Sunken Garden and Rain Water?

[Paulgwat]

Hi all,

I am hoping someone can sanity check my thoughts here, we are planning a 4M x 4M sunken garden area in our self-build. Due to the plot layout and height differences versus other properties we would overlook other gardens if the seating area was at garden level.

 

Digging down a metre means we are already in heavy clay so a soakaway isn’t an option.

 

I am trying to get my head around the sizing of a sump and pump for the sunken area as all of the off the shelf solutions I have found seem to be aimed at large basements where any amount of water needs to be contained and removed urgently, lifted many metres vertically and cost ridiculous sums of money given the context.

 

From a bit of googling it seems that 1mm of rain falling on a square meter is 1 litre  - for Bristol, where we are, typical rainfall amounts are 0.5mm to 4.00mm per hour with storms reaching 45-50mm /hr

 

Taking the worst case 50mm per hour and our roughly 16sqm sunken area is 800l/hr of rain fall.

 

Given that I am only going to be moving this water about 3M horizontally and less than 1M up (there is a handy storm water drain near by that I can connect into) I am planning on using a 24V submersible pump capable of something in excess of 3,000l /hr pumping @1M vertical as these are commonly available for boats and seems to me a good margin versus expected use and the cost differential to something smaller is negligible.

 

I am using 24V DC as the house has Loxone home automation and we have plenty of 24V already available locally to where the pump will be located

 

So my question is how big should I make the reservoir the pump sits in?

 

Rather than buy some huge tank and putting in perimeter channels I am thinking of using this https://vodaland.co.uk/collections/300x300-catch-basins-and-gullies - nominally it is about 25L capacity but with a couple of spacer rings that can be increased to about 45l quite easily

 

At 4mm/hr it would take about 5mins before there was sufficient water in the catch basin to turn on the pump which would then run for about 10 seconds then wait another 5mins for the catch basin to refill and so on

 

Even at 50mm/hr rainfall the pump would run for about 20 seconds per minute which is well within its specification.

 

I am not too worried if a sudden huge amount of water was to overwhelm the sump as I won't be out there getting my feet wet when it does!!!

 

Am I massively over thinking this or would you do something different?

 

Thanks 

 

Paul

[Gus Potter]

23 minutes ago, Paulgwat said:

Digging down a metre means we are already in heavy clay so a soakaway isn’t an option.

What depth are your house foundations sitting at? You don't want to compromise them if you essentially undermine them?

[Nickfromwales]

28 minutes ago, Paulgwat said:

Hi all,

I am hoping someone can sanity check my thoughts here, we are planning a 4M x 4M sunken garden area in our self-build. Due to the plot layout and height differences versus other properties we would overlook other gardens if the seating area was at garden level.

 

Digging down a metre means we are already in heavy clay so a soakaway isn’t an option.

 

I am trying to get my head around the sizing of a sump and pump for the sunken area as all of the off the shelf solutions I have found seem to be aimed at large basements where any amount of water needs to be contained and removed urgently, lifted many metres vertically and cost ridiculous sums of money given the context.

 

From a bit of googling it seems that 1mm of rain falling on a square meter is 1 litre  - for Bristol, where we are, typical rainfall amounts are 0.5mm to 4.00mm per hour with storms reaching 45-50mm /hr

 

Taking the worst case 50mm per hour and our roughly 16sqm sunken area is 800l/hr of rain fall.

 

Given that I am only going to be moving this water about 3M horizontally and less than 1M up (there is a handy storm water drain near by that I can connect into) I am planning on using a 24V submersible pump capable of something in excess of 3,000l /hr pumping @1M vertical as these are commonly available for boats and seems to me a good margin versus expected use and the cost differential to something smaller is negligible.

 

I am using 24V DC as the house has Loxone home automation and we have plenty of 24V already available locally to where the pump will be located

 

So my question is how big should I make the reservoir the pump sits in?

 

Rather than buy some huge tank and putting in perimeter channels I am thinking of using this https://vodaland.co.uk/collections/300x300-catch-basins-and-gullies - nominally it is about 25L capacity but with a couple of spacer rings that can be increased to about 45l quite easily

 

At 4mm/hr it would take about 5mins before there was sufficient water in the catch basin to turn on the pump which would then run for about 10 seconds then wait another 5mins for the catch basin to refill and so on

 

Even at 50mm/hr rainfall the pump would run for about 20 seconds per minute which is well within its specification.

 

I am not too worried if a sudden huge amount of water was to overwhelm the sump as I won't be out there getting my feet wet when it does!!!

 

Am I massively over thinking this or would you do something different?

 

Thanks 

 

Paul

Without a full topological survey of the entire area, it's impossible to advise. Some idea on possible permeability further down may offer a solution, but there is just not enough info here to even have a stab at it, sorry.

[saveasteading]

7 hours ago, Nickfromwales said:

just not enough info here to even have a stab at it, sorry.

Correct.

I don't really want to help you with this, sorry, other than to say I disagree with doing it at all.

It is complex anyway so you can't get great advice without a detailed design assessment.

Your assessment is under-rated ad you would have much more water than that.

 

Ethically it is wrong because we are all supposed to be reducing water to storm drains, not collecting it then pumping it in. it causes flooding or sewage overflows dowmstream, and diminishes the groundwater and aquifer.

You wouldn't only be dealing with your "own" water but collecting it from neighbours with potentially serious consequences.

Have you considered a soakaway or pond?

Rainwater harvesting?

 

If you do end up with a scheme, you must put it past building regulations and get the drain owner's approval.

 

I strongly suggest finding another means of privacy(fence, hedge).

 

Sometimes we have to compromise.

 

[BotusBuild]

9 hours ago, Paulgwat said:

Due to the plot layout and height differences versus other properties we would overlook other gardens if the seating area was at garden level.

It is to be commended that you are trying to be considerate to your neighbours, but, this is the way the gardens were designed in the first place, and those around you must already know this is a possibility.

Unless you have received some legal notification NOT to sit in your garden and NOT to overlook anyone, then avoid all this unnecessary cost and get on with enjoying YOUR outside space. 🙂 

[Paulgwat]

Interesting responses to what I thought was a technical question and definitely not one where I expected my ethics to be questioned!!!

 

I was asking a question on my calculations of water removal only but will fill in the other somewhat irrelevant facts for people

 

This area of garden is located around 10M from the house so unlikely to be causing any issues with the foundations - I would expect more of a concern for my foundations would be the neighbouring house which is approximately a metre away.

 

The main house roof does indeed go into a multi thousand litre rainwater harvesting tank that is also buried in the back garden which will be used for toilet flushing, washing machine and garden irrigation.

 

I have been advised by the manufacturer of the rain water harvesting system that collecting water from the sedum roof on the garage and any flat area of patio should NOT be drained into the rain water harvesting tank due to the high probability of the water being contaminated with debris etc.

 

The water from this area of garden would go into the storm drain (which is also where the overflow of the rain water harvesting tank goes) which will connect to the sites storm water drainage system that feeds into an attenuation pond at the bottom of the hill serving the other 200+ houses on the site and covers something like an acre.

 

There will be a low wall around the sunken area which will prevent run off from our garden (nearest up hill garden boundary 10M away) and in the 3 years I have owned the plot we have never had any issues with run off or standing water flooding on the plot or neighbouring ones.

 

I would be interested to understand where the additional volumes of water would come from? 

 

All of the rest of the ground around the plot will be laid to grass or made of permeable materials.

 

My Building regulations company are not interested as they view it the same as if the water was collected on any flat surface, and the response from planning was that it falls under general landscapping and even if it didn't it would be classed as permitted development. 

 

The storm drain is not adopted due to the private SUDS pond and they view it the same as if it was a patio or driveway with gulley drains.

 

The one thing the council have stipulated is that all boundary fences, planting etc be a maximum of 1.8M tall ( and they are already in place for houses which pre-date ours - the make up of the site means the fence is 1.8M on the other side but far less our side and also they own the fences) which is where all of this started as the other locations to put a table and seating area would mean zero privacy for us and our neighbours.

 

I will leave it at that as I believe I am doing more to mitigate the impact of my house on the site than most would

 

Regrads

 

Paul

 

[saveasteading]

OK that's helpful.

I would disagree re water from the roof. It should even be cleaner than straight off a roof or gutter. I think they are being cautious.

 

But the issues remain that you would be creating a very big sump, collecting water from the surrounding ground, including neighbours'.

 

You need structural design approval being close to the neighbours.

 

The pump part of the question  is the easy bit.

 

[-rick-]

1 hour ago, Paulgwat said:

The one thing the council have stipulated is that all boundary fences, planting etc be a maximum of 1.8M tall ( and they are already in place for houses which pre-date ours - the make up of the site means the fence is 1.8M on the other side but far less our side and also they own the fences) which is where all of this started as the other locations to put a table and seating area would mean zero privacy for us and our neighbours.

 

How wide is the plot? Just because your boundary fence height is limited doesn't mean you couldn't put something taller a bit away from your boundary.

 

boundary fence, low planting, path, high planting/fence, sitting area, high planting, path, low planting, fence. Will fit in a 4m wide seating are if plot is 8m wide.

 

Landscape designer will likely have lots of clever ideas for this that will be cheaper, easier to maintain and likely look better than a sunken area.

[saveasteading]

I'm thinking out how to conceal an ugly garage and get privacy from a neighbour's terrace. The plans are:

1. A raised bed of maybe 600mm with dwarf fruit trees, attractive in summer and not an issue in winter. Maybe some grasses.

2.A pergola and trellis with honeysuckle and clematis which are green all year.

These will be about 3m high when mature. Stunning flowers for 8 weeks.

[Paulgwat]

5 hours ago, saveasteading said:

 "I would disagree re water from the roof. It should even be cleaner than straight off a roof or gutter." - I am not sure what you mean here, if you are referencing the Sedum roof, even the supplier of the sedum roof says that in heavy rain fall soil will wash out of the tray system - The rainwater harvesting system we have has a control box with valves in it that will apparently easily get blocked by dirt so have added very fine filtration to the pipe route prior to the control box but I still don't see the point of adding contamination just for the fun of it

 

" But the issues remain that you would be creating a very big sump, collecting water from the surrounding ground, including neighbours'. " I am not following this at all, I am building retaining walls that will keep ground water out of the sunken area, sort of like a reverse swimming pool so I cannot understand how I am collecting water from surrounding ground? The bottom of the "sunken" bit is on clay and rock but the soil surrounding the walls is free draining so there won't be massive hydrostatic pressure on the perimeter walls which will either be foundation blocks or double skin concrete blocks with concrete infill and then externally tanked.

 

"You need structural design approval being close to the neighbours." - I have never said the sunken garden area is close to a neighbour - what I said was that a neighbouring property was approximately 1M from my house foundations - both structures are still 10M+ away from sunken garden and we don't require any structural design approval

 

" The pump part of the question  is the easy bit." Interesting that the pump was my question and nobody has seen fit to answer that bit and instead tell me everything I am apparently doing wrong.

 

To clarify the intent here is to build a roughly 4M x 4M patio and put an L shaped corner sofa on it with a firepit / table and possibly a barbecue if there is room  - the only difference to normal is that it is going to be between 750mm and 1M below the surface of the rest of the garden and some of that height change is a result of trying to get the garden something approaching gently sloping without having to resort to terracing.

 

To give some additional context, due to the nature of the plot ( the plot slopes left to right at the same time as sloping from front to back) our patio doors from our kitchen are 1.5M above outside ground level at one side of the house and only about 400mm above it on the other corner even after we have done our best to level the garden.

5 hours ago, saveasteading said:

 

 

[Paulgwat]

5 hours ago, -rick- said:

 

How wide is the plot? Just because your boundary fence height is limited doesn't mean you couldn't put something taller a bit away from your boundary.

 

boundary fence, low planting, path, high planting/fence, sitting area, high planting, path, low planting, fence. Will fit in a 4m wide seating are if plot is 8m wide.

 

Landscape designer will likely have lots of clever ideas for this that will be cheaper, easier to maintain and likely look better than a sunken area.

We are trying to "screen" the seating area from a property to our left that is uphill,  a property that is behind us that is significantly down hill where the fence on our side is only about 1.2M high but on their side something closer to 1.8M and a property to the right where the wall is about 1.6 -1.7M our side but probably nearer 2M on the other side.

 

The development is essentially 4 fields the slope into a small valley with the field boundaries left intact but the original developer did nothing to try and make the 4 build areas flatter.

 

I believe anything over the height would need to be a minimum of 1M in from the boundary and as we are trying to get privacy on all 3 sides of the garden we would have to sacrifice a good 20sqm of useable garden to achieve what you describe  - we are making use of hard landscapping and using high and low planting etc to disguise the boundaries as much as we can but we still have the privacy issue if the seating area is at ground level.

 

We have been trying to come up with garden plans for the last 3 years and unfortunately we keep coming back to the same conclusion that down is our best option

 

As for costs, the hole is already there so the costs to build are limited to a couple of pallets of blocks and some concrete for the base but this is being done at the same time as the garage floor so the concrete is almost free due to part load costs etc.