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Confluence: Inland Waters Vulnerable to Tsunami?
Winter 2012 | Vol. 1, Issue 1
How Far Could a Tsunami Penetrate Upriver?
By Nathan Gilles
What might happen if a nearshore tsunami caused by a local earthquake were to travel from the Oregon coast up the Columbia River? That was the focus of an Oregon Sea Grant research workshop in August 2011.
“We know tsunamis can penetrate along rivers for long distances,” said OSU tsunami expert Harry Yeh, the workshop organizer. But, said Yeh, tsunami penetration up rivers has been largely unexplored until now.
“In the coastal plain, they [tsunamis] can penetrate 2 to 5 kilometers [about 1- 3 miles], but with a river they can penetrate up to 10 kilometers [about 6 miles], no problem.”
The Oregon coast lies along the Cascadia subduction zone, a fault line that stretches from northern California to British Columbia. The Cascadia subduction zone is similar to the subduction zone that caused Japan’s recent earthquake and tsunami. A magnitude 9.0 earthquake occurs along the Cascadia subduction zone once every 300 to 500 years. The most recent such quake occurred in 1700. This expected earthquake would most likely be accompanied by a tsunami, which could affect the Columbia River, said Yeh, who himself had traveled to Japan soon after the March 2011 earthquake to inspect engineering damage in Sendai and elsewhere.
The 18 workshop attendees were asked by Yeh to model a section of the Columbia that stretches from the coast at Astoria, Oregon, all the way to the Bonneville Dam. Yeh said the idea was to gather general data that could be used in future tsunami modeling.
“We are not trying to do detailed models of, say, Astoria or Longview, for instance,” said Yeh, “though I think that’s the next step.”
Many Oregon coastal communities could one day feel the effects of a tsunami. The Oregon Department of Geology and Mineral Industries has mapped communities that could be flooded by tsunamis into inundation zones. But these inundation maps don’t include the Columbia River, said Yeh. Instead, they stop at Astoria, at the mouth of the Columbia. The workshop’s exploration of what might happen if a tsunami penetrated the Columbia could change how tsunami inundation maps are made in the future, said Yeh.
“This work is very theoretical, very academic, but it has direct consequences and real applications, which is why I feel like this is important work,” said Yeh.
Events at the recent workshop included presentations by Yinglong Joseph Zhang of Oregon Health and Science University on how far a tsunami could penetrate the Columbia, and a presentation by David Jay of Portland State University on the hydrodynamics of the river. Workshop discussions also included comparisons of different tsunami modeling techniques, including those used by the National Oceanic and Atmospheric Administration (NOAA) and those used by the Japanese government.
Of particular interest to Yeh is a stretch of the Columbia that starts at Astoria and follows the Columbia until it turns south toward Portland at Longview, Oregon. Yeh says the bathymetry - underwater depth or topography - in this area is very complex and includes marshes, islands, and other complex landforms, which makes modeling difficult.
Yeh’s workshop also examined the effect tidal movement could have on a tsunami traveling from the Pacific Ocean up the Columbia River. Whether the tide is coming in or out could be very important, said Yeh.
Of particular interest to researchers at the workshop was whether a tsunami could reach Portland.
“Even in the Portland area, residents could feel the effects of a tsunami,” said Yeh. “Although this effect is going to be very small, it is going to be measurable.”
- Japan's 60-foot tsunami toppled four-story concrete building, shattering safety and engineering strategies (The Oregonian, April 2011)