The effect of roughness elements on wind erosion threshold

Abstract

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 A. The prediction is R t -(1 -mrrA)-•/2(1 + m/3A) -•/2, where/3 is the ratio of the drag coefficient of an isolated roughness element on the surface to the drag coefficient of the substrate surface itself; rris 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.1.

Lateral vegetation growth rates exert control on coastal foredune hummockiness and coalescing time

“…Given an initial topography h(x, y) and a vegetation field, the model computes the bed shear stress perturbation due to the presence of a non-flat topography (Weng et al, 1991), modified by a separation bubble (when there is flow separation; Kroy et al, 2002) and the subsequent shear stress reduction due to vegetation (Raupach et al, 1993). From the bed shear stress field, the local nonuniform sand flux and sand flux divergence is then computed at every position (Kroy et al, 2002;Durán et al, 2010) -this determines the temporal change in topography.…”

Total vertical sediment flux and PM10 emissions from disturbed Chihuahuan Desert surfaces

“…In naturally vegetated landscapes, especially in humid regions, the vegetation protects the soil by absorbing part of the wind's shear force and reducing impact with the surface (Raupach, 1992;Raupach et al, 1993). Even in semi-arid and arid environments vegetation, although often sparse, protects much of the surface from the erosive forces of wind (Okin, 2008;Wolfe and Nickling, 1993).…”

Vegetation Against Dune Mobility

“…As a result, the total surface shear stress τ ≡ | τ | can be divided into two components, τ v acting on the vegetation and τ s on the sand grains. When plants are randomly distributed and the effective shelter area for one plant is assumed to be proportional to its frontal area, the absorbed shear stress τ v is proportional to the vegetation frontal area density λ times the undisturbed shear τ s [10]. Therefore, the fraction τ s of total stress acting on sand grains is reduced to…”

“…m is a model parameter [10,11]. If the plant frontal area density λ is zero there is no shear stress reduction.…”