Cumulative effects - all at sea?

8th April 2013


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Martin Broderick, Nick Medic and Alan Pearson on new guidelines aimed at helping developers perform cumulative effects assessments

The offshore wind sector is an industry of national importance that will help the UK secure its energy supplies, reduce dependence on imported fossil fuels, and protect the environment. It is a sector on which the country’s clean energy future rests. Delays to projects and planning permission rejections inhibit the UK’s ability to meet its 2020 targets for renewable energy.

One aspect that is causing major difficulty in the planning process for offshore wind farms is cumulative effects assessments (CEAs), including “in combination effects”, whereby multiple projects or activities create a cumulative effect greater than, or different from, that of the individual project.

Key to the overall CEA process are the requirements of the EU Directive on strategic environmental assessment (SEA) (2001/42/EC). Although SEA seeks to inform decision making on a particular action, it does so at a strategic level for local plans, programmes and strategies. Typically, planners or the regulator perform the SEA, while the developer undertakes the project level environmental impact assessment (EIA).

The SEA is an important step for the CEA because, at a project level, many strategic decisions have already been made, with questions over what type of development should take place and where, either decided or pre-empted by earlier policymaking processes.

The EIA focuses on a project’s direct effects, which makes it challenging to address cumulative effects at the project level. To help developers, a set of guiding principles for undertaking cumulative effect assessments for offshore wind farms in the UK (see below) has been developed by regulators and stakeholders from the sector.

Significant effects

All proposals for projects subject to the EIA Directive assessment (2011/92/EU) must be accompanied by an environmental statement describing the aspects of the environment likely to be significantly affected by the project.

The Directive requires an assessment of the likely significant effects of the proposed project on the environment, covering the direct effects and any indirect, secondary and cumulative effects. It requires that short-, medium- and long-term effects are all considered, as well as permanent and temporary effects, and positive and negative effects at all stages of the development. It also requires the inclusion of measures envisaged as necessary for avoiding or mitigating significant adverse effects.

Developers must ensure they consider both intra-project and inter-project cumulative effects. This Directive is implemented for offshore wind farms generating more than 100MW of electricity in the UK through the Infrastructure Planning (EIA) Regulations 2012 and the Electricity Works (EIA) (Scotland) Amendment Regulations 2008.

Separately, the EU Habitats Directive (92/43/EEC) requires that, where a plan or project is likely to have a significant effect on a Natura 2000 site – a special area of conservation or protection designated under the Habitats Directive or the Birds Directive (2009/147/EC) – either individually or in combination with other developments, it is subject to “appropriate assessment” of its implications for the site.

In-combination effects need to be considered for relevant habitats and species. The process of screening for likely significant effects and, where appropriate, the undertaking of an appropriate assessment is known as a Habitats Regulations Assessment (HRA).

The EIA and appropriate assessment regimes are separate: an EIA cannot substitute for an appropriate assessment, or vice versa. There is scope, however, for the processes to be undertaken in an integrated way.

The focal point of an EIA is the environmental statement, which is prepared by the developer. The decision-making body – for example, the planning inspectorate or the Scottish government – must take the statement into account, along with other information when approving or rejecting a project.

But, under the Habitats Directive, the appropriate assessment is “the competent authority’s own assessment” of the material effects on site integrity, and must be undertaken on a precautionary basis. If a proposal fails the integrity test it can only proceed in restricted circumstances.

The developer must provide sufficient information in the environmental statement and the CEA for the competent authority to undertake the appropriate assessment. Although acknowledging this crucial distinction, the new guiding principles apply to both the EIA and HRA processes, unless explicitly stated.

Meeting the challenge

Assessing cumulative effects is a regulatory requirement, but it can be challenging for several reasons. First, there is a lack of certainty over the process for undertaking a CEA, with inconclusive guidance and inconsistent definitions of scope and what should be considered “reasonably foreseeable”. There is also uncertainty over project-level effects, such as bird collision and displacement, which are compounded by a number of projects potentially contributing to the same effect.

Also, there are few existing definitive significance thresholds under which the cumulative effects of projects can be managed. Finally, there is potential for projects with larger environmental effects to be given consent before projects that may have lower environmental effects – using up important environmental capacities and potentially reducing the total generating capacity of projects that can gain consent. This is particularly difficult for project-level assessments to account for.

The newly-published guiding principles aim to tackle the first challenge and provide a framework that develops a consistency of approach in areas prone to uncertainty; not so much providing guidance as setting an expectation of standards. These principles do not seek to solve all CEA-related issues, however. Bird displacement and a better understanding of bird thresholds, for example, are areas that require further investigation. The guidelines aim to:

  • ensure that all stakeholders have the same expectations of the CEA process;
  • reduce uncertainty over the CEA process; and
  • promote streamlining of the consenting process.

The guiding principles were developed through a steering group comprising developers, the Crown Estate, Decc and Natural England. Other stakeholders submitted written comments – more than 300 were received – and participated in two development workshops. The guidance was also presented to the offshore renewable energy licensing group; and discussed with the planning inspectorate and IEMA.

A structured approach

The principles are structured to consider effects from additional changes caused by a proposed offshore wind farm in conjunction with other similar projects, or the combined effect of several developments.

The principles define cumulative effects as “those that result from additive effects caused by other past, present or reasonably foreseeable actions together with the plan, programme or project itself and synergistic effects (in-combination), which arise from the reaction between effects of a development plan, programme or project on different aspects of the environment”.

The focus of the guiding principles is on producing meaningful assessments, which strike the right balance between pragmatism and precaution. They must also provide a good quality analysis of the environmental effects of projects, while allowing a development to proceed in a timely fashion. The emphasis is on the assessment of potentially significant effects rather than cataloguing every conceivable effect.

Although the principles have been developed for project-level assessments in the UK offshore wind market, they may have wider relevance, including to future strategic environmental assessments.

Principles for cumulative effects assessments

General principles

  1. A cumulative effects assessment (CEA) is a project-level assessment, carried out as part of a response to the requirements of the EU Directives on habitats, birds and environmental impact assessment, designed to identify potentially significant impacts of developments and possible mitigation and monitoring measures.
  2. Developers, regulators and stakeholders will collaborate on CEA.


  1. Clear and transparent requirements for CEA are to be provided by regulators and their advisers
  2. Boundaries for spatial and temporal interactions for CEA work should be set in consultation with regulators, advisors and other key stakeholders, in line with best available data.
  3. CEAs will include early, iterative and proportionate scoping.
  4. Developers will use a realistic project design envelope.
  5. Developers will consider projects, plans and activities that have sufficient available information to undertake the assessment.


  1. The sharing and common analysis of compatible data will enhance the CEA process assessment.
  2. CEA should be proportionate to the environmental risk of the project and focused on key effects and sensitive receptors.
  3. Uncertainty should be addressed and, where practicable, quantified.

Mitigation and Monitoring

  1. Mitigation and monitoring plans should be informed by the results of the CEA.

To receive a complete version of the guiding principles containing the rationale and implementation proposals, contact or

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