Need help sizing a culvert.

Can you explain the flow difference between 2 24" corrugated metal and one 36" smooth bore. Is 36=2(24") or 3(24") pipes.
I've got 2 24" corrugated on hand as well as the 1 36" plastic.



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So why does a 36" smooth wall flow so much more than a 36" corrugated. Obviously part of it is the turbulent flow at the wall, but that can't account for the massive difference in flow rates. Is the inside diameter the same?
 
Fire departments deal with Friction loss. {The Corrugated doesn't help either} Not sure where to get the formula, but one 3" will flow the same or more, as two 2 & 1/2", if I recall correctly. Half the friction of flowing liquid in a larger line, than 2 lines of equal or more size.
 
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