Repairs begin on Howard Hanson Dam to protect Green River Valley

The Army Corps of Engineers has begun interim repairs to the Howard Hanson Dam aimed at controlling seepage through an earthen abutment and reducing the likelihood of winter flooding in the populous Green River Valley.

A contractor is scheduled to pour concrete pads today as a first step in building a 450-foot-long, 90- to 170-foot-deep "grout curtain" to slow the flow of water through the right abutment.

Because of that potentially destabilizing seepage, the Corps has restricted how much water can be held in the dam — meaning water released from the dam could overtop levees downstream.

A second contractor will start drilling a network of vertical and horizontal drains next week. The two contracts, to be mostly completed by Nov. 1, will be paid with $9.7 million in economic stimulus dollars.

Col. Anthony Wright, the Corps' district commander, said in a statement the grout curtain should reduce seepage and new drains should direct water "through a safe pathway. ... This work should increase our confidence in the dam's ability to reduce downstream flood risk."

Last month the Corps, King County and Kent began shoring up five sections of the Horseshoe Bend levee south of downtown Kent. Fourteen sinkholes appeared in the levee when it was damaged by storms in November 2006 and January 2009.

The Corps is also studying longer-term options for improving the dam.

Kent, Renton, Auburn, Tukwila and King County are urging thousands of residents and businesses to buy flood insurance and prepare for possible evacuation this winter.

The county plans to mail a brochure next week to 170,000 residents in the flood-prone area and on higher ground that could be affected by closures of highways, warehouses, stores and schools, and by disruptions of utility service.

County Executive Kurt Triplett said the Federal Emergency Management Agency has suggested that local authorities plan not just for a possible overtopping of the levees but also for the more serious possibility they will be breached. In that event, he said, "You're talking about water that's rocketing down the valley at the highest levels you've ever seen." Because of the high stakes, Triplett said he will seek to persuade the federal government to "predeclare" an emergency, paving the way for earlier federal help.

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