Vortex braids in the head wave formed as a dam breaks

The formation of flow-backing large vortex braids in a head dam-break wave running over the washed-out soil is revealed experimentally. Vortex dipoles braiding together with the captured soil are formed in the viscous near-bottom layer. Ascending and descending dipoles twist, forming a large near-bottom braid which rises to the water surface and forms a wall consisting of water and soil. The braids hold large soil particles if the flow velocity exceeds the critical value.