Roughness Values-Manning's Equation

Commonly used roughness values for different materials are:

Manning's Coefficient (n) for Closed Metal Conduits Flowing Partly Full

Channel Type and Description	Minimum	Normal	Maximum
a. Brass, smooth	0.009	0.010	0.013
b. Steel
1. Lockbar and welded	0.010	0.012	0.014
2. Riveted and spiral	0.013	0.016	0.017
c. Cast iron
1. Coated	0.010	0.013	0.014
2. Uncoated	0.011	0.014	0.016
d. Wrought iron
1. Black	0.012	0.014	0.015
2. Galvanized	0.013	0.016	0.017
e. Corrugated metal
1. Subdrain	0.017	0.019	0.021
2. Storm drain	0.021	0.024	0.030