrack—A structure (often constructed of metal or reinforced 

concrete) placed at the intake of a conduit, pipe or tunnel that 

prevents entrance of debris over a certain size. 

Valve—A device used to control the flow in a conduit, pipe, or 

tunnel that permanently obstructs a portion of the waterway. As 

distinguished from gates, valves are constructed so that the closing 

member remains in the water passageway for all operating positions. 
 Evolution of this Guide 

This “Pocket Safety Guide for Dams and Impoundments” was 

developed by a team led by the USDA Forest Service in 2012, 

and has been widely distributed and used since that time. 

In 2013 – 2014 a team from the Department of the Interior 

suggested revisions, as part of their Reduce Dam Safety Risk 

(RDSR) Enterprise Architecture effort. 

In 2015 FEMA and the National Dam Safety Review Board 

agreed to take on the responsibility of publishing and distributing 

the guide and solicited comments from other entities. 

Those comments have been incorporated into this version of the 

guide. 

Keywords: dam failure, dams, embankment, 

concrete, erosion, impoundments, low hazard dams, 

reservoirs, riprap, safety at work, spillways 

65 

Notes: 
Notes: 
Additional copies of this document 

may be ordered from: 

http://www.fema.gov/media-library-data/20130726-1651-

20490-4586/fema_publication_order_form_rev.pdf 

For additional information about dam 

safety, contact: 

https://www.fema.gov/dam-safety 

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Technical Manuals and Guides

The following manuals, guides, and reports provide procedures and

guidance for dam specialists and dam owners responsible for the

design, construction, inspection, maintenance, and repair of dams.

Evaluation and Monitoring of Seepage and Internal Erosion

(FEMA P-1032)

This document addresses evaluation and monitoring of

seepage and internal erosion in an effort to collect and

disseminate information and experience that is current and has

a technical consensus (2015).

Selecting Analytic Tools for Concrete Dams Address Key

Events Along Potential Failure Mode Paths (FEMA P-1016)

The purpose of this document is to stress the importance of

understanding the sequences of events leading to failure of

concrete dams and selecting analysis methods that address

these specific events (2014).

Technical Manual: Overtopping Protection for Dams

DVD (FEMA P-1015)

The National Dam Safety Program sponsored the development

of a technical manual, in conjunction with the Bureau of

Reclamation, to collect and disseminate useful and relevant

information regarding the design, construction, and

performance of overtopping protection alternatives for

embankment and concrete dams (2014).

Overtopping Protection for Dams Brochure (FEMA P-1014) 

A brochure that describes best practices for design,

construction, problem identification, maintenance, renovation,
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and repair (2014).

Federal Guidelines for Inundation Mapping of Flood Risks

Associated with Dam Incidents and Failures (FEMA P-946)

The purpose of this document is to provide dam safety

professionals with guidance on how to prepare dam breach

inundation modeling studies and conduct mapping that can be

used for multiple purposes including dam safety, hazard

mitigation, consequence evaluation, and emergency

management including developing Emergency Action Plans

(EAPs). This guidance is intended to provide a consistent

approach that can be applied across the country (2013).

Federal Guidelines for Dam Safety: Emergency Action Planning

for Dams (FEMA P-64)

This document is an update of the 2004 version and provides

guidance to help dam owners, in coordination with emergency

management authorities, effectively develop and exercise EAPs

for dams (2013).

Pocket Safety Guide for Dams and Impoundments (FEMA P911)

This guide was developed as a quick reference to help dam

owners and others assess low hazard dams and impoundments

(2012).

Technical Manual: Plastic Pipe Used in Embankment Dams

(FEMA P-675)

Procedures and guidance for best practices associated with

embankment dams (2007).

Emergency Action Planning for State Regulated High-Hazard

Potential Dams: Findings, Recommendations and Strategies

(FEMA 608)

An EAP developed to help prevent loss of life from dam failure

(2007).

Final Report on Coordination and Cooperation with the

European Union on Embankment Failure Analysis (FEMA

602CD) 
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An integration of the European Community and U.S. Research

findings over a 3-year period (2007).

Technical Manual for Dam Owners: Impacts of Plants on

Earthen Dams (FEMA 534) 

A manual that advances awareness of dam safety problems

associated with impacts on earthen dams (2005).

Technical Manual: Conduits through Embankment Dams

(FEMA 484) 

Procedures and guidance for best practices associated with

conduits through embankment dams (2005).

Technical Manual for Dam Owners: Impacts of Animals on

Earthen Dams (FEMA 473) 

A manual that provides guidance to dam specialists (2005).

Conduits through Embankment Dams Brochure (FEMA L-266)

A brochure that summarizes the procedures and guidance for

best practices associated with conduits through embankment

dams (2005).

Dam Owner’s Guide to Animal Impacts on Earthen Dams

Brochure (FEMA L-264) 

A brochure to help dam owners manage and reduce wildlife and

wildlife damages at earthen dams (2005).

Dam Owner’s Guide to Plant Impact on Earthen Dams

Brochure (FEMA L-263) 

A brochure that describes the dangers of problem vegetation on

earthen embankment dams (2005).

Many publications can be ordered from the FEMA Publications

Warehouse by calling 1-800-480-2520 (Monday to Friday from 8:00

a.m. to 5:00 p.m. EST) or sending a fax to 1-240-699-0252. You can

also email a request to FEMA-Publications-Warehouse@dhs.gov.

Please provide the title, FEMA publication number, and the quantity

of each publication along with your name, address, zip code, and

daytime telephone number.
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