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

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