In wetter climates it is vital that the path is drained properly.
As well as culverting small streams or drains that cross the path, its surface also needs to be drained. Water runs down the cut faces above the path, and if not collected and channelled away can cause serious erosion to the path surface and possibly to the foundations. Cut-off drains should be provided on the inside of the path along the bottom of the cut slope to collect water coming down it, and from the path itself. The path formation is best sloped inwards towards this drain. Culverts should be provided at intervals along the path to divert water collected in the side drains and reduce water flow. Silt traps, such as a basin below the entrance to the culvert, will prevent silt from finding its way into streams (see diagram).
On steep sections where storms might wash out the surfacing, cut-off drains or water bars should be laid across the path at frequent intervals. These are normally open channels, which can interfere with wheelchairs or buggies. Some can be narrow to reduce this problem, but they are more prone to
blockage and need more maintenance to keep them working. The types of cut-off drain can be as follows.
– A wooden board inserted into the surface across the path at a shallow gradient is functional, but can trip people.
– A log can be laid across the path, sunk into the surface. A V-shaped groove cut along this channels the water. It is simple and effective and less likely to trip people.
– A box drain can be constructed out of durable or preserved timber and set flush into the path surface. It can be partially enclosed to leave a narrow slit so that it impedes wheels less, but it is prone to blockage by leaves or larger stones.
– A similar open cross-drain can be constructed from local stone. Flat stones are laid on the bottom between the sides to keep them apart.
The design of cut-off drains or waterbars: (a) The simplest is a plank set on edge into the path. This can trip people up, and it is no good where wheelchairs or buggies are used. (b) A half-round log with a channel cut into it is simple, robust and easy to lay. (c) A box drain made from stone is very effective. This also presents problems for wheels. (d) A
wooden box drain is similar to the stone example, but it can be prefabricated off site.
Leading water under paths is best done using plastic culvert pipes. These are strong, light to carry into remote areas, and easily cut to lengths using a knife or saw. The pipe is laid in a trench dug across the path at a depth of 300 mm (1 ft) or more on a compacted base to prevent sharp stones from damaging it. The ends of the culvert pipe should be trimmed to match the sloping profile of the path.
If heavier loading is needed, concrete or galvanized pipes are stronger. These take more effort to lay, are heavier to transport, and require special cutting tools.
Box culverts made from durable timber such as elm or from slabs of stone are a traditional type. These have a place where natural materials are the first choice, but they require more skill to construct than laying a pipe.
If the path edge or the fill material of the base is unstable, a protective surround to the culvert ends should be built of dry or mortared stonework or timber to prevent erosion during heavy storms.
Where the drain across the path may have to cope with sudden, large amounts of stormwater, unless pipes are of large dimensions they can block with debris and cause flooding or washout of the path. In such cases, where early maintenance cannot be guaranteed, wider open culverts may be necessary, and walkers can be expected to step or jump across. If the path is in a remote location in rougher conditions, wheelchair access may be less appropriate, but users could be advised to carry crossing planks.