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| Basic concept
Engineering |
EngineeringThe underlying concept of the major headworks is to provide water for LUSIP without reducing the remaining flow of the Great Usuthu River, during the dry season. In the wet season, over a 7-month period between November and May, a part of the peak flows from the Usuthu River will be diverted. The water will be conveyed through a feeder canal to the Bovane reservoir. Run-off from the Mhlathuzane and Golome Rivers will also flow into the same reservoir. The yields of these catchments are expected to be relatively small. Water from the Bovane reservoir will then flow via two main canals to their respective irrigation areas for further distribution. WeirTo divert water from the Usuthu River during the rainy season a weir will be constructed at Bulungapoort. The weir and the intake on the Usuthu River will be combined into one compound structure to economise construction costs. The sole purpose of the weir is to block the flow of the river sufficiently so as to create a more or less constant water depth at the intake. Feeder canalFrom the intake the feeder canal, with a length of 23 km and a required discharge capacity of 13 m3/s, will convey water into the reservoir. In order to reduce the amount of sediments a sand trap will be incorporated into the feeder canal at the most advantageous location. ReservoirIn order to create the Bovane Reservoir, which will have a capacity of about 160 million m3, it is necessary to dam the Bovane ridge. This ridge consists of a massive dolerite sheet intruded into the sandstone. It is dissected by two gorges
containing the Mhlathuzane and Golome Rivers. DamsThe Mhlathuzane Dam will be built on the small Mhlathuzane River, which flows through the Bovane ridge. This dam will contain the main spillway and outlet pipes for the irrigation canals and will control the reservoir waters. The second dam will be the Golome Dam located on the Golome River that seldom has water except during flash floods in the rainy season. The third dam will be the South Saddle Dam, a comparatively modest structure. Emergency SpillwayIn addition to these three dams an emergency spillway, to handle the probable maximum flood, will be built between the Mhlathuzane and Golome Dams. All sites offer excellent foundation conditions for dam construction.
Main canalsWater will be released from the Bovane Reservoir into the two main canals, and conveyed to the irrigation Blocks: Secondary systemFrom the main canal sections a number of pipelines and lift stations will convey water to the distribution points. In the area below the main canal secondary pipelines and canals will follow ridges. For higher areas canals will be laid along contours. Using storage reservoirs, these canals will supply the fields located at higher altitudes. These reservoirs are filled via pipelines linked to lifting stations situated along the main canal. Canals are only used where soils are deep enough and gradients are less than 2%. Otherwise pipelines will be used.
On-farm irrigation systemWater will be delivered to blocks of 50 to 100 hectares that will be managed by WUG. On-farm irrigation will be provided by furrow irrigation and dragline sprinkler systems. Sprinklers are the system of choice for smallholder farmers who have already organised themselves into Associations and are drawing irrigation water direct from the Usuthu River. |
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