Conduit Type	Minimum	Normal	Maximum
Brass, smooth:	0.009	0.01	0.013
Steel:
Lockbar and welded	0.01	0.012	0.014
Riveted and spiral	0.013	0.016	0.017
Cast Iron:
Coated	0.01	0.013	0.014
Uncoated	0.011	0.014	0.016
Wrought Iron:
Black	0.012	0.014	0.015
Galvanized	0.013	0.016	0.017
Corrugated Metal:
Sub-drain	0.017	0.019	0.021
Storm drain	0.021	0.024	0.03
Cement:
Neat Surface	0.01	0.011	0.013
Mortar	0.011	0.013	0.015
Concrete:
Culvert, straight and free of debris	0.01	0.011	0.013
Culvert with bends, connections, and some debris	0.011	0.013	0.014
Finished	0.011	0.012	0.014
Sewer with manholes, inlet, etc., straight	0.013	0.015	0.017
Unfinished, steel form	0.012	0.013	0.014
Unfinished, smooth wood form	0.012	0.014	0.016
Unfinished, rough wood form	0.015	0.017	0.02
Wood:
Stave	0.01	0.012	0.014
Laminated, treated	0.015	0.017	0.02
Clay:
Common drainage tile	0.011	0.013	0.017
Vitrified sewer	0.011	0.014	0.017
Vitrified sewer with manholes, inlet, etc.	0.013	0.015	0.017
Vitrified Sub-drain with open joint	0.014	0.016	0.018
Brick:
Glazed	0.011	0.013	0.015
Lined with cement mortar	0.012	0.015	0.017
Sanitary sewers coated with sewage slime with bends and connections	0.012	0.013	0.016
Paved invert, sewer, smooth bottom	0.016	0.019	0.02
Rubble masonry, cemented	0.018	0.025	0.03
Chow, V.T. (1959). Open-Channel Hydraulics. McGraw-Hill.