Civil Engineering July 2021 | Vol 29 No 6

36 July 2021 Civil Engineering of a long curve 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 suf- ficient to operate the grit trap, but flood levels in the Mzimkhulu River require the abutment walls of the structure to be 15 m high. In the preliminary design, water from the abstraction works would be pumped 380 m via a 5.14 km pipeline to the ex- isting 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, hydro-cyclones and a 500 kℓ holding reservoir. From here the settled water would be pumped to a second reservoir 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. PRELIMINARY DESIGN REVIEW Due to the terrain, the proposed access road grades would be between 25% and 40% while traversing an extremely steep embankment with ground cross slopes above 35%. Alternative routes on the east bank were considered but were too expensive and would have a highly nega- tive impact on the environment. A route on the west bank through uMzimkhulu Village was feasible but the travel time and distance from the water treatment works would be too long. Moving the abstraction works was out of the question. After considering all options, including a funicular and a tunnel, the solution accepted by Umgeni Water was to use an existing track on the east bank down to the river, cross the river with a bridge, construct a new section of road 500 m down the west bank and cross the river again with a bridge onto the abstrac- tion works’ top deck. The flood profiles at the weir showed the lower bridge could be positioned slightly downstream of the weir where the 200-year flood would safely pass under the bridge. This route however bypassed the first high lift pump station. At the abstraction works, the top deck was set to 13.6 m above riverbed level for the location of the electrical and mechanical plant, leaving a void of 4 000 m 3 under the deck inside the outer flood wall. Instead of filling this space with selected fill, it was reconfigured to incorporate the first high-lift pump station, reservoir, hydro-cyclones and settling channels. This had significant benefits in terms of improved structural stability, reduced energy consumption, lower capital costs, reduced project footprint and environ- mental benefits. The sediment removed from the river water will be returned to the river at the point of abstraction and not accumulated in an inaccessible area where its disposal would be problematic. The energy saving stems from the reduc- tion in first stage pumping head from 95 m to 15 m, where water usage in the sediment removal process is expected to be as high as 20% of input volumes. HOW IT WORKS Inside the structure are three levels of 4 m high by 2 m wide sedimentation channels with a three-level gallery added on the Digital image of abstraction works, weir and access bridge Due to the terrain, the proposed access road grades would be between 25% and 40% while traversing an extremely steep embankment with ground cross slopes above 35%