Manning coefficient (n)for several commercial pipes

By European Small Hydropower Association (ESHA)
Over the years many empirical formulae, based on accumulated experience, have been developed. They are, in general, not based on sound physical principles and even, occasionally, lack dimensional coherence, but are intuitively based on the belief that the friction on a closed full pipe is:

1. Independent of the water pressure
2. Linearly proportional to its length
3. Inversely proportional to a certain power of its diameter
4. Proportional to a certain exponent of the water velocity
5. In turbulent flows it is influenced by the wall roughness

One of these formulae, widely used to estimate the flow in open channels, but also applicable to closed pipes, is that developed by Manning

Q= (1/n)* A^(5/3)* S^(1/2)* P^(-2/3)

Where n is the Manning roughness coefficient, P is the wetted perimeter (m), A is cross-sectional area of the pipe (m^2) and S is the hydraulic gradient or head loss by linear meter.

Applying the above formulae to a full closed circular cross section pipe:
S= 10.29 n^2 * Q^2 *D^(-5.333)
S= hf / L (head loss)
hf = Head loss (m)
Q = flow in penstock (m^3/s)
D = inside diameter of penstock (m)
L= lenght of pipe
Manning coefficient n for several commercial pipes

Types of Pipe	n
Welded steel Polyethylene(PE) PVC Asbestos cement Ductile iron Cast iron Wood-stave(new) Concrete (steel forms smooth finish)	0.012 0.009 0.009 0.011 0.015 0.014 0.012 0.014

Ref : ESHA (European Small Hydropower Association),”Layman’s Handbook on How To Develop a Small Hydro Site,”2nd ed, 1998. http://www.scribd.com/doc/8885765/Layman-Handbook-for-hydro-electric-power-plants