“An innovative design approach and engineering ingenuity in analysis using physical and numerical hydraulic and sediment modelling, among other tools, will ensure long-term operability of the Lower Thukela Bulk Water Supply Scheme,” – Stephan Kleynhans, Professional Civil Engineer, Aurecon
The Lower Thukela Bulk Water Supply Scheme supplies potable water to towns along the KwaZulu-Natal North Coast in South Africa. These range from Mandini Local Municipality in the north to KwaDukuza Local Municipality in the south, as far as Ballito.
Key project infrastructure components include: a weir across the Lower Thukela River (approximately 200 m in length); an abstraction works and low-lift pump station (ultimate capacity of 110 Ml/d) located on the south bank of the river; a desilting works; a water treatment works on the north bank of the river (current capacity of 55 Ml/d and upgradeable to 110 Ml/d); and a high-lift pump station linked to bulk supply pipelines (approximately 30 km in length with diameters of up to 900 mm) and associated water storage reservoirs (up to 30 Ml in size).
The feasibility phase of the study, initiated in 2009, determined the optimal location and process requirements of the proposed works. Final environmental authorisation was obtained from the Department of Agriculture and Environmental Affairs on 25 October 2013, with construction starting shortly thereafter. Umgeni Water appointed Aurecon for planning, design and procurement, together with M&E design evaluations of the R1.6-billion project.
One of the challenges on the project was the significant volume of sediment that the river transports, which has to be removed to produce water of potable quality. Sediment loading on the filters of the water treatment works was not to exceed a turbidity of 10 NTU, which led to the design preference for removing sediment from the raw water before it reached the treatment works. This was accomplished by optimising the location of the abstraction point from the river, and by designing the abstraction works to remove as much sediment as possible.
An innovative approach to the project design was required to enable the Lower Thukela Bulk Water Supply Scheme to remain operational even during major floods and droughts, despite high sediment loads in the river. Engineering ingenuity was displayed in analysis and design by careful assessment, using tools such as physical and numerical hydraulic and sediment modelling, to ensure long-term operability of the works.
Functional efficiency of the project was demonstrated by designing the abstraction works to remove as much silt as possible from the raw water and return it to the river before it reaches the treatment works. This reduced both the processing requirement in the treatment works, and the environmental impact of disposing of large quantities of waste silt.
Another challenge was the time constraints of the project. The urgency of the need for the additional water supply from this project led to it being fast tracked. The construction of the access road to the south bank of the Lower Thukela River and the weir across the river were critical to the programme. However, flow in the river is seasonal, with most high flow events occurring in the wet summer months and construction within the river could only take place in the drier five-month winter period.
The weir across the river was constructed during this low flow window period over two consecutive winters. Containing the raw water pipeline within the abstraction weir avoided the negative aesthetic and environmental impacts of a separate pipeline crossing the river. Our operator-centric approach to designing the water treatment works was also a notable advance in resolving some of the project’s engineering problems.
“The project was successfully commissioned in June 2017. The Lower Thukela Bulk Water Supply Scheme has improved the reliability of water supply in the area and provides sufficient water for planned new land developments to proceed, which will enable economic growth in the region to continue,” says Kleynhans.