Col. Anthony Wright jumped right into the middle of the depression on the abutment adjacent to the Howard Hanson Dam to show more than three dozen government officials and media members the depth and width of the hole.
Wright, commander of the Seattle district of the U.S. Army Corps of Engineers, led a private tour Thursday of the dam to deliver the message that flooding could strike the cities of Kent, Auburn, Renton and Tukwila next winter if less water than normal can be stored behind the facility because of the damaged abutment.
“We believe by the next flood season we will not be able to impound as much water,” Wright said to the tour. “We will not be able to use it as well as in the past.”
The dam served its purpose during the heavy rain in early January as the corps held back water to help keep the Kent Valley from flooding. The reservoir reached more than 75 percent of its capacity.
But the heavy rainstorm in January left a sinkhole about 10 feet wide and 6 feet deep on an embankment next to the dam, about 20 miles east of Kent.
The federal government built the a rock-and earth-fill dam in 1961 to control major flooding in the Green River Valley.
Army Corps officials plan to spend the next several months trying to figure out the reasons behind the depression and how the problem can be fixed.
“We’ll form a panel of experts for proposed actions and continue our investigations and monitoring to provide flood risk reduction,” said Mark Ohlstrom, chief engineer of construction for the Army Corps. “We don’t feel the integrity of the structure is in jeopardy. But we don’t want to get to the point where we have failure of the abutment and failure of the dam.”
A dye test of the depression last month showed seepage five hours later into one of the many monitoring wells next to the dam.
“We need to look at ways to minimize the seepage and provide a fix for the long-term integrity of the structure,” Ohlstrom said.
The potential fixes could include a grout curtain. Construction crews would drill holes to inject grout to the damaged area to help stop seepage. Such a project could take four or five months.
Another option might be for workers to install a concrete cutoff wall that no water could pass through. But cutoff walls are more expensive and take longer to build.
“I don’t want to give chemo if all we need is cold medicine,” Wright said.
John Hodgson, Kent’s chief administrative officer, toured the dam and came away impressed with how the Army Corps has handled the damaged abutment.
“I’m confident they know what they’re doing,” Hodgson said. “They’re bringing in outside experts. That’s a sign of how seriously they are taking it. They’re also looking at temporary fixes they can start making right away rather than waiting until they figure out a permanent fix.”
But if a major rainstorm strikes next winter before the problem is fixed and the Green River floods, Kent officials are working with the Army Corps, King County and the cities of Auburn, Renton and Tukwila to prepare evacuation and shelter plans.
“It’s an opportunity to get our emergency management services in shape and to coordinate with the other cities,” Hodgson said. “We have time as they (the Army Corps) try to find solutions.”
Mike Mactutis, a city of Kent environmental engineering manager, toured the dam and looks forward to finding out this summer what solutions the Army Corps finds to fix the potential leaks.
“This also gives us more urgency to the levee repairs we are planning to do,” Mactutis said.
Because the sinkhole sits so high on the abutment, Army Corps officials still expect to be able to store a large volume of water. But the storage level might not be enough to handle another storm like last January.
“But they still have significant capacity to use behind the dam,” Mactutis said.
An hour or so after Wright climbed out of the depression and stood on the charter bus that carried the private tour back to Seattle, he had one more message to deliver to residents and business owners near the Green River.
“They ought to buy flood insurance,” Wright said. “Even if everything was working perfectly, they should buy flood insurance.”
For more information, go to www.kingcounty.gov/floodplans or www.nws.usace.army.mil/. The county site links to the National Flood Insurance Program where residents can perform property searches on flood risk and insurance rates.
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