HR: 0800h
AN: P31B-0991    [Abstracts]
TI: Shear Stress Partitioning in Airflow over Rough Surfaces: Roughness Form Effects and Influence on the Distribution of Shear Stress
AU: * Gillies, J A
EM: jackg@dri.edu
AF: Division of Atmospheric Sciences Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512 United States
AU: Nickling, W G
EM: nickling@uoguelph.ca
AF: Wind Erosion Laboratory Department of Geography University of Guelph, Gordon Street, Guelph, ON N1G 2W1 Canada
AU: King, J
AF: Wind Erosion Laboratory Department of Geography University of Guelph, Gordon Street, Guelph, ON N1G 2W1 Canada
AB: Roughness elements distributed across a surface can significantly decrease the entrainment and transport of underlying fine-grained sediments by wind. The parameterization of roughness effects on wind erosion thresholds and sediment transport is critical to the development of models that can provide realistic predictions of sediment thresholds and fluxes due to wind erosion. Raupach et al. (1993) present a model for predicting the protective role of roughness elements in terms of a threshold friction velocity ratio as a function of the roughness geometry and the aerodynamic properties of the surface and roughness elements. The predictive capacity of this model remains uncertain and the work presented here represents part of an on-going effort of our group to improve the parameterization of the Raupach et al. (1993) model. To gain additional understanding of how roughness elements influence the magnitude and nature of the shear stress acting on the surface among the elements and evaluate strength and weaknesses of the roughness density parameter to characterize these effects, a wind tunnel study using model roughness arrays of similar roughness density composed of cube-shaped elements of different length dimensions was undertaken. Roughness density is defined as the total frontal area of all the elements to the total surface area that they occupy. Shear stress in the above element air flow was determined from vertical wind speed profile measurements. Point measurements of near surface shear stresses within the roughness array were made with simple omni-directional skin friction meters in order to investigate the partitioning of shear stress to the intervening surface. The results suggest that the roughness density parameter has severe limitations in describing the shear stress partitioning for these regularly arrayed rough surfaces. For surfaces with identical roughness densities, the surface composed of more and smaller elements was observed to have average and maximum shear stresses up to 20 percent lower than the surface composed of larger and less numerous elements, for similar freestream wind speeds. The results from the wind tunnel testing also suggest an explanation for the physical basis of the effect roughness has on particle transport thresholds due to alterations in the distribution of shear stress. Raupach et al. (1993) assumed that this effect was a result of spatial inhomogeneities in the surface shear however data from this study indicate the roughness modifies the frequency distribution of the range of shear stresses reaching the surface.
DE: 5415 Erosion and weathering
DE: 5445 Meteorology (3346)
DE: 3307 Boundary layer processes
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting