Rainwater infiltration

why should the soakaway only work when the cistern is full

Because I want to use the water from the cistern in the garden. In fact, it is irrelevant for infiltration; the soakaway must handle this alone, even if the cistern is full and there is a heavy rain.

: You write: "if you now put a cistern in front" That's nothing different from what I wrote. Or am I misunderstanding? The cistern is used as a "collection basin" (e.g., for irrigation) and everything that no longer fits flows over into the trench system for infiltration. How big is your property? If you have buried 11 of these things, it must be a pretty large area.

They are just over one meter long. The decisive factor for the size of the infiltration system is the roof area. You can also send them the data by e-mail, then they calculate how many tunnels should be installed.

so:

these graf parts are 1 meter long (and wide), can store 300 liters and have a huge discharge area (completely at the bottom and also around the edge). an advantage of these infiltration trenches would be, for example, that they can be installed very shallow with a high groundwater level (our problem), they are also suitable for heavy vehicles (trucks) with at least 0.5 meters of cover.
we have 11 pieces because we adopted the calculation from our house planner for his normal infiltration system. he calculated with 3200 liters storage but only 2 sqm discharge area (DN 1500 pipe a few meters deep). that didn’t fit due to the high groundwater, which is why we needed a different solution that the authorities couldn’t complain about. so area and volume are larger than originally planned. now our system is hopefully oversized, since the infiltration area is 10 times as large as needed.

You write: "if you now put a cistern in front"
That is nothing else than what I wrote. Or do I understand that wrong?

no, upstream means that the cistern serves as a reserve tank until the infiltration trench has gotten rid of the water. the connection to the infiltration trench would therefore have to be at the bottom of the cistern, not at the top. the top is only a one-time interim storage and certainly great in summer, but it doesn’t improve, for example, the storage volume in autumn or winter (because once full it stays full forever until you pump it out yourself, which probably doesn’t happen in winter).
it is about the fact that the thread starter can’t get rid of her water because of poor water absorption. so an interim storage is good so that during heavy rain the water can be temporarily stored and then released gradually through a large infiltration surface. the graf infiltration trenches already offer a certain volume here (300 liters per part).

this fun cost us:
11x 50€ for the infiltration trenches 550€
DN400 pipe with cement block + cover as inspection shaft directly in front of the infiltration trenches, a T piece, a few connecting pipes about 50€
endless earthworks ~2000€ with the garden landscaping contractor
gravel + fleece to cover the infiltration trenches (so they can get rid of their water) 500€

these are rough numbers from memory, but 3000€ is about right.

no, upstream means that the cistern serves as a reserve tank until the soakaway has gotten rid of the water. the connection to the soakaway should therefore be at the bottom of the cistern, not at the top. the top is only a one-time intermediate storage and certainly great in summer, but does not improve the storage volume in autumn or winter (because once full it stays full forever until you pump it out yourself, which probably does not happen in winter).
the point here is that the thread starter cannot get rid of her water due to poor water absorption. so an intermediate storage is good so that water can be temporarily stored during heavy rain and then gradually discharged through a large infiltration area. graf's soakaways already offer a certain volume here (300 liters per piece).

Interesting approach. Had never thought in this direction before.
OK - with us, retention with overflow into the sewer system is allowed - so I didn't have to think around like that.
But this explanation should be saved somewhere for future explanations. Also presented in a way easily understood even by a layman.

With a reasonably sized cistern, this is not an issue. However, if you only take 2000 liters, it is clear that it fills up quickly.