The Injaka dam, the Mooi-Mgeni river transfer scheme and the Tuva canal are some of the water projects currently under way in South Africa. The South African Committee on Large Dams outlines the work involved in each scheme
THE Injaka dam project focuses on the construction of a new dam on the Marite river, a major tributary of the Sabier river to the west of the Kruger National Park in the Mpumalanga Province of South Africa. The dam will deliver raw water to a treatment works, which is also currently under construction, and will supply potable water to a number of rural communities in the area. The dam will also augment low flows in the Sabie river which runs through the Kruger National Park.
Injaka dam comprises a 53m high and 550m long earthfill embankment with a central trough spillway of mass and reinforced concrete founded on shallow rock that occurs in the riverbed. The spillway design incorporates a 138m long horseshoe-shaped uncontrolled overflow structure that connects with a 256m long spillway chute and stilling basin. The total estimated concrete volume is 380,000 m3.
A zoned earthfill embankment with a clay core on either side of the spillway trough abuts onto valley flanks, consisting of erodible decomposed granite. The granite has been intruded by diabase dykes in which the grout curtain was formed, except for a 110m length on the left flank where a jet-grouted curtain was constructed to a maximum depth of 30m. The estimated volume of the embankment is 145M m3.
The upper horizon of the decomposed granite comprising the foundation of the embankment was found to be collapsible. Extensive investigations were undertaken to determine a suitable depth of excavation.
Construction of the dam began in January 1996. The construction period has been extended due to higher than average rainfall, additional excavation and associated additional volumes of concrete and earthfill.
The expected completion date is currently set for the end of April 2002, with the impoundment date scheduled for 15 October 2001.
Mooi-Mgeni river transfer scheme – phase I
The proposed Mooi-Mgeni river transfer scheme is situated 100km northwest of Durban in the KwaZulu-Natal Province of South Africa. The first phase of the scheme increases the transfer capacity of the existing emergency transfer scheme built in 1983. The scheme will involve:
• Building a new 8m high diversion weir on the Mooi river. It will be a mass concrete structure of 20,000m3 with a storage volume of 5.1M m3.
• Providing a standby pumping capacity at the existing Mearns pumping station. This will allow for the transfer of 3.2m3/sec.
• Raising the existing Midmar dam by 3.5m to increase storage capacity. This will be achieved by the installation of crest gates on the spillway.
The scheme will augment the yield of the Mgeni river system by 25M m3 per annum. The estimated capital costs are R87M (US$11.4M). Construction will commence in early 2001 with completion of all the components by late 2003. During phase I, transfer will only occur during the wet summer months when there is sufficient flow in the Mooi river.
The planning process has been characterised by a public participation process. Several environmental impact assessment studies have been done and these have influenced the design and final configuration of the scheme. Negative impacts will be further reduced through the implementation of mitigation measures. Environmental management plans for the different components will be prepared during the detailed design and construction phases to guide and guarantee the impact management process.
The largest bulk water user within the Mgeni river supply area, the Durban Metropolitan Council, has successfully introduced a water demand management programme that has seen its water demand remaining static for the last couple of years. The programme concentrated on reducing losses and the installation of water meters to control wasted consumption. A recycling plant will be commissioned early in 2001 that will further reduce demand from the system. The Mooi-Mgeni river transfer scheme is, however, still needed as the level of assurance of the system is below the required standard.
Phase II of the transfer scheme is currently in the planning process. This will consist of a large storage dam (approximately 140M m3) on the Mooi river and will allow transfers to be made for an average of about ten months of the year.
Upgrading Tuva canal
The Tuva canal, located in the Kwa-Zulu Natal Province of South Africa, forms part of the Drakensberg interbasin pumped storage project that provides water to the Vaal river system in the Gauteng Province. The 39.6km long trapezoidal canal is 3.35m deep with a top width of 14.75m, giving it a capacity of 20m3/sec.
The canal was relined with concrete and sealed to minimise leakage and the risk of untimely breakdown. The rebuilding commenced in June 1998 and will be commissioned during April 2001. The canal was machine lined using 100,000m3 of concrete. Earthworks amounted to 1M m3.
Special attention was given to the underdrainage of the canal to address the problem of dispersive soils. The formation of joints, and sealing them with a polyurethane sealant, allowed for expansion and prevented leakage.
Eighty-two stormwater crossings, using in situ precast concrete culverts, were provided to prevent the damming of stormwater on the uphill side of the canal from endangering the safety of the panels.
The final estimated cost for the Tuva canal project is R182.5M (US$24M).