# Rational method and nomograph



## jwatts (Sep 19, 2013)

Trying to understand the rational method for determining flow rate and applying it to nomograph to figure out the diameter of pipe I’ll need to drain the roof. All downspouts currently drain to a single point so I need to get it away from the house, but not sure how wide it needs to be for a big rain. The slope away from the house is pretty steep, probably 30 to 40%, which I’ll probably have to verify to nail this. 

Rational method -> Q = CIA
Q = volume of runoff in cf/s
C = coefficient of surface roughness (roof = 1)
I = intensity of the storm (using 2” of rain/hr, the historic high)
A = area of watershed (0.032 acres, or 1408 square feet of roof div by 43,560)
so Q = .064 cf/s

Well the lowest value on the nomograph for cf/s is 0.1, so I’m lower than that which doesn’t seem right at all. What am I missing here?


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## Fouthgeneration (Jan 7, 2014)

A little simple compared to real life, IMHO

the limiting factor is the moment the water flows over the opening(top of vertical drop/drain), the water has to accelerate (vertically) from Zero to sufficient to get out of its own way , thus you need a 'sump' around the drain entrance , or the water backs up some till there is enough head & time to At your 0.064/Cubic feet/second flow rate, = ~0.47 gallons/sec = ~28.72 gallons/minute, 1723.5 gal. /hr.

I'd use commercial roof drain/pan sets with the needed back up intake if your roof doesn't have a large enough scupper for when the main drain fails/is plug with foreign matter . Remember you must keep the water ponding to less then the live load capacity, 30 psf is less then 6 inches of water depth....

10 seconds on Google lead to a chart that says a 2inch per hour rain can be drained, up to 1440 sq.ft. with a 2" drain.....I go one inch big for freak weather.....


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