Experimental study of the scour regimes downstream of an apron for intermediate tailwater depth conditions

The local scour of a non-cohesive bed due to a 2-D submerged horizontal jet is investigated experimentally in presence of a protection apron. Previous researches conducted without protection apron demonstrate that, when 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. However, when the submergence is 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 scouring regime), determining shallow and elongated scour profiles. For relatively large tailwater depths, the jet remains attached to the channel bottom (bed jet scouring regime), leading to deeper and shorter scour 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 scour hole rapidly changes as a response of the jet mainstream position (bed-surface jet scouring regime). This paper aims to give an experimental description of the three mentioned regimes when a protection apron partly reduces the action of the flow on the loose bed. Scour hole profile evolution and velocity profile measurements obtained by LDA and ADV velocimetry are discussed.