The effect of roughness elements on wind erosion: The importance of surface shear stress distribution

Abstract

Representation of surface roughness effects on aeolian sediment transport is a key source of uncertainty in wind erosion models. Drag partitioning schemes are used to account for roughness by scaling the soil entrainment threshold by the ratio of shear stress on roughness elements to that on the vegetated land surface. This approach does not explicitly account for the effects of roughness configuration, which may be important for sediment flux. Here we investigate the significance of roughness configuration for aeolian sediment transport, the ability of drag partitioning approaches to represent roughness configuration effects, and the implications for model accuracy. We use wind tunnel measurements of surface shear stress distributions to calculate sediment flux for a suite of roughness configurations, roughness densities, and wind velocities. Roughness configuration has a significant effect on sediment flux, influencing estimates by more than 1 order of magnitude. Measured and modeled drag partitioning approaches overestimate the predicted flux by 2 to 3 orders of magnitude. The drag partition is sensitive to roughness configuration, but current models cannot effectively represent this sensitivity. The effectiveness of drag partitioning approaches is also affected by estimates of the aerodynamic roughness height used to calculate wind shear velocity. Unless the roughness height is consistent with the drag partition, resulting fluxes can show physically implausible patterns. These results should make us question current assessments of the magnitude of vegetated dryland dust emissions. Representing roughness effects on surface shear stress distributions will reduce uncertainty in quantifying wind erosion, enabling better assessment of its impacts and management solutions.

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

“…The schemes seek to establish the mean (or maximum) shear stress ratio in the absence of information about the heterogeneity of roughness on the land surface [87]. The spatial distribution of vegetation has been shown to affect horizontal sediment flux, even at low roughness densities [28,51,119,[144][145][146]. The roughness distribution effects on aeolian sediment transport are also nuanced, in that the sediment mass flux may not always be significantly affected as long as vegetation is not overly clumped together [147].…”

“…Brown et al [119] demonstrated that roughness configuration may have a small effect on the average drag partition when basic descriptive statistics (e.g., mean, standard deviation) are used to represent variability in surface shear stress measurements, but parameters of the Raupach et al [121] drag partition scheme are sensitive to roughness configuration. Webb et al [146] used Brown et al's [119] data to further examine the effects of roughness configuration and the performance of the Raupach et al [121] and Marticorena and Bergametti [118] drag partition schemes for estimating sediment mass flux. Both drag partition schemes were found to have difficulty in accurately, and realistically, representing sediment mass flux responses to changing free stream velocity (U f ), λ and z 0 .…”

“…Both drag partition schemes were found to have difficulty in accurately, and realistically, representing sediment mass flux responses to changing free stream velocity (U f ), λ and z 0 . This is due to uncertainties and potential inconsistencies in the parameter values and a lack of sensitivity of the schemes to roughness configuration when applied with invariant parameter values [146]. Drag partition schemes that explicitly represent the effects of roughness configuration on the distribution of surface shear stress are needed to improve the accuracy of wind erosion models [151].…”

Using solid element roughness to control sand movement: Keeler Dunes, Keeler, California

“…12 suggests that the non-uniform distribution of the straw bales did not greatly affect the reduction in sand flux compared to roughness placed in a staggered array pattern. The effect of the arrangement of roughness on NSF is likely to be noticeable when that arrangement produces a pattern that includes long open linear areas that mimic the sand transport corridors identified by Gillette et al (2006) as ''streets'' in mesquite dominated communities in the Chihuahuan Desert near Las Cruces, NM (Webb et al, 2014). When considering the use of roughness to control sand transport or dust emissions, distribution patterns with larger open areas should probably be avoided.…”

Controls on aeolian sediment dynamics by natural riparian vegetation in the Eastern Tarim Basin, NW China

“…We use a well-established physical based model to base upon the process rather than on empirical relationships. Nevertheless for drag partitioning as implemented in our model structure, there are some known issues related to the spatial distribution of the roughness elements (Okin, 2008;Webb et al, 2014). The partitioning scheme we selected is assuming regular distributed roughness elements across every grid cell, which does only occur approximately in natural environments.…”