Figure 25. Critical vertically averaged velocities for a plane bed (from van Rijn 1993)

Θcr ,s = 0.16

for D * > 10

(s - 1) gd50

ws is the sediment settling speed (which can be estimated for a given

grain size from charts or by Stokes law) and the dimensionless particle pa-

rameter, D*, is defined by:

 (s - 1) g 

1/3

D*= 



d50

 v



The value for the kinematic viscosity, n, is approximately 1 10-6 m2/s.

For determining the stability of a specific site, Figure 25 can be used to

indicate potential for site erosion when a distribution of the vertically av-

eraged velocities, bed grain-size distribution, and water depth are known.

If the velocities are frequently above ucr, then there is a potential for

some site erosion. There is a strong likelihood for severe erosion if the ve-

locities frequently exceed ucr,s. It should be emphasized that if there is

any question concerning site stability, i.e., Figure 25 does not clearly indi-

cate that erosion will not occur, more detailed data collection and model-

ing efforts should be undertaken to determine erosion potential.

Figure 25.

Critical vertically averaged velocities for a plane bed (from van Rijn 1993)

Chapter 8 Long-Term Cap Stability