Hydraulics & drainage · Stormwater & WSUD
Rational method peak flow calculator
Time of concentration (Bransby-Williams), peak flow for a design intensity and catchment.
Standard: ARR rational method Q = CIA/360 · Bransby-Williams tc
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Catchment area | 25 | ha |
| Runoff coefficient | 0.6 | |
| Design rainfall intensity (for tc) | 90 | mm/h |
| Main stream length | 0.9 | km |
| Equal-area slope | 2 | % |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| tc = 58 L/(A^0.1 S^0.2) | 52.20 | min |
| Q = C I A/360 | 3.75 | m³/s |
- ✓ Rational method suits A ≤ 25 km² (urban practice)
Method, formulas and assumptions
- tc = 58 L/(A^0.1 S^0.2) = 58*0.9/((25/100)^0.1*2^0.2) = 52.2 min
- Q = C I A/360 = 0.6*90*25/360 = 3.75 m³/s
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.