Local scour by submerged turbulent jets

The present study investigates with laboratory experiments the scour downstream of a sill, followed by a rigid apron, in clear water scour conditions. Nine experiments were run, changing the discharge and the tailwater depth of the flow. The temporal evolution of the scour was recorded by a video camera and measured using image analysis techniques. The velocity field was measured by an ultrasonic velocity profiler, both over the rigid apron and in the scour hole. The dimensionless scour profiles showed geometrical similarity for all the runs, if the maximum scour depth, Dsmax was used as length scale of the vertical and horizontal distances, respectively. The mean velocity profiles, measured over the rigid apron, were similar to a wall jet modified by the existence of a reverse flow close to the water surface. The mean velocity profiles, measured along the scour hole, were similar to an asymmetrical free jet, using as scales the maximum mean velocity, Um, and the distances from the plane of Um to the planes where the velocity was Um/2.