Flooding from the Green River could strike the cities of Kent, Auburn, Renton and Tukwila during a major rainstorm because of a damaged abutment at the Howard Hanson Dam.
“I’m not worried about the dam failing now,” said Col. Anthony Wright, commander of the Seattle district of the U.S. Army Corps of Engineers, at a briefing Monday in Seattle in front of the King County Flood District Board, which consists of the nine members of the County Council. “But I do not want to get in a position where I worry about it.”
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.
“We found a depression that shouldn’t appear and we’re trying to determine why,” Wright said. “It (the dam) is still capable of performing its mission. But the reduced capacity is a concern because of the damage.”
The federal government built the dam in 1961 to control major flooding in the Green River Valley.
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 Army Corps officials might have to keep the storage level low over the rainy season next fall until the problem is fixed.
“The real concern is we may have flows leaving the structure added to local flows (along the Green River) that exceed capacity (of the levees),” Wright said. “We’re working with the county on levee rehabilitation to strengthen the system. It would take an extraordinary change in the weather to approach the January level between now and October. That’s why this is a great time to fix the problem.”
Councilman Reagan Dunn, whose District 9 includes parts of east Kent, asked Wright how long the repairs would take.
“I’d be guessing at this point, so I hesitate to guess how long it would take,” Wright said. “If we need cutoff walls, that could take several years. But we’ll not be sure of the exact problem for another month or so. I won’t be shy about what we need to do to fix the problem once we have a better idea of what’s going on.”
The Army Corps will begin this spring to slowly allow water levels to rise at the dam’s reservoir. That will allow engineers to test and further investigate the sinkhole.
Councilwoman Julia Patterson, whose District 5 includes most of Kent, told Wright she wants to be sure the county avoids similar flooding that overflowed levees and heavily damaged New Orleans after Hurricane Katrina struck in 2005.
“My concerns are from what happened in New Orleans and the catastrophic failure of the levee system,” Patterson said. “And then every level of government pointed fingers at someone else.”
Patterson asked Wright the potential impact downstream if the Hanson Dam failed.
“If we had full dam failure, we estimate that 20 to 30 feet of water would be around IKEA,” Wright said, referring to the high-profile, home-furnishing store on Southwest 41st Street in Renton.
Dunn wanted Wright to describe the potential for flooding if no repairs are made to the sinkhole and a storm similar to the January rainstorm strikes again.
“Based on our scientific model, if we assume no levee failures, there would be 2 to 3 feet of water in some areas of Auburn and Kent,” Wright said. “If there is levee failure, there would be significantly deeper localized flooding. The deeper water would be upstream from Auburn.”
There’s also a chance no major flooding would overpower the Green River levees because of the sinkhole at Hanson Dam. But that’s not a chance Wright wants to take.
“The odds are it would not be a problem,” Wright said. “But I’m an engineer, not a gambler.”
County officials are working with city officials from Kent, Auburn, Renton, Tukwila and Seattle on an emergency evacuation and shelter plan in case major flooding strikes the Green River valley because of problems at the Hanson Dam.
“We’re coordinating with all of the cities and hope to have a draft plan by this July,” said Jim Morrow, incident commander for the Hanson Dam operation plan. “It would be used by all jurisdictions if an event were to happen.”
The county and Army Corps plan several public meetings over the next few months on what residents can do to prepare for a Green River flood. The first meeting is from 6 to 8 p.m. March 31 at the Lindbloom Student Center at Green River Community College, 12401 S.E. 320th St., Auburn.
“Ensuring the safety of downstream communities is a major priority until the Howard Hanson Dam is repaired and the risk of flooding is reduced,” said Dow Constantine, chairman of the County Council and the Flood District Board.
For more information, go to www.kingcounty.gov/floodplans or www.nws.usace.army.mil/.
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