Thickness of Riprap. The thickness of the riprap should be sufficient 

 to provide the desired protection of the slope. The minimum thickness should 

 be equal to the longest dimension of the largest rock or be 50 percent larger 

 than the median rock size, whichever is larger. This minimum thickness should 

 be increased by 50 percent if: 



• Wave action is possible. 



• Gradation is not as recommended. 



• Riprap is to be placed in flowing water. 



• A filter is not used when recommended. 



Filter Blankets. A filter blanket may be recommended for placement 

 beneath the rock riprap layer to prevent the loss of bank material through 

 the voids in the riprap. If the material washes out, cavities will form 

 beneath the riprap and failure of the riprap revetment can occur. The require- 

 ments for a filter depend on the size and gradation of the bank material and 

 on the voids in the riprap layer. If the composition of the bank material is 

 such that it is easily eroded, a filter layer is generally recommended. 

 Poor riprap gradation is also a reason to recommend a filter. Filters may 

 be well-graded gravel or a synthetic filter cloth. 



Gravel filters should use gravels ranging from about 5 mm to 90 mm 

 (Karaki et al. 1974). Filter thickness should be no less than 0.15 m; filter 

 thickness equal to half the riprap thickness is recommended. More than one 

 layer, of different gradation and median size, should be considered if there 

 is a very large difference in size between the bank material and the riprap 

 rock. Recommended guidelines for gradation of the filter are given by Karaki 

 et al. (1974); they are summarized in relations below. 



These relations should be applied to each layer in turn, starting with 

 the bank material as the fine material and using the needed filter material 

 as the coarse. The first filter selected then becomes the fine material for 

 the next filter layer computation. After determining the size and gradation 

 of each filter, these relations should be used with the last selected filter 



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