A once-weakened Howard Hanson Dam should operate at full capacity this winter.
So says a confident Col. Bruce Estok, commander of the Seattle District, U.S. Army Corps of Engineers.
Estok said the Corps can operate the 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,” Estok said on Friday. “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.
Local officials welcomed the good news.
“We are pleased that the Corps has been able to restore the original flood protection of the Howard Hanson Dam far ahead of the original time schedule,” said Sarah Miller, emergency preparedness manager for the City of Auburn. “Obviously there is still, and always will be, flood risk in the Green River Valley. But with the dam functioning within its design parameters, we are much more comfortable going into the upcoming winter season.”
Drainage improvement work is on schedule and is expected to be complete by the end of October, the Corps said. The Corps said it has already treated and tested drains through the area of highest concern, known as the short path seepage area.
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 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 said it had confidence that it 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., large flood events that involve use of the spillway).
Measures slated for completion in 2012, include:
• Installation of additional log booms to prevent debris from blocking the spillway.
• Further stabilizing the spillway by improving its anchoring 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 also is 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.
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More information about Howard Hanson Dam is available at www.nws.usace.army.mil You can also follow the situation at the dam on Twitter at twitter.com/HowardHansonDam.
