Corps lowers Green River flooding odds to 1-in-25

The Army Corps of Engineers says hurried repairs this summer at the Howard Hanson Dam have lessened the chances the Green River will flood this winter to 1-in-25.

Previously, the corps has said there was a 1-in-4 chance that a flood would inundate the heavily developed Green River Valley downstream. The corps has been working around the clock to prevent flooding, pumping tons of grout into the problem area, an abutment at the dam that was badly weakened by a torrential storm last winter.

Residents, businesses and local governments in the long, flat valley south of Seattle have been working feverishly to fortify their property and the levees along the winding river against a potentially catastrophic flood during the winter rainy season.

In recent weeks, some 40 miles of levees have been raised with sandbags, evacuation routes and emergency warning systems have been set up, and residents have been urged to assemble "go kits" — documents, medicine and other valuables they'll need if forced to flee on short notice.

Corps Col. Anthony Wright today said improvements were good news, but urged residents to continue to prepare for the possibility that floods may come.

"We've gone from a bad situation to one that's not so bad," he said. He added that the risk of serious flooding when the dam is working properly is usually no higher than 1 in 140.

The grouting — essentially pumping more than 470,000 gallons of concrete into an abutment to the dam — basically works like glue, helping bind earthen material to slow the seepage of water. Wright said the temporary fix has helped him increase capacity of the reservoir behind Howard Hanson from roughly 33 percent of normal to about half.

But he also said that if another "pineapple express" weather condition came through like the one last January, which brought enough rain to damage the abutment, he almost certainly would have to release enough water to cause a flood.

He also said the risks, while lower than they would have been without the grout, remain high for the hundreds of people who live upstream of Auburn, where there essentially are no levees and flooding can occur with far less water.

Dean Conti, who owns one of the dozens of precision machine shops in Kent where equipment is too heavy and expensive to move out of harm's way, was relieved by the Corps' new assessment.

"That's a huge a change," said Conti, who has worried since September about how best to protect his business. "I think it's a great day. It's certainly good news, and I don't find fault with anybody. They told us what they didn't know, and now they know more. We'll still be paying attention, though."

King County Executive Kurt Triplett had asked the council for permission to borrow up to $40 million to pay for flood preparations. While county officials emphasize that a risk of flooding remains, some of those plans will be re-evaluated.

King County spokeswoman Christine Lange said the county will postpone evacuation of its animal shelter in Kent and will take a second look at whether to move courts and inmates from the Maleng Regional Justice Center in Kent to other locations.

"It's much easier to evacuate an animal shelter ahead of a flood," Lange said.

Officials will study whether to go ahead with plans to move courts and inmates out of the justice center, which is now ringed with protective concrete blocks.

Election headquarters will remain at its temporary location near Boeing Field in Tukwila following its move from a flood-prone location in Renton, said Natasha Jones, spokeswoman for Triplett. Election offices won't move back to Renton until the dam has been permanently fixed, she said.

The Corps has said it could take three to five years to complete that fix — likely a concrete wall built within the abutment.

"The risk is still there," Jones said. "It's a little less, which is wonderful. We're glad all the collaborative work we've done to date has helped. It takes the pressure off, but there still is the risk."

The valley cities and King County have held scores of informational meetings, passed out hundreds of thousands of sandbags, posted extensive information on special Web sites and organized a "reverse 911" system to automatically call or message residents if an emergency is declared.

The Boeing Co. has erected a sandbag wall around its sprawling Space Center in Kent. Managers of a nearby Starbucks roasting plant are in constant communication with federal disaster officials. Many homeowners have installed one-way valves to prevent toilets backing up.

The valley floor has miles of malls, warehouses, and businesses small and large, including Recreational Equipment Inc.'s headquarters and Starbucks' regional roasting plant.

The four major cities in the Green River Valley — Auburn, Kent, Renton and Tukwila — all face the threat of flooding. An estimated 25,000 to 30,000 people live in flood zones, but Hillman Mitchell, the emergency management director in Tukwila, points out that 200,000 to 300,000 work or shop in the valley each day.

Seattle Times staff reporters Keith Ervin and Craig Welch contributed to this story.