NCEES Civil PE Specification V.B.: Stormwater collection and drainage
Problem
The pipe pictured below has an inner radius r of 18 inches and a flow depth h of 9 inches. Given a slope S of 1% and Manning’s constant n of 0.025, determine the flow rate Q in cubic feet per second.
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Solution
Answer: C) 4.8 cfs
Flow Depth:
$$ h=9in $$
Radius:
$$ r=18in $$
Calculate the angle theta for a partially filled pipe:
$$ \theta=2{\ast\cos}^{-1}\left(\frac{r-h}{h}\right)=2.09\ radians $$
Calculate area of flow:
$$ A={\frac{r^2\left(\theta-\sin{\theta}\right)}{2}}=1.38{\rm ft}^2 $$
