Artificially generating sediment incipient motion in natural conditions

Incipient motion thresholds for gravel bed rivers are studied in flumes and natural rivers. Flume studies allow variables such as channel slope, water velocity, water depth, sediment size and sediment composition to be controlled. Meanwhile, in the field, the incipient motion of particles is studied in natural conditions allowing the structure of the bed and the flood characteristics to be considered. Though much less developed, an intermediate possibility also exists. By artificially accelerating near bottom velocity of the water flow in a small portion of the bed, it is possible to initiate sediment transport. This allows sediment incipient motion to be observed in natural conditions while controlling the water velocities. The Cemagref (HHLY) has developed a device which confines water flow in a small tunnel on the bottom of the riverbed. It was developed in order to create a boundary layer similar to the one generated by natural flow. Water is injected into a filter which smoothes the flow before it enters a Plexiglas tunnel where sediment motion is observed. The flow is accelerated by two large pumps that allow flow velocities of up to 2.5 m/s in a small area 40 cm long, 20 cm wide and 12 cm high. As the water flow is confined, large scale turbulence similar to that occurring in natural rivers cannot be reproduced using the device. The velocity profile in the tunnel is stable and in equilibrium with the riverbed. Sediment motion was observed using this device on three Belgian gravel bed rivers (3.5 cm <D50<8 cm). The incipient critical velocities measured were coherent in the three rivers sampled, ranging between 1.3 m/s and 1.7 m/s. A progressive increase in velocity in the tunnel permitted size selective entrainment to be observed. Analysis of the particle entrainment schemes and associated velocities allowed the role played by the armoured layer and the Di/D50 ratio to be more fully understood. Critical velocities measured using the device were also compared to critical velocities observed in natural conditions, determined during several pebble tracing campaigns. This revealed that incipient motion velocities of the largest particles recorded in artificial conditions were systematically larger than those observed in natural conditions. This demonstrates the roles played by large scale turbulence and the vertical component of the flow velocity in single particle entrainment. Due to its inherent characteristics, both of these flow components are weak in the artificial conditions generated by the device. In conclusion, our study demonstrates that this type of device is a useful tool in observing and studying incipient motion processes and, in particular, the role played by the bed structure (armouring, protrusion, etc.). However, some improvements are still required in order to more accurately measure critical velocities, allowing critical discharges to be determined

Introduction to the special issue in honor of Prof. François Petit

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CARHYCE : Consolidation scientifique des connaissances et des modèles d’évaluation pour la caractérisation hydromorphologique des cours d’eau de métropole et d’Outre-mer

La Directive-cadre sur l’eau (DCE, 2000) et sa traduction dans la loi française (LEMA, 2006) définissent le contexte en matière de politique de l'eau. Elles impliquent une gestion globale et intégrée des milieux et écosystèmes aquatiques pour atteindre un « bon état écologique ». Cet objectif est défini sur la base de la situation des communautés biologiques et des caractéristiques physico-chimiques de l'eau mais pas uniquement. Les conditions hydrologiques et la morphologie des cours d'eau sont considérées comme les facteurs de contrôle, ou « supports », de la biodiversité aquatique au travers des habitats qu’elles contribuent à mettre en place. S’y ajoutent les conditions en matière de continuité des rivières, traduites par le thème de la « continuité écologique » dans la gouvernance française. S’intéressant tant à la forme des lits fluviaux, qu’aux sédiments qu’ils transportent, à la structure ou composition des berges ou encore à la qualité des corridors rivulaires végétalisés, l’ensemble est ainsi représenté par 3 grands éléments de qualité DCE (EQ) multi-compartiments (i-e Hydrologie, Continuité de la rivière, Morphologie).Dans ce rapport, il est tout d’abord proposé d’établir un état des lieux des données disponibles à l’issue des 4 mises à jour et des optimisations réalisées depuis le début de l’année 2019 au sein de l’Interface d’Exploitation des Données Carhyce (IED). Est présentée ensuite la nouvelle version du référentiel d’analyse dit « Référentiel v2.0 » basée sur les relevés récents d’une large majorité des stations de référence (au sens du fonctionnement hydromorphologique). Est proposée également une clé d’interprétation de l’Indicateur morphologique global (IMG) grâce à une comparaison avec le Morphological quality index (MQI), un indicateur morphologique utilisé en Italie. Enfin, cette étude conclut sur quelques éléments de réflexions sur les potentialités et perspectives du Lidar pour la caractérisation hydromorphologique des cours d’eau

Scale dependency in the hydromorphological control of a stream ecosystem functioning

Physical habitat degradation is prevalent in river ecosystems. Although still little is known about the ecological consequences of altered hydromorphology, understanding the factors at play can contribute to sustainable environmental management. In this study we aimed to identify the hydromorphological features controlling a key ecosystem function and the spatial scales where such linkages operate. As hydromorphological and chemical pressures often occur in parallel, we examined the relative importance of hydromorphological and chemical factors as determinants of leaf breakdown. Leaf breakdown assays were investigated at 82 sites of rivers throughout the French territory. Leaf breakdown data were then crossed with data on water quality and with a multi-scale hydro- morphological assessment (i.e. upstream catchment, river segment, reach and habitat) when quantitative data were available. Microbial and total leaf breakdown rates exhibited differential responses to both hydromorphological and chemical alterations. Relationships between the chemical quality of the water and leaf breakdown were weak, while hydromorphological integrity explained independently up to 84.2% of leaf breakdown. Hydrological and morphological parameters were the main predictors of microbial leaf breakdown, whereas hydrological parameters had a major effect on total leaf breakdown, particularly at large scales, while morphological parameters were important at smaller scales. Microbial leaf breakdown were best predicted by hydromorphological features defined at the upstream catchment level whereas total leaf breakdown were best predicted by reach and habitat level geomorphic variables. This study demonstrates the use of leaf breakdown in a biomonitoring context and the importance of hydromorphological integrity for the functioning of running water. It provides new insights for envi- ronmental decision-makers to identify the management and restoration actions that have to be un- dertaken including the hydromorphogical features that should be kept in minimal maintenance to support leaf breakdown

Le flottage du bois et ses conséquences écologiques, de l’Antiquité à l’époque contemporaine

Introduction : l’empreinte environnementale du transport du bois dans le passé Le flottage consiste à transporter de grands volumes de bois, de toutes dimensions (bûches, billots, troncs entiers) et par divers moyens (en vrac ou assemblés en radeaux ou en trains) sur la voie fluviale ou maritime. En effet, dès lors que les pièces transportées deviennent abondantes ou représentent de gros volumes, que les distances parcourues excèdent quelques kilomètres, ou encore que les centres de consommat..

Le flottage du bois et ses conséquences écologiques, de l’Antiquité à l’époque contemporaine.: Problèmes, matériel et méthodes pour une contribution à l’histoire environnementale.

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