Is attenuation always required

[MoDo]

Been told by our planning officer that we have to "demonstrate how existing surface water drainage rates will be reduced to existing greenfield runoff rates or 2l/s, whichever is greater". The greenfield runoff rate seems to be below 2l/s even for 1in200 year event:

 

Trying to understand if attenuation is required here. It would seem the development would cover between 50%-70% of the plot - depending if we use permeable paving. Is attenuation always required? I know a firm that has been mentioned on here will calculate how much attenuation is required FOC when buying their products - but they might be adverse to admitting if it were not required at all. Would be really grateful if anyone can advise.

[Gus Potter]

1 hour ago, MoDo said:

Trying to understand if attenuation is required here.

Take a pragmatic view.

 

You want to build a house and by doing so you stop the rain from slowly permeating into the ground in a natural way. If you cover the ground with a roof then water runs off quickly and that can flood houses down stream.

 

Thus get your head round the fact that you can't morally, not least, do this.

 

Now 2.0 litres per second equates to 2.0 x 3600 /1000 = 7.2m cube per hour.. which is actutally quite a lot of water off a domestic roof. over say a period of an hour.

 

My question is how big is your roof and how much water are we talking about.

 

You may find that this is easy to comply with.

 

If you can tell us how big you roof is and where the site is then some folk on BH may help you out.. but recognise that it takes a bit of time to do this so a donation to BH may be in order..

 

[Temp]

If you use soakaways to dispose of rainwater from roofs I imagine that doesn't count as "run off".

 

Only other thing I can suggest is preventing rainwater falling on your driveway or hard standing from running onto the road. Linear drains and soakaways should deal with that.

 

If you do the above why would run off increase beyond that of it's existing greenfield?

[joe90]

Our BC wanted attenuation but when I showed him we were on solid yellow clay and any “pit” would fill up and not drain away he allowed me to drain rain direct to a local drainage ditch. (Don’t rely on all BC,s to be this pragmatic).

[MoDo]

Thanks all for the responses. I was deliberately brief, perhaps overly. Cutting a very long story short, we are picking up the pieces (on a pathetically limited budget) from a contractor who overspent and erred at seemingly every opportunity. This submission of compliance with a condition of planning being one of them (which we just discovered had never been approved) - as no drainage improvement information was ever submitted. 

 

My understanding is that SuDs and attenuation are distinct - I may be wrong - but on this assumption…

 

Soakaways won't be possible - the site is too small and ground likely unsuitable anyway - we're in Manchester - very urban - no streams etc. 

 

The site is small, only a few meters to the road and almost nothing on the sides. So SuDs are unlikely to be viable (besides for the budget issue). The planning officer did also mention that it might be “possible to argue why SuDS cannot be incorporated into the scheme. However, it will still be necessary to demonstrate how existing surface water drainage rates will be reduced to existing greenfield runoff rates or 2l/s, whichever is greater”. The former part (in bold) leads me to believe that attenuation is non-negotiable (where runoff is increased beyond 2l/s) but that SuDs can potentially be omitted. Hence I'm left trying to calculate if a) we must have attenuation and b) how much do we need. 

I’ve been trying to work it out - mostly from this document https://assets.publishing.service.gov.uk/media/602e7158d3bf7f7220fe109d/_Rainfall_Runoff_Management_for_Developments_-_Revision_E.pdf but although I can see various tables titled “Attenuation storage volume as a function of QBAR/A and PIMP” I can’t work out which table to use!

 

Building footprint is approx 12x10m if that helps to calculate the roof size (and thus run-off)? I should add the site was previously a dilapidated residential garage so we aren't increasing runoff - however it seems this is irrelevant - we cannot exceed 2l/s regardless.

 

Here is the main part of the land registry location plan.

 

If anyone can help (short of the firms charging £1600+vat) I’m more than willing to make a donation to the forum (though also willing to pay a reasonable amount if someone on here can help professionally) and would be grateful to PM any further info.

[saveasteading]

OK it's easy. I could design all sorts of stuff with calculations. A common solution is underground crates and a flow control (2 litres per second in your case) so that the water is much delayed in reaching the drain or watercourse, after the peak has passed.

Instead propose a rainwater butt on two downpipes,  front and back, as big as you can manage. These can easily hold 300 litres each. Then turn the taps on to a dribble.

If those dribbles  go to flower beds then it is further slowed in its travels and will also  evaporate eventually.

Any excess will come out of the pipe or barrel and you can guide this to some sort of soakaway or a hollow in the ground.

 

Of course the barrels have to be left empty or this doesn't work. In summer you can use it for watering which is another environmental benefit.

 

Let's say that 10mm of rain falls very quickly. You have 120m2 of roof so that is 1200 litres. 600 litres held in the 2 tanks even if closed. 

This can be formalised with reference to storm predictions but perhaps they won't need that.

Driveway permeable as already stated.

 

As rainfall varies so dramatically around the country  your location will affect the amount of rain anticipated and the LA attitude to it.

London, eg,  is an extreme case, with low annual rainfall but coming in sudden downpours. 

 

Enough. More perhaps if this works for you.

[Redbeard]

Given the site restrictions it's the overflow which worries me. IIRC soakaways (if your ground is suitable to Soak Away anyway) need to be 5m from buildings and 2.5m from boundaries. That suggests a max dia of soakaway of 1115 if I have read the dwg properly. If you can get them now, how about one of the 1400 ltr(?) cylindrical recycled containers (which used to hold fruit juice concentrate) peppered with holes? I have not seen them on sale in a long time. You can see them in use (above and below ground) at CAT.

[Redbeard]

https://shop.smithsofthedean.co.uk/recycled-1520-litre--330-gallon-water-tank-751-p.asp

 

See dimensions - you'll have 15mm to keep the right side of the regs!!

 

(But yo do have to 'pass' a percolation test)

[saveasteading]

@MoDo

Does water run off the site as it currently is? What happens to the garage roof water?

Does the plot slope?

 

Even if they weren't imposing this condition you will be doing your bit for the world.

[MoDo]

Thanks, some interesting approaches. 

Yes the site slopes very gently so water likely already running off. However I get that we have to improve the situation not necessarily maintain it. I've done some calcs (well, the website tools do the calcs) using the Wavin and UKSUDS tools - both seem to indicate we'd need 2.5 - 3 m³ of storage which is minimal. I've no idea though if we'd get away with arguing that using permeable paving ought to be sufficient.

[saveasteading]

38 minutes ago, MoDo said:

using the Wavin and UKSUDS tools

I've never used these but it isn't a complex calculation (just tedious by hand) so they should be fairly accurate. 

OTOH they want to sell stuff so they won't be looking for economies.

 

The slope may be  your friend, as it takes your drain pipes and provides a possibility of ground level containment. Could you construct a pond or swale? (a hollow that can hold that amount of water.)

You could build  a small wall of kerbs or brick on the downside, and a hollow, then a controlled overflow to wherever it has to go. 

BUT your garden may be flooded from time to time.

I asked a naturalist about his thoughts on a pond that dried out in summer, and the death of creatures...he said not to worry about it.

Otherwise a crate container in the ground at greater cost.

 

I've redesigned some extensive crate systems and reduced the proposal by about half, and done some from scratch and not needed fancy kit at all, hence why I like ponds and swales.

Always accepted by the LA and/or EA so far.

My instinct and aim is to get most of the water into the ground, let the air take some of it, and only whats left go to watercourse or drain.

I've got the letters that a planner must accept, but you can try without and see if they are happy. To some extent they will care more about  a calculation in their file than about what you actually do.

But if you can do without any connection to the drains then it is good for you and the environment. And you get a big reduction on your water rates, for ever.

[saveasteading]

So I've run the Wavin thing. Very basic, location and area in, volume out.

The planner will accept it i'm sure.

for 120m2 of roof I got just over 1m3 as the capacity.  so that is not a big pond.

 overflow to a soakaway or the mains.

 

[MoDo]

Right yes 1m3 makes sense - I used a larger impervious area in case permeable paving isn't suitable for some reason so got the slightly higher figure. Perhaps I just need to accept that this amount of storage will be required. Can't use a pond or anything like that - just isn't the space, will probably have to be those crates or some underground tank. I'm still wondering if we could simply argue that 1 or 2m3 of storage can be mitigated (avoided) by using permeable paving or something along those lines. I guess no harm to draft some sort of report any try it. Has anyone written their own report? Assume it needs approving by the local water authority prior to submitting to planning?

[saveasteading]

4 minutes ago, MoDo said:

1 or 2m3 of storage can be mitigated (avoided) by using permeable paving or something along those lines.

It's an  option. If the drive is made of deep, single size stones as the base, so that there are lots of gaps, then water can fill the gaps. 4m3 of stone would hold 1m3 of water. then it can soak away over that big surface area and a bit of evaporation.

Or use plastic grids and fill the spaces with gravel.

Or slope the drive to one side and put in a stone trench for the water 

OR a skinny crate in the ground with the drive over it.

Add the barrels though as a bonus...they are often a planning condition so will be well received.

[George]

Driveway underlaid with type 3 aggregate wrapped with geotextile with a few overflow drains sounds good to me. Combined storage and soakaway.

[Gus Potter]

True story and I have the calcs / drawings to prove it.

 

I did this job for a company who renovate supermarket trolleys. They build big new shed and we had to limit run off into the local water course, from memory it was 5 litres per second.

 

We designed some storage capacity (attenuation) to hold the water and let some of the pollutants settle.

 

As they could weld etc we fabricated an orifice plate.. a bit of metal with a hole in it. But for this to pass EA / SEPA you need to screen it.. we used the old trolleys as a screen and put in place a maintenance schedule (also good to present).. it all passed and happy bunnies all round. If you have a look at the cost of a Vortex flow control valve.. you'll see it's a possible cost effective option.

 

Hint.. next time you are shopping take the trolley home.