Appendix A: Design Procedure for Riprap Armor
A.3 CHANNEL CHARACTERISTICS
A.3.1 SIDE SLOPES
The stability of riprap bank revetment is affected by the steepness of channel side
slopes. Side slopes should not be steeper than 1V on 1.5H, except in special cases where it
may be economical to use larger hand-placed stone keyed well into the bank. Side slopes
from 1V on 2H to 1V on 3H are recommended for riprap stability. The size of stone required
to resist the erosive forces of channel flow increases when the side slope angle approaches
the angle of repose of a riprap revetment. Rapid water-level recession and piping-initiated
failures are other factors in determining channel side slopes. Embankment stability analysis
should properly address soils characteristics, ground-water and river conditions, and probable
failure mechanisms.
A.3.2 CHANNEL ROUGHNESS, SHAPE, ALIGNMENT, AND INVERT SLOPE
As boundary shear forces and velocities depend on channel roughness, shape,
alignment, and invert slope, these factors must be considered in determining the size of stone
required for riprap revetment. Comparative cost estimates should be made for several
alternative channel plans to determine the most economical and practical combination of
channel factors and stone size. Resistance coefficients (manning's n) for riprap surfaces
should be estimated using the following form of Stricklers equation:
n ' K D90(min)1/6
(A.2)
where
D50(min) = size of which 90 percent of sample is finer, from minimum or lower limit
curve of gradation specification (ft),
k
= 0.036 average of all flume data,
K
= 0.034 for velocity and stone size calculation, and
= 0.038 for capacity and freeboard calculation.
The K values represent the upper and lower bounds of laboratory data determined for bottom
riprap. Resistance data from a large laboratory channel having an irregular riprap surface
similar to riprap placed underwater resulted in a 15% increase in Manning's n above the dry
placement values given above. These Manning n values represent only the grain resistance
of the riprap surface.
A-10
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