Physical Modelling Investigation of Rock Scour Extent Due to a Plunging Jet for Typical High Head Dams
Thesis (MEng)--Stellenbosch University, 2016.
ENGLISH ABSTRACT: Hydraulic structures can release high-velocity plunging jets, which can result in scouring of the rock downstream of the structure. The extent of the scour hole must be predicted and analysed to ensure the safety of hydraulic structures. Many researches had developed empirical or semi-empirical formulae based on physical or prototype assessment. These formulae have limitations in estimating the scour hole shape, i.e. determining the pressure propagation in fissure or joints of rock and the velocity of the jet as it travels through the plunge pool. A 1:40 scaled physical hydraulic model was constructed to determine the scour hole geometry in rock material. The main objective of the study was to determine the scour hole geometry formed in rock as a result of to an impinging jet by using a physical model and analysing the factors that cause scouring. The tests were conducted in two parts, namely the assessment of the scour hole geometry and secondly the measurement of the pressures inside the rock joints and the air concentration in the plunge pool. In this research, the Erodibility Index developed by Annandale(1995) was used to quantify the relative ability of rock to resist the scour capacity of water. The Comprehensive Scour Model developed by Bollaert (2002; 2010; 2012) was also used to evaluate the ultimate scour depth. The physical model data was obtained in the laboratory at Stellenbosch University, South Africa, and were compared to the predicted results based on scour prediction methods found in the literature, namely the Comprehensive Scour model (Dynamic Impulsion and Quasi Steady Impulsion methods) and the Erodibility Index method. There was a reasonable difference between the physical model results and the prediction method results. The differences could be readily attributed to limitations of the laboratory (available pumping capacity, physical model scale and the density of the PVC blocks used for replicating rock blocks was lower than that of rock).
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