Toward modeling the role of ecomorphodynamics in barrier island response to climate change

In barrier islands, coastal dune morphology is the most important factor determining the vulnerability to storm impacts, as low, sparse dunes are prone to storm-induced erosion and frequent overwash. Under strong winds and enough sand supply, sand dunes naturally emerge from the beach. However, in most situations they are small, highly mobile dunes with a seaward decreasing size, which means they provide a limited protection to the beach area. In this context, it is well known that the presence of vegetation, and grass in particular, is a crucial ingredient able to stabilize mobile dunes and to drive the formation of foredunes at the beach end, thereby increasing beach elevation and minimizing the effects of storms. Besides wind, storms are another important physical force to take into account in the evolution of barrier island relief, as they flatten the dunes and may disrupt the ecological and/or physical elements behind dune recovery. The long-term evolution of the barrier island relief can then be understood as the result of two competing processes: (i) dune formation/recovery, driven by the coupling of biological and geomorphic processes, in particular grass growth and aeolian sand transport, and (ii) dune destruction due to storm-induced overwash. These eco-geomorphic feedbacks driving the evolution of barrier island relief are particularly relevant under Climate Change, with a predicted increase in storm intensity and/or frequency, and also potential changes in grass species composition. To study the implications of such scenario, we have develop a morphodynamical model of the barrier island relief. We include the main biological and physical processes as well as physical elements such as the shoreline and watertable, and biological ones as different grass species and potential competition among them. As a result, we find that the dune morphology, and thus the island's degree of vulnerability to storms, depends on the grass specie, with some grasses building long dune ridges parallel to the coast, other creating sparse hummock-like dunes, while some may even hinder dune formation. We also find that the interaction between vegetation growth and the shoreline position is responsible for the saturation of the dune growth and a factor controling the final dune size. In a next step, we added the effect of storms to the model and study the recovery of the relief for different coastal morphologies. This allows us to get a better insight  into more complex eco-geomorphic feedbacks as dune formation and storm-induced overwash are not independent processes: the amount of dune erosion for a given storm depends on the dune morphology itself, in particular dune size, while changes in grass species composition and resilience as an ecological response to overwash have a negative impact in the subsequent dune recovery and thus increase the Island's vulnerability to the next storm.