06/24/2009

By TIM KLASS  / Associated Press

Leaks at Howard A. Hanson Dam have led the Army Corps of Engineers to reduce its storage capacity, potentially boosting the flood danger along the Green River in the suburbs between Seattle and Tacoma this fall and winter.

A temporary fix that may ease the risk will cost millions of dollars, and a long-term solution is likely years away, experts in the Army Corps of Engineers said Tuesday.

Besides the flood potential, reduced storage could mean the suburban municipalities between Seattle and Tacoma, as well as the city of Tacoma, will have access to less water for homes and businesses. Tacoma operates a water diversion system three miles downstream from the dam.

Since the water storage dam at Eagle Gorge was completed in 1961, flooding that once was common and sometimes caused extensive damage in Auburn, Kent, Renton and Tukwila has been eliminated.

In January, when a record 15 inches of rain fell in a 12-hour period in the upper Green River watershed, the pool behind the dam rose to 1,190 feet above sea level — also a record — and streamflow in Auburn was held within about 11,000 cubic feet per second, roughly 1,500 below the flood level.

At that time, engineers noticed increased seepage in the earthen dam's 1,200-foot-long right abutment, consisting of soil and rock left by a landslide that formed the gorge 10,000 years ago, said Casondra Brewster, a corps spokeswoman in Seattle.

In subsequent tests, dye placed in the ground has been detected in water passing through the abutment in as little as two hours, compared with 10 to 12 hours that is considered normal seepage, Brewster said.

The fastest leakage rate and highest degree of worrisome muddiness occurs when the reservoir level is highest, so the maximum summer storage level is being reduced from 1,167 feet elevation to 1,155 feet.

Increasing muddiness can signal a growth of hidden erosion conduits known as "piping," the biggest cause of catastrophic failure in earthen dams.

"That's what they're concerned about," Brewster said. "So far they don't feel that it's to the level of piping."

When the pool is above 1,155 feet, however, "we see a remarkable change" in leakage and turbidity, said Richard Smith, a corps geologist.

At that level, Smith said, the agency can continue to support salmon and steelhead runs but will have less for municipal and industrial use. In addition, Tacoma may give away less of its share of the water for additional fish enhancement than in past years, Smith said.

Even so, the reduced storage is more than the 1,147-foot summertime level that was maintained before a single-layer "grout curtain" about 300 feet long and 170 feet deep was installed along the abutment in 2002 to reduce seepage and allow additional water storage.

Next month the corps plans to issue a contract for a two-layer, more sophisticated grout curtain over the existing barrier and extending another 150 feet long. According to fedbizopps.gov, a Web site listing government business opportunities, the cost is projected at $10 million to $25 million.

Brewster said the temporary fix should be completed by Nov. 1, but it's unclear how much that will cut the fall and winter flood risk.