| Rational Method (For the areas less than 40 ha) |
 | | | Q | | Stormwater discharge [l/sec] | | I | | Stormwater intensity [mm/hr] | | F | | Catchment area [ha] | | Ø | | Run-off coefficient [-] |
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 | | Stormwater intensity is generally are taken from the 'Intensity-Duration-ReturnPeriod' diagrams of the project area.
'n' is the return period and is described mostly as in years. For example, 5 years return period means the effective rains which occur once in 2 years. Return period can be taken as 2 years for normal cases, 5 years for important plant + buildings and 50 - 100 years for river rehabilitations.
'T' is the duration of the rain and is described mostly as in minutes. For example, 15 minutes rain duration means the rain continues and finishes in 15 minutes. Short timed rains are always more effective than the long timed rains as it is seen in graph. |
 | | Catchment area (F) is the area for the design point (P) at which the rain water collected.
Run-off coefficient (Ø) is the ratio of stormwater discharge which flows on ground after infiltration. It mostly depends on the ground surface slope, pavement of the surface and permeability of the soil under the surface. It may be taken as around 0.1 - 0.30 for green areas and 0.5 - 1.0 for paved areas.
Multiplication of catchment area by run-off coefficient is commonly known as effective basin area: F' = F · Ø |
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