THE EFFECT OF ROUGHNESS ELEMENTS ON WIND EROSION THRESHOLD

A theory is developed to describe the dependence upon roughness density of the threshold friction velocity ratio R(t), the ratio of the threshold friction velocity of an erodible surface without roughness to that of the surface with nonerodible roughness present. The roughness density is quantified by the frontal area index lambda. The prediction is R(t) = (1 - msigmalambda)-1/2(1 + mbetalambda)-1/2 , where beta is the ratio of the drag coefficient of an isolated roughness element on the surface to the drag coefficient of the substrate surface itself; sigma is the basal-to-frontal area ratio of the roughness elements; and m (< 1) is a parameter accounting for differences between the average substrate surface stress and the maximum stress on the surface at any one point. The prediction is well verified by four independent data sets.