Effects of Projected Twenty-First Century Sea Level Rise, Storm Surge, and River Flooding on Water Levels in Puget Sound Floodplains and Estuaries

Abstract

Near coastal environments have been identified as some of the most likely to be impacted by climate change. Observed changes in Puget Sound sea level and flood magnitudes are in line with those projected by previous climate change impacts studies. Current understanding of the combined effects of these changes is relatively low and has prompted us to explore the ways in which their co-occurrence will influence near coastal ecosystems and infrastructure. This project examines the effects of climate change on the lower reaches of Puget Sound rivers by investigating changes in storm surge, sea level rise, and riverine flooding. The project utilizes numerical models to quantify the shifts in hydraulic conditions expected in the Skagit and Nisqually river basins. Global climate model simulations from the ECHAM-5 climate model were used as the climate forcings and were 1) statistically downscaled using the hybrid delta method, and 2) dynamically downscaled using the WRF regional climate model. Naturalized flows produced using the Variable Infiltration Capacity hydrology model were used to drive reservoir models that simulate flood control operations. Storm surge was calculated using a regression approach that included anomalous atmospherics forcings simulated by the WRF model. A 2D hydrodynamic model was used to estimate water surface elevations in the Skagit and Nisqually River estuaries using resampled hourly hydrographs keyed to regulated daily flood flows produced by a daily time step reservoir simulation model and tide predictions adjusted for SLR and storm surge. Combining peak annual storm surge with expected sea level rise, the historic (1970-1999) 100-yr peak tidal anomaly is found to be exceeded every year by the 2020s. By the 2050s, the extrapolated 100-yr riverine flood events are found to increase by 30% and 25% in the Skagit and Nisqually Rivers, respectively. In the Skagit River, the combined effect of sea level rise and larger floods yields increased areal flood inundation up to 80% relative to the present "100-year" flood.