Suds calculator

[Barryscotland]

Our drainage plan shows a filtration strip 25m long by 1m deep and 1m wide to stop water surging into the burn we are running our roof surface water into, our roof is roughly 220m2. It seems overkill to me, is there any simple calculation to see if it is needed before I phone the SE to see if it can be reduced any. 

[Kelvin]

I thought infiltration (or a soakaway in some cases) was pretty standard. As understand it it’s not just about surging water but also about removing potential pollutants from the water. I haven’t got everything back yet from the SE but I’m expecting something similar. We will have a soakaway for the treatment plant. It was also originally 1mx25m but I pointed out it was within 50m of my neighbour’s borehole so it’s been redesigned to be 9mx2m and however deep it needs to be. 

[Barryscotland]

8 minutes ago, Kelvin said:

I thought infiltration (or a soakaway in some cases) was pretty standard. As understand it it’s not just about surging water but also about removing potential pollutants from the water.

What pollutants are going to be picked up between the rain falling out the sky, running down my slates into a plastic pipe and into the burn, I could understand if it was run off from a septic tank but this is solely roof water so I think it’s just about stopping a surge? 

[Kelvin]

Roofs can be sources of concentrating pollutants that is subsequently picked up by water run off. I don’t know whether it’s of particular consideration here. There’s quite a lot of research done on it. 
 

Anyway, here’s the  regs for the domestic technical handbook (I’ve assumed Scotland)  which has a section on surface water drainage and soakaway sizing. My understanding, which isn’t much, is that a filter trench is much the same as a soakaway except that a certain amount of the runoff is allowed to drain to the water course via a pipe.

 

https://www.gov.scot/binaries/content/documents/govscot/publications/advice-and-guidance/2017/01/building-standards-2017-domestic/documents/building-standards-technical-handbook-2017-domestic/building-standards-technical-handbook-2017-domestic/govscot%3Adocument/Building%2Bstandards%2Btechnical%2Bhandbook%2B2017.pdf?forceDownload=true

 

 

 

Edited August 16, 2022 by Kelvin

[Gus Potter]

3 hours ago, Barryscotland said:

Our drainage plan shows a filtration strip 25m long by 1m deep and 1m wide to stop water surging into the burn we are running our roof surface water into, our roof is roughly 220m2. It seems overkill to me, is there any simple calculation to see if it is needed before I phone the SE to see if it can be reduced any. 

Barry.. unfortunately not.

 

If you want to learn a bit more about it then get yourself a copy of BRE digest 365 link below. Will cost you £28.00 but it will give you a grounding on this. I would give you my copy but will get hammered on copywrite if I post it.

 

https://www.brebookshop.com/details.jsp?id=327631

 

It takes a lot of stone to fill a trench like this, also you have to do something with the muck away, geotech membrane on top.

 

A few rough points. The trench will probably serve to act as a partial soakaway.. good in the summer when you have low rainfall but in concentrated bursts, more pollutants on the roof from birds and just general airborn pollutants. This is when you really damage the habitat. River / burn / stream flows are low so a small bit of highly polluted water off your roof during the start of rainfall can do a lot of damage to the ecology. In the winter flow rates in the water course are generally higher so more dilution, more regular rain washing the pollution off the roof in smaller concentration.

 

In the summer you can get really high rainfall in short spells, more so in England . The idea is that the first bit of water that comes off your roof gets stored in the trench, then as it fills it gets diluted before fully filling and entering into what you hope is a river / burn with an increased flow.

 

The trench is filled with stone with gaps. The gaps (voids) form the storage volume.. hence the apparently large trench.

 

Can you do a pond / retention basin / swale with storage. If you have a bit of space this could be a great addition to the garden with a bit of clever planting? Ponds don't need to be round and you can select plants that do well in both dry and filled pond conditions.

 

Once you get your head around the theory you can use your imagination. If you have a sloping site you can create bunds to a certain extent so a " pond" can be above the original ground level once you have regraded. Clay soil is your friend here. In the round have a look at what you have got and what you could do. You may save a bit, even if you need to pay your SE for another design. Then if you can go the pond way or say a swale you'll get your money back out of the enjoyment of the garden? I hope you are not intending to put your driveway say over the trench?

 

 

 

 

 

 

 

 

[Temp]

So 25 cubic meters full of stone can probably hold what 10% of that? or 2.5 cubic meters of water. Would some sort of surge attenuation tank be an option?

Edited August 16, 2022 by Temp

[Barryscotland]

13 minutes ago, Gus Potter said:

Barry.. unfortunately not.

 

If you want to learn a bit more about it then get yourself a copy of BRE digest 365 link below. Will cost you £28.00 but it will give you a grounding on this. I would give you my copy but will get hammered on copywrite if I post it.

 

https://www.brebookshop.com/details.jsp?id=327631

 

It takes a lot of stone to fill a trench like this, also you have to do something with the muck away, geotech membrane on top.

 

A few rough points. The trench will probably serve to act as a partial soakaway.. good in the summer when you have low rainfall but in concentrated bursts, more pollutants on the roof from birds and just general airborn pollutants. This is when you really damage the habitat. River / burn / stream flows are low so a small bit of highly polluted water off your roof during the start of rainfall can do a lot of damage to the ecology. In the winter flow rates in the water course are generally higher so more dilution, more regular rain washing the pollution off the roof in smaller concentration.

 

In the summer you can get really high rainfall in short spells, more so in England . The idea is that the first bit of water that comes off your roof gets stored in the trench, then as it fills it gets diluted before fully filling and entering into what you hope is a river / burn with an increased flow.

 

The trench is filled with stone with gaps. The gaps (voids) form the storage volume.. hence the apparently large trench.

 

Can you do a pond / retention basin / swale with storage. If you have a bit of space this could be a great addition to the garden with a bit of clever planting? Ponds don't need to be round and you can select plants that do well in both dry and filled pond conditions.

 

Once you get your head around the theory you can use your imagination. If you have a sloping site you can create bunds to a certain extent so a " pond" can be above the original ground level once you have regraded. Clay soil is your friend here. In the round have a look at what you have got and what you could do. You may save a bit, even if you need to pay your SE for another design. Then if you can go the pond way or say a swale you'll get your money back out of the enjoyment of the garden? I hope you are not intending to put your driveway say over the trench?

 

 

 

 

 

 

 

 

 

thanks for your explanation of why they ask for one it makes a bit more sense now.
Still an awful lot to dig out and haul away an I’m going to put in about 40t of gravel which will cost me about £500 to hold  about 2.2 cube of water, presume an attenuation tank of 2.2cube won’t be acceptable as it doesn’t filter or soakaway the same as a large strip?

pond isn’t an option as the drain runs down the middle of two future plots and yes maybe a drive on top of the drain in the future to access the plots

[Gus Potter]

You have a bit of thinking to do here.

 

Don't go sticking a trench under where you may want to put a future driveway, you'll get caught out when they ask how you are going to deal with the run off from the hard area of the drive. You may say.. permeable monoblock or say open textured asphalt, BC will say.. but there is a drain already using the infiltration capacity of the soil under... please provide calculations.

 

Also these things are supposed to be maintainable.. under a driveway with associated soil compaction.. thus reduced infiltration rates and so on..?

 

Have a word with your designer about your future plans, maybe a swale with some storage capacity either side of the future drive, could even try a simple orifice plate with filter at the ends to say reduce outflow to 5 or 2 litres per second into the water course. BC etc may accept this as it is a very small scheme? If you only are predicting 2.2 cube of water then this will equate to  small draft of water in a shallow swale of a reasoable length.

 

Let us know how you get on with your SE.

 

 

 

 

[saveasteading]

The advice above is good.

In a trench of round gravel, a third is air and the rest is stone. Anything less than round and the volume of air (hence water) decreases. Therefore your trench of 25m3 will only hold about 8m3 of water at best.

Perhaps that is enough. The calculation depends on location, rainfall intensity and duration, and is rather hard work, hence computer programmes are used as standard.

You haven't said much about the site: permeability, slopes, area.

 

This can be increased by using expensive crates instead of gravel, or by adding empty pipes to the trench, although that can be as expensive.

 

As Gus says, the trench will perform as a soakaway, depending on your ground permeability.

This can be increased by changing the shape to provide more edge area, if space allows.

Also you may be able to use the drain pipes as temporary storage, and soakaway area, if they are shallow slopes.

 

It is good practice, even if not enforced, to avoid any water at all running straight to the burn.

I don't think pollutants are an issue in your case, but it is good to do your bit to avoid increasing flooding downstream.

 

What happens to the water after your storage trench? Is it to soak away, or is there a controlled outlet to the watercourse?

 

Bottom line. Designers sometimes forget that rain has been falling on your site before your project. Your responsibility is to not make it worse for the environment. Soakaways, ponds and swales can be worked with to achieve this. The latter have the benefit of surface evaporation as well as soakage.

 

 

[Barryscotland]

I think I need to go back to the drawing board with this one. The top of our site where the house is is clay and this is the only place we did test digs so the SE suggested easiest option as the site is clay just run the surface water down to the burn. As with every other suggestion the SE and Architect made we just went with it as we had no other experience in house building and have been regretting lots of them in hindsite during the build. Gutters are connected up and have ran the drainage 18m downhill from the house and the ground is nice black soil and the rain from the thunderstorms we had a few nights ago all disappeared very quickly so I think I will get a perculation test done and see if we can just put in a soakaway where the pipe terminates at the moment.

Its the inconsistency that gets annoying as the chap at the bottom of the hill from us has just finished a build and he put all his surface water straight into the same burn with zero filtration or surge protection just piped straight in although maybe we could of done the same if the SE had shown it that way originally.

[saveasteading]

2 hours ago, Barryscotland said:

I will get a percolation test done

You can easily do this yourself. This will show if it is feasible at no cost. Then you may have to get your professional to watch you do it again and confirm.

It is not normally in SE or Architect skillset if it is a full assessment. But simple proof may be enough.

As I said, contamination isn't the issue....

It is reasonable however for the BCO to ask how you are disposing of the rainwater.

To a soakaway, any exceptional rain held back by a swale. None to burn if you can.

 

All my comments have to be related back to the reality of the site of course.

 

2 hours ago, Barryscotland said:

nice black soil

This will be on the surface. The test is dug beneath the soil, in the ground which has to allow the percolation. perhaps the same clay, but 18m is a long way down and maybe a different stratum there.

Which brings us back to a pond. The fish and frogs  will drink the water.

[Barryscotland]

Does anyone have there percolation test findings and soakaway design they can share with me so I can see what I need to present to BC?

[Kelvin]

My surface water drainage is straight into a water course. My soakaway design is for foul drainage. Here are screenshots of the relevant sections of the report:

 

 

 

 

 

Edited August 23, 2022 by Kelvin

[Gus Potter]

9 hours ago, Barryscotland said:

Does anyone have there percolation test findings and soakaway design they can share with me so I can see what I need to present to BC?

Hi Barry.

 

Attached is bit of one of my right in the background working spread sheets that I use from time to time to get a handle on how this works. It does not come out that well, some of parts of the graphs are on different pages and so on and also I don't want to give away too much of my IP. Tommy is the guy who dug the test holes. You'll probably clock that there is a bit work that that goes on behind the scenes and that is why I attach. They won't suit your site but I adapt my calcs and design philosophy on a site to site basis.. and tweak the spread sheets.. yes you can get software to do this but I like from time to time to use firstish principles and adapt to the geology of the site rather than just saying.. the computer says no.

 

On 16/08/2022 at 19:00, Barryscotland said:

I think it’s just about stopping a surge? 

Yes you are right.. partly.

 

On 17/08/2022 at 09:16, Barryscotland said:

Its the inconsistency that gets annoying as the chap at the bottom of the hill from us has just finished a build and he put all his surface water straight into the same burn with zero filtration or surge protection just piped straight in although maybe we could of done the same if the SE had shown it that way originally.

On 17/08/2022 at 09:16, Barryscotland said:

I think I need to go back to the drawing board with this one.

Yes give this a go, nothing to lose.

 

For folk to help on BH we need to know a lot more about the profile of your site, where you are.. assume you are in Scotland (use of burn rather than stream) then give us some local clues!

 

I would say to you.. don't focus on the calcs at this stage.. look at your ground, the topography, get a feel for it and rely on your common sense.. will serve you well. Clay soils do not drain well generally.. but there are ways around this if you have a bit of room to play with.

 

On 17/08/2022 at 09:16, Barryscotland said:

Its the inconsistency that gets annoying as the chap at the bottom of the hill from us has just finished a build and he put all his surface water straight into the same burn with zero filtration or surge protection just piped straight in although maybe we could of done the same if the SE had shown it that way originally.

Check this first.. has he worked a flanker or is this ok? Tell us what you know of the burn.. what do know about it? does it run almost dry in summer? here we want to know dry weather flow as does SEPA. Also if you are say located near some parts of the central belt / Perth catchment area for example they have a big problem with flooding as they have build schemes of houses in "flood planes" .. we need to know about this.

 

Once you get your head around this often a good solution jumps out. But you'll need to do the work first. Then you do the calcs to prove to BC etc you were right all along.. oh the satisfaction!

 

Hope this helps.

 

 

 

 

 

Rain water calcs prelim.pdf

[saveasteading]

Here is mine. 

NB this is exceptionally porous. Our soakaways are very small.

We happened to have deep pits dug for other reasons so tested within them. The test holes can be nearer the surface.

 

Soakaway design I am not publishing at present. See how you get on with percolation first. 

Then you have to do rainfall calculations before doing a soakaway proposals, which are not easy.

Edited August 23, 2022 by saveasteading

[Gus Potter]

Wow.. these show a really fast results.. you are lucky.. but did you check if they are too fast?

 

From memory there is an upper limit in the BRE design guide.

 

Will stand corrected though.

 

 

 

 

[Barryscotland]

 Thanks for the info shared, what I've taken from it is I will need to pay somebody to put together something to present to BC as its not something I can put together myself. The ground worker has done two percolation tests and given me a VP of 38 and recommended a soakaway of 4.5m3 if using crates but he is unable or unwilling to put it all together into something like Kelvin has shared above. 

 

So back to the SE to see if he will accept our tests and work from there or wants to come out and do his own and how much he will charge us for the pleasure.

 

Section 3.0 on kelvins report makes it clear to me that we should of been able to pipe straight into the burn without the need of the 25m3 of filtration strip. 

[Temp]

On 17/08/2022 at 09:16, Barryscotland said:

think I need to go back to the drawing board with this one. The top of our site where the house is is clay and this is the only place we did test digs so the SE suggested easiest option as the site is clay just run the surface water down to the burn.

 

Our site is clay and soakaways wernt going to work. So we proposed a rainwater storage tank with overflow to a ditch and it was accepted by the planners. Note this was just a buried storage tank not an attenuation tank. We use it to wash the car and water the garden. It's got a pump in it and we've even run water 400m to our allotment tank.

 

I'm not sure if the planners realised the difference between a storage and an attenuation tank but they approved it.

 

 

 

[saveasteading]

8 hours ago, Gus Potter said:

but did you check if they are too fast?

Yes, the stated limit is 15, so we are just outside that. However, I'm sure SEPA would have accepted a faster flow, as there is 15m of the same sand below that, so the water will be very well spread out before it hits the stone and runs off to the burn. The fast limit is to prevent erosion and sinkholes, and over-fast flow to watercourse, and neither would apply,

[saveasteading]

2 hours ago, Barryscotland said:

I will need to pay somebody to put together something to present to BC as its not something I can put together myself.

Good, that was rather my implication. I can do it but still find it hard. Just check /enquire of the Engineer if there is any natural way to reduce costs rather than crates.

Being pedantic here, drainage is Civil Engineering, not Structural. Your SE may very well be able to do this but might choose to sublet all or some of it...watch the costs, as I have seen 40 page reports where 3 would suffice.

 

The SE does not have to dig the holes or carry the buckets of water, so you should pay for observation time only, if they want to observe rather than accept your figures. Probably sensible to do this again, and it may even allow a reduction in caution, and soakaway size.

 

21 hours ago, Kelvin said:

My surface water drainage is straight into a water course.

That is surprising as it increases flood risk downstream. I thought there were always constraints.

If you haven't done it yet, I urge you to find a way to reduce this flow for the general benefit. It is all these small increases in flow that cause flooding problems.

Unless, of course, your water course ends up in the sea in a few minutes. Or into a reservoir would be good.

 

[Kelvin]

Our plan is a large storage tank as we have the space. But not plumbed into the house. 

[saveasteading]

7 hours ago, Kelvin said:

a large storage tank

Excellent.  What will you do with the water? In the rainy season will it be partially emptied for car washing etc, before the next rain storm?

I