COASTAL DUNE GEOMORPHOLOGY AND VULNERABILITY TO EROSION AND FLOODING IN THE U.S. PACIFIC NORTHWEST

We are investigating the geomorphological variability and relative vulnerability to erosion and flooding of coastal sand dunes in the Pacific Northwest. Coastal dunes protect lives, infrastructure, and ecosystems particularly during severe winter storms. Before the 1900’s, the native plant Elymus mollis was the dominant dune grass and dune morphology was largely determined by sediment supply (Cooper, 1958). The backshore consisted of small hillocks and open sand (Cooper, 1958). However, the introduction of invasive beach grasses in the 1920’s resulted in significant changes in dune geomorphology (Seabloom and Wiedemann, 1994). The grass Ammophila arenaria is linked with taller, narrower dunes while the grass Ammophila breviligulata is associated with shorter, broader dunes. The taller dunes are more resistant to overtopping and flooding but possibly more susceptible to erosion. The shorter dunes can withstand erosion but can increase vulnerability to flooding. We extract important dune features such as dune crest height, dune toe height, and dune face slope from lidar elevation data of Oregon and Washington coasts. This data is spatially compared with beach grass survey data to determine if a statistically significant correlation exists between beach grass type and dune morphology.

If the different grass species do affect dune morphology, it is important to understand how managing the different species might impact coastal vulnerability. To assess this we run a series of dune erosion models for several beaches with distinctly different dune heights and slopes. These models include a geometric model, an equilibrium profile model, and a calibrated wave impact model. The simulations are run with wave conditions from some of the most severe storms in the historical record. Broader dunes, such as those associated with the grass A. breviligulata, are expected to be more resistant to erosion. We also examine vulnerability to flooding by comparing dune heights with total water level (TWL) values from these storms. The TWL model includes wave setup, swash, and tides to predict the maximum elevation that water will reach during a storm (Ruggiero et al., 1996; Ruggerio et al., 2001). The taller dunes associated with A. arenaria are expected to provide more protection against flooding.