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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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Maps showing acoustic backscatter data collected, transitions and distances between areas of high and low backscatter, and graph illustrating the mean movement of backscatter transitions

Figure 4. Maps showing A, acoustic backscatter data collected using the multibeam echosounder in 2014 (Denny and others, 2015) and B, transitions and distances between areas of high- and low-backscatter margins of common sea-floor features digitized from 2011 (Schwab, Denny, and Baldwin, 2014) and 2014 backscatter data. C, Graph illustrating the mean movement (in meters) of backscatter transitions within four depth intervals between 2011 and 2014. Vertical bars indicate one standard deviation of mean value.

Map showing backscatter and bathymetry collected in 2011 and 2014 offshore of Fire Island, New York.

Figure 5. Maps showing A, backscatter and B, bathymetry collected in 2011 (Schwab, Denny, and Baldwin, 2014) and 2014 (Denny and others, 2015) in the vicinity of a borrow pit offshore of western Fire Island, New York. Location of maps shown on figure 4B. A, Sharp transitions between areas of high and low backscatter identified along the margins of discrete sediment distribution patterns, bedforms, and sedimentary structures from the 2011 and 2014 surveys are overlain on the backscatter data from the 2014 survey. B, Bathymetric data from each survey also illustrates significant change around the borrow pit area.

Map showing backscatter and bathymetry collected in 2011 and 2014 in an area of sorted bedforms offshore of Fire Island, New York.

Figure 6. Maps showing A, backscatter and B, bathymetry collected in 2011 (Schwab, Denny, and Baldwin, 2014) and 2014 (Denny and others, 2015) in an area of sorted bedforms offshore of eastern Fire Island, New York. Location of maps shown on figure 4B. A, Sharp transitions between high and low backscatter identified along the margins of discrete sediment distribution patterns and sedimentary structures from the 2011 and 2014 surveys are overlain on the backscatter data from the 2014 survey and aid in detecting change. B, Coincident bathymetry illustrates that change detection is prohibitive largely because of the subtlety of changes in this area. Change detection is further complicated because of artifacts in the data from the 2011 survey as described in the “Methods” section of this report.

Map showing change in modern sediment thickness betwween 2011 and 2014 survyes offshore of Fire Island, New York.

Figure 7. Map showing change in modern sediment thickness in meters between the 2011 (Schwab, Baldwin, and Denny, 2014) and 2014 surveys. A vertical resolution of 20 centimeters (cm) is assumed for the sediment volume calculation because of the resolution limits of the seismic system used. However, the change in sediment thickness is displayed with a less conservative estimate of 10 cm to better illustrate the sediment flux patterns. It is assumed that the majority of this change was a result of Hurricane Sandy.

Mapping Results

The patterns apparent in the backscatter data from the 2011 (Schwab, Denny, and Baldwin, 2014) and 2014 (Denny and others, 2015) surveys illustrate the primary sea-floor morphologies on the inner continental shelf, including sorted bedforms offshore of eastern Fire Island, a gravelly lag deposit offshore of central Fire Island, and a field of shoreface-attached sand ridges offshore of western Fire Island (fig. 4). Schwab and others (2013, 2014) provided detailed interpretations of these backscatter patterns, including how some are indicative of net southwestward sediment flux. The analysis of the sharp backscatter transitions in this report, which indicate minor variation in sediment texture and (or) structure, identified in the 2011 (Schwab, Denny, and Baldwin, 2014) and 2014 (Denny and others, 2015) surveys measures lateral offset distances ranging between about 1 and 450 m (with a mean of 20 m) during the 3-year period (fig. 4C) and depicts a dominantly southwestward movement of the transitions (figs. 5 and 6). Mean distances computed for changes within the aerial extents of four 5-m-depth intervals indicate that change occurred in water depths up to about 30 meters and tended to decrease with increasing water depth fig. 4C). The greatest change was detected in the area of a borrow pit (last excavated in 2009 for a beach nourishment project) in water depths of about 15 m where comparison of backscatter imagery shows that the southwest-facing flank of a shoreface-attached sand ridge migrated about 450 m southwest toward the borrow pit (fig. 5A). A comparison of the bathymetric data from the 2011 (Schwab, Denny, and Baldwin, 2014) and 2014 (Denny and others, 2015) surveys also shows deflation of this ridge flank (fig. 5B).

The comparison of modern sediment thickness mapped from the seismic-reflection data from the 2011 (Schwab, Baldwin, and Denny, 2014) and 2014 surveys closely agrees with results from the backscatter analysis, also illustrating what we interpret to be a net southwesterly migration of the shoreface-attached sand ridges. The isopach difference grid indicates a general pattern consisting of erosion on the northeast-facing ridge flanks and crests of the sand ridges and deposition on the southwest-facing ridge flanks and in the troughs adjacent to the southwest-facing ridge flanks (fig. 7). Statistics computed for the difference grid suggest that the modern sediment volume across the 81 km2 of common sea floor mapped in both surveys decreased by 2.8 million cubic meters, which is a mean thickness change of –0.03 m (below the resolution limit of the seismic-reflection systems used for mapping in this report). The largest magnitude change in sediment thickness (accretion of about 1.3 m) was observed in the same area where the largest magnitude morphologic change was observed in the backscatter analysis, which is along the eastern margin of the borrow pit area (fig. 5).

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