Page 321 - Fundamentals of Geomorphology

304 PROCESS AND FORM

Plate 12.4 Mega-ripples formed on a hard sebkha surface in the United Arab Emirates.
(Photograph by Dave Thomas)

Free dunes
of anchored dunes (Livingstone and Warren 1996, 75)
Dunes are collections of loose sand built piecemeal by (Table 12.3).
the wind (Figure 12.5). They usually range from a few Free dunes may be classed according to orientation
metres across and a few centimetres high to 2 km across (transverse) or form (linear, star, and sheet) (Figure 12.6).
and 400 m high.Typical sizes are 5–30 m high and spaced All types of transverse dune cover about 40 per cent
at 50–500-m intervals. The largest dunes are called draa of active and stabilized sand seas. The transverse vari-
or mega-dunes and may stand 400 m high and sit more ety (Table 12.3) is produced by unidirectional winds
than 500 m apart, with some displaying a spacing of up and forms asymmetric ridges that look like a series of
to 4 km. barchan dunes whose horns are joined, with their slip
Dunes may occur singly or in dune fields. They may faces all facing roughly in the same direction. Barchans
be active or else fixed by vegetation. And they may be are isolated forms that are some 0.5–100 m high and
free dunes or dunes anchored in the lee of an obstacle 30–300 m wide (Plate 12.5). They rest on firm desert
(impeded dunes). The form of free dunes is deter- surfaces, such as stone pavements, and move in the direc-
mined largely by wind characteristics, while the form tion of the horns, sometimes as much as 40 m/yr. They
of anchored dunes is strongly influenced by vegetation, form under conditions of limited sand supply and unidi-
topography, or highly local sediment sources. Classi- rectional winds. Other transverse dune types are domes
fications of dune forms are many and varied, with and reversing dunes. Domes lack slip faces but have an
local names often being used to describe the same orientation and pattern of sand transport allied to trans-
forms. A recent classification is based upon dune for- verse dunes. Reversing dunes, which have slip faces on
mation and identifies two primary forms – free and opposite sides of the crest that form in response to wind
anchored – with secondary forms being established coming from two opposing directions, are included in
according to morphology or orientation, in the case of the transverse class because net sand transport runs at
free dunes, and vegetation and topography, in the case right-angles to the crest.