Field measurement of the initiation of large bed particle motion in Blue Creek near Klamath, California

More than two-thirds of the field measurements of bed velocity necessary to initiate motion of coarse natural particles whose size, shape, specific gravity, and orientation angle were known agree within 20 percent of those velocities predicted from theory.The theory is based on balancing turning moments of the fluid forces of drag and lift with the resisting moment of the submerged particle weight.Initial motion seems to depend more on size and shape than on specific gravity or orientation angle.In fact, shape differences almost completely compensate for differences in specific gravity ranging from 2.65-3.00 and orientation angles ranging from 0°-25°.Bed velocities necessary to initiate motion of coarse bed material in Blue Creek are equaled or exceeded about 5 percent of the time.This fact and changes in channel topography and cross-sectional area emphasize the ability of perennial mountain streams to transport coarse bed material frequently.