Near-surface airflow field and aerodynamic characteristics of the railway-protection system in the Shapotou region and their significance

The effects of near-surface airflow on aeolian deposits across the Shapotou region's railway-protection system were studied. We found that iso-speed lines in the protective belt gradually become less dense with increasing distance along the wind direction due to the inclined topographic relief in that direction and to changes in surface properties (particularly dune undulation). In addition, wind speed at each elevation tends to decrease, with a negative velocity variation coefficient. This coefficient was positive on windward sides of fixed dunes and smaller than that on mobile dunes; its values were negative on the leeward slope of fixed dunes, averaging -1.48% m-1 below a height of 250 cm. Windward wind profiles followed a logarithmic relationship but deviated from this relationship leeward. The mean aerodynamic roughness length (z0) of artificially vegetated surfaces was 0.00543 m, versus 0.000118 m for mobile sands. The average air drag coefficient (Cd) of the artificial vegetation belt was 1.7 times that of mobile sands. The near-surface airflow field and aerodynamic characteristics control clay and silt contents in surface soils, which increase greatly (with decreasing median particle size) from the mobile sands upwind of the protective belt to the artificial vegetation further downwind within the belt, and sorting becomes increasingly poor. Silt content was negatively correlated with, and median particle size and sorting were positively correlated with the relative surface elevation in the protective belt. The relationship between clay content and landform was unclear. Silt content was high and sorting poor in the surface 5 cm; below this depth, silt content decreased and sorting increased. The particle-size profile varied among landforms. Estimated aeolian deposition rates around the crest of fixed dunes average 0.147 cm per year, versus more than 0.588 cm on the leeward side and in inter-dune depressions.