Not sure I understand your question or why it is posted on this forum. Are you interested in water demand flows? Will the Australian standards Stanier has cited be useful to you ? Are you interested in wet weather sanitary sewer flows, perhaps ?
Where is your project or practice and which kind of flows are you seeking?
Sewers are sized for peak hour flow. The ratio of peak hourly flow to daily average flow is given in empirical charts based on population. The larger the population, the lower the ratio. Here is such a chart:
As for wet weather flow, that is a good question. Wet weather flow depends on many factors such as the quality of design, quality of installation, depth of groundwater, age of your sewer system, etc., etc. Older systems tend to have more wet weather flow.
Wet weather flow is probably best determined by actual data from the sewer system in question.
