What is Manning Equation?

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	ft3/s
A	Cross-Sectional Area of Pipe	ft2
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.R2/3 (h/L) 1/2

Where,

Q	Flow Rate	m3/s
A	Cross-Sectional Area of Pipe	m2
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

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