|Dunes of the Namib Desert
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.
Namib dunes are the oldest in the world; 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
Desert Research Foundation of Namibia