Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility ControlsThe decrease in particle diameter causes a reduction in the roughness length (z 0 ), and so adecrease in wind speed (U) required to maintain u * > u *t . The threshold friction velocityreaches a minimum when particle diameter is approximately 0.08 mm (very fine sand). Afterthis point, a further decrease in particle size results in an increase in u *t . Bagnold (1941)suggested a mechanism determining this occurrence could be Reynolds number effects (anaerodynamic effect relating to a smoothing surface with decreasing grain size). Therelationship between particle size and u *t has been confirmed by a number of authors (e.g.Chepil and Woodruff, 1963; Belly, 1964; Iversen et al., 1976).Iversen and White (1982) used wind tunnel experiments to further examine the effects ofparticle size and Reynolds number on u *t under various atmospheric pressure conditions.They proposed that the minima in u *t (Figure 2.3) can be attributed to inter-particle cohesionforces between fine grains. Factors affecting the strength of these forces include moisturefilms, electrostatic charges, and van der Waals forces. The effects of these factors were notaccounted for in Bagnold’s (1941) expression to compute u *t (Equation 2.5). A number ofstudies have sought expressions relating particle cohesion, drag, lift and particle momentforces to u *t (e.g. Iversen et al., 1976; Iversen and White, 1982; Greeley and Iversen, 1985).Gillette (1988) used wind tunnel experiments to determine the effects of soil texture on u *t .The study reported increases in u *t with increasing soil clay content. This translates to a lowersoil erodibility for fine textured soils, however, u *t was found to decrease (increasing soilerodibility) as replicates of soils with cloddy, crusted and loose surface conditions wereexamined.Numerous studies have reported the effects of soil texture on wind erosion rates (Chepil,1953; Lyles, 1977; Gillette, 1978; Leys et al., 1996). In general, these studies have soughtempirical relationships between soil clay, sand and silt content and sediment transport atvarious wind speeds. Chepil (1953) presented one of the first comprehensive studies of thistype. The following expression describes the relationship between soil clay content anderosion rates (Chepil, 1953):(% clay)Q = a.(%clay)b . c(2.10)39