Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility Controlsu6sin 22cos F3dg sin * tw= u*tc+( s a)1(2.20)where the particle resting angle is defined by β and is approximately equal to 30°, andu6sin 23d ( 2cos + 1)( ) g sin* tw= u*tFc+s a1(2.21)where β is approximately 45°, ρ a is the air density and ρ s is the particle density (kgm -3 ), g isgravitational acceleration (ms -2 ), and d is the mean particle diameter (m).The model was calibrated by wind tunnel experimentation with a range of sand particle sizegroups, but was not tested for soils with high clay content. The model performance decreaseswhen adhesive forces become the primary forces holding water between particles, andcapillary forces, for which the model was developed to describe, become less important.Based on this outcome, McKenna-Neuman and Nickling (1989) proposed that tension may bea better parameter to describe the effects of soil moisture than gravimetric moisture content asit is more directly related to inter-particle cohesion.Gregory and Darwish (1990) reported a theoretical model to calculate soil moisture effects onu *t . The model was developed from wind tunnel experimentation and data presented in theliterature by earlier studies and uses a simpler model for grain shape and packing than theMcKenna-Neuman and Nickling (1989) model, using spheres rather than cones toapproximate grain packing angles. The following expression was obtained:u* tw= u* t6 a1+ w + gda+ exp gd214swa3ww1.5( w w )c(2.22)where ρ w is the density of water (kgm -3 ), w c is the moisture content attached to clay particlesor aggregates (kg kg -1 ), and a 1 , a 2 and a 3 are coefficients (a 1 in kgs -2 , a 2 in kg m-1s -2 and a 3 isdimensionless). The value for a 1 was derived from data reported by Greeley and Iversen(1985), while the values for a 2 and a 3 were developed from data presented in studies by Belly(1964), Bisal and Hsieh (1989) and McKenna-Neuman and Nickling (1989). As the contact47