WMS:FHWA Channel Flow

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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}}


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.

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