These take the flow of water across a path. They are used especially on paths in upland areas where there are often frequent points where intermittent or permanent flows cross the path, and where culverting is not practical, or necessary for path users.
Size
The following sizes are suggested to cope with various conditions on a path:
- Small. Drains with a diameter of about 100mm, of wood, concrete or clay channels. These are only sufficient to take small seepages of water which cause muddy patches after heavy rain. These are likely to be in lowland and low rainfall areas.
- Medium. Drains with a cross-section of about 200mm by 200mm, of stone or wood. These should be suitable for places where water flows across the path during heavy rainfall but not in sufficient volume to wash away the path.
- Large. Drains with a cross-section of over 300mm by 300mm, built of stone. These are constructed on upland paths where permanent or intermittent flows of water cross the path. In wet weather, these flows can be torrential, and cross-drains should be built as big and strong as possible, while still being negotiable by the walker. Fortunately these sort of flows are typical of areas where the stone to build them is available.
A wide and deep cross drain may discourage illegal use by bike riders, and on rough or upland paths may provide a useful forewarning to less able walkers of conditions ahead. Where a cross drain would be a barrier against use by elderly, disabled or those with young children on an otherwise suitable path, culverts should be built.
Position
If possible, walk the path in wet weather and mark the points where water is crossing or seeping onto the path. Otherwise, inspect the edges and surfacing carefully for signs of where water has crossed.
The drain should be positioned to catch the water effectively on the upper side of the path, and either disperse it away from the path on the lower side, or lead into a watercourse or drainage system. Cross drains on sloping paths will also take water draining down the path, acting as a cut-off.
Usually the drain will take the shortest route, straight across the path, but may need to be at an angle to follow the natural flow of the water.
The constructed drain should extend at least 500mm on either side of the path, connecting to ditches as necessary.
Wooden cross drains
These can be made in a workshop and then taken to the site to be installed. Pressure-treated timber should be used, as the timber has to withstand alternate wetting and drying, and contact with the soil. As well as making the drain strong and less likely to collapse due to scour, the solid bottom makes it easier to clean out. The spacers at the top are the weakest part of the construction, so use good pieces of wood, without knots or fault, and install with the grain as shown. If one does break in use, another can be nailed across the top. The advantage of these drains is that they can be quickly installed without major excavation. They should be set flush with the surface of the path, so they do not cause any hazard to walkers.
The disadvantage is that they need regular cleaning. They are not recommended for slopes where they will also act as a cut-off, as they rapidly block with debris.
The design illustrated is suitable for small flows. Use larger size timbers as necessary for larger flows.
Stone cross drains
Stone cross drains must be built of stones which are sufficiently large or well-bedded in to withstand the water pressure and the trampling of feet. The bottom of the stone drain should always be pitched with stone to resist scour.
Different designs of drain are shown below. The choice depends on the type of stone available or obtainable, and the size of the drain needed.
The procedure for construction is as follows:
- Dig a trench sufficiently large to take the side stones and the bottom stones. The drain should extend at least 500mm on either side of the path.
- Set the side stones in position, ensuring they butt neatly together with as even a top edge as possible. Large boulders will need to be levered into position using crowbars. They must be set solidly in the ground so there is no movement when they are trodden on.
- Pitch the bottom of the drain. Ensure the pitched stones extend sufficiently up against the side stones to make a strong, stable construction. Wedge further with smaller stones as necessary, leaving a smooth bottom to the drain which will not trap debris.
- Dig collecting and draining ditches as necessary.
Cross drains that drop steeply from the path should have a boulder with a slab, called a splash plate, to protect the drain from scour.
As well as taking torrential flows of water, stone drains can also be built to take seepage water from damp flushes on mountainsides. The stones placed at the inlet help keep the water table high in the flush during summer, and prevent material washing in and blocking the drain.
Stepped drains can be built which are easy to cross in normal conditions, but large enough to take storm flows.
Stream crossings
A few ideas are included here to cross flows of water which cannot easily be piped into a culvert, but which are not big enough to justify a bridge, or where a bridge would look intrusive.
Many stream crossings get eroded as walkers divert to either side of the original crossing to avoid getting wet or muddy. If large enough stones can be found, the profile of the stream can be narrowed to near its original width, and the eroded area built up again with stones and backfill. Some rearrangement of stones in the stream bed may be necessary to channel the flow.
Stone-pitched fords of the type shown below have been constructed on some paths in Scotland and the Lake District. The pitching prevents the crossing getting muddy and eroded. A large stepping stone may be needed in the centre, and this may need replacing or retrieving after storm flows.
Stepping stones can be used to cross small damp areas and flushes. Use the biggest stones available, and place them in such a way that several routes can be used.
