Surface water flow attenuation problem.

[G and J]

We have a surface water drainage problem!

 

We have just demolished a bungalow where rainwater was dumped to the ancient sewer.

 

We are required to reapply for our sewer connection.  After much too-ing and fro-ing Anglian Water have finally accepted that a soakaway won't work, as we have very poor infiltration rates.

 

So we are going to be allowed to connect our surface water outflow to the sewer, as long as the surface water only joins the foul drain at the last manhole before the road.

 

BUT!  They are telling us that we need to resubmit a drainage plan that chokes the flow to 1 litre per second.  In further discussion as we have very shallow drains they are minded to accept two outflows (one either side of our foul drain) each of which is choked to 1 litre per second.  They helpfully suggested that we need to design storage on site.  They also accept that as our drains are v shallow that will be rather challenging.

 

Having spent some time googling and reading past threads we are still none the wiser as to whether this means two water butts or a 20m3 underground tank (which would be impossible due to it being a narrow site and close neighbours with virtually no foundations).  

 

Help!  Where on earth do we start?

 

 

[nod]

More likely an underground tank would be what they are expecting 

[BadgerBodger]

Have you looked at a shallow crate system which could then have a hydrobrake on the exiting MH? 
 

[G and J]

1 hour ago, nod said:

More likely an underground tank would be what they are expecting 

I did wonder.  Trouble is, at the road we have an invert level of only 700mm, and the site has a slight slope downwards from the road.  So if we have any form of tank the outlet will have to be pumped, which is a pain.  
 

The site is only 7m wide, so with the drains going through one side and the garage covering the other I think we will struggle to make room for one.  How would I calculate how big a tank I must have?

 

[ETC]

I think I’d swop the WC and the sink over in the cloakroom.

[torre]

Perhaps you could explore a 'blue roof' - could the tank go on the garage roof maybe?

[G and J]

12 hours ago, ETC said:

I think I’d swop the WC and the sink over in the cloakroom.

 

Is this personal pref. Or a space concern?

 

If the latter we are not having a shower unit, just a glass panel between basin and shower and we are using the wall between cloakroom and coat cupboard to take cistern.....hopeful makes most of space (which tbh is a bit of compromise but works with rest if design vs £) and means we don't have to see the wc every time we wall past the door, but always willing to consider ideas....

 

 

[Temp]

On 30/04/2025 at 21:02, G and J said:

How would I calculate how big a tank I must have?

Perhaps..

 

https://www.cncbuildingcontrol.gov.uk/faqs/calculate-size-soakaway-required-extension/

[G and J]

I think that works for when the percolation rate is in bounds.  Outside that - nil points

[Alan Ambrose]

What’s the fall between ground level at the house and invert level at the pipe? Suggest you need about 1 in 100 on the pipe plus about 20cm for rainwater gullies.

 

Maybe get a suds analysis (uksuds.com) as the 1 l/s thing is a suds requirement. That calc uses your roof area and any non-permeable hard standing (so plan to have a permeable driveway), weather data, allowed drainage rate (2 l/s in your case), some climate assumptions and a bunch of stats to figure out the tank size you need. If you have the fall and room (under the drive maybe) you can use a suds tank. If not, above ground storage like butts. It’s possible your ground even in winter maybe take some water too. Storage can be an actual tank, crates, dug hole with shingle in (at 30% volume) etc.

I’m using a permeable drive as part of the ‘tank’.

 

Any place to drain at the rear of the plot too?

Edited May 4 by Alan Ambrose

[Alan Ambrose]

Sorry, not uksuds.com, actually (believe it or not) 'freeflush' ( https://www.freeflush.co.uk/products/suds-attenuation-sizing-design ). Cost is £120 now.

 

The calc looks something like this (I think they have updated it now). The key output number is the 'tank size' (the 95% is assuming a tank made from typical crates which are 95% void / 5% plastic). The 'tank' can be any shape, of course.

 

The key inputs are your location, the allowed drainage rate (1.9 l/s in my case), the horizontal area of your roof and any 'non-permeable surfacing' e.g. concrete driveway.

 

Actually, for a quick and dirty calc (as we're in a similar location weather-wise) just pro-rate my numbers using your own roof area etc.

 

Edited May 4 by Alan Ambrose

[G and J]

Crumbs, thank you for all that.  
 

In fairness I’ve calmed down a bit now (me meds finally kicked in!) and I’ve gone back to first principles.  
 

I’ve got to come up with a drainage plan that satisfies both Anglian Water and my BCO.

 

AW don’t care what I do with the rainwater as long as I squirt a maximum flow (hopefully 2 x 1 litre/s) down the sewer.

 

I’m guessing my BCO cares that I don’t flood anyone.  
 

Simples!  (I wish.)

 

If our new pad wasn’t there, rain would fall on the plot and what doesn’t soak in would trickle down the garden away from the road, and crucially, peep’s houses.  We are in a row of houses where each has a long, parallel back garden with the far end of the garden being circa 1m lower than the road.  Plus the light soil is a lot deeper at the far end of the garden, circa 1.2m deep.  Perfect for rainwater dissipation methinks.

 

My foul invert level is about 650mm at the road, and as the road is the highest point the invert level with respect to ground level gets progressively worse from there.   A couple of phone calls identified quickly that the requirements for a tank under the drive meant that any tank would be way too low to drain into the sewer without a pump.  Bugger.  No tanks then.

 

So, we are left with the notion that I have some form of overflow that takes excess storm surge rainwater and lets it wander off down the garden and stagnate and then seep away like me with a Pimm’s on a summer evening.  That’s what I’m currently working on.  So, no thanks to tanks.

 

If and when I get thumbs up from both AW and my BCO you’ll feel the earth shift with my sigh of relief.  

[Alan Ambrose]

>>> A couple of phone calls identified quickly that the requirements for a tank under the drive meant that any tank would be way too low to drain into the sewer without a pump.

 

Well, according to your drawing, your foul pipe is planned to drain that way, and the surface water need less of a fall, so it must be possible.

 

>>> too low to drain into the sewer without a pump

 

As above, not sure I agree. A pump is not ideal, but not impossible either - I'll have at least a couple in my build (and also a suds / rwh tank).

 

>>> far end of the garden being circa 1m lower than the road

 

Be interesting to see (maybe with a hosepipe) what happens to rain when it drains it that direction - it must go somewhere. A percolation test at the back of the plot?

 

Edited May 4 by Alan Ambrose

[G and J]

Pipes will have the fall to run to the road from the road end of the house no problem, except AW want a restriction which implies/requires storage or overflow.  Even though it’s a combined system they require separation until the last manhole, which makes the falls a bit more tricky.  
 

If I put a tank in the drive it would indeed need a pump and I’m rather keen to avoid that, because I’m tight, I’m lazy so want less maintenance and Sod’s Law dictates that such a system would go wrong when you most need it, e.g. a power cut during a mega storm.  (Seen it before, not going there).

 

And as for the end of the garden being able to soak it up, yes, that's a big part of the cunning plan I am hatching (maniacal laugh, as I stroke my Dick Dastardly style moustache!)