4" storm drain system capacity

[Rossn]

I have a surface drain system around my home, which gutters feed into. It is constructed of of 4" SDR 35 pvc sewer pipe, and I believe the installers were aiming for 1/8"/foot slope.

What's the flow capacity of this pipe at this grade?

I see several conflicting references on the internet. For instance, NDS says 135 GPM. Some other online calculators say closer to 70... that's a pretty big difference and my 100 year rain intensity probably brings me closer to 80ish GPM.

 

[Jeff H Young]

115 gpm at 1/8" fall

 

[Sponsor]

 

[Rossn]

Thanks, Jeff - that is great to hear! If I am looking around 80GPM, and have two branches come together like this, do you think I am still good?

 

[Jeff H Young]

I would trust 80gpm but you could have some over flows that normally wouldn't carry water but if the main gets backed up just shoots out away from building

 

[Rossn]

Thanks. I have grading that slopes towards the house, hence the drain system and the question. The good news is that there should be a little capacity before it would overflow into the home, and as long as I'm not dramatically exceeding the limits (or clogged), I should be good. Thanks for taking the time to respond!