Formation of sand dunes

For sand dunes to form, the following conditions must be achieved :

• A dried-up area with little vegetation and a large amount of loose sand.
• A gust of wind or breeze to move the sand.
• An obstacle of any size (rock, for example) to stop the momentum and settle the sand.

Sand dunes begin to form when the wind moves up to the top of the pile. Wind continues to move sand up to the top of the pile until the pile is so steep that it collapses under its own weight. The collapsing sand comes to rest when it reaches just the right steepness to keep the dune stable. This angle, usually about 30-34°, is called the angle of repose.





Source: http://www.desertusa.com/geofacts/sanddune.html

The methods by which the wind moves the sand are :

1. Saltation, whereby sand grains bounce in the wind (95% of sand is moved using this method).
2. Creep, whereby sand grains collide with other grains (clay, for example) causing them to move (4% of sand is moved this way).
3. Suspension, whereby sand grains are blown skywards before settling (1% of sand is moved using this method).

 
Source: http://geography.howstuffworks.com/…/sand-dune1.ht...