### 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:

1. http://www.fhwa.dot.gov/bridge/wsp2339.pdf
• This reference contains many images visually showing you the difference between multiple Manning's Roughness Coefficients

Main