Geoscience Reference
In-Depth Information
1.2
1.0
0.8
Kawata & Tsuchiya (1976)
0.6
Hotta (1984)
0.4
Belly (1964)
0.2
0.0
16
0
4
8
12
20
24
28
Moisture content (%)
Figure 18.16
Critical shear velocity for sediment entrainment as a function of sand moisture content (from Sherman, 1990).
present (McKenna-Neuman and Langston, 2006; Darke
and McKenna-Neuman, 2008; Darke, Robin and Oller-
head, 2009).
loess or dust (see Chapter 20). At the coarse end of this
spectrum, some material may be transported in modified
saltation (Hunt and Nalpanis, 1985), where saltation par-
ticle trajectories are affected by wind turbulence.
18.6
Modes of sediment transport
18.6.2
Creep
Once entrained into the airflow, sediment may be trans-
ported by any one of four mechanisms, principally depen-
dent upon the sediment grain size (Bagnold, 1941, and
Figure 18.17). The following modes of transport are not
discreet classes and the transition from one to another may
not be well defined.
Larger particles tend to be transported by one of the three
modes of bedload (or contact) transport: creep, reptation
or saltation. Surface
creep
describes the rolling action of
coarse particles (0.5-2.0 mm) as a result both of wind
drag on the grain surfaces and the impact of high-velocity
saltating grains. In the case of the finer particles in this
size range, a creep movement may become apparent im-
mediately prior to the onset of saltation (Nickling, 1983;
Willetts and Rice, 1985a). The difficulty in isolating creep
in experimental observations has made the task of defining
the relative importance of creep in terms of other trans-
port mechanisms problematic. Willetts and Rice (1985b)
estimate that creep accounts for approximately one quar-
ter of the bedload transport rate while Dong
et al.
(2002)
found in their wind tunnel tests that the creep transport
rate increased with wind speed but decreased with grain
size, while the creep fraction varied widely, ranging from
18.6.1
Suspension
Small particles of less than 62.5 µm (Goudie and Mid-
dleton, 2006) whose settling velocity may be very small
in comparison to the combined effects of wind lift and
drag might be transported in
suspension,
with the vertical
profile of flux decreasing with height and described by
a power function (Nickling, McTainsh and Leys, 1999;
Wang
et al.
, 2008). The turbulent motion of airflow can
keep very fine sediment suspended for many days, high
