Is impact ripple flattening caused by aeolian shear melting of the granular bed?

Ripples are common aeolian sand waves that can broadly be classified into megaripples, impact ripples and a newly proposed aerodynamic ripple type [1]. Grain scale numerical simulations and conventional theories associate impact ripple formation with a characteristic hop length for the grain trajectories over a quasi-static bed [2-5]. Based on original and literature data we show that impact ripple patterns vanish structurally with increasing shear stress not far from the transport threshold. We correlate the experiments with recent observations of the mass-flux scaling as a function of the shear stress and accompanying grain-scale transport models [6,7]. The comparison suggests that impact ripple vanishing may be associated with a crossover between two fundamentally different transport modes. Only at low wind speeds, near the transport threshold, a description in terms of grain hopping on a quasi-static dilatant sand bed applies, consistent with impact ripple formation. Under stronger winds, the bed-air interface becomes increasingly blurred, due to the formation of a collision-dominated fluidized transport layer that fails to support quasi-static short-wavelength ripples but remains susceptible to longer-wavelength hydrodynamic instabilities. Our observations and tentative physical explanations may contribute to a better understanding of impact ripple formation and provide a new criterion to discriminate between impact ripples and other, more stable ripple types.

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