2.0 Introduction to Drainage Techniques
An effective drainage system is essential for a well managed upland path. If the drainage system does not function properly erosion scars become severe, and any path surface work can be destroyed after one winter of rainfall, if not less.
There are three main sources of water on the path:
- rain falling directly onto and running down the path surface, or snow melting
- surface water from surrounding land flowing directly onto the path
- underground water running onto the path surface, in the form of springs or seepage
An assessment of the climate and altitude can give clues about these water patterns.
- naturally high rainfall e.g. western mountain areas
- altitude, and latitude, indicating the likelihood of snow cover
- seasonal snow fall and problems associated with potentially sudden snow-melt
ASSESSING THE SITE
The site must be assessed before any decisions can be made about which drainage features are appropriate, how many are needed and the positioning to achieve adequate protection of the path. At the same time the impact of changing the natural drainage system must be considered, particularly in environmentally sensitive areas where natural vegetation of ecological value is dependant on a particular source or level of water.
Assessment, or specification survey should preferably take place on a wet day, or just after a particularly wet period of weather. If this is not possible clues will be found on the path and the surrounding landscape.
The full length of the path should be walked, noting where water is coming from, both on the way up and on the way back down. These two perspectives should help to ensure that all aspects are considered. It is often the case that a drainage problem further up the path can be causing the problems below. For instance a stream at the top end of the path may have burst its banks or erosion debris diverted its course, directing flow straight down the path line.
The first clues come from the path itself. It should be possible to identify:
- damage caused by water flowing down or across the path - evident as gullies or eroded channels
- damage caused by water lying on the path - evident as puddles, boggy areas often saturated with water, signs of walkers skirting round and causing braiding
- where water has come from - evident by signs of springs, surface water, or water flowing onto the path from the slopes above
- where water is going - evident by signs of silt at the path edge, or vegetation may show signs of being flattened by water flowing off the path
The immediate landscape can also provide information about how the site reacts after periods of rain, and where and what drainage features are required.
- Geology of the area can indicate whether water flows close to the surface of the ground - evident by large areas of bedrock; or if it soaks away - evident as vegetated areas of deep soil or peat.
- Topography of the area can indicate where the water flows - quickly downhill if steep gradients are present and streams have formed; soaking away in flat areas of lush vegetation growth.
- Vegetation type can indicate areas of permanently wet ground by high presence of mosses, cotton grass or rushes. It can also show where water has flowed over it, flattening long grasses or deposited silt, inhibiting growth.
THE DRAINAGE SYSTEM
Having assessed the site, an appropriate and effective drainage system can be designed to combat the problems identified and protect the path.
There are two basic methods of drainage which are generally used in combination. Ditching is integral to both methods.
Off path drainage - to protect the path from water flowing onto it from the surrounding land.
- Ditching intercepts the water before it reaches the path and drains it away
- Culverts or cross-drains channel the intercepted water across the path
On path drainage - to divert water off the path surface.
- Cross-drains collect water at low points and channel it away into ditches
- Water-bars deflect running water off the path
- Letts drain away puddles that have formed on the path
Design and use of different techniques should also take account of the existing or expected path users. For instance a boxed culvert is preferable to a cross-drain if a path is used by ATVs for deer management; a cross drain with a narrower channel is preferable to a water bar if there is use by cyclists.
EXAMPLES OF SOLUTIONS
Some typical situations that arise on upland paths are illustrated below, showing a typical drainage problem and the potential solution, most of which will be in conjunction with path surfacing.
Boggy wet patch on path
Puddle of standing water on path
Gully caused by water flowing down path
Waterbar at top of gully
Burn crossing path
Water flowing on to path from slope on above path
Intercepting side ditch and Cross-drain
The main material used in the construction of drainage features is block stone. Whenever possible this should be found in the local area, and within easy reach of the path. The following points should be noted when looking for suitable stone.
- do not select stone from too close to the path - this will leave an obvious scar, and may damage the path edge
- be selective in choosing the right stone for the drainage feature - there is no point in moving stone to the path and then not using it
- choose natural weathered stone - lichen covered stone looks natural and will blend in with the surroundings
- collect stone randomly, and do not trample along the same line - this will quickly become visible particularly with repeated use by a power barrow
- turf over any scars or sockets where stone has been removed that are visible from the path
The nature of the available local stone will obviously affect the style of the constructed drainage feature. There is a variety of stone types found in Scotland including:
- Sandstone - provides good block stone with angular edges and flat faces
- Schists - thinner, slabby or slate-like stone; two stones may need to be placed together
- Granite - tends to be large but more rounded in shape, and lacking angular edges
PROBLEMS TO AVOID
There are three main reasons why a drainage feature will fail to solve the drainage problem.
- WRONG POSITION - misses the problem if drains too high or low on the path, or ditches not placed to intercept and disperse the flow
- WRONG SIZE - not able to cope with the highest water flows if too small or short; visually obtrusive if too big or long
- POOR CONSTRUCTION - not able to withstand the pressures of water and climate; stone becomes loose or ditches collapse
It is important to get the assessment, design and construction right, in order to avoid these problems occurring.
The following technical sheets give guidance on the drainage features that can be used and how to combat drainage problems. Each sheet has a section on positioning the feature, as well as the technical detail required for construction.