Mannings Roughness Coefficients (n):
Roughness coefficients represent the resistance to flood flows in channels and flood plains. The results of Manning's formula, an indirect computation of stream flow, have applications in flood-plain management, in flood insurance studies, and in the design of bridges and highways across flood plains.
One of the variables necessary to solve for Mannings Equation comes from Table 1 below. The table is grouped by pipe type and condition.
| Table 1: Mannings Roughness Coefficients (n)[1] | ||||
| Surface | Best | Good | Fair | Bad |
| Uncoated cast iron pipe | 0.012 | 0.013 | 0.014 | 0.015 |
| Coated cast iron pipe | 0.011 | 0.012 | 0.013 | 0.014 |
| Commercial wrought iron pipe, black | 0.012 | 0.013 | 0.014 | 0.015 |
| Commercial wrought iron pipe, galvanized | 0.013 | 0.014 | 0.015 | 0.017 |
| Coated cast iron pipe | 0.011 | 0.012 | 0.013 | 0.014 |
| Commercial wrought iron pipe, black | 0.012 | 0.013 | 0.014 | 0.015 |
| Commercial wrought iron pipe, galvanized | 0.013 | 0.014 | 0.015 | 0.017 |
| Smooth brass and glass pipe | 0.009 | 0.010 | 0.011 | 0.013 |
| Smooth lock bar and welded OD pipe | 0.010 | 0.011 | 0.013 | 0.014 |
| Smooth steel pipe | 0.012 | |||
| Riveted and spiral steel pipe | 0.013 | 0.015 | 0.017 | 0.018 |
| Vitrified clay sewer pipe | 0.011 | 0.015 | 0.015 | 0.017 |
| Common clay drainage tile | 0.011 | 0.012 | 0.014 | 0.017 |
| Glazed brickwork | 0.011 | 0.012 | 0.013 | 0.015 |
| Brick in cement mortar; brick sewers | 0.012 | 0.013 | 0.015 | 0.017 |
| Neat cement surfaces | 0.010 | 0.011 | 0.012 | 0.013 |
| Cement mortar surfaces | 0.011 | 0.012 | 0.013 | 0.015 |
| Concrete pipe | 0.012 | 0.013 | 0.015 | 0.016 |
| Wood-stave pipe | 0.010 | 0.011 | 0.012 | 0.013 |
| Corrugated Polyethylene (PE) with smooth inner walls | 0.009 | 0.015 | ||
| Corrugated Polyethylene (PE) with corrugated inner walls | 0.018 | 0.025 | ||
| PVC | 0.009 | 0.011 | ||
| Plank flumes: | ||||
| ► Paned | 0.010 | 0.012 | 0.013 | 0.014 |
| ► Unplaned | 0.011 | 0.013 | 0.014 | 0.015 |
| ► With battens | 0.012 | 0.015 | 0.016 | 0.017 |
| Concrete-lined channels | 0.012 | 0.014 | 0.016 | 0.018 |
| Cement-rubble surface | 0.017 | 0.020 | 0.025 | 0.030 |
| Dry-rubble surface | 0.025 | 0.030 | 0.033 | 0.035 |
| Dressed-ashlar surface | 0.013 | 0.014 | 0.015 | 0.017 |
| Semicircular metal flumes, smooth | 0.011 | 0.012 | 0.013 | 0.015 |
| Semicircular metal flumes, corrugated | 0.023 | 0.025 | 0.028 | 0.030 |
| Canals and ditches, no vegetation: | ||||
| ► Earth, striaght and uniform | 0.017 | 0.020 | 0.023 | 0.025 |
| ► Rock cuts, smooth and uniform | 0.025 | 0.030 | 0.033 | 0.035 |
| ► Rock cuts, jagged and irregula | 0.035 | 0.040 | 0.045 | 0.050 |
| ► Winding sluggish canals | 0.023 | 0.025 | 0.028 | 0.030 |
| ► Dredged earth canals | 0.025 | 0.028 | 0.030 | 0.033 |
| ► Canals with rough stony beds, weekd on earth banks | 0.025 | 0.030 | 0.035 | 0.040 |
| ► Earth bottom, rubble sides | 0.028 | 0.030 | 0.033 | 0.035 |
| ► Natural rock, cuts smooth and uniform | 0.025 | 0.030 | 0.033 | 0.035 |
| ► Natural rock, cuts jagged and irregular | 0.035 | 0.045 | ||
| ► Stone work, dressed ashlar | 0.013 | 0.014 | 0.015 | 0.017 |
| ► Stone work, dry rubble (riprap) | 0.025 | 0.030 | 0.033 | 0.035 |
| ► Stone work, cement rubble | 0.017 | 0.020 | 0.025 | 0.030 |
| ► Masonry, concrete finished | 0.011 | 0.012 | 0.013 | 0.014 |
| ► Masonry, concrete unfinished | 0.015 | 0.020 | ||
| ► Brick | 0.012 | 0.015 | 0.016 | 0.017 |
| Canals and ditches, vegetated | ||||
| ► Trapezoidal, more than 12" deep, good stand, longer than 24" | 0.090 | |||
| ► Trapezoidal, more than 12" deep, good stand, 10" to 24" | 0.060 | |||
| ► Trapezoidal, more than 12" deep, good stand, 6" to 10" | 0.040 | |||
| ► Trapezoidal, more than 12" deep, good stand, shorter than 6" | 0.035 | |||
| ► Wide shallow channels or swales more than 12" deep, good stand, longer than 24" | 0.300 | |||
| ► Wide shallow channels or swales more than 12" deep, good stand, 10" to 24" | 0.150 | |||
| ► Wide shallow channels or swales more than 12" deep, good stand, 6" to 10" | 0.080 | |||
| ► Wide shallow channels or swales more than 12" deep, good stand, shorter than 6" | 0.060 | |||
| NOTE: If stand is fair, use next lower grass length | ||||
| Natural stream channels: | ||||
| ► Clean, straight bank, full stage, no rifts or deep pools | 0.025 | 0.028 | 0.030 | 0.033 |
| ► Clean, straight bank, full stage, no rifts or deep pools, some weeds and stones | 0.030 | 0.033 | 0.035 | 0.040 |
| ► Winding, some pools and shoals, clean | 0.033 | 0.035 | 0.040 | 0.045 |
| ► Winding, some pools and shoals, clean, lower stages, more ineffective slope and sections | 0.040 | 0.045 | 0.050 | 0.055 |
| ► Winding, some pools and shoals, clean, some weeds and stones | 0.035 | 0.040 | 0.045 | 0.050 |
| ► Winding, some pools and shoals, clean, lower stages, more ineffective slope and sections, stony sections | 0.045 | 0.050 | 0.055 | 0.060 |
| ► Sluggish river reaches, rather weedy or with very deep pools | 0.050 | 0.060 | 0.070 | 0.080 |
| ► Very weedy reaches | 0.075 | 0.100 | 0.125 | 0.150 |
| ► Coarse gravel, weeds on banks | 0.025 | 0.033 | ||
| ► Fine, well-packed gravel | 0.020 | |||
References:
- http://www.fhwa.dot.gov/bridge/wsp2339.pdf
- This reference contains many images visually showing you the difference between multiple Manning's Roughness Coefficients