Dunes of the Namib Desert
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The Namib Desert is an extremely arid eco-region comprising of shifting sand dunes, gravel plains and rugged mountains. The world's oldest desert, the Namib Desert is completely devoid of surface water but bisected by several dry riverbeds.
The dunes between Lüderitz and the Kuiseb Rivers, the Southern Namib is made up of a belt of sand dunes 320 km long and about 120 km wide. These spectacular linear and crescent-shaped dunes reach elevations of 300 m.
A strip of low sand hummocks of about 2 m high runs between this sea of dunes and the ocean. The dunes march northwards, driven by prevailing southerly winds, and are then brought to an abrupt halt by vegetation of the Kuiseb riverbed, which forms a wind barrier.
TYPES OF DUNES:
Simple dunes represent a wind regime that has not changed in intensity or direction since the formation of the dune, while compound and complex dunes suggest that the intensity and direction of the wind has changed.
The most common dune form on Earth (and on Mars) is the crescentic. Crescent-shaped mounds generally are wider than longer. The slip face is on the dune's concave side. These dunes form under winds that blow from one direction, and they also are known as barchans, or transverse dunes. Some types of crescentic dunes move faster over desert surfaces than any other type of dune.
Straight or slightly sinuous sand ridges typically much longer than they are wide are known as linear dunes. They may be more than 160 kilometres long. Linear dunes may occur as isolated ridges, but they generally form sets of parallel ridges separated by miles of sand, gravel, or rocky inter-dune corridors. Some linear dunes merge to form Y-shaped compound dunes. Many form in bidirectional wind regimes. The long axes of these dunes extend in the resultant direction of sand movement.
Radially symmetrical, star dunes are pyramidal sand mounds with slip faces on three or more arms that radiate from the high centre of the mound. They tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally.
Oval or circular mounds that generally lack a slip face, dome dunes are rare and occur at the far upwind margins of sand seas.
U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. Sometimes these dunes are called U-shaped, blow-out, or hairpin dunes, and they are well known in coastal deserts. Unlike crescentic dunes, their crests point upwind. The elongated arms of parabolic dunes follow rather than lead because they have been fixed by vegetation, while the bulk of the sand in the dune migrates forward. The longest known parabolic dune has a trailing arm 12 kilometres long.
Occurring wherever winds periodically reverse direction, reversing dunes are varieties of any of the above types. These dunes typically have major and minor slip faces oriented in opposite directions.
Simple, Compound and Complex:
All these dune types may occur in three forms: simple, compound, and complex. Simple dunes are basic forms with a minimum number of slip faces that define the geometric type. Compound dunes are large dunes on which smaller dunes of similar type and slip face orientation are superimposed, and complex dunes are combinations of two or more dune types.
Desert Research Foundation of Namibia