# WMS:FHWA Channel Flow

Travel time for open channel flow segments is computed using the following form of Manning’s equation for open channel flow:

$T_t = \frac {Ln}{60KR^{\frac {2}{3}} \sqrt {S}}$

where:

Tt = travel time for open channel flow segments.
L = open channel flow length.
n = Manning’s roughness coefficient for channel flow. Suggested values are given in Table 3-4 of the FHWA HEC 22 manual and are repeated in the table below.
K = empirical coefficient equal to 1.49 for English units and 1.0 for Metric.
R = hydraulic radius (length, ft or m).
S = channel slope (length/length).

Values of Manning's coefficient for channels and pipes
Conduit Material n
Closed conduits
Asbestos-cement pipe 0.011-0.015
Brick 0.013-0.017
Cast iron pipe
Cement lined & seal coated 0.011-0.015
Concrete (monolithic) 0.012-0.014
Concrete pipe 0.011-0.015
Corrugated-metal pipe (0.5-2.5 inch corrugations)
Plain 0.022-0.026
Paved invert 0.018-0.022
Spun asphalt lined 0.011-0.015
Plastic pipe (smooth) 0.011-0.015
Vitrified clay
Pipes 0.011-0.015
Liner plates 0.013-0.017
Open channels
Lined channels
Asphalt 0.013-0.017
Brick 0.012-0.018
Concrete 0.011-0.020
Rubble or riprap 0.020-0.035
Vegetal 0.030-0.040
Excavated or dredged
Earth, straight and uniform 0.020-0.030
Earth, winding, fairly uniform 0.025-0.040
Rock 0.030-0.045
Unmaintained 0.050-0.14
Natural channels (minor streams, top width at flood stage < 100 feet)
Fairly regular section 0.03-0.07
Irregular section with pools 0.04-0.10

The hydraulic radius may be computed using the Channel Calculator. In this case, assume an approximate depth of flow or flow rate in order for the channel calculator to be able to compute the appropriate hydraulic radius.