Figure 26. Initiation of motion for waves over a plane bed based on critical velocity (from van Rijn 1993)

Figure 26.

Initiation of motion for waves over a plane bed based on critical velocity (from van Rijn

1993)

Example 1, Current-dominated environment: If the region of inter-

est is in 10 m of water, the median grain size (d50) is 500 mm, then the

critical velocity for initiation of motion from Figure 25 is approximately

44 cm/s, and the critical velocity for initiation of suspension is 70 cm/s

(these values can also be calculated from the equations in this section). If

the vertically averaged velocity for a particular storm frequently exceeds

50 cm/s with peak velocities around 65 cm/s, then it can be assumed that

the sediment bed will experience some erosion during the storm.

Example 2, Wave-dominated environment: The water depth is 5 m,

wave period is 7 s, wave height is 0.5 m, and d50 is 200 m.

For these conditions, it is determined that L = 46 m and k = 0.14 m-1.

Using the supplied equation, Uδ = 0.30 m. From Figure 26, for a d50 of

200 mm and wave period of 7 s, Uδ,cr is approximately 0.24 m/s. There-

fore, the bottom shear stresses generated by these conditions, represented

by Uδ = 0.30 m, are greater than the critical value of 0.24 m/s, and erosion

will occur under these conditions.

Chapter 8 Long-Term Cap Stability