New tsunami modeling for the Chehalis, Hoquiam, Willapa, and Wishkah Rivers

The Department of Natural Resources on Tuesday released new tsunami inundation and current speed maps for Grays Harbor and Pacific Counties. 

In their blog, DNR states that the maps follow the results from new data for a magnitude 9.0 Cascadia subduction zone earthquake scenario for the Chehalis, Hoquiam, Willapa, and Wishkah Rivers. 

Officials say that this tsunami modeling is a continuation of previous modeling published in 2018 by the Washington Geological Survey for areas surrounding Willapa Bay and Grays Harbor. That modeling was expanded as it did not cover upriver areas. 

The maps use the same earthquake scenario as the 2018 publication, with an updated Digital Elevation Model (DEM) to better understand how far up rivers the tsunami inundation would reach. 

“The seismic scenario used here generates subsidence that will affect all coastal elevations in this study area as a result of earthquake-induced land deformation.”

Officials say that the impact of subsidence on the first tsunami-driven wave arrival is significant in our study area, as it tends to mask a phase of a tsunami that can be observed in other parts of the Washington coastline. 

DNR notes that if the area were struck by a tsunami, it would arrive quickly—within 20 minutes at all river mouths modeled in the study area, leaving limited time for official tsunami alerts. 

“Maximum changes in tsunami water heights following the earthquake exceed ~10 feet (~3 meters) at all river mouths, with the highest height of ~15 feet (~4.5 meters) impacting the Willapa River. The tsunami also travels a minimum of ~8 river miles (13 kilometers) upriver in all four river valleys. The farthest modeled upriver flooding extent is up the Chehalis River, where the tsunami travels ~13 miles (21 kilometers) upriver. “

Although the majority of tsunami inundation in the study area would be confined to mapped floodplains, inundation impacting roadways may isolate communities upriver. 

The study is limited in that modeling does not account for variable tide stage, tidal currents, riverine flow, earthquake-induced landslides, seiches, liquefaction, or minor topographic changes that would locally modify the effects of tsunami waves.

“In addition, there are many assumptions associated with the scenario earthquake modeled here. Due to these limitations, this modeling may not be suitable for site-specific tsunami inundation assessment or for determining effects on the built environment.” 

This modeling can be used as a tool to assist with emergency preparations and evacuation planning prior to a Cascadia subduction zone event or to determine locations where a tsunami vertical evacuation refuge may be appropriate.

These maps show the modeled inundation from a Cascadia subduction zone earthquake scenario.