A Perspective of the USDA Watershed Erosion and Sedimentation Research

Keynote Lecture

Drag reduction on saltating sediment in shallow flow

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732Laboratory flume experiments of shallow overland flow with sediments consisting of sand particles of sizes (1000-1400 μm) and (600- 850 μm) and fine particles of sand (74-125 μm) revealed several features similar to gravity driven granular flow experiments with glass beads [Prasad et al. (2000)]. In the saltating flow regime the particle velocity was measured by cross-correlating signals obtained with twin photonic probes and particle concentrations by pre-calibrated single probe signals. In the very low particle concentration range, particle velocity showed an increased trend followed by a decreased trend at larger concentration values. A simple model was developed for the particles transport in water that showed drag reduction by so called "drafting". This "drafting" model is based on the superposition principle of two flow fields. First the drag coefficient is computed for the case of a single particle and the second part consists of a similar coefficient due to the flow field behind the leading particle. Thus, for a row of infinite number of spherical particles of diameter, ds, equally spaced at a distance, s, (center to center), the ratio of the drag coefficients (multiple to single particles) is found to be (1 - cα2) where α is the linear concentration, ds/s and c (a constant) depends on the particle Reynolds number