Quantitative Definition of Beginning of Bed Movement

The well-known Shields diagram (originally published in 1936) is widely accepted as a criterion for the beginning of bed movement of uniform non-cohesive sediment on a flat bed. However, the wide scatter of points on some recent revisions of the diagram is evidence that subjective assessment of beginning of bed movement is unsatisfactory. Physically, the difficulty is caused by the fact that near the so-called beginning of movement only the peaks of the turbulent fluctuations are competent to displace grains. Because these fluctuations are statistically distributed, rare grain movements can take place even at low values of the mean (time-average) shear. If for a given area of bed the number of grains displaced per unit time is plotted against mean shear, the curve will be quite flat at its lower end, so that it is virtually impossible to decide where to draw a line and call it beginning of movement. To say this is not necessarily to imply that a threshold of shear does not exist.