Transboundary sediment transfer from source to sink using a mineralogical analysis. Case study: Roseires Reservoir, Blue Nile, Sudan

Abstract

Reservoir sedimentation is restricting optimal water resources management of the Roseires Reservoir across the Blue Nile River, in Sudan. About 30% of its storage capacity has been lost by sedimentation before dam heightening (2012), despite regular sediment sluicing and flushing. At the same time, increasing soil erosion in the upper river basin in Ethiopia is significantly reducing land productivity. This paper shows the results of combining mineralogical X-Ray Powder Diffraction (XRPD) and cluster statistical analyses to identify the source areas of the sediments deposited inside Roseires Reservoir. The XRPD was used to determine the mineral content in the soil and rocks from the reservoir and from the upper basin in Ethiopia. The cluster analysis was used to determine the degree of similarity between sediments in the reservoir (sink) and the ones in the upstream subbasins (source areas). The results show that the sub-basins of Jemma, Didessa and South Gojam are the main sediment source areas. This is also supported by a sediment balance study at subbasin level showing that most of the sediment originates from these areas. The high sediment production in these two sub-basins could be related to the land use changes that occurred during the last 40 years, when natural forest soil cover decreased from more than 70% to less than 25% and agricultural crops increased from 30% to more than 70%. The results from the study are relevant for water resources planning and watershed management at transboundary level.

Water Value Flows Upstream

“…However, the current excessive concentrations silt-up reservoirs and clog irrigation canals, leading to costly measures to mitigate these problems [34][35][36]. The explanation of how this massive increase in silt-load occurred is complex [33,37], but it is evident that the source area is in the highlands of Ethiopia [38], and that the high erosion rates on farm lands not only resulted in low crop yields and poverty, but even changed the hydrology [39].…”