Drain pipe flow with two sections of different pitch

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rfb321
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Joined: Thu Sep 28, 2017 1:39 am

Drain pipe flow with two sections of different pitch

Post by rfb321 » Thu Sep 28, 2017 2:00 am

I'm trying to evaluate the flow rate of an existing 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 or flow rate does is this similar to? 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?

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