Why do sand dunes migrate?
Dunes are mounds or ridges of wind-deposited sand. Some move actively while others are stabilised by vegetation. Migration results from repeated transfer of grains from the windward to the lee slope.
Grain-by-grain movement
Wind carries grains by bouncing and rolling. Grains crossing the crest settle on the lee slope, where small slips follow when it becomes unstable. Repetition shifts the form. Dune shape depends on wind directions, sand supply and vegetation; not every dune is crescent-shaped.
What controls migration speed?
Small dunes with limited sand may move more quickly under some conditions, while moisture and vegetation can restrain movement. Storms and quiet periods differ. Dated aerial images measure progress; one image cannot supply a speed. Coastal dunes can buffer storm waves, but roads and vegetation removal may weaken that function.