Barchan Dunes Cruising Dune‐Size Obstacles

We 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. Plain Language Summary This paper is devoted to crescent‐shaped dunes, known as barchans, that are found on Earth, Mars and other celestial bodies, with roughly the same morphology but different scales. In some cases, barchans can approach an obstacle of comparable size, such as houses and buildings in the aeolian case, and bridge pillars and submarine structures in the aquatic case. In order to investigate that, we carried out experiments in a water channel where granular heaps developed into barchans that approached obstacles of different shape and size. We found that barchans can be blocked, bypass or pass over dune‐size obstacles, and that bypassing barchans can split in two or more bedforms, which means a significant redistribution of sand in space. Based on the experiments, we propose a classification map in which the barchan behavior depends on two dimensionless parameters. Our results shed light on how barchans behave in the presence of large obstacles, helping us to predict the outcomes of dunes interacting with objects in other environments, and design safer and eco‐friendly structures. Key Points We show that subaqueous barchans can be blocked, bypass, or pass over dune‐size obstacles In some cases, barchans can split in two or more bedforms We propose that the barchan behavior depends basically on two dimensionless parameters