Geoscience Reference
In-Depth Information
Transverse Dune
Seasonal reversal
of crest line
Reversing dune
with incipient
star dune arm
Figure 19.17
Model for linear dune development (after Bris-
tow, Bailey and Lancaster, 2000).
Development of arm
by secondary flow
major control on dune type is the directional variability of
the sand-transporting winds, as characterised by the ratio
between total potential sand transport (drift potential, or
DP) and resultant (vector sum) potential sand transport
(resultant drift potential, or RDP) (Figure 19.19). The di-
rectional variability or complexity of the wind regime in-
creases from environments in which crescentic dunes are
found to those where star dunes occur (Fryberger, 1979).
Crescentic dunes occur in areas where RDP/DP ratios
exceed 0.50 (mean RDP/DP ratio 0.68) and frequently
occur in unimodal wind regimes, often of high or mod-
erate energy. Linear dunes develop in wind regimes with
a much greater degree of directional variability and com-
monly form in wide unimodal or bimodal wind regimes
with mean RDP/DP ratios of 0.45. Star dunes occur in
areas of complex wind regimes with RDP/DP ratios less
than 0.35. Such a model is further supported by labora-
tory experiments and numerical simulations of bedforms
in directionally varying flows (Reffet
et al.
, 2008; Rubin
and Ikeda, 1990; Werner, 1995), which clearly show that
crescentic dunes are not stable in directionally variable
flows and that linear dunes are the product of bimodal
wind regimes in which the two modes are separated by at
least 90
◦
.
Accentuation of
arms by third wind
direction and secondary
flow
Figure 19.18
Formation of star dunes by modification of cres-
centic dunes (after Lancaster, 1989b).
Wasson and Hyde (1983a) confirmed that it was possi-
ble to discriminate between dune types on the basis of
the complexity of the wind regime (the RDP/DP ratio)
and the equivalent (or spread-out) sand. Barchans oc-
cur where sand supply is low and winds are unidirec-
tional, crescentic dunes are located where sand is more
abundant. linear dunes develop where sand supply is re-
lateively low but winds are more variable and star dunes
form in complex wind regimes with abundant sand sup-
ply. Similar relationships are evident in the Namib Sand
Sea, although the range of directional variability is lower
(Figure 19.19).
EST is, however, a measure of the volume of sand
contained in the dunes and may be a product of dune type,
19.7.3
Sand supply
The supply of sand has long been considered a factor in-
fluencing dune morphology (Hack, 1941; Wilson, 1972).
