Civil Engineering June 2022 | Vol 30 No 5

36 June 2022 Civil Engineering FLOOD HYDROLOGY AND SPILLWAY DESIGN The flood hydrological assessment of Ngwadini Dam included catchment deline- ation using GIS programs and comparison of calculation results for several determin- istic, statistical and empirical hydrological flood determination methods. Floods with different recurrent intervals were selected and routed through the dam and over the spillway to determine the optimal spillway length and dam freeboard height. The requirement is to have the most efficient and economical spillway layout to safely pass the routed safety evaluation flood without the dam failing. With the complex rapid changes of momentum and energy being dissipated, the flow patterns over and through spill- ways and associated structures should, ideally, be hydraulically modelled to con- firm that there is a stable free surface at all flows. With the relatively small size of the Ngwadini catchment together with a low flood unit discharge rate, the analysis could be safely limited to numerical modelling. The computational fluid dynamic (CFD) analysis took into account turbulence, air entrainment, angular vorticity and other physical parameters. DAM OUTLET WORKS DESIGN The outlet works of Ngwadini Dam com- prises a dry well concrete intake tower and a concrete conduit running below the base of the dam. The conduit is the only part of the initial river temporary diversion works to remain as permanent works (which extends through the whole site between the upper and lower cofferdams). Upon completion of the base of the tower, the conduit will be sealed off with a concrete plug. The intake tower has a dual-stack multi-level outlet system (dual for redundancy and multi-level for selecting optimum water quality when making releases). The vertical interval between the intakes with both stacks being used is 10 m. Both stacks can also be utilised to draw down the reservoir in emergency conditions. CONCLUSION The Durban North Coast is well-served by multiple interconnected bulk water supply schemes which provide good operational flexibility. The Durban South Coast is a lot more vulnerable when particular parts of the system have to be shut down for whatever reason. With the completion of the LUBWSS, the South Coast will enjoy much greater operational flexibility and a significant amount of demand from the Umgeni System will be freed-up for further growth in the greater Durban area. Article compiled by: Matt Funnell Pr Eng GIBB Design Engineer Mat Holmes Pr Eng GIBB Senior Design Engineer Duncan Shaw Pr Eng GIBB Lead Design Engineer Jacques du Plessis Pr Eng GIBB Technical Lead: Dams Figure 12 CFD model of side channel spillway and flip bucket Figure 14 Ngwadini Dam outlet works Figure 13 Spillway chute mean water surface profile extracted from CFD models Elevation (masl) 120 115 110 105 100 95 90 85 80 75 Distance along spillway chute CL (m) 200 150 100 50 0 CFD results – Spillway chute mean water surface profiles Spillway chute floor elevation 2Yr mean water surface elevation 20Yr mean water surface elevation 200Yr mean water surface elevation RMF+ mean water surface elevation PMF mean water surface elevation Top of spillway wall