French drain
From Wikipedia, the free encyclopedia
French drain refers to a ditch filled with gravel, rock that redirects surface and ground water away from an area. They are commonly used to prevent ground and surface water from penetrating or damaging building foundations.
French drains are also used to accommodate water that flows from the outlet of a typical septic tank sewage treatment system.
In addition to this; French drains are used behind retaining walls to relieve pressure from ground water.
The earliest forms of French drains were simple ditches, pitched from a high area to a lower one filled with gravel. An inclination of 1 in 100 to 1 in 200 is typical. Lining the bottom of ditch with clay pipe increases the volume of water that can flow through the drain. Modern French drain systems can be made with perforated pipe surrounded by sand or gravel and landscaping textile such as Bidim. Landscaping textiles are used to prevent dirt and roots from entering and clogging the drainage pipe. The perforated pipe provides some 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.
Some variations:
- filter drain is to drain groundwater
- 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.
In the US, 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.
French drains can be led 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] External links
- Graphical descriptions of French Drain installations
- An article about residential French drains and some history.
- Land Drainage for fields and gardens, AJ McCormick, [1] provides a simple explanation of the construction of french drains.
- Design Manual for Roads and Bridges, Volume 4 Drainage, Part 5 Determination of Pipe Bedding Combinations for Drainage Works, UK Highways Agency. Publication HA 40/01. November 2001. [2] provides a set of technical templates for the design of French Drains on pages B1 and B2.
- What's so French about French Drains? Where the name "French Drain" comes from.
- Non-residential French Drains are regulated in the U.S. if buried underground.
