How to build megaripples of sand

Wind that sweeps across beaches and deserts deposits and erodes sand grains to produce peanut-sized ripples and house-sized dunes. Less often, the interaction of wind and sand produces intermediate-scale sand formations called megaripples, shown in the middle of the photo, which was taken in the Sahara Desert. How the rare formations develop has long puzzled scientists. Now Klaus Kroy and Marc Lämmel of the University of Leipzig in Germany and colleagues have explained the conditions under which megaripples form.

Different aeolian processes lead to ripples and dunes. In the case of ripples, sand grains lifted a modest height by the wind fall back and strike the ground, which causes stationary grains to bounce up and hop small horizontal distances, usually about 10 cm. Those grains pile up around topographic bumps and propagate in a periodic pattern of corrugations defined by the hop length. Dunes form when wind accelerates over an obstacle, carrying more sand as it goes, until it dumps the sand just at the crest of the growing pile. Intermediate-size structures are not regularly observed because sand waves with wavelengths between the hop length and about 10 m get eroded rapidly.

Using grain size and morphological data from the literature and mathematical models, Kroy’s team showed that megaripples arise during steady winds that distribute sand as a layer of coarse grains armoring the sand bed. The wind transports fine, light grains while coarse, heavy grains accumulate. Bumps from the drifting grains provide the energy to make the heavy grains hop forward in tiny steps equivalent only to their diameter. Heavy grains thus respond to changes in wind speed on a much shorter length scale than lighter grains, so they can build dwarf dunes via the same mechanism as normal dunes. Unlike dunes, megaripples comprise only coarse grains, and they quickly erode if a storm causes larger grains to jump by themselves along the surface.

Refining models of dune formation can help scientists determine when dunes formed and moved; that information will provide insight into ancient wind and soil conditions. It could also help planetary scientists interpret formative wind regimes and grain sizes for megaripples that spacecraft have spotted in the deserts of Mars. (M Lämmel et al., Nat. Phys., in press, doi:10.1038/s41567-018-0106-z .)