Weekend rain had little effect on Green River dam

Heavy weekend rains in the Seattle area had little effect on the Green River's Howard Hanson Dam, the Army Corps of Engineers says.

Casondra Brewster, spokeswoman for the corps' Seattle District, said Monday the water level in the dam's reservoir is at 1,097 feet above sea level, just 22 feet above the point where the pool is considered empty.

The weekend rains - about 1 inch in a day at the dam - only slightly raised the amount of water flowing in, to about 800 cubic feet per second. The dam's outflow was about the same, only 200 cfs above its regulated outflow for this time of year.

"We're talking drops of water," Brewster said. "If you looked at the pool, you couldn't tell whether it is going up or down."

In contrast, the flood control dam in the Cascade foothills was releasing up to 8,000 cfs in the days after January's record rainstorm.

That storm, which dumped 15 inches of rain in 12 hours on Green's upper watershed, sent torrents into the 235-foot-high dam's reservoir. Emptied as usual at the start of winter, the reservoir quickly rose to six feet higher than ever before.

The dam itself remained sound, but engineers found serious signs of weakness in the right abutment, a 450-foot-wide pile of rock deposited by a prehistoric landslide. The corps immediately began drawing down the water to relieve the pressure, and later began a temporary fix of injecting grout into the abutment to reduce seepage.

Still, the corps has said there is a 1-in-4 chance it might have to release enough water this rainy season to cause flooding downstream in the heavily developed Green River Valley south of Seattle.

At present, the reservoir is 88 feet below the level where engineers become concerned about leakage through the abutment, Brewster said.

The corps actually increased the amount of water in the reservoir during the spring and summer, to ensure enough water flows down the Green to protect fish.

"October is not without its dry time," Brewster said. "We've been holding (water) and working with resource agencies to make sure the flow is maintained."

She said the corps is on track to finish the $8.9 million grouting project as scheduled on Nov. 1. Engineers are running tests on the work, and Seattle District commander Col. Anthony Wright has said he will better be able to estimate how much water the reservoir can hold when they are completed.

The Green River Valley has about 25,000 people and includes much of the cities of Kent, Auburn, Renton and Tukwila, huge shopping malls, major Boeing Co. facilities, plus one of the largest warehouse districts on the West Coast. The cities and King County are rushing to bolster levees along the river, Boeing is building an 8-foot-high wall around its Kent defense plant and Renton flight simulators, and residents have been given hundreds of thousands of sandbags.

The corps is planning a concrete wall as a permanent fix for the abutment but has said that would take more than two years.