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crests and braided patterns from vegetated linear dunes.
Recent work (Tsoar, 1989) has suggested that there is a
distinct class of linear dunes that are partially vegetated.
Their staight crests and orientation parallel to the dom-
inant wind result from the anchoring of dunes by veg-
etation, as well as the deposition of sand downwind of
vegetation.
Namib
Sand Sea
Star
Crescentic
Linear
Fryberger
(1979)
50
Star
Crescentic
40
19.7.5
Controls of dune size and spacing
30
Dunes exhibit a wide range of sizes, with close relation-
ships between dune height and crest-to-crest spacing, indi-
cating that the patterns are self-organised (Figure 19.20).
In many sand seas, there is also a spatial pattern of dune
size and spacing, such that small, closely spaced dunes
occur on the margins, with large dunes in areas of net
sand accumulation.
The controls on dune size and spacing have been de-
bated for many years, but recent work has clarified some
key issues. For example, the hypothesis of Wilson (1972)
that sand particle size controls dune spacing is not sup-
ported by empirical data (Wasson and Hyde, 1983a).
Likewise, there is no simple relationship between dune
size and sand transport rates (Elbelrhiti, Andreotti and
Claudin, 2008; Lancaster, 1988). In the Namib Sand Sea,
small dunes occur in areas of high sand transport rates
(as indicated by wind data), whereas large dunes occur
in areas of low net sand transport, as a result of their
growth in conditions of abundant sand supply and a wind
regime that promotes vertical accretion, rather than mi-
gration or extension. The size of superimposed dunes on
larger forms, however, scales with the local sand transport
rate (Havholm and Kocurek, 1988; Lancaster, 1988). The
minimum size of such simple dunes is a function of the
sand transport saturation length (Claudin and Andreotti,
2006), which on Earth is around 20 m.
Studies of dune patterns and numerical models indicate
that the spacing of crescentic dunes is a dynamic property
of the pattern (Elbelrhiti, Andreotti and Claudin, 2008).
The developed pattern results from merging and linking of
dunes with a highly variable spacing, size and migration
rate into fewer dunes with a much more regular pattern of
size and spacing (Figure 19.21), in a similar way to the
development of patterns in wind ripples and subaqueous
environments. Such 'pattern coarsening' leads to a more
ordered and stable pattern with fewer interactions between
dunes. Similar downwind pattern development has been
documented from crescentic dunes at White Sands (Ew-
ing and Kocurek, 2010b), as well as linear dunes in the
Simpson Desert (Fitzsimmons, 2007).
20
10
Linear
Barchan
0
0
0.1
0.2
Directional variability of wind regime (RDP / DP)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Figure 19.19
Relations between dune type, wind regime
variability (RDP/DP ratio) and sand thickness (RDP/DP data
from Fryberger, 1979, Wasson and Hyde, 1983a, and Lancaster,
1989).
with the dune type being influenced by the other factors,
especially the wind regime (Rubin, 1984). In the Namib
Sand Sea, dune types clearly occupy separate wind regime
domains and the EST data suggest that there is more sand
in complex linear and star dunes than in compound linear
and all types of crescentic dunes.
19.7.4
Vegetation
Vegetation affects the rate of sand transport via direct
protection of the surface, absorption of momentum and
partitioning of shear stress between the surface and plants
(Wolfe and Nickling, 1993). As a result, sand transport
rates decrease exponentially with vegetation cover and
sand surfaces are effectively stabilised when it exceeds
about 15 % (Lancaster and Baas, 1998; Wiggs
et al.
,
1994).
The effects of vegetation on dune morphology are less
well known. Hack (1941) suggested that in northeastern
Arizona crescentic dunes changed to parabolic dunes with
increased vegetation cover and that linear dunes occurred
in areas with less sand and lower vegetation cover than
parabolic dunes. In the Negev Desert, Tsoar and Møller
(1986) documented changes in the morphology of linear
dunes as vegetation cover decreased as a result of graz-
ing, including development of sharp sinuous 'seif' dune
