Although the Manning's Equation is most often used for calculating flow in open channels?, it can also be used to calculate flow in pipes.
Manning's Equation for Circular Pipes:
Q = \frac{1.49}{n}AR^{2/3}\sqrt{S}
where:
- A = cross-sectional flow area (ft2)
- R = hydraulic radius (ft)
- S = slope of energy grade line or slope of channel bed for uniform flow
- n = manning roughness factor (see Table of Manning's Roughness Coefficients)
Head Loss
h_F = \frac{4.637n^2L}{D^{5.333}}Q^2
where:
- hf = friction head loss (ft)
- n = Manning Roughness Coefficients
- L = length of pipe (ft)
- Q = discharge (ft3/sec)
- D = diameter of pipe (ft)