Proposed electricity generation developments: peat landslide hazard best practice guide

Introduction

1.1 Purpose

Peat landslides represent one end of a spectrum of natural processes of peat degradation. They have the potential to damage peatland habitats, affect biodiversity and deplete the peatland carbon store, which globally represents some 30% of the carbon stored in world soils (Immirzi et al., 1992). Human activity, including burning, farming (grazing), afforestation and construction may also act to damage the peat resource ( e.g. Lindsay and Bragg, 2004).

In the wake of widely reported peat landslide incidents in 2003 ( e.g. Dykes and Warburton, 2008; Lindsay and Bragg, 2004; Mills et al., 2007) and given the number of onshore wind farms and small hydro schemes being developed, there has been an increasing focus on peatlands when considering future Section 36 applications seeking consent under the Electricity Act 1989 (Scottish Government et al., 2014; SNH and FCS, 2010; Scottish Renewables and SEPA, 2012). While this guidance is primarily focused on S36 applications for electricity generation projects, the principles apply equally to S37 applications for above-ground overhead lines which pass through peatland environments. This guidance, first published in December 2006 (Scottish Executive, 2006), has been developed to provide best practice information on methods for identifying, mitigating and managing peat landslide hazards and their associated risks.

Following a ten year period of practice this guidance has been updated to reflect new research and publications. The revision takes account of practical experience of working with the guidance from developers, statutory bodies and the Scottish Government Energy Consents Unit against a backdrop of increasing efforts to safeguard peatlands via guidance documents issued in related fields (Scottish Renewables and SEPA, 2012; SEPA, 2010; Scottish Natural Heritage, 2016).

1.2 Guidance objectives

The objectives of this guidance are to:

• Promote best practice and raise awareness of potential peat landslide hazards and their associated risks;
• Provide guidance on identifying natural peat landslide hazards and elevated landslide hazard associated with construction and operation activities in relation to any proposed development;
• Provide guidance on the required scope of site investigations at various stages in the planning process for proposed electricity generation developments; and
• Provide advice on potential mitigation options for detailed feasibility assessment in the planning of upland electricity generation developments in order to reduce peat landslide hazard and risk.

1.3 Context

Blanket bog is the most widespread peatland type in Scotland, particularly in the uplands, and is the one most commonly affected by electricity generation developments. However, raised bogs, intermediate bogs and fens are also sometimes affected, directly or indirectly. All of these habitats are of high value for nature conservation due to their rarity and/or vulnerability and all are particularly susceptible to changes to their hydrology. Blanket bog, raised bog and some types of fen are on Annex 1 of the EC Habitats Directive, meaning that some examples are worthy of designation as Special Areas of Conservation ( SACs).

Lowland raised bog, blanket bog and upland flushes, fens and swamps are included on the Scottish Biodiversity List ( SBL), which includes all habitats that Scottish Ministers consider to be of principal importance for biodiversity conservation in Scotland. The SBL was published in 2005 to satisfy the requirement under Section 2(4) of The Nature Conservation (Scotland) Act 2004. The list has been updated several times since its original publication to take into account changes to the UKBAP priorities list, and most recently has been revised into 'Categories for Action' which relate to the different types of activities that public bodies carry out to deliver their Biodiversity Duty.

Peat landslides are a characteristic landscape response of peat uplands to intense rainfall events, and the importance of understanding their impacts and the potential for their occurrence is now well understood. It is estimated that Scotland's peatlands hold approximately 50% of the UK's total soil carbon store (Cummins et al., 2011), and as infrastructure pressure on peat uplands increases, the potential impacts of wind farm developments must be considered alongside their potential benefits.

In recent years, costs and benefits have been considered not just in terms of the stability of the peatland (this guidance), but the volumes of peat excavated, reused and potentially lost during construction (Scottish Renewables and SEPA, 2012) and the carbon balance of the wind farm (Scottish Government, 2011). In recognition of the importance of peat as a carbon store, a number of policy documents and national plans make clear the Scottish Government's intention to protect, manage and restore degraded peatlands to their natural functions, biodiversity and benefits, and in so doing create a source of carbon sequestration (Scottish Government, 2017; Scottish Natural Heritage, 2015a).

Just as wind farms and their associated infrastructure may be affected by or cause peat landslides, other infrastructure such as road networks, flood defences, drainage, power lines, residential areas and farmland may also be affected. Terrestrial habitats in the path of a peat landslide may be damaged by ground displacement and by burial by debris, and aquatic habitats damaged by incorporation of landslide debris in watercourses (McCahon et al., 1987). In addition, the displacement and break-up of peaty debris after a landslide event will ultimately result in small scale depletion of the terrestrial carbon store (Nayak et al., 2008).

Typically, slope instability and landslide hazard assessments have followed a standard approach, detailed in a number of statutory and guidance documents ( e.g. BS5930, 1999; Department of Environment, 1990; 1996). However, previous investigations have illustrated that the geotechnical controls of peat landslides differ from landslides in mineral soils (dry peat is typically 90% - 95% organic matter) and that pre-conditions for failure are not well accounted for by site investigation methods detailed in existing documentation. For example, peat has special hydrological properties (90% water content), it has a very low density and is often very fibrous in nature (Hobbs, 1986, 1987). Therefore, this guidance has been developed to ensure that appropriate and reliable peat landslide hazard and risk assessments can be undertaken during the planning of upland electricity generation developments such as wind farms. Although the Derrybrien slide in Ireland remains the only widely publicised peat landslide event to have occurred during wind farm construction, other failures have occurred close to (but not necessarily in association with) wind farms ( e.g. Long et al., 2011).

1.4 Information requirement

At the project level, large engineering projects involving peat should be planned and carried out using national best practice. This includes geotechnical risk management as discussed in the joint publication by The Institution of Civil Engineers ( ICE) and the Department of the Environment, Transport and the Regions ( DETR) publication "Managing Geotechnical Risk" (Clayton, 2001).

The Energy Consents Unit ( ECU) looks for a peat landslide hazard and risk assessment ( PLHRA) that addresses the guidelines of The Electricity Works (Environmental Impact Assessment) (Scotland) Regulations, Schedule 4, "Content of an Environmental Statement". These Regulations are intended to cover all aspects of an EIA. Part II of these regulations sets out in general terms the requirements that a PLHRA must satisfy:

• The data required to identify and assess the potential impacts that the development is likely to have on the environment;
• A description of the development comprising information on the site, design and size of the development;
• A description of the measures envisaged in order to avoid, reduce and, if possible, remedy significant potential adverse impacts.

The ECU expects developers to demonstrate that site specific peat stability information has been properly recorded, analysed and presented. For example if a developer's site investigation/survey identifies any area of high or medium risk in relation to a potential peat landslide incident, then it is expected that the submitted information will include detailed proposals of mitigation measures that reduce those risk levels to an acceptable or manageable level (see Table 5.4). The EIA should also address the risks associated with peat landslides following construction and after restoration works are complete.

The PLHRA is carried out by the developer as part of the Environmental Impact Assessment and will be assessed on behalf of the ECU by their appointed assessor. A written assessment of the developer's submitted PLHRA will be prepared by the ECU. This assessment is known as a 'Checking Report'. The Assessor's 'Checking Reports' will be succinct, providing clear and justified conclusions, detailing those issues which require addressing to ensure a satisfactory assessment while also providing recommendations for minor revisions to improve clarity or content where appropriate. If considered necessary by the Assessor, draft conditions of consent addressing specific recommendations will be provided as an annex to the Checking Report.

ECU acknowledges that in complex cases, some iteration of reports submitted under the guidance may be necessary to resolve technical aspects of the PLHRA. Where this is the case, clear guidance will be provided by the ECU, facilitated by the Assessor.

1.5 ECU assessment services

Most Section 36 applications will be assessed in relation to their potential to generate peat landslide risks. Exceptions would be those sites where there is no peat or where peat is highly localised and demonstrably outwith areas proposed for infrastructure. Section 37 applications should also be assessed for peat landslide risk where infrastructure is proposed in peatland areas.

The scope of each Checking Report will be relative to the scale, complexity, and topography of each development site. Information that has been referenced but not submitted will not be reviewed. A site visit may be carried out in order to help the Assessor prepare the written assessment, and will be arranged through the developer, if required.

Each Checking Report will include a summary of the findings of the developer's PLHRA and will confirm whether or not adequate and appropriate field survey, peat sampling and analytical methods have been employed to assess peat stability and associated landslide risks. The report will provide a summary outcome as follows:

• The PLHRA is considered to be satisfactory: the checking report has determined that the PLHRA is sufficiently robust in all respects; although some recommendations may be made for clarity, no further revisions are required.
• The PLHRA requires minor revisions: although much of the PLHRA is sound, one or two key elements are considered to be insufficiently robust to support the PLHRA conclusions and minor revisions are required; areas for attention will be advised in the review findings and may be progressed by the developer through either an appendix to the original submission or by clarification letter.
• The PLHRA requires resubmission: there are significant shortcomings throughout the PLHRA and reworking of the PLHRA report is required to support a robust assessment; areas for attention will be advised in the review findings and outline guidance offered to support the developer in preparing a satisfactory PLHRA.

Outline contents of a typical PLHRA are presented in Appendix D.

Developers should note that the Scottish Government has employed the checking services under contract. The ECU will review the Checking Reports before issuing them to each developer. Developers are asked to submit responses directly to the ECU and not to the assessment contractor acting on behalf of the Scottish Government.

ECU staff can consider brokering one-to-one dialogue between the assessment contractor and the developer, in order to discuss and clarify further information requirements and/or agree technical solutions. It is accepted that the developer must be permitted to make the decision on what data are used to support a particular development and for this reason ECU is prepared to review and comment on any approach that treats geotechnical risk management in a reasonable fashion.

1.6 Developer design team

Detailed assessments of peat landslide hazard as a precursor to risk assessment require an understanding of geology, peat hydrogeology and ecology, and the geotechnical properties of peat and the underlying materials. Accordingly, assessments of peat landslide hazard and risk require a competent, multidisciplinary team comprising at least three of the following disciplines:

(i) Engineering geologist;

(ii) Geomorphologist;

(iii) Geotechnical engineer;

(iv) Hydrogeologist/hydrologist;

(v) Ecologist.

These team members should be led by a Competent Person who will be chartered through an appropriate professional institution ( CEng, CGeol, CIWEM, MICE or equivalent) with a minimum of 5 years demonstrable experience in managing geotechnical risk and undertaking upland geohazard assessments and/or surveys, specifically in peatland environments. Given the complexity of peatlands, the qualifications and experience of the team should be clearly stated early in the developers' reports.

1.7 Checklist for peat landslide hazard and risk assessments

Figure 1.1 provides a pathway from an initial scoping study through to delivery of a detailed site assessment of peat landslide risk in the form of a PLHRA. Once the area of interest has been identified, the scoping study, supported by an initial site reconnaissance survey, should provide the basis for a first pass assessment of potential peat landslide hazards.

Should the site indicate potential for peat landslide hazards, guidance follows to assist the specification of detailed site investigation to obtain critical information to inform peat landslide hazard and risk assessment and engineering mitigation of associated risks. Exit points from the hazard and risk assessment process are provided at appropriate stages. For example, if only minimal peat cover is identified after the desk study stage and confirmed by site reconnaissance, the option to exit the hazard assessment process is made available.

The structure of this document follows the assessment process shown in Figure 1.1. A brief review of peat landslide mechanisms and indicators follows to provide context for those unfamiliar with peat landslide hazards. This is followed by detailed guidance on preparation of a front-end desk-study, initial site assessment and accompanying site reconnaissance survey, criteria for detailed ground conditions assessment thereafter, and an overview of the principles of hazard and risk assessment.