Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility Controlsproportional to the cube of its speed (Bagnold, 1941). Therefore, a slight decrease in windspeed will result in a significant reduction in its energy and capacity to erode (Liu et al.,1990). Factors determining the degree of protection or potential momentum reductionafforded by standing vegetation include vegetation or non-erodible element size, geometry,spacing (density), lateral cover, flexibility and porosity. In line with this momentumreduction, vegetation displaces the surface roughness length (Equation 2.3). Thisdisplacement shelters the soil surface in the lee of elements, and increases boundary layerturbulence (Shao, 2000).Chepil (1950b) and Chepil and Woodruff (1963) introduced the critical surface constant tomodel the effects of roughness elements, including both soil surface roughness and standingcover. They reported that the relationship between roughness element height (H) dividend bythe distance between elements (d) was constant at the point at which wind erosion iscontrolled by roughness. Lyles and Allison (1981) found that this was not in fact constant,but changed as a function of u * . They determined that as surface roughness increases thesurface stress absorbed by the roughness elements also increases. Marshall (1971) andMarshall (1972) further examined the effects of roughness element density and distributionon surface drag. The concepts explored in this work led to the development of schemes toadjust u *t for bare surfaces to account for the presence of non-erodible roughness elements.Gillette et al. (1989) presented a method to quantify the effect of surface roughness on the u *tthrough the threshold friction velocity ratio R t = u *tS /u *tR . The method computes the ratio ofthe threshold friction velocity of a bare surface (u *tS ) to that of one covered with roughnesselements (u *tR ). Like the SLR, their model decreases from 1 as roughness increases over abare surface. Raupach (1992) and Raupach et al. (1993) developed this model to predict R tbased on shear stress partitioning between the roughness elements and the surface. Thepremise of the model was that provided by Marshall (1971), that “the attenuating effect ofroughness on erosion is closely related to momentum absorption by roughness, which isclosely controlled by the frontal area…of the roughness elements” (Raupach et al., 1993:3023). The frontal area index (λ) is defined by the expression:nbh =(2.29)s58