Well, you need a minimum of 2 ft/sec velocity for gravity pipe system to transport organic material and some municipality requires a minimum 8" dia pipe. Periodical flushing of manhole is an option but sometimes maintanence cost of periodical flushing over a 50 to 70 years of time is not...
Ok. How about 5 yr storm. Most of the municipality design their storm sewer to handle 5 yr storm. What is the probabality of getting 5 yr storm in any given year?
