Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility Controlscalled the roughness height (z 0 ). For bare sandy surfaces this effect can be attributed to themean particle dimensions:z = 10D30(2.1)where D is the mean diameter of particles on the surface. The distance between particles andother roughness elements will also affect the roughness length (Lancaster, 1995). The shearvelocity (u * ) describes the total drag force imparted by the airflow on the roughness elementsand surface. The shear velocity is related to wind speed and roughness length by:kUu*= (2.2)ln( z / z0)where k is the von Karman constant (approximately equal to 0.4); U is the wind velocity atheight z, and z 0 is the height at which velocity is zero. When large roughness elements arepresent (e.g. soil clods, stones, vegetation), the roughness length may be displaced upwards.This new region of zero velocity is called the zero plane displacement height (d), and is afunction of roughness element height, density, distribution, flexibility, and permeability. Theshear velocity can then be described by:u*kU= (2.3)ln( z d / z0)The shear stress (τ) exerted by the wind on the surface can be related to the shear (drag)velocity and to the density of air (ρ a ) by the following expression:u *=(2.4)aIntuitively, the expression implies that both the shear velocity and shear stress increase aswind velocity (U) increases. As the wind velocity increases, grains on the surface may beginto move. At this point, the shear velocity (u * ) overcomes a ‘fluid threshold’ which defines the34