The crisis at a California dam has raised concerns about evacuation plans and risks from quakes and overflows in the Puget Sound area.
The unfolding crisis at California’s Oroville Dam is prompting local officials to take a closer look at dams in King County.
King County Councilmember Reagan Dunn is calling for a detailed analysis of existing evacuation plans, as well as a review of the risks of dam failure caused by heavy storms and earthquakes.
“On virtually every major river system in King County, you’ve got a dam, most of them built in the 1960s,” Dunn said. ”There’s a lot of people who live in those valleys … all of which are potentially at risk.”
According to King County’s Regional Hazard Mitigation Plan, there are 122 dams in the county that hold at least 10 acre-feet of water. The four with the potential to cause countywide emergencies if they fail are: Howard Hanson Dam on the Green River; Tolt River Dam, above Carnation; Masonry Dam on the Cedar River; and Mud Mountain Dam on the White River.
Other dams that could cause significant damage include Lake Youngs Outlet Dam, near Covington and Culmback Dam. The latter, on the Sultan River, mostly poses a hazard to Snohomish County communities like Sultan and Monroe, but could also flood Duvall and a swath of unincorporated King County that’s home to more than 3,000 people.
All of the major dams in the county have emergency plans and are inspected regularly, officials said. The county-mitigation plan ranks the risk from dam failure as “low.”
“I don’t think there’s any significant danger in the near future,” Dunn said, “But that’s what they thought in Oroville. We need to be prepared to get people out of harm’s way if the worst-case scenario happens.”
Nearly 200,000 people were evacuated from below Oroville Dam after heavy rains raised the water to dangerous levels and both spillways were damaged, causing concern that the country’s tallest dam could fail. Roads out of the area were clogged for hours during the chaotic evacuation and many people didn’t know where to take shelter.
The Puget Sound area faced a similar threat in 2009, when a leak at Howard Hanson Dam sparked fears of downstream flooding in Kent, Auburn, Tukwila and Renton. Much of the area was sandbagged, and local communities beefed up their emergency and evacuation plans.
Dunn said those efforts are a model for the type of analysis he wants to extend across the county.
He’s also concerned about risks to the dams from earthquakes. “We need to see if we have the potential for a catastrophic event that we will have to react to,” he said.
Analyses show that the earthen Howard Hanson and Mud Mountain dams aren’t expected to fail in a major quake, said Richard Smith, dam-safety program manager for the U.S. Army Corps of Engineers Seattle District. After the 2009 leaks, the Corps spent $40 million to upgrade Howard Hanson and has replaced the outlets at Mud Mountain to make them more seismically sound, he added.
In 2009, the town of Pacific was flooded when the Corps released water from Mud Mountain without warning residents. Now, the agency works more closely with local governments, said Michelle Clark, of the King County Flood Control District.
The city of Seattle owns the Tolt River, Masonry and Lake Youngs Outlet dams, which are used for drinking water and hydropower. A statement from the city’s utilities said there are also four drinking-water reservoirs and two stormwater-retention ponds in Seattle that are classified as high-hazard dams because of the potential for damage or loss of life if they fail.
Evacuation plans exist for most major dams, said Timothy Doyle, program manager for dam safety at the King County Office of Emergency Management. The city of Carnation, where about 1,800 people live downstream from the Tolt River Dam, conducts regular drills that include schoolkids, Doyle said. Sirens and warning systems are in place to sound the alarm, if needed.
But in other places, many residents may not even be aware they live downstream from a dam, Doyle said.
Dunn said he hopes to identify gaps in emergency planning, raise awareness of evacuation routes and designate emergency shelters.
The analysis, which will cost about $100,000, will be a collaborative effort between the county and the flood-control district, which Dunn chairs. He plans to introduce authorizing legislation in both bodies, and expects it to be approved.
The report will be due in six months, so fixes can be implemented before the next rainy season, Dunn said.