Sterkfontein Dam is situated in the very upper reaches of the Vaal Dam catchment on the Nuwejaarspruit, a few kilometres from the edge of the Drakensberg Escarpment. It has a very small catchment area with negligible natural inflow with the result that it requires no spillway. The dam receives its water via the Tugela-Vaal Project which is a pumped-storage scheme involving the net transfer of up to 630 million m3 of water from KwaZulu-Natal. The water from KwaZulu-Natal is stored in Sterkfontein Dam and released to Vaal Dam via the Wilge River when needed. Due to the favourable storage and climatic characteristics of Sterkfontein Dam, it is beneficial to store water in the deep cool Sterkfontein Dam and only release water to the shallow Vaal Dam when needed. The evaporation losses from Sterkfontein Dam are approximately 35 million m3/a which represent approximately 10% of the losses that would be experienced from Vaal Dam for a similar volume.
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Sterkfontein Dam was commissioned in 1977 and comprised 69m high earthfill embankment 2 290m long with no spillway. It was subsequently raised in 1980 to its current height of 93m with a crest length of 3 060m and a full supply capacity of 2 656 million m3. At full supply, it has a surface area of only 70km2 which combined with an MAP of our 800mm and an annual evaporation of 1300mm makes it a highly efficient storage unit.
The dam wall contains 17 million m3 of material making it the largest dam wall in South Africa with regard to volume while the reservoir formed by the dam is currently (1977) the third largest in the country.
A small section of the Sterkfontein Dam reservoir is cut-off by the construction of Driekloof Dam which forms a small reservoir with a capacity of 35.6 million m3. This reservoir forms the upper storage reservoir for the Tugela-Vaal pumped storage scheme and is connected to the lower storage reservoir of Kilburn Dam located some 500 meters below in the Tugela catchment.
Water is pumped up from Kilburn Dam at a rate of 174m3/s by four 250MW pump/turbines into Driekloof Dam during period of low power demand. During period of peak demand, typically during the morning and early evening, the water is released from Driekloof Dam at a rate of 312 m3/s to generate up to 1 000 MW of power.
The scheme is operated in such a way that there is a net pumping of up to 630 million m3/annum depending upon the water availability in the Tugela catchment (Woodstock Dam) as well as the need for augmentation in the Vaal Dam catchment.