D.8: Assessing the Significance of Impacts

Key Stages and Steps in the EIA Process
Stage 1: Before Submission of the Environmental Statement
  • Deciding whether EIA is required
  • Requiring submission of an Environmental Statement
  • Preliminary Contacts and Liaison
  • Scoping the Environmental Statement
  • Information Collection
  • Describing Baseline Environmental Information
  • Predicting Environmental Impacts
  • Assessing the Significance of Impacts
  • Mitigation Measures and Enhancement
  • Presenting Environmental Information in the Environmental Statement
Stage 2: Submission of Environmental Statement and Consideration of Environmental Information
  • Submission of Environmental Statement and Project Application for consent
  • Consultation and Publicity
  • Requiring more Information
  • Negotiating modifications to the Project
  • Considering the Environmental Information
Stage 3: Making the Decision
  • Making the Decision
  • Guaranteeing compliance
Stage 4: Implementation
  • Implementation of mitigation and compensation measures
  • Monitoring
  • Review, reassessment and remedial measures
  • Reporting

[See Also Figure 2, Sections C.3, D.4 and D.7, Case Studies 1 - 5 and Appendices 1 - 8]

SNH's Role

SNH will need to assess whether the effects on the natural heritage are significant to SNH's interests.

Statutory Provisions

The Environmental Statement should include a description of the nature, scale and significance of the effects, so this is a necessary procedure for the developer. It will also be a necessary procedure for SNH to consider the significance of the effects in order to make representations to the Competent Authority. Guidance on this stage is also provided in PAN 58 at paragraphs 47 - 52.

The Significance of Impacts

Whereas prediction of impacts should be a largely objective step, assessing the significance of impacts relies, at least in part, on value judgements, including placing weight or value on the environment likely to experience the change. The significance of impacts at this stage should relate back to the impacts deemed to be significant at the scoping stage (section D.4 above). It is also possible that new environmental effects may come to light in the assessment process because it should be iterative. Essentially, the EIA was undertaken to address impacts that were then deemed to be significant, has it revealed that the impacts will occur and if so how important will they be?

As with impact prediction, area staff may often need help from specialist advisers, especially for assessing the significance of landscape and visual impacts and impacts on earth heritage features, soils, natural systems and countryside recreation and access.

The significance of change is also related to the duration, timing and extent of effects, the degree of certainty in the prediction of impacts and the likelihood of irreversible changes occurring. For example, an effect which is unlikely, or the likelihood of which is uncertain, may nevertheless be significant if it would be a very serious or irreversible adverse effect, if it did occur. This is the basis of the "precautionary principle", see Section F.1 below.

The significance of the effects of a proposed development should be considered in the context of changes that will occur regardless of whether the project goes ahead or not, the "do nothing" alternative. The "do-nothing" comparison, or in some cases, such as road improvements, the "do-minimum" comparison, is a projection of the existing data to provide a baseline for comparison to show how the site would change if the project did not go ahead. The "do-nothing" comparison examines trends currently occurring at the site, including likely management, land use changes or other interventions, and assesses the significance of these changing conditions. The "do-nothing" comparison, however, should be used in a reasonable way, genuinely predicting likely change and not taking the best possible comparison for the purpose of the Environmental Statement.

Alternative solutions, if the project went ahead in a different form or at a different location, should normally be considered in an Environmental Statement. This will reveal the full picture of the project's effects and the least damaging option. If alternatives have been considered they should be included in the Environmental Statement along with the main reasons for choosing the final option.

Box D.8.1
Factors Affecting Significance of Impacts

The significance of an impact is derived from an analysis of:-

  • The sensitivity of the natural heritage resource to change, including its capacity to accommodate the kinds of changes the project may bring about;
  • The amount and type of change, often referred to as the impact magnitude which includes the timing, scale, size and duration of the impact;
  • The likelihood of the impact occurring - which may range from certainty to a remote possibility;
  • Comparing the impacts on the natural heritage resource which would result from the project with the changes that would occur without the project- often referred to as the “do-nothing” comparison; and
  • Expressing the significance of the impacts of the project, usually in relative terms, based on the principle that the more sensitive the resource, the more likely the changes and the greater the magnitude of the changes, compared with the do nothing comparison, the greater will be the significance of the impact.

A matrix can be used for considering the significance of impacts. This may combine the work previously undertaken for the assessment in respect of baseline information about the resource and impact prediction. The sensitivity of the resource can be analysed from the baseline information and may be summarised and classified in a matrix, an example of which is given in Figure 6 below.

The significance matrix can combine the information about the sensitivity of the resource, in this case the landscape resource, with the information previously compiled about the magnitude of impacts, of the kind shown in Figure 5 above. Combining the two sets of analysis, from Figures 5 and 6, enables a simple matrix of significance to be compiled as shown in Figure 7.

Figure 6
Example of Scale of Sensitivity of Landscape Receptors

High Sensitivity Key characteristics and features, identified by systematic landscape character assessment, which contribute significantly to the distinctiveness and character of the landscape character type. Designated landscapes eg. National Parks, NSAs and AGLVs and landscapes identified as having low capacity to accommodate proposed form of change.
Medium Sensitivity Other characteristics or features of the landscape that contribute to the character of the landscape locally. Locally valued landscapes which are not designated. Landscapes identified as having some tolerance of the proposed change subject to design and mitigation etc.
Low Sensitivity Landscape characteristics and features that do not make a significant contribution to landscape character or distinctiveness locally, or which are untypical or uncharacteristic of the landscape type. Landscapes identified as being generally tolerant of the proposed change subject to design and mitigation etc.

Figure 7
Example of a Matrix Showing Impact Significance Related to Sensitivity and Magnitude of Change

Significance of Impact Sensitivity of Receptor Magnitude of Change
Substantial / High High High
High Medium
Medium High
Moderate / Medium High Low
Medium Medium
Low High
Slight / Low Medium Low
Low Medium
Low Low
No Change High Medium or Low No Change

The construction of the matrix for weighing the significance of landscape and visual impacts should be adapted to fit individual cases or types of cases. For example, a significance matrix for natural heritage impacts may look like the example in Figure 7. The impacts are individually ranked for their significance on the basis of the sensitivity of the resource and the magnitude of the change, a high sensitivity resource and high magnitude of change would result, self evidently, in a high or "substantial" significance of the impact.

Beneficial and adverse impacts should be treated in the same way.

Box D.8.2
SNH Approach to Impact Significance

SNH should seek to ensure that all Environmental Statements:

  • clearly set out the sensitivity of the natural heritage resource;
  • the magnitude and likelihood of change, compared with at least the baseline information but preferably compared with the do nothing alternative; and
  • explain the significance of all relevant impacts on the natural heritage in a systematic, impartial, consistent and rational way that is clearly described in the Environmental Statement.

Predicting impact significance is partly objective and partly subjective. It relies on the professional judgement of landscape architects, ecologists and others who may place varying weight on the many factors involved. This naturally leads to differences of opinion. The Environmental Statement should therefore set out the basis of these judgements so that others can see the weight attached to different factors and can understand the rationale of the assessment. The Environmental Statement should clearly explain how the impact significance has been derived.

Box D.8.3
SNH Comments on Significance

SNH should not seek to criticise an Environmental Statement merely because it expresses conclusions which do not accord with SNH's conclusions.

Wherever possible, comments should identify why the conclusions are different so the Competent Authority may judge the basis of the two different assessments.

SNH should indicate how and where impact prediction in the Environmental Statement is inappropriate, eg. Where:-

  • inappropriate predictive techniques have been used;
    • impacts have been omitted;
  • the sensitivity of the resource is under-estimated, (eg. Insufficient attention has been paid to reasons why areas have been designated);
  • any aspect of the timing, scale, size or duration of the impact has been omitted or inappropriately applied to the assessment;
  • the impacts are not compared adequately or appropriately with the do nothing or other relevant alternatives;
  • the scale of impact significance is unclear, inconsistent, inappropriate or partial.