'Very fast' seepage found at dam upriver from Kent, Renton, Auburn, Tukwila

The Army Corps of Engineers is lowering the water level stored behind the troubled Howard Hanson Dam after finding signs that water is moving through an earthen abutment "very fast," the agency announced this morning.

The dam, which provides flood protection for the Green River Valley cities of Kent, Renton, Auburn and Tukwila, has been under intense study since January, when engineers found several "anomalies" that suggested excessive amounts of water might be seeping through the right abutment and causing erosion.

A recent dye test showed water moving quickly through the abutment at higher-than-expected levels. "This phenomenon continues to be troubling," Hanson Dam program manager Mamie Brouwer was quoted in a news release.

"These tests indicate that there are preferential flow paths within the natural materials of the right abutment that could lead to internal erosion ... if water is held at these higher elevations for extended periods of time."

The Corps, which operates the dam and recently raised the water level to 1,169 feet above sea level, is now lowering the water level to a safer 1,155 feet — below the normal summer level for supporting steelhead and salmon runs and providing drinking water for Tacoma, Kent and Covington.

The agency is hiring contractors to study the geology of the abutment, install a "grout curtain" to reduce seepage, and drill additional vertical and horizontal drains in the abutment before the next rainy season in November. Those measures are expected to cost more than $20 million.

Dam-safety experts from around the country are advising the Corps on those interim measures and possible long-term solutions. Until permanent repairs are made, the dam is likely to be operated to hold less water than in the past, increasing the risk of flooding in the industrialized cities downstream.

Seepage through the right abutment — built on soil and rock from a landslide that changed the river's course 10,000 years ago — has caused concerns for much of the dam's 48-year life. In 1965, after excessive seepage was detected, the Corps of Engineers installed a drainage tunnel with wells and horizontal drains.

In 2002, a grout curtain was installed to allow water storage at higher levels during the summer.

The anomalies found in the abutment in January were higher-than-expected water levels in the abutment; heavy, muddy water flow in one drainage well; and a 10-foot-wide, 6-foot-deep depression in the soil.

The reservoir behind the dam, at its highest level since it went into operation in 1961, was quickly drawn down to a lower level in response those disturbing observations. The water level has been raised slowly over recent weeks as engineers and scientists have studied water flow through the abutment.

Bolstering the right abutment and determining how much water the dam can hold next winter are among the Corps of Engineers' top priorities nationally, district officials say.

Officials with the valley cities, King County and the Corps have been urging businesses and residents in the Green River Valley to obtain flood insurance and have drawn up plans for informing and evacuating residents in the event of flooding.

"We don't want to alarm the public, but at the same time we have a problem," Tukwila Public Works Director and flood incident commander Jim Morrow said in a recent interview. "The Corps has done such a marvelous job since the dam was built of controlling floods, people who live in the valley or have a business in the valley have no memory that that's a floodplain."

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