Dunes

Dunes are depositional landforms, comprising accumulations of sediment (sand, silt, or clay), typically deposited by wind (eolian) action. They may be defined according to the type of sediment they comprise (sand dunes, clay dunes), their morphology (linear dunes, barchan dunes, star dunes, parabolic dunes, transverse dunes), their position relative to other geomorphic features in the landscape (barrier dunes, lunette dunes, headland bypass dunes, climbing-falling dunes), the time of their formation (contemporary dunes, paleo- or relict dunes), or even according to their genesis (natural dunes, artificial dunes).

Dunes vary in extent and, typically, reach heights of between 5 m (meters) and 30 m. Megadunes reach 100 m or, in extreme cases, 400 m or more in height. Dunes occur typically, but not exclusively, in hot deserts and coastal environments. The sediment comprising a dune is usually quartz sand, with a carbonate component in coastal environments and a gypsum component in certain arid environments. The formation of sand dunes requires a plentiful supply of sand and a consistent wind direction. An obstacle, commonly vegetation, may retard the flow of air near the surface, initiating deposition. Once a [Page 801]dune is established, it will continue to grow, provided that the upwind sand supply is maintained. Its ultimate height and form are a product of factors described below.

Figure 1 Dune cross-section. Note successive slip faces, the product of sand migration up and over the windward slope.

Source: Redrawn from Christopherson, R. W. (2009). Geosystems: An introduction to physical geography (7th ed.). Upper Saddle River, NJ: Pearson Education. Copyright © 2009 Pearson Education. Reprinted with permission of Pearson Education, Inc., Upper Saddle River, NJ.

Dunes may have a complex internal structure, particularly where the wind direction is highly variable. Dune morphology (shape or form), internal structure (bedding), and composition (the nature of the sediment comprising the dune, and its inherent physical and geochemical properties) reveal evidence regarding the formation and growth of the dune and regarding the source of its sediment. Ground-penetrating radar may reveal the internal structure of the dune. Radiometric dating techniques such as optically stimulated luminescence (OSL) facilitate the determination of the time when the dune sediment was last exposed to sunlight. Effectively, this is the last time the dune was active.

The morphology of dunes underpins the most common means of classification. The amount of sand available, variability of wind direction, wind strength, presence or absence of vegetation cover, and nature of the terrain interact to determine the type of dune that will form. Much of our understanding of dune formation and dune-building processes is derived from the classic work of R. A. Bagnold in the 1940s. When wind blows over a surface, frictional resistance between moving air and the surface results in a complex pattern of turbulence, vortices, and eddies. Dune building is the result of an intricate and dynamic interplay between wind, sediment, and obstacles or irregularities on the surface.

Barchans are the most common dune type, and so their formation is described here as an example of dune processes. Barchans form where there is a limited sand supply and sparse vegetation cover. Barchans are asymmetrical, with a gently sloping (∼ 5° to ∼10°) windward (stoss) face and a steeper (∼30° to 35°, which is the angle of repose of dry sand), concave lee or slip face. The horns of these crescent-shaped dunes point downwind. Under unidirectional wind conditions, sand grains advance up the windward face of the dune through a process of short hops (saltation). As the top of the lee face oversteepens through the accumulation of more and more sand, it eventually fails and sand moves down the face till the angle of repose is reestablished. The process repeats itself as long as there is a supply of windblown sand off the windward face, onto the top [Page 802]of the slip face. The result is twofold; clearly defined foreset beds occur on the slip face, and the dune advances downwind. Barchans may migrate downwind over long distances. Figure 1 shows a dune cross-section.

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