APPLICATION OF GEOLOGIC MAPPING OF THE INNER CONTINENTAL SHELF TO COASTAL EVOLUTION AND NEARSHORE SEDIMENT TRANSPORT PROCESSES RESEARCH, AN EXAMPLE FROM FIRE ISLAND, NEW YORK

The inner continental shelf adjacent to Fire Island, New York, a barrier island along southern Long Island, was mapped in 1996-1997, 2011 and 2014 using bathymetric, sidescan-sonar and seismic-reflection techniques to evaluate the influence the geologic framework has on coastal evolution and sediment transport processes. The common area mapped during all the surveys is 50 km long by 8 km wide extending from Moriches to Fire Island Inlets in water depths ranging from 8 to 32 m. Mapping results show that the inner-shelf morphology and sediment distribution patterns are determined by erosion of Pleistocene outwash sediments during the ongoing Holocene marine transgression. Remnants of an outwash lobe define a submerged headland offshore of central Fire Island. Northeast of the headland, older outwash is exposed over much of the inner shelf and covered by sorted bedforms interpreted to indicate erosion and southwest transport of reworked sediment. Erosion of the northeast flank of the headland during marine transgression yielded modern sand that was transported southwest and reworked into a field of shoreface-attached sand ridges. Comparison of the sand ridge morphology with the morphology of the underlying Holocene transgressive unconformity and comparison of modern sediment thickness mapped in the three surveys show that the sand ridges move toward the southwest, presumably driven by event-scale waves and currents. Littoral transport on this coast is primarily directed alongshore from northeast to southwest. Mapping results suggest that some component must also be directed shoreward, as erosion of the inner shelf provides sediment volumes required to balance the coastal sediment budget. To evaluate the physical processes that mobilize and transport sediment on the inner shelf, a coupled ocean-atmosphere-wave-sediment transport model (COAWST) simulated several time periods, all verified with oceanographic observational data. Results show: 1) different storms drive varying patterns but maintain a net southwest sediment flux alongshore; 2) storm-driven flows to the southwest promote growth/stability of the shoreface-attached sand ridges; 3) the inner shelf morphology modifies waves that drive alongshore-variable currents in the nearshore which control the observed persistent shape of the shoreline.