Surface drainage... Help... What does that mean?

In our property purchase contract it states that we must retain a certain amount of rainwater and discharge it into a sewer with a time delay.

It also says that infiltration must occur on the property. Interesting ...

What is referred to here as a cistern, in my opinion, means a soakaway shaft or soakaway pit. This is essentially a cistern without a bottom (in fact, the bottom is a permeable layer). In your case (retention, then discharge into a sewer), you can imagine it like this: When it rains (heavily), more water flows into the shaft per hour than can infiltrate at the bottom at the same time. The water level in the shaft rises and eventually overflows. This is the retention and it relieves the sewer and the central infiltration basin of the development.

A (real) cistern is a water storage tank. You can install it, but you don’t have to. If you do, the overflow of the cistern must then be connected to a retention/infiltration system (see above). The cistern water is also available for use in the garden or house.

For pure infiltration, besides the soakaway shaft there are other options, for example, trenches or infiltration areas. What is suitable depends on the property (soil survey) and of course the requirements; apparently you only need to retain, not fully infiltrate.

Hi,

what confuses me a bit there is the word "Des Weiteren". It can also be understood as meaning that both a retention cistern _and_ an infiltration cistern must be installed.

Rainwater retention, calculated with 10 liters/m^2, quickly amounts to a good 2-3 m^3. However, it only has an overflow, no infiltration. Do you really have to bury two cisterns there? That would be a bit excessive.

Best regards,

Andreas

If I were Milanni, I would call the building authority of the municipality and ask what is meant by that or have it confirmed in writing, and that’s it. This way, one doesn’t have to speculate for so long :)

Best regards Andi

Hello everyone! Thank you very much for your answers!

I think Andi is right... I'll call them tomorrow and ask what they actually want. :D
...I almost fear that they don't know themselves. :rolleyes:

Let me explain what is meant by 10 L/m² and whether there might be a time reference missing (10 L/m²/h).

But the quote above is from your purchase contract, which you have already signed, right? Doesn't one check unclear points before signing?

I read it as you have to reserve a volume of 10l of retention volume for every sqm of buildable area. So for 200sqm of buildable area, that would be 2000l. The buildable area of the property is specified in the development plan, if available. However, the passage about 25 percent of the buildable area as the size for the infiltration area is not quite clear to me, in my example that would be 50(!) sqm... So no... Are you building in a water protection area?

Providers of infiltration trenches (stormwater infiltration systems) usually calculate the required trench area and size for you based on a hydrogeological report (approx. 350-400 EUR). We ended up spending 4500 euros for the whole thing, but there are definitely much cheaper solutions. However, we could not realize them due to requirements of the lower water protection authority. Many thanks for that ... It would have been around a maximum of 2000 euros. Without piping. I'm curious what you will find out tomorrow! Best regards t