Barrier islands and back-barrier marshes respond dynamically to climate change, and it is important to study the two as a coupled system to properly understand the trajectories of future barrier system behavior. The fundamental coupling between the two environments is that sea level rise drives barrier island migration over back-barrier marshes through the process of storm-driven overwash. We developed a coupled barrier island-marsh model (GEOMBEST+) to simulate the morphological behavior of barrier island evolution over a back-barrier environment, where morphology varies as a result of sea level rise, overwash deposition, bay erosion, and marsh accretion. We use this new model to assess the impact of overwash deposition on back-barrier marsh morphology, and the impact of marsh morphology on rates of island migration. Results indicate that overwash deposition provides back-barrier marshes with an important source of sediment that maintains existing narrow marshes in a long-lasting metastable state (~500 m wide). Marshes, in turn, provide barrier islands with a platform over which to migrate which causes a reduction in island landward migration rates.
To apply our model results to a real world back-barrier marsh, we developed a field experiment to simulate the effect of overwash deposition on a Spartina alterniflora marsh on Hog Island, Virginia. In a total of 72 experimental plots, marsh vegetation was buried under varying thicknesses of sand (0, 5, 10, 15, 30, and 60 cm) to simulate an overwash deposition event. We monitored vegetation growth and recovery over the course of a growing season. Our preliminary results suggest that there is a threshold thickness of overwash deposition (15-30 cm) that will result in marsh die-off. Our combination of modeling and field studies provide an expansive look into how barrier islands and back-barrier marshes will evolve together, ecologically and geomorphologically, as a response to climate change.