The field of fluvial seismology has undergone significant advances over the past decade. The development of dedicated physical theories and their applications in various contexts have allowed separating the respective contributions of turbulent flow and bedload transport, such that physical parameters like flow depth and sediment flux may be inferred from seismic observations. However, the quantitative link between signal characteristics (amplitude, frequency) and the underlying physics yet involves simplified considerations that do not necessarily apply to more complex situations, such as for example under rough flow conditions or during extreme floods. In this talk I will present results from laboratory experiments that we designed specifically in order to quantify the seismic signature of flow turbulence and intense bedload transport under a range of conditions using force sensors coupled to the river bed. On one hand, I will show that existing theory regarding turbulent flow properly captures the main characteristics of the seismic source, but that additional dependencies on flow conditions and particle-wake development need to be included for more accurate predictions. On the other hand, I will show that existing theory regarding bedload transport fails at capturing the main characteristics of the seismic source under intense bedload transport conditions associated with complex changes in internal flow dynamics. In this case the seismic source appears to be a decreased function of solid concentration, as opposed to an increased function such as considered in current theories, which we suggest is due to grain impacts being agitation-controlled rather than bed-roughness controlled. Finally, I will discuss possible ways towards building more generic theories of ground motion induced by sediment transport.

To monitor the effects of rapid changes in climate and land use on sediment export from erodible environments, it is crucial to accurately quantify highly fluctuating suspended sediment concentrations (SSCs) in contrasted river systems that drain small to mesoscale catchments. To this end, we investigate the turbidity-based quantification of SSCs in the range of 0.05-100 g/L through laboratory experiments performed with 7 different types of turbidity sensors and sediments from 10 watersheds. We find that measurements of scattered light from multiple angles may allow for: (1) an extended monitoring range with SSCs up to 10-100 g/L, where enhanced uncertainty may occur near the transition in the effective operational ranges of the underlying signals (typically somewhere in the range of 1-10 g/L); and/or (2) a slightly reduced sensitivity to sediment properties. The specific turbidity of the investigated sensors is inversely related to particle diameter (D10) for SSCs up to 1-5 g/L. Backscatter and combined-signal sensors also show a dependency on sediment colour (CIE a*), which becomes particularly prominent at SSCs above 10 g/L. We relate this increase in colour dependency with SSC to the expected effect of cumulative near-infrared light absorption associated with multiple scattering. We discuss covarying physical properties of naturally occurring river sediment that can dampen or enhance measurement sensitivity and result in turbidity-based SSC rating curves that may strongly differ in magnitude and form from curves derived for industrially prepared material that is often used for sensor calibration. Although the differences in SSC per sensor among sediment types are generally less than one order of magnitude, the systematic errors and uncertainties associated with high SSCs are typically greater than one order of magnitude and may disproportionally affect the quantification of sediment loads during large-magnitude flow events.

Monitoring bedload transport in rivers is a challenging research domain teeming with technical innovations and methodological developments aimed at improving our knowledge and models of bedload processes at different spatial–temporal scales. Radio frequency identification (RFID) technology has improved sediment tracking, allowing the characterisation of transport processes of individual particles at flood‐event scales. Meanwhile, geophone sensors have enabled the long‐term continuous monitoring of seismic signals that can provide surrogate measures of bedload fluxes at local scales, during flood events and at sediment‐pulses. The combination of these two techniques could allow sediment transport processes to be linked with both flood events and sediment pulses. In this study, we used a combination of active ultra‐high frequency RFID technology and geophone monitoring stations to link the virtual velocity of tracers with seismic activity, hydraulic forcing, and the properties of the tracked particles. Single and multiple regression models show that seismic activity best explained the observed variance (81%) of the virtual velocity of particles, in comparison with discharge (58%) and stream power (63%). Furthermore, when several control variables (seismic activity and particle properties) were combined in an empirical model, the model explained 89% of the variance and allowed quantification of the portions of the variance explained by hydraulic forcing, geophonic activity and tracked particles. These results show the high potential of these combined monitoring techniques for future in‐field experiments to investigate bedload processes at different spatiotemporal scales in rivers of different morphologies.

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.

On 2 October 2020, the Maritime Alps in southern France were struck by the devastating Storm Alex, which caused locally more than 600 mm of rain in less than 24 h. The extreme rainfall and flooding destroyed regional rain and stream gauges. That hinders our understanding of the spatial and temporal dynamics of rainfall–runoff processes during the storm. Here, we show that seismological observations from permanent seismic stations constrain these processes at a catchment scale. The analysis of seismic power, peak frequency, and the back azimuth provides us with the timing and velocity of the propagation of flash-flood waves associated with bedload-dominated phases of the flood on the Vésubie River. Moreover, the combined short-term average to long-term average ratio and template-matching earthquake detection reveal that 114 local earthquakes between local magnitude ML=-0.5 and ML=2 were triggered by the hydrological loading and/or the resulting in situ underground pore pressure increase. This study shows the impact of Storm Alex on the Earth's surface and deep-layer processes and paves the way for future works that can reveal further details of these processes.

Rainfall is a key driver of geomorphological processes ranging from impacting drops that lead to the small-scale dislodgement of soil particles to large-scale morphogenic floods and rainfall-induced hillslope processes. Although rainfall has been identified in seismic records, the associated power spectral density and its quantitative relation to the underlying physical processes have not yet been studied. Here, we analyze nearly 2 years of combined seismic and optical disdrometer measurements, where the latter enables the drop-based quantification of rainfall physical properties. Our measurements confirm the broadband observation of ground velocity power spectral density due to rainfall, allowing the seismic identification of rainfall at intensities as low as 1 mm/h. Seismic power, P, shows a power-law scaling with rainfall intensity, I, and kinetic energy, E: P proportional to I-2.1 and P proportional to E-1.6. The observed scaling relations are consistent between the three monitored sites although there are absolute differences in seismic power of about 1 order of magnitude, which are likely due to variability in landcover and subsurface seismic properties. With a physical model, we demonstrate that the observed power-law relations are set by an underlying linear relation between seismic power and rainfall impulse power, and that the associated exponent values of I and E are due to the covariance of the raindrop size distribution with the total number of drops. The largest raindrop fractions, whose relative contribution increases with rainfall intensity, dominate the seismic signal where, in our case, 90% of the seismic power is attributed to drops larger than 3 mm. Using our model, we estimate the contributing area of rainfall to seismic observations to be within a radial distance of -5-25 m. The spatially integrated nature of the seismic measurements and their sensitivity to large raindrops, which control the disaggregation and the mobilization of soil particles, make seismic records well-suited for the investigation of soil erosion processes. More generally, our work provides a basis for the temporally-resolved seismic quantification of rainfall that drives the dynamics of various hydro-geomorphological processes.

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.

On 2-3 October 2020, the Maritime Alps were struck by storm Alex, a violent meteorological event that triggered heavy rainfall in southeast France, more generally referred to as a "Mediterranean Episode". The Mediterranean episode generated cumulative 24-hour rainfall rate locally exceeding yearly averages (>500 mm per 24 hours). The torrential rains triggered hazardous sediment-transporting floods of an intensity never documented in the area causing several casualties, and large infrastructure and economic damage. Rain and stream gauges’ measurements during the episode are incomplete and highly uncertain due to threshold saturation and destruction of measuring devices, and changes in the stream bed. However, 11 regional seismological stations of the French permanent network recorded continuous ground shaking during and after the episode. Significant ground unrest was generated by the geomorphological phenomena providing additional information on their temporal and spatial dynamics. Here, we present results of the combined efforts in environmental and crustal seismology to better understand the spatiotemporal dynamics of the sediment-transporting floods and hydrological forcing on the solid Earth during and after the episode. For that, we first analyze seismic power, peak frequency, and dominant noise directions of seismic signals generated by sediment-transporting floods to infer bedload transport dynamics. Moreover, by using template matching we detect 93 small earthquakes that were triggered during the Alex episode exactly in the area where rainfall was maximum. This exceptional seismic swarm is possibly triggered by overpressure due to the water load in karsts, or changes in pore fluid pressure. Our results illustrate that seismological observations allow for better understanding and quantifying of the geomorphological impact of extreme weather phenomena in mountainous settings and the related hydro-geomorphological hazards.

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. References Gimbert, 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/10.1002/2014JF003201 Lamb, 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

The monitoring of coarse sediment transport is a domain teeming with technical innovations and methodological developments aimed at improving the characterization of bedload processes at different spatial and temporal scales. Radio frequency identification (RFID) technology has improved sediment tracking experiments, allowing the characterization of processes at the individual particle and flood-event scales. Meanwhile, geophone sensors have enabled the continuous monitoring of bedload intensity and variations in sediment fluxes at intra-flood event and sediment-pulse scales. The combination of these two techniques allows these scales and processes to be linked. In this study, we used a combination of active ultra-high frequency RFID and geophone stations to link the virtual velocity of tracers with channel geophone activity, hydraulic forcing, and tracked particle properties, analyzing the results with single and multiple regression models. Our results show that compared with discharge or stream power, the geophone activity best explained the variance (81%) of the virtual velocity observed. Furthermore, when all the control variables tested were combined in an empirical model, the model explained 95% of this variance, and allowed quantification of the portions of the variance explained by hydraulic forcing, geophonic activity, and tracked particles. This shows the high potential of such monitoring combinations for future in-field experiments to investigate bedload processes in river systems of different morphologies at different spatiotemporal scales.

Fine sediment transport results from the complexity of the interactions between the different modes of transport and the variety of possible sediment sources, from the river bed stocks remobilization to hillslopes erosion. From a 2‐year period in an Alpine catchment, we show how the combined use of continuous turbidity and seismic measurements can help to address these issues. In the studied catchment, the signals are more strongly correlated during the high flows of the snowmelt period than during the summer period when the river bed is stable and the hillslopes are no longer protected by a snow cover during storms. This sheds light on the seasonal control exerted by the river bed mobility and the snow cover on suspended sediment dynamics in mountainous catchments. It also questions the potential shift of this dynamics from river bed to hillslope dominated in a context of global warming.

Alpine environments are responding to accelerated climate warming through the release and mobilization of large amounts of unconsolidated sediment. Sediment fluxes delivered to Alpine streams may be buffered, filtered and/or modulated as they pass through braided river reaches, which play a key role in the downstream transfer and dynamics of bed material. The functioning of these braided reaches is however still poorly understood, particularly during high magnitude events whose effects are very difficult to monitor but play an ever more prominent role in river system evolution. In this study, we investigate the transfer of bedload material and river bed morphological change in a braided reach of the Séveraisse River (France), over the course of the melt season and two large flood events with an estimated return period of 5 and 50 years. To quantify braided reach dynamics, a multi-physical approach is employed that combines both temporally and spatially resolved techniques. We use bank-side geophones and locally derived parameters that describe seismic wave propagation in the subsurface to accurately quantify bedload transport and gain a unique insight in its temporal dynamics, particularly during the flood events. River bed elevation changes are determined from intermittent UAV-based LiDAR and photogrammetric acquisition. These are complemented with hourly (daytime) time-lapse images that register planform changes during the flood events. Our results show strongly contrasting morphodynamic behavior with different flow conditions. During ‘normal’ bedload transport conditions driven by annual snow-melt, channel aggradation occurs leading to progressively lower bedload export from the reach for a given discharge. During the flood with a 5 year return period, which occurred at the end of the melt season, the braided riverbed morphology is rearranged and net sediment export took place. Most interestingly, in the autumn an extreme flood event led to the development of a single channel, meandering planform with significant outer bend erosion on alternating banks. Although this morphological change may be only temporary, i.e. a braided configuration may be expected to be gradually re-instated, it has important implications on the general functioning and morphological evolution of the reach and the downstream transfer of sediment.

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) 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 (between 0.01 and 1 kg/m/s), demonstrating that seismic measurements can provide an indirect measure for bedload flux with an uncertainty of less than one order of magnitude. Larger deviations may be due to the contribution of turbulent flow effects and remaining uncertainties in particle impact mechanics (elasticity and velocity). When constraining these uncertainties, seismic monitoring may provide an accurate and continuous means to investigate bedload dynamics in gravel‐bed streams.

Braided gravel‐bed rivers show characteristic temporal and spatial variability in morphological change and bedload transport under steady flow and sediment supply rates. Their morphodynamic behavior and long‐term evolution in response to nonstationary external forcing is less well known. We studied daily morphological changes in a well‐constrained reach of an Alpine braided river that is subject to regulated sediment‐laden flows, associated with hydroelectric power exploitation, as well as occasional floods. We found that net reach erosion and deposition were forced by upstream sediment supply, albeit in a nonlinear fashion. The spatial distribution of morphological change and inferred spatially‐distributed sediment transport rates varied strongly along the braided reach and between successive sequences of flushing. Local morphological change was driven by two factors: (1) local relief, leading to the preferential filling of topographic lows and erosion of highs, particularly during longer duration floods, which allow braided dynamics to be maintained; and (2) system memory, leading to a negative autocorrelation in bed level changes where erosion was followed by deposition of similar magnitude and vice versa. This effect was associated with the temporary storage of high sediment loads from flushing due to the abrupt on‐off nature of these flows and reveals the relatively efficient transport of sediment in a river that is heavily impacted upon by flow abstraction. In general, the internal morphodynamics of the braided river condition their own response to external forcing events and thus sediment transfer.

Research in the 1990s showed that bed-material transport rates could be estimated at the reach scale in both one-dimension and, over small spatial scales (10s of m), in two-dimensions. The limit on the latter was the spatial scale over which it was possible to obtain distributed data on morphological change. Here, we revisit the morphological method given progress in both topographical data acquisition and hydraulic modelling. The bed-material transport needed to conserve mass is calculated in both one and two dimensions for a 1600 m x 300 m Alpine braided river "laboratory". High-resolution topographical data were acquired by laser scanning to quantify Digital Elevation Models (DEMs), and morphological changes caused by the flushing of the water intake were derived from repeated surveys. Based on DEMs of differences, 1D bed-material transport rates were calculated using the morphological method. Then, a 2D hydraulic model was combined with a topographic correction to route sediment through the network of braided channels and to obtain a spatially variable estimate of transport in both downstream and cross-stream directions. Monte Carlo simulation was applied to the routing model parameters, allowing identification of the most probable parameter values needed to minimize negative transport. The results show that within-section spatial compensation of erosion and deposition using the 1D treatment leads to substantial local errors in transport rate estimates, to a degree related to braiding intensity. Even though the 2D application showed that a large proportion of the total transport was actually concentrated into one main channel during the studied low flow event, the proportion of transport in secondary anabranches is substantial when the river starts braiding. Investigations of the effects of DEM resolution, competent flow duration and survey frequency related to 'travelling bedload' and sequential erosion-deposition emphasized the critical importance of careful data collection in the application of the morphological method. (c) 2019 John Wiley & Sons, Ltd.

Understanding the interactions between bedload transport and morpholdynamics in braided streams has important applications in river management and restoration. Direct field measurements addressing this question are however scarce as they are often challenging to perform. Here, we report an extensive two-month field campaign in an Alpine braided reach (La Severaisse river, French Alps) that experienced predictable daily peak discharge (48 events observed) generating significant bedload transport and morphological changes during the melting season. We monitored these processes using a wide range of direct and indirect techniques (bedload sampling, continuous seismic measurements, pebbles tracking, topographic surveys, remote sensing using ground control cameras and drone flights). Doing so, surrogate measurements allowed to extend temporally discrete manual bedload sampling, and to extend spatially local riverbed cross section measurements. These measurements provide unique complementary constraints on the targeted physics, at various spatial and temporal scales which enabled us to draw robust conclusions. Data showed a progressive decrease in bedload transport for a given flow rate along the two months period. Simultaneously, river morphology in the braided sections changed from an incised to a more distributed configuration which led to a decrease of local maxima in dimensionless shear stresses in the braided reach for similar flow conditions. This control of bedload transport by maximum local shear stresses was in line with tracked pebble surveys indicating that coarse bedload particles were mostly transported in the main active channel. At the reach scale, this transport was found to be more efficient in laterally confined sections than in braided ones which has important implications in terms of bedload estimation in alternative confined and braided (unconfined) rivers. Finally, this study highlight the interest to combine a large variety of traditional and innovative measurements techniques to better understand complex sediment transport processes in the field. (C) 2019 Elsevier B.V. All rights reserved.

Sediment export from glaciated basins involves complex interactions between ice flow, basal erosion and sediment transfer in subglacial and proglacial streams. In particular, we know very little about the processes associated with sediment transfer by subglacial streams. The Haut Glacier d'Arolla (VS, Switzerland) was investigated during the summer melt season of 2015. LiDAR survey revealed positive surface changes in the ablation zone, indicating glacier uplift, at the end of the morning during the period of peak ablation. Instream measures of sediment transport showed that suspended load and bedload responded differently to diurnal flow variability. Suspended load depended on the availability of fine material whereas bedload depended mainly on the competence of the flow. Interpretation of these results allowed development of a conceptual model of subglacial sediment transport dynamics. It is based upon the mechanisms of clogging (deposition) and flushing (transport/erosion) in sub-glacial channels as forced by diurnal flow variability. Through the melt season, the glacier hydrological response evolves from being buffered by glacier snow cover with a poorly developed subglacial drainage system to being dominated by more rapid ice melt with a more hydraulically efficient subglacial channel system. The resultant changes in the shape of diurnal discharge hydrographs, and notably higher peak flows and lower base flows, causes sediment transport to become discontinuous, with overnight clogging and late morning flushing of subglacial channels. Overnight clogging may be sufficient to reduce subglacial channel size, creating temporarily pressurized flow and lateral transfer of water away from the subglacial channels, leading to the late morning glacier surface uplift. However, without further data, we cannot exclude other hypotheses for the uplift. (c) 2018 John Wiley & Sons, Ltd.