1.3 Star

Radially symmetrical, star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound. They tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally. They dominate the Grand Erg Oriental of the Sahara. In other deserts, they occur around the margins of the sand sea s, particularly near topographic barriers. In the southeast Badain Jaran Desert of China, the star dunes are up to 500 meters tall and may be the tallest dunes on Earth.

1.4 Dome

Parabolic dune partially stabilized by marram grassSee text Marram grass is a genus of perennial grass, used to prevent or stop erosion of dunes. It is a pioneer species which acts as a sand-binder and dune-builder, with an extensive root system and a coarse, stiff structure. The roots contain rhizhomes w

Oval or circular mounds that generally lack a slipface, dome dunes are rare and occur at the far upwind margins of sand seas.

1.5 Parabolic

U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. Sometimes these dunes are called U-shaped, blowout, 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 kilometers long.

1.6 Combined types

Occurring wherever winds periodically reverse direction, reversing dunes are varieties of any of the above types. These dunes typically have major and minor slipfaces oriented in opposite directions.

All these dune types may occur in three forms: simple, compound, and complex. Simple dunes are basic forms with a minimum number of slipfaces that define the geometric type. Compound dunes are large dunes on which smaller dunes of similar type and slipface orientation are superimposed, and complex dunes are combinations of two or more dune types. A crescentic dune with a star dune superimposed on its crest is the most common complex dune. 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.

2 Coastal dunes

Dunes form on coasts where the backshore can support and onshore winds encourage the accumulation of sand blown inland from off a beach. Any part of the upper beach, once dry, can lose sand to the wind, especially if the sand is fine, and dune formation proceeds in the direction towards which the predominant wind direction is blowing.

Dunes provide privacyPrivacy is the ability of a person to control the availability of information about and exposure of him- or herself. It is related to being able to function in society anonymously (including pseudonymous or blind credential identification). According to E and shelter from the wind.

2.1 Succession on coastal dunes

The fore dune and first yellow dune at Studland, England

As a dune forms, plant succession occurs. The conditions on an embryo dune are harsh, with salt spray from the sea carried on strong winds. The dune is well drained and often dry. Rotting sea weed brought in by storm waves adds enough nutrients to allow pioneer species to colonise the dune. These pioneer species are marram grass, sea wort grass and other sea grasses in England. These plants are well adapted to the harsh conditions of the fore dune, typically having deep roots which reach the water table, rhibozomes which produce nitrogen compounds, and protected stoma, reducing transpiration. The deep roots also bind the sand together, and the dune grows into a fore dune as more sand is blown over the grasses. The grasses add nitrogen to the soil, meaning other, less hardy plants can then colonise the dunes. Typically these are heathers and gorses. These too are adapted to the low soil water content and have small, prickley leaves which reduce transpiration. Heathers add humus to the soil, but have a pH of lower than 7, so make the soil slightly acidic. Heathers are usually replaced by coniferous trees which can tollerate the low pH. Coniferous forests and heathland are common climax communities for sand dune systems.

Young dunes are called yellow dune s, dunes which have high humus content are called grey dune s. Leaching occurs on the dunes, washing humus into the slacks, and the slacks may be much more developed then the exposed tops of the dunes.

For the snow analogue to a sand dune see sastruga.

3 Sub-Aqueous dunes

Sub-Aqueous ( underwater) dunes form on a bed of sand or gravel under the actions of water flow. They are ubiquitous in natural channels such as rivers and estuaries, and also form in engineered canals and pipelines. Dunes move downstream as the upstream slope is eroded and the sediment deposited on the downstream or lee slope.

These dunes most often form as a continuous 'train' of dunes, showing remarkable similarity in wavelength and height.

Dunes on the bed of a channel significantly increase flow resistance, their presence and growth playing a major part in river flooding.

4 References & Links

• The Physics of Blown Sand ( 1941) by Ralph Bagnold

5 Examples

5.1 Sand dune plains

(large expanses of dunes)

• Great Sand Dunes National Park
• Mesquite Flat Dunes , USA
• Western Sahara
• White Sands National Monument
• Rig-e Jenn in the Central Desert of Iran
• The Great Sand Dunes of southwest Saskatchewan

5.2 Sand dune systems

(coastal dunes featuring succession)

• Studland, Dorset, England

6 See Also

• Earth science
• List of landforms

Landforms Deserts

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