Aeolian sediment transport over a granule ripple surface and its implications for geomorphological evolution

Aeolian sediment transport causes a range of sand hazards and serious air pollution, but also sculpts intricate landscapes in arid and semi-arid areas. Previous studies of aeolian transport have mainly focused on the particle transport rate or flux density above various surfaces, such as sandy and gravelly beds, or have linked these factors to the dynamic processes that govern fine-grained bedforms, including dunes and sand ripples. Granule ripples have been studied less. Their sediments exhibit a bimodal size distribution, and they are armored by coarse grains at the crest. The aeolian transport characteristics above their surface are crucial for understanding their morphodynamic processes, but have been rarely studied in the field. To provide this missing data, we conducted six observations using a blowing-sediment trap on the surface of granule ripples in the northern part of China's Kumtagh Desert to elucidate the geomorphological effects of aeolian transport. The flux density decreased exponentially with increasing height, but with substantial differences in the vertical profiles of flux density for different grain-size groups. The transported particles were predominantly fine to medium sands, although coarse sands to very fine gravels also underwent saltation at high wind speeds. Wind strength significantly influenced the aeolian transport process, with particles of different sizes responding to different threshold velocities and engaging in different transport modes. We related selective, bimodal, and unimodal transport processes to the morphological evolution of granule ripples and the formation of their sedimentary structure. We suggest that the saltation of coarse particles should be fully considered to refine the theoretical framework for granule ripple evolution.