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Hautamecki18 Apr 2022 14:29Hello dear housebuilding community,
I am new to this forum but still hope to receive many answers.
We recently received our building permit / planning permission from the authorities, which of course was reason for initial joy.
Unfortunately, there was a major condition that is currently somewhat overwhelming us.
We stated that we want to discharge our rainwater into the nearby village stream. This is common practice among all neighbors and does not incur any fee since it is a public watercourse. The requirement from the local water authority is that the restricted outflow must not exceed 1 liter per second per 1000 m² (10,764 ft²) of connected impervious surface area, and the stored volume must be completely emptied after the rain event ends.
In other words, we are supposed to install a retention tank and calculate the restricted outflow according to our sealed surface area. We calculated based on 180 m² (1,938 ft²) of sealed area, including the house, terrace, and carport, resulting in a restricted outflow of 0.18 liters per second. Then we were almost shocked when the calculation showed we need to plan for an 8,000-liter (2,114-gallon) retention tank. That would be fine if we could use the rainwater for the garden, but the volume must be fully drained afterwards. If we also planned for usable volume, the size required would be estimated at 12 m³ (3,170 gallons) or more.
This naturally involves significant costs: the 8,000-liter version costs about €3,000, and the 12,000-liter about €5,000. On top of that, there are excavation costs which I cannot estimate yet.
An additional difficulty is that we cannot dig infinitely deep, as the drain connection needs to be feasible in terms of height.
However, I feel I still cannot fully assess all options due to a lack of knowledge.
I hope for many responses—I am at my wit’s end.
I am new to this forum but still hope to receive many answers.
We recently received our building permit / planning permission from the authorities, which of course was reason for initial joy.
Unfortunately, there was a major condition that is currently somewhat overwhelming us.
We stated that we want to discharge our rainwater into the nearby village stream. This is common practice among all neighbors and does not incur any fee since it is a public watercourse. The requirement from the local water authority is that the restricted outflow must not exceed 1 liter per second per 1000 m² (10,764 ft²) of connected impervious surface area, and the stored volume must be completely emptied after the rain event ends.
In other words, we are supposed to install a retention tank and calculate the restricted outflow according to our sealed surface area. We calculated based on 180 m² (1,938 ft²) of sealed area, including the house, terrace, and carport, resulting in a restricted outflow of 0.18 liters per second. Then we were almost shocked when the calculation showed we need to plan for an 8,000-liter (2,114-gallon) retention tank. That would be fine if we could use the rainwater for the garden, but the volume must be fully drained afterwards. If we also planned for usable volume, the size required would be estimated at 12 m³ (3,170 gallons) or more.
This naturally involves significant costs: the 8,000-liter version costs about €3,000, and the 12,000-liter about €5,000. On top of that, there are excavation costs which I cannot estimate yet.
An additional difficulty is that we cannot dig infinitely deep, as the drain connection needs to be feasible in terms of height.
However, I feel I still cannot fully assess all options due to a lack of knowledge.
- For example, is it possible to install two restricted outlets in such a retention tank, allowing a higher combined outflow and thus reducing the required volume? I imagine the second outlet could lead to an infiltration trench or similar. According to the soil survey, the soil has limited infiltration capacity (permeability coefficient kf of approximately 10^-5 to 10^-7), so we cannot infiltrate everything there.
- Could a downstream split after the retention tank be installed as mentioned in the first point, dividing flow between the stream and infiltration, thereby reducing the retention tank volume needed?
- Or another idea: Would it help to have a normal tank upstream, where overflow then flows into a much smaller retention tank?
- Or a completely different idea?
I hope for many responses—I am at my wit’s end.
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