HEC added the Riprap and Scour calculator to HEC-RAS version 6.1. This tool computes riprap size and gradation and the potential toe scour that the design must account for. An introductory video and the files used in that video are included below.
Riprap Calculator: Sizing Rock Gradation
Scour Calculator: Computing Launchable Volume
Step 7: Select a Reasonable Scour Depth Based on Calculations, Regional Geology, and Engineering Judgement