Army: Slightly less chance of Green River flood

Work to strengthen a reservoir wall and to raise levees means there is a slightly lower chance of flooding this winter in the Green River Valley south of Seattle, the district commander for the Army Corps of Engineers said Monday.

However, Col. Anthony Wright told the King County Council that even with interim repairs to the hillside next to Howard Hanson Dam, he still can't risk filling the dam's reservoir to capacity. The dam in the Cascade foothills southeast of Seattle controls flooding from the river in the heavily developed Green River Valley.

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.

The dam held the water back and remained sound. But engineers found excess seepage and signs of weakness within the dam's right abutment, a 450-foot-wide pile of rock deposited by a huge landslide 10,000 years ago.

Since then, crews have been injecting grout into the abutment to form a shield that will help strengthen the hillside. Wright told the council that those interim repairs are on track to be completed by Nov. 1, and that work is under way to raise the height of the levees in the Green River Valley.

He said those efforts have lowered the possibility of catastrophic flooding from the 1-in-3 chance he gave earlier this year to 1-in-4.

Still, Wright cautioned the council that the grout shield will only reduce but not eliminate the seepage, and he can't risk filling the reservoir to capacity. Corps officials hope to construct a concrete wall in the abutment as a permanent fix, but have said that work will take at least three years.

A news release from the County Council said Wright praised efforts by the county and cities in the valley to prepare for possible flooding. He said 750,000 sandbags were being delivered for distribution, including 750 "supersacks" - large plastic and fiber bags filled with rock and gravel.

While the sandbags will help prevent levees from being overtopped, Wright warned that communities should not get into what he called "levee wars" - building levees higher than those elsewhere on the river system.

"This creates the risk of damaging the levees under their own weight" and causing them to collapse, Wright said. "Building the system up to capacity is an appropriate balance to prevent overtopping while still not placing too high of an overburden."

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.

Officials have urged residents to buy flood insurance, stow valuables in safe places and be ready to flee if a heavy rainstorm hits.