Yard/storm drain pitch question

[pat bowen]

I have a backyard storm drain system that runs from a catch basin at the back of the yard to a storm drain manhole in the street with the following:

Total length - 188 feet

Pipe and size - 8" PVC SDR 26

Pitch or slope - 2.5% for the first 65 feet starting at the back yard catch basin, and the dropping off to .5% for the next or last 123 feet to the street.

So, my question is, what real slope does this represent. I'm thinking that the first 1/3 of the run at 2.5% will provide a push to the remaining 2/3 of the run (at .5%) that will add to the volume of water able to be carried in that .5% run. But, will the total flow capacity be greater than if it were just a .5% slope for the entire 188 feet?

 

[Wrenched]

This might be more of an engineering question than a plumbing question. 8"(200mm) is a good size pipe(very large for a backyard catch basin), close to as big as most plumbers work on. Projects that would need that size would usually be engineered. My code tables say it can handle a hydraulic load of 345gpm at 0.5% and a load of 690gpm at 2%. I'm not sure if any plumbing codes give a hybrid max. load based on average slope. Our inspectors/checkers would probably base the capacity of the entire run on the section with the least slope.

What's the situation that brings up the question?

 

[Sponsor]

 

[pat bowen]

This might be more of an engineering question than a plumbing question. 8"(200mm) is a good size pipe(very large for a backyard catch basin), close to as big as most plumbers work on. Projects that would need that size would usually be engineered. My code tables say it can handle a hydraulic load of 345gpm at 0.5% and a load of 690gpm at 2%. I'm not sure if any plumbing codes give a hybrid max. load based on average slope. Our inspectors/checkers would probably base the capacity of the entire run on the section with the least slope.

What's the situation that brings up the question?

Mostly curiosity. This system is already in place and it works under a fairly heavy spring rain. I have yet to see it in one of those 50 year rains that happen every five years now. I know it will work as well as a 123' run at .5%. But, does that first 65' running down hill at 2.5% provide a push to help exceed the 345 gpm? You are right that this is a question for a sewer and water engineer, but I can't seem to find that type of forum. I'm guessing that it probably runs similar to a .65% or .75% rate

 

[Jadnashua]

The 1/4"/foot applies to pipe smaller than 4" ID...the pipe will drain properly with a lower slope when the pipes get larger. At 1/4"/foot, municipal drains could end up WAY underground, so it obviously works. I don't know where it is in the codes...