Sand Dunes and Ripples

• As soon as wind velocity drops wind deposition occurs.
• The heaviest material is deposited first while the finer material and dust is carried further before being dropped.
• As a result loess (which consists of fine particles) is sometimes deposited thousands of kilometers from deserts.
• Large mounds of sand result from sand depositions within the desert.
• These result in the formation of erg landscapes such as those found in the Sahara.
• Three major types of features result from wind deposition and form part of the erg landscape:
• sand ripples, barchan dunes and seif dunes.

Sand ripples

Sand ripples. Image credit allposters.com

• These are small wave-like features which develop on sand which move easily.
• They range from a few centimeters to about a meter in height
• They are often temporary and suffer destruction when the wind changes direction.

Sand Dunes

• These are hills of sand which are found in a variety of shape,size and direction.
• Dunes develop when sand grains moved by saltation and surface creep are deposited (remember suspension material forms loess which is deposited outside deserts).
• Some dunes, but not all, form around obstacles such as trees, bushes, rocks, a small hill or even a dead animal.
• Most dunes form on areas that are flat and sandy rather than those areas that are rocky and uneven.
• Dunes vary in size from a few meters to over a 100 meters in height.
• Although they take many shapes, there are two common types of dunes:
• Barchan and Seif dunes.

Barchan Dunes

Barchan dune. Image credit Media Wiki

• A barchan dune is a small crescent shaped dune.
• It has a height can range from a few meters to about 30 meters in height and it can be 400 meters wide..
• They lie at right angles to the prevailing wind.
• It has its “horns” pointing downwind.
• They usually form around an obstacle such as a rock, piece of vegetation or even a dead animal.
• As the mound, which is wind ward grows due to continued sand depositions,
• Its leading edges are slowly carried forward in a downwind direction.
• The windward slope of the dune is gentle.
• The downwind side is steep and slightly curved.
• This is caused by eddies that are set up by the prevailing wind.
• A barchan dune moves as grains of sand are moved up the windward slope to fall onto the leeward side.
• They can occur both singly or in groups.

Barchan Dune cross section. Image credit Revisionworld.com

Sief Dunes

• Are also known as transverse dunes, linear dunes or draa.
• They are ridge-shaped with steep sides and lie parallel to the prevailing wind.
• They are also formed and appear parallel to each other.
• A seif dune has a sharp crest which may be a 100 meters in height and they can stretch for up to 150 kilometers in length.
• They are separated by flat corridors which are between 25 and 400 meters wide.
• These corridors are swept clear of sand by the prevailing wind.
• Eddies blow up against the sides of dunes and drop deposit sand that is added to the dunes.
• They usually develop from small sand ridges.
• They slowly move forward in the direction of the prevailing wind as they move forward.
• They feature in parts of the Namib Desert and the Sahara Deserts as well as other deserts.

Sief Dunes.

To access more topics go to the Geography Notes page.