RESERVOIRS WATER BALANCE ANALAYSIS, JOINT OPERATION AND OPTIMAL OPERATION RULES CURVE FOR SYSTEM PERFORMANCE (A Case Study of Proposed Reservoirs on the main Blue Nile River, Ethiopia)

Abstract

This study has been conducted on the envisaged reservoirs along the main stem of the Blue Nile River on Ethiopian highlands. The study sites (i.e. Karadobi, Mandaya and Border reservoirs) water shed is located in western part of Ethiopia, between 7° 45' and 11° 12'50"N latitude, and 35°05'40" and 39°45'E longitude. Approximately all over basin here is in after called Blue Nile River Basin (BNRB).See fig 1.1 Management of available water resources and its conservation in terms of quantity and quality is necessary to study the source of water and appropriate method of utilization. This research has applied Spread sheet excel and HEC-ResSim technique to determine the aspects of reservoir water balance, monthly and daily reservoirs operation using Spread sheet and HEC-ResSim respectively in the case of proposed reservoir on the main Blue Nile in Ethiopia have calculated from 1972 to 2002. The spreadsheet model uses monthly evaporation, surface runoff, and precipitation as an input and groundwater component is neglected, Based on the calculation results it is found that evaporation, rainfall, surface runoff inflow to dam, regulated released, storages increments or declined and reservoir spillage from the reservoirs constitutes 46.40, 43.63, 1505.81, 1185.13, 34.2 and 352.52 Mm3 of the annual average Karadobi reservoirs water balance respectively, 38.50, 33.154, 2982.95, 2849.72, 244.79 and 183.22Mm3 annual average for Mandaya reservoir and finally 51.26, 41.89, 4198.66, 3855.65, 32.05 and 306.86 Mm3 in case of annual average Border reservoir. Due to the climatic condition and topography evaporation is the highest when we compare the relative volume-area of all reservoirs. The regulation of upstream reservoirs cause transference of flow pattern modification of the downstream reservoir (i.e. the regulation at Karadobi reservoir causes elevation curve to calculate the head for power generation subsequently the mean power r are determined 2010.1, 1349.23, 740.57 MW from Karadobi, Mandaya and Border reservoirs respectively. Finally the HEC-ResSim model uses the reservoir physical characteristics (Elevation­ Capacity-Area curve), net monthly evaporation from the reservoirs (mm), daily inflow (m3 /s) operation rule adjusted i.e. tandem operation for Karadobi and Mandaya reservoir finally released operation rule for the Border reservoir, the alternative released guide curve of elevation maximum capacity operation level and maximum elevation relation of dam appurtenant structures such as spillway, penstocks and tail water elevation. The outputs of tandem operation results an operation rule based on the reservoir water levels of the three reservoirs that maximize the net release volume of water for system maximum hydropower generation. HEC-ResSim has also used to determine the optimal release rule of the maximum power generation by simulation/optimization technique, which is based on successive trials. The analysis considered system power release pattern and individual evaporation and spillage-losses minimization and analysis optimal release alternative for four different system levels for the maximum system conservative zone utilization up to 90 percent. The advantages and disadvantages of the other alternatives were evaluated. From the released pattern alternatives, the simulation/ optimization operation was suggested considering relatively the system minimum losses and maximum power generation of average 908.63, 1592.22 905.58 MW respectively in case of Karadobi, Mandaya and Border reservoirs respectively. These indicate the intention of the optimal system performance