A mesoscale sediment budget for Long Island, New York

Abstract

A sediment budget encompassing the 140-km shoreline from Montauk Point to Fire Island Inlet, New York, derived longshore transport rates for 25 coastal cells using comparative surveys. The study area along the south shore of Long Island, a generally straight coast of mainland and barrier island beaches, includes the eroding headland at Montauk Point, three jettied inlets, two groin fields, and three barrier beaches. Bridging between small-scale (quantitative) surveys and large-scale regional geology, the sediment budget provides a mesoscale view and quantitative estimates of key coastal processes in the area. Results integrate changes between the foredunes and lower foreshore to the estimated depth of closure (−9 to −12 m below mean sea level) for a 24.5-year period (1955–1979). Beach fills and losses to washovers and breach channels are incorporated in the analysis.The results confirm net westerly transport and provide the first evidence that shoaling in Fire Island Inlet (long estimated to be ~ 400,000 m3/yr) is due primarily to cannibalization of the immediate updrift compartments, particularly erosion of the foreshore below mean low water. This lowers estimates of longshore transport rates along most of the study area compared with previous estimates. The offshore source attributed by others to account for Fire Island's stability appears to be trailing ebb shoals lagging behind inlet migration. Jettied inlets in the central study area intercept some longshore transport but since stabilization (circa 1955) have been ebb dominant, preserving the littoral budget seaward of the strandline; littoral losses to the lagoons by inlets or washovers were insignificant over the period. However, breach channels, if not closed soon after storms, were found to offset years of ebb dominance in jettied inlets by depositing major flood deltas. Beach fills had measurable and positive effect on the sediment budget along the western half of the study area through the early 1970s. The volume of beach fills since 1975 has diminished to about 10% of the earlier rate. A field of 15 groins in the central study area is shown to trap 100% of the longshore transport and produce a local drift reversal. Rates of accretion within the groin cells and erosion along the downdrift compartment dwarf the average rates of change for the majority of littoral compartments in the study area.