Effects of non-uniform flow in the mixing layer in compound channels

Abstract

The alluvial valleys of rivers usually have a compound channel cross section during flood events. This configuration leads to much more complex flow mechanisms when compared to a single channel flow namely with the existence of a mixing layer and large-scale coherent structures in the interface between the main channel and the floodplain (i.e. subsections). Uniform flow in compound channels has been extensively studied (e.g. Shiono and Knight 1991). Due to the non-prismatic nature of rivers, non-uniform flow is the most common situation such as converging or diverging floodplains or meandering channels. Non-uniform flows can also be observed in prismatic geometries with unbalanced inflow conditions. This last configuration is studied in the present paper. The influence of the non-uniformity of the flow in a straight compound channel is carefully analysed in an experimental study carried out for different relative depths and subsection discharges. For each flow case, the longitudinal profile of water depth and the longitudinal development of the flow mechanisms (e.g. shear layer, turbulence intensities) are characterized.