Outdoors & maps
Manning open-channel flow
Estimate uniform flow in a rectangular or trapezoidal channel.
A clear answer. Here’s how.
In SI, Q=(1/n)·A·R^(2/3)·sqrt(S), where A=(b+zy)y and wetted perimeter=b+2y√(1+z²), so R=A/P. Zero side slope gives a rectangle. Uses the supplied roughness and energy slope under steady uniform open-channel assumptions. Does not infer backwater profiles, critical depth, freeboard or flood capacity.
See a worked example
Worked example
A fixed demonstration, separate from the values you enter above.
- Bottom width (m)
- 2
- Water depth (m)
- 0.5
- Side slope: horizontal / vertical
- 0
- Manning n (SI)
- 0.015
- Energy slope (m/m)
- 0.001
Example result
- Flow area (m²)
- 1
- Wetted perimeter (m)
- 3
- Hydraulic radius (m)
- 0.333333333333
- Uniform flow (m³/s)
- 1.01350968813
- Mean velocity (m/s)
- 1.01350968813
- Top width (m)
- 2