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Orencio Duran

MARUM, University of Bremen, MARUM, Post-Doc

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University of Bristol

Utrecht University

Uppsala University

Armando Marques-Guedes

UNL - New University of Lisbon

Gloria I. López

University of Haifa

Gabriel Gutierrez-Alonso

University of Salamanca

University College London

Rice University

Hungarian Academy of Sciences

Fabien Graveleau

Université des Sciences et Technologies de Lille (Lille-1)

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Papers by Orencio Duran

Genesis of coastal dune fields with vegetation

Vegetation greatly affects the formation and dynamics of dune fields in coastal areas. In the pre... more Vegetation greatly affects the formation and dynamics of dune fields in coastal areas. In the present work, we use dune modelling in order to investigate the genesis and early development stages of costal dune fields in the presence of vegetation. The model, which consists of a set of coupled equations for the turbulent wind field over the landscape, the saltation flux and the growth of vegetation cover on the surface, is applied to calculate the evolution of a sand patch placed upwind of a vegetated terrain and submitted to unidirectional wind and constant sand influx. Different dune morphologies are obtained, depending on the characteristic rate of vegetation growth relative to wind strength: barchans, transverse dunes with trailing ridges, parabolic dunes and vegetated, alongshore sand barriers or foredunes. The existence of a vegetation-free backshore is found to be important for the nucleation time-scale of coastal dune generations. The role of the sand influx and of the maximum vegetation cover density for the dune shape is also discussed.

Measurements and numerical simulations of

1 the activation degree and vegetation cover on 2 parabolic dunes in north-eastern Brazil Abstrac... more 1 the activation degree and vegetation cover on 2 parabolic dunes in north-eastern Brazil Abstract 16 In this work we present measurements of vegetation cover over parabolic dunes with 17 different degree of activation along the north-eastern Brazilian coast. We extend 18 the local values of the vegetation cover density to the whole dune by correlating 19 measurements with the relative brightness index C of high resolution QuickBird 20 panchromatic satellite image of the dune field. Next, we introduce the vegetation 21 data into a continuous model for vegetated dunes, coupling sand erosion and vegeta-22 tion growth, and perform simulations of parabolic dunes. Finally, from the compar-23 ison of both, the measurements and the simulation results, we show that the model 24 is able to predict the dune shape and the vegetation distribution of real parabolic 25 dunes as result of the evolution of a blow-out. 26 Manuscript 28

Barchan dune's size distribution induced by collisions

Barchans are isolated mobile dunes often organized in large dune fields. Dune fields seem to pres... more Barchans are isolated mobile dunes often organized in large dune fields. Dune fields seem to present a characteristic dune size and spacing, which suggests a cooperative behavior based on dune interaction. In Duran et al. (2009), we propose that the redistribution of sand by collisions between dunes is a key element for the stability and size selection of barchan dune fields. This approach was based on a mean-field model ignoring the spatial distribution of dune fields. Here, we present a simplified dune field model that includes the spatial evolution of individual dunes as well as their interaction through sand exchange and binary collisions. As a result, the dune field evolves towards a steady state that depends on the boundary conditions. Comparing our results with measurements of Moroccan dune fields, we find that the simulated fields have the same dune size distribution as in real fields but fail to reproduce their homogeneity along the wind direction.

Reply to “Comment on ‘Minimal size of a barchan dune

Physical Review E, 2007

We reply to the comment by Andreotti and Claudin (submitted to Phys. Rev. E, arXiv:0705.3525) on ... more We reply to the comment by Andreotti and Claudin (submitted to Phys. Rev. E, arXiv:0705.3525) on our paper "Minimal Size of a Barchan Dune" [Phys. Rev. E 75, 011301 (2007)]. We show that the equations of the dune model used in our calculations are self-consistent and effectively lead to a dependence of the minimal dune size on the wind speed through the saturation length. Furthermore, we show that Meridiani Planum ripples are probably not a good reference to estimate the grain size of Martian dune sands: the soil in the ripple troughs at the landing site is covered with nonerodible elements ("blueberries"), which increase the minimal threshold for saltation by a factor of 2.0. We conclude that, in the absence of large fragments as the ones found at the landing site, basaltic grains of diameter d = 500 ± 100µm that compose the large, typical dark Martian dunes [K. S. Edgett and P. R. Christensen, J. Geophys. Res. 96, 22765 (1991)] probably saltate during the strongest storms on Mars. We also show that the wind friction speed u * ≈ 3.0 m/s that we found from the calculations of Martian dunes is within the values of maximum wind speeds that occur during Martian storms a few times a decade [R. E. Arvidson et al., Science 222, 463 (1983); H. J. Moore, J. Geophys. Res. 90, 163 (1985); R. Sullivan et al., Nature 436, 58 (2005); D. J. Jerolmack et al., J. Geophys. Res. 111, E12S02 ]. In this manner, the dune model predicts that Martian dunes can be formed under present Martian conditions, with no need to assume other conditions of wind and atmosphere that could have prevailed in the past.

Measurements and numerical simulations of the degree of activity and vegetation cover on parabolic dunes in north-eastern Brazil

Geomorphology, 2008

In this work we present measurements of vegetation cover over parabolic dunes with different degr... more In this work we present measurements of vegetation cover over parabolic dunes with different degrees of activation along the north-eastern Brazilian coast. We extend the local values of the vegetation cover density to the whole dune by correlating measurements with the relative brightness index C of high resolution QuickBird panchromatic satellite images of the dune field. We then introduce the vegetation data into a continuous model for vegetated dunes, coupling sand erosion and vegetation growth, and perform simulations of the evolution of the morphology and vegetation cover of parabolic dunes. Finally, from the comparison of both, the measurements and the simulation results, we show that the model is able to predict the dune shape and the vegetation distribution of real parabolic dunes as result of the evolution of a blow-out.

Transformation of Barchans into parabolic dunes under the influence of vegetation

Barchan dunes were found to transform into parabolic dunes and vice versa when the amount of vege... more Barchan dunes were found to transform into parabolic dunes and vice versa when the amount of vegetation on and around them changes. This work presents the first numerical simulation of this effect. We propose a continuum model for the density of vegetation. An established sand transport model is used for simulating the evolution of the dunes.

Modelling of saturated sand flux

Journal of Statistical Mechanics-theory and Experiment, 2006

We derive a saturated sand flux model based on the previous models of Sauermann et al (2001 Phys.... more We derive a saturated sand flux model based on the previous models of Sauermann et al (2001 Phys. Rev. E 64 0313005) and Sørensen (2004 Geomorphology 59 53) and determine its parameters, as a function of the grain and fluid properties, from a comparison with wind tunnel data. We also show that dunes simulated with the new sand transport model compare well with observations of Moroccan dunes.

Breeding and solitary wave behavior of dunes

Physical Review E, 2005

Beautiful dune patterns can be found in deserts and along coasts due to the instability of a plai... more Beautiful dune patterns can be found in deserts and along coasts due to the instability of a plain sheet of sand under the action of the wind. Barchan dunes are highly mobile aeolian dunes found in areas of low sand availability and unidirectional wind fields. Up to now modelization mainly focussed on single dunes or dune patterns without regarding the mechanisms of dune interactions. We study the case when a small dune bumps into a bigger one. Recently Schwammle & Herrmann (Solitary wave behavior of sand dunes. Nature 426, 610 (2003)) and Katsuki et al.(Collision dynamics of two Barchans dunes simulated by a simple model. cond-mat 0403312 ) have shown that under certain circumstances dunes can behave like solitary waves. This means that they can "cross" each other which has been questioned by many researchers before. In other cases we observe coalescence, i.e. both dune merge into one, breeding, i.e. the creation of three baby dunes at the center and horns of a Barchan, or budding, i.e. the small dune, after "crossing" the big one, is unstable and splits into two new dunes.

Vegetated dunes and barchan dune fields

$Figure 3.7: Proportionality between the basal density p, = A,/A and the frontal area density \ = A;/A. The proportionality constant of the fit (solid line) is A,/A; = 1.48. Each point represents a measured area in the dunes placed in A quiraz, showed in Fig. 3.4 top, (red points) and the others dunes, Fig. 3.4 bottom,(green points).$

Modeling transverse dunes with vegetation

Physica A: Statistical …, Jan 1, 2009

In the present work, we use dune modeling in order to investigate the evolution of transverse dun... more In the present work, we use dune modeling in order to investigate the evolution of transverse dunes in the presence of vegetation. The vegetation is allowed to grow up to a maximum height H max with a growth rate R that oscillates in time. We find that the presence of the vegetation establishes a maximum height for the transverse dunes. If the transverse dune is larger than this maximum size, then the vegetation traps a considerable amount of sand, leading to the formation of vegetation marks at the upwind of the dune. We also investigate the formation of the transverse dune fields from a flat sand beach under saturated sand flux and vegetation growth. We find that the behavior of the field is determined by the maximum height, H max of the vegetation cover.

Numerical simulation of turbulent sediment transport, from bed load to saltation

Arxiv preprint arXiv:1111.6898, Jan 1, 2011

Sediment transport is studied as a function of the grain to fluid density ratio using two phase n... more Sediment transport is studied as a function of the grain to fluid density ratio using two phase numerical simulations based on a discrete element method (DEM) for particles coupled to a continuum Reynolds averaged description of hydrodynamics. At a density ratio close to unity (typically under water), vertical velocities are so small that sediment transport occurs in a thin layer at the surface of the static bed, and is called bed load. Steady, or 'saturated' transport is reached when the fluid borne shear stress at the interface between the mobile grains and the static grains is reduced to its threshold value. The number of grains transported per unit surface is therefore limited by the flux of horizontal momentum towards the surface. However, the fluid velocity in the transport layer remains almost undisturbed so that the mean grain velocity scales with the shear velocity u * . At large density ratio (typically in air), the vertical velocities are large enough to make the transport layer wide and dilute. Sediment transport is then called saltation. In this case, particles are able to eject others when they collide with the granular bed, a process called splash. The number of grains transported per unit surface is selected by the balance between erosion and deposition and saturation is reached when one grain is statistically replaced by exactly one grain after a collision, which has the consequence that the mean grain velocity remains independent of u * . The influence of the density ratio is systematically studied to reveal the transition between these two transport regimes. Based on the mechanisms identified in the steady case, we discuss the transient of saturation of sediment transport and in particular the saturation time and length. Finally, we investigate the exchange of particles between the mobile and static phases and we determine the exchange time of particles.

How barchan dunes distribute over deserts

Geophysical …, Jan 1, 2007

Abstract We derive an equation for fields of barchan dunes, which relates the fundamental macrosc... more Abstract We derive an equation for fields of barchan dunes, which relates the fundamental macroscopic properties characterizing a dune field, namely the inter-dune spacing and the first and second moments of the dune size distribution. This equation is obtained from a mean-field approach and is fulfilled for both simulations and real dune fields. Further, we show that the dune size distributions are log-normal and dunes tend to distribute uniformly in space. Finally, based on simulations, we derive the relationship between these properties ...

Bedforms in a turbulent stream: ripples, chevrons and antidunes

Journal of Fluid …, Jan 1, 2012

The interaction between a turbulent flow and a granular bed via sediment transport produces vario... more The interaction between a turbulent flow and a granular bed via sediment transport produces various bedforms associated to distinct hydrodynamical regimes. In this paper, we compare ripples (downstream propagating transverse bedforms), chevrons and bars (bedforms inclined with respect to the flow direction) and anti-dunes (upstream propagating bedforms), focusing on the mechanisms involved in the early stages of their formation. Performing the linear stability analysis of a flat bed, we study the asymptotic behaviours of the dispersion relation with respect to the physical parameters of the problem. In the subcritical regime (Froude number F smaller than unity), we show that the same instability produces ripples or chevrons depending on the influence of the free surface. The transition from transverse to inclined bedforms is controled by the ratio of the saturation length L sat , which encodes the stabilising effect of sediment transport, to the flow depth H, which determines the hydrodynamical regime. These results suggest that alternate bars form in rivers during flooding events, when suspended load dominates over bed load. In the supercritical regime F > 1, the transition from ripples to anti-dunes is also controlled by the ratio L sat /H. Anti-dunes appear around resonant conditions for free surface waves, a situation for which the sediment transport saturation becomes destabilising. This resonance turns out to be fundamentally different from the inviscid prediction. Their wavelength selected by linear instability mostly scales on the flow depth H, which is in agreement with existing experimental data. Our results also predict the emergence, at large Froude numbers, of 'anti-chevrons' or 'anti-bars', i.e. bedforms inclined with respect to the flow and propagating upstream.

Sand transport on Mars

Computer Physics …, Jan 1, 2009

Whether the winds of the present Mars are shaping the variety of dune forms observed on the Marti... more Whether the winds of the present Mars are shaping the variety of dune forms observed on the Martian surface has remained an open question that challenged planetary scientists for decades. In order to elucidate this issue, we have studied sand transport and dune formation from two different points of view. The first approach consists of solving the equations of aeolian transport at the level of the particles, whereas we study the trajectories and the velocities of sand grains during Martian wind storms. Next, we adapt a well established continuum model for sand dunes, which successfully reproduces the shape of Earth dunes, in order to study the formation of dunes under atmospheric conditions of Mars. Based on our results, we provide estimates for the wind strength, dune velocity and the timescale of changes in wind directions on Mars.

Size distribution and structure of Barchan dune fields

Arxiv preprint cond-mat/ …, Jan 1, 2007

On aeolian transport: Grain-scale interactions, dynamical mechanisms and scaling laws

Aeolian Research, Jan 1, 2011

Aeolian processes involve the wind action on a sedimentary substrate, namely erosion, sand transp... more Aeolian processes involve the wind action on a sedimentary substrate, namely erosion, sand transport and deposition. They are responsible for the emergence of aeolian dunes and ripples but also erosive structures like yardangs. In this review, we discuss the physics of aeolian sediment transport from a physical point of view. Relevant time and length scales associated to turbulent wind fluctuations

The dune size distribution and scaling relations of barchan dune fields

Granular Matter, Jan 1, 2009

Model for the genesis of coastal dune fields with vegetation

Geomorphology, Jan 1, 2011

Vegetation greatly affects the formation and dynamics of dune fields in coastal areas. In the pre... more Vegetation greatly affects the formation and dynamics of dune fields in coastal areas. In the present work, we use dune modeling in order to investigate the genesis and early development stages of coastal dune fields in the presence of vegetation. The model, which consists of a set of coupled equations for the turbulent wind field over the landscape, the saltation flux and the growth of vegetation cover on the surface, is applied to calculate the evolution of a sand patch placed upwind of a vegetated terrain and submitted to unidirectional wind and constant sand influx. Different dune morphologies are obtained, depending on the characteristic rate of vegetation growth relative to wind strength: barchans, transverse dunes with trailing ridges, parabolic dunes and vegetated, alongshore sand barriers or foredunes. The existence of a vegetation-free backshore is found to be important for the nucleation timescale of coastal dune generations. The role of the sand influx and of the maximum vegetation cover density for the dune shape is also discussed.

A continuous model for sand dunes: Review, new developments and application to barchan dunes and barchan dune fields

Earth Surface Processes …, Jan 1, 2010

Basically, sand dunes are patterns resulting from the coupling of hydrodynamic and sediment trans... more Basically, sand dunes are patterns resulting from the coupling of hydrodynamic and sediment transport. Once grains move, they modify the surface topography which in turns modifies the flow. This important feedback mechanism lies at the core of continuous dune modelling. Here we present an updated review of such a model for aeolian dunes, including important modifications to improve its predicting power. For instance, we add a more realistic wind model and provide a self-consistent set of parameters independently validated. As an example, we are able to simulate realistic barchan dunes, which are the basic solution of the model in the condition of unidirectional flow and scarce sediments. From the simulation, we extract new relations describing the morphology and dynamics of barchans that compare very well with existing field data. Next, we revisit the problem of the stability of barchan dunes and argue that they are intrinsically unstable bed-forms. Finally, we perform more complex simulations: first, a barchan dune under variable wind strength and, second, barchan dune fields under different boundary conditions. The latter has important implications for the problem of the genesis of barchan dunes.

Constitutive relations for the isotropic deformation of frictionless packings of polydisperse spheres

Comptes Rendus Mecanique, Jan 1, 2010

The isotropic compression of polydisperse packings of frictionless spheres is modeled with the di... more The isotropic compression of polydisperse packings of frictionless spheres is modeled with the discrete element method (DEM). The evolution of coordination number, fraction of rattlers, isotropic fabric, and pressure (isotropic stress) is reported as function of volume fraction for different system parameters. The power law relationship, with power ≈ 1/2, between coordination number and volume fraction is confirmed in the jammed state for a broad range of volume fractions and for different (moderate) polydispersities. The polydispersity in the packing causes a shift of the critical volume fraction, i.e., more heterogeneous packings jam at higher volume fractions. Close to jamming, the coordination number and the jamming volume fraction itself depend on both history and rate. At larger densities, neither the deformation history nor the loading rate have a significant effect on the evolution of the coordination number.

Genesis of coastal dune fields with vegetation

Vegetation greatly affects the formation and dynamics of dune fields in coastal areas. In the pre... more Vegetation greatly affects the formation and dynamics of dune fields in coastal areas. In the present work, we use dune modelling in order to investigate the genesis and early development stages of costal dune fields in the presence of vegetation. The model, which consists of a set of coupled equations for the turbulent wind field over the landscape, the saltation flux and the growth of vegetation cover on the surface, is applied to calculate the evolution of a sand patch placed upwind of a vegetated terrain and submitted to unidirectional wind and constant sand influx. Different dune morphologies are obtained, depending on the characteristic rate of vegetation growth relative to wind strength: barchans, transverse dunes with trailing ridges, parabolic dunes and vegetated, alongshore sand barriers or foredunes. The existence of a vegetation-free backshore is found to be important for the nucleation time-scale of coastal dune generations. The role of the sand influx and of the maximum vegetation cover density for the dune shape is also discussed.

Measurements and numerical simulations of

1 the activation degree and vegetation cover on 2 parabolic dunes in north-eastern Brazil Abstrac... more 1 the activation degree and vegetation cover on 2 parabolic dunes in north-eastern Brazil Abstract 16 In this work we present measurements of vegetation cover over parabolic dunes with 17 different degree of activation along the north-eastern Brazilian coast. We extend 18 the local values of the vegetation cover density to the whole dune by correlating 19 measurements with the relative brightness index C of high resolution QuickBird 20 panchromatic satellite image of the dune field. Next, we introduce the vegetation 21 data into a continuous model for vegetated dunes, coupling sand erosion and vegeta-22 tion growth, and perform simulations of parabolic dunes. Finally, from the compar-23 ison of both, the measurements and the simulation results, we show that the model 24 is able to predict the dune shape and the vegetation distribution of real parabolic 25 dunes as result of the evolution of a blow-out. 26 Manuscript 28

Barchan dune's size distribution induced by collisions

Barchans are isolated mobile dunes often organized in large dune fields. Dune fields seem to pres... more Barchans are isolated mobile dunes often organized in large dune fields. Dune fields seem to present a characteristic dune size and spacing, which suggests a cooperative behavior based on dune interaction. In Duran et al. (2009), we propose that the redistribution of sand by collisions between dunes is a key element for the stability and size selection of barchan dune fields. This approach was based on a mean-field model ignoring the spatial distribution of dune fields. Here, we present a simplified dune field model that includes the spatial evolution of individual dunes as well as their interaction through sand exchange and binary collisions. As a result, the dune field evolves towards a steady state that depends on the boundary conditions. Comparing our results with measurements of Moroccan dune fields, we find that the simulated fields have the same dune size distribution as in real fields but fail to reproduce their homogeneity along the wind direction.

Reply to “Comment on ‘Minimal size of a barchan dune

Physical Review E, 2007

We reply to the comment by Andreotti and Claudin (submitted to Phys. Rev. E, arXiv:0705.3525) on ... more We reply to the comment by Andreotti and Claudin (submitted to Phys. Rev. E, arXiv:0705.3525) on our paper "Minimal Size of a Barchan Dune" [Phys. Rev. E 75, 011301 (2007)]. We show that the equations of the dune model used in our calculations are self-consistent and effectively lead to a dependence of the minimal dune size on the wind speed through the saturation length. Furthermore, we show that Meridiani Planum ripples are probably not a good reference to estimate the grain size of Martian dune sands: the soil in the ripple troughs at the landing site is covered with nonerodible elements ("blueberries"), which increase the minimal threshold for saltation by a factor of 2.0. We conclude that, in the absence of large fragments as the ones found at the landing site, basaltic grains of diameter d = 500 ± 100µm that compose the large, typical dark Martian dunes [K. S. Edgett and P. R. Christensen, J. Geophys. Res. 96, 22765 (1991)] probably saltate during the strongest storms on Mars. We also show that the wind friction speed u * ≈ 3.0 m/s that we found from the calculations of Martian dunes is within the values of maximum wind speeds that occur during Martian storms a few times a decade [R. E. Arvidson et al., Science 222, 463 (1983); H. J. Moore, J. Geophys. Res. 90, 163 (1985); R. Sullivan et al., Nature 436, 58 (2005); D. J. Jerolmack et al., J. Geophys. Res. 111, E12S02 ]. In this manner, the dune model predicts that Martian dunes can be formed under present Martian conditions, with no need to assume other conditions of wind and atmosphere that could have prevailed in the past.

Measurements and numerical simulations of the degree of activity and vegetation cover on parabolic dunes in north-eastern Brazil

Geomorphology, 2008

In this work we present measurements of vegetation cover over parabolic dunes with different degr... more In this work we present measurements of vegetation cover over parabolic dunes with different degrees of activation along the north-eastern Brazilian coast. We extend the local values of the vegetation cover density to the whole dune by correlating measurements with the relative brightness index C of high resolution QuickBird panchromatic satellite images of the dune field. We then introduce the vegetation data into a continuous model for vegetated dunes, coupling sand erosion and vegetation growth, and perform simulations of the evolution of the morphology and vegetation cover of parabolic dunes. Finally, from the comparison of both, the measurements and the simulation results, we show that the model is able to predict the dune shape and the vegetation distribution of real parabolic dunes as result of the evolution of a blow-out.

Transformation of Barchans into parabolic dunes under the influence of vegetation

Barchan dunes were found to transform into parabolic dunes and vice versa when the amount of vege... more Barchan dunes were found to transform into parabolic dunes and vice versa when the amount of vegetation on and around them changes. This work presents the first numerical simulation of this effect. We propose a continuum model for the density of vegetation. An established sand transport model is used for simulating the evolution of the dunes.

Modelling of saturated sand flux

Journal of Statistical Mechanics-theory and Experiment, 2006

We derive a saturated sand flux model based on the previous models of Sauermann et al (2001 Phys.... more We derive a saturated sand flux model based on the previous models of Sauermann et al (2001 Phys. Rev. E 64 0313005) and Sørensen (2004 Geomorphology 59 53) and determine its parameters, as a function of the grain and fluid properties, from a comparison with wind tunnel data. We also show that dunes simulated with the new sand transport model compare well with observations of Moroccan dunes.

Breeding and solitary wave behavior of dunes

Physical Review E, 2005

Beautiful dune patterns can be found in deserts and along coasts due to the instability of a plai... more Beautiful dune patterns can be found in deserts and along coasts due to the instability of a plain sheet of sand under the action of the wind. Barchan dunes are highly mobile aeolian dunes found in areas of low sand availability and unidirectional wind fields. Up to now modelization mainly focussed on single dunes or dune patterns without regarding the mechanisms of dune interactions. We study the case when a small dune bumps into a bigger one. Recently Schwammle & Herrmann (Solitary wave behavior of sand dunes. Nature 426, 610 (2003)) and Katsuki et al.(Collision dynamics of two Barchans dunes simulated by a simple model. cond-mat 0403312 ) have shown that under certain circumstances dunes can behave like solitary waves. This means that they can "cross" each other which has been questioned by many researchers before. In other cases we observe coalescence, i.e. both dune merge into one, breeding, i.e. the creation of three baby dunes at the center and horns of a Barchan, or budding, i.e. the small dune, after "crossing" the big one, is unstable and splits into two new dunes.

Vegetated dunes and barchan dune fields

$Figure 3.7: Proportionality between the basal density p, = A,/A and the frontal area density \ = A;/A. The proportionality constant of the fit (solid line) is A,/A; = 1.48. Each point represents a measured area in the dunes placed in A quiraz, showed in Fig. 3.4 top, (red points) and the others dunes, Fig. 3.4 bottom,(green points).$

Modeling transverse dunes with vegetation

Physica A: Statistical …, Jan 1, 2009

In the present work, we use dune modeling in order to investigate the evolution of transverse dun... more In the present work, we use dune modeling in order to investigate the evolution of transverse dunes in the presence of vegetation. The vegetation is allowed to grow up to a maximum height H max with a growth rate R that oscillates in time. We find that the presence of the vegetation establishes a maximum height for the transverse dunes. If the transverse dune is larger than this maximum size, then the vegetation traps a considerable amount of sand, leading to the formation of vegetation marks at the upwind of the dune. We also investigate the formation of the transverse dune fields from a flat sand beach under saturated sand flux and vegetation growth. We find that the behavior of the field is determined by the maximum height, H max of the vegetation cover.

Numerical simulation of turbulent sediment transport, from bed load to saltation

Arxiv preprint arXiv:1111.6898, Jan 1, 2011

Sediment transport is studied as a function of the grain to fluid density ratio using two phase n... more Sediment transport is studied as a function of the grain to fluid density ratio using two phase numerical simulations based on a discrete element method (DEM) for particles coupled to a continuum Reynolds averaged description of hydrodynamics. At a density ratio close to unity (typically under water), vertical velocities are so small that sediment transport occurs in a thin layer at the surface of the static bed, and is called bed load. Steady, or 'saturated' transport is reached when the fluid borne shear stress at the interface between the mobile grains and the static grains is reduced to its threshold value. The number of grains transported per unit surface is therefore limited by the flux of horizontal momentum towards the surface. However, the fluid velocity in the transport layer remains almost undisturbed so that the mean grain velocity scales with the shear velocity u * . At large density ratio (typically in air), the vertical velocities are large enough to make the transport layer wide and dilute. Sediment transport is then called saltation. In this case, particles are able to eject others when they collide with the granular bed, a process called splash. The number of grains transported per unit surface is selected by the balance between erosion and deposition and saturation is reached when one grain is statistically replaced by exactly one grain after a collision, which has the consequence that the mean grain velocity remains independent of u * . The influence of the density ratio is systematically studied to reveal the transition between these two transport regimes. Based on the mechanisms identified in the steady case, we discuss the transient of saturation of sediment transport and in particular the saturation time and length. Finally, we investigate the exchange of particles between the mobile and static phases and we determine the exchange time of particles.

How barchan dunes distribute over deserts

Geophysical …, Jan 1, 2007

Abstract We derive an equation for fields of barchan dunes, which relates the fundamental macrosc... more Abstract We derive an equation for fields of barchan dunes, which relates the fundamental macroscopic properties characterizing a dune field, namely the inter-dune spacing and the first and second moments of the dune size distribution. This equation is obtained from a mean-field approach and is fulfilled for both simulations and real dune fields. Further, we show that the dune size distributions are log-normal and dunes tend to distribute uniformly in space. Finally, based on simulations, we derive the relationship between these properties ...

Bedforms in a turbulent stream: ripples, chevrons and antidunes

Journal of Fluid …, Jan 1, 2012

The interaction between a turbulent flow and a granular bed via sediment transport produces vario... more The interaction between a turbulent flow and a granular bed via sediment transport produces various bedforms associated to distinct hydrodynamical regimes. In this paper, we compare ripples (downstream propagating transverse bedforms), chevrons and bars (bedforms inclined with respect to the flow direction) and anti-dunes (upstream propagating bedforms), focusing on the mechanisms involved in the early stages of their formation. Performing the linear stability analysis of a flat bed, we study the asymptotic behaviours of the dispersion relation with respect to the physical parameters of the problem. In the subcritical regime (Froude number F smaller than unity), we show that the same instability produces ripples or chevrons depending on the influence of the free surface. The transition from transverse to inclined bedforms is controled by the ratio of the saturation length L sat , which encodes the stabilising effect of sediment transport, to the flow depth H, which determines the hydrodynamical regime. These results suggest that alternate bars form in rivers during flooding events, when suspended load dominates over bed load. In the supercritical regime F > 1, the transition from ripples to anti-dunes is also controlled by the ratio L sat /H. Anti-dunes appear around resonant conditions for free surface waves, a situation for which the sediment transport saturation becomes destabilising. This resonance turns out to be fundamentally different from the inviscid prediction. Their wavelength selected by linear instability mostly scales on the flow depth H, which is in agreement with existing experimental data. Our results also predict the emergence, at large Froude numbers, of 'anti-chevrons' or 'anti-bars', i.e. bedforms inclined with respect to the flow and propagating upstream.

Sand transport on Mars

Computer Physics …, Jan 1, 2009

Whether the winds of the present Mars are shaping the variety of dune forms observed on the Marti... more Whether the winds of the present Mars are shaping the variety of dune forms observed on the Martian surface has remained an open question that challenged planetary scientists for decades. In order to elucidate this issue, we have studied sand transport and dune formation from two different points of view. The first approach consists of solving the equations of aeolian transport at the level of the particles, whereas we study the trajectories and the velocities of sand grains during Martian wind storms. Next, we adapt a well established continuum model for sand dunes, which successfully reproduces the shape of Earth dunes, in order to study the formation of dunes under atmospheric conditions of Mars. Based on our results, we provide estimates for the wind strength, dune velocity and the timescale of changes in wind directions on Mars.

Size distribution and structure of Barchan dune fields

Arxiv preprint cond-mat/ …, Jan 1, 2007

On aeolian transport: Grain-scale interactions, dynamical mechanisms and scaling laws

Aeolian Research, Jan 1, 2011

Aeolian processes involve the wind action on a sedimentary substrate, namely erosion, sand transp... more Aeolian processes involve the wind action on a sedimentary substrate, namely erosion, sand transport and deposition. They are responsible for the emergence of aeolian dunes and ripples but also erosive structures like yardangs. In this review, we discuss the physics of aeolian sediment transport from a physical point of view. Relevant time and length scales associated to turbulent wind fluctuations

The dune size distribution and scaling relations of barchan dune fields

Granular Matter, Jan 1, 2009

Model for the genesis of coastal dune fields with vegetation

Geomorphology, Jan 1, 2011

Vegetation greatly affects the formation and dynamics of dune fields in coastal areas. In the pre... more Vegetation greatly affects the formation and dynamics of dune fields in coastal areas. In the present work, we use dune modeling in order to investigate the genesis and early development stages of coastal dune fields in the presence of vegetation. The model, which consists of a set of coupled equations for the turbulent wind field over the landscape, the saltation flux and the growth of vegetation cover on the surface, is applied to calculate the evolution of a sand patch placed upwind of a vegetated terrain and submitted to unidirectional wind and constant sand influx. Different dune morphologies are obtained, depending on the characteristic rate of vegetation growth relative to wind strength: barchans, transverse dunes with trailing ridges, parabolic dunes and vegetated, alongshore sand barriers or foredunes. The existence of a vegetation-free backshore is found to be important for the nucleation timescale of coastal dune generations. The role of the sand influx and of the maximum vegetation cover density for the dune shape is also discussed.

A continuous model for sand dunes: Review, new developments and application to barchan dunes and barchan dune fields

Earth Surface Processes …, Jan 1, 2010

Basically, sand dunes are patterns resulting from the coupling of hydrodynamic and sediment trans... more Basically, sand dunes are patterns resulting from the coupling of hydrodynamic and sediment transport. Once grains move, they modify the surface topography which in turns modifies the flow. This important feedback mechanism lies at the core of continuous dune modelling. Here we present an updated review of such a model for aeolian dunes, including important modifications to improve its predicting power. For instance, we add a more realistic wind model and provide a self-consistent set of parameters independently validated. As an example, we are able to simulate realistic barchan dunes, which are the basic solution of the model in the condition of unidirectional flow and scarce sediments. From the simulation, we extract new relations describing the morphology and dynamics of barchans that compare very well with existing field data. Next, we revisit the problem of the stability of barchan dunes and argue that they are intrinsically unstable bed-forms. Finally, we perform more complex simulations: first, a barchan dune under variable wind strength and, second, barchan dune fields under different boundary conditions. The latter has important implications for the problem of the genesis of barchan dunes.

Constitutive relations for the isotropic deformation of frictionless packings of polydisperse spheres

Comptes Rendus Mecanique, Jan 1, 2010

The isotropic compression of polydisperse packings of frictionless spheres is modeled with the di... more The isotropic compression of polydisperse packings of frictionless spheres is modeled with the discrete element method (DEM). The evolution of coordination number, fraction of rattlers, isotropic fabric, and pressure (isotropic stress) is reported as function of volume fraction for different system parameters. The power law relationship, with power ≈ 1/2, between coordination number and volume fraction is confirmed in the jammed state for a broad range of volume fractions and for different (moderate) polydispersities. The polydispersity in the packing causes a shift of the critical volume fraction, i.e., more heterogeneous packings jam at higher volume fractions. Close to jamming, the coordination number and the jamming volume fraction itself depend on both history and rate. At larger densities, neither the deformation history nor the loading rate have a significant effect on the evolution of the coordination number.