Pre-soakaway attenuators calculations

[Olf]

When visiting to sign off footings trenches, the BC was a bit shocked how much clay was there and requested percolation test rather than stock hole for soakaway. On top of that all the new roofs served will be total of 120 m², so with insufficient absorbing rate the hole could get bigger than the area available.

However, we're planning a green roof on the extension (40 m²)  and the remaining 80 m² will be going through a surface rainwater harvesting system (2 or 3 IBCs).

As much as plain soakaway calculation process is covered in BRE Digest 365, is there a similar source showing how to calculate either reduction of necessary capacity when slowing/delaying rainwater runoff, or adjustments to the parameters taken for BRE soakaway calculations (effectively achieving the same)?

I can see some companies selling products that promise exactly this (eg https://suds-solutions.co.uk/rain-activ/), so there must be a way of either convincing or dazzling BC to approve reduced soakaway volume.

[joe90]

I was lucky with my BC officer, he told me I needed a soakaway and I said “you mean swimming pool”, I explained that if I dug a hole in my clay it would fill with water and stay there, so he asked me what I proposed and I told him I would pipe it straight into a nearby ditch that normally takes all the ground water from that site and he agreed. I am sure it did not meet regs but he was a practical sort of chap.

[Olf]

Well there is a rainwater sewer right under the extension the builders 'found it' and have fun repairing, but in BC's words we can only connect there when all the other options fail.

Possibly again, if I can show calculations that green roof and rainwater harvesting limit the output massively, may be a good bargaining chip.