Wyoming Flood Awareness Week

Thursday - NWS Response to Dam Breaks

The Wind River floods a home west of Riverton in 2011

Index Flash Flooding Snowmelt / Run-off Flood Safety Dam
AHPS / Drought
Social Media

Swift Creek
Swift Creek Dam 2011


The National Weather Service (NWS) will be notified by dam operators or owners of the dam (U.S. Bureau of Reclamation, Bureau of Indian Affairs, State of Wyoming, or other entity), and/or local emergency officials (sheriff department, emergency managers or town/city officials) that there is a potentially hazardous or imminently disastrous situation with a dam. This situation may involve partial breaching or total failure of a dam, which will likely threaten lives and property.

NWS Response:

NWS forecasters will immediately begin to decide what information to distribute to the public to warn of the impending hazard.

  • Flash Flood Watch - indicates a 50 to 80 percent chance of dam failure within 12 hours. This product will delineate an area downstream from the dam break, which might be affected by dangerous rises of water over the following 6 hours.
  • Flash Flood Warning - means dam failure is imminent or an 80 percent chance or greater of a dam failure within 6 hours. It will delineate an area downstream from the dam break, which will be affected by dangerous rises of water over the following 6 hours. If the reservoir with a potential for complete failure has an Emergency Action Plan (EAP), then NWS forecasters will use a warning software package that can immediately transmit a Flash Flood Warning within minutes of notification of a total failure of a dam. This warning software can also be used for smaller reservoirs that do not have EAPs. The warning software will automatically plot preformatted Lat/Lon pairs that define the immediate impact areas downstream within 6 hours travel time of initial dam break. The warning will include estimated heights of the flood wave at point locations along the river as well as estimated times of arrival for the flood wave.


Updates to Flood Warnings:

NWS forecasters will continue to update the public with new information on the flooding until emergency officials tell the NWS that the immediate threat is over. Forecasters will use Flash Flood Statements to keep the public apprised of new information.

Actions to Protect Life and Property:

In the event of a dam break which poses serious threats to lives and/or residential, business or public property, state and local emergency management officials will issue statements to the public concerning actions to be taken, including evacuations, if necessary. These messages will be communicated through the media and other emergency notification systems. The NWS may be requested to issue Civil Emergency Messages (CEM's) for broadcast over NOAA All-Hazards Weather Radio regarding protective actions to be taken by the public.

Individuals living within a flood plain downstream of a dam are encouraged to have a NOAA All-Hazards Weather Radio receiver in their home. When activated by the local NWS forecast office, the receiver will automatically warn users of impending hazards, such as a potential dam failure.

Example of a Dam Break Incident - Fort Washakie Dam Leak on 6/1/2000:

Construction for repairs on Fort Washakie Dam was completed in March of 2000. Minimal seepage by the dam was being experienced, but this was reported to be normal. On June 1, 2000, excavation work was being performed at the bottom of the dam to help alleviate the seepage. A series of pipes was being put in at the base of the dam so the seepage would drain into the South Fork of the Little Wind River. While doing this project, it was noticed that there was some ponding of water that was occurring on the left side at the base of the dam. After further investigation and digging, water was seen "flowing" from this area. At this point the dam operator began releases and the Emergency Action Plan (EAP) was put into effect. The NWS issued a Flash Flood Warning as emergency officials had recommended evacuation of residents along the South Fork of the Little Wind River. The warning continued until officials had alleviated the potential for dam failure

Washakie Reservoir Spillway
Washakie Reservoir Spillway (Note: Not taken during 6/1/2000 event)

For additional information, contact your local NWS office:

Cheyenne, WY Riverton, WY Billings, MT Rapid City, SD Salt Lake City, UT
800-269-6220 800-211-1448 406-652-0851 605-341-7531 801-524-5133

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