WATER & WASTEWATER
Design innovation shapes Mhlabatshane Bulk Water project
A novel concept and digital twin for the design of the uMzimkhulu River Abstraction Works for Phase 2 of the Mhlabatshane Bulk Water Supply Scheme paves the way for its construction. By Graham Simpson*, Kyle Holmes** & Richard Jones***
T
he Mhlabatshane Bulk Water Supply Scheme (BWSS) is owned and operated by Umgeni Water – one of the top suppliers of bulk water to communities in South Africa. The maximum yield of the Mhlabatshane Dam ser ving Phase 1 of the Mhlabatshane BWSS has been reached and Umgeni Water is now implementing Phase 2, where water from the uMzimkhulu River will augment the water from the dam, enabling the capacity of the water treatment works to be increased from 6 Mℓ/day to 12 Mℓ/day. The location of the weir on the uMzimkhulu River was determined by Stellenbosch University for Umgeni Water, following model testing to find the best place to minimise the intake of the high sediment load in the uMzimkhulu River during the summer rainfall season. There
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is not enough run-of-river yield in the winter months and the dam will provide the yield during this period. The location selected is in the middle of a long cur ve in the river, with a high, steep rock face on the east
(left) bank and steep, but flatter ground on the west (right) bank. The rock exposed in the riverbed and left bank is hard but fractured tillite. The abstraction design provides for a grit trap on the left bank, flushed by a radial gate, with two intake slots in the abutment wall to two parallel 25 m long sediment settling channels, each with a submersible pump and sluice gate at the end. The weir is 2.1 m high, which is sufficient to operate the grit trap, but flood levels in the uMzimkhulu River require the abutment walls of the structure to be 15 m high. In the preliminar y design, water from the abstraction works would be pumped up 380 m via a 5.14 km pipeline to the existing water treatment works. With a 95 m head limit on the submersible pumps, a further two pumping stages were required. These would be located along the access road to the abstraction works at the river. The first high-lift pump station would include a second set of settling channels, hydrocyclones and a 500 kℓ holding reser voir. From here, the settled water would be pumped to a second reser voir and high-lift pump station, and then on to the water treatment works. Due to environmental constraints, the raw water pipeline was to follow the proposed access road.
