A sand dune collapses when its angle becomes too steep to support its own weight, primarily on the windward side.
Here's a more detailed explanation:
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Windward Side Build-Up: Wind carries sand particles up the windward (upwind) side of the dune.
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Increasing Angle: As sand accumulates, the slope of the windward side increases.
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Angle of Repose Exceeded: Eventually, the angle of the slope exceeds the angle of repose. The angle of repose is the steepest angle at which a material (in this case, sand) will remain stable. This angle depends on factors like the size and shape of the sand grains, as well as their moisture content.
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Collapse Initiation: When the angle is too steep, the sand grains at the top are no longer supported by the grains below.
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Avalanche Effect: The sand then begins to slide down the slope in a small avalanche, causing the dune to partially collapse. This movement redistributes the sand, reducing the slope angle until it is once again below the angle of repose, thereby stabilizing the dune until more sand is deposited and the process repeats.
In summary, sand dune collapse is a result of the relentless addition of sand by wind exceeding the dune's structural capacity to support its own weight at an increasingly steep angle, leading to avalanches and slope readjustment.
