The armoring sub-layers of aeolian megaripples

Megaripples are large aeolian ripples where their crest is covered by an armoring layer composed of coarse grains. This armoring layer reduces wind erosion at the crest which further allows its growth. Grain size analysis was conducted on sand cores taken from a megaripple crest near Torra Bay, Namibia. The internal structure of the top 5 cm was examined using 0.5 cm slices. The results reveal that the armoring layer can be divided into three zones with distinct grain size distribution based on the skewness. The upper active layer has the largest portion of coarse grains and acts as a cap of the armoring layer where deposition and erosion processes take place. The median grain diameter decreases downwards to the middle layer where small grains can percolate through the spaces between the coarse grains of the upper layer. The bottom layer of the armoring layer contains more finer material that builds the bulk of the megaripple; a further decrease in the median grain diameter characterizes this layer. This internal structure is a result of three main segregation processes. The first is due to the different transport modes of the fine and the coarse grains. The fine grains move in saltation with long ballistic trajectories whereas the coarse grains move in reptation and creep by a succession of incremental small grain steps. The second mechanism is segregation by shape. Coarse grains are less circular than fine grains, making them less prone to rolling down. Consequently, they tend to accumulate at the bottom of the megaripple. The third mechanism is the percolation of finer grains through the pores between the coarser grains. Our results show a linear relation between the median grain diameter in the armoring layer and the armoring layer thickness. These new results will improve our understanding of the development and dynamics of megaripples.