Drainage
From GPG
Contents |
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 "Footpaths a Practical Handbook" (1996) BTCV, illustrates some common problems and their usual remedies:
Problem
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.
Alternative solutions
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.
Maintenance commitment
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
Further information on drainage is given in drainage of public rights of way
