Author(s): Bahaeldeen A. Zaid; Abdalla M. T. Shigidi; Katinka Koll
Linked Author(s): Katinka Koll
Keywords: River Bed; Degradation; Bed load; Aggradation; 1D Model
Abstract: Morphological changes in the River Nile due to large dams such as Merowe Dam are taking place due to changes in the flow and sediment regime. The typical downstream changes vary from the bank erosion, river bed degradation and changes of bed material. In order to investigate the river bed changes downstream of Merowe Dam, 1D numerical model calculation for a downstream reach of about 313 km was carried out. MIKE11 software package was used to predict the river bed changes over 50 years of the dam operation. The hydrodynamic and sediment model has been calibrated for the conditions before dam. Due to the large reservoir volume [12. 4 Billion m3] the bed load will be deposited in the reservoir and part of the suspended sediment will be transported to the downstream river reach. The long term simulation was carried out using the mean outflow from the dam as indicated by the operation study of the dam and assuming that no bed load will be released from the dam. Van Rijn formula for bed load and suspended load was applied for the sediment calculation. During the simulation the river bed update was assumed to be uniform throughout the cross-section. The result of the model has shown that the river reach from the dam to Dongola some 313 km downstream, can be fairly divided into three zones depending on the morphological changes taking place. A degradation zone extends from the dam site down to some 50 km after 5 years and 100 km after 15 year of the dam operation and remains with no further increase towards the downstream. The degradation zone is followed by an aggradation zone which extends about 50 km right on the big bend of the river Nile around Al-Daba town. The aggradation zone increases with time towards the downstream. Following the aggradation zone there is more or less a stable bed level zone that decreases with time extending from chainage 150km to Dongola. The average degradation in the first 10 km after 50 years is about 5 m and is associated with water level drawdown of about 3m. The average degradation from chainage 19. 5 km to chainage 100km is 0. 85m. The aggradation zone extends from chainage 100km to chainage 200km after 50 years and is associated with an average increase of about 0. 94 m in the bed level. The rest of the modeled river reaches is more or less stable with some sedimentation taking place from chainage 228km to chainage 263km. The high degradation process predicted by the model in the first 100 km downstream Merowe Dam was found to be sensitive to high discharges, accordingly reducing the high flood peaks will significantly reduce the degradation downstream the dam.