Geoscience Reference
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Horn
(h)
(g)
Walvis Bay
Avalanche face
Stoss
Lee
Brink
Crest
Avalanche face
To e
RDP/DP = 0.73
Figure 19.3
(
Continued
)
compared to 0.09 m/yr to 0.35-0.40 m/yr for the main
dunes (Sharp, 1979).
Where sand supply increases, crescentic ridges or trans-
verse dunes occur (Figure 19.3). Worldwide, such dunes
occupy about 10 % of the area of sand seas. Good ex-
amples of simple crescentic ridges can be found at White
Sands, New Mexico (Kocurek
et al.
, 2007; McKee and
Douglass, 1971) and in the Skeleton Coast dune field of
Namibia (Lancaster, 1982a). At White Sands, the dunes
are typically sinuous crescentic ridges and barchanoid
dunes, 8-12 m high, with a crest length of about 250 m
and an average spacing of 136 m. These dunes are migrat-
ing towards the ENE at 1-7 m/yr. The Skeleton coast dune
field is characterised by straight to slightly sinuous cres-
centic dunes with a spacing between 150 and 300 m and
a height ranging between 4 and 30 m, migrating towards
the north or northeast. Dune size increases northwards
(downwind) and the eastern side of the dune field is char-
acterised by smaller crescentic dunes that are changed to
barchans as the sand supply decreases.
Compound or crescentic megadunes are characterised
by the superimposition of smaller crescentic dunes on
their stoss (windward) slopes (Figure 19.4(d) and (e)).
These smaller dunes migrate across the main form at an
order of magnitude greater than that of the main dune.
Classic examples of crescentic megadunes can be found
in the Algodones dune field of southeast California (De-
rickson
et al.
, 2008; Havholm and Kocurek, 1988), the
Liwa area of the UAE (El-Sayed, 2000; Stokes and Bray,
2005) and along the coastal areas of the Namib Sand Sea
(Lancaster, 1989c). In the Algodones, the main dunes are
40-60 m high with a mean spacing of 1025 m, whereas
the superimposed coalesced crescentic dunes are 12-20
m high with a mean spacing of 93 m (Derickson
et al.
,
19.3.2
Linear dunes
Dunes of linear form (Figure 19.4 and Figure 19.5) are
widespread in deserts worldwide and dominate in Aus-
tralia and the Kalahari, as well as in Namibia, Arabia
and the southern and western Sahara. Linear dunes oc-
cur in simple, compound and complex varieties (the latter
two at megadune scales). There are two main varieties of
the simple linear dune: (1) sinuous, relatively short dunes
(also known as seif dunes) and (2) longer, straight, vege-
tated linear dunes such as those that occur in the Simpson
Desert of Australia and the Kalahari.
Sinuous simple linear dunes (seif dunes) have been doc-
umented from many desert regions (Figure 19.4). They
typically consist of a sinuous ridge with alternate peaks
and saddles and a more or less triangular cross-section, the
sinuosity increasing with dune height. Such dunes are typ-
ically a few hundred metres long. In many areas, it appears
that dunes of this type are very young, comprising dunes
that have developed in modern wind regimes as a modifi-
cation of older patterns, e.g. in the Azefal Sand Sea, Mau-
ritania (Lancaster
et al.
, 2002), the Wahiba sands (Warren
and Allison, 1998) or representing the extension of larger
linear dunes (Bristow, Bailey and Lancaster, 2000).
Vegetated linear dunes in the southwest Kalahari and
the Simpson-Strzelecki dune fields (Figure 19.4(c) and
(d)) are very similar in form and provide good exam-
ples of this dune type. These dunes typically comprise
