Sediment Transport in the New York Bight, North American Atlantic Shelf

Abstract

Vincent, C.E., Swift, D.J.P. and Hillard, B., 1981. Sediment transport in the New York Bight, North American Atlantic Shelf. Mar. Geol., 42: 369-398. Aanderaa current-meter data collected during the course of NOAA's New York Bight Project are sufficiently extensive in time (135000 observation hours) and space (41 stations) to permit an estimate of the sediment transport in this area. The analysis is based on mean flow observations; the role of wave oscillatory currents in resuspending sediment is considered but not explicitly included in the sediment transport calculations. An estimate of the potential sediment transport rate Q B at each station for three different threshold velocities U TH (12, 16, and 20 cm s -1 ) were made using a Bagnoldtype transport equation: Q B = const ( U c - U TH ) 3 for U c > U TH Q B = 0 for U c < U TH where U c is the mean current speed. Comparison was made between the results from this simple equation and other bedload and suspended-load transport rates (which include wave data) for a limited subset of the Bight data. This comparison indicates that the Bagnold results are reasonable for the along-isobath transport estimates, but are not reliable for the cross-isobath transport. In the cross-isobath direction, waves appear to play a dominant role in determining the transport rate. The comparison also indicates that the Bagnold approach tends to overestimate transport in deep water. In shallow water, resuspension of sand and rotor pumping of current meters both may become important, and sediment transport calculations may not be accurate. Fine-sediment suspended transport is approximately double the potential bedload transport for fine sand (0.02 cm), but does not dominate the total transport as is believed to occur on the Washington Shelf (Smith and Hopkins, 1972). The data show that the New York Bight is a storm-dominated shelf, with the general southwestward drift of water (∼6 cm s -1 ) modulated by more intense storm-induced flows. The transport calculations indicate that the bulk of sediment transport occurs during the winter months when wind stress is at a maximum and the water column is well mixed. Two major transport provinces are present; (a) the Hudson Shelf Valley, and (b) the rest of the shelf. The Shelf Valley acts as a sink and a barrier to the continuity of the mainly southwestward transport of sediment (∼10 4 g cm -1 yr -1 ) along the shelf. Within the valley the potential transport of material is of the order of l0 5 g cm-1 yr-1 along the valley axis. Progressive vector transport paths for fine sediment indicate that removal of sediment out of the ends of the valley can occur rapidly. However, consider-ation of the finesediment budget for the whole Hudson Shelf Valley and of the attenua-tion of wave activity with depth show that these results are not inconsistent with the presence of high mud concentrations in the valley. Up-valley sediment rates predominate during the period for which data are available.