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Army Corps plans to extend grout curtain at Hanson Dam until permanent fix found
The U.S. Army Corps of Engineers revealed plans Thursday to extend a grout curtain as a temporary fix for the next several years as engineers design and construct a permanent fix to stop a leak through a damaged abutment next to the Howard Hanson Dam.
The corps still needs to acquire federal funds for the proposed $44 million project that would add 650 feet in length to the grout curtain installed last fall to help protect the cities of Kent, Auburn, Renton and Tukwila from Green River flooding. The corps spent $8.9 million last fall on the grout curtain.
"We may be looking at another flood season or more before we complete this," said Col. Anthony Wright, commander of the Seattle district of the Army Corps.
Wright presented an update about the Hanson Dam to local elected officials and the media at corps district headquarters in Seattle.
Local officials emphasized in a press conference after Wright's presentation that Congress needs to act quickly to fund the extended grout curtain.
"Time is of the essence," said King County Executive Dow Constantine. "Congress must address the need to move more quickly than what might be standard procedure for the corps."
The corps expects to have the design for a permanent fix, such as a concrete cutoff barrier, by the end of June in order to apply for funding from Congress in 2012 to start the project.
"We'll have perhaps another five flood seasons before it is complete," Wright said about a permanent fix.
Wright said the expanded grout curtain would allow the corps to store more water in the Eagle Gorge reservoir behind the dam and could return the odds of flooding in the Green River Valley to the 1 in 140 chance when the dam operates at full capacity.
The risk of flooding this winter sat at a 1 in 33 chance because the leak in the abutment reduced the storage capacity behind the dam.
Wright emphasized the grout curtain would only serve as a temporary fix for the next several years and that a permanent fix still would be needed.
Local officials from the state, King County and the cities of Kent, Auburn, Renton and Tukwila will now try to help the corps pursue the $44 million needed this year from Congress to start construction of the extended grout curtain.
"I was very encouraged by the colonel's remarks," said Kent Mayor Suzette Cooke in a press conference after the presentation by Wright. "The temporary fix of extending out the grout curtain and making it deeper adds a lot of assurance. We will not know the chance of risk until it is fully tested but it is at a 1 in 33 chance and it could be 1 in 140 so that is reassuring news."
Wright said the extended grout curtain could be completed in time for the next flood season if funds are found soon enough. He also will know how much capacity the dam's reservoir can handle after the current grout curtain's performance is tested this spring and summer by filling the reservoir.
"At the end of the summer, we will know how to plan for the next flood season," Wright said.
The abutment was formed nearly 10,000 years ago by a landslide. The federal government built the rock-and earth-fill Hanson dam in 1961 next to the abutment to control major flooding in the Green River Valley. The dam is about 25 miles east of Kent.
Problems with water storage behind the dam were discovered by the corps when a 10-foot-wide depression formed on the embankment next to the dam after heavy rain in January 2009. The corps stored a record amount of water in the reservoir during that storm to prevent flooding.
The corps rates the Hanson Dam as No. 1 in terms of highest-risk dams because of the damaged abutment, Wright said.
The lack of heavy rainstorms this winter kept the Green River from flooding. Wright credited the cities for placing giant sandbags along the levees for added protection in case heavy rains struck.
Several alternatives are being looked at as far as permanent fixes. Wright said a design for a permanent fix would be ready in late June in order to meet deadlines by Congress to consider the project for funding in 2012. Wright did not yet have an exact cost for a permanent fix or an exact timeline of when construction would be completed. He estimated a permanent fix would cost several hundred million dollars.
"All the solutions we are looking at have a fairly lengthy construction period once funds are appropriated," Wright said. "It would take several years for the fixes to be complete."
Auburn Mayor Pete Lewis, Renton Chief Administrative Officer Jay Covington, Tukwila Mayor Jim Haggerton and County Councilwoman Julia Patterson also attended the dam briefing. Covington said he sat in for Renton Mayor Denis Law because Law was in Washington, D.C., to meet with the state's Congressional delegation about funding for the Hanson Dam.