If conditions at a dam under our jurisdiction prompt an emergency declaration or the involvement of local/state emergency response staff, we track it here. This does not necessarily mean that a failure is imminent. In these cases, we work with local emergency management and the dam owner to take steps to reduce the risks posed by the dam.
Eightmile Lake Dam
Nov. 18, 2019
We're providing technical and regulatory support to reduce the risk posed by Eightmile Lake Dam in Chelan County. The dam is 90 years old and located in the Alpine Lakes Wilderness. After a 2017 wildfire, the area was left vulnerable to erosion, flash flooding, and possible dam failure. In spring 2018, the Chelan County Department of Emergency Management and the dam owner, the Icicle-Peshastin Irrigation Districts (IPID), each declared emergencies to notify the public and initiate remedial actions.
The IPID completed temporary repairs to the dam in summer 2018 including reshaping and armoring the dam crest and replacing part of the low-level outlet pipe. We installed monitoring equipment around the dam and conditions are normal.
The dam’s outlet pipe was held open through the 2018-2019 winter to keep storage volume down and increase the dam’s capacity to handle rain and snowmelt. The dam performed well during the 2019 spring runoff.
The IPID would like to rebuild the dam. They're developing options for review by all stakeholders through the State Environmental Policy Act (SEPA) process. That process will likely result in an Environmental Impact Statement to assess a dam rebuild.
Dam failures in Washington
Some incidents can lead to a catastrophic release of water which may cause loss of life, damage to property, and environmental harm. We refer to these events as failures. Below is a list of notable failures and some details on recent events.
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