The influence of shelf bathymetry and beach topography on extreme total water levels: Linking large-scale changes of the wave climate to local coastal hazards

Total water levels (TWLs) at the coast are driven by a combination of deterministic (e.g.,
tides) and stochastic (e.g., waves, storm surge, and sea level anomalies) processes. The contribution of each process to TWLs varies depending on regional differences in climate and
framework geology, as well as local-scale variations in beach morphology, coastal orientation,
and shelf bathymetry. Large-scale changes to the climate altering the frequency, direction,
and intensity of storms, may therefore propagate to the nearshore differently, amplifying or
suppressing local coastal hazards and changing the exposure of coastal communities to extreme
TWLs. This study investigates the hydrodynamic and geomorphologic factors controlling
local TWLs along high-energy United States coastlines where wave-influences dominate
TWLs. Three study sites in the states of Washington, Oregon, and California are chosen
to explore how regional and local differences in beach topography and wave transformation
over shelf bathymetry drives variations in the magnitude and impacts of extreme TWLs. Results
indicate that TWLs are most influenced by wave transformation processes in locations
with steep beach slopes (which drive larger relative contributions of wave runup) and complex
offshore bathymetry, while beach topography influences the severity of coastal impacts.
Once the relative morphologic controls on TWLs are better understood, hypothetical future
climate scenarios are explored to assess how changes to the average deepwater wave climate
(height, period, and direction) may alter local TWLs when compared to estimates of likely
sea level rise and future coastal management strategies. Changes to the wave climate are
found to be as detrimental to the coastline as sea level rise in some locations, where small
variations of the TWL drive large, nonlinear changes in hours of impact to the backshore
beach. Overall, this study develops an approach for quantifying the range of hydrodynamic
and morphologic controls on the magnitude of TWLs which will ultimately better prepare
coastal communities for uncertain changes to the global climate.