Green River Basin flood plan: Addressing possible increase in flood risk
Following a record high level of water behind Howard Hanson Dam in January 2009, the U.S. Army Corps of Engineers restricted flood storage behind the dam after they became concerned about two depressions on the right abutment, increased water levels in groundwater monitoring wells, and the appearance of silty water entering the abutment drainage tunnel. To date, engineers have excavated the depressions, installed additional monitoring equipment and conducted tests while a summer conservation pool was stored at the dam.There is no imminent risk of dam failure. However, what the Corps has found so far hasn’t yet increased confidence in using the full flood storage capacity of the dam. Should a major flood event occur with the temporary restriction of pool elevation, it is possible that levees in the lower valley could be overtopped. The Corps has been working closely with King County and the downstream cities of Auburn, Kent, Renton, Tukwila and Seattle to prepare for flood season, should higher-than-standard flows be necessary from the dam.
Preparations for the upcoming flood season include constructing an interim seepage barrier wall and improving the drainage tunnel to control seepage through the most critical part of the right abutment, continuing to monitor the abutment, and repairing segments of levee along the Green River. Simultaneously, the Corps has initiated test borings, geotechnical modeling and analysis to support planning for a long term repair project to address seepage.
The Corps of Engineers will continuously reassess the pool restriction as conditions change and may change the maximum pool elevation after careful deliberation.
Public safety is the number-one priority. This Web page is dedicated to the flood planning effort and will serve as a primary communication link for accessing current information and resources for personal preparedness.
More information about Howard Hanson Dam from the U.S. Army Corps of Engineers
Howard Hanson Dam Pool Restriction Fact Sheet (July 2009; 2 pages)
Howard Hanson Dam Reduced Storage Capacity Frequently Asked Questions (July 2009; 3 pages)
What residents, business and property owners can do
- Review your flood insurance polices for structure and contents coverage.
- Don't have insurance? Contact your insurance representative or visit www.floodsmart.gov.
- Consider purchasing an inexpensive NOAA weather radio which will relay urgent weather alerts day and night.
- Establish a family emergency plan and know your evacuation routes should a significant flood event occur.
- If severe weather is predicted, monitor area news media for information.
- Subscribe to www.RPIN.org to receive emergency alerts and other breaking news.
Media releases, headlines and blogs
Conservation pool levels are being brought down as engineers are concerned the right abutment is showing signs of internal erosion.
The Army Corps of Engineers is lowering the water level stored behind Howard Hanson Dam after finding signs that water is moving through an earthen abutment "very fast".
Water storage behind the Howard A. Hanson Dam on the Green River is being reduced for two weeks because of leaks in the earthen dam. King TV video on-line
The speed at which water is seeping through a flank of the Howard Hanson Dam raises the risk of flooding for the cities in the valley below.
Of the state's total of 43 Presidential disaster declarations, 31 have involved flooding. Not all flooding can be prevented - insurance available to National Flood Insurance Program participating communities.
Most homeowner and business insurance does not cover flood damage.
Informational videos
Howard Hanson Dam and Green River safety
(Video length 01:28:00)
Flood Control District Briefing
(Video length 01:11:00 - Briefing begins at agenda item #7)
For more information, contact your local Emergency Management office.