TABLE OF CONTENTS Many engineering problems, such as the calculation of diffusion rates from sewage outfalls, delineating of mixing zones, and the estimation of pollutant spreading from accidental spills are related to some aspects of river mixing processes. If a passive pollutant, e.g. dye is injected into a river, the dye patch will be carried downstream at the local mean velocity. At the same time, the patch will increase in volume and change its shape. A number of physical processes are involved in the transport and mixing of the pollutant, namely: advection, molecular and turbulent diffusion and differential advection. The molecular diffusion controls the spreading of substances in water only in laminar flows. However, when the flow is turbulent, the effect of molecular diffusion is negligible compared to turbulent diffusion. Flows in open channels and rivers are always turbulent, therefore transport by molecular mechanism is usually neglected. When velocity distributions are not uniform and possess velocity gradients in both vertical and horizontal directions, the spreading of pollutant becomes more complicated. In this case an additional phenomenon of mixing should be considered. It is called dispersion in hydraulics literature. This phenomenon occurs only in 2D and ID approaches of the mixing process analysis. Fortunately, in the three-dimensional approach there is no need to account for the process of dispersion.
One should stress a role of secondary currents in the mixing processes.
