PROJEC T FOCUS
Xhora off-channel Storage Dam
assist with water security
Dam approaching completion, with the rock quarry in foreground
Located in the Elliotdale District of Mbhashe Local Municipality in the Eastern Cape, the Xhora Off-channel Storage Dam was built to give greater security of supply to the Xhora Water Supply Scheme (Xhora WSS), which provides potable water to 70 000 people.
the river diversion works, water abstraction facilities and associated pipework, as well as a pedestrian bridge across the spillway he Xhora Off-Channel Construction for the local community. Dams can cause loss of life and extensive Storage Dam is 230 m long, A double-barreled diversion culvert damage should they fail, and a spillway has a 29.4 m crest and is was constructed to allow for floodwater is a key component of a dam for the classified as a category 2 dam. during construction. The diversion culvert safe discharge of flood waters. A 25 m It is a zoned earth fill embankment dam provides passage for the two outlet pipes, long ogee side-channel spillway on the on an unnamed tributary of the Xhora a domestic supply to the water treatment right flank with a 5 m wide channel was River and has a gross storage capacity works, and for emergency emptying of constructed. It has been designed for a of 2.7 million m3. the dam, as well as making provision for 1:100 return period flood of 87 m3/s and environmental releases. On completion of Earth filled embankment dam safety evaluation flood of 165 m3/s. the dam, the culvert was closed through Materials are very important to the design The spillway discharge curve was the installation of two sets of precast and construction of an earth embankment obtained using computational fluid reinforced concrete planks, with the dam. Hard rock and crushed rock products dynamics software. Due to the proximity void between the planks filled with mass are required for the rip rap, as well as filters of the water treatment works downstream concrete post-grouted through a tube and drains within the dam. Therefore, a of the spillway, the design of the stilling manchette system. hard rock quarry was developed within basin needed to be accurate. The novel In the initial design process, the team the basin primarily utilising the igneous design was developed and model tested debated how to abstract water for granophyre rock with the overburden by Stellenbosch University. domestic consumption. The preferred indurated siltstone. The dam was fitted with outlet pipework, water for domestic consumption should be The embankment was constructed the stilling basin, twin box culvert as part of drawn from the surface waters of the dam, with different zones, where which are oxygenated. Typically, the central core acted as the an intake tower is constructed PROJECT STATS low-permeability zone of the dam with inlets at various levels to be – available in the reservoir basin. operated so that water can be • 109 local people employed over the contract However, materials available abstracted close to the surface as • R5.2 million spent on local wages and local contractors in the basin had a fairly low the reservoir level moves up and • 295 000 m3 ear thworks volume plasticity so, for backfilling of the down. As the water demand on 3 • 12 000 m concrete volume cut-off trench, a more flexible clay the scheme is relatively small, the
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material was imported from a decomposed dolerite borrow pit about 8 km away.
