Several studies investigated the Grand Ethiopian Renaissance Dam (GERD) operation using simulation techniques, yet the optimum operation policy of GERD is not investigated. This study presents a nonlinear optimization model for GERD operation using natural historical inflow series (1912-2011). The model aims to find the optimum firm hydropower production along with the resultant impacts on the Blue Nile flow. Several formulations of the objective function were investigated, but the most stable GERD firm energy were obtained when minimizing the sum of squared deviation from the firm hydropower production. Due to the nonlinearity of the objective functions and several constraints, extended Lingo 17.0 algorithm was used to solve the nonlinear optimization model. The results of model applications showed that GERD mean hydropower production is about 14,650 GWH/year. The firm energy is on the order of 12,900 GWH/year that represents 24.5% of dam installed hydropower capacity. It is found that the GERD operation, if only used for power production, will permanently reduce the Blue Nile flow to the downstream countries on the average of 3.2%. It will also extend the downstream drought periods 200- 300%, compared to the case before GERD.
The announced configuration of GERD would control 1.4 times the mean annual flow of the Blue Nile. A prudent dam operation strives to maximize the operational benefits while maintaining a minimum associated environmental risks, most importantly, protecting the natural flow rights. In order to arrive at better downstream conditions, less dam active storage capacities are tried and results are analyzed. For instance, the results showed that decreasing 40% of the GERD live storage, yields 90% of the mean annual hydropower production of the announced capacity, while the Blue Nile flow deficit would be limited to 2.5%. This alternative size can be adapted even if the construction stages did not permit the other storage reduction means of the dam. The findings of this study would help resolving the related outstanding issues of the GERD.
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