No 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...

No abstract

Several generations of the ancestral Pee Dee River system have been mapped beneath the South Carolina Grand Strand coastline and adjacent Long Bay inner shelf. Deep boreholes onshore and high-resolution seismic-refl ection data offshore allow for reconstruction of these paleochannels, which formed during glacial lowstands, when the Pee Dee River system incised subaerially exposed coastal-plain and continental-shelf strata. Paleochannel groups, representing different generations of the system, decrease in age to the southwest, where the modern Pee Dee River merges with several coastal-plain tributaries at Winyah Bay, the southern terminus of Long Bay. Positions of the successive generational groups record a regional, southwestward migration of the river system that may have initiated during the late Pliocene. The migration was primarily driven by barrier-island deposition, resulting from the interaction of fl uvial and shoreline processes during eustatic highstands. Structurally driven, subsurface paleotopography associated with the Mid-Carolina Platform High has also indirectly assisted in forcing this migration. These results provide a better understanding of the evolution of the region and help explain the lack of mobile sediment on the Long Bay inner shelf. Migration of the river system caused a profound change in sediment supply during the late Pleistocene. The abundant fl uvial source that once fed sandrich barrier islands was cut off and replaced with a limited source, supplied by erosion and reworking of former coastal deposits exposed at the shore and on the inner shelf.