THE INFLUENCE OF INNER SHELF MORPHOLOGY ON THE MORPHODYNAMICS OF THE BEACH AND BAR SYSTEM, FIRE ISLAND, NEW YORK

The inner shelf off Fire Island, New York has a well-defined ridge-and-trough topography. The ridges are comprised of Holocene sand overlying a ravinement surface. Variable geomorphology of the barrier island and imbalances in the regional sediment budget suggest that some material from the sand ridges supplies the beaches along western Fire Island. The mechanism of exchange is unknown and understanding has been limited by a lack of: 1) nearshore bathymetry to establish that the ridges connect to the shoreface, 2) an understanding of the nearshore bar dynamics, and 3) a detailed analyses of shoreline change patterns and how they relate to the inner shelf morphology.

Single-beam bathymetric data were collected in the nearshore off Fire Island in 2007 and 2009. These surveys provide the first continuous bathymetric data for the 2-12 m water depth range at Fire Island. Multibeam data extending from 12 to 24 m water depth were also collected in 2009, overlapping with the single-beam data. The resulting bathymetric surface indicates that the inner-shelf ridges and troughs are continuous into the nearshore and appear to be connected to the bar system. Comparisons of the repeated single-beam surveys show that the locations of the ridge and trough axes do not appear to change in the short-term (2007 to 2009). The fixed nature of the nearshore morphology is further supported by time series camera data collected in the winters of 2007 and 2008 that indicate persistent bars with breaks that re-form in the same locations. Additionally, long-term (~70-yr) shoreline change analyses suggest that the inner shelf morphology is exerting control on the behavior of the beach. Distinct zones of erosion and accretion are pervasive over time scales greater than a half-century, and their length-scales are similar to the spacing of the offshore ridge-trough system.

We propose that the evidence from the nearshore bathymetry that the ridges and troughs connect to the shoreface, coupled with the complex but consistent bar morphology and persistent shoreline change patterns demonstrate that there is a feedback occurring between the regional geologic framework and modern processes. The first-order framework is responsible for the existence and locations of the ridges and troughs, which then influence the morphodynamics of the beach and bar system.