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Nzhelele Dam
Makhado (Louis Trichardt) district, Limpopo
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Click to view large map Coordinates: | Also called Njelele Dam It is an arch type dam located on the Nzhelele River in Limpopo Province, South Africa. It has a capacity of 55.3 million m³. (Wikipedia). NJELELE IRRIGATION DAM. The Njelele River, which drains the northern slopes of the Zoutpansberg range, is a perennial tributary of the Limpopo. Irrigation development along most of the streams in South Africa, had outstripped the resources provided by the dry season flow. The problem caused by a shortage of water was, moreover, accentuated by overstocking and uncontrolled veld-burning in the catchment, which resulted in a weakening of the winter flows. The 35 riparian owners along the Njelele River sought to solve their difficulties, by applying to the Water Court for a legal apportionment of the normal flow. In 1936, the Water Court apportioned the normal flow but naturally this step did not make available more water, and in spite of this ruling the irrigators found themselves in the position that existed before the Court order. It was necessary, therefore, to approach the Government with a request that consideration be given to the suggested provision for water storage. Meanwhile, during the year 1932, the Irrigation Department had established a gauging station and was in a position to make an assessment of the total water resources. Reconnaissance surveys were commenced and an ideal dam site was located on the farm Nairobi. In the year 1939, during the months of September and October, the foundations were tested by diamond drill when a total of ten boreholes were sunk. Afterwards, surveys of the soil were undertaken by the Division of Chemical Services and a project was ultimately designed and submitted to the Minister in February 1945. The project, reviewed and recommended by the Irrigation Commission and accepted by the Minister in April 1945, comprised the construction of a dam to regulate the flow of the river in order to ensure at all times, adequate water supplies to the existing diversion furrows. The provision of new canals was not contemplated. Obstacles to the Project. In considering the obstacles to the realisation of the project, it was apparent that the chief one was in the Order made in 1936, by the Water Court. It was an obvious fact that the proposed dam would unavoidably cause the normal flow to mix with the conserved surplus water, and it would be virtually impossible to effect the apportionment stipulated by the Court. This problem, together with others, was thrashed out at several meetings between farmers and officials of the Irrigation Department, and eventually, the riparian owners agreed to vest their rights in an irrigation board which would undertake the responsibility of effecting an equitable distribution of the conserved supply. The Minister thereupon promised to recommend the construction of the dam by the Government. Capital amounting to £150,000 was placed on the Loan Estimates of the Irrigation Department for 1945, and immediate steps were taken to build up an organisation from the Department's depleted staff, to tackle the preliminary work on the project. The construction of an access road was commenced in August 1945 under the direction of the Circle Engineer, Northern Transvaal. Erection of houses for the staff and workmen then followed, and in November the late Mr. P. C. Jackson, at that time Controller of Reconnaissance, was appointed Resident Engineer on the scheme. Prefabricated asbestos and steel houses were erected for European [sic] workmen, […]. The Dam. Situated on the farms Aerial and Nairobi, some 35 miles [~55km] by road north-east of the town of Louis Trichardt, the site - a narrow poort with practically vertical sides of solid rock- stood literally "asking" to be closed by an overspill arch dam. A mass concrete constant-radius arch dam of 120 feet [36,0m] radius, with vertical upstream face and 5 : 1 downstream batter, was designed with an overspill crest 105 feet [31,5m] above the level of the river-bed. The thickness of the wall is 29.8 feet [~12,0m] at the lowest foundation and 6 feet [1,8m] at the crest, which will permit the height of the dam to be increased by 10 feet [3,0m], without adding to the thickness of the wall. The overspill section of the crest is confined by abutments to 140 feet [31,2m], […]. The outlets, are set in the arch at the left flank; constructed between stiffening buttresses. These consist of a pair of cast iron pipes, 36-inch [900mm] diameter, bell-mouthed at the upstream face and transitioned to 2 feet [600mm] square section on the downstream site and fitted with radial gate valves. The valves discharge on to dispersers which aerate the issuing jet, and at the same time deflect it upwards. In this way the jet is broken up into a fine spray which falls with little violence, on the river bed below. The plumes of spray produced when the valves are in operation provide a magnificent spectacle. All gate valves and dispersers are similar to those installed at Vaal Dam, and are of a special type developed by mechanical engineers of the Irrigation Department. Two additional pairs of outlet pipes at intervals of 15 feet [4,5m] above the bottom pair, are provided for use in case siltation should interfere with the operation of the lower valves. These spare outlets are meanwhile blanked off. Excavation. Excavation of the foundations for the dam, necessitated the removal of 6,900 cubic yards [5 275 m³] of material, a task which took 5 months to complete. A coffer dam was constructed and by-passing of the ordinary flow of the river was effected, by the simple expedient of providing a corrugated iron flume to convey 70 cusecs [~2 100 l/sec] across the excavation. The concrete-mixing station, comprised of two three-quarter-yard mixers complete with the necessary bins, was located at the top of the left flank krantz overlooking the work. Stone required for concrete was quarried nearby, crushed in an 8-inch [203mm] McCully Crusher and conveyed in 6-yard [5,5m] dumper wagons to the bins above the mixing station. Sand was carted by lorry from the Mtambaspruit, a few miles from the job. Concrete. Considerable difficulty was experienced with cement supplies owing to the general shortage throughout the country. At one period cement destined for almost all other works under the control of the Irrigation Department, was diverted to Njelele in order to tide over a critical period. Concrete was lowered from the mixing station to the site of the work by means of chutes, assembled zig-zag fashion to avoid slopes greater than 45°. These chutes delivered concrete of slump not exceeding half an inch [12mm]. The design […] satisfied the requirements [to optimise the use of cement] of the specification and a strict control of the mix resulted in a gratifying saving in cement during a country-wide shortage. Altogether 44,927 bags of cement were used for the production of the 16,230 cubic yards [12,4 Ml] of concrete in the dam. The total cost of the work amounted to approximately £135,000. Employment. The number employed on this scheme varied from month to month, [from 293 to 195 person over the year of construction]. The works were completed in March, 1948. It is interesting to note that throughout the period of the contract no labour difficulties were experienced. Administration. The Njelele Irrigation District Amendment Act was passed by Parliament during 1946, vesting in the Njelele Irrigation Board control of water in the Njelele River from the farm Nairobi down to the confluence with the Limpopo River. This act came into effect by Proclamation 88 of 1948 as from the 1st May, 1948. By controlling the river flow during the latter stages of construction the Dam was almost full when completed - in fact, it spilled over in April 1948, about one month after completion. Adequate water should therefore be available for the winter irrigation season. The lower Njelele Valley has developed into an important winter vegetable producing area. In the main, tomatoes, gem and other squashes, marrows, cucumbers and such like vegetables are grown, also grapes and tropical fruits. […]. The dam was officially opened on 8th May, 1948, by Senator A. M. Conroy, then Minister of Lands. (PWSA July 1948 Vol IX No 56:14-18). |
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