An introduction to garden drainage… from an expert

Here at ALDA Landscapes, garden drainage is dear to our hearts, particularly after a succession of years with spells of prolonged wet weather.  How to deal with surface water is a critical consideration in so much of what we do. Here’s our whistle stop guide (starting with the simple stuff, before moving on to some more complex challenges):

1. Ask, Ask, Ask!

When having any landscaping undertaken, you should ALWAYS ask your chosen contractors what they are intending to do regarding drainage.  No question about it.  Don’t be afraid to ask!

2. Paths & Patios

There is one main aim here – to prevent water sitting (or running) where it isn’t wanted.  This includes:

  • preventing puddling
  • stopping water pooling next to the retaining walls of a sunken patio
  • and probably most importantly, preventing a build-up of surface water against the wall of the house.

Where possible, paths and patios should always be built with a fall (a very gentle gradient) to direct water to a lawn or border, where it can be gently absorbed into the soil.

Where the land slopes towards the house (or where there is no lawn or border to drain to), additional drainage gulleys and channels (e.g. Aco drains) may be required to capture & disperse the water – either by feeding it into a strategically located soakaway, or – and where permissible – the household storm (rainwater) drain system.   Rainwater should never be mixed with the foul drainage system.

3. Retaining Walls

Retaining walls are another area where drainage is vital – to prevent the build-up of water behind the retaining wall (& potentially, subsequent collapse of said wall!) The drainage here normally takes the form of weep holes or vents in the wall and / or a perforated pipe set in shingle, angled to take the water into a soakaway or other outlet.

4. Front Drives

Front drives bring a whole new set of issues of their own… house owners now need planning permission before building a new or replacement drive that:

  • is over 5 square metres
  • uses an impermeable material (e.g. standard block paving)
  • and has no facility for the drive to drain to a porous / permeable area.

Assuming you don’t want to hassle of having to apply for planning permission (who does?… besides, permission may well not be granted anyway) the drive either has to:

  • drain to a border / lawn / gravel area, where the water can be absorbed naturally into the soil (drainage might be simply by means of a fall, or by way of drainage gulleys / pipework / soakaway)
  • or be constructed using permeable materials (gravel, permeable paving, or porous asphalt for example) so that the water can drain through it.

Full government guidance on what you can and can’t do with a front garden can be found here at the time of writing (or try Googling ‘paving front gardens – government guidance’).

 


That’s the simple stuff covered. But when most people talk about garden drainage, they aren’t talking about patios and retaining walls. Rather, they are referring to the problems caused by having a frequently soggy or waterlogged garden (or part thereof!) – often due simply to the local topology surrounding the garden.

Last month’s blog on how to cope with wet gardens discussed ways of easing these problems by de-compacting the soil, using plants that are happy in wet soils etc. But often, if an area of the garden is permanently wet, the only real solution may be to install some form of land drainage system.


5. Land Drainage Systems

Whilst land drainage systems come under various different guises and names (French drains, Fin drains, dispersal systems – e.g. the leach fields used with sceptic tanks, etc), they all work on the same basic principle:

Rainwater (or rather groundwater) always follows the path of least resistance, and it will flow down even the gentlest of slopes.

A typical land drainage system comprises a herringbone layout of perforated plastic pipe, laid in trenches on a bed of gravel, and encased in coarse gravel. The pipe network is laid so that it slopes down, such that the water is transported away from the problem area, normally (but not always) into a soakaway, from which it can gradually seep away into the surrounding soil.

Before the trenches are backfilled, a geotextile membrane is normally used to prevent soil particles getting into the trench, and also help to prevent root encroachment from surrounding trees.

All this sounds tremendously wonderful in theory (well, it does if you have a waterlogged garden). But in practice, the installation of land drains can be somewhat problematical.

Problem 1:

If a sufficient enough fall cannot be “manufactured” for the water to drain away by gravity, it might be necessary to install a pump. So far so good – a bit of a pain, but there’s a solution at least.

Problem 2:

And the single biggest issue associated with land drainage – where to drain the water to.

It is not always possible to find a suitable location for a soakaway – the garden may be at the bottom of a natural slope and naturally collect all the rain from the surrounding land. A soakaway here would be almost permanently full during wet weather, with the water simply backing back up the perforated pipe. (Additionally, heavy clay soils are not always a good match for soakaways, as the water struggles to drain through).

If there is no place for a soakaway, it might be possible to drain the water elsewhere – neighbouring waste ground, or a drainage stream (if permissible). But if you are surrounded by other houses / gardens, your neighbours won’t be very amused if you simply drain your garden into theirs!

Lateral thinking required!

As a company, ALDA Landscapes has a lot of garden drainage experience. A simple land drainage system plus soakaway is achievable in most gardens. But in some cases it really is difficult, if not impossible, to achieve an economically viable solution.

And that’s when some lateral thinking is required – and often this way, a sensible and pragmatic solution can be devised. We are currently considering building up a large section of a garden, so that a soakaway can be located beneath the built up area (with a raised patio above). Part of the thinking here is that the additional soil brought in will help to absorb the excess ground water. Watch this space; we’ll let you know how this project develops!

In Conclusion

This is of course only an introductory guide, but hopefully one which will give you a good overview of the subject – and give you the sense that drainage problems can be overcome!  Let us know if you have questions.

In the meanwhile, another very useful and extensive resource can be found on the PavingExpert website: http://www.pavingexpert.com/drainage.htm.

 

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