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often they are not. The spatiotemporal dynamics of these
sand streamers in differing wind conditions is not known
and so the impact they have on measurements of salta-
tion activity and flux provided by sand traps and impact
responders is also uncertain. The controls on the forma-
tion and development of sand streamers is unclear, but it
seems likely that they are governed by near-surface tur-
bulent structures in the wind (Baas, 2008).
mobile sand dunes (Castro and Wiggs, 1994; Wiggs, Liv-
ingstone and Warren, 1996; Walker and Nickling, 2002,
2003; Weaver and Wiggs, 2010) and sand streamers (Baas,
2008).
The practical difficulties inherent in measuring
high-frequency turbulence in sand-laden airflows has
resulted in a moderate rate of advance in research. In
his wind tunnel experiments, Butterfield (1991, 1993,
1999) investigated the impact of temporally varying
winds (of the order of seconds) on saltation behaviour.
He discovered that mass flux and aerodynamic roughness
responded within about 1 second to an acceleration in
flow, but that the response was several seconds longer in
a decelerating flow due to the momentum of the grains.
These findings led Butterfield (1993) to conclude that
with naturally fluctuating winds over a sand bed, the
grain-laden boundary layer is in constant adjustment
and may rarely achieve an equilibrium state. With the
increasing application of sonic anemometers in the
field (Walker, 2005; van Boxel, Sterk and Arens, 2004;
Weaver, 2008) and the use of high-frequency grain impact
sensors (Schonfeldt and von Lowis, 2003; Baas, 2004),
investigations have now begun to observe the relationship
between saltation dynamics and instantaneous turbulent
peaks in wind velocities at much higher frequencies (1 to
>
18.9
The role of turbulence in aeolian
sediment transport
As discussed above, there is an increasing understanding
that sediment entrainment and transport can be highly in-
termittent under many environmental conditions. While
variability in surface conditions can account for some ir-
regularity in entrainment, it is now clear that, in a similar
manner to river flows (Bennett and Best, 1996; Kostaschuk
and Villard, 1996; Venditti and Bauer, 2005), turbulence
in the boundary layer is also a major driving force behind
aeolian sediment entrainment and transport (Livingstone,
Wiggs and Weaver, 2007; Weaver, 2008; Sterk, Jacobs
and van Boxel, 1998; Schonfeldt and von L owis, 2003).
Indeed, it has also been shown that the influence of peak
instantaneous turbulent stresses on sand transport dynam-
ics is required to explain the observed development of
10 Hz). While the relationship between turbulent wind
flow and sand transport is complex at such frequencies
(Figure 18.25) the association between the parameters
1600
12
1400
10
1200
8
1000
800
6
600
4
400
2
200
0
0
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
6000
6500
Time (10 Hz intervals)
Sand transport
Wind velocity
Figure 18.25
Time series of horizontal wind velocity (using a sonic anemometer) and saltation impacts (on a grain impact
sensor) measured at 10 Hz. The association is complex but the turbulent frequencies in both series question the application of
