Modelling wind-blown sediment transport around single vegetation elements

Abstract

Wind erosion is an important soil erosion and hence a soil degradation problem in the Sahelian zone of West Africa. Potentially, the characteristic dryland vegetation with scattered trees and shrubs can provide for soil erosion protection from wind erosion, but so far adequate quantification of vegetation impacts is lacking. The aim of this study was to develop a model of windblown soil erosion and sediment transport around a single shrub-type vegetation element. Starting with the selection of a suitable transport equation from four possible sediment transport equations, the effects of a single vegetation element on wind speed were parameterized. The modified wind speed was then applied to a sediment transport equation to model the change in sediment mass flux around a shrub. The model was tested with field data on wind speed and sediment transport measured around isolated shrubs in a farmer's field in the north of Burkina Faso. The simple empirical equation of Radok (Journal of Glaciology 19: 123-129, 1977) performed best in modelling soil erosion and sediment transport, both for the entire event duration and for each minute within an event. Universal values for the empirical constants in the sediment transport equation could not be obtained because of the large variability in soil and roughness characteristics. The pattern of wind speed, soil erosion and sediment transport behind a shrub and on either side of it was modelled. The wind speed changed in the lee of the vegetation element depending on its porosity, height and downwind position. Wind speed was recovered to the upstream speed at a downwind distance of 7·5 times the height of the shrub. The variability in wind direction created a 'rotating' area of influence around the shrub. Compared to field measurements the model predicted an 8% larger reduction in sediment transport in the lee of the vegetation element, and a 22% larger increase beside the vegetation element.

A coupled vegetation/sediment transport model for dryland environments

“…The total amount of sediment eroded from a given cell is calculated deterministically as a function of the cell ' s wind velocity, using the semiempirical flux relationship of Dong et al []: where Q is the predicted mass flux for the given time interval (kg m −1 s −1 ), u is the mean horizontal wind velocity over the given time interval (m s −1 ), u t is the horizontal velocity threshold for sediment entrainment (m s −1 ), ρ air is the density of air (1.25 kg m −3 ), g is acceleration due to gravity (9.81 m s −2 ), and a D is an empirically fitted constant. Dong et al 's [] model has been shown to perform well over a variety of averaging intervals using field‐based wind velocity data [ Leenders et al ., ; Mayaud et al ., ].…”

Vegetation and sedimentation on coastal foredunes

“…Arens et al, 2001b;Dong et al, 2008;Dupont et al, 2014;Lancaster and Baas, 1998;Leenders et al, 2011;Luo et al, 2014;Okin, 2008;Van Dijk et al, 1999), it remains difficult to predict patterns of erosion and deposition for varying wind conditions and on irregularly vegetated surfaces, such as foredunes Leenders et al, 2011). However, such predictions are a prerequisite for modelling foredune development, as erosion and sedimentation patterns eventually determine foredune morphology and evolution (Hesp, 1988).…”

The effect of vegetation patterns on wind-blown mass transport at the regional scale: A wind tunnel experiment

“…A large number of studies have reported that vegetation has the capacity to decrease soil loss by wind as it reduces wind speed and soil erodibility, and increases the entrapment of eroded material (Van De Ven et al, 1989;Sterk et al, 1998;Udo and Takewaka, 2007;Abdourhamane Toure et al, 2011;Leenders et al, 2011;Munson et al, 2011). Research on stabilization of moving sand dunes has also linked reduction of sand movement to the increased trapping capacity of vegetation cover (Hesse et al, 2004;Chang et al, 2011;Drenova, 2011;Floyd and Gill, 2011).…”