Army Corps Lowers Flood Risk on Green River to 1:60; Permanent fix anticipated by 2012

FOR IMMEDIATE RELEASE: August 10, 2010

Contact:
Dana Hinman, Public Affairs Manager
City of Auburn
253-931-4009
dhinman@auburnwa.gov

AUBURN, Wash. - August 10, 2010 - The U.S. Army Corps of Engineers, Seattle District, provided an update yesterday regarding the status of the Howard Hanson Dam and the potential risk of flooding in the Green River Valley. The Corps is now reducing the risk of flooding on the Green River to 1:60, down from the last estimate of 1:33. The reduction of risk comes from the results of testing of the temporary measures put in place this year to address concerns about seepage through the dam’s north abutment.

According to Corps of Engineers, this spring the dam was able to hold a pool of 1,167 feet above sea level (48 percent of full) for more than two months without seeing any problems. This has provided additional confidence in the storage capacity of the dam for this coming flood season.

The reservoir at Howard Hanson Dam reached 1,170 feet in elevation and was held at an elevation at or above 1,167 feet May 9 through July 12, allowing engineers to test the interim seepage barrier with additional reservoir volume. During the conservation pool, the Corps conducted in-depth investigations of the dam’s current effectiveness and has determined the risk to be approximately 1:60 for the upcoming flood season. At full capacity, the odds of flooding are 1:140. Flood season in Western Washington is typically November – February.

Officials are recommending that residents and businesses continue to be prepared for the increased risk of flooding as weather experts are predicting a wetter weather pattern this fall and winter. This includes purchasing flood insurance if your home or business is in the potential impact area, having an emergency kit at home and in your car, and signing up for emergency notifications. For more information on how to prepare, visit www.auburnwa.gov/disaster or call 253-876-1925.

Background:
During January 2009 flooding symptoms of internal erosion were observed, prompting an operational limitation of the dam at the headwaters of the Green River. This operational limitation raised the flood risk for those communities in the Green River Valley downstream to levels not seen for 50 years.

After much analysis and review, the U.S. Army Corps of Engineers has determined that the most effective risk reduction measures to construct at Howard Hanson Dam would be to rehabilitate the existing drainage tunnel, add a branch to that tunnel through the area of highest concern to intercept seepage, and to install additional vertical and horizontal drains to channel more water from the abutment into the tunnel. Once these measures are in place, the Corps believes that the dam could be operated at full capacity.

Funding recently approved for the Corps will allow construction to begin before the end of the year with estimated completion by summer 2012.

The Corps operates the Howard Hanson Dam project, which provides flood damage reduction to the cities of Auburn, Kent, Renton and Tukwila, as well as low-flow augmentation to the Green River for environmental benefits. More details and history about Howard Hanson Dam and the right abutment issue can be found at www.nws.usace.army.mil.

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City of Auburn
25 West Main St.
Auburn, WA 98001
253-931-3000
www.auburnwa.gov