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Engineering ToolBox > Manning Formula for Gravity Flow
Manning's equation can be used to calculate cross-sectional average velocity flow in open channels
v = kn/n R2/3 S1/2 (1)
where
v = cross-sectional average velocity (ft/s, m/s)
kn = 1.486 for English units and kn = 1.0 for SI units
A = cross sectional area of flow (ft2, m2)
n = Manning coefficient of roughness
R = hydraulic radius (ft, m)
S = slope of pipe (ft/ft, m/m)
The volume flow in the channel can be calculated as
q = A v = A kn/n R2/3 S1/2 (2)
where
q = volume flow (ft3/s, m3/s)
A = cross-sectional area of flow (ft2, m2)