Effects of Transverse Groynes on Meso-Habitat Suitability for Native Fish Species on a Regulated By-Passed Large River: A Case Study along the Rhine River

Abstract

River regulations ultimately degrade fluvial forms and morphodynamics and simplify riparian and aquatic habitats. For several decades, river restoration actions have been performed to recover geomorphic processes and diversify these habitats to enhance both river biodiversity and ecosystem services. The objective of this study is to provide quantitative feedback on the experimental restoration of a large regulated and by-passed river (the Upper Rhine downstream of the Kembs Dam, France/Germany). This restoration consisted of the construction of two transverse groynes and the removal of bank protection. A monitoring framework composed of topo-bathymetric surveys as well as flow velocity and grain size measurements was established to assess the channel morphodynamic responses and evaluate their effects on habitat suitability for five native fish species using habitat models. A riverscape approach was used to evaluate the landscape changes in terms of both the configuration and the composition, which cannot be considered with classic approaches (e.g., Weighted Usable Area). Our results show that the two transverse groynes and, to a lesser extent, bank erosion, which was locally enhanced by the two groynes, increased habitat diversity due to the creation of new macroforms (e.g., pools and mid-bars) and fining of the bed grain size. Using a riverscape approach, our findings highlight that the restoration improved eel and juvenile nase species due to slowing down of the current and the deposition of fine sediments downstream of both groynes. As a consequence, the restoration improved the habitat suitability of the studied reach for more fish species compared with the pre-restoration conditions. This study also demonstrates that the salmon habitats downstream of the restored reach were improved due to fining of the bed grain size. This finding highlights that, for restorations aimed at fish habitats, the grain size conditions must be taken into consideration along with the flow conditions. Furthermore, the implementation of groynes, while not a panacea in terms of functional restoration, can be a strategy for improving fish habitats on highly regulated rivers, but only when more functional and natural options are impossible due to major constraints.

Bank re‐erosion action to promote sediment supply and channel diversification: Feedback from a restoration test on the Rhine

“…Post‐restoration geomorphological evolution induced positive ecological effects in the short term, such as an increase in the diversity and richness of macroinvertebrate, fish and macrophyte communities (Aelbrecht et al, 2014; Staentzel et al, 2018, 2019). An ecological habitat‐modeling approach also highlighted a significant increase in habitat suitability for several lentic fish species (Chardon et al, 2020).…”

The Rhine Catchment: A Review of Sediment-Related Knowledge, Monitoring, and a Future Research Perspective

“…The sediment deficit is responsible for the undermining of bridge piers and the coarsening of the substrate, which consequently affects habitat conditions by, e.g., leading to a lack of spawning grounds [62]. In the Old Rhine, morphological and hydrological conditions have changed in such a way that aquatic habitats were disturbed or even vanished, changing riverine communities and biodiversity [80]. Due to a lack of morphodynamics in this river section, an armoured bed layer formed, which was impossible to mobilize during renaturation tests [81].…”

“…The University of Strasbourg is conducting several studies on how to implement restoration measures most effectively and on how to assess the hydromorphological and ecological effects of these actions [84][85][86][87][88]. "Électricité de France" (EDF) implemented multiple restoration measures in the Od Rhine with the objective of improving morphological and ecological processes [80,[88][89][90][91][92][93][94]. Measures included artificial gravel nourishments, increasing instream flow, removal of bank protections, and groyne rearrangement [88].…”

Development of a Framework for Cost–Benefit Analysis of I-Head and T-Head Groynes Based on Scour and Turbulent Flow Characteristics

“…Additionally, groynes gradually shift the thalweg in a bend of a channel away from the outer bank and towards the centerline, which is the more desired location. On the other hand, a reduction in velocity fields and thus sediment deposition in the groyne fields combine for a favorable habitat for the growth of fishes and other aquatic biotas [1][2][3][4]. Traditional methods for protecting stream banks, such as riprap, concrete paving, or gabion walls, stifle the growth of vegetation on stream banks.…”