SUSPENDED COHESIONLESS MATERIAL IN A 2D SCOUR POTHOLE DUE TO PLUNGING JETS

The intimate details of the erosion mechanisms occurring downstream of hydraulic structures still remain elusive. Only recently the differences between dynamic and static scour hole features at equilibrium have been uncovered. Specifically, the dynamic equilibrium configuration is characterized by the jet still acting on the bed, and by a large amount of suspended (and rotating) material inside the scour cauldron. When the jet action ceases, the suspended/rotating material falls back to the bed (i.e., this is the static equilibrium configuration). In this way, a significant reduction in both maximum scour depth and scoured volume appears for the static condition. This aspect is crucial for a correct assessment of the geometric dimensions of the scour hole and, consequently, it becomes essential to obtain an estimation of the amount of sediment in suspension during the dynamic process. To the best of authors’ knowledge, no study in the open literature has analyzed such important aspect. Therefore, this paper is to the best of our knowledge the first to present a comprehensive analysis of such phenomenon for the case of 2D equilibrium scour morphologies. In this framework, special tests were conducted in a dedicated laboratory flume, under different hydraulic conditions and with granular cohesionless bed materials. For each test, the amount of suspended sediment during the jet action was quantified. Experimental tests revealed that the amount of suspended material strongly depends on the water discharge, tailwater level, jet angle, etc., and that it contributes to modify the shear stress distribution on the scour hole surface.