The Manning formula is an empirical formula for open channel flow, or flow driven by gravity. It was developed by the Irish engineer Robert Manning.

$V\; =\; \backslash frac\{1\}\{n\}\; R\_h\; ^\backslash frac\{2\}\{3\}\; \backslash cdot\; S\; ^\backslash frac\{1\}\{2\}$

Where:

• V - cross-sectional average velocity (m/s)
• n - Manning coefficient of roughness
• R_h - hydraulic radius (m)
• S - slope of pipe/channel (m/m)

When using English units, the 1/n may be simply replaced by 1.486/n in order to correct for different units, in which cases the units of meters above will be in feet.

The hydraulic radius is:

$R\_h\; =\; \backslash frac\{A\}\{P\}$

• A - cross sectional area of flow (m²)
• P - wetted perimeter (m)

The hydraulic radius is not half the hydraulic diameter, despite what the similarity in the names may suggest. It is actually one quarter the hydraulic diameter. In wide rectangular channels, the hydraulic radius is approximated by the flow depth.

See also

• Hydraulics

External links

• Manning roughness coefficient - values for some common materials
• History of the Manning Formula
• Manning formula - online calculator
• Notes on basic hydraulics - describes the difference between open channel flow and closed channel flow
• The Manning Formula
  
  

Fluid dynamics | Hydrology | Piping

Fórmula de Manning | משוואת מנינג