Resolving the problem of poor drainage
Visit the site in wet weather to determine where water is flowing from and to. Look for signs of discolouring or staining to indicate blocked outfalls or flooded ditches. Look for puddling, poaching or outwash fans to indicate where water has collected or may collect. Check old maps, aerial photographs or seek local advice as to where old drainage systems may be located.
If the water is from a source outside the boundary of the PRoW e.g. adjoining land, consider whether it is possible to prevent it from flowing onto the route. The drainage of a path should fit in with the wider drainage network. It is recommended that officers consult landowners on drainage problems and in identifying suitable solutions.
Some muddy problems can be resolved without drainage, or by diverting the path. On very wet sites such as peat bogs drainage may be impractical. In such cases boardwalks or large stone flags may be the solution, or a floating path on a raft of geotextile.
The list below, based on the TCV practical handbook for Footpaths, illustrates some common problems and their usual remedies:
Muddy route in woodland or scrub
- remove overgrowth to let sun and wind dry out surface
- clear existing drains
- make camber or cross fall to shed water
- lay sub base if necessary
Soft, deep mud
- Find source of water and divert it away from route
- Make camber or cross fall to shed water into side drains
- Lay geotextile, or other sub-base, and surfacing
Soft mud deposited over firm base
- Scrape off mud and dig drains to prevent water running onto route
Route crosses patch of boggy ground
- Consider ecological value of bog, either build raised surface, float path on surface or if necessary dig side drain.
Water flowing across route
- Divert water at source
- Build cross drain or culvert
- Build Irish bridge or ford paved with stone setts or pitched stone
Water running down route
- Divert water at source
- Build cut offs to divert water off route
Repair existing drainage systems.
Clear existing ditches. Take into account changes in water flow and enlarge any ditch where necessary.
Blocked pipe sections should be located and unblocked by rodding or that section dug out and replaced. Ditches should be dug where necessary at the initial outfall of pipes.
Recreating the camber of the path is the single most effective way of restoring drainage. All paths whose surface has been restored or upgraded should be cambered or have a crossfall of at least 50mm. Bridleways, RBs and BOATs should have a camber of at least 75mm.
Sometimes path clearance work may be all that is required, as the increased light and wind will act to dry the route out. Building boardwalks (footpaths only), bridges or floating paths is the only solution where the poorly drained soil is characteristic or ecologically valuable and there is no desire to drain the site. The maintenance commitment for such constructions will need careful consideration before such work is undertaken.
Ponds are very useful for controlling and disposing of large volumes of water in areas prone to a sudden influx of storm water. Where space allows an appropriately sized pond can be dug. The ditch or pipe should be directed into the upstream side of the pond and a weir constructed on the downstream side, from where the drainage should be continued.
In areas where drainage is a perennial problem and the route holds strategic importance certain tree species can be planted to dry out the soil. Seek ecological advice before carrying out any works of this nature.
Creating new drainage systems.
It may be necessary or advantageous to design an entirely new system that can intercept, collect, transport and dispose of water away from the path. A site survey will be required to discover how the water reaches the path and assess whether it is surface or subsurface flow.
Surface flow is easier to deal with, as subsurface flow often occurs over a wider area. Look for where water flow comes to the surface, where flow can be intercepted, diverted and finally taken away from the path. Visit the site in wet weather to determine where flood water is flowing. Mark with a peg where any storm drains flow onto the path.
Consider where the water will go once it has been drained off the path. It may be possible to drain it into an existing drainage system. Consult the adjacent landowner as you may be creating an unintentional problem on their land or creating another problem elsewhere on the path.
It may be appropriate when considering drainage works to consult the Environment Agency or drainage board, especially where water will drain into existing water courses.
Selection of drainage system
In choosing a system consider what it will be designed for; to collect, intercept, carry or disperse water? The type and size of the system will be dependent on its function and also the amount and type of flow. Careful consideration must be given before undertaken drainage works. Potential solutions include open ditches, piping, French drains, or cross-drains, cut-offs, and culverts or indeed any combination of the above.
Open ditches are quickest and cheapest to construct, they encourage users to stick to the path, although they may spoil the appearance of the area. Ditches require regular maintenance, but this is normally simple to do.
It may be necessary to create a more unobtrusive system in the form of piping or French drains. This can be especially useful when space is restricted in an enclosed route and where open ditches would take away too much of the usable width. Pipe-laying is more expensive, but provides an unobtrusive system. However, if problems do occur, they are troublesome to correct. French drains can also be used, and form unobtrusive and low maintenance systems.
In more rural environments, the provision of a simple cross-drain or cut-off on a PRoW is the most appropriate construction. On routes that could expect use by wheelchairs, pushchairs, horses or bicycles any cross-drains must be a minimal obstruction or preferably fully covered.
Where the path is sloping at a greater angle than 10% a drain may be provided at each side of the path if it is cambered, or on the lower side of the path if it has a cross-fall. Where a surface adjacent to a path slopes down to that path at an angle greater than 15%, a drain may be provided between the slope and the path.
Poor drainage is the cause of many problems in path management. However, even well designed and constructed drainage systems require regular maintenance in order to keep them functioning properly.
A regular system of maintenance checks should be initiated, although this is clearly an expensive and time consuming job. The frequency of maintenance will be dependent on the category of the route. It will be required of all high priority routes that drainage is regularly maintained.
Problems with drainage systems are both more obvious and more damaging during wet weather, so maintenance checks should be planned accordingly. Maintaining drainage systems should include the following:
- Remove build-up of silt, stones and other debris from open ditches, drains, cut-offs and culverts.
- Check for scouring or erosion at the outfall ends of drains and ditches.
- Replace loose or missing stones from culverts, drains and cut-offs.
- Remove any large stones or debris that may effect the drainage.
- Observe areas of scouring on the path surface which may require additional drainage.
(text from Hertfordshire County Council's good practice website - http://enquire.hertscc.gov.uk/ROWGuide/default.htm
Open ditches are traditionally the most simple of all drainage systems being basically trenches that collect, intercept or disperse water. The simplest form of open ditch runs along one or both sides of the route and collects surface water from its camber or crossfall.
Care should be taken to avoid switching the ditch from side-to-side of the route as crossovers will increase the maintenance commitment of the route. Other types of open ditch could transport water to another drainage system, to a soakaway or a storage "broken ditch" for seepage.
In digging open ditches always remember the safety aspect, especially on enclosed routes. It may be more appropriate to dig a French drain or install piping.
Ditch sizes can be calculated by considering the amount of run off, but this is usually only necessary for the design of large carrier systems. Most usually it would be hoped to keep the size in the order of 0.6m x 0.6m. Size will have to be increased if the water is to be dispersed into the subsoil by seepage as the ditch will have to initially hold the water.
Ditch digging procedure
- For most soil types ditches require sloping sides for stability, with the top of the ditch at least twice as wide as the bottom. Except where the soil is sufficiently stable, ditches should be V-section with sides 45? (Ratio 1:1).
- The gradient of the ditch will be dictated by the lie of the land.
- Minimum gradients of 1% are required, gradients steeper than 3% should be avoided or erosion can occur. Erosion of steep ditches can be reduced by building check dams to slow the flow. Keep the gradient as even as possible. The ditch line should be as straight as possible.
- Where a subsidiary ditch joins a main ditch it should do so at a gradual angle to prevent erosion and the bottom should be slightly higher than the main ditch.
- Always work from the lowest point up hill so you are not working in a flowing ditch.
- The practice of using spoil to recamber the path, filling holes or restoring eroded areas is not recommended.
Open ditches may need checking, as and when necessary, to avoid damage occurring to the PRoW due to deteriorating drainage. The frequency with which these works will occur will depend on the category of the route. Works are likely to involve the removal of branches, leaf litter and other such obstructions.
It is sometimes necessary to run a drain below ground, and piping can be used to achieve this. This is particularly useful when space is restricted, where open ditches would prove dangerous or where a drain has to pass beneath a PRoW. Piping can be permeable or impermeable depending on whether it is designed to collect or transport water.
Permeable or perforated pipes shall be used whenever water needs to be collected, intercepted or dispersed and are used with permeable backfill such as stone chippings.
Impermeable pipes (unperforated) are used when water is to be carried into another drainage system or across and under a route. Backfill which is stone free and of a good compaction factor is preferred.
Plastic piping is most suitable for the majority of drainage work over 0.25m diameter. It is good to work with being inexpensive, easily cut and joined with adapters. Plastic piping is considered 'flexible' and is moderately resistant to cracking resulting from irregularities on the ditch bottom, which could damage other 'rigid' types of piping such as concrete or clay.
Pipe size is dependent on expected conditions of water flow. Piping should be of a sufficient diameter to be able to cope with the maximum possible water volume that the site is likely to produce.
Pipe access points
Where pipe systems are more extensive, provision should be given for rodding points or inspection pits so that pipes can be inspected or cleaned. Such points should be provided at every change of:
- pipe size
- pipe type
Wherever several pipes join together an access point should be provided. For extensive lengths of piping, rodding points should be provided every 50-75m and inspection pits every 100-150m.
Pipe laying procedure
- Follow procedure for ditch digging above, but dig trench as narrow and as steep sided as possible. (When using permeable piping there is a need to create sufficient space to surround the pipe in washed gravel or similar material.)
- The base should be as smooth and firm as possible having removed all the rocks, tree roots and filled any holes with backfill of 30mm down to 5mm. Ensure that the gradient is steep enough for the pipe to be self cleaning (minimum 1%). As a general rule the gradient should be half the diameter of the pipe (in mm). i.e. 100mm pipe should be laid with a minimum gradient of 1 in 50 (2%).
- Pipes should be laid as soon as possible after the trench has been dug, checking for alignment. The trench should be dug in an uphill direction. d. Backfilling should begin from the outfall end, the type of backfilling varying with the type of piping.
- For permeable pipes the backfill should be either washed gravel or stone chippings of size 30mm down to 5mm. It should not be powdery or smaller than 5mm as this leads to pipe blockage. The backfill should not be compacted over permeable pipes.
- For impermeable pipes the backfill can be compacted and so should be stone free.
- Enough backfill should be initially added to secure the pipe in position, compact then add the rest in 150mm layers tamping down carefully. Mound to allow for settlement.
- When constructing permeable drains do not cover the top if surface water is to be collected, if not then turf can be used to cover the top. If designed to cross below a route then wood shuttering or stone slabs covered in topsoil can be placed on top.
Pipes require little regular maintenance, however it is suggested that they should be checked annually. This may involve simply clearing the pipe out and digging away debris from the ends of the pipe. The frequency of maintenance will depend on the category of the right if way.
Where a French drain is installed, the following specification is to be implemented:
- French drains are stone filled ditches that may have a porous pipe laid along the bottom. They are useful where space is restricted and open drains may prove dangerous i.e. too deep, or where water tends to form sub-surface flows.
- Size will be dependent on flow conditions but can be smaller than open ditches. A size of 0.3m x 0.3m is useful when transporting water under a route. This can be increased to 0.6m x 0.6m where appropriate.
- The top of the drain should be left uncovered unless appearance is important, in which case the drain can be covered with turf, wood shuttering or stone slabs with topsoil.
- When something prevents the construction of parallel ditches, discrete pits can be dug (at least 1.2m depth) and filled with stone chippings in order to hold storm water until natural seepage occurs.
- Follow procedure for ditch digging above.
- To prevent clogging, the drain can be lined with a geotextile mat, filled with backfill then the geotextile wrapped over. This filters out fine particles that would otherwise clog between the chippings.
- Fill the ditch with coarse permeable backfill of clean gravel, stones or stone chippings graded in size from 75mm down to 5mm. Particles smaller than 5mm block the drain as does powdery backfill and so should not be used.
A cross-drain is a system to convey water across a path or route. Open ditches are not recommended, but cross-drains may be used where necessary.
The size of cross-drain will depend on the local conditions. Within Hertfordshire small drains should be sufficient for most purposes. These shall be 0.1m x 0.10m, constructed of concrete, wood or clay piping. Sites anticipating heavy rain may require a drain of about 0.2m x 0.2m, built of stone or wood.
Where a cross-drain would be a barrier to the elderly, disabled or those with young children, a culvert should be built instead.
A cut-off drain is a more durable form of cross-drain suitable for footpath use. Construction is the same as for cross-drains above, the main advantage being that they are flush with the path's surface. Concrete cut-off drains should be used on paths designed for wheelchairs and prams, as their narrow opening prevents the wheels getting stuck. An alternative is to use precast clay halfpipes set flush into the path surface.
A cut-off is a surface feature designed to divert water from the surface of a path or route.
Board and log cut-offs
These are simple constructions capable of diverting surface runoff, albeit only a limited volume. They typically consist of an embedded bar of stone or wood crossing the route. The choice of timber or stone will depend on the location.
The bar must be high enough to divert the flow, but not so high as to present an obstruction or danger to path users. The bar must not be more than 0.05m from the surface of the path on the lower side. It is recommended that the downslope side is flush with the bar wherever possible, this will require infill with soil.
The bar should be placed between 20º and 45º to the path and the channel gradient should be a minimum of 5%. Furthermore, it is recommended that the bar be extended approximately 0.5m on either side of the path to discourage users from going around the ends.
Culverts are structures designed to take water under a path, and are built where the flow is too great for a cross-drain, or where vehicular access is needed. Culverts have similar considerations to piping (above) plus the following.
As large a diameter pipe as possible should be used to avoid silting up (Minimum 0.25m). The pipe should be laid at the bottom of an open ditch and surrounded with concrete or granular fill. Where vehicular traffic is expected they should be covered by at least 0.3m of soil. A pit should be dug at each point where the pipe extends beyond the overlying route. This is to prevent the end’s of the pipe from becoming blocked.
Culverts that are expected to be crossed by heavy farm machinery will be constructed with pipes complying to BS 65/540 (Clay pipes) or BS 556 (Concrete pipes) and will need to be covered by at least 0.9m of backfill or 0.6m concrete. They will also require the construction of a headwall minimum of 0.2m thick with a weather-proof capping. The headwall shall be built on concrete foundations and shall be built using bricks, blocks or concrete bags, 0.45m into the banks.
These can be constructed of concrete, stone or wood. They should be used where the profile of the route is wide and shallow, and where there is limited scope for covering a pipe with sufficient material to support vehicular traffic.
See also Drainage 2