Seasonal morphological evolution and migration of granule ripples in the Sanlongsha Dune Field, northern Kumtagh Sand Sea, China

Granule ripples are aeolian landforms that are armored against erosion by coarse grains at their crest. Their bimodal grain-size distributions and large dimensions distinguish them from normal wind ripples. The morphodynamics of granule ripples have attracted considerable attention because they provide a good example of coarse-grained aeolian bedforms. Previous studies have mainly focused on the morphological characteristics and formation process of granule ripples, but their evolution has been less studied. Using unpiloted aerial vehicle photogrammetry and real-time kinematic positioning, we conducted five field observations in the Sanlongsha Dune Field of northern China's Kumtagh Sand Sea. We analyzed the seasonal morphological evolution and migration of granule ripples, and the corresponding wind regimes. The morphometric parameters and migration rate showed distinctly different changes in different periods. The lowest height, the fastest migration, and most of the disintegration of granule ripples occurred in the season with the strongest wind. Although the proportions of wind speeds that exceeded the impact and fluid threshold velocities of coarse grains were low, these wind events significantly influenced the morphodynamics of granule ripples. Our findings and theoretical analysis demonstrate that the evolution of granule ripples is closely related to bimodal and unimodal transport events. To enhance our understanding of the morphodynamics of granule ripples, we must focus future research on the reptation and saltation of coarse grains.