Coastal Dune Building Capabilities of Native and Exotic Beach Grasses: Initial Results From a Moveable Bed Wind Tunnel Experiment

Coastal dune morphology results from the interplay between wind, waves, sediment supply, and vegetation. Over the last 100 years in the U.S. Pacific Northwest, two exotic beach grasses (Ammophila arenaria and A. breviligulata) have replaced much of the native beach grass (Elymus mollis), and formed large continuous, and relatively stable, foredunes where open low-lying dynamic dune systems had previously dominated. Field measurements demonstrate a correlation between dominant beach grass species and foredune height, suggesting the potential for an ecological control on coastal vulnerability. Over a 20 year period, and across a broad range of sediment supply along the coast, A. breviligulata is associated with lower foredune heights than A. arenaria. Based on these correlations, we hypothesize that the A. arenaria is more effective at entrapping and stabilizing sand which results in steep and tall foredune morphology along reaches where this species dominates. A moveable bed wind tunnel experiment is being performed at Oregon State University's O.H. Hinsdale Wave Research Laboratory to test this hypothesis. Our experiments will quantify the relative sand trapping capabilities among the three beach grass species across different planting densities and wind speeds. Results from the experiment will help parameterize a computational fluid dynamics model describing dune building capabilities of different species, species' densities, wind speeds, and sediment supplies. Wind tunnel experiments and modeling results will be combined with field data, both biological and physical observations, to inform coastal protection measures and dune ecosystem management.