Civil Engineering June 2022 | Vol 30 No 5

Civil Engineering June 2022 33 system (which can be de-activated if de- sired) prevents settling out by immediately draining the entire contents of the rising main back to the river. The automated system also allows for re-filling the rising main at a controlled rate to avoid damaging the pumps due to insufficient backpressure. Although not necessary for operational reasons, variable frequency drives are necessary to limit the start-up currents (the Eskom supply comes all the way from Craigieburn and has certain limitations). NGWADINI DAM CHOICE OF DESIGN As there is very limited material with a high enough clay content to construct a conventional earthfill embankment dam, the Ngwadini Dam has been designed as a concrete face rockfill dam (CFRD). As the Ngwadini catchment is very small, the design flood can easily be accommodated by a side channel ogee crest spillway with a trapezoidal concrete spillway chute, flip bucket and plunge pool. The intake tower (which incorporates multi-level offtakes and three outlet sleeve valves) is accessed via a bridge which can be used by con- struction/maintenance vehicles. Table 1 indicates the Ngwadini Dam and appurtenant structures’ characteristics. NGWADINI DAM GEOLOGICAL AND GEOTECHNICAL ASPECTS Dam site and quarry / borrow pit inves- tigations from a number of studies since 1996 included rotary core drilling, test Figure 9 Ngwadini Dam design overview Table 1 Ngwadini Dam characteristics Component Value Catchment area 1710 ha Mean annual precipitation 910 mm Mean annual evaporation 1 050 mm Dam category III Dam capacity Total storage volume 10.6 million m 3 Dam embankment Type CFRD Height 52 m Crest length 300 m Crest width 8 m Upstream slope 1V:1.4H Downstream slope 1V:1.6H Fill volume 625 000 m 3 Face slab area 12 000 m 2 Spillway Side channel spillway, chute with downstream flip bucket Recommended design flood peak (1:200) 430 m 3 /s Safety evaluation flood peak (RMF+) 714 m 3 /s Probable maximum flood peak 810 m 3 /s Spillway ogee crest length 40 m Chute width 10 m River diversion 1:20 year flood 160 m 3 /s Outlet conduit diameter 4.5 m Outlet conduit length 230 m Outlet works Type and size of intake tower 8 m x 11 m rectangular 49.3 m high concrete tower Intake offtakes 5 x 1000 mm diameter staggered levels Pipe system Dual vertical stacks Pipe diameter 800 mm Pipe length and discharge 54 m 1.7 m 3 /s Downstream control 3 x sleeve valves (2 x 500 NB, 1 x 200 NB) Emergency draw down FSL to MOL in 46 days Side channel spillway CFRD dam wall Intake tower Diversion tunnel Outlet works Flip bucket Chute Spillway bridge