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Peat Landslide Hazard and Risk Assessments: Best Practice Guide for Proposed Electricity Generation Developments

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Please refer to the second edition of this guidance (April 2017) published at http://www.gov.scot/Publications/2017/04/8868.

 

THE FOLLOWING GUIDANCE HAS BEEN SUPERSEDED BY THE SECOND EDITION PUBLISHED 26 APRIL, 2017. THE VERSION FROM JANUARY 2007 WILL BE RETAINED FOR ARCHIVAL PURPOSES.

 

1 Introduction

1.1 Purpose

In the wake of widely reported peat slide incidents and the increasing number of on-shore wind farms being developed, there is a greater focus on peat hazard risk assessment in considering future section 36 applications seeking consent under the Electricity Act 1989. This guidance has been developed to provide best practice information on the methods for identifying, mitigating and managing peat slide hazards and their associated risks.

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 the required scope of any preliminary site investigations for proposed electricity generation developments;
  • Provide guidance in identifying potential upland peat landslide hazard and risk prior to and during the planning of upland 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 and all are the subject of Habitat Action Plans (HAPs) under the UKBAP.

The Scottish Biodiversity Strategy adopts the UK HAP targets, which include no loss of current habitat extent and an improvement in the condition of what remains. Under terms of Section 1 of the Nature of Conservation (Scotland) Act 2004, the Scottish Executive and planning authorities are required, in all their decisions, to have regard to this Strategy.

Peat landslides are a characteristic landscape response in peat uplands to intense rainfall events. Failures initiate by sliding and may degenerate into peaty flows of debris before becoming incorporated in stream channels as peaty debris floods. The importance of understanding peat landslide mechanisms and the potential for their occurrence has increased as pressure for renewable energy technologies and development sites in peatlands has increased. Wind farms, applications for which are often concentrated in upland and peat covered areas, are seen by many as the means by which carbon emissions and the UK's reliance on fossil and nuclear fuels might be reduced. However, the high environmental value afforded to peat uplands requires that the benefits of wind farm developments are evaluated against their potential negative consequences for local peat areas and their often diverse and unique habitats. 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 impingement of landslide debris on watercourses. 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.

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; DoE, 1990; 1996). However, previous investigations have illustrated that the geotechnical controls of peat landslides are distinct to organic 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, supplementary guidance is required to ensure that accurate and realistic peat slide hazard and risk assessments can be undertaken during the planning of upland electricity generation developments such as wind farms.

1.4 Scope of document

This document provides guidance on:

  • identifying existing, potential and construction induced peat landslide hazards;
  • suitable intrusive and non-intrusive methods of ground investigation;
  • suitable methods for assessing peat landslide hazard and associated risk;
  • the management and mitigation of potential peat landslide hazard and risk to electricity generation development sites.

1.5 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 and the Transport Regions (DETR) publication "Managing Geotechnical Risk (Improving Productivity in UK building and construction).

The Energy Consents Unit (ECU) looks for a peat stability risk assessment 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 very reasonable requirements of what a peat stability assessment should address:

  • The data required to identify and assess the main effects 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 adverse effects.

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 of peat landslide incident, then it is expected the submitted information will include detailed mitigation measures, that brings those high and medium areas into the low category of risk.

The peat landslide hazard report is made by the developer as part of the Environmental Impact Assessment and will be assessed on behalf of the ECU by their appointed assessor. The ECU assessment reports will be succinct and focused on analysis rather than description and will provide clear and justified conclusions, complete with recommendations.

ECU acknowledge that in complex cases, some iteration may be necessary to resolve technical aspects of the proposals. The methods adopted by the developer are discretionary, but preferably confined to this guidance.

1.6 ECU Assessment Services

Most Section 36 applications will be assessed for the risk of a peat landslide incident. The assessment will be produced following a site visit which will be arranged through the developer. The site visit will help the appointed assessor to prepare a written report for the ECU. The scope of each assessment report will be relative to the scale, complexity, and topography of each development site.

Each report will include a summary of the findings, complete with recommendations, and will be presented within 1 month of receipt of the submitted site information. The report will confirm whether or not adequate and appropriate field survey, peat sampling and analytical methods have been employed to provide a sound basis for assessing peat stability and the risk of peat landslides.

The assessor can make recommendations to the ECU that further data is required or that the risk remains too high and is therefore not suitable for construction.

The assessment report will take account of the following work areas:

  • Schedule of work;
  • Site selection;
  • Sampling equipment and strategy;
  • Techniques and methodology;
  • Sample analysis;
  • Risk assessment/register;
  • Recommendations on mitigation measures;
  • Proposals for further investigation;
  • Summary of requirements;
  • References used.

Refer to section 5.6 Reporting (page 36) for further information.

Developers should note the Scottish Executive has employed the peat risk assessment services under contract. The ECU will appraise the assessment reports and issue them to each developer. Developers are asked to submit responses to the assessment reports, directly to the ECU and not to the assessment contractor, acting on behalf of the Executive.

ECU staff can consider brokering one-to-one dialogue between the assessment contractor and the developer, in order to reduce costs and 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 collection systems are to be used on a particular development and for this reason ECU is prepared to review, and comment on, any system that treats geotechnical risk management in a reasonable fashion.

1.7 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 qualities 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:

(i) Engineering geologist;

(ii) Engineering geomorphologist;

(iii) Geotechnical engineer;

(iv) Hydrogeologist/hydrologist;

(v) Ecologist.

These team members should be chartered (CEng, CGeol, CIWEM, MICE or equivalent) with demonstrable experience in managing geotechnical risk and undertaking upland geohazard assessments and/or surveys, specifically in peatland environments.

1.8 Checklist for peat landslide hazard assessment

Figure 1.1 provides a flow diagram checklist for peat landslide hazard assessment. This provides a clear route from initial desk study and site reconnaissance, through detailed site investigation and on to quantitative risk assessment (QRA). Once the area of interest has been identified, a desk study, supported by an initial site reconnaissance survey provides a basis for a first pass assessment of potential peat landslide hazards.

Should the site indicate potential for peat landslide hazard, guidance follows for detailed specification of a targeted site investigation to better quantify and provide mitigation for peat landslide risks. Exit points from the hazard and risk assessment process are provided at appropriate stages. For example, if, after the desk study stage only minimal peat cover is identified, the option to exit the hazard assessment process is made available.

The structure of this document follows the hazard assessment process delineated 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 and accompanying site reconnaissance survey, criteria for detailed site investigation thereafter, and an overview of the recommended approach to hazard and risk assessment.