Source: Journal of Geophysical Research: Earth Surface
Barchans are crescent-shaped sand dunes that form under unidirectional wind or water flows on surfaces where little sand is available, such as bedrock. Barchans rarely move in isolation but rather collectively migrate in organized barchan fields, where binary barchan-barchan interactions constitute an important size- and migration speed-regulating mechanism.
In the past, barchan-barchan interactions were predominantly studied numerically, using dune models that couple local sand motion with turbulent flow along complex topography. While such models have been shown to make some predictions in agreement with observations at the dune scale, it has remained largely unclear how they perform at smaller scales. Assis and Franklin  have finally provided experiments that can, in principle, support or falsify existing models down to the grain scale.
By means of experiments carried out in a water channel, the authors resolved the motion of individual sand grains travelling along each barchan and from one barchan to the other during different kinds of binary barchan interactions. They revealed a diffusion-like spreading motion of barchan-crossing grains.
The experiments may lead to improved dune models, which are not limited to subaqueous barchan fields but can, in principle, be applied to any fluvial and aeolian environment, and thus may lead to an improved understanding of aeolian dune fields on Earth, Mars, and beyond.
Citation: Assis, W. R., & Franklin, E. M. . Morphodynamics of barchan-barchan interactions investigated at the grain scale. Journal of Geophysical Research: Earth Surface, 126, e2021JF006237. https://doi.org/10.1029/2021JF006237
—Thomas Pähtz, Associate Editor, JGR: Earth Surface