Hydrodynamics and Sediment Transport Along and Across Shoreface-Connected Ridges: Fire Island, NY.

Fire Island is a 50 km long barrier island located along the south shore of Long Island, NY. The inner continental shelf of the island is characterized by field of shore-face connected ridges (SFCR). Repeated geophysical studies have mapped the sediment distribution and morphology of the inner continental shelf and identified areas of erosion and deposition associated with ridge migration, as well as areas of sediment deposition on the inner shelf not associated with ridge mobility. This study describes storm driven flows along and across the inner continental shelf to investigate the hydrodynamic forcings and the possible maintenance mechanisms of these SFCR in the region using numerical experiments and field observations. We applied a nested grid, three-dimensional hydrodynamic model considering storm forcing and integrating sedimentological data using the COAWST model, which couples the oceanographic model (ROMS), the wave model (SWAN), and the Community Sediment Transport Modeling System (CSTMS) set of sediment transport modeling routines. The coarsest grid covers the entire US East Coast and grid refinement is used to nest down to the study zone along Fire Island with a final horizontal resolution of 100-m. Wave, flow, and sediment transport observations at 9 sites offshore of Fire Island from January to April 2014 were used to test the model. Results reveal the variability along the inner continental shelf during different storm events as well as a strong relation between currents (surface and bottom, alongshore and cross-shore) and wind patterns. Storms with northeast winds drive flow that promoted SFCR maintenance. Other storm types resulted in upwelling and circulation patterns which are in the opposite sense and may not be a supporting mechanism for ridge maintenance. This analysis supports storm-generated northeast wind-driven forcing play an important role controlling movement of the SFCR.