Bedload transport drives morphological changes in gravel-bed streams and sediment transfer in catchments. The large impact forces associated with bedload motion and its highly dynamic spatiotemporal nature make it difficult to monitor bedload transport in the field. In this study, we revise a physically-based model of bedload-induced seismic ground motion proposed by Tsai et al. (2012, htpps://doi.org/10.1029/2011GL050255) and apply it to invert bedload flux from seismic measurements alongside an Alpine stream. First, we constrain the seismic response of a braided river reach with a simple active experiment using a series of large-rock impacts. This allows the characterization of surface wave propagation and attenuation with distance from the impact source. Second, we distinguish bedload-generated ground vibrations from those caused by turbulent flow using frequency-based scaling relationships between seismic power and discharge. Finally, absolute bedload transport rates are quantified from seismic measurements using inverse modeling based on a simplified formulation of bedload particle motion. The results are verified with a large data set of bedload samples, demonstrating that seismic measurements can provide an indirect measure for bedload flux with uncertainties within a factor of 5 ±1 for instantaneous measurements (between 0.01 and 1 kg/m/s). Larger deviations may be caused by uncertainties in the contribution of turbulent flow effects, particle impact velocity, and especially particle size that may vary with sediment supply and flow conditions. When constraining these uncertainties, instream sediment transport measurements are no longer necessarily required and seismic monitoring may provide an accurate and continuous means to investigate bedload dynamics in gravel-bed streams.

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Bedload transport is recognized as a key process in the development of river channel forms; however, most rivers suffer from an absence of data. Performing bedload measurements to document bedload transport rates is a challenge, as the deployment of traditional bedload samplers is time consuming and risky in floods. Consequently, bedload measurements are rarely executed. Alternative techniques are being developed to complement the use of traditional bedload measurements and to provide continuous monitoring. Passive acoustic measurements are made with hydrophones, measuring the underwater sounds naturally generated by bedload impacts in rivers. This paper proposes an innovative deployment of hydrophones to record bedload sounds at the scale of a cross section. The measured acoustic signals are interpreted with bedload samplings and with hydraulic and river bed parameters. Field experiments were done in 14 different sites, exploring a diversity of rivers. Bedload flux was observed to be the most consistent variable explaining the monitored acoustic power. Based on 25 experiments on 14 rivers, the cross‐section‐averaged acoustic power was related to the specific bedload flux and showed a good agreement (60% of bedload flux estimated within a factor of 2). The robustness of the obtained calibration curve remains to be tested. However, the potential of passive acoustic profiles to provide a continuous measurement of bedload sounds that could be used in the development of bedload gauging stations is shown.

The transport of suspended sediment is associated with important social, economic, and environmental issues. It is still unclear, however, how suspended sediments eroded on hillslopes are transferred downstream through the river system. In this study, we aimed to investigate this process by applying a sediment budget approach to a typical 3.5-km-long Alpine braided reach. Using high-frequency suspended load measurements combined with Monte Carlo simulations for uncertainty propagation, we observed a significant buffering behavior of the reach studied. Thirty-two of the 48 events observed during the 2-month campaign showed significant differences between upstream and downstream mass transported as suspension, despite the reach studied was short compared to the upstream drainage area (130 km 2 ). These differences at the event scale varied widely within an envelope comprised between a net erosion equivalent to 74% of upstream suspended mass and a net deposition equivalent to 71%. Budgets were found to be controlled at a nearly instantaneous time scale by the liquid discharges and the suspended sediment concentrations in an opposite way: for low upstream concentrations, net erosion increased when the discharges increased, while above a certain concentration, net deposition increased when the concentrations increased. Moreover, coarse particles mobility in the reach (characterized via bedload transport measurements) appeared to have a strong influence on the availability of suspended particles as both quantities evolved concomitantly through time. These observations have important implications for our understanding and modeling of the transfer of suspended particles in gravel bedded streams.

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Suspended load transport can strongly impact ecosystems, dam filling and water resources. However, contrary to bedload, the use of physically based predicting equations is very challenging because of the complexity of interactions between suspended load and the river system. Through the analysis of extensive data sets, we investigated extent to which one or several river bed or flow parameters could be used as a proxy for quantifying suspended fluxes in gravel bed rivers. For this purpose, we gathered in the literature nearly 2400 instantaneous field measurements collected in 56 gravel bed rivers. Among all standard dimensionless parameters tested, the strongest correlation was observed between the suspended sediment concentration and the dimensionless bedload rate. An empirical relation between these two parameters was calibrated. Used with a reach average bedload transport formula, the approach allowed to successfully reproduce suspended fluxes measured during major flood events in seven gravel bed alpine rivers, morphodynamically active and distant from hillslope sources. These results are discussed in light of the complexity of the processes potentially influencing suspended load in a mountainous context. The approach proposed in this paper will never replace direct field measurements, which can be considered the only confident method to assess sediment fluxes in alpine streams; however, it can increment existing panel tools that help river managers to estimate even rough but not unrealistic suspended fluxes when measurements are totally absent. © 2019 John Wiley & Sons, Ltd.

Bedload Self-Generated Noise (SGN) measurements consist in deploying an underwater microphone (i.e. a hydrophone) in the river and to record the ambient noise. The use of hydrophones to measure bedload characteristics (flux, spatial distribution, granulometry) could be of interest as it can be more easily and rapidly deployed than physical samplers in rivers. Several measurement campaigns where conducted during spring and summer 2017 in 5 alpine rivers with contrasted transport conditions (bedload D50 between 1 and 40 mm) and varying slopes (0.05 to 1 %). Physical sampling measurements were done from a bridge along the river cross section for specific bedload flux varying between 10 and 150 g.m-1s-1. Bedload SGN measurements were obtained with a small board equipped with a hydrophone and deriving downstream the bridge within a 10 to 50 m long river section. For 2 of the 5 rivers, acoustic Doppler current profilers (ADCP) were also deployed along the river cross-section to provide a surrogate measurement of apparent bedload velocity. As a result, we have been able to draw an acoustic 1D-map of the river bottom, derived from the SGN sub-surface measurements obtained with the deriving board. The results show a coherent relation between the riverbed acoustic maps and the physical samplings for 3 rivers over 5. Bedload profile were less consistent with SGN measurements when bedload transport was localized in a narrow channel. The apparent bedload velocities obtained with ADCP for 2 rivers are consitent with the physical samplings (bedload location and flux distribution) but a slight bias was observed and is attributed to grain-size sorting effects along the cross-section. Finally, when plotting together 4 over 5 rivers, an almost linear relation can be established between bedload discharge (computed with physical samplings data) and the average acoustic response (i.e acoustic power averaged over the cross-section). This result suggests that a generalized calibration curve could exist between bedload SGN and bedload discharge. The existence of an outsider is interpreted as a problem related to propagation effects. Further researches should therefore concentrate their effort on deconvoluting SGN signals from propagation effects to give a better confident proxy for bedload discharge measurement in different rivers types.