A Spatial and Temporal Assessment of Dune/Beach Volume Change along a Continuous Surface: Fire Island, New York

Volumetric change assessments are often used to study the responses of coastal barrier systems to natural variations and anthropogenic modifications. However, many analyses utilize volumes estimated from profiles, and thus introduce spatial interpolation errors into the assessments. This study presents a 3D assessment of spatial and temporal change for the dune-beach system at two study sites at Fire Island, New York, using continuous topographic surfaces generated from lidar and RTK-GPS surveys. The topographic surfaces are interpolated using GIS software and directly compared through raster subtractions. Initial results show that the more eastern of the two study sites is erosional over a decadal timescale, and may be more vulnerable to future storm events.

Fire Island is centrally located in a northeast-southwest trending barrier system along the south shore of Long Island. A decadal-scale volumetric change analysis of the dune-beach system at Fire Island shows that from 1998 to 2008, the chronically eroding eastern site decreased in volume (-200,000 m3) by a similar order of magnitude as the increase at the western site (300,000 m3). This is consistent with the westward longshore sediment transport direction. Seasonal changes at both sites show expected volumetric increases during the summer and decreases during the stormier winter. Annual data, however, which include an extreme storm (the 2007 Patriots Day nor'easter), show that the western site is relatively stable in the short-term, even considering the impacts of an extreme storm event (annual loss of ~18,500 m3). Over this same period, the eastern site lost material at a rate nearly 20 times that of the western site. These results suggest that the western portion of the island is presently stable enough to withstand severe storm events, but that the eastern site is highly vulnerable and may present substantial management challenges in the near future.