as the fine material and the riprap as the coarse material. If the results 

 are within the indicated limits, an additional filter layer is not needed. 



D-. (coarse) 

 50 



D^^ (fine) 

 50 



< 40 



D,^ (coarse) .^ 

 5 15 < 40 



D.^ (fine) 

 I 5 



D,^ (coarse) 

 15 < 5 



D (fine) 



Where D is median diameter, D is the diameter particle of which 15 per- 

 cent of the material is finer, and D is the diameter particle of which 

 85 percent of the material is finer. An example of filter gradation design 

 is given in Figure C-4. 



Filter cloths have been used with success for more than a decade. They 

 can support large riprap material with no damage to the cloth. A disadvan- 

 tage of filter cloths is that the riprap must be placed with care to prevent 

 damage to the cloth. 



End and Toe Protection. The ends of the riprap revetment along the 

 channel may be subject to erosion. The erosion could remove material from 

 behind the riprap and cause failure of the riprap. Extending the riprap 

 revetment to areas not having erosive velocities is a recommended end protec- 

 tion (Figure C-5a). If this is not possible, the thickness of the riprap 

 layer should be increased to twice that otherwise needed. This extra thick- 

 ness should be placed in a recess cut into the bank to maintain a uniform 

 riprap face (Figure C-5b). 



123 



