Geoscience Reference
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5.2.4.2 Transport processes
Although individual saltating grains can reach
3 m into the air (Pye & Tsoar 1990), most (80%)
of all such transport takes place within 2 cm
of the ground surface (Butterfield 1991). The
impact of saltating grains on the surface may
lead to reptation, or, where sufficient momentum
is transferred, may induce mass transport by
saltation in a cascading system (Nickling 1988).
Transport will be dependent on a number of
factors including grain shape. For example,
Willets (1983) showed that platy grains have a
tendency for lower and longer trajectories than
more spherical grains.
As with water, sediment movement will occur
only where the erosivity of the wind is greater
than the erodibility of the surface materials. The
erodibility will be determined by a number of
factors. Vegetation will enhance surface rough-
ness and modify local wind-velocity profiles. In
dune systems this can reduce ground-surface
velocities by as much as 200% (Wiggs et al.
1994) thus limiting entrainment and encour-
aging deposition. Surface slope will have an
impact on both the threshold of sediment move-
ment and the rate of sediment transport, and
may be more important than initial theories sug-
gest (Hardisty & Whitehouse 1988). Moisture
increases surface tensions in sediments, reduc-
ing erodibility, although the impact is strongly
affected by the grain-size characteristics of
the sediment (McKenna-Neumann & Nickling
1989). In the case of moisture the most rapid
changes for erodibility occur at about 8% mois-
ture content. Thus near-surface groundwater
tables may limit sand transport (Stokes 1968)
as can periodic flooding, both of which are
particularly common factors in playa, sabkha
and coastal sand-sea situations (Fryberger et al.
1988). Moisture contents up to 14%, however,
will have no real impact on sediment already
in transport (Sarre 1989, 1990). Salt and algal
crusts (section 5.3.5) may also protect surfaces
from erosion.
Once entrained the sediments may be trans-
ported by a number of different modes. Suspen-
sion applies to smaller particles (
5.2.4.3 Landforms created by aeolian transport processes
Ventifacts are common in areas dominated by
wind erosion, such as adjacent to slopes. They
are generated mainly by saltating, rather than
suspended grain movement. The latter tend to
be swept around an object rather than impact-
ing with it. Sand-grade material in saltation thus
accounts for most wind erosion on the wind-
ward side. Smaller particles of dust, although
having less impact energy, can, in sufficient load
(Whitney & Dietrich 1973), erode on the lee
side of objects, utilizing wind eddies (McCauley
et al. 1977, 1979). There is, however, much
controversy on the dominance of dust versus
suspended load and its role in abrasion (see
discussion in Breed et al. 1997). Larger scale
versions of abrasion structures are known as
'yardangs'. These have the form of an inverted
boat, with a high windward side and stream-
lined lee side. They range in size from a few
metres to several kilometres. The largest yardangs
on Earth are up to 30 km long and developed
in the Tibesti Plateau of the central Sahara
(Peel 1970).
0.6 mm). Very
fine sediment may remain suspended for many
days and travel great distances. Creep involves
larger particles (0.5-2 mm), which roll as a
result of wind drag and the impact of saltating
grains. It has been estimated that creep accounts
for 25% of aeolian bedload transport rate (Willets
& Rice 1985). Reptation represents the transi-
tional state between creep and saltation and is
generated by the physical impact of high-velocity
saltating grains on near-surface grains. Saltation
is the most researched mode of transport. Saltat-
ing sediments (typically 0.06 - 0.5 mm) move in
a number of steps using a ballistic trajectory.
<
5.3
SEDIMENT ACCUMULATION PROCESSES
Sediment deposition can occur temporarily within
transport zones, or more permanently in areas
of net accumulation (sediment storage; Fig. 5.1).
This section will examine the sediment accumu-
lation processes that affect these zones.
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