New insights into hydropeaking mitigation assessment from a diversion hydropower plant: The GKI project (Tyrol, Austria)

Abstract Hydropeaking is an anthropogenic regime that results from the strong flow variations in the receiving waters of storage hydropower plants. These variations are due to flexible energy production, i.e. the sudden increase of electricity demands or from its decrease, affecting the fish assemblages of that river stretch. We evaluated the mitigation potential of the hydropeaking diversion hydropower plant (hd-HPP) named Gemeinschaftskraftwerk Inn - GKI, located in the Inn River at the Alpine region (Tyrol, Austria). This HPP will use a weir reservoir for the retention of hydropeaking coming from upstream HPPs, together with the diversion of peak flows, which intends to mitigate the impacts of hydropeaking along the residual flow stretch while allowing for hydropower production. Our study aims to evaluate and mitigate the stranding potential of the European grayling (Thymallus thymallus) within the time range of the species' larvae life-stage in the Inn river's studied stretch. To do so, we addressed a set of high-resolution historical hydrological data and, afterwards, the implementation of mitigation operational scenarios, for wet and dry years. The performance of the historical status and the mitigation scenario was assessed based on 1D unsteady modelling in a set of 26 cross-sections along a 6.5 km stretch, by quantifying the downramping rate and flow ratio events over the grayling larvae's tolerance thresholds to stranding (0.2 cm/min) and the Austrian legal threshold for flow ratio (1:3), respectively. Furthermore, we developed a new eco-hydromorphological assessment scheme, including topographic features that have been confirmed as stranding increase factors, besides the hydrological parameter's assessment. A qualitative classification was defined (Very High, High, Medium, Small, Low) for the species' larvae stranding potential, gathering the hydrological and morphological parameters assessment, represented in a map with the cross-section's location and assessment class. Results confirmed that the operational scenario highly reduced the number of events where downramping rate and the flow ratio were over the thresholds in all sections, with higher effectiveness in the wet year analysis, where tributaries influence showed a major role. Afterwards, for the qualitative assessment, in the historical records analysis the cross-sections status were included in the High or Medium classes of stranding potential. In the mitigation scenarios, most of sections' status shifted to the Low or Small classes. With this study, we confirmed that the GKI hydropower scheme may be considered as an effective hydropeaking mitigation strategy. Additionally, we presented a novel 1D-based approach for stranding potential assessment, grounded on a cost-effective and integrative classification scheme.