|Turbulent exchange of fine sediments in tidal flow|
van der Ham, R. (1999). Turbulent exchange of fine sediments in tidal flow. Communications on Hydraulic and Geotechnical Engineering, 99-1. TU Delft. Faculty of Civil Engineering and Geosciences: Delft. xi, 167 pp.
Part of: Communications on Hydraulic and Geotechnical Engineering. Delft University of Technology. Department of Civil Engineering: Delft. ISSN 0169-6548, more
Sediments; Tidal flow; Turbulent exchange
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The morphodynamic and biodynamic behaviour of estuaries and coastal regions is of great economic and environmental interest. Accumulations of fine cohesive sediments in harbours and navigation channels may hinder navigation and form a source of potential pollution as heavy metals and pesticides are adsorbed by cohesive sediments. Deficiencies in predictive capability of cohesive sediment transport models, which are used as tools in estuarine and coastal management, arise from, among other things, the lack of knowledge of cohesive sediment properties and sediment-induced buoyancy effects on turbulence, inadequate and too short records of hydrodynamic and sedimentary parameters at too few sites.
This study addresses the role turbulence plays in erosion, deposition, and small-scale mixing of cohesive sediment in tidal flows, with emphasis on sediment-induced density stratification resulting in dampening of turbulence. The approach is to quantify the turbulence properties of water and sediment motions in relation to vertical distributions of mean velocities and sediment concentrations. To that end, in-situ high-frequency measurements were made of velocities and suspended sediment concentrations at three levels close to the bed at one location in a tidal channel during a few tidal cycles in June and August 1996.
Flux Richardson and gradient Richardson numbers were calculated for selected parts of the measuring periods. The results showed that effects of sediment-induced density stratification on the turbulence properties occurred at higher levels in the water column and during those parts of the tidal cycle when flow velocities were relatively small or concentrations were relatively high. Effects of density gradients also showed in a decrease in the streamwise integral length scale and a decrease in correlation between streamwise and vertical velocity fluctuations.
Computations with a IDV mathematical model indicated that rapid settling of suspended sediment towards slack water can be explained from buoyancy effects. The computations also indicated a limited availability of sediment from the bed during high flow velocities. Laboratory experiments carried out in a rotating annular flume with bed material from the research area showed that limited availability was a result of high bed strengths.