Things to Be Considered When Designing a New Forest Road

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FOREST ROAD DESIGN

Roger Hay ¹

1 Engineering Consulting Co., United Kingdom.

Abstract

Without road access most of the production of a forest would not be available. The construction of a forest road has the most potential of any forest harvesting operation to cause damage to the amenity and environment of the forest and, yet, is a most essential part of the strategic and tactical harvesting and other forest operations. This paper examines the impact of road construction on the environment of the forest and suggests ways in which the effect can be reduced and environmental benefits could be obtained.

Forest road design

It is the need to fit these two contrary requirements that are the core of any forest road manual. It as necessary to ensure that all of the elements which go into the successful combination of:

· a road which is right for the harvesting operation;

· a road which is capable of safely carrying the haulage traffic;

· a road which does not interfere with the natural drainage patterns;

· a road which does not damage the landscape;

· a road which takes account of the flora and fauna;

· a road which can be satisfactorily and economically maintained.

are included.

To do this requires an input from those involved with the preparation and implementation of the strategic and tactical plans for the forest area. They will include specialists in forest planning, landscape, conservation, harvesting, and engineering and may have additional input from fishing, game and soil specialists where required.

All of these people are experts in their own field but, unless they understand the reasons for the views of others, the various inputs could be counterproductive and result in a less than optimum solution.

As an engineer it is my purpose, in preparing a code of practice for the use of those who specialize in road building, to make sure that the principles of good road design and construction are paramount, but to take on board the need to have:

· good landscape design;

· road drainage design that deals adequately with demands of erosion prevention and fishing requirements, but does not compromise the absolute need to drain the road effectively;

· road embankments and cuts which allow harvesting to take place without hindrance but which remain structurally stable;

· cleared road widths which are minimal consistent with good road design and the ability for the sun and wind to reach the road to assist in drying;

· the possibility of using the construction of the road to enhance the conservation environment. The change in soil structure arising from road building allows the growth of species which require that soil and the light arising from the tree felling on the road line. Wildlife can also be encouraged with the changes in vegetation at the road edge.

Good quality roads can add to the recreation potential of the area by allowing easier walking and cycling in parts of the forest previously closed and can allow easier game retrieval.

By providing a code for such work, which can be discussed with and agreed by all of the participants in forest strategy, the need for discussion at the implementation level is reduced. Those who are actually building the road know the criteria on which they should proceed and against which they will be judged.

The increasing concerns of conservation bodies and the general public to changes in their environment (and building of forest roads is certainly that), requires a framework in which the engineers who build the roads can operate and a Code of Practice which has been agreed and into which the input of engineers has ensured that the fundamental engineering principles have not been compromised is a way forward to prevent friction and to allow foresters to maximize their asset utilization

There is always a tension between the natural desire to maximize the benefits of having an access road and the need to keep the basic engineering design in place. The use of tertiary roads for timber extraction is a case in point. To have any real purpose these roads must be very cheap to construct and are likely to be a bulldozer cut and not much else. The unfortunate consequences of poor drainage from such a track can be as serious as for a primary road and the lack of proper geometric design and surface metal compounds these problems. It is therefore questionable whether the real economics of poorly designed and constructed secondary and tertiary roads are fully judged. The difficulty is that once constructed it is extremely expensive to reinstate the pre-road situation and the criticism of "outsiders" is impossible to counteract.

Selection of the route corridor

Landscape appraisal

When choosing a road line and surveying potential routes, an appraisal of the landscape issues that may be involved should also be carried out. There may be views onto the area to be roaded which may be important - major settlements, tourist routes, long-distance footpaths for example. These may mean that more than usual care needs to be taken over the location of the route, the effect of road line cut-and-fill.

Photographs may be useful to check what factors are important and eventually used to illustrate options for route corridors. Views put from the road line which may eventually be used as a recreation route should not be ignored and their potential should be identified wherever possible. Natural features of interest can also be identified and the road line, where possible, routed to take advantage of them.

Landscape, habitats, heritage and local opinions should be considered at all stages of road development, maintenance and reconstruction as well as the requirements of operational access. Forest roads create lines in the forest, which must follow the principles of landscape design if they are not to appear intrusive.

General concepts

The more natural qualities of the landscape should be reflected in careful planning of the general route, in design of the alignment and in detailed design and management of cuttings and embankments. The intention is not to disguise the road and other structures completely but to reflect both the scale and forms of the landscape in their design by following landform in road alignment and re-establishing vegetation on large areas of excavation and fill.

Where forest roads are planned and constructed to take account of their environment as well as operational needs from an early stage, the additional costs are often modest and the environmental benefits can be substantial. Mistakes can be very expensive to put right.

Visual impact

Roads crossing unplanted ground and young trees are highly visible in the broader landscape. Although roads constructed through older trees only appear initially as shadowy lines in the trees they are eventually revealed by felling. The geometry of the road, bare areas of fills, large-scale cuts and associated bridges and culverts all increase the artificial qualities of the forest especially in close views.

Additional effort is needed where roads cross open ground, e.g. for access across farmland, and the visual impact should be considered prior to negotiations for purchase. It may be less appropriate for verges and embankments to become colonized by vegetation in these locations than in the forest, and finding less visible routes is important.

Route selection

The general location of forest roads should be planned to avoid important habitats and appear unobtrusive in the landscape. In agricultural landscapes access roads can be hidden behind hedgerows, dykes and walls or in hollows screened by landform.

Landscape of particular quality or genius loci should be avoided, for example focal views, water edges, and waterfalls; watercourses of particular quality should be crossed at the least visible point. High standards of design and construction must be achieved in these sensitive locations and measures taken to reduce visual impact as far as possible.

The visible parts of the road should be in scale with the landscape for example, by not running close to the skyline for long stretches and avoiding large cuts and areas of fill in narrow valleys. The general alignment should run diagonal to the slope and not horizontally, as far as technical constraints permit.

Roads should cross skylines as near as possible to the lowest point or in a slight hollow. The steeper slopes and summits of hills should be avoided. Where there are a number of possible routes, they should all be illustrated in sketches from main viewpoints and compared.

Design of the vertical and horizontal alignment

Like other lines in the landscape, the shape made by a forest road has a major effect on its appearance. The line should curve gently from side to side and up and down the slope. Straight lines in either dimension tend to cut awkwardly across landform and generally horizontal alignments look very artificial. In order to blend with landform, roads should be inflected gently downwards on convex slopes and rise slightly in hollows and valleys.

In flat landscapes, roads have tended to be straight in the past. This can lead to awkward and intrusive visual effects and can be very daunting to walkers. Curving alignments work better and look more natural, taking advantage of any landform there is present.

Landings and turning points, which require additional space, should be located as far as possible where natural gradients provide space and should not be positioned prominently, e.g. on rides and spurs. This is particularly important on steep slopes where large areas of fill would be needed.

Treatment of the road line scar

Cuttings and areas of spoil produced by road construction on steep slopes often appear intrusive because of their geometric shapes, light raw colours and large scale. Alignments should be planned to minimize cut and fill to reduce visual impact as well as lowering road costs.

In soft rocks areas of cuts should be made into more rounded banks while in hard rock it is better to imitate natural crags. Occasionally an irregular combination of these two treatments can be used to break the artificial line of a long cut.

In softer rock natural processes tend to erode the top of the slope back into natural levels until the angle of repose is reached while the toe of the slope is extended by the collection of eroded debris. Where slopes are eroding beneath overhanging turf, the line of shadow appears particularly intrusive as it runs generally parallel to the road. In sensitive landscape road cuttings should be made less intrusive by imitating a more natural profile with a curved section at top and bottom. The intervening slope should be as near to the angle of repose of loose material as possible. These measures allow vegetation to become established on the slope and remove the intrusive shadow of overhanging turf.

It is not necessary to completely cover cuttings with vegetation except in the most sensitive landscapes but rather to create more natural shapes in scale with the landscape. Additional planting of trees and shrubs may also be necessary to reduce the scale of cuttings further where ground conditions permit. In the most sensitive landscapes turf may need to be fixed on banks with netting.

When cuts are made in harder rocks precisely even faces should be avoided but strongly irregular shapes more similar to natural outcrops adopted. This allows the natural bedding of the rock to show through, introduces more variations of light and shadow in the surface and provides ledges where debris can collect and plants become established. In sensitive landscapes this process can be artificially accelerated by turfing raw rock ledges.

Some geological formations exposed by road making are of great scientific interest or support unusual flora and should not be modified without wide consultation.

Because of the expense and technical difficulty of establishing vegetation on spoil produced by road making, care in design will not only produce more natural patterns but can also save money by identifying which are the most cost-effective areas to plant. Seeding an area of spoil completely with grass may simply turn an intrusive shape and scale from grey to bright green. Unless the surrounding vegetation is identical in colour, the visual effect is unlikely to improve significantly. Irregular shapes of darker vegetation, e.g. heather, bracken, gorse, sallow which extend across the spoil and into the natural herbage are far more likely to be effective. An interlocking pattern of shapes will also help to integrate the road line with the landscape.

Where spoil from road making is too coarse and open for plants to grow, occasional pockets of finer material should be incorporated into banks near the surface so that turfs, bracken rhizomes, shrubs or even trees can be planted. Wherever possible the impact of roads should be reduced. Where there is no space for storage, every opportunity should be taken to transport these materials directly from where they are being stripped to completed areas of fill.

Culverts

Culverts to take water off the road can have quite important effects on hydrology and water quality of nearby streams. Silt traps may be needed in some areas and roads should be located away from streams.

Culverts to carry the road over a stream need care in the design of the surrounding area, e.g. the road embankment. Where gabions or similar retention is required massive vertical faces should be avoided in favour of stepped profiles with the gabions set at angles rather than in straight rows to emulate more natural rock formations and to promote vegetation growth.

Road materials

Wherever possible road materials should be of local origin, ideally from the same location. This will ensure that colour and texture blend with the surrounding landscape. Bright coloured or white stone can look very intrusive in many landscapes.

In some instances where forest roads cross-areas of particular value to nature conservation, certain kinds of rock may cause serious harm. The use of limestone over an acid bog is one example. If there is any doubt about this then expert opinion should be sought before going ahead.

Maintenance

The verges of forest roads are often colonized by a wide range of plants, which in turn may support a wide and often locally rare range of animals, especially insects. Care is needed during maintenance of the verges to avoid damaging this. If possible the read profile should be designed to allow a proportion of vegetation to remain while some is lost at certain times. In some places maintenance can be happily carried out once plants have seeded and insects gone dormant or moved on to other hosts for the winter. Mowing of vegetation after seeding is to be preferred to chemical control.

General issues

Care in working practice at road building times is needed to avoid undue disturbance to animals and humans e.g. bird nesting periods and popular times for recreation. Care is also needed to avoid spillage of petrol, oil and lubricants near streams, ditches or culverts.

Conclusion

As a civil engineer I am anxious that the long understanding of good road design practice is followed in the design and construction of forest roads. The forest engineer who sets out the alignment of the new forest road must be aware of the requirements of all of the parties interested in the site, and the Road Manual or Code of Practice must contain sufficient advice to allow the engineer to be able to meet these requirements, in the main without further reference. It is my contention, however, that the use of good engineering design and practice will meet the demands of the conservation and environmental lobbies in the best way.

References

Bell, Simon. 1993. Elements of visual design in the landscape. E & F Spon. London.

Forestry Commission. Civil engineering code. Great Britain. Unpublished.

Forestry Commission. 1994. Forest & water guidelines. HMSO. London.

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Things to Be Considered When Designing a New Forest Road

Source: https://www.fao.org/3/x0622e/x0622e04.htm

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