The Manning's equation finds its roots in the Chezy formula for steady uniform flow:
Where the coefficient C is
This gives the more familiar form:
Where:
-
n = Manning's Coefficient
-
R = Hydraulic Radius
-
S = Slope in ft/ft.
Note:
For metric units use 1.0 in place of 1.49 in the above equation.
There are numerous sources for Values of Manning's n.
|
Type and description of conduits |
N Values |
|
Min |
Design |
Max |
|
Channels, Lined |
|
Asphalt concrete, machine placed |
|
0.014 |
|
|
Asphalt, exposed prefabricated |
|
0.015 |
|
|
Concrete |
0.012 |
0.015 |
0.018 |
|
Concrete, rubble |
0.016 |
|
0.029 |
|
Metal, smooth (flumes) |
0.011 |
|
0.015 |
|
Metal, corrugated |
0.021 |
0.024 |
0.026 |
|
Plastic |
0.012 |
|
0.014 |
|
Shotcrete |
0.016 |
|
0.017 |
|
Wood, planed (flumes) |
0.009 |
0.012 |
0.016 |
|
Wood, unplanned (flumes) |
0.011 |
0.013 |
0.015 |
|
Channels, earth |
|
Earth bottom, rubble sides |
0.028 |
0.032 |
0.035 |
|
Drainage ditches, large, no vegetation |
|
<2.5 hydraulic radius |
|
0.04 |
|
|
2.5-4.0 hydraulic radius |
0.035 |
|
0.04 |
|
4.0-5.0 hydraulic radius |
0.03 |
|
0.035 |
|
>5.0 hydraulic radius |
0.025 |
|
0.03 |
|
Small drainage ditches |
0.035 |
0.04 |
0.04 |
|
Stony bed, weeds on bank |
0.025 |
0.035 |
0.04 |
|
Straight and uniform |
0.017 |
0.0225 |
0.025 |
|
Winding, sluggish |
0.0225 |
0.025 |
0.03 |
|
Natural Streams |
|
(a) Clean, straight bank, full stage, no rifts or deep pools |
0.025 |
|
0.033 |
|
(b) Same as (a) but some weeds and stones |
0.03 |
|
0.04 |
|
(c ) Winding, some pools and shoals, clean |
0.035 |
|
0.05 |
|
(d) Same as (c ), lower stages, more ineffective slopes and sections |
0.04 |
|
0.055 |
|
(e) Same as (c ), some weeds and stones |
0.033 |
|
0.045 |
|
(f) Same as (d), stony sections |
0.045 |
|
0.06 |
|
(g)Sluggish river reaches, rather weedy or with very deep pools |
0.05 |
|
0.08 |
|
(h) Very weedy reaches |
0.075 |
|
0.15 |
|
Pipe |
|
Asbestos cement |
|
0.009 |
|
|
Cast iron, coated |
0.011 |
0.013 |
0.014 |
|
Cast Iron, uncoated |
0.012 |
|
0.015 |
|
Clay or concrete drain tile (4-12 in.) |
0.01 |
0.0108 |
0.02 |
|
Concrete |
0.01 |
0.014 |
0.017 |
|
Metal, corrugated |
0.021 |
0.025 |
0.0255 |
|
Steel, riveted and spiral |
0.013 |
0.016 |
0.017 |
|
Vitrified sewer pipe |
0.01 |
0.014 |
0.017 |
|
Wood stave |
0.01 |
0.013 |
|
|
Wrought iron, black |
0.012 |
|
0.015 |
|
Wrought iron, galvanized |
0.013 |
0.016 |
0.017 |
|
|
|
|
|
Typical Manning's Values, (Design Hydrology and Sedimentology for Small Catchments, Haan, Barfield, Hayes, 1994