Appendix A: Design Procedure for Riprap Armor
(d) In some cases, a large part of the channel perimeter is covered with riprap;
the average channel velocity, depth, and riprap size are dependent upon one
another; and the solution becomes iterative. A trial riprap gradation is first
assumed and resistance coefficients are computed using Equation (A.2).
Then the four steps described in paragraph (b) are conducted. If the
gradation found in Step 4 is equal to the assumed trial gradation, the
solution is complete. If not a new trial gradation is assumed and the
procedure is repeated. Example 2 demonstrates this procedure.
a. Problem. Determine stable riprap size in a bend of a trapezoidal
channel with essentially uniform flow. Bank slope is 1V on 2H and
both the bed and banks will be protected with the same size of
riprap. The bottom width is 140 ft, slope is 0.0017 ft/ft, and the
design discharge is 13,500 cfs. Use 1D100(max) thickness and the
same quarry as in Problem 1. Bend radius is 500 ft and bend angle
is 120 deg.
b. Solution. In this problem the solution is iterative; flow depth,
velocity, and rock size depend on each other. Use Strictler's
equation n = 0.036 (D90(min))0.166 to estimate Manning's resistance
coefficient. Bend velocity is determined using Figure A.2. Assume
trial gradation and solve for riprap size as shown in Tables A.2 and
A.3. Use uniform flow computations to determine the following:
Table A.2 Uniform Flow Computations
From iterative solution of Manning's equation Q/A = (1.49/n)R2/3S1/2.
Use velocity estimation and riprap size equations to obtain riprap
size in Table A.3: