Page 320 - Fundamentals of Geomorphology

AEOLIAN LANDSCAPES 303

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 Transverse
dunes
saltating grains, which are unable to rebound on impact supply Stars
as easily as they are on the surrounding stony surface. Linear
This mechanism works only if the sand body is broader sand dunes
than the flight lengths of saltating grains. A critical lower
width of 1–5 m seems to represent the limiting size for Increasing Networks
dunes. On the leeside of the dune, airflow separates and Barchans
decelerates. This change enhances sand accumulation
and reduces sand erosion, so the dune increases in size. Unimodal Complex
The grains tend to be trapped on the slip face, a process Wind direction variability
aided by wind compression and consequent acceleration
over the windward slope. The accelerated airflow erodes Figure 12.4 Dune types in relation to the variability of
wind direction and sand supply.
thewindwardslopeanddepositsthesandontheleeslope. Source: Adapted from Livingstone and Warren (1996, 80)
As the sand patch grows it becomes a dune. Eventually,
a balance is reached between the angle of the windward complex dunes; and (3) compound and complex dunes
slope, the dune height, the level of airflow acceleration, or draa.
and so the amount of erosion and deposition on the
windward and lee slopes.The dune may move downwind
(Figure 12.3). Ripples
Figure 12.4 is a speculative model of the conditions
conducive to the formation of different dune types, Wind ripples are the smallest aeolian bedform. They
which are discussed below (Livingstone and Warren are regular, wave-like undulations lying at right-angles to
1996, 80). The two axes represent the two main factors the prevailing wind direction.The size of ripples increases
controlling dune type.The first represents an unspecified with increasing particle size, but they typically range from
measure of the amount of sand available for dune forma- about 10 to 300 mm high and are typically spaced a few
tion, while the second axis represents the variability of centimetres to tens of metres apart (Colour Plate 15,
wind direction. 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.
Dune types
Seemingly simple aeolian bedforms, ripples have with-
Some researchers believe that aeolian bedforms form a stood attempts to explain them. Several hypotheses have
three-tiered hierarchy. Nicholas Lancaster (1995) identi- been forthcoming, but most are flawed (see Livingstone
fied three superimposed bedforms, the first two of which and Warren 1996, 27). According to what is perhaps the
occur in all sand seas: (1) wind ripples; (2) individual most plausible model (Anderson 1987; Anderson and
simple dunes or superimposed dunes on compound 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
Time 1 Time 2 Time 3 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
Figure 12.3 The downwind progress of a transverse dune. bore coarser particles at their crests, as is ordinarily the
Source: Adapted from Livingstone and Warren (1996, 73) case in actual ripples.