Kees Nederhoff, Sean Crosby, Nathan VanArendonk, Eric Grossman, Babak Tehranirad, Tim Leijnse, Wouter Klessens, Patrick Barnard

The Puget Sound Coastal Storm Modeling System (PS-CoSMoS) is a tool designed to dynamically downscale future climate scenarios (i.e., projected changes in wind and pressure fields and tem-perature) to compute regional water levels, waves, and compound flooding over large geographic areas (100s of kilometers) at high spatial resolutions (1 m) pertinent to coastal hazard assessments and planning. This research focuses on advancing robust and computationally-efficient approaches to resolving the coastal compound flooding components for complex, estuary environments and its application to the Puget Sound region of Washington State (USA) and greater Salish Sea. The modeling system provides coastal planners with projections of storm hazards and flood exposure for recurrence flood events spanning the annual to 1-percent-annual chance flood, necessary to manage public safety and the prioritization and cost-efficient protection of critical infrastructure and valued ecosystems . The tool is applied and validated for Whatcom County, Washington, includes a cross-shore profile model (XBeach) and overland flooding model (SFINCS) and is nested in a regional tide-surge model and wave model. Despite uncertainties in boundary conditions, hindcast simulations performed with the coupled model system accurately identified areas that were flooded during a recent storm in 2018. Flood hazards and risk are expected to increase ex-ponentially as sea level rises in the study area of 210 km of shoreline. With 1 meter of sea-level rise, annual flood extents are projected to increase from 13 to 33 km2 (5 and 13% of low-lying Whatcom County) and flood risk (defined in USD) is projected to increase fifteenfold (from 14 to 206 million USD). PS-CoSMoS, like its prior iteration in California (CoSMoS), provides valuable coastal hazard projections to help communities plan for the impacts of sea level rise and storms.