Reason in checking the ratio (i.e. design flow to full-bore flow > 0.5) in circular pipe design

For checking of self-cleansing velocity for pipes, there is another criterion to check design flow Q to full bore flow Q_full> 0.5. If this criterion is met, it may be assumed that design flow is always greater than self-cleansing velocity.

The reason behind it is that from the chart of circular pipes when Q/ Q_full>0.5 then ratio of design velocity V to full bore velocity V_full>1. After confirming V_full>1m/s then it leads to V>1m/s. Therefore minimum velocity at full bore flow must be checked.