8.1 Conditions for dune formation
8.1.1 Aeolian transport
The movement of sand by wind results from momentum transfer from air to sediment (Sherman and Hotta, 1990). Simple deterministic models have been developed to quantify this concept in order to predict aeolian transport rates across ideal surfaces. One of the most frequently used is the equation of Bagnold (1936):
where q is the rate of sediment transport, C is an empirical constant ranging from 1.5 (nearly uniform sand) to 2.8 (sand of a very wide range of grain sizes), ρa is air density, g is acceleration due to gravity, D is grain diameter, Dr is a reference grain diameter of 0.25 mm, and u* is shear velocity. Of note is that the sediment transport rate according to eqn. 8.1 is proportional to the velocity cubed and that the equation disregards the existence of a threshold velocity. Models of aeolian transport are primarily based on assumptions that: (1) the wind field is unidirectional, fully turbulent, uniform and steady, implying that the vertical velocity profile can be readily defined; (2) sediments available for entrainment and transport are uniform in size and composition; and (3) the surface is planar, horizontal, dry and unobstructed (Sherman ...