Chapter 16 - Reservoir operation analysis for Ribb reservoir in the Blue Nile basin

Abstract

Reservoir operation is an important part of the planning and management of water resources. It is a complex problem that involves many decision variables, conflicting objectives, and considerable risk and uncertainty that lead to significant challenges for operators when making operational decisions. An operating policy is developed to guide the reservoir operators in decision making of releasing or storing water. The primary purpose of this study is to establish operating guide curves that include the probability of water shortage and water spillage by employing HEC-ResSim model for Ribb reservoir. Ribb reservoir is located in south Gonder zone of Amhara region mainly to irrigate an area of 19,925ha. This study is to evaluate alternatives of reservoir operations and develop operating guidelines. Simulation was performed based on three defined alternatives. Alt-1, Alt-2, and Alt-3, that is, 100%, 90%, and 80% of the irrigation water demand fulfilled. The input data for the analysis were: inflows, reservoir characteristics, irrigation and domestic water demands, and reservoir water losses. The analysis revealed that the catchment, upstream of Ribb dam could yield 237.12Mm3 (million cubic meter) of water annually including rainfall on the reservoir. The outputs of the model were the difference in inflow and outflow of the reservoir system. Maximum release is 223.4Mm3 including domestic and water losses. Storage capacity of the reservoir is 233.7Mm3 and water losses are 50Mm3 per year via spilling. The study also assessed the performance of the reservoir using reliability, resilience, and vulnerability (R-R-V) criteria. Based on the outputs, the study provided a reservoir operating guide curves (upper, lower, and operating rule curves). The analysis deduced that the available water source in the Ribb watershed has sufficient capacity to develop the 100% of the irrigable land (19,925ha). But, in order to increase the reservoir performance (reliability) and for full utilization of the available water source in the watershed, the study proposes additional storage dam. Dry years are likely to occur during El Niño years at a confidence level of 90%. Low and high flow analysis for selected tributaries and flow at the Blue Nile River flow shows different recurrence intervals of the high and low flows.