Quantcast STONE SIZE (Cont.)

Appendix A: Design Procedure for Riprap Armor
D30  = riprap size of which 30 percent is finer by weight
Sf   = safety factor (see (3) below),
Cs   = stability coefficient for incipient failure, thickness =
ID100(max) or 1.5D50(max), whichever is greater,
D85/D15 = 1.7 to 5.2,
= 0.30 for angular rock,
= 0.375 for rounded rock,
Cv  = vertical velocity distribution coefficient,
= 1.0 for straight channels, inside of bends,
= 1.283 - 0.2 log (R/W), outside of bends (1 for (R/W) >
= 1.25, downstream of concrete channels,
= 1.25, ends of dikes,
CT  = thickness coefficient (Figure A.8),
= local depth of flow (length),
W  = unit weight of water (weight/volume),
= local area velocity, usually VSS (length/time),
K1  = side slope correction factor (see below), and
= gravitational constant (length/time2).
This equation can be used with either SI (metric) or non-SI units.
(3) Safety Factor. The basic equation for stone size as defined by Equation (A.3)
produces a rock size that is at incipient failure for Sf = 1. To produce a
competent riprap design, the characteristic rock size must be increased in size to
resist hydrodynamic and a variety of nonhydrodynamic imposed forces and/or
uncontrollable physical conditions. The size increase can best be accomplished
by including the safety factor coefficient, which will be a value greater than unity.
The minium safety factor is Sf = 1.0. The basic safety factor may have to be
increased in consideration for the following conditions:
(a) Imposed impact forces resulting from logs, uprooted trees, loose vessels,
ice, and other types of large floating debris. Impact will produce more
damage to a lighter weight riprap section than to a heavier section.
Therefore, consideration of an added safety factor should be given to the
lighter weight riprap design. For moderate debris impact, it is unlikely that
an added safety factor should be used when the blanket thickness exceeds
18 inches.
(b) The basic stone sizing parameters of velocity, unit weight of rock, and depth
need to be determined as accurately as possible. The ability to determine
exact values is highly unlikely; therefore, a safety factor should be included
to compensate for small inaccuracies in these parameters. If conservative


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