Initial motion of streambeds composed of coarse uniform sediments

Results of an experimental study of the initial motion of coarse uniform se diments 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 use d and they ranged in size from 1 mm to 14 mm, The purpose of the experiment s was to determine flow conditions associated with the initiation of bed se diment motion. To :eliminate the subjectivity in defining the beginning of streambed movement, the concept of critical-bed state is linked to he inten sity of sediment motion. The experiments reveal that critical-bed shear-str ess for sediment motion depends not only on the grain size, but also on the bed lope. The latter fact is explained by the effect of relative depth (de pth to grain-size ratio) on overall flow resistance. It is also shown that the value of critical dimensionless bed shear-stress is not constant or rou gh turbulent flow, as is usually assumed, The same conclusion follows m the measurements of turbulence characteristics near the surface of the bed, re vised Shields diagram relating critical tress, grain Reynolds number and be d slope is derived for different intensities of sediment motion. Finally, a diagram relating critical flow depth, bed slope and grain size is presente d in a form suitable for use study design engineers.