The development of a new framework is looking to enhance the dual functionality of some of the US Army Corps of Engineers’ (USACE) reservoirs. With single-purpose flood control reservoirs described as ‘falling short’ when it comes to addressing the escalating demands for diverse water resource management, the study aims to ensure those reservoirs initially designed for flood control, will also be able to support water supply through the determination of maximum safe water levels (MSWLs).
The findings by Mingda Lu and Venkatesh Merwade in in the Journal of the American Water Resources Association, illuminate the intricate balance required between managing flood risks and enhancing water supply, indicating that precise operational adjustments can significantly improve reservoir sustainability and efficiency.
“This method,” the authors state, “offers a viable pathway to convert single purpose reservoirs into multi-purpose reservoirs, meeting growing water demands while ensuring robust flood mitigation, and making a step toward better water utilisation.”
With most of the Army Corp’s dams and reservoirs being constructed since the 1900s, with an average age of 61 years, over time there have been significant impacts on sediment transport, runoff coefficient, and regional evapotranspiration. These have led to reservoir underperformance due to changes in watershed and climate conditions. In turn, such underperformance is likely to result in the degradation of water resources management for flood control, hydropower generation, and water supply and can further socioeconomically impact stakeholders and investors.
USACE has proposed to enhance floodwater utilisation by using the available water to meet other water supply needs. The study looked to modify and optimise the reservoir operations of 15 medium and large sized reservoirs, operated by the Louisville District, Great Lakes and Ohio River divisions, and originally built for flood risk reduction purposes downstream of the Ohio River basin. These possess potential for multipurpose use due to their larger capacities compared to small sized reservoirs.
The findings of the study indicate that some of the reservoirs have the potential to be used for water supply purposes in the future. These were namely, Cagels Mill Lake, Cave Run Lake, Nolin River Lake, and Taylorsville Lake.
The authors added that the framework can be implemented to reservoirs with similar data availabilities and functionalities despite the disparities in size and location. However, the design events may undergo modifications depending on the specific performance requirement stipulated by different stakeholders and decision-makers involved in reservoir operation and management.
“This study,” Lu and Merwade explained, “focused on single extreme inflow/flood events with sufficiently large peak flow rates and long durations for the determination of MSWLs. The total design flood volumes not only exceed any single extreme event that was observed historically, but also greater than the majority of cascade/continuous multiple extreme flow events. However, the statistical possibility of cascade/continuous multiple extreme flow events, which may result in larger flood volumes and potential pressure of the dam cannot be disregarded.”
Irrigated hydropower
Other research in Renewable and Sustainable Energy Reviews has found there is major potential for storage-fed irrigation in many places where dams are currently mostly used for hydropower. This includes Europe, Northern parts of the Americas, and Africa. While there is little potential to increase hydropower in developed economies, basins in the global south have the potential to increase both hydropower and storage-fed irrigation, Rafael Jan Pablo Schmitt and Lorenzo Rosa claim.
As a result of this, a clear pattern of surpluses and deficits emerges. Notably, hydropower deficits abound most clearly in Europe and parts of Asia in India and the Indo-Pacific. This seems counterintuitive, as many European countries have strong hydropower portfolios. However there is very little potential for more hydropower to meet the projected increases in demand. Spatial patterns of deficits for storage-fed irrigation are more complex, the authors go on to explain, with hotspots in Europe, and South Asia, but also in Sub-Saharan Africa and the Middle East. Thus, Europe and South Asia are some hotspots where gaps in both irrigation storage and hydropower are projected to challenge food and energy systems in the future.
Motivated by the observation that both sectors depend on, and compete for the same service, namely water storage, this research set out to study future growth of hydropower and irrigation. It identified the challenges this competition will pose, and determined geographic locations where it will be most pronounced. As a result, the authors provided the first comprehensive analysis of future joint growth rates and potentials for hydropower and storage-fed irrigation and thus critical sectors in the water-energy-food nexus.
Compromises in Hungary
Reservoir management in Hungary should be considered as a multi-criteria decision problem, where a best compromise solution for satisfying the needs of water users, flood control, ecology, and recreation has to be searched for.
A characteristic part of the hilly landscape of Hungary, reservoirs were constructed to serve different goals such as fish farming, flood control, and recreation, as well as industrial, agricultural, and communal water supplies. Over time though, as István et al explain, the economic and livelihood conditions in the Hungarian countryside have undergone significant changes.
With industrial activities and fish farming declining, the recreation sector has been enhanced with many reservoirs gradually being transformed into lakes serving recreational purposes only, such as angling, swimming, and boating. To optimise the hydrological conditions for these activities, water level fluctuation in these reservoirs has been mitigated to a great extent. As a result, the capacities of these lakes to serve purposes other than recreation have been constrained severely.
As the authors explain in their paper presented at the 10th International Symposium on Hydraulic Structures, all of this is happening at a time when the need for certain traditional reservoir services, such as flood control and communal/agricultural water supplies, has started to grow again.
Sustained conflict has ensued between different users of the reservoirs. The authors give the example of the Maconka Reservoir on River Zagyva which was designated primarily for flood control, and for water supply as a secondary function with recreational and sport fisheries considered only as additional uses.
With the local fishing association demanding continuous and stable high water levels in the reservoir, it compromises the reservoir’s flood control and water supply functions. Increased evaporation losses have further enhanced water shortage problems significantly in the hilly basins of Hungary.
Model-based analysis has demonstrated that constructing a new reservoir on the Zagyva River has the potential to improve flood control, water supply, and ecological conditions in the river basin while providing satisfactory conditions for recreation in the reservoir itself. In addition, this new reservoir would also contribute to mitigating the conflict over the use of an existing reservoir downstream.
The authors claim “the general experience in Hungary is that as soon as significant water cover stays in a reservoir for longer periods, the conquest of this reservoir by anglers and other recreational users begins inevitably”. To prevent future conflict, they recommend applying a model-based approach for identifying reservoir configurations and operations that satisfy all interested parties. It is also important to break with the previous unilateral practice in such a way that all eligible users agree before constructing a new reservoir or modifying an existing one. In this agreement, the partners approve the best compromise design and operation of the flow control structures identified during the model-based analyses.
