The variation of the critical shear stress for beginning of motion as a function of sediment
size given by various investigators is shown in Figure 3.17 (Chien 1954). The spread of the
representative curves shows the diversity of experimental and theoretical results.
Nevertheless, equations, tables and figures given in this section provide useful tools for
determining beginning of motion for sediment particles and the design of riprap.
Figure 3.17. Comparison of critical shear stress as a function of grain diameter
(after Chien 1954).
3.5.5 Equations for Flow and Sediment Variables for Beginning of Motion
Particles start to move in steady, uniform flow when the shear stress applied by the flow
equals the resistance to movement of the particles. A useful relation can be developed
between the flow velocity, depth, and resistance and bed material size by equating the
applied shear stress to a resistance to motion shear stress. In equation form it is as follows: