Statistics of bedload transport over steep slopes: Separation of time scales and collective motion

Heyman, Joris; Mettra, François; Ma, Hsi-Pin; Ancey, Christophe

doi:10.1029/2012gl054280

Cited by 47 publications

(87 citation statements)

References 18 publications

(45 reference statements)

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“…These data come from earlier experimental campaigns conducted on steep slopes, thus in the supercritical regime (Böhm et al 2004;Ancey et al 2008;Heyman et al 2013), but the theory applies to subcritical regimes as well. Only essential details of methodology, data processing and results are presented here.…”

Section: Applicationsmentioning

confidence: 95%

A microstructural approach to bed load transport: mean behaviour and fluctuations of particle transport rates

2014

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This paper concerns a model of bed load transport, which describes the advection and dispersion of coarse particles carried by a turbulent water stream. The challenge is to develop a microstructural approach that, on the one hand, yields a parsimonious description of particle transport at the microscopic scale and, on the other hand, leads to averaged equations at the macroscopic scale that can be consistently interpreted in light of the continuum equations used in hydraulics. The cornerstone of the theory is the proper determination of the particle flux fluctuations. Apart from turbulence-induced noise, fluctuations in the particle transport rate are generated by particle exchanges with the bed consisting of particle entrainment and deposition. At the particle scale, the evolution of the number of moving particles can be described probabilistically using a coupled set of reaction-diffusion master equations. Theoretically, this is interesting but impractical, as solving the governing equations is fraught with difficulty. Using the Poisson representation, we show that these multivariate master equations can be converted into Fokker-Planck equations without any simplifying approximations. Thus, in the continuum limit, we end up with a Langevin-like stochastic partial differential equation that governs the time and space variations of the probability density function for the number of moving particles. For steady-state flow conditions and a fixed control volume, the probability distributions of the number of moving particles and the particle flux can be calculated analytically. Taking the average of the microscopic governing equations leads to an average mass conservation equation, which takes the form of the classic Exner equation under certain conditions carefully addressed in the paper. Analysis also highlights the specific part played by a process we refer to as collective entrainment, i.e. a nonlinear feedback process in particle entrainment. In the absence of collective entrainment the fluctuations in the number of moving particles are Poissonian, which implies that at the macroscopic scale they act as white noise that mediates bed evolution. In contrast, when collective entrainment occurs, large non-Poissonian fluctuations arise, with the important consequence that the evolution at the macroscopic scale may depart significantly that predicted by the averaged Exner equation. Comparison with experimental data gives satisfactory results for steady-state flows.

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Section: Applicationsmentioning

confidence: 95%

A microstructural approach to bed load transport: mean behaviour and fluctuations of particle transport rates

2014

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“…Although the movement of grains in natural channels has advantages over simplified flume experiments, unless those grains have radio-tags or embedded accelerometers (see Section 2.2) it is very difficult to know the exact travel distances, rest times, travel velocities, and shear stresses at entrainment or disentrainment. Laboratory experiments using videos or impact plates have allowed for such details to be accurately measured for relatively simple bed conditions (e.g., Heyman et al, 2013). For example, experimental observations have shown that the number of moving particles increases faster with excess shear stress than particle velocities or step lengths and suggests that bedload fluxes are largely driven by particle activity (e.g., Lajeunesse et al, 2010;Roseberry et al, 2012;Furbish and Schmeeckle, 2013).…”

Section: Grain Scale Transport Mechanicsmentioning

confidence: 98%

Taking the river inside: Fundamental advances from laboratory experiments in measuring and understanding bedload transport processes

2015

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Bedload transport in gravel-bed rivers impacts channel stability, the lifespan of hydraulic structures, physical components of aquatic habitat, and long-term channel evolution. Field measurements of bedload transport are notoriously difficult, which precludes understanding many of the processes and mechanics associated with grain motion. Such uncertainties are exacerbated when using bedload transport equations, most of which were derived using data from a single river or set of laboratory flume experiments. Recently, laboratory experiments have focused on better quantifying the processes that impact bedload fluxes, which can then be used to improve sediment transport predictions. We highlight recent advances in laboratory instrumentation that can be used in bedload transport studies. In particular, more accurate ways to measure bedload fluxes, near-bed turbulence, bed grain sizes, and topography hold great promise. Laboratory experiments have also fundamentally improved our understanding of the influence of sediment supply and armoring processes on bedload fluxes and channel conditions. The importance of flow hydrographs in controlling total bedload transport rates and bedload hysteresis has also been demonstrated using flume experiments. Finally, many details about the mechanics of grain motion including flow turbulence, bed arrangement, and particle transport statistics are only possible through laboratory investigations, and we feature key knowledge gaps that can be improved with further study.

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“…While suspended sediment represents the largest fraction of mass exiting the landscape (Milliman and Syvitski, 1992;Willenbring et al, 2013), it is coarse bed load transport that sets the limiting rate of landscape incision through its control on bedrock erosion and channel geometry in gravel rivers (Sklar and Dietrich, 2004;Snyder et al, 2003;Parker et al, 2007). The rate of bed load transport is known to vary both spatially and temporally due to turbulence and granular phenomena such as clustering, bed forms, bed compaction, grain protrusion/hiding, and collective motion (Gomez, 1991;Kirchner et al, 1990;Schmeeckle et al, 2001;Strom et al, 2004;Ancey et al, 2008;Zimmermann et al, 2010;Marquis and Roy, 2012;Heyman et al, 2013), which makes predictions difficult (Recking et al, 2012) and point measurements highly variable (e.g., Gray et al, 2010). Bed load is especially difficult to predict near the threshold of motion (Recking et al, 2012), where transport is highly intermittent, often resulting in partial bed load transport, in which only a fraction of the bed is mobilized during a transporting event (Wilcock and McArdell, 1997).…”

Section: Introductionmentioning

confidence: 99%

Dynamics and mechanics of bed-load tracer particles

Phillips

Jerolmack

2014

Earth Surf. Dynam.

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Abstract. Understanding the mechanics of bed load at the flood scale is necessary to link hydrology to landscape evolution. Here we report on observations of the transport of coarse sediment tracer particles in a cobblebedded alluvial river and a step-pool bedrock tributary, at the individual flood and multi-annual timescales. Tracer particle data for each survey are composed of measured displacement lengths for individual particles, and the number of tagged particles mobilized. For single floods we find that measured tracer particle displacement lengths are exponentially distributed; the number of mobile particles increases linearly with peak flood Shields stress, indicating partial bed load transport for all observed floods; and modal displacement distances scale linearly with excess shear velocity. These findings provide quantitative field support for a recently proposed modeling framework based on momentum conservation at the grain scale. Tracer displacement is weakly negatively correlated with particle size at the individual flood scale; however cumulative travel distance begins to show a stronger inverse relation to grain size when measured over many transport events. The observed spatial sorting of tracers approaches that of the river bed, and is consistent with size-selective deposition models and laboratory experiments. Tracer displacement data for the bedrock and alluvial channels collapse onto a single curve -despite more than an order of magnitude difference in channel slope -when variations of critical Shields stress and flow resistance between the two are accounted for. Results show how bed load dynamics may be predicted from a record of river stage, providing a direct link between climate and sediment transport.

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Statistics of bedload transport over steep slopes: Separation of time scales and collective motion

Cited by 47 publications

References 18 publications

A microstructural approach to bed load transport: mean behaviour and fluctuations of particle transport rates

A microstructural approach to bed load transport: mean behaviour and fluctuations of particle transport rates

Taking the river inside: Fundamental advances from laboratory experiments in measuring and understanding bedload transport processes

Dynamics and mechanics of bed-load tracer particles

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