Controls on and effects of armoring and vertical sorting in aeolian dune fields: A numerical simulation study

Abstract

Unlike ripples, there are only few numerical studies on grain size segregation at the scale of dunes in aeolian environments. Here we use a cellular automaton model to analyze vertical sorting in granular mixtures under steady unidirectional flow conditions. We investigate the feedbacks between dune growth and the segregation mechanisms by varying the size of coarse grains and their proportion within the bed. We systematically observe the development of a horizontal layer of coarse grains at the top of which sorted bed forms may grow by amalgamation. The formation of such an armor layer controls the overall sediment transport and availability. The emergence of dunes and the transition from barchan to transverse dune fields depend only on the grain size distribution of the initial sediment layer. As confirmed by observation, this result indicates that armor layers should be present in most arid deserts, where they are likely to control dune morphodynamics.

Large grains matter: contrasting bed stability and morphodynamics during two nearly identical experiments

“…While it may seem that an immobility threshold of 5% is small, this value is comparable to the results reported by Gao et al. (), who found that aeolian dunes do not form when the initial proportion of coarse (i.e. immobile) grains within the bed is greater than 4–5%.…”

Controls on the aerodynamic roughness length and the grain‐size dependence of aeolian sediment transport

“…Desert landscapes with abundant sand supply and a wide grain-size distribution often exhibit evidence of vertical grain sorting, suggesting that armor layers should be present in most arid landscapes, as confirmed by recent field observations (Gao et al, 2016). Armor layers in natural landscapes can vary greatly in depth and typically become thicker near the ripple crest but can be as thin as a monolayer at the surface (Bagnold, 1941;Sharp, 1963).…”

“…Other studies have suggested that changes in surface roughness and saltation threshold can lead to the development of armor layers at the surface in the presence of saltating particles (Rasmussen and Sørensen, 1999;Gao et al, 2016). At values of u * > 0.55 m s -1 , the flux density of particles ranging in size from 0.125 to 0.25 mm remained fairly constant or decreased slightly at the measurement height of 0.185 m. These trends were only observed for the smallest size fraction when shear velocities were large.…”

“…At values of u * > 0.55 m s -1 , the flux density of particles ranging in size from 0.125 to 0.25 mm remained fairly constant or decreased slightly at the measurement height of 0.185 m. These trends were only observed for the smallest size fraction when shear velocities were large. However, the mechanistic effects of armor layers on sediment transport and availability is not well understood because of the complexities that wide grain-size distribution introduces in the sediment transport process (Dong et al, 2008;Frey and Church, 2009;Gao et al, 2016;McKenna Neuman and Bédard, 2017). Other studies have suggested that changes in surface roughness and saltation threshold can lead to the development of armor layers at the surface in the presence of saltating particles (Rasmussen and Sørensen, 1999;Gao et al, 2016).…”

“…Armor layers in natural landscapes can vary greatly in depth and typically become thicker near the ripple crest but can be as thin as a monolayer at the surface (Bagnold, 1941;Sharp, 1963). Although armor layers are abundant in many desert landscapes, their overall control on sediment transport and availability is poorly understood in most natural landscapes because of the complexity that wide grain-size distribution introduces in the sediment transport process (Dong et al, 2008;Frey and Church, 2009;Gao et al, 2016;McKenna Neuman and Bédard, 2017).…”