Open Channel Flow Calculator
Calculate flow rate in open channels with Manning’s equation from roughness, area, radius, and slope.
How it works
- 1Select the channel surface material to set Manning’s roughness coefficient n.
- 2Enter the cross-sectional flow area A (m²), hydraulic radius R (m), and bed slope S (m/m).
- 3The calculator applies Q = (1/n)·A·R^(2/3)·√S and shows discharge in m³/s, L/s, m³/h, and mean velocity.
Use cases
- Sizing irrigation canals and drainage ditches for a target flow.
- Checking a stream section can carry a design flood without overflowing.
- Validating storm-sewer and culvert capacity during drainage design.
Frequently asked questions
What is Manning’s n and why does it matter?
It is a dimensionless roughness coefficient. Smooth concrete (n ≈ 0.013) carries more flow than a weedy earth channel (n ≈ 0.030); since the formula divides by n, small changes have a large effect on Q.
How do I find the hydraulic radius R?
R = A ÷ P, where A is the wetted area and P the wetted perimeter. For a wide rectangular channel of width w and depth d, R ≈ w·d / (w + 2d).
Worked example: A = 2 m², R = 0.6 m, S = 0.002, smooth concrete?
Q = (1/0.013)·2·0.6^(2/3)·√0.002 ≈ 4.89 m³/s. Manning’s equation assumes steady, uniform, turbulent flow — verify design work with a hydraulic engineer.
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