Hautamecki
2022-04-18 14:29:32
- #1
Hello dear house-building community,
I am new here in the forum, but still hope for many answers. We recently received our building permit from the authorities, which was of course a reason to be happy at first. Unfortunately, there was a major requirement that is currently overwhelming us a bit.
Specifically, we stated that we want to discharge our rainwater into the nearby village brook. All the neighbors do this and in this case it does not incur any fee, because it is a public water body. The demand from the Lower Water Authority is that the throttled outflow must not exceed 1 l/s per 1000 m² of connected runoff-effective area and that the retained volume must be completely emptied again after the end of the rain event. In other words, we are supposed to install a retention cistern and calculate the throttled outflow according to our sealed surface. We have now calculated with 180 m² of sealed surface, i.e. house, terrace, and carport, resulting in a throttled outflow of 0.18 l/s. Then we were almost shocked because the calculation showed that we would have to plan for an 8000 l retention cistern. That would still be okay if we could use the rainwater for the garden, but the volume must be completely emptied. If we were to also plan usable volume, we would need an estimated size of 12 m³ or more. This, of course, is also associated with immense costs, the 8000 l variant costs about €3000, the 12000 l about €5000. Then there are earthworks, which I cannot estimate at all yet. Another complicating factor is that we cannot go infinitely deep either; the outflow must also be connectable in terms of height. But I have the feeling that I cannot yet assess all possibilities because I simply lack the knowledge.
I hope for as many answers as possible; I am at my wit’s end.
I am new here in the forum, but still hope for many answers. We recently received our building permit from the authorities, which was of course a reason to be happy at first. Unfortunately, there was a major requirement that is currently overwhelming us a bit.
Specifically, we stated that we want to discharge our rainwater into the nearby village brook. All the neighbors do this and in this case it does not incur any fee, because it is a public water body. The demand from the Lower Water Authority is that the throttled outflow must not exceed 1 l/s per 1000 m² of connected runoff-effective area and that the retained volume must be completely emptied again after the end of the rain event. In other words, we are supposed to install a retention cistern and calculate the throttled outflow according to our sealed surface. We have now calculated with 180 m² of sealed surface, i.e. house, terrace, and carport, resulting in a throttled outflow of 0.18 l/s. Then we were almost shocked because the calculation showed that we would have to plan for an 8000 l retention cistern. That would still be okay if we could use the rainwater for the garden, but the volume must be completely emptied. If we were to also plan usable volume, we would need an estimated size of 12 m³ or more. This, of course, is also associated with immense costs, the 8000 l variant costs about €3000, the 12000 l about €5000. Then there are earthworks, which I cannot estimate at all yet. Another complicating factor is that we cannot go infinitely deep either; the outflow must also be connectable in terms of height. But I have the feeling that I cannot yet assess all possibilities because I simply lack the knowledge.
[*] Is it for example possible to install two throttled outlets in such a retention cistern, whereby the summed outflow would be higher and the volume could be reduced? The second outlet could then, in my imagination, go into an infiltration shaft or similar. According to the soil report, we also only have limited infiltration-capable soil (kf value of about 10^-5 - 10^-7), so I cannot infiltrate everything there either.
[*] Maybe it is possible to install a "split" downstream of the retention cistern that then divides the flow—as in the first point—towards the brook and infiltration, and thereby reduce the volume of the retention cistern again.
[*] Or another consideration: Does it make sense to precede with a normal cistern, where the overflow then flows into a significantly smaller retention cistern?
[*] Or a completely different idea?
I hope for as many answers as possible; I am at my wit’s end.