scholarly journals Discrete and continuum modelling of grain size segregation during bedload transport

2020 ◽  
Vol 895 ◽  
Author(s):  
Rémi Chassagne ◽  
Raphaël Maurin ◽  
Julien Chauchat ◽  
J. M. N. T. Gray ◽  
Philippe Frey
Keyword(s):  
Grain Size ◽  
Bedload Transport ◽  
Size Segregation ◽  
Continuum Modelling ◽  
Image Position

Continuous modeling of grain size segregation in bedload transport

2020 ◽  
Author(s):  
Hugo Rousseau ◽  
Rémi Chassagne ◽  
Julien Chauchat ◽  
Philippe Frey
Keyword(s):  
Grain Size ◽  
Mixture Theory ◽  
Bedload Transport ◽  
Force Balance ◽  
Size Segregation ◽  
Small Particles ◽  
Class Model ◽  
Advection Diffusion Equation ◽  
Advection Diffusion ◽  
And Diffusion

<p>Rivers carry sediments having a wide grain size distribution, ranging from a few hundreds microns to meters. This leads to grain size segregation mechanism that can have huge consequences on morphological evolution.  Accurate comprehension and modeling of this mechanism with continuous equations is a key step to upscale segregation in sediment transport models.<br><em>Thornton et al. (2006) </em>developed continuous equations for bidisperse segregation in the context of the mixture theory. Based on the momentum balance of small particles, a simple advection-diffusion equation for the volumetric concentration of small particles was derived. This equation enables to explicit the advection term, that tends to segregate the different particle sizes and the diffusive term, that tends to remix the particles. However, this approach does not immediately provide the physical characteristics of the granular flow in the advection and diffusion terms.</p><p>Recently, <em>Guillard et al. (2016)</em> showed, using a Discrete Element Method (DEM), that the segregation force on a large intruder in a bath of small particles, can be seen as a buoyancy force proportional to the pressure. In addition, <em>Tripathi and Khakhar (2011)</em> showed that a large particle rising in a pool of small grains experiences a Stokesian drag force proportional to the granular viscosity.<br>These new results enable to infer a force balance for a single coarse particle in bedload transport. Solving this force balance showed that the large particle rises with the accurate dynamics, meaning that this force balance is relevant to model grain-size segregation.</p><p>Based on these new forces, a continuous multi-class model has been developed to generalize to the segregation of a collection of large particles. The concentration and the segregation velocity of the small particles have been compared with coupled-fluid DEM bedload transport simulations from <em>Chassagne et al. (2020)</em> and show that the accurate dynamics of segregation can be modeled using this continuous model.<br>Based on this continuum multi-class model, a similar advection-diffusion equation as <em>Thornton et al. (2006) </em> has been obtained. The latter appears to provide the physical origin of the advection and diffusion terms by linking them to the parameters of the flow.</p><p> </p><p>Chassagne R., Maurin R., Chauchat J., and Frey P. Discrete and continuum modeling of grain-size segregation during bedload transport. J. Fluid Mech. 2020 (in revision).</p><p>Gray J. M. N. T., and  Chugunov V. A. Particle-size segregation and diffusive remixing in shallow granular avalanches. J. Fluid Mech. 569: 365-398, 2006.</p><p>Guillard F. Forterre Y., and Pouliquen O. Scaling laws for segregation forces in dense sheared granular flows. J. Fluid Mech. 807, R1, 2016.</p><p>Thornton A. R., Gray J. M. N. T., and Hogg A. J. A three-phase mixture theory for particle size segregation in shallow granular free-surface flows. J. Fluid Mech. 550: 125, 2006.</p><p>Tripathi A., and Khakhar D. V. Numerical simulation of the sedimentation of a sphere in a sheared granular fluid: a granular stokes experiment. Phys. Rev. Lett. 107, 108,001, 2011.</p>

scholarly journals Experiments on grain size segregation in bedload transport on a steep slope

2020 ◽  
Vol 136 ◽  
pp. 103478
Author(s):  
P. Frey ◽  
H. Lafaye de Micheaux ◽  
C. Bel ◽  
R. Maurin ◽  
K. Rorsman ◽  
...  
Keyword(s):  
Grain Size ◽  
Bedload Transport ◽  
Steep Slope ◽  
Size Segregation

scholarly journals Bridging the gap between particle-scale forces and continuum modelling of size segregation: application to bedload transport

2021 ◽  
Vol 916 ◽  
Author(s):  
Hugo Rousseau ◽  
Remi Chassagne ◽  
Julien Chauchat ◽  
Raphael Maurin ◽  
Philippe Frey
Keyword(s):  
Bedload Transport ◽  
Size Segregation ◽  
Continuum Modelling

Abstract

Controlled experiments and finite element simulations with the Swiss plate geophone bedload monitoring system: particle size identification and transport mode

2021 ◽  
Author(s):  
Zheng Chen ◽  
Siming He ◽  
Tobias Nicollier ◽  
Lorenz Ammann ◽  
Alexandre Badoux ◽  
...  
Keyword(s):  
Particle Size ◽  
Grain Size ◽  
Finite Element ◽  
Quantitative Agreement ◽  
Bedload Transport ◽  
Material Structure ◽  
Flume Experiments ◽  
Fem Simulations ◽  
Transport Mode ◽  
Signal Response

<p>The Swiss plate geophone (SPG) system is an indirect bedload transport monitoring device that records the acoustic signals generated by bedload particle impacts, with the goal to derive the bedload flux and grain size distribution. Particle drop experiments with quartz spheres in quiescent water in a flume setting were performed to investigate the dynamic signal response of the SPG system impacted by particle-like objects varying in size and impact location. Systematic flume experiments with natural bedload particles in flowing water were conducted to study the effects of impact angle and transport mode (saltating, rolling and sliding) on the SPG signals. For each impact caused by a single particle, the number of signal impulses, the amplitude, the positive area surrounded by the signal envelope, and the centroid frequency were extracted from the raw geophone monitoring data. The finite element method (FEM) was used to construct a virtual model of the SPG system and to determine the propagation characteristics of the numerical stress wave in the material structure. The experimental and numerical results showed a qualitative and partially quantitative agreement in the changes of the signal impulses, the amplitude, and the envelope area with increasing colliding sphere size. The centroid frequencies of the SPG vibrations showed qualitatively similar dependencies with increasing particle size as some field measurements for the coarser part of the investigated range of impact sizes. The effects of variable particle impact velocities and impact locations on the geophone plate were also investigated by drop experiments and compared to FEM simulations. In addition, the signal response for different bedload transport modes and varying impact angles were explored. In summary, the FEM simulations contribute to the understanding of the signal response of the SPG system and the findings in this study may eventually result in improving the bedload grain size classification and transport mode recognition.</p>

Development of Model for Acoustic Noise Generated from Bedload in Rivers 

2021 ◽  
Author(s):  
Mohamad Nasr ◽  
Thomas Geay ◽  
Sébastien Zanker ◽  
Recking Alain
Keyword(s):  
Grain Size ◽  
Size Distribution ◽  
Impact Velocity ◽  
Acoustic Noise ◽  
Acoustic Power ◽  
Bedload Transport ◽  
Empirical Formulas ◽  
Impact Rate ◽  
Flux Measurements ◽  
The Impact

<p>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, we</p><p>The 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.</p><p>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.</p>

scholarly journals Geometry of meandering and braided gravel-bed threads from the Bayanbulak Grassland, Tianshan, P. R. China

2016 ◽  
Vol 4 (1) ◽  
pp. 273-283 ◽  
Author(s):  
François Métivier ◽  
Olivier Devauchelle ◽  
Hugo Chauvet ◽  
Eric Lajeunesse ◽  
Patrick Meunier ◽  
...  
Keyword(s):  
Grain Size ◽  
Scaling Laws ◽  
Sedimentary Basin ◽  
Bedload Transport ◽  
Braided River ◽  
Statistical Distributions ◽  
Meandering Rivers ◽  
Gravel Bed ◽  
Threshold Theory ◽  
Gravel Bed Rivers

Abstract. The Bayanbulak Grassland, Tianshan, P. R. China, is located in an intramontane sedimentary basin where meandering and braided gravel-bed rivers coexist under the same climatic and geological settings. We report and compare measurements of the discharge, width, depth, slope and grain size of individual threads from these braided and meandering rivers. Both types of threads share statistically indistinguishable regime relations. Their depths and slopes compare well with the threshold theory, but they are wider than predicted by this theory. These findings are reminiscent of previous observations from similar gravel-bed rivers. Using the scaling laws of the threshold theory, we detrend our data with respect to discharge to produce a homogeneous statistical ensemble of width, depth and slope measurements. The statistical distributions of these dimensionless quantities are similar for braided and meandering threads. This suggests that a braided river is a collection of intertwined threads, which individually resemble those of meandering rivers. Given the environmental conditions in Bayanbulak, we furthermore hypothesize that bedload transport causes the threads to be wider than predicted by the threshold theory.

Total and fractional bedload transport in alpine streams approximated by different surrogate measurement systems

2021 ◽  
Author(s):  
Lorenz Ammann ◽  
Tobias Nicollier ◽  
Alexandre Badoux ◽  
Dieter Rickenmann
Keyword(s):  
Grain Size ◽  
Measurement Techniques ◽  
Bedload Transport ◽  
Accelerometer Sensor ◽  
Protection Measures ◽  
Measurement Systems ◽  
Size Classes ◽  
Alpine Streams ◽  
Surrogate Measurement ◽  
Using Data

<p>Knowledge about bedload transport in rivers is of high importance for many hydraulic engineering applications, in particular related to flood protection measures. Passive acoustic surrogate measurement techniques provide useful continuous estimates of bedload transport in terms of total mass, as well as for different grain-size classes.</p><p>We compare different surrogate measurement systems regarding their performance in quantifying total and fractional bedload transport in three alpine streams. The investigated measurement systems are the well-established Swiss plate geophone (SPG), an equivalent system in which the geophone sensor is replaced by an accelerometer sensor, and the miniplate accelerometer (MPA) system. The latter is a more recent device and consists of four small square metal plates embedded in elastomere elements. While the signal recorded with the SPG is known to be proportional to the transported bedload mass, we find that the MPA-signal shows a non-linear dependency. In addition, the MPA reacts more sensitively to small grain size classes than the other two systems, indicating a possible alternative to improve the quantification of bedload transport consisting of those classes.</p><p>Based on the raw signal recorded with the SPG and the MPA in a flume experiment, we test the ability of different empirical models to predict the known weight of the impacting particle. We show that it is possible to identify the particle weight with high accuracy with relatively simple models using data of either of the two measurement systems. One remaining challenge is to account for the site-to-site variability in the (amount of) signal caused by the combination of differing numbers of plates in the measurement setup and the lateral transmission of the signal across multiple plates, especially for the SPG system.</p>

Effects of episodic sediment supply on experimental step-pool channel morphology, bedload transport and channel stability

2021 ◽  
Author(s):  
Jiamei Wang ◽  
Marwan A. Hassan ◽  
Matteo Saletti ◽  
Xingyu Chen ◽  
Xudong Fu ◽  
...  
Keyword(s):  
Grain Size ◽  
Morphological Changes ◽  
Mass Movement ◽  
Channel Morphology ◽  
Bedload Transport ◽  
Sediment Supply ◽  
Primary Control ◽  
Channel Stability ◽  
Flume Experiments ◽  
Small Magnitude

<p>Steep step-pool streams are often coupled to adjacent hillslope, directly receiving episodic sediment supply from mass movement processes such as landslides and debris flows. The response of step-pool channels to the variations in sediment supply remains largely unexplored. We conducted flume experiments with a poorly sorted grain-size distribution in an 8%-steep, 5-m long flume with variable width at the University of British Columbia, to study the effects of episodic sediment supply on channel evolution. After a conditioning phase with no feed, the channel was subjected to sediment pulses of different magnitude and frequency under constant flow discharge. High-resolution data of hydraulics, bedload transport, bed surface grain size, and channel morphology were collected every 10-20 minutes and an additional time at the end of each pulse.</p><p>In response to sediment pulses, we recorded an increase in bedload transport rates, channel aggradation, bed surface fining, and continuous step formation and collapse. In between pulses, bedload rates dropped by several orders of magnitude, net erosion occurred, the bed surface gradually coarsened, and steps became more stable. The small-magnitude high-frequency pulses caused smaller but more frequent spikes in bedload transport, bed surface evolution, and thus step stability. Instead, the large-magnitude low-frequency pulses cause larger changes but provided a longer time for the channel to recover. This suggests that in step-pool channels pulse magnitude is a key control on channel rearrangement, while pulse frequency controls how fast and strong the recovery is.</p><p>The frequency and stability of steps varied as a function of local channel width, showing that channel geometry is a primary control on step formation and stability even under episodic sediment supply conditions. Instead, the effect of sediment pulses is less important because the total number and average survival time of steps were similar among runs with different pulses. The critical Shields stress decreased following sediment pulses, then increased immediately after, and fluctuated until the next pulse. The variations in sediment supply caused cycles in bedload transport rate, surface and bedload texture, thus controlling the variability in the threshold for motion.</p><p>Our results indicate that episodic sediment supply is a primary control on the evolution of step-pool channels, with sediment feed magnitude affecting mostly morphological changes, and sediment feed frequency controlling channel stability.</p>

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