The Northwestern Division of the U.S. Army Corps of Engineers — which encompasses the Portland, Seattle, Walla Walla, Omaha and Kansas City districts — led an exercise in mid-June to prepare its emergency planners, operators and engineers for the possibility of a severe earthquake.
Seismologists predict that the Pacific Northwest has a 40% chance of experiencing a severe earthquake — potentially as large as magnitude 9.2 — during the next 50 years from the Cascadia Subduction Zone, a 700-mile “megathrust” fault line stretching from northern California into British Columbia. The last time an earthquake occurred in this fault was Jan. 26, 1700. It measured an estimated magnitude 9.0. According to the Oregon Office of Emergency Management, the quake caused the coastline to drop several feet and triggered a large tsunami that crashed into the land.
The exercise scenario was dire. A rupture along the fault line on the Oregon coast, near the city of Florence, caused a magnitude 9.0 quake, resulting in a tsunami up to 80 feet high. Thousands of people were killed. More than a million buildings were damaged. There were widespread power outages. A total loss of communications occurred for the Corps’ Portland and Seattle districts, with 70% of the two districts’ employees impacted.
It’s a situation Portland District Catastrophic Disaster Response Program Manager Mark McKay sums up with a single word: devolution. It means the damage is bad enough to essentially take the Portland and Seattle districts out temporarily. So the Walla Walla District, which lies far enough east to remain unaffected by the quake, takes over command and control for its sister entities. Portland and Seattle set up fully functional command posts outside of the earthquake’s impact zone and spent most of the three days taking accountability for their teams and infrastructure.
The major lift for the two districts? Checking on the nearly 30 dams they operate and maintain.
For the exercise, Portland District dispatched local damage assessment teams — small groups of engineers who live near the dams — to perform safety inspections. According to McKay, these teams were just implemented within the past year.
The district also put several battery-powered, briefcase-sized satellite units into operation throughout its 13-dam Willamette Valley System — in this scenario, the hardest hit area of Corps locations — which the teams used to relay data and information to an emergency operations center at Walla Walla District.
Walla Walla sent in a five-person team of engineers by helicopter to inspect Portland District’s Cougar and Blue River dams — remotely operated, isolated structures in Oregon’s McKenzie River Valley, east of Eugene, that Portland District teams might not be able to make it to. While any needed repairs to the structures would be a long way off, immediate assessments would allow the district to prioritize management of the dams and reservoirs toward the best outcome for communities living downstream.
Erik Petersen, the district’s Willamette Valley operations project manager, uses the situation across the valley, brought on by late-season rainfall in May and June, as an example of how difficult those decisions could be. Corps reservoirs are the fullest they’ve been in the past few years. A major seismic event, Petersen says, would complicate the district’s ability to keep rivers from flooding downstream. Dams in need of evacuating the water stored behind them would be opened for releases, while others would remain closed. Easier said than done, of course.
“If and when Cascadia happens, the whole West Coast is going to experience upset conditions,” Petersen said. “Things are going to be upside down. And we don’t want to make the situation worse by not doing everything we can do to protect the populations at risk downstream of our facilities.”
The somewhat good news is that dams have performed extremely well during seismic events. Only one concrete dam in modern history has failed as the result of a seismic event: Shih-Kang Dam in Taiwan. According to Northwestern Division Dam Safety Program Manager Ross Hiner, even for earthen embankment dams, which could be more vulnerable to an earthquake, failure is rare and remains unlikely.
Nevertheless, in recent years, the Portland District has launched seismic studies and risk assessments at each of its dams to better understand the structures’ expected performance in the event of a major earthquake. Findings from those studies will determine long-term measures the district may need to implement to improve the seismic safety of its dams.
