Sand Dunes and their classification

Sand dunes are the build up of sand formed due to water or wind process under the influence of gravity. It typically takes the shape of a ridge, mound or hill that far from the tidal influence. The area which is associated with the dunes is called dune system or dune complex. A collection of dunes is called dune field and a comparatively large dune field is called as erg.

Ripple vs Dune : Ripples are smaller with height of 5cm if more than 5cm it will be termed as dune.

Dune morphology :-

Sand dunes can be of many types as follows :-

1. Barchans : These are sand dune with shape of C. The windward side is wide and small slip face or leeward side. Horn points downwind direction. They are the most common and fast moving dunes.
2. Parabolic dunes : They are mirror image of Barchans and have small windward side. Horns points upwind direction.
3. Linear dunes : There are many dunes which are not crescent shaped but they are shaped like a straight line, called linear dunes. Linear dunes can be of two types :-
   • Longitudnal dunes : Longitudnal dunes are formed parallel to the wind flow direction.
   • Transverse dunes : Transverse dunes are formed perpendicular to the wind flow direction.
4. Star dunes : Star dunes shaped as star with a central point ridge with multiple slip faces.
5. Dome dunes : They have pointed ridge without any slip or leeward slop. They are the rarest types of dunes. The wind can blow material from any side.

Free dunes	Anchored dunes
There are the dunes can form anywhere without need of an obstacle. They can change their position with time.	These are the dunes which formed where there is availability of an obstacle. Anchored dunes doesn’t change their position.
Transverse (Transverse, Barchan, Dome, Reversed )	Vegetated (Parabolic, Blowout, Nebkha)
Linear (Linear, seif) Star (Star, Network)	Topographic (Climbing, Falling, Lee, Sand ramp)

Other special terminology :

• Impeded Sand Dunes : These are the dunes in which vegetation plays a major role in the dune development.
• Climbing dunes : Ramp up against hills and scarps with slopes of 30º – 50º.
• Echo dunes : Form upwind of obstacles with slopes >50º, and are created by a vortex return-flow from the obstacle.
• Flank dunes : Form downwind of obstacles; they are noticeable because winds accelerating around the sides of obstacles clear a corridor between the obstacle and the dune
• Reversed dunes : It is the intermediate between transverse dune and star dune.

REFERENCES :

• https://education.nationalgeographic.org/resource/dune/
• E-content module (Department of Geology G.G.M Science College Jammu)
• Geomorphology of DESERT DUNES 2nd ed. by Nicholas Lancaster
• https://ag.arizona.edu/~lmilich/dune.html