Threshold Velocities and Initial Particle Motion as Influenced by Air Turbulence
S particles move in response to the dynamic forces generated by fluid flow. A wind strong enough to produce soil-particle movement is always turbulent (Chepil and Woodruff, 1963). Sutherland (1966) has stated that grains cannot be lifted from the bed in water without the presence of turbulent fluctuations adjacent to and directed toward the bed. Although it has been recognized that velocity and pressure fluctuations influence threshold or critical values of drag or of velocity for initiating sand or soil movement, quantitative information is limited. Kalinske (1943) has suggested that the local longitudinal-turbulence intensity, <ru/uz, is about 33 percent near the bed. Assuming a normal distribution of velocity fluctuations, instantaneous velocities would be twice the mean, and momentary values of drag would be four times the mean. Earlier, White (1940) had made a similar observation.