Rational Method Stormwater Calculator: Peak Flow and Pipe Size by Manning's Equation
Stormwater flow and pipe size for a small catchment.
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How this calculator works
Enter the runoff coefficient and area of each surface and the design rainfall intensity.
Enter the pipe gradient and choose the pipe material.
The calculator gives the weighted coefficient, the peak flow in cubic metres per second and litres per second, the pipe size and the full-bore velocity.
Formulas used
- Q = \frac{CiA}{360}
- C = \frac{\sum C_iA_i}{\sum A_i}
- Q = \frac{1}{n}\,A\,R^{2/3}\sqrt{S}
Worked example
With the default numbers in the calculator above:
- Total catchment area: 2 ha (20000 m2)
- Weighted runoff coefficient: 0.54
- Peak flow by the rational method (area in hectares, intensity in mm per hour): 0.3 m3/s (300 L/s)
- Smallest circular pipe that carries it flowing full (Manning's equation): 458.67 mm
- Next standard pipe size: 525 mm
- Full-bore capacity and velocity of that pipe: 0.4301 m3/s and 1.99 m/s
Change the numbers in the calculator to see the full working, every step, for your own case.
Common questions
What is the rational method?
A simple way to find the peak flow from a small catchment: the runoff coefficient times the rainfall intensity times the area, with the right units.
What rainfall intensity should I use?
The intensity for a storm that lasts as long as the time of concentration, at the frequency your council requires, from the local rainfall data.
What velocity should a stormwater pipe have?
Between about 0.6 m/s, so silt does not settle, and about 5 m/s, to avoid wear.
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Got a question about this? Ask an engineering question and get a plain-language answer with references.
Further reading: Any hydraulics text (rational method). Use Australian Rainfall and Runoff and the council's drainage rules for design.
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