incipient motion; if S.F. is less than unity, the riprap is unstable. Problem F.1 in Appendix F
illustrates how to determine the stability of riprap.
Simplified Design Aid For Side Slope Riprap. When the velocity along a side slope has no
downslope component (i.e., the velocity factor is along the horizontal), some simple design aids
can be developed.
For horizontal flow along a side slope, the equations relating the stability factor, the stability
number, the side slope angle, and the angle of repose for the rock are obtained from
Equations 6.4 and 6.6 with λ = 0.
η tan φ
β = tan -1
2 sin θ
(6.7)
and
1 + sin β
η′ = η
(6.8)
2