Appendix A: Design Procedure for Riprap Armor
Find Vss using Figure A.2.
Find D30 using Equation (A.3).
Find gradation having D30(min) $ computed D30.
A PC-based computer program of this procedure is available from the U.S.
Army Engineer Waterways Experiment Station.
(c) This procedure can be used in both natural channels with bank protection
only and prismatic channels having riprap on bed and banks. Most bank
protection sections can be designed by direct solution. In these cases, the
extent of the bank compared to the total perimeter of the channel means that
the average channel velocity is not significantly affected by the riprap.
Example 1 demonstrates this procedure.
a. Problem. Determine stable riprap size for the outer bank of a
natural channel bend in which maximum velocity occurs at bank-
full flow. Water-surface profile computations at bank-full flow
show an average channel velocity of 7.1 feet per second and a
depth at the toe for the outer bank of 15 feet. The channel is
sufficiently wide so that the added resistance on the outer bank will
not significantly affect the computed average channel velocity (true
in many natural channels). Quarries likely to be used for the
project have rock weighing 165 pounds per cubic foot and
routinely produce the 12-, 18-, and 24-inches D 100(max) gradations
shown in Table A.1. A bank slope of 1V on 2H has been selected
based on geotechnical analysis. A blanket thickness of 1D100(max)
will be used in this design. Bend radius is 620 ft and water-surface
width is 200 ft.
b. Solution. Using Figure A.2, the maximum bend velocity (Vss) is
1.48 (7.1) or 10.5 feet per second. The side slope depth at 20
percent up the slope from the toe is 12 ft. Using Equation (A.3),
the required D30 is 0.62 ft. From Table A.1, the 18-in. D100(max)
gradation has D30(min) = 0.73 ft is the minimum routinely produced
gradation that has D30(min) greater than or equal to 0.62 ft. In this
example, the actual safety factor of (1.1(0.73/0.62)) = 1.3 results
from using standard gradations to avoid the extra production costs
incurred by specifying a custom gradation for every design