Tsunami surge study produces sobering results

When a major earthquake strikes the Northwest, destructive shaking won’t be the only thing the region has to worry about.

“The big one” could also trigger a tsunami that threatens communities up and down the Washington and Oregon coasts. And the surge could also push miles inland along the Columbia River, raising water levels as much as 13 feet at the river’s mouth, according to a new study by researchers at Oregon State University.

The study found that the Columbia would rise by about 6.6 feet at river Mile 12, within 3 miles of Astoria, Ore. At Welch Island, just downstream of Cathlamet, the river would rise by about 1.6 feet. It wouldn’t be until Longview — some 50 miles upstream — that the tsunami would largely dissipate and cause no measurable rise on the Columbia, according to the study. The Portland-Vancouver area would also be unaffected by the surge.

The study also found the tsunami’s severity would be impacted by ocean tides more than the amount of water flowing in the river at the time.

“We were a little surprised that the river’s water flow didn’t really matter that much,” David Hill, an associate professor of civil engineering at OSU’s College of Engineering, said in a released statement. “The maximum reach of a tsunami on the Columbia will be based on the tidal level at the time, and of course the magnitude of the earthquake causing the event.”

Researchers assumed a 9.0-magnitude earthquake for their study. That’s on par with the monster temblor that struck Japan and caused a devastating tsunami there in 2011. That event also saw the tsunami reach far inland through local rivers and cause damage, according to the study. That’s why researchers are paying more attention to tsunami risks facing residents along such rivers, including the Columbia.

“There have been previous models of Columbia River run-up as a result of a tsunami, but they had less resolution than this work,” Hill said.

As part of the study, researchers created a map detailing the specific areas on the Columbia that could see tsunami-caused flooding. The study was recently published in the Journal of Waterway, Port, Coastal and Ocean Engineering. It was authored by Hill and OSU graduate student Kirk Kalmbacher.

The Northwest’s next major earthquake is expected to come from the Cascadia Subduction Zone. That’s the fault scientists believe is due for another large event causing widespread damage across the region. The Cascadia fault sits off the Pacific coast, and stretches roughly from Vancouver Island to northern California.

Though a tsunami wouldn’t push far enough inland to pose any risk to Clark County, people should still be prepared for an earthquake or any type of disaster, said Eric Frank, emergency management coordinator for the Clark Regional Emergency Services Agency. Residents should have adequate water, nonperishable food and other supplies on hand in case basic utilities are unavailable for an extended period of time, he said. Phone systems could be overwhelmed; firefighters and first responders could be out of reach, he added.

“Realistically we just do not know what types of resources will be available,” Frank said.

When preparing for any disaster, including an earthquake, residents should also be mindful of medication and other needs specific to them, he said.

“That means different things to different people,” Frank said.