Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility ControlsSaleh and Fryrear (1995) identified a number of deficiencies in the early studies of moistureeffects on particle mobilization by wind. A particular limitation is that the expressions do notaccount for processes that affect particle entrainment once u * exceeds u *t and the saltationprocess is established. Subsequently, two expressions were developed for abrading (2.17) andnon-abrading (2.18) surface conditions where:( TWC / W ') 0.375( TWC / ) 2u ta= +(2.17)*0.205+ 0.182W '( TWC / W ') 0.506( TWC / ) 2u t= +(2.18)*0.305+ 0.222W 'where u *t is the threshold friction velocity under non-abrading conditions, u *ta is the thresholdfriction velocity under abrading conditions, TWC is the soil surface threshold water content(the moisture content at which particle entrainment is initiated), and W’ is the gravimetricwater content of the soil. Their results suggest that under abrading conditions, because of theerosive impact of saltating particles, less wind energy is required to initiate entrainment.Recent developments in quantifying the effects of soil moisture on the process of soilmobilization by wind have combined wind tunnel experimentation with conceptual models ofinter-particle cohesion. The focus of the research moved from trying to develop regressionequations to quantifying the binding forces responsible for inter-particle cohesion. McKenna-Neuman and Nickling (1989) presented a theoretical model for the effect of moisture on u *t .The research developed the notion that inter-particle contacts can be modelled by accountingfor capillary forces between particles with angular contact geometries. The basis for themodel, the inter-particle capillary force (F c ) is defined by:T= G(2.19)PF c2where T is the surface tension of the water, P is the pressure deficiency, and G is a nondimensionalcoefficient. This function was incorporated into Bagnold’s (1941) staticthreshold model (Equation 2.5). Two expressions were developed to account for open-packed(Equation 2.20) and closed-packed grains (Equation 2.21):46