Appendix A: Design Procedure for Riprap Armor
A.6 REVETMENT TOP AND END PROTECTION
A.6.1 REVETMENT TOP
When the full height of a levee is to be protected, the revetment will cover the
freeboard, i.e., extend to the top of the levee. This provides protection against waves,
floating debris, and water-surface irregularities. Factors which determine whether protection
can be terminated below design flowline are discussed in 6.2.5 of the main text. Figure A.6
provides general guidance for velocity variation over channel side slopes that can assist in
evaluating the economics of reducing or omitting revetment for upper bank areas. Gradation
and thickness reduction should not be made unless a sufficient quantity is saved to be cost
effective. A horizontal collar or key at the top of the riprap can be provided to protect
against escaping and returning flows by adapting the end protection methods illustrated in
Figure A.9.
A.6.2 REVETMENT END PROTECTION
The upstream and downstream ends of riprap revetment should be protected against
erosion by increasing the revetment thickness T or extending the revetment to areas of
noneroding velocities and relatively stable banks. The following guidance applies to the
alternative methods of end protection illustrated in Figure A.9.
Method A.
For riprap revetments 12 in. thick or less, the normal riprap layer
should be extended to areas where velocities will not erode the natural
channel banks.
Method B.
For riprap revetments exceeding 12 in. in thickness, one or more
reductions in riprap thickness and stone size may be adopted for a
distance a (Figure A.9) in which velocities decrease to a noneroding
natural channel velocity, if savings in stone quantity justify the extra
expenses of using more than on gradation.
Method C.
For all riprap revetments that do not terminate in noneroding natural
channel velocities, the ends of the revetment should be enlarged, as
shown in Figure A.9. The dimensions a an b should be 3 and 2 times
the layer thickness, respectively. The decision to terminate in erosive
velocities should be made cautiously, since severe erosion can cause
the revetment to fail by progressive flanking.
A-27
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