Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility Controlsresistance of the surface to mobilisation (Bagnold, 1941). This fluid threshold is described bythe threshold friction velocity (u *t ):up a= A g D(2.5)*t.awhere ρ a is the density of air; ρ p is the density of particles; g is the acceleration due to gravity,and D is the particle diameter. A is an empirical coefficient related to Re p (the particleReynolds number). The Reynolds number can be described by the shear velocity, particlediameter and kinematic viscosity v:DRep= u*.(2.6)vRe p provides a measure of turbulence around a particle. Particle movement, and wind erosion,therefore occur when u * > u *t . Of note, is that Equation (2.5) describes the threshold frictionvelocity in terms of the soil properties of particle diameter and packing density. In practice,this may only be applicable to loose sandy soils. Equation (2.5) does not account for theeffects of inter-particle cohesion (by soil moisture or crusting), roughness effects (e.g. bystone cover or vegetation), or how these may vary over soils of different texture.In addition to wind stress effects, bombardment by grains may also initiate movement of newgrains, and results in the ability for sediment movement to be maintained at wind velocitieslower than u *t . The lower threshold is the dynamic or impact threshold u t : 30 u t= 680 D.log(2.7) D where D is the mean surface grain diameter. Once the wind friction velocity exceeds theentrainment threshold velocity (i.e. u * > u *t ) particle mobilisation will occur. Movement ofsediment by wind occurs in a number of modes. These modes are linked and occur during theevolution of the erosion process. The modes include creep, reptation, saltation andsuspension, and are illustrated in Figure 2.2. The modes in which particles are moved are afunction of grain size and weight, and the wind friction velocity and transport capacity.35