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Federal Way prepares its own Hanson Dam flood plan

The Howard Hanson Dam, located in South King County, restricts water flow to the Green River. Concerns with water leaking through the right abutment have caused the U.S. Army Corps of Engineers to announce it will keep the reservoir at a lower level, allowing more water into the river, as way to prevent structural damage to the dam. Cities in the Green River Valley are preparing plans to address possible winter flooding. - Courtesy of the U.S. Army Corps of Engineers
The Howard Hanson Dam, located in South King County, restricts water flow to the Green River. Concerns with water leaking through the right abutment have caused the U.S. Army Corps of Engineers to announce it will keep the reservoir at a lower level, allowing more water into the river, as way to prevent structural damage to the dam. Cities in the Green River Valley are preparing plans to address possible winter flooding.
— image credit: Courtesy of the U.S. Army Corps of Engineers

Howard Hanson Dam flood precautions are not limited to cities in the Green River Valley.

While the U.S. Army Corps of Engineers continues to address problematic seepage in the dam's right abutment, Federal Way is preparing to serve as a safe haven for evacuees fleeing the valley, should flooding occur. The city is also preparing itself to operate with a limited supply of resources.

The Hanson Dam, built in 1961, hugs a mound of land called the right abutment. The structure was left by a landslide. Together, the abutment and the man-made architecture restrict water flow in the Green River.

Boulders, sand, clay and a variety of materials make up the abutment, said Patricia Graesser, U.S. Army Corps of Engineers Seattle District spokeswoman. Water has naturally worn a path through the land mass since the dam's construction, she said.

"When you have that kind of material, there's gaps," Graesser said.

In January, the U.S. Army Corps of Engineers, which manages the dam, noticed water leaking through the abutment at a rate faster than it was comfortable with. The Corps became concerned the water could find a direct path through the mound, taking soil with it and compromising the dam's integrity. Fearing the dam's effectiveness is weakened, the Corps announced it will keep the reservoir behind the dam at a lower level following massive rainfalls. Larger quantities of water will flow through the river below.

Flood precautions

More water in the Green River means cities such as Auburn, Tukwila, Renton and Kent must be prepared for flooding. Many of the municipalities have begun planning for such. They are not alone in their efforts.

Federal Way staff and public officials are also preparing for winter flooding in the Green River Valley. Federal Way emergency management staff started communicating with the valley cities concerning the issue in March, emergency management coordinator Ray Gross said.

"It's not too early to think about (the possibility of flooding)," he said. "We're working with the valley cities on what exactly they plan on doing."

Staff is also coordinating with the American Red Cross to identify locations in Federal Way that could serve as large sheltering facilities, Gross said. The city expects flooding in the valley will cause evacuees to seek higher ground in Federal Way.

In addition to an increased population, Federal Way residents will likely see impacts to the city's transportation system, he said. The roadways will not be shut down completely, but there will be obstacles to overcome.

"I don't think we'll be completely isolated, but as far as transportation being impacted, yeah, if the worst case scenario happens...," Gross said.

City council member Jim Ferrell, who is an attorney and works at Kent's Regional Justice Center, said that facility is also planning for flooding. Finding a place to transport the jail's prisoners during such an event is a main concern, he said.

Federal Way's role now is to wait for jurisdictions in the Green River Valley to solidify a flood evacuation plan, Gross said. Flooding could require people in the valley and in Federal Way to survive with little assistance for up to a week, he said.

"We're going to wait until they develop a message so we're pretty much saying the same thing," Gross said.

Finding a solution

Meanwhile, the Corps are working to establish both a short-term and permanent solution to the leaking abutment. Tests on its ability to restrict water flow are ongoing, Graesser said. A grout curtain is expected by November. Holes will be drilled into the earth near the abutment. They will be filled with slurry-type material, she said. The material will serve as a curtain, helping to contain water in the reservoir, Graesser said.

Improvements were made to address a leakage problem in 1965 and in 2002. Technology has come a long way since that time, she said. The Army Corps expects the curtain to hold, but will not know its success rate until the rainy season hits. As a precaution to the valley cities, it is working with King County and flood-endangered cities to prepare for possible flooding.

"We're trying to keep everybody as fully informed as we can," Graesser said.

Check it out

Learn more about the dam and the Corps' efforts to stabilize the right abutment at www.usace.army.mil.

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