Dynamics of shallow lateral shear layers: Experimental study in a river with a sandy bed

Shallow lateral shear layers forming between flows with different velocities, though essential for mixing processes in natural streams, have been examined only in laboratory settings using smooth, fixed?bed channels. This paper reports the results of an experimental study of a shear layer in a straight reach of a natural river where the layer, in contrast to the two?dimensional structure observed in the laboratory, is highly three?dimensional. The study included pronounced transverse pressure gradients, which influenced shear layer structure compared to flume experiments. It also introduces an analysis that complements conventional theory on mixing layers. The lateral velocity gradient between the flows downstream from a splitter plate placed in the river, the principal controlling factor, was adjusted for three experimental runs to determine the influence of different gradients on shear?layer dynamics. In each run, detailed three?dimensional measurements of mean and turbulent characteristics were obtained at five cross sections downstream from the splitter plate. Although experimental results agreed with conventional mixing?layer theories with respect to turbulence, the dynamics of the shear layers were dominated by the mean lateral fluxes of momentum. After re?examining the governing equations, we developed a parabolic equation describing the shear layer evolution and several scaling relations for essential terms of the energy budget: mean and turbulent lateral fluxes of momentum, turbulent kinetic energy, and dissipation rates. The study also provides insight into the spectral dynamics of turbulence in the shear layer and clarifies previous observations reported for confluences in natural streams.Hydraulic EngineeringCivil Engineering and Geoscience