ALONGSHORE GEOMORPHOLOGICAL EVOLUTION OF GREAT KILLS, GATEWAY NATIONAL RECREATION AREA

Sediment mobility at Great Kills, part of the Staten Island Unit of the Gateway National Recreation Area, is impacting the cultural and natural resources of the Park. Ongoing monitoring is generating a database of geomorphological response describing the vectors of change. High-resolution topography, shoreline position, and beach profile surveys are being conducted along the park. In addition, an on-site high-frequency radar unit (CODAR) provides a measure of the surface currents and the associated longshore transport in the area.

In the early 1940s, land fill and sand dredged from Great Kills Harbor were used to rebuild the barrier spit that had been eroding and eventually breached in the early 20th century. The rebuilt shoreline extended considerably seaward, filling the coastal marsh that existed there, creating the boat basin of Great Kills Harbor, and building the surrounding upland to build a bathhouse and parking lots for recreational purposes. However, the presence of groins and other shore-perpendicular structures updrift of Great Kills along with a projected headland created an obstruction to the alongshore transport. The wave diffraction downdrift of the headland accelerated the loss of sediment, causing the shoreline to erode, resulting in a cliffed coast with a logarithmic spiral form. Shoreline erosion rates varied between 2 and 16 meters per year during the period between 1951 and 1984, retreating into the older marsh habitat, and resulting in the removal of the bathhouse in 1998. Recent rates point to a persistent cliff retreat between 3 and 5 meters per year. The sediment released by the cliff erosion is being transported downdrift of the former bathhouse, and has filled the catchment area of the Crookes Point jetty. Further transport is carried downdrift, building a narrow beach on the downdrift side, and moving towards the federal navigation channel for Great Kills Harbor.

A holistic sediment management approach that includes understanding the locus of erosion and the subsequent sites of deposition at Great Kills represents a challenge to the Gateway National Recreation Area administration.