Can lateral mobility be restored along a highly domesticated low-energy gravel-bed river?

Fluvial engineering works such as weirs, rip-rap, groynes, and dykes have constrained for decades and more the lateral mobility of rivers, one of the key drivers of aquatic and riparian diversity. Preserving or restoring a sufficient space for river mobility has therefore become a major river management focus. Because the success and relevance of management actions are conditioned by the level of energy and sediment supply of rivers, such actions are generally considered unsuitable for low-energy rivers. However, some low-energy rivers have numerous ancient engineering works along their length, especially bank protections, suggesting a potential capacity for bed migration. In this context, it is essential to determine to what extent planform dynamics is disturbed, and if lateral mobility can be restored. Herein, a case study was done on a 146 km stretch of the low-energy meandering gravel-bed Cher River (France). The goal of the study was to estimate the remnant shifting capacity, identify the factors controlling the location and intensity of lateral erosion, determine the potential for preserving and restoring lateral mobility, and examine management measures that could be implemented to this end. For that, field surveys, analysis of existing databases, aerial photographs, and laser imaging detection and ranging digital elevation model (LiDAR DEM) data were combined. The study revealed a strong longitudinal fragmentation of the river, with most of it laterally constrained due to the presence of anthropogenic structures such as bank protections, former gravel pits in the alluvial plain, bridges, and weirs. The river is now composed of a string of constrained and unconstrained reaches, and the space available for river shifting has been dramatically reduced. Due to these fluvial engineering works and anthropogenic legacies, the potential for lateral movement of the riverbed, and, therefore, diversification of riparian and aquatic habitats, is limited. Furthermore, lateral mobility could be preserved or restored only for very short sections of the river. It is therefore highly unlikely that good ecological status could be achieved on the entire river corridor through removal of bank protections. Nevertheless, a possible solution could be combining bank protection removals with a series of gravel augmentations close to each other.

Restoring fluvial forms and processes by gravel augmentation or bank erosion below dams: A systematic review of ecological responses

Aquatic biological communities have directly undergone human-induced changes. Altered hydrological and morphological processes in running waters have caused the degradation of main habitats for biotas and have disturbed ecosystem functionality. The latest advances in river restoration concerned the rise in far-reaching hydromorphological restoration actions that have been implemented below dams to reverse well-known negative impacts of anthropogenic pressures. Some authors emphasized the enhancement of sediment supply and habitat diversity using gravel augmentation or bank erosion to restore morphodynamics, and thus improve biodiversity. We explored the Web of Science database for empirical research papers that specifically addressed such hydromorphological river restoration actions. Articles were examined using a text-content analysis tool to determine the major concepts or ideas they deal with. It has also been proved as useful in defining interrelationships and degree of interdisciplinary. Results showed that a low number of published scientific articles exist about such projects, mainly condensed in the North hemisphere. Divergent ecological issues were highlighted by the word co-occurrence networks: (i) gravel augmentation was used to improve spawning habitats for fish of economic interest whereas (ii) erodible corridor was designed to safeguard natural riparian systems, approaching morphological goals of channel widening. Overall, ecological responses were consistent with those expected, leading however rather to functional shifts than richness increase. Gravel augmentation or bank erosion were not usually combined with in-channel structure management. However, this might be an option to consider since the biological communities seem to be sensitive during first restorations with such combination. This review demonstrates the value of word co-occurrence networks in exploring a high number of previous publications, keys for formulating guidance to manage gravel augmentation or bank erosion along ecological purposes.