• Dikes should be built beyond the limits of the low-flow buffer. 



BANK PROTECTION BY REVETMENTS 



A revetment is a layer of erosion resistant material placed on a bank 

 or embankment to armor against erosion. Methods and materials for revetments 

 other than riprap are available but are not discussed here because they 

 generally are unacceptable for environmental reasons. 



Riprap 



The most common form of revetment is riprap, a layer of rock which 

 may be dumped, hand-placed, or grouted. Dumped rock riprap is most commonly 

 used, although grouted rock riprap may be applicable if the available ma- 

 terial is not large enough to meet the requirements of dumped riprap. Rock- 

 filled wire baskets (gabions) may also be used when available materials are 

 of insufficient size to meet dumped riprap requirements. There are several 

 factors important in the design of dumped rock riprap; these include: 



• Shape, size, and gradation of the rock. 



• Density and durability of the rock. 



• Velocity and depth of flow near the rock. 



• Steepness of the slope being protected. 



• Thickness of the riprap layer. 



• Filter blanket presence and design. 



• End and toe protection. 



These factors are discussed briefly in the following sections. 



Shape, Size, and Gradation . The shape, size, and gradation of the rock 

 riprap are the primary properties in resisting erosion. The shape should be 

 angular to provide an interlocking of the rocks. Large rock is more erosion 

 resistant than small rock. Selection of the proper rock size is a complex 

 function of flow characteristics and slope of the embankment being protected. 

 Karaki et al. (1974) present a method for estimating rock size. Nei I I (1973) 



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