INCIPIENT MOTION OF SAND-GRAVEL SEDIMENT MIXTURES

Experiments on the incipient motion of gravel and sand mixtures of 0:100, 10:90, 25:75, 45:55, and 100:0 percent ratios of gravel and sand, respectively, were made in a laboratory flume. A series of transport experiments was made for each of the five sediment beds and the critical shear stress for motion was calculated for 12 size ranges by interpolating or extrapolating the bed shear stress for a very small transport rate. The sand in each of the five bed mixtures began to move at nearly the same bed shear stress. All gravel sizes began to move at nearly the same bed shear stress in the 100% gravel experiments, but in the sand-gravel experiments the gravel showed an increase in critical shear stress with increasing size. Reasons for this change in the initiation of motion of the gravel from the 100% gravel to the sand-gravel mixtures may result from the abundant sand-sized sediment inhibiting the formation of a coarse bed surface layer in the sand-gravel experiments.