Now that the role of SEA assessment and mitigation in the development of the strategic action has been discussed, the next few pages go into more depth about the process of predicting, evaluating and mitigating impacts.
Impact prediction aims to identify environmental/sustainability changes that would result from the strategic action. It compares the likely state of the sustainability indicators with and without the strategic action. A strategic action's impacts can be:
- positive or negative;
- large or small;
- short-term or long-term;
- reversible or irreversible; and
- likely or unlikely to occur.
SEA normally deals with large areas, large numbers of potential projects/actions, and uncertainty about the strategic action's likely effects. Given these factors, and that SEA aims to help decision-making rather than achieving perfection, the predictions in SEA can 'look' quite different from those in EIA.
The interactive animation below shows types of impact predictions for an air pollution scenario.
A range of prediction techniques are used in SEA. The figure below shows some of these techniques, and a link to further information provided in the SEA-Wiki.
Possibly the most commonly-used prediction technique is expert judgement: one or more knowledgeable people decide whether they think the impact is likely to be positive, negative etc. This is a rapid and cost-effective technique, but generally gives only qualitative answers, and is open to bias.Another very common SEA technique is the impact matrix, based on an SEA framework. This can be used to identify and compare the impacts of sub-components of the strategic action, or alternative approaches to implementing the strategic action.
How much detail is needed?
The aim of the prediction stage is to get a good enough idea about the strategic action's likely impacts so that the competent authority can evaluate the impacts' significance, consider appropriate mitigation measures, and determine whether the strategic action should go ahead or not. The aim is NOT to give a perfect, detailed forecast, unless perfection is needed for the decision.
A broad distinction can be made between prediction techniques that are quantitative, detailed and rigorous (typically adapted from project EIA) and those that are qualitative, broad-brush and rapid (typically adapted from policy appraisal, public consultation approaches etc.). Clearly, rigour is necessary:
"The quantification of impacts, which is generally accepted practice at the project level, can also be valuable for informing the decision makers about the future ramifications of [strategic actions]. Quantification is also an important tool for comparing plan alternatives" (Bass, 1998).
On the other hand, many SEAs so far have tended to be excessively detailed:
"The level of abstraction for impact analysis should be balanced with the strategic level of information analysis, especially if details will be further elaborated in a tiered system of SEA/EIA... In the authors' opinion, the causes of the tendency towards unnecessary detail are that the environmental experts are not used to dealing with such a degree of uncertainty and that the public is too impatient to wait for more detailed impact predictions in later stages of decision-making" (DHV, 1994)
"quantitative assessment works fairly well when comparing similar alternatives, but if there are differences in principle, or a possibility of shifts in paradigm, measurement becomes nearly useless" (Jansson, 1999)
At the programme level, or where it is important for legal reasons, the comprehensive methods are likely to be appropriate. However it may be worthwhile starting with the faster, qualitative methods and only carrying out comprehensive analysis for those impacts where it really is necessary.