Civil Engineering December 2021 | Vol 29 No 11

Civil Engineering December 2021 35 T he Kruisvallei Hydropower Project involved the engineering and construction of two small run- of-river hydropower plants on the Ash River, between the towns of Bethlehem and Clarens in the Free State. The plants harness the highly regulated flow from the Lesotho Highlands Water Project to produce renewable energy interconnected into the national grid. The project was part of Round 4 of South Africa’s successful Renewable Energy Independent Power Producer Programme. The Power Purchase Agreement (PPA) was awarded to Zevobuzz as the Independent Power Producer, which is owned in majority by Red Rocket and H1 Holdings. OVERALL DESIGN CONSIDERATIONS The two power stations are each able to operate with a rated flow of 37 m 3 /s, passing through vertical Kaplan turbines and producing a combined 4 MW at rated condition. Use of existing weirs in the river provided the available head, specifically 7.2 m and 4.9 m for Middle and Lower Kruisvallei respectively. Based on the site topography and river hydrology creating a high flow and low head scenario, conventional vertical Kaplan turbines were selected as the optimum choice and are believed to be the first of their kind installed in South Africa. Being a low head scheme, literally every centimetre of head loss has an associated value. Consequently, it was imperative to design the scheme mini- mising hydraulic losses through the water conveyance. Optimisation of costs versus returns in the financial model achieved a maximum hydraulic loss of 0.5 m and 0.7 m respectively at the Middle and Lower Kruisvallei sites. RESOLVING KEY DESIGN AND CONSTRUCTION ISSUES Extensive Computational Fluid Dynamics analyses were undertaken to optimise waterway designs and to ensure uniform, turbulence-free flow entering the tur- bines. The bypass canals were designed to discharge into the tailrace channels rather than directly back to the river, where energy dissipation structures would have been required, also allowing for a more compact site. The use of a vertical Kaplan turbine required a significantly deeper excava- tion than for a horizontal turbine but resulted in a more efficient scheme overall. In addition, the turbine is fitted with a step-up gearbox, significantly reducing the size, weight, and costs of the generator. Hydromechanical equipment design was optimised in terms of cost and flex- ibility. Radial gates were employed instead of the traditional stoplogs at the upstream and downstream ends of the waterways. Active Spillway Gates® were used to con- trol flow into the bypass channels in the event of an unplanned shut-down. Hyson cells were utilised as a flexible and permeable concrete liner. The use of a concrete liner significantly decreased the friction losses in the canals, aiding the hydraulic efficiency of the scheme. The plastic geocells acted as permanent form- work and the concrete was cast in situ, ac- celerating construction. The plastic layers between the square interlocked concrete cells act as a bond breaker, allowing the release of hydrostatic pressure when the canal is emptied and eliminating the need for a subsurface drainage system. CONCLUSION With a total construction cost of approxi- mately R252 million and completion in 22 months, the project was an exceptional feat of engineering and construction con- sidering the extremely tight construction programme and budget. The teamwork approach between the contractor, client and three main contrac- tors resulted in the successful completion of a project that all parties are extremely proud of, and which will have immense benefit for the people of South Africa for years to come.  SANRAL SA I CE 2021 Nat i onal Awards Category: Techn i cal Excellence FINALIST Technical Excellence category (Sponsored by Maccaferri SA) KEY PLAYERS Client: Red Rocket EPC Consultant: Zutari Contractors: Red Rocket EPC, Eigenbau, Tractionel Enterprise & Scotta Kruisvallei Hydropower Project Extensive analyses were undertaken to optimise waterway designs The project will generate 4 MW of power