Horizontal mixing of quasi-uniform straight compound channel flows

The generation and evolution of large-scale vortices with vertical axis (macro-vortices) in a straight compound channel under quasi-uniform flow conditions is investigated. We discuss possible similarities and clear differences with free shear layer flows induced by the meeting of shallow streams of different speeds. An experimental investigation based on particle image velocimetry (PIV) measurements of free-surface velocities forms the basis for an analysis of both the specific features of macro-vortices and of the related mean flow characteristics. Dynamical properties strongly depend on the ratio rh between the main channel flow depth (h*_mc) and the floodplain depth (h*_fp), and three flow classes can be identified. Shallow flows (rh > 3) are dominated by strong shearing and large macro-vortices populating the transition region between the main channel and the floodplains. The mean streamwise velocity induced in intermediate flows (2 ≤ rh ≥ 3) is characterized by a dip in the transition region, while it closely resembles that occurring in a rectangular channel in the case of deep flows (rh < 2). For both the latter cases the shear in the transition region decreases and the macro-vortices are also generated in the wall boundary layer of the floodplains. The typical size of the quasi-two-dimensional macro-vortices, generated at the transition region, is found to be independent of the streamwise coordinate. This and the non-monotonic behaviour of the mean streamwise velocity suggest that in straight compound channels the topographic forcing is so dominant that conceptual models interpreting these flows as free shear layers may largely fail to describe the physics of compound channels flows.