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Suggested Roughness Coefficient "n" for Manning's Formula (for site drainage)
| Materia of pipe | From | To | Used | |
|---|---|---|---|---|
| Pipes | ||||
| Asbestos-cement pipe | 0.011 | 0.015 | 0.013 | |
| Corrugated metal pipe | 0.022 | 0.02 | 0.024 | |
| Cast iron pipe | 0.011 | 0.015 | 0.013 | |
| Concrete pipe | 0.011 | 0.015 | 0.013 | |
| Ductile iron (cement lined) | 0.011 | 0.015 | 0.013 | |
| Plastic pipe, all kinds | 0.009 | 0.015 | 0.01 | |
| Steel pipe | 0.012 | 0.02 | 0.015 | |
| Vitrified clay pipe | 0.011 | 0.015 | 0.013 | |
| Ditches (open channels) | ||||
| Paved invert corrugated metal steel pipe | 0.02 | |||
| Cast iron, ductile iron, concrete, and steel pipe | 0.013 | |||
| unpaved corrugated metal steel pipe | 0.024 | |||
| Small plant ditches (no vegetation, fair condition) | 0.015 | |||
| Main plant ditch (grass, some weeds, fair condition) | 0.03 | |||
| Natural ditches (dense weeds, high as flow depth poor condition) | 0.12 | |||