The Brew Cabin

Posted: **Sun Oct 13, 2013 8:28 am**

Hi all, great site here, I've been reading up on attenuation cells here:

Attenuation Cell Building

I'm not in the trade myself, but the house I am purchasing has an Attenuation 'tank' underneath the garden of the house and my next door neighbor. (the back gardens face each other)

In all documentation it is refereed to as Attenuation 'tank' rather than 'cells' - It's one of my questions to the solicitors to find out exactly what kind of tank it is.

Further research suggests tanks/cells are fairly common for new build projects, to meet SUDS regulations, it's a group of just 9 houses which feed into this tank. Which were finished in November 2012.

My main reason for posting is I've not been able to find any details on an overflow for these systems?

If you have heavy sustained waterfall for long periods of time, much more so than normal, what happens if these tanks become full? My understanding is that the water drains away at a certain rate, so one would assume if the drain rate is massively exceeded for a long period of time the tank could become full?

What happens in this instance? Would the pressure force the water away quicker? Or would water be 'backed up' along the drain pipes? I just want to ensure my back garden would become a muddy bog!

Plans show this tank is only for surface drainage of the close, nothing more.

Additionally, in the back garden there does appear to be pair of inspection hatches. Which maybe suggests it's not a cell type build?

Small cropped photo of said hatches here, if that helps:

Thanks in advance for any further information/suggestions.

Posted: **Sun Oct 13, 2013 9:22 am**

the chances are its a tank type construction, to store water after rain, inside there is a device to let the water into the ground at a controlled rate, they will have been designed by a water engineer and drainage authority,
are you in a in a heavy soil area or clay,which will not absorb water freely? that will be one of the reasons
unless you go to the original builder or possible the building control for the area you wont know the construction and the flow rates etc etc.
maybe ask the other owners if they have any issues with the system not working MSH

Posted: **Sun Oct 13, 2013 10:16 am**

Thanks for the quick reply.

The soil is not of clay type, it's fairly soft. At least going by my Grandmothers back garden which is 1/2 mile or so away. I'm not sure if it's heavy soil or not, the area is Camberley in Surrey if that helps.

The road is un-adopted, so maybe the tank was a council request for the small development? ???

So just to confirm, there is no over-flow or other form of release if it gets full?

Thanks again

Posted: **Sun Oct 13, 2013 10:45 am**

I would not say , yes or no to overflow. the whole idea is to control outflow so having a by pass over flow would defeat the object. MSH

Posted: **Sun Oct 13, 2013 10:49 am**

msh paving wrote:I would not say , yes or no to overflow. the whole idea is to control outflow so having a by pass over flow would defeat the object. MSH

Ok thanks again.

I'll ask about any previous issues, that the current owners may have experienced. But these systems are generally reliable?

Posted: **Sun Oct 13, 2013 12:58 pm**

have you had the inspection hatches up?
we have come across 1.8m concrete rings 5 m deep in a back garden in bushmead luton for rainwater
LLL

Posted: **Sun Oct 13, 2013 1:05 pm**

lutonlagerlout wrote:have you had the inspection hatches up?
we have come across 1.8m concrete rings 5 m deep in a back garden in bushmead luton for rainwater
LLL

Wow, 1.8m by 5m? That's a lot of water!

I've not had the inspection hatch up, as on the viewing I did not know what it was for

Posted: **Sun Oct 13, 2013 1:57 pm**

These attenuation cells are able to overflow, basically spill out during when they are completely surcharged - otherwise something would eventually give way. Well....the one we installed (300 crates) had this facility designed into it.

Posted: **Sun Oct 13, 2013 2:36 pm**

Dave_L wrote:These attenuation cells are able to overflow, basically spill out during when they are completely surcharged - otherwise something would eventually give way. Well....the one we installed (300 crates) had this facility designed into it.

Thanks for this.

Assume as these systems are low enough in the ground it would sink down rather than waterlog the garden?

Posted: **Sun Oct 13, 2013 7:38 pm**

not sure if the same thing but a site in basingstoke we just finished had these the ground workers devines i think dug 10 of these 5m x5mx5m the ground was lime or chaulk not sure

but if i remember rightly they had some sort of overflow bit like a toilet over flow in case they became full which ran too drains

Posted: **Mon Oct 14, 2013 3:14 pm**

gonchy wrote:not sure if the same thing but a site in basingstoke we just finished had these the ground workers devines i think dug 10 of these 5m x5mx5m the ground was lime or chaulk not sure

but if i remember rightly they had some sort of overflow bit like a toilet over flow in case they became full which ran too drains

Thanks for responding, thats positive to hear.

The plans I have indicate this too, as it shows the tank being connected to the street.

Today I've asked specific questions on the tank, and it's over-flow. Hopefully will hear back soon

Posted: **Wed Oct 16, 2013 7:32 pm**

How refreshing to see members of the public start asking questions about such soakaway / drainage systems. Your right to do.

It sounds like the back end of the houses drain to a single soakaway which has been put in your back garden. Such systems are not fit and forget.

This was a standard kind of arrangement in the past, which caused serious problems for people. It was often the cheapest solution for the builder. In the end, with so many builder "designed" soakaways flooding the Government changed the rules, so if you have more than one house going to a soakaway it needs to be adopted(from next year). The adoption authorities usually only agree to them been located in an "accessible location" usually a front garden or common open space.

Before you purchase consider asking the following to the developer:
a) Understanding - Can I have a drawing showing the arrangement?
b) Risk of flooding -What design return period was it designed to? 1:10 year? 1:100? etc
c) Legal liability - When (not if) it needs to be replaced what is the situation regarding ownership and payment. How do my neighbours who use it contribute to maintaining / replacing it?
d) Maintenance - What is the recommended maintenance for it?

As to the practical elements, some soakaways do have overflows other don't - as suggested it depends all on the specific location and the permeability within a estate can be completely different.

If for example it was designed for the minimal 1:10 year, then it would surcharge i.e. the water would flood in the back gardens. The biggest issue is if to gets to a level in which it can get into the house as it would start causing major damage.

Of course it could have been designed professionally and build by an experience contractor in such a way as the risk is minimal.

Posted: **Thu Oct 17, 2013 11:11 am**

stephen gibson wrote:a) Understanding - Can I have a drawing showing the arrangement?
b) Risk of flooding -What design return period was it designed to? 1:10 year? 1:100? etc
c) Legal liability - When (not if) it needs to be replaced what is the situation regarding ownership and payment. How do my neighbours who use it contribute to maintaining / replacing it?
d) Maintenance - What is the recommended maintenance for it

Thanks for responding, some useful points raised there which I will ask. In response to the above.

a) I already have a plan layout, it shows fowl & drainage water, for the whole close. All drains from each house including (hopefully) mine, lead into this tank. It also shows a line from the tank, going to the street.

b) This i don't know, will find out.

c) It's owned by the maintenance company, the close is an un-adpoted road. The close, is made up of 9 houses, each of which are a member on "The Close" - Management Company Ltd. There is an annual fee, which covers everything communal in the close, grass, road, fences ect, this includes the tank. If it needs repairing, it's payed out of that fund.

d) Already finding this out, there are inspection hatches as pictured above.

thanks again

Posted: **Wed Oct 23, 2013 5:31 pm**

can someone tell me the size of cell i would need to drain area of approx 100m2

Posted: **Wed Oct 23, 2013 5:52 pm**

^^^^ its not that easy, soil type and conditions are the factor ,its for a qualified man to design MSH

The Brew Cabin

Attenuation cell - overflow?