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Aeolian sand sorting and megaripple formation

Abstract

Sand is blown across beaches and deserts by turbulent winds. This seemingly chaotic process creates two dominant bedforms: decametre-scale dunes and centimetre-scale ripples, but hardly anything in between. By the very same process, grains are constantly sorted. Smaller grains advance faster, while heavier grains trail behind. Here, we argue that, under erosive conditions, sand sorting and structure formation can conspire to create distinct bedforms in the ‘forbidden wavelength gap’ between aeolian ripples and dunes. These so-called megaripples are shown to co-evolve with an unusual, predominantly bimodal grain-size distribution. Combining theory and field measurements, we develop a mechanistic understanding of their formation, shape and migration, as well as their cyclic ageing, renewal and sedimentary memory, in terms of the intermittent wind statistics. Our results demonstrate that megaripples exhibit close similarities to dunes and can indeed be mechanistically characterized as a special type of (‘reptation’) dune.

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Fig. 1: Sand sorting and megaripples.
Fig. 2: Model and field data for wind-driven sand sorting.
Fig. 3: Bulk sand polydispersity and coarsening rate.
Fig. 4: Dune-type versus ripple-type analysis of megaripple morphology and migration.
Fig. 5: Transient sand sorting and megaripple evolution under variable winds.

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Acknowledgements

We thank E. Schmerler for his kind help with the sand sample preparation and analysis. This research was supported by a grant from the GIF, the German-Israeli Foundation for Scientific Research and Development (no. 1143-60.8/2011). We also acknowledge the hospitality of the KITP in Santa Barbara and the MPI-PKS in Dresden, where this work was started, and financial support by the National Science Foundation under grant no. NSF PHY-1125915, the MPI-PKS Visitors Program, and the German Academic Exchange Service (DAAD) through a Kurzstipendium (for M.L.).

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M.L. and K.K. developed the theory; M.L., A.M., H.Y., H.T. and I.K. designed and conducted the experiments and field surveys; M.L., A.M., I.K. and H.Y. analysed the data; M.L. and K.K. wrote the paper. All authors discussed the results and implications and commented on the manuscript at all stages.

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Correspondence to Klaus Kroy.

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Supplementary Figures 1–4, Supplementary Tables 1 and 2, Supplementary References 1–7

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Lämmel, M., Meiwald, A., Yizhaq, H. et al. Aeolian sand sorting and megaripple formation. Nature Phys 14, 759–765 (2018). https://doi.org/10.1038/s41567-018-0106-z

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