French drains

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French drains are essentially trenches that have been filled with clean free draining stone and can be used to drain surface water or intercept groundwater. They may have a perforated pipe installed to increase the volume of water that can be drained. The name comes from the ‘inventor’, Henry French, rather than the country. 

  • French drains can be used where open ditches are considered to be an unacceptable hazard or where available space is limited with no room for an open ditch along side the path

  • Where a French drain is required to intercept surface water runoff from a path or a slope the clean free draining stone fill should reach the surface. However, mud and silt can easily block the open spaces between the stones and encroaching vegetation can allow surface water to bypass the drain

  • To reduce clogging by silt and mud, a permeable geotextile sheet can be used to line the excavated trench, which acts as a filter

  • For a French drain to work effectively the minimum gradient should be 2% or 1:50

  • French drains can be used on steep slopes

A French drain


Starting at the lowest point in the drain line, excavate a 300mm wide trench to 300mm - 500mm depth below ground level with a continuous gradient
Dispose of the excavated spoil on-site at designated locations
If required, line the trench with lightweight permeable geotextile sheet, and allow for 300mm overlap either side
Fill the bottom of the drain trench with graded aggregate - single size clean gravel or quarry stone (20 - 40mm) with no fines
Lay 150mm PVC perforated land drain pipe (sometimes referred to as Flexicoil or Wavincoil) in the base of the trench, on top of the laid graded aggregate. Use 100mm for small drains. Ensure that there is a continuous gradient down slope
Fill drain trench with graded aggregate to 250mm – 450mm below top edge of trench sides using single size clean gravel or quarry stone (20-40mm) with no fines. Alternatively, use screened ‘as dug’ gravel without fines, recycled crushed brick or railway ballast without fines
If a geotextile liner has been used, wrap the sheet over the top of the aggregate before filling the trench as follows:
  • For intercepting water on the surface and ground water, fill the remaining trench with graded stone or gravel

  • For intercepting ground water only, fill the remaining trench with excavated materials. A geotextile sheet at the base of this layer will help prevent clogging of the drain

  • Provide an inspection / sediment pit at regular intervals (or where there is a junction or bend)


Annual inspection of French drains should be a high priority
Encroaching vegetation should be removed and any tree seedlings/saplings near the line of the drain should be removed
Pipes can be cleaned using drainage rods and silt dug out of the bottom of inspection pits


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