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:
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).
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 you will have to 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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