This thesis investigates the ecological impact of hydropeaking in the Dutch Meuse and explores the effectiveness of retention as a mitigation strategy. While the presence of hydropeaking in the Dutch Meuse is known, there is limited knowledge about the ecological effects of these hydropeaks. In addition, no mitigation measures have been explored yet. This research aims to address these knowledge gaps.

Hydropower plants in the Belgian Meuse are operated to accommodate fluctuating energy demands and release fluctuating discharges as a result. These short-term fluctuations — called hydropeaks — propagate into the Dutch Meuse, where they disrupt aquatic ecosystems.

In this study, hydropeaks are isolated from the hydrograph using a wavelet transform to determine their magnitude and frequency. The wavelet transform method is also used to determine how far downstream hydropeaks reach into the Dutch Meuse. The impact of hydropeaking is assessed by looking at the degree to which protected habitats and species are affected by a median and a 95th percentile hydropeak. The hydropeaks are simulated in D-HYDRO, and the ecological consequences are assessed using three established ecological indicators: disconnected pool formation, wetted river area variation, and bed shear stress fluctuation. These indicators function as objective metrics that show how suited the river is for protected species and habitat types in response to hydropeaking. These species are river lampreys, chabot bullheads, macroinvertebrates, and the habitat type is large pondweed. Finally, the potential of retention is explored by incorporating a retention basin into the D-HYDRO model.

In the Dutch part of the Meuse, hydropeaking is mostly present in the Upper Meuse and the Common Meuse. Median hydropeaks do not cause significant harm to ecology in those reaches, whereas 95th percentile summer hydropeaks do exceed critical ecological thresholds associated with the studied indicators. The wetted river area variation threshold is exceeded at four one-kilometre reaches during an extreme hydropeak, destabilising potential spawning grounds of chabot bullheads and habitats of river lampreys. Retention reduces the number of reaches exceeding the wetted area variation threshold by 75%. At two out of three studied gravel bars, notable disconnected pool areas form that may endanger chabot bullheads, which are reduced by 61% and 67% when implementing retention. Bed shear stresses fluctuate substantially at two out of three gravel bars, compromising macroinvertebrate and chabot bullhead habitats. Retention reduces drift-prone areas at those gravel bars by 54% and 89%.

These results suggest that hydropeaking significantly affects ecology in the Meuse, and that substantial improvements can be achieved when retention is adopted as a mitigation strategy. However, hydropeaking is only one of several factors influencing the river's ecology. Therefore, measures that address hydropeaking impact alone are unlikely to bring the ecosystem to its desired state. An integrated approach is required, focusing on water quality, climatological developments, and ecosystem connectivity, alongside specific measures targeting protected habitats and species as defined by Natura 2000 guidelines (Rijkswaterstaat, 2023). Furthermore, while retention has been explored in a theoretical context in this study, its practical implementation poses significant challenges, making it unlikely to serve as a feasible and effective solution on its own.

The study concludes that the southern part of the Dutch Meuse is adversely affected by extreme hydropeaks, resulting in harm to the local ecosystem. The application of retention could improve ecological conditions. However, further research is required to optimise hydropeaking magnitude predictions, enabling retention based on real-time data. Additionally, impounding using the weir of Borgharen may be considered as an alternative mitigation measure. Enhanced communication between Rijkswaterstaat and Belgian hydropower plant operators can improve effective mitigation, regardless of the type of measure. Any measure should be embedded into a broader strategy which addresses all ecological stressors, such that a sustainable solution is achieved. Finally, as large summer hydropeaks are particularly detrimental to ecology, efforts to limit the formation of such hydropeaks at their source may reduce the need for downstream mitigation measures.