read the result
Peak runoff depends on the rainfall basis as much as the arithmetic.
The Rational Method relates a catchment runoff coefficient, rainfall intensity, and area to a preliminary peak flow. The intensity should come from the selected duration and return-period basis, normally tied to the catchment response time.
Governing relationship
With i in mm/h and A in hectares, Q = 2.7778 C i A in L/s. The same result is 0.0027778 C i A in m³/s.
Do not stop at peak flow.
- Confirm the IDF source, return period, duration, time of concentration, and any climate-change allowance.
- Check routing, storage, attenuation, inlet capacity, surcharge, tailwater, blockage, and downstream hydraulic control separately.
- Use a suitable catchment subdivision and runoff basis where land use, slope, soil, or response time varies materially.