Optimising reservoir operations

8 November 2019

Filling period performance may impact the acceptability of new dams in large systems. If release rules are allowed to change during system expansion, performance improves but not updating rules during expansion can lead to 25% losses in financial benefits. New research, using the Blue Nile as a case-study, demonstrates how evolving operating rules during multi-reservoir system expansion and planning can increase the ability to identify best performance plans

Meeting growing food and energy demands in many regions will require the expansion of water resources infrastructure. Planning the expansion of multi-reservoir systems is a complex task but, as Robel T. Geressu and Julien J Harou claim in new research, the ability to change operating rules during expansion is often overlooked.

In their paper called “Reservoir system expansion scheduling under conflicting interests” published in Environmental Modelling and Software, the authors investigate the impact of allowing different extents of reservoir rule changes during multi-reservoir system expansion on design recommendations and their scheduling over time.

“The paper shows that operating rules of interdependent reservoirs should ideally change during new reservoir filling to accommodate its impact on inflow to other reservoirs and improve performance,” they state.

In some multi-reservoir systems, the greatest hindrance to further development is their downstream impact during the period when new reservoirs fill, which can last several years.

“New reservoirs in a river reach could also impact other new reservoirs downstream of them depending on the relative position, scale, and sequence of new infrastructure implementation. Delayed filling and excessive downstream impact can make dam projects financially infeasible and politically unacceptable,” Geressu and Harou write.

The proposed approach investigates the development of new reservoirs on the Blue Nile, and takes the filling period of dams into consideration

The largest tributary of the Nile River which contributes more than half of the annual Nile flow at the High Aswan Dam, the Blue Nile River basin hosts large potential for hydropower production in Ethiopia. However, because of long-standing disputes on water use rights in the Nile basin and Ethiopia's lack of capacity to self-finance large projects, the basin remains under-developed.

According to the authors, the Blue Nile presents an interesting case for evaluating multi-reservoir system investment scheduling because of the number of proposed reservoirs still on the drawing board, their large sizes, and their close-proximity which requires consideration of the impact of flow attenuation by upstream dams on downstream ones.

In this study, alternative scheduling options are assessed considering the reduction in downstream flow in filling periods. The study investigates best options (choice of reservoirs, the timing of their activation, and their operating rules that adapt to system expansion) for various balances of benefit from new reservoirs and their downstream impacts.

The study evaluates the impact of alternative problem formulations on decision support and trade-offs between conflicting performance of interest to upstream and downstream countries. Visualising the benefits and impact of alternative plans and performance trade-offs of the best plans can help foster understanding and help parties negotiate benefit sharing. However due to limited number of future hydrological projections, the results of this study is “meant to demonstrate the approach rather than guide actual infrastructure development decisions”.
Optimised operations

In their conclusions, the authors say their approach is demonstrated using the Blue Nile hydropower reservoir system, identifying dam activation schedules that achieve efficient trade-offs for various conflicting objectives including discounted net benefits, reliable downstream releases and energy generation. Multi-reservoir system expansion scheduling formulations with three different levels of operating rule responsiveness to expansion are compared for performance and computational requirements. Results show benefits increase when release rules change as the multi-reservoir system expands, with failure to optimally adapt operating rules loosing up to 25% of net present value.

The proposed scheduling approach enables planning without having to depend on initial assumptions about filling rate, operating rules, sequence of implementations and future priorities (how reservoirs may be operated in future).

Results show that assessing the scheduling of reservoirs assuming fixed operating rules overestimates the compromises required in agreeing on a rate of dam filling. Optimising operating rules for each reservoir and for each unique system expansion stage reveals the best compromise designs (ie whether a change in physical or operating rule design is needed to minimise impact and maximise benefits).

While increasing the detail with which operating rule flexibility is represented is computationally costly, it allows a more accurate identification of efficient development plans and reveals less dramatic trade-offs between performance objectives of upstream and downstream stakeholders.  

As Geressu and Harou conclude: “Failure to consider the coordinated use of a multiple reservoirs achieved through re-operation of rules as the reservoir systems expands under-estimates benefits and could lead to suboptimal design recommendations.”

Reservoir system expansion scheduling under conflicting interests by Robel T.Geressu and Julien J.Harouba. Environmental Modelling & Software. Volume 118, August 2019, Pages 201-210. https://doi.org/10.1016/j.envsoft.2019.04.002

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