The Green River could flood next rainy season, so people should have a plan to deal with it.
And they should buy flood insurance.
Those were the messages the U.S. Army Corps of Engineers, King County and the King County Flood Control District left with some 280 residents of the communities of Auburn, Kent, Renton and Tukwila who came to Green River Community College on Tuesday night to learn the latest about the situation at Howard Hanson Dam.
Mark Ohlstrom, chief of engineering and construction for the Seattle District of the Corps of Engineers, said the Corps is trying to learn all it can about the two depressions that opened on the dam’s right abutment during the mid-January floods.
Engineers need to find answers. Any undermining of the earth and rock abutment, the product of a 10,000-year-old landslide, could cause water to flow through and around the structure, potentially weakening it and the dam itself.
Until engineers know what’s going on, they will continue to restrict the storage capacity of the reservoir behind the dam to avoid stressing the abutment.
Ohlstrom said engineers don’t expect to have everything figured out and fixed before the onset of the next rainy season. The upshot is that starting in mid-October, potentially high waters could top the river’s levees in the event of a heavy snow melt or a major storm event.
“We want to make people aware that there (will be) this time of increased risk of higher flows downstream next year depending on the flood event,” Ohlstrom said. “We will have less ability, potentially, to store water behind the dam until we have greater confidence in the integrity of the abutment structure.”
Ohlstrom did have some good news. He said excavations last week of the depressions show that their causes are localized, not indicative of a systemic problem. The smaller depression, he said, appears to be the product of weathering of the rip rap and engineered filter blanket material used to provide some form of protection to the abutment. Excavation of the larger depression revealed that materials had settled into a tunnel 30 feet below the abutment.
Ohlstrom added that the dam itself is safe and poses no immediate danger to people and property.
Jeff Bowers, assistant director of the King County Emergency Coordination Center, said King County and the cities of Auburn, Kent, Tukwila, Renton and Seattle, have formed a Green River Basin flood planning group to develop a strategy for public notification and coordinating flood response efforts.
Bowers said the county will work with transportation planners to identify the best flood evacuation routes, and it will publish this information this spring and summer. He said the county will also develop new public warning and notification systems.
Bowers encouraged people to make arrangements in advance of a flood to stay with family or friends.
“Trust me, it’s much better than shelters,” Bowers said. “We will have sheltering available, of course, but that should be the last resort for everybody. What’s most important is that you plan ahead so that each family member knows exactly where to go should there be a flooding situation and what to do.”
Katy Vanderpool, Green-White River Basin coordinator for the Flood Warning Center, said the center will open earlier this year when major storms are on the way to ensure speedy dissemination of information.
But the key, said Vanderpool, is to be prepared.
“Whether it’s a dam or a levy or some other emergency that has nothing to do with the river, be prepared … If you live in a flood plain, you should have flood insurance,” Vanderpool said.
Roger Campbell, who has lived in Auburn since 1939 and is now a resident of Rio Verde Mobile Estates on 22nd Street Northeast, a stone’s throw from the river, suggested one way of cutting the flood risk: dredging the river.
“There’s trees growing up in the river like mad. You can see it. You’ve created islands out there, logjams and stuff.
“…You’re afraid of killing the fish or something like that. God bless the fish, I like fish as much as anybody else. But who comes first? If we’re under water, the fish will still be around,” Campbell said.
Ohlstrom said that dredging alone would not solve the problem.