Barchan sand dunes are highly mobile, crescent-shaped dunes that occur in areas where sand is sparse and the wind is unidirectional. Here we show mathematically how two such loose-grained dunes are able to pass through one another while still preserving their shape. The crucial parameters for this solitary-wave behaviour, which is consistent with field observations, are the heights of the two colliding dunes. Because of their remarkable mobility, barchan dunes can disrupt structures such as roads and pipelines in arid, sandy regions. The velocity of a dune can reach several tens of a metres per year and is proportional to the reciprocal of its height, meaning that small dunes move faster than large ones. However, the dynamics and evolution of dunes are difficult to assess because of the large time scales involved, with field measurements having to be compiled over several decades.
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