Manning Open Channel Flow Calculator
Flow and velocity in a rectangular open channel, with the hydraulic radius and wetted area.
Results
Flow rate
2.2667
m³/s
Speed
1.4167
m/s
Hydraulic radius
0.44444
m
Area
1.6000
m²
Flow rate
2266.7
L/s
What this tool does
Manning's n is a roughness coefficient taken from published tables: around 0.011 to 0.013 for smooth concrete, 0.016 for rough concrete, 0.022 to 0.030 for an earth channel, and 0.05 or more where there is vegetation. The equation assumes steady uniform flow in a channel long enough to settle, so it is a design tool rather than a description of a flood.
Formula
v = (1 ÷ n) R^(2/3) S^(1/2) ; Q = A v
Variables
| Symbol | Meaning | Unit |
|---|---|---|
bw | Channel width | m |
yd | Water depth | m |
nm | Manning roughness n | — |
sl | Channel slope | m/m |
QQ | Flow rate | m³/s |
VV | Speed | m/s |
RH | Hydraulic radius | m |
AA | Area | m² |
QL | Flow rate | L/s |
Worked example
- Channel width2 m
- Water depth0.8 m
- Manning roughness n0.013
- Channel slope0.001 m/m
- Flow rate2.2667 m³/s
- Speed1.4167 m/s
- Hydraulic radius0.44444 m
- Area1.6000 m²
- Flow rate2266.7 L/s
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.