Rough Boundary Layers Research Articles

Estimates of Sand Transport on the New York Shelf Using Near-bottom Current Meter Observations

ABSTRACT 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 meterological 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.

Surface Roughness and Internal Boundary Layer Near a Coastline

Abstract Wind profile observations obtained at two sites near the upper Texas coast during onshore winds in June 1968 are used to determine local toughness parameters. Values of 3–4 cm are obtained at a 32 m tower site 90 m inland and 0.81–1.5 cm at 4.8 km inland; both are in close agreement with results of others for similar terrain. The portion of the wind profile obtained at heights between 6.7 and 27 m near the beach is extrapolated downward to obtain a roughness between 0,0001 and 0.0003 cm for the Gulf of Mexico. An internal boundary layer was detected from the data obtained 90 m inland. The mean height was observed to he 6.7 m— a value associated with a mean slope of about 1:13 for an internal boundary originating at the upwind shoreline roughness discontinuity. Daytime values for the internal boundary height are somewhat higher than the mean, averaging 7.2 m, while the nighttime mean is 5.9 m. The data also suggest a wind speed dependence on the height of the internal boundary layer.