Wave-driven effects and geomorphic controls on bay-shelf exchange in a back-barrier estuary

Exchanges between semi-enclosed bays and adjacent shelves are mainly controlled by frictional effects predominantly within inlets. During normal conditions, tidal forcing drives the exchange variability. However, during storms additional factors such as storm surge, winds, waves, and geomorphic changes (i.e., breaches, overwash and changes to the inlet geometry) modify bay dynamics and flushing. Enhanced apparent roughness caused by waves in the vicinity of inlets can have a large impact on the exchange. Additionally, wave setup and storm surge increase water depth and intensify the barotropic pressure gradient between shelf and bay, which further modifies inlet currents. During extreme storms, barrier island breaching and overtopping create new avenues for shelf waters to enter the bays and shift the exchange balance between existing inlets. In the present study, we used the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system to analyze differences in bay-shelf transfers in Great South Bay and adjacent bays along the south shore of Long Island, NY. The relative contributions of the factors modifying inlet frictional control during normal and extreme conditions (e.g., Hurricane Sandy) are quantified. Breaching (through the inclusion of the Wilderness Breach across Fire Island) and overtopping are also considered. The results demonstrate the need to include wave frictional and setup contributions and geomorphic changes in the prediction of bay dynamics and shelf-bay exchanges during large storms. Properly characterizing these exchanges supports informed decision-making in the prediction of coastal flooding, hazard response, and changes in bay water quality.