Geoscience Reference
In-Depth Information
have no moving parts prone to attack from saltating particles. Third, and most importantly, they can simultaneously
measure the three high-frequency (
10 Hz) components of wind velocity in the horizontal streamwise component
(
u
), the lateral or spanwise component (
v
) and the vertical component (
w
). From these instantaneous velocity
measurements it is possible to determine turbulence statistics including the
uw
covariance, the Reynolds stress and
coherent flow structures, all of which are now thought to be relevant in saltation dynamics. It has taken some time
for investigators to devise protocols for using the anemometers in complex field situations and also to develop
analytical schemes to make best use of the huge volumes of data that can be collected. However, the field application
of sonic anemometers now offers a new paradigm in aeolian geomorphology.
>
tionship assumes a homogeneous surface and well-sorted
sediment. Furthermore, it ignores the effect of roughness
element spacing and scale where the aerodynamic rough-
ness might be determined by larger particles on a stone
pavement. For example, Greeley and Iversen (1985) noted
that
z
0
may reach a maximum value of
d
/8 for widely
spaced elements, before returning to a
d
/30 ratio as ele-
ment spacing increased further (see Figure 18.5). Further,
it is clear that
z
0
also varies in relation to changes in mi-
crotopography and this suggests that mean particle size
may not be a good determinant of
z
0
for surfaces with a
large range in particle sizes (Lancaster, Greeley and Ras-
mussen, 1991) or changing surface patterning (e.g. sand
ripples).
For this reason aerodynamic roughness has often been
determined from the intercept of velocity profiles with the
height axis, as shown in Figure 18.2. This intercept clearly
describes
z
0
as defined as the depth of air at the surface
with an effective zero velocity. When plotted in the manner
of Figure 18.2,
z
0
can be determined from the slope and
intercept components of the regression equation by
Given some of the difficulties in assessing the erosivity
of the wind by measuring various windflow characteristics
(as decribed above) an alternative approach is to measure
the surface shear stress (
τ
0
) directly. This can be achieved
by using calibrated Irwin sensors or rugged drag balances
placed directly on the sediment surface. Irwin sensors are
small, omnidirectional skin friction meters that measure
the near-surface vertical pressure gradient (Irwin, 1980),
which, once calibrated, can be used to measure high-
frequency surface shear stresses (Wu and Stathopoulos,
1994). A drag balance, buried with the measuring ele-
ment flush to the surface or attached to a roughness ele-
ment, directly measures the force applied by the wind and
records it on a sensitive load cell. Both technologies have
been successfully tested in wind tunnels and dryland field
locations to measure wind forces on bare sediment and
vegetated surfaces in a variety of flow conditions and sur-
face roughness configurations (Gillies
et al.
, 2000; King,
Nickling and Gillies, 2005; Gillies, Nickling and King,
2007; Walker and Nickling, 2003).
z
0
=
−
/
exp(
n
m
)
(18.5)
18.2.3
Measuring aerodynamic roughness (
z
0
)
The aerodynamic roughness (
z
0
) of desert surfaces varies
widely both temporally and spatially. Typical values
for
z
0
may be 0.0007 m for stationary sand surfaces
(Stull, 1988), 0.003 m for surfaces with moving sand
(Rasmussen, Sørensen and Willetts, 1985) to 0.2 m and
greater for vegetated or semi-vegetated desert surfaces
(Wiggs
et al.
, 1994). Owing to its importance in determin-
ing wind shear velocity, it is a vital parameter to calcu-
late correctly in studies of aeolian processes (Levin
et al.
,
2008). However, on many desert surfaces it is a parameter
that is difficult to resolve effectively because of its sen-
sitivity to the frequently changing scales of nonerodible
elements and surface roughness.
Bagnold (1941) found a relationship between
z
0
and
surface roughness. He noted that
z
0
=
z
0
~
~
d
/30
d
z
0
~
~
d
/8
d
z
0
d
/30
~
~
d
Figure 18.5
Aerodynamic roughness height (
z
0
) as a func-
tion of roughness element spacing (after Greeley and Iversen,
