SALEM — A proposed redrawing of the tsunami inundation line on the Oregon Coast increases the amount of urban land at risk for massive flooding by 30 to 40 percent, according to the state Department of Geology and Mineral Industries.
Earth Science Information Officer Ali Ryan said the new tsunami boundary will place 28,000 structures on the seaward side of the line, up from 20,000.
Better technology, better maps and better risk assessment are spurring the adjustment — not a change in real danger, scientists say.
In 1995, when the department first tried to model the undersea earthquakes that cause destructive tidal waves, computers were unable to simulate the wave after it reached the shoreline. In their place, the department relied on contour maps from the U.S. Geological Survey in order to estimate where the water would stop during a tsunami. Those maps were accurate only to the nearest 20, sometimes 40, feet.
But times have changed. Recently, the department used LIDAR, a plane-mounted laser that can take 500,000 measurements a second, see through vegetation and is accurate to the nearest half inch, in order to create their own tsunami maps.
In conjunction with better simulations, geologists have created five possible tsunami lines — small, medium, large, XL and XXL. Each size corresponds to a line drawn on the map of Oregon’s coast. The bigger the size, the farther inland the wave will strike.
Government scientists use the XXL line, which encompasses the sort of massive sea wave that occurs only once or twice every 10,000 years, when planning tsunami evacuation routes.
But city planners still use the 1995 boundary when searching for building sites for “essential structures” — mostly places like schools, hospitals, police stations and other emergency operation centers.
Oregon bans the construction of new essential facilities on the western side of the tsunami inundation line, with exceptions and exemptions for pre-existing buildings.
“If you want to have no risk, you use the XXL, because it’s not likely (for water) to come any higher than that,” Interim State Geologist Ian Madin said in an interview. “But when you talk about risk, well the question arises, how much risk are you willing to tolerate. (And the XXL line) in many communities, is the entire community.”
A department advisory committee selected the “large” category on Aug. 5, 2013; tsunamis with enough power to surpass this ranking occur about once every 2,475 years. It’s the same level of strictness used by Oregon’s building code for earthquakes, and scientists say it equates to about a 2 percent chance of major damage to structures — and people — every 50 years.
Madin updated the department’s governing board on the proposed line change during a public meeting on May 15. He said it would take at least a year to get through the required period for public comment and input.
“The public dialogue costs money that the agency did not have. So that process stalled,” he said. “I am trying to reopen that process, because I think it’s important.”
Ryan, the department’s spokeswoman, said options for public engagement wouldn’t be ready for the board’s approval until the fall.
And when will the agency actually adopt the new tsunami line?
“I can not even hazard a guess,” Ryan said.
The rule change will have ramifications for private developers, too. Builders are legally required to consult with the department before planning high-occupancy structures, like a hotel or retirement home, within the inundation zone.
But builders don’t have to listen to the state’s recommendations, and the department has no formal method of monitoring compliance.
“Nobody has totally blown us off,” Madin said. “But informally, it seems like they are paying attention.”
Madin said the department has been asked to consult about seven times in the last 20 years, reflecting the slow pace of development on the Oregon Coast.
No matter when the redrawing happens, the rule change will affect different towns in different ways.
In low-lying areas like Seaside, a worst-case scenario tsunami would flood the city hospital, fire department and municipal airport, as well as every public school, according to the Seaside Signal. In contrast, Newport’s unelevated South Beach is already zoned for flooding under the outdated water line, while houses on the bluffs would remain unaffected by the new rule.