Shoreface-connected sand ridges control on decadal-scale shoreline morphology

Fire Island, NY, is a 50-km long barrier island that is oriented along WSW-ENE direction. Historical analysis of shoreline positions over the past 80 years was conducted using 35 shorelines. The positions vary over time but along the western half of the island they maintain a persistent undulating shape with cross-shore amplitudes of 20-40 m. The inner continental shelf offshore of these persistent shoreline undulations is characterized with a series of obliquely-oriented shoreface-connected sand ridges (SFCR). Through a series of oceanographic deployments, geologic framework investigations, and numerical simulations, we have evaluated mechanisms that can relate the SFCRs to these shoreline undulations. We used a process-based model (COAWST) to investigate the formation of these persistent features using a nested grid modeling configuration which starts with the entire US East Coast and scales down to resolve the Fire Island nearshore. This nested scheme simulates the nearshore effects of waves, currents, and sediment transport driven by both distantly-generated swells and locally generated wind waves. The model was tested against field measurements of waves collected at 9 sites along the western end of Fire Island during a large-scale field effort in 2014, and historical wave data from nearby buoys. Alongshore sediment fluxes and shoreline change rates were calculated from model-computed bedload and suspended load. To isolate the influence of the SFCRs on the generation of the persistent shoreline shape, simulations were performed with a linearized nearshore bathymetry to remove alongshore transport gradients associated with shoreline angle variations. The model was able to predict the alongshore scale of the persistent shoreline undulations; the positions of relative embayments and promontories are predicted accurately in one area but not in another. Net onshore sediment flux along the western end of the island obtained from the model and observations are analyzed to investigate these discrepancies.