What is Manning Equation?

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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 Equation

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 MaterialResistance Factor
PVC0.009
Very smooth0.010
Cement-lined ductile iron0.012
New cast iron, welded steel0.014
Old cast iron, brick0.020
Badly corroded cast iron0.035
Wood, concrete0.016
Clay, new riveted steel0.017
Canals cut through rock0.040
Earth canals average condition0.023
Rivers in good conditions0.030
Manning Index- Reference: Pipping Calculations Manual (1)

What is the Manning Equation in US Units

Q=(1486 / n ) A.R 2/3 (h/L) 1/2
Manning Equation

Where,

QFlow Rateft3/s
ACross-Sectional Area of Pipeft2
RHydraulic Radiusft
nManning index, or Roughness Coefficient, Dimensionless
hFriction Lossft of water
LPipe Lengthft

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.R2/3 (h/L) 1/2
Manning Equation

Where,

QFlow Ratem3/s
ACross-Sectional Area of Pipem2
RHydraulic Radiusm
nManning index, or Roughness Coefficient, Dimensionless
hFriction Lossm of water
LPipe Lengthm

Where, R = D/4 (Circular Pipes)

D is inside diameter of pipe in m

References

  1. 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

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