The rapid economic expansion of the greater Johannesburg area in the 1960s and 70s put its water supply under long term threat and it was decided to redirect water from the large Tugela River which was going largely un-used into the sea and store it in large strategic water storage reservoir with very low evaporation. The initial site chosen for this project was in the adjacent valley to the west on the Elands River. This was the preferred option from an engineering aspect because it would involve a smaller dam wall. However shortly before construction was about to start the site on the Nuwejaarspruit was selected for political reasons because it avoided the partial flooding the newly planned Apartheid eraBantustan of Qwaqwa in Phuthadichaba. Under the peculiar politics of the time it potentially could have meant this strategic national resource could have been located in a "foreign country" which would have been unacceptable as seen from the perspective of Apartheid Government.
Construction
Construction began in 1969 and the first phase was commissioned in 1977 and comprised a high earthfill embankment long with no spillway. After completion of the first phase the decision was made to immediately continue with the final phase and it was subsequently raised in 1980 to its current height of with a crest length of and a full supply capacity of. At full supply, it has a surface area of no more than 70 km2. The extended dam was commissioned in 1980, and has a capacity of, with a surface area of only, the dam wall is high. The total construction period was 11 years. The Sterkfontein Dam was designed and constructed by the Department of Water Affairs. The dam mainly serves for domestic and industrial use and its hazard potential has been ranked high.
Naming
The dam was named after one of the many farms expropriated in its construction. The temporary township for the construction workers was built on the Sterkfontein farm. The word sterkfontein means strong fountain when translated from Afrikaans.
The wall is an earth gravity type. The dam wall contains of material making it the largest dam wall in South Africa with regard to volume. The reservoir is impounded by two walls. The second wall is long and contains of material.
No spillway
This dam is rather unique in that very little of its water is sourced is from rainfall and instead, it is filled by diverting water from the upper reaches of the Tugela river, which is underutilized and the water would otherwise flow unused into the ocean. The dam was not built on a river but instead, it is sited on what can at best be classed as a large stream in the upper reaches of a valley on the Nuwejaarspruit. The catchment area of the dam is less than three times the water surface area of the dam and therefore not risk to a flood. For this reason the main dam wall does not have the need for a spillway. Any excess rainwater can be safely ejected from its outlet valves. The advantage of this location is the Dam is a highly effective reservoir, since it has the depth to store a large amount of water, with very little loss to evaporation. The Vaal Dam lake by comparison has a large surface area and is relatively shallow, which results in a higher rate of evaporation. It would take 17 years of rainfall to fill the dam. But capturing rainfall was not an important intent behind this project. It was built to capture water from the Tugela river and deliver it into the Vaal system.
The dam receives its water via the Tugela-Vaal Water Project later renamed the Drakensberg Pumped Storage Scheme which is a 1,000 MW pumped hydro storage scheme involving the net transfer of up to of water from KwaZulu-Natal. This is stored in the Sterkfontein Dam and released to the Vaal Dam via the Wilge River when needed.
A dam within a dam
The Driekloof Dam which is inside the Sterkfontein Dam, is regarded as a separated dam and reservoir, and is not included in the above figures. It is essentially a dam within a dam. Driekloof Dam is the upper storage reservoir of the Drakensberg Pumped Storage Scheme. If the Sterkfontein Dam were to be empty the Driekloof reservoir would retain the water for the pumped storage scheme to operate unimpeded. When Sterkfontein Dam is at full capacity the Driekloof Dam is submerged up to the top of its spillway.
Third largest dam in South Africa
This is the 3rd largest dam in South Africa in terms of water capacity and it is slightly larger than Vaal Dam.
