Scour development downstream of aprons with and without end-sill

This paper focuses on the local bed scour downstream of low-head spillways with stilling basins. Experiments were carried out in a 1 m wide rectangular channel. Free overfall jets from an ogee-crested spillway plunged into a positive-step stilling basin or a plain horizontal apron. Four nearly uniform bed sediments were used. All the experiments were performed under steady approach flow conditions and runs lasted three days on average to achieve a quasi-equilibrium state and well developed scour pits. Tail water depths were set to minimize the combination of local and general scour phenomena. Based on the experimental data, salient features on the spatial and temporal evolution of the local bed morphology are provided highlighting the effects of the end-sill. It was found that the main parameters governing the scouring process are the tailwater densimetric Froude number, the relative tailwater depth, and a dimensionless time. The temporal variation of the scour hole geometry can be estimated by using simple power-type equations that relate the scour geometry to the controlling scour parameters. Polynomial-type equations are also given for the temporal axial scour profiles.