Peat Stability Risk Assessment

Transcription

1 Contents 9A Peat Stability Risk Assessment September 2010 Corriemoillie Wind Farm Environmental Statement

2 Appendix 9A Peat Stability Risk Assessment Introduction Overview 9A.1 E.ON is currently progressing proposals for a Wind Farm within Corriemoillie Forest. 9A.2 This report provides a Peat Stability Risk Assessment to support the Corriemoillie Wind Farm EIA. This document outlines the peat stability risk assessment methodology, along with the analysis performed and results obtained. The outcome of this assessment is presented in mapping and tabular form, identifying areas assessed as having a risk of a peatslide occurring. Based on the results of the analysis recommendations are made for the future development of the site. 9A.3 RPS believes this technical assessment is appropriate for informing the EIA and planning process of all the relevant matters. However this does not constitute a detailed engineering design, and detailed site investigations and geotechnical assessments will be required prior to and during construction activities. Objectives 9A.4 This assessment follows the guidance as outlined by the Scottish Government (2006) - Landslide Hazard and Risk Assessments. Best Practice Guide for Proposed Electricity Generation Developments. This is a guidance document designed to address the requirement for peat stability assessments as part of The Electricity Works (Environmental Impact Assessment) (Scotland) Regulations 1999 (EIA Regulations). 9A.5 Scores were attributed to the key factors that have the greatest influence on peat stability. Hazard scores were determined which, when combined with an assessment of vulnerability of target areas, could be developed into an assessment of risk. This is described in section 9A.54 onwards. 9A.6 This study therefore comprises a generally non-intrusive investigation that has been compiled based on available information, a review of the existing peat stability knowledge and a subsequent site reconnaissance exercise including a peat depth survey and engineering geomorphological assessment. The main objectives of this assessment were as follows: Conduct a desktop study of the site with regard to peat stability; Undertake a site walkover and engineering geomorphological assessment of the site within the vicinity of the proposed wind farm and surrounding area; Identify potential areas of instability and assess the impact future infrastructural development will have on peat stability; Identify peat depths across the site from peat probing and Provide recommendations for further work or specific construction methodologies and mitigation measures to suit the ground conditions at the study area and reduce the potential for peat instability. Ground Assessment Desktop Study 9A.7 In order to gather baseline information, a desktop study was undertaken in order to: Describe surface water hydrology, including watercourses and springs; Collect soil, geological and hydrological information; Collate information relating to ground stability of the site; Identify land use and future land use of the site and Identify any areas of recent and/or historic peat failures from aerial photography. 9A.8 A list of the reference literature consulted in the compilation of this report is provided at the end of this report. 9A.9 Data sets used in the assessment include: 1:25,000 Ordnance Survey Topographic Mapping; Google Maps; 1:50,000 British Geological Survey (BGS) Superficial Geology (digital data); 1:50,000 British Geological Survey (BGS) Bedrock Geology (digital data); Aerial Photography and 50m Digital Terrain Model (DTM). Walkover Survey 9A.10 The main aims of the site reconnaissance survey were to verify the information gathered during the desktop study, and to record targeted peat depth information associated with the planned location of the site infrastructure. 9A.11 RPS carried out an initial walkover survey with peat probing in May This was augmented with a second walkover survey in October Once it was determined that the shared access would be used to access the site, a further visit looking at land to be covered by the link road between the main site boundary and the shared access track was carried out in June As the shared access track has been consented, the route of this was not covered in the assessment. During these surveys, hand peat probing was undertaken as well as shear vane tests and an engineering geomorphological assessment. Probing was undertaken across the site and within areas of the site that will be affected by the proposed Wind Farm. Peat probing locations were predefined and uploaded onto a handheld GPS to ensure accuracy of the probing locations. Peat Probe records are contained in Appendix A. During the site walkovers, notes were taken regarding topography, landforms, vegetation cover, hydrology, drainage and the presence and nature of peat deposits. 9A.12 Photographs were taken during the site visits to record the site features and a number of peat probing locations. The photographs are contained in Appendix B. Corriemoillie Wind Farm Environmental Statement Appendix 9A Page 1

3 Geological Setting 9A.13 The site forms a bowl between the peaks of Meall nan Coarach, Meall Mhic lomhair and Meallan Mhuthaidh Mor to the west, Sidhean nan Cearc and Beinn nan Cabag to the east and Beinn a Bhric to the south. As a result the majority of streams rising in these hills flow towards the site as discussed in Chapter 9 (Hydrology, Geology and Hydrogeology) and shown in Figure A.14 On the southern and eastern slopes of Meall Mhic lomhair which is to the west of the proposed Corriemoillie Wind Farm, Lochluichart Wind Farm has been granted planning permission. It should be noted that if peat failures occur on this site they may impact on the southwestern portion of the proposed site, and trigger further failures. Superficial Geology 9A.15 The steeper slopes tend to have a thinner covering of blanket bog which overlies a thin layer of Pleistocene Till deposits and bedrock. Within the lower flatter areas the peat is thicker and is more akin to fen bog due to the very high water input and saturation of the ground in these areas. The 1:50K series indicates that the peat in these central areas of the site are underlain by Alluvium (Clay, Silt, Sand and Gravel). This is shown in Figure 9.3. Bedrock Geology 9A.16 The 1:50k Bedrock series indicates the site is dominated by Psammite. Granitic Gneiss are indicated to the south of the site in a small band running northeast to southwest. Inter-bedded Breccia and Sandstone are also shown touching the boundary of the site at the southern end. There is also a normal fault running north-east to south-west (inferred) across the southern portion of the site on a line close to turbines 4, 2, 15 and 18. Bedrock outcrops on the adjoining hills and localised high points within the site. These are predominantly the Psammite. However, the Granitic Gneiss is believed to outcrop along the Allt Coire Mhuillidh just north of the Dam. This is shown in Figure 9.4. Hydrology 9A.17 The site is typically orientated north south with slope aspects predominantly orientated east and west. The central part of the site forms a low point or sink for water flowing from the adjoining hills. As a result the area has been heavily influenced by hydrological processes and forms the source zone for a number of streams. Toward the centre of the site is Lochan Dubh Mor. From this the Allt Giubhais Beag flows north. Approximately 180m to the south of this is the source of the Allt Coire Mhuilidh which flows south. On site, the differentiation between the two catchments for each of these rivers is difficult to define as the area between them is dominated by large flushes of saturated peat with surface water. There are many more surface streams present on site which are not mapped or named on the Ordnance Survey 1:25,000 topographic mapping. These have caused extensive gully erosion as they have cut meandering paths through the peat. Associated with these streams are very extensive flushes and networks of peat pipes. These typically flow toward the central low area of the site. Along the western boundary of the site is Lochan Dubh Beah at the southern end, an unnamed Loch in the middle and Loch a Mheallain Chaorainn at the northern end. The Allt Coire Mhuilidh flows south east along the western boundary and the Allt Beithe (a tributary of the Allt Giubhais Beag) flows northeast along the northern boundary of the site. The Allt Giubhais Mor acts as the receptor for the catchment of the shared access track. The hydrological features of the site are shown in Figure 9.1 in Chapter 9. Engineering Geomorphology 9A.18 The main characteristic of the site is the saturated nature of the ground. The majority of the site is very wet with significant bog pool systems and peat hags on the upper exposed areas, incised gully systems, peat pipe networks and significant peat flushes throughout the site. A number of relic failures (primarily slides) were also noted. 9A.19 Due to the heavily forested nature of the site, access was restricted and the forest may have masked many similar geomorphological features that were noted in other areas of the site. 9A.20 Table 1 below provides a record of features noted during the walkover survey and should be read in conjunction with Figure 9A.1(Geomorphology). Where photographs were taken of the features, these are shown in Figure 9A.2. Table 1 Features recorded during the walkover survey (October 09) (number indicates photographs, see Table 2) Index Description 1 Relic failure, stream flowing into head of failure, peat 0.5m thick. Very weak, surface water. 6 2 Flush peat pipes, water audible under surface. Formed in saddle between two peaks, concentration of flows. 9, 10 3 Overland flow saturated peat. 50m wide flows down into flush Saturated peat, convergence of flow lines, quaking bog, Failure of gully walls, Peat Pipes Relic failure, 60m wide behind fence line, 0.5m back scarp. 22, 23 6 Relic failures noted in hill m deep peat, surface water, flush, 100m long. 8 Saturated peat, flush 9 Near surface rock, outcrop Convex break of slope Peat pipe Peat pipe. 13 Vane shear strengths 4kPa, 8kPa, 11kPa, 14kPa. 33, Rock/boulder clay outcrop. 15 Surface water ponding Flush Bog pool/ saturated peat Relic failure 10mx20m, convex break of slope. 19 Flush/stream Stream & saturated peat Peat pipe Possible relic failure- flush Bog pools & flush. 24 Gully formed by steam, 1m deep erosion. 57, Peat hags formed by water erosion. 59, Concentric rings - possible spreading or formed by water erosion. 62, 63 Appendix 9A Page 2 Corriemoillie Wind Farm Environmental Statement

4 Index Description 27 Large relic failure, stepped back scarps Head of streams Saturated Ground Peat hags flooded between Collapsed peat pipes. 77, Flush & stream 33 Bog pools Bog pools. 35 Rock outcrop Multiple flushes leading to river. 37 Quaking bog 38 Relic failure - tension cracks 39 Collapsed peat pipes Collapsed peat pipes, draining down slope Convergent flow lines, collapsed peat pipe and flush Flushes & broken hummocky ground Near surface rock, outcrop. 44 Flush and gully system 45 Possible relic failure, very wet A.21 Table 2 below provides a photographic record of features noted during the walkover survey and should be read in conjunction with Figure 9A.2 (Index of Photos October 2009) and the photographs in Appendix B. Table 2 Photographs taken during the walkover survey (October 09) Index Photo Comment 1 CIMG2650 View south 2 CIMG2651 View east upslope 3 CIMG2652 Overland flow, wet peat 4 CIMG2653 Looking north 5 CIMG2654 Looking south 6 CIMG2655 Relic Failure, saturated peat 7 CIMG2656 Flush, converging flow lines, peat pipes looking down slope 8 CIMG2657 Flush, converging flow lines, peat pipes looking up slope 9 CIMG2658 Flush, peat pipes and collapsed peat pipes 10 CIMG2659 Convergent flow lines, hummocky topography, concentration of drainage, saddle between two hills 11 CIMG2660 Flush, overland flow 12 CIMG2661 Flush 13 CIMG2662 Peat pipe 14 CIMG2663 Peat pipe 15 CIMG2664 Peat pipe 1m diameter Index Photo Comment 16 CIMG2665 Peat pipe 17 CIMG2666 Gully, convergent flow lines, quaking bog 18 CIMG2667 Gully, convergent flow lines, quaking bog, looking north 19 CIMG2668 Peat pipe 20 CIMG2669 Peat pipe 21 CIMG2670 Looking north 22 CIMG2671 Relic failure outside fence 23 CIMG2672 Relic failure outside fence 24 CIMG2673 View south - failures in hillside present 25 CIMG2674 View north - failure in hillside outside fence line 26 CIMG2675 Rock outcrop 27 CIMG2676 View east, stream and saturated peat in middle ground 28 CIMG2677 Relatively dry well vegetated, little sign of instability 29 CIMG2678 Multiple flushes running west to east 30 CIMG2679 Near surface rock on hill 31 CIMG2680 Peat pipe 32 CIMG2681 View west towards river 33 CIMG2682 Gully erosion 34 CIMG2683 Gully erosion/stream 35 CIMG2684 Gully erosion stream 36 CIMG2685 Peat pipe, just below access road 37 CIMG2686 Peat soil interface on access road 38 CIMG2687 Stream, saturated peat flush 39 CIMG2688 Lochan Dubh Beag looking north 40 CIMG2689 Lochan Dubh Beag looking north 41 CIMG2690 Lochan Dubh Beag looking north 42 CIMG2691 Lochan Dubh Beag looking north 43 CIMG2692 Looking southeast 44 CIMG2693 Stream, saturated peat flush 45 CIMG2694 Flush 46 CIMG2695 Flush 47 CIMG2696 Peat pipe 48 CIMG2697 Peat pipe 49 CIMG2698 Peat pipe 50 CIMG2699 Peat pipe 51 CIMG2700 Bog pools & flushes 52 CIMG2701 View south 53 CIMG2702 Flush, collapsed peat pipe 54 CIMG2703 Flush, collapsed peat pipe 55 CIMG2704 Flush, collapsed peat pipe 56 CIMG2705 Flush, saturated peat 57 CIMG2706 Gully stream, upslope Corriemoillie Wind Farm Environmental Statement Appendix 9A Page 3

5 Index Photo Comment 58 CIMG2707 Gully stream, downslope 59 CIMG2708 Gully, heavily eroded peat 60 CIMG2709 Gully, heavily eroded peat 61 CIMG2710 Inter connecting flushes, hummocky ground, saturated peat 62 CIMG2711 Possible spreading, gully erosion 63 CIMG2712 Possible spreading, gully erosion, concentric gullies 64 CIMG2713 Loch a Mheallain Chaorain 65 CIMG2714 Relic failure 66 CIMG2715 Relic failure 67 CIMG2716 Relic failure 68 CIMG2717 Relic failure 69 CIMG2718 Relic failure 70 CIMG2719 Eroding banks of peat around Loch a Mheallain Chaorainn 71 CIMG2720 Stream, gully erosion 72 CIMG2721 Stream, gully erosion 73 CIMG2722 Gully erosion, peat hags 74 CIMG2723 Gully erosion, peat hags 75 CIMG2724 Bog pools & flushes 76 CIMG2725 Bog pools & flushes 77 CIMG2726 Collapsed peat pipe 78 CIMG2727 Collapsed peat pipe 79 CIMG2728 Collapsed peat pipe 80 CIMG2729 Flushes and gullies 81 CIMG2730 Flushes and gullies 82 CIMG2731 Flushes and deep peat 83 CIMG2732 Flushes and deep peat 84 CIMG2733 Bog pools and flushes 85 CIMG2734 Large flush 86 CIMG2735 Large flush 87 CIMG2736 View south 88 CIMG2737 Bedrock outcrop at river 89 CIMG2738 View north 90 CIMG2739 View north 91 CIMG2740 View north 92 CIMG2741 View south towards dam 93 CIMG2742 View east 94 CIMG2743 Peat pipe 95 CIMG2744 View east of drainage line 96 CIMG2745 View upslope of drainage line 97 CIMG2746 Collapsed peat pipe leading to flush 98 CIMG2747 Collapsed peat pipe 99 CIMG2748 Gully, collapsed peat pipe Index Photo Comment 100 CIMG2749 Gully, collapsed peat pipe 101 CIMG2750 Collapsed peat pipe, 3m width 102 CIMG2751 Looking north, saturated ground and stream 103 CIMG2752 Looking west, stream saturated peat 104 CIMG2753 Looking northwest across Lochan Dubh Beag 105 CIMG2754 Broken ground, hummocky and flushes 106 CIMG2755 Unnamed Loch north of Lochan Dubh Beag 107 CIMG2756 Unnamed Loch north of Lochan Dubh Beag 108 CIMG2757 Looking north, flushes leading into stream, eroding stream banks 109 CIMG2758 Looking south, flushes leading into stream, eroding stream banks 110 CIMG2759 Hummocky broken ground, water flowing across firebreak, unstable ground. 111 CIMG2760 Flushes and gullies, looking towards river 112 CIMG2761 Relic failures below crag 113 CIMG2762 Relic failures below crag 114 CIMG2763 Relic failures below crag 115 June 2010 Edge of site showing broken ground and flushes Surface Hydrology and Presence of seeps, pools, springs, flushes 9A.22 Research has shown that peat instability can be triggered along natural drainage lines (flushes, streams, pipe networks) or in association with artificial drainage. Areas of limited drainage, such as blanket bog, are considered more susceptible to instability than better drained areas due to higher groundwater tables. 9A.23 Groundwater seeps and springs are controlled by rainfall. Fluctuations in rainfall may increase the rate of discharge. If this occurs following an extended period of dry weather where there has been significant drying of the peat mass it may increase the risk of failure (Scottish Executive, 2006). 9A.24 Bog pools often form in areas of impeded drainage on relatively flat ground. Bog pool complexes often indicate areas of deep and saturated peat. 9A.25 The walkover survey identified significant flushes throughout the site. Toward the base of the slopes through the central part of the site the flat areas represent deep flushes. These are aligned with the Allt Giubhais Beag and Allt Coire Mhuilidh which flow north and south respectively. These flushes may not pose a significant peat stability risk. However, they do pose a significant constraint to construction and access. On the slope leading down to the central part of the site there are multiple flushes. These typically follow the fire breaks aligned parallel to the fall line of the slope. However, there may be more present which were not visible as they were masked by the forestry. Flushes represent drainage pathways which have low permeability and are charged from the head of water upslope. These areas indicate zones of saturated and weak peat and have been associated with the trigger point for many peat failures. The peat is generally very weak with a shear strength <4kPa. 9A.26 Construction works which cut through a flush may cause a blockage to the natural flow of water down slope. This in turn can lead to increased pore water pressures at the point of blockage or diversion and concentration of water into other areas leading to failure. Appendix 9A Page 4 Corriemoillie Wind Farm Environmental Statement

6 9A.27 Alternatively, construction plant trafficking across the peat surface, the construction of floating roads, or side casting or loading of the peat can cause bearing failure with subsequent translational failure of the peat mass down slope. Artificial Drainage 9A.28 Water can be concentrated into zones of potential instability by networks of artificial drainage. Should these ditches be partly infilled and vegetated it is also likely that they will act as a store of water from upslope rather than facilitating the rapid removal of water. 9A.29 Artificial drainage ditches have the potential to instigate peat instability for a number of reasons: the removal of peat at the break of slope can decrease the support on the upslope peat mass and potentially cause the peat to fail; critical level of pore water pressure exceeded within the upslope peat mass during heavy and/or intense rainfall; and liquefaction of the basal peat by increased water content. 9A.30 The site is extensively forested. As part of the forestry operations, furrows were ploughed perpendicular to the contours at approximately 3m spacings to aid drainage. These furrows can form potential rupture points in unstable areas as they cut through the upper acrotelm layer of the peat which provides the majority of the strength to the peat mass. This was the case in Derrybrien in 2003, where the size of peat failure was exacerbated by the presence of forestry furrows. 9A.31 In many places across the site, the furrows allow the rapid drainage of water down slope to areas of intact bog. These are often the fire breaks between areas of forestry. The intact bog provides a blockage to the flow of water and becomes saturated. These saturated zones are weaker and therefore represent areas susceptible to failure. 9A.32 As the area has been forested it has had a serious impact on the peat. Once the area is felled, the water uptake provided by the trees will no longer be there and may result in increased runoff and erosion of the peat. Presence of subsurface drainage networks or water bodies 9A.33 The relationship between peat and water is strongly intertwined and impacts on this relationship can incur long-tem implications for any developments in these areas. 9A.34 Changes to the hydrology of blanket bogs have been significant to the initiation of many failures. The active acrotelm is affected by a fluctuating water table, a high hydraulic conductivity and variable water content. The permeability of the catotelm peat is moderate to low in the range of 10-5 to ms-1 and decreasing significantly with humification (Boylan et al, 2008). Hydraulic conductivity tends to be small however macro-pores and pipes tend to provide pathways for significant amounts of runoff and can potentially represent a significant risk of peat failure. 9A.35 There is a high concentration of peat pipes across the site. These are typically associated with flushes and streams. In many instances fast flowing water could be heard beneath the surface of the peat with little or no surface expression. In other areas of the site, peat pipes had collapsed and water emerged to the surface as springs or simply cut a new pipe. 9A.36 During intense rainfall the capacity of these subsurface drainage pathways may be exceeded. This can result in the generation of high-pore-water pressures leading to failure of the peat mass. 9A.37 Construction works which cut through a peat pipe may cause a blockage to the natural flow of water down slope. Alternatively, construction plant trafficking across the peat surface or the construction of floating roads can collapse the ceiling of these pipes and cause blockages to the flow of water. This in turn can lead to increased pore water pressures at the point of blockage or diversion and concentration of water into other areas leading to failure. Evidence of Wind and Water Erosion 9A.38 Peat hags can provide an indication of past and current peat erosion and site drainage. Peat Hags were noted to the east of Turbine 6 and northwest of Turbine 16 within an exposed area void of forestation. Water filled many of hollows between the peat hags in this area. The hags range in height from 1m to 2m deep. A large volume of water was noted ponding within the hag system. These areas represent zones where there is connectivity between surface drainage and the peat/impervious subsoil interface and/or saturation of the lower basal peat which tends to be weakest. 9A.39 Erosion gullies were also noted across other areas of the site. These were often associated with preferential flow paths, flushes and peat pipe networks. 9A.40 The peat hags represent a significant constraint to access across the area, either by foot or tracked excavator. Construction works through these areas may cause a blockage to the natural flow of water down slope. This in turn can lead to increased pore water pressures at the point of blockage or diversion and concentration of water into other areas leading to failure. Evidence of relic and present failure scars 9A.41 These features are often difficult to identify in the landscape as they may have a subdued relief and are often quickly re-vegetated (Scottish Executive, 2006). Peat is >90% water, therefore once it fails and breaks down the water is liberated from the peat mass. This makes it highly mobile but apart from the back scarp of the failure they often leave little surface expression as would be associated with other failures once re-vegetated. 9A.42 A number of failure scars were noted within the site and on the hills outside the site boundary. It is unclear as to the true number of failures present across the site as the ground is masked by the forestry. Given failures are present on the hills surrounding the site and the very high moisture of the peat within the site, it is likely relic failures are present within the forestry areas. 9A.43 The presence of existing failure scars in a development area indicates the local site conditions of that area are conducive to future peat instability. Not only do these features provide an indicator of future peat landslide hazard in the area but they may also be re-activated by both natural and more probable anthropogenic factors. 9A.44 Loading of these relic failures either by floating road construction, side casting arising or stockpiling can remobilise the failure. Alternatively, excavations for roads, borrow pits, and foundations which cut through the toe of a failure can reduce the confining stress resulting in remobilisation. Presence of pre-failure indicators 9A.45 Incipient failures usually indicate where a failure may be due to occur. Tension cracks, bulging and compression ridges can often be the precursors to a larger failure. Tension cracks are often perpendicular to the fall line of the slope and may extend part way or entirely to the base of the peat. As such they provide a pathway for rapid infiltration of water to the substrate and the generation of excess pore-water pressures at depth. Often they may be filled with water and may Corriemoillie Wind Farm Environmental Statement Appendix 9A Page 5

7 extend for several metres. These features may be found as single isolated features or multiple features which may intersect one another. They provide an indication of movement and a potential precursor to failure. Figure 9A.3 Model Process Tree 9A.46 Tension cracks were noted at the back of a relic failure close to the location of Turbine 14 (see point 38 on Figure 9A.1). Peat Stability Assessment Qualitative Peat Landslide Hazard Assessment 9A.47 Most reported failures appear to have been triggered by heavy or prolonged rainfall but the exact mechanism of failure is not always known. The properties of the peat are thought primarily to account for its instability, the key properties being the degree of humification, shearing and tensile strength, water content and permeability, and the bulk physical properties and structure of the peat mass. Extrinsic causes of failure include human disturbance of the original peat mass by peat cutting/extraction/stockpiling, pre-forestry ploughing, burning and cutting of boundary and drainage ditches amongst others (Yang & Dykes, 2006). 9A.48 Sites of peat mass movement share the following characteristics that predispose them to failure (Warburton, 2004), A peat layer overlying an impervious or very low permeability clay or mineral base; A convex slope or a slope with a break of slope at its head; Proximity to local drainage either from seepage, groundwater flow, flushes, pipes or streams and Connectivity between surface drainage and the peat/impervious interface. 9A.49 The analysis was conducted using the Spatial Analyst extension of ArcGIS 9.1. This is a qualitative approach utilising available data sets within a multi-criteria analysis. 9A.50 It is important to note that this study only focuses on peat soils and the criteria used is specifically tailored to the key factors affecting peat stability. As such it does not account for the stability of other mineral soils or rock. Input Data Sets 9A.51 The input data sets used for the analysis were as follows: 50m Digital Terrain Model (DTM); Ordnance Survey Vector Rivers; and Site Survey Information. Methodology 9A.52 Determination of the level of hazard is not presented e.g. for peat depth, and in these instances professional judgement has been applied. 9A.53 Six input layers were generated and each layer was reclassified based on their value ranges and influence on peat stability. These were then standardised by multiplying each layer by its respective maximum value. The resulting standardised grids then contained values which ranged from 0 to 1. Each of the 6 grids was then weighted based on their overall influence on peat stability and to each other (see Table 3). The analysis process is shown in Figure 9A.3 below. Layers & Rankings Relic Failures Layer 9A.54 Past failure scars present on site indicate the area has been unstable in the past and therefore is likely to be unstable in the future. From the walkover survey relic failure scars were recorded and the location logged in a GPS. These were then digitised and ranked as follows. Failures Ranking Absent 0 Present 1 Slope Angle Layer 9A.55 The limiting factor governing the formation of thick peat deposits is topography. In the case of blanket peat it tends to be deepest in closed depressions and typically thin as the slope angle increases. Appendix 9A Page 6 Corriemoillie Wind Farm Environmental Statement

8 9A.56 A slope angle raster was generated from the DTM with a 50m cell resolution. This was calculated as degrees and ranked as follows. Slope Angle Ranking >20 5 9A.57 The rankings increase with increasing slope angle. Peat Depth Layer 9A.58 Peat thickness is seen as one of the key factors associated with peat stability. Typically, the deeper the peat the more humified it is and therefore potentially weaker and unstable it is. Peat depths were recorded during the site walkover surveys in May and October These were recorded using a handheld GPS. The data was then interpolated using Inverse Distance Weighting Interpolation (see Figure 9A.4). The depth raster was then reclassified as follows. Peat Depth (m) Ranking > >0 1 9A.61 The ranking was based on the curvature values obtained from the back scarps of the Pollatomish failures which had an average of Negative values indicate convex slope, positive values concave slopes and values close to 0 indicate even/flat slopes. The ranking is weighted toward convex slopes. Rivers Layer 9A.62 Peat failures are known to originate in stream valleys. The Ordnance Survey 1:50k rivers was augmented with streams identified during the walkover survey and on aerial photography. The rivers were rasterised on a 50m grid cell which provides a 25m buffer either side of the river course. The raster was then standardised and a weighting applied as shown in Table 3. Rivers Ranking Absent 0 Present 1 Weightings 9A.63 Each layer was weighted on its perceived influence on peat stability and to each other. The weightings are shown below in Table 3.1. Flushes, Pools, Pipes etc Layer 9A.59 From a review of the aerial photography and walkover survey the location of flushes, pools, peat pipes and areas of saturated peat were recorded. These were then digitised and rasterised on a 50m grid. Flushes, Pools, Pipes, etc Ranking Absent 0 Present 1 Table 3 Susceptibility Model Weightings Layer Weightings Presence of Failures 6 Slope Angle 5 Presence of Flushes, Pools, Springs, Peat Pipes etc 3 Peat Depth 3 Convex Slopes 4 Streams & Rivers 2 Convex Slope Layer 9A.60 From the 50m DTM the convex slope layer identified concave, convex and flat slopes. Typically steep gradients tend to have higher curvature values, hence the increasing curvature rankings would be related to increased slope angles. The curvature raster was ranked as follows. Curvature Ranking Hazard Summary 9A.64 The qualitative assessment takes into account many of the factors which predispose a site to peat failure. As shown on the Peat Landslide Hazard Map (Figure 9A.5) the assessment indicates that the hazard scores for the site range from Low to High with the majority of the site assessed as Low- Medium to Medium-High. The majority of the access roads are within medium hazard zones but also cross areas of medium-high and high hazard potential. This represents the baseline hazard potential of the site based on a qualitative assessment. Corriemoillie Wind Farm Environmental Statement Appendix 9A Page 7

9 Semi-Quantitative Peat Hazard Assessment 9A.65 Peat failures in upland blanket bog typically resemble translational planar slides. An infinite slope analysis, as given in the Scottish Executive guide (2006) is a simple method for assessing the factor of safety of this situation. Cu F z cos sin F = Factor of Safety C u = undrained shear strength, kpa. = bulk unit weight of saturated peat = 12kN/m 3. z = is peat depth in direction of normal stress, m. = is the angle of the slope to the horizontal, degrees. 9A.66 The equation above ignores the passive resistance provided by material at the toe of a slope. This means that the stability of the peat is considered independent of whether the toe is undermined or not, as would be the case for the excavation of foundations, borrow pits, ditches and access roads. 9A.67 Low shear strength values in areas of high water content have also been adopted and a factor of safety of 1.5 given the unpredictable properties of peat. 9A.68 Based on published literature and shear vane tests carried out on site the following shear strength values were adopted for the various peat depths and features on site. Peat Depth (m) Cu kpa >2 2 Flushes 2 Relic Failures 1 The qualitative assessment indicates that the hazard potential of the site is Medium to High. In order to significantly reduce the hazard associated with development and therefore the associated risk a Semi- Quantitative assessment has been carried out. The subsequent semi-quantitative assessment looks at two scenarios: 1. Loaded conditions during construction This would occur from the stockpiling of equipment, floating road construction or the side casting of excavated spoil. peat upslope, acting as a gravity dam. Roads in cut through the underlying till and sub-soil should not initiate a failure in areas with a factor of safety of >1.5. Stability Analysis Loaded Conditions During Construction (Floating Roads, Stockpiles, Sidecasting) 9A.69 Figure 9A.6 represents the factor of safety associated with the site during construction. This assumes a load of 20kPa is applied to the peat surface. This is equivalent to m of fill used to construct floating roads, the side-casting of arising or stockpiling spoil as would be typical on any site. 9A.70 It is evident from this analysis that large portions of the site have a factor of safety of <1 therefore peat slides are likely to be triggered as a result of construction which applies a loading to the peat surface. Other areas of the site indicate factors of safety of <1.5 which is considered unsatisfactory. The blue areas indicate potentially acceptable conditions where the peat can be gradually loaded to 20kPa with a factor of safety >1.5. This is subject to a detailed bearing capacity and stability assessment. 9A.71 From this analysis as with the Qualitative assessment it is clear the construction works must not apply loads to the peat over much of the site as this will initiate peat failures. Therefore, floating roads should not be the preferred method of access road construction on this site. Access roads should be founded on the mineral soil or bedrock beneath the peat and any spoil excavated should be removed to designated areas which will undergo the rigours of a detailed stability assessment. However, floating roads may be used in localised areas on site where the ground is flat and the area poses significant constraint to conventional road construction. Again, it is imperative that any floating road or element that loads the peat undergoes the rigour of a full detailed bearing and stability assessment prior to construction and should be continually assessed during construction. Stability Analysis Unloaded Conditions During Construction 9A.72 In order to reduce the hazard and associated risk any elements which results in a load being applied to the peat should be relocated or redesigned. Once there is no load being applied to the peat surface the hazard of bearing failure and subsequent translational failure of the peat mass down slope as a result of construction works is greatly reduced. This does not reduce the baseline level or risk associated with the site. 9A.73 To prevent loading of the peat, access roads and other elements must be founded directly on the underlying sub-soil or bedrock. Figure 9A.7 represents a semi-quantitative assessment of the factor of safety associated with the site in its natural state. This is based on an infinite slope model and does not account for changes in groundwater level or other environmental factors which may influence stability (i.e. rainfall etc). Construction elements which are founded on the sub-soil should have little influence on peat stability as the analysis does not take into account the passive resistance provided by the toe of material at the base of the slope. This is the same situation as would be the case with excavate and replace roads founded on the underlying mineral soil or bedrock. 2. Unloaded conditions during construction By founding access roads on the mineral subsoil or bedrock it will substantially reduce the hazard and risk associated with failure. Access roads which require fill will provide support to Appendix 9A Page 8 Corriemoillie Wind Farm Environmental Statement

10 9A.74 Evidence to support this is an existing forestry track which currently runs for over 2km from the A835 in the east through plantation forest to the proposed site. The existing track on site, marked on Figure 1.2 of the main ES, is part of this forestry track. This track is cut into the underlying glacial till providing a suitable access road for forestry traffic. Along its route it cuts through both the peat and mineral soils in areas of high and medium hazard and risk as identified in the qualitative assessment. From the walkover survey, it would appear that no failures have occurred along its route that have resulted in any adverse impacts. Where roads are constructed on embankments these will act as a gravity dam and provide additional passive resistance to the peat upslope of them. 9A.75 Areas with a factor of safety <1 are considered unstable in their current natural state. These areas should be avoided by all works with a sufficient up and down slope buffer distance. 9A.76 This is a preliminary assessment based on a 50m grid. It is important that a detailed stability assessment is carried out prior to detailed design. 9A.77 Areas with a factor of safety of <1.5 are areas which are potentially unstable and should also be avoided by all works with a sufficient buffer distance. Risk Assessment 9A.78 The level of risk allocated to a particular area relates to the presence of peat, the likelihood of failure occurring (the hazard) and the consequences of such a failure (the exposure). The risk assessment discussed in the following sections is based on a scoring system, where the hazard and exposure scores are multiplied to produce a final risk score. Hazard x Exposure = Risk 9A.79 The following sections detail the methodologies for determination of the appropriate exposure and risk. Consequences of Peat Failure 9A.80 The effects of peat failures are felt locally, both in the long and short term, but they also have wider off-site implications. 9A.81 A key part of the risk assessment process is to identify the potential scale of peat failure, should it occur, and identify the potential environmental effects as well as the receptors of such an event. 9A.82 Predicting the size of a failure and the distance it may travel is very difficult. The high moisture content of peat makes it especially mobile once it fails and the structure of the peat breaks down. If a peat slide enters a watercourse this can mobilise the slide further and have impacts many kilometres beyond the bounds of the site. In many instances minor slumps are localised and have little or no impact. Other failures may travel a m and those entering watercourses many miles as was the case of the Derrybrien failure in Co.Galway, Ireland in A.83 Peat failure in the proposed Wind Farm development area could affect the following key receptors: the proposed Wind Farm, including infrastructure and turbines; site workers and plant (risk of injury/death or damage to plant); land based and aquatic ecological effects (damage to habitats); effects on the quality of on-site and downstream watercourses; site drainage (blocked drains/ditches leading to localised flooding and/or erosion); designated sites; and visual amenity (scarring of landscape). Exposure 9A.84 In order to assess exposure, the site has been divided into north and south. The Allt Glubhais Beag flows north and has a shallower gradient and flow velocity within the site than the Allt Coire Mhuilidh which flows south. Therefore the run out distance of a failure on entering a water course is potentially greater in the southern portion of the site than the north. 9A.85 Both watercourses within the proposed wind farm site drain into the River Conon catchment. The River Conon is a designated salmonid fishery under the Freshwater for Fish Directive (78/659/EEC). The Black Water to the north and east of the site is a tributary of the River Conon also. 9A.86 Private water supplies were identified at the Aultguish Inn, (the supply being approximately 800m to the north of the site boundary) and at Corriemoillie Farm, Corriemoillie Lodge, Tigh Tioram and Glenview Cottage to the South approximately 2km south of the site. There are private properties which are close to the Allt Coire Mhuilidh which may fall within the corridor if a large peat failure was to occur and enter this watercourse. 9A.87 The potential impact from a peat failure on entering either the Allt Coire Mhuilidh is seen to be greater than the Allt Glubhais Beag. Therefore, an exposure score of 3 has been applied to the north and 4 to the south as shown below in Table 4 and on the Peat Landslide Exposure Map (Figure 9A.8). Risk Table 4 Impacts and Exposure Impacts Exposure Score Loss of life, Major damage to property, Major Pollution Incident High 5 Blockage of public Roads, minor damage to public property. Short to Medium term pollution incident. Medium High 4 Damage to rural lands, localised pollution incident. Loss of access road. Medium 3 Blockage of site access roads. Low Medium 2 Minor Restoration of works. Low 1 9A.88 In order to assess risk, the hazard scores are multiplied by the exposure scores. The resulting Peat Landslide Risk Map (Figure 9A.9) indicates that the majority of access roads and turbines fall within zones of Medium and High Risk. Localised areas indicate Very High Risk. This is deemed an unacceptable level of risk. In order for the development to proceed the level of risk should be reduced to medium or less through relocation of the roads and turbines (micrositing) or the implementation of specific mitigation measures. However, this should be done at the detailed design stage due to the coarse level of accuracy of this model (50m), which will ensure that final locations will have acceptable risk. Figure 9A.9 indicates that the few small areas of very high risk are located within larger areas of lower risk, suggesting that the heterogeneity of the site will ensure potential for such micrositing and mitigation. Corriemoillie Wind Farm Environmental Statement Appendix 9A Page 9

11 Action Risk Score Avoid Project Development At These Locations Very High Project Should Not Proceed unless hazard can be avoided or mitigated at these locations, without significant environmental High impact, in order to reduce hazard ranking to medium or less. Project may proceed pending further investigation to refine assessment and mitigate hazard through relocation or re-design Medium 5 10 at these locations. Project should proceed with monitoring and mitigation of peat landslide hazards at these locations. Low 1 4 9A.89 The Qualitative Stability Assessment indicates that the baseline level of risk associated with the site is Medium, High and Very High as shown on Figure 9A.9. Therefore, the method of construction and mitigation put in place must not elevate the risk across the areas outside the immediate boundary of the construction works. The areas directly affected by the construction works must have or reduce the level of risk associated with them to medium or low risk. 9A.90 As discussed previously peat slides can occur in a number of ways, however the majority of peat slides triggered by construction works are caused by rapid loading of the peat mass which results in bearing failure and subsequent translational failure down slope. This can be caused by the construction of floating roads, side casting of spoil or the stockpiling of spoil or plant. This means works are carried out on top of the body of peat. 9A.91 From the Semi-Quantitative assessment the impact of construction could be assessed as shown on Figure 9A.6. Under a loading scenario, the infinite slope stability model indicates that over the vast majority of the site peat slides would be triggered. In many areas of the site this would elevate the baseline level of risk. Therefore, in many areas across the site, any construction which loads the peat would be unacceptable and should not proceed. The analysis also indicates that localised areas may be capable of taking gradual loading subject to a detailed bearing and stability analysis. As such these areas may be potentially suitable for stockpiling, side-casting and floating road construction. 9A.92 In order to reduce the hazard and the risk, associated with construction, the majority of construction elements across the site should be founded directly on the sub-soil or bedrock. By doing this the hazard of peat failure caused by loading the peat is avoided as the peat is removed. This must be combined with best practice construction methodologies, management and mitigation in order to achieve this. The analysis indicates that by cutting roads through the peat and removing the peat to designated disposal areas the risk associated with the construction works is significantly reduced. It should be noted that this does not reduce the overall baseline level of risk (as shown on Figure 9A.9) associated with other areas of the site which are outside the immediate footprint of the construction works. 9A.93 Figure 9A.10 presents a risk assessment associated with the construction works. This indicates that construction elements can be constructed across the majority of the site with an acceptably low level of risk. A few locations still have elevated levels of risk associated with them. As such access roads and turbines have been relocated to avoid the majority of these. Recommendations for relocating; redesign and mitigation are discussed from paragraph 9A.96. Limitations of the Analysis Access was typically limited to fire breaks between areas of forestry and information gathered through observation within the forestry area itself was limited. The analysis assumes when the forest was planted it was located in areas avoiding unstable or particularly wet areas; The DTM used for the analysis has a 50m cell resolution. Therefore, particular subtleties within this 50m will not be identified, for example, sharp breaks of slope. Therefore the output analysis has a 50m grid cell resolution; A small number of pixels may have been discarded by geo-referencing errors, but the area that these represent is considered small; The model focuses on key trigger points, hence does not define zone s susceptibly to landslide run-out or avalanche corridors, hence areas defined as low may be within the fall line of a potential landslide initiated in a hazard area. The hazard map output considers the peat in a natural state as per the data sets used; It does not account for other anthropogenic influences such as a change in drainage pattern, deforestation, rainfall etc; The analysis only focuses on peat stability. Hence, the criteria used will not account for areas that may be prone to other forms of mass movement such as rock fall, debris flows or other landslides within mineral soils or rock; The Infinite Slope Model is based on assumed shear strength values for the peat based on research literature and the limited number of shear vane tests conducted on site; and Due to the forestry access, any observational data which would otherwise be recorded was restricted. Therefore the assessment assumes that the flushes and other features of constraint noted on site were also present before the forestry was planted and that the forestry was planted to avoid these areas. Discussion & Recommendations 9A.94 A detailed geotechnical design for each turbine location, access track, borrow pit, laydown area and construction compound will be undertaken prior to construction. This will also include a detailed stability and bearing capacity assessment for all construction elements including (but not limited to) roads, stockpiles, side-casting and foundations. The stability analysis should also look to re-address the hazard and level of risk associated with peat stability across the site using detailed survey information and geotechnical parameters obtained from comprehensive testing. 9A.95 As part of the detailed design process, a geotechnical risk register will be maintained and incorporated into the overall site risk register. The register will show the degree of risk attached to various elements of the proposed Wind Farm construction and operation. The purpose of the register is to provide and outline a description of the hazards, identify the likely cause, describe the consequence or impact of the hazard and identify the design and construction controls to be implemented in order to reduce the probability to a tolerable level. The overall application of the risk register will allow the management of geotechnical risk. The register will be actively used during the design and construction stages of the project and will be updated continually based on information gathered on site. The register will take note of the mitigation measures suggested in this document. Appendix 9A Page 10 Corriemoillie Wind Farm Environmental Statement