Hanson Dam: Prepare now for possibility of winter floods, Army Corps says

Mamie Brouwer

Mamie Brouwer

Mamie Brouwer, a project manager with the U.S. Army Corps of Engineers, didn’t hesitate to answer when an area resident asked her if the Howard Hanson Dam, which controls flooding on the Green River, would fail because of a damaged abutment.

“The Corps of Engineers has never lost a dam in its history and we’re not planning to start with the Howard Hanson Dam,” Brouwer said firmly.

A crowd of more than 120 packed a meeting room Monday night at the Kent Station campus of Green River Community College to learn from a panel of government experts about flood safety, flood insurance and the potential impact in Kent of the 10-foot-wide depression that formed on the embankment next to the dam after heavy rain in early January.

Flooding from the Green River could still strike the Kent Valley this winter even without a dam failure, if corps officials determine the dam must store less water than normal behind the facility because of the damage to the structure.

Residents wanted the panel to give them as much detail as possible about the potential risk of flooding.

“If we have the same type of weather system that we had at the beginning of this year, we’ll have a flood,” said Dominic Marzano, division chief for Kent Emergency Management, who is helping to oversee regional emergency coordination plans in case of a flood. “We don’t know the final outcome of the dam yet, but more likely than not we’ll have flood events. The dam is there for a reason because the valley always floods and Mother Nature has the upperhand.”

That’s the reason Kent city officials organized the public meeting. They want residents to be prepared with flood insurance and evacuation plans just in case the river does flood the valley.

“Plan ahead, start now,” Marzano said. “This summer we will get out a lot more information.”

Emergency shelters would be set up if people had to flee their homes, Marzano said. But he encouraged residents to make plans to stay with friends or family outside the area.

Steve Bleifuhs, section manager for King County river and floodplain management that oversees 500 levees, encouraged people to buy flood insurance whether they are in a floodplain or not. A floodplain consists of low-lying ground adjacent to a river that is subject to flooding.

“Anybody can get flood insurance,” Bleifuhs said. “You might not be in a mapped floodplain, but a lot of the lower Green River Valley could be wet.”

Bleifuhs explained how closely the corps and county monitor the levels of the Green River. A flow of 9,000 cubic feet per second would trigger a phase three alert from the county flood warning system. County crews would be sent out to monitor the river. The levees can handle a river flow of 12,000 to 14,000 cubic feet per second before flooding would start to occur.

“After 14,000 to 15,000 cubic feet per second, we have concerns about the ability of the levees to hold water,” Bleifuhs said. “But we’ve exceeded 12,000 cubic feet per second only three times since the dam was built.”

The federal government built the rock-and earth-fill Hanson dam in 1961 to control major flooding in the Green River Valley.

“If we have to run 15,000 or 16,000 cubic feet per second, there would be a significant flood impact,” Bleifuhs said.

Corps officials are uncertain at this point how much water they will be able hold back behind the dam.

They are slowly raising the level of the pool of water over the next several weeks to test the damaged abutment. The elevation of the pool reached 1,151 feet on Monday.

“We are not seeing anything out of the ordinary on our instrumentation,” Brouwer said.

Engineering experts continue to monitor the damaged abutment to see how it behaves, as well as to figure out how to repair the depression.

Col. Anthony Wright, commander of the Seattle district of the Army Corps, approved raising the pool to 1,157 feet next week, Brouwer said. After another two weeks of testing, Wright will decide whether to go to 1,167 feet.

“If we can reach 1,167 feet, it’s much more infrequent that flood pools need that,” Brouwer said. “That will give us more breathing room.”

Corps officials expect to decide in July how well they will be able to manage flood control for this winter. By fall, they’ll begin to determine a long-term fix for the damaged abutment.

Crews could install a low-permeability barrier within the abutment this fall to reduce seepage of water from behind the structure. But a long-term solution could still be a few flood seasons away, the corps says.

“While the dam does not present an immediate danger of failing, there is an increased risk to the downstream communities for higher flood levels until such time that the seepage issues with the right abutment have been resolved,” Wright stated in a corps media release Wednesday.

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.

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