Mohale Dam is a concrete faced rock-fill dam in Lesotho. It is the second dam, under Phase 1B of the series of dams of the proposed Lesotho Highlands Water Project (LHWP), which will eventually include five large dams in remote rural areas of Lesotho and South Africa.^[1] The project has been built at a cost of US$1.5 billion.^[2]

The Mohale Dam was awarded the 2005 Fulton Awards by the Concrete Society of South Africa as having the "Best Construction Engineering Project and Best Construction Technique." ^[3]

The dam is built across the Senqunyane River below its confluence with the western tributary, the Likalaneng River.^[4] Investigation of the dam site in the "Highlands of Lesotho" has revealed "rounded, steep sided valleys" where the soil cover is thin and basaltic lava is the dominant formation.^[5] The dam site has been identified with two geological features of faults which needed treatment measures.^[6] The dam is located 100 kilometres (62 mi) to the east of Maseru, which is the capital of Lesotho.^[7] The dam drains a catchment area of 938 square kilometres (362 sq mi) over a river length of 60 kilometres (37 mi) with an elevation variation of 1,050 metres (3,440 ft).^[4]

The importance of the large water resources potential of the country, one of the various natural resources of the land-locked country. Subsequently, Director of Public Works, requested an engineer to examine the potential of the project as a possible means to supplement the water supply needs in South African gold mines.^[8][9]

In 1983, the World Bank began implementing the project. Phase I covered some of the key strategic components of the project, while Phase IB's financing covered not only engineering design and supervision of the main works but an institutionalized framework with a panel of experts in the field of engineering and the environmental and social aspects of the project. The Development Bank of Southern Africa (DBSA), the European Investment Bank (IIB), and Export Credit agencies were also approached to finance the project.^[10]

The Lesotho Highlands Water Project (LHWP) was established, with the objective of supplying water to the Gauteng region of South Africa and for hydroelectric power to meet the needs of Lesotho. A treaty signed between the government of Lesotho and South Africa gave the green light for the project, and the team was set up to implement the project.^[11][12] The treaty is limited to Phases IA and IB only.^[13] The Trans-Caledon Tunnel Authority (TCTA), vested with the operation and maintenance aspects of the project is guided by the larger establishment of the Lesotho Highlands Development Authority (LHDA).^[14] The project envisages diverting water in the highlands of the country through three dams; two under Phase I (PhaseI A -Katse Dam and Phase II - Mohale Dam) and the Polihali Dam under Phase II. Construction of the Phase I dams were completed in 2003 and formally commissioned in 2004.^[11][12]

The Mohale Dam under Phase I B of the LHWP, is designed to divert about 70 cubic metres (2,500 cu ft) per second of storage to the Katse Dam reservoir. The water supply meets the needs of the Gauteng region which encompasses the mining and industrial cities of Johannesburg and Pretoria.^[7] The regulated flow from the reservoir is 9.5 cubic metres (340 cu ft) per second which is transferred as gravity flow through the interconnecting concrete lined tunnel of 32 kilometres (20 mi) length with an internal diameter of 3.4 metres (11 ft).^[15]

Mohale Dam is categorized as a high dam, and at 145 metres (476 ft) in height it is said to be the highest rock-fill concrete-face dam (CFRD) in Africa. It is built as an embankment dam with a rock-fill concrete-face on the upstream side. The volumetric content of the dam structure is 7.5 million m³. The dam has a length of 700 metres (2,300 ft) with a 12 metres (39 ft) wide crest. The spillway, located on the left bank of the dam, is not gated and flood is routed over the crest of the ogee shaped spillway through a concrete lined chute with a flip bucket and a stilling basin at its terminus. The concrete spillway is designed for a flood discharge of 6,000 cubic metres (210,000 cu ft) per second.

The project's reservoir submergence involved some of the most fertile agricultural land in the area. The project affected 390 households and some 1700 people.^[17]