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.