Chapter 12: Cumulative, Synergistic and Secondary Environmental Effects
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Contents
Part 1 - Introduction
Part 2 - Legislative requirements
- The relevant statutory provisions
Part 3 - Cumulative, synergistic and secondary environmental effects
- Defining cumulative, synergistic and secondary environmental effects
- Methods for assessing cumulative, synergistic and secondary environmental effects
- Further guidance on cumulative, synergistic and secondary effects
Part 1 - Introduction
Introduction
12.1.1 Many environmental problems result from the accumulation of multiple, small, often indirect, effects. Examples include climate change, reduction in air quality, changes in the landscape and loss of habitat. These cumulative effects are difficult to assess at the project level, but may be more effectively captured at the strategic assessment level.
12.1.1 The purpose of this chapter of the Tool Kit is to provide an introduction to the key requirements and methods relating to the assessment of cumulative, secondary and synergistic effects. A list of additional, more detailed guidance is provided at the end of this chapter and it is recommended that practitioners refer to these documents when considering cumulative, synergistic and secondary effects.
Part 2 - Legislative requirements
The relevant statutory provisions
Screening |
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Section 8(1) - The responsible authority shall determine whether or not … a plan or programme …is likely to have significant environmental effects. Section 8(3) - In making a determination under subsection (1), the responsible authority shall apply the criteria specified in schedule 2. Schedule 2 (2) - Characteristics of the effects and of the area likely to be affected, having regard, in particular, to (b) the cumulative nature of the effects. |
12.2.1 A plan, programme or strategy ( PPS) subject to screening (and/or pre-screening) should include the cumulative nature of the effects as one of the criteria used to determine whether a significant environmental effect is likely to arise (see Chapter 4, Parts 2 and 3).
Environmental Report |
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Section 14(3) - The [environmental] report shall include such of the information specified in schedule 3 as may reasonably be required … Schedule 3(6) - The likely significant effects on the environment, including (e) secondary, cumulative and synergistic effects. |
12.2.2 Schedule 3 of the Environmental Assessment (Scotland) Act 2005 ('the Act') sets out the information to be included in Environmental Reports ( Chapter 6). This information includes the likely significant effects on a number of environmental issues, including secondary, cumulative and synergistic effects.
Part 3 - Cumulative, synergistic and secondary environmental effects
Defining cumulative, synergistic and secondary environmental effects
12.3.1 Schedule 3 of the Act refers to secondary, cumulative and synergistic effects. These terms are not mutually exclusive: often the term "cumulative effects" is taken to include secondary and synergistic effects. They are defined as follows:
- Secondary effects are effects that are not a direct result of the PPS, but occur away from the original effect or as a result of a complex pathway. An example of secondary effects is a development that changes a water table and thus affects the ecology of a nearby wetland.
- Cumulative effects arise, for instance, where several developments each have insignificant effects but together have a significant effect, or where several individual effects of the PPS have a combined effect. An example is the gradual loss of natural areas such as woodlands or Green Belt through discrete developments.
- Synergistic effects interact to produce a total effect greater than the sum of the individual effects, so that the nature of the final impact is different to the nature of the individual impacts. An example is the combination of SOx and NOx to produce smog.
12.3.2 Cumulative effects can arise from the combined effects of more than one PPS. They can also result from the interaction of individual policies within one PPS, e.g. a policy designed to improve transport flow and a housing policy may, when taken together, result in significant effects on noise levels and on landscape. Cumulative effects can result from a combination of past, present or (foreseeable) future actions which, although not necessarily significant in themselves, may together result in significant environmental effects. They can often happen as habitats, resources or human communities reach capacity. For instance, a wildlife habitat can become progressively fragmented with perhaps only limited effects on a species until the last fragmentation makes the area too small to support the species at all.
Methods for assessing cumulative, synergistic and secondary environmental effects
12.2.3 Methods and techniques are still developing for assessing cumulative, synergistic and secondary effects. The SEA Practical Guide ( ODPM et al, September 2005) lists a number of key principles underpinning the assessment of cumulative, synergistic and secondary effects:
- Consideration of cumulative/synergistic/secondary effects as an integral part of the SEA and PPS preparation process, rather than a separate stage.
- Focussing on identifying the total effect of both direct and indirect effects on receptors. These may include natural or historic resources ( e.g. air, water, soil, landscape, monuments), sections of the population or ecosystems and species.
- Consideration of the nature, extent and sensitivity of the receptors, such as ecosystems and communities, avoiding restricting the assessment to administrative boundaries.
- Consideration of effects that may arise from interaction with proposals within the PPS as well as those within other PPS.
- Take account of whether any effects will bring receptors such as a wildlife habitat close to their capacity or threshold to remain productive or sustainable.
- Being aware of the level of uncertainty in identifying cumulative, secondary or synergistic effects and ensuring that, where uncertainty exists, this is documented.
12.3.3 There are various tools available for the assessment of cumulative, secondary and synergistic effects; note that many of these are also used generally in SEA. A list of these is provided in Figure 12.1.
Figure 12.1. Examples of assessment tools (from Cooper, 2004)
Tools | Application |
|---|
Impact models, networks or systems models (includes network diagrams and causal chains) | Detailed assessment of cause-effect relationships between actions and valued resource and impact interactions |
Trends analysis | Assesses the status of a resource and changes in the occurrence or intensity of stresses over time. |
Indicators of change | Select indicators of environmental quality for an area and establish an allowable target or threshold for this indicator, which is then used to evaluate the cumulative effects of existing and future PPS. |
Carrying capacity | Measures cumulative effects against threshold |
Scenario analysis | Predicting outcomes of various scenarios |
Computer modelling | Quantifies cause-effect relationships leading to cumulative effects ( e.g. air, hydrological, water quality, noise, transport) |
Spatial analysis using Geographic Information System ( GIS) or overlay mapping | Can analyse biodiversity, landscape and historic environment parameters and identify where effects are worse. It can quantify results of actions ( e.g. area of land cleared) and changes to landscape features ( e.g. loss of woodland) |
Tables and matrices | Evaluation and comparison of variables |
Checklists | Can show whether the PPS has an impact and impact type; ensures coverage of topics |
12.3.4 It may be helpful to present the assessment in tabular format. Several example tables are provided overleaf. Note that, when assigning overall impact, negative and positive effects should not be automatically offset against each other. The conclusions regarding overall impact are likely to be based on expert opinion and professional judgment. It is good practice to set out how professional judgment is used for assigning impact, to ensure that the method is entirely transparent.
Figure 12.2 Example of a table for recording cumulative effects and their causes
(Figure 23 from the SEA Practical Guide ( OPDM et al, September 2005)
Cumulative Effect | Affected Receptor | Causes |
|---|
Habitat fragmentation | Wildlife habitats | Use of land for flood management, transport infrastructure and new development |
Climate change | Worldwide | Greenhouse gas emissions ( e.g. from industrial development, increases in traffic volumes) |
Loss of tranquillity | Countryside | New development and associated increases in traffic volumes |
Figure 12.3 (Source: Levett-Therivel Sustainability Consultants)
The following approach may be helpful for identifying the cumulative effects of the PPS across the SEA issues/topics: 
In the example above, each part of the PPS has a neutral or positive overall impact (reading 'down'), but the policies cumulatively have a significant adverse effect on climatic factors (reading 'across'). In such a case, it is good practice to review to see whether they cannot be changed so as to reduce their joint effect. Negative and positive effects should not be assumed to cancel each other out. + = positive, - = negative and 0 = neutral effect. |
12.3.5 Figure 12.4 is similar to Table 7 in the interim Environmental Report produced for the Midlothian and Shawfair Local Plans 1 The table provides a useful overview of negative effects and where they are concentrated. It summarises the environmental assessment of individual land allocations. A commentary would also be required on how these concentrations of negative effects may lead to significant cumulative, synergistic and secondary effects.
Figure 12.4 Check List for Cumulative and Synergistic Effects
(examples are illustrative- the criteria used in will depend on the PPS)
SEA topic | SEA Objectives/Criteria (examples) | Areas/sites with negative assessments |
|---|
biodiversity/flora/fauna | avoid all designated nature conservation sites | |
population | avoid land in the green belt | |
human health | avoid land take from open green space and recreational areas | |
soil | avoid prime agricultural land | |
water | avoid pollution of bathing waters and shellfish waters in accordance with legislation | |
air | avoid inappropriate/unlawful emissions | |
climatic factors | avoids inefficient energy consumption | |
cultural heritage | avoid inappropriate development in designated areas/sites | |
material assets | make full and appropriate use of urban brownfield land or other urban refill | |
landscape | avoid inappropriate development in designated landscapes | |
Further guidance on cumulative, synergistic and secondary effects
A Practical Guide to the Strategic Environmental Assessment Directive (Office of the Deputy Prime Minister, Department of the Environment in Northern Ireland, Scottish Executive, Welsh Assembly Government) http://www.communities.gov.uk/index.asp?id=1501988
Guidelines for the Assessment of Indirect and Cumulative Impacts as well as Impact Interactions (European Commission Europa website) http://europa.eu.int/comm/environment/eia/eia-studies-and-reports/guidel.htm
Study on the Assessment of Indirect and Cumulative Impacts … Research Study and Findings (European Commission - Europa website) http://europa.eu.int/comm/environment/eia/eia-studies-and-reports/volume2.htm
Cooper, L. M. (2004) Guidelines for Cumulative Effects Assessment in SEA of Plans, EPMG Occasional Paper 04/ LMC/ CEA, Imperial College London http://www.env.ic.ac.uk/research/epmg/CooperCEAGuidelinesJuly04FINAL.pdf
Environment Agency web pages on assessing cumulative effects. These web pages include links to other sites and a Literature Review/Scoping Study on Cumulative Effects Assessment and the SEA Directive produced for EA by TRL Ltd. http://www.environment-agency.gov.uk/aboutus/512398/830672/831980/832006/?version=1&lang=_e