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Army corps reveals new Hanson Dam repair plans to protect Green River Valley
The U.S. Army Corps of Engineers has decided to install additional drains as well as extend an existing drainage tunnel to repair the Howard Hanson Dam rather than extending a temporary grout curtain.
The corps revealed the new plans at a press conference Friday in Seattle organized by U.S. Sen. Patty Murray, D-Wash., to announce how the corps would spend the $44 million approved for repairs as part of a $59 billion emergency supplemental spending bill approved earlier this month by Congress and signed July 29 by President Obama.
Crews expect to start work this fall to install new drains to help stop a leak through a damaged abutment next to Hanson Dam. The dam helps protect the cities of Kent, Auburn, Renton and Tukwila from Green River flooding. Construction is expected to take about one year. That means one more winter
for residents and businesses under temporary dam repairs completed last year and the giant sandbags that line the river banks.
"We've been working steadily trying to determine the best method to restore Howard Hanson Dam back to its design capacity," said Col. Anthony Wright, commander of the Seattle district of the Army Corps. "We believe we have the best solution based around a series of drains that controls the water inside the embankment and the abutment of the reservoir."
Wright said the ability to control water through drains and tunnels will keep the embankment from internal erosion and the potential collapse of the dam.
"After completion of these drains, I believe I can operate the dam at its full design capacity," Wright said.
The odds of flooding in the Green River Valley are a 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.
The abutment next to the dam 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.
Wright expects the drains and tunnel to work well.
"We believe that this is a longer duration fix than the grout curtain that has been discussed in the past," Wright said of plans revealed by the corps in March to extend a grout curtain by 650 feet. "Grout curtains are subject to wear as water goes up and down against them. The current grout curtain is performing well, but it has a (short) lifespan. We want to go with something that has a longer duration."
The corps spent $8.9 million to install a temporary grout curtain last fall that increased the storage capacity in the reservoir behind the dam.
It is possible the newly revealed fix of drains and an extended tunnel might even serve as the long-term fix for the dam rather than the construction of a concrete cutoff curtain that was expected to cost as much as $500 million.
"We have an ongoing dam safety modification study to determine how the corps approaches a longer term fix," Wright said. "This fix has a considerable duration and we're considering it as one of the alternatives. With that study not yet approved, I can't say this is the final answer but it is going to accomplish the mission we need to do to reduce the risk downstream."
Wright said water will drain into a tunnel before it is slowly released into the river a small amount at a time.
"We can try to stop (water) or control it and this is a method to control it," Wright said.
Engineers decided a concrete cutoff wall might not work at Hanson Dam.
"A concrete cutoff wall is technically challenged in this material this deep," said Wright, who added its unlikely a concrete cutoff would be built as the permanent fix. "The drains are in the running as the top fix."
With that fix a year away, residents and businesses will face a flooding risk this winter similar to last year.
"We were fortunate to have a fairly dry season last year," said King County Executive Dow Constantine at the press conference. "We expect a wetter season this year. Our message will be to get ready and stay ready."
Constantine, Kent Mayor Suzette Cooke, Renton Mayor Denis Law and Auburn Mayor Pete Lewis attended a briefing about the dam by Wright before the press conference and then thanked Murray for her efforts in helping to get the $44 million for the repairs.
"You have to view this as an incredible win at the federal level," Constantine said.
Lewis praised the efforts of the mayors and county officials to work with the state's federal delegation to get money for the dam to protect the Green River Valley.
"We still have a season to go," Lewis said. "We still have to be vigilant. But what we've heard today about the short period of time before the corps begins new work, we can look forward to a brighter future for us all and remembering that it is not just the four cities by the Green River but the entire region economically that could have been affected."
Dave Komendat, chief security officer at Boeing, was invited to the press conference to talk about how the dam repairs will help the company's facilities in Kent, Renton and Tukwila. Boeing constructed 8-foot berms around a Renton facility as well as the Kent building that houses its national defense programs.
"We put a lot of time and effort last year into mitigation plans and processes to make sure we would not be adversely impacted," Komendat said. "Sen. Murray's work to help fix the dam ensures that Boeing and hundreds of other businesses and thousands of families in the Green River Valley will be protected in the future."
For more information, go to www.nws.usace.army.mil.