NWREporter November 2009

King County Tells Residents:
Prepare Now For Flooding in Auburn, Kent, Renton & Tukwila

"Serious flooding may occur in some areas of King County this winter. Homes, farms and businesses in the Green River Valley are particularly at risk. Residents should prepare now for the winter season." That's the word from King County Government following discovery of problems with Howard Hanson Dam on the Green River.

According to King County, residents, businesses and farms below the Howard Hanson Dam in the Green River Valley should prepare now for a higher risk of flooding.

Parts of Auburn, Kent, Renton, South Seattle and Tukwila—could be flooded and evacuations in some communities are possible." Again, according to King County, "Evacuation plans and routes are under development now." Residents are encouraged to check the County's website (www.metrokc.gov) in early September for detailed information.

Early estimates made public by the County indicate the cost to residents and businesses from flooding impacts could exceed $2 billion. The County anticipates $10 million could be required just to deal with minimizing the impacts of potential flooding issues at County facilities. Sewage overflows into homes and businesses are a possibility depending upon the amount of flooding.

Some cities are already preparing to train volunteers to help fill and place sandbags. For example, Renton has already begun recruiting and training volunteers to assist the City with sandbagging, and dyke building. Some early reports indicate perhaps as many as 10,000 volunteers might be needed to assist throughout the Green River Valley, depending upon how much flooding occurs.

Basic infrastructure could also be in jeopardy depending upon water levels. Emergency planners and elected officials are hopeful that El Niño (mild winters) might help by resulting in milder conditions and less rainfall during the rainy season that typically begins in November, but could run from October to March.

The concerns of flooding arise in connection with problems discovered at Howard Hansen Dam in January of this year. The County says that 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 the Corps became concerned about two depressions on the right earthen 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.

The County says 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.

County and city Emergency Management Professionals are urging residents to prepare now for the increased flooding risk. Their recommendations include:

• Make an emergency plan and assemble an emergency kit. For assistance see: http://www.govlink.org/3days3ways/makeaplan.html and http://www.govlink.org/3days3ways/buildakit.html
  
• Buy flood insurance now. For information see: www.floodsmart.gov.
  It takes 30 days for a policy to take effect. Review your current policy; a standard insurance policy will not cover flood damage. Contact your insurance agent or use the link above.
• Purchase an inexpensive AM radio or NOAA weather radio with batteries in case of power outages for urgent news day and night.
• Monitor area news media for information if severe weather is predicted.
• Listen for alerts about evacuation routes or find the latest evacuation route information on the website.
• Monitor local road conditions and obey closure signs.
• Take medications and supplies with you, for those with medical needs.
• Be ready to relocate animals and livestock out of harm's way.
• Minimize flood damage:
  
  • Store valuables and electronics higher on the ground floor, on the second story or in your attic.
  • Store chemicals above possible flood levels; recycle them or dispose of them at the King County Wastemobile.
  • Ensure underground storage tanks are sealed and secure.
  • Learn how to use sandbags and locate where you can buy or get them ahead of time if needed for flood fighting.
  • Move vehicles and equipment to high ground before flood waters rise.
  • Keep storm drains free of leaves and other debris.
  • Know who to call and how to protect yourself if flooding leads to sewage overflows into homes and businesses. For assistance see: http://www.kingcounty.gov/safety
    /FloodPlan/EmergencyContacts.aspx

More information about Howard Hanson Dam is available from the U.S. Army Corps of Engineers at: http://www.kingcounty.gov/safety/FloodPlan/GreenRiverValley.aspx