La mobilité de la charge de fond des rivières torrentielles méditerranéennes

La mobilité de la charge de fond de trois affluents de la Drôme (Barnavette, Béoux et Esconavette) a fait l’objet d’un suivi événementiel entre 1997 et 2002, au moyen d’un dispositif de terrain relativement simple permettant de mesurer les phénomènes de creusement et de remblaiement du lit, ainsi que la mise en mouvement des particules. Trois stations de mesure comprenant un ou plusieurs profils en travers pour le suivi topographique, un ensemble de chaînes d’érosion, des traceurs colorimétriques et un équipement d’enregistrement des hauteurs d’eau maximales ont été installés à proximité de l’exutoire des trois affluents. Cet équipement a permis de caractériser la fréquence du charriage (6 à 7 crues mobilisatrices par an en moyenne), les distances de transport de la charge de fond (en moyenne entre 450 et 470 m par an) et les seuils de mise en mouvement. Les résultats obtenus sont en adéquation avec les valeurs communément observées dans les rivières à forte pente, avec des forces tractrices critiques adimensionnelles comprises entre 0,031 et 0,076.Event-based bedload mobility of three tributaries to the Drôme River (Barnavette, Béoux and Esconavette) was monitored between 1997 and 2002, with field devices allowing the measurement of scour and fill processes, and individual particles displacements. Three field sites including one or two resurveyed cross sections, a set of scour chains, painted tracers and one crest stage gauge were installed near the outlet of the streams. This equipment allowed the characterization of the frequency of bedload transport, the hydraulic conditions for incipient motion and the distances of transport of individual grains. We observed that an average of 6 to 7 competent flow events per year occurred during the study period. The annual distances of transport of coarse particles range between 450 and 470 m. The thresholds of motion for bedload transport are similar to the values published for small gravel-bed rivers, with dimensionless critical shear stress between 0.031 and 0.076

Focus : Le torrent de Faucon (Ubaye)

Le torrent du Faucon, dans la vallée de l'Ubaye, est connu pour ses nombreux phénomènes torrentiels. Les auteurs nous présentent ici l’étude du bilan sédimentaire du torrent du Faucon à travers l’exemple de la lave torrentielle du 5 août 2003

Dense and long-term monitoring of Earth surface processes with passive RFID -- a review

Billions of Radio-Frequency Identification (RFID) passive tags are produced yearly to identify goods remotely. New research and business applications are continuously arising, including recently localization and sensing to monitor earth surface processes. Indeed, passive tags can cost 10 to 100 times less than wireless sensors networks and require little maintenance, facilitating years-long monitoring with ten's to thousands of tags. This study reviews the existing and potential applications of RFID in geosciences. The most mature application today is the study of coarse sediment transport in rivers or coastal environments, using tags placed into pebbles. More recently, tag localization was used to monitor landslide displacement, with a centimetric accuracy. Sensing tags were used to detect a displacement threshold on unstable rocks, to monitor the soil moisture or temperature, and to monitor the snowpack temperature and snow water equivalent. RFID sensors, available today, could monitor other parameters, such as the vibration of structures, the tilt of unstable boulders, the strain of a material, or the salinity of water. Key challenges for using RFID monitoring more broadly in geosciences include the use of ground and aerial vehicles to collect data or localize tags, the increase in reading range and duration, the ability to use tags placed under ground, snow, water or vegetation, and the optimization of economical and environmental cost. As a pattern, passive RFID could fill a gap between wireless sensor networks and manual measurements, to collect data efficiently over large areas, during several years, at high spatial density and moderate cost.Comment: Invited paper for Earth Science Reviews. 50 pages without references. 31 figures. 8 table

Volume, Peak discharges and Froude Number of Debris-Flow Surges: 10 Years of Monitoring on the Réal Torrent (France)

This work presents a summary of data on debris-flow monitoring stations focusing on the surge scale rather than full-scale debris-flow event (several fronts and surges with intermediate diluted flows). Surge-scale debris-flow data are not easily accessible for modellers but would be very beneficial for the community. A summary of the data processing protocol is offered, and its application to the monitoring station of the R?al Torrent is described (drainage area: 2 km2, SE France). Investigated bulk surge features are volume, front height, peak discharge, and Froude number. This investigation leads to statistical distributions of these parameters on 34 surges gathered from 2011 to 2020. Their volumes are typically a few thousand cubic metres, their peak flow height is 1 to 2 m, their peak discharge is a few dozens of cubic metres per second and their Froude number is near critical. Results drawn from this work will be a great asset for modellers to better feed their numerical experiments with realistic, field-driven features

Major floods of the V\'esubie and Roya Rivers (Alps, France) in October 2020: hydrogeomorphological caracterisation and management perspectives

On October 2nd, 2020, under the combined effect of the winter Alex storm formed off the Brittany coast, and a strong Mediterranean episode, very intensive rainfalls affected in the south eastern France, both Roya and V{\'e}subie catchments (locally up to 600 mm in 24h). This paroxysmal event with a heavy human toll (10 dead, 8 missing) generated extreme flash floods over a large part of the hydrographic network. The result is an almost generalized fluvial metamorphosis of rivers, from sinuous single-thread channels to braided channels. The characterization of morphological effects of these floods is based on a diachronic aerial picture analysis highlighting a strong increase of the active channel width (up to 900%) reaching -- or even pushing back in few sectors -- front limits of the valley bottom. In the V{\'e}subie, the 2D morphological effect of the Alex storm was 10 times higher than that of the 100-yrs return period flood of November 1997. Comparison of digital terrain models (DEM) before- and after-flood also allows us to foresee the altitudinal variations (erosion/deposition) that affected beds and their riverine margins. The analysis of the impacts caused by these floods changes the perception of the ``freedom space'' of these alpine rivers, which now must be taken into account in the perspective of resilient reconstruction.Comment: in French language. IS Rivers 2022, ZABR; GRAIE, Jul 2022, Lyon, Franc

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

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Évaluation des apports sédimentaires depuis les sites miniers de Nouvelle-Calédonie à partir de la largeur de la bande active des cours d’eau

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

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