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Dam safety incidents

Despite our best efforts to promote dam safety and assist owners in maintaining their dams in a safe manner, failures sometimes occur.

Why dams fail

Nationally, the main reasons that dams fail are:

  • Overtopping — 34 percent of all failures
    • Inadequate spillway design
    • Debris blocking the spillway
    • Settlement of dam crest
  • Foundation defects — 30 percent of all failures
    • Differential settlement
    • Sliding and slope instability
    • High uplift pressures
    • Uncontrolled foundation seepage
  • Piping and seepage — 20 percent of all failures
    • Internal erosion through the dam caused by seepage — "piping"
    • Seepage and erosion along hydraulic structures, such as an outlet
    • Conduits or spillways, or leakage through animal burrows
    • Cracks in the dam
  • Conduits and valves — 10 percent of all failures
    • Piping of embankment material into conduit through joints or cracks
  • Other — 6 percent of all failures

Dam failures in Washington

Bonasa Breaks Ranch Dam

The most recent incident occurred in Asotin County near Anatone in April 2017. An unpermitted dam impounding 36 acre-feet of water suddenly breached, releasing about 29 acre-feet of water (over 9 million gallons). The water traveled down Rattlesnake Creek and released into the Grand Ronde River about 6.5 miles away. Although no lives were lost, the resulting flood caused extensive damage to public infrastructure, private property, and critical stream habitat.

Wenner Lakes Dams

In August 2014, five dams on the Wenner Lakes in the recently-burned Benson Creek watershed in Okanogan County were overwhelmed by considerable flooding. This flooding caused the closure of State Highway 153 near Twisp, the failure of three dams, and the overtopping of the two other dams. Fortunately, there were no fatalities, injuries, or missing persons. 

Hydraulic modeling showed that the burned area resulting from the Carlton Complex Fire greatly amplified the rainfall runoff anticipated for the watershed and exceeded the dams’ capacity. Our engineering analysis of these failures can be viewed in the following technical reports: