Shear stress partitioning for idealized vegetated surfaces

A. M. Thompson, B. N. Wilson, B. J. Hansen

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29 Scopus citations


Vegetation and other surface roughness materials partition the shear force of flowing water into a portion acting on the vegetation (vegetal shear) and the remainder acting on the intervening soil surface (particle shear). The fraction acting on the soil surface is directly involved in subsequent particle detachment. The purpose of this study was to directly measure the components of shear stress and to quantify the shear partition for various densities of idealized elements representative of non-submerged rigid vegetation in overland flow. Insight into the magnitude of particle shear and vegetal shear is necessary 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 were used to model vegetation. Detailed spatial and temporal particle shear measurements were made using a unique hydraulic flume and hot-film anemometry. Drag force was measured on individual elements within test arrays. This combination of measurements allowed for direct determination of the shear partition. The tests were conducted on three uniform element densities at discharges of 0.005 and 0.01 m3/s. Element width-to-spacing ratios ranged from 0.04 to 0.20. Over the range of densities studied, particle shear accounted for 13% to 89% of the total shear, indicating that complete surface coverage is not required to significantly reduce the shear stress acting on soil particles. Existing shear partitioning theory, in which the partition is a function of the ratio of element to surface drag coefficients and the roughness density, was found to represent the observed partition reasonably well (mean squared error = 0.036). The results from this study are important for selecting appropriate plant species and densities for erosion control systems.

Original languageEnglish (US)
Pages (from-to)701-709
Number of pages9
JournalTransactions of the American Society of Agricultural Engineers
Issue number3
StatePublished - May 1 2004


  • Drag
  • Hot-film anemometry
  • Overland flow
  • Vegetal elements


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