Beyond Size Sorting - Shape segregation in aeolian megaripple

Aeolian megaripples develop on bimodal sands and are stabilized by an armoring layer enriched in coarse grains that developed on the crest. While size-selective transport is central to the segregation mechanism involved in megaripple formation, recent field observations indicate that shape segregation may also contribute to megaripple formation (Saban et al, Geosci. Lett., 2025). Here, we quantify the shape contrast between fine and coarse fractions of megaripples across multiple sites worldwide and investigate the physical mechanism that may explain it. Additionally, we investigate how ripple formation is affected by shape segregation through controlled wind tunnel experiments.

We analyzed samples from megaripple crests at multiple sites. Each sample was divided into sub-samples of fine fraction (<355µm) and a coarse fraction (>710µm), which represent the bimodal grain size distribution (GSD) of all the samples. Grain shape was quantified using a Circularity index (isoperimetric quotient), computed from a 2D projected grain outline derived from microscopy images. Grain outlines were produced by automated segmentation and were manually validated to ensure accuracy. Mineralogical composition and GSD were also measured and used as proxies for mechanical durability and abrasion history contrasts between the size fractions.

Across most sites, the coarse fraction is more angular (less circular) than the fine fraction, indicating a robust shape contrast between size fractions. To explain this pattern, we used a physically motivated combined index that accounts for the size contrast and the quartz contrast between the fine and coarse fractions. Sites where the fine grains are both relatively finer and more quartz-rich compared to the coarse fraction show a stronger shape contrast (i.e., fines are more circular). This suggests that abrasion history and mechanical durability influence grain shape.

Finally, we designed a wind tunnel experiment to isolate the role of shape segregation in the formation of nascent megaripple. We used mixtures of angular natural sand and spherical glass beads with the same grain size. These mixtures were subjected to wind above the fluid threshold until ripple formation. Spatial distribution analysis of grain shape at the end of the experiments reveals clear sorting patterns, driven solely by shape segregation, where angular grains accumulate on the crest and form an armoring-like layer.