WATER TRANSFER SCHEMES IN THE MIDDLE ORANGE
Bloemfontein is the sixth largest city in South Africa with a population of approximately 300 000. It is situated in the Modder River catchment which has insufficient water resources to meet the growing water requirements. The water supply to Bloemfontein is therefore augmented from the adjacent Caledon River by means of the Caledon - Modder River Government Water Scheme (CMRGWS)
It was originally envisaged that Bloemfontein would receive Orange River Water from Vanderkloof Dam and the outlet works for the transfer scheme still exist at the dam wall. For various reasons, however, it was decided to construct a smaller scheme on the Caledon River which could be implemented before completion of the Vanderkloof Dam. The Welbedacht Dam on the Caledon River was constructed as the main storage element of the CMRGWS and water is abstracted from this dam for transfer to Bloemfontein and various smaller users along the way.
The Welbedacht Dam is a concrete barrage-type dam on the Caledon River which was designed and constructed by the Department of Water Affairs. The dam has a catchment area of some 15 245 km2 with a natural MAR of approximately 1210 million m3/a (1920 to 1987) and was completed in 1973. Its purpose was to supply water to the city of Bloemfontein via the 115 km long Caledon-Bloemfontein pipeline which has a capacity of ~1.157 m3/s. Due to the high sediment concentration in the water, the transfer from Welbedacht Dam is first purified at the Welbedacht Purification Plant which is located just downstream of the Dam. The purification plant has a capacity of 1.68 m3/s
Due to siltation, the storage capacity of the Welbedacht Dam reduced rapidly from the original 115 million m3 to approximately 16 million m3 during the twenty years since completion. This reduction in storage created problems in meeting the Bloemfontein demand at an acceptable level of reliability and as a result, the 50 m high Knellpoort Dam was completed in 1988. It was the first arch gravity Roller Compacted Concrete (RCC) dam in the world and comprises almost 64 600 m3 rollcrete and 14 200 m3 concrete with a gross storage capacity of 137 million m3.
In order to prevent similar siltation problems to those experienced at the Welbedacht Dam, the Knellpoort Dam functions as an off-channel storage dam with a relatively small catchment area of only 798 km2 and corresponding MAR of approximately 20 million m3/a . Water from the Caledon River is pumped to Knellpoort Dam from the Tienfontein Pumping Station via a 2 km long canal which is equipped with a silt trap to reduce siltation in the main reservoir.
The Knellpoort Dam is currently used to supplement the water in the Welbedacht Dam for abstraction to Bloemfontein via the Caledon Bloemfontein pipeline. As the Bloemfontein demands increase, however, the capacity of the pipeline will become the limiting factor. Water will then be transferred directly from the Knellpoort Dam into the Rustfontein Dam catchment via the Novo Transfer Scheme which is currently in the detailed design stage..
The Novo Transfer Scheme which will transfer water from the Knellpoort Dam to the Modder River, will be developed in stages in accordance with the needs of Bloemfontein and is expected to reach a maximum transfer capacity of 150 million m3/a by the year 2030.
