Options for surface water run off - soak pit or alternative?

Steekers · 27-05-2021 2:14pm #1

Hi all,

After a lot of years of my drive flooding from surface water from the road running in, the council have installed a drain to collect the water and have piped it into the field that we own beside us. It is then up to us to deal with that water either by connecting it to a soak pit or some other alternative.

Now, what i am thinking is, if it can be done, is instead of digging a standard soak pit was to install a chamber that would collect the water piped from the drain and disperse it into the ground but which would also give me access to clean it out when needed which a soak pit obviously doesn't therefore giving me a better long term solution. It might be a septic tank that i'm thinking of but maybe not.

If anyone has any ideas i'd really appreciate it.

Thanks,
Stephen.

Calahonda52 · 27-05-2021 3:08pm

If you own the field and have access for machinery, just get a soak pit dug and a load of crushed stone.
Whats your thinking on not doing a soak pit?

PMBC · 27-05-2021 3:17pm

I think most of your problem, as you have identified, will be blockage of the percolation area/soakpit from the fine material - grit, sand and soil - in the road water.
You need to have some idea of what area of road is draining through the 'water-cut' that the Council made. If, say, its a 6m wide road and its draining 50 m. length thats 300 sqm of surface run off you have to deal with.
A rule of thumb that I came across recently is to multiply the rainfall rate in metres per hour by the area and divide by three. In this case, assuming a rainfall rate of 0.050 mm (2 inches) per hour the calculation goes
300 x 0.050 divide by 3 gives a volume of soakpit of 5 cubic metre - below the inlet pipe to the soakpit.
This is all assuming the ground has good soakage/permeability.
The next element is the clogging up effect. My solution way back was to use large single size (no fine materials or lesser sizes) 50mm or 75 mm stone wrapped in a geotextile preferably in a (number of) manhole rings on permeable base. To prevent the fine material, or most of it, from getting ot the percolation area, construct a manhole in advance of the filter with the outlet set 150mm above the inlet so that it acts as a stilling basin - you could use a road gulley pot for this. The manhole will have to be pierced or if in blockwork leave the vertical joints open.
Best of luck and I will be interested to see the other solutions offered
There is also a BRE Digest setting out ways of calculating for soakpits - but Ive never read it.

chooseusername · 27-05-2021 4:11pm

A catchpit before the soakaway that you can clean out regularly
would keep it from blocking up.
Any old tank would do .

Steekers · 27-05-2021 4:25pm

Calahonda52 wrote: »

If you own the field and have access for machinery, just get a soak pit dug and a load of crushed stone.
Whats your thinking on not doing a soak pit?

Exactly as PMBC said, the water is bringing a lot in grit/sand etc and it deposits it on the drive when drying so presume the same will happen to the soak pit meaning it will clog fairly quickly.

Steekers · 27-05-2021 4:25pm

PMBC wrote: »

I think most of your problem, as you have identified, will be blockage of the percolation area/soakpit from the fine material - grit, sand and soil - in the road water.
You need to have some idea of what area of road is draining through the 'water-cut' that the Council made. If, say, its a 6m wide road and its draining 50 m. length thats 300 sqm of surface run off you have to deal with.
A rule of thumb that I came across recently is to multiply the rainfall rate in metres per hour by the area and divide by three. In this case, assuming a rainfall rate of 0.050 mm (2 inches) per hour the calculation goes
300 x 0.050 divide by 3 gives a volume of soakpit of 5 cubic metre - below the inlet pipe to the soakpit.
This is all assuming the ground has good soakage/permeability.
The next element is the clogging up effect. My solution way back was to use large single size (no fine materials or lesser sizes) 50mm or 75 mm stone wrapped in a geotextile preferably in a (number of) manhole rings on permeable base. To prevent the fine material, or most of it, from getting ot the percolation area, construct a manhole in advance of the filter with the outlet set 150mm above the inlet so that it acts as a stilling basin - you could use a road gulley pot for this. The manhole will have to be pierced or if in blockwork leave the vertical joints open.
Best of luck and I will be interested to see the other solutions offered
There is also a BRE Digest setting out ways of calculating for soakpits - but Ive never read it.

Thanks for the detail, i will look into this.

Steekers · 27-05-2021 4:27pm

chooseusername wrote: »

A catchpit before the soakaway that you can clean out regularly
would keep it from blocking up.
Any old tank would do .

Didnt know of a catch pit but i guess that what was in my head. Thanks, will check it out!

chooseusername · 27-05-2021 4:29pm

Steekers wrote: »

Didnt know of a catch pit but i guess that what was in my head. Thanks, will check it out!

Basically what PMBC said, but he put it much better.

Metric Tensor · 27-05-2021 6:13pm

What PMBC describes is often called a "Silt Trap" and does exactly as he describes - it catches the silt and only allows water through to your soakpit. You then have to clean it out every now and then to stop if from silting up. But provided you leave a manhole cover it's a relatively easy job.

BRE 365 is the document PMBC mentions in case you feel the need to look it up!

Markcheese · 27-05-2021 7:56pm

I suppose one of the things you'll want to know is what the water table is like in your field , especially in winter - there's no point digging a massive soak pit , that's going to fill with ground water in winter and leave you back to square one .
I'm assuming there's no open drains or gully that you could pipe to ?
Or if the ground water in the field is low and the drainage is good , could you just put in the silt trap and run some percolation pipes in several directions ?
Failing that , stick in a shallow sided pond ,plant it with reeds and make it a wildlife haven...

stampydmonkey · 27-05-2021 9:31pm

A swale is your friend if the water table is low.

27-05-2021 10:47pm

stampydmonkey wrote: »

A swale is your friend if the water table is low.

+1 swale (vegetated bioswale)
Low maintenance

Steekers · 28-05-2021 9:09am

Markcheese wrote: »

I suppose one of the things you'll want to know is what the water table is like in your field , especially in winter - there's no point digging a massive soak pit , that's going to fill with ground water in winter and leave you back to square one .
I'm assuming there's no open drains or gully that you could pipe to ?
Or if the ground water in the field is low and the drainage is good , could you just put in the silt trap and run some percolation pipes in several directions ?
Failing that , stick in a shallow sided pond ,plant it with reeds and make it a wildlife haven...

Open drain was an option a few years ago when the resurfaced the road but decided against piping it to a stream 200m away!
It is a bit of a low point to does tend to catch a lot of the drainage from the field at the point where the soakpit would be located.
Building a pond not an options either as it would block access to the field.

Options for surface water run off - soak pit or alternative?

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