Calculation of flow and pollutant dispersion in meandering channels

Experiments on and calculation methods for flow and pollutant spreading in meandering channels are reviewed. The shortcomings of existing calculation methods are discussed in the light of the complex three-dimensional nature of the flow situation. A mathematical model is presented which takes full account of the three-dimensionality of the flow and pollutant concentration fields. This model is based on the solution of the momentum equations governing the flow in the lateral, vertical and longitudinal directions with a three-dimensional numerical procedure together with the continuity equation. The turbulent stresses appearing in the momentum equations are calculated with a version of the k -ε turbulence model that accounts for streamline curvature effects on turbulence. The pollutant concentration field is subsequently obtained from a solution to its transport equation. The model is tested by application to three different meander situations for which velocity and concentration measurements are available from the literature, with channel width-to-depth ratios in the range 4–20, smooth and rough beds and various pollutant-discharge locations. Detailed comparisons of the velocity and concentration fields show generally good agreement. The effect of streamline curvature on the turbulent mass fluxes was found to be important only in the narrow channel with a smooth bed. Bed-generated turbulence appears to overrule this in the other two cases of a wide channel with a smooth bed and a narrow channel with a rough bed. The flow patterns show the presence of a single large eddy at most cross-sections in these cases, whereas the predictions indicate the presence of usually more than one eddy in the former case.