Regional assessment of the multi-decadal changes in braided riverscapes following large floods (Example of 12 reaches in South East of France)

Abstract. The district of the France Rhône basin is characterised by several braided reaches, preserved from the widespread disappearing occurred in the 20th century. Even if the overall pattern is evolving through a general river narrowing, some reaches have shown to be still active and have widened. The preliminary results suggest that differences in width pattern could be related to several factors, such as high magnitude and low frequency floods, and geographical position in the catchment which influences bedload delivery conditions and vegetation recruitment related to climate. From an initial set of 53 braided reaches, we selected 12 sites, distributed into four main hydro-geographical regions. Reaches were selected to be representative of the overall study area. We analysed the braiding width pattern and the vegetation pattern dynamic among five observation periods dating from the 1950s to the 2000s. We hypothesised that a comparative analysis of a detailed temporal trajectory (i.e. five dates) of a set of rivers within several hydro-geographical contexts would allow us to better distinguish the relative role of floods (in terms of magnitude and duration) and other controlling factors acting at the regional scale. We showed that active channel width is controlled mainly by Q10 flood and secondarily by bedload availability whereas island pattern is in large part independent of flood series characters. Moreover a clear regional differentiation, constant over time, in terms of riverscape response is observed, mainly opposing south-western and south-eastern reaches. This opposition depends on several concurring factors, i.e. the flood characters, the river activity, the human influence and the climate. Finally, these findings allowed us to highlight those sectors in which the braided pattern could disappear, and those sectors in which the braided pattern is still active, because critical processes responsible of channel dynamic are still present

Étude des changements morphologiques d'une rivière en tresses par LiDAR aéroporté

International audienceDes levés LiDAR aéroportés multi-temporels sont utilisés pour reconstruire les changements morphologiques d'un lit en tresses suite à une crue morphogène. Ces levés ont été acquis en octobre 2008 et avril 2010 sur un linéaire de 7 km du Bès, un affluent de la Bléone dans les Préalpes du Sud. Au cours de cette période, une crue de récurrence 15 ans a été enregistrée en décembre 2009. Les deux MNT ont été soustraits pour obtenir une carte d'érosion et de dépôt. Des mesures topographiques au dGPS ont été réalisées sur des surfaces aux caractéristiques différentes pour évaluer la variabilité des erreurs dans les MNT. La carte d'érosion et de dépôt permet d'étudier les changements morphologiques à l'échelle des macroformes et de calculer un bilan sédimentaire. Ces changements morphologiques sont enfin replacés dans leur contexte historique, étudié à partir des campagnes de photographies aériennes (1948-2010) pour lier la morphologie du lit actuel avec les changements morphologiques à long terme

Génie végétal contre l'érosion des berges de cours d'eau dans un contexte de changement climatique : quelles nouvelles espèces utiliser ?

National audienceL'érosion des berges est un phénomène naturel participant activement aux processus de dynamique fluviale. Toutefois, afin de protéger des enjeux riverains (habitations, infrastructures de transport,) il est parfois nécessaire de fixer les berges avec des ouvrages de protection. Pour se faire, le génie végétal est aujourd'hui de plus en plus employé. Il s'agit de mettre en place des matériaux végétaux vivants qui vont stabiliser les berges. Ces ouvrages comprennent notamment des boutures de saules, dont les racines vont fixer la zone érodée. Les saules, sont des espèces qui apprécient l'eau et restent inféodées à la nappe phréatique. Or la sécheresse est une des causes majeures d'échec des ouvrages de génie végétal, et ces dernières devraient s'aggraver avec le changement climatique. Il apparaît donc particulièrement intéressant de tester de nouvelles espèces présentant à la fois de bonnes capacités en termes de génie végétal et une meilleure résistance à la sécheresse. Myricaria germanica et Tamarix gallica, deux espèces pionnières des bords de cours d'eau, présentent à priori de telles caractéristiques, mais leurs patrons de développement ainsi que leur capacité de résistance à la sécheresse sont peu connues. Les comportements de boutures de ces deux espèces ont été comparés avec celle d'un saule dans trois expérimentations en laboratoire et sur le terrain. On a ainsi pu déterminer les dynamiques de croissance des parties aériennes et souterraines en fonction de différentes modalités de sécheresse. Myricaria germanica et Tamarix gallica montrent généralement un taux de reprise au bouturage élevé et proche de celui du saule, et un investissement dans les racines supérieur en cas de sécheresse, Cependant la croissance de Myricaria germanica apparaît nettement plus faible que celles des deux autres espèces Ces deux espèces présentent un fort potentiel pour le génie végétal, notamment dans un contexte de changement climatique. Elles s'adaptent bien aux conditions de sècheresse et disposent de bonnes capacités de stabilisation. Leur utilisation dans ce domaine devrait contribuer à augmenter la biodiversité des ouvrages

Morphodynamics of the exit of a cutoff meander: experimental findings from field and laboratory studies

The morphological evolution of the entrances and exits of abandoned river channels governs their hydrological connectivity. The study focusses on flow and sediment dynamics in the exit of a cut-off meander where the downstream entrance is still connected to the main channel, but the upstream entrance is closed. Two similar field and laboratory cases were investigated using innovative velocimetry techniques (acoustic Doppler profiling, image analysis). Laboratory experiments were conducted with a mobile-bed physical model of the Morava river (Slovakia). Field measurements were performed in the exit of the Port-Galland cut-off meander, Ain river (France). Both cases yielded consistent and complementary results from which a generic scheme for flow patterns and morphological evolution was derived. A simple analogy with flows in rectangular side cavities was used to explain the recirculating flow patterns which developed in the exit. A decelerating inflow deposits bedload in the downstream part of the cavity, while the upstream part is eroded by an accelerating outflow, leading to the retreat of the upstream bank. In the field, strong secondary currents were observed, especially in the inflow, which may enhance the scouring of the downstream corner of the cavity. Also, fine sediment deposits constituted a silt layer in a transitional zone, located between the mouth of the abandoned channel and the oxbow-lake within the cut-off meander. Attempts at morphological prediction should consider not only the flow and sediment conditions in the cavity, but also the dynamics of the main channel

Morphologic adjustments of actively evolving highly curved neck cutoffs

Neck cutoffs and their resultant oxbow lakes are important and prominent features of riverine landscapes. Detailed field-based research focusing on the morphologic evolution of neck cutoffs is currently insufficient to fully characterize cutoff evolution. High-resolution bathymetric data were collected over 3 years for the purpose of determining channel morphology and morphologic change on three actively evolving neck cutoffs. Results indicate the following general trends in morphologic adjustment: (1) a longitudinal bar in the upstream meander limb that develops near the entrance to the abandoned bend; (2) a deep scour hole in the downstream meander limb immediately downstream of the cutoff channel; (3) erosion of the bank opposite the cutoff in the downstream meander limb; (4) a cutoff bar in the downstream meander limb at the junction corner of the cutoff channel and the downstream meander limb; and (5) perching of the exit of the abandoned bend above the cutoff channel due to channel bed incision. The results presented herein were used to develop a conceptual model that depicts the morphologic evolution of highly curving neck cutoffs. The findings of this research are combined with recent analyses of the three-dimensional flow structure through neck cutoffs to provide a mechanistic explanation for the morphodynamics of neck cutoffs. (c) 2019 John Wiley & Sons, Ltd

River widening in mountain and foothill areas during floods: Insights from a meta-analysis of 51 European Rivers

River widening, defined as a lateral expansion of the channel, is a critical process that maintains fluvial ecosystems and is part of the regular functioning of rivers. However, in areas with high population density, channel widening can cause damage during floods. Therefore, for effective flood risk management it is essential to identify river reaches where abrupt channel widening may occur. Despite numerous efforts to predict channel widening, most studies have been limited to single rivers and single flood events, which may not be representative of other conditions. Moreover, a multi-catchment scale approach that covers various settings and flood magnitudes has been lacking. In this study, we fill this gap by compiling a large database comprising 1564 river reaches in several mountain regions in Europe affected by floods of varying magnitudes in the last six decades. By applying a meta-analysis, we aimed to identify the types of floods responsible for more extensive widening, the river reach types where intense widening is more likely to occur, and the hydraulic and morphological variables that explain widening and can aid in predicting widening. Our analysis revealed seven groups of reaches with significantly different responses to floods regarding width ratios (i.e., the ratio between channel width after and before a flood). Among these groups, the river reaches located in the Mediterranean region and affected by extreme floods triggered by short and intense precipitation events showed significantly larger widening than other river reaches in other regions. Additionally, the meta-analysis confirmed valley confinement as a critical morphological variable that controls channel widening but showed that it is not the only controlling factor. We proposed new statistical models to identify river reaches prone to widening, estimate potential channel width after a flood, and compute upper bound width ratios. These findings can inform flood hazard evaluations and the design of mitigation measures

The Achievement of a Decentralized Water Management Through Stakeholder Participation: An Example from the Drôme River Catchment Area in France (1981–2008)

International audienceDifferent water Acts (e.g., the European Water Framework Directive) and stakeholders involved in aquatic affairs have promoted integrated river basin management (IRBM) over recent decades. However, few studies have provided feedback on these policies. The aim of the current article is to fill this gap by exploring how local newspapers reflect the implementation of a broad public participation within a catchment of France known for its innovation with regard to this domain. The media coverage of a water management strategy in the Drôme watershed from 1981 to 2008 was investigated using a content analysis and a geographic information system (GIS). We sought to determine what public participation and decentralized decision-making can be in practice. The results showed that this policy was integrated because of its social perspective, the high number of involved stakeholders, the willingness to handle water issues, and the local scale suitable for participation. We emphasized the prominence of the watershed scale guaranteed by the local water authority. This area was also characterized by compromise, arrangements, and power dynamics on a fine scale. We examined the most politically engaged writings regarding water management, which topics each group emphasized, and how the groups agreed and disagreed on issues based on their values and context. The temporal pattern of participation implementation was progressive but worked by fits and starts

Closing the gap between science and management of cold-water refuges in rivers and streams

Human activities and climate change threaten coldwater organisms in freshwater eco-systems by causing rivers and streams to warm, increasing the intensity and frequency of warm temperature events, and reducing thermal heterogeneity. Cold-water refuges are discrete patches of relatively cool water that are used by coldwater organisms for thermal relief and short-term survival. Globally, cohesive management approaches are needed that consider interlinked physical, biological, and social factors of cold-water refuges. We review current understanding of cold-water refuges, identify gaps between science and management, and evaluate policies aimed at protecting thermally sensitive species. Existing policies include designating cold-water habitats, restricting fishing during warm periods, and implementing threshold temperature standards or guidelines. However, these policies are rare and uncoordinated across spatial scales and often do not consider input from Indigenous peoples. We propose that cold-water refuges be managed as dis-tinct operational landscape units, which provide a social and ecological context that is relevant at the watershed scale. These operational landscape units provide the founda-tion for an integrated framework that links science and management by (1) mapping and characterizing cold-water refuges to prioritize management and conservation actions, (2) leveraging existing and new policies, (3) improving coordination across jurisdictions, and (4) implementing adaptive management practices across scales. Our findings show that while there are many opportunities for scientific advancement, the current state of the sciences is sufficient to inform policy and management. Our proposed framework pro-vides a path forward for managing and protecting cold-water refuges using existing and new policies to protect coldwater organisms in the face of global change. behavioral thermoregulation, climate change adaptation, lotic ecosystem management, refugia, salmonids, temperature, thermal heterogeneity, thermal refugespublishedVersio

Restauration morpho-dynamique et redynamisation de la section court-circuitée du Rhin en aval du barrage de Kembs (projet INTERREG / EDF)

National audienceThe Upper Rhine River has been heavily impacted by channelization for flood protection and navigation, and then by damming for hydropower generation. In normal non flooding conditions, most of the flows are diverted in a canalized section whereas the regulated “old Rhine” bypassed reach runs a minimum flow. Between Huningue and Neuf-Brisach, engineering works induced simplification and stabilization of the channel pattern from a formerly braiding sector to a single incised channel, hydrological modifications, bottom armouring due to bedload decrease, and thus ecological alterations. Two complementary and interdisciplinary projects have been initiated to restore alluvial morphodynamics: i) the international “INTERREG IV - Redynamisation of the old Rhine” project (2009-2012) coordinated by the Alsace region, France; ii) the left bank “controlled erosion” project launched by Electricité de France (EDF) within Kembs hydroelectric station relicensing process since 2003-2004. The purpose of these projects is to evaluate the feasibility of an important hydro-morphological and ecological restoration plan on a 45 km long reach, through both field testing of bank erosion techniques at favourable locations, and artificial sediments input from right bank excavations. This will help define possible long term prospective scenarios, in order to restore sustainable sediment transport, morphodynamics variability and associated ecological functions. The study will involve historical analysis, hydro-morphological / hydraulic physical and numerical modelling, physical and ecological monitoring, and sociological aspectsLe Rhin alsacien-allemand a enregistré de profondes modifications morphologiques et hydrologiques à la suite de sa correction et de sa régularisation pour la protection contre les crues et la navigation, puis après la construction de barrages hydro-électriques. Les aménagements réalisés entre Huningue et Neuf-Brisach ont engendré une simplification et une stabilisation du style fluvial. Un fleuve en tresses a cédé la place à un chenal unique incisé. Le fond de chenal est devenu pavé à cause d’une diminution des apports de charge de fond et des altérations écologiques ont été observées (simplification des habitats aquatiques et riverains). Deux projets complémentaires et interdisciplinaires ont été engagés afin de restaurer une dynamique des formes alluviales : i) le projet international INTERREG IV – Redynamisation du Vieux Rhin (2009-2012) sous l’impulsion de la région Alsace ; ii) le projet d’érosion maitrisée des berges de la rive gauche conduit par Electricité de France (EDF) dans le cadre du renouvellement de la concession de l’aménagement de Kembs. L’objectif des deux projets est de définir un plan de restauration hydro-morphologique et écologique conduisant à la redynamisation d’un tronçon de 45 km. L’étude repose sur une analyse historique, l’exploitation de modèles à la fois physiques et numériques, et les suivis morphologiques in situ d’une recharge artificielle en sédiments et d’érosions de berge contrôlées. Ces études de faisabilité sont complétées par des analyses écologique et sociologique pour apprécier l’impact socio-environnemental de ces projets

Simple large wood structures promote hydromorphological heterogeneity and benthic macroinvertebrate diversity in low-gradient rivers

This work has been carried out within the SMART Joint Doctorate Programme ‘Science for the MAnagement of Rivers and their Tidal systems’ funded by the Erasmus Mundus programme of the European Union