Appendix A: Design Procedure for Riprap Armor
Some aspects of riprap design are not precisely defined. In order to provide more
specific guidance, the Corps of Engineers has conducted studies to determine stability of
various riprap gradations and thickness, velocity estimation in bends, impinged flow in braided
streams, and design of toe and end protection. Many of these studies were conducted in a
large outdoor test channel having a capacity of 200 cfs. For information about these studies,
contact the Hydraulics Laboratory at the U.S. Army Waterways Experiment Station, 3909
Halls Ferry Road, Vicksburg, Mississippi 39180-6199, USA.
The references contain descriptions of some other approaches to riprap design. The
designer may want to examine these other approaches to determine if they would be useful
in some applications to supplement the guidance presented here.
The following provides guidance on stone shape, size/weight relationship, unit weight,
gradation, and layer thickness. Reference EM 1110-2-2302 for additional guidance on riprap
material characteristics and construction.
Riprap should be blocky in shape rather than elongated, as more nearly cubical stones
"nest" together best and are more resistant to movement. The stone should have sharp, clean
edges at the intersections of relatively flat faces. Cobbles with rounded edges are less
resistant to movement, although the drag force on a rounded stone is less than on sharp-edged
cubical stones. As graded cobble interlock is less than that of equal-sized angular stones, the
cobble mass is more likely to be eroded by channel flow. If used, the cobbles should be
placed on flatter side slopes than angular stone and should be about 25 percent larger in
diameter. The following shape limitations should be specified for riprap obtained from quarry
The stone shall be predominately angular and subangular in shape.
Not more than 30 percent of the stones reasonably well distributed throughout
the gradation shall have a/c greater than 2.5.
Not more than 15 percent of the stones reasonably well distributed throughout
the gradation shall have a/c greater than 3.0.


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