Sewers must be checked for minimum velocities at their minimum hourly flows which is equal to:
A sewer must be sufficiently self-cleansing of an average hourly rate of flow. Normally sewers are designed for the max rate of flow and verified with min rate of flow but atleast they must be self-cleansing at the average rate of flow.
Sewers are designed on a peak flow basis, however, the minimum flow passing through the sewer is also important in the design of a particular sewer because at low flow the velocity will be reduced considerably which may cause silting.
So the sewers must be checked for minimum velocities at their minimum hourly flows. Generally, for a moderate area, the following minimum flows may be assumed.