Appendix A: Design Procedure for Riprap Armor
Table A.3 Velocity Estimation and Riprap Size
VSS1
Computed D302
D30(min) of Trial3
Trial D100(max)
(in.)
(fps)
(ft)
(ft)
0.48
0.59
9.9
12
0.73
0.53
9.5
18
0.97
0.48
9.2
24
1
From Figure A.2 using trapezoidal channel.
2
From Equation (A.3).
3
From gradation information given in Table A.2.
This example demonstrates that the increasing rock size for the three
trial gradations results in increasing depth and decreasing velocity. The
minimum acceptable routinely produced gradation is the 18-in.
D100(max).
(e) In braided streams and some meandering streams, flow is often directed into
the bank line at sharp angles (angled flow impingement). Precise guidance
is lacking on determining the imposed force for this condition. Until better
guidance can be developed, a local velocity of 1.6 times the average channel
velocity at the impingement point is recommended for use in riprap design.
The discharge used for design conditions should correspond to a stage at or
just above the tops of the mid-channel bars. A velocity distribution
coefficient CV of 1.25 is typical of low R/W bends and should be used for
impingement points in Equation (A.3).
(f) Transitions in channel size or shape may also require riprap protection. The
procedures presented here are applicable to gradual transitions where flow
remains tranquil. In areas where flow changes from tranquil to rapid and
then back to tranquil, riprap sizing methods applicable to hydraulic
structures should be used (Hydraulic Design Criteria, Chart 712). In
converging transitions, the procedures based on Equation (A.3) can be used
unaltered. In expanding transitions, flow can concentrate on one side of the
expansion and design velocities should be increased. For installations
immediately downstream of concrete channels, a vertical velocity
distribution coefficient of 1.25 should be used due to the difference in
velocity profile over the two surfaces.
A.5 FILTER REQUIREMENTS
Guidance for filter requirements for riprap and other armors is given by USACE
(1986), and Pilarczyk (1984).
A-26
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