3.1 Structure of drifting sand flux with no ridges
Under the condition of no ridges, the sand transport rate within the height of 0~70 cm above the sand bed decreases with the increase of height and continues to increase with the increase of friction velocity (Table 1). More than 97% of the sand transport is concentrated in the height of 0~10 cm, and the sand transport decreases rapidly in the 10~20 cm layer, accounting for 1.6% of the total sand transport on average. With the increase of height, the sand transport decreases gradually, but the decreasing rate between layers is getting smaller and smaller, and the sand transport in each layer above 20 cm is no more than 1%. With the increase of friction velocity, the proportion of sand transported in the drifting sand flux layer near the bed decreases relatively, and the proportion of sand transported in the upper layer increases accordingly. Based on the measurement results of sand transport, the sand transport rate at each height level (the average values of upper and lower heights are taken for each height level) is calculated (Fig. 2), and the correlation between the sand transport rate and height is analyzed. The results show that the sand transport rate decreases with the increase of height in a power function, which can be expressed by the functionq =Ah-B , and the correlation coefficients are all above 0.941.