Geoscience Reference
In-Depth Information
Physical crusts form where limited rainfall impacts on
a soil with at least 5 % clay content (Rajot
et al.
, 2003),
although Rice and McEwan (2001) state that the greater
the proportion of fine (cementing) material, the greater
the strength of the crust with surfaces containing
2.0
1.8
u
crb
/
u
cro
1.6
12 %
fines being easily eroded. Hupy's (2004) field investiga-
tion found that thick, silty, physical crusts could be as
protective of a surface as a cover of gravel or grass. In
contrast, McKenna-Neuman and Maxwell (2002) under-
took a wind tunnel test of the breakdown of microphytic
(moss) crusts and found that they were weak compared
to physical crusts and could deteriorate under the impact
of saltating grains even at low wind velocities. However,
they also found that in isolated cases the elasticity of such
microphytic crusts could protect the soil beneath under
heavy saltation bombardment if the integrity of the fila-
ment net in the crusts was strong.
On silt/clay surfaces a crust may significantly protect
the underlying fine, dust-sized particulates from erosion.
However, where such a sediment bed is of mixed size and
includes sand-sized particles that are prone to saltation,
the impact of saltating particles at relatively low wind
velocities can break down the crust and induce intense
dust entrainment. Such bombardment is thought to be the
primary process in the entrainment of dust-sized mate-
rial (Houser and Nickling, 2001a), with the most severe
sources of dust emission from playa surfaces being those
associated with a surface cover of erodible sand (Cahill
et al.
, 1996).
<
1.4
1.2
0.8
0.6
0.4
0.2
30
20
10
0
10
20
30
Upslope
Downslope
Figure 18.14
The effect of bedslope on threshold of grain
entrainment: solid line
=
theoretical models of Allen (1982)
and Dyer (1986), circles
=
experimental data (from Hardisty
and Whitehouse, 1988).
of climbing a 20
◦
slope under normal wind conditions,
explaining the accumulation of large particles at the base
of dune windward slopes.
Surface slope may not only have an important effect on
the
threshold
of sediment movement but also on the
rate
(or flux) of sediment transport. Despite much discussion
of the potential effect (Hunt and Nalpanis, 1985; Nalpa-
nis, 1985; Hardisty and Whitehouse, 1988; Whitehouse
and Hardisty, 1988; Iversen and Rasmussen, 1994), con-
clusions are far from complete. Bagnold (1941) derived
a simple geometric relationship to describe the effect of
bedslope on the sand transport rate, although Howard
et al.
(1977) found that the Bagnold formula had only a minor
influence on transport rate predictions and characterised
the actual sand transport rate no better than if bedslope was
not taken into account. In contrast, Hardisty and White-
house (1988) found that the sand transport rate was much
more dependent upon surface slope than predicted by the
Bagnold relationship (Figure 18.15).
18.5.2
Bedslope
Despite the potential importance of surface slope on the
threshold of entrainment and saltation trajectories of par-
ticles (some windward dune slopes may reach angles
aproaching 30
◦
), there has been relatively little empirical
research in this area. Theoretical analyses have been pre-
sented by Allen (1982) and Dyer (1986) while Hardisty
and Whitehouse (1988), Iversen and Rasmussen (1994)
and Rasmussen, Iversen and Rautahemio (1996) have
used portable field and tilting laboratory wind tunnels.
Hardisty and Whitehouse (1988) found good agreement
between theory and practice, with results indicating a
relative increase in critical threshold on positive slopes
(upslope) and a decrease on negative slopes (downslope)
(Figure 18.14). Computational fluid dynamics (CFD)
modelling by Huang, Shi and van Pelt (2008) confirmed
this influence of dune slope angle on particle entrainment
while the field work, modelling and wind tunnel testing
of Tsoar, White and Berman (1996) and White and Tsoar
18.5.3
Moisture content
It is clear that a high moisture content in surface sed-
iment can substantially increase entrainment thresholds
and reduce transport potential, except where the erosive
