Civil Engineering June 2022 | Vol 30 No 5

and flushed to the river while the other compartment continues to operate. A flushing system has also been designed for the two raw water storage reservoirs up the hill. One reservoir may be isolated, drained and the accumulated fine sediment scoured and flushed to the river while the other continues to operate. The high-lift pumpstation has four duty and one standby 1 000 kW pumpsets. The limitations of the Eskom power supply capacity, together with the need to operate from one to eight hydrocyclones (at a fixed design operating pressure) to accommodate low to maximum raw water demand at the waterworks, variable frequency drives at both the low-lift and high-lift pumpstations had to be used. The rising main to the two 12.5 Mℓ reservoirs and gravity main from the reservoirs to the waterworks traverse mainly private properties which are either undeveloped or used for commercial vegetable growing. Most of the pipeline route is over steep terrain with many air and scour valves despite judicious cutting through some high spots to minimise their number. NGWADINI SYSTEM The Ngwadini System comprises an almost identical abstraction and low-lift pumpsta- tion as Goodenough, but without hydrocy- clones. The three duty pumps deliver up to 1 m 3 /s directly into the off-channel storage dam via a 500 m long, 900 mm diameter steel pipeline and 250 m long, 1.5 m di- ameter pipe jacked precast concrete sleeve at the dam’s full supply level through the intervening watershed. Like the Goodenough abstraction works, the position, height and orientation of the Ngwadini abstraction works were determined and its performance verified by hydraulic and numerical modelling by Stellenbosch University. The pumps only operate when the river is not carrying a heavy sediment load, as virtually all sediment transferred into the dam will ultimately settle out. In the long term, this will reduce its effective storage capacity. When the pumps are switched off (either because the dam is full or because the river water is carrying a high sediment load), it will be for a prolonged period and much of the sediment larger than about 10 µm will settle out in the rising main. As this would not easily be re-suspended when pumping next resumes, an automated Figure 8 Goodenough upper site layout (high-lift pumpstation, balancing tanks and hydrocyclones) Figure 7 Hydrocyclone de-silting works and balancing tanks 32 June 2022 Civil Engineering Figure 6 Isometric view of the abstraction works Sand trap / pump canal Trash rack Crump weir: higher notch Flushing channel Gravel trap Fish ladder Crump weir: low notch Pump intakes (not shown) Dry well pumpstation RCC platform walls High intake wall Submerged intake Boulder trap