Variation with height of aeolian mass flux density and grain size distribution over natural surface covered with coarse grains: A mobile wind tunnel study

The vertical distributions of aeolian mass flux density and grain size with height over the natural surface covered with coarse grains were investigated using a mobile wind tunnel in the Yardang Geological Park, Dunhuang, China. Results reveal that the mass flux density of mix-sized sediments decays exponentially with increasing height. However, the mass flux density profiles of separate grain-size groups depend on size grading: those of coarse sand groups (0.8-2.0 mm) can be described by exponential decay functions while those of fine sand groups (0.063-0.8 mm) can be expressed by Gaussian distribution functions. Therefore, the mass flux density profile of mix-sized saltating particles is a superposition of many different profiles. In addition, results of grain-size analysis show that the size distribution of eroded sediments close to the ground reflects that of the loose material in the parent soil. The mean grain size first deceases remarkably with increasing height in the near-surface region, this trend is reversed at heights of 60-80 or 120-140 mm above the bed and then there is a coarsening trend of mean size with height. Sorting only improves with height in the region above 80 or 100 mm, but it declines in the near-bed region. These experimental data are significant for studies of heterogeneous saltation and numerical modeling of grain trajectories.