gravity flow rates

velocity should be checked when flowing full

I am going to disagree. It depends on what the design standards are for your project location.

Every jurisdiction I have dealt with in California sizes sewers to flow half full at peak dry weather flow. For examples, the City of Los Angeles: Section F250. They also want a minimum velocity of 3 fps (Section F253). Some agencies, such as the City of San Diego, allow trunk sewer to flow deeper, but typically no more than 75%. The City of San Diego Sewer Design Guide, Section 1.3.3.3, says, "New sewer mains 15 inches and smaller in diameter shall be sized to carry the projected peak wet weather flow at a depth not greater than half of the inside diameter of the pipe (dn/D not to exceed 0.5). New sewer mains 18 inches and larger shall be sized to carry the projected peak wet weather flow at a depth of flow not greater than 3/4 of the inside diameter of the pipe (dn/D not to exceed 0.75)." (
I don't know if Los Angeles set these criteria because it has a lot of combined sewers (it's been more than 30 years since I dealt with any sewers in LA), but I see the same standards in other jurisdictions that don't have many (if any) combined sewers.

The moral of the story is: check with the agency having jurisdiction.



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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

Agree with cvg regarding his comment which applies for municipal wastewater collection systems. fel3 makes some good points in regards to municipal wastewater collection systems in hot and arid regions where there will be concerns with hydrogen sulfide.

It must be recognized that decreased slopes may cause additional sewer maintenance expense and special linings or materials should be considered for corrosion protection.

However, you appear to be dealing with a service sewer. Service sewers are the drains that run from the building to the municipal sewer in the street. Service sewers generally have a minimum slope of 1%. You should check this service sewer requirement with the local AHJ.

The simple, general requirement for any mechanical construction is that it must be constructed to the purpose and load it is meant for. eg. for maximum load.

The answers in previous posts point out some guidelines. Worst case: what happens if the receiving municipal pipeline gets clogged or run full?

This is a service line. In a nutshell, if I have an X" pipe at X% slope, flowing half full, I know a velocity and discharge. As long as what is coming out of my building is less than that half full discharge, it is ok. My concern was designing a pipe with a velocity that matched the outflow from the building.

The 1% slope for a sewer service should provide a higher velocity than 2 f/s, probably over 3 f/s.

Most municipalities require a minimum size sewer service for a residential development to be 6-Inches in diameter and the minimum size sewer main for commercial and industrial developments to be 8-Inches in diameter.

The reason for over-sizing service sewers is for ease of cleaning.

@65nerd - It also depends on the type of occupancy that will take overtime. Since that is not in our hands or the agency, they design conservatively. So if the design criteria is set for min 8" pipe @ 0.035 slope to achieve 2 fps velocity when pipe is 70% full, then that will dictate the design. If you design the pipe to match with the current outflow from the building now, that can and will change in the future. 8" PVC SDR 35 or 26 is fairly inexpensive in grand picture. Hope that helps.