Leaky dam built on old landslide causes flood problems in Seattle area.

King County has declared a state of emergency as the Howard Hanson Dam began showing disturbing signs of vulnerability after a torrential rainstorm in January. The dam, located in the Cascade foothills east of Seattle, has prevented major floods in the Green River valley since it was completed in 1962. However, that changed when last winter's heavy rains weakened a hillside next to the dam. A record 15 inches of rain fell in 12 hours on the Green River's upper watershed in January, sending torrents into the 235-foot-high dam's reservoir. The reservoir rapidly filled 6 feet higher than ever before.

In this Sept. 23, 2009 photo, the downstream side of the Howard Hanson Dam is shown as viewed from the top of the dam, near Ravenswood, Wash. Residents face the all-too-real prospect that the river will gush past a leaky upstream dam and swallow up their homes once the rainy season starts in November, devastating a heavily developed area in the Seattle suburbs that is a vital hub of commerce. (AP Photo/Ted S. Warren).

The dam held the water back and remained sound. But at the high water levels, engineers saw worrying signs within the dam's right abutment, a 450-foot-wide pile of rock deposited by a huge landslide 10,000 years ago.

Workers use a drill to make holes that they will inject grout into to strengthen the Howard Hanson Dam.

As a temporary fix, the Army Corps of Engineers is spending $8.9 million to inject grout into the abutment, forming a shield to lessen the seepage. Without such work, Col. Anthony Wright, the corps' Seattle District commander, said there would be a 1 in 3 chance this winter for flooding in the Green River Valley. As permanent measures, the corps hopes to create a concrete wall the length of the abutment and reaching down to bedrock — designed within a year and finished after two years of construction — a project that could cost up to $500 million.

About 25,000 people live on the valley floor, which includes parts of the cities of Kent, Auburn, Renton and Tukwila, but hundreds of thousands work, shop or travel there daily. State officials say 22,000 people might have to be evacuated in a flood.

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