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Open-File Report 2015-1238


Assessing the Impact of Hurricanes Irene and Sandy on the Morphology and Modern Sediment Thickness on the Inner Continental Shelf Offshore of Fire Island, New York


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Summary

Hurricane Sandy made landfall in southern Long Island, New York, and surrounding areas in October 2012. The U.S. Geological Survey has been involved in research in the region since the mid-1990s to evaluate the influence of the regional geologic framework on coastal evolution and formulate a conceptual model of sediment flux in the coastal ocean. In May 2011, the USGS conducted a high-resolution marine geophysical survey of the lower shoreface and inner continental shelf offshore of Fire Island, a barrier island on the southern side of Long Island, using interferometric sonar and seismic-reflection techniques. The 2011 survey served to document conditions on the inner continental shelf offshore of Fire Island approximately 6 months before the passage of Hurricane Sandy. Additional geophysical surveys in 2014 document conditions after the storm and serve to determine the impact of Hurricane Sandy on the inner continental shelf morphology and modern sediment distribution and to broaden the baseline geospatial framework for sediment transport and coastal change model development.

This report documents the changes in seabed morphology and modern sediment thickness measured between the mapping investigations from the 2011 and 2014 surveys and compares these changes with those detected from mapping surveys conducted in 1996-97 and 2011. Hurricane Sandy produced sustained winds of 25.1 meters per second and wave heights as sizeable as 9.7 meters (m), measurements about 25 and 50 percent higher, respectively, than most other large storms during the previous 17 years, with the exception of Hurricane Irene (August 2011), which impacted the area shortly after the 2011 survey and produced sustained winds of 19.2 meters per second and wave heights of 7.9 m.

Morphologic analyses on backscatter data for the periods from 1996-97 to 2011 and from 2011 to 2014 indicate predominantly southwestward movement of the sediment distribution patterns, bedforms, and sedimentary structures. The only significantly contrasting observations made from the two analysis periods occurred along the southwestern margins of shoreface-attached sand ridges in water depths less than about 15 m.

From 1996–97 to 2011, erosional scarps defining the seaward margins of the ridges had predominantly migrated landward; in places these scarps define the seaward extent of the toe of the shoreface. From 2011 to 2014, the margins were mostly observed to have moved seaward, but landward motion was also detected at discrete locations along the lengths of the margins. Analysis of backscatter transitions indicates predominantly western or southwestern bedform migration and a general decrease in the magnitude of change relative to water depth. The measured lateral offset distances of backscatter transitions identified in the 2011 and 2014 surveys ranges between about 1 and 450 m (with a mean of 20 m) during the 3-year period before and after Hurricane Sandy, and depicts a dominantly southwestward movement of the transitions. Mean distances computed for changes indicate that change occurred in water depths as large as about 30 meters and tended to decrease with increasing water depth.

Calculations of the differences for the periods from 1996–97 to 2011 and from 2011 to 2014 indicate net southwesterly migration of the shoreface-attached sand ridges, generally showing erosion on the eastern flanks and crests of the ridges and deposition on the western flanks and in the troughs between them. For 199697 to 2011, analysis indicated substantial accretion along the lower shoreface and adjacent inner continental shelf offshore of western Fire Island, and statistics computed on the difference grid suggested that the modern sediment volume across the about 274 square kilometers of commonsea floor mapped increased by 20.2 million cubic meters during the 15-year period, a mean thickness change of +0.07 m. These results suggest that some part of the predominantly southwestward alongshore net sediment flux is directed shoreward and that erosion of the Pleistocene outwash and lower Holocene channel-fill deposits exposed at thesea floor continues to yield the modern sediment required to balance the coastal sediment budget and maintain the shoreface-attached sand ridges.

The combined impacts of Hurricanes Irene and Sandy caused erosion of the sand ridges, with sediment transported in a general southwesterly direction. For the period from 2011 to 2014, analysis indicated substantial erosion on the inner continental shelf, and statistics computed on the difference grid suggested that the modern sediment volume across the 81 square kilometers of commonsea floor mapped in both surveys decreased by 2.8 million cubic meters, which is a mean thickness change of 0.03 m. Surveys of the shoreface and subaerial components of Fire Island indicate that the eroded sediment was not transferred to the shoreface or adjacent barrier island and thus was transported southwest out of the study area and (or) distributed in a veneer over the glaciofluvial and lower Holocene channel-fill deposits beyond the resolution detected by the seismic-reflection techniques, which is about 20 centimeters.

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