EROSION AND ACCRETION OF SEDIMENT AT PLUMB BEACH IN THE GATEWAY NATIONAL RECREATION AREA

The study deals with the examination of accretion and erosion process as a function of wave energy (via wind speed) and season at Plumb Beach (40.5867545 N, 73.9275318 W). The beach is located in The Gateway National Recreation Area of The National Parks Service in Brooklyn, New York. Plumb Beach is sheltered and highly impacted by human development. Erosion is a concern because of the proximity of the Belt Parkway, a major transportation route in Brooklyn. Elevation surveys were taken in the central area of Plumb Beach where erosion is most severe. The surveys were conducted on ten dates in 2009 from July 26^th to September 23^rd and along five transects. The total accretion or erosion from one survey to the next was small and showed no relationship to wind speeds on the one to two week time scale. The exception was the accretion found on august 26^th after a storm that occurred a few days before the survey. However, more noticeable changes in accretion and erosion were observed from the beginning of the study to the end. The general trend showed accretion on transects one through four and erosion at transect five. The amount of sediment accreted throughout this study was approximately 1.1 m³ per meter width of beach. The first clear change in profile shape occurred on August 26^th (after the storm), with accretion mostly accounted for in the foreshore region of the beach. This resulted in a shallower beach profile typical of higher wave energy conditions. A well developed berm was present on the following survey taken September 9^th (just two weeks after the storm profile) which is typical of a summer profile but not often seen on Plumb Beach. The storm profile was not present on the survey of September 9^th as it was quickly eroded, indicating that it was not in equilibrium with the beaches’ normal conditions. The average mean grain size found was about 0.27 mm and the slope ranged from 1:8.9 to 1:12.5. This is consistent with the average slope of protected beaches.