WORLD FOR CIVIL

Manning’s Equation For Open Channels

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).