Calculations of cohesionless bottom-sediment movement within the New York Bight have been made by applying the transport formulae of Bagnold (1941, 1956, 1963), Einstein (1950), and Yalin (1963) to near-bottom current meter and surficial sand size observations. Current data were drawn from the records of eighteen long-term Savonius rotor current meter deployments at various locations within the Bight during Fall of 1973 and Spring of 1974.
The assumptions underlying the calculations are that wave activity was minimal at recording sites, that a drag coefficient of 3 × 10−3 reflecting small scale roughness and large boundary layer Reynolds number is suitable to convert measured currents to friction velocities, and that laboratory threshold velocities apply in the marine environment.
The calculations suggest that oceanic bottom sediment movement reaches maximum intensity during the fall and winter due to the added energy input from strong meteorological events. Calculated transport quantities on the inner shelf tend to decrease as depth and distance from shore increase. However, during the Fall of 1973, the deep waters near the head of the Hudson Shelf Valley exhibited current flows directed to the north in excess of 40 cm/sec. These up-channel flow events appear to be in response to strong, sustained westerly winds. The maximum sediment transport rate caused by these current velocities is two orders of magnitude greater than that occurring at much shallower depths along the New Jersey coast during the same measurement period. The Spring 1974 current velocity field yields transport rates of lesser magnitudes relative to the previous fall, with a net down-channel sediment flux at the head of the Hudson Shelf Valley. The New York Harbor entrance appears to be the site of near-continual sediment transport generated by swift tidal currents. |
