Parution d’un article dans la revue Geomorphology : The anthropogenic nature of present-day low energy rivers in western France and implications for current restoration projects

Laurent Lespez, Vincent Viel, Anne-Julia Rollet et Daniel Delahaye ont publié dans la revue Geomorphology un article intitulé « The anthropogenic nature of present-day low energy rivers in western France and implications for current restoration projects« . Voici ci-dessous le résumé en anglais de ce travail ainsi que quelques illustrations  (téléchargeable sur la page Researchgate des auteurs)Abstract : As in other European countries,western France has seen an increase in river restoration projects. In this paper, we examine the restoration goals,methods and objectives with respect to the long-term trajectory and understanding of the contemporary dynamics of the small low energy rivers typical of the lowlands ofWestern Europe. The exhaustive geomorphological, paleoenvironmental and historical research conducted in the Seulles river basin (Normandy) provides very accurate documentation of the nature and place of the different legacies in the fluvial systems we have inherited. The sedimentation rate in the Seulles valley bottom has multiplied by a factor of 20 since the end of the Bronze Age and has generated dramatic changes in fluvial forms. Hydraulic control of the rivers and valley bottoms drainage throughout the last millennium has channelized rivers within these deposits. The single meandering channel which characterizes this river today is the legacy of the delayed and complex effects of long termexploitation of the river basin and the fluvial system. Bring to light that the “naturalness” of the restored rivers might be questioned. Our research emphasizes the gap between the poor knowledge of the functioning of these rivers and the concrete objectives of the restoration works undertaken, including dam and weir removal. Account of the long-termhistory of fluvial systems is required, not only to produce a pedagogic history of the “river degradation” butmore fundamentally (i) to situate the current functioning of the fluvial systemin a trajectory to try to identify thresholds and anticipate the potential turning points in a context of climate and land use change, (ii) to understand the role ofmorphosedimentary legacies on the current dynamics, (iii) to open the discussion on reference functioning or expected states and (iv) to open discussion on the sustainability of ecological restoration. To conclude, we point out the necessity to take into account the hybrid nature of low energy rivers in rural environments and to develop specific evaluation protocols which would include both biophysical processes and usual human activities which could be a way to share the evaluation and overcome conflicts between socioeconomic needs and environmental issues.

Schematic sketch of the morphosedimentary changes of the rivers of the Seulles catchment. Slopes : 1. Steppe landscape; 2. Patchy environment: forest and grassland; 3. Mixed oak forest (oak, hazel); 4. Cultivated slopes and colluvial deposits; Valley bottom : 5. Wooded valley bottom; 6. Tufa barrage; 7. Wooded wetland; 8. Meadows on silt overbank deposits; and 9. Stream (permanent, temporary) and water mill.
Fig. 3 – Schematic sketch of the morphosedimentary changes of the rivers of the Seulles catchment. Slopes : 1. Steppe landscape; 2. Patchy environment: forest and grassland; 3. Mixed oak forest (oak, hazel); 4. Cultivated slopes and colluvial deposits; Valley bottom : 5. Wooded valley bottom; 6. Tufa barrage; 7. Wooded wetland; 8. Meadows on silt overbank deposits; and 9. Stream (permanent, temporary) and water mill.

 

Fig-4-Comparison-of-the-planform-of-the-Seulles-River-along-the-Vaux-sur-Seulles
Fig. 4. Comparison of the planform of the Seulles River along the Vaux-sur-Seulles reaches from the 18th c. to the current situation. A. Topographical map of 2010 @IGN; B. Extract of the Plan terrier of the Vaux-sur-Seulles lordship of 1743. Colour map (43 × 55 cm). Dep. Arch. Calvados, H 106: 1. Leat and mill reach; 2. Main mill; and 3. Comparison of the main sinuosity forms
Fig-5-Schematic-sketch-of-the-legacy-and-the-restoration-projects-for-the-small-rivers
Fig. 5. Schematic sketch of the legacy and the restoration projects for the small rivers of low energy of Western Europe based on the research in the Seulles catchment. A. The legacy of the anthropogenic practices and their lifetime; B. The restoration projects and some of the questions that emerge from the long-term studies.
Fig-6-Photographs-of-the-lower-Dan-River-valley-bottom-near-Caen-and-its-alluvial
Fig. 6. Photographs of the lower Dan River valley bottom near Caen and its alluvial forest and swampy landscapes. Photographs by L. Lespez.
Fig-8-Schematic-fi-gures-showing-some-example-of-connections-between-agricultural
Fig. 8. Schematic fi gures showing some example of connections between agricultural practices and banks and channel dynamics along the Seulles River (Heuzé and Chouain sites). A. Pruning and management of the alluvial forest line and woody debris; B. Ford and maintenance of banks, fences and alluvial forest line; C. Ramp and watering places; D. Landscaped ramp and watering places (AESN and local authorities, subsidized); E. Nose pumps and fence maintenance (AESN and local authorities, subsidized).
Fig-9-Accomplished-and-projected-river-restoration-operations-in-the-Calvados-for-the
Fig. 9. Accomplished and projected river restoration operations in the Calvados for the period 2012 – 2015. 1. Dam; 2. Weir; 3. Completed project; 4. Calvados Department; and 5. Main river basin; Main stream.