MAPPING FLOOD HAZARDS FOR FUTURE SEA LEVEL RISE ALONG RUSTON WAY, TACOMA, WA

Coastal flooding is driven by extreme water levels and such anomalies are expected to increase exponentially with projected sea level rise. In Puget Sound, high water anomalies are associated with passing low pressure systems (barometric effect) that elevate the sea surface up to 1 m and bring strong winds to the region. While extreme still water levels have the potential to flood low-lying coastal regions, wind-generated waves further elevate the water surface by inducing wave setup and run-up. Flood hazard maps produced by the Federal Emergency Management Agency (FEMA) are currently the primary tool used by local municipalities and coastal planners to guide development and mitigate hazards in the coastal zone. FEMA’s Flood hazard maps are based on numerical wave models and parameterized wave run-up modeling of the 100-yr extreme event under current sea level. While simple run-up models provide a useful first-order approximation, they are uncalibrated across the mixed sediment beaches of the region.

A case study along Ruston Way in Tacoma, WA was completed to estimate flooding owing to extreme water levels and waves using the numerical wave model SWAN (Simulating WAves Nearshore) and hydrodynamic model XBeach. A 60-yr hindcast of waves, tides, and total water levels was developed using a look-up-table approach with SWAN and an empirical run-up model. The 10-yr extreme total water level was statistically derived from the hindcast, then modeled in XBeach under both current and future sea levels. Model output illustrates a bimodal flood hazard regime driven by barometric effects during south gales and wind waves and wave run-up during north gales. Modeled run-up from XBeach for a 10-yr event predicts landward inundation of 1 – 3 meters with multiple locations showing > 3 meters of inundation under moderate SLR scenarios by 2100. Mapped outputs of predicted flood extents, depth of water and change in recurrence frequency of today’s 10-yr event under projected sea level rise were generated, providing additional planning information to complement FEMA.