The Rational Method
Q = C × i × A. Peak runoff (m³/s) equals runoff coefficient × rainfall intensity (mm/hr) × area (ha), with unit conversion.
Runoff coefficient C: 0.10 for forest, 0.30 for lawn, 0.80 for concrete, 0.95 for asphalt.
How much water your roof sheds
A 150 m² roof in a 25 mm/hr storm sheds ~3,500 litres per hour. A rain garden sized at 10% of impervious area typically captures a 25-mm design storm.
Frequently asked questions
- Do I need permits for a rain garden?
- Usually not for small residential systems under ~50 m². Larger stormwater controls may need municipal approval.
stormwater runoff flooding hydrology