Doctoral Degrees (Civil Engineering)
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Browsing Doctoral Degrees (Civil Engineering) by Author "Bosman, Adele"
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- ItemHigh dam scour hole geometry prediction for fully developed jets plunging into shallow pools on bedrock(Stellenbosch : Stellenbosch University, 2021-03) Bosman, Adele; Basson, G. R.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: The safety of dams is often endangered by rock scour formation near the dam’s foundation due to a high-velocity free falling jet from the spillway plunging into the plunge pool. The prediction of the scour hole geometry (ultimate depth and maximum extent) is essential in the hydraulic design of high head dams to ensure the stability of the dam. Rock scour is a complex physical phenomenon that is normally predicted by analytical-empirical formulae and methods, which combine laboratory and prototype observations. The empirical formulae however, cannot describe all the hydrodynamic and geo-mechanic effects involved in rock scour. 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 knowledge on the prediction of the equilibrium scour hole geometry downstream of a high head dam. The rock scour process was investigated via physical and numerical models, as well as by evaluation of scour prediction methods from literature. The physical model investigated the hydrodynamic effects of a plunging jet and the subsequent scour of a rectangular, open-ended jointed, movable rock bed. Equilibrium scour hole geometries for various fissured dimensions (simulated with tightly hand-packed rectangular concrete blocks), for a range of flow rates, dam heights, plunge pool depths, rock sizes, and joint structure orientation scenarios were experimentally established with 31 model tests. The experimental results indicated that greater scour occurs if the deposited rocks downstream of the scour hole are removed by floodwaters. For the first time the transient pressures at the joint opening due to a high-velocity plunging jet were measured for a movable bed with a complex joint structure. Additionally, the research studied the scour hole geometry formed by a low-frequency turbulent jet issuing from a rectangular horizontal canal and not discharging from a nozzle. The dynamic pressures at the water-rock interface and the velocity distribution of the wall jet in the plunge pool were recorded and evaluated. Non-dimensional formulae were developed using ordinary least squares regression analysis on the physical model results. The experimental scour results from this study were compared to various analytical methods found in literature. The scour prediction methods yielded a wide range of scour depths for the same input conditions. No single analytical method is superior, but 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 and Mason and Arumugam’s empirical formula. A three-dimensional, multi-phase, transient numerical model, in combination with the developed scour depth regression formula, was used to simulate the equilibrium scour hole geometry. The numerical model was calibrated against the physical model results. The numerical simulation results were satisfactory and representative of the model pressures in the open-ended joints. The proposed three-dimensional numerical model, in conjunction with the scour depth regression formula developed in this study, is capable of simulating the scour hole geometry, which includes both hydrodynamic and geo-mechanical rock scour aspects, if the diffusion of the jet through the air and plunge pool is modelled accurately. Further research is required to improve the numerical simulations to automatically calculate the deformation of a movable bed using the approach developed by this research.