Rational runoff, then stop
The rational method estimates a peak flow from a coefficient, a rainfall intensity and an area. In the units used here, Q in litres per second is 2.78 × C × i (mm/h) × A (hectares). One millimetre per hour on one hectare is 2.78 litres per second, which is why the factor appears.
Worked catchment
C 0.8, 50 mm/h, 0.4 ha gives 44.5 L/s. That is a screening figure for a small paved area. It is not a flood estimate. It assumes the whole area contributes at once, and that intensity lasted at least as long as the time of concentration. Neither was checked.
Where practice goes next
Highway drainage in the UK is tied to the Design Manual for Roads and Bridges. Sewer adoption looks to the water company’s standard, often Sewers for Adoption or its successor. Flood estimation for larger catchments uses FEH, not this equation. SuDS storage is a volume problem, not a peak from the rational method alone.
Open the rational runoff calculator.
How to use the figure in a review
Quote the inputs with the result. A pad pressure without the load and the area cannot be reviewed. A peak flow without the coefficient, the intensity and the area cannot be reviewed. A gear ratio without the tooth counts cannot be reviewed. The page is the screen. The project document is the limit.
Do not paste the result into a published container as design information. Published means the project state, not this site. A preliminary number stays preliminary until the appointed party issues it under the appointment.
Last checked 3 October 2026. Original guidance. Standards text stays with the publisher.