Sand Dunes: Saltation, Migration and the Slip Face

Saltation: sand does not roll, it hops

Wind rarely moves sand grains by rolling them along the surface. Above a threshold wind speed, grains are lifted briefly into the airflow and carried a short hop downwind before gravity pulls them back down — a motion called saltation. When a falling grain strikes the surface, it does not just stop: the impact can eject several more stationary grains into the air (the splash effect), each of which then saltates in turn. This makes saltation self-sustaining once it starts, and it is the dominant mechanism moving sand in both deserts and along beaches.

Two very different slopes on the same dune

A dune's windward face, the stoss slope, is gentle, because saltating and creeping grains are continually pushed up it by the wind and rarely accumulate steeply. Grains that reach the crest and are carried just past it fall into the wind's shadow, where the airflow separates and slows sharply. Sand piling in that shelter builds up until it reaches the angle of repose for dry sand — typically around 30 to 34 degrees — at which point any further addition triggers small avalanches (grain flows) down the face. This steep downwind face is the slip face, and it is close to the maximum angle loose, dry sand can hold before gravity wins.

Why the dune as a whole migrates

Because sand is continuously eroded from the stoss slope and deposited on the slip face, the entire dune profile shifts downwind over time even though no single grain travels the whole distance in one trip — each grain typically saltates up the windward face, gets buried by later avalanches on the slip face, and re-emerges only after enough of the dune has migrated past its position. Migration rate depends on wind strength and consistency, dune height (taller dunes contain more sand and move slower for a given wind), and sand supply.

stoss slope (windward): gentle, ~10-15°, net erosion, saltation dominates
crest: flow separation, sand launched into wind shadow just beyond it
slip face (leeward): steep, ~30-34° (angle of repose), net deposition + avalanches

Wind pattern decides the dune shape

A single, dominant, steady wind direction with limited sand supply tends to build crescent-shaped barchan dunes, with the horns of the crescent pointing downwind because the thinner edges of the sand pile move faster than the taller centre. Abundant sand under a unidirectional wind builds long transverse dune ridges perpendicular to the wind. Winds that regularly shift between two dominant directions build linear (seif) dunes aligned roughly along the resultant wind direction, while complex, multidirectional wind regimes with ample sand produce star dunes with several radiating arms and comparatively little net migration, since opposing winds cancel out much of the one-directional creep.

What stops a dune: vegetation and moisture

Vegetation interrupts saltation directly — plant stems and roots slow near-surface wind speed below the transport threshold and physically trap grains, and roots bind sand in place against further erosion. A dune colonised faster than it is buried becomes fixed, eventually developing an irregular, vegetated (parabolic or nebkha) shape rather than the clean geometric forms of bare, actively migrating dunes. Moisture has a related stabilising effect, since damp sand grains stick together and resist being lifted into saltation far more than dry grains, which is part of why beach dunes above the high-tide line migrate far more slowly once a stabilising crust or vegetation gets a foothold.

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