One of the more popular of the numerous equations developed for determination of flow in an open channel is Manning’s variation of the Chezy formula:

V=C ?RS

where
R= hydraulic radius, ft (m)

V =mean velocity of flow, ft/s (m/s)

S= slope of energy grade line or loss of head due to friction, ft/linear ft (m/m), of channel

C =Chezy roughness coefficient


Manning proposed:

C=(1.486^1/6)/n

where
n= is the coefficient of roughness in the Ganguillet–Kutter formula.
When Manning’s C is used in the Chezy formula, the Manning equation for flow velocity in an open channel results:

V=(1.486R^2/3S^1/2)/n

Because the discharge Q=VA, this equation may be written:

Q=(1.486AR^2/3S^1/2)/n

where A =area of flow, ft² (m²)
and Q= quantity of flow, ft³/s (m³/s).