Water managers have begun slowly filling the reservoir at Howard Hanson Dam at the headwaters of the Green River.
As is typical, Army Corps of Engineers slowly begins annual spring fill of the reservoir in early March, allowing water levels to rise within Howard Hanson Dam’s reservoir. During the roughly three-month refill period, the Corps will conduct testing and in-depth investigations of the dam’s current effectiveness, according to a Corps media release.
The current plan targets a maximum pool elevation of 1,167 feet above sea level. Throughout the reservoir fill, the Corps will constantly monitor the operations of the facility including special observations and tests to measure the effects of the pool on the right abutment.
A 1,167-foot elevation pool has been the spring refill target since 2007. Visual observations and instrumentation monitoring made during these 1,167-foot elevation pools have indicated no serious seepage concerns. The right abutment interim repairs completed in 2011 are expected to improve the seepage performance of the right abutment, thus further reducing risk for a conservation pool.
As a result, it is recommended that a 1,167-foot conservation pool be stored again this year. A monitoring plan will be developed prior to the pool reaching elevation 1,147 feet to monitor the performance of the drainage tunnel improvements completed in early fall of 2011.
In 2011, the Corps invested around $40 million to improve the safety of the dam, making repairs due to seepage issues discovered after a flood event in 2009.
The rainfall and flow rate during February’s flooding landed in the top 10 events in the dam’s history, but many may not have realized the potential impact because the dam functioned according to design. Without Hanson Dam holding back flows, estimated flows on the Green River would have been near 22,000 cubic feet per second, nearly double the river’s maximum design flow of 12,000 cfs measured in Auburn.
Water stored during the spring refill of the reservoir will be used to provide water supply to the city of Tacoma and its water supply partners, including the city of Kent.
Additionally, at least three-fifths of the reservoir’s supply will be used to augment downstream flows to help endangered species of fish, including Chinook and steelhead. Project managers believe that the regulated downstream flows will aid spawning and other life-cycle events for the fish.
The Corps operates the Howard Hanson Dam project, which provides flood damage reduction to the Green River Valley, low-flow augmentation to the Green River for environmental benefits and water supply.
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