Reservoir Sedimentation as a function of land use and land cover change
Mavima, Godwin. A.
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In Zimbabwe, sediment load has exceeded normal design limits in many reservoirs, thus reducing storage capacity and shortening their useful life for human benefit. The study sought to investigate the impact of land use and land cover change on reservoir sedimentation. Two reservoir catchments, Sebastopol Dam (MAR: 135 mm yr-1) and Chesa Causeway Dam (MAR: 129 mm yr-1) in the Upper Manyame and Upper Ruya subcatchments (Manyame and Mazowe Catchments), respectively, were studied. Sedimentation rates during the 2009-2010 rainfall season at both sites were quantified using hydrographic surveys and grab sampling methods. The study also examined the driving factors for land use and land cover change and how they have changed over time (1991 - 2009) using Landsat TM images. Sedimentation analysis showed that the current sediment specific yield at Sebastopol is 390 tkm-2yr-1-using the grab sampling method and 258 tkm-2 yr-1 from hydrographic survey while that at Chesa Causeway is 774 tkm-2 yr-1 from the grab sampling method and 503 tkm-2 yr-1: from hydrographic survey. Projections based on current sediment loading indicate that Sebastopol dam will last for another 11 years while Chesa Causeway will be silted up in 9 years. Contrary to common belief, satellite images show that vegetated land is increasing while bare land is reducing at Chesa Causeway dam catchment. Both sediment quantification methods confirmed that Chesa Causeway dam is in a less conserved catchment than Sebastopol dam and also that Chesa Causeway is a small dam built in a large catchment area with a very small design gross storage ratio of 0.01; land use activities influence the lifespan of reservoir and, in this case, the less conserved Chesa site which is characterised by alluvial gold panning activities will have a much less useful lifespan of 25 years compared with Sebastopol Dam (40 years). From the projection made, if this trend continues at the current rate of sediment loading, alternative water sources need to be explored immediately at both study sites for the communities who depend on both dams for sustenance of livelihoods. The study recommends that hydrographic surveys and the grab sampling method should be both used for estimating sedimentation rates in reservoirs as they can be effective and useful tools for decision making in integrated catchment management; all catchment councils should adopt and enforce comprehensive catchment management plans.