Stone pitched drains can be used to protect footpaths when there is evidence of erosion occuring. Erosion can occur as a result of high levels of rainfall, because a path is very regularly used, or because the ground is steeply inclined and covered in sandy or peaty soil which is prone to washing away. In most situations it is some combination of all of these factors.
If left unchecked, eroded paths can become wider and wider as people walk different routes to avoid mud and uneven ground. Eventually they can be an unsightly blot on the landscape, damage habitat and make public access difficult or unsafe.
Stone pitched drains are suitable when there is a need to get water off a path without much of the path washing away. They are supposed to slow the water down and direct it away from paths being walked upon.
This stone pitched drain construction technique has been used successfully in the Brecon Beacons National Park in Wales and similar techniques are used in National Parks and nature reserves throughout the UK. This type of drain is said to be based on traditional methods dating back to Roman times.
These drains are long lasting but very labour intensive to create. The materials required can also be very expensive, particularly transporting stone by helicopter. In less wet areas, or paths that are not intensively used, or on gentle slopes then quicker or cheaper solutions may be available.
Planning the Project
This method requires a lot of good quality stone. It may be necessary to sort the stone into pieces of a suitable size and shape beforehand, particularly if they are expensive to transport. A lot of small stones will not be very useful. The sizes of stones which are most useful are:
- Long, thin stones to form the front edge. The ideal stone for this job is perhaps about 60mm x 60mm x 140mm.
- Heavy flat stones to form the base of the channel. These should be large and heavy and have at least one flatish side. The ideal size here is probably 140mm x 200mm x 60mm.
- Irregular stones for the sloping back wall of the channel. A much wider range of sizes is possible here, but maybe stones of approximately 90mm x 60mm x 60mm are typical.
The stones should be quarried locally and match the natural stone of the area. Importing a lot of stone from a different area can affect the soil, for example if lime were imported into an area with lots of plants which favour acid soils.
Every 20-30m a culvert will be needed to take the water across the path and down the hillside. These are built to a similar method to the drain but with both sides like the “path-side” of a drain. They are typically built using the best quality larger stones available and with even more care to make them robust.
Occasionally culverts have large stones bridging them for the benefit of walkers, but normally they are just a spade’s width and so can easily be stepped over. They should always be slightly wider than the blade of a spade so that they are easy to clear out.
It is common for the path itself to be stone-pitched for about 1m on either side of the culvert. This helps to strengthen the culvert and prevent the water escaping down the path on the lower side. Culverts typically go diagonally across the path, to ensure there is a good incline for the water to flow downwards.
- Dig ditch along the higher side of the path. The front edge of the ditch should be near vertical. The line of the ditch should be approximately straight, or follow the curve of the path smoothly.
- Lay front vertical stones.These stones are placed on the path-side of the ditch. The ends are dug in to varying depths and the tops should all be to the same height above the final height of the path. Typically the path is surfaced as a last job so the front stones may protrude until the final surfacing is done with the left-over soil from the ditch.
- Lay the base stones up to the front stones.The base stones are supposed to be large and heavy to stop them being lifted by the effects of frost and flow of water. If there are not large enough stones to form the base of the drain, then smaller stones should be placed side on, in the line of the drain, but this will clearly require more stones. Side-on stones should also be used if required to ensure that the drain is a comfortable spade-width wide. It is essential that the drain be wide enough to fit a spade blade, because they will have to be regularly cleared. The base stones should touch the front stones to hold them in place. If there are gaps then wedges should be used to prevent the front stones wobbling.
- Lay the rear stones into the bank. The rear stones serve three purposes: they support the bank, they prevent run-off eroding the bank and they hold the base stones in place. For this reason they must be dug into the bank, so that run off water runs down the stones and not behind them. They should slope at approximately 45 degrees but this can be varied to match the slope of the bank. One end should tuck under base stones, it should be a tight fit. Hollows behind the stones should avoiding by filling in with soil.
- Fill in the gaps with soil and turf. The gaps between the stones should be stuffed with soil or turf. Turf is much better and any cut in the course of the previous steps should have been carefully preserved so it can be re-used at this stage. Soil should be thoroughly tamped down, but even then there is a serious risk it washes away before vegetation can colonise it. Sometimes soil is planted straight away with grass seed, but great care should be taken to source a suitable strain for the local conditions.
- Level path up to front stones.This will add a lot of strength and is very important to stop them being accidently kicked out by people walking by. In time, with luck, vegetation will grow in the cracks and help to bind the stones together.
- Check inflow. The inflow at the top of the section of drain, should be a gentle slope covered in turf. This normally carries little water, as the water from the previous section will have been directed across the path in a culvert.
Regular inspection and clearing will be needed to remove mud and stones in both the drains and the culverts. The only tool required for this job is a a spade and a long length of path can be walked with occasional stops when a problem is encountered. The frequency of clearing will vary, but once a year is probably a good starting point.
Running repairs will almost certainly be needed and it might be a good idea to schedule a repair session a few months after a new section has been created to patch up any sections that have failed after the ground has settled.
- String or rope. – Used to mark out the route.
- Spade – For digging ditch and cutting turf.
- Pick or mattock – For digging ditch where ground is hard or rocky.
- Small trowel – For accurately digging holes for individual stones.
- Lump hammer – For breaking stone and tamping in stones.
- Kneeling mat – For protecting knees while working.