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Lawn Drainage

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Reproduced by kind permission of www.gardenzine.co.uk

drainage in the garden can have many causes. If you have a high
seasonal water table you will find that the water sits on or close
to the surface during periods of high rainfall.

If your soil
structure is heavy, this too can reduce soil permeability and
cause poor drainage. Low points in the garden, can allow water to
collect and drain poorly.

Walking on your poorly drained lawn when
it’s wet can further add to the problem because the wet soil
compacts and loses its structure.

surface water on poor draining soil

If the problem exists in only a
small low point, it might be worthwhile considering turning a problem into a
design feature by digging out a border and filling it with plants that love
their feet to be wet. But if you want a lawn you can actually walk on after
the rain without it turning into a mud bath, then you should install an
underground drainage system of land drains.

Land drains are trenches
consisting of single walled corrugated PVC-U pipe supplied perforated for the
collection and removal of surface and sub surface water and gravel;  they
work by encouraging the water to
enter the trench because the gravel provides less resistance than the
surrounding soil. (Water will always travel to the area of least resistance if
encouraged to do so). The trenches then carry the water away from the problem

First you need to take a
good look at your site and decide where you want to drain the water to. The
thing to remember here is that water will not travel uphill so your trenches
must finish downhill from where they start. It may be possible to ‘daylight’
your pipe into an area beyond your garden but there are certain
restrictions attached to this. Even waste ground belongs to somebody so you
have to check that the owner of the ground you plan to drain onto doesn’t
object. You can’t drain onto a road either because water running off your
property onto the road surface may cause cars to skid, nor should you drain
into a river or stream as nutrients from your garden may upset the ecosystem
and you definitely cannot drain into your neighbour’s property thus
transferring the problem onto them. You can daylight you drain if it is
possible to slope the trench downhill until it comes up above ground and it’s
certainly the easiest way of dealing with the excess water.

lawn drainage drain diagram


you can’t daylight the drain, you can create a soak away. Dig a big hole – 1m
x 1m by around 75cm deep would be fine. Lead your trenches into the hole and
fill it full of gravel. The soak away serves to hold the water underground
until it can drain away slowly through the sides and bottom of the soak away
pit. Soakaways should be sited, if possible, in an area of the garden that is
unused. Soakaways are fine for a small amount of water but they won’t cope
with large volumes so, if you are coping with water run-off from adjoining
land, the chances are that, during periods of heavy rainfall, the soak away
will fill up too fast and the water will back-up along the trench. Soakaways
are a good choice if your garden slopes away from the house and, if this is
your only option, you may want to think about having more than one soak away
if you’re coping with a lot of water.

If you don’t have a place
to put a soak away, or if the garden slopes towards the house, you will have
to direct the water into the rainwater drain. This involves following the rain
gutter pipe from your house until you can find a suitable place to break into
it. You will need to put in a silt trap so that you don’t block your drain
and, preferably, a rodding point so that the drains can be cleared if there is
a problem in the future. Be sure it is the rainwater drain and not the sewage
pipe! This kind of work is best carried out by an expert. Ask a suitably
qualified builder or landscaper to set this system up for you and check with
your local council that it’s okay to divert excess water in this way.

Once you have set up a
place to which to divert the excess water, you will need to decide on the
layout of your system. The most popular layout is a herringbone pattern with
one or two vertical drains which have diagonal drains running into them.
Remember all of the trenches should slope downhill towards the main trench.

lawn soakaway diagram


herringbone pattern is good because it allows the trenches to catch water all
the way across the lawn but you can use any convenient pattern of drain as
long as all the trenches slope downhill. You don’t need too much of a slope.
(The bubble on your spirit level touching the line will do fine). When
planning your system, go with the lie of the land as much as possible – in
other words, try to avoid digging a trench that goes against the surface
slope. The distance between the trenches depends on the structure of your
soil. In general, you can expect water to drain to around 2m on either side of
each trench but this will be less in heavy soil.

Once you have your plan,
start digging the trenches – 30-50cm deep and around a spade’s width will do
fine. Check the slope by putting a straight piece of wood onto two bricks in
the bottom of the trench and put your spirit level onto the wood.

After the trenches have
been dug and you are happy with the slope, etc., you need to put a layer of
pea gravel into the bottom of the trench. Next, put in your perforated land drain
pipe  e.g. WavinCoil, which is a flexible, perforated pipe
and you can get it from any builders merchants. A common misconception about
land drain pipe is that it acts like a closed pipe and ‘carries’ water just like a
closed pipe. Yes, it does carry water to an extent but, remember, it is
perforated and therefore can’t hold water as such. Its real purpose is to
create a space within the trench into which water can flow but keep in mind
it’s the whole trench that carries the water and that perforated land drain
pipe just isn’t
effective on its own, nor will it carry water anywhere if the trench itself
isn’t properly sloped.


trenches for lawn drainage pipe installation for lawn drainage

partially filled in trench after lawn drainage pipe installed

finished lawn drainage installation

you have placed the pipe in the trench, add more pea gravel until it is completely covered. The pea gravel around the
pipe helps
to keep the trench open and acts as a filter for silt. When ordering your pea
gravel, you should assume around 1 tonne of pea gravel per 15 metres of
trench. Cover up your drains with a good layer of topsoil. If your drainage
problem is caused by heavy soil then don’t put the old soil on top of the
drains. Use a nice graded sandy topsoil such as the type sold by turfing
companies or you will undo some of the good you have done by impeding the
surface water’s entry to the drain. To further assist your lawn to drain, it
is a good idea to returf over a layer of sand. The sand helps to improve
surface drainage, and you can get a good level lawn this way.

Your drains may take a
while to begin working to their full capacity. While you will probably notice
an immediate improvement to your surface water problem (especially if you
returf over sand), until the soil dries out and the air gets back into it, you
won’t appreciate the full effect.


Originally this
article mentioned “waving coil” , however, it has been brought
to my attention by Mr. David Carey that this is incorrect and,
therefore, the wording has been corrected accordingly apart from in the
diagram. There is
apparently a product called “WavinCoil”.





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