ABSTRACT
Rock scour formation near the foundation of a dam due to a plunging jet could compromise the safety of the structure. It is therefore essential to predict the geometry of the equilibrium scour hole during the hydraulic design of the dam. Rock scour is normally predicted by analytical-empirical formulae and methods. Despite extensive research since the 1950s, presently there is no universally agreed method to accurately predict the equilibrium scour hole dimensions caused by plunging jets at dams. The main purpose of the research is to contribute to the body of knowledge in predicting the equilibrium geometry of a scour hole in bedrock downstream of a high dam caused by a fully developed rectangular jet plunging into a shallow plunge pool. Both physical and numerical modelling were used to investigate the hydrodynamic and geo-mechanical aspects of rock scour. The physical model investigated the equilibrium scour hole geometries of an open-ended jointed, movable rock bed for different discharges, dam heights, plunge pool depths, rock block sizes, and joint orientations. Novel contributions to science made by the research were the measurement of the dynamic pressures at the joint openings at the water-rock interface of a movable pool bed, while the jet was issued from a rectangular horizontal canal and not a nozzle. From the experimental results, non-dimensional formulae for the scour hole geometry were developed using multi-linear regression analysis. The experimental scour results from this study were compared to various analytical methods found in literature. The equilibrium scour hole depth established in this study best agrees with that predicted by the Critical Pressure method, followed by the Erodibility Index Method.A three-dimensional, multi-phase numerical model, in combination with the developed scour depth regression formula, was used to simulate the equilibrium scour hole geometry in an iterative manner. The proposed three-dimensional numerical model is capable of accurately simulating the scour hole depth, and to a lesser extent the scour length and width.
