By MARK FREEMAN
TRAIL — Federal safety officials this year plan to study Lost Creek dam’s ability to withstand a catastrophic earthquake much more intense than those previously modeled at the Rogue River Basin’s largest reservoir.
The U.S. Army Corps of Engineers has Lost Creek on its list of Pacific Northwest dams set for seismic safety studies in the coming months. For the first time, the Corps will look at how Lost Creek would fare during an event similar to 2011’s 9.0-magnitude quake in Japan, Corps officials said.
Such a quake, which geologists say is possible in the Cascadia Subduction Zone off the Oregon Coast, likely would cause ground disturbances greater than past studies at Lost Creek ever envisioned, says spokesman Scott Clemans from the Corps’ Portland District, which includes the Rogue Basin.
“We feel fine that our dams would do fine in a magnitude-4 or magnitude-5 earthquake,” Clemans says. “Basically, our dams would be OK in a normal earthquake, if there is such a thing. But we haven’t looked at a Cascadia-type event.”
Experts doubt a massive Cascadia event would cause the kind of complete failure that would send a wall of water rumbling down the Rogue in doomsday fashion, he says.
“The nightmare scenario that everybody talks about — a complete, abrupt failure — is the least likely scenario and the most easily dismissed scenario by dam-safety people,” Clemans says.
The more likely scenarios would involve sections of a dam shifting or being “bent out of shape” rather than all or part of the dam suddenly falling down, Clemans says. Also, the shifting ground could damage gates controlling water flow through the powerhouse or render inoperable some water-regulation gates, he says.
Other possibilities include the impounded water finding a route under, through or around the dam, causing erosion from within, he says.
Lost Creek dam went online in 1977, and Applegate Dam followed three years later, both well before discussions of the Cascadia Zone quakes were part of the municipal safety lexicon.
“Back when we built those dams, we knew a lot less about the seismic potential of the Pacific Northwest, no question about it,” Clemans says.
“Our dams are really in pretty good shape,” he says. “They may not be up to the same standards as if we were building those dams today, but we’re pretty confident they can withstand some pretty significant events.”
Scientists believe the Cascadia zone is subject to major shifts about every 500 years, producing magnitude-9.0 quakes. That’s the same magnitude as Japan’s devastating March 2011 earthquake.
An Oregon State University research report also suggests that the southern end of the fault — a portion that skirts approximately the southern third of the Oregon Coast — slips far more frequently, producing earthquakes in the lower, magnitude-8.0 range about every 240 years.
In comparison, the Loma Prieta earthquake in the San Francisco Bay Area in 1989 was a magnitude-6.9 quake. An 8.0-magnitude quake is 10 times more powerful.
Seismologists measure the effect of earthquakes and other ground disturbances on structures in relation to the force of gravity, with gravity considered 1.0 on the G scale.
At the time, Lost Creek dam was built to withstand a peak ground acceleration measured as 0.1 G when the earthquake potential for the area was measured by the U.S. Geological Survey as 0.06 G to 0.08 G, Corps data shows.
Since then, studies involving earthquakes within the Cascadia zone’s potential suggest the possibility of ground accelerations in the range of 0.17 G to 0.19 G in the vicinity of Lost Creek and Applegate dams, according to the Corps.
During the upcoming study, the Corps’ dam-safety experts will inspect the structure and then run its data through a series of computer models representing different types of seismic scenarios, Clemans says. The results likely would not be available until next year, he says.
A similar study is planned in 2015 for Applegate Dam, the Corps’ other Rogue Basin dam.
Reach reporter Mark Freeman at 541-776-4470 or email firstname.lastname@example.org.
This story originally appeared in Medford Mail Tribune.
OPB | Feb. 22, 2017