Land drainage layout and installation

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The obvious purpose of land drainage is to remove excess water from fields, public grassed areas such as sports fields and football pitches and, last but not least, the humble domestic garden.

Land drainage seems a simple enough task on the surface but there are pitfalls for the unwary.

Many land drainage schemes can become ineffective over time as pipes and trenches get clogged up with sediment, pipework gets damaged by tree roots or drainage channels are not properly formed.

Check for a proper functioning outfall

One of the first considerations in setting up a land drainage scheme is deciding where the surplus water is to go. There is little point in creating or maintaining a land drainage system if there in no outfall.

Without somewhere to go water will keep on flooding the land no matter how much pipework is laid down. The outfall could be a ditch, a culvert, brook or public drain.

It is always a good idea to check with the local authority to confirm that the outfall does not need any special permissions. Land drains, for example, should never be connected to a foul drain without express permission and where a connection is allowed it must also incorporate a trap.

Very often the cause of regularly waterlogged land is nothing more than a blocked ditch or culvert. This is particularly the case on farmland where ditches can often become overgrown.

Land drainage systems can actually be in very good working order but a blocked outflow cause will have caused the surface water to back up. Check watercourses are free of debris, that banks have not collapsed and that existing soakaway are working effectively, before making a start on a land drainage system.

Types of land drainage

There are two basic types of land drainage, groundwater and shallow systems, and which to choose depend very much on the characteristics of the soil.

Which to choose depends on whether there is am impermeable layer beneath the surface. Sometimes land can be waterlogged on the surface but quite dry underground.

This happens when an impermeable layer forms just below the soil surface and can happen from compacting or overworking the soil. Farmland can suffer this problem on soils that are over-tilled or have heavy tractors regularly rolling over the fields.

The impermeable layer forms an 'artificial' water table which prevents water draining away. This is why farmland kept as permanent pasture will often dry quickly after heavy rains while surface pools quickly form on a neighbouring field that is regularly tilled for crops.

If there is no impermeable layer then a piped drainage system will easily cure any waterlogging problems but if such as layer is found then it is also necessary to improve the drainage capacity of the soil with a shallow drainage scheme such as mole or gravel mole drainage to break up the layer.

Make land drainage a downhill task

The second most important consideration is the slope of the land. The most efficient system is one that allows water to collect and flow to the lowest elevation point from the site.

Although surface conditions can certainly be a factor, the slope of the site is an extremely important issue A gradual fall is all that is needed for adequate land drainage and a fall of between 1-50 and 1-100 is fine.

In areas that are to be 'built' such as drives or car parks, it is easy to ensure good surface drainage by simply sloping the land gently towards an area where the water will either drain naturally or into a storm sewer or gutter.

On open grassed areas, a 'sale' or shallow channel can be created to carry water to lower ground.

How land drains work

The most efficient way to get rid of waterlogged areas is to lay perforated drainage pipe underground. Most drainage pipes are made from PVC and have small perforations to allow water to seep inside.

Pipes are often bedded in shingle to aid drainage. In a small garden pipes are often laid in a herringbone pattern with a distance between drainage pipes of two to three metres. This is impractical for large areas such as sports fields where 10 metres is more economical.

Trenches are dug around 750mm deep and wide enough to allow at least 150mm of granular shingle each side of the pipe and 100mm of stone beneath it. Trenches are normally backfilled with 150mm of shingle, 300mm of course sand and a final surface layer of 100mm of soil.

Sometimes contractors will wrap pipes in a textile membrane to speed up water flow and prolong the life of the system by preventing the build up of fine particles.


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