Figure 6.18. Windrow revetment, definition sketch (after U.S. Army Corps of Engineers 1981).
The velocity and characteristics of the stream dictate the size of stone that must be
used to form a windrow revetment. The size of stone used in the windrow was not significant
as long as it was large enough to resist being transported by the stream. An important design
parameter is the ratio of the relative thickness of the final revetment to the stone diameter. It
was found that large stone sizes will require more material than smaller stone sizes to produce
the same relative thickness. Since a filter cannot be used, a well-graded stone is important to
ensure that the revetment does not fail from leaching of the underlying bank material. The
stream velocity was found to have strong influence on the ultimate side slope of the revetment.
It was determined that the initial bank slope was on the average approximately 15 percent
steeper than the final revetment slope. In general, the greater the velocity, the steeper the side
slope of the final revetment.
Filters. Filters are used under riprap to allow water to drain easily from the bank without
carrying out soil particles. Filters must meet two basic requirements: stability and permeability.
The filter material must be fine enough to prevent the base material from escaping through the
filter, but it must be more permeable than the base material. Section 6.7 provides general
specifications for granular and geosynthetic filters. HEC-11 (Brown and Clyde 1989) contains
more detailed design guidance.
6.5.7 Riprap Failure Modes
In a preliminary evaluation of various riprap design techniques, Blodgett and McConaughy
(1985) concluded that the procedures based on velocity as a means of estimating stresses on
the boundary provide the most reliable and consistent results. The following procedures were
investigated: The 1967 version of FHWA HEC-11, FHWA HEC-15, USACE (EM-1601),