This article is about the sand formations, for other meanings see Dune (disambiguation)
In physical geography, a dune is a hill of sand built by eolian (wind-related) 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.
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.
Types of dune
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.
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.
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.
Oval or circular mounds that generally lack a slipface, 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, 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.
Complex dune: Dune 7 in Namib desert, one of the tallest in the world.
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.
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 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, root nodules that 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 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.
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.
Longitudinal and Transverse Dunes
Longitudinal dunes elongate parallel to the prevailant wind, possibly caused by a larger dune having its smaller sides blown away (feel free to edit this because I'm not sure what causes them). A transverse dune is horizontal to the prevailing wind, probably caused by a steady buildup of sand on an already existing minuscule mound.
References & Links
Sand dune plains
- (large expanses of dunes)
Sand dune systems
- (coastal dunes featuring succession)