Oregon Sea Grant

Coastal foredunes protect lives, infrastructure, and ecosystems during severe winter storms. In the U.S. Pacific Northwest (PNW), coastal foredune geomorphology is determined by both physical and ecological mechanisms. Before the 1900's, the native plant Elymus mollis was the dominant dune grass and dune morphology was largely determined by sediment supply and other physical factors. The introduction of two different invasive beach grasses in the first half of the 20th century resulted in significant changes to PNW foredune geomorphology. Recent work has shown that the invasive beach grass Ammophila arenaria is associated with taller, narrower foredunes while the grass Ammophila breviligulata is associated with shorter, broader foredunes. We hypothesize that this may impact coastal vulnerability as the taller, narrower foredunes may be more resistant to overtopping, but less resistant to erosion, while the shorter, broader foredunes may be more resistant to erosion, but less resistant to overtopping.

The work presented in this thesis is part of a larger, interdisciplinary effort to determine the relative importance of physical and ecological mechanisms in controlling coastal foredune geomorphology and vulnerability in the PNW. Airborne topographic lidar data is used to characterize the regional variability in foredune geomorphology within several littoral cells, subcells, and beaches of the PNW. We present and discuss techniques to automatically and accurately extract foredune morphometric parameters from a lidar elevation data set of the coasts of Oregon and Washington. If the different grass species do affect foredune geomorphology, it is important to understand how managing them might impact coastal vulnerability. Therefore, we use the geomorphological parameters and measured hydrodynamic parameters during a major winter storm to estimate vulnerability to overtopping and erosion at beaches with distinctly different foredunes. Estimates of overtopping and erosion are normalized by the foredune heights and widths to elucidate how the different foredune types, and possibly the associated beach grasses, affect vulnerability. A series of simple foredune erosion models including a geometric model, an equilibrium profile model, and a wave impact model are evaluated for applicability to the PNW coast.