MODELING THE FORMATION OF DUNES ON BARRIER ISLANDS

Coastal dune morphology is the most important factor determining barrier island vulnerability to storms—low and sparse dunes are prone to frequent overwash and inundation during storms whereas high and continuous dunes are more likely to minimize storm impacts. However, there is a poor understanding of the mechanisms behind coastal dune formation and recovery following storms, in particular the influence of the beach and shoreline. Dune formation in coastal areas is partially driven by the coupling of biological and geomorphic processes, in particular, grass growth and aeolian sediment transport, while the beach controls the aeolian sediment input. These processes were implemented in a computational model, including physical elements such as the shoreline and water table, as well as different grass species and potential competition among them. Consistent with field observations, we find that the dune morphology, and thus island vulnerability to storms, depends on the grass species, with some grasses building long dune ridges parallel to the coast and others creating sparse hummock-like dunes, while some may even hinder dune formation. We also find that the interaction between vegetation growth and shoreline position is responsible for the stabilization of dune growth and is a factor controlling final dune size. In a next step, we added the effect of storms to the model and studied the recovery of relief for different coastal morphologies. This allows us to gain insight into more complex eco-geomorphic feedbacks as dune formation and storm-induced overwash are not independent processes. For example, the degree of dune erosion during a given storm depends on dune size and morphology, while changes in grass species composition and resilience, following an overwash event, may negatively impact dune recovery thus increasing island vulnerability to subsequent storms.