Army Corps: Green River flood risk over for winter

The flood risk for the Green River Valley south of Seattle appears over for the winter, but another round of temporary repairs to the Hanson Dam's reservoir upstream is needed quickly to avoid a repeat of the danger next fall, U.S. Army Corps of Engineers officials said Thursday.

A wall of the flood-control dam's reservoir was weakened during record rains in January 2009. Since then, the corps has limited the amount of water the reservoir can hold, greatly increasing the flood danger in the heavily developed valley below.

Col. Anthony Wright, the corps' Seattle District commander, told valley mayors and King County officials at a briefing Thursday that the reservoir could be returned to roughly its normal capacity by the next flood season if the new temporary repairs are completed by then. But he warned it could be difficult to get Congress to quickly approve the $44 million needed for the work.

A relatively mild winter brought on by the El Nino weather phenomenon kept the reservoir in the Cascade foothills southeast of Seattle nearly empty. As to the danger of a possible flood this late in the season, "I would say we are out of it," Wright said.

Millions of dollars were spent last summer and fall to sandbag the Green River's meandering channel through the valley and protect homes and businesses. Besides the suburbs of Auburn, Kent, Renton and Tukwila, the valley is one of the largest warehouse and manufacturing areas on the West Coast.

The corps hurriedly drilled holes and injected grout into the weak abutment, spending $12.5 million to build a 450-foot-long "grout curtain" to temporarily hold back the water. Still, Wright warned last fall there was a 1-in-25 chance that heavy rains might force him to release enough water to cause flooding in the valley.

The next step, Wright said Thursday, is to extend the grout curtain by another 650 feet toward a more stable hillside, buying time until permanent repairs can be made.

The corps wants to get the interim project under way as soon as possible, in hopes of having it at least partially completed by late this year. But while Washington's congressional delegation and the corps' senior leadership are behind the proposal, Wright said it's late in the federal fiscal year that ends Sept. 30 to get the money approved.

"Unless I find it, and I don't consider it likely," the valley can expect to go through another flood season with the same level of risk, Wright said.

King County Executive Dow Constantine urged local politicians to keep pressing for the money, saying $44 million is cheap insurance against the estimated $3.8 billion in damage from a major flood.

Wright cautioned that the temporary repairs likely could survive only a few major flood situations and that it would be at least five flood seasons before the permanent fix could be completed.

The corps has said that work could cost an estimated $500 million and likely would involve building a deep concrete wall the length of the abutment. If money is approved by Congress for the next fiscal year, the corps estimates preliminary work could start in late 2011, with completion as early as late 2014.

Wright would not estimate what the flood risk would be for next winter if the new temporary repairs aren't finished, saying he needed to see results from extensive tests the corps is performing at the dam this summer.