Garden drainage problems ?

What causes poor drainage?

There are several reasons for poor garden drainage. On new housing estates, it is often caused by compacted soil as a result of builders' lorries and trucks. This combined with the mixing of sub-soil and topsoil when all the trenches and foundations were excavated often leads to a heavy, wet plot.

In cases where new estates have been built on old arable land that has been cultivated annually for centuries, a "plough pan" may have developed. This is an impervious layer of more compact soil just below the maximum ploughing depth, caused by years of polishing by the sole of the plough.

Of course, drainage problems need not be caused by any human activity, the soil may by its nature (e.g. clay soils) be water retentive or, if it is low lying, be affected by the natural water table of the area.

On clay and other water retentive soils, the flow of water though the soil is very slow, this can be exacerbated if the contours of the land form depressions in which the surface water can collect.

On land which has a very shallow water table, not much can be done to avoid water logging after heavy rain as the water table can rise to the surface of the land or actually above it, in this latter case a pond will naturally form.

How to know if a garden needs better drainage?

Wet winter weather will show how good or bad the natural drainage is. Patches of water on the surface are the most common signs of problems. To check how well the land drains, dig a hole about 600mm (24 inch) deep and 300mm (12 inch) square, then fill it half full with water. Leave it for 24 hours in which time it should empty on well-drained soil. On very wet land, the hole may actually fill.

What sort of drainage system should be used?

If the top-soil is very poor, it may be worth stripping off the top 150mm (6 inch) and importing new topsoil Where the topsoil is just very water retentive, the problems may be reduced by double-digging and incorporating large amounts of bulky organic materials, but where the soil is inherently heavy and waterlogged, some form of drainage will be necessary.

The type of drainage used depends largely on the extent of the problem. If the water logging is not severe and there is only excess surface water, it may be possible to overcome the problem by shaping the garden surfaces so that the water flows off into ditches. These ditches should be 0.9-1.2m (3 to 4 ft.) deep, with sloping sides.

Alternatively, creating a soakaway may be the answer. This requires a large hole at the lowest point of the garden and filled with rubble or broken bricks, and then covered with 300mm of soil. The soakaway will need to be quite large to have the desired effect - up to 1.8m (6 ft.) deep and the same across. There is not always a need for underground pipes to a soakaway although on land that has insufficient gradient to drain into a soakaway naturally, underground drain pipes of some sort will be required. Both clay and plastic pipes are available - the latter being easier to use and lighter to handle.

land drain pipeModern plastic land drain pipes (example right) consist of crush-resistant plastic with perforations, it's usually fairly stiff byut flexible so that the line of the drain need not be straight. The pipes can be supplied in lengths of up to 15m (50 ft) which can just be laid into a trench and surrounded by gravel. With a large area to drain, the most efficient way is to lay a number of pipes in a "herring-bone" pattern.

Underground pipes should always be laid above the local water table otherwise the pipes will just be moving the groundwater instead of draining the topsoil On level ground, the pipes should be sloped underground to encourage water to drain away quickly and on sloping land, the main drain should run down the slope. Drained water obviously needs somewhere to go so all pipes should terminate in a soakaway or ditch.