Wednesday, February 24, 2016

Bit of South Africa History in pictures: Hartbeespoort Dam (Harties)

The possibility of building a dam in the Hartbeespoort, where the Crocodile River cuts through the Magaliesberg, had already been considered since 1902 although the water of the Crocodile River has been used since the Stone Age. For nine hundred years the Iron Age people utilized the clay, iron ore and flora for man and animal. In 1836 the white pioneers “discovered” the potential of the water. Fountains, brooks and rivers were harnessed for household and agricultural purposes and for mechanical power for mills. Fords and bridges were constructed and the rivers became the focal point of attention.

In those years already water was gathered from rivers and brooks and taken to the nearby fertile grounds by furrows. Sometimes small weirs were built in the streams and often retention dams were constructed in convenient positions to regulate the supply of water over dry periods. In the Brits and De Kroon area there were seven unlined furrows and weirs in the Crocodile River alone before 1863 which irrigated about 2000 ha of riverside land. These ‘old furrows’ were operated and maintained by owners until recently.

In 1898 Hendrik Schoeman (later general) built the first “grand scale” dam of stone and cement and dammed the whole Crocodile River in the Witwaterspoort. This ambitious scheme, which he called the Sophiasdam after his wife, cost him £10 000. In those years it was the biggest dam in the southern hemisphere. Unfortunately the engineer made a mistake in his calculations and the dam was washed away in a flood. Johan Schoeman, Hendrik Schoeman’s son, in 1902 revived the idea by rebuilding the furrows fed by a smaller weir higher up in the river. With this he irrigated 3000 ha of land. 

Between 1905 and 1910, various preliminary investigations were undertaken by the then Department of Irrigation of the Republic of the Transvaal, under Mrrs WL Strange and FA Hurley. With the establishment of the Union of South Africa in 1910, the investigations were temporarily suspended. However increased public interest resulted in the Hartbeespoort Irrigation Scheme (Crocodile River) Act (Act 32 of 1914). Act 32 of 1914 authorised the construction of the Hartbeespoort Dam on the farm Hartbeesfontein. Construction of the dam was however postponed due the outbreak of the First World War (1914 - 1918). 

Building of the dam started in 1918 when ground was bought or expropriated, roads, a suspension footbridge with a span of 60m, staff quarters, offices, power house, cement shed were built. A 15km cocopan railway line was built from the Brits West siding to transport cement, stone and sand to the Poort. The cocopans were drawn by oxen, horses and mules. Work also started on the building of the two cofferdams to dam up the river so that the dam wall foundations could be poured. These cofferdams however washed away during the flood of March 1921.

After the flood the department employed the engineer, FW Scott, who revised the plans by replacing the gravity structure (the same type of wall that was used at Sophiadam) with a varying radius arch structure, which would be supported against the rock faces on both sides of the Poort. This was done primarily to be able to complete the foundations and get the dam level above riverbed in one dry season and also to save costs on the volume of cement required. Initial estimates were that 800 000 bags of cement would be required but by changing the design of the wall this amount was reduced to 250 000 bags.

The cofferdams were reinforced in April 1921 but still more water that could be handled by pumps moved through the working areas. It was then decided to divert the river through an 80m long tunnel with dimensions of 1.8 x 3.6m. The water was guided into the tunnel from a weir upstream. The river was successfully diverted through the tunnel on 24 May 1921 making it possible for the foundations to be excavated. 

The first concrete was poured into the  foundations of the wall by 29 July 1921 and by 7 September 1921 7220 m3 of concrete had been placed, raising the wall 2m above the riverbed. The floods of the 1922/23 season were impounded and the wall proper completed in April 1923. The road over the wall, now the new main road between Pretoria and Rustenburg, was opened to the public in September 1923. The dam overflowed for the first time in 1925.

In 1970 the dam supply level was raised by 2,44 m by the installation of 10 crest gates on the spillway. Each crest gate is 10,06 m long and 2,44 m high.

- Hartbeespoort Environment and Heritage Association (HEHA) CD-set - SA Irrigation Department Magazine: Hartbeespoort Dam, November 1923- Hartbeespoort Government Water Scheme, 1991 (DWAF)- The Damming of The Poort: The construction of the Hartbeespoort Dam, 1991 

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