The largest part of the catchment area is situated on the Highveld plateau at an altitude of more than 1 500m above mean sea level; the remaining part is on slopes of the plateau in the Lowveld. The most important towns in the catchment area are Middelburg, Bethal, Delmas, Witbank and Bronkhorstspruit.
The mean annual runoff is approximately 451 million m3. The catchment area of the dam is 12 300km2 and at full supply level its surface area is 2350 ha. The dam basin is underlain by felsitic rock which is extensively jointed.
Description
The Loskop Dam was completed in 1938 and the wall was raised in 1979. It comprises a mass concrete gravity wall with an ogee crest spillway. The present wall is 54m high. The total crest length, of which the spillway section is 244m long, is 506m. Owing to the poor foundation conditions on the right bank, a portion of the right flank was built at an angle to the dam wall on better foundations. The original Loskop Dam was the first dam where splitters were introduced on the downstream face of the wall to dissipate the kinetic energy of the overflowing water. The splitter idea was developed by Lieut.-Col. D.F. Roberts (the then Resident Engineer) in the Department's very first hydraulics laboratory on the banks of the Olifants River at the dam site.The raising of a concrete dam wall needs to be done with great care. When such a wall is raised, thickening of the existing structure is generally also required. Ideally such a raising should take place while the existing structure is not under load (i.e. when the dam is empty). The new structure should be able to shrink freely without loading the existing structure, but the two structures should eventually react as a monolith i.e. as a bonded whole. In practice this is rarely possible as precious stored water cannot simply be released to facilitate such a raising. Thus the loading caused by the water pressure on the old structure coupled with the stresses due to concrete shrinkage in the new structure can lead to very undesirable stresses in the total structure unless special precautions are taken to limit their effects.
At Loskop Dam this was achieved by casting the section being added in three distinct phases. For the first phase the old concrete face was chipped along the lowest section of the wall to effect a strong bond with the new concrete. The new concrete was then cast to the predetermined level and allowed to harden, thus allowing for concrete shrinkage to take place. In the lower section of an existing dam wall the normal stresses are such that shrinkage along a bonded joint causes no serious problems.
For the second phase of the construction a sliding joint was created on the joint between the original and the new concrete by painting whitewash on the face of the old structure. A drainage system was also provided to discharge any water leakage into the joint so preventing pressure build-up between the existing and the added concrete. After the concrete of phase two had been placed no further concrete was placed for some six months while concrete shrinkage was allowed to take place. The sliding joint prevented the stresses caused by shrinkage from being transmitted to the original structure and causing stresses in both parts.
For the third and final phase the original concrete face was chipped as was the case in phase one, and the new concrete cast against it to form a monolithic structure. The final product was a raised dam wall meeting all modern technical requirements. In the design of the dam provision was made for crest gates in order to facilitate the raising of the wall by another four meters at a future stage.
The full supply capacity of the raised dam is 362 million m3. The dam has been built to accommodate a design flood of 2 886 m3/s. This is the 1:200 flood. The RMF (Regional Maxium Flood) is 7 750 m3/s and the PMF (Probable Maximum Flood) is 14 500 m3 / s. In 1989 (Stemet et al. 1989) is was established that the overflow will be able to handle the RMF without flooding the N.O.K. Outlet works were built on both flanks of the dam wall. Water is released into a canal system which provides water inter alia to irrigate an area of approximately 25 600 ha. The total length of the canals is approximately 480 km.
Purpose
The dam was initially built to provide for the irrigation needs of farmers in the Olifants, Moses and Elands River valleys. Development in the Witbank-Middelburg area necessitated the raising of the dam wall so that the portion of the assured yield which had in the meantime become affected as a result of the construction of upstream dams such as the Rondebosch, Witbank and Doornpoort Dams on the Highveld, could be restored.General
During the middle of the previous century the first Voortrekkers established themselves in the Kruis River Valley, not far from where the Loskop Dam is today. The first farms in the Olifants River Valley were demarcated in 1886 and the farmers cultivated wheat under dry-land conditions. In 1917 the first private dam was completed on the farm Rooikraal. Thanks to irrigation, the wheat crop on this farm increased from 150 to 8 000 bags per year. Around 1925, after the successes of the small irrigation schemes. the Hereford Irrigation Board was founded to supply irrigation water to an area of about 2 140ha which was situated a few kilometers downstream of the present Loskop Dam. The early success of this scheme gave rise to a petition which resulted in studies of the Hereford Scheme, as well as in a soil survey and a topographical survey of the dam basin. This paved the way for the commencement of the construction of the dam in 1934. The dam was completed in 1938. The Loskop Water Scheme was one of the most important of a large number of irrigation schemes developed during the thirties, in which mainly manual labour was made use of.The ashes of Lieut.-Col. D.F. Roberts, the pioneer Resident Engineer, were originally buried on an island in the dam. As the raising of the wall was to result in the inundation of the island, the ashes were removed and reentered in a niche on the left flank of the dam wall. At present, the Loskop Water Scheme consists of 667 properties with an average scheduling of 25.7ha each. A full water quota amounts to 7 700m3 per hectare per year. Wheat, vegetables, tobacco, peanuts, cotton and citrus fruit are cultivated. Furthermore, water from the dam supplies the Hereford Irrigation Board, the Olifants River Irrigation Board as well as the Groblersdal and Marble Hall Municipalities.
Recreation
The Loskop Dam is a popular picnic and holiday resort. It is a paradise for fresh-water angling in a lovely subtropical climate. The Loskop Dam falls within a nature reserve and crocodiles, hippopotami, rhinoceroses, kudu and impala can be seen.A popular holiday resort is operated by Aventura Forever Resorts on the bank of the dam. With the raising of the dam, parts of the resort were threatened by inundation and were demolished. The resort was then re-planned and new facilities were erected to replace those lost.
Summary
|
Year of completion |
1938; Raised in 1979 |
|
Purpose |
Irrigation, Domestic, Industrial |
|
River |
Olifants River |
|
Nearest town and province |
Groblersdal, Mpumalanga |
|
Type |
Mass concrete |
|
Net storage capacity |
348 million m3 |
|
Wall height above lowest foundation |
54 m |
|
Crest length |
506 m |
|
Material content of dam wall - original and raised wall |
415 000 m3 |
|
Type of spillway |
Uncontrolled |
|
Capacity of spillway |
7 750 m3/s |
|
Surface area of dam at full supply level |
2 350 ha |
|
Owner, design and construction |
Department of Water Affairs |