3.2 Drifting sand flux Structures with microtopography of
ridges
The features of underlying surface and wind velocity are the main
factors affecting the drifting sand flux (Lancaster et al., 1998;
Butterfield, 1999). The fluctuation of underlying surface changes the
saltation trajectory of sand particles (Anderson and Hallet, 1986; Tsoar
and White, 1996; Frank and Kocurek, 1996; Wiggs, 2001), while the change
of wind velocity changes the movement properties of sand particles in
the drifting sand flux, which eventually leads to the change of the
drifting sand flux structure (Huang and Zheng, 2007). The experimental
results show that the total sand transport rate at 0~70
cm height with different ridges increases continuously with the increase
of wind velocity, but the change of sand transport rate with height can
be divided into two cases. One is that the sand transport rate decreases
with the increase of height, and the fitting relationship between
surface sand transport rate and height deviates from the power function
law with no ridges, but mostly obeys the exponential function law of
particle distribution of reaction saltation. The other is that the sand
transport rate increases with the increase of height below a certain
height, and decreases with the increase of height above the certain
height, showing the ”elephant nose” effect similar to drifting sand flux
structure in Gobi desert. The distribution of the sand transport rate
with ridges under different heights can be divided into two sections,
the lower section has no obvious law, and the upper section still
follows the exponential function law (Fig. 3).
Mostly, the variation of sand transport rate with ridge structures (the
combination of height and spacing) changing with height near surface
conforms to the first case. The sand transport rate is largest in
0~10 cm layer, and it decreases rapidly in
10~20 cm layer, while the decreasing rate of sand
transport rate above 20 cm height goes smaller. Taking a ridge height of
5 cm as an example (Table 2), the sand transport rate of
0~10 cm layer accounts for 58.4% of the total sand
transport rate, 10~20 cm layer rapidly decreases to
21.2%, 20~60 cm layer keeps decreasing with a smaller
reducing rate, and in 60~70 cm layer, the sand transport
rate decreases to 1.9%. Compared with the condition with no ridges, the
sand transport rate under different ridge structures decreases obviously
in 0~10 cm layer, while increases significantly in
20~60 cm layer, and increases slightly in the uppermost
layer. The maximum height of sand saltation also increases accordingly.
Only when the friction velocity is small, except for a few ridge
structures, the sand saltation reaches the maximum height within 70 cm
height, the others do not reach the maximum height.
The surface sand transport rate of some ridge structures conforms to the
second case, and the higher the ridge height is, the more common this
phenomenon is. Taking a ridge height of 15 cm and a ridge spacing of 15H
as an example, when the friction velocity is 0.68
m·s-1 and the height layer increases from
0~10 cm to 30~40 cm, the sand transport
rate continuously increases from 0.0742
g·cm-2·min-1 to 0.2284
g·cm-2·min-1, and the sand transport
rate at 30~40 cm is the largest. When the height is
above 40 cm, the sand transport rate decreases gradually, and the height
of 60~70 cm decreases to 0.0229
g·cm-2·min-1. Compared with the
relative sand transport at different heights under different friction
wind velocities, the sand transport at 0~40 cm height
increases gradually with the increase of height from 10.6% to 26.1%,
and the sand transport at 40~70 cm height decreases
gradually from 17.0% to 3.9%. The boundary height is approximately 40
cm which increases with the increase of ridge height and ridge spacing.
When the ridge height is 7.5 cm, the boundary height is
10~20 cm. When the ridge height is 10 cm, the boundary
height increases from 10~20 cm at the ridge spacing of
5H to 20~30 cm at the ridge spacing of 15H. When the
ridge height is 12.5 cm, the ridge spacing increases from 5H to 25H, the
boundary height increases from 10~20 cm to
30~40 cm. When the ridge height increases to 15 cm, the
boundary height increases from 30~40 cm at the ridge
spacing of 15H to 40~50 cm at the ridge spacing of 25H.
The reason of the above changing is that, the influence of ridge on
low-level drifting sand flux increases with the increase of ridge
height, and its influence height also increases gradually. The influence
mechanism of ridge spacing on the drifting sand flux structure is not
clear for now.