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International audience; Les compilations mondiales des charges sédimentaires des rivières montrent que les zones montagneuses produisent une forte proportion des sédiments transportés vers les océans. Cependant, en raison de l'effort impliqué dans la mesure des flux de sédiments dans les systèmes fluviaux de montagne, les charges de ces rivières sont généralement inconnues. Ici, nous présentons des estimations des charges sédimentaires contemporaines de 16 rivières à lit de gravier drainant le massif des Ecrins-Pelvoux dans le sud-est de la France. La production de sédiments dans cette partie des Alpes est relativement élevée et de nombreux segments fluviaux sont soit errants, soit tressés. Nous modélisons les flux de sédiments et les rendements annuels de sédiments en couplant des relations basées sur l'hydraulique pour le transport des sédiments avec des relations basées sur l'hydrologie pour la fréquence des flux. Les flux de charriage sont modélisés pour une gamme de décharges à l'aide d'une fonction qui relie les taux de transport à la contrainte de cisaillement excessive. Les flux sont ensuite pondérés par la fréquence des rejets individuels et additionnés pour obtenir la charge de fond annuelle pour chaque site. La charge suspendue est estimée empiriquement comme une fraction de la charge de fond. Les résultats suggèrent que les flux de charge de fond aux écoulements formant des canaux évoluent presque linéairement avec les augmentations en aval du débit. De plus, il semble que les charges sédimentaires annuelles (charge de fond + charge en suspension) évoluent linéairement avec la zone de drainage. Une relation complémentaire pour le rendement spécifique des sédiments suggère que la charge par unité de surface de drainage est constante dans l'ensemble des bassins étudiés. Les rendements sédimentaires modélisés sont comparables aux estimations antérieures sur le terrain des rendements sédimentaires modernes, et généralement inférieurs aux estimations élaborées à partir des analyses de radionucléides cosmogéniques.

International audience

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In the upper part of mountain river catchments, large amounts of loose debris produced by mass wasting processes can accumulate at the base of slopes and cliffs. Sudden destabilizations of these deposits are thought to trigger energetic sediment pulses that may travel in downstream rivers with little exchange with the local bed. The dynamics of these exogenous sediment pulses remain poorly known because direct field observations are lacking, and the processes that control their formation and propagation have rarely been explored experimentally. Here we carry out flume experiments with the aims of investigating (i) the role of sediment accumulation zones in the generation of sediment pulses, (ii) their propagation dynamics in low-order mountain channels, and (iii) the capability of seismic methods to unravel their physical properties. We use an original set-up where we supply with liquid and solid discharge a low slope storage zone acting like a natural sediment accumulation zone, and connected to a downstream 18 % steep channel equipped with geophones. We show that the ability of the self-formed deposit to generate sediment pulses depends on the sand content of the mixture. In particular, when a high fraction of sand is present, the storage area experiences alternating phases of aggradation and erosion strongly impacted by grain sorting. The upstream processes also influence the composition of the sediment pulses, which are formed by a front made of the coarsest fraction of the sediment mixture, a body composed of a high concentration of sand corresponding to the peak of solid discharge, and a diluted tail that exhibits a wide grain size distribution. Seismic measurements reveal that the front dominates the overall seismic noise, but we observe a complex dependency between seismic power and sediment pulses’ transport characteristics, which questions the applicability of existing simplified theories in such context. These findings challenge the classical approach for which the sediment budget of mountain catchments is merely reduced to an available volume, since not only hydrological but also granular conditions should be considered to predict the occurrence and propagation of such sediment pulses.

Quantifying bedload transport is important for many applications such as river management and hydraulic structures protection. Bedload flux measurements can be achieved using physical sampler methods. However, these methods are expensive, time-consuming, and difficult to operate during high discharge events. Besides, these methods do not permit to capture the spatial and temporal variability of bedload transport flux. Recently, alternative measuring technologies have been developed to continuously monitor bedload flux and grain size distribution using passive or active sensors. Among them, the hydrophone was used to monitor bedload transport by recording the sounds generated by bedload particles colliding on the river bed (referred as self-generated noise SGN). The acoustic power of SGN was correlated with bedload flux in field experiments. To better understand these experimental results and to estimate measurement uncertainties, we developed a theoretical model to simulate the SGN. The model computes an estimation of the power spectral density (PSD)by considering the contribution of all signals generated by impacts between bedload particles and the riverbed, and accounting for the attenuation of the acoustic signal between the source and the hydrophone position due to river propagation effects,. In this model, weThe energy of acoustic noise generated from the collision between two particles is mainly dependent on the transported particles' diameter and the impact velocity. We tested different empirical formulas for the estimation of the number of impact (impact rate) and the impact velocity depending on particle size and hydraulic conditions. To characterize the acoustic power losses as a function of distance and frequency, we used an attenuation function which was experimentally calibrated for different French rivers.We tested the model on a field dataset comprising acoustic and bedload flux measurements. The results indicate that the PSD model allows estimating acoustic power (in between a range of one order of magnitude) for most of the rivers considered. The model sensitivity was evaluated. In particular, we observed that it is very sensitive to the empirical formulas used to determine the impact rate and impact speed. In addition, special attention should be kept in mind on the assumption of the grain size distribution of riverbed which can generate large variability in some rivers particularly in rivers with a significant sand fraction.

Over the last decade, seismic techniques have provided unique observational constraints on Earth surface processes. In particular, dense seismic array monitoring has recently allowed the detailed investigation of noise sources and their spatiotemporal dynamics. Despite their large potential, these approaches have not yet been applied for the monitoring of fluvial processes. In a context where traditional methods often do not provide data with adequate temporal and spatial resolution, the use of dense arrays could allow the identification and tracking of different sources of river-induced seismic ground vibrations (e.g. turbulence and bedload transport), which would provide insight in river functioning and morphological evolution.Here, we study the potential of dense seismic array monitoring by analysing data from a 4-month long field survey, which we conducted in summer 2019 along a 600-m long braided reach of the Séveraisse river (French Alps). We installed a network of 40 to 80 seismometers on both river banks, predominantly deployed in 4-seismometer subarrays, and we supplement these seismic observations with flow gauging measurements and time-lapse imagery covering the study area. We present a preliminary analysis that focuses on a high-flow event that occurred at the end of the melt season. During this event, we observe impulsive signals that are coherently detected over the array, and which we interpret as being associated with the bedload transport of clusters of coarse grains. Through phase-delay analysis we are able to locate episodes of motion at high temporal resolution and investigate their spatiotemporal dynamics with respect to river morphology and morphological changes observed from the time-lapse images. Our work demonstrates the unique capability of using dense seismic arrays to better understand the fluvial processes that play an important role in storing and transferring sediments in braided rivers.

Seismic measurements are used to study various processes that shape the Alpine landscape, including rock falls, debris flows, bedload transport and turbulent water flow. Here, we focus on the seismic quantification of turbulent flow conditions which is particularly useful for the remote monitoring of channels that are inaccessible (e.g. subglacial conduits) and/or highly dynamic (e.g. actively braiding river reaches). We test a physically-based model (Gimbert et al., 2014) to quantify force spectra generated by turbulent flow in flume experiments performed by Lamb et al. (2017) and subsequently apply the model to estimate river flow depth from continuous seismic measurements in the field.In the flume, we assess near-bed flow velocity spectra and resulting drag and lift force spectra experienced by particles (D=0.075-0.20 m) on the cobble bed for a wide range of channel gradients (S=0.004-0.3) and submergence levels (h/D50=1-9.6). These measurements are used to test our model, and to quantify wake (interaction) effects and fluid-dynamic admittance on force spectral amplitude. Based on the conservation of turbulent energy in the Kolmogorov inertial subrange, we predict lift and drag force spectra to within ±5 dB rel. N2/Hz (frequency ~10-25 Hz) of the measured values.We apply the calibrated model to bank-side geophone measurements from an Alpine stream (Séveraisse River, France). Using locally-derived seismic parameters, riverbed particle-size distribution and bed roughness, we can invert for water depth over a range of flow conditions, including flows with bedload transport (bedload transport dominates the seismic signal at higher frequencies). This allows us to monitor changes in flow depth during the course of a high-magnitude flood (October 2019). During the falling limb, the inferred flow depths progressively deviate from independently made water level measurements, indicating local riverbed aggradation of approximately 0.5 m, which is in agreement with post-flood observations. Through insights in near-bed turbulent flow conditions and their seismic signature, we can study flow-bedload transport interactions and the effects of extreme flow events on river morphodynamics.ReferencesGimbert, F., Tsai, V. C. & Lamb, M. P. (2014). A physical model for seismic noise generation by turbulent flow in rivers. Journal of Geophysical Research: Earth Surface, 119(10), 2209-2238. http://dx.doi.org/10.1002/2014JF003201Lamb, M. P., Brun, F. & Fuller, B. M. (2017). Direct measurements of lift and drag on shallowly submerged cobbles in steep streams: Implications for flow resistance and sediment transport. Water Resources Research, 53(9), 7607-7629. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2017WR020883

International audience; Studying sediment transport partitioning between bedload and suspended load has been a major research topic for decades (Bagnold, 1966; Einstein et al., 1940; Turowski et al., 2010). This partitioning is crucial for the understanding of sediment transport processes and the transfer of sediment in catchments. Coarse particles (boulders to sand) are often considered to be transported as bedload by sliding, rolling, or saltating and thus strongly interacting with the river bed and its morphology (Ashmore, 1988; Hoey, 1992). Finer particles (sand to clay) are often considered to be transported over longer distances in suspension throughout the water column without necessarily interacting with the bed (Einstein et al., 1940). Due to these long transport distances, the fine fraction generally originates from various sources and processes including rainfall and runoff erosion of nonvegetated hillslopes (Cheraghi et al., 2016; Mohamadi & Kavian, 2015) or rapid mass movements and debris flows suddenly connecting source areas to the river system (Fryirs, 2013). The fine fraction may also originate directly from the river bed resuspension (Misset, et al., 2019), although the extent to which this fraction interacts with the river bed is often unknown (Walling et al., 1998). Quantifying this is crucial for the understanding of landscape evolution (Ludwig & Probst, 1998), river system, and reservoir management planning (Kondolf et al., 2014) or optimizing water resource quality

Suspended load often represents the most significant fraction of sediment load in rivers. The transfer of these fine particles produced on hillslopes and transported through the river system is poorly understood in Alpine rivers. However, fine particles transport is associated with social, economic, and ecological issues such as pollutant transfer, riverbed clogging, vegetation growth, or reservoir siltation. In this context, the objective of this study is to characterize the interactions between suspended load and the riverbed of La Severaisse (Ecrins Massif), a typical Alpine river. To this aim, we applied a short-term sediment budget approach based on high frequency suspended load measurements on a 3.5-km reach. In addition, we measured bedload transport through direct sampling. The results indicate that suspended load is significantly buffered in the studied reach during the two-month campaign. Moreover, conditions leading to the mobility of coarse particles were found to evolve concomitantly with conditions leading to fine particle mobilization. These observations bring new insights in understanding the transfer of suspended material in Alpine rivers.

International audience; While the finest fraction of suspension has long been considered to have limited interactions with the river bed, several recent studies based on flume and field observations raise questions about this hypothesis which is fundamental for suspended load modeling and river management. In this study, we report a large field campaign in which we quantify the river bed stocks of fine particles in 7 contrasted Alpine catchments. Using a simple protocol, we performed more than 300 riverbed measurements of the local surface and subsurface stocks. Results indicate that even when the river bed surface contains no fine particles, significant quantities can be found in the subsurface layer which is in most cases the layer having the higher stocks. We also observed that stocks highly depend on the facies considered suggesting that storage processes are strongly driven by the local hydraulics and river bed characteristics. By integrating these local stocks at the catchment scale, we estimated that they could represent more than 50% of the mean annual suspended load in catchments having large alluvial braided sections. On the opposite, these stocks could be as small as 1% in highly eroded head water catchments. This suggests that the bed of large alluvial Alpine rivers can be considered as a significant source of fine particles. These observations were confirmed by using a simplified vertical scouring model to estimate conditions for these stocks to be released in the flow. However, the use of this model suggests that other bed reworking processes (channel widening and migration) have to be considered. Finally, these observations suggest that interactions between particles transported as suspension and gravel beds are far from being negligible processes in catchments having large alluvial sections typically found in Alpine environments.

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.

International audience; This paper uses flume experiments to investigate the influence of lateral width confinement on channel morphology, sediment sorting, and bedload transport. Three runs of about 60 hr were carried out under constant feeding rate equal to 0.6 l/s and 8 g/s, with a bimodal mixture of natural sediments, a fixed flume slope of 3%, and width imposed by lateral walls from 0.12 to 0.50 m in order to model three different flow confinement configurations. Despite the three runs transporting at the same rate on average, they presented different gravel-bed river morphologies and different degrees of bed complexity. The three runs also presented differences in bedload transport rate fluctuations associated with different magnitude and mechanisms of bed storage and release of sediments. The intermediate width configuration (Run 2) was found to have a minimum storage and release of sediments compared with both the wider configuration (Run 1) and the narrower configuration (Run 3). Runs 1 and 3 were characterized by a significant storage and release of sediments resulting in a highly fluctuating bedload transport rate; however, while Run 1 (braiding morphology) stored and released sediments through lateral deposits and bed variations with planimetric sorting, Run 3 (single-thread channel) stored and released sediments through variations in bed elevation accompanied by vertical sorting. We extrapolated this concept to the full set of gravel-bed river morphologies, thus speculating about the channel morphology for optimal sediment transfer.

Bedload transport modelling in rivers, which defines the threshold for pebble movement, takes into account the size and density of pebbles, but does not formally consider particle shape. The lack of analyses evaluating the influences of shape and density on particle mobility presents a major deficiency. To address this issue and to compare the relative roles of the density and shape of particles, we performed original sediment transport experiments in an annular flume using molded artificial pebbles equipped with a radio frequency identification tracking system. The particles were designed with four distinct shapes and four different densities while having the same volume, and their speeds and distances traveled under constant hydraulic conditions were analyzed. The results show that particle shape has more influence than particle density on the resting time between particle displacement and the mean traveling distance. For all densities investigated, the particle shape systematically induced differences in travel distance that were strongly correlated (R² = 0.94) with the Sneed and Folks shape index. Such shape influences, although often mentioned, are here quantified for the first time, demonstrating why and how they can be included in bedload transport models.S

Le présent document correspond au rapport synthétique du programme « Immila, Qualification et quantification des flux de la mine au lagon et de leurs impacts ».- Le texte de ce rapport synthétique est volontairement réduit pour faciliter sa lecture. Il renvoie à des paragraphes détaillés, des figures et des tableaux de données qui sont présentés dans le rapport scientifique final et le volume d’annexes (contributions complètes, consultables sur : https://cnrt.nc/bassin-versant/rapport-scientifique-lie-au-projet-immila-de-la-mine-au-lagon-edition-2020/).- Ce programme s’insère dans un ensemble de deux projets sur le « Transport solide dans les bassins versants miniers » composé de :• IMMILA, Impact de la mine au lagon, 2 volumes : Rapport scientifique et Annexes (contributions complètes) • GESTION DU PASSIF de l’activité minière et remédiation, 2 volumes : Rapport scientifique et Guide méthodologique.- Cet ensemble de projets traite des causes et des mécanismes de mobilisation des matériaux et propose des mesures de remédiation aux engravements anciens ou plus récents des rivières calédoniennes.

International audience