The self-sustained turbulent shear or mixing layer that develops at the interface between a channel and a lateral cavity is the leading mechanism that drives the transfer of momentum and mass in these open-channel flows. Therefore, quantifying the interactions between large-scale vortical structures and the enhanced velocity fluctuations at the interface is critical to understand the physical processes which control the exchanges between the cavity and the main channel. In this investigation, we carry out hydrodynamic experiments in a straight, rectangular channel with a lateral square cavity. We measure the velocity field in a horizontal plane using particle image velocimetry to study the dynamics and statistics of the mixing layer, including the effects of the adverse pressure gradient at the downstream corner. By combining proper-orthogonal decomposition with a vortex identification technique, we investigate the motion of coherent structures and calculate the histograms of their trajectories, capturing also additional phenomena such as the vortex splitting, and the interaction of the mixing layer with inner vortices formed inside the cavity. We finally quantify the mass transport capacity of the mixing layer, from the statistics of the transverse velocity at the interface.

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