Rainwater soakaway size

[GCarnegi]

Scottish water have asked me to provide a rainwater attenuation system as my new extension is approx 6sqm larger in area than the previous extension is replaces. They have asked that the max flow be attenuated so that it does not exceed the max flow from before the development.

The thing is my architect has specified a 6sqm x 1.2m deep trench filled with pebbles, which has now been approved by building control.

Surely 6sqm of extra roof area doesnt require 6sqm of pit area, does it? Especially as it's only there to ATTENUATE the water, and not 100% soakaway, ie it will still be connected to the mains drain to drain the soakaway, albeit in the overflow condition.

That's going to be a hell of a lot of digging!

Three questions:
1. Do I really need 6sqm to attenuate the rainwater?
2. If I don't then am I committed to do it anyway as it has been approved by planning?
3. Does it really have to be filled with "large pebbles" as per the plan, or will demolition rubble do the job? I can't imagine where I will find 6cubic metres of large pebbles!

FYI, I'm half way up a suburban hillside with no clay to speak of in the soil (ie, good drainage)

thanks,
Guy

 

[studips]

you could ask your man why he's chosen not to use aquacell crates / attentuation cells - they're something like 95%+void - filling any hole with large pebbles is going to give you maybe 30% void if your lucky (at a guess).

building control are quite human - you can ask them direct for their opinion.

 

[jeds]

Not 100% sure about Scottish regs but in my area (which is generally clay and does not drain that well) the average pit is between 1m³ and 1.5m³ for areas less than 25m².