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Impact Prediction Comparison of Alternatives and Determination of Significance

Prediction should be based on the available environmental baseline of the project data. Such predictions are described in quantitative or qualitative terms.

Considerations for Impact Prediction

Magnitude of Impact: this is defined by the severity of each potential impact and indicates whether the impact is irreversible or, reversible and estimated potential rate of recovery. The magnitude of an impact can not be considered high if a major adverse impact can be mitigated.

Extent of Impact: the spatial extent or the zone of influence of the impact should always be determined. An impact can be site-specific or limited to the project area (SP); a locally occurring impact within the watershed of the proposed project (L); a regional impact that may extend beyond the watershed (R); and a national impact affecting resources on a national scale (N).

Duration of Impact: environmental impacts have a temporal dimension and needs to be considered in an EIA. Impacts arising at different phases of the project cycle may need to be considered. An impact that generally lasts for only three to nine years after project completion may be classified as short-term (St). An impact which continues for 10 to 20 years may be defined as medium-term (Mt), and impacts that last beyond 20 years are considered as long-term (Lt). The type of impacts produced during the construction phase are of generally of short-termed.

Example

Nepal: Impact identification of Langatang Kohla Hydropower project

Uncertainty in Impact Prediction...

When the impact of some activity is predicted, the assessor gives an impression that it is going to happen certainly. In most of the cases, however, it is universally true that all the predictions made have some elements of uncertainties about social, physical and economical environment; uncertainties about guiding values such as policies, priorities and legislation and uncertainties about related decisions such as planning, negotiation, coordination, etc., usually affect the accuracy of prediction in EIA process.

However, in resolving the question of uncertainty, the issue of probability of occurrence and confidence limit of impact prediction has to be addressed. All predictions should be expressed such that each of the outcomes should be within a certain range of percentage of confidence. For example, a new industrial unit might emit a noise level of 65-70 DBA, and this is concluded at the 95 per cent confidence interval. This means that only five times out of 100 would be the noise level be predicted to exceed the expected range.

Comparison of Alternatives...

Assessment of alternatives in EIA has been considered as the "heart" of environmental impact reports. In order to achieve systematic decision-making in the choice of alternatives, it is desirable to use trade-off analyses, which typically involve the comparison of a set of alternatives relative to a series of decision factors. The following formal and informal approaches can be used to carry out the comparative analysis:

Qualitative approach: in which descriptive information on each alternative is presented,

Quantitative approach: in which quantitative information on each alternative is presented,

Ranking, rating or scaling approach: in which the qualitative or quantitative information, on each alternative is summarised through the assignment of rank, rating or scale value usually based on the characteristics of the impacts (severity, reversibility, etc.),

Weighting approach: in which the importance in weight of each alternative is presented in view of the relative importance of the decisive factors, and

Weighting-ranking/rating/scaling approach: in which the relative importance of either environmental factors, or impacts are determined and numerical weights are assigned to each factor or impact. The important weight is multiplied by the ranking/rating of each alternative, then the resulting products for each alternative are summed up to develop an overall composite index or score for each alternative.

Example

Nepal: Prediction and Determination of Significant impacts
Prediction of Impacts of the implementation of Langatang Khola Hydropower Project Nepal - An example from Nepal - A step wise consideration

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