Wind-Tunnel Modelling of the Influence of Vegetation Structure on Saltation Threshold

Abstract

The influence of vegetation structure on saltation threshold was investigated using uniformly spaced arrays of non-erodible roughness elements on a bed of erodible sand in an wind tunnel. Structural variables tested using arrays of solid cylinders included element aspect ratio (height/diameter) and lateral cover (total frontal-silhouette area per unit ground area). In agreement with previous studies, increase in saltation threshold above the value for bare sand was strongly related to lateral cover. Increasing aspect ratio from 0.25 to 4 tended to enhance the increase in saltation threshold at a given lateral cover. An approximate fit to the results could be obtained Porous elements were constructed as clusters of narrow, vertically oriented cylinders (model 'stems'), forming porous model plant bodies that were cylindrical in overall shape. Low-porosity elements were found to be approximately 50 per cent more effective in increasing saltation threshold than either solid or high-porosity elements. The ratio of plantbody frontal-silhouette area (based on overall dimensions) to total stem frontal-silhouette area was found to be a useful measure of plant-body porosity for wind erosion studies. Some conventional measures of vegetation amount fail to account for changes in vegetation structural attributes that may strongly influence aeolian processes.

Vegetation in Drylands: Effects on Wind Flow and Aeolian Sediment Transport

“…Taylor [92] found that drag coefficients decreased exponentially as the porosity of a two-dimensional barrier increased, but Grant and Nickling [91] demonstrated that a peak in drag coefficient occurred at an intermediate optical porosity (~20%) for three-dimensional trees. It has been shown that elements with a porosity less than 20% act to enhance wind speed recovery in their lee [68,[93][94][95], so elements with intermediate porosity strike a compromise between shear stress reduction and the downwind distance for which it is effective [96,97]. However, even porosity values of 50% can cause wake effects to extend to at least 50 h downwind of a windbreak [98].…”

“…However, even porosity values of 50% can cause wake effects to extend to at least 50 h downwind of a windbreak [98]. The sediment trapping mechanism of plants (see Section 2.1.3) also depends on plant porosity [53,54,96], with wind tunnel (e.g., [97]) and field (e.g., [91]) experiments showing that the trapping effect of vegetation is maximised at intermediate values of porosity (20-40%). Using wire mesh structures, Gillies et al [86,99] showed that porous elements have a greater potential for modulating sediment flux than solid elements in large spatial arrays.…”

“…Shear stress (or drag) partitioning is traditionally used to quantify the proportion of the wind shear stress or shear velocity (u * ) that is acting on a soil surface, compared to the proportion acting on vegetation elements (e.g., [48,96,115]). The theoretical basis for partitioning comes from the work of Schlichting [116], who proposed that the total drag on a roughened surface (τ) can be expressed as:…”

“…This deficiency is minimised in large homogeneous areas of randomly or regularly spaced vegetation of the same height, but becomes problematic in natural environments displaying structural anisotropy [27,58]. The λ parameter has also been shown to inadequately describe three-dimensional objects, due an intrinsic bias in using the frontal silhouette area [90,96]. Chappell and Webb [148] provided an in-depth analysis of the limitations of the drag partition scheme with respect to Raupach's [57] assumptions about how the sheltering afforded by roughness elements can be approximated by the λ parameter (denoted L c in their text).…”

Aeolian sand transport: a wind tunnel model

“…Musick et al, 1996), where strips of double-sided sticky tape were placed flat on the bed surface near the downwind end of the test tray to capture moving grains by adhesion. This initiation of sand grains was observed visually.…”