Geologists and politicians alike predict that an offshore earthquake – like the one that hit Japan – would have catastrophic consequences in Oregon. They say schools would collapse. Roads would buckle. Oregon is also home to hundreds of dams. As Rob Manning reports, officials have a lot of work to do to determine the seismic risks of many of those dams.
A few years ago, Oregon’s earthquake experts at the Department of Geology and Mineral Industries surveyed the seismic condition of schools and fire stations all over the state. But agency scientist, Ian Madin, says there’s one reason dams have not been studied.
Ian Madin: “Well, no one has asked us to do it and paid us to do it.”
Madin says there’s a more technical reason, too. He says doing what’s called a “Rapid Visual Screening” that works with buildings doesn’t work for dams.
Ian Madin: “There is an established, published methodology for doing Rapid Visual Screening of buildings. There is no such methodology for doing Rapid Visual Screening of dams. It just doesn’t exist.”
Oregon has more than 1,300 dams. Three-quarters of them are so small or remote, they’re not considered very dangerous if they fail. That leaves more than 300 that emergency officials believe could threaten human life or property, if something went wrong.
Discovering the likelihood of something actually going wrong requires literally drilling down into the dam and the ground it sits on.
And that is time-consuming and expensive.
The Army Corps of Engineers supervises 20 Oregon dams out of its downtown Portland district office. The agency is supposed to do seismic reviews of each dam, every 15 years. But the Corps is way behind. The Corps has reviewed only seven of its 13 dams in the Willamette Valley. And two of those studies are so old, they need to be done over.
Jeremy Britton leads the Corps’ Portland geo-technical team.
Jeremy Britton: “The way the Corps does it, is they look at all the risks in the country, and they try to rank them, and they fund things based on a ‘worst-first’ kind of approach. So sometimes these seismic safety reviews that we budget for, they end up not receiving funding because there are other higher, critical projects.”
Out of the seven seismic reviews the Army Corps has done of Willamette Valley dams, they’ve raised sufficient questions – in every case – to merit a deeper study. Most of the follow-up reviews are incomplete.
The Corps has finished one for the Fern Ridge reservoir near Eugene. That study found pockets of fill material. Geologists say that sustained shaking from a subduction zone earthquake could reach the Willamette Valley and turn that fill material to jelly.
Jeremy Britton: “If the entire deposit were to liquefy, it wouldn’t be able to support the dam embankment, and the dam could move upstream or downstream, and we could have enough loss of height that the worst case would be that the reservoir overtops the dam, and it fails. So, we’ve identified a potential problem.”
Britton says a “Phase Two” review could determine how much fill there is – and where it is – at Fern Ridge. Britton says such reviews take a year or two. But it’s been ten years since the earlier study found potential problems. The Phase Two review is still incomplete.
Britton says money to finish the study competes with more pressing day-to-day problems – like fixing a problematic gate.
Jeremy Britton: “Usually when there’s a problem under normal operating conditions, as compared to these extreme event-type things, in combination with there’s a known deficiency – where we know there’s a problem with the gates versus the dams, where we’re not necessarily sure there’s a deficiency from a seismic point of view, the gates just leap above.”
It is a bit of a chicken-and-egg issue. Officials acknowledge that since seismic studies haven’t been funded in the past, there may not be enough information available to prove they’re worth spending money on, in the future.
What studies have been done aren’t readily available. Federal officials say national security means reviews and emergency plans aren’t usually publicized.
But OPB’s Vince Patton was able to observe while seismic tests were done at one dam. He visited the Scoggins Dam in Washington County as part of a story he did for Oregon Field Guide in 2009.
Vince Patton: “They drill through every layer of the dam, into the bedrock of the valley 165 feet down. Every core sample from each key depth is tested.”
The Scoggins Dam is one of 22 dams owned by the federal Bureau of Reclamation. It’s one of three the Bureau is investigating for seismic risk. The Bureau has upgraded three dams to a higher seismic standard since the early 1990s.
Reclamation officials say they often identify seismic problems through comprehensive inspections they do every six years. But with increasing concerns about the dangers of a subduction zone earthquake, officials expect seismic safety to be a bigger focus in the future.
The Army Corps and Bureau of Reclamation own some of the biggest dams in Oregon. An additional 28 hydropower dams in Oregon have a federal requirement to withstand a “maximum credible earthquake.”
That leaves more than 200 dams that are located in areas where federal regulators say they could cause significant harm to people and property. The Department of Water Resources inspects dams with the greatest potential for damage every year.
Department engineer Keith Mills says the state is caught up on those inspections, but he says the inspections don’t dig into seismic problems. The last time the state thoroughly reviewed dams for earthquake risk was 30 years ago. Mills wants to update that.
Keith Mills: “Obviously, we didn’t know the earthquake risk at that time, but we do have some good basic information on those dams, and right now, I’m working with my intern to make a priority list that we need the most new information on.”
Mills intends to do initial assessments on 40-or-so dams close to the Cascadia Subduction Zone off the Oregon Coast. Further studies would likely follow.
Fixing what they might find – and paying for it - is another matter.
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