EIA Quality Mark
Article
‘What are the chances of that?’ - Probability and its Role in Determining Impact
Significance.
A key purpose of Environmental Impact Assessment
(EIA) is to assess the ‘likely significant effects’;
however there is little explanation as to what this
term actually means. The term implies there are also
“unlikely significant effects” and “likely and unlikely
non-significant effects”; however it could be asking
us to reflect our confidence in the prediction of
significance. We would like to focus on what the
term ‘likely’ means, and how it should, or should
not, affect our assessment of the significance of
effects. Two aspects of ‘likelihood’ are:
1) The probability of an effect occurring (e.g.
an accident at power plant could be
catastrophic for the environment, but the
probability of it occurring may be very
small).
2) Our confidence in judgments made of the
sensitivity or value of the receptor, or
whether there is a receptor at all (e.g. the
potential for buried archaeology of
importance to be present is low therefore
significant effects are unlikely).
Previous articles in The Environmentalist (Duckett,
D. 2012; Parsons, D et al 2012) have presented
useful discussions about taking a risk based
approach, mostly where there is uncertainty about
the receptor. This article focuses on situations
where the probability of a source or pathway
existing in the first place is considered unlikely.
Schedule 3 of the EIA Regulations gives the first
insight. When considering the potential for
significant effects of Schedule 2 development the
regulations require the character of the
development to be considered including:
“the risk of accidents, having regard in particular to
substances or technologies used” and, also the
characteristics of the impact including “the
probability of the impact”
Once it has been determined that a development
does have the potential to cause significant effects
and is ‘EIA Development’, Schedule 4 of the
Regulations deals with the required content of an
ES. Point 1.3 requires
“A description of the aspects of the environment
likely to be significantly affected by the
development”
and point 1.4
“A description of the likely significant effects of the
development on the environment”
and in part 2.3
“The data required to identify and assess the main
effects which the development is likely to have on
the environment.”
Note that there is no longer any reference to
probability; we now have the term ‘likely’ included
before the term ‘significant’ and, importantly,
separated from the main effects in point 2.3. This
could be interpreted as meaning that probability
should not be a factor in deciding significance, but
should be applied after significance is defined. Given
that the term ‘probability’ only occurs in Schedule 3,
it may be that consideration of the probability of an
effect occurring is deemed to be of greater
importance at the screening and scoping stage.
This is highlighted again in the Part 4.13 (6c) which
state that in developing its scoping opinion the
relevant planning authority must take into account
“the environmental features likely to be affected by
the development”. If the probability of the effect
occurring is very small, can we really say that a
feature is likely to be affected? The implication
seems to be that once we are undertaking EIA and
presenting the ES, we are dealing with those effects
that are likely to happen.
Tests of the term ‘likely significant effects’ in case
law are limited. In Regina v Rochdale Metropolitan
Borough Council, Ex Parte Milne, Sullivan J appears
to define ‘likely’ as a factor of probability. The
following extract is interesting, in that it again
appears to separate probability from significance:
“ ..one is identifying and mitigating the “likely
significant effects”, not every conceivable effect,
however minor or unlikely”
It could be argued that whilst the EIA directive is
intended to deal with the likely effects of a
development, other directives, such as SEVESO II1
are intended to deal with the potential
environmental consequences of major accidents.
However an AC-IMPEL publication of 20011 had this
to say:
“..The scope of the information required under the
EIA procedure is the widest. It should therefore be
the starting point for information for all (and
frequently subsequent) procedures..”
Uncertainty surrounding how probability should
influence screening, scoping and EIA reporting is
evident when reviewing Environmental Statements.
There are a wide range of responses to dealing with
the consequences low probability effects. Some
completely avoid the issue of accidents/nonstandard operating conditions; others consider them
only in terms of human safety effects and a few take
a full environmental risk assessment approach.
Perhaps the best option may be to separate out
effects that could result from an accident, or non
standard operating conditions, into a separate
chapter or volume of the ES, or to reference other
documents where such information can be found.
Consideration also needs to be given to keeping
source and pathway probability separate from
significance e.g. “Although the effect would be
significant, as the magnitude is large and the
sensitivity of the receptor is high, it is not likely, as
the probability of it occurring is extremely low…”
Given the wide range of approaches being adopted,
perhaps it is time for further debate on the role of
probability in EIA, and whether it can and should
help determine significance, or should be applied
separately.
Prepared by Nigel Pilkington and Jane Knowles of
AECOM.
1
Directive 2003/105/EC of the European Parliament and of the
Council of 16 December 2003 amending Council Directive
96/82/EC on the control of major-accident hazards involving
dangerous substances.
1
AC-IMPEL 2001. Review of the Interrelationship Between IPPC,
EIA, SEVESO Directives and EMAS Regulation.
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