Diffusive Component of the Granular Motion during Subaqueous Barchan-Barchan Collisions

Abstract:

Barchans are formed in areas where prevail one-directional fluid flow and limited amount of granular material. They are usually organized in barchan fields, where barchan interactions regulate their dynamics. In a previous study [Assis and Franklin, Geophys. Res. Lett., submitted], we investigated barchan-barchan interactions (including barchan collisions) in the subaqueous case and found five different patterns for both aligned and off-centered bedforms. We now investigate the diffusive component of granular motion during the collisional process between two barchan dunes. In the present communication, we focus specifically on two collision patterns named Exchange and Merging. For that, experiments were conducted in a water channel of transparent material where controlled grains were poured inside, forming pairs of granular piles in aligned position. A high-speed camera placed above the test section acquired images of the bedforms, which were afterward processed with a numerical code that tracked several of the moving grains. As a result, we observed small fluctuations during the trajectories of grains associated with collisions at the grain scale, from which we computed diffusive length and time scales. Those scales are correlated with the transverse spreading of grains observed at the macroscopic level after the upstream barchan collided with the downstream one. Our results are fundamental to understand how different patterns emerge from barchan-barchan interactions.