Geoscience Reference
In-Depth Information
direction subject to the maximum gross bedform normal
transport across the crest (Figure 19.22
(
a)), so that sand
transport from all directions contributes to bedform de-
velopment. These experiments and subsequent numerical
simulations show that the type of dune that forms is de-
termined by the divergence angle between the dominant
and subordinate transport vectors and the ratio between
the two primary transport directions (transport ratio). A
trend parallel to or normal to the resultant direction of
sand transport is purely coincidental.
This model is supported by empirical data on dune
trends and winds. There is a close agreement between ob-
served and predicted gross bedform normal orientations in
the case of many barchans, crescentic dunes, simple linear
dunes and star dunes (Figure 19.22(b)), suggesting that all
major dune types are oriented to maximise gross bedform
normal sediment transport and therefore are dynamically
similar (Lancaster, 1991). This approach can also be used
to identify dune trends that are out of equilibrium with the
modern sand transporting wind regime, as demonstrated
for Mauritania, and to suggest wind regimes that could
have produced the trends of dunes constructed in the past.
(Lancaster
et al.
, 2002).
(a)
30
°
20
°
Lake
Chad
10
°
0
1000
km
Sand sea
Sand transport
vector
0
°
10
°
0
°
10
°
20
°
30
° Ε
48° E
(b)
0
600 km
Nafud
24°N
Liwa
Wahiba
19.8
Dune patterns
Arabian Sea
Sand Seas
Dune systems exhibit clear patterns of dune morphol-
ogy, size, spacing and crestline alignment on several spa-
tial scales: (1) regional, (2) within a sand sea and (3)
within an area of dunes of similar morphology. The pat-
tern of dunes is a reflection of the external and internal
factors that determine the present and past dynamics of the
system.
On a regional scale, mapping of dune trends shows a
pattern that is determined by regional atmospheric cir-
culations (Figure 19.23), especially the pattern of winds
outblowing around anticyclonic cells, as in the Kalahari
(Lancaster, 1981) and Australia (Wasson
et al.
, 1988).
In the Sahara, the pattern of dune trends corresponds
broadly to the of the Trade Wind circulation (Mainguet,
1984), modulated in some areas by topography and the
development of low-level jets (Washington
et al.
, 2006),
while in Arabia, dune trends follow the Shamal circulation
(Glennie, 1998).
Within individual sand seas and dune fields, mapping of
dune patterns from aerial photographs and satellite images
shows distinct spatial patterns of dune type, size and spac-
ing (Breed and Grow, 1979; Ewing, Kocurek and Lake,
2006). In part, these patterns are the product of regional
(c)
20
°
Great Sandy
Desert
Tanami Desert
Simpson Desert
Gibson
Desert
Strzelecki Desert
30
°
Great Victoria Desert
Mallee
Dunefield
500
0
km
130
°
142
°
118
°
Figure 19.23
Dune morphology patterns - regional: (a) sand
transport patterns in the Sahara; (b) dune trends in the Arabian
Peninsula; (c) linear dune trends in Australia.
