2.3 Stone Waterbar
The waterbar is used in a range of styles of path construction, on low ground as well as upland. In forested areas waterbars are often built using timber. Stone waterbars are predominantly used on upland gravel, soil, or grass paths. On steeper gradients they are an integral part of a stone-pitched surface.
The key function of a waterbar is to divert running surface water off a sloping path. Without them the path surface scours and gradually becomes so rough, gullied and wet that walkers will not use it. Waterbars can also help to stabilise the path surface, by providing a solid anchor. A waterbar does a different job from crossdrains, which are generally used to take water from uphill ground, across the path.
Bill of Quantities (example)
Use local, weathered stone to construct a waterbar, between 30°- 45° to the path line. Bar depth should be a minimum 100mm rising to approximately 150mm. Liner should provide a draining fall of 5° minimum. Extend by 300mm on each path side. Include splash plate if ground drops steeply. Re-construct path at least 2 metres above and below the waterbar.
Positioning bars for effective collection and dispersal of surface water is essential. Key positions include path corners and above a steepening in the path. The location of other bars will depend on the source of water coming on to the path; the volume expected; and the scope for dispersal. Waterbars should deflect surface water off the path at the earliest available point, and then frequently to prevent the flow gathering volume and erosive force. Frequency will depend on the path gradient, and the associated mobility of the path surface. The following table gives a general guide.
|Gradient material||Mobility of path surface||Distance|
|steep > 10º||high||< 10m|
|slight 5º||low||> 25m|
Once the position is determined waterbars should be angled to deflect water away from the path and not flow back on further down. It may be necessary for extra ditching, to catch and re-direct the water, especially in the case of a zigzag path line.
Note needs to be taken of obstructions such as bedrock, boulders etc. These may prevent building a bar in the best position; alternatively they may provide a natural barrier or channel.
The classic waterbar has two key components:
- the shedding bar
- the front liner, or channel
The shedding bar comprises a line of stones placed together at an angle across the path forming an upstand above the liner. Its purpose is to:
- form a low barrier to the water flow
- shed the water across the path to the downhill side and away
The front liner comprises one or possibly two courses of stone set in front of the bar face on the uphill side. Its purpose is to:
- prevent the bar stones from being undermined by the water flow
- provide a smooth channel that is relatively self cleansing, but easy to clear of silt and debris
Section through waterbar
Depending on the downhill slope, a waterbar may need a splash plate at the outflow, to prevent edge erosion. A ditch for the outflow will ensure that water is dispersed away from the path edge and does not come back on to the path lower down.
- the angle of the waterbar across the path should provide an adequate fall and be between 30°- 45° to the path
- the draining fall in the liner across the path should be no less than 5°, and up to 15°
- the bar upstand above the liner should effectively catch and disperse the water and be a minimum of 100mm depth at the upper path edge rising to approximately 150mm at the lower edge, but not present a barrier to path users
- the top surface of the bar stone should be flush with the downhill surface
- the surface of the liner stones should be flush with the uphill surface and slightly angled down to the bar stone
- the bar should extend approximately 300mm either side of the path, as the site allows, to prevent water flowing back onto the path, and walkers from walking around and damaging the path edges
Local stone selected should be in its natural form, preferably weathered. The amount of stone needed will depend on the path width. The following points should be noted when selecting stone.
- block stone for the bar 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 it will be too small
- bar stones should be deep enough for half the depth to be below the liner level, and to provide the required upstand depth
- the front face of the bar stone should have no protrusions and provide an even surface with adjacent bar stones
- the top face, or tread, of bar stone should be large enough and suitable for walkers to step onto
- liner stones can be smaller, but must be at least 200mm deep to prevent under-mining and movement by heavy water flow
- the upper surface of liner stones should have no protrusions and provide an even surface with adjoining liners
Method of Construction
Excavate a trench across the path.
- dig the trench wide and deep enough to allow for the liner width and depth, and the required upstand of the bar stone with tread flush with the downhill path surface
- the line for the trench should be at an angle to provide the required fall and disperse the flow of water
Position the bar stones ensuring that the required angle and fall is maintained.
- set the bar stones vertically, butted tightly together, to provide an even face and even tread surface, level with the path surface
- wedge and pack gaps with smaller stone, allowing no movement of the bar
Position the liner stones, starting at the outflow end and working up, to achieve at the required draining fall.
- set the liner stones, tightly butted up to the bar stones, to give the required bar upstand depth above the liner; this will be higher at the outflow end for effective flow catchment and dispersal
- join liner stones tightly with top faces even to provide a clear water run
- liner stone joins should be off-set from bar stone joins, to provide a stable construction and reduce the risk of water getting through
- wedge and pack all gaps with smaller stone, so that the whole waterbar is solid and immovable once completed and will not allow water into the structure
Rebuild or grade the path surface
- grade the path surface below the bar, and compact the surface to be flush with the bar stone surface; this is a common erosion point at waterbars
- grade the path surface above the waterbar over at least 2m, falling gradually to be flush with the liner; compact soundly to the required level, to prevent loose material washing onto the liner
A splash plate or extension of the liner may be needed where there is a steep drop, or soft ground at the path edge. Ditching may be required to channel the water away from the path edge, and to ensure it does not flow back onto the path, further down.
The area around the waterbar should be restored if there is any damage or erosion. The path edges may require turfing up, particularly below the bar where it breaks the path edge; but NOT immediately above the drainage channel.
Key points to watch out for:
- maintain a good draining angle on the bar to prevent it silting up
- avoid gaps or low points on the bar stones - they let water through
- make sure there are no protrusions - they should be carefully hidden around the path catch and hold dead grass and other debris
- make sure the water-bar extends far enough beyond the path edge to collect and shed away all the water flow
- if there is a risk that the bar will push walkers off the path line, install a blocking boulder or a landscaped mound below the bar at the path edge
Where there is sedimentary geology, it may be necessary to use upstanding slab stones to create the bar. These can be butted end to end, placed in a double row, or overlapped.
High user numbers on a mobile surface may cause rapid silting-up of the waterbar and compaction on the downhill side of the bar. It may be necessary to pitch 1-2 metres either side to provide a harder wearing path surface, and prevent mobile material blocking the waterbar.
Where the path is used by cyclists consideration should be given to using narrow channel cross-drains as an alternative to waterbars.
MAINTENANCE TASKSWaterbars are the most likely of all drainage features to stop functioning due to the channel and outflow becoming blocked with silt and debris. It is essential that the following maintenance tasks are carried out on a regular basis.
- clear out debris and silt from the bar channel and the outflow ditch
- re-pack the path surface above and below the waterbar, particularly where there has been compaction or scouring and erosion
- check the stability of all stonework; re-pack where there is movement or visible gaps
- check the landscaping around the water bar and path edge, re-instate as required
- cleared silt or gravel can be re-used in resurfacing and packing; spare material
- select local stone away from the path edge and out of sight; if this is unavoidable ensure that holes left are reinstated
- use surplus turf and spoil from excavations for site restoration, or hide discretely
HEALTH AND SAFETY HAZARDS
- take particular care moving large stone; when manoeuvring stones into position beware of crushing fingers
- with water flowing down the path the trench can become very wet and slippery; erect a temporary bar further up the path or take extra care
- do not under estimate the amount of water that can flow down a path, and the number of waterbars required to disperse its erosive power.