Measurements of the Reynolds stress components close to a marine sand bank

Observations of turbulence have been made at a height of 65 cm above the sea bed in water 42 m deep on the edge of the Skerries sand bank off southwest England. The observations extended over a 4-h period from the time of maximum flood velocity to slack water. Electromagnetic current meters were used to measure the streamwise (μ), cross-stream (υ), and vertical (ω) velocity components, while simultaneously a conventional current meter measured the speed and direction 5 m below the water surface. Ratios of (μ2)12, (υ2)12, and (ω2)12 to shear velocity agreed with accepted values for laboratory and atmospheric flows. Near to the surface the current was found to flow across the bank while near the bed it ran nearly parallel to the isobaths. Consequently the bottom current was directed 48° clockwise of the surface current. The azimuth of the Reynolds stress (ϱμω, ϱυω) near the bed lay between these directions, and was directed about 6° anticlockwise of the near-bed current. This balanced the cross-stream as well as the streamwise shear. A lateral shear stress ϱμυ associated with the bank was observed. Taken together with previous measurements of the velocity distribution over the bank, this yielded a value for lateral eddy viscosity of 1.2·105 cm2 s−1. The scale over which this acts is of the order 1 km and the value is appreciably smaller than reported values deduced for larger scales. The veering of the current acts to lessen the bottom current convergence associated with Coriolis and curvature effects, and may contribute to lateral sorting by preferentially carrying finer sediment toward the crest of the bank. The lateral eddy viscosity coefficient may be useful in assessing the diffusion of sediment on or off the bank.