SuDs

[dnoble]

I've just secured planning (hooray) after sensible counsillors unanimously voted to allow against planning officer advice.

The decision has arrives with a massive list of jargon-heavy conditions which I am slowly translating into plain English

I am making a new driveway; Plan is to remove topsoil, porous membrane, crushed stone + gravel (so free draining).

This paragraph below outlines the requirement. I'm sure my plan fulfils it, but can anyone point me rto a proforma or way of formatting my plan so it conforms to this?

Any help gratefully received

Dan

 

Sustainable Drainage System (SuDS)
The development hereby approved shall not commence until a Sustainable Drainage
Strategy and associated detailed design, management and maintenance plan of
surface water drainage for the site using SuDS methods has been submitted to and
approved in writing by the Local Planning Authority. The approved drainage system
shall be implemented in accordance with the approved Sustainable Drainage
Strategy prior to the use of the building commencing and maintained thereafter for
the lifetime of the development.
Reason: To prevent the increased risk of flooding by ensuring the provision of a
satisfactory means of surface water disposal is incorporated into the design and the
build and that the principles of sustainable drainage are incorporated into this
proposal and maintained for the lifetime of the proposal.

[Jeremy Harris]

If I laid a drive like that here it would never drain, and would just flood the lane, I'm afraid.  Compliance with the Sustainable Urban Drainage requirements depends very much on your sub-soil, specifically on how permeable it is.  The best bet is to do a quick test to check this (easy, dig a hole, pour water in, time how long it takes to drain away - there is a standard procedure) and then work from there.  Once you know how permeable the underlying sub-soil is you can look at the options you have.

 

We're close to being a worst case, and I ended up having to fit 20 off, 196 litre, heavy duty Aquacell drainage crates, wrapped in terram, in a large excavation below our drive sub-base in order to act as surge storage, to prevent water from heavy rainfall washing out into the lane.

[dnoble]

Hello Jeremy.

You very kindly showed my wife (Timna) and I your great house a year or so ago if you  remember? Its taken 2 years to sort get this planning!

 

The drive is going over some existing lawn and I'd have thought would be as permeable as that was. Obviously more complicated than I anticipated.

 

Does this mean I'll need a certified drainage person to do a report (along with the arboriculturalist, the ecologist, the contatminated land-ist etc etc), I'm not sure the LPA will accept my hole+ bucket of water test (though if I can do this myself I'd be more than happy to?)

It's becoming apparent that there are many hoops to jump through each needing a report form a consultant or specialist..

 

[Jeremy Harris]

Welcome, sorry I didn't recognise your user name here!

 

No need for any experts, all you really need is an idea as to how well the sub-soil drains, and it's a pretty easy DIY test.  The standard test is called a percolation test, and is done in accordance with the procedure on page 32 of Part H2 of the Building Regs: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/442889/BR_PDF_AD_H_2015.pdf

 

Here's the quote from the relevant section, to save having to look it up (ignore the bit about the Environment Agency - it's not normally applicable to rain water run off, and don't take too much notice of the bit in section 1.33, as it's mainly aimed at sewage treatment plant drains):

 

Quote

Ground conditions

 

1.31  Well drained and well aerated subsoils are usually brown, yellow or reddish in colour.  Examples of subsoils with good percolation characteristics are sand, gravel, chalk, sandy loam and clay loam. It is important that the percolation characteristics are suitable in both summer and winter conditions. Poorly drained or saturated subsoils are often grey or blue in colour. Brown and grey mottling usually indicates periodic saturation. Examples of subsoils with poor percolation characteristics are sandy clay, silty clay and clay.

 

1.32  A preliminary assessment should be carried out including consultation with the Environment Agency and local authority to determine the suitability of the site. The natural

vegetation on the site should also give an indication of its suitability for a drainage field. 

 

1.33  A trial hole should be dug to determine the position of the standing groundwater table.  The trial hole should be a minimum of 1m² in area and 2m deep, or a minimum of 1.5m below the invert of the proposed drainage field pipework.  The groundwater table should not rise to within 1m of the invert level of the proposed effluent distribution pipes. If the test is carried out in summer, the likely winter groundwater levels should be considered. A percolation test should then be carried out to assess the further suitability of the proposed area.

 

1.34  Percolation test method

 

– A hole 300mm square should be excavated to a depth 300mm below the proposed invert level of the effluent distribution pipe. Where deep drains are necessary the hole should conform to this shape at the bottom, but may be enlarged above the 300mm level to enable safe excavation to be carried out. Where deep excavations are necessary

a modified test procedure may be adopted using a 300mm earth auger. Bore the test hole vertically to the appropriate depth taking care to remove all loose debris.

 

1.35  Fill the 300mm square section of the hole to a depth of at least 300mm with water and allow it to seep away overnight.

 

1.36  Next day, refill the test section with water to a depth of at least 300mm and observe the time, in seconds, for the water to seep away from 75% full to 25% full level (i.e. a depth of 150mm).  Divide this time by 150. The answer gives the average time in seconds (Vp) required for the water to drop 1mm.

 

1.37  The test should be carried out at least three times with at least two trial holes. The average figure from the tests should be taken.  The test should not be carried out during abnormal weather conditions such as heavy rain, severe frost or drought.

 

 

 

 

Also, don't take too much heed of the suggestion that the test should be carried out three times.  In my experience, BCOs only usually bother to check once, if that.  Depending on the Vp, you can then start designing a scheme to comply with SuDS, and it shouldn't need any experts - if in doubt we can help on here with ideas.

[Vijay]

Might be of some help http://www.pavingexpert.com/drain12.htm

[dnoble]

Thanks very much for that really helpful reply Jeremy.

Pavingexpert is a useful resource too, Vijay

The percolation test looks straightforward.

Not sure I fancy digging a 1m2 x 2m deep hole for the ground conditions test, though!

 

The frustrating part if this is that if I'd just decided to make this same driveway for the existing house outside of a planning app I wouldn't have needed consent or any conditions. They really seem to've chucked he book at us as it's part of a new build

[Russell griffiths]

Do you also have to show where your rainwater off your roof is going. 

We have just had to calculate this and show what we are doing with it. 

[Jeremy Harris]

If you replace an old driveway with a new one that's made of non-permeable material then you still have to comply with the SuDS regs, even though you may not need planning permission.

 

It makes sense to try and combine the roof rain water drainage solution with the drive/path/patio one if you can, as that saves cost and effort. 

 

Unless you suspect that the water table may be quite high, then I'd not bother doing the water table test for the quick and dirty initial assessment, just the percolation test. 

[dnoble]

1 hour ago, Russell griffiths said:

Do you also have to show where your rainwater off your roof is going. 

We have just had to calculate this and show what we are doing with it. 

 

Actually, though it doesn't specify that, I suspect this will also be required for the new build so I probably do need to cover both aspects, thanks both for pointing that out...

I was planning to save rainwater for toilet flushing and then garden watering but hadn't drilled down into details about how this would happen yet.

[Oz07]

I wouldn't bother trying to bring it back into the house. Watering the garden though yeh

[dnoble]

Well I've dug a hole and done the percolation test.

Neighbours were pretty jealous of my new hobby of watching a hole of water very slowly empty

There was solid clay about 25cm down so it was pretty slow. 500sec which comes out at 3.3sec/mm drop

I assume this indicates fairly poor drainage and will mean more serious measures needed.

I'm struggling to work out from the guidance/info how to translate this number into a course of action for draining the driveway.

It is about 50m2, previous lawn. I was planning to dig out the topsoil, put a membrane downs, hardcore/crushed stone and then gravel/shingle on top.

The main issues I think will be avoiding any drainage onto the pavement and onto my neighbours driveway which is slightly down hill (there's as step down of about 30cm onto the level of his front driveway; also gravel.

Presumably some kind of French drain or filled gully along those 2 borders. How would I know if I need a larger sump (attenuation cell?) like you used, Jeremy. Is there some kind of formula which indicates it or should I just assume the worst, belt and braces it?