Army Corps would operate Howard Hanson Dam to full capacity
Sept. 16, 2011
Seattle - Today Col. Bruce Estok, commander of the Seattle District, U.S. Army Corps of Engineers, announced that he is confident that he can operate Howard Hanson Dam to its full flood storage capacity as designed now that important risk reduction measures are nearing completion.
Seattle District engineers and scientists have concluded that the Corps can operate the dam this season to hold its design full pool – an elevation of 1,206 feet above sea level – with low risk to the dam.
“All indications are that the new measures will perform as intended to full flood storage capacity,” said Col. Estok. “However, until the dam experiences a flood above summer pool level [1,167 feet] there isn’t 100 percent certainty of how the new measures will perform.”
The Corps will continue to monitor the dam to ensure effectiveness of the corrective measures, especially during high flow events.
Drainage improvement work is on schedule and is expected to be complete by the end of October 2011. Drains through the area of highest concern, known as the short path seepage area, have already been completed and tested. The reservoir at Howard Hanson Dam reached a summer elevation of 1,167 feet above sea level in June, allowing engineers, geologists and other scientists to run a battery of tests to gather more data on the right abutment drainage improvements that are completed and underway.
The Corps of Engineers constructed a seepage barrier (grout curtain) in November 2009 to reduce seepage and improved the drainage collection system of the right abutment by installing drains that more effectively direct seepage into the drainage tunnel. Last flood season the Corps had confidence that we could safely store water to at least elevation 1,170 feet. Testing showed that the work was successful, however, the 2009 grout curtain is not considered a permanent repair.
Along with controlling seepage in the right abutment by installing additional vertical and horizontal filtered drains this year, the Corps is pursuing projects to increase confidence that the dam can safely operate during extreme flood events (i.e., very large flood events that involve use of the spillway). These measures, to be completed in 2012, include:
* Installation of additional log booms to prevent debris from blocking the spillway
* Further stabilizing the spillway by improving how it is anchored to bedrock
* Placing additional rock along the upstream face of the dam to protect it against erosion from fast-moving water in the event the spillway is used.
The Corps is also making good progress on a detailed dam safety study to ensure all significant and credible risks have been evaluated. The study is scheduled to be completed by the end of 2013.
More information about Howard Hanson Dam is available at the Seattle District’s website at: http://ping.fm/vc3ET You can also follow the situation at the dam on Twitter at: http://ping.fm/5C9o4