Geoscience Reference
In-Depth Information
Figure 18.3
Photograph of a 6 m high anemometer tower incorporating eight logarithmically spaced cup anemometers to
measure changing shear stress (
u
∗
) and aerodynamic roughness (
z
0
) on the Skeleton Coast in Namibia. The array also consists of
four wedge-shaped sand traps, two saltation impact responders and a vertical array of three sonic anemometers (photo: author).
where
Livingstone and Warren, 1996; also see Chapter 19). In
such cases, calculated values of
u
∗
measured above a
κ
=
von Karman's constant (
≈
0.4)
dune's surface become meaningless.
Corrections to measured values of
u
∗
also have to be
made for the effect of thermal stability or instability, which
can significantly alter the shape of velocity profiles. If
strong thermal heating or atmospheric stratification oc-
curs, both of which are common in arid environments
when wind velocities are low, then wind turbulence may
be driven by buoyant forces rather than mechanical forces
and the velocity profile may again become non-log-linear
(Frank and Kocurek, 1994). Rasmussen, Sørensen and
Willetts (1985) found that omitting stability corrections
could lead to errors in the measurement of
u
∗
of the order
of 15-25 %, with subsequent estimates of sand transport
in error by as much as 15 times (Frank and Kocurek,
1994). Correction factors involve the calculation of the
Richardson number (RI), a measure of the relative strength
of buoyancy to shear generated turbulence (Oke, 1987),
and require knowledge of the local atmospheric tempera-
ture gradient. However, Rasmussen, Sørensen and Willetts
(1985) suggested that errors in measured velocity due to
such instability was a function of height and that at ele-
vations below
m
=
gradient statistic of the regression equation
The averaging time required to measure values of
u
∗
meaningful for sediment transport calculations using this
least squares approach is a matter of debate. While for re-
gional scale wind erosion modelling a
u
∗
calculated over
several hours may be sufficient, many geomorphologists
investigating relationships between changing
u
∗
and sed-
iment transport flux have calculated
u
∗
on the order of
seconds or minutes. However, at high measurement fre-
quencies (
1 second) it is clear that the log-linear nature
of the turbulent wind velocity profile collapses and it is not
possible to calculate a time-averaged
u
∗
(Bauer, Sherman
and Wolcott, 1992; Wiggs, Atherton and Baird, 2004).
The highest frequencies for successful
u
∗
calculation are
of the order of 30-40 seconds (Wiggs, Atherton and Baird,
2004; Weaver, 2008), although Namikas, Bauer and Sher-
man (2003) report success with measurement frequencies
as high as 10-20 seconds.
Fluctuating topography also hinders the successful cal-
culation of
u
∗
. Process research on sand dunes has been
particularly hampered by the recognition that acceleration
of wind flow up the stoss slopes of dunes results in the
disruption of the log-linear velocity profile (thus negating
the calculation of
u
∗
), with maximum acceleration oc-
∼
∼
0.5 m the correction required was unim-
portant. From their measurements in White Sands, New
Mexico, Frank and Kocurek (1994) also found that cor-
rection factors were not necessary at wind speeds higher
than
∼
10 m/s.
An additional measure of stress within the airflow is
