TM 5-820-3/AFM 88-5, Chap. 3
velocity and turbulence, movement of the natural
bed material through the riprap resulting in
slumping of the blanket, and undercutting and
raveling of the riprap by scour at the end of the
blanket. Therefore, in design, consideration must be
given to selection of an adequate size stone, use of
an adequately graded riprap or provision of a filter
blanket, and proper treatment of the end of the
blanket.
d. Expanding and lining the channel downstream
from a square or rectangular outlet for erosion
control can be with either sack revetment or
cellular blocks as well as rock riprap, as placed
shown in figure 5-14. The conditions of discharge
and tailwater required to displace sack revetment
with length, width, and thickness of 2, 1.5, and
0.33 feet, respectively (weight 120 pounds);
cellular blocks, 0.66 by 0.66 foot and 0.33 foot
thick (weight 14 pounds); or riprap with a given
thickness are shown in figure 5-15. The
effectiveness of the lined channel expansion relative
to the other schemes of riprap protection described
c. Three ways in which riprap can fail are move-
previously is shown in figure 5-10.
ment of the individual stones by a combination of
5-15
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