Description
These dunes are arc-shaped individual mounds or arc-shaped segments of sand ridges consisting of loose, well-sorted, very fine to medium sand. The upwind slope, which is usually less than 15°, is wind packed, and firm; the slip face, or lee slope, is composed of unstable, loose sand at its customary angle of repose of about 32°. Two arms, also called horns, extend from the main body of the dune mound or from each segment of a dune ridge; they point downwind. Sizes of individual simple barchans range from a meter or so to perhaps a hundred meters from horn to horn.
Compound barchans are large basal mounds with a single proportionately large slip face and an upwind slope covered with many smaller barchans or barchanoid ridges with proportionately smaller slip faces, all oriented in the same direction as the main dune. Such dunes have a "two-story" aspect and commonly grow to sizes of 1 or 2 km from horn to horn, with heights of 30 m or more. These large compound dunes are often called "megabarchans." Individual barchans or megabarchans commonly occur in elongate chains or trains that merge with coalesced dunes in fields or ergs. Barchans and megabarchans are highly migratory: small barchans typically move several meters to tens of meters per year, at speeds inversely proportional to their size. Megabarchans move more slowly and commonly "shed" smaller barchans off their horns, as at Pur-Pur Dune north of Trujillo in coastal Peru (Simons [1]).
Barchans and megabarchans frequently occur in coalesced form as highly curved segments in continuous dune ridges more or less perpendicular to the wind direction. Although this coalescence tends to obliterate the pattern of the arms, the main characteristics of barchans--arcuate slip faces and more gentle upwind slopes persist. Because these characteristics have been retained, such dunes are called barchanoid or megabarchanoid ridges. These wavy, barchanoid forms contrast with the straight or slightly curved segments of transverse ridges. Like transverse dunes, they typically occur as repeated, parallel ridges that can extend for hundreds of kilometers.
Of all dune forms, barchans and megabarchans have the best understood, least ambiguous relations to the directions of the winds that form them. The slip faces on these dunes are maintained by virtually unidirectional winds, and the arms (horns) of the dunes point downwind, unlike parabolic dunes, whose arms trail behind and point upwind. The presence of barchans, with their typically crisp, fresh outlines, indicates that strong, sand-moving winds blow frequently from one quarter. Where occasional or seasonal winds blow from an opposite direction, barchans and megabarchans can develop smaller, secondary slip faces oriented in a reverse direction from the main slip faces.
The grain size of these loose, well-sorted, very fine to medium sands is about 0.06 to 0.5 mm. In fields where barchans or megabarchans are isolated on a bare desert floor (bedrock or sand sheet), movement is generally easy, both in a down-field and cross-field direction. Movement becomes much more difficult where dunes are coalesced into a network pattern and interdunal spaces are enclosed. The best route from one barchanoid or megabarchanoid ridge to the next is along the horns that commonly extend downwind from one ridge to the next, thus avoiding the interdunal basins. The surfaces of the gentler slopes on the upwind sides of these dunes are wind-packed sand and are trafficable. The surfaces of the steep lee slopes are loose sand that will avalanche easily. Slip faces higher than a meter or two should be avoided. Descents straight down such short slip faces are possible, but they should begin very slowly from the dune brink to avoid the separation of the vehicle from the dune surface, with consequent crash-landing. This type of descent is not feasible for large or heavy vehicles, because sand avalanches will result that can cause overturning.
Depending on the size of the dune and wind conditions, the floor inside the cusp, near the edge of the slip face, can be a good place to camp or effect repairs. If the wind picks up to above 20 kph some of the fine particles can settle out into the lee area. It also provides concealment from downwind travelers, because they must look back into the cusp to see the area. Trenching in these dunes is not generally practical unless they happen to be wet.
(common names are in bold) Crescent dunes, sand hills, barkans, demkhas, giant crescent, bourrelets, draas (for the large forms), megadunes.
1. Simons, F.S. 1956. A note on Pur-Pur Dune, Viru Valley, Peru. Journal of Geology, v. 64, pp. 517-521.