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(a)
(b)
Blowout
Parabolic dune
(c)
(d)
Lunette
Nebkha
Falling
dune
Echo
dune
Fore
dune
Fig. 5.14 Morphology of the main impeded,
simple dunes described in Table 5.3. (Adapted from
Summerfield 1991.)
(e)
(f)
Lee dune
Climbing dune
shows that the largest dunes on Earth, star dunes,
form in areas with greatest wind variability and
greatest sediment supply. The calibre of available
sediment is also important in the generation of
dune types. For example, coarse sand is usually
associated with dunes with low-angle morphology
and limited height (typically up to 4 m). These
dunes may be linear (zibar dunes) or mound-like
(dome dunes) depending on wind regime. Strong
wind velocities are required for transport, and
movement of the dunes is limited.
Initial dune-sand accumulation may be initi-
ated by a surface roughness or by temperature
gradients in the atmosphere (sand accumulation
tends to occur where airflows are ascending and
relatively slower). The accumulation and move-
ment of sand will eventually develop a stoss-
and slip-face. Where there is abundant supply
of suitable sediment and unidirectional wind,
transverse ridge dunes will develop (Fig. 5.15).
If two opposing prevailing winds of similar
strength and duration exist, reversing dunes may
develop as active slip faces change with wind
direction. As topographically lower dunes can
move more quickly than higher dunes, a sinuos-
ity develops, creating a sinuous, barchanoid ridge
50
40
Star
30
Transverse
20
10
Linear
Barchan
0
0
0.2 0.4
Wind direction variability
Multidirectional
0.6
0.8
1
Unidirectional
Fig. 5.15 Relationship between wind variability and equivalent
sand thickness (thickness of sand if it were spread evenly over an
area). For explanation see text. (After Wasson & Hyde 1983.)
wind. A low value indicates a highly variable wind
regime. Equivalent sand thickness represents the
thickness of dune sand in an area if it were spread
evenly, so the greater the thickness, the greater
the overall volume of the dune sand. Figure 5.15
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