Dunes are an accumulation of sand grains that take the shape of a ridge or sometimes a mound due to the wind subjected to the influence of gravity. Dunes are located in areas like abandoned or eroded farms in a semi-arid region or on beaches, deserts, or wherever loose sand is windblown. Sand dunes can be of various sizes or shapes, such as crescents, stars, or even repeated lines. Some dunes can go up to 4000 feet or 1200 metres high, above the waves' usual maximum reach. Sahara is an example of a dune dessert.
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Types of Sand Dunes
There are various types of dunes created or formed across the globe. However, the various types of dunes result from the different wind patterns and the presence or lack of vegetation.
Star dunes are exciting and rare types of sand dune structure that pepper landscapes in sandy deserts. They are formed when multiple and alternating winds from varied directions pile sand in a particular location leading to a dune peak, with many arms protruding from the centre. The slip face of star dunes keeps shifting focus depending on the flow of wind. They form in large groups in a dune field or a star dune. Places that exhibit star dunes are the Grand Erg Oriental of the Sahara desert, Gran Desierto de Altar in Mexico, Badain Jaran Desert in China, and more.
Parabolic dunes, also known as U-shaped or hairpin dunes or coastal sand dunes, are formed at locations where vegetation covers the sand. Parabolic dunes are created when strong winds erode a section, pushing the sediment leeward as the vegetation will hold back the dune's arms, leading to its leeward direction. Coastal sand dunes are the most common example of parabolic dunes.
Barchan dunes are commonly known as Crescentic dunes and are formed like a crescent or half-moon shaped. Barchan dunes are formed where conditions are ideal and require a flat landscape with limited sand, and the wind blows in only one direction. Barchan dunes are commonly found in all types of deserts as these dunes point against the wind. Barchan dunes have steep faces, but the trailing is not and often joins other dunes to form barchanoid ridges.
Longitudinal dunes are also known as linear dunes. These dunes are giant and present a parallel needle-esque feature on the landscape. Unlike the typical dunes, the longitudinal dunes are long and straight. A linear dune forms when sand is dearth or when not in excess and when the wind blows in a particular direction. When subjected to an extended period, longitudinal dunes will migrate in the direction the wind is propelling.
When Barchan dunes become aligned together along a plane perpendicular to the wind where the lines become straight with ridges marching forward, the dune is known as a transverse dune. Transverse dunes progress forward as their leeward side sand and an avalanche at a time. Transverse dunes are usually found in sediment-rich hyper-arid areas like the Sahara Desert.
Nebkha or Coppice Dunes
Nebkha dunes are also addressed as coppice dunes. These are simple dunes that form around vegetation, primarily on the sand sheet. The vegetation around like- the clumps of shrubs and grass begin to gather the windblown sand; as the sand gets more profound, the plants also grow taller, allowing more sand to gather around them. The Death Valley National Park, California, is the most common example of Coppice dunes.
Growth and Formation of Dunes
A dune formation process is complex, mainly where multiple dunes are grown side-by-side in a sand region or dune desert. Most dunes are a result of winds that move as a mass of jumping or saltating grains. The coaster particles move towards the surface slowly and remain in motion partly by shelling the saltating grains. Smoother or modest hollow patches reduce the sand amount carried by the wind and lead to the initiation of sand patches.
The winds adjust the velocity gradient on reaching the sand patch, and this adjustment takes place over several metres, resulting in a build-up. The growth of a dune cannot continue indefinitely, and the windward slope gets eventually adjusted. This leads to an increase in the near-surface velocity to compensate for the drag imposed by the surface.
As a dune grows, the smooth leeward slope steepens until the wind is restricted to deflect downwards. The wind then separates from the surface, leaving a 'dead zone'. Here, the sand built up by the windward slope falls. When the slope gets steepened to an angle of repose - 32 degrees, this angle maintains, and the added sand slips down the slope.