Environmental Engineering Reference
In-Depth Information
5.3.3
Impact Significance Criteria
Virtually any activity will impact, to some degree, the environmental
resource touched by the activity, but the challenge of environmental impact
analysis is to determine the level of impact and its significance. The level of
impact determination is dependent on the specific action, resource, and level
of intensity, and as discussed in this chapter, methods are available or can
be developed to measure the impact. But the measurement is only part of the
picture, and it is necessary to understand the significance of the predicted
measurement to fully address the goals of environmental impact analysis.
The level of impact can range from no observable change to collapse of an
environmental resource on a regional scale, and the position on this scale of the
predicted impact is the useful outcome of comprehensive impact prediction.
Assigning objective significance criteria identifying the threshold between
insignificant and significant impact levels is an approach proven successful in
positioning the degree of impact on a common scale among resources and alter-
natives (Figure 5.11). The thresholds or significance criteria vary for different
environmental resources and types of impacts, but applying the significance
criteria to every resource places impact prediction on a common impact scale.
Determining the level of significance on such a scale has multiple advantages:
r It is useful to decision makers with little or no expertise in the envi-
ronmental resources at issue. For example, informing a decision
maker that a particular alternative will generate an additional 20 trips
on the adjacent highway or decrease dissolved oxygen by 0.5 mg/l is
virtually useless unless the predicted change is placed in context.
16
Impact
significance
criteria
Unacceptable
range
14
12
10
8
6
4
2
0
0
5
10
15
20
Ideal
range
Environmental response
FIGURE 5.11
Impact significance criteria for defining relative level of impact.
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