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UNESCO-IHE Institute for Water Education, Delft (2011)

Sedimentation of sand and silt in Roseires reservoir : vertical and horizontal sorting with time

Omer, A.Y.A.

Titre : Sedimentation of sand and silt in Roseires reservoir : vertical and horizontal sorting with time

Auteur : Omer, A.Y.A.

Université de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science (MS) 2011

Dams in rivers and the reservoirs behind them represent an area of concern, since sediment deposition has negative social, economical and ecological impacts on water resources. Roseires Reservoir is located on the Blue Nile, in Sudan. It is the first trap to the sediment coming from the upper catchment in Ethiopia, which suffers from high erosion and desertification problems. The reservoir lost already one third of its storage capacity due to sedimentation in the last four decades. This is a big economical loss to Sudan in addition to the high maintenance costs of the sediment clearance in front of the turbines to facilitate the hydropower production. This problem could be mitigated by appropriate management of the upper catchment, where sediment coming from. In order to do that, the areas provide the highest sediment volume 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 historical sedimentation and deposition of sand and silt in Roseires Reservoir, with the aim of identifying in when sediment from a certain area deposited. The identification of area of origin is based on sediment chemistry, the identification of deposition time from stratification inside the reservoir. This requires a combination of modelling and soil coring. From the model study suitable coring location can be distinguished as well as deposition volumes. The work also investigated the effect of dam heightening on future deposition rates, life span of the reservoir and the backwater effect of the new dam level as first attempt. The data was analyzed in order to understand the characteristics of the reservoir region and the trends of discharge, sediment and sedimentation in the reservoir with time. Delft3D software package was used to prepare a 2D-model with a capability to record the bed changes in Z-direction. The model was calibrated Hydrodynamically for one year with morphological factor (MF) equals one and validated Hydrodynamically for another year ( with MF=1). The calibrated hydrodynamic model was used to prepare a morphodynamic model for seven years (MF=30.4) from 1985 to 1992. The model was calibrated for the former period and validated for another fifteen years from 1992 to 2007. In addition to two future scenarios executed to see the effect of the dam heightening to Roseires Reservoir sedimentation.

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