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Rainwater, soakaway and all manner of other potential complications


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Good evening.

Bear with me on this one, I might just be over complicating the situation...

My current thought process which I'm trying to figure out surrounds dranage and rainwater etc. I might be thinking too in depth given that this isn't going to be happening until next year at the earliest but like that song that you can't get out of your head, I've got this currently stuck in mine!

 

I have planning for an extension on the house, but I also want to knock down and re build the garage. The planning for the extension requires a soakaway to be fitted (relatively straight forward...) and the garage will require the same, so two birds with one stone. The boss also wants a big veg garden so I am looking at if I can justity rainwater harvesting.

Question 1: Would fitting a rainwater harvisting tank allow for a smaller soakaway? If so, how do I figure out the ammended soakaway size?

 

Next, the drive will eventually be either gravel or possibly some form of permiable surface (I'd rather have something permiable to avoid complicating the dranage further than I am currently thinking, but I would like something more solid and stable than gravel. But that's going to be a discussion for another day as this will be the bitter end of the project and as we're yet to start, who knows when the end will be! I digress...)

I am looking to run a 110mm drain in a trench down the length of the drive and into the garden, then onto the soakaway (and perhaps rainwater tank) to collect the water from the front & side roofs.

Question 2: Does a drive count as a 'light road' and so require 900mm covering or is it more 'Garden' so can it be 600mm covering? I suspect the former just to make my digging more difficult.

 

Finally, the house is 1600s ish so likely to be no or very little foundations, it also has a little damp in the wall along the drive. The dirve is slightly higher than the inside floor level in places, and I hope to reduce this to floor level where I can in the long run, but this might not be practical. So I'm considering putting a french drain in the drive, around 1m way from the house wall so as to not be too close to the probable lack of foundations, but hopefully help with some of the damp.

Question 3a: What are peopls thoughts on a french drain in a dive way? What depth should I be thinking about digging this to? 

Question 3b: I know I can't put surface water or water from an open Aco type drain into a rainwater harvesting tank, but can a french drain be directed into one if the trench is lined with geotextile etc?

I'm leaning towards 'No' on this as it might pick up some silt. On the other hand I'm thinking that with a lined trench this should be minimal and adding a silt trap of some form at the end of the french drain should catch the last of the silt so it should be ok...

 

That is enough of my waffel for now I suspect, hopefully I've made some sense there.

Thanks!

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1. Ask bro but I doubt it. It's only a buffer and when full achieves nothing.

2.I see some drains as close as 2 or 300 to surface on some jobs. I've always concreted (encased) them when anything less than 600 ish on a driveway

3. sounds good, doubt you can re use water though

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15 minutes ago, Oz07 said:

1. Ask bro but I doubt it. It's only a buffer and when full achieves nothing.

2.I see some drains as close as 2 or 300 to surface on some jobs. I've always concreted (encased) them when anything less than 600 ish on a driveway

3. sounds good, doubt you can re use water though

Thanks.

 I can imagine there are a few 'interpretations' for the various depths in the building regs...

If I can get away with 600 deep then it means a little less digging and back filling to do!

The plans have a patio with a gully and aco drains which will have to bypass the rainwater harvesting if I do fit it, so the safest bet is probably to just take the French drain to that (again, if I do out one in)

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N.B. You shouldn't use any ground level drainage for harvesting as it may get contaminated with whatever it picks up off the ground - run that straight to a soak away.

 

Roof drainage can go to RWH tank but once full, excess will go to soak away.

 

RWH tanks are usually buried as being dark and cool inhibits nasties to grow, however this makes them expensive. You can DIY with IBCs (as I did) but mine are below ground level in a big concrete box covered with a deck that we needed to build for the basement anyway. You cannot bury IBCs without substantial protection as they will get crushed.

 

 

 

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27 minutes ago, Bitpipe said:

N.B. You shouldn't use any ground level drainage for harvesting as it may get contaminated with whatever it picks up off the ground - run that straight to a soak away.

 

Roof drainage can go to RWH tank but once full, excess will go to soak away.

 

RWH tanks are usually buried as being dark and cool inhibits nasties to grow, however this makes them expensive. You can DIY with IBCs (as I did) but mine are below ground level in a big concrete box covered with a deck that we needed to build for the basement anyway. You cannot bury IBCs without substantial protection as they will get crushed.

 

 

 

 

That was the general feeling I had RE the French drain into rain water harvesting. Thought I'd get some second opinions though as runing it to the soakaway will be a little more complicated/expensive. Makes perfect sense though!

And I need a second of drainage going directly to the soakaway anyway so that will just tee into it.

 

I think I've read your posts about the IBCs (I've been going quite a bit of reading up on RWH the past few days)

my original plan was IBCs behind the garage and gravity feeding a tap for the garden, but a buried tank will be much better for many reasons. Unfortunately I think if I can make the costs work out sensibly then the extra cost of a tank will outweigh the hassle of burying IBCs.

Then with a proper tank I can pump it for WCs in the house as well as the garden/garage tap.
Lots to think about and cost up!

 

 

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14 hours ago, WestcountryWonderer said:

Would fitting a rainwater harvisting tank allow for a smaller soakaway? If so, how do I figure out the ammended soakaway size?

I was after the same, though it appeared that high water table prevents from building any soakaway and so war with Thames Water to connect to existing rain sewer is on.

I'd recommend you dig a control hole, that answers many of your questions. Using it for percolation test will give rough idea of volume ( and so cost) of soakaway to start with. Or - like in my case - possibility of having one at all.

To answer the question it is yes and no. Yes, as there are companies doing just that, but not sure what methodology they use. I suspect it is based on HR Wallingford tools, so something not publicly available and requiring some skills to do the calculations. I'd believe there are consultancies using the same method without selling any product, but still the effort may be more costly than building 'standard' soakaway capable of capturing all the rainwater, and adding water harvesting system as a bonus

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12 hours ago, WestcountryWonderer said:

 

That was the general feeling I had RE the French drain into rain water harvesting. Thought I'd get some second opinions though as runing it to the soakaway will be a little more complicated/expensive. Makes perfect sense though!

And I need a second of drainage going directly to the soakaway anyway so that will just tee into it.

 

I think I've read your posts about the IBCs (I've been going quite a bit of reading up on RWH the past few days)

my original plan was IBCs behind the garage and gravity feeding a tap for the garden, but a buried tank will be much better for many reasons. Unfortunately I think if I can make the costs work out sensibly then the extra cost of a tank will outweigh the hassle of burying IBCs.

Then with a proper tank I can pump it for WCs in the house as well as the garden/garage tap.
Lots to think about and cost up!

 

 

 

There is a bit of a leap from a simple RWH system for gardening to servicing internal systems (toilets etc). The latter needs mains fed buffer tanks to allow for drought, pumps and must be WRAS compliant - also needs independent plumbing to the WC. Some people have raised potential health concerns about virus in bird droppings that may get washed from roof into the system and be aerosolised when the loo is flushed.

 

Realistically you can't bury IBCs so forget that, I stored mine underground which is a better way to think of it. However once you move below ground you need a decent pump to make it useful.

 

In my experience, RWH was an easy way to meet the water usage calcs.

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Our relatives in Australia get ALL their water from rainwater and use it for everything including drinking.

 

The big difference is they have a "wet season" and get a lot of rain in a short time.  when it rains they let it run off for a bit to clean the bird muck etc off the roofs, before diverting it to the tanks.

 

It is never going to work so well here when so much of our rain is short showers or drizzle so no chance to clean the roof before starting collection.

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16 hours ago, Oz07 said:

1. Ask bro but I doubt it. It's only a buffer and when full achieves nothing.

2.I see some drains as close as 2 or 300 to surface on some jobs. I've always concreted (encased) them when anything less than 600 ish on a driveway

3. sounds good, doubt you can re use water though

 

1. Absolutely not. It could already be full when the next rainstorm comes.

2. as Oz07

3b, yes if for garden only, and even with membrane around it you will get silting so need access to clear it. But as above (BitPipe) , what if weedkiller or oil or detergent from the drive gets into the tank and hence to the garden....so best not.  Use soakaway.

 

Proper garden barrels are not too expensive, esp if you keep your eyes out for seasonal bargains. eg the identical barrels are half the price at Wickes as from a garden centre I saw selling them. You sometimes find food product barrels available, in blue or green, and make your  own lid and connections.

 

If you have space and distance from house and boundaries, you can use a french drain to the soakaway and add that to the soakage area, so the pit size is reduced. It is better fro the ground too as it disperses the water.

 

aco to the perimeter of the house....that could help dewater the ground as you suggest. It will only catch what runs there though, so needs a runoff from the house wall. Ensure that it does not do the opposite  by strict control of levels. at connections.

 

 

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1 hour ago, ProDave said:

so much of our rain is short showers or drizzle

Got me thinking.

 

I was surprised recently to rediscover the differences in rainfall around the UK.

I am used to SE England projects. In calculating for the Highlands it is staggering difference between west and east.

Also, I had not realised that the intensity varies dramatically and that is what affects capacities and causes flooding.

 

I realised I was overdesigning the drainage system for the East Highlands and wondered why...so looked up some key figures.

 

Rainfall in Inverness is low, totalling 723 mm in a typical year. Precipitation is distributed evenly, falling over about 143 days per year. 

Rainfall in Fort William is high, totalling 1504 mm in a typical year. Precipitation tends to fall more in the winter, falling over about 181 days per year.

London averages approximately 106 rainy days each year and receives a total of 583.6 mm of precipitation annually.

Royal Tunbridge Wells. The rainfall here is around 737 mm | over about 99 days

Newquay Cornwall The average amount of annual precipitation is: 1021.0 mm in 260 rainfall days.

 

 

Apologies for not reformatting these in the same order.

This tends to explain why  Tunbridge Wells suffers from flooding recently, ditto London. Intense rain. 

 

The moral: do not be stingy with gutter, drain or soakaway sizes, especially in the areas with high intensity.

 

 
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5 hours ago, saveasteading said:

 

 

aco to the perimeter of the house....that could help dewater the ground as you suggest. It will only catch what runs there though, so needs a runoff from the house wall. Ensure that it does not do the opposite  by strict control of levels. at connections.

 

 

 

This is one other through I have been considering.

I'd like to drop the drive level down to first floor level or slightly below to awoid any dampness in the ground tracking through to cause damp in the wall. In doing this though, I might end up with a drive that actually slopes towards the house wall which would be more detrimental than leaving the ground level as it is... If that turns out to be the case ( I need to get out and do some actual measurements) then I either leave the ground level slightly higher outside and do the best I can with it. Or I purposfully make the drve slope towards the house to get the ground level lower and put an aco drain along the house (set away slightly) to catch any runoff.

 

This does though run the risk that in heavy rain, the channel would be overwealmed and I could cause more damage than good. Also it means even more digging and planning drains....

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The current updated plan then is looking like 110mm drain dug into  the drive 600 ish deep to take the gutter outlets into a rain water tank then the outlet of that into the soakaway via a silt trap.

 

French drain also along the drive, tee'd into the chamber for the patio drains and then directly into the silt trap and then soakaway.

If I put any channel drain along the house/drive edge this would also drop into this run and so bypass the tank.

 

Use the rain water tank only for garden use (and probably add a pump at a later date to reduce the initial cost).

 

I'm figuring that the french drain would also want to be down around 600mm as it would be in the drive and any shallower than that would probably need some form of concrete capping over the top.

Does that sound about right?

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Can I just suggest that you don't push your luck. Water is not sympathetic.

Also I work it all out myself as have not found many in the industry, certainly not groundworkers and plumbers, who really understand how water works. 

Easy, it runs down-hill. 

Not so easy, is that it builds up, goes under and through, and stops and reverses when there is an obstruction.

 

What I do, even if people laugh, is once I  have worked it all out, I go back to the start of the run and imagine a rainstorm.

And I use a golf ball (snooker better) to check where the water will actually go, not the theoretical flow.

 

best of luck...report back when successful.

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48 minutes ago, saveasteading said:

Can I just suggest that you don't push your luck. Water is not sympathetic.

Also I work it all out myself as have not found many in the industry, certainly not groundworkers and plumbers, who really understand how water works. 

Easy, it runs down-hill. 

Not so easy, is that it builds up, goes under and through, and stops and reverses when there is an obstruction.

 

What I do, even if people laugh, is once I  have worked it all out, I go back to the start of the run and imagine a rainstorm.

And I use a golf ball (snooker better) to check where the water will actually go, not the theoretical flow.

 

best of luck...report back when successful.

 

That all makes sense! Nothing like mother nature to throw a spanner in the works once you've meticulously planned what is suppsed to happen.

 

Is there anything particular so far that might be pushing my luck? I'm really at the beinging of a long journey of learning here. Happily as an engineer alot seems quite logical, but that also often leads to over engineering job and not being able to see the wood for the trees.

 

I will indeed report back as things progress. It will be a long process though! And there'll be a lot of other questions on many topics along the way!

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The other thing that has just sprung to mind is that I assume that drainage is something that building control will be quite interested in?

How much are they likely to want to see?

I know that each inspector will be slightly different, but are they likely to want to see all the trenches open with pipework before they are filled?

How about soakaways, are they likely to want to see that all in place before being covered in?

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  • 2 weeks later...

The next question that has sprung to mind on this topic is that I have a rain water outlet that is noted as 'assumed' to be connected to a combined sewer via a manhole (there is a soil stack running next to it.)

I suspect that the reality is that the rain water pipe just goes into a hole in the ground rather than anywhere meaningfull.

 

Am I right to assume that I wouldn't get away with reinstating this and actually running it into the combined sewer if it isn't already?

What about it it turns out that it does currently go into the sewer, can I reinstate it to the sewer then?

 

And I guess that I am also right to assume that if I move the rain water outlet from the front roof which currently discharges into the road (and therefore eventually the sewer) to the other end of the house, then this will also not be allowed to discharge into the combined sewer? And must go to the new soakaway?

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