LOCAL SCOUR DOWNSTREAM OF GRADE CONTROL STRUCTURES IN URBAN STREAM RESTORATION

摘要

The local scour downstream of a grade control structure with a rigid apron was experimentally investigated in the hydraulics laboratory of the University RomaTRE. The evolution of the scouring process was recorded by a CCD camera and the profiles of the scour hole were obtained using image analysis techniques. The experiments show that the temporal evolution of the maximum scour depth is very rapid during the first minutes of the run and then decreases with increasing time. Velocity profiles, both the horizontal and the vertical components, were measured in the centre of the flume when the equilibrium scour depth was reached. The velocity measurements show the presence of a wall jet, which moves downstream along the rigid apron, and is related to the submerged hydraulic jump. Such a wall jet is responsible for the initial rapid growth of the scour. As soon as the scour increases, a transformation of the wall jet into a free jet occurs and the scouring process tends to attain an equilibrium state, because the scouring effectiveness diminishes. A formula obtained by interpolating the measured maximum scour depths is proposed. Such a formula indicates that the maximum scour increases with the discharge and decreases with the downstream tailwater depth. As a concluding remark it can be stated that the present work has showed the main phenomenological characteristics of the local scour process. In particular, it highlighted that the presence of the rigid apron influences the maximum depth of the scour as it reduces the magnitude of the velocity corresponding to the mobile bed. The length of the apron is then one of the most important parameters for designing the structure, when the need to reduce the scour downstream highly important. Further development of this work could consist in the development of a numerical model for simulating the temporal evolution of the scour hole or investigations of the rigid apron's length as an experimental parameter, in order to derive quantitative relations between the apron length and the maximum scour depth.