The Nature of Saltation and of Bed-Load Transport in Water
01 Jan 1988-pp 292-322
TL;DR: In this article, it has been shown that suspension by fluid turbulence of mineral solids larger than those of medium sands does not become appreciable until the bed shear stress is increased to a value exceeding 12 times its threshold value for the bed material considered.
Abstract: Owing to observational difficulties the distinction between a ‘suspended’ load of solids transported by a stream and a ‘ bed-load ’ has long remained undefined. Recently, however, certain critical experiments have thrown much light on the nature of bed-load transport. In particular, it has been shown that bed-load transport, by saltation, occurs in the absence of fluid turbulence and must therefore be due to a separate dynamic process from that of transport in suspension by the internal eddy motion of a turbulent fluid. It has been further shown that the forward motion of saltating solids is opposed by a frictional force of the same order as the immersed weight of the solids, the friction coefficient approximating to that given by the angle of slip. The maintenance of steady motion therefore requires a predictable rate of energy dissipation by the transporting fluid. The fluid thrust necessary to maintain the motion is shown to be exerted by virtue of a mean slip velocity which is predictable in the same way as, and approxim ates to, the terminal fall velocity of the solid. The mean thrust, and therefore the transport rate of saltating solids, are therefore predictable in terms of the fluid velocity close to the bed, at a distance from it, within the saltation zone, of a ‘centre of fluid thrust’ analogous to the ‘centre of pressure’. This velocity, which is not directly measurable in water streams, can be got from a knowledge of stream depth and mean flow velocity. Thus a basic energy equation is obtained relating the rate of transporting work done to available fluid transporting power. This is shown to be applicable to the transport both of wind-blown sand, and of water-driven solids of all sizes and larger than that of medium sand. Though the mean flow velocity is itself unpredictable, the total stream power, which is the product of this quantity times the bed shear stress, is readily measurable. But since the mean flow velocity is an increasing function of flow depth, the transport of solids expressed in terms of total stream power must decrease with increasing flow depth/grain size ratio. This considerable variation with flow depth has not been previously recognised. It explains the gross inconsistencies found in the existing experimental data. The theoretical variation is shown to approximate very closely to that found in recent critical experiments in which transport rates were measured at different constant flow depths. The theory, which is largely confirmed by these and other earlier experiments, indicates that suspension by fluid turbulence of mineral solids larger than those of medium sands does not become appreciable until the bed shear stress is increased to a value exceeding 12 times its threshold value for the bed material considered. This range of unsuspended transport decreases rapidly, however, as the grain size is reduced till, at a certain critical size, suspension should occur at the threshold of bed movement.
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TL;DR: In this paper, an analytical model of aeolian sand transport is presented, which quantifies the momentum transfer from the wind to the transported sand by providing expressions for the thickness of the saltation layer and the apparent surface roughness.
Abstract: We present an analytical model of aeolian sand transport. The model quantifies the momentum transfer from the wind to the transported sand by providing expressions for the thickness of the saltation layer and the apparent surface roughness. These expressions are derived from basic physical principles and a small number of assumptions. The model further predicts the sand transport rate (mass flux) and the impact threshold (the smallest value of the wind shear velocity at which saltation can be sustained). We show that, in contrast to previous studies, the present model's predictions are in very good agreement with a range of experiments, as well as with numerical simulations of aeolian saltation. Because of its physical basis, we anticipate that our model will find application in studies of aeolian sand transport on both Earth and Mars.
54 citations
01 Jan 2013
TL;DR: In this paper, the authors review some of the historical developments in the study of wind-blown sand, focusing on the major topics of: initiation of motion; modes, models, and complexities of transport; and measuring sand transport.
Abstract: In this chapter we review some of the historical developments in the study of wind-blown sand, focusing on the major topics of: initiation of motion; modes, models, and complexities of transport; and measuring sand transport. We emphasize field-based investigations although some reference to laboratory work is essential. The focus is on the fine-scale, physics-based approach established by Bagnold. We consider spatial and temporal unsteadiness in transport, effects of surface slope and moisture content as factors complicating transport processes; a related discussion of atmospheric boundary-layers is found in Chapter 11.2 . This discussion considers the transport of sand-sized particles only.
23 citations
Dissertation•
01 Jan 2009
TL;DR: In this paper, the morphodynamique and morphologie des structures sedimentaires are investigated in the presence of a debit de sediments, a debit d'eau and de la taille du sediment.
Abstract: Cette these a pour objet l'etude experimentale de la morphodynamique des structures sedimentaires dans des conditions d'apport limite en sediments. Cette etude s'inscrit dans un contexte ou il s'agit de prendre en compte la complexite naturelle des rivieres a l'echelle d'un dispositif experimental afin de mieux apprehender les processus d'erosion et de deposition de sediments mis en jeu a l'echelle d'un troncon de riviere. Pour cela, nous avons mis au point un chenal experimental permettant de controler le debit de sediments de facon independante du debit d'eau. Nous avons, grâce a ce dispositif, caracterise la dynamique et la morphologie des depots sedimentaires (hauteur, espacement, rapport d'aspect, ...) en fonction du debit de sediments, du debit d'eau et de la taille du sediment. Apres un regime transitoire, les depots finissent par adopter un etat d'equilibre dont la morphologie et la taille dependent crucialement du debit de sediments, en plus du debit d'eau. Nous avons mis en evidence deux regimes de transport : i) un premier regime controle par l'apport en sediments ou la hauteur des formes d'equilibre varie lineairement avec le debit de sediments, et ii) un second regime gouverne par l'ecoulement ou la hauteur des depots devient independante du debit de sediments. Par ailleurs, dans le premier regime, nous montrons que la hauteur d'equilibre des depots obeit a une loi d'echelle simple faisant intervenir le debit de sediments, le cube de la vitesse de frottement et la taille des sediments. Enfin, nous avons aborde le cas d'un apport en sediments bidisperses qui represente une premiere etape dans la prise en compte du caractere polydisperse des rivieres naturelles. Nous montrons, en autre, que la dynamique des depots bidisperses ne peut s'expliquer par une simple superposition des resultats obtenus en monodisperse, indiquant ainsi que les mecanismes de segregation jouent un role preponderant.
9 citations
Cites background from "The Nature of Saltation and of Bed-..."
...Dans le transport par charriage, Bagnold (1973) [9] distingue les trois types de mouvement suivants : la saltation, le roulement et le glissement....
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01 Dec 2018
TL;DR: In this article, the authors derived the bulk transport cessation threshold by extrapolating the transport load as a function of the dimensionless fluid shear stress (Shields number') to the vanishing transport limit.
Abstract: Using particle-scale simulations of non-suspended sediment transport for a large range of Newtonian fluids driving transport, including air and water, we determine the bulk transport cessation threshold $\Theta^r_t$ by extrapolating the transport load as a function of the dimensionless fluid shear stress (`Shields number') $\Theta$ to the vanishing transport limit. In this limit, the simulated steady states of continuous transport can be described by simple analytical model equations relating the average transport layer properties to the law of the wall flow velocity profile. We use this model to calculate $\Theta^r_t$ for arbitrary environments and derive a general Shields-like threshold diagram in which a Stokes-like number replaces the particle Reynolds number. Despite the simplicity of our hydrodynamic description, the predicted cessation threshold, both from the simulations and analytical model, quantitatively agrees with measurements for transport in air and viscous and turbulent liquids despite not being fitted to these measurements. We interpret the analytical model as a description of a continuous rebound motion of transported particles and thus $\Theta^r_t$ as the minimal fluid shear stress needed to compensate the average energy loss of transported particles during an average rebound at the bed surface. This interpretation, supported by simulations near $\Theta^r_t$, implies that entrainment mechanisms are needed to sustain transport above $\Theta^r_t$. While entrainment by turbulent events sustains intermittent transport, entrainment by particle-bed impacts sustains continuous transport. Combining our interpretations with the critical energy criterion for incipient motion by Valyrakis and coworkers, we put forward a new conceptual picture of sediment transport intermittency.
8 citations
Dissertation•
01 Jan 2008
TL;DR: In this paper, a modele conceptuel de transport par suspension dans les ejections is proposed, in fonction du flux de quantite de mouvement autour des particules, de leur position and temps de residence dans the ejections.
Abstract: Le role joue par la turbulence de paroi dans le transport de particules est etudie a l’aide de deux types d’experiences. Les premieres ont porte sur un ecoulement a surface libre charge en billes de verre et de ceramique transportees par suspension, pour 4 conditions hydrauliques donnees. Les secondes experiences ont ete realisees en turbulence controlee pour des conditions de transport tres proches du seuil de suspension. Afin de mesurer simultanement le champ de vitesse de l���ecoulement ainsi que la position et la vitesse des particules, nous avons utilise la PIV (Velocimetrie par Image de Particules). Nous pouvons ainsi analyser l’ecoulement autour de chaque particule et mettre en evidence l’action locale du fluide sur les particules. L’analyse de l’ecoulement se fait dans le voisinage de chaque particule. L’analyse par quadrants montre que les ejections sont fortement impliquees dans le transport, notamment dans la montee des particules. Elle indique aussi que certaines particules qui descendent sont dans des ejections. Nous cherchons donc a determiner pourquoi une ejection est capable ou non de faire monter les particules. On observe que plus le flux de quantite de mouvement (-rho u’v’) des ejections est important, plus les particules montent. Un seuil de suspension, adimensionalise par (g delta rho d) est determine. Il est de l’ordre de 0,5 et est quasiment constant en fonction du nombre de Reynolds, de la hauteur et des caracteristiques des particules. Un modele conceptuel de transport par suspension dans les ejections est propose, en fonction du flux de quantite de mouvement autour des particules, de leur position et temps de residence dans les ejections.
8 citations
References
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Book•
01 Jun 2008
TL;DR: The Intergovernmental Panel on Climate Change (IPCC) Technical Paper Climate Change and Water draws together and evaluates the information in IPCC Assessment and Special Reports concerning the impacts of climate change on hydrological processes and regimes, and on freshwater resources.
Abstract: The Intergovernmental Panel on Climate Change (IPCC) Technical Paper Climate Change and Water draws together and evaluates the information in IPCC Assessment and Special Reports concerning the impacts of climate change on hydrological processes and regimes, and on freshwater resources – their availability, quality, use and management. It takes into account current and projected regional key vulnerabilities, prospects for adaptation, and the relationships between climate change mitigation and water. Its objectives are:
3,108 citations
TL;DR: In this article, a large number of spherical grains of diameter D = 0.13 cm were sheared in Newtonian fluids of varying viscosity (water and a glycerine-water-alcohol mixture) in the annular space between two concentric drums.
Abstract: Dispersions of solid spherical grains of diameter D = 0.13cm were sheared in Newtonian fluids of varying viscosity (water and a glycerine-water-alcohol mixture) in the annular space between two concentric drums. The density σ of the grains was balanced against the density ρ of the fluid, giving a condition of no differential forces due to radial acceleration. The volume concentration C of the grains was varied between 62 and 13 %. A substantial radial dispersive pressure was found to be exerted between the grains. This was measured as an increase of static pressure in the inner stationary drum which had a deformable periphery. The torque on the inner drum was also measured. The dispersive pressure P was found to be proportional to a shear stress λ attributable to the presence of the grains. The linear grain concentration λ is defined as the ratio grain diameter/mean free dispersion distance and is related to C by λ = 1 ( C 0 / C ) 1 2 − 1 where C 0 is the maximum possible static volume concentration. Both the stresses T and P , as dimensionless groups T σ D 2 /λη 2 , and P σ D 2 /λη 2 , were found to bear single-valued empirical relations to a dimensionless shear strain group λ ½ σ D 2 (d U /d y )lη for all the values of λ C = 57% approx.) where d U /d y is the rate of shearing of the grains over one another, and η the fluid viscosity. This relation gives T α σ ( λ D ) 2 ( dU / dy ) 2 and T ∝ λ 1 2 η d U / dy according as d U /d y is large or small, i.e. according to whether grain inertia or fluid viscosity dominate. An alternative semi-empirical relation F = (1+λ)(1+½λ)ηd U /d y was found for the viscous case, when T is the whole shear stress. The ratio T/P was constant at 0·3 approx, in the inertia region, and at 0.75 approx, in the viscous region. The results are applied to a few hitherto unexplained natural phenomena.
2,445 citations
TL;DR: In this article, a method is presented which enables the computation of the bed-load transport as the product of the saltation height, the particle velocity and the bed load concentration.
Abstract: A method is presented which enables the computation of the bed-load transport as the product of the saltation height, the particle velocity and the bed-load concentration. The equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity. Experiments with gravel particles (transported as bed load) are selected to calibrate the mathematical model using the lift coefficient as a free parameter. The model is used to compute the saltation heights and lengths for a range of flow conditions. The computational results are used to determine simple relationships for the saltation characteristics. Measured transport rates of the bed load are used to compute the sediment concentration in the bed-load layer. A simple expression specifying the bed-load concentration as a function of the flow and sediment conditions is proposed. A verification analysis using about 600 (alternative) data shows that about 77% of the predicted bed-load-transport rates are within 0.5 and 2 times the observed values.
1,653 citations
TL;DR: The physics of aeolian saltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission, the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scale vortices are reviewed.
Abstract: The transport of sand and dust by wind is a potent erosional force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols This article presents an extensive review of the physics of wind-blown sand and dust on Earth and Mars Specifically, we review the physics of aeolian saltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission, the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scale vortices We also discuss the physics of wind-blown sand and dune formation on Venus and Titan
1,175 citations
TL;DR: In this paper, a review of the development of ideas in the fields of geomorphology/Quaternary geology vs. sedimentary geologies is provided, and key processes that operate to produce alluvial stratigraphic records over time-scales of 103−106 years.
Abstract: Summary
Fluvial landforms and deposits provide one of the most readily studied Quaternary continental records, and alluvial strata represent an important component in most ancient continental interior and continental margin successions. Moreover, studies of the long-term dynamics of fluvial systems and their responses to external or ‘allogenic' controls, can play important roles in research concerning both global change and sequence-stratigraphy, as well as in studies of the dynamic interactions between tectonic activity and surface processes. These themes were energized in the final decades of the twentieth century, and may become increasingly important in the first decades of this millennium.
This review paper provides a historical perspective on the development of ideas in the fields of geomorphology/Quaternary geology vs. sedimentary geology, and then summarizes key processes that operate to produce alluvial stratigraphic records over time-scales of 103−106 years. Of particular interest are changes in discharge regimes, sediment supply and sediment storage en route from source terrains to sedimentary basins, as well as changes in sea-level and the concept of accommodation. Late Quaternary stratigraphic records from the Loire (France), Mississippi (USA), Colorado (Texas, USA) and Rhine–Meuse (The Netherlands) Rivers are used to illustrate the influences of climate change on continental interior rivers, as well as the influence of interacting climate and sea-level change on continental margin systems.
The paper concludes with a look forward to a bright future for studies of fluvial response to climate and sea-level change. At present, empirical field-based research on fluvial response to climate and sea-level change lags behind: (a) the global change community's understanding of the magnitude and frequency of climate and sea-level change; (b) the sequence-stratigraphic community's desire to interpret climate and, especially, sea-level change as forcing mechanisms; and (c) the modelling community's ability to generate numerical and physical models of surface processes and their stratigraphic results. A major challenge for the future is to catch up, which will require the development of more detailed and sophisticated Quaternary stratigraphic, sedimentological and geochronological frameworks in a variety of continental interior and continental margin settings. There is a particular need for studies that seek to document fluvial responses to allogenic forcing over both shorter (102−103 years) and longer (104−106 years) time-scales than has commonly been the case to date, as well as in larger river systems, from source to sink. Studies of Quaternary systems in depositional basin settings are especially critical because they can provide realistic analogues for interpretation of the pre-Quaternary rock record.
1,125 citations