Research papersThe impact of Hurricane Sandy on the shoreface and inner shelf of Fire Island, New York: Large bedform migration but limited erosion
Introduction
“Superstorm” Sandy made landfall as a post-tropical cyclone, with 70-knot maximum sustained winds, near Brigantine, NJ, on October 29, 2012 (Fig. 1, Fig. 2). Its sustained winds were ~25% higher, and significant wave heights ~50% higher than most other large storms over the previous 17 years (Fig. 2). Its unusual shoreward trajectory and massive size created record storm surges for longer periods along the heavily populated New Jersey and New York coastlines (Fig. 1; http://www.nhc.noaa.gov/data/tcr/AL182012Sandy.pdf). Infrastructure in the New York City metropolitan area was heavily damaged, and the Long Island barrier island system was both breached in places and seriously eroded (Hapke et al., 2013).
The impacts of this storm on the shoreface and inner shelf, which are permanently submerged and therefore primarily accessible only through acoustic mapping, are harder to observe. However, although the shoreface and inner shelf are neither populated nor veneered with human infrastructure, they are nevertheless critical to both people and their structures, because they are the first line of defense of barrier island systems against a naturally retreating, or “transgressing,” coastline. Under rising sea level conditions, the natural condition today along most of the U.S. east coast, barrier islands will back-step (retreat landward) by erosion on the seaward side and deposition on the landward side (Bruun, 1962, Swift and Thorne, 1991, Thorne and Swift, 1991). Large storms, with consequent high waves, strong currents and above-normal tidal ranges/surges, are thought to be primary drivers of such shoreface erosion (Swift, 1968, Swift and Thorne, 1991). Such storms are also considered important contributors to landward aggradation through overwash deposition (Lentz et al., 2013), although island breaching and inlet formation/closure are also major drivers over the short term (Leatherman, 1985).
There are few observational studies of large-storm impacts on the shoreface that can help constrain the storm-driven sediment budget for any barrier system. Logistically, such studies are difficult to organize because they require rapid mobilization of survey assets as soon after the storm as possible. Some luck is also required in order to have available any recent pre-storm surveys of like kind against which post-storm data can be compared. Furthermore, comparative studies done to date have resulted in different conclusions about hurricane impacts. After Hurricane Ike in 2008, for example, Goff et al. (2015) documented a widespread “storm” event involving up to 1 m of erosion on the Bolivar Peninsula, TX, shoreface. In contrast, after Hurricane Ivan in 2004, Kraft and de Moustier (2010) found up to 1 m of deposition along the shoreface of Santa Rosa Island, FL. These examples suggest that how a storm impacts the shoreface is likely to be dependent in part on local factors, such as wave/current history, abundance of mobile sand, and shoreface/barrier morphology. Multiple before-and-after storm maps of the seafloor and shallow subsurface, as well as access to key observational data (waves, currents, sediment transport processes), are required before we can constrain physics-based models of processes driving sediment flux on the shoreface and inner shelf during storms.
A comprehensive survey of the Fire Island, NY, lower shoreface and inner shelf was conducted by the U.S. Geological Survey (USGS) in June, 2011 (Fig. 1, Fig. 2; Schwab et al., 2013, Schwab et al., 2014), approximately a year and a half prior to Sandy. This pre-storm survey provides an important baseline for quantifying seabed changes during this time period. The same area was also surveyed by the USGS in 1996 and 1997 (Fig. 2; Schwab et al., 2000), providing a longer-term rates-of-change baseline to compare against short-term (i.e., baseline+storm-induced) changes. To complement these pre-storm data sets, we mounted a collaborative post-Sandy survey in early January, 2013, aboard the R/V Seawolf to collect multibeam bathymetry and backscatter, CHIRP (compressed high-intensity sonar [previously radar] pulse) acoustic reflection data, and sediment grab samples offshore of part of southern Long Island. Two survey patches, “Fire Island West” (FIW) and “Fire Island East” (FIE), overlap the 2011 USGS survey (Fig. 1), and are the focus of the results presented in this paper. The intervening time between 2011 and 2013 surveys also included the passage of Irene, which impacted the Mid Atlantic Bight as a tropical storm in late August, 2011 (http://www.noaa.gov/extreme2011/irene.html). Irene was a lesser storm in terms of winds and waves than Sandy (Fig. 2). Its peak sustained winds were on par with more typical large storms in the region, but its significant wave heights were larger (Fig. 2). Although our primary focus is on the larger storm, Irene could also have contributed to any observed “storm-induced” component of change.
The seabed offshore of Fire Island undergoes a significant change in morphology between the eastern and western ends of Fire Island (Fig. 1; Schwab et al., 2000, Schwab et al., 2013, Schwab et al., 2014). To the west, the seabed morphology is dominated by shoreface-attached sand ridges, large (~1-6 m high, ~1-3 km wide) dune-like bedforms angled ~20°–50° to shore (acute angle to the east). To the east, shoreface-attached bedforms also exist, but they are smaller and narrower (<0.5–1.5 m high and ~0.2–1.0 km wide), angled ~60°–70° to shore (acute angle to the east), and are classified as “sorted bedforms” rather than as sand ridges (Schwab et al., 2013). This terminology refers to the pronounced segregation of grain sizes into coarse and fine sand regions, with coarser grain sizes flooring basins to stoss (upcurrent) slopes (Murray and Thieler, 2004). Schwab et al., 2000, Schwab et al., 2013 link this change in shoreface and inner-shelf morphology to the abundance of modern marine sand; much more is available offshore of the western half of Fire Island, because sand is being transported westward from a large Pleistocene sand unit mid-island by longshore currents. Schwab et al., 2000, Schwab et al., 2013 also note that the coastline of western Fire Island is presently stable or advancing, whereas to the east it is retreating. To explain this correlation, they hypothesize that shoreward advection of sand from the inner shelf contributes to the replenishment of shoreface sand off western Fire Island; they further suggest that storms contribute to this process. Consequently, the 2013 western and eastern post-Sandy surveys described in this paper provide an opportunity to investigate the shoreface response to Sandy in different settings in terms of sand thickness and seabed morphology, and to explore possible linkages to coastline stability.
Section snippets
Multibeam bathymetry and backscatter
The post-Sandy bathymetric and backscatter data were collected in January, 2013, (Fig. 2) using a hull-mounted Kongsberg EM 3000D multibeam echosounder. This system operates at 300 kHz and utilizes two transducers to image a swath that has a width up to 10 times water depth. Vessel heave, pitch, roll and heading were recorded continuously. Sound velocity profiles were collected when possible, since these profiles could only be collected during breaks in the CHIRP survey lines, which could only
Backscatter
Multibeam backscatter data for the post-Sandy FIW and FIE surveys are presented in Fig. 4, Fig. 5, respectively. The backscatter map in both cases is dominated by the primary bedforms: sand ridges within FIW and sorted bedforms within FIE. Schwab et al., 2000, Schwab et al., 2013, Schwab et al., 2014 suggest that both types of bedforms are, over the long term, migrating to the west or southwest, as evidenced by coarser grain sizes and higher backscatter on the stoss (northeast) flanks. We
Bedform migration in response to storm forcing
Our primary conclusion is that superstorm Sandy caused sand ridges and sorted bedforms along the shoreface and inner shelf off Fire Island to migrate to the southwest. The amount of migration is more than observed in prior years (Schwab et al., 2013), but otherwise fully consistent with prior observations and with expectations of southwest migration based on the lee/stoss relationship of grain size (Schwab et al., 2000, Schwab et al., 2013, Schwab et al., 2014). This conclusion confirms what
Conclusions
Over the long term, barrier islands migrate landward in response sea level rise by erosion on the seaward side and deposition in the bay side; large storms are assumed to be primary drivers of this process (Swift and Thorne, 1991, Thorne and Swift, 1991). The stability of a barrier island is linked to the extent of seaward erosion which, along modern shores, excavates antecedent sediments, and forms an erosional unconformity, or ravinement, generally underlying a modern marine sand layer. The
Acknowledgements
We thank the captain and crew of the R/V Seawolf, owned and operated by Stony Brook University, for their tireless work and dedication. S. Saustrup at UTIG helped to process the CHIRP profiles. This work was funded primarily by a rapid response Grant from the Jackson School of Geosciences, The University of Texas, Austin. UTIG contribution number 2839.
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