DAMS, RESERVOIRS & STORAGE
The Polihali Dam will create a reservoir with a surface area of 5 053 hectares (Credit: Matla a Metsi JV)
POLIHALI DAM AND TUNNEL PROGRAMME UPDATE
O
nce completed, the Polihali Dam will add approximately 2 325 million cubic metres in storage capacity to the Lesotho Highlands Water Project (LHWP) in Phase II of the programme. This will increase the project’s current annual supply rate capacity from 780 to 1 270 million cubic metres – a welcome addition towards meeting South Africa’s rising water needs. Simultaneously, the additional flow of water from Polihali will increase the volume of hydropower generated within Lesotho by approximately 40% and is another step towards meeting the country’s domestic electricity needs and reducing its dependence on imported power. The dam site is located approximately 130 km north-east of Lesotho’s capital, Maseru, 20 km west of the highlands town of Mokhotlong, and just a kilometre downstream of the confluence of the Khubelu and Senqu (Orange) rivers.
Key elements The Polihali works include:
Dam
and
appurtenant
-1 66 m high concrete faced rockfill dam (CFRD) forming the main dam on the Senqu River - 48 m high CFRD saddle dam - 98 m long spillway with 40 m wide concrete chute and flip bucket discharging into an excavated plunge pool in rock - access bridges - compensation outlet system with intake tower and a compensation outlet pipe - outlet house with hydropower station - river gauging stations in the upper reaches of the Khubelu, Senqu, Moremoholo, Mokhotlong and Sehonghong Rivers with associated access roads. The construction programme is focused on the impoundment date and for this reason both the rockfill embankment and concrete face slab of the Polihali Dam will be staged. The Polihali Dam will create a reser voir with a sur face area of 5 053 hectares. Its waters will be conveyed by gravity through the Polihali transfer tunnel – the second main works feature of the Phase II water transfer component – to the Katse reser voir.
TRANSFER TUNNEL - 38 km long - 5 m nominal bore - Links Polihali reservoir to Katse Dam - Transfers water by gravity - Construction commences in 2022
Milestones Construction procurement is advanced for both the Polihali Dam and the Polihali transfer tunnel, with construction expected to commence this year. Anticipated milestones according to the current Phase II master programme are: - Impoundment (first stage): 2025 - Dam complete (civil works): 2027 - Dam complete (including final wet testing): 2028 - Tunnel complete: 2027 - Water deliver y: 2027/28. In December 2021, the Lesotho government confirmed the Oxbow Hydropower Scheme as the preferred option for Phase II. The procurement of a ser vice provider for the detail design and super vision is under way.
IMIESA July 2022
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