Geoscience Reference
In-Depth Information
has led many to question the efficacy of 'mean' measures
of erosivity, such as mean wind velocity or shear velocity
(
u
∗
) to predict sediment fluxes (Stout, 1998; Baas and
Sherman, 2005; Weaver, 2008).
Research is now focused towards incorporating in-
stantaneous flow velocities into assessments of shear
stress and turbulence structure. By using two- or three-
dimensional sonic anemometers, the instantaneous veloc-
ity deviations from mean values can be assessed in both
the horizontal (
u
component) and vertical (
w
component).
These data can be used to determine Reynolds shear stress
(rather than mean
u
∗
) to assess the magnitude of turbulent
shear stresses or in quadrant analysis to assess the exis-
tence of structure in the turbulence signal (Lu and Will-
marth, 1973). Quadrant analysis divides the flow into four
discrete turbulent events depending on the relative signs of
instantaneous velocity deviations away from mean values
in both the horizontal and vertical flow components (Fig-
ure 18.26). Sweeps and ejections contribute to a 'bursting
process' in the flow where low-speed ejections of fluid
away from the bed are followed by high-speed sweeps of
flow towards the bed, and this process has been found to
be important in accounting for sediment transport in rivers
(Best, 1993). However, field investigations by Sterk, Ja-
cobs and van Boxel (1998), Schonfeldt and von L owis
(2003), van Boxel, Sterk and Arens (2004), Leenders, van
Boxel and Sterk (2005) and Weaver (2008) have found that
the majority of instances of high aeolian saltation flux are
rather associated with sweeps and outward interactions,
i.e. flow events that have instantaneous horizontal veloc-
ities greater than the mean (Figure 18.26). The reduced
significance of the vertical flow component in air (in com-
parison to water) is likely to be due to its lower density.
Much further research is required into the relation-
ships between turbulence structures, sediment transport
mechanics and evolving bed topographies. In the aeolian
case, it seems likely that initial future research directions
will focus on developing sediment transport equations that
incorporate some aspect of the instantaneous horizontal
wind speed or establishing probabilistic analyses of poten-
tial sediment flux given wind turbulence characteristics.
18.10
Conclusions
Continued investigations over the last decade of aeolian
sediment mobilisation using fieldwork, wind tunnels
and mathematical modelling have significantly enhanced
our understanding. Huge progress has particularly been
made regarding the impacts of vegetation on erosivity
and erodibility, intermittency in entrainment thresholds
and the role of turbulence in sediment transport rate de-
termination. Much of this work has necessarily involved
a small-scale approach and the continuing challenge is
to apply our new knowledge at this scale to research
questions at landscape and global scales. With growing
concerns about the potential impact of global warming
on dryland landscapes, knowledge of aeolian processes,
through which much landscape change will occur, will
be of increasing importance.
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w'
> 0
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Outward
interaction
Ejection
u'
< 0
u'
> 0
Inward
interaction
Sweep
w'
< 0
Figure 18.26
Quadrant plot of the four coherent turbulent
flow structures, based on instantaneous horizontal (
u
)and
vertical (
w
) velocity fluctuations from the mean (from Best,
1993). Bars represent the occurrence (% time) of saltation in
