Drag partition for regularly-arrayed rough surfaces

Vegetation and other roughness elements distributed across a surface can provide significant protection against wind erosion by extracting momentum from the flow and thereby reducing the shear stress acting at the surface. A theoretical model has previously been presented to specify the partition of drag forces for rough surfaces and to predict required vegetation density to suppress wind erosion. However, the model parameters have not yet been constrained and the predictive capacity of the model has remained uncertain. A wind-tunnel study was conducted to measure the drag partition for a range of roughness densities and to parameterise the model in order to improve its range of potential applicability. The drag forces acting on both an array of roughness elements and the intervening surface were measured independently and simultaneously using new drag balance instrumentation. A detailed measure of the spatial heterogeneity of surface shear stresses was also made using Irwin sensors. 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 = C-R/C-S (where C-R and C-S are roughness element and surface drag coefficients, respectively) and a constant proportional difference between the mean and maximum surface shear stress was found. The results of this experiment suggest that the definition for m, the surface shear stress inhomogeneity parameter, should be revised, although the theoretical and physical reasons for including this parameter in the model appear to be valid. Best-fit values for m ranged from 0.53 to 0.58.