Critical shear stress for incipient motion of sand/gravel streambeds

Results of an experimental study of the incipient motion of streambeds comp osed of sand/gravel sediment mixtures are reported and compared with the ea rlier findings for uniform sediments. The experiments were conducted in an 8 in long by 0.30 in wide glass-walled tilting flume and an 18 in long by 0 .80-1.10 in wide trapezoidal concrete channel. A reference transport method is used to define the beginning of bed material movement. The experiments demonstrate that the incipient motion of individual size fractions within a mixture is controlled by their relative size with respect to median size ( intergranular eff.-cts), mixture standard deviation (effect of the shape of grain-size distribution), absolute value of median size (absolute size eff ect), and bed slope (effect of relative depth on overall flow resistance). The shear stress at incipient motion of median-sized grains in mixtures is found to be the same as for uniform sediment of this size. The present find ings are consistent with available flume and field data. A technique for ca lculating the critical shear stress of different grain sizes in coarse unif orm sediments and unimodal/weakly bimodal sediment mixtures is proposed.