Mesoscale Mapping of Sediment Source Hotspots for Dam Sediment Management in Data-Sparse Semi-Arid Catchments

Reservoir sedimentation and spatiotemporal land use changes in their watersheds: the case of two sub-catchments of the White Volta Basin

Reservoir storage is compromised by sedimentation for which reason it has become an important matter in reservoir operation and management. While many studies have investigated sediment deposition rate in reservoirs, few have analyzed reservoir sedimentation from their catchment's land use change perspective. Based on bathymetric survey conducted on two reservoirs in the White Volta Basin in 2020 and analysis of four Landsat satellite imagery (1986, 1996, 2006, and 2020) within their watersheds, this study assessed the land cover change within the watersheds to draw inferences on the rate of sedimentation of the reservoirs located downstream of their catchments. The results revealed rapid sedimentation in the small-sized reservoir (Vea), with an annual sedimentation rate of 0.304% and a nominal sedimentation rate of 0.17% for the mid-sized reservoir (Tono). Furthermore, the savannah forest within the Vea catchment declined drastically from 29.4% (1985) to 9.9% (2020) influenced by the rapid expansion of farmlands from 18.7% to 47.9% within the same period, respectively. On the other hand, the savannah forest within the Tono catchment declined from 34.7% (1985) to 21.6% (2020) due to farmland expansion from 19.2% to 39% within the same period, respectively. The higher sedimentation rate observed in the small-sized reservoir was observed to be worsened by extensive tree cover removal in its catchment. Therefore, land cover characteristics within a watershed have a significant bearing on the rate of sedimentation in the reservoirs located downstream of their catchment. Hence, adopting a multi-sectorial approach to dealing with land use management is necessary to sustain reservoirs' storage.

Drivers, Impacts and Mitigation of Increased Sedimentation in the Hydropower Reservoirs of East Africa

Hydropower reservoirs are essential for the climate-neutral development of East Africa. Hydropower production, however, is threatened by human activities that lead to a decrease in water storage capacity of reservoirs. Land use/land cover and climatic changes are driving accelerated soil erosion in semi-arid East Africa, which ultimately increases reservoir sedimentation and decreases energy production. Sediment delivery dynamics at the catchment scale are complex, involving the interaction of multiple factors and processes on different spatial and temporal scales. A lack of understanding of these processes and their interactions may impede the efficiency of sediment mitigation and control strategies. A deep understanding of the processes of erosion and connectivity of the land to river channel, as well as storage of eroded material within hillslopes and floodplains, and sediment accumulation in the reservoirs supports selection of future dam locations and sustainable management of reservoirs. The sediment budget approach can provide such a holistic perspective by accounting for the various sediment sources, transport, sinks, and redistribution when the sediment is routed through that catchment. Constructing sediment budgets is challenging, but the potential for integrating a number of different techniques offers new opportunities to collect the required information. In East Africa, the spatial planning of dams is mainly dominated by political and financial motives, and impacts of land use and climate on the sediment transport dynamics are not adequately considered. Production of sediment budgets under different scenarios of land use and climate change should be an essential step when deciding the location and management strategies for dams. Selection of new hydroelectric reservoir sites must consider long-term scientific data on climate change, and the sediment budget components for sustainable land management planning, hydropower sustainability.