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I want to put a drain pipe under a sidewalk for rain drainage. I am
trying to determine the volume of a 6 inch pipe and a 4 inch. I know
that one 4 inch wont be enough, but am wondering if TWO 4 inchers
carry as much or more water than a 6 inch pipe. I have never been any
good at mathematics. I think I have to multiply 4 or 6 by 3.14, but I
am not sure. I will either use a 6 inch or two 4 inchers, which ever
holds more water. Or I may even go to an 8 incher. Alot of water runs
down the hill and keeps putting mud over the sidewalk. I need lots of
drainage volume.

Can anyone help.

Assume similar pipe types such as tile or concrete and a slope that is the
same for both the 4-inch and the 6-inch pipes.

The maximum (pipe full) flow for the 6-inch pipe will be about 15 percent
greater than for the pair of 4-inch pipes.

I computed on the basis of Manning's Formula. I'll spare you the details
but you can Google it if you're curious. Been a long time since I did a lot
of that as a young engineer so I checked a nomogram in one of my old
handbooks and got verification. Interesting exercise but I had to scrape
through a lot of mental rust.