Assessing the rate of sedimentation of the Lubovane reservoir and the implications on the lifespan of the Lusip Project in Sphofaneni, Swaziland

Abstract

An average of 19% of the reservoir storage volumes has been lost to sedimentation in Africa. The Lubovane reservoir in Swaziland, having been commissioned in 2009, is not an exception to sedimentation as all reservoirs globally lose about 1% of their storage due to sedimentation annually. The Lubovane reservoir was constructed as part of the Lower Usuthu Smallholder Irrigation Project(LUSIP) aimed at intensifying and commercializing agriculture for smallholder farmers. The study aimed at determining the rates of sedimentation of the Lubovane reservoir, with sediment deposited from Mhlatuzane River and a canal that conveys water from the Usutu River to the reservoir. Suspended sediment concentration was determined through grab sampling 300mm below the water surface in the canal and the river. The Revised Universal Soil Loss Equation(RUSLE) combined with GIS and remote sensing were used to predict sediment yield for the Mhlatuzane micro-catchment and it was validated using data that was collected from four erosion monitoring plots. A bathymetric survey was conducted in the Lubovane reservoir to determine the sediment that has already settled in the reservoir and the annual capacity loss due to deposition of sediment. The average sediment yield was found to be 26.8, 24.3, 8.83, 29.8 and 8.99 t.ha-1.yr-1 for years 1995, 2000, 2005, 2009 and 2015 respectively. Concentration of suspended sediment was found to be 2231 mg/l and 3436 mg/l for Mhlatuzane River and the Feeder canal respectively. The sediment yield of the Mhlatuzane River derived from the sediment concentration was 2.98 t.ha-1.yr-1 and the combined sediment yield from the sediment concentration monitoring, was 0.82x106m3 leading to a 0.52% of storage capacity. The bathymetric survey data showed that there has already been 14.7x106m3 of sediment that has settled in the reservoir to date, with an annual capacity loss of 1.36%. Suspended sediment monitoring underestimated the sediment loading in the reservoir since bedload was not sampled and hence, this method cannot be used to without sampling bedload toadequately monitor siltation of the reservoir. The study then concluded that if no measures are taken to reduce sedimentation and the sedimentation rates remain at this level, the reservoir life will be reduced from 100 years (design lifespan) to 81 years.The loss of storage capacity will lead to an annual loss in 20% yield of 1.23%.