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Fire Island Coastal Change

Storm Impact and Recovery: Hurricane Sandy - Shoreline Change

In order to examine the dynamics of the shoreline from Hurricane Sandy and monitor the continued response and recovery, surveys of continuous alongshore differential global positioning system (DGPS) data were collected in conjunction with cross shore profiles. Multiple shore-parallel and shore-oblique tracks are driven to capture the base of the dune, the mid-beach, and the upper and lower foreshore along the length of the island from Fire Island lighthouse to the western side of the Wilderness breach at Old Inlet.

Initial surveys were conducted one day prior to landfall and immediate post-storm surveys were conducted over the three days following Sandy. The beaches and dunes were resurveyed monthly from December 2012 through April 2013, and periodic surveys have been collected or planned through the fall of 2015. The USGS is continuing to monitor the beaches and dunes to evaluate how much of the sand removed by Sandy returns to the beach via natural beach-building processes.

In addition to field data presented here, the USGS has collected various forms of lidar data (CLARIS, EARRL) for Fire Island, and as those correlate with some of our datasets, they have been presented here for comparison with the field collected data. As the dates are not exact – some offset (especially post storm) is expected, but is still included to provide alongshore data, that is otherwise missing from the field data collection.

Where noted, lidar data was used in conjunction with field data to provide continuous along shore coverage for change statistics.

Uncertainty for all field collected shorelines is calculated by following the methods described in Henderson et al, 2015. More information available at http://pubs.usgs.gov/ds/0931/ For the shorelines used in this analysis, individual shoreline position uncertainty ranged from 1.2 to 5.1 m, with an average uncertainty of 2.6 m.  For the analysis of net shoreline movement (which is calculated using two shoreline pairs) the uncertainty was calculated by taking the square root of the sum of the squares of both shoreline position uncertainty values, yielding an average uncertainty of 4.2 m, with a range of 1.8 – 5.4 m.

Shoreline Change Plot
Shoreline Change Plot

Statistics plotted include the net shoreline movement (NSM), which is the total movement between the two shoreline positions. This measurement of change uses only two dates, and thus is considered an end point measure of change, as it does not use any other information between dates.

Sandy impact (blue line, top plot): The net shoreline movement following Hurricane Sandy (Oct 27 Nov 05) was progradational and averaged 9.9 m but movement ranged from +43.7 m to -22.4 m.

Post-Sandy winter (red line, top plot): From Nov 5 - Mar 13, the average shoreline change was highly erosional (-35.4 m) with a maximum landward shift of -66.9 m.

Recovery (green line, center plot): after the winter months, and through the summer (Mar Sept 2013), substantial recovery occurred along the majority of the shoreline (97% of transects were accretional during this time), with average change of + 30.4 m.

The Post-Recovery (light blue line, center plot): shoreline change from September 2013 to September 2015 (~24 months) indicates the system has transitioned to a state of increased or sustained erosion with an average change of -17 m.

Total Change (gray line, lower plot): The average net shoreline movement from immediately before Hurricane Sandy to the most recent survey date is erosional (-15.0 m). Although there are distinct zones of erosion and accretion along the length of the island, nearly 87% of the coast eroded over this time period. [larger version]

Revised March 2016 R. Henderson

    Shoreline Change Statistics: Hurricane Sandy and Beyond The red and green bars in the table represent the approximate percent of the measured coast that was found to be accreting (green) or eroding (red) for each time period. The use of lidar data in lieu of field data is noted by italicized font with an asterisk. Revised Mar 2016 R. Henderson.

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