Modeling Study of a Proposed Vertical Evacuation Structure Site for the Shoalwater Bay Tribe Final Project Report

Abstract

A Maximum Considered Tsunami (MCT) scenario was developed that includes projected sea level rise of 0.47 m over 75 years and a magnitude 9 tsunamigenic earthquake on the Cascadia Subduction Zone; development of the scenario is compliant with our current understanding of the 2016 American Society of Civil Engineers (ASCE) Building Code 7, i.e., ASCE 7-16, and was reviewed by a SEFT Peer Review Team and Peer Review Advisory Panel for the project. The results of the numerical simulation support the design of a vertical evacuation structure, a tower located within the city of Tokeland, WA on land purchased by the Shoalwater Bay Indian Tribe. Estimates of key hazard design parameters at the site include 75 years of sea level rise to 0.47 m above the current level, seismic subsidence of -2.5 m, maximum flooding of 5 m and maximum current speed of 6.7 m/s, all referenced to mean high water (MHW). The arrival times of the leading edge and the maximum amplitude of the first wave to arrive at the site are 29 and 36 minutes, respectively. Hazardous tsunami waves continue to arrive for the 6 hours of the simulation and would likely continue for as many more. Most of the peninsula will be flooded twice daily at mean high water to a depth of 1-3 m for decades, assuming that post-seismic uplift eventually raises the land above mean high water.