Manning's n Coefficients
Standard roughness values for common channel materials.
What is the Manning Roughness Coefficient?
The Manning Roughness Coefficient (n) is a dimensionless value that represents the resistance to water flow in channels, streams, and pipes. Selecting the correct 'n' value is a critical step in hydraulic modeling, as it directly affects the computed velocity, discharge capacity, and water surface elevations.
The table below provides standardized minimum, normal, and maximum values for various natural and artificial channel materials. These values have been derived from extensive field observations, empirical studies, and established hydraulic engineering literature.
View the Table| Material | Minimum | Normal | Maximum |
|---|---|---|---|
| Concrete (Unfinished) | 0.014 | 0.015 | 0.017 |
| Concrete (Finished) | 0.011 | 0.012 | 0.013 |
| Earth (Straight, Uniform) | 0.018 | 0.022 | 0.025 |
| Earth (With Grass) | 0.025 | 0.030 | 0.033 |
| Gravel (Firm) | 0.020 | 0.023 | 0.026 |
| Natural Stream (Clean) | 0.025 | 0.030 | 0.033 |
| Natural Stream (Stones/Weeds) | 0.035 | 0.040 | 0.050 |
| Riprap (Loose Stone) | 0.035 | 0.045 | 0.055 |