Preliminary Modeling Study of a Vertical Evacuation Structure Site for the Aberdeen School District

Abstract

A Maximum Considered Tsunami (MCT) scenario was developed for a magnitude 9 tsunamigenic earthquake on the Cascadia Subduction Zone. The development of this MCT scenario is compliant with our current understanding of the American Society of Civil Engineers (ASCE) Building Code 7 published in 2016, i.e., ASCE 7-16. The results of the numerical simulation support the design of a vertical evacuation structure (VES) as part of the new Stevens Elementary School in Aberdeen, WA. Estimates of key hazard design parameters at the site include seismic subsidence of -2.76 m, 75 years of sea level rise to 0.37 m above the current level, maximum flooding of 2.75 m, and maximum current speed of 1.2 m/s. The arrival times of the leading edge and crest of the first wave to arrive at the site are 80 and 91 minutes, respectively, but several waves of increasing amplitude follow; the largest wave arrives almost 4 hours after the earthquake to create the maximum flooding and hazardous tsunami waves may continue to arrive for several hours beyond the 6-hour simulation. Substantial Aberdeen land area will be lost due to permanent flooding at levels that vary twice a day from 0 m (no flooding) at MLW to a maximum of about 3 m at MHW, depending on the location. It must be kept in mind that the simulated MCT scenario is probabilistic in nature, a single realization of an event with an estimated 2,475-year mean recurrence interval, so that the results discussed in this report cannot be taken literally as an accurate, detailed prediction of what will happen; rather, valuable general guidance is provided on what may happen, but significant uncertainties exist in any seismic or tsunami model result.