Manning's n Coefficients
Reference roughness ranges to assess against project conditions.
What is the Manning Roughness Coefficient?
Manning's roughness coefficient (n) represents resistance to flow. In the SI form of Manning's equation, its unit is s/m1/3. The selected value affects computed velocity, discharge capacity and water-surface levels.
The ranges below are reference values for a preliminary assessment. Material type and a single table value do not determine a project-specific n on their own: vegetation, channel irregularity, obstructions, season and water level also matter.
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 |
Using roughness in a flood model
Justify Manning n using the observed surface, cross-section and vegetation conditions. Different flow paths in a channel and floodplain may need different values. Sensitivity tests with several reasonable n values and comparison with available water-level observations show how much the results depend on this assumption. This table is one of the starting tools for a project-specific engineering assessment.
Manning equation and units: USACE Hydraulic Reference Manual, Chapter 2