Figure 6.14. Stability numbers for a 1.5 stability factor for horizontal flow along a side slope.

The curves in Figure 6.14 are computed in the following manner: (1) Select an angle of repose

φ. For example φ = 45; (2) Select a side slope angle. For example θ = 25; (3) Compute Sm

from Equation 6.11:

tan 45

Sm =

= 2.14;

tan 25

Compute η from Equation 6.12 with S. F. = 1.5

(4)

(2.14)2 - (1.5)2

η=

cos 25 = 0.31

(1.5) (2.14)2

Repeat the above steps for the full range of interest for φ and θ.

(5)

If the shear stress τo on the side slope is known, then, from Equation 6.5, the riprap size

required is obtained from:

21τo

Dm =

(6.13)

(S s - 1) γ η

where the stability number η is obtained from Figure 6.14.

In riprap design, it is often desirable to relate the tractive force (shear stress) acting on the

riprapped bed or bank to the fluid velocity in the vicinity of the riprap. For fully turbulent flow,

the reference velocity Vr at a distance D50 above the bed is determined from Equation 6.14.

6.24

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