Reproduced by kind permission of www.gardenzine.co.uk
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 zone.
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.
If 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.
The 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.
After 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. There is apparently a product called "WavinCoil".
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