French drain

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A recently completed and poorly functioning French drain after a rainfall.

A French drain or land drain is a ditch filled with gravel, rock that redirects surface and ground water away from an area. French drains are common drainage systems, primarily used to prevent ground and surface water from penetrating or damaging building foundations. Alternatively, the French drain technique may be used to distribute water, such as that which flows from the outlet of a typical septic tank sewage treatment system. French drains are also used behind retaining walls to relieve ground water pressure.

1 Construction
2 Legal issues
3 Gallery
4 See also
5 References
6 External links

[edit] Construction

A diagram of a French Drain

The earliest forms of French drains were simple ditches, pitched from a high area to a lower one filled with gravel. Ditches may be dug by hand or with a trencher. An inclination of 1 in 100 to 1 in 200 is typical.^[1] Lining the bottom of ditch with clay or plastic pipe increases the volume of water that can flow through the drain. Modern French drain systems can be made with perforated pipe (weeping tile) surrounded by sand or gravel and geotextile or landscaping textile. Landscaping textiles are used to prevent migration of the drainage material as well as preventing dirt and roots from entering and clogging the drainage pipe. The perforated pipe provides a minor underground storage volume but the prime purpose is for the perforations to drain the area along the full length of the pipe and to discharge any surplus water at its end. The direction of percolation will depend on the relative conditions inside and outside the pipe.

Variations on the French drain model include:

filter drain is to drain ground water
collector drain (or inteceptor drain) combines groundwater drainage with the interception of surface water or run-off
dispersal drain distributes the waste water from a septic tank
fin drain uses a perforated pipe with a thin vertical section (the fin) of drainage composite above. The advantage is that the fin drain is narrower (200mm or 7 inches) than a traditional French drain (450mm or 17 inches and up), and cheaper to build.

French drains can lead to dry wells or environmentally-friendly rain gardens where the extra water is held and absorbed by plants, when city water systems, or other waste water areas can not be used.

[edit] Legal issues

In the U.S., municipalities may require permits for building drainage systems as federal law requires water sent to storm drains to be free of certain contaminants and sediment.

In the UK, local authorities may have specific requirements for the outfall of a French drain into a ditch or watercourse.