Rating of discharge at monitoring station affected by backwater effects - El Deim station in the Blue Nile

Abstract: On the Blue Nile in Sudan, near the Ethiopian border, there is a measurement station named El Deim. The discharge assessments carried out at this station are crucial for the water resource management in Sudan. Due to changed conditions, caused by a heightening of the downstream-located Roseires dam, new methods for discharge assessment are needed. The objective of the present study was to examine possibilities and methodologies to assess the discharge at this station. The flow dynamics was examined through steady state as well as dynamic hydraulic modeling by use of the Mike 11 modeling software package. By simulating possible future scenarios, in the aspect of discharge variations in the Blue Nile and water level variations in the reservoir, the effects from the raised dam on El Deim could be studied. The model was based on bathymetrical data in form of cross sections. As boundary conditions for the simulation, measured and synthetic data series of discharge and water levels were used. The known measured water levels at El Deim were compared with the simulated water levels at El Deim for the same discharge scenarios. The modeled value corresponds well to the measured values. The existing discrepancies between the simulated and measured values are likely caused by insufficient bathymetrical data.   Simulation results show that the flow dynamics at El Deim are highly dependent on the water level of the reservoir and the discharge’s rate of variation. Accordingly, rating curves were created for a range of water levels at the reservoir. With the use of these curves, and tables/equations based on them, the discharge can be rated by knowing the water level at the Roseires dam and El Deim. However, the results from this study are more a description of the principles of how the discharge ratings could be performed. If the methodology and rating tools from this study are planned to be implemented the model must be improved with more bathymetrical data. The improvements are needed to create more accurate curves, tables and equations for discharge rating. Discharge ratings can then be produced and enable better operation of Roseires dam and a more efficient use of the valuable water resources in Sudan. In order to test the applicability of the created model and produced rating tools they should be compared with new measurement data from El Deim with the heightened Roseires dam fully implemented.   It is possible to assess the discharge at El Deim even when backwater effects affect the station. The methodology developed in this thesis would be applicable for similar studies at other locations.