by Eric Barker
OROFINO -- Like the water behind it, Dworshak Dam’s safety rating is slowly climbing.
Officials from the U.S. Army Corps of Engineers announced Monday that Dworshak is now ranked at “low risk” of failure -- or 4 on a scale of 1 to 5, with 5 being the most safe. Risk on the scale is calculated by multiplying the probability of dam failure by the consequences of failure. Corps officials made the announcement at a Clearwater County Commission meeting.
In 2007, the dam was rated as “high risk” -- 2 on the scale -- and said to be “unsafe or potentially unsafe.” Corps officials at the time downplayed the risk and said the rating stemmed from water leaking through and under the dam that had the potential to undermine the 45-year-old structure if it were allowed to get substantially worse, combined with the potential downstream devastation that would be unleashed if the dam were to fail.
The agency has slowly worked to mitigate the leaks and clear clogged drains that help monitor uplift pressure on the 26-billion-pound structure.
In 2012, the safety rating was upgraded to “moderate risk” -- 3 on the scale -- and said to be “conditionally unsafe” following a study that was prompted by the 2007 finding. The evaluation confirmed the “robustness” of the dam’s design and its historic performance since being completed in 1972.
A recently completed second phase of that study confirmed the dam likely would withstand an earthquake and prompted the latest safety upgrade.
“Under the type of seismic events we would expect to have in the Dworshak area the risk is very low,” said Donna Street, dam safety officer at the Corps of Engineers Walla Walla District. “It’s safer.”
The 700-foot-high dam holds back more than 1 trillion gallons of water in a 55-mile-long reservoir that would devastate downstream communities if it were ever to fail. Because of that, Dworshak will always score poorly on the consequence-side of the formula used to calculate risk, which means it’s not likely to ever reach the highest safety rating possible.
“I would not expect this one to go any higher,” Street said.
The agency also announced Monday that Mill Creek storage dam near Walla Walla, Ice Harbor Dam upstream of the Tri-Cities and Lower Monumental Dam near Kahlotus, Wash., were all upgraded from “moderate” to “low” risk. The levee system at the Tri-Cities also was reclassified to “low risk.”
“Public safety is our highest priority, and safety is a daily focus at our dams,” said Lt. Col. Damon Delarosa, commander of the Walla Walla District, in a news release. “We can’t completely eliminate risk, but we can reduce risk of dams and dam-related levees by continuing to inspect, assess, maintain or upgrade the dams we manage.”
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