Mise en oeuvre d'un outil d'alerte et de cartographie temps réel des aléas naturels liés aux précipitations dans les régions montagneuses et méditerranéennes du Sud-Est de la France

International audienceDue to its mountainous topography and its Mediterranean climate, the Provence-Alpes-Côte d'Azur (PACA) region in Southeastern France is particularly prone to flash floods, debris flows and mass movements (landslides and rockfall). A mapping system for these rainfall induced hazards has been tested by local and regional authorities and Government agencies since 2011 as part of the RHYTMME project. This system allows, thank to radar rainfall estimation and rainfall-runoff modelling, the real-time warning and monitoring of flash floods wherever they may occur in the PACA territory. It is also intended to enable, during intense rainfall events, the localisation of the streams susceptible to generate debris flows and of the slopes the more likely to trigger landslides and/or rockfalls

Soil and water bioengineering: practice and research needs for reconciling natural hazard control and ecological restoration

Soil and water bioengineering is a technology that encourages scientists and practitioners to combine their knowledge and skills in the management of ecosystems with a common goal to maximize benefits to both man and the natural environment. It involves techniques that use plants as living building materials, for: (i) natural hazard control (e.g., soil erosion, torrential floods and landslides) and (ii) ecological restoration or nature-based re-introduction of species on degraded lands, river embankments, and disturbed environments. For a bioengineering project to be successful, engineers are required to highlight all the potential benefits and ecosystem services by documenting the technical, ecological, economic and social values. The novel approaches used by bioengineers raise questions for researchers and necessitate innovation from practitioners to design bioengineering concepts and techniques. Our objective in this paper, therefore, is to highlight the practice and research needs in soil and water bioengineering for reconciling natural hazard control and ecological restoration. Firstly, we review the definition and development of bioengineering technology, while stressing issues concerning the design, implementation, and monitoring of bioengineering actions. Secondly, we highlight the need to reconcile natural hazard control and ecological restoration by posing novel practice and research questions

A high-frequency, long-term data set of hydrology and sediment yield: the alpine badland catchments of Draix-Bléone Observatory

Draix-Bléone critical zone observatory was created in 1983 to study erosion processes in a mountainous badland region of the French Southern Alps. Six catchments of varying size (0.001 to 22 km2) and vegetation cover are equipped to measure water and sediment fluxes, both as bedload and suspended load. This paper presents the core dataset of the observatory, including rainfall and meteorology, high-frequency discharge and suspended-sediment concentration, and event-scale bedload volumes. The longest records span almost 40 years. Measurement and data-processing methods are presented, as well as data quality assessment procedures and examples of results. All the data presented in this paper are available on the open repository https://doi.org/10.17180/obs.draix (Draix-Bleone Observatory, 2015), and a 5-year snapshot is available for review at https://doi.org/10.57745/BEYQFQ (Klotz et al., 2023).</p

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

(trad auto)Géomorphologie et transport sédimentaire des systèmes fluviaux alpins : des torrents à forte pente aux rivières à lit de gravier du piémont

This report proposes a synthesis of my research activity since my assignment at the ETNA research unit of Irstea Grenoble. My works concern the geomorphology and sediment transport of alpine rivers, from steep-slope headwaters to large gravel-bed piedmont rivers. They can be structured around three thematic fields that are presented as successive chapters in the present report. The first chapter is dedicated to the geomorphic responses of headwaters. Results concerning the regional detection of the most dangerous debris-flow prone torrents are presented, as well as outcomes concerning rainfall triggering conditions of debris flows, coming from recently deployed monitoring stations in the French Alps. The second chapter deals with the sediment supply effect on channel forms and sediment transport processes. The debris-flow sediment cascade of a bedrock-dominated catchment of the Northern Prealps is analysed through intensive and repetitive topographic surveys of hillslopes and channels. The sediment supply effect on bedload responses has been investigated in a small experimental catchment at Draix, where a seasonal hysteresis has been detected and interpreted as a consequence of periodic sediment pulses from hillslopes. The longterm morphological trajectories of alpine braided rivers were reconstructed and confronted to present-day sediment supply conditions in the French Alps. The third chapter concerns bedload transport assessment and monitoring in alpine environments. Examples of applications of (1) repetitive airborne LiDAR imagery for sediment budget reconstruction of a large braided channel, (2) passive RFID tags for the tracing of bedload transport along a high sediment load torrent, and (3) automatic bedload slot-sampler under conditions of ultra-high suspended sediment concentrations are successively presented. The last chapter presents research perspectives of the three thematic fields for the next 5 years. My activities in term of research animation are presented in the last part of the report.Ce rapport présente une synthèse des travaux que j'ai réalisé depuis mon affectation au sein de l'unité de recherche ETNA de l'Irstea de Grenoble. Ils portent sur la géomorphologie et le transport solide des rivières alpines, depuis les têtes de bassin à fortes pentes jusqu'aux corridors fluviaux de piémont. Ces travaux sont structurés autour de trois axes thématiques, qui sont présentés successivement sous forme de chapitres. Le premier chapitre est consacré aux réponses géomorphologiques des têtes de bassin. Il présente les résultats obtenus en matière de détection régionale des torrents susceptibles de produire des laves, et en matière de caractérisation des conditions pluviométriques de déclenchement des laves étudiées à partir de stations expérimentales déployées récemment dans les Alpes. Le deuxième chapitre s'intéresse aux effets de la recharge sédimentaire sur les formes et les flux solides. La cascade sédimentaire associée aux événements de lave survenus dans un bassin rocheux des Préalpes du Nord est présentée à partir des résultats d'un suivi topographique répétitif et intensif des versants et des chenaux torrentiels. L'effet de la recharge sédimentaire du lit sur le transport solide par charriage a été étudié sur le site du Moulin à Draix, où nous avons mis en évidence une hystérèse saisonnière du flux charrié sous l'effet de la propagation de vagues sédimentaires annuelles. La recharge sédimentaire est aussi appréhendée à l'échelle des tresses alpines, dont les trajectoires morphologiques ont été reconstituées à partir de données historiques et confrontées aux conditions actuelles de fourniture sédimentaire. Le troisième chapitre concerne la mesure du transport solide par charriage en contexte alpin, à partir d'approches innovantes. Des exemples d'application de l'imagerie LiDAR aéroportée en contexte de lit en tresses, du traçage par RFID de la charge de fond en contexte torrentiel, et de la mesure haute-fréquence du charriage à l'aide d'une trappe à fente en contexte de forte concentration en fines, sont présentés successivement. Le dernier chapitre présente les perspectives de recherche à 5 ans pour chaque thème de recherche. Mon expérience en matière d'animation de la recherche est présentée de manière factuelle en fin de mémoire

Sediment dynamics in Alpine basins

International audienceThe special issue consists of 13 papers. The 13 papers can be grouped according to the following four main themes: i) Sediment cascade and budget in steep headwater catchments; ii) Bedload transport monitoring in Alpine streams and rivers; iii) Effects of structures on sediment transport continuity: characterization and restoration; and iv) Tools and approaches for sediment management in Alpine basins

Active channel width as a proxy of sediment supply from mining sites in New Caledonia

International audienceAlthough the channel morphology of upland fluvial systems is known to be strongly controlled by sediment supply from hillslopes, it is still difficult to isolate this effect from the other controlling factors of channel forms, such as the sediment transport capacity (depending notably on the size of the catchment) and local conditions (e.g. confinement, riparian vegetation, valley-floor slope). The rivers in New Caledonia offer an interesting field laboratory to isolate the morphological effect of contrasted sediment supply conditions. Some of these rivers are known to be highly impacted by the coarse sediment waves induced by the mining of nickel deposits that started in the early 1870s, which was particularly intensive between the 1940s and 1970s. The propagation of the sediment pulses from the mining sites can be traced by the presence of wide and aggraded active channels along the stream network of nickel-rich peridotite massifs. A first set of 63 undisturbed catchments in peridotite massifs distributed across the Grande Terre was used to fit a classic scaling law between active channel width and drainage area. A second set of 86 impacted sites, where the presence of sediment waves was clearly attested by recent aerial imagery, showed systematically wider active channels, with a width ratio around 5 (established from the intercept ratio of width-area power laws). More importantly, this second set of disturbed sites confirmed that the residual of active channel widths, computed from the scaling law of undisturbed sites, is statistically positively related to the catchment-scale relative area of major mining sediment sources. It is therefore confirmed that the characterization of sediment supply conditions is crucial for the understanding of spatial patterns of active channel width, and this should be more thoroughly considered in morphological studies of rivers draining environments with contrasted geomorphic activities on hillslopes

Debris-flow susceptibility of upland catchments

International audienceOver the last three decades, many regional studies in mountain ranges under temperate climate revealed that it is possible to discriminate debris-flow and fluvial fans from morphometric indicators measured at the scale of the catchment and the fan itself. The most commonly used indicators are the Melton index (R), a normalized index of the gravitational energy of the catchment, and the fan slope (S). A wide range of thresholds have been proposed for discriminating purpose, but these are generally based on a small population of catchments and may be highly influenced by ambiguous fans included in the data set. A database of 620 upland catchments from several mountain ranges under temperate climate was compiled from the literature to propose robust discriminant morphometric thresholds for debris-flow versus fluvial responses. Linear discriminant analysis (LDA) and logistic regression (LR) were performed using the whole data set, and a leave-one-out cross-validation was used to evaluate performances of the models. Sensitivity and specificity scores obtained for LDA and LR were 0.96 and 0.73, and 0.95 and 0.75, respectively. It is also shown that the channel slope above which debris-flow is observed decreases with the gravitational energy of the catchment. Limitations of the morphometric discrimination are discussed

ECSTREM, une approche pratique pour prédire la production sédimentaires des torrents des Alpes françaises

International audienceQuantification of sediment transport volumes in small upland catchments is an essential step for the protection and prevention against flood hazards in mountainous terrains. This contribution presents a method dedicated to the prediction of the sediment yield of small basins of the French Alps. Data on sediment yield from 72 catchments have been collected from sediment retention basins registry, historical documents from archives, scientific papers and technical reports. These data were used to develop and test different predictive models. Multivariate regression techniques were used to correlate sediment yield magnitude to morphometric parameters, topographic characteristics of the basins and spatial frequency of sediment sources in the catchment. The main originality of the proposed statistical models is that they integrate the surface of active erosion terrains connected to the stream network; this parameter is highly correlated to the sediment yield. We also proposed specific models for fluvial and debris-flow dominated catchments. The main interests of these predictive tools are that they are calibrated on French alpine torrents and they satisfy the need of regional tools for predicting sediment.Dans les études de bassins versants torrentiels, la quantification des apports solides constitue une étape essentielle et incontournable du diagnostic d’un site. Parmi les modèles utilisés en France dans le domaine de l'ingénierie, très peu ont toutefois été calés sur des données observées dans les Alpes françaises. L'objectif de cet article est de présenter une méthode de prédiction qui repose sur une série de modèles statistiques multivariés calés à partir d'un jeu de données original portant sur 72 bassins torrentiels des Alpes françaises. Ces modèles permettent d'estimer la production sédimentaire événementielle de torrents dont la pente est comprise entre 3 et 40% et dont la superficie est inférieure à 50 km² environ. La principale originalité des modèles proposés est de prendre en compte la surface en érosion active connectée au réseau hydrographique, qui est une variable fortement corrélée à l'intensité des apports solides, de même que le type de processus de transport solide le plus prépondérant pour le torrent. L'intérêt de ces modèles est aussi d’avoir été calés sur des données représentatives des torrents des Alpes françaises et de présenter un niveau de performance qui s'avère plutôt satisfaisant, comparativement au degré d'incertitude généralement accordé à d’autres approches applicables dans un tel contexte. La mise en ½uvre de ces modèles utilise les outils usuels de l'ingénierie et nécessite, sur un bassin donné, la détermination de certains paramètres morphométriques et topographiques, ainsi que le repérage et la délimitation des sources sédimentaires