Sediment transport on the Long Island shoreface, North American Atlantic Shelf: Role of waves and currents in shoreface maintenance

Abstract

Data from a 12-day deployment of a boundary layer sensing package have been used to estimate the magnitude and directions of several components of the total sediment transport at a single site on the Long Island inner shelf where the water depth is 10 m. Bedload transports were calculated by three separate methods, using the formulations of Madsen and Grant (1977, American Society of Civil Engineers, 2, 1093–1112); Vincent et al. (1981a, Marine Geology, 39, 11–80); and (for comparison) a simple Bagnold-type steady current exceedance method. Suspended coarse sediment transport was estimated from the sediment profiles of an acoustic concentration meter included in the package. Suspended fine sediment transport was computed from the surface wave energy and the steady current ( Lesht et al., 1980, Geophysical Research Letters, 2, 1049–1052). Two mild storms occurred during the deployment. A considerable onshore component was found in both the bedload and suspended coarse sediment transports (∼40° to shoreline orientation), while the near-bed current drift and fine sediment transport were more nearly shore parallel (∼20° to shoreline orientation). During the second storm event all the transport components with the exception of the Madsen and Grant(1977) bedload transport, have a slight offshore component. Within the likely accuracy of the measurements and calculations, bedload transport and suspended fine transport were the dominant components of the total sediment transport at this site on the inner shelf, while suspended sand transport rates were two orders of magnitude less. Two typical wave and current conditions are examined for the Long Island inner shelf and both are shown to produce bedload transport rates with onshore components. This mechanism may be important in the maintenance of coarser material on the shore face.