Sgurr Energy Ltd. Linfairn Wind Farm Appendix 9.3 Outline Peat Management Plan. September This Document is of UK Origin

Transcription

1 Sgurr Energy Ltd Linfairn Wind Farm Appendix 9.3 Outline Peat Management Plan September 2013 This Document is of UK Origin

2 EnviroCentre Document No EnviroCentre Project No j Status Final Project Reviewer Project Manager Lucila Lantschner Hannah Barker Date of Issue September 2013 Filename linfairn_outline_pmp_final.docx EnviroCentre Craighall Business Park Eagle Street Glasgow G4 9XA t f w e info@envirocentre.co.uk Offices Glasgow Banchory This document has been prepared for the Client named on this front cover. EnviroCentre accept no liability or responsibility for any use that is made of this document other than by the Client for the purpose of the original commission for which it has been prepared. This Document is of UK Origin

3 Linfairn Wind Farm Outline Peat Management Plan Table of Contents 1. Introduction Terms of Reference Objectives Site Design Process Suitability of Peat for Reuse Peat Volume Calculations Handling and Storage of Excavated Peat Conclusions & Recommendations... 7 List of Tables Table 3.1: Volumes of Excavated Peat... 4 Table 3.2: Reinstatement and Reuse Requirements... 5 Table 3.3: Balance of Excavated Peat and Reuse Requirements... 5 i

4 1. INTRODUCTION 1.1 Terms of Reference EnviroCentre Ltd has been commissioned by SgurrEnergy Ltd to provide an outline Peat Management Plan (PMP) for the proposed Linfairn Wind Farm (the Project), near to Straiton, South Ayrshire. This report forms a Technical Appendix to Chapter 9 (Hydrology, Hydrogeology & Soils) of the Environmental Statement (ES) for the Project and should be read with reference to this chapter. The outline PMP has been produced in accordance with the Scottish Environment Protection Agency (SEPA) Regulatory Position Statement Developments on Peat and guidance produced by Scottish Renewables and SEPA on the assessment of peat volumes, reuse of excavated peat and the minimisation of waste 1. It will remain a live document and will be updated at the post-planning consent and pre-construction stage to incorporate detailed construction method statements and any further site investigation information that is made available. The outline PMP draws on information presented in Chapter 9 of the ES (Hydrology, Hydrogeology & Soils) and the Peat Landslide Risk Assessment (PLRA) (Appendix 9.2 of ES) produced for the Linfairn Wind Farm, including: Site layout and design; Topographical, hydrological and geological information; Peat depth, determined from peat probing surveys; and Physical characteristics of peat, including degree of humification, bulk density and carbon content which have been determined through in-situ and laboratory testing of peat samples. For further details regarding this information please refer to Chapter 9 of the ES and the PLRA. Organic soils of less than 0.5 m depth are not classified as peat in current guidance, and this definition has been used throughout this assessment. 1.2 Objectives The Scottish Renewables and SEPA guidance highlights two key guiding principles for avoiding or minimising the generation of peat waste: Prevent and/or minimise the quantity of peat which will be excavated, as far as is practicable, through site investigation and design; and To only reuse peat where it is suitable for an identified and required use. The first of these principles is discussed in section 1.3 (Site Design Process). The second of these principles is addressed in this outline PMP through the following scope of works: Detail the volumes of peat expected to be excavated by construction activities associated with the proposed wind farm; Identify volumes of peat expected to be reused for reinstatement and landscaping purposes; Describe the characteristics of the peat at the site and assess whether it will be suitable for the reinstatement requirements; Outline whether any excavated peat will not be suitable for reuse and the options available for treatment; and Outline how the reuse potential of excavated peat will be maximised through methods of handling and storage. 1 Scottish Renewables and SEPA Developments on Peatland: Guidance on the assessment of peat volumes, reuse of excavated peat and the minimisation of waste. 1

5 1.3 Site Design Process The proposed infrastructure layout has been designed through an iterative process to minimise the potential impacts of the Project on the environment, including consideration of minimising the quantity of peat requiring excavation. Peat excavation has also been minimised through engineering design, such as the use of floating track construction. British Geological Survey (BGS) drift geology data suggested that peat deposits at the site were localised in extent, and therefore the initial peat depth survey focused on the construction envelope of the first draft layout design (25wind turbine generators (WTGs)). Surveyed peat depths ranged from <0.5 m to >4 m, and a number of localised areas of deeper peat were identified: To the west of the current position of WTG4 to the north of the site; In a valley bottom to the north-east of the current position of WTG6 to the north of the site; and On the flat, plateau area between the current positions of WTG17 and WTG18. The peat depth data was taken into consideration when reviewing the draft layout and proposed WTG locations were moved to avoid these areas of deeper peat. On-site access tracks were also routed around the areas of deeper peat wherever possible. Two further phases of peat depth probing were carried out following updates to the proposed layout. The final infrastructure layout has therefore been designed to minimise the quantity of peat to be excavated as follows: 12 of the 25 WTGs are situated completely out with peat. The average peat depth is less than 1 m at all the remaining WTGs, including the working area which is assumed to have a diameter of 31 m (see Table 3.1); Ten of the hardstandings are located on peat, however none are located on deeper peat (>1 m) and the average peat depth at the hardstandings is 0.6 m; The temporary construction compounds and control building have been located out with areas of peat; Of the 15.9 km of access track, only 2.6 km are located on peat. Floating road construction will be used where peat depths are >1 m, which accounts for a length of only 430 m; and Cable trenches will follow the on-site tracks. 2. SUITABILITY OF PEAT FOR REUSE The characteristics of the excavated peat will determine how suitable it is for reuse, for example unconsolidated and saturated peat will have limited use for reinstatement works. Suitability for use will be determined by the supervising engineer. Peat decomposition was assessed through the vertical profile using the Von Post classification at three locations where peat depth exceeded 1 m. The results of the Von Post classification suggest that peat at these locations is generally moderately decomposed (H5 H7). Moisture content generally increased with depth and the measured shear strength ranged from 6 14 kpa. The acrotelm is the upper layer of peat consisting of living and partially decayed material with a higher hydraulic conductivity and variable water table. It contains viable plant parts to assist in the regeneration or restoration of peatland vegetation and is responsible for many of the distinctive peatland processes such as carbon sequestration. The catotelm layer consists of more highly decayed material with lower hydraulic conductivity and anaerobic conditions. It tends to be less stable and cohesive than acrotelmic. The full establishment of a suitable acrotelm is required to stabilise the peat mass and its hydrological conditions (e.g. control of erosion and evaporation during dry periods). 2

6 The depth of the acrotelm layer varies and can be difficult to define in practice, typically ranging between 0.2 m and 0.5 m in hummocks 2. The information obtained from the field surveys suggests that the acrotelm at the site has a maximum depth of 0.5 m at most locations, and may be shallower in places. Where infrastructure encroaches onto peat, these areas are generally shallower (mostly <1 m) than those sampled. For the purposes of this management plan, it is assumed the top 0.5 m of peat will be stripped with the vegetation as the acrotelm layer to ensure that its structure is retained. The following assumptions are made regarding the characteristics of the peat in relation to intended reuses at the site: Acrotelmic peat, including vegetated turves, will be suitable for the dressing off of the top layer of all reinstatement. More fibrous catotelmic peat will be suitable for reinstatement below the surface layer (i.e. if reinstatement depths are >0.5 m), for example at trackside verges and cable trenches. Calculations of excavated peat for each infrastructure element, as discussed in section 3, have shown that average depths of peat requiring excavation are less than 1 m. 2 Rydin, H. and Jeglum, J The Biology of Peatlands. Oxford University Press. 3

7 3. PEAT VOLUME CALCULATIONS The volume of excavated peat associated with each construction element has been calculated based on the following general assumptions: Soils of less than 0.5 m depth are not considered to be peat, in-line with current definitions. Peat at the site is localised and the infrastructure tries to avoid peat wherever possible. As a result, many of the individual infrastructure elements, i.e. each hardstanding or WTG foundation area, only partially encroach onto peat. A GIS method has been used to determine the area of encroachment for each infrastructure element and the associated average peat depth based on an interpolation of the peat probing data; 12 WTGs, 15 hardstandings, 13.3 km of track, both construction compounds and the control building will require no excavation of peat (as shown in Figures 9.8a f, Volume 3 of the ES); No excavation of peat will be required for floating tracks; Acrotelmic peat is considered to be 0.5 m depth for the purposes of the calculations; and Excavation or reuses associated with drainage ditches and areas of cut and fill for on-site access tracks have not been calculated as the dimensions of these construction activities will not be available until the detailed design phase (post-consent). Additional assumptions for the dimensions of infrastructure elements have been provided by the project engineers and are included in Table 3.1 along with the expected volume of excavated peat. Table 3.1: Volumes of Excavated Peat Infrastructure Element Volumes of Excavated Peat (m 3 ) Acrotelmic peat Catotelmic peat Total WTG foundations 1, ,545 Crane hardstandings On-site tracks (excavated) Assumptions 18 m diameter base with a 1 m working space and 1:2 side slopes; typical 3 m depth foundation, resulting in a 31 m diameter area at surface ,095 Hardstandings 45 x 25 m 4,894 1,725 6,619 Cable trenches 1,738 1,280 3,018 Total 8,725 3,552 12,277 Total length of track where peat will be excavated = 2.2km; track width 5m Cable trench assumed 1.5 m wide x 1 m deep Table 3.2 identifies the reinstatement and reuse requirements for the Project and the total volume of peat to meet these requirements. These proposed reuses are in line with the guidance produced by Scottish Renewables and SEPA (2012). It has been assumed that peat will only be reused at or in close proximity to the areas from which it is excavated. 4

8 Table 3.2: Reinstatement and Reuse Requirements Reuse Details Volumes of Peat Required (m 3 ) Assumptions Acrotelmic Peat Catotelmic Peat Total Reinstatement at WTG foundations Reinstatement at crane hardstandings Excavated track verges 1, ,176 5, ,625 2, ,937 Reinstatement of cable trenches 1,738 1,280 3,018 Total 11,476 1,280 12,756 Reinstatement of the acrotelm that was excavated - for all WTGs this area was less than the full working area minus the diameter foundation. Top layer of peat and vegetation will be reinstated across whole hardstanding area following completion of construction. Up to 0.5 m depth of reinstatement across shallow verges up to 3 m in width. Cable trench assumed 1.5 m wide x 1 m deep. Table 3.2 shows that an approximate balance between excavated peat and reinstatement uses can be achieved at the site. The calculations assume that excavated track verges will be up to 3 m in width however these can be narrower if a deficit of peat in experienced. For most infrastructure elements the acrotelmic layer will be stripped separately with the vegetation and reused as the top layer of reinstated peat at the same location to minimise transport and handling of the peat. Table 3.3 shows that there will be insufficient acrotelmic peat to provide the top layer of reinstatement for all of the proposed reuse requirements. Where catotelmic peat is used for the top layer of reinstatement, seeding of the peat will be carried out in consultation with SNH and erosion control matting will be used if required. Additional reinstatement requirement will be met using other soil material excavated at the site. Table 3.3: Balance of Excavated Peat and Reuse Requirements Volume of Peat (m 3 ) Acrotelmic Peat Catotelmic Peat Total Excavated peat 8,725 3,552 12,277 Reuse requirements 11,476 1,280 12,756 Balance -2,751 2,

9 4. HANDLING AND STORAGE OF EXCAVATED PEAT Prior to and during construction, this outline PMP will be updated to include detailed methods and timings of handling of excavated peat, its temporary storage and use in reinstatement. The volumes of excavated peat and reuse requirements will also be updated once detailed engineering designs are available. The updated PMP will be included in the Environmental Management Plan (EMP) which will be submitted to SEPA for approval prior to works commencing. This section outlines the principles on which the detailed method statements should be based to ensure that the existing structure and integrity of the excavated peat is retained and its reuse potential is maximised. These measures will also minimise the oxidation of peat and associated carbon losses. Pollution prevention measures to ensure that peat handling and storage does not cause pollution of the water environment are also outlined: The vegetation and surface layer of peat (i.e. the acrotelm) will be stripped separately. Peat will be stored separately from any other soil, drift deposit or rock material. Mixing during excavation and transport will be avoided. Excavated peat will be stored and replaced, where possible, in the location from which it was removed. For example, peat excavated from cable trenches should be used to infill these upon reinstatement. Where this is not possible, the distance that excavated peat is transported should be minimised to reduce the impact on its structure, and should be transported directly from excavated location to reinstatement location where possible. Temporary storage time of excavated peat should be minimised with reinstatement carried out as soon as practicable. Suitable areas for temporary storage should consider areas of lower gradient, lower ecological value, low peat landslide risk and at least 20 m from watercourses. The locations of the temporary storage areas will be determined post-consent based on the updated volume calculations and will be included in the updated PMP. Where peat is stockpiled in large amounts, piles will be bladed off at the side to minimise the available drying area. The stockpiles will be sprayed if necessary to prevent drying. The stockpiles will be checked and findings recorded by the Ecological Clerk of Works (ECoW). Areas of peat restoration will be considered as potential pollution sources until fully restored. Sustainable Drainage System (SuDS) principles will be used to minimise pollution risk. Runoff from stockpiled peat will be caught in swales or blind ditches. Any larger areas of peat storage may require further SuDS measures in place. Clean surface water runoff will be diverted around stockpiles. A continuous layer of acrotelm/ vegetated turves will be used for surface reinstatement. There will be a deficit of acrotelm/ vegetated turves for all surface reinstatement. In these areas, regeneration of vegetation cover (e.g. seeding, heather shoots & seeds, natural regeneration) will be carried out in accordance with SHN guidance 3. Fencing of seeded areas to keep herbivores out of the restoration site will be carried out if considered appropriate. Depending on the time of year when reinstatement is carried out, erosion control matting may be required to stabilise bare peat prior to seeding in spring. Reinstated areas will be monitored to ensure that environmental/ ecological objectives are realised including ecological surveys. The monitoring period may extend beyond completion of the construction works. 3 SNH Constructed Tracks in the Scottish Uplands.. 6

10 5. CONCLUSIONS & RECOMMENDATIONS The infrastructure layout of the Project has been designed to minimise the volume of excavated peat by siting infrastructure out with areas of deeper peat wherever possible and through the engineering design, for example through the use of floating roads where tracks cross deeper peat. Volume calculations have shown that a balance between excavated and reused peat can be achieved at the site. The total volume of excavated peat associated with the Project has been calculated as 12,277 m 3 based on the average peat depth at each infrastructure element encroaching onto peat, informed by peat depth surveys. Proposed reuses of the excavated peat are in line with the Scottish Renewables and SEPA guidance (2012) and include reinstatement of the working around area around turbines, at hardstandings and cable trenches, and use in track verges. No additional treatment of the peat is anticipated to be required, although methods to encourage regeneration of vegetation cover may be needed in some areas due to insufficient acrotelmic peat to provide the top layer of reinstatement for all reuses. An approximate balance can be achieved at the site between excavated peat volumes and the volumes required for reinstatement. Methods of handling and storing excavated peat have been outlined to ensure its reuse potential is maximised and any carbon losses are minimised. Monitoring of the reinstated areas will be carried out to ensure that the environmental objectives are realised. This outline PMP will be updated prior to and during construction to incorporate detailed site investigation information when available and detailed method statements. 7