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.