Water leaking through a damaged abutment at the Howard Hanson Dam still puzzles the U.S. Army Corps of Engineers and increases the risk of flooding this winter in the Green River Valley.
Corps officials saw last week that the abutment remained weakened when they raised the pool level behind the dam to 1,169 feet.
Dye testing showed water is moving through the right abutment very fast at higher pool elevations, according to a Corps media release Tuesday. If water is held at that high of a level for an extended period, internal erosion could occur within the right abutment.
“This phenomenon continues to be troubling,” said Mamie Brouwer, a Corps program manager, in a media release. “However, there is no visual distress of the right abutment that has been observed.”
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 rains 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 if corps officials store less water than 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.
Dominic Marzano, division chief for Kent Emergency Management, said the latest update from Corps officials makes the regional emergency coordination effort among local cities and King County even more important because of an increased risk of flooding.
“It’s disappointing, but it doesn’t put us in a position of not being ready,” Marzano said in a phone interview Wednesday. “We’re still making the final plans from when we first heard (early in the year) about the concerns of flooding.”
Emergency planners from local cities and the county are working on evacuation routes in case water overflows the Green River levees. The emergency group expects to brief elected officials next month on the plan and begin emergency exercises and drills later in the year.
“We don’t want two cities to evacuate and then run into each other going opposite directions,” Marzano said. “We need to make sure everybody works together.”
Col. Anthony Wright, commander of the Seattle district of the Army Corps, said the flood risk remains higher this winter until issues with the right abutment are resolved.
“I can’t stress enough our No. 1 mission here is public safety,” Wright said in a media release. “We will continue to keep Green River Valley leaders and first responders informed.”
As far as the seepage through the abutment, data collected by the Corps below 1,157 feet in the reservoir seems to stay within normal ranges. Water stored at the higher elevations causes more seepage and uncertain water paths through the abutment.
The Corps decided to release water from the dam over the next two weeks to bring the pool level down to 1,155 feet. The reservoir outflows are not expected to exceed 1,200 cubic feet per second, so residents along the river are not expected to see significant changes in the river flows over the next two weeks.
Pool levels as high as 1,167 feet would give the Corps a bit of breathing room because flood pools during the winter rarely need to go that high, Brouwer said at a Hanson Dam flood meeting in May in Kent.
Corps officials continue the around-the-clock monitoring.
“We do not understand how the water is traveling through the abutment,” Brouwer said. “We know that what we may be seeing fits the traditional definition of internal erosion.”
Corps officials expect to decide later this summer how well they will be able to manage flood control for this winter and will begin to figure out in the fall a long-term fix for the damaged abutment.
Crews plan to install a low-permeability barrier within the abutment this fall to reduce seepage. But a long-term answer could still be a few years away.
“They don’t believe they can hold back the maximum amount,” said Kent Mayor Suzette Cooke in a June 16 update on the Hanson Dam to the City Council after she and other local mayors received a briefing from the Corps. “They strongly recommend flood insurance for residents.”
For more information, go to www.kingcounty.gov/floodplans or www.nws.usace.army.mil/. Click on National Flood Insurance Program on the county site for property searches on flood risk and insurance rates.