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This report presents the results from a study on shear stress partitioning for vegetation. The project involved partitioning the shear stress from overland flow into one component that acts on the vegetation (form shear) and the remainder that acts on the intervening soil particles (particle shear). Particle shear is important for predicting soil erosion. The study used idealized shapes to represent vegetal elements. Researchers designed and constructed a unique laboratory hydraulic flume, which they used in conjunction with hot-film anemometry to measure particle shear. They also designed and constructed instrumentation to measure the form shear on individual rigid vegetal elements, taking detailed spatial and temporal shear stress measurements for three element densities. Form shear was measured on each element within the test array. The study investigated a total of 16 test scenarios. Particle shear accounted for 13 to 89 percent of the total shear. Shear partitioning theories developed for wind erosion adequately represent the observed data and can be used to determine an appropriate vegetation density for a threshold particle shear.

This report presents the results of a field study on the performance of erosion control products under artificial rainfall conditions, bed shear partitioning using a hydraulic flume, and regression analysis of previously published data. Ninety-six runs of different plot lengths, erosion control treatments, vegetative cover, and initial moisture contents were gathered and analyzed. Above-ground biomass varied substantially within the growing season, runoff was most strongly influenced by initial moisture content, and sediment load was substantially reduced using erosion control products and mulches. In the hydraulic flume experiments, the percentage of the total shear acting on soil particles was less than 13.2% for all tests. Failure to obtain a large data base of product characteristics greatly limited the use of regression analysis to explain the performance of erosion control blankets.