Subaqueous Barchan Research Articles

Local Similarity Between Aeolian Barchan Dunes and Their Downsized Subaqueous Counterparts

AbstractAeolian barchan dunes on Earth and other planets have been widely investigated. Much of the understanding of barchan dune morphodynamics comes from field observations, numerical simulations, and downsized water‐tunnel experiments as well. Many of the evolution of barchan dunes in water‐tunnel experiments are similar to those of aeolian cases, although they have notable differences in scale, sand particle motion and hydrodynamic characteristics. Here, we first review the literature on the local similarities between aeolian and downsized subaqueous barchan dunes, focusing on (a) dune formation, (b) dune morphology, (c) particle‐scale characteristics, and (d) sand/dune emission at horns. A comprehensive description of double‐dune interaction modes is then presented to illustrate the local similarity of barchan dune morphodynamics. Specifically, as the interaction mode undergoes a process of “merging‐splitting‐chasing,” the similarity between the interaction modes of aeolian and downsized subaqueous dunes continuously decreases. Furthermore, we summarize the significance and limitations of downsized water‐tunnel experiments for barchan dunes, and highlight the focus for future investigation.

Experimental study of the stability of crescent barchan shape in subaqueous dune evolution in unsteady water flow

A water tunnel experiment adopting the frequency conversion technology is conducted to investigate the evolution of subaqueous barchan dune driven by the periodic water flow. The kinematic and morphological characteristics of the barchan dune evolution are derived through the imaging processing. Results exhibit a good correlation between the dune kinematics and the periodic variation of water flow, which indicates that the unsteadiness is transmitted from the fluid phase to the sand phase with nearly no inertia. The stable crescent barchan shape under unsteady flow is found. Meanwhile, this crescent barchan shape breaks and reconstructs periodically during the variation of unsteady flow, which implies a flexible compromise between the dune system and the propelling force. This study suggests that the stability of crescent barchan shape can be robust under unsteady flow, which brings new understandings of the morphological stability of sand dune evolution.

Barchan Dunes Cruising Dune‐Size Obstacles

AbstractWe investigate the behavior of subaqueous barchans reaching dune‐size obstacles by carrying out experiments where we varied the obstacle shape and size, the flow strength, and the grains' properties. We found that a subaqueous barchan can pass over or bypass a dune‐size obstacle, or even be blocked, with some intermediate situations. In the bypass cases, the original barchan can split in two or more bedforms, redistributing sand in space. Finally, we propose a classification map in which the barchan behavior depends basically on two dimensionless parameters. Our results represent a step toward understanding how barchans behave in the presence of large obstacles, such as retaining walls, tubes and bridge pillars.

Grain-scale computations of barchan dunes

Barchans are crescent-shaped dunes commonly found in diverse environments and scales: from the 10-cm-long barchans found under water to the 1-km-long barchans on Mars, passing by the 100-m-long dunes on Earth's deserts. Although ubiquitous in nature, there is a lack of grain-scale computations of the growth and evolution of those bedforms. In this paper, we investigate the values of grain properties (coefficients of sliding friction, rolling friction, and restitution) necessary to carry out numerical simulations of subaqueous barchans with CFD–DEM (computational fluid dynamics–discrete element method), and how the values of those coefficients change the barchan dynamics. We made use of LES (large eddy simulation) for the fluid, varied the coefficients of sliding friction, rolling friction, and restitution in the DEM, and compared the outputs with experiments. We show (i) for the case of glass spheres, the values of coefficients for correctly obtaining the dune morphology, timescales, trajectories of individual grains, and forces experienced by grains; (ii) the LES meshes allowing computations of bedload while capturing the main disturbances of the fluid flow; (iii) how different values of coefficients affect the morphology of barchans; and (iv) that spheres with higher coefficients of rolling friction can be used for simulating barchans consisting of angular grains. Our results represent a significant step for performing simulations that capture, at the same time, details of the fluid flow (large eddies) and grains' motion (individual particles).

Morphodynamics of Barchan‐Barchan Interactions Investigated at the Grain Scale

AbstractCorridors of size‐selected crescent‐shaped dunes, known as barchans, are commonly found in water, air, and other planetary environments. The growth of barchans results from the interplay between a fluid flow and a granular bed, but their size regulation involves intricate exchanges between different barchans within a field. One size‐regulating mechanism is the binary interaction between nearby dunes, when two dunes exchange mass via the near flow field or by direct contact (collision). In a recent Letter (Assis & Franklin, 2020, https://doi.org/10.1029/2020GL089464), we identified five different patterns arising from binary interactions of subaqueous barchans and proposed classification maps. In this paper, we further inquire into binary exchanges by investigating the motion of individual grains while barchans interact with each other. The experiments were conducted in a water channel where the evolution of pairs of barchans in both aligned and off‐centered configurations was recorded by conventional and high‐speed cameras. Based on image processing, we obtained the morphology of dunes and motion of grains for all interaction patterns. We present the trajectories of individual grains, from which we show the origin and destination of moving grains, and their typical lengths and velocities. We also show that grains from the impacting dune spread with a diffusion‐like component over the target barchan, and we propose a diffusion length. Our results provide new insights into the size‐regulating mechanisms of barchans and barchanoid forms found on Earth and other planets.

Growth of barchan dunes of bidispersed granular mixtures

Barchans are dunes of crescentic shape found on Earth, Mars, and other celestial bodies, growing usually on polydisperse granular beds. In this Letter, we investigate experimentally the growth of subaqueous barchans consisting of bidisperse grains. We found that the grain distribution within the dune changes with the employed pair, and that a transient stripe appears on the dune surface. We propose that observed patterns result from the competition between fluid entrainment and easiness of rolling for each grain type, and that grains segregate with a diffusion-like mechanism. Our results provide new insights into barchan structures found in other environments.

Force distribution within a barchan dune

Barchan dunes, or simply barchans, are crescent-shaped dunes found in diverse environments such as the bottom of rivers, Earth’s deserts, and the surface of Mars. In our recent paper [“Shape evolution of numerically obtained subaqueous barchan dunes,” Phys. Rev. E 101, 012905 (2020)], we investigated the evolution of subaqueous barchans by using the computational fluid dynamics-discrete element method, and our simulations captured well the evolution of an initial pile toward a barchan dune in both the bedform and grain scales. The numerical method having shown to be adequate, we obtain now the forces acting on each grain, isolate the contact interactions, and investigate how forces are distributed and transmitted in a barchan dune. We present force maps and probability density functions for values in the streamwise and spanwise directions and show that stronger forces are experienced by grains at neither the crest nor the leading edge of the barchan but in positions just upstream the dune centroid on the periphery of the dune. We also show that a large part of grains undergo longitudinal forces of the order of 10−7 N, with negative values around the crest, resulting in decelerations and grain deposition in that region. These data show that the force distribution tends to route a large part of grains toward the crest and horns of subaqueous barchans, being fundamental to comprehend their morphodynamics. However, to the best of the authors’ knowledge, they are not accessible from the current experiments, making our results an important step toward understanding the behavior of barchan dunes.

A Comprehensive Picture for Binary Interactions of Subaqueous Barchans

AbstractWe investigate experimentally the short‐range interactions occurring between two subaqueous barchans. The experiments were conducted in a water channel of transparent material where controlled grains were poured inside, and a camera placed on the top acquired images of the bedforms. We varied the grain types (diameter, density, and roundness), pile masses, transverse distances, water flow rates, and initial conditions. As a result, five different patterns were identified for both aligned and off‐centered configurations, and we propose interaction maps that depend basically on the ratio between the number of grains of each dune, Shields number, and alignment of barchans. In addition, we show experimental indications that an ejected barchan has roughly the same mass of the impacting one in some cases and that in wake‐dominated processes the asymmetry of the downstream dune is large. The present results shed light on the size regulation of barchans found on Earth and other planets.

On the genesis and evolution of barchan dunes: Hydrodynamics

Barchans are an important signature of turbulent atmospheric and aquatic flows on parts of the Earth’s surface where the supply of sediments is scarce. Khosronejad and Sotiropoulos [“On the genesis and evolution of barchan dunes: Morphodynamics,” J. Fluid Mech. 815, 117–148 (2017)] demonstrated that high-fidelity hydro-morphodynamic large-eddy simulation (LES) can replicate sub-aqueous barchan fields exhibiting the geometric features and morphodynamic interactions observed in nature. Herein, we first validate the ability of our LES method to simulate mean flow and turbulence statistics over a frozen barchan dune created in a control laboratory experiment. We then analyze our high-fidelity LES datasets to elucidate the hydrodynamic phenomena that drive the previously identified morphodynamic interactions at various stages of subaqueous barchan dune evolution, from flatbed to quasi-equilibrium. We uncover and quantify the hydrodynamic mechanisms that cause (i) the initiation of crescent microfeatures on the initial flatbed; (ii) the development of three-dimensional barchan dunes; and (iii) several other phenomena that occur during the barchan field maturation, such as the birth of new small barchans and the exposure of the underlying bedrock in the wake of the barchans. Defining a dimensionless timescale, i.e., the saturation timescale, as a function of the saturation length scale, we attempt to categorize for the first time various temporal stages of barchan field development. We show that the barchan field has specific characteristics corresponding to each of the barchan stage developments in terms of both the flow field and the bed topology. Comparing the simulation results of transverse and barchan dunes, we illustrate that the two dune types have similar topographic and hydrodynamics characteristics until bedrock exposure occurs.

Shape evolution of numerically obtained subaqueous barchan dunes.

In the realm of granular bedforms, barchan dunes are strong attractors that can be found in rivers, terrestrial deserts, and other planetary environments. These bedforms are characterized by a crescentic shape, which, although robust, presents different scales according to the environment they are in, their length scale varying from the decimeter under water to the kilometer on Mars. In addition to the scales of bedforms, the transport of grains presents significant differences according to the nature of the entraining fluid, so that the growth of barchans is still not fully understood. Given the smaller length and time scales of the aquatic case, subaqueous barchans are the ideal object to study the growth of barchan dunes. In the present paper, we reproduce numerically the experiments of Alvarez and Franklin [Phys. Rev. E 96, 062906 (2017)2470-004510.1103/PhysRevE.96.062906; Phys. Rev. Lett. 121, 164503 (2018)PRLTAO0031-900710.1103/PhysRevLett.121.164503] on the shape evolution of barchans from their initiation until they have reached a stable shape. We computed the bed evolution by using the computational fluid dynamics-discrete element method, where we coupled the discrete element method with large eddy simulation for the same initial and boundary conditions of experiments, performed in a closed-conduit channel where initially conical heaps evolved to single barchans under the action of a water flow in a turbulent regime. Our simulations captured well the evolution of the initial pile toward a barchan dune in both the bedform and grain scales, with the same characteristic time and lengths observed in experiments. In addition, we obtained the local granular flux and the resultant force acting on each grain, the latter not yet previously measured nor computed. This shows that the present method is appropriate for numerical computations of bedforms, opening new possibilities for accessing data that are not available from current experiments.

Experimental study on the stable morphology and self-attraction effect of subaqueous barchan dunes

The evolutionary process of isolated dunes under the action of a unidirectional steady water flow is recorded from a bottom-up visual angle by optical synchronization imaging measurements. The morphological characteristics of dunes formed from different initial sand pile configurations are analyzed by an image-processing method. Results show that the trend of reproducing the crescent shape of the dune is shown in all sand piles, which is independent of the initial configuration. The triangular initial sand pile is found to be the first to stabilize to a crescent shape, whereas the square initial sand pile takes the longest time to form a stable crescent shape. This finding is essentially due to the shape effect of the triangle, which effectively reduces the amount of transverse moving sand particles. Based on the aspect ratio of the barchan dune that is stable at approximately 1, we determine that the evolution time required for the final stability of barchan dunes reproduced from different initial sand piles is approximately 200 s within the restriction of the present closed-water-tunnel experimental condition. Moreover, a dimensionless comparison of the profile curves of barchan dunes’ stable morphologies reveals a “streamwise–spanwise” dimension of the dune, which essentially synthesizes characteristic information about windward face and two horns. This “streamwise–spanwise” dimension ultimately presents the self-attraction effect of the crescent shape.

Horns of subaqueous barchan dunes: A study at the grain scale.

Many complex aspects are involved in the morphodynamics of crescent-shaped dunes, known as barchans. One of them concerns the trajectories of individual grains over the dune and how they affect its shape. In the case of subaqueous barchans, we proposed [C. A. Alvarez and E. M. Franklin, Phys. Rev. Lett. 121, 164503 (2018)PRLTAO0031-900710.1103/PhysRevLett.121.164503] that their extremities, called horns, are formed mainly by grains migrating from upstream regions of the initial pile, and that they exhibit significant transverse displacements. Here, we extend our previous work to address the dynamics of grains migrating to horns after the dune has reached its crescentic shape and present new aspects of the problem. In our experiments, single barchans evolve, under the action of a turbulent water flow, from heaps of conical shape formed from glass beads poured on the bottom wall of a rectangular channel. Both for evolving and for developed barchans, the horns are fed up with grains coming from upstream regions of the bedform and traveling with significant transverse components, differently from the dynamics usually described for the aeolian case. For these grains, irrespective of their size and the strength of the water flow, the distributions of transverse and streamwise components of velocities are well described by exponential functions, with the probability density functions of their magnitudes being similar to results obtained from previous studies on flat beds. Focusing on moving grains whose initial positions were on the horns, we show that their residence time and traveled distance are related following a quasilinear relation. Our results provide new insights into the physical mechanisms underlying the shape of barchan dunes.

Velocity fields and particle trajectories for bed load over subaqueous barchan dunes

This paper presents an experimental investigation of moving grains over subaqueous barchan dunes that consisted of spherical glass beads of known granulometry. Prior to each test run, a pre-determined quantity of grains was poured inside a closed conduit, and the grains settled on its bottom wall forming one conical heap. As different turbulent water flows were imposed, each heap evolved to a barchan dune, which was filmed with a high-speed camera. An image processing code was written to identify some of the moving grains and compute their velocity fields and trajectories. Our results show that the velocity of grains varies along the barchan dune, with higher velocities occurring close to the dune centroid, and that grains trajectories are intermittent. Depending on the region over the dune, we found that the velocity fields present values within 1 and 10 % of the cross-sectional mean velocity of the fluid. Considering the average trajectories of grains moving over a given dune, their mean displacement varies within 30 and 60 grain diameters and their characteristic velocities within 10 and 20 % of the cross-sectional mean velocity of the fluid. The displacement time varies between 30 and 90 % of the settling time, and it seems to have two asymptotic behaviors: one close to bed load inception and other far from it. When compared with bed load over a plane bed, we observe that grains have the same mean velocity, but they travel distances up to 5 times larger, with higher densities of moving grains.

Role of Transverse Displacements in the Formation of Subaqueous Barchan Dunes.

Crescentic shape dunes, known as barchan dunes, are formed by the action of a fluid flow on a granular bed. These bedforms are common in many environments, existing under water or in air, and being formed from grains organized in different initial arrangements. Although they are frequently found in nature and industry, details about their development are still to be understood. In a recent paper [C. A. Alvarez and E. M. Franklin, Phys. Rev. E 96, 062906 (2017)PRESCM2470-004510.1103/PhysRevE.96.062906], we proposed a timescale for the development and equilibrium of single barchans based on the growth of their horns. In the present Letter, we report measurements of the growth of horns at the grain scale. In our experiments, conical heaps were placed in a closed conduit and individual grains were tracked as each heap, under the action of a water flow, evolved into a barchan dune. We identified the trajectories of the grains that migrated to the growing horns, and found that most of them came from upstream regions on the periphery of the initial heap, with an average displacement of the order of the heap size. In addition, we show that individual grains had transverse displacements by rolling and sliding that are not negligible, with many of them going around the heap. The mechanism of horns formation revealed by our experiments contrasts with the general picture that barchan horns form from the advance of the lateral dune flanks due to the scaling of migration velocity with the inverse of dune size. Our results change the way in which the growth of subaqueous barchan dunes is explained.

Experimental study on the formation of subaqueous barchan dunes in closed conduits

The present paper reports the formation of subaqueous barchan dunes by analyzing the temporal evolution of their main geometrical characteristics (width W, length L and horn lengths Lh). After certain time, the dunes reach an equilibrium state and it is possible to study the relation between W versus L, and the dependence of the dune velocity on L. The barchan dunes were formed from spherical glass and zirconium beads. An initial conical heap of beads was placed on the bottom wall of a rectangular channel and it was entrained by a water turbulent flow. The evolution of the dunes was filmed with a CCD camera placed above the channel and mounted on a traveling system. Our results show that after a characteristic time the dune shape does not change and it travels with a roughly constant velocity. Once the equilibrium state is reach, W and L are measured, showing linear dependence. Furthermore, we show that the dune velocity Vd scales with the inverse of the dune length.

Birth of a subaqueous barchan dune.

Barchan dunes are crescentic shape dunes with horns pointing downstream. The present paper reports the formation of subaqueous barchan dunes from initially conical heaps in a rectangular channel. Because the most unique feature of a barchan dune is its horns, we associate the time scale for the appearance of horns to the formation of a barchan dune. A granular heap initially conical was placed on the bottom wall of a closed conduit and it was entrained by a water flow in turbulent regime. After a certain time, horns appear and grow, until an equilibrium length is reached. Our results show the existence of the time scales 0.5t_{c} and 2.5t_{c} for the appearance and equilibrium of horns, respectively, where t_{c} is a characteristic time that scales with the grains diameter, gravity acceleration, densities of the fluid and grains, and shear and threshold velocities.

Subaqueous Barchans and Plane Beds from Deposition of Quartz Silt

AbstractThe suspension flow of quartz silt (geometric mean grain size of 4.15 μm), in unfavorable conditions for deposition, is the motivation for the development of different bed morphologies. Particles deposit over a plane nonerodible surface and develop well-defined barchan ripples, barchanoids, and plane beds in flume experiments. Bedload transport of quartz silt by bedform migration is several orders of magnitude smaller than the suspended transport. The final bed morphology is controlled by the suspended sediment concentration and running time of the experiment. The average dimensions of the bedforms after 1-day experiments are 4.93 cm wide, 10.33 cm long, and 0.45 cm high. Cohesive plane beds appear after 2-day and 3-day experiments with very high sediment concentrations (≥22.5 kg/m3). Viscous effects are deemed relevant for the formation of the beds.

Subaqueous barchan dunes in turbulent shear flow. Part 2. Fluid flow

AbstractWe report an experimental study of the turbulent flow above a barchan dune in a channel, from particle image velocimetry measurements, for Reynolds numbers ranging from 9000, just below the threshold for particle motion, up to 24 000, where the dune moves. Two calculations of the speed-up over the dune are compared, the usual ‘same-elevation’ and the more relevant ‘Lagrangian’, showing that the latter is smaller by a factor of two. The two-layer structure of the flow disturbance – an essentially inviscid outer layer and a turbulent inner layer of thickness ${\delta }_{i} $ – is assessed. In the outer layer, streamline curvature is shown to be responsible for half of the Lagrangian speed-up, from the comparison of the velocity measurements with two Bernoulli calculations. In the inner layer, detailed measurements of the velocity and stresses are provided, down to ${y}^{+ } \approx 1$, and the momentum budget is discussed. The Reynolds shear stress decreases monotonically towards the dune surface, according to the standard mixing-length closure, whereas the total shear stress increases strongly in the viscous sublayer. Along the dune surface, the shear stress increases up to the crest where it reaches twice its unperturbed value. A good estimate of the surface stress is provided by a parabolic fit of the inner velocity profile matching the outer flow at ${y}_{d} \approx {\delta }_{i} $. Doubling the Reynolds number, the surface shear stress and the speed-up decrease by ∼30 %. The implications of these results on the dune motion, presented in Part 1 of this study (Franklin & Charru, J. Fluid Mech., vol. 675, 2011, pp. 199–222), are finally discussed.

Flow effects on the morphology and dynamics of aeolian and subaqueous barchan dunes

Barchans are aeolian sand dunes that form where unidirectional winds blow on firm ground with limited sand supply. Although they have been analyzed for decades, the difficulty of conducting experiments in the field, due to the large lengthscale and timescale involved, is a considerable obstacle to understand their dynamics. However, crescentic patterns also appear underwater on a much smaller scale, allowing easier study. It is shown here that down‐scaled solitary barchans can be created in the laboratory using the periodic and asymmetric motion of a plate underwater to simulate the effect of a unidirectional flow. Those subaqueous barchans compare very well to their aeolian relatives. Their speeds scale as the cube of the speed of the flow times the inverse of their typical dimension. Finally, the exploration of new topics, such as the influence of the directionality of the wind or that of binary collisions, illustrate that barchans can eventually adapt to any external perturbations by emitting one or several small barchans.

Barchan‐shaped ripple marks in a wave flume

AbstractBarchans, isolated crescent‐shaped bedforms, are believed to be formed under almost unidirectional wind or water flows and limited sand supply. The formation of barchan morphologies under the action of purely oscillatory wave motion has not yet been fully investigated. The present study attempted to form barchan topography in a wave flume and to compare this with barchans in the field. Barchan morphologies of ripple size, called the barchan ripples, were generated from a flat bed by the action of waves. The horn width, the distance between horn tips, of the barchan ripples increased linearly with an increase in the total length, the overall length projected on the centre line of the barchan, with a coefficient common to barchan dunes in deserts. The ratio of horn length to horn width of the barchan ripples was smaller than that of barchan dunes, but similar to that of subaqueous barchans in the field. The longer the wave period was, the larger the ratio of the body length to horn width became. Most subaqueous barchans formed under waves (in the laboratory) and unidirectional flows (in the field) had blunter horns than subaerial barchans. The shape of the barchan ripples changed with wave period. The outer rim became rounder with increasing wave period. The relationship between the base area and the height of barchan morphologies seems to be linear, with a constant coefficient for the scale from ripples to dunes. The barchan ripples showed a linear relationship between the height and the horn width, similar to that for barchan dunes. The migration speed of the barchan ripples was proportional to the cube of the flow velocity and was inversely proportional to height. The same relation with a different value of the coefficient was obtained for barchan dunes. Copyright © 2004 John Wiley & Sons, Ltd.