Assessing Coastal Vulnerability to Storm Surge and Wave Impacts with Projected Sea Level Rise within the Salish Sea

Sea level rise (SLR) in the Salish Sea, a large inland waterway shared between Canada and the United States, is expected to be 0.3 to 1.8 m by the year 2100. Uncertainty in greenhouse gas emissions, global ice sheet loss, and other controls such as vertical land movement all contribute to this range. Valuable property, infrastructure, and critical habitats for shellfish and threatened salmon populations are at risk to coastal changes associated with SLR. Additionally, development in Washington State is expected to accelerate through the end of the 21st century adding extra pressure on protecting ecosystems and people from natural hazards along the coast. Global climate models (GCMs) predict increases in temperature and changes in precipitation, yet little is known about the impacts of climate change on the local wave climate. Understanding the dynamic interactions that SLR and climate change will have on the wave climate and coastal systems within the Salish Sea is vital for protecting these resources and planning for the future. In support of the Washington Coastal Resilience Project and the United States Geological Survey Coastal Change Impacts Project, I modeled historic and potential future waves in the Salish Sea to evaluate the extent that wave energy reaching the shore may change with 0.3, 0.6, and 0.91 m of SLR. I also assessed potential changes in future wind conditions that drive wave generation projected by the publicly available MACA (Multivariate Adaptive Constructed Analogs) downscaled NOAA GFDL-ESM2M (Geophysical Fluid Dynamics Laboratory Earth Systems Model) GCM. Lastly, I modeled wave runup to assess potential flood and wave impacts along the shore to the year 2100 as part of a case study in support of the City of Tacoma’s climate adaptation planning for parks, sensitive habitats and significant commercial development along Ruston Way. This project generated the first regional wave model and historical hindcast within the Salish Sea to define the recurrence frequency of a range of extreme events and resolve their variability alongshore at spatial scales relevant for planning. Existing models of future climate indicate little change in extreme wind speeds, but potential changes in wind direction that could affect waves. Model results indicate that annual extreme deep water waves (-10 m NAVD88 depth) may increase up to 30 cm under 0.91 m of SLR with the greatest change occurring in shallow embayments and large river deltas where higher water levels will reduce depth limitation and influence fetch. Wave runup modeling along the demonstration site of Ruston Way in Tacoma, showed that extreme coastal water levels reaching and exceeding the Federal Emergency Management Agency 100-yr Base Flood Elevation (BFE) will significantly increase under 0.85 m of SLR, the 50% probabilistic estimate by 2100 for the city of Tacoma. While the dominant exposure of shorelines to flooding is along south-facing coasts, wave runup modeling elucidated that extreme water levels causing flooding are sensitive to waves and wind stress, especially important along north facing shorelines. Equally important is the finding that intermediate disturbances driving flooding will significantly increase in frequency with sea level rise; today’s 10-yr recurrence storm event under 0.85 m of SLR was projected to exceed FEMA’s 100-yr BFE across more than 50% of locations modeled along Ruston Way, suggesting that FEMA’s BFE may be biased low for projected future sea level change. In the Salish Sea, SLR is expected to drive an increase in coastal flooding extent and frequency where waves amplify the impacts of higher static water levels and further elevate the water surface