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

Abstract

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 in Alvarez and Franklin [Phys. Rev. Lett. 121, 164503 (2018)] 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 water turbulent flow, from heaps of conical shape formed from glass beads poured on the bottom wall of a rectangular channel. Both for evolving and 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 strength of 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 quasi-linear relation. Our results provide new insights into the physical mechanisms underlying the shape of barchan dunes.

Force distribution within a barchan dune

“…LES, although less accurate than DNS and needing turbulence models, is able to compute the flow around dunes at a much lower computational cost. The simulations captured well the evolution of an initial pile toward a barchan dune in both the bedform and grain scales, with the same characteristic time and lengths observed in previous experiments 7,8,18 . In addition to reproducing accurately previous experimental data, the numerical results revealed in detail quantities not accessible from experiments, such as the resultant force acting on each grain.…”

“…The large time scales of aeolian and martian barchans compared to the aquatic case make of subaqueous barchans the ideal object of study. Therefore, experiments have been conducted in water tanks and channels, where measurements were made at both the dune 7,9,12,[14][15][16] and grain scales 8,17,18 , and from which length and time scales of barchans and typical trajectories and velocities of moving grains were obtained. In particular, Wenzel and Franklin 17 and Alvarez and Franklin 8,18 are the only experimental studies showing the distribution of grain velocities over the barchan surface and trajectories of grains migrating to different parts of the barchan dune.…”

“…Therefore, experiments have been conducted in water tanks and channels, where measurements were made at both the dune 7,9,12,[14][15][16] and grain scales 8,17,18 , and from which length and time scales of barchans and typical trajectories and velocities of moving grains were obtained. In particular, Wenzel and Franklin 17 and Alvarez and Franklin 8,18 are the only experimental studies showing the distribution of grain velocities over the barchan surface and trajectories of grains migrating to different parts of the barchan dune. For the forces on individual grains within a barchan dune, there are currently no experimental results, the acquisition of these data being highly difficult.…”

A Comprehensive Picture for Binary Interactions of Subaqueous Barchans

“…For the trajectories, grains move mainly by rolling and sliding and follow closely the fluid flow in the subaqueous case, being susceptible to small vortices and other small structures of the flow. This has been shown to be especially important for the grains migrating to the barchan horns (Alvarez & Franklin, 2018, 2019). When the fluid is a gas, grains move by saltation and reptation, and those in saltation follow basically a straight line in the main flow direction (Bagnold, 1941), being undisturbed by the small structures of the flow.…”

Morphodynamics of Barchan‐Barchan Interactions Investigated at the Grain Scale

“…With that, we can estimate the overall transport of grains in the interdune space (migrating from one dune to another or being entrained further downstream), which is also presented in the supporting information (in terms of mass flow rates). In addition, trajectories of grains leaving an isolated subaqueous barchan are also available in the supporting information, from which we can observe that all grains leave the dune through their horns (a great part of them coming from upstream regions and going around the dune before reaching the horns, as shown by Alvarez and Franklin (2018) and Alvarez and Franklin (2019)).…”