METRICS OF COASTAL RESILIENCE AND RECOVERY AT FORT TILDEN, GATEWAY NATIONAL RECREATION AREA, POST-HURRICANE SANDY
Hurricane Sandy greatly impacted Fort Tilden, eliminating the existing foredune, causing major volumetric loss, and destruction of a shore parallel road. Starting in December 2014, topographic surveys of transects were conducted seasonally using geodetic-grade GPS units. LiDAR datasets from August 2010 and November 2012 were used to establish pre- and post-storm conditions of the beach-dune system. The survey data are used to create digital elevation models (DEMs). Difference rasters are created by comparing DEMs to measure the change in elevation, provide metrics of volumetric gain and loss, and describe the spatial distribution of changes.
A contributing factor to the recovery is a beach nourishment project that occurred in 2014 when 2,217,200 m3 of sediment was placed updrift in the Rockaways and Jacob Riis. Post storm recovery has shown a trend of gain low on the beach, with little recovery in the dune area. The beach area has gained 69,120 m3 from December 2014 to November 2016, compared to the net gain of 14,760 m3 in the dune area. The shoreline has expanded seaward since the first survey, with a mean displacement of +6.75 m. A new foredune line was established post-Sandy ~30 m inland of the 2010 location.
Input from the updrift beach nourishment has influenced the spatial recovery of coastal features, causing greater rates of sediment accumulation at the updrift margin and decreasing to very little recovery at the downdrift margin. Geomorphologically, the dune feature has stabilized in position and form throughout the site, whereas the beach segments, separated into compartments by groins, has an alongshore gradient in recovery. Importantly, the areas of the profile that have greatest recovery are also the most likely sites for re-mobilization and impacts to the existing metrics of change. The spatial distribution of volumetric gain and loss as a measure of resilience can help inform decisions about infrastructure located within Fort Tilden.