The effects of suspended sediment on turbulence within an estuarine turbidity maximum

Predictions of cohesive sediment transport are hampered by poor understanding of the effects of sediment suspensions upon the nature of near-bed turbulence. In this study, current velocities, associated turbulence and flow characteristics were measured within an estuarine turbidity maximum during two spring ebb tides. Measurements at two heights, both within and above a near-bed high concentration layer, show the importance of intermittent internal waves. The ratio of vertical to horizontal turbulence intensities exceeded 1.0 when waves were present. The waves were more significant above the layer than within it. Comparison between the turbulent kinetic energy and the Reynolds stresses showed 'inactive turbulence' occurring at stresses less than about 0.2 N m(-2). During these periods negative suspended sediment fluxes occurred, indicating that the turbulence could enhance settling of the sediment. The effect of the intermittent internal waves on the turbulent bursting structure, and their interaction with the concentration profile appears to increase the variability of the Reynolds fluxes of suspended sediment and cause downwards Reynolds fluxes, especially beneath the lutocline. (C) 2003 Elsevier Ltd. All rights reserved.