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Page Title: Figure 26. Initiation of motion for waves over a plane bed based on critical velocity (from van Rijn 1993)
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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.
89
Chapter 8 Long-Term Cap Stability

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