TRENDS OF DUNE CREST DISPLACEMENT 1976-2012, FIRE ISLAND, NEW YORK

Long-term quantifications of sediment storage are key components towards a better understanding of the function and evolution of coastal geomorphological systems. Utilizing the change in foredune crest location as a readily identifiable, simply defined proxy for local sediment budget, spatiotemporal patterns of dune crest migration were examined for intervals between 1976 and 2012 at Fire Island National Seashore, a barrier island off Long Island, New York. Considering sediment budget to be the primary determinant of shoreline position change, the vectors of crestal displacement calculated for 5-10 year intervals serve as a trend of shoreline displacement during those periods, absent larger fluctuations of the beach itself. The spatially segregated sum of those parts, equating to the total displacements in this long-term analysis, revealed patterns of periodic erosion and accretion in some areas and consistently landward or seaward vectors of displacement in others.

Revisiting previous analyses of dune crest migration at Fire Island dating to 1976, beach/dune profiles, stereo pair orthophotos, and LiDAR elevation datasets were examined to derive dune crest locations. The several data sources provided feedback confirming results where they overlapped. A mean displacement of -25 m from 1976 to 2012 was calculated for the length of the island between Robert Moses State Park and Moriches Inlet, including maximum landward and seaward displacements of 175 m and 111 m, respectively. Periodic erosion and accretion were observed within that time, with particularly large fluctuations occurring as a result of the 1992 storms and subsequent replacements/replenishments. Between 2000 and 2011, the mean crestline displacement was approximately 1 m seaward for the entire length of the island and the largest landward vectors were located in the Old Inlet area; a location that subsequently breached under the stress of Hurricane Sandy. Sandy accounted for an average displacement of -15 m along the entire island. The foredune crest was almost systematically eliminated by the storm, either replaced in form and function by a secondary dune that was previously situated landward of the active foredune or eliminated entirely as a result of overwash. Approximately 17% of the foredune was eliminated without replacement.