MODELING DYNAMIC INTERACTIONS AND FEEDBACKS BETWEEN BARRIER ISLANDS AND FRINGING BACK-BARRIER MARSHES

Efforts to model barrier island evolution under changing conditions have demonstrated the importance of sea level rise rates, sediment-loss rates, antecedent topography and underlying stratigraphy in controlling rates of island migration, couplings between barrier and back-barrier environments have yet to be explored in a modeling framework. Here, we modify an existing morphological-behavior model of barrier island evolution (GEOMBEST) to incorporate a dynamic back-barrier environment in order to investigate couplings between barriers and their fringing marshes. To simulate back-barrier processes in greater detail we have added model components representing overwash deposition, bay erosion, marsh accretion and marsh erosion. Marsh growth is limited by the amount of available fine-grained sediment. If sea level rise outpaces the input of sediment into the lagoon, new marsh may not form. Alternatively, if sea level does not rise, and there is sufficient input of sediment, the entire lagoon will fill with marsh. Overwash deposition can also help to create new marsh, by filling the bay with sand to a depth at which the marsh can grow.

Our initial experiments are based on Metompkin Island, located on the Virginia Eastern Shore. Preliminary results indicate that if back-barrier conditions are conducive to fringe marsh development, island migration rates initially decrease due to the reduction of accommodation space. However, once the marsh has existed long enough for it to be exposed at the shoreface (as a stratigraphic layer), island migration rates increase because there is less sand available in the shoreface (since the marsh unit is made up predominantly of fine-grained sediment). In this case, more of the shoreface must be eroded to liberate the same amount of sand. Initial results also suggest the potential for climate change to alter the feedbacks between barrier islands and back-barrier marshes. For example, as storm intensity increases, the delivery of more sand to fringing marshes through overwash may increase the rate at which they aggrade and prograde. At the same time, a higher rate of sea level rise will increase erosion at the fringing marsh edge. Model experiments suggest that which processes ultimately win out will depend on factors such as sea level rise rate, antecedent morphology and marsh composition.