2016 – Design Considerations for the Stortemelk Hydropower, South Africa

BJ Rochecouste Collet, PC Blersch, AL Olivier

The paper shows how multidisciplinary engineering can create future-proofed solutions for dam management and how innovation is bringing those advancements to life. It includes the retrofitting of a small hydropower station to an existing dam structure, enhancing the use of the dam. The paper also reveals design, technical and project finance considerations underpinning the multi-disciplinary services.

Water supply in South Africa’s economic hub of Gauteng journeys from the Lesotho Highlands Water Project (LHWP) through three river systems that converge in to the Vaal Dam from which the water is treated and pumped for domestic usage. One of those systems delivers water originating in Lesotho to discharge into the Ash River. The Ash River was, by origin, a small river with an environmental reserve flow of 50 l/s. However, with the LHWP significantly adding flow, the annual average increased to 24,500 l/s (24.5 m3/s) and is set to increase further with future phases of the scheme. To mitigate significant erosion caused by the greatly increased flow, several structures were erected along the river, including the Botterkloof Dam. Whilst the energy was dissipated in the dam’s spillway, a private developer studied if the water could be used for energy generation. The river also offered some rapids some 1.6 km downstream, which also showed potential for hydropower generation. An option to combine the two sites was also considered.

Aurecon conducted the feasibility study in 2010 for both sites, including the combined option, which concluded that there would be significant benefits in the implementation the projects in two separate schemes. This boasted many advantages including reduced capital investment, reduced social impact (canoeist), reduced geotechnical risks, and lesser land acquisition leading to a better return on investment. Aurecon are currently providing engineering, procurement and construction management (EPCM) services for the entire project. Construction of the 4.4 MW hydropower station commenced construction in September 2014 and was commissioned ahead of time and under budget.

The founding conditions under the proposed hydropower station location, comprising interbedded sandstone and mudstone was fairly poor with the mudstone effectively decomposing in less than two days. The Botterkloof dam was build on a thick layer of sandstone which dips quite steeply towards the right bank. The right bank on the other hand comprises an old paleo channel. The Boston A dam, located on the left bank immediately adjacent to the Botterkloof Dam, is founded on the weather mudstone and the spillway is grass lined. The power station construction was constructed in the narrow space in between the two existing dams – the Botterkloof Dam (owned by the Department of Water and Sanitation (DWS), Government of South Africa) and the adjacent privately-owned Boston A Dam. Permission had to be obtained from the respective owners and all regulatory permits approved before the project could be submitted to the South African Renewable Energy Independent Power Producer Programme (REIPPP) implemented by the Department of Energy (DoE) of the South African Government.

Another significant challenge in the construction itself included the need for deep excavations through the left embankment of the Botterkloof Dam and adjacent to the spillway stilling basin whilst such construction needed to be done without affecting operations and stability of either of the two dams.

The solution was a shallow intake, followed by a cut and cover concrete penstock leading to a compact hydropower station housing a single 4.4 MW vertical “Compact Axial Turbine” Kaplan turbine ending in the tailrace, which was rotated at 90 degrees.

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