Impact of channel bar form and grain size variability on estimated stranding risk of juvenile brown trout during hydropeaking

Abstract

Hydropeaking leads to artificial fluctuations in discharge and corresponding water levels with pronounced dewatering areas between base and peak flow along gravel bars and channel banks. In the present study, 16 hydropeaking reaches in Austria were investigated to assess possible differences in the estimated stranding risk for young of the year brown trout according to different gravel bar types and differences in microtopography roughness. Based on hydrodynamic-numerical modelling, a predictive habitat modelling approach was implemented in the study design. Accompanied by grain size sampling along the various channel bars, a conceptual stranding risk model (SRM) was developed. The results showed that a high variability in estimated stranding risk exists for the tested sites considering discharge ratios of 1:3, 1:5 and 1:10. With respect to the discussion of establishing legal thresholds for ramping ratios in discharge, it was documented that, exemplarily, a discharge ratio base flow/peak flow of 1:5 (winter base flow conditions) could cause minor differences in the spatial extent of dewatering areas and the related estimated stranding risk for juvenile brown trout compared to a ratio of 1:2 for summer base flow conditions. Microtopographic roughness was addressed due to sampling and analysis of grain size distributions. Statistical testing of grain size distributions revealed significant differences between the surface material compositions of the investigated gravel bars. Those differences are evident, particularly for the coarser fraction (d 90 ), which is important as cover for young of the year brown trout. These aspects of grain size in habitat use and hydraulics have been addressed in the conceptual SRM. The results showed that point bar morphology, in particular, was less sensitive to the risk of stranding compared to, for example, alternating gravel bars. Considering the multiple pressures for alpine rivers, the improvement of structural features due to bar formation and related self-forming processes is discussed as a possible alternative for future mitigation measures to reduce the negative impacts of hydropeaking. igure 3. Various hydro-morphological units A-J investigated in examining the impact of different hydropeaking scenarios along gravel bars; XS = cross-sections. This figure is available in colour online at wileyonlinelibrary.com

Eco‐hydraulic modelling of the interactions between hydropeaking and river morphology

“…The proposed modelling approach allows highlighting trends in the eco‐hydraulic response of a river reach along a gradient of different channel morphologies, from almost flat bed to braiding. Overall, the eco‐hydraulic response shows a high degree of nonlinearity in behaviour, in qualitative agreement with the findings of Hauer et al ().…”

“…They simulated almost identical hydropeaking intensities (1:3, 1:5 and 1:10) to the present analysis, occurring over several sites with different bar morphologies in regulated Austrian rivers. The outcome of the stranding risk model of Hauer et al () highlights a qualitatively analogous contrasting effect of gravel bar morphology with respect to the one emerging from the present analysis: simpler morphologies, found in more regulated reaches, featured the smallest reduction in wetted area, coherently with the blue region in our Figure c. However, the same morphologies are also characterized by the absence of minimum suitable habitats, as it also emerges from our Figure a,b at both base and peak flows.…”

“…When moving to single‐thread morphologies, alternate bars are predicted to be an extremely sensitive environment to macroinvertebrate drift, to offer safer regions from stranding, and also are not affected by relevant diversity loss, although their diversity is never particularly valuable in ecological terms (HMID usually <9). In a recent study, Hauer et al () analysed the stranding risk associated with different types of gravel bars, by combining information on peak‐to‐base change in wetted area, on shallow habitat availability and on substrate grain size composition to develop and apply a conceptual stranding risk model. They simulated almost identical hydropeaking intensities (1:3, 1:5 and 1:10) to the present analysis, occurring over several sites with different bar morphologies in regulated Austrian rivers.…”

Topo-Bathymetric LiDAR for Monitoring River Morphodynamics and Instream Habitats—A Case Study at the Pielach River

“…In such semi-natural or regulated rivers there are huge declines in available and suitable habitats if the wetted width has already reached the banks. The increase in fast run habitats (flow velocity > 1 ms −1 ) with increasing flow is dominantly reducing shallow water-and backwater-areas to almost zero due to the regulated banks [101] or is causing excessive downstream drift of juvenile fish. Thus, a stable overall habitat distribution at the reach scale is an indicator for the quality of the investigated study area as a natural self-forming reach of the Pielach River.…”

Débits écologiques : la place des modèles d’habitat hydraulique dans une démarche intégrée

“…En particulier, le modèle numérique Evha ne fait plus l'objet de suivi et ne permet pas de modélisation numérique bidimensionnelle (2D) des écoulements. Or, les modélisations 2D sont particulièrement utiles dans certaines situations comme la prédiction des effets de restaurations physiques, les études comportementales ou la modélisation des impacts des éclusées (Capra et al, 2011 ;Hauer et al, 2014). Plusieurs pistes sont envisagées pour faire évoluer le panel de modèles d'habitat disponibles.…”