The Howard Hanson Dam performed well during recent heavy rains to help control flooding along the Green River, according to the U.S. Army Corps of Engineers.
The corps held water behind the dam that pushed the reservoir level to a peak of 1,155 feet above sea level. That is above an elevation (1,147 feet) that triggers increased on-site monitoring to ensure dam safety.
Despite the extra water storage, engineers said the dam performed well, according to a Jan. 19 corps media release.
The corps has held Hanson Dam outflows to around 7,000 cubic feet per second, while inflows peaked above 20,000 cfs. The corps is operating the dam to keep flows along the Green River at Auburn about 10,500 cubic feet per second, below the 12,000 cfs level of concern for levees from Auburn downstream to Kent, Tukwila and Renton.
Levee patrol teams remained out Jan. 19 in the Green River basin monitoring conditions and communicating what they see back to the Army Corps Seattle District’s reservoir control center and emergency operations center.
Flows above or near levels of 9,000 cfs along the Green River trigger the corps to monitor levees 24 hours a day.
There is a risk of flooding for property owners along the non-leveed middle Green basin, the rural portion of the river upstream from Auburn to Hanson Dam. Flows above 9,000 cubic feet per second as measured at the Auburn gage, triggered a flood warning from the National Weather Service due to flooding concerns in the Middle Green River.
These flows do not present flooding concerns for the more highly populated areas downstream, including Auburn, Kent, Tukwila or Renton.
The corps works to keep the reservoir empty at Hanson Dam along the Green River until storage is required for flood risk management. The dam had an empty reservoir on Jan. 14.
The corps has lowered the threat of Green River flooding this winter to 1 in 60 from 1 in 33 last year because of installation of a grout curtain that stood up to summer tests to allow for more storage capacity in the reservoir behind the dam.
Repairs were needed at the dam after a January 2009 storm damaged an abutment next to the dam.
The odds of flooding are 1 in 140 when the dam is fully operational. An additional $44 million in repairs, including more drains and the extension of the existing drainage tunnel to put the dam back at its top level, will not be completed until late 2012.
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