A wind-tunnel investigation of drag partition

Vegetation distributed across a surface may provide significant protection against wind erosion by extracting momentum from the flow and reducing the shear stress acting at the surface. Raupach et al. (1993) proposed a theoretical model to specify the partition of drag forces for rough surfaces and to predict required vegetation density to suppress wind erosion. However, the model parameters beta and m have not yet been constrained and the predictive capacity of the model has remained uncertain.; This study was conducted to measure the drag partition for a range of roughness densities and to parameterise the Raupach et al. (1993) model and improve its range of potential applicability. New drag balance instrumentation was used in a recirculating wind-tunnel to measure independently and simultaneously the drag forces acting on both an array of roughness elements and the intervening surface. A detailed measure of the spatial heterogeneity of surface shear stresses was also made. The data agreed well with previous results and confirmed the general form of the model. Analysis of the drag partition confirmed the parameter definition beta = CR/C S. Measured spatial distributions of surface shear stress were negatively skewed and a constant proportional difference between the mean and maximum surface shear stress was found. The definition for m was found to be incorrect, although the theoretical and physical reasons for including this parameter in the model remain valid. Best-fit values for m ranged from 0.53 to 0.58.