Trench infiltration: the correct dimensioning

If you want to build a trench infiltration yourself, you must of course pay attention to the correct dimensioning. Only if the storage block of the trench is large enough can the accumulating water volume be absorbed. You can find out in our article how the dimensioning of a trench works correctly from a technical point of view.

Basis for the dimensioning

It is crucial that the trench can completely absorb all of the rainwater that accumulates. A sufficiently large infiltration area must be available for this.

  • Also read - Trench infiltration: the basic structure
  • Also read - Build your own infiltration system for rainwater - this is how it works
  • Also read - rainwater infiltration - how does the calculation work?

What you should know for the calculation of the trench is:

  • the amount of rainwater that accumulates
  • the infiltration capacity of the soil
  • the number of heavy rain events in the respective area

Determine the amount of rainwater

Determining the amount of rainwater is not as easy as it seems. First of all, the roof area that is being drained must be known. If additional land areas are to be drained into the trench (e.g. a driveway), these must of course also be added.

What is absolutely crucial is the topping the respective area. With the roof, for example, it makes a difference whether there is a load (e.g. gravel). Extensive or intensive greening also changes the amount of water from the roof considerably.

DIN 1986-100 forms the basis for this calculation. The basic calculation methods and the deviations due to certain roof coverings are precisely defined there.

Calculation according to DIN 1986

The rainwater runoff in l / s (liters per second) is calculated according to DIN as follows:

Rainfall x runoff coefficient x effective runoff area x 1/10.000

For the rain donation, the values according to the KOSTRA-DWD 2000 (coordinated heavy precipitation regionalization evaluations of the German Weather Service) for the respective location are used. The runoff coefficient is a parameter that results from the roof structure:

  • Roof area with a slope of more than 3 °: Discharge coefficient = 1
  • Gravel roof with a slope of up to 3 °, discharge coefficient = 0.5
  • Extensive green roof discharge coefficient = 0.3 (with more than 10 cm green roof height)

Soil infiltration capacity

You can at least roughly determine the infiltration capacity of a soil yourself.

1. Dig a pit (20 x 20 cm, around 20 - 40 cm deep, in the depth of the planned infiltration system)
2. Cover the pit floor 1 - 2 cm with fine gravel
3. Pre-water the pit for around 1 hour
4th. Put the folding rule in the pit
5. Fill the pit 20 cm high, measure the time until a change can be seen

You then have a value for the infiltration rate in cm / hour., which you can base your calculation on. Less than 1 cm / hour. are consistently very problematic (loamy soil), between 1 and 10 cm / hour. the construction of the trench is significantly less complex.

Heavy rain events

If you use the rain donation according to KOSTRA-DWD 2000, you do not necessarily have to consider heavy rain events separately. However, you can add a small "safety buffer" to your calculation in order to be prepared for very heavy rainfall.

In practice, values that can be read from statistics are used as a basis for heavy rain events. For the calculation of heavy rain, rainwater quantities are used for heavy rain events that occur once every five years.

Calculation method

The calculation of the trench area is now relatively simple: the trench volume results from the amount of rainwater that accumulates, which can be determined as described above, minus the percolation capacity of the soil in relation to the trench base area. Then the required storage volume can easily be calculated. It is best to use a nominal width of 300 mm for the pipe.

Tips & Tricks If you want to be sure about the dimensioning, it is best to ask a specialist whether he or she will help you with the calculation. He can also help you plan the construction of the infiltration system.