Influence of spillway height and discharge on rock scour, Stellenbosch (South Africa)

Since the start of my Bachelor Civil Engineering I have had the dream to conduct a water research project somewhere abroad. Unfortunately I could not manage to go abroad during my Minor phase so I was extra motivated to find something for my Bachelor Thesis. During my holiday last year in South Africa I arranged a short meeting with a teacher at the Stellenbosch University, Adele Bosman. After chatting for a short while, she offered me the chance to write my Bachelor Thesis in Stellenbosch this year and to help her with her PhD project.

My research was about the influence that the spillway height of large dams and the discharge has on rock scour behind dams. The most well known case of this is The Kariba Dam on the border of Zimbabwe and Zambia, a water-retaining structure that faces the consequences of rock scour already for many years. The scour hole that has been formed is two-thirds of the total dam height and is a big hazard for the dam foundation.

In the hydraulic laboratory of the University of Stellenbosch a 1:20 scale model was set up to simulate the general case of water falling from a certain height into a plunge pool. In order to be able to do research about the influence of the height the inlet channel could be adjusted to three different heights. Besides this the discharge could also be adjusted to three different values and the water level in the basin was either 1 or 0.5 meter. Twelve layers of cobblestones (0.1m x 0.1m x 0.05m) were placed on the bottom of the basin to simulate the rock formations behind dams. After every test with a certain height, discharge and tailwater level a scour hole was formed by blocks that had been scoured away by the water flow.

Nine different tests have been executed with three different discharges (98, 126 and 228 l /s ), two different drop heights (3 and 4 m ) and two different tailwater levels (0.5 and 1 m ). Increasing the discharge resulted in an increase in the scour depth, length and width, corresponding with was found in the literature. An increase in drop height resulted in an increase in scour depth. However, it is expected that at higher discharges increasing the spillway height would result in a decrease in the scour depth, due to the consequences of air entrainment, decreasing the scour potential of the jet.

Halfway through my research in Stellenbosch I visited the Kariba Dam in Zimbabwe myself. Here I had the opportunity to see the Dam from inside and to speak with the Swiss company that is currently doing the maintenance work at the dam. Besides talking about their strategy to fill up the scour hole and the consequences for the downstream part of the river, the chief engineer offered me an internship for next year at the Kariba Dam!

Altogether it was a life changing experience for me! Not only did I learn a lot about dams and especially its downstream part, I also learned about working in a hydraulic laboratory and a very beautiful bonus is the opportunity for me to do an internship in Zimbabwe next year.

Thomas Grolleman

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