Modeling Extreme Water Levels in Puget Sound

Coastal Puget Sound is home to more than 5 million people that, along with important industries and sensitive nearshore habitats, are increasingly vulnerable to projected Sea-Level Rise (SLR) impacts. Storms generate non-tidal residual (NTR) water levels that increase the extent and frequency of coastal flooding in combination with SLR. Despite the importance of coastal storms on flood hazards, a lack of direct observational data has limited our understanding of the spatial heterogeneity and dominant processes contributing to NTR water levels in Puget Sound. We developed a hydrodynamic model of the Puget Sound using Delft3D-Flexible Mesh to evaluate the relative importance of atmospheric pressure, wind, upwelling and downwelling, as well as seasonal and inter-annual anomalies on NTRs. On the oceanic boundary, the model was forced with astronomic tidal constituents as well as offshore NTRs from HYbrid Coordinate Ocean Model (HYCOM) to capture downwelling effects. Fluvial discharge from 25 nearshore rivers were included, and spatially varying wind and pressure fields from the Pacific Northwest National Laboratory meteorological data were applied to the free surface.

Model simulations were performed for the top 10 recorded extreme water-level events in Puget Sound from 1984 to present. We analyzed model results to investigate the spatial and temporal variability in the magnitude, phasing, duration, and the major driving factors of NTRs. These results suggest that the effect of wind setup varies greatly, accounting for as much as 30% of the total NTR in enclosed basins that are aligned with the wind trajectory. Large-scale downwelling events originating in the Pacific Basin propagate throughout the model domain, in some cases elevating water levels by as much as 30 cm in the entire Puget Sound. Through providing a better understanding of processes that control NTRs in Puget Sound, this work will help to improve our estimates of future coastal flooding and storm impacts.