The reservoir at Howard Hanson Dam reached an elevation above 1,167 feet above sea level May 9, allowing engineers to test the interim seepage barrier installed in November 2009 with additional reservoir volume.
While the Corps brought the pool up above 1,167, outflow from the dam has been running at about 500 cubic feet per second, and now water managers must release significantly higher outflows to lower the pool back to elevation 1,167 by May 16.
Reservoir outflows will increase significantly beginning late evening on May 12 and are expected to remain high through May 15. During this period, releases from the dam are expected to be mostly in the range of 1,500 to 2,500 cfs. By May 16, reservoir outflows will return to near the inflow rate, which is currently forecast to be about 1,200 cfs.
Those using the Green River from Thursday May 13 through Saturday May 15 are advised that flows will be significantly higher than they have been over the past week, so caution is advised. Outflows are expected to remain well below flood stage.
During the conservation pool, the Corps has been testing and conducting in-depth investigations of the dam’s current effectiveness. During the fill, if there are any indications from readings or observations that the dam is behaving in any way not considered expected or normal, the reservoir level can be reduced appropriately.
Since April 21, when the reservoir reached the benchmark of 1,147 feet, the U.S. Army Corps of Engineers has conducted intensive monitoring, including scientists on the ground around the clock. Since early March, the Seattle District of the Corps has been slowly filling Howard Hanson Dam’s reservoir for its annual conservation pool.
Preliminary data from the testing results suggest that the grouted seepage barrier is reducing flow through the north abutment. However, this data is preliminary and should be considered provisional, as it has not been fully analyzed. 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 downstream to levels not seen for 50 years.
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 http://www.nws.usace.army.mil.
