THE FIRST TSUNAMI VERTICAL EVACUATION STRUCTURE IN THE UNITED STATES: OCOSTA ELEMENTARY SCHOOL, WASHINGTON

The Washington coast lies adjacent to the Cascadia subduction zone, where a magnitude 9+ earthquake and ensuing tsunami will imperil communities on low-lying spits of the Columbia River littoral cell. Paleoseismic investigations demonstrate that this area has been subject to repeated tsunami attack. Three different numerical models simulating three different earthquake scenarios demonstrate that the communities of Long Beach, Westport and Ocean Shores would be inundated in a few tens of minutes, and sufficiently high ground is generally not accessible in that time.

The National Tsunami Hazard Mitigation Program (NTHMP), a federal-state partnership administered by NOAA, sought to address that issue by developing modeling, siting, load calculation, and structural design guidance for structures to both survive a near field earthquake and be tall and robust enough to be used for local evacuation from tsunamis. The result was entitled “Guidelines for Design of Structures for Vertical Evacuation from Tsunamis” (FEMA P646), published jointly by NOAA and FEMA. It was updated in 2012 to incorporate information from the Tohoku tsunami.

A multi-agency planning team led by University of Washington researchers and graduate students and Washington Emergency Management Division, including Washington Department of Natural Resources, NOAA, FEMA, USGS, county and tribal emergency management officials, created a community-driven process to identify potential sites for vertical evacuation in these at-risk areas.

On the Westport peninsula, the principal site identified was the location of Ocosta Elementary School, which is approximately at the limit of modeled inundation but also at risk from earthquake-induced ground failure of an adjacent protective ridge. Two previous attempts to pass bond issues to replace the school failed. The current plan to replace the school called for making part of the new school a tsunami vertical evacuation structure that could host as many as a thousand people. This time the bond issue passed…by a 70/30 majority.

The gym is designed to be 30 feet above grade and 55 feet above sea level following earthquake-induced subsidence. Its roof will be accessed from the outside of the four corners and will be capable of holding more than 1,000 people. Ground breaking for the new school is scheduled for fall 2014.