Abstract
A group of barchans, crescent sand dunes, exhibit a characteristic flying-geese pattern in deserts on Earth and Mars. This pattern implies that an indirect interaction between barchans, mediated by an inter-dune sand stream, which is released from one barchan's horns and caught by another barchan, plays an important role in the dynamics of barchan fields. We used numerical simulations of a recently proposed cell model to investigate the effects of inter-dune sand streams on barchan fields. We found that a sand stream from a point source moves a downstream barchan laterally until the head of the barchan is finally situated behind the stream. This final configuration was shown to be stable by a linear stability analysis. These results indicate that flying-geese patterns are formed by the lateral motion of barchans mediated by inter-dune sand streams. By using simulations we also found a barchan mono-corridor generation effect, which is another effect of sand streams from point sources.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Satellite photographs of the Earth's surface have become available to the public through the Internet, and we can inspect large-scale patterns on the Earth's surface in the laboratory and at home. Barchans are certainly the most impressive dunes seen in deserts. Seeing satellite photographs of the barchan field of the west Sahara, for example, one notices that a flock of barchans exhibits a characteristic flying-geese triangular pattern. We performed computer simulations in order to understand the mechanisms underlying this pattern.
Main results. In this paper, we showed that the inter-dune sand streams play a critical role in the pattern formation in barchan fields. Our findings are summarized as follows: (1) a sand stream from a point source moves a downstream barchan laterally, which indicates that the flying-geese pattern is actually formed by inter-dune sand streams; and (2) a barchan mono-corridor observed in a desert is generated from a point sand source.
Wider implications. The importance of inter-dune sand streams has not been widely recognized until now. This recognition will provide useful information regarding the geographical conditions and environmental settings of the surfaces of other planets, such as Mars.
Figure. Snapshots of the lateral motion of a barchan behind the point source. The amount of sand supplied is the same as that out of the numerical field. The barchan moves laterally toward the sand stream.