Chézy and Manning's Equation Method and Calculator

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& Manning's Roughness Coefficient

Manning's equation, which is based on Chezy's equation, is a common method to calculate the free surface flow or fluid flow within a channel.

For Manning's equation, the roughness coefficient in Chezy's equation is calculated as:

C = k [ R(1/6) / n]


C = Chezy's roughness coefficient (ft 1/2/sec, m 1/2/sec)
R = Hydraulic Radius (ft, m)
n = Manning's roughness (s/m1/3)
k = Constant (1 m1/3/m1/3, 1.49 ft1/3/ft1/3)

Substituting the calculated roughness into Chezy's equation, one can obtain Manning's equation:

Q = [k / n] A R2/3 S1/2


Q = Discharge (ft3/sec, m3/sec)
k = Constant ( 1.49 ft1/3/ sec, 1.00 m1/3/ sec)
n = Manning's roughness coefficient
A = Flow area (ft2,m2)
R = Hydraulic Radius (ft, m)
S = Slope Friction (ft/ft, m/m)