A saltation cellular automaton of a sand sea. Wind above Bagnold's threshold picks up grains, slip faces (34°) trap them, vegetation anchors the sand, and supply + wind variability decide whether barchans, transverse ridges, linear or star dunes emerge. Watch them migrate.
Threshold friction velocity for saltation: u*_t = A·√(g·d·(ρs−ρ)/ρ) (Bagnold, A ≈ 0.1); grains coarser than ~0.1 mm need stronger wind. Sand flux follows the (u − u_t)³ law, q = C·(u − u_t)³. Grains erode windward faces, deposit on the downwind slip face (angle of repose 34°), so dunes migrate at c ≈ q/(ρ·h). Wind directional variability with low supply produces barchans, medium supply transverse ridges, bimodal winds linear dunes and multi-directional winds star dunes.