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Sewage Systems

Rainwater drainage

The precipitation values are climatically conditioned and regionally very different. Depending on their frequency, the rainfall donations that occur are differentiated into:

r5/2 Five-minute rainfall, which statistically must be expected once in 2 years
r5/100 Five-minute rainfall, which statistically must be expected once in 100 years

DIN 1986-100 lists the values for several German cities as examples. The values differ from r5/2 =200 to 250 l/(s ha) or r5/100 = 800 l/(s ha) [1 ha = 10,000 m²].

Information on rain events is available from the local authorities or alternatively from the German weather service. Reference values are given in DIN EN 1986-100 Annex A.

If no values are available, rT(n)=200 l/(s ha)should be assumed. For economic reasons and in order to ensure the self-cleaning capability, pipe systems and the associated components of the rainwater drainage system shall be dimensioned for a mean rainfall event.

In the scope of DIN 1986-100, the calculated rain is an idealized rain event (block rain) with a constant rain intensity over 5 minutes. The annuality (Tn) to be used for each design case is determined by the task definition. Rainfall events above the calculated rainfall (r5/2) are to be expected as planned.

Qs - Rainwater drainage (calculated flow)

rT(n) - Dimensioning value according to statistical events

C - Characteristic coefficient for type of drainage area

A - Size of drainage area

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