There are a wide range of techniques and methods for impact assessment, which are available to undertake EIA. Most have been developed during the 1970s in response to NEPA. Many of the more complex methods were initially developed by US government agencies that often dealt with large numbers of similar projects. Since their original design, many of these methods have been refined or altered and applied to other types of development. There are numbers of possibilities how to divide methods and techniques used in EIA processes. They can be divided into those that are analytical or quantitative in nature and those that are planning orientated. The following are some examples of some techniques belonging to each of these groups: analytical methods are spatial analysis; network analysis; biogeographic analysis; interactive matrices; ecological modeling; and expert opinion. Planning methods are multicriteria evaluation; programming models; land suitability evaluation; and process guidelines.
Other possibility how to distinguish techniques and methods used in EIA is if they are predictive or evaluating. The first group, predictive methods, is used during the scoping and impact identification phase of an EIA. Predictive methods can be subdivided into five distinct categories:
1. Checklists are the easiest of all methods to use, consisting of a list of various factors that may be affected by the development. Checklists are useful in identifying impacts generally, ensuring that impacts are not overlooked. The main disadvantage of the checklist is that they must be exhaustive if no serious impact is to be overlooked. However, checklists do not identify relationships between impacts and are therefore very limited in their application to indirect and cumulative impacts as well as impact interactions.
2. Matrices are the most commonly used methods in EIA. Matrices display in two-dimensional format the relationship between project actions and the environmental factors. The first matrices were used by Leopold, who created complex matrices with 88 environmental parameters along one axis and 100 development characteristics along the other. The magnitude and significance of the impact is assessed on a 10-point scale and the score is included in each cell indicating where a likely impact is anticipated. Recently, there are numbers ofmodifications of Leopold matrices. Matrices have been modified to display not only direct relationships between development actions and the environment but also to give indications of impact magnitude through impact-weighting systems. However, there are major problems with such weighted matrices, not least being the problem of subjectivity in attaching numerical values to different impact types. Additionally, conventional matrices deal only in direct impacts and are not, therefore, appropriate to the assessment of indirect and cumulative impacts as well as impact interactions.
Condensing information on complex environmental variables into some manageable form is a problem of assessment. Aggregation, sometimes called weighting and scaling, methods represents a technical fix to this problem. Advocates favor combing numerical values indicative of individual impacts into a surrogate reflecting overall impact. These methods have two elements, scaling and weighting. In the 'environmental evaluation system', value functions have been concocted to translate the state of individual environmental parameters into arbitrary, environmental quality indices all expressed on the same scale. Environmental quality scores are multiplied by the appropriate weightings and added to give a total score of environmental quality for each option under consideration. Many subjective elements are subsumed within both the weighting scheme and the value functions. In effect, the basis for a decision is created by the method. The preferred option and, hence, the only decision that logically can be taken, is the one with the highest score for environmental quality.
3. Quantitative methods cover a broad spectrum of techniques, from mathematical and numerical models to sophisticated computer models. Fundamentally, quantitative techniques attempt to compare impacts and producing a relative, composite index. Despite the appeal of quantitative techniques through their ability to provide numerical evidence to support impact assessment they have many weaknesses such as their complexity and can be easily manipulated by changing assumption underlying the model. In terms of assessing direct and cumulative aspects as well as impact interactions, quantitative techniques can be used to identify impact relationships but only the relevant parameters are known and included in the model. Moreover, these techniques reduce environmental components to discrete units, often losing a great deal of information in translation to numerical form.
4. Network methods are, theoretically, the most appropriate. Network method is a hybrid approach developed by Sorensen. Such methods recognize that environmental systems are composed of complicated, interrelated components and attempt to model these interactions. By following development impacts through the web of environmental relationships the effects of these impacts can be predicted through changes in the model. The drawbacks of using network are that they are very time consuming in development and require highly specialized knowledge to accurately create a network for each environment under consideration.
5. Overlay maps have been in use for a considerably long time in environmental planning being ideally suited for the consideration of spatial aspects, before even EIA was a recognized technique. By using a series of annotated base maps each reflecting a different environmental component of the development a composite picture of the developments impacts can be generated. A total impact can be shown by this technique. The advance of computer graphics and geographical information systems (GISs) has allowed weightings to be given to different types of information and more data to be analyzed with this technique. The disadvantage of this method is that at their most complex they are very capital- and skill-intensive whereas at their most basic level they are limited to a small number of impacts that can be overlain successfully, about a dozen, although photographic and hierarchical clustering approaches overcome this practical constraint. Computer development has revealed the full potential of overlay approaches. The raw data files can be differently manipulated and using different software can give good identification of locations with the highest impact and the minimal impact. Moreover, overlays will not identify secondary impacts and requires that the user has already identified the individual impacts before the technique can be used.
The second group of EIA methods, described as evaluation methods, can be used to assess the significance of identified impacts. Evaluation techniques can be classified into two groups. To the first group belong 'cost-benefit analysis' (CBA) techniques. CBA techniques rely on assigning monetary values to resources and calculating whether the economic gains of a development will outweigh the economic losses throughout the life span of the development. CBA techniques, when used solely for the purpose of EIA, have fundamental drawback in that many environmental resources are intangible and, therefore, cannot be priced in a meaningful way, for example, air quality or the value of endangered species or landscape. This factor prevents CBA being used as a comprehensive tool for impact evaluation in EIA. The inability of CBA to accommodate intangibles had led to the emergence of other monetary valuation techniques based on CBA that claim to be able to include intangible resources within their calculations. The valuation of intangible resources can be achieved through a variety of methods which measure, either directly or indirectly, the preferences of consumers of environmental resources. There are many pitfalls in utilizing these methods and their complexity is such that their use is confined to academic research projects and large-scale public sector development rather than project EIA. The second major group of EIA evaluation techniques termed multicriteria methods seeks to overcome some of the strictly monetary deficiencies of CBA by giving weight not only to tangible resources but also allocating weight to the differing views and goals from within society at large concerning environmental change. Similar to the weighted-matrix predictive techniques detailed above the scoring systems used in most multicriteria analyses are open to subjective interpretation and manipulation. There are two most popular multicriteria methods. The first one is the 'multi-attribute utility theory' (MAUT) which relies not just on the assignments of arbitrary units to value impacts, but also attempts to incorporate the values of key interest parties. Consultation with key interest parties, such as local group, has been identified as an important factor that is often overlooked in the identification of indirect and cumulative effects as well as impact interaction. Taking this concept a stage further is the second method, the Delphi method, which attempts to build the views of key parties into the evaluation process by the collection of expert opinion and gaining consensus on the issues being considered.
In practice, the application of all these methods for the identification and assessment of impact is either limited or has not been developed to its full potential. It is widely accepted that a single method would be unlikely to meet all the criteria required for effective assessment of impact. It would be expected that various techniques and methods in an adaptive approach would be combined to perform individual assessment. The most suitable combination of methods will depend on the nature of the problem, purpose of the analysis, access to and quality of data, and available resources.
There are numbers of methods which can be used in EIA process, but most of them use one of the approaches listed above.
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