These are typically constructed along the side of contour paths in upland areas, or along sunken paths where water cannot easily be drained away from the path. They intercept surface and sub-surface flow and carry it away downhill.
They may also be constructed by paths that run directly downslopes, and which may otherwise themselves become watercourses.
Material from the ditch can be used to build up the surface of the path, or scattered below the path. Re-use any turf on eroded parts of the path.
Dig the ditch as close as possible to the edge, whilst ensuring that the side of the path will not collapse into the ditch. To prevent this happening, side drains can either be lined or filled.
Lined drains
The design will depend on the stone available, and the expected flows. The construction technique is similar to building a cross drain, as described here.
As well as for side drains, this method can be used generally on slopes down very steep gradients of up to 30degrees. This method of drain construction has been successfully used in Snowdonia and elsewhere, on sites where fairly drastic action is needed to stabilise rapidly eroding rocky slopes. Drains of this type take the water off the slope, and allow the construction of pitched paths and the restoration of vegetation. As the slope stabilises and revegetates, the drain should no longer be needed, and can be allowed to fill with debris and revegetate (see Chapter 11 – Erosion control and vegetation restoration).
Where suitable stone is available, a narrow drain can be built to the design shown below, to take moderate flows of water.
Drains near paths on steep slopes may incorporate stone revetments immediately above them.
French drains
These are ditches filled with permeable backfill of clean gravel, stones, stone chippings, slag or clinker, graded from diameter 50-5mm. It is important that the material contains no particles smaller than 5mm, or it will rapidly block.
French drains are not generally successful in upland areas, where they cannot take the flows resulting from high rainfall and large catchments.
When deciding on open or French construction, consider how the side drain works. If it is a carrier for water from a spring, or if it intercepts surface flow, an open ditch is recommended. If it intercepts sub-surface flow, or carries seepage water from a bog, a French drain should be more effective. Open drains and long French drains (over 10 metres) should lead into watercourses which take the water away. Short French drains can lead into soakaways.
Although French drains cannot take as much flow as open drains, they are resistant to scour and collapse, and so are useful on steep slopes (as seen on motorway embankments), or for side drains which will receive little maintenance. If good appearance is important, they can be hidden by covering with turf, but this lessens their ability to intercept surface flows.
The effectiveness of French drains can be greatly increased by laying a permeable uPVC pipe inside the drain.
The ‘permeable fill’ of French drains does tend to become less permeable as fine particles are washed in. This can be prevented by lining the drain with a permeable membrane such as Terram 500, which acts as a filter. This can be cost effective as it allows a smaller amount of permeable fill to be used to make the same capacity drain, and lengthens the life of the drain. Also, coarser ungraded fill can be used, as it only needs to carry water, and does not need to function as a filter.
