Gravel saltation: 1. Experiments

Niño, Yarko; García, Marcelo H.; Ayala, Luis F.

doi:10.1029/94wr00533

Cited by 176 publications

(202 citation statements)

References 17 publications

(14 reference statements)

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“…(52), whereas the resulting equation is then solved for 1 − dU dV (V s ) −1 -this is the term involving dU/dV which we need to compute L s in Eq. (44). In this manner, we finally obtain a closed expression for the saturation length, which we present and discuss in the next subsection.…”

Section: A Derivationmentioning

confidence: 99%

Analytical model for flux saturation in sediment transport

et al. 2014

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The transport of sediment by a fluid along the surface is responsible for dune formation, dust entrainment and for a rich diversity of patterns on the bottom of oceans, rivers, and planetary surfaces. Most previous models of sediment transport have focused on the equilibrium (or saturated) particle flux. However, the morphodynamics of sediment landscapes emerging due to surface transport of sediment is controlled by situations out-of-equilibrium. In particular, it is controlled by the saturation length characterizing the distance it takes for the particle flux to reach a new equilibrium after a change in flow conditions. The saturation of mass density of particles entrained into transport and the relaxation of particle and fluid velocities constitute the main relevant relaxation mechanisms leading to saturation of the sediment flux. Here we present a theoretical model for sediment transport which, for the first time, accounts for both these relaxation mechanisms and for the different types of sediment entrainment prevailing under different environmental conditions. Our analytical treatment allows us to derive a closed expression for the saturation length of sediment flux, which is general and can thus be applied under different physical conditions.

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Section: A Derivationmentioning

confidence: 99%

Analytical model for flux saturation in sediment transport

et al. 2014

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“…In a more detailed analysis, particle momentum balance, and in particular relaxation effects involving, e.g., particle inertia (Parker, 1975;Charru, 2006), should be included. In so far as a step length generally consists of many individual particle saltations (Nino et al, 1994), however, the present kinematic formulation may be sufficient for a firstorder analysis. One other way to formulate the problem is in terms of mass and momentum conservation of two sediment phases; i.e., a static bed phase and a moving bedload phase above it, with exchange between the two (e.g., Charru, 2006).…”

Section: Theoretical Frameworkmentioning

confidence: 99%

Morphodynamics of river bed variation with variable bedload step length

Pelosi

Parker

2014

Earth Surf. Dynam.

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Abstract. Here we consider the 1-D morphodynamics of an erodible bed subject to bedload transport. Fluvial bed elevation variation is typically modeled by the Exner equation, which, in its classical form, expresses mass conservation in terms of the divergence of the bedload sediment flux. An entrainment form of the Exner equation can be written as an alternative description of the same bedload processes, by introducing the notions of an entrainment rate into bedload and of a particle step length, and assuming a certain probability distribution for the step length. This entrainment form implies some degree of nonlocality, which is absent from the standard flux form, so that these two expressions, which are different ways to look at same conservation principle (i.e., sediment continuity), may no longer become equivalent in cases when channel complexity and flow conditions allow for long particle saltation steps (including, but not limited to the case where particle step length has a heavy tailed distribution) or when the domain of interest is not long compared to the step length (e.g., laboratory scales, or saltation over relatively smooth surfaces). We perform a systematic analysis of the effects of the nonlocality in the entrainment form of the Exner equation on transient aggradational/degradational bed profiles by using the flux form as a benchmark. As expected, the two forms converge to the same results as the step length converges to zero, in which case nonlocality is negligible. As step length increases relative to domain length, the mode of aggradation changes from an upward-concave form to a rotational, and then eventually a downward-concave form. Corresponding behavior is found for the case of degradation. These results may explain anomalously flat, aggradational, long profiles that have been observed in some short laboratory flume experiments.

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“…However, there are several indications that Bagnold's formulation may be flawed or crude. First, for this scaling to match experimental observations, the bulk particle friction coefficient introduced by Bagnold must be fitted to non-physical values (Fernandez Luque & van Beek 1976;Niño, Garcìa & Ayala 1994). Secondly, for conditions that depart from steady uniform flow, Bagnold's model yields poor results, notably for flows over arbitrarily sloping beds (Seminara et al 2002) or at low levels of solid discharge (Fernandez Luque & van Beek 1976;Nelson et al 1995).…”

Section: Introductionmentioning

confidence: 99%

Entrainment and motion of coarse particles in a shallow water stream down a steep slope

Ancey

Davison

Böhm³

et al. 2008

J. Fluid Mech.

177

365

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We investigate the entrainment, deposition and motion of coarse spherical particles within a turbulent shallow water stream down a steep slope. This is an idealization of bed-load transport in mountain streams. Earlier investigations have described this kind of sediment transport using empirical correlations or concepts borrowed from continuum mechanics. The intermittent character of particle transport at low-water discharges led us to consider it as a random process. Sediment transport in this regime results from the imbalance between entrainment and deposition of particles rather than from momentum balance between water and particles. We develop a birth-death immigration-emigration Markov process to describe the particle exchanges between the bed and the water stream. A key feature of the model is its long autocorrelation times and wide, frequent fluctuations in the solid discharge, a phenomenon never previously explained despite its ubiquity in both nature and laboratory experiments. We present experimental data obtained using a nearly two-dimensional channel and glass beads as a substitute for sediment. Entrainment, trajectories, and deposition were monitored using a high-speed digital camera. The empirical probability distributions of the solid discharge and deposition frequency were properly described by the theoretical model. Experiments confirmed the existence of wide and frequent fluctuations of the solid discharge, and revealed the existence of long autocorrelation time, but theory overestimates the autocorrelation times by a factor of around three. Particle velocity was weakly dependent on the fluid velocity contrary to the predictions of the theoretical model, which performs well when a single particle is moving. For our experiments, the dependence of the solid discharge on the fluid velocity is entirely controlled by the number of moving particles rather than by their velocity. We also noted significant changes in the behaviour of particle transport when the bed slope or the water discharge was increased. The more vigorous the stream was, the more continuous the solid discharge became. Moreover, although 90 % of the energy supplied by gravity to the stream is dissipated by turbulence for slopes lower than 10 %, particles dissipate more and more energy when the bed slope is increased, but surprisingly, the dissipation rate is nearly independent of fluid velocity. A movie is available with the online version of the paper.

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Gravel saltation: 1. Experiments

Cited by 176 publications

References 17 publications

Analytical model for flux saturation in sediment transport

Analytical model for flux saturation in sediment transport

Morphodynamics of river bed variation with variable bedload step length

Entrainment and motion of coarse particles in a shallow water stream down a steep slope

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