Aeolian Geomorphology
Barchan dunes are crescent-shaped sand dunes forming in areas with a constant wind direction and limited sand supply.
Migration speed ∝ q/H where q is sand flux and H is dune height. Smaller dunes overtake larger ones, causing collisions and merging or splitting.
The angle of repose (~34°) sets the slip-face slope. Sand avalanches when the slope exceeds this angle, maintaining the shape.
Barchans found: Sahara, Namib, Peru coast, and on Mars (photographed by NASA rovers). Sand transport formula: Q ∝ u³ − u_t³ (Bagnold 1941).