Developed by the National Technology & Development Program, Missoula, MT.
Dams are essential to the infrastructure of the United States. They are built to store water for human consumption, power generation, flood control, irrigation, stock watering, wildlife, and recreation.
Our nation currently has about 85,000 dams. The Forest Service owns or has under its special use authorization around 1,800 dams. More than 50 percent of these Forest Service dams are 50 years or older. The average design life of a Forest Service dam is 75 to 100 years. Due to their advanced age and limited maintenance, dams need to be assessed regularly to ensure they will not fail.
A failure can cause devastating losses to life and property. Dam safety is critical.
Dam owners play a major role in dam safety. This program will help land management employees and special use owners contribute to the safety of low hazard dams or impoundments. The program has five parts:
The owners of a dam are responsible for safely operating and maintaining their dam.
There are two types of dams on National Forests, those owned and operated by the Forest Service and those owned and operated by holders of special use authorizations, pre-Forest Service easements, and Ditch Bill easements.
The extent of an owner's liability varies from State to State and depends on statutes and case law precedents. An owner could be held liable for any failure of a dam and all the damages resulting from its failure. Maintaining a safe dam is critical to preventing failures and limiting an owner's liability.
Dams may either be manmade or natural. Manmade dams, also known as impoundments, are artificial barriers that impound or divert water on a temporary or long-term basis. Naturally occurring dams may include beaver dams, landslides, or glacial deposits.
The two most common manmade dams include gravity and embankment dams. Gravity dams are usually constructed of concrete or masonry. Embankment dams are constructed of earth, rock, or industrial waste materials. This program addresses embankment dams constructed of earth, normally called earthen dams.
Some of the essential features of a dam are:
Dams have many failure modes. Failure modes can include slope failures, consolidation, differential settlement, drain failures, outfall failure, spillway failure…the list is extensive.
Failures occur in varying degrees, from minor to catastrophic. Minor failures are not an immediate threat to the overall stability or integrity of the dam. However, minor failures are progressive and often lead to failure, if untreated.
Major failures involve the sudden and uncontrolled release of water. Major failures are often described as a catastrophic since the damage they cause is often catastrophic. The top three categories of earthen dam failures are piping, overtopping, and structural.
Piping is caused by the progressive development of internal erosion by seepage. Piping is evident by the excessive growth of grass or plants, wet spots, or salt deposits on the downstream dam face. Depressions or sink holes in the dam face can also be an indicator since they are often locations where dam material has been lost internally.
Overtopping of dams may be caused by an inadequate principal spillway design or clogging of the spillway by debris. This may be evident by water troughs running across the top of the dam from past flooding events.
Structural failures typically occur as a result of substandard foundation investigations during the design phase. Such failures are usually the result of seepage paths caused by inadequate foundation materials (such as sand or gravel) and foundation settlement. A boil at the downstream toe of the dam is a sign of seepage under the foundation.
Problems that May Cause Dam Failure
Each dam has a hazard potential classification. The rating indicates the potential impact the dam may have if it fails. It is not based on its current structural condition. There are three hazard classifications:
High hazard—A dam failure would likely result in the loss of at least one human life.
Significant hazard—Loss of human life is possible but unlikely in the event of a dam failure. However, significant loss of property or environmental destruction would likely occur.
Low hazard—Loss of human life and property and/or environmental damage is unlikely in the event of a dam failure.
The earthen dams covered in this program are low hazard dams.
Dress in appropriate field clothing and sturdy boots.
Make sure to bring a camera to photograph any suspected problems. Another item to bring is the "Pocket Safety Guide for Dams and Impoundments." The guide will serve as a reminder of important features of dams and common problems. You may also want to bring a notebook, pen or pencil, measuring tape, and 6-foot ruler. The ruler will add scale to the photographs.
A visual condition assessment of a dam should be conducted following a consistent sequence. This will reduce the chance that a problem will be missed. One way is to start at the downstream side of the dam and work your way toward the reservoir.
There are two types of visual condition assessments, a cursory and hands on. The cursory assessment involves looking at the dam as a whole and identifying possible defects or problems.
The hands-on assessment involves getting within arm's reach of the feature looking for potential problems. If you feel you can't assess a feature safely, error on the side of caution and don't do it.
If you think you have found a problem, photograph it, try to determine a possible cause, and follow the recommended actions in the pocket safety guide. The recommended actions you take will depend on your experience level. Inexperienced individuals need to report the problem to the appropriate Federal or State agency official or the dam owner.
Let's take a look at some common problems you may see when visibly assessing the condition of earthen dams.
SINKHOLES—surface depressions. Sinkholes are caused by piping or the internal erosion of embankment materials or the foundation. A cave-in or a small hole in the wall of the outlet pipe or drain can also cause a sinkhole.
Piping will cause a dam embankment or foundation to fail if not repaired.
SLIDE, SLUMP or SLIP—the unplanned movement of earth or rock down the upstream or downstream face of a dam. This can be caused by foundation movement, excessive seepage, seismic loading, drain failure, over steepened slopes, or a combination of those factors.
If a slide, slump, or slip is not corrected, the principal spillway may become blocked or the dam may fail all together.
BROKEN DOWN or MISSING RIPRAP—damage to the large stone, precast blocks, bags of cement, or other material on the upstream face of a dam. Riprap protects slopes against wave action, erosion, or scour. Riprap can be damaged through poor quality material, wave or ice action, or rocks rolling out.
The dam itself may erode if broken down or missing riprap is not replaced.
TREES or OBSCURING VEGETATION—the unwanted growth of trees and bushes on the dam. Grass is the desired vegetation on a dam to stabilize slopes. Trees and bushes grow when dam faces have not been properly maintained.
Trees and bushes that are not removed from a dam can impede inspection and attract damaging rodents. In addition, the roots may allow seepage paths to develop and large trees can blow down during storms causing damage to a dam or even failure.
Dam Failure Caused by a Tree
RODENT ACTIVITY or ANIMAL IMPACT—the unwanted presence of burrowing rodents or other animals on the dam. This activity is frequently caused by poor vegetation maintenance. An overabundance of vegetation, such as cattail-filled areas and trees close to the reservoir, provide an ideal habitat and foraging area for rodents and other animals.
When rodents and other animals are not controlled, they can dig holes, tunnels, and caverns. Tunnels reduce the required length of the seepage paths and could cause piping problems or may even lead to a dam failure.
LIVESTOCK and CATTLE TRAFFIC—the unwanted crossing or grazing of livestock on a dam. Livestock traffic is usually caused by damaged fences or barriers.
Livestock on the dam may create paths, possibly reducing erosion protection and causing erosion channels. Grazing livestock may also create bare areas where water can pool. As these pools evaporate, drying cracks may result.
TRANSVERSE CRACKING—where the embankment of a dam has separated along a line perpendicular to the crest of the dam. This kind of cracking can be caused by uneven movement between adjacent zones within the embankment. It can also be caused by structural stress or instability.
If transverse cracking is not corrected, the weakened area may cause additional movement, further deformation, or even failure. In other instances, water may enter the crack and saturate the surrounding area leading to a localized failure.
LONGITUDINAL CRACKING—where the embankment of a dam has separated lengthwise. Longitudinal cracking is caused by uneven settlement between the adjacent sections or zones of the embankment. It can also be caused by a foundation failure or embankment slide.
If longitudinal cracking is not corrected, the weakened area may cause additional movement, further deformation, or even failure. In other instances, water may enter the crack, saturate the surrounding area, and lead to a localized failure.
LOW AREA IN THE CREST OF A DAM—a depression on the top of a dam. Low areas are caused by uneven settling of the embankment or foundation, internal eroding of the embankment, spreading of the foundation, or improper grading of a road on the crest after construction.
Low areas can reduce the available freeboard, the distance between the normal reservoir elevation and the crest of the dam. Reduced freeboard may allow water to spill over the top of the dam. Overtopping can erode the downstream face and toe of the dam and an eventual dam failure.
EXCESSIVE QUANTITY or MUDDY WATER EXITING FROM A POINT—when water and sediment flows or leaks out of the downstream face of a dam. Rodents, frost, tree roots, and poor construction contribute to internal eroding of the embankment that may create pathways, channels, and piping through the dam. Leaks can also develop when seepage causes excessive pressure in the downstream face or toe of the dam.
If leaks are not fixed, water may saturate parts of the embankment potentially causing embankment erosion, slides, or a dam failure.
SEEPAGE WATER EXITING AS A BOIL IN THE FOUNDATION—when water bubbles out from underneath the foundation of a dam. Boils may be caused by a sand or gravel layer in the foundation that provides a pathway for water to flow.
Unfixed boils can cause the foundation to erode and lead to a dam failure.
SEEPAGE WATER EXITING AT THE ABUTMENT CONTACT—when water flows between the interface of the dam's embankment and the valley's side slope. This seepage is caused when water flows through pathways in the native abutment soils or the dam embankment.
Flows at the abutment contact must be monitored over the long term to detect the relationship between the flow and the reservoir elevation. Abutment seepage transporting soil (looks like cloudy water) must be reported immediately.
SEEPAGE WATER EXITING FROM A POINT ADJACENT TO THE OUTLET PIPE—when water flows through a dam along the side of the outlet. This seepage can be caused by a break in the outlet pipe, a hole in the pipe, or when a water pathway (piping) has developed along the outside of the pipe due to poor compaction.
If seepage along the outlet is not corrected, the embankment may erode and the dam may fail.
FAILURE OF CONCRETE or ROCK OUTFALL STRUCTURES—when the structure around the end of the outlet pipe has broken or the rock headwall has failed. Failure of concrete outfall structures may be caused by poor concrete quality or too much pressure on the nonreinforced concrete. Failure of rock outfall structures may be caused by blockage from rocks that roll down too steep of slopes.
If the outfall structure is not repaired, the embankment may erode from exposure to outlet releases and lead to dam failure.
OUTLET RELEASES ERODING THE TOE OF THE DAM—when reservoir water flowing through the outlet pipe causes a scour hole to develop underneath the outlet pipe. This erosion can be caused by an outlet pipe that is too short. It also can be caused when the end of the outlet pipe does not have an energy-dissipating pool or structure.
If the toe of the dam erodes, the downstream face may become too steep or progressive sloughing may occur and lead to dam failure.
EXCESSIVE VEGETATION or DEBRIS IN THE SPILLWAY CHANNEL OR AROUND THE INLET—the unwanted presence of plants or other natural material in a dam's principal spillway channel or around the inlet. This problem is caused by dirt sliding into the channel, vegetation growing in the channel, or dead trees collecting in the channel or around the inlet.
If the principal spillway channel or inlet is partially or completely blocked, it may overflow, possibly causing flow through the emergency spillway or the dam to overtop. Most earthen dam crests are not armored so overtopping can lead to significant structural damage and failure.
EROSION OF SPILLWAY CHANNELS—the wearing away of the manmade channel that transports water from the reservoir to the natural downstream channel, especially during intense rainstorms or flows from the reservoir. In concrete-lined spillways, erosion occurs after the lining fails exposing the bed material. When this material is exposed, erosion hollows out the spillway foundation resulting in large voids and the eventual collapse of the spillway lining. Livestock hooves can also trigger damage by erosion especially in areas subject to flowing water like spillways. Livestock should never be allowed to graze on any part of a dam for any reason.
If this erosion is not corrected, slides, slumps, or slips can occur reducing the spillway's capacity, leading to the dam overtopping and possibly failing as a result.
In this program, you learned about dam owner responsibilities, common dam terms and features, possible dam failures, visual condition assessments of dams, and some of the most common dam problems. Most importantly, remember this program does not make you an expert but it will help you recognize potential problems that could result in dam failures.
For additional information, including useful terms and references, please refer to the pocket safety guide.
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U.S. Forest Service Missoula Technology and Development Center
U.S. Forest Service Washington Office of Engineering
U.S Department of Homeland Security, Federal Emergency Management Agency,
National Dam Safety Program
PROJECT LEADER: James Scott Groenier
SOCIOLOGIST: Lisa Outka-Perkins
EDITOR: Geraldine Wolf
DESIGNER: Amanda Determan
ILLUSTRATOR: Ed Jenne, Jenne Illustration
The Forest Service, United States Department of Agriculture (USDA), 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 the Department of any product or service to the exclusion of others that may be suitable.
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