Upland Pathwork

Construction Standards for Scotland

Contents

• Home
• Preface
• Section 1. Introduction to Upland Pathwork
• 1.1. Principles of Upland Pathwork
• 1.2. Environmental Impact
• 1.3. Path Assessment
• 1.4. Path Survey
• 1.5. Maintenance
• 1.6. Working Practice
• 1.7. Tools and Equipment
• 1.7a. Mechanised Equipment
• 1.8. Health and Safety
• Section 2. Upland Path Drainage
• 2.0. Introduction to Drainage Techniques
• 2.1. Ditching
• 2.2. Stone Lett
• 2.3. Stone Waterbar
• 2.4. Stone Cross-drain
• 2.5. Stone Culvert
• 2.6. Piped Culvert
• 2.7. Stone Ford
• Section 3. Upland Path Surfaces
• 3.0. Introduction to Surface Techniques
• 3.1. Aggregate Path
• 3.2. Geotextiles: Aggregate Path on Peat
• 3.3. Anchor Bars: Aggregate Path on Slopes
• 3.4. Stone Pitched Path
• Section 4. Upland Path Site Restoration
• 4.0. Introduction to Restoration Techniques
• 4.1. Materials and Use
• 4.2. Defining Path Alignment
• 4.3. Containing Path Use
• 4.4. Bank and Slope Stabilisation
• 4.5. Restoring Vegetation
• Glossary of Symbols
• Glossary of Terms
• Further Reference

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.

FUNCTION

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.

POSITIONING WATERBARS

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
moderate 5º-10º	medium	10m-25m
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.

CONSTRUCTION

Components

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

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.

Dimension Guidelines

• 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

Materials

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

Step 1

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

Step 2

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

Step 3

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

Step 4

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

Step 5

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.

Step 6

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.

TROUBLE-SHOOTING

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

VARIATIONS

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.

On steeper paths the use of anchor bar stones, or stone-pitching below the bar can stabilise the structure.

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 TASKS

Waterbars 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

next » 2.4. Stone Cross-drain