Should angular or rounded stones be used in riprap channel?

Rock used for riprap should be blocky and angular, with sharp edges and flat faces. Angular stones proved to be effective to withstand external forces. Rounded stones have a high tendency to roll and inadequately protect the channel bed and bank. The ratio of length to thickness of angular stones should be less than 2.

If rounded stones have to be used, they should not be placed not steep embankments. Moreover, the size of rounded stones shall be increased (say 25%) with the corresponding increase in thickness of riprap layer.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.