Initial motion of streambeds composed of coarse uniform sediments Available to Purchase

Results of an experimental study of the initial motion of coarse uniform sediments are presented in this paper. The experiments were conducted in an 8 m long, 0·3 m wide by 0·3 m deep tilting flume. Natural sediments were used and they ranged in size from 1 mm to 14 mm. The purpose of the experiments was to determine flow conditions associated with the initiation of bed sediment motion. To eliminate the subjectivity in defining the beginning of streambed movement, the concept of critical-bed state is linked to the intensity of sediment motion. The experiments reveal that critical-bed shear-stress for sediment motion depends not only on the grain size, but also on the bed slope. The latter fact is explained by the effect of relative depth (depth to grain-size ratio) on overall flow resistance. It is also shown that the value of critical dimensionless bed shear-stress is not constant for rough turbulent flow, as is usually assumed. The same conclusion follows from the measurements of turbulence characteristics near the surface of the bed. A revised Shields diagram relating critical stress, grain Reynolds number and bed slope is derived for different intensities of sediment motion. Finally, a diagram relating critical flow depth, bed slope and grain size is presented in a form suitable for use by design engineers.