DAMS & WATER STORAGE
Xhora Dam showcases design innovation
The complete dam, stilling basin at rest water level, downstream face of dam with crushed stone, and access road to crest
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.
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he Xhora Off-Channel Storage Dam is 230 m long, has a 29.4 m crest and is classified as a category 2 dam. It is a zoned earth fill embankment dam on an unnamed tributary of the Xhora River and has a gross storage capacity of 2.7 million m3.
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Earth filled embankment dam
The embankment was constructed with different zones, where the central core acted as the low-permeability zone of the dam – available in the reservoir basin. However, materials available in the basin had a fairly low plasticity so, for backfilling of the cut-off trench, a more flexible clay material was imported from a decomposed dolerite borrow pit about 8 km away.
Materials are very important to the design and construction of an earth embankment dam. Hard rock and crushed rock products are required for the rip rap, as well as filters and drains within the dam. Therefore, a hard rock quarry was developed within the basin primarily utilising the igneous granophyre rock with the overburden indurated siltstone.
Dams can cause loss of life and extensive damage should they fail, and a spillway is a key component of a dam for the safe discharge of flood waters. A 25 m long ogee side-channel spillway on the right flank with a 5 m wide channel was constructed. It has
IMIESA January 2022
Construction
