Riprap Filter Design Using FHWA Hydraulic Toolbox

To do a Riprap Filter Design with the FHWA Hydraulic Toolbox, you need to understand the underlying criteria the program uses to accept or reject a filter design.  The criteria are taken from National Cooperative Highway Research Program Report 568: Riprap Design Criteria, Recommended Specifications, and Quality Control. Section 3.2.2, pages 62 and 63.

Riprap Gradation

D100/D50 and D50/D20 between 1.5 and 3.0 is acceptable. 
(FHWA HEC-15 Design of Roadway Channels. Section 6.4.2

Filter Layer Particle Size Distribution

According to NCHRP Report 568 Section 3.2.2, page 62, "As a rule of thumb, the gradation curve of the granular filter material should be approximately parallel to that of the base soil. Parallel gradation curves minimize the migration of particles from the finer material into the coarser material. Heibaum (2004) presents a summary of a procedure originally developed by Cistin and Ziems whereby the d50 size of the filter is selected based on the coefficients of uniformity (d60/d10) of both the base soil and the filter material.  With this method, the grain size distribution curves do not necessarily need to be approximately parallel. Figure 3.16 provides a design chart based on the Cistin–Ziems approach." 

Hydraulic Conductivity

"The permeability of the filter should be at least 10 times the permeability of the underlying material." (National Cooperative Highway Research Program Report 568: Riprap Design Criteria, Recommended Specifications, and Quality Control. Section 3.2.2, page 62)