The U.S. Army Corps of Engineers expects to finish construction of a grout curtain by Nov. 1 to slow the water leaking through a damaged abutment at the Howard Hanson Dam.
But it remains to be determined how much more capacity the $8.9 million temporary fix will give the corps to store water at the reservoir behind the dam in order to reduce the risk of flooding this winter in the Green River Valley. The Corps plans to construct a concrete cutoff wall as a permanent fix within the next three to five years.
“There is a 1 in 3 chance this flood season for flooding without a grout curtain,” said Col. Anthony Wright, commander of the Seattle district of the Army Corps, during a media tour Wednesday of the dam.
Wright expects the grout curtain to lessen the chance of flooding, but he wasn’t ready to make that call yet.
“After testing of the grout curtain in November, it’ll give me more confidence if I can take a larger pool behind the dam in the reservoir,” Wright said. “But I can’t determine how much water I can take until it’s complete.”
Even if the grout curtain allows Wright to store more water behind the dam, the curtain remains a temporary fix and won’t stop the risk of flooding. Wright expects he will need to release more water than normal this winter through the dam because of less storage capacity.
“It will reduce the risk but not eliminate it,” Wright said. “That will take a larger fix and we are working on that design now.”
The problems with water storage behind the dam started when a 10-foot-wide depression formed on the embankment next to the dam after heavy rain in early January. The dam is about 20 miles east of Kent.
Flooding from the Green River could strike the cities of Kent, Auburn, Renton and Tukwila this winter because corps officials will not be able to store as much water as normal behind the facility because of the damaged abutment. The federal government built the rock-and earth-fill Hanson dam in 1961 to control major flooding in the Green River Valley.
Crews were busy working on three drills Wednesday along the abutment. Workers drill holes about 160 feet deep and then pump gallons and gallons of grout, a mixture of clay, cement and other materials, to fill the voids and gaps that allow water to seep through. Crews started constuction of the grout curtain in July.
When finished, the twin-layer grout curtain will be about 20 feet wide, 450 feet long and between 90 to 160 feet deep depending on the location along the abutment.
The abutment is a natural feature that was formed nearly 10,000 years ago by a landslide. Construction crews built the earthen dam nearly 50 years ago next to the abutment.
“I’m not fixing a dam, I’m fixing a piece of landslide material,” Wright said.
Wright said the grout curtain cannot be fully tested until next spring when the corps can raise the level of the reservoir to near capacity.
“We don’t want to raise the water up this year because we don’t want a full pool of water right before the flood season,” Wright said. “We will do a spring refill and that will give us a higher confidence of how the grout curtain is performing. But there is no time this year for a full raise.”
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 is moving through the right abutment very fast at pool elevations above 1,155 feet. The level will be lowered to 1,075 feet at the beginning of the flood season.
But Wright did not want to commit to what elevation level he might be able to use this winter with a grout curtain.
“Anything I told you now would be guessing,” he said.
For more information, go to www.kingcounty.gov/floodplans or www.nws.usace.army.mil/.
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