Keeping water off the path

PDF Print

One of the most important aspects of managing lowland paths is dealing with water. Whether this is rain falling directly onto the path, surface water from adjacent land or groundwater seeping onto the path from below, water has the capacity to cause damage to paths and misery for users.

Keeping Water Off The Path

Intercepting surface water approaching a path...

Wherever possible, water should be kept away from the path by intercepting it before it reaches the surface and diverting it away so that it does not cause problems elsewhere. Take a careful look at the topography (the shape of the ground) and watch or work out where the water is likely to drain. Bear in mind that water will always look for the easiest, if not shortest, route across the ground. You may need to intercept water from springs or surface flow.

The shape of the path surface can be used to 'shed' water and is one of the most important points to consider before installing any drainage features into the surface. You can use a camber, which provides a sloping surface with raised centre to drain water to either side of the path or a crossfall, which is a level surface that slopes to one side. On very narrow paths, you can add a short dip in the surface to reverse the direction of the gradient (called grade reversal) to force water off the path.

You can intercept and divert surface and ground water away from the path using:

  • A ditch - an open channel

An open ditch

  • A French drain (or filter drain) -  a channel filled with stone and a pipe

frenchdrain-unlabelled

There are pros and cons of open ditches and French drains, and you will need to think about your local circumstances before deciding which to use. For example, ditches can look more natural and are easier to maintain. French drains, on the other hand, can be hidden from view, do not attract litter and prevent open water hazards. However, if a French drain becomes blocked it may require complete excavation and replacement. If you decide to use French drains, an added complication is that tree roots will grow towards the flowing water and will quickly damage any permeable liner and block pipes. When thinking about the positioning of open ditches or closed drains bear in mind that excavation works close to trees can damage their roots and in the long term may cause a tree to die.

Cut-off drains, water bars, culverts and cross drains may be needed to divert the water off or across the path. These features need to be correctly positioned so that once the water has been removed it should not be allowed back on to the path. This may mean that ditches need to be dug to divert the water away from the path, with a soakaway to help disperse it into the ground.

  • A cut-off drain can catch surface water flowing down, or drain water across, the path

Cut-off drain

  • A water bar intercepts flowing water on the path surface but adds a step, creating a barrier and trip hazard across the path

Waterbar

  • A culvert allows water to drain under the path

culvert-unlabelled

 

  • A cross drain allows water to cross the path but introduces a open gap in the surface, creating a barrier and trip hazard

Cross drain

In some cases you may be able to run ditches or pipes to existing burns to discharge the water from along the path. Take care not to change the overall drainage pattern – your ditches and drains should allow the water to pass through the path and resume its original flow. Diverting all collected water into a burn could cause problems downstream, such as flooding. 

If there is an existing drainage system, such as agricultural land drains, you may be able to make improvements or repairs, with the landowner's agreement, rather than starting from scratch. Take care not to damage existing drains and check that, if you want to divert surface water into the existing system, there is enough spare capacity to cope with the extra water.

Where a path has to cross a watercourse – a stream, river or drainage channel – you will need to construct a culvert or bridge. You will need to check with your local Scottish Environment Protection Agency (SEPA) office regarding the Water Environment (Controlled Activities) (Scotland) Regulations (2011) as work within a watercourse may be restricted or prohibited. These regulations are often simply referred to as the CAR regulations.

Bridges can be costly, but will allow a much greater volume of water to pass unrestricted along the watercourse. They are essential for any sizeable burn or river. Planning, designing and constructing a bridge is a potentially complex subject, and is beyond the scope of this Guide. For more information about planning, designing and building small-span bridges see the Path Bridges Guide.

Culverts are generally a pipe or pipes laid in the bed of a small burn with stone headwalls to retain the material laid over the pipe and hide the pipe ends. Pipe culverts are cheap, in comparison with bridges, and easy to install, but will have a greater effect on a watercourse than a bridge, so size is critical.

In order to design an appropriate drainage system, you will need to work out the potential surface run-off that your system might need to cope with. There are lots of 'factors' to take into account, such as the area being drained, slopes, the permeability of the underlying soil, rainfall intensity and duration. There are complex calculations that you can perform if you want to have an accurate estimate of the amount of water likely to be flowing during a storm.

Find more information about drainage here

Accessibility and water

Choosing drainage features to install in to the path surface, such as water bars and cross drains, will create a barrier for most users on a path. This makes access for most users more difficult or not possible, particularly people with restricted mobility and on wheels e.g. wheelchair user. This means that grade reversals, closed cut-off drains and piped culverts are better options for keeping water off the path as they provide a continuous surface. Steep cambers or cross falls, whether natural or man-made, can also create problems for many people.

When planning a route, remember to look at the overall accessibility of the path and install drainage features that are appropriate to the situation – if the terrain limits what is technically feasible, select drainage features that match the accessibility.

<< Back to top

Β© 2014 Paths for All - Registered Scottish Charity No: SC025535, Company Limited by Guarantee No: 168554 inc. 19 Sept 1996 at Companies House, Edinburgh