3-D computation of sediment transport at water intakes

The three-dimensional numerical computational fluid dynamics computer program SSIIM was used to predict the flow field and the sediment transport at the Kapunga water intake in Tanzania. It solved the Reynolds-averaged Navier-Stokes equations in three dimensions to compute the water flow and used the finite-volume method as the discretisation scheme. The model was based on a three-dimensional, non-orthogonal, structured grid with a non-staggered variable placement. The k-ε model was used to predict the turbulence and the SIMPLE method to compute the pressure. The suspended sediment transport was calculated by solving the convection-diffusion equation with the bed boundary condition provided by an empirical formula for the reference concentration. The results from the numerical model were verified using observed performance ratios at the water intake. These ratios were calculated from the sediment concentrations in the river upstream of the intake and passing into the water intake. The computed performance ratios corresponded well with those derived from field measurements. Sensitivity tests showed that the use of a fine grid in combination with a second-order upstream scheme gave the best results. Varying the bed roughness showed that the results were not unduly sensitive to the method used to determine that parameter. The results from the SSIIM model and the field measurements were also compared to the results of an earlier numerical model study in which the commercial computational fluid dynamics program PHOENICS had been used.