Technical Abstract: Physical model tests were conducted to determine criteria for the size and placement of riprap downstream of SAF stilling basins to ensure basin integrity. Relationships were developed to determine the depth, width, and length for preformed scour holes as a function of the Froude number and riprap size. Results show that the riprap size required for stability increases exponentially with the Froude number. Smaller riprap is required for stability if the riprap is recessed below the elevation of the basin floor.