Appendix A: Design Procedure for Riprap Armor
Method D.
An extremely useful technique where water levels prohibit excavation
for a toe section is to place a launchable section at the toe of the bank.
Even if excavation is practicable, this method may be preferred for cost
savings if the cost of extra stone required to produce a launched stone
revetment is exceeded by the cost of excavation required to carry the
design thickness T down the slope. This concept simply uses toe scour
as a substitute for mechanical excavation. This method also has the
advantage of providing a "built-in" scour gage, allowing easy
monitoring of high-flow scour by visual inspection of the remaining toe
stone after the high flow subsides, or by surveyed cross sections if the
toe stone is underwater. Additional stone can then be added for
reinforcement if necessary. This method is also readily adaptable to
emergency protection, where high flow and the requirement for quick
action make excavation impractical.
Shape of the stone toe is not critical. For trench-fill revetments, the height of the
stone section is generally one-half to one times the length. For weighted riprap toes, heights
of 2.5 to 4.0 times the bank protection thickness should be used. Providing an adequate
volume of stone as discussed below is critical.
To compute the required launchable stone volume for Method D, the following
assumptions should be used:
Launch slope = 1V on 2H. This is the slope resulting from rock launched on
non-coehesive material in both model and prototype surveys. Launch slope
is less predictable if coehesive material is present, since cohesive material may
fail in large blocks.
Scour depth = existing elevation - maximum scour elevation.
Thickness after launching = thickness of the bank revetment T.
To account for the stone lost during launching and for underwater placement, the
increases in stone volume listed in Table A.4 are recommended. Using these assumptions, the
required stone volume for underwater placement for vertical launch distance less than 15 ft
equals
Required Stone Volume
= 1.5T times launch slope length
= 1.5T times scour depth 5
= 3.35T (scour depth)
A safety factor should be added if data to compute scour depth are unreliable, if
cohesive bank material is present, or if monitoring and maintenance after construction cannot
be guaranteed. Widely graded ripraps are recommended because of reduced rock voids that
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