Past and present controls on bed evolution, sediment distribution, and transport capacities in the Rhône River

Un siècle et demi d’intenses modifications humaines ont modifié drastiquement les capacités de transport par charriage dans le Rhône. Une étude basée sur le couplage de la modélisation hydraulique 1D, des équations de transport par charriage, et une large base de données granulométriques et bathymétriques a été menée afin d’estimer les capacités de transport à l’état actuel tout au long du chenal principal du Rhône en France. Nous comparons ces estimations avec celles obtenues pour un régime hydraulique non aménagé et une granulométrie de lit plus fine. Ces résultats apportent une vision générale sur la variabilité spatiale des capacités de transport par charriage dans le Rhône et une comparaison avec les capacités de transport avant les grandes phases d’aménagement du fleuv

Past and present controls on bed evolution, sediment distribution, and transport capacities in the Rhône River

Un siècle et demi d’intenses modifications humaines ont modifié drastiquement les capacités de transport par charriage dans le Rhône. Une étude basée sur le couplage de la modélisation hydraulique 1D, des équations de transport par charriage, et une large base de données granulométriques et bathymétriques a été menée afin d’estimer les capacités de transport à l’état actuel tout au long du chenal principal du Rhône en France. Nous comparons ces estimations avec celles obtenues pour un régime hydraulique non aménagé et une granulométrie de lit plus fine. Ces résultats apportent une vision générale sur la variabilité spatiale des capacités de transport par charriage dans le Rhône et une comparaison avec les capacités de transport avant les grandes phases d’aménagement du fleuv

Past and present controls on bed evolution, sediment distribution, and transport capacities in the Rhône River

Un siècle et demi d’intenses modifications humaines ont modifié drastiquement les capacités de transport par charriage dans le Rhône. Une étude basée sur le couplage de la modélisation hydraulique 1D, des équations de transport par charriage, et une large base de données granulométriques et bathymétriques a été menée afin d’estimer les capacités de transport à l’état actuel tout au long du chenal principal du Rhône en France. Nous comparons ces estimations avec celles obtenues pour un régime hydraulique non aménagé et une granulométrie de lit plus fine. Ces résultats apportent une vision générale sur la variabilité spatiale des capacités de transport par charriage dans le Rhône et une comparaison avec les capacités de transport avant les grandes phases d’aménagement du fleuv

Contribution of grain sorting to the bedload active layer dynamics

During the last 20 years, numerous sediment transport flume experiments have shown that grain sorting generates bed level fluctuations and bedload pulses. In this work, we first propose a new analysis of these experimental data. From this analysis, we derive a model for gravel bed rivers where both local slope and bedload are known to largely fluctuate through space and time, in the socalled 'bedload active layer'. The model considers a maximum slope for local armoring equals to the mean bed slope affected by a coefficient which expresses the difference of mobility of the coarse fraction considered alone or in a mixture, and a minimum local slope for bed erosion equals to the mean bed slope corrected by a coefficient that depends on the armor ratio Ar (ratio of the surface to the sub-surface grains diameter) and the reach-averaged transport rate. The model is then compared with a compilation of scour-fill depths measured in the field. Results suggest that the slope fluctuations in 1D flume experiments are consistent with in-channel bed-level fluctuations associated to scour-fill processes in the active layer, with a maximum scour depth which is slope dependent. For the pulse intensity, we provide a justification to the simplified squared slope equation used to compute solid concentration C = Qs/Q proportionnal to S² (with Qs the solid discharge, Q the water discharge and S the slope), which has often been used in place of standard bedload equations for modelling highly concentrated bedload transport events in mountain streams

Geomorphological patterns of grain size sorting in a braided gravel bed river (Vénéon River, French Alps)

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Effects of continuous embankments and successive run-of-the-river dams on bedload transport capacities along the Rhône River, France

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Contrôles géomorphologiques sur la dispersion de sédiment en chenaux de montagne

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Using UAS optical imagery and SfM photogrammetry to characterize the surface grain size of gravel bars in a braided river (Veneon River, French Alps)

International audienceThis paper explores the potential of unmanned aerial system (UAS) optical aerial imagery to characterize grain roughness and size distribution in a braided, gravel-bed river (Veneon River, French Alps). With this aim in view, a Wolman field campaign (19 samples) and five UAS surveys were conducted over the Veneon braided channel during summer 2015. The UAS consisted of a small quadcopter carrying a GoPro camera. Structure from -Motion (SfM) photogrammetry was used to extract dense and accurate three-dimensional point clouds. Roughness descriptors (roughness heights, standard deviation of elevation) were computed from the SfM point clouds and were correlated with the median grain size of the Wolman samples. A strong relationship was found between UAS-SfM-derived grain roughness and Wolman grain size. The procedure employed has potential for the rapid and continuous characterization of grain size distribution in exposed bars of gravel-bed rivers. The workflow described in this paper has been successfully used to produce spatially continuous grain size information on exposed gravel bars and to explore textural changes following flow events. (C) 2017 Elsevier B.V. All rights reserved

Experimental bed active-layer survey with active RFID scour chains: Example of two braided rivers (the Drac and the Vénéon) in the French Alps

An innovative scour chains device composed of active UHF RFID tags is proposed to survey the bed active layer. This device is tested on three cross-sections deployed along two large Alpine braided rivers: the Drac and the Vénéon. A specific field deployment procedure is developed, using a technique of drilling with a tube and a jack-hammer. After each flood, the device allows recording presence/absence of active tags inside a scour chain column. This provides the maximum scouring depth of the bed at the position of the column, and the topographic resurvey of the channel provides the net fill depth. Results show that bed active layer can reach up to 1.43 m during high flow conditions and remains <0.1 m during low and moderate flows. The main advantage of this technology against traditional scour chains is the rapidity and easiness with which the scouring depth is obtained after a flow event, notably under conditions of massive net deposition after the flow. This technology is therefore particularly suitable for large braided rivers, where traditional scour chains necessitate too much field efforts for their relocation, and can be rapidly lost under deep active layers

Particle transport in gravel-bed rivers: Revisiting passive tracer data

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