Dunes are accumulations of loose, well sorted, windblown sand grains (mostly very fine to medium) in wave-like mounds or ridges whose characteristic shapes are maintained by periods of wind-induced, grain-by-grain movement.
Dunes occur wherever topographic and climatic conditions permit the deposition of sandsized, wind-borne material; they are usually found in arid regions where wind erosion is less inhibited by vegetation. Dunes tend to migrate from sources of sand, such as riverbeds, beaches, and playas, to sites of accumulation that are topographically controlled, such as basins and valleys. Individual mounds or ridges are meters to hundreds of meters high. They have one or more gentle upwind slopes of about 10° to 20°, one or more steep lee slopes of about 32° (the angle of repose of loose, dry sand), and may have extending arms of various shapes and sizes (see photo illustration). The upwind surfaces are usually hard packed and smooth, locally cut by minor flutes, grooves, and hollows (blowouts). The lee slopes are soft and unstable, and are called slip faces, or avalanche faces.
Dunes can occur as solitary features, but are more common in groups called fields. Regionally extensive fields are called sand seas or ergs. Any given dune field, or erg, usually consists of individuals of similar shapes repeated across the center of a field; variations in shape are common along the margins. Within a field or erg, individual dunes are separated by areas of desert floor (interdune areas) whose shape and extent are related to the dune type. In areas of occasional rainfall, the base of a dune may store enough moisture to nourish grasses in the interdunal flat immediately adjacent, and seepage of moisture may occur around the dune perimeter.
The types of dunes listed below are classified on the basis of the external shape of the dune and the arrangement of its slip faces, if any, relative to its shape in plan view. These characteristics are primarily functions of wind direction and sand supply, although local topography and presence of vegetation are also important factors.
All have variant forms called simple, compound, and complex:
The physiographic variations of dunes result from the interaction of winds of various strengths and directions with collections of various particles whose sizes allow them to be sorted and moved by wind. Consequently, these sand patterns can tell much about the characteristics of both historical and present winds. Details, where known, are given in the appropriate dune sections. The most reliable clue to wind direction is that the slip face is always on the most recent lee (downwind) side of the dune. Horns or arms that extend from the dune can point either upwind or downwind. They normally point downwind unless anchored by vegetation, as in the case of parabolic dunes.
Sand grains of all dune forms described here (except those of lunettes) are very fine to medium, about 0.06 to 0.5 mm. On dunes with asymmetric slopes, the gentler upwind slope is wind compacted and can usually support foot and light vehicular traffic. The steep lee slope, or slip face, will not support either foot or vehicular traffic without avalanching. Although skillful and experienced drivers can take some kinds of vehicles (dunebuggy, light truck, etc.) down lesser lee slopes, it can be a dangerous procedure and should be avoided. On dunes of any size, these vehicles must travel straight down the 32° slip face to its abrupt junction with the interdune desert floor. A better route for descent can usually be found down the flank of the dune. The interdunal floor is usually relatively flat and compacted, and easy to travel across unless occupied by seasonally wet playas or smaller dunes. The floor may contain patches of loose sand, which should be avoided, and it may be partly or fully enclosed by dunes so that upslope escape is impossible.
In general, fixed-wing aircraft operations are not appropriate to dune areas because of the uneven topography and blowing sand, but rotary-wing aircraft can operate on the interdune floors in many areas.
None.
McKee, E.D. (ed.). 1979. A study of global sand seas. U.S. Geological Survey Professional Paper 1052, 429 pp.
Pye, K. and H. Tsoar. 1990. Aeolian sand. Cambridge, MA: Unwin Hyman, 396 pp.
Description
