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).

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:

3.39

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