Management of Extreme Sedimentation in a Reservoir in a Semi-Arid Region

The paper presents the extreme sedimentation case study of Welbedacht Reservoir, South Africa, and the respective RESCON 2 analysis for screening of sediment management alternatives. The reservoir is located in a semi-arid region characterized by increased hydrologic variability and therefore theprovided flow regulation services aremuch needed for the reliable Municipal and Industrial water supply in the wider project area. Welbedacht Reservoir has lost 95% of its initial capacity within the first 45 years of operation due to excessive sedimentation. The reservoir is hydrologically small and therefore it is able to accommodate the water losses associated with flushing despite the semi-arid climate conditions. The dam, however,does not embody a low-level outlet and therefore flushing over the gated spillway crest has a limited deposit removal capacity being, thus, able to sustain only 5% of the original storage capacity, despite the favorable narrow geometry of the reservoir. Another constraint is the limited flushing duration which is imposed by the requirement for continuous operation of the adjacent water treatment plant that receives water from the reservoir. The calibrated RESCON 2 model was able to reproduce the historically observe storage loss based on limited data. The analysis indicates that the efficiency of flushing could be improved with a larger water level drawdown and a prolonged duration. If low-level outlets were foreseen in the dam design and effective flushing was implemented from the first years of operation significant cost savings could have been achieved and a more reliable water supply could have been ensured. Retrofitting hydraulic structures for sediment management purposes in existing facilities is possible but associated with significant costs can and is technically demanding.