Dune

[13] Subaqueous dunes can form from the action of water flow (fluvial processes) on sand or gravel beds of rivers, estuaries, and the sea-bed.

[22] A draa is a very large aeolian landform, with a length of several kilometers and a height of tens to hundreds of meters, and which may have superimposed dunes.

Dunes are made of sand-sized particles, and may consist of quartz, calcium carbonate, snow, gypsum, or other materials.

A group of dunes moved more than 100 metres per year between 1954 and 1959 in China's Ningxia Province, and similar speeds have been recorded in the Western Desert of Egypt.

In the southern third of the Arabian Peninsula, a vast erg, called the Rub' al Khali or Empty Quarter, contains seif dunes that stretch for almost 200 km (120 mi) and reach heights of over 300 m (980 ft).

These hills appear to have been formed during the last ice age under permafrost conditions dominated by sparse tundra vegetation.

In plan view, these are U-shaped or V-shaped mounds of well-sorted, very fine to medium sand with elongated arms that extend upwind behind the central part of the dune.

In inland deserts, parabolic dunes commonly originate and extend downwind from blowouts in sand sheets only partly anchored by vegetation.

They can also originate from beach sands and extend inland into vegetated areas in coastal zones and on shores of large lakes.

Most parabolic dunes do not reach heights higher than a few tens of metres except at their nose, where vegetation stops or slows the advance of accumulating sand.

Complex parabolic dunes include subsidiary superposed or coalesced forms, usually of barchanoid or linear shapes.

A type of extensive parabolic dune that lacks discernible slipfaces and has mostly coarse grained sand is known as a zibar.

[30] The dunes are small, have low relief, and can be found in many places across the planet from Wyoming (United States) to Saudi Arabia to Australia.

[40][41] Models of coastal dunes suggest that their final equilibrium height is related to the distance between the water line and where vegetation can grow.

Over the course of time coastal dunes may be impacted by tropical cyclones or other intense storm activity, dependent on their location.

[47][48] During a storm event, dunes play a significant role in minimizing wave energy as it moves onshore.

[49] To counteract the damage from tropical activity on coastal dunes, short term post-storm efforts can be made by individual agencies through fencing to help with sand accumulation.

[50] How much a dune erodes during any storm surge is related to its location on the coastal shoreline and the profile of the beach during a particular season.

Rotting seaweed, brought in by storm waves adds nutrients to allow pioneer species to colonize the dune.

In Europe these plants include: creeping willow, cotton grass, yellow iris, reeds, and rushes.

These include: Plants have evolved many adaptations to cope with these pressures: In deserts where large amounts of limestone mountains surround a closed basin, such as at White Sands National Park in south-central New Mexico, occasional storm runoff transports dissolved limestone and gypsum into a low-lying pan within the basin where the water evaporates, depositing the gypsum and forming crystals known as selenite.

The crystals left behind by this process are eroded by the wind and deposited as vast white dune fields that resemble snow-covered landscapes.

These types of dune are rare, and only form in closed arid basins that retain the highly soluble gypsum that would otherwise be washed into the sea.

Sub-aqueous (underwater) dunes form on a bed of sand or gravel under the actions of water flow.

Ripple marks preserved in sedimentary strata in the geological record can be used to determine the direction of current flow, and thus an indication of the source of the sediments.

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

Cross-bedded layers of stacks of lithified dunes can produce the cross-hatching patterns, such as those seen in Zion National Park in the western United States.

A slang term, used in the southwest US, for consolidated and hardened sand dunes is "slickrock", a name that was introduced by pioneers of the Old West because their steel-rimmed wagon wheels could not gain traction on the rock.

[63] Preventing sand dunes from overwhelming towns, villages, and agricultural areas has become a priority for the United Nations Environment Programme.

Due to widespread human population expansion, dunes face destruction through land development and recreational usages, as well as alteration to prevent the encroachment of sand onto inhabited areas.