Army corps: Summer work on Seattle-area dam has lessened Green River flooding chance

Corps: Dam work lessens Seattle-area flood chance

SEATTLE — Hurried repairs at a badly weakened flood-control reservoir have greatly reduced but far from eliminated the risk of flooding this winter in the Green River Valley near Seattle, the U.S. Army Corps of Engineers said Thursday.

The region has been preparing for flooding ever since a torrential storm in January weakened an abutment to an important upstream dam that holds back the Green River. Residents and businesses have been piling up sandbags and the corps has been working around the clock on repairs to shore up the abutment. Most of the repairs were completed within the past week.

Col. Anthony Wright, the corps’ Seattle district commander, provided an update Thursday on the dam repairs. He said there was now a 1-in-25 chance that a storm would force the corps to release enough water from the dam’s reservoir to cause a flood downstream in the Green River Valley. The odds of widespread flooding in the valley improve to 1-in-32 when all the sandbagging and flood-protection efforts are factored in.

Previously, the Corps of Engineers said the chance of widespread flooding was 1-and-4. While that is “a substantial change in the risk for the people downstream,” the danger is still very high and the preparations in the valley were critical, Wright said.

“It was really bad before and it’s now just bad,” he said.

Residents, businesses and local governments in the long, flat valley 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. The corps has estimated that beyond the human cost, a flood could cause $4 billion in damage.

Wright said he could safely accept more water behind the dam than he was comfortable with before — about 50 percent full compared to the earlier restriction of about 30 percent. But flooding is still much more likely than when the dam can operate at capacity, when there’s about a 1-in-140 chance of flooding.

If there’s a repeat of the January rainstorm that dumped 15 inches on the area around the dam in 12 hours, “I cannot handle that flood and that would result in widespread flooding downstream,” he said.

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 buy flood insurance and assemble “go kits” — documents, medicine and other valuables they’ll need if forced to flee on short notice.

The valley is an important center of commerce, including Boeing Co. facilities and a Starbucks roasting plant. Boeing workers put an up 8-foot-high floodwall to protect their sprawling Space Center in Kent.

The abutment to the corps’ dam on the upper reaches of the Green River was found to be seriously weakened after the record January rains. To reduce the danger the abutment might fail, the corps immediately lowered the water in the reservoir and within months began the $8.9 million project to inject grout into the abutment, a massive pile of rock and dirt left by a 10,000-year-old landslide.

Eventually, the corps plans to build a concrete wall as a permanent fix, but the valley still faces several winters of flood danger before that can be completed.

“The Corps of Engineers tells us to be prepared for inches to feet,” says Capt. Kyle Ohashi of the fire department in Kent. “All we can do is assume the worst, prepare for it and hope it never gets to that point.”