M6 Wind Transport and Dune Systems

Topic

Aeolian sediment transport is the movement of sand, silt, and dust particles driven by the wind across surfaces that lack vegetation or have very little of it—such as deserts, beaches, bare agricultural fields, or tundras.

Saltation is the dominant mechanism of sand movement: grains are lifted into the wind as soon as the wind's shear stress on the surface exceeds the Shields threshold for the aeolian environment; these grains then follow ballistic trajectories—rising roughly ten to one hundred centimeters and traveling thirty to one hundred centimeters horizontally—before falling back to the surface. It is precisely this impact upon landing that mobilizes neighboring grains, a phenomenon known as the "splash" effect. Creep, on the other hand, involves the movement of grains too heavy to saltate; instead, they are pushed forward by the repeated impacts of saltating grains striking them.

The rate of aeolian sand transport described by Bagnold's law is proportional to the cube of the friction velocity—specifically, the velocity exceeding the mobilization threshold—meaning that small increases in friction velocity result in much larger increases in the volume of sand transported. The aeolian mobilization threshold itself depends on grain size: for typical dry beach or desert sand, this threshold corresponds to a friction velocity of between 0.2 and 0.5 meters per second, which equates to a wind speed of five to eight meters per second at a height of ten meters above the surface. The flux of mineral dust—composed of particles smaller than seventy micrometers in diameter—is of global significance: the Sahara Desert alone emits between four hundred and two thousand teragrams of dust annually, which ultimately fertilizes both the Amazon rainforest and the Atlantic Ocean with the iron and phosphorus it carries.

Dunes are the characteristic sedimentary structures resulting from this aeolian transport, and their shapes reveal the conditions under which they formed: crescent-shaped barchan dunes appear where sand is scarce and the wind blows consistently in one direction; transverse dunes, conversely, form where sand is abundant; longitudinal dunes—also known as seif dunes—align with oblique winds; and star dunes develop where the wind blows from multiple directions throughout the year.

The behavior of this entire system involves dune migration speeds that are inversely proportional to dune height, meaning shorter dunes move faster than taller ones; it also encompasses dune stabilization by vegetation—a process that occurs once plant cover extends over fifteen to twenty percent of the dune's surface, as this cover reduces near-ground wind speed below the threshold required for particle mobilization. Thus, from the initial particle movement described at the beginning of this text to the final stabilization by vegetation—and encompassing saltation, Bagnold’s law, mineral dust flux, and diverse dune shapes—aeolian transport and dune systems sculpt the arid landscape, grain by grain, through the force of the wind sweeping across it.

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