The Role of Ecomorphodynamic Feedbacks and Landscape Couplings in Influencing the Response of Barriers to Changing Climate

Abstract

Because barriers are low-lying and dynamic landforms, they are especially sensitive to changing environmental conditions. The continued existence of barriers will depend on the degree to which these landforms can maintain elevation above sea level while also migrating landward. We are increasingly learning that ecomorphodynamic interactions (i.e., interactions between morphology, fluid dynamics, and/or sediment transport with biological processes) as well as couplings between barrier and back-barrier environments play a critical role in determining how barrier systems will evolve as sea level rises, storm intensity increases, and the species composition of coastal vegetation changes in the future. For example, the effectiveness of storms in building elevation and moving a barrier landward is determined, in large part, by the morphology of the coastal foredune (i.e., the seaward-most dune), which is itself a product of couplings between vegetation and sediment transport processes. The cross-shore and alongshore shape of coastal foredunes, in the presence of shoreline erosion or shoreline accretion, is influenced by the distance from the shoreline that vegetation can grow, the rates of lateral and vertical vegetation growth of dune-building vegetation, as well as the dependence of vegetation growth on dune slope. In addition, as storm frequency increases relative to the rate at which dunes can grow, dunes, and therefore local barrier elevation, may become bistable, tending to be in either a high dune/barrier or low dune/barrier state. When dunes are low, storms can effectively increase barrier elevation and move a barrier landward over time leading also to the potential for increased connectivity to back-barrier marshes, which are vulnerable to drowning as sea level rises. In this case, sand delivered to back-barrier marshes can, for a time, allow back-barrier marshes to persist under conditions in which they would otherwise disappear, thereby benefitting the entire barrier-marsh system. Here we provide a synthesis of model results—tested against observations—that demonstrate these findings, illustrating the importance of feedbacks between vegetative and sediment transport processes, and couplings between landscape units, in influencing the future evolution of barrier-marsh systems.