Browsing by Author "Saayman, Ruben Lourens"

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    Energy dissipation at uncontrolled high dam spillways by flare
    (Stellenbosch : Stellenbosch University, 2020-03) Saayman, Ruben Lourens; Basson, G. R.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: Spillways have been used for approximately 3500 years to discharge water from dams. A growing world population has made the construction of dams a matter of critical importance. Several different types of spillways are used on different dams. Ogee spillways attached to stepped spillways are one of the oldest spillway types and with the advancement of new technologies and building techniques, such as Roller Compacted Concrete, the popularity of this spillway type has increased significantly. However, cavitation damage poses a large threat to a spillway and therefore, the safe discharge capacity of spillways has been limited to prevent cavitation damage to the structure itself. It is estimated that future climate change impacts on extreme flood runoff could increase flood peaks by 50% to even 100% by the end of this century in some parts of the world. Dam spillways will therefore have to deal with larger unit discharges. The prevention of cavitation is possible by increasing the air concentration to at least 8% in the flow at the pseudo-bottom of a spillway. Flaring Gate Piers (FGP) were developed in China as an aeration structure. To ensure aeration of the flow, flares are installed on the downstream end of a standard Waterways Experiment Station (WES) spillway. The flares are located upstream of the steps on a smooth ogee. All the flares tested in this study was fitted to a 1:50 model scale of the Dachaoshan dam spillway in China. Research on the effect of the FGP’s has been previously conducted by (Koen, et al., 2019). This research focuses on the removal of the gate piers to determine the effect of cavitation of the flares and the effects of the water jets that form over the flares. The two different types of FGP’s that are investigated are the X-Shape and the Y-Shape FGP. The effect and efficiency of the flares are investigated by taking pressure readings on the flares to identify areas of cavitation for different discharges. The flare type with the least or no areas of cavitation, along with the flare that aerates the flow the most significantly to prevent cavitation, is selected as the best flare type for this study. The water jets caused by the high discharge over the flare were also investigated to determine their effect downstream of the flares. After all 6 flares were tested for cavitation and the water jets were analysed, it was possible to determine which flare type design preformed the best. No pressures severe enough to cause cavitation damage was measured on any of the flares tested in this study. This means that all the flares are save from cavitation damage up to a unit discharge of 200 m2/s. The effects of the water jets were analysed to determine which flare type design preformed the best in this study. Y-shaped flare 3 preformed the best of all the flares tested during this study. This flare design projected the water further and higher than any of the other flare designs. Projecting the water further can lead to the jets being projected into tail waters downstream of the spillway. Deep tail waters can dissipate all the energy of the jets before it causes damage to the riverbed. The higher projected jets collide in the air with one another and dissipate fractions of the energy of the jets. The remaining energy of the jets are further dissipated by the resistance of the air on the water. Further optimization of the flares can be performed by altering the design of the side flares to ensure that the jets from the middle flares do not project over the jets from the side flares.