Dunes occur in different shapes and sizes, formed by interaction with the flow of air or water. Most kinds the physics of blown sand and desert dunes pdf dunes are longer on the windward side where the sand is pushed up the dune and have a shorter “slip face” in the lee of the wind. A “dune field” is an area covered by extensive sand dunes.
Dominant wind direction would be from left to right. Crescent-shaped mounds are generally wider than they are long. The slipfaces are on the concave sides of the dunes. They may be composed of clay, silt, sand, or gypsum, eroded from the basin floor or shore, transported up the concave side of the dune, and deposited on the convex side. Examples in Australia are up to 6. 5 km long, 1 km wide, and up to 50 metres high. They also occur in southern and West Africa, and in parts of the western United States, especially Texas.
The distance between each dune is 1. Straight or slightly sinuous sand ridges typically much longer than they are wide are known as linear dunes. 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.
China, the star dunes are up to 500 metres 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. Schematic of coastal parabolic dunes. U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes.
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These dunes are formed from blowout dunes where the erosion of vegetated sand leads to a U-shaped depression. Unlike crescent shaped dunes, their crests point upwind. The bulk of the sand in the dune migrates forward. 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. There are slipfaces that often occur on the outer side of the nose and on the outer slopes of the arms. These dunes often occur in semiarid areas where the precipitation is retained in the lower parts of the dune and underlying soils.
The stability of the dunes was once attributed to the vegetative cover but recent research has pointed to water as the main source of parabolic dune stability. The vegetation that covers them—grasses, shrubs, and trees—help anchor the trailing arms. 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. Simple parabolic dunes have only one set of arms that trail upwind, behind the leading nose. Compound parabolic dunes are coalesced features with several sets of trailing arms. Complex parabolic dunes include subsidiary superposed or coalesced forms, usually of barchanoid or linear shapes. Parabolic dunes, like crescent dunes, occur in areas where very strong winds are mostly unidirectional. Although these dunes are found in areas now characterized by variable wind speeds, the effective winds associated with the growth and migration of both the parabolic and crescent dunes probably are the most consistent in wind direction.
The grain size for these well-sorted, very fine to medium sands is about 0. Parabolic dunes have loose sand and steep slopes only on their outer flanks. The inner slopes are mostly well packed and anchored by vegetation, as are the corridors between individual dunes. Because all dune arms are oriented in the same direction, and, the inter-dune corridors are generally swept clear of loose sand, the corridors can usually be traversed in between the trailing arms of the dune.