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dunes, so promoting an overall increase in dune size and
spacing downwind (pattern coarsening).
At best, these studies span the last few decades. A
longer-term (centuries to millennia) perspective on dune
dynamics can now be gained through the application of
high-resolution optically stimulated luminescence (OSL)
dating of dunes, especially when used in combination
with ground penetrating radar (GPR) studies of dune sed-
imentary structures. Crescentic dunes superimposed on
the southern edge of a north-south-oriented linear dune
in the northern part of the Namib Sand Sea have migrated
to the west at an average rate of 0.12 m/yr over the past
1570 years (Bristow, Duller and Lancaster, 2005), while a
nearby 60 m-high linear dune shows evidence of episodic
development and lateral migration at rates of up to 0.13
m/yr to the east over the past 6000 years (Bristow, Duller
and Lancaster, 2007) and 20-m high crescentic dunes in
theLiwaareaoftheUAEhavemigratedtothesouthover
the past 320 years at an average rate of 0.78 m/yr, with
a more rapid migration of 0.91 m/year between 220 and
110 years ago (Stokes and Bray, 2005).
Following Tsoar (1989), vegetated linear dunes are
nucleated by vegetation, and extend downwind from
nebkhas. Vegetated linear dunes can also develop from
source bordering dunes; closely spaced dunes link by
Y-junctions
to
form
fewer,
larger
dunes
downwind
(Wasson, 1983).
The formation of sinuous linear dunes (seif dunes) by
elongation of one arm of a barchan as it migrates into a bi-
modal wind regime was first proposed by Bagnold (1941)
and supported by field observations in the Namib (Lan-
caster, 1980). Strong winds from an oblique direction add
sand to one horn and gentler winds from the original direc-
tion extend the horn (Figure 19.16(a)). A different model
is suggested by observations in the Sinai (Tsoar, 1984),
where winds from both the primary and secondary direc-
tions extend the horn on the side of the dune opposite to
the secondary wind direction (Figure 19.15(b)). The linear
element of the dune extends more rapidly than the original
barchan can migrate, and the dune evolves to a linear form.
Linear dunes develop by extension downwind and by
slow accretion of sand. Bristow, Bailey and Lancaster
(2000) present a new model for development of sinuous
linear dunes based on GPR visualisation of sedimentary
structures. In this model (Figure 19.17), secondary flows
and form-flow interactions become more important as the
dune increases in size, leading ultimately to the establish-
ment of superimposed bedforms on the dune flanks.
Star dunes appear to develop mainly by modification of
existing dunes (Lancaster, 1989b; Nielson and Kocurek,
1987), either as they migrate or extend into areas ex-
periencing multidirectional winds (Figure 19.18) or as a
result of changes in wind regime as a result of climate
change, as in star dunes developed on linear dunes in the
Gran Desierto (Beveridge et al. , 2006) and the southeast
UAE.
Many megadunes possess superimposed dunes, which
are in equilibrium with the current wind regime (e.g. on
Algodones and Namib compound crescentic dunes). The
slopes of these large dunes present an effectively planar
surface on which sand transport takes place. Therefore,
variations in sand transport rates on megadunes in time or
space will lead to the formation of superimposed dunes if
the major dune is sufficiently large (Andreotti et al. , 2009;
Elbelrhiti, Claudin and Andreotti, 2005).
In some areas, however, the larger primary (mostly
linear) dunes are clearly a product of past wind regime
and sediment supply conditions, as demonstrated by OSL
dating of the major form (Lancaster, 2007). Such dunes
have considerable inertia (represented by the reconstitu-
tion time, or the time required for the dune to migrate its
length in the direction of net transport) and require sig-
19.6
Dune development
The origins and growth of individual dunes and the de-
velopment of dune patterns are intimately linked, because
dunes rarely occur in isolation.
Dune initiation is a poorly understood process, but is
likely to involve localised reductions in sand transport
rates by convergence of streamlines, changes in surface
roughness (e.g. vegetation cover), surface particle size or
by variations in microtopography (slope changes, relict
bedforms), leading to deposition of sand, which forms a
nucleation point for a protodune (Kocurek et al. , 1992;
Lancaster, 1996). There is a minimum size for a proto-
dune, determined by the saturation length or the distance
required for actual sand mass flux to reach the capacity
of the wind to transport sand. The elementary dune wave-
length is
600 m on Mars (Claudin
and Andreotti, 2006). Once initiated, growth of crescentic
dunes occurs by merging of smaller, faster-moving dunes
with larger, slower-moving dunes, in a series of construc-
tive interactions.
The primary response of sand surfaces to bedform de-
velopment is to form a asymmetric flow-transverse dune
with a convex stoss slope. Crescentic dunes dominate in
unidirectional wind regimes; small dunes in multidirec-
tional wind regimes are also of this form, because they
are small enough to be completely reworked in each wind
20 m on Earth and
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