Impact significance - a risk-based approach

24th February 2012


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Derek Duckett from Xodus gives the practitioner's view on using a risk-based approach to assessing the significance of impacts

There are various methods of determining environmental impact significance which have a range of benefits, but also some limitations. Developing an approach to significance assessment that incorporates impact likelihood can help to resolve some of the potential limitations.

Identifying the issues

The first step is to identify which issues should be included in the assessment and to justify exclusions.

An effective method of identifying issues for significance assessment is to conduct an environmental issues identification (ENVID) workshop. This workshop should be undertaken during scoping and revisited throughout the project as designs are refined and mitigation strategies developed. The workshop should include relevant project staff including engineers and environmental personnel, for example.

The selected staff can work through all project aspects and make an assessment of each in terms of potential significant impact. This process should be formalised to ensure that is facilitated and recorded correctly.

The output of the workshop process is a matrix which records assessment of each impact’s significance in terms of its consequence and likelihood.

Consideration of likelihood

The more likely the change, the greater the significance will be. Consider, for example, the following definition: “Receptor sensitivity X potential impact magnitude = potential environmental consequence.”
Essentially, the likelihood of the potential overall consequence of the impact occurring is then explicitly taken into account to derive the impact significance.

One of the challenges in this process is to effectively and transparently communicate an impact significance that incorporates an assessment of likelihood. For some projects consequences can be very severe, but they will also have low probability of occurring. Impact significance assessment is often supported by modelling and this likelihood can be explicitly included in significance assessment.

Stakeholder perception and regulatory compliance can be combined with environmental consequence, where appropriate, to give an overall consequence. The potential overall consequence is then combined with likelihood to give the impact significance (see figure 1 below). The inclusion of regulatory, stakeholder and likelihood considerations provides an integrated assessment of the significance of risk or impact.

Figure 1: Impact significance flow chartFigure 1: Impact significance flow chart

Figure 1: Impact significance flow chart

There are, however, a number of challenges in assessing impact significance including:
• The double counting of impacts through confusion around impact magnitude, consequence and likelihood.

• An inconsistency of definitions across different industry disciplines.

• Ensuring consistency across different environments and assessment regimes.

Good communication and setting clear, explicit boundaries to incorporate varying definitions and assessment methods are key to resolving these challenges.

Example: Tidal turbine impact on seals

Underestimation of impact significance:

  • The impact magnitude for fatal seal-turbine interaction is assessed as negligible as the impact is considered extremely unlikely.
  • The overall consequence is then assessed as low because the “negligible magnitude” has brought down the high receptor-sensitivity, regulatory and stakeholder ratings.
  • The impact significance is therefore assessed as negligible because the “low overall consequence” has been combined with a “negligible likelihood”.

More accurate assessment:

  • The impact magnitude assessed as severe as the assumption is made that impact will occur.
  • The overall consequence is then assessed as high as receptor-sensitivity is high and there are also high ratings for stakeholder/regulatory requirements.
  • The impact significance is assessed as moderate (ie requiring mitigation) as the “negligible likelihood” of impact is combined with a “high consequence”

Lessons learned from the oil, gas and marine energy sector

Our experience in the oil and gas sector, and on a recent marine energy project, has reinforced the importance of effective communication and the engagement of all stakeholders to ensure the significance of impacts is accurately assessed.

EIA practitioners must ensure the development of consultation strategies and facilitation of stakeholder engagement throughout the EIA process, including the full involvement of engineering, design and management teams into the ESIA using ENVID workshops.

Practitioners should provide environmental advice throughout the project life cycle including best available techniques assessments.

For renewable energy projects involving new technologies an integrated approach is required to EIA and the design process. It is important to ensure clear communication with the developer, the regulator and the local community. Baseline survey methodologies must also be fit for purpose not only for your EIA but also for monitoring once installation completed and for future EIA.

Using a method of assessing impact significance that explicitly takes account of environmental, stakeholder and regulatory potential consequence and then combines this overall consequence with likelihood to give the impact significance, teases apart the various aspects of significance assessment.

The strength of this method is that each step deals with one aspect of significance at a time and, when used correctly, allows stakeholders to follow the logical steps that have led to the impact significance rating.


This article was written as a contribution to the EIA Quality Mark’s commitment to improving EIA practice.

Derek Duckett is a senior environmental consultant at Xodus

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