| Abstrakt: |
Corridors 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. Plain Language Summary: Barchans are dunes of crescentic shape that are commonly found on Earth, Mars, and other celestial bodies. Although of similar shape, their scales vary with the environment they are in, going from the millennium and kilometer for Martian barchans, down to the minute and centimeter in the aquatic case, passing by hundreds of meters and years for eolian barchans. Other common characteristic is that barchans are organized in dune fields, where barchan‐barchan collisions are an important mechanism for their size regulation. We took advantage of the smaller and faster scales of subaqueous dunes and performed experiments in a water channel, which allowed us to determine the trajectories of individual grains while two barchans interacted with each other, something unfeasible from field measurements on terrestrial or Martian deserts. We show typical lengths and velocities of individual grains, and that, in case of barchan collisions, grains from the impacting barchan spread with a diffusive component over the other barchan. Our results provide new insights into the evolution of barchans found in water, air, and other planetary environments. Key Points: We determine the trajectories of individual grains during barchan‐barchan interactionsWe show the origin and destination of moving grains and typical lengths and velocitiesWe find the spreading rate of grains over the target barchan once dune‐dune collision has occurred [ABSTRACT FROM AUTHOR] |
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