Earth Dam Failures

Contents

Overtopping Failures

Seepage Failures

Structural Failures

Combined Failures

Resources

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Owners of dams, operating personnel, and maintenance personnel must be knowledgeable of the potential problems which can lead to dam failure. These people regularly view the structure and, therefore, need to be able to recognize potential problems so that failure can be avoided. If a problem is noted early enough, an engineer experienced in dam design, construction, and inspection can be contacted to recommend corrective measures, and such measures can be implemented.

If there is any question as to the seriousness of an observation, an engineer experienced with dams should be contacted.

Acting promptly may avoid possible dam failure and the resulting catastrophic effect on downstream areas.

Since only superficial inspections of a dam can usually be made, it is imperative that owners and maintenance personnel be aware of the prominent types of failure and their telltale signs. Earth dam failures can be grouped into three general categories: overtopping failures, seepage failures, and structural failures. A brief discussion of each type follows.


 

Overtopping Failures

Overtopping failures result from the erosive action of water on the embankment. Erosion is due to uncontrolled flow of water over, around, and adjacent to the dam. Earth embankments are not designed to be overtopped and therefore are particularly susceptible to erosion. Once erosion has begun during overtopping, it is almost impossible to stop. A well vegetated earth embankment may withstand limited overtopping if the dam's crest is level, the downstream slope of the dam is uniform with a consistent slope gradient, and there are no bare areas or undulations along the surface of the dam. The owner should closely monitor the reservoir pool level during severe storms.

 


Seepage Failures

All earth dams leak to some extent and this is known as seepage. This is the result of water moving slowly through the embankment and/or percolating slowly through the dam's foundation. This is normal and usually not a problem with most earthen dams if measures are taken to control movement of water through and under the dam. If uncontrolled, seepage can progressively erode soil from the embankment or its foundation, resulting in failure of the dam. Typically, erosion of embankment soil begins at the downstream side of the dam and progressively works toward the reservoir eventually developing a path to the reservoir which is referred to as "piping." Piping action can be recognized by an increased seepage flow rate, the discharge of muddy or discolored water, sinkholes on or near the embankment, and possibly a whirlpool at the surface of the reservoir. Once a whirlpool (eddy) is ovserved, failure of the dam may follow. As with overtopping, fully developed piping is virtually impossible to control and will likely cause failure.

Seepage can also cause dam failure by saturating the embankment, thus weakening the dam, or by increasing internal pressure within the embankment. Saturation and internal pressure within the dam are difficult to determine without proper instrumentation.


 

Structural Failure

Structural failure typically refers to the collapse of non-earthen embankment dams such as those made from concrete, masonry, or other materials not consisting of a soil matrix. In addition, failure of a dam's appurtenant structures such as a concrete chute spillway slab, gate structures and components, or other such features may lead to failure of the dam itself. Earthen dams to not tend to collapse or fail catastrophically on their own except where earthquakes of significant magnitude are prevalent or other erosive forces weaken the structure. Large cracks in an earthen embankment, major settlement, and major slides may require emergency measures to ensure safety, especially if these problems occur suddenly. If this type of situation occurs, the lake level should be lwoered, the appropriate state and local authorities notified, and professional advice sought. If the observer is uncertain as to the seriousness of the problem, a qualified professional engineer with experience in dam safety should be contacted immediately.


 

Combined Failures

The three types of failure previously described are often interrelated in a complex manner. For example uncontrolled seepage may weaken the soil and lead to a structural failure. A structural failure may shorten the seepage path and lead to a piping failure. Surface erosion may result in structural failure.

Minor defects such as cracks in the embankment may be the first visual sign of a major problem which could lead to failure of the structure. The seriousness of all deficiencies should be evaluated by someone experienced in dam design and construction. A qualified professional engineer can recommend appropriate permanent remedial measures.


 

Resources

ASDSO Resources

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Free Webinar
Introduction to Inspecting Dams for Owners and Operators-2013

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Sample Inspection Forms I
Sample Inspection Forms II
Sample Inspection Forms III

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Dam Emergency Intervention Toolbox (Funded by a grant to Montana and Wyoming by the FEMA National Dam Safety Program)
Dam Owner Academy: Emergency Intervention Toolbox Webinar: ON-DEMAND

The ASDSO website houses national guidelines on dams. Go to:
DamSafety.Org/ManualsandGuidelines

For case studies and lessons learned from historic dam failures, go to:
DamFailures.Org

For more information, videos, and tools for dam owners go to:
DamOwner.Org

Watch for training in you area sponsored by ASDSO or your State Dam Safety Office. Access your state's Dam Safety Program by clicking your state at:
DamSafety.org/States

 

DHS/FEMA Resources

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Pocket Safety Guide for Dams and Impoundments (FEMA National Dam Safety Program)

Dam Safety: An Owners Guidance Manual (FEMA National Dam Safety Program)

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Identification of Visual Dam Safety Deficiencies (FEMA Training Aids for Dam Safety [TADS])
Dam Safety Awareness (FEMA Training Aids for Dam Safety [TADS])
How to Organize a Dam Safety Program (FEMA Training Aids for Dam Safety [TADS])
How to Organize an Operation and Maintenance Program (FEMA Training Aids for Dam Safety [TADS])

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US Fish and Wildlife Service Standing Operating Procedures for Low Hazard Dams

FEMA Fact Sheet on Emergency Action Planning (FEMA National Dam Safety Program)

FEMA Fact Sheet on Emergency Operations Planning (FEMA National Dam Safety Program)

FEMA Fact Sheet on Risk Communication for Dams (FEMA National Dam Safety Program)

Emergency Action Planning for Dam Owners (FEMA 64) (FEMA National Dam Safety Program)

Emergency Action Planning  (FEMA National Dam Safety Program)

FERC Emergency Action Plan Program

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Form Fillable EAP Template

To encourage and facilitate development of EAPs at all high and significant hazard potential dams, ASDSO and the EAP Workgroup of the NDSRB have developed simple and low-cost tools for creating and implementing an EAP at their dam(s). One of the resources was a EAP fillable form template. This template can be easily edited for specific dams and the needs of local emergency management agencies.
Fact Sheet 
EAP Template
Instructions

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Simplified Inundation Mapping

In 2009, a task group under the National Dam Safety Program developed the recommendation in the methodology and fact sheet below. The recommendations are provided to assist states and dam owners in developing reduced cost Simplified Inundation Mapping (SIMS) for EAPs. They are not a substitute for engineering judgment nor do they alleviate the need to comply with state or federal regulatory requirements.
SIMS Fact Sheet
SIMS Methodology     

The Department of Homeland Security and FEMA make several publications and videos available through their websites. Visit their websites below and search "dam safety" for more information.
FEMA.gov
DHS.gov

 

Videos

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Perfect to Imperfect Dam - Maintenance (YouTube, Download)
Overtopping Failure (YouTube, Download)
Piping Failure (YouTube, Download)
Slope Failure  (Youtube, Download)
Identification of Visual Dam Safety Deficiencies
 (FEMA Training Aids for Dam Safety [TADS])


 

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