Local scour downstream of an apron for intermediate tailwater depth conditions

Local scour due to 2-D submerged horizontal jets is a well-known topic of hydraulic research. If the jet is directly injected on the erodible bed and the tailwater depth is either deep or shallow, the equilibrium state characteristics of the scoured bed profile are mainly a function of the densimetric Froude number. If the submergence lays between these two extremes, at fixed Froude number, three different scour regimes are possible. For relatively shallow tailwater depths, the jet mainstream directs towards the free surface (surface-jet scourhole regime), determining shallow and elongated scourhole profiles. For relatively large tailwater depths, the jet remains attached to the channel bottom (bed-jet scourhole regime), leading to deeper and shorter scourhole profiles. For intermediate conditions, the flicking of the jet between the erodible bed and the water free-surface is possible. When this instability occurs, the shape of the scourhole rapidly changes as a response of the jet mainstream position (bed-surface jet scourhole regime). This paper aims to give an experimental description of the three mentioned regimes, in presence of a protection apron. Scourhole profile evolutions and velocity profile measurements on the apron are briefly discussed.