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


Hamman, J.J.  2012. Master's Thesis, University of Washington.


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. Using numerical simulation models the project examines the projected effects of climate change on water levels and inundation in the lower reaches of the Skagit River in western WA due to the combined effects of changes in storm surge, sea level rise, and riverine flooding. 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 and regulated flow during extreme events. Storm surge was calculated using a regression approach that included atmospheric pressure patterns simulated by the WRF model and ENSO. A 2D hydrodynamic model was used to estimate water surface elevations in the Skagit River estuary and floodplain using resampled hourly hydrographs keyed to regulated daily flood flows produced by the 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.