Obama approves $44M to patch Hanson Dam; Corps say fix may be permanent

Permanently repairing a damaged flood-control dam on the Green River could cost half a billion dollars less than originally thought, the Army Corps of Engineers said Friday.

The corps is studying whether a $44 million interim fix of Howard Hanson Dam approved by Congress and signed by President Obama on Thursday might end up being the permanent solution.

"That's entirely possible. We do think once this is complete we'll be able to operate this dam at its full capacity," corps spokeswoman Casondra Brewster said.

Construction, funded as part of an emergency supplemental appropriations bill, will begin this fall and will be finished next year.

Until recently, engineers thought they would have to build a concrete cutoff wall — at a possible cost of $450 million to $500 million — to control the flow of water through the dam's porous right abutment.

The dam, which went into operation in 1961, tamed the Green River in a once-rural valley and transformed Kent, Auburn, Tukwila and Renton into one of the state's most important warehousing and manufacturing centers.

But after severe storms in January 2009, corps staffers discovered two depressions in an abutment of the earth-and-rock dam, and saw indications that water was washing through the abutment in an uncontrolled way. The corps restricted the amount of water it would store behind the dam during storms, leading to a heightened risk of downstream flooding.

The corps built an underground "grout curtain" to reduce seepage through the hillside abutment. When engineers conducted extensive tests over the past two months, they found the grout curtain had reduced some measures of seepage by "three to four orders of magnitude," Hanson Dam Program Manager Mamie Brouwer said Friday.

"We're so thrilled with how well it's working," Brouwer said.

It's working so well, the corps has revised its strategy for strengthening the abutment and the dam.

Instead of thickening, deepening and widening the grout curtain, engineers have decided it would be more effective to extend an existing drainage tunnel and add more pipes carrying water into the tunnel. They also plan interim improvements to stabilize the dam's spillway and the embankment around it.

The corps is studying whether these improvements could be the permanent solution to the seepage problem. A report addressing that question is due in November.

Sen. Patty Murray, in remarks prepared for a Friday-afternoon news conference in Seattle, called the $44 million improvement plan "a huge step forward for families, businesses and the local economy. ...

"I pushed the administration day after day, hearing after hearing after hearing — and I spent so much time talking with fellow senators about this that I bet they never want to hear the words 'Howard Hanson Dam' again."

Rep. Adam Smith said in a statement, "The livelihoods of tens of thousands of people are at risk if flooding was to occur, and the interim measures announced today provide the residents of the Green River Valley the protection and assurance they deserve."

Keith Ervin: 206-464-2105 or kervin@seattletimes.com