Abstract
To meet targets for climate change, energy transition towards renewable electricity generation is urgently needed. Most of the remaining feasible renewable energy potential is found in variable resources from wind and solar power. Hydropower with storage capacity is crucial to meet the need for integrating this variable renewable power generation into the grid, and there is a strong need for modernization of the European hydropower fleet to meet this challenge. At the same time, freshwater biodiversity and ecosystem services from rivers and lakes have seen a rapid decline in the last decades, where impacts from hydropower are one of the key drivers. Modernization of hydropower must therefore combine the needs for increased energy services and improving the ecological status of affected rivers and lakes. Modernization includes both increasing flexibility and storage options to meet new demands in the energy system as well as adapting to modern requirements to meet ecological targets as well as societal needs such as flood control and water management. We focus on some guiding principles and methods to achieve more sustainable hydropower generation, giving also some examples of how research and innovation are contributing to real cases of improving both hydropower generation and ecological status using ecohydraulics principles and tools. Examples from new developments in Norway for retrofitting existing hydropower infrastructure with pumped storage and capacity expansions in the EnviPump project will be given as well as examples from the EU-funded innovation and demonstration project ReHydro about sustainable refurbishment of hydropower.
Keywords: Ecohydraulics; hydropower; modernization; ecology; sustainability
1. Introduction
To meet targets for climate change, energy transition towards renewable electricity generation is urgently needed. Most of the remaining feasible renewable energy potential is found in variable resources from wind and solar power. Hydropower with storage capacity is crucial to meet the need for integrating this variable renewable power generation into the grid, and there is a strong need for modernization of the European hydropower fleet to meet this challenge (IHA 2023; Quaranta et al. 2021). At the same time, freshwater biodiversity and ecosystem services from rivers and lakes have seen a rapid decline in the last decades, where impacts from hydropower are one of the key drivers. Modernization of hydropower must therefore combine the needs for increased energy services and improving the ecological status of affected rivers and lakes. Modernization includes both increasing flexibility and storage options to meet new demands in the energy system as well as adapting to modern requirements to meet ecological targets (Arthington et al. 2018) as well as societal needs such as flood control and water management.
2. Research projects
The main objective of the EnviPump project is to analyze the effects on physical and biological conditions from developing pumped storage hydropower in existing reservoirs, as well as to find effective mitigation measures. EnviPump will use hydro-physical modelling to explore impacts on water temperature, ice cover and flow. Further, key ecological impacts on fish populations of hydro-physical variations will be investigated.
The ReHydro project will demonstrate how biodiversity can be improved with new fish-friendly turbines, re-establishment of environmental flows, improving the thermal regime in downstream rivers and monitored using new tools such as eDNA and remote sensing. Smarter use of water resources will also be implemented, so that more services can be delivered to the power markets, while multi-purpose use of the water resources for navigation and recreation can be expanded and the ability to mitigate flooding and draughts are strengthened.
3. Ecohydraulics principles and tools
The main principles of Ecohydraulics are the combination and interaction of hydraulic conditions and effect on the ecosystem, including assessments of hydro-morphological conditions and biological requirements. Ecohydraulic principles must incorporate the effects of man-made infrastructures and river regulation such as alterations in hydrology, disturbance of morphological processes and the introduction of dams, weirs, barriers and other structures. When modernizing and retrofitting hydropower, there is an opportunity to improve also the ecological status of hydropower-affected rivers and lakes (reservoirs). Classic and modern ecohydraulics tools are therefore needed to understand how hydropower has impacted rivers and lakes historically, as well as to model and predict how modernization and retrofitting actually can improve the status.
4. Results
The first steps in designing more eel-friendly turbines show a potential to improve survival of downstream eel migration at the Belver power plant on the Tagus River in Portugal with a potential for application at several power plants in the Rhône River in France.
In the Ain River in France, a pumped storge plant will be built between the lower Coiselet reservoir and the Saut-Mortier reservoir upstream. As the small Saut-Mortier reservoir already has a pumping facility to pump water to the larger upper Vouglans reservoir, it is possible to pump water in two steps from Coiselet to Vouglans reservoirs. The main reason for introducing the new pumped storage plant is to better manage the inflow and reduce the need for hydropeaking. In addition, studies to investigate how releases from the upper Vouglans reservoir can contribute to decrease water temperature in the Ain River in periods with environmentally harmful high temperatures.
Across all scenarios, pump storage clearly modifies the thermal profile (Figure 1). In Dry conditions, pump storage leads to a noticeably cooler surface layer and a more uniform vertical temperature distribution, indicating enhanced mixing. Under Normal conditions, the effect remains significant, with reduced surface temperatures and a shallower thermocline compared to the non-pump scenario. In Wet conditions, the differences are subtler, but pump storage still contributes to slightly lower surface temperatures and diminished stratification. Overall, the figure demonstrates that pump storage operations play a key role in mitigating thermal stratification, especially during drier periods, thereby enhancing the reservoir’s thermal resilience and supporting more adaptive water management strategies.
In two rivers that are bypassed by power plants in the Røldal-Suldal hydropower schemes in Norway, environmental flow for the survival of a special population of large brown trout will be provided by a small hydropower plant in the Roalkvamsåna River, and by pumping water from the downstream Suldalsvatn lake to the Brattlandsdalsåna River. If the pump for Brattlandsdalsåna had been installed over the last 20 years, it would have been necessary to run the pump for 62-165 days per year to guarantee at least 1 m3/s in the lower part of the river (Figure 2).
Figure 1. Thermal profiles of the Coiselet dam without pumped storage (above) and with pumped storage (below) in a dry (left), normal (middle) and wet (right) year.
Figure 2. Left: Yearly operation days (in blue bars), yearly pumping volume in Mm3 (red bars). Right: Pumping rate averaged over operation days/year for 1 m3/s
Retrofitting reservoir hydropower to pumped storage hydropower will enhance the flexibility and storage capacity in power systems but introduces also some new environmental challenges (Harby et al. 2013). Pumped storage hydropower systems affect the environment primarily through two mechanisms. First, by transferring water between reservoirs, these systems mix water masses with different physio-chemical properties. Secondly, the operation of these systems results in fluctuations of water levels within the reservoir, which can influence shoreline erosion, habitat availability, and ecosystem structure. These fluctuations can disrupt natural thermal stratification by modifying the thermocline, affecting temperature profiles, and mixing processes. These processes increase turbidity and alter nutrient dynamics, leading to subsequent ecological changes.
We have analyzed expected water level fluctuations in the Holmavatn reservoir in a future power system using Holmavatn reservoir as the upper reservoir in retrofitting conventional hydropower with pumped storage capacity. Reservoir levels were derived from a power system model using future power price estimates combined with inflow and weather data from weather years 1989-2010. Figure 3 shows large variations between years as some years have quite stable and high-water levels for several months in early winter, whereas other years have very low water levels in early winter. Advanced hydro-dynamic modelling tools must be used to assess the impacts on hydro-physical conditions such as water temperature, stratification, currens and ice cover, in order to understand the biological and ecosystem impacts of changes in hydro-physical conditions.
5. Conclusion
The European hydropower fleet will have to go through a process of modernization to increase flexibility and adapt to future power system needs, as well as meeting sustainability goals and improving both the geomorphological and ecological status of hydropower rivers and reservoirs. To meet this challenge, it is crucial to include ecohydraulic tools and knowledge in the early stages of planning modernization and retrofitting. This presentation will show more results and examples of how ecohydraulics tools have contributed to ensure that modern hydropower can meet both power system and sustainability requirements,
Figure 3. Water levels in Holmavatn reservoir with modelled pumped storage system for the months October-July for weather years 2001-2010.
6. Acknowledgements
The ReHydro project is funded by the EU under the Grant Agreement 101147310. The EnviPump project is funded by the Research Council of Norway under the Contract 358659.
7. References
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Harby, A., Sauterleute, J., Korpås, M., Killingtveit, Å., Solvang, E. and Nielsen, T. 2013. Pumped storage hydropower. In Stolten, D. and Sherer, V. (eds) (2013). Transition to Renewable Energy Systems. Wiley-VCH. Pp 597-618. https://doi.org/10.1002/9783527673872.ch29
IHA (2025). World Hydropower Outlook. Sector trends and insight. https://www.hydropower.org/publications/2025-world-hydropower-outlook
Quaranta, E., Aggidis, G., Boes, R.M., Comoglio, C., De Michele, C., Patro, E.R., Georgievskaia, E., Harby, A., Kougias, I., Muntean, S., Pérez-Díaz, J.I., Romero-Gomez, P. Rosa-Clot, M., Schleiss, A.J., Vagnoni, E., Wirth, M. and Pistocchi, A. 2021.Assessing the energy potential of modernizing the European hydropower fleet. Energy Conversion and Management, Volume 246. https://doi.org/10.1016/j.enconman.2021.114655