These barriers are built to reduce the erosion of slopes and gullies caused by trampling, in order that vegetation can re-establish. It is essential to keep walkers off the slope or gully, and therefore any scheme must include the building of a new path which takes a more resistant line, together with fences and publicity to encourage walkers to keep to the new path.
Gully erosion
Gully erosion can occur on slopes of sand, chalk, or on much harder rocks in mountain areas.
The gullies considered here are those caused when trampling up and down a narrow zone destroys vegetation, exposing the thin soil and underlying rock to erosion by water. The basic method of repair involves installing barriers of wood or stone across the gully.
In lowland areas of chalk or sand it is also possible to install drains to divert water from flowing down the gully. In upland areas, the high rainfall, high run-off, large catchment areas and the lengths of the gullies makes the construction of drains impractical.
Erosion barriers are built in a similar way to revetments, but as they will eventually be partly or wholly buried, the appearance is not too important. The main requirement is for a simple and robust construction which can be easily positioned and secured. Working on steep and eroded slopes is very difficult, tiring and can be dangerous, and the work should be planned in such a way that the barriers can be positioned with the minimum of fuss.
The need to fill behind the barrier and grade the slope depends on the situation and the resources available. On very steep and rocky slopes it may be impractical to do
anything more that let the barriers fill as material moves down the slope by natural erosion, or by the action of walkers. Further barriers can be built on top as necessary, until the gully is filled. The resulting slope should then be stabilised or revegetated (see below), and walkers kept off it. Where the gully reaches to the top of the slope and there is no way that material can naturally fill it, the barriers must be back filled and the slope graded by hand or machine. In other places grading may be necessary to speed the restoration process, or for cosmetic reasons.
It is usually best to start placing barriers from the bottom of the slope, so that any material dislodged during construction is trapped by a barrier below.
Stone barriers
In shallow gullies on eroded hillsides, low stone barriers can be made by building rough walls from nearby stone. Neat stone walls tend to attract walkers, so pile the stones up roughly to look as unobtrusive as possible. Associated drainage work, stone pitching or steps must be done to divert people and water away from the eroded gully. If backfill is available, the gullies can be partly filled and seeded as desired. Where backfill is not available, the gullies will have to be left to stabilise over time.
Wooden barriers
Simple barriers made of the easiest obtainable material are suitable. Fencing stakes with planks of about the dimensions shown are appropriate. Pressure treated timber is not essential, as the barrier should be buried and the slope stabilised before the timber rots.
Locally cut timber can be used where available, with round timber used instead of planks. Where a wooded slope is desirable, and willow is locally typical, freshly cut timber can be easily rooted. Use branches up to 100mm diameter, knocked into the ground the same way as they were growing.
Grassland slopes
On grassland slopes, and especially on ancient monuments, barriers must be carefully constructed and backfilled so that the repair merges invisibly with the rest of the slope. Many techniques have been tried, but the simplest methods appear to be the best. The following advice is based on very successful restoration work done at Badbury Rings, Dorset, a chalk downland hill fort. Scrub encroachment over the hill fort and heavy visitor pressure had resulted in the formation of many gullies, one to five metres wide, up to a metre deep, and about five metres long from top to bottom of the chalk embankments.
The barriers are set very close, about 300mm apart, with the ends matched to the terracettes or sheep tracks. The ends are butted up to the edge of the eroded area, with as little disturbance as possible to the uneroded edge. The tops of the stakes show at ground level, but soon get covered with grass and eventually rot. The barriers are backfilled with chalk, and then the top is covered with molehill soil gathered from nearby. This is carefully spread no more than a finger thick. Any thicker than this, and the growth of rank grasses and bramble is encouraged. The molehill soil contains an ample seed-bank, and germination is successful at most seasons, though slow in summer. No seed is bought in, and turf is not used. In the past, turf was cut from nearby to repair gullies, but although the turfing was successful, the source area took many years to recover.
Repairs are made as soon as bare patches or ‘pigeon holes’ develop. Work is done as soon as the need arises, at any season. Repaired areas are covered with polypropylene netting to keep sheep and people off. If there is a problem with children using the netting as a slide, this can be discouraged by sticking branches into the ground. More stout barriers such as sheep hurdles tend to be self-defeating, as the sheep use them as rubbing posts.
Gully barriers with drainage
This scheme, at Cannock Chase, Staffordshire, involved the construction of a complicated series of erosion barriers and large-scale cut-off drains up a 100 metre long sandstone gully with a slope of 1 in 2.3. The barriers were made of sawn timber (green oak) and pine logs cut locally. They were secured by wooden stakes where possible, or by metal pins where the rock was resistant.
Slope erosion
Eroded slopes occur where trampling is spread over a wide area, and flow of water is not concentrated in gullies, but extends over the whole slope. In some situations the slope will naturally stabilise and re-vegetate if a new path is provided and people keep off the slope, but in serious cases erosion barriers may be needed to stabilise the slope long enough for vegetation to take a hold. Other methods of stabilising slopes are described the remaining pages of this chapter.
Wooden barriers
Wooden barriers of the type described above have been used in several areas to stabilise slopes, either set in lines across the width of the eroded area, or randomly across the slope.
At Box Hill, Surrey, severe trampling through scrub on a steep chalk slope had resulted in a wide zone of erosion (Mackworth-Praed, 1991). The aim was to repair the slope with wooden barriers, backfilled with chalk and topsoil, and allow the regrowth of scrub. The eroded zone was fenced to prevent access, and walkers were channelled up two flights of steps at either end. At the top of the eroded section, a sawn timber revetment was first put in to support the track above. Two different methods of repair were used.
On the first section, about 1000 stakes 500mm long, cut locally from any available wood, were inserted in roughly straight lines across the eroded area, by making a hole with a crowbar and knocking each stake in about 200mm. The lines were spaced 2-3m apart. Brushwood was then woven between the stakes. Topsoil was brought in by digger and dropped over the revetment onto the slope below, and then raked down to give a covering of about 200mm depth.
The second section was repaired by making higher barriers of round larch thinnings, instead of brushwood. These were backfilled with a much thicker layer of chalk topped with topsoil (about 500mm in total), to give an instant repair nearer to the original profile. Planting with purchased shrubs was not very successful, partly due to drought. The thick layer of chalk surpressed any natural growth from underneath.
The first method was cheaper and more successful, and remnant scrub quickly recovered. A decade later, the area with the thicker backfill remained fairly open.
Post and netting
These barriers were constructed at Arnside Knott, Cumbria, on a rapidly eroding limestone slope, and successfully stabilised the slope so that vegetation was able to re-establish.
Sediment traps
A sloping area eroded of its covering of peat on Pen-y- Ghent, Yorkshire, was repaired by building a series of low stone walls. The traps were lined with a semi-permeable membrane to trap peat and other sediments as they washed down the slope. As each trap filled, the wall was raised by a course of stones, and the membrane lifted over. Eventually the area will be regraded and seeded. To be successful, this technique requires a source of sediment, and also frequent attention is needed to keep the traps filling, and ensure erosion is not occurring around the ends of the walls.
A similar scheme (The National Trust, 1993), is to plant cotton grass behind low barriers of stone, to help trap peat which would otherwise be washed away. A semi-permeable membrane would not be used. Cotton grass, with the addition of lime and fertiliser, is able to colonise bare peat.
Transporting materials
Gully and slope restoration can involve the moving of a large amount of material for construction and backfill. The use of a Hymac or other machine with a skilled operator is the easiest method if the site is suitable. It may be possible to have the material dumped at the top of a slope, and then to move it by hand downhill. In the method shown below, the backfill is shovelled onto a ‘toboggan’ of heavy tarpaulin, which is then pulled at speed down the plastic chute by four nimble volunteers. It is not possible to simply push material down a polythene chute except on excessively steep slopes (1 in 1.5 or steeper). This method can be hazardous, because it must be done at speed. Care must also be taken that erosion or damage is not caused in one area in order to repair another.
Other methods include building chutes of scaffold planks, secured by metal pins or similar. The backfill will need to be pushed down the chute with shovels. Barrow runs, built across a slope, are described here. Builders’ tube, available from tool hire companies, is another possibility. On remote sites, helicopter airlifts may be needed.
If large groups of volunteers are available, it may be possible to move the material by small amounts in buckets and fertiliser sacks. There are sites where visiting walkers have been asked to carry up a small contribution. This needs checking from time to time to ensure that material is being put where you want it, and that you do not find yourself with an embarrassing surplus.
