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MOHALE DAM

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When completed, the Mohale Dam will form the main storage element in Phase 1B of the LHWP and will be located on the Senqunyane River immediately downstream of the confluence with the Jorodane River.

The dam will be approximately 144 m high with a total capacity of 938 million m3. The catchment area for the dam is 938 km2 with an MAR in the order of 312 million m3 with the result that it is a 3 MAR dam.

Various options were considered for the Mohale Dam including rockfill, concrete arch and Roller Compacted Concrete (RCC). After thorough investigations it was decided to construct a concrete faced rockfill dam (CFRD)and with a height of 145 m, the Mohale Dam will be one of the top ten highest dams of its type in the world. One of the considerations leading to the selection of the CFRD is the fact that such dams are very stable when earthquakes or seismic activity are experienced as can be the case in Lesotho. The rockfill construction will also make good use of the basalt materials which are readily available in the surrounding Lesotho mountains. The CFRD is also less prone to time delay risks such as adverse weather conditions and the design is therefore better suited to the tight 5-year construction programme. The dam is scheduled to deliver water to Katse Dam in the year 2003 and must be able to impound the water two years earlier in 2001.

The dam will require approximately 7 million m3 of rockfill material and in order to achieve the programme goals of impoundment by the year 2001, the thin concrete face on the dam wall will be constructed in two phases. Instead of casting the concrete face once the dam reaches the full height, an interesting design feature of the Mohale dam is the placement of a portion of the face once the dam is approximately two thirds of the final height. As the lower slab is being placed, construction on the upper third will continue with the result that the dam will be able to impound water at least one year earlier than would otherwise have been the case.

Foundation excavation to form the base of the dam is expected to be limited and a geotechnical investigation is currently underway to confirm the foundation conditions. Further investigations are also underway to source the 7 million m3 of basalt for the dam wall as well as to find suitable aggregate for the 100 000 m3 of concrete needed for the face of the dam wall. A fleet of 50 large trucks will be needed to haul the enormous amount of rockfill up the steep roads which have yet to be blasted into the Senqunyane Valley. The haul roads and quarries will be located below full supply level whenever possible to minimise the environmental impact of the construction on the surrounding area.

The intake structure for the Mohale - Katse transfer tunnel will be located approximately 5 km upstream of the dam wall. With a minimum operating level of 2005 m above sea level, the Mohale reservoir will have an active storage of 856 million m3. The spillway of the dam, located on the left flank will be an ogee type with a concrete lined chute and a flip bucket at the bottom with a design capacity of 2 600 m3/s.

Preliminary estimates for the dam put the cost at between $135 million and $225 million at 1997 prices and exchange rates. These estimates do not include finance charges.