Measurements of suspended sediment transport parameters in a tidal estuary

Results are reported on detailed measurements of suspended sediment concent ration and particle size profiles, collected using a multi-frequency acoust ic backscatter system in the river Taw estuary, UK, over a sandy rippled be d. The acoustic data were augmented with electromagnetic and rotary current meter measurements of the flow and in situ pumped samples of the suspended sediments. Combining the conventional flow and pumped sampled measurements with the high-resolution multi-frequency acoustic data, an examination of sediment diffusivity profiles, reference concentrations and mean suspended sediment concentration profiles has been conducted. Utilising the high vert ical spatial resolution of the acoustic system, profiles of the sediment co ncentration and sediment diffusivity have been used to obtain beta the rati o of sediment diffusivity to eddy viscosity. These results show beta to be independent of the height above the bed, with consistent values typically i n the range beta t approximate to 1-2. A new empirical expression has been formulated for fl using available tidal field data. Employing the capabilit y of acoustics to obtain unobtrusive very near-bed concentration measuremen ts, estimates for the reference concentration based on the van Rijn (J. Hyd raulic Engng 110 (11) (1984) 1613) definition have been obtained. The measu rements are compared with predictions and the results show that the predict ions are typically within a factor of two of the measurements. Finally usin g the sediment diffusivity results, a two-layer representation for the mean suspended sediment concentration profile is suggested. This representation has been used to generate suspended sediment concentration profiles and th ese are shown to be in good agreement with the measured profile. (C) 2001 E lsevier Science Ltd. All rights reserved.