5-12 Sand Movement on a Human-influenced, Reflective Shoreline: Case Study from Staten Island, NY

Session: Nearshore and Estuarine Research: Dynamics and Future Resiliency in the Coastal Zone (Posters)

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Abstract:

The east coast of Staten Island is a wave-dominated, reflective shoreline that has been altered by human activity. This, along with its location in the mouth of the Hudson Estuary, makes it extremely vulnerable to inundation during storm surge events such as Superstorm Sandy. Understanding how sand moves in this environment prior to the planned construction of a buried seawall will provide a baseline for monitoring future changes to the beach and effectiveness of the coastal defenses. Twelve transects were surveyed to measure the beach morphology, and samples from each transect were collected to analyze grain size distribution. These surveys were repeated 4 times over the course of one year, followed by two further morphology surveys two years later. Results confirmed an average beach face slope of 4.5° - 6°, beach face sediment composed of medium to very coarse sand, and a ridge of sandy gravel at the low tide mark; all consistent with the typical characteristics of a wave-dominated, reflective shore. The beach face is a dynamic zone, with the greatest variation in grain size distribution from one survey to the next. The backshore has fine sand that is moderately sorted, and varies little over the course of the year. Sand is redistributed along the beach by longshore currents, and morphology surveys confirm the build-up of sand trapped by groins, widening the beach while the beach face slope remains relatively constant. Areas down current of groins and the point where the longshore currents naturally diverge are particularly vulnerable to erosion. When the artificial dunes at these locations are eroded during storms, they feed the beach with sand of a finer grain size than the natural sand on the beach, influencing the grain size of the beach face and low-tide ridge. The low tide ridge moves to reflect the changing shape of the beach, but does not appear to move with the cycle from spring to neap tides. Additionally, the beach is regularly mechanically raked during the summer, flattening the backshore area, and sometimes bringing sand from the beach face to the back shore, altering grain size distribution. This information is important for understanding the response of the beach to storm events and predicting future vulnerabilities for the communities in the adjacent low-lying areas.

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