The Role of Ecomorphodynamics in Barrier Island Response to Climate Change

Coastal dune morphology is the most important factor determining barrier island vulnerability to storms—low relief, sparse dunes are prone to frequent overwash and inundation during storms whereas high relief, continuous dunes are more likely to minimize storm impacts. The evolution of island relief, then, is a function of two competing processes: (i) dune formation/recovery, driven by the coupling of biological and geomorphic processes, and (ii) dune destruction due to storm-induced overwash. These two processes are not independent, as the degree of dune erosion during a given storm depends on dune size and morphology. In addition, changes in grass species composition and resilience, following an overwash event may negatively impact dune recovery thus increasing island vulnerability to subsequent storms. Climate-change induced sea level rise, increases in storm intensity and shifts in species composition have the potential to amplify these eco-geomorphic feedbacks. In a first attempt to analyze quantitatively the effects of climate change on barrier island relief, we have developed an "ecomorphodynamic" model of island evolution. Consistent with observations, we find that the dune morphology primarily depends on grass species, with some grasses building long dune ridges and others creating sparse hummocky dunes, while some grasses may even prevent dune formation. In a next step, we added the effect of storms to study the evolution of island relief under a series of storm impacts. By changing average storm intensity and impact frequency, it is then possible to mimic some of the potential effects of climate change. For a sufficiently low storm frequency, and constant, moderate storm intensity, we find that dunes are able (on average) to recover from an impact before the next storm occurs. In this case, dunes are only partially eroded during a storm, allowing them to eventually re-attain their potential maximum size for a given set of external conditions. Under such a scenario, the island reaches a relatively stable "high elevation" state having minimum vulnerability to storms. In contrast, for a sufficiently high storm frequency, the island enters a feedback of ever more-widespread overwash and increasing dune erosion, leading to an ever weaker dune recovery. Hence, the island ends up in a "low elevation" state having maximum vulnerability to storms. Therefore, by increasing storm frequency from one run to another as an initial proxy for different climate change scenarios, the island transits from a "high" to "low" state nonlinearly, such that a small increase in frequency leads to an abrupt change in vulnerability.