Dune

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In physical geography, a dune is a hill of sand built by eolian processes. Bare dunes are subject to shifting location and size based on their interaction with the wind. The "valley" or trough between dunes is called a slack. A "dune field" is an area covered by extensive sand dunes.

Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach. In most such cases the dunes are important in protecting the land against potential ravages by storm waves from the sea. Although the most widely distributed dunes are those associated with coastal regions, the largest complexes of dunes are found inland in dry regions and associated with ancient lake or sea beds.

Dunes also form under the action of water flow (alluvial processes), on sand or gravel beds of rivers, estuaries and the sea-bed.

The word 'dune' derives from a medieval germanic or norse word - "dun", a hill.

Dune shapes

Crescentic

The most common dune form on Earth (and on Mars) is the crescentic. Crescent-shaped mounds generally are wider than long. The slipface 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. A group of dunes moved more than 100 meters per year between 1954 and 1959 in China's Ningxia Province; similar rates have been recorded in the Western Desert of Egypt. The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than 3 kilometers, are in China's Taklamakan Desert.

Linear

Straight or slightly sinuous sand ridges typically much longer than they are wide are known as linear dunes. They may be more than 160 kilometers long. Linear dunes may occur as isolated ridges, but they generally form sets of parallel ridges separated by miles of sand, gravel, or rocky interdune 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.

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 seas, 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.

Dome

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

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.

Longitudinal and transverse dunes

Longitudinal dunes, also called Seif dunes, elongate parallel to the prevailing wind, possibly caused by a larger dune having its smaller sides blown away. Seif dunes are sharp-crested and are common in the Sahara. They range up to 300 m (900 ft) in height and 300 km (200 mi) in length.

Seif dunes are thought to develop from barchans if a change of wind direction occurs. The new wind direction will lead to the development of a new wing and the overdevelopment of one of the original wings. If the prevailing wind then becomes dominant for a lengthy period of time the dune will revert to its barchan form, with one exaggerated wing. Should the strong wind then return the exaggerated wing will further extend so that eventually it will be supplied with sand when the prevailing wind returns. The wing will continue to grow under both wind conditions, thus producing a seif dune. On a seif dune the slip face develops on the side facing away from the strong wind, while the slip face of a barchan faces the direction of movement. In the sheltered troughs between highly developed seif dunes barchans may be formed because the wind is unidirectional.

A transverse dune is horizontal to the prevailing wind, probably caused by a steady buildup of sand on an already existing minuscule mound.

Reversing dunes

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

All these dune shapes 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.

Dune types

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.

Lithified dunes

A lithified (consolidated) sand dune is a type of sandstone that is formed when a marine or eolian sand dune becomes compacted and hardened. Once in this form, water passing through the rock can carry and deposit minerals, which can alter the hue of the rock. Cross-bedded layers of stacks of lithified dunes can produce the cross-hatching patterns, such as those seen in Zion National Park.

A local slang term used for these consolidated dunes is "slickrock", a name that was introduced by pioneers of the old west because their steel-rimmed wagon wheels could not gain purchase on the rock.

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 privacy and shelter from the wind.

Succession on coastal dunes

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 colonize 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, root nodules that produce nitrogen compounds, and protected stoma, reducing transpiration. Also, the deep roots 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 colonize the dunes. Typically these are heathers and gorses. These too are adapted to the low soil water content and have small, prickly 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 tolerate the low pH. Coniferous forests and heathland are common climax communities for sand dune systems.

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

For the snow analogue to a sand dune see sastruga.

Desertification

One of the biggest problems posed by sand dunes is their encroachment on human habitats. Sand dunes move through a few different means, all of them helped along by wind. One way that dunes can move is through saltation, where sand particles skip along the ground like a rock thrown across a pond might skip across the water's surface. When these skipping particles land, they may knock into other particles and cause them to skip as well. With slightly stronger winds, particles collide in mid-air, causing sheet flows. In a major dust storm, dunes may move tens of meters through such sheet flows. And like snow, sand avalanches, falling down the steep slopes of the dunes that face away from the winds, also moving the dunes forward.

Sand threatens buildings and crops in Africa, the Middle East and China. Drenching sand dunes with oil stops their migration, but this approach hurts the environment and uses a finite resource. Sand fences might also work, but researchers are still trying to figure out the best possible fence design. Preventing sand dunes from overwhelming cities and agricultural areas has become a priority for the United Nations Environment Programme.

Examples

The Kelso Dunes, in the Mojave desert of California.

(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
Southeastern Shore of Lake Michigan (Indiana Dunes)
Algodones Dunes near Brawley, California
Guadalupe-Nipomo Dunes, Central Coast California, see Dunes Center, Guadalupe, CA

World's Highest Dunes

World's Highest Altitude Dunes?

**Note: This table is based on estimates and incomplete information.**
Dune	Height from Base feet/meters	Height from Sea Level feet/meters	Location	Notes
Average Highest Area Dunes	1,526/465?	~6,500/~1,980?	Isaouane-n-Tifernine Sand Sea, Algerian Sahara	Highest in Africa
Big Daddy/Dune 7	1,256/383	?	Sossuvlei Dunes, Namib Desert, Namibia
Star Dune	>750/230	~8,950/2,730	Great Sand Dunes National Park, Colorado, USA	Highest in North America,
Dune of Pilat	~345/105	?	Bay of Arcachone, Aquitaine, France	Highest in Europe
Mount Tempest	~920/280	~920/280	Moreton Bay, Brisbane, Australia	Highest in Australia
Ming-Sha Dunes	?	5,660/1,725	Dunhuang Oasis, Taklamakan Desert, Gansu, China
Medanoso Dune	?	4,921/1,500 or 7,923/2,415?	Atacama Desert, Chile	Highest in South America?
Badain Jaran Dunes	~1640/500	~6640/2,020	Badain Jaran Desert, Alashan Plain, Inner Mongolia, Gobi Desert, China	World's Tallest Dunes?

Sand dune systems

(coastal dunes featuring succession)

References

The Physics of Blown Sand (1941) by Ralph Bagnold

External links

Magilligan Dunes, Northern Ireland
Dune pattern identification, U.S. Army
Treading Lightly: Minimum Impact Dune Hiking
Dune Racers of the Empty Quarter - video of sand dunes in the United Arab Emirates

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