Threshold friction velocities for large pebble gravel beds

Abstract

Experiments have been performed in a high‐speed wind tunnel to determine threshold friction velocities for the onset of continuous particle motion (the fluid threshold velocity per Bagnold [1941]) for two types of gravel soil beds. The 50% finer‐by‐weight particle diameter for the fine gravel sample was ∼6.35 mm and for the coarse gravel bed was 42 mm. Measured threshold friction velocities ( u * t ), as determined with rake‐mounted Pitot‐static probes, were 3.1 and 9.9 m/s for the fine and coarse gravel soils, respectively. Corresponding free‐stream threshold velocities ( u ∞ t ) were 41.8 and 94.3 m/s. Summarized results from the current measurements and other natural soil experiments [ Gillette , 1978, 1988] vary approximately with the square root of particle diameter, in agreement with the prediction of Bagnold [1941] but exceed his results by a factor of 2–3.