The impact of Hurricane Sandy on the shoreface and inner shelf of Fire Island, New York: Large bedform migration but limited erosion

Abstract

14We investigate the impact of superstorm Sandy on the lower shoreface and inner shelf 15 offshore the barrier island system of Fire Island, NY using before-and-after surveys involving 16 swath bathymetry, backscatter and CHIRP acoustic reflection data. As sea level rises over the 17 long term, the shoreface and inner shelf are eroded as barrier islands migrate landward; large 18 storms like Sandy are thought to be a primary driver of this largely evolutionary process. The 19 "before" data were collected in 2011 by the U.S. Geological Survey as part of a long-term 20 investigation of the Fire Island barrier system. The "after" data were collected in January, 2013, 21 ~two months after the storm. Surprisingly, no widespread erosional event was observed. Rather, 22 the primary impact of Sandy on the shoreface and inner shelf was to force migration of major 23 bedforms (sand ridges and sorted bedforms) 10's of meters WSW alongshore, decreasing in 24 migration distance with increasing water depth. Although greater in rate, this migratory behavior 25 is no different than observations made over the 15-year span prior to the 2011 survey. 26Stratigraphic observations of buried, offshore-thinning fluvial channels indicate that long-term 27 erosion of older sediments is focused in water depths ranging from the base of the shoreface 28 (~13-16 m) to ~21 m on the inner shelf, which is coincident with the range of depth over which 29 sand ridges and sorted bedforms migrated in response to Sandy. We hypothesize that bedform 30 migration regulates erosion over these water depths and controls the formation of a widely 31 observed transgressive ravinement; focusing erosion of older material occurs at the base of the 32 stoss (upcurrent) flank of the bedforms. Secondary storm impacts include the formation of 33 ephemeral hummocky bedforms and the deposition of a mud event layer. the New York City metropolitan area was heavily damaged, and the Long Island barrier island 46 system was both breached in places and seriously eroded . 47The impacts of this storm on the shoreface and inner shelf, which are permanently 48 submerged and therefore primarily accessible only through acoustic mapping, are harder to 49 observe. However, although the shoreface and inner shelf are neither populated nor veneered 50 with human infrastructure, they are nevertheless critical to both people and their structures, 51 because they are the first line of defense of barrier island systems against a naturally retreating, 52or "transgressing," coastline. Under rising sea level conditions, the natural condition today along 53 most of the U.S. east coast, barrier islands will back-step (retreat landward) by erosion on the 54 seaward side and deposition on the landward side (Bruun, 1962;Swift and Thorne, 1991; Thorne 55 and Swift, 1991). Large storms, with consequent high waves, strong currents and above-normal 56 tidal ranges/surges, are thought to be primary drivers of such shoreface erosion (Swift, 1968; 57 Swift and Thorne, 1991). Such storms...

Sorted bedforms off Western Long Island, New York, USA: Asymmetrical morphology and twelve‐year migration record

“…() and Goff et al . () off Fire Island. Bathymetry is derived from National Geophysical Data Center (NGDC) Coastal Relief Model ( http://www.ngdc.noaa.gov/mgg/coastal/startcrm.htm).…”

“…(A) Sustained wind speeds and (B) significant wave height from 2000 through to 2013 at National Oceanic and Atmospheric Administration (NOAA) buoy 44025 ( http://www.ndbc.noaa.gov/station_page.php?station=44025); modified after Goff et al . (). See Fig.…”

“…Previous studies (e.g. Goff, ; Goff et al ., ) have linked development of this surface with migration of major bedforms. This study therefore provides a new opportunity to investigate that relationship.…”

Persistent Shoreline Shape Induced From Offshore Geologic Framework: Effects of Shoreface Connected Ridges

“…Oceanographic and geologic investigations along Fire Island have focused on the aspects of SFCR maintenance (Warner et al, ) and SFCR effects on evolution of Fire Island coast (Kana et al, ; Lentz et al, ; List et al, ; Rosati et al, ; Schwab et al, ; Swift et al, ). Fire Island has recently been subject to increasing attention also due to the impact of Hurricane Sandy in October 2012 (Goff et al, ; Schwab et al, ; Warner et al, ) which identified the need to further understand the variations of impacts from storm events.…”

Inner-shelf ocean dynamics and seafloor morphologic changes during Hurricane Sandy

“…J.C. Warner et al Continental Shelf Research 138 (2017) 1-18 Understanding the form and magnitude of seafloor morphologic change is critical to developing coastal sediment budgets, as well as increasing our knowledge of how these processes are time-integrated into the decadal and longer timescales of continental margin sedimentation (e.g., Riggs et al, 1995;Schwab et al, 2013;Miselis and McNinch, 2006;Denny et al, 2013). Observations of impacts to the seafloor in the coastal zone (including shoreface and inner continental shelf) from hurricanes are sparse due to the logistical and technological difficulty in data collection required for measuring seafloor changes (e.g., Goff et al, 2015;Schwab et al, 2016). Observations of oceanographic and sediment transport conditions during hurricanes are also typically rare, and often observed by chance (Wren and Leonard, 2005;Teague et al, 2007).…”