Blue Nile River Course Morphology Change Response to Watershed Treatments: Lake Tana to Grand Ethiopian Renaissance Dam

Abstract

Within the Blue Nile River (BNR) system, large amounts of riverine deposits and influx of hill slope materials provide the sediment for the channel to transport. The amount of material that the river system can transport is directly related to the discharge and slope, which in turn are driven by watershed management, climate and river cross section lithology. In this study, the channel morphology change is particularly described as the change in elevation of the riverbed level. The possible variations in sediment supply under different scenarios of watershed management practices and the corresponding changes in the riverbed level were computed. Three different watershed management (agronomic practices, mechanical measures and biological measures) scenarios were tested for their possible effect on the BNR morphology. The analysis for each of the scenarios effect on the riverbed level change was obtained from a calibrated finite element model for sediment and diffusive wave transport models. The current sediment supply and river sediment transport capacity were first evaluated. This was achieved by first calibrating the model with observed average monthly flows and sediment concentration data of two stations located at Kessie Bridge and Ethiopia-Sudan border. The performance of the model was measured by NSE, PBIAS and RSR values for which the model has resulted in acceptable performance level. The morphologic characters of the river (riverbed elevation, cross section and slope) used in the river hydraulics and sediment transport analysis were obtained from 30 m digital elevation model. The results indicated a decrease in the average riverbed level during flood rising time (June to July) and an increase during flood recession time (August to September). The decrease in the riverbed level was due to the reduction in the sediment concentration which gives the flow a higher erosive power than the current situation. The scoured bed in the flood rising period has created a larger cross section for the flow in the recession time, which resulted in a decrease in the river competence that ultimately creates deposition of sediments. The result has shown a relatively high change in the riverbed level between 300 and 550 km downstream of Lake Tana. In between these two distances, from Lake Tana along BNR, Muger, Guder and Fincha Rivers join the main BNR course. However, the effect is becoming smaller while moving downstream. The river morphology around the location of the Grand Ethiopian Renaissance Dam is also significantly affected by the watershed treatments; this is mainly due to the treatments made on the Dabus River catchments, rather than the other main rivers upstream. Among the scenarios developed for improving the watershed, scenario 3 (biological measures) has resulted in the highest riverbed level change in both flood peaking and recession periods. Thus, this scenario is expected to bring about lesser sediment for the river to transport and the greatest change in the river morphology when compared to the other scenarios.