Surface water / soakaway advice

[JackOrion]

One of our planning conditions is to provide details of surface water drainage. The LPA and water company will only allow foul water into the sewer, so we need engineering calcs and drawings showing the soakaway design and location.

 

Site details:

 

• Mainly sloping site

• Soil is very gravelly clay

• Test pits (400 × 400 × 400mm) drained quickly (in under 7 minutes)

• Roof area approx. 120m²

• Paths/driveways will all be permeable

• A few old Victorian garden buildings are on site with lots of old land drains dotted around (it's an old walled garden). No drainage issues anywhere and we've owned the plot a good few years.

 

From what I understand this probably means we need around 3m³ soakaway capacity.

 

We’ve had two drainage engineers visit and both suggested a single soakaway in a flat area (about 5m²) in front of the house. However, that spot isn’t 5m from the house foundations nor is it 5m from a retaining wall further downslope (neighbour’s property), so it seems like it will not meet the usual guidance, and could prove problematic in future.

 

On the other side of the house we have about ½ acre of sloping woodland, with plenty of space but more difficult digging (tree roots present, and will need to be by hand), and then we have a more gently sloping footpath below that.

 

I’m wondering whether splitting the drainage into several smaller / shallower soakaways in the wooded / footpath area might make more sense, which would keep everything well clear of structures.

 

Does that sound like a sensible approach?

Has anyone used any drainage engineers who may be a little more inventive than those who have suggested a single large soakaway, or perhaps have more progressive SUDS experience?

 

Any recommendations / thoughts welcomed.

 

 

Edited Sunday at 19:56 by JackOrion

[Bonner]

I think you need to size for a 10 year historical storm with a climate change rainfall intensity factor (40% in my case) so 3m³ soakaway seems a bit small. I have 10m³ for about 200 sq.m roof, good infiltration rate. You don’t need to have it all in one place, I have four, roughly one at each corner of the property. I am convinced this is a better solution anyway, more surface area for infiltration and shorter drain runs. My drainage engineer has retired unfortunately but I found him (and other ideas!) by looking through local planning applications.

[JackOrion]

Thanks for this. I think in our area we need to allow for a 1 in 100 year storm event, so yes, 3m³ would likely need to be increased.

 

Inspired by some other threads on here and reading online, we're now looking at multiple SuDS planters, a substantial amount of rainwater harvesting, and splitting into smaller soakaways from that point.

 

Hopefully we can find an engineer on the same page.