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UPDATED STORY: Flood risk goes down for Hanson Dam; Green River Valley
The repairs at the Howard Hanson Dam have helped, but the risk of flooding remains high this winter in the Green River Valley.
That was the word Thursday at an Army Corps of Engineers press conference in Seattle. The corps had encouraging words about a reduced risk of flooding - a 1-in-32 chance, thanks to measures that have been taken to aid the dam and river levees - but noted that's still a lot less than when the dam's reservoir was fully operational.
A new grout curtain constructed by the U.S. Army Corps of Engineers to slow a leak through a damaged abutment at the Hanson Dam will reduce the risk of flooding this winter in the valley to a 1 in 25 chance. Without that fix, the valley would see a 1 in 3 chance of the winter floods.
And the addition of thousands of giant sandbags along the Green River levees reduces the chance of flooding even more: to 1 in 32 chance, said Col. Anthony Wright, commander of the Seattle district of the Army Corps.
"That may sound like a big reduction," Wright said. "But it is still quite a bit less than the 1 in 140 chance when Howard Hanson Dam is operating at design capacity."
A heavy rainstorm similar to what struck the Green River Valley last January would overtop the levees and cause flooding in the cities of Auburn, Kent, Renton and Tukwila because the corps will not be able to store as much water as normal at the Eagle Gorge reservoir, the pool of stormwater the corps retains behind the dam.
"We've gone from bad to not so bad," Wright said. "There is still a high level of flood risk."
The corps pumped more than 400,000 gallons of cement to form a grout curtain in the abutment as part of its $8.9 million temporary fix to reduce the risk of flooding. The curtain is about 20 feet wide, 450 feet long and between 90 to 160 feet deep, depending on the location along the abutment.
"We're not out of the woods yet, but we're a little bit closer to getting out of the woods," Wright said. "I'm generally happy with the results. This is not a slam dunk. It's not like this stops all of the water and there is no water that can get around. What we have done is an interim measure that had to be done by flood season."
The corps plans to construct a concrete cutoff wall as a permanent fix within the next three to five years. Crews have started the design process for the concrete wall but the design isn't expected to be completed until next year.
The abutment was formed nearly 10,000 years ago by a landslide. The federal government built the rock-and earth-fill Hanson dam in 1961 next to the abutment to control major flooding in the Green River Valley. The dam is about 25 miles east of Kent.
Problems with water storage behind the dam were discovered by the corps when a 10-foot-wide depression formed on the embankment next to the dam after heavy rain in early January. The corps stored a record amount of water in the reservoir during that storm to prevent flooding.
The full-storage capacity behind the dam is 1,206 feet. The level reached 1,189 feet during the heavy rain last January. Dye testing in June showed that water was moving through the right abutment very fast at pool elevations above 1,155 feet. The level of the reservoir has been lowered to 1,075 feet for the flood season from November through March.
Mike Mactutis, a Kent city environmental engineering manager who has helped to oversee flood preparation work by the city, attended the press conference.
"I agree with Col. Wright that it helps, but it doesn't alleviate all of the concerns," Mactutis said after the briefing. "There is still an elevated risk of flooding. But to go from a 1 in 3 chance to 1 in 32 - that's significant."
Wright praised the work of the cities of Kent, Auburn and Tukwila to place giant sandbags along the levees. The thousands of 3,200-pound sandbags increased the height of the levees to handle a river flow of 13,900 cubic feet per second compared to 12,000 cubic feet per second without the bags.
"I'm really impressed with what the cities have done," Wright said. "There is a tremendous amount of effort going on."
Wright said that extra height could keep the levees from overtopping if he has to release more water than normal from behind the dam during a storm.
"These measures that the cities are doing directly reduced that risk of the levees overtopping and thereby significantly reduced the risk of flooding," Wright said.
If the corps does have to release water that would cause flooding, Wright said a minimum warning of at least eight hours would be given to local emergency officials who might have to order evacuations. It takes eight hours for water released from the dam to reach Auburn.
For more information, go to www.kingcounty.gov/floodplans or www.nws.usace.army.mil/.