Measurements of suspended sediment transport parameters in a tidal estuary

Results are reported on detailed measurements of suspended sediment concentration and particle size profiles, collected using a multi-frequency acoustic backscatter system in the river Taw estuary, UK, over a sandy rippled bed. 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 vertical spatial resolution of the acoustic system, profiles of the sediment concentration and sediment diffusivity have been used to obtain β the ratio of sediment diffusivity to eddy viscosity. These results show β to be independent of the height above the bed, with consistent values typically in the range β≈1–2. A new empirical expression has been formulated for β using available tidal field data. Employing the capability of acoustics to obtain unobtrusive very near-bed concentration measurements, estimates for the reference concentration based on the van Rijn (J. Hydraulic Engng 110 (11) (1984) 1613) definition have been obtained. The measurements are compared with predictions and the results show that the predictions are typically within a factor of two of the measurements. Finally using 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 these are shown to be in good agreement with the measured profile.