Following an emergency repair to the breach at Desimone levee on Dec. 15 along the Green River in Tukwila, the U.S. Army Corps of Engineers, Seattle District, will resume normal flood water outflows from Howard Hanson Dam.
In response to the Desimone levee breach, the Army Corps initiated a 1,000 cubic feet per second reduction in releases from Hanson Dam on Dec. 15, which lowered flood elevations by 1 to 2 feet at the breach location, according to a Dec. 16 Army Corps news release. The dam is on the upper Green River and helps control flooding in Auburn, Kent, Renton and Tukwila by holding water back in a reservoir.
Following emergency repairs at noon on Dec. 16, the Army Corps will return to the pre-breach target flow of 10,000 cubic feet per second at the Auburn gage.
At Desimone levee, these changes will result in a 1-to-2-foot increase in water surface elevation after about 20 hours, according to the Army Corps. By returning outflow from Hanson Dam to pre-breach levels, the Army Corps will regain space in the reservoir ahead of upcoming storms.
“We understand rivers are stressed with continued high flows and conditions on the ground may be changing,” said Travis Ball, Seattle District chief of hydraulic and coastal engineering. “We’ll continue to evaluate outflows as we work to regain space in the reservoir in anticipation of the upcoming storm event this week, as well as provide downstream flood risk reduction as safely as possible.”
The reservoir reached a record capacity Dec. 12 of 44,400 acre feet, which is about 74% of capacity, said Kyle Comanor, Army Corps Seattle District senior water manager, in a Dec. 16 media call. As of Dec. 16, storage was 26 feet down from the record capacity, with 43% of storage utilized.
Comanor said that levee safety is one of the primary concerns, so the Army Corps watch that carefully. Staff is looking at a target flow of 10,000 cubic feet per second (cfs) at the Auburn gage along the Green River.
“We look at local flows and storms and rain and how much water we can safely release to not put too much stress on these levees as we come out of this flood event,” Comanor said.
Ball said during the media call that the Army Corps have enhanced monitoring during high flow events.
“When the (reservoir) pool was approaching record peak elevation we had 24/7 surveillance at Hanson Dam,” Ball said.
Ball said the Hanson Dam and the Mud Mountain Dam along the White River are in good condition.
“We have no concerns about dam safety,” Ball said. “The dams are operating as designed. We are not seeing any issues at Mud Mountain or Hanson Dam right now.”
In addition to managing Hanson and Mud Mountain dams, the Army Corps provides technical support as well as materials and supplies to assist counties (at their request) to help control flooding.
The public is urged by the Army Corps to remain vigilant as flood conditions can change rapidly. Residents seeking resources or assistance should contact their local government offices.
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