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can induce aerodynamic effects that encourage deposi-
tion. Deposition may produce a sand patch. Once a sand
patch is established, it may grow into a dune by trapping
saltating grains, which are unable to rebound on impact
as easily as they are on the surrounding stony surface.
This mechanism works only if the sand body is broader
than the flight lengths of saltating grains. A critical lower
width of 1-5 m seems to represent the limiting size for
dunes. On the leeside of the dune, airflow separates and
decelerates. This change enhances sand accumulation
and reduces sand erosion, so the dune increases in size.
The grains tend to be trapped on the slip face, a process
aided by wind compression and consequent acceleration
over the windward slope. The accelerated airflow erodes
the windward slope and deposits the sand on the lee slope.
As the sand patch grows it becomes a dune. Eventually,
a balance is reached between the angle of the windward
slope, the dune height, the level of airflow acceleration,
and so the amount of erosion and deposition on the
windward and lee slopes. The dune may move downwind
(Figure 12.3).
Figure 12.4 is a speculative model of the conditions
conducive to the formation of different dune types,
which are discussed below (Livingstone and Warren
1996, 80). The two axes represent the two main factors
controlling dune type. The first represents an unspecified
measure of the amount of sand available for dune forma-
tion, while the second axis represents the variability of
wind direction.
Transverse
dunes
Stars
Linear
dunes
Networks
Barchans
Unimodal
Complex
Wind direction variability
Figure 12.4 Dune types in relation to the variability of
wind direction and sand supply.
Source: Adapted from Livingstone and Warren (1996, 80)
complex dunes; and (3) compound and complex dunes
or draa.
Ripples
Wind ripples are the smallest aeolian bedform. They
are regular, wave-like undulations lying at right-angles to
the prevailing wind direction. The size of ripples increases
with increasing particle size, but they typically range from
about 10 to 300 mm high and are typically spaced a few
centimetres to tens of metres apart (Colour Plate 15,
inserted between pages 208 and 209; Plate 12.4). Wind
ripples develop in minutes to hours and quickly change
if wind direction or wind speed alters.
Seemingly simple aeolian bedforms, ripples have with-
stood attempts to explain them. Several hypotheses have
been forthcoming, but most are flawed (see Livingstone
and Warren 1996, 27). According to what is perhaps the
most plausible model (Anderson 1987; Anderson and
Bunas 1993), ripple initiation requires an irregularity in
the bed that perturbs the population of reptating grains.
By simulating the process, repeated ripples occurred after
about 5,000 saltation impacts with a realistic wavelength
of about six mean reptation wavelengths. In a later ver-
sion of the model (Anderson and Bunas 1993), two grain
sizes were included. Again, ripples developed and these
bore coarser particles at their crests, as is ordinarily the
case in actual ripples.
Dune types
Some researchers believe that aeolian bedforms form a
three-tiered hierarchy. Nicholas Lancaster (1995) identi-
fied three superimposed bedforms, the first two of which
occur in all sand seas: (1) wind ripples; (2) individual
simple dunes or superimposed dunes on compound and
Time 1
Time 2
Time 3
Figure 12.3 The downwind progress of a transverse dune.
Source: Adapted from Livingstone and Warren (1996, 73)
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