On the slip face of a barchan, sand avalanches occur when sand piles up beyond the angle of repose, which is just over 30 degrees.
Formation and movement of a barchanDuring its formation, a barchan is constantly shaped by the wind. It has one steep slope on the leeward side (see diagrams below) and a gentle gradient on the windward side of roughly 12 degrees. Some of the sand deposited by the wind on the gently sloping surface is swept forward over the crest of the dune and comes to rest at an angle of 30 to 34 degrees.
From the study of ancient sand dunes even seasonal changes in wind direction have been detected.
Related category GEOGRAPHY
Source: Great Sand Dunes National Park, NPS
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