Surface water disaposal in groundwater source protection zone 1

[Drellingore]

What do you do when there's not enough space for soakaways and you can't drill a deep bore soakaway for surface water?

 

Four years in, we're still in the planning phase of the conversion of two barns in groundwater source protection zone one. The drainage consultant we're using is very alarmed that there may be no viable drainage strategy for surface water. Because the barns are quite large, there's a large area of rainwater that's collected. This currently just falls off all the eaves, or in some cases goes along gutters, down downpipes, and into underground pipes of unknown quality, diameter, incline and destination.

 

I knocked up a quick (and inaccurate, but better-than-nothing) estimate of the amount of roof area that we'll need to deal with based on Approved Document H3:

 

The lowest point on the site is sandwiched between the barns and a road. Approved Document H3 asks for soakaways to be 5m away from buildings and roads.

 

The drainage consultant has suggested getting BRE365 infiltration tests done, and taking things from there. However, his tone seems to imply that he thinks that it's going to be unpossible to get a viable solution that will pass building control and the approval of the EA (who in turn delegate the decision to the LPA). I'm going to be flabbergasted if after four years and several hundred thousand pounds of plot purchase, the development is snookered because of rain falling on the site. I mean, we're not going to make it rain more. We are going to need to direct that somewhere, but it can't be an intractable problem.

 

So: what options are there? Does anyone have experience of doing things like pumping surface water elsewhere? Would wanging ponds into the wildlife/landscaping scheme help?

 

The drainage consultant seems very thorough and knowledgeable, and already has context on the site. I'm considering getting second professional opinions though just because I can't be doing with the figurative sucking of teeth when what I need is solutions rather than proclamations about how tough the situation is.

 

P.S.

Some folks may remember my questions about drainage and sewage treatment. We got the EA permit for discharge-to-ground for that, despite every professional thinking it was impossible. So I was quite surprised to have surmounted that obstacle only to find that chuffing rain water would be a potential showstopper  

[Redbeard]

This? 

 

[Alan Ambrose]

I think 5m is, in practice, an ambition rather than a hard rule. What's the best you can do?

[Gus Potter]

On 12/04/2024 at 10:55, Drellingore said:

The drainage consultant has suggested getting BRE365 infiltration tests done, and taking things from there.

Do a soakaway test.

 

Your drainage Consultant is flagging this up and rightly so.

On 12/04/2024 at 10:55, Drellingore said:

he drainage consultant we're using is very alarmed

You use the word "alarmed" and if that is what they are indicating then there probably is a problem with a normal soakaway.

 

On 12/04/2024 at 10:55, Drellingore said:

I'm going to be flabbergasted if after four years and several hundred thousand pounds of plot purchase, the development is snookered because of rain falling on the site.

You seem to have a propensity for understatement in this case? But if that amount of money is at stake then there may be an Engineering solution available or a deal with a neighbour... yes it will cost you but you won't lose all your dosh.

 

I find this stuff fascinating, the challenge, investigating and finding a solution. If you want a bit of help from BH folk.. there are a few folk that can help that know do their stuff but to get the best help.. then you'll need to post a lot more info... be careful before you do as it may compromise your position later.. as you know.

 

If you do want to post info then basically the full shebang is required. Site plan (dimensioned) and address, any geotechnical info you have, the building footprint, pretty much everything you have and then you'll get some free advice.

On 12/04/2024 at 10:55, Drellingore said:

and you can't drill a deep bore soakaway for surface water?

Yep a clever option. But are you over a major aquifer that supplies drinking water? You are in a zone one! Are you over the North Downs main aquifer?

 

On 12/04/2024 at 10:55, Drellingore said:

Does anyone have experience of doing things like pumping surface water elsewhere?

Good solution if you can get the neighbour or Water company to take it for a fee?. With a bit of attenutation you could actually win a watch as you could supply flow to say ditches that suffer from dry weather / intermitant flow. Just mulling over ideas.

 

Ponds / swales etc can work if you know enough about the ground, flood risk, ground water levels etc..

 

What if you had no roof gutters and drained the water into a swale, raised planters ( attenuation), wide French drain or use a louvred edge roof detail. Or you can have gutters which is more traditional and do the same. Now your 5.0 m from the building no longer applies so long as you can show the structure won't be affected. There are lots of options.

On 12/04/2024 at 10:55, Drellingore said:

Approved Document H3 asks for soakaways to be 5m away from buildings and roads.

Yes this is a general rule to avoid compromising the founds ect.. as above

 

On 12/04/2024 at 10:55, Drellingore said:

I can't be doing with the figurative sucking of teeth when what I need is solutions rather than proclamations about how tough the situation is.

 

I'm thinking... just say we look at all other options and nothing works or to do a deal with a neighbour costs too much. If there is this much money at stake then why not say deepen the founds and let the rainwater soak away under the building? like it did before the building was there? In reality what happens is the neighbour may see the pound notes flashing.. you say.. hey we have another solution.. push too hard and you'll get nothing..

 

The above is a bit of a glib statement.. there are many challenges to doing this, geotechnical and SE wise. If you are on chalk then is has up and down sides but fundamentally it should work.. but it will take a lot of work to convice all parties involved that it does.

 

I would look at all the other simple options first. Some you can use to compliment the Architecture and look great if well designed. No gutters can be a problem as the splash zone impacts on cladding design etc.

 

Hope this raises your spirits a bit... if required.

 

 

 

[saveasteading]

+1 for what @Gus Potter says.

Don't despair. 

 

Your rainfall seems to be exaggerated by taking the worst combinations of slopes. That works for individual drains serving those slopes' but you don't have to add them together. When all aggregated the rain fall on the site is the rainfall on the site. The wind driven rain onto one side is not happening on the other side simultaneously.

 

403m2 of roof, plus the site itself is 'all' I would propose you have to allow for at outfall/storage., whatever Doc H says.

You're not getting runoff from off site I hope.

 

If your drainage consultant is struggling then maybe drainage isn't their main thing.  We don't all know everything. Some consultants don't think outside of the box.

On the chance that it might give you some encouragement, I did a project where no water leaves the site, even though there are sewers available. 

Why? showing off. Saving money.

 

Boreholes probably aren't a good idea where I think you are, plus they can clog after 10 years and need reboring.

Water going through chalk erodes it and the flow increases. Swallow holes can be a problem.

But soakaways are usually ok. Even then they might want a test pit to be filled with a few m3 of water to check it doesn't rush away.

 

On 12/04/2024 at 10:55, Drellingore said:

despite every professional thinking it was impossible.

 

Wrong professionals then if someone then resolved it for you?

 

The solution will be utterly site specific.

The BCO will not be expert, and will accept what your consultants advise, assuming they are qualified and can justify it.

 

 

It will cost money but can be done. An elegant solution will cost a lot. An ugly one not so much.

 

"rainwater harvesting, build hub taught me no.." 

I wouldn't discount anything in your situation.

 

On 12/04/2024 at 10:55, Drellingore said:

despite every professional thinking it was impossible.

You've said it again.   Except the one who made it work perhaps? Is there a reason for having had lots of professionals look at it?

[SteamyTea]

7 hours ago, saveasteading said:

Your rainfall seems to be exaggerated by taking the worst combinations of slopes.

Should/can it be calculated on the RMS value (root mean squared), or is simple terms, for 3 measurements, 0.71 of the sum of maximum flows (which may be the design flow rates).

Just a mathematical solution, not a real life one.

 

 

[saveasteading]

I'm simply saying that when rain is lashing against one slope, and concentrating the amount if water from that drain, at thd same time there is a reduced amount on the opposing face. The amount of water reaching the main sewr or other disposal system is not factored. 10mm of rain is 10mm to deal with.

It's still a lit of water., and needs storm modelling for the worst case.

 

As the OP says, the rain is already landing on site and going somewhere.  But it must not be allowed to run off site causing problems elsewhere.

[ProDave]

It seems somewhat illogical that you have a EA permit for foul water disposal, and are now struggling with rainwater disposal.  I would not have thought rainwater soaking into the ground would be a bother even in protection zone 1.

 

Have you actually submitted a design to the EA and had it refused?

[Drellingore]

Wow, lots of comments to consider. Thanks for your consideration and input, and for also trying to cheer me up. It was certainly a bit of a surprising knockback (I'm sure there'll be many more to come) and the missus was on holiday visiting the in-laws so I didn't have anyone else to vent to!

 

On 12/04/2024 at 14:06, Alan Ambrose said:

I think 5m is, in practice, an ambition rather than a hard rule. What's the best you can do?

 

Aye - I think it's one of those areas in which there are alternative means of satisfying the building regs, so 5m is a rule of thumb to keep things simple on ordinary sites. I'm currently waiting on the missus to add a 5m exclusion zone on the site plan so we can work out how much space we have to play with.

 

11 hours ago, Gus Potter said:

Do a soakaway test.

 

Your drainage Consultant is flagging this up and rightly so.

 

Agreed on both points. We've asked them to proceed with a quote for the BRE infiltration tests. I do absolutely value that the consultant is raising this - the information is invaluable, and it's much better to receive it sooner rather than later. I do wish that they were providing a little more in the way of potential solutions and little less teeth-sucking though  

 

11 hours ago, Gus Potter said:

you over a major aquifer that supplies drinking water? You are in a zone one! Are you over the North Downs main aquifer?

 

Correct! So quite rightly we've got to be very careful. The pumping station is 300m away.

 

Thanks for the various suggestions @Gus Potter, those were all things I hadn't thought of. Thanks also to @saveasteading for the point about the calculations - so to recap, you're saying that individual gutters/drains need to be sized taking angle into account (in case the rain is blowing in sideways), but the overall amount being disposed just needs to consider the plan area? If so, that makes a lot of sense and reduces my levels of astonishment at the building regs!

 

1 hour ago, ProDave said:

It seems somewhat illogical that you have a EA permit for foul water disposal, and are now struggling with rainwater disposal.  I would not have thought rainwater soaking into the ground would be a bother even in protection zone 1.

 

Tell me about it!  In my IT consultant life we often talk about designing things with "the principle of least astonishment." This scenario has a non-minimal level of astonishment. In the planning application from the previous owners that got approved, the LPA mandated oil interceptors from all areas of hardstanding. This is particularly annoying as 1) the site has had tractors on boats on it for years, without any interceptors; 2) we only drive electric cars. Of course delivery vans and future owners might drive things that use oil, but still...

 

1 hour ago, ProDave said:

Have you actually submitted a design to the EA and had it refused?

 

That's a good question. No, we haven't. In prior planning applications the EA have been a statutory consultee, and have delegated approval back to the LPA with a set of conditions. So we might end up with the LPA making that decision on the EA's behalf, which I'm not sure will be better or worse. It took four months for the EA to grant as a permit for sewage... Maybe I should start looking into permits for rainwater now.

[saveasteading]

5 minutes ago, Drellingore said:

you're saying that.....

Yes.

 

6 minutes ago, Drellingore said:

oil interceptors from all areas of hardstanding

Gravel parking will suffice. Details upon request.

Anyway, modern cars don't drip oil. They must have shares in interceptors or simply not understand or care about good design.

[Drellingore]

There is a surface water 'sewer' that was installed on the property when it was a farm. It's basically a metal pipe of something like 8-10cm diameter, that pops up on the tarmac drive. I rented a cat-and-genny when looking into the sewage solution a while back, and it appears to run to an unlisted storm drain on the other side of the road, that roadside drains also flow into. The farmer that used to work the site said that the water company (that does the extraction) built those drains/sewers and connected the barns up in the 60s.

 

Given what @saveasteading said about my misunderstanding about the pitch factor, the sum plan area of buildings is 403sqm, so that should yield a flow rate through a pipe of 8.8litres/second and Diagram 3 of Document H3 makes it look like a pipe of 75mm diameter should be able to handle that.

 

I'm tempted to go back to the drainage consultant and ask for a quote on a CCTV survey of the pipes to see if they can be used. If they turn out to be fine, then it'd be daft not to use them.

 

[Drellingore]

21 minutes ago, saveasteading said:

Gravel parking will suffice. Details upon request.

 

Consider this a request politely being made  

[Drellingore]

If anyone's feeling even more generous with their time and brainpower than y'all have already been, could you help me understand bits of this planning condition that as on the decision notice of the previously-approved scheme?

 

Quote

Prior to the commencement of the development a detailed scheme for the disposal of foul

sewage, waste water and surface water shall be submitted to and approved in writing by the

local planning authority. The scheme shall demonstrate that all surface water drainage from

parking areas and hardstandings shall be passed through an oil interceptor before entering the

ground. Any downpipes discharging into the ground shall be sealed against any other source

of discharge. Any unsealed downpipes shall drain into a foul sewer. The scheme shall include

a programme for implementation and long term maintenance.

 

Quote

Any downpipes discharging into the ground shall be sealed against any other source

of discharge.

 

What exactly do they mean by downpipes here, do you think? Just those that gutters run into? Is there any other meaning of the term? I can see the logic in this, in that if there's a pipe that discharges into the ground, if it could be contaminated by some other source of discharge, then those contaminents would be fast-tracked into the ground. Which would be bad.

 

Quote

Any unsealed downpipes shall drain into a foul sewer.

 

The intention here is clear - if it's unsealed, treat it as foul water to be on the safe side. What counts as an unsealed downpipe though? Is the downpipe that gutters run into 'unsealed'? Does the seal refer only to the bit where the pipe joins the ground?

[saveasteading]

17 minutes ago, Drellingore said:

Any unsealed downpipes shall drain into a foul sewer

No this is crazy. They aren't paying attention that you are using a digester.

It would send clean rain water through the digester and this messes up the digestion process plus sends extra water to the digester soakaway.

 

21 minutes ago, Drellingore said:

What counts as an unsealed downpipe though?

 

Presumably this is to stop you pouring old sump  or cooking oil into a rainwater hopper. But if you connect the dp direct into the drains, with no hopper, you are satisfying this.

[Drellingore]

Thanks for the help @saveasteading. The missus is currently looking into hardstanding/concrete pavers/oil interceptors and the like, so if you had any words of wisdom on that front, I can owe you a virtual pint!

[saveasteading]

OK. A virtual pint is what the doctor prefers.

 

Gravel in a gravel grid catches any oil drips over their large surface area and exposes them to bacteria. and the oil gets consumed.

 

But what if you have a major spill?  Most will stick to the gravel and the rest to the membrane beneath it, which wont allow oil through. In extremis it then washes down to the side where you collect it to a storage area. As long as your catchment is on the surface like a ditch or lagoon, and not connected to springs or soakaways, then it will rise to the top and you can skim it off if the plants don't consume it first. Phew.

Or you could add a small weir / catchment between pipes and lagoon, so that water goes through a half height pipe and oil stays on top. £300 instead of thousands but that's still excessive.

 

This is to be preferred to a hidden oil interceptor where oil gathers unseen until it washes through.

 

This is all provable somewhere but I can't remember where. Anyway I've done it loads of times for schools and offices ( 40 spaces or so) , and has always been approved. and I know it works. 

You must state it all with utter confidence as a complete strategy.

 

Block pavers with gaps for drainage also work, if you have open stone beneath. but the gaps clog quite quickly.

 

 

Edited April 16 by saveasteading