Integral model of shallow mixing layers

Mixing layers that develop downstream of the confluence between two Rows of different velocity determine the lateral exchange of pollutants and sedime nt between both Rows. The shallowness of most flows in the natural environm ent restricts the development of those mixing layers in two ways. First, th e bottom friction has a stabilizing influence on the generation of large-sc ale turbulent structures in the mixing layer and in this way reduces the gr owth of the mixing layer. Second, the bottom friction leads to a decrease o f the velocity difference between both Rows, also leading to a reduced grow th. This reduced growth results in a reduced lateral exchange between both flows which has consequences for the pollution acid sedimentation of harbou rs and Rood plains and for the longitudinal dispersion in rivers. To predic t the development of mixing layers in shallow flows a simple integral model was developed in which both reduction mechanisms are incorporated. The mod el gives a fair reproduction of the downstream evolution of the width and t he transverse displacement of investigated shallow mixing layers on laborat ory scale. Simulation of river confluences yields very narrow mixing layers which corresponds to the often observed phenomenon that merging rivers har dly mix, but appear to Row alongside each other over a long reach.