DISASTERSFlood Risk is Increasing in Pacific Northwest

A powerful earthquake, combined with rising sea levels, could significantly increase flood risks in the Pacific Northwest, impacting thousands of residents and properties in Northern California, Oregon, and Washington, according to a new study published this week in the Proceedings of the National Academy of Sciences.

The study, titled “Increased flood exposure in the Pacific Northwest following earthquake-driven subsidence and sea-level rise,” found that a major earthquake could cause coastal land to sink up to 6.5 feet. This would expand the federally-designated 1% coastal floodplain, an area with a 1-in-100 chance of flooding each year, by 35-116 square miles.

Harvey Kelsey, a research associate in the Geology department at Cal Poly Humboldt and a co-author of the study, explained that the research is the result of several decades of fieldwork aimed at understanding the extent of flooding after a subduction zone earthquake.

“The expansion of the coastal floodplain following a Cascadia subduction zone earthquake has not been previously quantified, and the impacts to land use could significantly increase the timeline to recovery,” said Professor of Geosciences at Virginia Tech, Tina Dura, lead author of the study and former postdoctoral fellow at Cal Poly Humboldt (2017-18).

The research shows the most severe effects would hit southern Washington, northern Oregon, and Northern California.

Researchers generated tens of thousands of earthquake models to estimate the potential range of earthquake-driven subsidence—sinking land—that can be expected from the next large Cascadia earthquake. Then, using geospatial analysis, the team quantified the earthquake-driven expansion of the 1% floodplain at 24 estuaries and communities along the Cascadia subduction zone. Because the timing of the next large earthquake is uncertain, the team modeled the impacts of an earthquake striking today or in 2100, when climate-driven sea-level rise will further amplify the impacts of earthquake-driven subsidence.   

The study estimates that following an earthquake today, an additional 14,350 residents, 22,500 structures, and 777 miles of roadway would fall within the post-earthquake floodplain, more than doubling flood exposure. Potential flooding would affect five airports; 18 critical facilities, including public schools, hospitals, police stations, and fire stations; eight wastewater treatment plants; one electric substation; and 57 potential contaminant sources, including animal feeding operations, gas stations, and solid waste facilities.