Contents

Manning Equation helps to calculate the open channel flow or sometimes used in pipe flow. The formula we use here is to help calculate the open channel flow when pressure drops due to friction in water piping systems.

## General Information

The Manning formula is known in Europe as the Gauckler–Manning formula, or Gauckler–Manning–Strickler formula. In the United States, is Known as Manning’s equation.

It was first introduced by the French specialist Philippe Gauckler in 1867, and later re-created by the Irish architect Robert Manning in 1890.

The Manning Formula uses the Manning Index* n *or roughness coefficient which rely on the type and internal condition

of the pipe.

## Manning Index

Pipe Material | Resistance Factor |

PVC | 0.009 |

Very smooth | 0.010 |

Cement-lined ductile iron | 0.012 |

New cast iron, welded steel | 0.014 |

Old cast iron, brick | 0.020 |

Badly corroded cast iron | 0.035 |

Wood, concrete | 0.016 |

Clay, new riveted steel | 0.017 |

Canals cut through rock | 0.040 |

Earth canals average condition | 0.023 |

Rivers in good conditions | 0.030 |

## What is the Manning Equation in US Units

Q=(1486 / n ) A.R ^{2/3} (h/L) ^{1/2} |

Where,

Q | Flow Rate | ft^{3}/s |

A | Cross-Sectional Area of Pipe | ft^{2} |

R | Hydraulic Radius | ft |

n | Manning index, or Roughness Coefficient, Dimensionless | – |

h | Friction Loss | ft of water |

L | Pipe Length | ft |

Where, R = D/4 (Circular Pipes)

D is inside diameter of pipe in ft

## What is the Manning Equation in SI Units

Q= (1/ n) A.R^{2/3} (h/L) ^{1/2} |

Where,

Q | Flow Rate | m^{3}/s |

A | Cross-Sectional Area of Pipe | m^{2} |

R | Hydraulic Radius | m |

n | Manning index, or Roughness Coefficient, Dimensionless | – |

h | Friction Loss | m of water |

L | Pipe Length | m |

Where, R = D/4 (Circular Pipes)

D is inside diameter of pipe in m

## References

- E. Shashi Menon, P.E., Piping Calculations Manual, 2005, McGraw-Hill

Other Equations may be of interest Colebrook–White | Darcy-Weisbach | Hazen Williams

Pipe Friction Factor App for Android Mobiles