Author(s): Robert Booij; Jan Tukker
Abstract: Mixing layers that develop downstream of the confluence between two flows of different velocity determine the lateral exchange of pollutants and sediment between both flows. The shallowness of most flows in the natural environment restricts the development of those mixing layers in two ways. First, the bottom friction has a stabilizing influence on the generation of large-scale turbulent structures in the mixing layer and in this way reduces the growth of the mixing layer. Second, the bottom friction leads to a decrease of the velocity difference between both flows, also leading to a reduced growth. This reduced growth results in a reduced lateral exchange between both flows which has consequences for the pollution and sedimentation of harbours and flood plains and for the longitudinal dispersion in rivers. To predict the development of mixing layers in shallow flows a simple integral model was developed in which both reduction mechanisms are incorporated. The model gives a fair reproduction of the downstream evolution of the width and the transverse displacement of investigated shallow mixing layers on laboratory scale. Simulation of river confluences yields very narrow mixing layers which corresponds to the often observed phenomenon that merging rivers hardly mix, but appear to flow alongside each other over a long reach.