desert dunes

desert dunes Deserts are often imagined to be blanketed in sand dunes, but although almost half of the Australian deserts are sand covered, globally only about 20 per cent of drylands actually reflect the classical image of blowing sand sheets and dunes. Indeed, only 1 per cent of desert areas in the Americas are composed of sand deposits. Despite this, where sand deposits do occur they commonly exist as extensive sand seas (or ergs) and so form an impressive large-scale landscape. The nature of these sand seas is highly variable. Many (about 60 per cent) are dune-covered, the type and form of dunes being controlled by variations in sand supply and wind direction. Others may be dune-free and consist of low-profile sand sheets often with some vegetative cover. The chief areas of mobile and stabilized dunes are shown in Fig. 1.

Dune types

Dune sand is generally derived from sources where it has been concentrated by marine or fluvial action (e.g. coastal beaches and dry river valleys) and then transported by desert winds to regions of aeolian accumulation. These winds are not necessarily stronger than winds in humid regions but are more effective in transporting sand because of the lack of surface vegetation in drylands. Sand-sized material (0.04–1.0 mm) also has a low shear velocity threshold and is preferentially transported when compared to larger or smaller grain sizes. Unvegetated (or free) dunes can be classified in relation to their morphology and formative wind characteristics.

Barchan dunes are characterized by their crescentric plan form, with the dune moving in the directions of the horns, as shown in Fig. 2. They are found in regions of limited sand supply and unidirectional winds, resulting in relatively small and highly mobile stores of sand migrating in a downwind direction. Sizes are highly variable, ranging from 1 m in height and 5 m in width to over 30 m high and 300 m wide. Their speed of movement is related to their size; smaller dunes can move at rates of over 40 m a year. Barchan dunes are found within aeolian sand transport pathways and are common in dryland coastal regions such as the Gulf States, North Africa, Namibia, Peru, and California. Transverse (or barchanoid) ridges have a higher sand supply than barchans but also exist in unidirectional winds. In morphology they resemble a series of barchan dunes in which the horns have coalesced into a ridge. Transverse ridges are a very common dune form and can be found throughout the North African, Middle Eastern, and Chinese sand seas.

Linear dunes form in regions with bidirectional wind regimes and relatively limited sand supply. They take the form of linear ridges, between 20 m and 200 m high and over 1 km apart, with steep flanks and a sharp crest. Dunes such as these tend to be accumulating forms, trapping windblown sand from two wind directions and slowly accumulating and extending linearly in the resultant wind direction. They are common in the Namib sand sea. Vegetated varieties occur in Australia and the Kalahari. With complex wind regimes and high sand supply, star dunes may form, with a number of ‘arms’ of sand radiating from a central peak. Each ‘arm’ corresponds to a different wind direction. These are accumulating forms with no lateral movement. Star dunes can reach heights of 300–400 m. They are common in the central Saharan sand seas and also in Namibia, China, and Iran.

The classification given above can be complicated by such factors as vegetation and topography. Examples are provided by parabolic dunes and echo dunes. Parabolic dunes are similar in plan-form to barchans but have lower relief, with the horns fixed in position by vegetation. The central part of the dune moves freely downwind and the horns face upwind (in contrast to barchans). Echo dunes accumulate in the lee of topographic barriers where the lee-side airflow may oscillate above the surface, providing regions of low wind velocity where sand masses may accumulate. Owing to their semistabilized nature, parabolic, echo, and vegetated linear dunes may have been fixed in position for thousands of years, providing a useful sedimentary sequence for environmental reconstruction.

Dune initiation and dynamics

The processes by which sand dunes are formed are still little understood. It is clear that a reduction in the shear velocity (u*) of the wind is required so that grains fall out of the transporting wind and are deposited at the surface. Obstacles ranging from large hills to small shrubs may initiate deposition in their lee, and small hollows in the surface may encourage flow expansion and subsequent deposition. Reduction in shear velocity may also occur because of subtle reductions in the aerodynamic roughness of the desert surface caused by changes in surface grain size. Once a patch of sand has been formed it may grow into a dune by trapping more sand and developing a slip face. As the wind progresses up the windward slope of the dune form, streamlines are compressed by the dune body, causing an acceleration of flow towards the crest, as shown in Fig. 3. This accelerated wind erodes sand from the windward slope and deposits it beyond the dune brink. The lee slope is characterized by low wind velocities in a reverse-flow vortex where sand is deposited on the slip face, which lies at an angle of 32–4°, the natural angle of repose of sand. With erosion on the windward slope and deposition in the lee, the dune body moves in a downwind direction. As the dune develops, an equilibrium is achieved between the angle of the windward slope, the height of the dune, the degree of airflow acceleration, and hence the rates of erosion and deposition on the windward and lee slopes.

Desert dune management and control

Active dunes may pose a hazard to human activity where the blowing sand comes into contact with communication lines, buildings installations, or oases. In semi-arid areas, dunes which are stabilized by vegetation (with more than 10–15 per cent vegetation cover) may also cause problems at a local scale where they become re-activated because of the removal of vegetation owing to farming pressures, drought, or fire. In these cases careful management and control of dunes is required. In the semi-arid context, dunes can revegetate quite naturally and quickly once the initial disturbance has been removed. Grazing management in these instances is quite simple, although the effects of a longer-term change in climate may be more serious. Actively mobile dunes in arid areas can be more problematic, especially in the Middle East where infrastructure for the oil industry is commonly placed within or across actively moving dunefields. In these situations the dunes may be controlled with the use of fences or they may be removed with a long-term and work-intensive programme. The simplest and most effective solution, however, is to use careful planning and avoid hazardous areas completely.

Giles F. S. Wiggs

Bibliography

Cooke, R. U.,, Warren, A.,, and and Goudie, A. S. (1993) Desert geomorphology. UCL Press, London
Lancaster, N. (1995) Geomorphology of desert dunes. Routledge, New York.
Thomas. D. S. G. (ed.) (1997) Arid zone geomorphology: process, form and change in drylands (2nd edn). John Wiley and Sons, Chichester.