Bio-barriers are structures that incorporate planting as an integral part of their design. They are being researched and developed across Europe, particularly in theNetherlands. Some early bio-barriers proved unsatisfactory for a number of reasons, such as the need for maintenance and irrigation, but newer designs have addressed these problems. In the UK, however, there is continuing resistance to bio-barriers precisely because of these early teething troubles and the need for continued maintenance (Figures 5.68 and 5.69).
The question of irrigation and maintenance raises fundamental concerns aboutattitudes towards barrier provision. Where the decision has been made to provide a barrier to meet legal or design objectives, the appropriate barrier should be chosen and the concomitant maintenance must be regarded as an essential part of the scheme. The need for periodic maintenance should not inhibit this choice. A range of natural-looking bio-barriers has been developed which offers an alternative to earth mounds. These have the advantage that they do not require the space needed for a mound, in effect creating a living barrier on a narrow strip of land (Figure 5.70). As well as reducing land-take, these bio-barriers act as wildlife corridors creating habitats for small mammals and insects.
Experience has shown that the successful appearance of a bio-barrier depends on:
› compatibility of plant species with soil conditions and type (soils must be
› analysed for fertility, acidity, salinity, contaminants, organic content and
› drainage);
› density of planting (plants should not compete with one another);
› provision of irrigation or watering during plant establishment;
› provision of irrigation and watering through dry periods;
› establishment of an appropriate plant maintenance regime, including weed control, pruning, application of fertilisers and replacement of dead plants.
For ease of categorisation, bio-barriers may be divided into four generic types,
the names of which reflect the main structures or principles of the design:
› A-frame and vertical corten steel bio-barriers;
› box wall bio-barriers;
› woven-willow bio-barriers;
› stack and crib bio-barriers.
Note: In many situations the planting medium will tend to dry out when there is little
rainfall. Supplementary irrigation may then be required. A-frame and vertical corten steel bio-barriers The A-frame barrier consists of two slightly corrugated corten steel sheets which are splayed at the base, anchored to the ground with treated timber staves and angled to form an apex. The corten steel, which forms an anticorrosive rust on the surface to protect the inner core of the steel, acts as a reflective barrier and is expected to have a useful life of more than 20 years. Plant material is placed immediately adjacent to both sides of the barrier at regular intervals and trained up it using loose rubberties. Care is taken that the plants do not scrape or chafe against the plates in the wind. Before the planting is established, and during the winter where deciduous plantin is used, the steel plates give the barrier a rust colour. This natural colour blends easily into rural/semirural areas. In summer and once the planting is established, the steel is mostly screened and the barrier gives the appearance of a dense, tapering hedge (Figure 5.71 and see Figure 5.68(a)). Thus, deciduous planting allows the appearance of the barrier to change colour according to the season, in keeping with its surroundings. Planting can be varied according to the location and species such as willow, alder, ash, field maple, privet, lime and ivy have been used in the Netherlands. The vertical corten steel barrier comprises a single sheet of corrugated corten steel which may be placed vertically or splayed slightly. The steel plates are supported by a timber frame to the rear. Planting is usually placed on both sides of the barrier, screening the steel sheets on the front and the frame to the rear. Care should be taken to provide a stable structure as any movement disturbs the plants’ roots and inhibits growth (see Figure 5.68(b)). Both A-frame and vertical barriers are installed by Mostert and De Winter in the Netherlands and are registered as ‘Geluid Groeischerm’ (Soundproofing Growing Screen).
In the Netherlands, willow, the most commonly used species with this type of barrier, provides a suitable biotope for some insects. The suitability of willow species, however, should be assessed according to the ground conditions. In areas of low rainfall, irrigation may be needed. Some willows, too, may be sensitive to salt spray and salt in the ground, and susceptible to some diseases and pests, especially in the initial stages of growth. The issue of spray from roads that have been de-iced using salt and the resulting accumulation of salt in the soil is an important consideration for all roadside and barrier planting. Finally, although these barriers are welcome additions to the designers’ portfolio, they are relative newcomers only and there are still a few problems associated with structures, fixings, the long-term establishment of some planting, and damage to plants in high winds and other extreme weather conditions. Close planting is not an option since subjects fail due to root competition. Maintenance of the planting and its fixings is an issue complicated by the choice of plant species, the need to train or prune some subjects and to maintain the form of the original design.
Filed under: Uncategorized, Barriers, BioBarriers, Data, Enviroment, Highway, Materials, Reaserch
