2.4 Stone Cross-drain
The stone cross-drain is a traditional, and versatile, drainage feature, sometimes referred to as an open culvert, or a stone lined ditch. The elements of the design used today remain relatively unchanged from those used on stalkers paths and hill tracks.
The main purpose of the cross-drain is to channel water from above the path to the lower side. The source of the water may be from small streams, springs, mossy flushes, areas of uphill surface water or seepage. Cross-drains are also used to collect and disperse path surface water at low points on the path, or on sloping paths where water bars are not suitable for the path use.
Bill of Quantities (example)
Use local weathered stone to construct a stone cross-drain with a minimum channel depth and width of 300mm. Extend by 300mm on each path side. Stone line the full length of the drain base, with a gradient of 5° minimum. Allow for an outflow splash plate and approximately 10 metres of in and out flow side ditch. Construct path at least 2 metres either side of the drain.
POSITIONING OF THE CROSS-DRAIN
When assessing the location for cross-drains it must be remembered that an inadequate drainage system with too few or wrongly positioned drains can lead to extensive path damage. Frequent cross-drains are needed where the path is traversing a slope, and consequently all water flows have to cross the path.Typical locations where drains will be required are:
- where a continuous water flow crosses the path - a burn or stream
- where water flows onto the path from above after heavy rainfall - surface water intercepting ditches will also be needed
- where there is underground water seepage from above or under the path - path side protective ditching may also be required
- where path surface water collects at low points and cannot drain away naturally
Positioning should also be:
- at the best place for water to cross the path, normally at the lowest point, or where the flow is intercepted
- connected directly, and frequently with protective side ditching
- where there is scope for dispersing water away from the path, and not back on to it lower down
- away from obstructions such as bedrock, unless it can form the drain side or channel - more often than not it will cause a problem rather than help
The cross-drain has two main components - side walls and a lined channel base. They provide a solid channel across the path which is easy to clear of silt and debris, and is relatively self cleansing.
- side walls provide the channel width and depth, and are comprised of two lines of block stone across the path, placed with faces to channel the water flow - essential ‘stone extensions’ of drainage ditch or water course sides
- lined channel base is comprised of a row of liner stones, between the side walls, which helps to stabilise the side stones and prevents undermining by water
A splash plate stone extending the liner stones at the outflow may be required to prevent erosion, especially where there is a steep drop, or soft ground is present.
Inflow ditches collect the water flow to be taken across the path from the water source. The ditch for the outflow may connect with the drainage system, or lower water courses, and will ensure that water is dispersed away from the path edge.
These will vary according to the nature, source and volume of water to be channelled, and the direction and dispersal of waterflow.
- the cross-drain is normally at a shallow angle across the path, depending on the nature and direction of flow; the angle may need to be increased in order to provide an adequate fall in the channel
- the draining fall in the channel should be no less than 5°, and up to 10°, to ensure a clear run
- the channel width and depth can be variable, but will normally be a minimum of 300mm deep and 300mm wide; this will allow room for a spade during maintenance, and less chance of being choked with larger debris.
- the channel should not be so wide as to provide an obstacle to path users
- the top surface of the side stones should be flush with the path surface, to allow collection of path surface water, and to provide a tread surface for walkers stepping across the channel
- the cross-drain should extend approximately 300mm either side of the path, as the site allows, to protect path edges and prevent water flowing onto the path
Large block stone is required, preferably available, from within reach of the path. It should be large enough to withstand the pressure of path use, the greatest waterflow, and frost heave. If it can be moved and lifted easily by one person it is probably too small.
It should be used in its natural form, preferably weathered (see Section 2.0), although it may be necessary to shape the stone slightly by chipping off minor protrusions. The quantity of stone required will depend on the size of cross-drain to be built and the path width.
Points to note when selecting stone.
- side, or face stones should be deep enough for at least 1/3 to be below the surface of the liner, and to provide the required channel depth above the liner
- faces forming the channel side should be as even as possible, with no protrusions that may hamper water flow or collect debris
- tread faces should be as even as possible, with no protrusions for walkers to trip on
- the shape should match evenly and tightly with the adjacent side stones
- liner stones can be smaller, but must be wide enough for the required channel width, and at least 1/3 of the depth of the side stones; also large enough to prevent undermining by fast and high volumes of water
- upper surfaces should provide an even channel surface with adjoining liners, and have no protrusions to hamper water flow and collect debris
Method of Construction
Excavate a trench across the path.
- dig the trench deep enough for the liner stones and the required channel depth, and wide enough for both rows of side stones and the required channel width
- the angle of the trench across the path must provide the required fall
- the angle and depth of the trench must complement the drainage ditching or watercourse.
Construct the drain side walls.
- set the side stones vertically, or angled slightly back from the channel - stones leaning into the channel will be unstable and make cleaning difficult
- allow at least 1/3 of the stone to be below the liner level
- set top faces at a height that will be flush with the re-constructed path surface, with treads as level and flat as possible
- butt stones tightly to form even front faces to the water channel, with no protrusions
- wedge and pack gaps firmly with smaller stones so that each side wall is solid and immovable once the liner is in position
Position the liner stones
- set the liner stones to fit tightly between the side stones and give the required channel depth
- maintain the drainage fall by working from the lower to the higher end
- join liner stones tightly, with joins off-set from the side stones to provide a stable construction and reduce the risk of water getting through
- adjoining stones should have top faces even, with no protrusions
- if required add a splash plate at the outflow, set slightly lower than the channel, to prevent erosion
- wedge and pack all gaps firmly with smaller stones so that whole drain construction is solid and immovable, and will not allow water to sink in
On completion of the cross-drain, connecting ditching should be completed, or re-dug, to ensure effective inflow and outflow away from the path.
Re-construct the path on either side of the cross-drain. Grade the path surface, over approximately 2m length, down to the level of the side stones, and compact well to prevent surface debris washing into the drain.
Restore any damaged areas, particularly the path edges immediately above and below the cross-drain, turfing slightly over the side stones to ensure that the feature appears as natural and unobtrusive as possible.
Key points to watch out for:
- make sure the angle and depth complement the ditching
- avoid over-hanging face stones - they make the channel unstable and too narrow to clean
- make sure there are no protrusions - they cause debris to block the drain
- don’t be tempted to use thin liner stone - it will quickly be under-mined and wash out
Variations will depend on the stone available, and the volume of water expected. Where a high flow is common, for instance a small stream in spate, a double cross-drain may be constructed with two channels, flowing around a central stepping stone.
Specific path use may dictate a change in dimensions, particularly a narrower channel width if the path is used by cyclists, or quad bikes.
Cross-drains may be incorporated in stone-pitching, with the side stones forming part of the pitched path surface.
Cross-drains can be subjected to very fast flowing and high volumes of water particularly after snow melt or periods of heavy rain. This can result in undermining of stonework, and drain blockage by large debris. It is essential that the following tasks are regularly carried out.
- clear out debris and silt blocking the channel and connecting ditching
- check the stability of stonework - re-pack where there is any movement or gaps
- re-pack surfacing behind drain stones if there has been compaction or erosion of the surface
- check the landscaping around the crossdrain and path edge, re-instate as required
- cleared silt or gravel can be re-used in packing and re-surfacing; any spare material should be carefully hidden
- select local stone away from the path edge and out of sight; if this is unavoidable ensure that any visible scars are reinstated
- use surplus turf and spoil from excavations for site restoration, or hide discretely
- restore damage from cross-drain construction to look as natural as possible
HEALTH AND SAFETY HAZARDS
- take particular care moving heavy side stones and when manoeuvring into position; use safe manual handling techniques; beware of crushing fingers or dropping stone onto feet
- when wedging large face stones, watch out for stone movement that may cause crush injuries
- the trench of a cross-drain will become very wet and slippery; divert or block water flow during construction, if feasible
- do not under estimate the amount of water to be channelled in the wettest conditions. Too few, or inadequate size drains can lead to extensive damage to the path surface.