# Installing drain pipe for rain run-off

I've got to install a drain pipe from a low spot to the street gutter - about 100 feet. Is there a formula to figure the maximum flowrate that a 4-in pipe will carry using the 1/4 per foot slope assuming the inlet is full?
Sam
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Yep.
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GPM = 0.0408 x pipe ID ^2 x distance / water velocity
0.0408 * 16 sq.in. * 100 ft. --------------------------- = ~520 gal/minute 2 ft./sec.
re: DeWalt Plumbing Professional Reference
I've got to install a drain pipe from a low spot to the street gutter - about 100 feet. Is there a formula to figure the maximum flowrate that a 4-in pipe will carry using the 1/4 per foot slope assuming the inlet is full?
Sam
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Thanks, Dennis.

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500 gpm seems like a rather high flow rate for gravity flow in a 4" plastic pipe with a drop of 1/4" per foot.
If my calcs are correct....500 gpm thru a 4" pipe yields about 12 ft/ sec not 2 ft/sec
at 2 ft/sec, completely full, the 4" pipe, the pipe will flow ~80 gpm....
I haven't done the gravity flow calcs for a 4" pipe at 1/4" per ft but I'm guessing its more like 50 gpm max (just a WAG).
Like I said, I haven't done the calcs, but my 3" drainline with ~1/4" per foot drop easily handles the ~15 gpm output of a garden hose but I doubt if it would handle 15x that flow.
After a lot of searching I found
http://vylonpipe.com/brochures/plasticpipe.pdf
page 15 gives the Manning formula for gravity flow......stumbling through the calcs at midnight, I got
~70 gpm for a 4" PVC line with a 1/4" drop per foot
somebody please double check these numbers?
cheers Bob
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I have no idea how I came up with that, correct answer would be around 33 gpm.
(New calculator, still have figured the blasted thing out. Somehow it took 16 and squared it, when 16 was already a square. Everything seems backward - best excuse I can think of....)
The Dewalt formula is simply listed as gallons per minute through a pipe. It doesn't give the type of pipe (roughness or corrosion) so I would tend to believe that the .0408 takes the worst case into consideration which would correlate to the low figure being generated.

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