Turbulent mixing layers associated with streamwise uniform and nonuniform flows in compound channels (main channel with adjacent floodplains) are experimentally investigated. The experiments start with uniform flow conditions. The streamwise nonuniformity is then generated by imposing an imbalance in the upstream discharge distribution between main channel (MC) and floodplains (FPs), keeping the total discharge constant, which results in a transverse depth‐averaged mean flow. This study first aims at assessing the effect of a transverse flow on the mixing layer and coherent structures that form at the MC/FP interfaces. A wide range of initial velocity ratio or dimensionless shear between MC and FP is tested. The study second aims at assessing the effect of this velocity ratio on the mixing layer, for a fixed vertical confinement of flow. The total discharge was then varied to quantify the confinement effect. The results show that, far from the inlet section, Reynolds‐stresses increase with local velocity ratio for a fixed confinement and decrease with confinement for a fixed velocity ratio. It is also shown that, irrespective of confinement, the existence of quasi‐two‐dimensional coherent structures is driven by velocity ratio and the direction and magnitude of transverse flow. These structures cannot develop if velocity ratio is lower than 0.3 and if a strong transverse flow toward the MC occurs. In the latter case, the transverse flow is the predominant contribution to momentum exchange (compared with turbulent mixing and secondary currents), convex mean velocity profiles are observed, preventing the formation of quasi‐two‐dimensional structures.

The reported experimental study assesses the effects of flow non-uniformity on the momentum flux in straight compound channels. Two flumes were used, featuring vertical and sloping banks. Starting with uniform flow condition, various imbalances in the upstream discharge distribution were introduced. This resulted in a time-averaged lateral flow and advective transport of momentum, which interacted with the shear-layer turbulence generated by the compound geometry. To investigate this interaction, the three contributions to transverse momentum flux (depth-averaged flow, shear-layer turbulence and dispersive term of spanwise velocity) are assessed. The first two contributions were strengthened by the sloping banks, while the third becomes important for the case of the vertical bank. With a lateral flow towards the main channel, the first contribution rises at the expense of the second. With a lateral flow towards the floodplain, the first two contributions have the same order of magnitude, and the Boussinesq approach is invalidated.

No abstract

7The effect of the shallowness on meandering jets in a shallow rectangular reservoir is investigated. Four 22The analysis of the stability regime finally reveals that the sinuous mode is convectively unstable and may 23 become absolutely unstable at the end of the reservoir when the friction number is small. 24 25

The present study deals with turbulent flows in an asymmetrical compound channel with an embankment set on the floodplain, perpendicularly to the longitudinal direction. The main purpose of this study was to assess how a rapidly varied flow affects interaction between the floodplain flow and the main channel flow. In addition to rapid changes in the water level and velocity across the compound channel that have a great influence on the boundary shear stress distribution, the embankment, through two recirculation zones developing upstream and downstream, is also responsible for strong lateral mass exchange between the main channel and the floodplains (channel sub-sections). The lateral velocity can indeed reach 50% of the longitudinal velocity, which modifies the characteristics of the mixing layer developing between the channel sub-sections. Depth-averaged Reynolds shear stresses five times greater than those measured for reference