Cut-offs are bars or dips constructed across paths on slopes to divert any water flowing down the path to run off at the side. They are also known variously as diversion drains, waterbreaks, jarnocks, by-sets, grips, waterstops, and waterbars.
The usual method of making a cut-off is to embed a bar of stone or wood across the path, so that it catches the water on the uphill side, and diverts it off the path. Various styles are used depending on the materials available, the slope, and the preference of the builder. A channel is made on the upper side of the bar, and the bar can either be flush to the path surface, or stand proud, as shown. A bar flush with the surface is usually preferable as it does not present a step to uphill users. Most cut-offs need periodic maintenance to clear them of debris.
Cut-offs can also be made by the same method as a cross- drain, either of stone, or installed ready-made from wood, clay tile, slate or concrete. These type of cut-offs need frequent cleaning as they block with debris.
Stone cut-offs and cross drains are an integral part of restoring eroded rocky slopes and building stone pitched paths, which is a method of building paths on slopes by embedding stones to form the path surface. As well as the details below, further information on drains for pitched paths is included in Chapter 8 – Surfacing.
Position
Look carefully along the edge of the path for fans or scatterings of pebbles or silt, which indicate where water flows off during high rainfall. The path immediately above will probably show signs of gullying, where the material has been washed away. Look uphill to try and find whether the water is coming from a particular source, or whether it is from general run-off from the slope and path. If it’s the former, it should be possible to divert the water from flowing onto the path, or to take it across the path at a higher point. Where a source is not identifiable, cut-offs will be necessary. Install one at the point where gullying begins, and some more at intervals of 3m to 5m down to the point where water flows off.
Choose a position where there is already a slight dip, or where there is a boulder or tree root against which a barrier can be wedged. Make sure there are no obstructions off the edge of the path which will block the flow of water, but beware of directing the water off a steep edge of unconsolidated material, as it will quickly erode.
A cut-off should be installed just above the angle of a bend. Take care that the outflow does not look like a gullied path, or walkers may follow it. A short, deep trench should take the water off without encouraging people to follow.
Angle
The angle should be set so that the cut-off is self cleaning, and neither erodes nor fills with debris. The angle will depend on the steepness of the path, and the lie of the ground to either side of the path. An angle of 45 degrees to the line of the path is average, decreasing as the path becomes less steep. The gradient of the channel should be a minimum of five degrees.
Height
The bar must be high enough to divert the flow, but not so high that walkers regard it as a barrier, and try to go around the ends.
In general it is best to put in a higher number of cut-offs of a lower height on any given stretch of path. This spreads the load of water and minimizes the sections between each cut-off where erosion of the path surface can occur. Walkers are less likely to walk around the ends where cut-off height is low. In these situations, a height of 75-100mm may be suitable.
In other situations where flows are greater, or there are only a few points where cut-offs can be installed, the height will have to be greater. To be secure in the ground, the size of the stone or timber will have to be larger. These are likely to be paths which require stone pitching to remain stable, and where a cut-off height of 125-175mm will not present a barrier, compared with the roughness of the adjacent path.
Most styles of cut-off are not visible from below, but merge into the profile of the path. Where the barrier does stand proud, it should be no more than 50mm from the path surface on the lower side.
Length
Extend the cut-off by about 500mm on either side of the path, to discourage walkers (and mountain bike riders) from going around the ends. The extended ends can be raised, as further discouragement. However, beware of extending the cut-off too far on the upper side of the path, so that water is caught unnecessarily. The extended ends can be protected by mounding soil and turf, or by boulders.
Where possible, it is best to allow the water to drain naturally away from the end of the cut-off, preferably into a flattish area, a non-eroding boulder field or a natural watercourse. Avoid digging ditches at the ends of the cut- offs, as these form a potential site for erosion, as well as looking unsightly on open hillsides. Cut-offs may accumulate a delta of debris at their lower ends, which will need clearing or returning to the path if it begins to block the cut-off, or smother the vegetation. If a delta is not accumulating, watch out for signs of erosion at the end of the cut-off. Extra cut-offs or other drainage may be necessary to reduce the flow.
Where necessary to join with existing watercourses, or to direct the water away from lower sections of the path, dig a ditch about 300mm wide with as steep sides as possible, and at least a metre long. Where suitable stone is available, this can be stone-lined.
In an emergency, when water is flooding down an unconsolidated path, it is worth using any tool available, spade, stick or boot, to dig out a rough drain and to try to divert the water off the path.
Stone cut-offs
Stone is the most commonly used material for cut-offs, due to its advantages over other materials (see above).
The preference is to build with fairly even size and shape stone, of at least 100mm thickness, and about 400mm depth. The upper edge needs to be flat, so that the top of the cut-off can be as even as possible, for maximum water catchment.
The channel on the upper side should be pitched with stone, to prevent scour and facilitate cleaning. The size of stone for the channel would normally be smaller and less regular than those for the cut-off bar, but will depend on what’s available.
The general procedure for building a cut-off is as follows:
- Select suitable stone for the cut-off bar.
- Mark the line of the cut-off, and dig a trench to allow the stone to be set in 75% to 100% of its depth, relative to the original ground level. The choice depends on the treatment of the path above and below the cut-off, and the preference of the builder. Keep the trench face neat on the lower side, and allow sufficient width on the upper side for the channel stones.
- Starting at the lower end, set the cut-off stones in the trench, so that the top edges are as even as possible.
- Starting at the lower end, set in the channel stones, packing tightly as necessary with smaller stones to give an even finish.
- Grade the path above and below as necessary to give an even finish.
Depending on the slope and surface of the path, a section of stone pitching may be needed above and below the cut- off to protect it from erosion. The general procedure is as follows:
- Select suitable stone for the cut-off bar.
- Dig out the area to be pitched above and below the cut-off.
- Mark and dig the trench to the required depth for the cut-off bar.
- Set in large ‘key stones’ at the downhill end of the pitched section, to securely anchor the section.
- Set in the stones for the cut-off bar, keeping the top edge as even as possible.
- Pitch the remaining section below the cut-off, wedging the stones in to make a tight fit.
- Pitch the section above the cut-off, working upwards.
- Brush the pitching and tidy the site.
For more details on pitching, see Chapter 8 – Surfacing.
Where the stone is not quite so regular in shape, a staggered row can be made with the stones overlapping. Where only thinner stone is available, a double staggered row can be built.
The channel stones can be set in various ways, again according to size of stone and personal preference.
Old fence slates or specially cut pieces of slate can be used for the bar where available. They should be set in with a stone channel as shown above.
Timber cut-offs
Sleepers or other heavy planks (175mm x 50mm minimum) can be used for cut-off bars, held in position by stakes. Use stakes of timber 50mm x 50mm, knocked in on the lower side only of the plank. Weather the tops of the stakes, and nail them to the plank. Use square or half-round stakes in preference to round, as they give a stronger joint.
Sleepers can be nailed, but are better secured by angle iron or metal pins. The strongest anchorage is made by drilling the sleeper with an auger and bit, and knocking the metal pins through it. Alternatively, the timber can be drilled part-way, and then positioned over the accurately secured pins. If using angle iron, nail it to the lower corners of the plank with 50mm galvanised clout nails.
Round timber
These are the type of cut-off commonly built on trails in America, where timber is abundant.
- Peel the log with a peeling spade or light axe, as the bark increases the speed of timber decay if left in place. Stakes can be made of timber up to about 75mm diameter, cut into 450mm lengths.
- Dig a trench to a depth of half the diameter of the log, and a width of double its diameter.
- Notch the uphill side of the log to accept the stakes so they will not clog the cut-off.
- Seat the log securely in the trench, and drive the stakes in at an angle to hold the log. Saw off flush any extra that cannot be driven in.
- Score or recess the top of the log to give a rough surface that boots will grip.
- Pack excavated soil and rock on the lower side of the log.
Boulders placed at either side will give extra strength and discourage walkers from going around the ends. Seat the boulders in carefully so they appear ‘natural’. Dig the outflow ditch.
Cut-off drains
Cut-off drains can be made out of wood, slate or stone. The method of construction is the same as for a cross drain, and is described on page 68. Pre-cast concrete and clay channels are also available. The main advantage of cut-off drains is that they are flush with the path surface and so present no barrier to the walker. Concrete cut-offs should be used on paths that are designed for wheelchairs and prams, as their narrow opening prevents the wheels getting stuck. Concrete cut-offs need clearing out frequently.
