Author(s): Helder Guta; David Hurther; Julien Chauchat

Linked Author(s): Helder Guta, David Hurther, Julien Chauchat

Keywords: Sediment transport; Turbulent mixing; Bed-load effects; ADVP; ACVP

Abstract: A process-based analysis of a new experimental dataset is carried out based on co-located 2C velocity, sediment concentration and sediment flux profile measurements obtained with an Acoustic Concentration and Velocity Profiler (ACVP). The experiments were carried out in energetic open-channel flows using the LEGI tilting flume, with light-weight PMMA of two diameters dp=3mm (covering the range suspension numbers, given as the ratio of settling velocity ws and friction velocity Uf 0.8≤ws/Uf≤1.3 and Shields number 0.35≤θ≤0.85,) and dp=1mm (0.4≤ws/Ut≤0.6 and 0.4<θ<1). Four sediment-load conditions are studied for a given hydraulic flow varying from clear water, 2 non-capacity to the full transport capacity conditions. Three hydraulic flow conditions are investigated for each particle diameter. All sediment-laden runs have one clear-water run as reference, acquired before each sediment-laden run. This allows the analysis of the particle effects on a wide variety of hydrodynamic quantities. With the larger particle experiments (dp=3mm), it is shown that the bed-load layer has important impact on the turbulent boundary layer. The log-layer is found to prevail only from around the edge of the bed-load layer upwards. Inside this layer, turbulent momentum mixing is highly reduced, compared to the corresponding clear-water flow. It is seen that this change in flow structure should be taken into account in the turbulent mixing length model, affecting both the law of the wall and the Rouse formulation, for modelling velocity and concentration distribution, respectively. As result, a modified analytical solution that includes the bed-load effects for the concentration profile was derived. Concerning the experiments with dp=1mm, less pronounced effects of turbulent mixing length and eddy viscosity were observed. A parametrization of the depth-averaged ratio of sediment and momentum diffusivity β based on the present experimental results was proposed, allowing to extend the van Rijn (1984) model to a wider range of suspension numbers (0.1≤ ws/Ut ≤1.5). Finally, the differences in the flow structure in the sediment-laden with the two particle diameter is further illustrated through some terms in balance of turbulent kinetic energy and by coherent turbulent structures (specifically ejections and sweeps).

DOI: https://doi.org/10.3850/IAHR-39WC252171192022784

Year: 2022