Surface water drainage help - How to improve drainage in garden

[DeanT]

I have a problem with my back garden which suffers badly from waterlogging. Can anyone offer some advice please.

The surface water is unable to drain away. The property is about 7 years old and there is very little decent top soil in the garden, it is just solid clay which absorbs the water. I recently dug a hole about 2 feet deep and it is just clay all the way, this hole is full of rain water which takes several weeks to soak away. If you walk onto my lawn your feet just sink in the lawn.

The garden is about 14 metres deep and 17 metres wide and there is a very slight slope from the back left of the garden to the front right. Most water collects at the back of the garden where our plants are dying, and in the centre of the lawn.

I have got 2 people out so far to look.

One suggested french drains with the water from these drains feeding into the storm drain at the side of our house (the storm drain is situated near the front right of the garden). They said it would take about 15 tonne of gravel to fill the trenches and would cost £1200. Does this sound right? My only concern here is that silt/clay from the drains could clog up the storm drain. How best to avoid this?

Another suggested digging a soakaway with a pipe near the top of the soakway which takes excess water to the storm drain. I've heard that soakaways are a bad thing in clay soil so is this idea best to avoid?

Both the people I have got out so far are landscape gardeners. I guess what I really need is advice from a drainage specialist. How best do I find one? I had a look in the yellow pages under groundwork contractors which lists various people - is that the best place to look?

Any advice/help appreciated,
Thanks.

[Tony McC]

The second option is the better, but what you need is an attenuation system rather than a simple soakaway with an overflow. These allow you to create relatively huge underground storage voids that 'stockpile' excess surface/ground water and allow it to seep into the ground over a much longer time span. They're very simple to construct but you must have suitable site, where the water table is sufficiently low and the ground sufficiently permeable to allow the system to function properly.

If there is a concern that the system could overflow, then a surcharge pipe could be used to divert excess water to the existing drainage system, but it really does need someone to work out the flow regimes and potential for soaking away before spending more money. You can usually find such specialists listed as drainage contractors or drainage consultants in the YP, but if you have no luck, try asking at your local Builders' Merchant, as they often know who does what locally.

You could also try contacting some of the manufacturers as they usually maintain a list of suitably capable contractors in each area, but you'll need to bear in mind that most contractors are geared up to work on commercial installations, rather than "back gardens", even large-ish back gardens such as yours, so be prepared to be 'knocked back' a few times before you find someone willing to take on the project.

[elu150]

Do you have any follow up on your issue. I'm currently digging out my back garden and plan on installing plastic pipe along side the retaining wall from next door. I live side on to quite a steep hill and the area nearest to the highest neighbour is very wet. I now understand why the previous owner had concreted the whole area!!! I have exactly the same problems as you due to the clay only being a foot or 2 under the top soil. Easter weekend coming up, lets prepare to dig!!!

[Tony McC]

If you install a collector/interceptor drain, do you have anywhere to discharge?

[elu150]

I have a gully at the back of the house. I plan on discharging into here. I realise that I don't have much of a decline for the water to flow, but anything is better than nothing.

[Tony McC]

For short runs (less than around 10 metres) even if a land drain is aid flat, it will function due to what is known as the hydrostatic gradient. Water entering the pipe at the wet end will "push" the already collected water towards the outfall point.

So, you plan should work. Have you read the Land Drainage section of the website?

[elu150]

Thanks for the advice. I have read the Drainage section - on more than one occasion!!! Finished work for the day, have moved an incredible amount of clay and got most of my trenches dug. I now need gravel, but haven’t ordered any yet as I hadn’t planned on getting as far as I have done today. Tried to order some but have been told it will take up to 2 weeks for delivery (B&Q). Do you know of any contacts that might get it to the Bolton area quicker??? Am I right in asking for 10-20mm clean gravel?? I’m going to try some builders merchants first thing tomorrow.

[Tony McC]

If I'm not too late, try Cooper Clarke in Farnworth - 01204 862222