Barchans

Barchans are crescent-shaped sand dunes that form in arid regions under relatively unidirectional winds and with a limited supply of loose sand. They occur where there is enough sand to build dunes but not so much as to produce linear dune forms, and where vegetation is sparse. The horns of the crescent point downwind, and the dune migrates slowly as sand is transported from the windward side toward the leeward slip face.

Morphology and structure: A barchan has a gentle windward (stoss) slope and a steep leeward (slip) face, with the crescent’s horns extending downwind. The slip face typically sits on the dune’s lee side, and sand avalanches along this face maintain the dune’s shape. Dune height can range from several meters to tens of meters, with widths from tens to hundreds of meters, though sizes vary with wind conditions and sand supply. The horns grow as sand is preferentially removed from the central crest and deposited at the ends.

Formation and dynamics: Barchans form under steady, predominantly single-direction winds that transport sand via saltation and deposition. A limited, sand-rich supply allows the crescent to stabilize into a distinct shape rather than forming more continuous ridge dunes. Dune migration occurs as material moves over the crest and down the slip face, shifting the dune downwind. Interactions among barchans—such as collisions, horn growth, or fragmentation—can alter their size and spacing.

Distribution and significance: Barchans are common in the world’s deserts, including parts of the Sahara, the Arabian Peninsula, Central Asia, and certain arid regions of North America and Australia. They typically appear in dune seas or erg landscapes and serve as key indicators of prevailing wind regimes and sediment supply in desert environments.