Geoscience Reference
In-Depth Information
entrained only by the drag and lift forces of the moving
air. However, one of the most significant influences on the
entrainment of grains is the existence of grains already in
motion. In particular, the impact of saltating grains on the
bed surface imparts momentum to previously stationary
grains, hence 'splashing' them into saltation (see the later
discussion). A second threshold of grain entrainment can
therefore be identified: the
impact
(or
dynamic
) threshold,
which refers to the threshold of entrainment where other
grains are already in motion. Anderson and Haff (1988)
note that the impact threshold is about 0.8 of the static
threshold (see Figure 18.11) because saltating grains bring
extra momentum gained from higher in the velocity profile
towards the surface. It is the action of impacting grains
that drives the saltation process in this case, rather than
the lift and drag forces of the wind.
This impact mechanism of entrainment is seen to be
particularly important in the emission of dust-sized parti-
cles (
80
60
40
20
0
<
62.5 µm; see Goudie and Middleton, 2006). Many
highly emissive sources of dust are often found where
the entrainment of surface sand at relatively low wind ve-
locities results in the emission of dust via the impact of
saltating sand grains on the surface, rather than by direct
fluid entrainment (Rice, Willetts and McEwan, 1996; Lu
and Shao, 1999; Goudie and Middleton, 2006).
Once entrainment of grains has been accomplished,
however, the relative importance of the two processes of
grain entrainment is still not entirely clear (Ungar and
Haff, 1987). Rice (1991) has shown that when impact dis-
lodgement becomes established, direct fluid entrainment
still accounts for a significant proportion of erosion. How-
ever, the consensus from empirical observations (Bagnold,
1941; Willetts and Rice, 1985a) and theoretical examina-
tions (notably by Anderson and Haff, 1988; Werner, 1990;
Haff and Anderson, 1993; McEwan and Willetts, 1994)
is that grain impact is the principal mechanism by which
saltation is maintained. Hence, once grains are entrained
at the fluid threshold, a reduction in wind velocity will
not necessarily result in a reduction in sand transport so
long as the velocity remains above the impact threshold,
as shown in Figure 18.11.
0
0.04
0.08
0.40
0.80
1.20
1.60
Grain diameter (mm)
Figure 18.11
The idealised relationship between grain diam-
eter (
d
) and threshold shear velocity (
u
∗
ct
) showing the fluid
and impact thresholds (after Chepil, 1945). See text for details.
Figure 18.11 shows the idealised relationship between
fluid threshold shear velocity and grain size. From this
figure, it can be seen that, in general, larger particles have
a higher threshold of entrainment. However, smaller parti-
cles (with diameters less than about 0.07 mm) also require
higher shear velocities to entrain them. This is because
particles in this size range and smaller tend to have ad-
ditional molecular and electrostatic forces of cohesion.
They also have an affinity for the retention of moisture,
often become protected from erosion by larger particles
and frequently rest in the zero-velocity layer (beneath the
height of
z
0
). The most susceptible grain size for entrain-
ment is seen to be between about 0.07 and 0.08 mm, i.e.
fine textured sand. The heightened mobility of sediment in
this size range in drylands enables large volumes of sand
to be moved and accumulate, resulting in the extensive
dunefields present in many drylands (see Chapter 17).
The relationships shown in Figure 18.11 tend to result
in a downwind fining of aeolian sediment both at the scale
of the individual ripple and across whole sand seas. Such
a downwind sorting can be used to identify process-form
relationships; sand surfaces subject to aeolian erosion tend
to consist of a coarse lag material (Nickling and McKenna-
Neuman, 1995), while those subject to aeolian deposition
are commonly composed of fine material, such as loess.
The discussion above refers to the value of the
static
or
18.4
Determining the threshold
of grain entrainment
The determination of the wind velocity (
u
t
) or shear veloc-
ity (
u
∗
t
) threshold for grain entrainment is a fundamental
necessity in the understanding and prediction of the ae-
olian sediment transport system. There are many means
by which threshold values can be ascertained, although
