The Manning's roughness coefficients must be entered for each culvert type. HECRAS uses Manning's equation to compute friction losses in the culvert barrel, as described in the section entitled "Culvert Hydraulics" of this chapter. Suggested values for Manning's n values are listed in Table 6-1 and Table 6-2, and in many hydraulics reference books. Roughness coefficients should be adjusted according to individual judgment of the culvert condition.
Table 6-1 Manning's "n" for Closed Conduits Flowing Partly Full
Type of Channel and Description
Minimum
Normal
Maximum
Brass, smooth:
0.009
0.010
0.013
Steel:
Lockbar and welded Riveted and spiral
0.010 0.013
0.012 0.016
0.014 0.017
Cast Iron:
Coated Uncoated
0.010 0.011
0.013 0.014
0.014 0.016
Wrought Iron:
Black Galvanized
0.012 0.013
0.014 0.016
0.015 0.017
Corrugated Metal:
Subdrain Storm Drain
0.017 0.021
0.019 0.024
0.021 0.030
Lucite:
0.008
0.009
0.010
Glass:
0.009
0.010
0.013
Cement:
Neat, surface Mortar
0.010 0.011
0.011 0.013
0.013 0.015
Concrete:
Culvert, straight and free of debris Culvert with bends, connections, and some debris Finished Sewer with manholes, inlet, etc., straight Unfinished, steel form Unfinished, smooth wood form Unfinished, rough wood form
0.010 0.011 0.011 0.013 0.012 0.012 0.015
0.011 0.013 0.012 0.015 0.013 0.014 0.017
0.013 0.014 0.014 0.017 0.014 0.016 0.020
Wood:
Stave Laminated, treated
0.010 0.015
0.012 0.017
0.014 0.020
Brickwork:
Glazed Lined with cement mortar Sanitary sewers coated with sewage slime with bends and connections Paved invert, sewer, smooth bottom Rubble masonry, cemented
0.011 0.012 0.012 0.016 0.018
0.013 0.015 0.013 0.019 0.025
0.015 0.017 0.016 0.020 0.030
[Chow, 1959]
Table 6‑2 Manning's “n” for Corrugated Metal Pipe
Type of Pipe and Diameter
Unpaved
25% Paved
Fully Paved
Annular 2.67 x 2 in. (all diameters)
0.024
0.021
0.021
Helical 1.50 x 1/4 in.:
8 inch diameter 10 inch diameter
0.012 0.014
Helical 2.67 x 2 inc.:
12 inch diameter 18 inch diameter 24 inch diameter 36 inch diameter 48 inch diameter 60 inch diameter Annular 3 x 1 in. (all diameters)
0.011 0.014 0.016 0.019 0.020 0.021 0.027
0.015 0.017 0.020 0.019 0.023
0.012 0.012 0.012 0.012 0.012
Helical 3 x 1 in.:
48 inch diameter 54 inch diameter 60 inch diameter 66 inch diameter 72 inch diameter 78 inch & larger
0.023 0.023 0.024 0.025 0.026 0.027
0.020 0.020 0.021 0.022 0.022 0.023
0.012 0.012 0.012 0.012 0.012 0.012
Corrugations 6 x 2 in.:
60 inch diameter 72 inch diameter 120 inch diameter 180 inch diameter