9. Predicting cumulative impacts

SEA involves assessing the impacts of a strategic action on various receptors: it focuses on the strategic action.  Cumulative impact assessment (CIA), instead, cuts in the opposite direction: it focuses on the receiving environment - the receptors.  It considers, for each relevant receptor, the impacts of the strategic action, together with other past, present and future actions: its cumulative impacts

Synergistic impacts are a subset of cumulative impacts, where impacts interact to produce a total impact greater than the sum of the individual impacts.  For instance, NOx and volatile organic compounds (VOC) each have impacts on human health; but when they combine to form ozone, they have different, often more negative, impacts.  Synergistic impacts often happen as habitats, resources or human communities get close to capacity.  For instance a wildlife habitat can become progressively fragmented with only limited effect, until the last fragmentation makes the areas too small to support the species at all: at that stage the species becomes extinct in that areas - a very significant synergistic impact.

Cumulative impact assessment

CIA has been required as part of many countries' project EIA systems for years, and is supported by a range of guidance internationally (e.g. CEAA 1998, 1999; Court et al. 1994; ODPM 2005, USCEQ 1997).  However this often focuses on project-level rather than strategic CIA.  Generally, CIA is still an underdeveloped and too rarely applied part of EIA and SEA.

Example of CIA: Appropriate Assessment under the EC Habitats Directive

Article 6.3 of the EC 'Habitats Directive' (Directive 92/43/EEC) requires that

"Any plan or project not directly connected with or necessary to the management of [a Special Area of Conservation (SAC) for habitats or a Special Protection Area (SPA) for birds] but likely to have a significant effect thereon, either individually or in combination with other plans or projects, shall be subject to appropriate assessment of its implication for the site in view of the site's convervation objectives.  In the light of the conclusions of the assessment... the competent national authorities shall agree to the plan or project only having ascertained that it will not adversely affect the integrity of the site concerned and, if appropriate, after having obtained the opinion of the general public."

The requirement to consider a plan's impacts "in combination with other plans and projects" makes this a CIA requirement.

The receptors here are SPAs and SACs.

The test of impact significance is whether the "in combination" impacts affect site integrity.

The main steps in strategic-level CIA are (Therivel and Ross, 2006):

  1. identify the affected receptors (scoping)
  2. determine what past, present and fugure human activities have affected or will affect these receptors, and what has led to these activities (context) 
  3. predict the impacts on the receptors of the strategic action in combination with the impacts of other human activities, and determine the significance of the impacts (prediction)
  4. suggest how to manage the cumulative impacts (mitigation).

Clearly these should be carried out alongside the relevant SEA stage.  They are discussed separately here because CIA involves a different, and often more subtle and complex, mind-set than SEA.

Some ideas for how to carry out each of these steps are given below.  Therivel and Ross (2006) provide more detailed information.


The CIA scoping stage aims to identify those cumulative impacts that require further assessment. Good starting points are:

  • Existing problems identified at the baseline stage.  These are generally cumulative impacts that have built up over time.
  • Cumulative impacts and problems identified in the SEA/CIAs of higher-level strategic actions. These should often by "imported" to the lower-level SEAs. For example if a habitat is shown to be particularly affected at the regional level, then this should be taken into account at the local CIA scoping stage, even if the habitat is not rare or endangered at the local level.
  • Issues that are not yet problems but are likely to become so in time, notably climate change and flooding.
  • Receptors that are consistently negatively impacted by the strategic action. For instance, the sub-components of the strategic action may all be basically sustainable, but may all have a negative impact on air quality: air quality would then be 'scoped in' as a cumulative impact.

This should result in a draft list of cumulative impacts which can be fine-tuned - added to and removed from - during subsequent stages of CIA.


The baseline stage in CIA aims to provide the background data needed for CIA prediction and mitigation. For each scoped-in impact, it should

  • Identify what past, present and likely future activities lead to that impact. The historical evolution of environmental problems is often particularly useful in suggesting effective mitigation measures.
  • Explain, where appropriate, how the baseline situation compares with relevant targets.
  • Identify other organisations that contribute to the impacts and/or need to be involved in managing them.


The prediction stage aims to predict the cumulative impacts of the strategic action, and other past, present and future actions.  It is this "and other..." clause that differentiates CIA from SEA.

Typically the prediction stage involves identifying 1. the total impacts on a receptor arising from different components of the strategic action (intra-strategic action impacts), and 2. the total impacts of the strategic action in combination with those other activities (inter-strategic action impacts). The two figures below show simple techniques for identifying these two different types of cumulative impacts.

Cumulative impact predictions should:

  • be based on reasonable assumptions, for instance about people's future behaviour, technologies and fuel price and availability;
  • consider long-term as well as shorter-term impacts, and not abbreviate the temporal or spatial horizons so as to miss out key cumulative impacts;
  • consider the interactions of the proposed strategic action and a good range of other past, present and future actions;
  • aim to identify overall trends in impacts. For instance, does the environment consistently lose out to social concerns? Are economic targets always phrased very precisely while social ones are much vaguer?


The aim of CIA mitigation is to avoid or reduce those cumulative impacts that are not already mitigated as part of the SEA process. Two ways of dealing with 'deaths by 1000 cuts' are to 1. reject the activities and/or 2. make the cuts/impacts smaller, i.e. to mitigate the insignificant impacts. Mitigation measures for cumulative impacts often focus on behavioural change, given that many cumulative impacts are due to people's behaviour in the first place.

The CIA prediction stage often shows just how little impact a given strategic action has on a receptor, relative to the sum total of all the impacts on that receptor.  People's behaviour (choices of where to live, when to drive v. walk, how warm to keep their houses etc.) and past actions often seem to account for a great proportion of a given cumulative impact; yet it is difficult to change these factors. This could suggest that a competent authority only has a relatively limited ability to do anything about a cumulative impact. The "apportionment of blame" is a common problem, and one that is often used to justify not doing much about a problem.  Arguably, any strategic action that makes a significant cumulative impact worse should at minimum fully mitigate its own impacts.

Mitigation measures are discussed further in the next few pages. The example below shows how mitigation measures can be devised specifically for cumulative impacts: in this case, the cumulative impacts of dog-walkers on ground-nesting birds in a heathland area (Surrey Heaths) in south-east England.

Example of buffer zones as mitigation for cumulative impacts: Surrey Heaths Special Protection Area (SPA)

The Surrey Heaths are a series of sites designated as Special Protection Areas under the EC Birds Directive because of their ground-nesting heathland birds.  SPAs require appropriate assessment under the Habitats Directive- see the box above.

Recreational disturbance - particularly the impact on ground-nesting birds of people walking their dogs - is a key impact on the Surrey Heaths.  Recreational impacts are already significant, and considerably more houses are planned near the Surrey Heaths for the next 20 years.

The UK government body responsible for biodiversity, Natural England, has proposed the following rules for mitigating these cumulative recreational impacts.  If agreed - they are contentious and even the draft rules have already been the subject of court cases by housing developers - the rules would be incorporated into the land use plans of all 11 local authorities around the heaths:

  • no housing within 400 metres of an SPA
  • for new housing 400 metres - 2km from an SPA, provision of new or improved open space at a minimum of 16 hectares per 1000 (new) population
  • for new housing 2 - 5km from an SPA, provision of new or improved open space at a minimum of 8 hectares per 1000 (new) population.

The map below shows these buffer zones.

The new open space would aim to provide alternative recreational facilities, to reduce recreational impacts on the SPA of both current users and the residents of any new housing.  Provision of the alternative open space must take into account not only the distance of housing from the heaths, but also the quantity of new open space provided, the size of the new sites (minimum sizes would be required), their distance from housing developments, and their quality.  New sites must be easily accessible, local, provide for letting dogs off leads, and provide a qualitatively similar experience to the Surrey Heaths site - i.e. they should be semi-natural and informal.

June 13, 2006 Uncategorized — brendan @ 2:20 pm

1 Comment »

  1. Our environment stayed for long recieving and acting as a sink of any wastes released from various sources, it is our grant and security to depend on, however the damage to our home is increasing from time to time. Because of this various stakeholders from differnet area are on work to halt the problem and to get alternative meanses that have a potential replace, restore, rehabilitate the room where we are in. As one of big stakeholder SEA course module provide highly valuable concepts and application techonologies that are on use through its enclosure. I as student of natural science (Ecological studies) i found comprhensive and integrative works, hence it will be the pioneer source of information for students wishing to work on environmental care!!

    Comment by Haylemikael Tafese — July 13, 2010 @ 5:30 am

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