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KATSE DAM

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The key to the Lesotho Highlands Water Project is the breathtaking Katse Dam which has recently (1997) been completed in the Malibamatso Valley just below the Bokong River confluence.

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The Katse Dam is a major international undertaking and is now the highest dam in Africa with a height of 185m. The dam was designed by a consortium of South African and overseas consultants and the construction contract was awarded to Highlands Water Venture, a consortium of Italian, French, South African, German and British contractors on 14 December 1990.

 

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The dam forms part of the first phase of the Lesotho Highlands Water Project (LHWP) and is usually referred to as Phase 1A of the LHWP. It forms the main collecting storage in Lesotho from which all transfers to South Africa will be made through approximately 80 km of concrete lined tunnels.

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The reservoir formed by the Katse Dam is very narrow, winding and deep with a surface area at full supply level of only 35.8 km2. The relatively small surface area together with the low evaporation rate at the dam result in very low evaporation losses from the reservoir water surface - approximately 10% of the evaporation experienced at Vaal Dam for the same storage.

 

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The intake tower is located approximately 18km north of Katse Dam and has been designed to accommodate 70 m3/s which was the maximum transfer originally envisaged for full implementation of the LHWP. The intake tower is 23m in diameter and 98m high to accommodate the large range in reservoir storage elevation as well as to facilitate the draw-off at four distinct lake levels. This will enable the quality of the transferred water to be controlled at all times. Due to the position and design of the intake tower, there is a certain volume of water in the reservoir that cannot be accessed via the transfer tunnels. This dead storage is approximately 430 million m3 with the result that the effective live storage behind Katse Dam is reduced from the total storage capacity of 1 950 million m3 to 1520 million m3.

The Katse Dam is a double curvature concrete arch dam with a crest length of 710m and contains 2.3 million m3 of concrete. The catchment area of the dam is 1867 km2 and the natural Mean Annual Runoff (MAR) is approximately 560 million m3/a - the Katse reservoir therefore has a live storage of approximately 3 MAR. The overflow crest spillway comprises 10 ungated bays each 16m wide. The jet from the spillway will be dispersed before dropping into a plunge pool created by a 17m high tailwater dam.