Appendix A: Design Procedure for Riprap Armor
Riprap protection for open channels is subjected to hydrodynamic drag and lift forces
that tend to erode the revetment and reduce its stability. Undermining by scour beyond the
limits of protection is also a common cause of failure. The drag and lift forces are created by
flow velocities adjacent to the stone. Forces resisting motion are the submerged weight of
the stone and any downward and lateral force components caused by contact with other
stones in the revetment. Characteristics of the available stone and the designer's experience
play a large part in determining size of riprap. This is particularly true on projects where
hydraulic parameters are ill-defined and the total amount of riprap required is small.
Stone size computations should be conducted for flow conditions that produce the
maximum velocity at the riprapped boundary. In many cases, velocities continue to increase
beyond bank-full discharge; but in some cases backwater effects or loss of flow into the
overbanks results in velocities that are less than those at bankfull. Channel bend riprap is
conservatively designed for the point having the maximum force or velocity. For braided
channels, bank-full discharges may not be the most severe condition. At lesser flows, flow
is often divided into multiple channels. Flow in these channels often impinges abruptly on
banks or levees at sharp angles. Precise guidance is lacking in defining design conditions for
braided channels, although a correction factor for velocity is suggested.
The method presented here for determining stone size uses depth averaged local
velocity, since a designer will be better able to estimate local velocity than local boundary
shear. Local depth-averaged velocity and local flow depth are used in this procedure to
quantify the imposed forces. Riprap size and submerged unit weight quantify the resisting
force of the riprap. This method is based on a large body of laboratory data and has been
compared to available prototype data (Maynord, 1988). This method defines the stability of
a wide range of gradations if placed to a thickness of 1D100(max). Guidance for thickness
greater than 1D100(max) is presented. The method is applicable to side slopes of 1V on 1.5h
or flatter.
(1) Velocity Estimation. The characteristic velocity for side slopes VSS is the depth-
averaged local velocity over the slope at a point 20% of the slope length from the
toe of slope. The 20% location was selected because it represents the point of
maximum side slope shear in straight channels. Various methods exist to
estimate local depth-averaged velocity for use in this design procedure.
Numerical methods include two-dimensional depth averaged models. Physical


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