Geoscience Reference
In-Depth Information
Fig. 5.2 Mechanisms for
dust emission. ( a )Dust
emission by aerodynamic lift,
( b ) by saltation bombardment
and ( c ) through
disaggregation (Adapted from
Shao et al. 2011 )
have highlighted the importance of saltation as a prerequisite for dust emission (Sow
et al. 2009 ;Shaoetal. 2011 ). Once produced, particles smaller than
70 m can
be entrained by suspension because their terminal fall velocity is lower than the
vertical wind velocity; the smallest fraction ( D < 20 m) can easily be uplifted and
transported long-range.
In the following sections, the physical processes involved in the main stages
of dust emission and the soil and surface properties or environmental factors that
influence them are described.
5.3
Erosion Threshold
The threshold for particle motion results from the balance between the wind shear
stress, , and the forces acting on the soil particles to hold them on the surface. These
forces include the particle weight P , interparticle cohesion forces I p and cohesion
forces induced by soil moisture F c . U t is thus a function of the parameters that
control these forces, mainly the particle size and density and the soil moisture.
The generic expression “wind erosion threshold” encompasses several different
concepts. First, it corresponds either to a wind velocity at a given height U ( z )or
to a wind friction velocity U * , both defined and measured at the erosion threshold.
Second, the definition of the threshold can depend on the way it is experimentally
determined. Theoretically, it should be the threshold of movement of the first
particles set in motion by the action of the fluid (also called the “fluid threshold”).
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