Browsing by Author "Calitz, Jan Albertus"

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    Investigation of air concentration and pressures of a stepped spillway equipped with a crest pier
    (Stellenbosch : Stellenbosch University, 2015-12) Calitz, Jan Albertus; Basson, G. R.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The evolution of roller compacting concrete has led to stepped spillways becoming increasingly popular over recent decades, mainly credited to the fact that the stepped profile of the downstream dam wall can be incorporated into a spillway chute. However, the discharge over stepped chutes in current use has been limited, due to the risk of local cavitation damage to the concrete of stepped chute structures. A general accepted practice to combat cavitation is to aerate the flow. Would it be possible, when adding a pier to the spillway crest, to introduce air into the flow upstream of the inception point in order to reduce the cavitation potential on the chute, and subsequently allow discharges greater than the current recommended values to be safely passed? A physical hydraulic model was constructed at a scale of 1:15 to investigate the air concentration along the pseudo-bottom and minimum pressures at the upper vertical step face for areas on the spillway chute where cavitation could be imminent for large discharges. The tests were conducted using a conventional stepped spillway with no pier as the control, to which the test results of two different pier configurations fixed on the spillway crest were compared. The recorded results showed an increase in air concentration and minimum pressures downstream of the pier for both tested crest pier designs. The Type 1 pier is recommended over the Type 2 pier due to the increased ability of the former pier to aerate the flow that consequently alleviates minimum pressures found on the spillway chute. In summary, the literature recommends a maximum discharge of 18 m²/s, but the experimental study has shown through a cavitation evaluation of the air concentration and minimum pressures that, for a no-pier stepped spillway with a chute angled at 51.3° and a prototype step height of 1.5 m, a maximum discharge of up to 25 m²/s can be allowed. For a spillway equipped with a Type 1 pier, Method A suggested that a unit discharge of at least 30 m²/s could be safely passed.