Appendix A: Design Procedure for Riprap Armor
tend to prevent leaching of lower bank material through the launched riprap. Launchable
stone should have D85D15 $ 2.
Table A.4 Increase in Stone Volume for Riprap Launching Sections
Vertical Launch Distance
Volume Increase Percent
(ft)1
Dry Placement
Underwater Placement
# 15
25
50
> 15
50
75
1
From bottom of launch section to maximum scour.
A.8 DELIVERY AND PLACEMENT
The common methods of riprap placement are hand placing; machine placing, such
as from a skip, dragline, or some form of bucket; and dumping from trucks and spreading by
bulldozer. Hand placement produces the most stable riprap revetment if the long axis of the
riprap particles are oriented perpendicular to the bank. It is the most expensive method
except when stone is unusually costly and/or labor unusually cheap. Steeper side slopes can
be used with hand-placed riprap than with other placing methods. This reduces the required
volume of rock and may be an attractive alternative where rights of way are restricted.
However, the greater cost of hand placement usually makes machine or dumped placement
methods and flatter slopes more economical. Also, geotechnical considerations also dictate
channel side slope design. In the machine placement method, sufficiently small increments
of stone should be released as close to their final positions as practical. Rehandling or
dragging operations to smooth the revetment surface tend to result in segregation and
breakage of stone. Stone should not be dropped from an excessive height as this may result
in the same undesirable conditions. Riprap placement by dumping and spreading is not
recommended as significant segregation and breakage can occur. However in some cases,
it may be economical to increase the layer thickness and stone size somewhat to offset the
shortcomings of this placement method. Smooth, compact riprap sections have resulted from
compacting the placed stone sections with a broad-tracked bulldozer. This stone must be
quite resistant to abrasion. Thickness for underwater placement should be increased by 50
percent to provide for the uncertainties associated with this type of placement. Underwater
placement is usually specified in terms of weight of stone per unit area, to be distributed
uniformly in a "grid" pattern established by survey control.
A.9 ICE AND DEBRIS
Ice and debris create greater stresses on riprap revetment by impact and flow
concentration effects. Ice attachment to the riprap also causes a decrease in stability. One
rule of thumb is that thickness should be increased by 6-12 in., accompanied by appropriate
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