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Recent studies have shown that hydrophone sensors can monitor bedload flux in rivers by measuring the self-generated noise (SGN) emitted by bedload particles when they impact the riverbed. However, experimental and theoretical studies have shown that the measured SGN depends not only on bedload flux intensity but also the propagation environment, which differs between rivers. Moreover, the SGN can propagate far from the acoustic source and be well measured at distant river positions without bedload transport. It has been shown that this dependency of the measured SGN data on the propagation environment can significantly affect the performance of monitoring bedload flux by hydrophone techniques. In this article, we propose an inversion model to solve the problem of the SGN propagation and integration effect. In this model, we assume that the riverbed acts as SGN source areas with intensity proportional to the local bedload flux. The inversion model locates the SGN sources and calculates their corresponding acoustic power by solving a system of linear algebraic equations, accounting for the actual measured cross-sectional acoustic power (acoustic mapping) and attenuation properties. We tested the model using data from measured bedload SGN profiles (acoustic mapping with a drift boat) and bedload flux profiles (direct sampling with an Elwha sampler) acquired during two field campaigns conducted in 2018 and 2021 on the Giffre river in the French Alps. Results confirm that the bedload flux measured at different verticals on the river cross-section correlates more with the inversed acoustic power than measured acoustic power. Moreover, it was possible to fit data from the two field campaigns with a common curve after inversion, which was not possible with the measured acoustic data. The results of the inversion model, compared to measured data, show the importance of considering the propagation effect when using the hydrophone technique and offer new perspectives for the calibration of bedload flux with SGN in rivers.

Abstract Sediment flows generate ground vibrations by exerting force fluctuations on the riverbed. Linking force fluctuations to properties of highly concentrated sediment flows, however, remains particularly challenging due to complexities arising from grain‐to‐grain interactions. Here, we conduct downscaled flume experiments in which we specifically measure force fluctuations and local seismic vibrations together with flow properties of highly concentrated sediment flows at high spatial and temporal resolution. We observe hysteresis behaviors between force fluctuations amplitude and flow surface elevation and mass that occur during complex changes in internal flow dynamics. By contrast, force fluctuations amplitude exhibits a unique negative relationship with solid concentration. We suggest this is due to the rheology of dense granular flows, where solid concentration is a proxy for particle agitation. We therefore advance that solid concentration should be incorporated in seismic models of such sediment flows as a key parameter describing inter‐particle collisions and impacts to the bed.

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. 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 setup wherein we supply liquid and solid discharge to a low-slope storage zone acting like a natural sediment accumulation zone that is connected to a downstream 18 % steep channel equipped with geophones. We show that the ability of the self-formed deposit to generate sediment pulses is controlled by the fine fraction of the mixture. In particular, when coarse grains coexist with a high content of finer particles, 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 pulse transport characteristics, which questions the applicability of existing seismic theories in such a 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.

Bedload transport can fluctuate considerably over relatively short periods of time and for a given quasi‐constant flow rate. What are the implications of replacing the fluctuating signal with a smoothed signal when calculating bedload transport using averaged values, as is common practice? This question was investigated with the BedloadR code, which allows 1D bedload calculation as well as Monte Carlo simulations using a new data set collected in the Severaisse River (French Ecrins massif). Four bedload equations (Camenen & Larson, 2005, https://doi.org/10.1016/j.ecss.2004.10.019; Meyer‐Peter & Mueller, 1948; Parker, 1990, https://doi.org/10.1080/00221689009499058; Recking, 2013a, https://doi.org/10.1061/(asce)hy.1943‐7900.0000653) were selected for their performance relative to the measured bedload (except for and Meyer‐Peter and Mueller) and because each equation has a different mathematical form and degree of nonlinearity. They were used in a Monte Carlo approach, with input probability distributions fitted to the measured river width, slope, bed grain‐size distribution, and to the associated (computed) Shields stress. The results show that accounting for natural variability in the calculation reproduces bedload fluctuations well. But overall, when calculating the bedload volume transported by a flow event, accounting for variability systematically leads to higher estimated volumes (of the order of 20%) than those obtained with a deterministic approach using average input parameters. This is a direct consequence of the nonlinearity of the equations. Plain Language Summary: Transport of bedload in a river, that is, sediment in motion that stays close to the riverbed, fluctuates a lot over relatively short time periods for a given near‐constant discharge. In this paper, we try to reproduce this natural process. To do this, we use a protocol that involves running a bedload equation several thousand times, slightly and randomly modifying the input data with each run. The results show that by taking into account the natural variability of sediment size or channel geometry, we obtain realistic modeling of bedload transport fluctuations. Key Points: A new field data set is used for computing bedload in a Monte Carlo approachBedload modeling accounting for input variability reproduces bedload fluctuation wellBecause of non‐linearity, volumes computed with input variability are 20% higher [ABSTRACT FROM AUTHOR]

Abstract Bedload transport modelling in rivers takes into account the size and density of pebbles to estimate particle mobility, but does not formally consider particle shape. 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.

Dense seismic array monitoring has recently allowed the detailed investigation of sources of ground vibrations and their spatiotemporal dynamics. In a context where traditional monitoring techniques for fluvial processes 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). Here, we study the potential of dense seismic array monitoring by investigating a high-flow event that occurred in summer 2019 along a 600-m-long braided reach of the Séveraisse River (French Alps). We use a network of 80 seismometers deployed on both river banks, and we supplement the seismic observations with flow gauging measurements and time-lapse imagery. 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 (blocks). Through phase-delay analysis we are able to locate these seismic events on the bend apex of an active branch of the reach. These results demonstrate the capability of such a method to locate bedload activity at high spatiotemporal resolution, providing crucial information for geomorphological investigations and natural risk management.

Recent studies have shown that hydrophone sensors can monitor bedload flux in rivers by measuring the self-generated noise (SGN) emitted by bedload particles when they impact the riverbed. However, experimental and theoretical studies have shown that the measured SGN depends not only on bedload flux intensity but also the propagation environment, which differs between rivers. Moreover, the SGN can propagate far from the acoustic source and be well measured at distant river positions without bedload transport. It has been shown that this dependency of the measured SGN data on the propagation environment can significantly affect the performance of monitoring bedload flux by hydrophone techniques. In this article, we propose an inversion model to solve the problem of the SGN propagation and integration effect. In this model, we assume that the riverbed acts as SGN source areas with intensity proportional to the local bedload flux. The inversion model locates the SGN sources and calculates their corresponding acoustic power by solving a system of linear algebraic equations, accounting for the actual measured cross-sectional acoustic power (acoustic mapping) and attenuation properties. We tested the model using data from measured bedload SGN profiles (acoustic mapping with a drift boat) and bedload flux profiles (direct sampling with an Elwha sampler) acquired during two field campaigns conducted in 2018 and 2021 on the Giffre river in the French Alps. Results confirm that the bedload flux measured at different verticals on the river cross-section correlates more with the inversed acoustic power than measured acoustic power. Moreover, it was possible to fit data from the two field campaigns with a common curve after inversion, which was not possible with the measured acoustic data. The results of the inversion model, compared to measured data, show the importance of considering the propagation effect when using the hydrophone technique and offer new perspectives for the calibration of bedload flux with SGN in rivers. [ABSTRACT FROM AUTHOR]

For over 150 years, humans have tried to limit the geomorphic activity of mountain watercourses, and the related damage, using torrent control works. Check dams are likely the most emblematic civil engineering structures used in mountain system soil conservation programs. Modern mountain societies have inherited thousands of these structures built in upland gullies and streams. The expected qualitative effect of check dams on river systems is referred to their function. Check dam function must be clearly understood to help define their effectiveness and decisions concerning their maintenance or new project designs. The next steps concern quantitative assessments of each function on the flood features and propagation of all effects through the sediment cascade. The present understanding of these sometimes old structures’ functions can be complicated because the societal and environmental contexts in which the original structures were built may have changed. To bridge this gap, this paper traces the purposes for which check dams were built, through a detailed analysis of French archives. We first analyze chronologically how each function was theorized and applied in the field. In the nineteenth century, engineers developed a thorough empirical and conceptual knowledge of mountain soil erosion, torrential geomorphology, and sediment transport processes as well as check dam interactions with these natural processes. The second part of this paper synthesizes conceptual descriptions of the check dams’ functions, in the light of more than 150 years of experience. The French experience is compared to other countries’ pioneering works. Finally, the next steps and remaining research challenges toward a comprehensive analysis of check dams’ efficiency in torrent hazard mitigation are presented. This analysis is proposed to remind how, conceptually, check dams may influence geomorphic systems, bearing in mind the knowledge represented in pioneer guidelines and recent works on the subject.

Entre terre et eau, les berges sont des milieux d'une grande richesse floristique et faunistique qui jouent un rôle majeur de corridors biologiques. Si l’érosion des berges est un phénomène naturel qui contribue à la dynamique et à la diversité des cours d’eau, des ouvrages de protections restent néanmoins nécessaires lorsque des enjeux humains doivent être protégés. La protection des berges par des ouvrages de génie végétal est une solution écologique et économique, mais la faiblesse des éléments de dimensionnement de ces techniques reste encore un frein à leur développement. En s'appuyant sur la littérature et sur l’analyse d’une vingtaine de sites, cet article s'intéresse ici à l'amélioration des méthodes de dimensionnement des ouvrages de génie végétal en berges de cours d'eau par une approche empirique.

Encore peu utilisées sur les cours d'eau dynamiques comme les rivières de montagne, les techniques de génie végétal représentent pourtant une solution écologique et économique pour la protection des berges. En s'appuyant sur le retour d'expérience de plusieurs chantiers pilotes en France et en Suisse, cet article s'intéresse à la capacité des techniques de génie végétal à résister sur des rivières de montagne associant contraintes climatiques, végétation et hydrologie particulière avec d'importantes contraintes physiques liées à l'eau et au transport solide.