The Riprap Calculator in the current version of HEC-RAS follows the approach described in the USACE Engineering Manual EM 1110-2-1601: Hydraulic Design of Flood Control Channels (USACE, 1994). The design approach is outlined in Chapter 3 of EM 1601 and is based on the work of Dr. Stephen Maynord.  

See also, Maynord (1992) "Riprap Resistance Tests From a Large Test Channel"
and
Maynord S., Ruff, J., and Abt, S. (1989) Riprap Design, ASCE Journal of Hydraulics..

The recommended approach is based on a series of experiments conducted at the Engineering Research and Development Center (ERDC, formerly Waterways Experimentation Station – WES) Riprap Test Facility, which measured rock stability in a series of bends in a mesoscale flume (pictured below).


The general workflow of the USACE riprap design approach is illustrated in the figure below:



Future versions of HEC-RAS will include other Riprap sizing methods.

Limitations

The EM 1601 design guidance applies to riprap design for open channels not immediately downstream of stilling basins or other highly turbulent areas (for stilling basin riprap, use HDC 712-1, EM 1601 Plates 29 and 30).

Transitions in size or shape may also require riprap protection. The procedures in this calculator are applicable to gradual transitions where flow remains tranquil. In areas where flow changes from tranquil to rapid and then back to tranquil, riprap sizing methods applicable to hydraulic structures (HDC 712-1) should be used. In expanding transitions, flow can concentrate on one side of the expansion and design velocities should be increased. For installations immediately downstream of concrete channels, a vertical velocity distribution coefficient of 1.25 (an option in the calculator) should be used due to the difference in velocity profile over the two surfaces.

Both the EM 1601 riprap computation and HEC-RAS model application are suitable for shallow slopes with gradually varied flow. As the Froude number approaches 1.0 (critical depth), the user should strongly consider alternative methods. For steep slopes ranging from 2 to 20 percent where unit discharge is low, EM 1601 presents a separate design process that is not included in the riprap calculator.  A typical application is a rock-lined chute (EM 1601 section 3.7 (e)).