A “significant” effect may be broadly defined as one that should be brought to the attention of those involved in the decision-making process, and could result in a condition on the planning permission or even refusal of planning permission. This definition is prescribed to varying degrees by statute, EU and national guidelines and standards, and precedents established in previous EIAs.
Guidance on significance is mainly of a generic nature and practitioners have been obliged to develop definitions for specific topics and projects based on the locality. It is broadly accepted, however, that significance reflects the relationship between a number of factors:
- the magnitude or severity of an impact (i.e. the actual change taking place to the environment)
- the importance or value of the affected resource or receptor
- the duration involved
- the reversibility of the effect
- the number and sensitivity of receptors
The severity of an impact is often quantifiable in terms of, for example, extent of land-take or predicted change in air quality. The sensitivity, importance or value of a resource or receptor is normally derived from:
- Its designated status within the land use planning system.
- The number of individual receptors, such as residents.
- An empirical assessment on the basis of characteristics such as rarity or condition.
- Its ability to absorb change.
Significant effects are likely to occur where valuable or sensitive resources, or numerous receptors, are subject to impacts of considerable severity. Effects are unlikely to be significant where low value or non-sensitive resources, or a small number of receptors, are subject to minor impacts. In practical terms significance is often an issue of degree. Within the methodology this is often recognised by adopting the five levels of significance as described in the draft EIA guide to good practice and procedures.
These five levels of significance are:
- Severe: These effects represent key factors in the decision-making process. They are generally, but not exclusively associated with sites and features of national importance and resources/features which are unique and which, if lost, cannot be replaced or relocated.
- Major: These effects are likely to be important considerations at a regional or district scale but, if adverse, are potential concerns to the project, depending upon the relative importance attached to the issue during the decision making process.
- Moderate: These effects, if adverse, while important at a local scale, are not likely to be key decision making issues. Nevertheless, the cumulative effect of such issues may lead to an increase in the overall effects on a particular area or on a particular resource.
- Minor: These effects may be raised as local issues but are unlikely to be of importance in the decision making process. Nevertheless, they are of relevance in the detailed design of the project.
- Neutral: no effect, not significant. Irrespective of other effects.
Allocation of the significance of effects in intermediate situations will be a matter for professional judgement in each topic area. Within this ES, the following generic matrix (Table 1.2) is used to define the level of significance of effects. In some cases analogous matrices for the various specialist topics are used, and where these use different assessment criteria this is clearly stated within the relevant chapter.
