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Likely flow for storm water

Calculate the likely flow of stormwater from a surface.

Likely flow

Input Result
Rain intensity is=
Horizontally projected area A=
Surface coefficient C=
Risk factor ɣ=
Likely rainwater flow q=


Rain intensity

Is it the flow of the rain that lasts for 10min and occurs once every 5 years the current resort
For areas of less than 10,000 m 2, the rainfall intensity can be set to 0.013 l/s m 2 for the entire country.

Surface coefficient

Surface coefficient is a value that shows how much a surface permits water.

Type of surface Area m2 ɣ
Garden plot <1500 0,3
Gravel-covered surface <1500 0,6
Grass surface untreated land. <1500 0,1
Surface with dense coating such as roof, asphalt or concrete. - 1,0

Risk factor

A factor that takes into account the consequence of the stormwater system not being able to handle the flow.

Type of drainage ɣ
External drainage 0,3
External drainage where flooding can create problems such as over entrances to public buildings. 1,5
Internal drainage or where unusually heavy rain can create leakage into the building. 2,0
Internal drainage where high protection is required such as hospitals, buildings that are critical for communication, buildings with precious art, warehouses with goods that form toxic gases or become flammable on contact with water. 3,0

Horizontally projected area

The projected area corresponds to the area that is visible straight from the top.

Likely rainwater flow

The calculated total amount of water delivered in from the calculated area.


Likely rainwater flow: Likely rainwater flow