Understanding Long-term, Large-scale Shoreline Change and the Sediment Budget on Fire Island, NY, using a 3D hydrodynamics-based model
Jeffrey H List1, Ilgar Safak2, John C Warner3, William Charles Schwab3, Cheryl J Hapke4 and Erika E Lentz3, (1)U.S. Geological Survey, Woods Hole, MA, United States, (2)Dolfen Engineering and Consultancy, Istanbul, MA, Turkey, (3)USGS Coastal and Marine Science Center Woods Hole, Woods Hole, MA, United States, (4)U.S. Geological Survey, St. Petersburg, FL, United States
Abstract:
The processes responsible for long-term (decadal) shoreline change and the related imbalance in the sediment budget on Fire Island, a 50 km long barrier island on the south coast of Long Island, NY, has been the subject of debate. The estimated net rate of sediment leaving the barrier at the west end of the island is approximately double the estimated net rate of sediment entering in the east, but the island-wide average sediment volume change associated with shoreline change is near zero and cannot account for this deficit. A long-held hypothesis is that onshore sediment flux from the inner continental shelf within the western half of the island is responsible for balancing the sediment budget. To investigate this possibility, we use a nested, 3-D, hydrodynamics-based modeling system (COAWST) to simulate the island-wide alongshore and cross-shore transport, in combination with shoreline change observations.
The modeled, net alongshore transport gradients in the nearshore predict that the central part of Fire Island should be erosional, yet shoreline change observations show this area to be accretionary. We compare the model-predicted alongshore transport gradients with the flux gradients that would be required to generate the observed shoreline change, to give the pattern of sediment volume gains or losses that cannot be explained by the modeled alongshore transport gradients. Results show that the western 30 km of coast requires an input of sediment, supporting the hypothesis of onshore flux in this area.
The modeled cross-shore flux of sediment between the shoreface and inner shelf is consistent these results, with onshore-directed bottom currents creating an environment more conducive to onshore sediment flux in the western 30 km of the island compared to the eastern 20 km. We conclude that the cross-shore flux of sediment can explain the shoreline change observations, and is an integral component of Fire Island’s sediment budget.
