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rougher surface (such as coarse-grained ripples or stone
pavement). At the smooth-rough transition, airflow close
to the surface is quickly decelerated by the additional drag,
and the aerodynamic roughness ( z 0 ) and surface shear ve-
locity ( u 0 ) increase rapidly in response. As the internal
boundary layer responding to the rougher surface grows
downwind, shear velocity steadily decreases to reach an
equilibrium level. It should be noted that such a rise in u
may not necessarily lead to an increase in sediment trans-
port or erosion because the increase in surface roughness
may result in more grains lying below the height of z 0 and
hence in a zone of zero wind velocity, although velocity
may be enhanced around individual, large, nonerodible
roughness elements and result in additional local erosion
(Ash and Wasson, 1983).
From Figure 18.6, it can be seen that at points marked X
the wind is still in equilibrium with the smooth upstream
surface, while at point Y the wind is responding to the
rougher downstream surface. Hence, z 0 and u measured
at any particular point in a velocity profile are functions
not only of the size and spacing of surface roughness
elements in the immediate locality but also of changes in
surface roughness within the fetch of the wind (Blumberg
and Greeley, 1993).
Clearly, although z 0 is a critical value to determine in
terms of assessing aeolian sediment transport, there are
many difficulties in its successful and effective calcula-
tion. There has been some success in interpreting values
and spatial variations in z 0 from remote sensing. Greeley
et al. (1997) found good correlations between subregional
variations in z 0 and radar backscatter in the Mojave and
Namib Deserts, demonstrating the potential to map z 0 for
large vegetation-free areas from orbit using radar systems.
Such an approach may make it easier to determine the
temporal variation in large-scale z 0 values with changing
climatic and environmental conditions, but often the scale
of interest in aeolian processses is small enough that field
and wind tunnel studies are the only means of providing
the relevant data.
(a)
Isolated-Roughness flow
(b)
Wake Interference
(c)
Skimming flow
Figure 18.7 Flow regimes associated with different rough-
ness element spacings and geometries. The shaded areas rep-
resent zones of reduced wind speed (after Wolfe and Nickling,
1993).
trolling wind erosion (Ash and Wasson, 1983; Wasson
and Nanninga, 1986; Wolfe and Nickling, 1993; Wiggs
et al. , 1994, 1995). However, the varying geometry, spa-
tial organisation and density of such roughness elements
are difficult to account for in models of aeolian sediment
transport (King, Nickling and Gillies, 2005).
The way in which nonerodible roughness elements
might interact with the wind flow was demonstrated the-
oretically by Wolfe and Nickling (1993) and is shown
diagrammatically in Figure 18.7. With an increasing den-
sity of roughness elements the flow regime changes from
one where the elements act individually on the flow (iso-
lated roughness flow) to a regime where the interaction
between the wakes downwind of the elements is such that
the flow skims across the top of the elements (skimming
flow). In semi-arid areas the distribution of plants is often
such that the airflow is responding to the isolated rough-
ness or wake interference regimes (Wolfe and Nickling,
1993).
It is clear from Figure 18.7 that an increasing rough-
ness element density reduces near-surface flow velocities
and this is reflected in the corresponding velocity pro-
files. Using vegetation as an example, Figure 18.8 demon-
strates that the additional drag on the airflow provided by
a vegetated surface not only increases the aerodynamic
roughness ( z 0 ) but also displaces it upwards by a value d ,
resulting in a greater depth of flow with zero velocity at
the surface. The greater z 0 of the vegetated surface also
18.2.4 The effect of nonerodible roughness
elements on velocity profiles
The distribution of cobbles, rocks and vegetation on other-
wise homogeneous desert surfaces act as a surface rough-
ness, providing significant drag on overlying airflow and
considerably altering velocity profile parameters (King,
Nickling and Gillies, 2008; Gillies et al. , 2000; Wiggs
et al. , 1994, 1996). By altering values of u
and z 0 such
nonerodible elements can have a considerable role in con-
 
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