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Behavior of coastal systems on time scales ranging from single storm events to years and decades is controlled by both small-scale sediment transport processes and large-scale geologic, oceanographic, and morphologic processes. Improved understanding of coastal behavior at multiple time scales is required for refining models that predict potential erosion hazards and for coastal management planning and decision-making. Here we investigate the primary controls on shoreline response along a geologically-variable barrier island on time scales resolving extreme storms and decadal variations over a period of nearly one century. An empirical orthogonal function analysis is applied to a time series of shoreline positions at Fire Island, NY to identify patterns of shoreline variance along the length of the island. We establish that there are separable patterns of shoreline behavior that represent response to oceanographic forcing as well as patterns that are not explained by this forcing. The dominant shoreline behavior occurs over large length scales in the form of alternating episodes of shoreline retreat and advance, presumably in response to storms cycles. Two secondary responses include long-term response that is correlated to known geologic variations of the island and the other reflects geomorphic patterns with medium length scale. Our study also includes the response to Hurricane Sandy and a period of post-storm recovery. It was expected that the impacts from Hurricane Sandy would disrupt long-term trends and spatial patterns. We found that the response to Sandy at Fire Island is not notable or distinguishable from several other large storms of the prior decade. [ABSTRACT FROM AUTHOR]

On October 29, 2012, storm surge and large waves produced by Hurricane Sandy resulted in the formation of a breach in eastern Fire Island, NY. The goals of this study are to gain a better understanding of the physical processes that govern breach behavior and to assess whether process-based models can be used to forecast the evolution of future breaches. The wilderness breach grew rapidly in size during the first winter following formation. Growth of the breach was accompanied by the formation of a complex of flood shoals inside Great South Bay, a primary channel that flowed through the eastern part of the flood shoals, and an ebb shoal on the ocean side of the breach. From the summer of 2013 through late 2015, the breach continued to change and evolve, albeit at a much slower pace than in the first year after formation. A hybrid combination of Delft3D and XBeach models is used to hindcast the morphodynamic evolution of the wilderness breach over the first three years after formation. The formation of the breach during Hurricane Sandy is not part of the simulations. Model simulations are initiated with a post-storm topography in which the breach is already present. The models are capable of hindcasting the main morphodynamic changes of the wilderness breach. The spatial patterns, as well as the bulk statistics, such as breach geometry and sediment volume changes, are reasonably well reproduced by the model. The model sheds light on previously unknown processes of breach evolution, especially regarding sediment transport and flow regimes within the breach complex. • A hybrid combination of Delft3D and XBeach models hindcasts the morphodynamic evolution of a barrier island breach. • Model results are validated against a wide range of field observations. • The models shed light on previously unknown processes of breach evolution regarding sediment transport and flow regimes. [ABSTRACT FROM AUTHOR]

The inner-continental shelf off Fire Island, New York was mapped in 2011 using interferometric sonar and high-resolution chirp seismic-reflection systems. The area mapped is approximately 50 km long by 8 km wide, extending from Moriches Inlet to Fire Island Inlet in water depths ranging from 8 to 32 m. The morphology of this inner-continental shelf region and modern sediment distribution patterns are determined by erosion of Pleistocene glaciofluvial sediments during the ongoing Holocene marine transgression; much of the shelf is thus an actively forming ravinement surface. Remnants of a Pleistocene outwash lobe define a submerged headland offshore of central Fire Island. East of the submerged headland, relatively older Pleistocene outwash is exposed over much of the inner-continental shelf and covered by asymmetric, sorted bedforms interpreted to indicate erosion and westward transport of reworked sediment. Erosion of the eastern flank of the submerged Pleistocene headland over the last ~ 8000 years yielded an abundance of modern sand that was transported westward and reworked into a field of shoreface-attached ridges offshore of western Fire Island. West of the submerged headland, erosion of Pleistocene outwash continues in troughs between the sand ridges, resulting in modification of the lower shoreface. Comparison of the modern sand ridge morphology with the morphology of the underlying ravinement surface suggests that the sand ridges have moved a minimum of ~ 1000 m westward since formation. Comparison of modern sediment thickness mapped in 1996-1997 and 2011 allows speculation that the nearshore/shoreface sedimentary deposit has gained sediment at the expense of deflation of the sand ridges. [ABSTRACT FROM AUTHOR]