Cumulative effects assessment: a call to arms

A clear, commonly agreed and replicable approach to cumulative effects assessment is needed within the environmental impact assessment community, argue Andy Mitchell and Joanna Wright 

In environmental impact assessment (EIA), cumulative effects assessment (CEA) considers the potentially significant environmental effects of a proposal together with those of “other existing and/or approved projects, taking into account any existing environmental problems relating to areas of particular environmental importance likely to be affected or the use of natural resources”. This is the latest wording within the UK EIA Regulations.

Definitions and guidance

Specific reference to CEA appeared in the first EU EIA Directive 85/337/EEC. Ever since, it has proved something of an enigma, an aspect of EIA practice that can vary widely by sector, practitioner, and even by regulatory expectations. There have, therefore, endured underlying question marks over the adequacy of the assessment unless and until the related consent is secured, and beyond. 

References to CEA exist in the Directive, but offer only clues to what it requires. There is also general, sector-specific and topic-specific guidance on how practitioners should approach CEA, from sources including IEMA, IAIA, the European Commission, Scottish Natural Heritage, the Planning Inspectorate, Highways England, Renewable UK, the Landscape Institute and CIEEM – but this is disjointed, with no clear, agreed approach.

In the absence of this agreed approach, CEA fosters uncertainty and nervousness among practitioners, regulators and decision-makers. Without clear and specific guidance on exactly what to assess and how to present it, there are a number of challenges facing the EIA community.

Key challenges

One issue associated with the undertaking of CEA is the differing definitions. Do cumulative effects equate to the total combined effects of all projects considered, the contribution of the project in question to the total effects, or both of these? Do cumulative effects encompass the inter-related effects of a single project on a single receptor, or should this be presented separately? Should lodged planning applications be taken into account or even projects at EIA scoping stage? Should existing developments simply form part of the baseline description?

Another problem is uncertainty. While it may sound logical to assign differing levels of confidence to assessment findings depending on how far projects have advanced through the planning process, in practice, this can be difficult to in a manner that genuinely aids decision-making. Should a range of possible scenarios be defined and assessed, or is this too complicated and cumbersome? How do you deal with ‘approved’ projects that are the subject of a multi-staged consent and so lack detail at earlier stages? What about projects that don’t require EIA and are supported by limited information, but which may still merit consideration? 

There is also the issue of confidentiality and impartiality. Cumulative assessments are likely to play a key role in development hotspots and/or particularly sensitive receiving environments where there is need for a robust discussion regarding the overall level and ‘shape’ of development to be consented. In challenging commercial circumstances, developers may be unwilling to share information before it reaches the public domain, or even supporting information that has not been made publicly available. Is it possible to verify that an unbiased CEA has been undertaken as the responsibility of a single developer? In theory, a more strategic level assessment can help to overcome these problems, but in reality, assessment findings often have to be heavily caveated due to the lack of individual project detail. Furthermore, strategic decision-making authorities only have limited resources available to undertake or commission this work themselves. 

Defining significance also becomes challenging when considering cumulative effects. For example, how do you ensure that you identify where an accumulation of individual non-significant effects could, in combination, trigger a significant effect such as exceeding an established threshold? If an EIA practitioner focuses too narrowly on identifying likely significant effects, there is a danger that cumulative non-significant effects might be missed. If a significant cumulative effect arises from a combination of individual non-significant effects, how is appropriate mitigation identified and apportioned? Should effects upon a single receptor be counted twice if there is a significant standalone effect and also a significant cumulative effect, or is this double counting (and if so, does this matter)?

As a minor, but important, point, the cumulative assessment is traditionally tackled at the end of chapter drafting, when the author is running out of time and energy.  This could mean that, despite best intentions, the cumulative component of the assessment may not get the attention it deserves.

The way forward

In the EIA community, there is building momentum for sharing our concerns, ideas and approaches around CEA. Recent events have brought practitioners together to help define the problems, and vehicles such as IEMA webinars and the newly published Impact Assessment Outlook Journal provide further opportunities for this to continue. This continual dialogue is essential to developing practice in CEA, but it is not enough.

Given the status afforded to CEA through the Directive and UK regulations, and the importance of consistency in ensuring valid assessments, a commonly agreed and replicable approach is needed. As the representative body of the EIA profession, IEMA is best placed to take on the mantle of defining what CEA should be and how it should be done. Whether this needs further research, casting a wider net than just the UK, or even a primer for an agreed ultimate methodological guidance note on CEA, in an ever more complex world of EIA and the drive for user-friendly assessment and proportionality, the time for that surely is now. 

The EIA community must push for what it needs. The IEMA Impact Assessment Steering Group can make the case for more research, a primer on CEA, or even the development of technical guidance – but it is incumbent on all of us to encourage, support and inform this process.

Andy Mitchell is an associate at Arup and leads the Glasgow environment team.

Joanna Wright is a director of environmental planning for environmental consultancy LUC.






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