Phenomena

Sand Dunes

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Sand dunes form with a gentle incline facing the wind and a steeper slip face pointing away from the wind. Most slip faces are angled at about 30 to 34 degrees–called the angle of repose. The shape is determined by the dune’s ability to support its own weight; add more sand and it will cascade down the slip face in a miniature avalanche. Similarly, if you disturb sand on the slip face by digging a hole at the base, you get the cascading collapse seen in this video. By removing sand, the dune’s equilibrium is broken and it can no longer support its weight. This makes sand flow down the slip face until enough is shifted that the dune can support itself. Being a granular material, the sand itself appears to flow much like a fluid, with waves, ripples and all. (Video credit: M. Meier; submitted by Boris M.)

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