Aeolian shear stress partitioning within sparse vegetation

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King, James Stephen
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University of Guelph

This thesis investigates the application of shear stress partitioning theory to wind erosion over vegetated surfaces within and and semi-arid environments. Its purpose is to examine the available partitioning models for wind erosion, expose their dependencies with previously collected data, investigate the limitations of the best available model through a laboratory wind tunnel study, and increase the application of the model through the refinement of the previously undefined variables with a field study. The Raupach et al. (1993) model was superior because of the incorporation of non-erodible element geometry and the alterations to the flow they can cause, while the Marticorena and Bergametti (1995) approach displayed a scaling dependency making it ineffective outside of a laboratory setting. Limitations of the Raupach et al. (1993) model included a dimensionless spatial variable that displayed a height dependency and the use of an undefined variable to characterize the distribution of surface stresses. A laboratory wind tunnel study was conducted to ascertain the height dependency of the Raupach et al. (1993) model, while the measurement of the surface and total shear stress were also investigated to evaluate the applicability of fitting the logarithmic profile to measurements within sparsely-spaced elements. The direct measurement of the shear stress within the shear stress ratio and the logarithmic profile determination of the shear velocity corresponded very well with previous field and laboratory measured shear stress ratios, respectively suggesting only the cautionary use of the logarithmic profile without prior knowledge of the constant stress layer. Field-based measurements collected in the Chihuahuan desert indicated that the aerodynamic properties of a mesquite-dominated surface were attributable to the alignment of the vegetation with the dominant wind direction. A relationship was found between the roughness length of the surface and the Raupach et al. (1993) model for a given wind direction. This linear relationship showed an increase with an increase in roughness length. This result contributes insight into the credibility of using a parameter to quantify the maximum surface shear stress and provides a solution to the Raupach et al. (1993) model.

shear stress partitioning, sparse vegetation, shear stress partitioning theory, wind erosion, semi-arid environments