Sand Dunes and Ripples

Sand Dunes and Ripples

Seif dunes. Image credit 7-themes.com

Seif dunes. Image credit 7-themes.com

ZIMSEC O Level Geography Notes: Sand Dunes

  • 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

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

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.
Features of barchan dunes. Image credit Revisionworld.com

Features of barchan dunes. Image credit Revisionworld.com

Barchan Dune cross section. Image credit Revisionworld.com

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.

Sief Dunes.

To access more topics go to the Geography Notes page.

By |2017-01-17T11:22:00+00:00June 13th, 2015|Hot Deserts, Notes, O Level Geography, Ordinary Level Notes|Comments Off on Sand Dunes and Ripples

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He holds an Honours in Accountancy degree from the University of Zimbabwe. He is passionate about technology and its practical application in today's world.
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