skip navigation
Support KUOW
Listen to News


question mark


Earthquake Study Raises Risk Potential Around Central Wash. Dams

Tom Banse


WANAPUM DAM, Wash. - Central Washington was considered at low risk for earthquakes back when big hydropower dams went up on the Columbia River many decades ago. But a recently completed seismic hazard assessment has found greater earthquake potential for the area than previously thought.

Now the dam owners have to figure out how to respond. Seismic retrofits could cost ratepayers across the region hundreds of millions of dollars.

I'm sitting on the Columbia River shore just across from the massive concrete and rock face of Wanapum Dam. When I drove over here -- over the Cascade Crest from the Puget Sound Basin -- I thought I was leaving Earthquake Country behind. Turns out, I'm wrong.

But you know, the folks who designed this dam more than half a century ago -- and the nearby dams up and downriver -- they too didn't consider this Earthquake Country. That according to the engineers who now have to figure out what if anything to fix.

"The potential always existed. You know, we're just more aware of it and we have a better understanding now of what the potential loads are," says Grant County PUD Hydro Engineering Manager Kevin Marshall.

The "better understanding" comes from a newly released seismic hazard reassessment of the Mid-Columbia region. It took four years to produce and was commissioned by three Central Washington public utility districts. The new study covers six large hydropower dams they own.

If you follow the curves of the Columbia River downriver through central Washington, you'll pass by Wells Dam, Lake Chelan Dam, Rocky Reach, Rock Island, Wanupum and Priest Rapids Dam.

"Our Chelan and Rock Island structures were designed and built in the 1920's and early 30's," says Bill Christman, Marshall's counterpart at Chelan County PUD.

"Back then, people didn't have a good understanding of the earthquake potential. And if there was an earthquake, they didn't consider it to be catastrophic necessarily, because the region was virtually unoccupied."

Now there are a lot more people here. And the estimate for how strongly the ground could shake from a local earthquake has tripled or quadrupled.

Consulting seismologist Ivan Wong says the risk comes primarily from crumpling of the earth's crust roughly between the Oregon-Washington border, Yakima, Ellensburg and Wenatchee.

"Those folds, those geologic structures we see in the Yakima Fold Belt are underlain by faults that can rupture in large magnitude earthquakes," he says. "Probably in the range of the upper magnitude sixes to maybe more than magnitude seven."

So here's the question ... could the Mid-Columbia dams survive that kind of big earthquake?

"For the most part, I think the dams are going to be very robust," says Marshall.

"They will survive, but there will be some triage necessary," Christman adds. "They won't survive without some damage. We don't think there will be any uncontrolled rapid release of water, which is one of the standards we use to try to decide what if anything do we need to do before an earthquake to really protect those structures."

Last year's killer quake in Japan showed that large dams can withstand just about anything you can throw at them. This according to the dam safety chief for the federal Bureau of Reclamation, Brian Becker.

"In Japan, there was a report of one failure and I think seven dams that were damaged," he says. "In general, I think dams fared relatively well. So maybe there's some comfort and solace in that."

Yet the Mid-Columbia dam owners and their federal regulator are taking an approach that has very little tolerance for risk. After all, lives are at stake. They say they're prepared to spend money to protect against an event that may only happen once every 10,000 years.

Grant and Chelan County PUD started to analyze specific vulnerabilities at each of their dams even before the ink was dry on the regional seismic study. The next round of studies promise to open a tough debate about what price for what degree of safety.

"When you get into dam issues where you have to do remediation, it gets very expensive very quick," Marshall says.

"A good example I think is we've got these big transformers on the deck of the dams that could fall over or be misplaced," Christman says. "Just tying things down and responding in a way that keeps things from falling over in a big ground shaking event costs a lot of money."

In the end, the multimillion dollar cost for earthquake retrofits comes out of ratepayer pockets. The three Mid-Columbia public utility districts sell surplus power throughout the Northwest, so pretty much all of us could be chipping in small amounts in coming years.

The new information about earthquake potential in central Washington has prompted the U.S. Department of Energy to launch its own seismic risk update for the Hanford site and its sensitive nuclear facilities.

Separately, the Eugene Water & Electric Board has ordered up a similar comprehensive seismic reevaluation of its hydropower dams on the McKenzie River in the Oregon Cascade foothills.

On the Web:

Link to Probabilistic Seismic Hazard Assessment for Mid-Columbia Dams (very large pdf file):

Grant County PUD PowerPoint presentation – includes estimated cost of remedial work: