DESIGN AND MANAGEMENT PART 2 HA 56/92 NEW ROADS PLANTING, VEGETATION AND SOILS SUMMARY

Transcription

1 DESIGN MANUAL FOR ROADS AND BRIDGES VOLUME 10 SECTION 1 ENVIRONMENTAL DESIGN AND MANAGEMENT NEW ROADS PART 2 HA 56/92 NEW ROADS PLANTING, VEGETATION AND SOILS SUMMARY This Advice Note gives guidance on the environmental design of planting, vegetation and soils for new roads. INSTRUCTIONS FOR USE 1. Remove existing title page, content page and General Preface page on the Goods Roads Guide series of Advice Notes. 2. Insert new title page. 3. Archive this sheet as appropriate. Note: New contents pages for Volume 10 containing reference to this document are available with HA 55/92. February 2001

2 DESIGN MANUAL FOR ROADS AND BRIDGES HA 56/92 THE HIGHWAYS AGENCY THE SCOTTISH EXECUTIVE DEVELOPMENT DEPARTMENT THE NATIONAL ASSEMBLY FOR WALES CYNULLIAD CENEDLAETHOL CYMRU THE DEPARTMENT FOR REGIONAL DEVELOPMENT* New Roads Planting, Vegetation and Soils * A Government Department in Northern Ireland Summary: This Advice Note gives guidance on the environmental design of planting, vegetation and soils treatment for new roads.

3 Volume 10 Section 1 Part 2 HA 56/92 Registration of Amendment REGISTRATION OF AMENDMENTS Amend Page No Signature & Date of Amend Page No Signature & Date of No incorporation of No incorporation of amendments amendments December 1992

4 Registration of Amendment Volume 10 Section 1 Part 2 HA 56/92 REGISTRATION OF AMENDMENTS Amend Page No Signature & Date of Amend Page No Signature & Date of No incorporation of No incorporation of amendments amendments December 1992

5 DESIGN MANUAL FOR ROADS AND BRIDGES VOLUME 10 ENVIRONMENTAL DESIGN SECTION 1 THE GOOD ROADS GUIDE - NEW ROADS PART 2 HA 56/92 THE GOOD ROADS GUIDE NEW ROADS PLANTING, VEGETATION AND SOILS Contents General Preface to the Good Roads Guide series of Advice Notes Chapter 1. Planting, Vegetation and Soils: Introduction 2. Screening with Vegetation 3. Off-site Planning 4. The Right Vegetation for the Countryside 5. Formal Planting and the Urban Fringe 6. Retaining Existing Vegetation 7. Establishing Woodland 8. Scrub and Tree Groups 9. Hedges 10. Grass and Heathland 11. Steep Slopes 12. Effective Tree and Shrub Establishment 13. Soils 14. Enquiries December 1992

6 Volume 10 Section 1 Part 2 HA 56/92 General Preface The Good Roads Guide GENERAL PREFACE TO THE GOOD ROADS GUIDE SERIES OF ADVICE NOTES Structure of the Guide 0.1 The Good Roads Guide is the name given to the series of documents contained in Sections 1, 2 and 3 of Volume 10 of the Design Manual for Roads and Bridges. The Guide is written in nine Parts each of which is published as an Advice Note. The Guide is written to be read as a whole. The Parts of the Good Roads Guide are as follows:- Section 1 December 1992 NEW ROADS Part 1 HA 55/92 Landform and Alignment Part 2 HA 56/92 Planting, Vegetation and Soils Part 3 HA 57/92 Integration with Rural Landscapes Part 4 HA 58/92 The Road Corridor Part 5 HA 59/92 Nature Conservation Part 6 HA 60/92 Heritage Part 7 HA 61/92 Contract and Maintenance Implementation Section 2 MOTORWAY WIDENING Part 1 HA 62/92 Environmental Design Widening Options and Techniques Section 3 IMPROVING EXISTING ROADS Part 1 HA 63/92 Environmental Design Improvement Techniques How to use the Good Roads Guide 0.2 Many of the design ideas put forward in Section 1 - New Roads are also relevant to the other Sections and cross references have been provided. 0.3 The first Chapter of each Part of the Guide reviews the issues and topics covered. The subsequent chapters deal with a particular topic. Within each chapter, the key issues are first listed and then discussed with illustrations drawn from roads throughout the UK. 0.4 The Good Roads Guide is not a step-by-step guide on how to build a road or a substitute for professional advice. It is intended to be used by the designer to help in the identification of areas and issues where careful consideration of environmental factors is required. The division of the Guide into Parts and the Parts into topics has been done to aid this process. 0.5 Environmental design of roads is a matter of respecting the special character of each individual location. The illustrations included show solutions devised to meet the requirements of specific sites. The use of standard solutions, irrespective of the location, is not appropriate. Implementation 0.6 The principles set out in this Advice Note should be taken into account in the preparation of all schemes for the construction and improvement of trunk roads, including motorways. 0.7 Where conflicts exist between environmental design, costs, engineering feasibility and safety requirements, and competing options are available, the Design Organisation will need to advise the Overseeing Department accordingly. Application in Wales 0.8 Requirements in Wales are primarily covered by the publications "Roads in Upland Areas: Design Guide" (published by the Welsh Office 1990) and "Roads in Lowland Areas: Design Guide" and "Rock Profiling and Vegetation Re-establishment" (both due for publication by the Welsh Office in 1993). This Advice Note supplements these Design Guides. Application in Scotland 0.9 The Scottish Office Roads Directorate endorses the practice given in the Good Roads Guide. More specific guidance is provided by the Roads Directorate's Landscape Officer The Scottish Office discussion document published in February 1992 "Roads, Bridges and Traffic in the Countryside" addresses related issues. 0/1

7 General Preface The Good Roads Guide Volume 10 Section 1 Part 2 HA 56/92 Application in Northern Ireland 0.11 The principles set out in this Advice Note are endorsed as good practice by the Department of the Environment (NI). The guidance will be taken into account in preparing schemes for the construction or improvement of all roads in Northern Ireland. Acknowledgements 0.12 The following photographs have been reproduced with permission:- Chapter 11 Page 1 Timber retaining wall with concrete piers, planted with willow previously published in Use of Vegetation in Civil Engineering published by CIRIA/Butterworths. Chapter 11 Page 2 Two photographs supplied by MMG Erosion Systems: Soil restored by honeycomb geotextile before hydroseeding and after. Photograph showing the use of fibre mat by permission of Richards, Moorehead and Laing Ltd, previously published in Landscape Design. Two photographs supplied by Comtec (UK) Ltd showing hydroseeding before and after. 0/2 December 1992

8 CHAPTER 1 PLANTING, VEGETATION AND SOILS: INTRODUCTION 1.1 SCOPE l This Part gives guidance on the environmental design of planting, and vegetation, and soil treatment for new roads. 1.2 MAIN ISSUES l The pattern and species composition of vegetation in the English landscape often changes dramatically over short distances. This pattern and species grouping needs to be reflected in design. Good practice in planting aims to integrate the road with adjacent landscape and not create a separate roadside landscape. l Planting and seeding need clear objectives. Design of woodland planting, for instance, should take into account amenity, wildlife or screening. Different types of grassland are needed for specific functions such as nature conservation interest or return to agriculture. l Soil conditions are the most important elements in establishing vegetation. If soil structure is damaged or if the wrong soil type is reinstated, it may be impossible to establish suitable vegetation. l Long-term management must always be considered in planting design. This has not always been the case on road schemes. By such practices as the use of high shrub densities and low tree densities at planting, maintenance can be concentrated in the most effective areas. l There is often public pressure for instant landscapes using large trees. Such landscapes are difficult to establish and are often unsuccessful. l Planting outside the highway boundary may be necessary to ensure integration with the surrounding landscape and this can be achieved by using the Department of Transport s off-site planting powers. 1.3 EFFECTS OF ROAD DEVELOPMENT ON VEGETATION AND SOILS l Direct loss of vegetation l Creation of unsuitable soil conditions and adverse gradients that are too steep for new planting. l Disruption of the existing pattern of vegetation in the landscape. l Indirect effects on retained vegetation through changes in drainage pattern and exposure of mature woodland to windblow. l Exposure of the road to views, such as those from properties, which require screening. l Opportunities for restructuring the vegetation of degraded landscapes. l Opportunities to reintroduce predominantly native planting in areas such as intensive arable land at the urban fringe. 1.4 DESIGN OBJECTIVES l To define areas needed for effective mitigation by planting. l To restore as much of the pre-existing pattern of field boundaries, woodland, heathland and moorland, as possible. Retaining land adjacent to the highway should always be considered, in order to provide integration with the landscape. l To establish a clear design objective and maintenance regime for each area of vegetation established. l To reinstate soil to the highest possible standard by stripping, storing and reinstating it in line with current best practice. l To ensure soil restoration using matching soil types wherever possible. l To mitigate secondary impacts on retained vegetation. 1.5 MITIGATION l Design for effective long-term maintenance. l Provide adequate conditions for plant growth through preparation of uncompacted soils to appropriate depths. Where these cannot be provided, an alternative solution should be sought. l Recognise the limitations of prevailing conditions: eg large trees cannot be established on compacted 1:2 embankments. l Use a high standard of maintenance for rapid establishment. l Design for easy maintenance. l Allow adequate areas for planting for specific purposes. For example, a tall screen needs to be at least 10 m deep. An eye-level shrub screen can be achieved with a minimum width of 5 m. l Respect the existing pattern of vegetation and use mainly native species in rural areas. l Design earthworks with the type and extent of planting in mind: the shallower the gradient the easier it is to establish vegetation. 1.6 STATUTORY BODIES l Within this Part, reference to the Department of Transport, English Nature, English Heritage and the National Rivers Authority should also be read as referring to the appropriate statutory authority or adviser for Wales, Scotland and Northern Ireland. 1/1

9 CHAPTER 2 SCREENING WITH VEGETATION 2.1 PRINCIPLE l Planted screens can be effective in restricting both views of the road and views from the road. Good design will achieve both of these, while reflecting local character, maintaining good views and creating a screen which is not itself an intrusion into the landscape. 2.2 KEY ISSUES l The most effective screen is one close to the viewer rather than to the object viewed. A screen at the bottom of the garden is more effective than one at the roadside. l A minimum thickness of 10 m is required for a tree screen which can be adequately maintained. A shrub screen needs to be at least 5 m wide. l Vegetation needs to be at least 4.5 m tall to screen heavy goods vehicles. l Plants vary greatly in their foliage density and branching habit. Hawthorn, for example, makes a good, dense screen; ash does not. l Variations in width and height are essential when fitting a screen into an established vegetation pattern. l When used in combination with walls or with closed-boarded fences, planting should reflect differences in the landscape character on the two sides of the screen. l Vegetation does not reduce noise significantly, but it may reduce people s perception of noise. l Best design practice provides screening while maintaining attractive, long-distance views. 2.3 LOCATION AND DENSITIES OF SCREENS Good practice: A52, Nottingham This dense, well-maintained hawthorn screens the settlement very well from this busy road 2.4 SPECIES FOR SCREENING For screening, useful trees are beech, holly, whitebeam, hornbeam and field maple. Hawthorn, blackthorn and dogwood are useful shrubs. Choice of species does, however, depend on the character of the surrounding vegetation: see for example, Ch SCREENING A ROAD CORRIDOR THROUGH DEVELOPMENT A2, Kent Typical screening problems where the road runs close to property A12, Essex Housing, road and railway are close together in a narrow corridor with little screening or planting structure Good practice Typical arrangement of a well-considered screen Improvement by screen planting 2/1

10 CHAPTER 2 SCREENING WITH VEGETATION 2.6 SCREENING WHILE MAINTAINING VIEWS A typical integrated design that maintains views 2.7 SCREENING FOR FLAT LANDSCAPES The problem: M40, Oxfordshire When this planting matures an attractive view, in which the road is not a dominant element, will be lost. It might have been better either to have accepted the view or to have broken it up with intermittent planting Screen on flat landscapes can be difficult. They often have very little vegetation cover, so that planting near the road merely draws attention to the problem. It is essential, therefore, to design planting around existing features. A17, Norfolk In such an open landscape screening needs to be based around existing tree groups and settlement Improvement 2/2

11 CHAPTER 3 OFF-SITE PLANTING 3.1 PRINCIPLE l Off-site planting is used to screen property and public places at some distance from the road. Designers need to be fully aware of their off-site planting powers and the maintenance required to get full benefit from this planting. 3.2 KEY ISSUES l Under Section 253 of the Highways Act 1980 the Department of Transport can enter into agreement with a landowner affected by a new trunk road for planting on his/her land at the Department of Transport s expense. After a minimum three-year maintenance period, the landowner is obliged to maintain the planting for 25 years. l Off-site planting is most often achieved on the land of people who wish to be screened from the road. Planting on third-party land where the owner receives no benefit from it can be difficult to arrange. l Off-site planting is an opportunity not only to provide mitigation by screening but also to integrate the road with the landscape, as discussed in Pt LOCATION OF OFF-SITE PLANTING Typical off-site planting benefits and constraints Points of good practice showing off-site planting used in a variety of ways to conserve or enhance landscape character Good practice: M40, Banbury Bypass shows off-site planting used to screen both property and public open space Good practice: M40, Warwickshire Off-site planting on the Burton Dassett Hills has conserved the character of a country park and provided screening for local settlement 3/1

12 CHAPTER 4 THE RIGHT VEGETATION FOR THE COUNTRYSIDE 4.1 PRINCIPLE l Before preparing detailed planting and seeding strategies (these are dealt with in Chs 7-13) it is essential to gain a clear understanding of the type of vegetation and species composition appropriate to the setting of the road. Integration with the landscape (see Ch 3), provision of nature conservation benefits and driver interest are essential considerations. 4.2 KEY ISSUES l Large-scale tree and shrub planting may not be the best landscape strategy for a new road. Where screening is not an issue, it may be appropriate to create grassland, heathland or scrub, which may be better suited to their surroundings. l Native species should generally be used in rural areas. In addition, they must be native to the region and occur naturally on the soil type adjacent to the road. For example, beech is not native north of the midlands and should not be planted in woodlands beyond this region, but it is widely used in designed landscapes throughout the country. Similarly, shrubs of limestone such as the wayfaring tree should only be used on these soils. l The choice of seed for wildflower mixes presents a very complex problem, and advice should be sought from the Wild Flower Handbook published by the Department of Transport as well as from English Nature. l Although many semi-natural plant communities contain a lot of species, others such as beechwood or heathland, have only a few. Good design must work with, not against, the composition of natural vegetation types and consider the use of natural groupings, not arbitrary mixtures, grids or matrices, in preparing planting layouts. 4.3 GRASSLAND Grassland is often the right vegetation type to fit the existing landscape and must be established as a site-specific type. Rank, coarse grassland is typical of much highway land and stands out in marked contrast to the surrounding agricultural land, rather than blending with it Good practice: M6, Cumbria Apt use of low-maintenance grassland on the right soil conditions has ensured a fit to the landscape, as well as providing nature conservation interest 4.4 HEATHLAND landscape 4.5 SCRUB Heathland is a rapidly-windling resource, easier to establish than many other vegetation types. The Department of Transport can therefore make a positive contribution to nature conservation and character in the right circumstances. Scrub communities can be just as varied as woodland, but they can also be large, uniform areas of common species like hawthorn and blackthorn. Careful appraisal of local conditions is needed and arbitrary introduction of species is not good practice. 4.6 WOODLAND Most woodlands are products of either a long period of management of self-regenerating trees and shrubs or of deliberate planting. They do not have a natural distribution of species. New planting is an opportunity to create a more natural woodland type which will give a special character to the area and be of high wildlife interest. The important points to remember are: l oak is a common woodland tree but is not the dominant species it is often believed to be. Small-leaved lime, ash, field maple, and other species were originally equally common in some areas. Good practice dictates that the right range of local species is established for a project by consultation with English Nature and other bodies; l natural woodland structure is a mosaic of groups of the same species responding to local changes in soil, topography and drainage. Beech woodland, for instance, is almost entirely dominated by one species, while ash with field maple woodland is more varied. 4/1

13 CHAPTER 5 FORMAL PLANTING AND THE URBAN FRINGE 5.1 PRINCIPLE Establishing avenues in rural areas l Most inter-urban roads run through the countryside where they need to blend with their surroundings in the ways illustrated in Chs 2, 3 and 4. However, three are some situations where a distinctive roadcorridor landscape, or the use of non-native species, is appropriate because it provides a sense of place and a distinctive style. 5.2 KEY ISSUES l An avenue can give a distinctive character to a road corridor. Large trees should be placed at least 7.5 m away from the edge of the carriageway. l Bold planting can give structure to the urban fringe and bring character and a sense of place to urban backlands. l Well-sited planting can highlight landmarks, signal the approach to settlement and focus the driver s attention. l Good design can signal the change from rural to urban environments. 5.3 AVENUES Oxford ring road For roads through cities, avenues give a distinctive character to the roadside landscape Good practice: A40, Buckinghamshire Near the historic landscape of West Wycombe Park this well-maintained lime avenue is entirely appropriate to its setting 5/1

14 CHAPTER 5 FORMAL PLANTING AND THE URBAN FRINGE 5.4 APPROACH TO SETTLEMENT 5.5 THE NEED FOR PLANTING Structure planting can buffer the road from development, improve the road corridor and structure views from the road: see Pt 4, Ch 12 M25, Waltham Abbey section Structure planting would have made all the difference to this road corridor Warrington New Town An adequate width of planting between the industrial units and the road provides a pleasant environment Western Avenue, A40 Planting on the roadside edge here would break up the scale of the built environment and focus views on the buildings of interest 5/2

15 CHAPTER 5 FORMAL PLANTING AND THE URBAN FRINGE 5.6 CHANGING TO AN URBAN ENVIRONMENT 5.7 BACKLANDS New roads can cut through the backlands of urban and suburban areas where structure planting may be necessary to improve the environment for both drivers and residents. Signalling change Planting at roundabouts (see also Pt 1, Ch 13) can signal change from rural to urban areas and provide landmarks. Simple ground cover, shrub and specimen tree planting is inexpensive to maintain Chesterfield Bypass, Derbyshire Urban planting: Milton Keynes More formal planting, using robust ornamental species tolerant of roadside conditions is appropriate where roads pass through urban areas. See also Chs 1 and 12 Improvement Planting of highway land and adjacent derelict areas could greatly improve the character of the road corridor 5/3

16 CHAPTER 6 RETAINING EXISTING VEGETATION 6.1 PRINCIPLE l Retaining existing vegetation can play an essential part in maintaining landscape character and local landmarks. Making the best use of established vegetation needs to be considered both when a route s alignment is being selected and in order to safeguard individual trees near the carriageway. 6.2 KEY ISSUES l Where trees are to be retained as landscape features, inspection by a qualified arboriculturalist is needed to ensure that the effort is worthwhile. In general, a tree should last at least 15 years after completion of the road. l Young trees and scrub require as much attention as mature trees. They may not be much to look at but they will develop to maturity well in advance of new planting. l Alignment near groups of trees and woodland edges must conform with the safe distances for development given in BS 5837:1991. l Retaining an existing tree near the road can be an expensive and time-consuming process. It must be tackled in the right way. l Where the edge has been taken off a wood, special attention must be paid to the effects of windblow and storm damage on the remaining trees. 6.3 ALIGNMENT NEAR TREES AND WOODLAND Careful alignment allows maximum use to be made of existing woodland, trees and hedges to improve the landscape setting of a road. 6.4 EXCAVATION AND TREES Most tree roots lie in the top 600 mm of soil and 90 per cent are in the top metre. Even very shallow disturbance can cause irreparable damage. Poor practice: A22, Sussex The root system has been severed by the road construction, damaged further and eventually suffocated by the construction of a bund Improvement It should have been accepted that it was not feasible to retain the tree and a new landscape scheme devised The principle The road has been aligned to fit within a landscape of mature parkland trees Good practice: A16, Boston, Lincolnshire A fine specimen tree was retained by good practice. It was necessary to construct a scaffolding cage to prevent damage during construction 6/1

17 CHAPTER 6 RETAINING EXISTING VEGETATION 6.5 INSTANT MATURITY 6.6 EXPOSING A NEW WOODLAND EDGE Where a road has to pass through a woodland the trees at the newly exposed centre, which are usually tall and slender with narrow crowns, can be blown over in strong winds. Good practice: A69, Brampton Bypass, Cumbria Mature trees have been retained within the working area of viaduct construction by co-operation between designers and contractors Poor practice: new residential road, Hampshire Exposure of the dense, leggy plantation has given a very unattractive result Good practice Careful thinning in advance of road construction has produced an attractive and more stable edge A23, West Sussex Mature trees have been retained in a wide central reserve, giving instant maturity to landscape: see also Pt 1, Ch 12 Points of good practice are llustrated here. Scarification can play an important part in preparing ground intended to develop as woodland edge 6/2

18 CHAPTER 7 ESTABLISHING WOODLAND 7.1 PRINCIPLE l The objectives of woodland planting for road development are to integrate the road with the landscape, to provide visual interest and to provide wildlife benefits. Commercial forestry objectives are not appropriate. Native trees and shrubs should generally be used. 7.2 KEY ISSUES l Trees and shrubs grow best in small blocks of the same species (this is the way they are found in nature), and not in apparently random mixtures. Planting plans must, therefore, create simple, realistic layouts that can be achieved and supervised by planting on a grid. l The structure and species composition intended for the mature woodland must be established as clear design objectives using species native to the locality. l There are several types of woodland structure. It is important to choose the one that fits in with adjacent woodland and is appropriate for the maintenance regime proposed. l Correct planting distances are essential for effective maintenance and good establishment. If trees are allowed to grow too close together they become very tall and slender-stemmed, with very narrow crowns. l Different species grow at different rates. If poorly-designed mixtures are used, some species will be suppressed and the wrong balance of surviving trees will result. l Successful establishment is achieved by the rigorous application of basic horticultural principles. See Pt 7, Ch VARIETY AND GROUPING 7.4 WOODLAND TYPE High forest has trees providing Coppice with standards This is Scrub woodland: scrub with a closed canopy with little or no the traditional method of English emergent trees. The scrub is shrub layer. Regular plantations woodland management with unmanaged and gradually such as those of poplar, or about 40 trees/ha and shrubs dies out as trees grow up. species with dense shade like cut (coppiced) on a 7- to 25-year This can be a very effective beech look like this and should cycle. It is appropriate in many means of woodland be imitated if they occur nearby lowland situations because it will establishment, creating marry in with existing woodland. significant wildlife interest The standard trees have single without major maintenance stems, and the coppice is multistemmed. implications. It is the method generally recommended for new woodland planting, with a ratio of 8 shrubs to 1 tree: see Pt 7, Ch 5 Good practice: M27, Hampshire A variety of species has been used. The numbers of individuals in each species group and the size of the clumps, are in keeping with the scale of the road. 7/1

19 CHAPTER 8 SCRUB AND TREE GROUPS 8.1 PRINCIPLE l The use of scrub and small groups of trees is often essential in softening abrupt edges of woodland planting and integrating the road with pre-existing landscape character. Clumps of trees can be an essential element for integrating the road with parkland. 8.2 KEY ISSUES l Scrub is an effective way of giving a more natural edge to woodland and increasing its wildlife interest. It is also the present vegetation character of a number of places which have thin, poor soils. l Tree clumps and single trees need particular attention for successful establishment and effective maintenance. l Intermittent planting is particular important for landscapes like downland and wetland, where large-scale planting is usually inappropriate. 8.3 SCRUB AND WOODLAND EDGES Scrub can be a significant element in softening new woodland when planted at the same time, or in softening newly-exposed edges. It can be maintained easily by rotational coppicing as set out in Pt 7, Ch 5. An example is shown below. 8.4 TREE GROUPS Tree clumps are necessary in the reinstatement or construction of parkland landscapes or in providing a broken edge to a woodland to fit in with local character, as shown below: Keeping local character: for this tree groups are best established by the traditional method ofplanting in a matrix of scrub which is eventually shaded out or removed. This reduces maintenance requirements and allows better establishment, thus: Planting with scrub Principles put into practice 8.5 INTERMITTENT PLANTING Maturing clump Intermittent planing is appropriate where a strong vegetation pattern would over-emphasise the road. Typical intermittent planting on an open landscape with sparse vegetation Good practice: Using scrub to soften a woodland edge Successful intermittent planting in the open landscape of the Pevensey levels A27, Brighton Bypass Here hawthorn scrub is a longestablished vegetation type. New scrub is the most appropriate planting to absorb the road into the landscape 8/1

20 CHAPTER 9 HEDGES 9.1 PRINCIPLE l Hedges can be essential for integrating the road with agricultural landscapes. They need to be planted and managed in a way that reflects local character. 9.2 KEY ISSUES l Hedges are normally provided as part of accommodation works by negotiation with the farmer. Their layout should ensure that he/she can maintain them easily employing methods used elsewhere on the farm. l Hedgerow trees are being lost gradually in many parts of the country. Roadside hedges offer an opportunity to provide replacements and enhance landscape character as a result. l Species composition should reflect that of neighbouring hedges. Although overgrown hedges of mixed species may be appropriate in intensively farmed areas, hedges in many areas are hawthorn-dominated. l Low machine-cut hedges have very little wildlife interest and the emphasis in intensively farmed landscape, where these are predominant features, should be on integration with existing landscape pattern. 9.3 ESTABLISHMENT Cultivate a strip 600 mm wide and 300 mm deep at a distance of 300 mm from the fence line and incorporate a 75 mm layer of bulky organic matter. Plant in two lines 300 mm apart to form a staggered, double row. The transplants in each line should be 450 mm apart, giving a total of five plants per running metre. 9.4 FENCING A standard post-and-rail fence is unnecessary and inappropriate where a hedge is to form a boundary. Light metal posts and stockproof wire are the best solution and allow the hedge to grow through the fencing on the highway side. On the farmer s side it will usually be necessary to put up a temporary stockproof fencing to allow the hedge to become established. 9.5 OVERGROWN HEDGES Hedges are normally best planted outside highway land as accommodation works (see Ch 3). However, overgrown hedges with mature trees are frequently found in the countryside and maintaining them can be a good way of integrating the road with the landscape and of screening property. Integrated planting 9.6 HEDGEROW TREES Hedgerow trees can be planted with the staggered double row of transplants when a hedge is planted. They must be indicated clearly by marker stakes so that they are not cut accidentally during hedge trimming. However, their retention relies on the goodwill of the farmer and the attention of his hedging contractor, and it may be advisable to offset the trees from the hedges so that they lie just within the highway land. M26, Kent Light metal posts and stockproof wire provide a more-than-adequate fence which does not impede hedge growth Well-formed hedgerow trees can help integrate the road with the surrounding landscape. It is worth solving the problems of their establishment and maintenance 9/1

21 CHAPTER 10 GRASSLAND AND HEATHLAND 10.1 PRINCIPLE l Grassland and heathland of nature conservation and amenity interest can be developed in many road landscapes. Clear design and management objectives are necessary for each area. These should take advantage of current practice in amenity and nature conservation grassland establishment and management KEY ISSUES FOR GRASSLAND l The lack of clear objectives in the establishment and management of grassland in the past has led to the domination of extensive areas by vigorous, coarse species. l Semi-natural, species-rich grassland have developed over a long time and with complex management requirements. A realistic objective for roadside grassland is not to attempt to recreate these grasslands but to develop extensive areas of moderate species richness and diversity. l Control of soil fertility is the key to establishing a diverse grassland with low maintenance requirements. Conventional topsoil is required only in some circumstances, and then sparingly. Where there are naturally-fertile soils it is realistic to aim for grassland of moderate species diversity, made up of plants with wide tolerances. l Unmanaged grassland eventually turns to scrub. However low its fertility, a management programme is required. l On steep slopes, rapid establishment may be necessary to avoid surface erosion. In general, the use of short-term nurse crops like Italian rye-grass is not recommended and a site-specific solution, such as the use of emulsions to hold the surface while cover establishes, should be sought. l The types of grassland described in this section are slower to establish than conventional rye-dominated mixtures and can look untidy in autumn and winter. Education of the public is therefore a significant requirement MANAGEMENT IMPLICATIONS A45 The steep-sided bund in the central reservation is difficult to get to and cannot be cut with conventional mowing equipment. A dense hedge would have been better, or the use of a low-fertility, low-maintenance design. Any grassland needs cutting to maintain it. The frequency and height of cutting depend on the objective, the site s fertility and flowering period of the plants present. These must be taken into account in specifications and are discussed in Pt 7, Ch PUBLIC ACCEPTABILITY 10.3 CLEAR OBJECTIVES Wildflower grassland does not establish as easily or as rapidly as a lawn. Informing the public can help make such temporary features more acceptable Poor practice This grassland, dominated by false oat grass, has very little wildlife interest or amenity value. It has developed from rye grass sown on topsoil, a technique more appropriate for heavily used public open space Tidiness or naturalness? French motorways have a neat and tidy appearance. However, the close mowing is inimical to wildlife interest 10/1

22 CHAPTER 10 GRASSLAND AND HEATHLAND 10.6 ESTABLISHMENT AND FERTILITY l Advice on establishment is to be found in Pt 7, Ch 4. l Subsoil (see Ch 13) must be separated from topsoil, spread after the completion of the earthworks and cultivated to a depth of 100 mm. On nutrient-poor subsoils, such as chalk, a layer of topsoil, no deeper than 50 mm, should be spread as a seed bed. l Cultivation of the surface 50 mm to provide a seed bed is required, but a fine tilth such as that required for the seeding of lawns, is not needed. No fertiliser should be used. l While many grasses will germinate in spring or autumn over a long period, many broadleaf plants have more exacting requirements, so it is recommended that seeding takes place in August/early September: whenever practical. l A range of standard seed mixes is available. However a site-specific mix, prepared with specialist advice, is usually the best option. Good practice: A22, Willingdon Roundabout On low-fertility chalk soils an attractive flora can be developed using the right preparation, seeding and management 10.7 NATURALLY-FERTILE SOILS On naturally-fertile soils, such as most clays, a mixture of robust and colourful species can be used to give a pleasing effect. Many of these species form stable and long-lasting herbaceous vegetation which requires limited maintenance. A selection of species which have been used with some success is given in the table opposite, together with some of their requirements. Taller species must be set back from the verge and kept out of sight lines. Key Persistence: * - **** Geographical distribution: W-widespread; S-below 55º Showiness: * - **** Moisture demand: * - *** 10/2

23 CHAPTER 10 GRASSLAND AND HEATHLAND 10.8 NATURALLY INFERTILE SOILS Good practice Re-use of poor moorland soil seeded with appropriate grass species has allowed the re-establishment of heathland vegetation including heather, tormentil and lady s mantle Chalk grassland: A27 Chalk grassland is disappearing rapidly as a result of farming improvements and the abandoning of grazing. Chalk grassland can become established on suitably treated highway land KEY ISSUES FOR HEATHLAND l Heathland is a declining habitat nationally and most significant areas are now under statutory protection. Where new roads are put through former heathland their construction is an opportunity to develop new heathland habitats. l The best examples of heathland are the results of complex management regimes. These cannot be created on roadside landscapes, but there are opportunities for developing simpler systems. l There is often a shortage of suitable seed sources for heathland and best effects are usually achieved by using chopped-up fragments of heathland vegetation and surface soil (blading) from site clearance. l Management must always be borne in mind, since heathland on well-drained sites (ie most highway land) is particularly vulnerable to invasion by birch and pine. l Widening into existing heathland is discussed in Section 2: Pt 1, Ch MAKING THE MOST OF OPPORTUNITIES Road development can create conditions for species-rich grassland, a rapidly disappearing habitat. A21, Kent Heathland has established on the poor, acid soils of the Tunbridge Wells Sand Wetland: M6 Low ground at centres of roundabouts can be much moister than the surrounding land. Here, wet grassland may be created. A similar habitat can be established around balancing ponds If bladed material is available this should be spread in a prepared seed bed of subsoil as described for grassland, if seed alone is to be used, it may be best to use a mulch and emulsion to stabilise the light sandy soils while vegetation established. More information on heathland establishment is contained in Pt 5, Ch 2. 10/3

24 CHAPTER 11 STEEP SLOPES 11.1 PRINCIPLE l Steep slopes are sometimes necessary in road development, although they should be avoided wherever possible. Establishing vegetation on them can be a difficult process, but it is often essential in fitting the road to the landscape and in stabilising soil. On some continental and North American roads, willow, which will grow vigorously from stem cuttings (setts) has been used to stabilise clay banks. Brushwood can also be used with gabions or a timber slope grating KEY ISSUES l Woody vegetation can help stabilise material with shallow-seated instability (less than 2 m), especially on clay soils, but it has no effect on deep-seated instability. l The use of geotextiles plays an integral part in vegetating steep slopes. New materials continually arrive on the market and the solutions given here are only indicative. l Part 1, Ch 18 deals with rock exposures. Sometimes it is appropriate to establish vegetation on them, in which case the use of geotextiles and other techniques can create the right microsites for natural regeneration. l Some of the most satisfactory results can be obtained by the use of vegetation together with modular retaining systems like gabions. l Hydroseeding is a widely-used method of establishing grass on steep slopes. The principle of establishing grassland for nature conservation and amenity, set out in Ch 10, should be applied to the issues raised by vegetating steep slopes STABILISING WITH VEGETATION Vegetation can help surface stability where it can penetrate to the bedrock. Impenetrable bedrock Vegetation does not help to stabilise this Penetrable bedrock eg clay. Roots can act as toe piles and can be a major element in the slope s stability Brushwood combined with timber slope grating and Simple methods of establishing concrete piers can make a visually attractive roadside shrub cover with brushwood edge 11/1

25 CHAPTER 11 STEEP SLOPES 11.4 GEOTEXTILES Honeycomb geotextiles or matting is appropriate where a rapidly-established, dense grass cover is required HYDROSEEDING Hydroseeding is widely used to establish good grass cover. The choice of emulsion, organic matter and seed used as well as their proportions in the mix are, however, crucial for good results. A precise specification chosen for the site and a properly-qualified, specialist subcontractor are essential. Soil restored by honeycomb geotextiles before hydroseeding Good practice The geotextile has established a stable surface, permitting natural regeneration by ash Good practice A steep slope has been successfully grassed over by hydroseeding 11.6 ROCK OUTCROPS Rock outcrops can vegetate naturally. This is usually the most effective method, and certainly gives the best visual effect. Biodegradable geotextiles can be used for surface stability while seeding becomes established, as shown in the photograph below: A470, Glamorgan Natural colonisation usually provides the best visual effects Where weathering is likely to cause many fragments to fall down the surface, vegetation can be established by creating microsites to be colonised naturally Temporary surface stability on a site with self-evident risk of erosion 11/2

26 CHAPTER 12 EFFECTIVE TREE AND SHRUB ESTABLISHMENT This section is a brief aide memoire for details of current planting and establishment DISTANCE FROM THE CARRIAGEWAY Shrubs must be not planted within 3 m of the carriageway and trees not within 5 m of it PLANT SIZES Bare-root transplants, mm high, for shrubs and trees are the recommended size and specification, except for evergreens which should be container-grown and can be used at smaller sizes. Plants taller than 900 mm are not recommended except where light, feathered trees are needed for early impact. Trees taller than this will provide short-term benefit only. The special case of the use of larger stock in relation to motorway widening is dealt with in Section 3: Pt 1 and avenues are discussed in Ch STOCK HANDLING Bare-root plants must be protected from drying out right up to the time that they are put into the ground. Forestry bags are recommended PREPARATION The growing medium needs to be cultivated to a depth of 450 mm to allow adequate tree and shrub growth. Where this is not possible, there is no point in creating planting pits extending into compacted layers WEED CONTROL The importance of a high standard of weed control cannot be over-emphasised. A 750 mm diameter area around each plant should be weed free at the start of the growing season and be maintained that way. Where ornamental planting is appropriate, such as at roundabouts, a blanket mulch of course bark or similar material may be required. In rural areas chemical weed control is recommended, in preference to mulch mats or other physical methods PEST CONTROL It is necessary to protect woody transplants on sites where rabbits, hares, voles or deer are present. The type of protection to be used (eg individual guards or fencing-off planting blocks) will depend on the size of area and abundance of the pests. Guidance can be found in the relevant Forestry Commission publications STAKING Staking is unnecessary for the sizes recommended and prevents proper establishment of smaller trees and shrubs. It should only be used for larger sizes. Stakes should be no taller than one quarter the height of the tree PLANTING AND THINNING GRIDS 12.5 PRE-PLANTING WEED CONTROL Before planting into a dense grass sward or weed-infested area, a herbicide spray must be used to give a weed-free planting area of at least 750 mm diameter for each plant TIME OF PLANTING Planting is best carried out in November and December. Planting from January to March can be much less satisfactory and it should not take place after the end of March FERTILISERS On most disturbed soils, an overall base application of a balanced granular fertiliser should be given at the manufacturer s recommended rate, either incorporated into the surface, or as a top dressing PLANTING METHODS Pit planting is generally recommended. Pits should be 300 x 300 x 300 mm with the sides and bottom broken up. The back-fill material should be 75 percent topsoil, or approved subsoil (see Ch 13) and 25 percent bulky organic matter. The latter will vary from one part of the country to another because of local availability TREE SHELTERS These should generally be used only on exposed sites or where rapid establishment is imperative. 12/1

27 CHAPTER 13 SOILS 13.1 PRINCIPLE l The handling, storage and preparation of soil is the most important factor in establishing vegetation, since mistakes can rarely be put right. Engineers are interested in the physical properties of soil as a construction material, so detailed information about the mechanical properties of the available soil is obtained for constructing a road. However, as soil is also a complex biological system supporting plant growth, an equal understanding of its biological system supporting plant growth, an equal understanding of its biological properties is needed for successful vegetation establishment KEY ISSUES l For each soil type to be stripped and respread, the topsoil and subsoil layers need to be identified and clearly defined. A strategy for their use and storage must be developed before the contractor moves onto a site. l Topsoil is needed to grow trees and shrubs, but usually in only very limited quantities for grassland establishment. l Soils for reinstatement should be those derived from the site and no soil should be moved from the site until it is clear that all the resoiling requirements can be met from this source. If soil has to be imported it must be the right one for the vegetation being planted or sown. l Clear guidance on soil-handling constraints, particularly under wet conditions, must be available to the contractor and resident engineer. l The risk of surface soil slippage can be reduced by producing a very rough finish to the formation to allow the surface soil to key into it, and by ensuring that the soil is sufficiently uncompacted to allow roots to penetrate to the full depth of the soil profile and have maximum lateral spread within it (see Steep Slopes, Ch 11) UNDERSTANDING TOPSOIL AND SUBSOIL Soils vary greatly in their characteristics, depths and vertical structures. A soil under mature woodland or permanent grassland consists of layers, referred to as its A, B and C horizons, over bedrock. Horizons A and B are suitable for tree and shrub growth and in some circumstances, especially when mixed with ameliorants, C may be suitable. The materials from some B and C horizons are suitable for establishing species-rich grassland (or heathland on sandy soils). However, soils vary greatly from this typical pattern and each site must be investigated. It is essential that the characteristics of all the soils along the line of the route are taken into account with the advice of a soil scientist and horticulturalist. Definition of soil should not be based on arbitrary depths USING THE RIGHT TOPSOIL Soils can vary considerably along the line of a route. It is essential to keep these different soils separate and restore them to the right parent area. This will ensure that the roadside vegetation marries with its surroundings AMELIORATING THE SOIL When a subsoil has insufficient structure to allow plants to establish (eg a heavy clay), incorporation of other material to ameliorate it can be much more effective than the expensive process of importing new topsoil. Suitable ameliorants can, under appropriate circumstances, range from bulky organic matter, such as approved sewage sludge, to inert materials such as pulverised fuel ash, which open out the subsoil s structure SOIL STRIPPING AND STORAGE The subsoil and topsoil should be stripped and stored separately under favourable weather conditions so that a proper soil profile can be re-established. Storage-mound depth is related to the method used to make it. Loose tipping is the ideal way to make a storage mound, when it may be up to 4 m high at the centre of the tipped heaps. Otherwise, mounds should not exceed 2 m in height The structure of clay soil, in particular, is irreversibly damaged by stripping when it is too wet. The contract should specify that soils with a clay fraction of more than 25 per cent should not be handled when their moisture content exceeds a specified level. Once completed, soil mounds must not be trafficked by any machinery and should be kept free of injurious weeds by the application of herbicide if necessary. If they are to be left for longer than nine months they should be sown with annual rye-grass or another appropriate species SOIL SPREADING The formation on which soil is spread must have the surface roughened to 150 mm deep, to allow it to key in properly. The same constraints apply to spreading soil as apply to stripping it. Appropriate machinery with the lightest possible footprint must be used. Compaction must be avoided ALLEVIATION OF COMPACTION Sites that have become compacted, such as contractors compounds, require breaking up before tree planting can take place. They should be ripped with tines not further apart than 750 mm to be depth of 450 mm in two passes at right angles. If this is not possible, shallower ripping should be carried out and consideration given to shrub or grass establishment only. Excavation of pits into compacted surfaces is not recommended, since plants roots will only grow within the area of the excavated pit. This is likely to become waterlogged and the roots will die SOIL DEPTHS The standard depth for soil for tree and shrub establishment is 300 mm over the 150 mm roughened formation surface. This will not allow very large trees to develop in the way that they would on natural soils and this must be taken into account in design and management. In special circumstances, such as the rapid establishment of screening on level sites a greater depth may be possible. This is described in Section 3: Pt 1. Topsoil is generally not required for grass establishment (see Ch 10). However, where a dense sward that will require regular mowing is necessary, topsoil depth should not exceed 100 mm. Deeper soil leads to excess growth and hence the need for increased maintenance RESTORATION TO AGRICULTURE Restoration to productive agriculture is an essential part of good practice. Land is acquired under license with the compulsory purchase order and offered back to the farmer on completion. The restoration needs to be to a high standard not only to ensure it fits with the surrounding landscape, but also to ensure that the landowner will take it back. A widely accepted code of practice has been developed by the minerals extraction industry. 13/1

28 Volume 10 Section 1 Part 2 HA 56/92 Chapter 14 Enquiries 14.ENQUIRIES All technical enquiries or comments on this Advice Note should be sent in writing as appropriate to:- Head of Highways Policy and Environment Division The Department of Transport J ROBINS 2 Marsham Street Head of Highways Policy London SW1P 3EB and Environment Division The Deputy Chief Engineer The Roads Directorate Scottish Office Industry Department New St Andrew s House Edinburgh EH1 3TG J INNES Deputy Chief Engineer Head of Roads Engineering (Construction) Division Welsh Office Y Swyddfa Gymreig Government Buildings Ty Glas Road Llanishen Cardiff CF4 5PL B H HAWKER Head of Roads Engineering (Construction) Division Superintending Engineer Works Department of the Environment for Northern Ireland Commonwealth House Castle Street Belfast BT1 1GU D O HAGAN Superintending Engineer Works Orders for further copies should be addressed to: DOE/DOT Publications Sales Unit Government Buildings Block 3, Spur 2 Lime Grove Eastcote HA4 8SE Telephone No: December /1