Geomorphic effects of gravel augmentation on the Old Rhine River downstream from the Kembs dam (France, Germany)

Abstract

During the last 30 years, river restoration activities aiming to improve the functionality of degraded fluvial ecosystems increased markedly. For large rivers, it remains difficult to evaluate restoration efficiency and sustainability due to the lack of standardized monitoring metrics. From 2010 to 2016, three gravel augmentations were performed on the Old Rhine, a by-passed reach downstream from the Kembs dam (France- Germany). A geomorphic monitoring combining topo-bathymetric surveys, bedload tracking and hydraulic modelling allows to evaluate the successfulness of these actions. Results show that, to be mobilized, artificial sediment deposit should be located in concavity rather than convexity areas, due to higher shear stresses for moderate floods (Q2). Sediment starvation appeared rapidly on the restored reaches once the sediment wave moved downstream, as a consequence of limited upstream sediment supply. Bathymetric homogenization was observed along and downstream from the restored reaches without creation of new fluvial forms. This research highlights that future actions should include channel enlargement downstream of gravel augmentations, which would promote sediment deposition and habitat diversification. Sediments excavated during artificial widening could be stored and injected progressively into the upstream part of the Old Rhine to benefit the downstream sections.

Effects of Transverse Groynes on Meso-Habitat Suitability for Native Fish Species on a Regulated By-Passed Large River

“…These effects were notably observed along a 50 km by-passed reach parallel to the Grand Canal d'Alsace (GCA) downstream of Basel [38] (Figure 2.b). Since 2010, several restoration actions, including an increase in instream flow [39], gravel augmentations [40,41] and controlled bank erosion with the implementation of two transverse groynes [42], have been performed along this by-passed reach to enhance the bedload transport and morphodynamics therein and thus diversify aquatic and riverine habitats. In December 2010, the instream flow entering the Old Rhine increased from a quasi-constant discharge of 20-30 m 3 .s −1 to a seasonally varying discharge of 52-150 m 3 .s −1 in accordance with the life cycle of native fish species [43].…”

“…Finer grain sizes may be related to the deposition of fine sediments provided by bank erosion [42] and/or by the scouring of the two pools (Figure 7.c). These finer particles may also originate from the upstream reaches due both to probable local bank erosion and to break-up of the pre-existing bed armour layer following the intense flood of May-June 2013 (Q 15 ) [41]. Indeed, the break-up of this armour layer induces the entrainment of the subsurface layer, which contains particles that are finer than those within the surface armour layer [62].…”

“…These results raise questions regarding environmental ethics related to the increased use of artificial features to enhance ecological conditions. To restore geomorphic processes in a more sustainable way and achieve improvements to aquatic and riverine habitats efficiently, [38,41,66] recommended combining gravel augmentations and channel widening. More specifically, channel widening should be performed downstream of gravel augmentations, promoting the deposition of some injected sediments and potentially diversifying aquatic and riverine habitats [40,67].…”

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

“…These effects were notably observed along a 50 km by-passed reach parallel to the Grand Canal d'Alsace (GCA) downstream of Basel [39] (Figure 1b). Since 2010, several restoration actions, including an increase in instream flow [40], gravel augmentations [41,42], and controlled bank erosion with the implementation of two transverse groynes [43], have been performed along this by-passed reach to enhance the bedload transport and morphodynamics therein and thus diversify aquatic and riverine habitats. In December 2010, the instream flow entering the Old Rhine increased from a quasi-constant discharge of 20-30 m 3 /s to a seasonally varying discharge of 52-150 m 3 /s in accordance with the life cycle of native fish species [44].…”

“…Finer grain sizes may be related to the deposition of fine sediments provided by bank erosion [43] and/or by the scouring of the two pools (Figure 7c). These finer particles may also originate from the upstream reaches due both to probable local bank erosion and to break-up of the pre-existing bed armor layer following the intense flood of May-June 2013 (Q 15 ) [42]. Indeed, the break-up of this armor layer induces the entrainment of the subsurface layer, which contains particles that are finer than those within the surface armor layer [63].…”