SHEAR STRESS PARTITIONING FOR IDEALIZED VEGETATED SURFACES

Abstract

Vegetation and other surface roughness materials partition the shear force of flowing water into a portion actingon the vegetation (vegetal shear) and the remainder acting on the intervening soil surface (particle shear). The fraction actingon the soil surface is directly involved in subsequent particle detachment. The purpose of this study was to directly measurethe components of shear stress and to quantify the shear partition for various densities of idealized elements representativeof non-submerged rigid vegetation in overland flow. Insight into the magnitude of particle shear and vegetal shear isnecessary for understanding the role of vegetation in reducing particle shear and, consequently, reducing potential erosion.Circular cylinders and idealized elements with differences in the rate of change in upstream frontal area with flow depth wereused to model vegetation. Detailed spatial and temporal particle shear measurements were made using a unique hydraulicflume and hot-film anemometry. Drag force was measured on individual elements within test arrays. This combination ofmeasurements allowed for direct determination of the shear partition. The tests were conducted on three uniform elementdensities at discharges of 0.005 and 0.01 m3/s. Element width-to-spacing ratios ranged from 0.04 to 0.20. Over the rangeof densities studied, particle shear accounted for 13% to 89% of the total shear, indicating that complete surface coverageis not required to significantly reduce the shear stress acting on soil particles. Existing shear partitioning theory, in whichthe partition is a function of the ratio of element to surface drag coefficients and the roughness density, was found to representthe observed partition reasonably well (mean squared error = 0.036). The results from this study are important for selectingappropriate plant species and densities for erosion control systems.