Velocity and turbulence measurements in a scour hole at different scouring stages

Velocity and turbulence measurements downstream of a sill followed by a rigid apron were performed. Three runs were conducted at different scouring stages: flat bed, 50% of the maximum scour depth and 100% of the maximum scour depth. For all the scouring stages, the vertical distribution of the horizontal velocity compo-nent, measured over the rigid apron, was found to be similar to a wall jet. When the bed is flat, the wall jet ve-locity distribution changes into a free surface velocity distribution, as the distance from the apron increases. When a scour hole develops downstream of the apron, the wall jet velocity distribution changes into an “asymmetric free jet” velocity distribution, till the abscissa of the maximum scour depth. Turbulence intensi-ties, along both the vertical and the horizontal axis, were found to decrease as the distance from the sill in-creased, for all the scouring stages. A similar behavior was observed for dimensionless Reynolds stresses over a flat bed. When a scour hole develops downstream of the apron, dimensionless Reynolds stresses show posi-tive values close to the bottom both inside the scour hole and downstream of the dune.