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Crews begin filling Howard Hanson Dam reservoir
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.
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 of 1,167
foot elevation. As a result, it is recommended that a 1,167 foot
conservation pool be stored again this year according the established
practice for Howard Hanson Dam. 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
Howard Hanson 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 ten events in Howard Hanson Dam's history, but many may not
have realized the potential impact because the dam functioned according to
design. Without Howard 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, Wash.
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.
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 to the City of Tacoma and its
water supply partners.