Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility Controlswhere n is the number of roughness elements, b and h are mean width and height of theroughness elements (giving frontal area bh), and s is the surface area. The method uses thethreshold friction velocity approach, whereby the total shear stress (τ) on a land surface canbe partitioned into that which is incident on roughness elements (τ R ), and that which isincident on the substrate surface (τ S ). From Equation 2.4, this can be presented as:2 = u= R+*(2.30)Swhere ρ is the air density, and u * is the friction velocity. Figure 2.9 illustrates the dragpartitioning model for a bare surface (a), surface covered with sparse vegetation (b), and adensely vegetated surface (c). The underlying assumption in this theory is that the surfacestress deficit behind isolated roughness elements can be described by an effective shelterarea. This shelter area can be characterised by the geometry of the elements and bulk flowproperties (Raupach et al., 1993). Importantly, the model allows for the effective shelter areasin the lee of non-erodible elements to be superimposed.Figure 2.9 Illustration of the effects of vegetation on surface roughness and the drag partitioningmodel (after Chepil and Woodruff, 1963). (a) shows the relationship for a bare surface, (b) for asurface with sparse vegetation cover, and (c) for a densely vegetated surface.Following from Equation 2.30, the total threshold shear stress (τ) on a surface covered withroughness elements, and the threshold shear stress (τ S ) of a bare surface can be defined by:59