Starting up; early stages - Introduction to Environmental Impact Assessment - page 122

decisions away from decision-makers (Skutsch & Flowerdew 1976). The methods are
difficult for lay people to understand, and their acceptability depends on the assumptions,
especially the weighting schemes, built into them. 3 People carrying out assessments may
manipulate results by changing assumptions (Bisset 1978). Quantitative methods also
treat the environment as if it consisted of discrete units. Impacts are related only to
particular parameters, and much information is lost when impacts are reduced to
numbers.
Networks
Network methods explicitly recognize that environmental systems consist of a complex
web of relationships, and try to reproduce that web. Impact identification using networks
involves following the effects of development through changes in the environmental
parameters in the model. The Sorensen network was the first network method to be
developed; it aimed to help planners reconcile conflicting land uses in California. Figure
4.16 shows a section of the network dealing with impacts on water quality. Water is one
of the six environmental components, the others being climate, geophysical conditions,
biota, access conditions and aesthetics.
The Sorensen method begins by identifying potential causes of environmental change
associated with a proposed development action, using a matrix format; for instance,
forestry potentially results in the clearing of vegetation and the use of herbicides and
fertilizers. These environmental changes in turn result in specific environmental impacts;
in the example, the clearing of vegetation could result in an increased flow of fresh water,
which in turn could imperil cliff structures. The analyst stops following the network when
an initial cause of change has been traced through all subsequent impacts and changes in
environmental conditions, to its final impacts. Environmental impacts can result either
directly from a development action or indirectly through induced changes in
environmental conditions. A change in environmental conditions may result in several
different types of impact. Sorensen argues that the method should lead to the
identification of remedial measures and monitoring schemes (Sorensen 1971).
A simpler version of this technique is used in the development of many UK Local
Transport Plans. Causal chain analysis (or cause-effect diagrams) are drawn by planners
to identify how one action—say maintenance, renewal and improvements to
carriageways and junctions (Figure 4.17)—leads to changes in social, economic and
environmental conditions. They also identify what preconditions are needed to achieve a
positive outcome, and problems to avoid.
Network methods do not establish the magnitude or significance of interrelationships
between environmental components, or the extent of change. They can require
considerable knowledge of the environment. Their main advantage is their ability to trace
the higher-order impacts of proposed developments.
Overlay (or constraints) maps
Overlay maps have been used in environmental planning since the 1960s (McHarg 1968),
before the NEPA was enacted. A series of transparencies is used to identify, predict,
assign relative significance to and communicate impacts. A base map is prepared,
showing the general area within which the project may be located. Successive transparent
overlay maps are then prepared for the environmental components that, in the opinion of