TM 5-820-4/AFM 885, Chap 4
material may dictate the gradation of riprap to be
for protection as a function of the Froude number.
used. In such cases the gradation should resemble
(See TM 5-820-3AFM 88-5, Chapter 3. Two curves
as closely as possible the recommended mixture.
are given, one to be used for riprap subject to
Consideration should be given to increasing the
direct attack or adjacent to hydraulic structures
thickness of the riprap blanket when locality dic-
such as side inlets, confluences, and energy dis-
tates the use of gradations with larger percents
sipators, where turbulence levels are high, and
of small stone than recommended. If the grada-
the other for riprap on the banks of a straight
channel where flows are relatively quiet and par-
tion of the available riprap is such that movement
allel to the banks. With the depth of flow and av-
of the natural material through the riprap blan-
ket would be likely, a filter blanket of sand, crushed,
erage velocity in the channel known, the Froude
rock, gravel, or synthetic cloth must be placed under
number can be computed and a stone size deter-
mined from the appropriate curve. Curves for de-
the riprap. The usual blanket thickness is 6 inches,
but greater thickness is sometimes necessary.
termining the riprap size required to prevent scour
downstream from culvert outlets with scour holes
of various depths are also available. The thickness
5-5. Riprap design. An ideal riprap design would
of the riprap blanket should be equal to the long-
provide a gradual reduction in riprap size until
est dimension of the maximum size stone or 1.5
the downstream end of the blanket blends with
times the stone diameter (50 percent size), which-
the natural bed material. This is seldom justified.
ever is greater. When the use of very large rock
However, unless this is done, turbulence caused
is desirable but impractical, substitution of a
by the riprap is likely to develop a scour hole at
grouted reach of smaller rock in areas of high ve-
the end of the riprap blanket. It is suggested that
locities or turbulence maybe appropriate. Grouted
the thickness of the riprap blanket be doubled at
riprap should be followed by an ungrouted reach.
the downstream end to protect against undercut-
ting and unraveling. An alternative is to provide
a constant-thickness rubble blanket of suitable
5-4. Riprap gradation. A well-graded mixture
length dipping below the natural streambed to the
of stone sizes is preferred to a relatively uniform
estimated depth of bottom scour.
size of riprap. In certain locations the available
5-2
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