Appendix A: Design Procedure for Riprap Armor
A.7 REVETMENT TOE SCOUR ESTIMATION AND PROTECTION
A.7.1 GENERAL
Toe scour is probably the most frequent cause of failure of riprap revetments. This
is true not only for riprap, but also for a wide variety of protection techniques.
A.7.2 REVETMENT TOE PROTECTION
Toe protection may be provided by either of two general approaches:
(1) Place the lower extremity below the expected scour depth or found it on
noneroding material. This is the preferred method but it can be difficult and
expensive when underwater excavation is required.
(2) Place sufficient launchable stone at the toe to arrest toe erosion before
geotechnical instability occurs. This approach has been successfully used on
many streams. It has experiences some failures where flow abruptly impinges on
the bankline, perhaps as a result of the design not adequately accounting for this
condition.
Four specific applications of these two general approaches are illustrated in Figure
A.10. Methods A and B are intended to extend to the maximum depth of scour. Method C
is suitable where significant toe scour is not expected. Method D can be adapted to a wide
range of site conditions and scour depths. Constructability and the designer's judgement
determine which method is preferable for a specific project.
Method A.
When toe excavation can be made in the dry, the riprap layer may be
extended below the existing groundline a distance exceeding the
anticipate depth of scour. If excavation quantities are prohibitive, the
concept of Method D can be adapted to reduce excavation.
Method B.
When the bottom of the channel is non-erodible material, the normal
riprap should be keyed in at streambed level.
Method C.
When the riprap is to be placed underwater and little toe scour is
expected (such as in straight reaches that are not downstream of
bends), the toe may be placed on the existing bottom of height a and
width c equal to 1.5T and 5T, respectively. This compensates for
uncertainties of underwater placement (see A.8). When the anticipated
erosion depth will undermine the stone toe, Method D should be used.
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