Journal of Environmental Planning and Management,
45(5), 747-764, 2002
Taylor & Francis Group
POLICY AND PRACTICE
Strategic Routeing and Environmental Impact
Assessment for Overhead Electrical Transmission Lines
ROSS MARSHALL & ROSS BAXTER
SP PowerSystems, New Alderston House, Dove Wynd, Strathclyde Business Park, Bellshill ML4 3FF, UK.
(Received March 2002)
ABSTRACT High-voltage, high-capacity overhead lines are the economic and reliable choice for the bulk
transmission of electricity throughout the world. The routeing of transmission lines is a complex process,
and in the UK requires a balance to be struck between statutory obligations, engineering requirements,
economic viability, land use and the environment. Transmission line routeing projects can rightly generate
considerable public interest and debate, but issues often focus on local effects such as visual amenity
rather than the wider benefits of the project to society as a whole. Given the extent of their public and
regulatory scrutiny, the environmental statements of such schemes must be objective and transparent in
the approach adopted to the routeing strategy. The approach outlined is based on the premise that the
major effect of an overhead transmission line is visual and the degree of visual intrusion can be reduced
through careful routeing.
The purpose of overhead transmission lines is to transmit electricity in large quantities between the points
of generation (power stations) to an electricity substation close to load (demand) centres where the
electricity is consumed. In the UK, this is commonly achieved at voltages of 132 kV, 275 kV or 400 kV.
Transmission at 400 kV offers much higher capacity. One 400 kV double-circuit quad-conductor
transmission line is, for instance, equivalent to three 275 kV double-circuit twin-conductor lines or twenty-
one 132 kV double-circuit singleconductor lines. Transmission lines also provide interconnection between
power stations, load centres and other supply systems, forming what is commonly termed 'the grid'. At the
electricity substation, the power is either transmitted on to another substation, or transformed to a lower
voltage-33 kV or 11 kV-and distributed through the distribution network to the customers.
In an alternating current supply system, three continuous wires (called conductors) are required to form
an electrical circuit. The conductors are suspended at a specified height above ground, supported on
lattice steel towers or wooden poles, spaced at intervals. In order to maintain a safety clearance between
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748 R. Marshall & R. Baxter
the 'live' conductor wires, activities taking place on the ground and objects such as trees, the conductors
are suspended above the ground in a catenary curve between the supporting structures.
Overhead transmission lines are large linear elements in the landscape. They are likely to affect, to
varying degrees, visual and other environmental aspects of the area through which they run. The scale of
a transmission line relative to objects in close proximity, for example houses and trees, is such that the
major effect is usually the visual intrusion of the towers on the area through which the line is routed. The
towers and conductors may be visible from houses, roads, tourist attractions and other important
locations and may alter the character of the landscape in which they are sited. In addition, the towers'
construction may disturb, for example, archaeological remains or sites of nature conservation interest,
require the construction of temporary access tracks and through their life cause periodic disturbance
through maintenance operations.
Local populations are often hostile to the proposed routeing of new transmission lines, as they often
perceive no direct benefit from their installation and have concerns that property values may suffer if the
scheme is granted consent. Thus applications by an electricity supply company for consent to construct
and operate a new transmission line may bring it into direct conflict with the very customers whose
electricity supplies it seeks to secure.
ScottishPower's Transmission System
ScottishPower's SP Power Systems Ltd (PowerSystems) business is one of three companies in the UK
which has a legal duty to develop and maintain an efficient, co-ordinated and economical transmission
system of electricity supply in accordance with the Electricity Act 1989.
PowerSystem's transmission network comprises some 3850 km of double-circuit and single-circuit
overhead lines and 235 km of underground cables and supplies an area of 37 500 km2 across southern
Scotland, north Wales and the north-west of England. The network has 400 switching and/or transforming
stations (substations), the former facilitating the switching of current between lines and the latter being the
points at which primary transmission voltages are transformed to a lower voltage and delivered to the
distribution centres at 33 kV or below. The company is required under the Electricity Act 1989 to fulfil its
supply duties whilst paying due regard to environmental interests.
Legislation and Consents
A number of legal provisions apply in the UK to the development of overhead electricity transmission
lines, principally the Electricity Act 1989, the Electricity Works (Environmental Impact Assessment)
(England and Wales) Regulations 2000 and the Electricity Works (Environmental Impact Assessment)
(Scotland) Regulations 2000 (the Environmental Impact Assessment (EIA) Regulations). Section 9 of the
1989 Act requires PowerSystems to "develop and maintain an efficient, co-ordinated and economical
system of electricity transmission". Section 37 of the Act stipulates that consent is required from the
necessary decision-making authority for the installation of any non-exempted overhead electricity line
greater than 20 kV. All transmission licence holders are required by Schedule 9 of the 1989 Act
Strategic Routeing and EIA for Electrical Transmission Lines 749
to take account of the following factors in formulating any relevant proposals (for the installation of
overhead transmission lines):
(a) to have regard to the desirability of preserving natural beauty, of conserving flora, fauna and
geological or physiographical features of special interest and of protecting sites, buildings and objects
of architectural, historic or archaeological interest; and,
(b) to do what he reasonably can to mitigate any effect which the proposals would have on the natural
beauty of the countryside or on any such flora, fauna, features, sites, buildings or objects.
PowerSystems interprets the words "reasonably can" to mean that it should make every effort to mitigate
the environmental effects, whilst bearing in mind the technical constraints imposed by overhead
transmission line technology, and its duties under Section 9 of the Act.
EIA and Overhead Transmission Lines
The EIA Regulations implement within the UK the requirements of European Community (EC) directives
85/337 and 97/11 (European Economic Community, 1985; EC 1997) on the assessment of the effects of
certain private and public projects on the environment in relation to overhead lines. Under the provisions
of these regulations, the preparation of an environmental statement is mandatory for an electric line
installed above ground with:
• a voltage of 220 kV or more; and
• a length of more than 15 km, the installation of which (or the keeping installed of which) will require a
Section 37 consent under the Electricity Act 1989 (Schedule 1.2).
Applications for development covered by Schedule 2 of the regulations:
• an electric line installed above ground with a voltage of 132 kV or more, the installation of which (or the
keeping installed of which) will require a Section 37 consent (Schedule 2.3); and
• an electricity line installed above ground in a sensitive area, the installation of which (or the keeping
installed of which) will require a Section 37 consent but which is not a Schedule 1 development (Schedule
2.4) or Schedule 2.3 development;
will require an EIA where the proposed development is likely to have significant effects on the
environment. It is important to remember that in the consenting process associated with new electrical
transmission systems, the 'need' for a new transmission line has to be proven. In this respect such
development is different from more speculative forms of development, such as power stations, retail parks
and football stadia.
The Approach to the Routeing and EIA of Overhead Transmission Lines
The importance of careful routeing. The most likely major effect of any proposed overhead
transmission line is the visual effect on the people who live, work, recreate in and visit the corridor
750 R. Marshall & R. Baxter
through which it is proposed. Specific visual effects will relate to the individual or cumulative visibility of
towers or poles, the insulators and the conductors along this linear route. The significance of the visibility
experienced by a viewer will vary at specific locations within the route corridor due to parameters such as
viewer distance, cumulative impact, tower design type, skyline positioning or backdrop. Since no technical
way of reducing overhead line visibility other than choice of towers exists, and only limited ways of
achieving screening through planting for reasons of time and land ownership are possible, the most
effective way of reducing visual disturbance is through careful routeing.
Routeing strategy. The approach adopted by PowerSystems to line routeing and EIA for overhead
transmission projects can be described as two separate processes involving:
• route selection-a strategic environmental assessment and evaluation of route options to select a
preferred route, and following consultation with stakeholders to identify the proposed route;
• EIA of the proposed route.
PowerSystems is required under the Electricity Act 1989 to consider environmental, technical and
economic matters, and reach a balance between them. This means that the proposed route will be the
one, selected after extensive consultation and evaluation of a number of route options, which balances
technical feasibility and economic viability with the least disturbance to people and the environment. After
the selection of a proposed overhead transmission line route, it is necessary to undertake EIA of the likely
environmental effects, and to reduce and offset significant adverse effects, where practicable.
The Importance of Consultation
Since the introduction of EIA into the UK, PowerSystems has attached importance and commitment to
consultation with those parties who have a statutory right to consultation and with those stakeholders
whose interests are likely to be affected. There is no requirement for widespread consultation within the
provisions of the EIA Regulations or the Electricity Act 1989; however, the company believes that a formal
and ongoing consultation process assists all parties. Table 1 summarizes the consultation procedure
adopted by the company.
Where appropriate the consultation document highlighted below is issued to all statutory consultees,
copies are placed at council offices and in public libraries and its availability to the public is advertised in
the press. Local exhibitions and/or public meetings are commonly arranged if the project has attracted or
is likely to attract significant local interest.
The adoption and application of this approach are intended to ensure that:
• overhead transmission line routeing is carried out in a systematic manner, resulting in the selection of a
route which is technically feasible and economically viable and causes the least disturbance to people
and the environment;
• EIA is undertaken in an objective manner based on a thorough review of potential environmental effects;
Strategic Routeing and EIA for Electrical Transmission Lines 751
Table 1. ScottishPower's consultation process for overhead transmission line routeing
Stage Phase Actions and objectives
1 Pre-project notification Meetings with local planning authorities, environmental agencies and other statutory
consultees to introduce the need for the project and what is likely to be required.
2 Information collection All statutory and identified consultees provided with project information and formally
invited to provide information and comments on the proposed route corridor. Baseline
constraint maps produced by independent environmental consultants recording all
potentially significant environmental and social aspects.
3 Discussion with statutory
consultees Discuss development of project proposals with all key statutory
consultees and interested parties to review project thoroughness.
4 Document for public
consultation Preparation of a Consultation Document recording all identified environmental issues,
comparing route options, and the justification for the selection of the preferred route.
First community consultation exercises and exhibition held at local centres
5 Appraisal of consultation
feedback The results of the consultation process and public exhibitions are considered,
evaluated and used to reappraise the preferred route. Personal responses to all public
comments prepared. Feedback is provided to stakeholders on proposed route
Second community consultation exercises and exhibition held at local centres to outline significant routeing
6 Appraisal of consultation
feedback The results of the consultation process and public exhibitions are considered,
evaluated and used to reappraise the preferred route. Personal responses to all public
comments prepared. A further round of discussions with statutory consultees may take
place and following this the proposed route is nominated.
A proposed route is identified, on which EIA is then performed by independent
7 Publication of the
EIA is reported in the form of an environmental statement.
• the information in the consultation document on how the preferred route was selected can be presented
in a logical, comprehensive and objective manner;
• the information in the environmental statement is presented in a clear, comprehensive and objective
Routeing an Overhead Transmission Line
This section describes the process that PowerSystems follows to select a proposed overhead
transmission line route.
The Approach to Route Selection
Transmission lines are linear projects between two determined points; in this case these are substations.
Whilst the selected option is not necessarily the shortest route between these two points, deviation still
requires continuity of function along the linear corridor(s) selected. PowerSystems's approach to route
751 R. Marshall & R. Baxter
selection is based on the premise that the major effect of an overhead transmission line is visual and that
the degree of visual intrusion can be reduced by careful routeing. Reduction in visual intrusion can be
achieved by routeing the line to fit the topography, by using topography and trees to provide screening
and/or background and by routeing the line at a distance from settlements and roads. In addition, a well-
routed line will take into account other environmental and technical considerations and will avoid,
wherever possible, the most sensitive and valued natural and man-made features. The approach to route
selection is summarized in Figure 1. The approach is iterative and the steps may be revisited several
times before a balance is achieved between technical, economic and environmental considerations.
Consultation is carried out throughout the process. Professional judgement is used to establish explicitly
the balance between factors. Each of the steps is described below.
The Objective of Route Selection
The objective of route selection is to identify a technically feasible and economically viable overhead
transmission line route, between specified points, which causes the least disturbance to people and the
Established Practice for Overhead Transmission Line Routeing
The Holford Rules. Broad principles for overhead transmission line routeing, known in the UK as the
'Holford Rules' (Holford, 1959), have been established within the electricity supply industry and are
summarized in Table 2.
The Holford Rules were formulated by the late Lord Holford, Professor of Town Planning, University
College London, in 1959 and published by the Royal Society of Arts. The Holford Rules remain the
starting point for routeing electricity transmission lines in the UK. However, it is now recognized that the
Holford Rules are a product of a specific time and set of circumstances. At the time the Holford Rules
were written, the area of land designated within the UK for amenity value was far smaller than now and
the design of routes to avoid such areas was easier as a result.
National Grid Supplementary Interpretation Notes
The Holford Rules provide a valuable basis for an approach to transmission line routeing, but give no
guidance on how to reconcile routeing to avoid areas of amenity value where this would have a greater
visual intrusion due to the proximity of the line to people. This limitation has been recognized by other
transmission companies and a set of supplementary notes has been proposed by the National Grid
Company PLC when interpreting the Holford Rules (National Grid Company, 1992) (Table 3).
Forestry Commission guidance. Guidance has also been produced by the UK's Forestry
Commission to provide basic design guidance for integrating power line corridors through woodlands and
forests (Forestry Commission, 1994). The Forestry Commission guidance is summarized within Table 4.
Strategic Routeing and EIA for Electrical Transmission Lines 753
Figure 1. Approach to the routeing and EIA of overhead transmission lines.
Determination of Likely Effects
Overhead transmission lines are large linear elements in the landscape. They are likely to affect, to
varying degrees, visual and other environmental aspects of the area through which they run (Figure 2).
754 R. Marshall & R. Baxter
Table 2. The Holford Rules
1 Avoid altogether, if possible, the major areas of highest amenity value, by so planning the general
route of the line in the first place, even if the total mileage is somewhat increased in
2 Avoid smaller areas of high amenity value or scientific interest by deviation, provided that this can
be done without using too many angle towers, i.e. the more massive structures which are used
when lines change direction.
3 Other things being equal, choose the most direct line, with no sharp changes of direction and thus
fewer angle towers.
4 Choose tree and hill backgrounds in preference to sky backgrounds wherever possible; and when
the line has to cross a ridge, secure this opaque background as long as possible and cross
obliquely when a dip in the ridge provides an opportunity. Where it does not, cross directly,
preferably between belts of trees.
5 Prefer moderately open valleys with woods where the apparent height of towers will be reduced,
and views of the line will be broken by the trees.
6 In country which is flat and sparsely planted, keep the high-voltage lines as far as possible
independent of smaller lines, converging routes, distribution poles and other masts, wires and
cables, so as to avoid a concatenation or 'wirescape'.
7 Approach urban areas through industrial zones, where they exist; and when pleasant residential
and recreation land intervenes between the approach line and the substation, go carefully into the
comparative costs of undergrounding, for lines other than those of the highest voltage.
Table 3. The National Grid supplementary notes on interpreting the Holford Rules
1 Residential areas: avoid routeing close to residential areas as far as possible on grounds of
2 Designations of county, district and local value: where possible choose routes which minimize the
effect on special landscape areas, areas of great landscape value and other designations of
county, district or local value.
3 Alternative tower designs: in addition to adopting appropriate routeing, evaluate where
appropriate the use of alternative tower designs now available where these would be
advantageous visually and where the extra cost can be justified.
The environmental effects of an overhead line are likely to include interactions with visual amenity, the
landscape, nature conservation (flora and fauna), land use and agriculture, archaeology, cultural heritage,
amenity and tourism. The scale of a transmission line relative to objects in close proximity, for example
houses and trees, is such that the major effect is usually the visual intrusion of the towers in the area
through which the line is routed. Impacts arising from construction works and through maintenance
removal operations over the life of the transmission line have also to be considered.
The likely visual and physical effects are those relating to the transmission towers. The towers occupy a
ground area and require below-ground foundations which may disturb, for example, archaeological
remains or sites of nature conservation interest. Their location and siting in respect of other physical
features may impact on avian migration and movement. Construction of the transmission line may require
temporary access tracks to be built. Conductors strung between towers require clearance from trees and
other objects. The towers and conductors may be visible from houses, roads, tourist attractions and other
important locations and may alter the character of the landscape in which they are sited.
Strategic Routeing and EIA for Electrical Transmission Lines 755
Table 4. Forestry Commission guidance on routeing transmission lines
1 Route transmission lines to follow open space and to run alongside, not through, woodland.
2 Where there is no alternative route; a power line through the forest should:
§ avoid areas of landscape sensitivity;
§ avoid the line of sight of important views;
§ be kept in valleys and depressions;
§ not divide a hill into two similar parts where it crosses over a summit;
§ cross skyline or ridges where they drop to a low point;
§ follow alignments diagonal to the contour as far as possible;
§ be inflected upwards in hollows and downwards on ridges.
3 In the design of the transmission line corridor, the transmission line within forests should seem to
pass through a series of irregular spaces. The forest should appear to meet across the open
space in some places so that the corridor does not split the forest completely. The aim should be
a corridor of varying character and width, swinging from one side of the line to the other, taking
care to avoid irregular but parallel edges, or irregular but symmetrical spaces. Exit points should
be gently asymmetrical bell-mouths. Felling areas should be planned to link with and cross the
power line corridor and create greater irregularity.
In seeking to determine likely effects, a framework is established, identifying the individual effects most
likely to occur and the likely interaction between the effects.
Figure 2. Environmental aspects commonly encountered during overhead line construction.
NATURAL RESOURCES MAN MADE RESOURCES
756 R. Marshall & R. Baxter
Routeing Considerations and Collection of Background Information
The main technical and environmental considerations which should be studied in order to route a
transmission line with the least visual intrusion and least disturbance to people and the environment are
determined from a study of potential effects and established routeing practice. These 'routeing consider-
ations' include topography, landscape character and areas of amenity value and scientific and historical
A 'study area' is first defined, and information on the main environmental considerations within it is
gathered and categorized into three groups:
• relating to the character of the landscape and its development;
• relating to the protection of the environment;
• relating to the potential for environmental and landscape change.
This method of categorizing information is intended to provide an understanding of the environment of the
study area. In addition, information is gathered on the technical considerations which apply. Consultation
is undertaken to obtain additional, up-to-date information on pertinent considerations.
Considerations which are likely to constrain routeing are mapped together on a 'constraints map', which is
a key part of the routeing process. This mapping process allows an interactive overview to be taken of all
pertinent routeing issues, identifying all major environmental constraints in their relative locational context.
The landscape character of the study area is also mapped. This can be based on general published
studies, but is often refined for detail within the route selection process. Theoretical visibility of route
options (excluding vegetation) is also mapped and compared.
Routeing considerations and established practices are then used to develop a 'routeing strategy' which
establishes considerations that are used to identify broad corridors (routeing issues) and those which are
used to modify routes within corridors (deviation issues). Routeing issues are generally of a strategic
nature and extensive in area; deviation issues tend to be of local importance and smaller in scale.
Development of Route Options
A number of 'route options' are developed, based on the routeing strategy (Figure 3). On some projects,
where the length of line is considerable, it has been helpful to extend this process further, and to identify
and assess a number of corridor options prior to developing and evaluating route options.
Evaluation of Route Options
As each route option is developed, its effect on the routeing considerations is recorded. At this stage a
route option may be rejected, modified or studied in more detail.
Strategic Routeing and EIA for Electrical Transmission Lines 757
Figure 3. Excerpt from an environmental statement identifying route options within the eastern corridor of
a proposed new transmission line in Scotland.
It is important that the process remains flexible and responsive to information provided by consultees. In
conjunction with the collection of relevant data and the evaluation of route options, the routeing
considerations may be reappraised and updated as new information becomes available. Route options
may then be rejected or modified, or new route options developed.
The broad routeing principles are applied to the study area to establish a number of possible route
options. This process involves the avoidance wherever possible of designated areas of high amenity or
nature conservation value. By definition, the route of the line must be continuous and, as a consequence,
the environmental advantages for routeing in one area of terrain may be offset by the disadvantages of
routeing through an adjoining area. A second application of the broad routeing principles establishes the
relative importance of these advantages and disadvantages by reference to constraint mapping and
assessment criteria. Options which perform poorly in this initial evaluation are rejected; the remaining
route options are then further refined and re-evaluated.
The objective of this process remains that of identifying the route which has the least likely adverse
environmental effects of the options considered whilst being technically feasible and economically viable.
Selection of a Preferred Route
After the comparative evaluation of route options, a preferred route option is selected and a document for
consultation prepared for issue to interested parties.
757 R. Marshall & R. Baxter
The consultation document records all identified environmental issues and compares route options, with
the justification for the preferred route's selection.
Modification of the Preferred Route
The preferred route is subjected to further evaluation in response to consultation responses, and may be
further modified as a direct response to these consultations.
Selection of the Proposed Route
The preferred route, modified to take into account consultations and the consideration of specific local
issues which may result in a minor deviation, is then promoted as the 'proposed route'.
The proposed route is subjected to further detailed assessment to determine and quantify its likely effect
on the environment. During this 'environmental assessment process', further modifications may be made
to the proposed route. Preliminary measures are identified to reduce or remedy adverse effects of the
proposed route. In the final evaluation, the selection of the proposed route requires a balance to be struck
between environmental effects and cost whilst ensuring that technical constraints are properly
EIA of an Overhead Transmission Line
As stated earlier, overhead transmission lines fall into the category of linear EIA developments. It is
important to accept that subtle differences in EIA approach exist between such linear developments and
the more commonly encountered site-specific developments. These primarily relate to how the scale of
impacts and magnitude of impacts along the route of the proposal are assessed. This is influenced by the
extended geographical siting of the development, and the fact that receptor impacts for specific
environmental aspects at specific locations can be multi-faceted, for example near-distance, mid-distance
and long-distance views of the same development can be experienced from one location.
This section describes the process that is followed in order to assess the likely environmental effects of a
proposed overhead transmission line. The strategic environmental assessment of possible route corridors
and route options now gives way to the identification and prediction of significant adverse effects along
the proposed route. In building the EIA process from the initial strategic assessment, it will be apparent
that the company is already in receipt of extensive information concerning the likely interactions between
development and the receiving environment.
Determination of Likely Main Effects
Schedule 4 within the EIA Regulations states which environmental components should be assessed for
impact in an environmental statement. The list is general, and is closely aligned with the parent EC
Strategic Routeing and EIA for Electrical Transmission Lines 759
In general, the main effects of an overhead transmission line most likely to occur are the effects on visual
amenity (particularly on people living and working in an area) and on the landscape resource. Other
significant effects may relate to nature conservation, agriculture, archaeology, cultural heritage, recreation
and tourism. It is often necessary for PowerSystems to retain independent consultants to assess the
specific effect on these and other topics such as noise and electro-magnetic fields. It should be noted that
the effect on visual amenity (on human beings) and the effect on the landscape (resource) are listed
separately in the European Union's model for EIA.
A particular overhead transmission project may or may not have impacts, in varying degrees, on some or
all of the listed environmental components. Therefore, it is necessary to examine the list of components in
relationship to the specific project under consideration along all parts of the route, and to determine the
possible impacts of the development on these environmental components where they are identified. It is
then necessary to predict which of the possible impacts are likely to occur. Following the prediction of the
likely impacts on environmental components, the likely main effects can be determined by reference to
assessment topics, such as visual amenity, agriculture and archaeology.
The UK's EIA Regulations require that the likely main effects are assessed. To assist readership,
PowerSystems formats the environmental statement so that it contains separate chapters reporting the
assessment of the significance of the likely main effects on each of the identified topics. Typical examples
of some of the environmental statement chapters for an overhead transmission line are as follows:
• chapter 1: effect on visual amenity;
• chapter 2: effect on habitats (flora and fauna) and nature conservation;
• chapter 3: effect on tourism and recreation;
• chapter 4: effect on the landscape resource;
• chapter 5: effect on agriculture;
• chapter 6: effect on forestry plantations and woodlands.
To ensure compliance with the EIA Regulations, it is PowerSystems's policy to show how the topics
discussed in the environmental statement relate to the requirements of the EIA Regulations. This allows
the company to clearly demonstrate that all the required possible effects on environmental aspects have
been assessed. If this cannot be demonstrated, then it is likely that the challenge can be raised that the
environmental statement does not comply with the requirements of the EIA Regulations. Table 5 gives an
example of how it is possible to demonstrate compliance with the regulations.
It can be seen from Table 5 that specific environmental statement chapters will assess environmental
effect across several environmental components listed in the EIA Regulations. It can also be seen that
interactions between effects may exist for specific topics. For example, effects on visual amenity and
cultural heritage may interact to highlight an indirect effect on tourism and thus on the tourism economy of
the area. The UK's EIA Regulations also require this interaction to be assessed.
Guidance on Assessment Methodology
The EIA regulations do not specify any particular methodology for the assessment process.
760 R. Marshall & R. Baxter
Table 5. Relationship between the environmental statement and the environmental components
listed in the EIA Regulations and their interaction
amenity Habitats and nature
conservation Tourism and
Human beings X X
Flora and fauna X X
soil X X
Water X X
e X X X
Cultural heritage X X X
Governmental advice and guidance (Department of the Environment (DoE), 1988, 1989; Scottish
Development Department, 1988; Department of Trade and Industry, 1992; Department of Transport et al.,
1993; Forestry Commission, 2001) can be summarized as stating that an environmental assessment
must be undertaken in a rigorous manner, using a systematic approach, and the information must be
presented in a comprehensive, clear and objective manner. PowerSystems seeks to ensure that, as the
developer of a probably contentious project, these objectives are met in a consistent manner for all
Collection of Baseline Information
The information already obtained at the route selection stage is supplemented with more detailed
information for the proposed route. This will commonly involve specialist studies such as an
archaeological survey and walk over the route, or the drawing up of habitat survey maps. This process
enables the EIA project managers to consult further with statutory and other consultees, not only to obtain
the most up-to-date environmental data, but also to discuss the proposed route in detail.
Assessing the Nature of an Effect
Although EIA is generally treated as a decision-making tool which identifies adverse effects associated
with new development, its corollary is that beneficial effects may equally be associated with the proposed
development. For example, a new overhead transmission line may have a probable adverse effect on
visual amenity within the location in which it is proposed. If the new line replaces an existing line, as part
of transmission system redesign and reconfiguration, the overall effect on visual amenity may be
beneficial through a reduction in the network length. Assessment of whether the effect of a proposed
overhead transmission line on any particular topic is likely to be adverse or beneficial is a matter of
professional judgement. To assist this categorization into adverse/ beneficial, PowerSystems requires its
consultants to use a set of criteria which assesses, for every effect its nature, i.e. whether it is adverse or
Strategic Routeing and EIA for Electrical Transmission Lines 761
Assessing the Significance of an Effect
The EIA Regulations state that "The specified information [to be included within the environmental
statement] is... (c) a description of the likely significant effects, direct and indirect, on the environment of
the proposed development" (Schedule 3).
The subsequent clause in Schedule 3(d) states that "where significant adverse effects are identified ... a
description of the measures envisaged to prevent, reduce any significant adverse effects" is required.
Therefore, at this stage of the EIA process, the EIA Regulations require a distinction to be drawn between
those effects which are 'significant adverse' and those that are merely 'adverse'. The question as to what
is, or is not, a significant adverse effect has been accepted by other commentators as one of the most
difficult areas of EIA to define (Fortlage, 1990) and it is often inevitable that developmental impact
significance will reflect the anthropocentric bias of EIA.
The significance of a likely effect is a function of its character (magnitude and duration, etc.) and the value
of the resource being affected. Assessing significance is a three-stage process:
• selecting the criteria against which effects will be assessed (the assessment criteria);
• establishing thresholds of significance (significance thresholds);
• comparing effect characteristics with the thresholds for each criterion.
Specific EIA topics of assessment, such as air quality and noise, have established international or
national guidelines or legislative criteria which may be used to identify threshold values for significance
determination; however, the significance assessment of more subjective environmental parameters, such
as visual effect or amenity, is often more difficult. In such circumstances, effect has to be measured using
a combination of quantitative and qualitative criteria. This approach necessarily involves the use of
professional judgement, based on criteria that are often applied on a case-by-case basis, these criteria
being dependent on the particular characteristics of the overhead transmission line project being
It has been found to be useful, when judging what constitutes a 'significant' effect, to first identify, for the
relevant quantitative/qualitative criteria, the extremes of the possible range of effects (i.e. what would
constitute the least effect and the greatest effect). Within these extremes of the range, it is helpful to
identify combinations of criteria that would cause an effect to be placed, using professional judgement,
into one of the following three categories, these being of increasing 'significance'.
For the purpose of PowerSystems 'effect' has been considered to be:
• none: no detectable change to the environment;
• minor: a detectable but non-material change to the environment;
• moderate: a material but non-fundamental change to the environment,
• major: a fundamental change to the environment.
These categories apply equally to both adverse and beneficial effects. Any effect of a proposed
transmission line assessed as 'major' or 'moderate' (in terms of the criteria defined above) would be
considered to be 'significant' within the terms
761 R. Marshall & R. Baxter
of the EIA Regulations. Any effect assessed as 'minor' would not be considered as 'significant' within the
terms of these regulations.
Defining mitigation. The measures referred to in Schedule 3(d) above to "prevent, reduce and where
possible offset" significant adverse effects are often referred to collectively as mitigation, although that
term is not used in the EIA Regulations. However, the term 'mitigation' is accepted in everyday EIA
'Mitigation' is used in Schedule 9.3(1)(b) of the Electricity Act 1989, in connection with the wider
obligations of a transmission licence holder towards the environment. The Act states that "a licence
holder ... shall do what he reasonably can to mitigate any effect". Thus 'mitigation', as used in the UK's
Electricity Act, presupposes the existence of an effect.
It can be argued that 'mitigate', as used in the Electricity Act, should be interpreted as encompassing the
terms 'reduce and where possible offset any significant adverse effects on the environment' as used in
the EIA Regulations. 'Prevent', as used in the EIA Regulations, should be interpreted as preventing any
effect from occurring, and is therefore not strictly mitigation, because, as stated above, mitigation
presupposes the existence of an effect. As stated earlier in this paper, the main method of avoiding
significant adverse effects associated with an overhead transmission line is by careful routeing.
Identifying Mitigation Measures
The consideration of mitigation measures is an integral part of the PowerSystems approach to overhead
transmission line routeing. Mitigation is embodied in the detailed design of the proposed route, in fine
adjustments in routeing, in the placement of individual towers within the route corridor and, possibly, in
selected locations, the management of existing vegetative cover and new tree, shrub and hedgerow
planting. These mitigation measures are consistent with the terms in which measures to reduce, remedy,
avoid or mitigate effects are considered in the Electricity Act, the EIA Regulations and the EC directive on
Proposals for other mitigation measures, such as off-site planting, woodland management and forestry
corridor design, depend on the co-operation of the relevant landowner or organization. UK legislation
does not grant powers of compulsory acquisition to utilities to allow these works.
Where mitigation measures are proposed, their likely effectiveness must be examined, and the
significance of the effect should be reassessed and adjusted, as appropriate.
Reporting the Assessment
The assessment criteria and significance thresholds which have been used, first, to establish whether an
effect occurs, and secondly, to determine whether it is a significant adverse effect are central to the
validity of the whole environmental statement. It is PowerSystems' policy that every environmental
statement should contain a chapter explaining how the methodology, assessment criteria, significance
Strategic Routeing and EIA for Electrical Transmission Lines 763
thresholds and the document as a whole satisfy the requirements of the EIA Regulations. Each
assessment chapter should also detail the assessment methodology used to assess the likely effect on
the topic in question.
New overhead transmission line proposals have the potential for significant environmental effect within
the areas in which they are proposed. The EIA Regulations recognize this fact and require the production
of an environmental statement and its review by the relevant decision-making authority prior to the
granting of consent within the UK. In addition, UK transmission licence companies are required by the
Electricity Act 1989 to have regard to the desirability of the preservation and conservation of amenity and
to endeavour to mitigate any effects of their proposals on amenity. For most transmission proposals, the
formal assessment of their likely environmental effects is thus a requisite rather than an option.
High-voltage, high-capacity overhead lines are the economic and reliable choice for the bulk transmission
of electricity, and PowerSystems has a firm commitment to providing new overhead transmission lines
which are technically feasible and economically viable whilst causing least disturbance to people and the
environment. The approach adopted towards overhead transmission line routeing is based on the
premise that the principal way of mitigating environmental effect is by the selection of the most
Integral with this approach is the realization that the process followed must be objective and
comprehensive to the ultimate decision-making authority and those stakeholders whose interests are
affected by the proposed development. This requires the company to ensure that an iterative, systematic
evaluation of a number of route options (alternative routes) is incorporated, and that the views of statutory
consultees and the local community are considered and evaluated. Ultimately, however, the proposed
route must reflect a professional judgement on what comprises technical feasibility and economic viability
with the least disturbance to people and the environment.
Environmental Assessment of an Overhead Transmission Line
Following the selection of the proposed route, the formal statutory requirement for an EIA is undertaken.
The methodology outlined ensures that this requirement is performed in an objective manner that
assesses the likely environmental effects of the preferred scheme, and where options to reduce and
where possible offset any significant adverse effects on the environment are considered.
The evaluation of route options, the EIA of the proposed route and proposed mitigation measures are
reported in an environmental statement. The approach adopted by PowerSystems is designed to ensure
that the information presented in the environmental statement is clear in context, comprehensive and
objective in its conclusions.
764 R. Marshall & R. Baxter
This paper summarizes a proprietary methodology contained within ScottishPower (2001). It is hoped that
this summary paper has provided interested readers with an overview of:
§ the particular properties of overhead transmission lines, a form of linear development that has specific
characteristics at EIA that distinguishes it from site-specific forms of development,
§ the legal framework within which UK power transmission companies operate, and the duties and
responsibilities on such companies, which do not apply to developers promoting other types of
§ PowerSystems' approach to both overhead transmission line route selection and EIA of the
environmental effects of a proposed overhead line;
§ How the requirements of the EIA Regulations pertaining to overhead transmission lines are
incorporated within the submitted environmental statement.
This paper is based on a paper presented at the International Association for Impact Assessment's
Annual Conference, Cartagena, Columbia, 26 May-2 June 2001. The authors acknowledge the
contribution made in developing PowerSystems' transmission routeing strategy and EIA process by Mark
Turnbull of the Turnbull Jeffrey Partnership, Edinburgh, and Brian Evans of the Gillespie Partnership,
Glasgow. All errors and omissions remain the responsibility of the authors. The process is proprietary.
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