*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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