Eulerian and Lagrangian coherent structures in a positive surge

In this paper, a positive surge is studied in the laboratory at Froude 1.5, using stereo time-resolved particle image velocimetry (stereo-PIV). To understand the flow dynamics, the coherence of the flow is studied using Lagrangian and Eulerian tools. It is analyzed using flow statistics calculated in a frame of reference moving with the surge front. Just after the breaker, the boundary layer thickens, and strong 3D perturbations appear just behind. The shear layer under the free surface cannot follow the breaking front, giving rise to a flow instability that develops rapidly and invades the whole flow, interacting with the boundary layer. The analysis of the coherent structures sheds new light on the flow organization. New quantities can be measured, such as coherent structures maximal penetration or velocity. Graphical Abstract