Coastal dune geomorphology results from the interplay between wind, waves, sediment supply, and vegetation. Over the last 100 years in the U.S. Pacific Northwest, two invasive beach grasses (Ammophila arenaria and A. breviligulata) have replaced much of the native beach grass (Elymus mollis). Large, continuous, and relatively stable foredunes have resulted at the land-sea interface, where open, low-lying and dynamic dune systems had previously dominated. Field measurements demonstrate a correlation between dominant beach grass species and foredune geomorphology, suggesting the potential for an ecological control on coastal vulnerability to both flooding and coastal change hazards. Over a 20 year period, and across a broad range of sediment supply along the coast, A. breviligulata is associated with lower and wider foredunes than A. arenaria. Based on these correlations, we hypothesize that after accounting for biomass and density, A. arenaria is more effective at capturing and stabilizing sand, resulting in steep and tall foredune geometry along reaches where this species dominates. A moveable bed wind tunnel experiment was performed at Oregon State University’s O.H. Hinsdale Wave Research Laboratory to test this hypothesis across a range of grass species, densities, and wind speeds.
Results/Conclusions
Our physical model test results indicate that across species, density, and wind speed, the native E. mollis had higher sand capture efficiency than either of the two invasives; a result likely due to its morphological characteristics. Among the two invasive species, A. breviligulata had higher sand capture efficiency across density and wind speed, also likely due to morphological characteristics. These results provide key understanding of the biotic mechanisms responsible for dune geomorphology along the coast. When combined with biological (vegetation surveys) and physical (sediment supply and dune morphology measurements) field measurements, these data are helping to parameterize models describing the distribution of grass species and associated dune geomorphology along the coast, ultimately informing both coastal protection and dune ecosystem management measures.
