Abstract
Abstract. Roseires Reservoir, located on the Blue Nile River in Sudan, is the first trap to the sediments coming from the vast upper river catchment in Ethiopia, which suffers from high erosion and desertification problems. The reservoir has already lost more than one-third of its storage capacity due to sedimentation in the last four decades. Appropriate management of the eroded soils in the upper basin could mitigate this problem. In order to do that, the areas providing the highest sediment volumes to the river have to be identified, since they should have priority with respect to the application of erosion control practices. This requires studying the sedimentation record inside Roseires Reservoir in order to assess when and how much sediment is deposited and to identify its source. This paper deals with the identification of deposition time and soil stratification inside the reservoir, based on historical bathymetric data, numerical modelling and newly acquired soil data. The remoteness of the study area and the extreme climate result in coring campaigns being expensive and difficult. Therefore, these activities need to be optimised and coring locations selected beforehand. This was done by combining bathymetric data and the results of a depth-averaged morphodynamic model recording the vertical stratification in sediment deposits. The model allowed for recognising the areas that are potentially subject to neither net erosion nor bar migration during the lifespan of the reservoir. Verification of these results was carried out by analysing sediment stratification from the data collected during the subsequent field campaign.
Highlights
The construction of dams and reservoirs represents a great achievement for the management of the water resource, but at the same time it creates a relevant disturbance to the river ecology and morphology
By recording erosion and deposition during the development of the bed topography, the model would allow for the best areas to be recognised. Another advantage of using a numerical model lies in the possibility of better analysing the sedimentation process in Roseires Reservoir, Lesser et al (2004) extensively describe the open-source Delft3D code which is applied in the current study
Two promising coring areas inside Roseires Reservoir were selected by combining bathymetric data analysis with the results of a 2-D morphodynamic model including horizontal and vertical sorting
Summary
The construction of dams and reservoirs represents a great achievement for the management of the water resource, but at the same time it creates a relevant disturbance to the river ecology and morphology. At that specific location the deposited sediment is mainly sand, most probably settling only during the highflow seasons, which does not allow for the yearly sequences of deposition to be recognised This preliminary field campaign showed the necessity of returning to the field, but this time only after having identified the best coring locations. This would maximise efficiency and minimise costs, considering that the area is difficult to access and that coring can be carried out only at the end of the low-flow season, when large parts of the reservoir bed become dry. This allowed us to study the sediment depositing in the reservoir and gave a unique opportunity to validate the model
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