OSE3506 Chapter 4
Ore Farm Wind Energy Project
Geology, Superficial Deposits and Hydrology
Orkney Sustainable Energy Ltd 6 North End Road Stromness Orkney KW16 3AG
01856 850054 [email protected] www.orkneywind.co.uk
Orkney Sustainable Energy Ore Farm Wind Energy Project Report OSE/3506 Chapter 4 Orkney Sustainable Energy Document OSE/3506 Chapter 4 March 2011
Report OSE/3506 Chapter 4 March 2011
Client: Orkney Wind Farms Ltd Haybrake Farm Lyness
Assessment: Richard Gauld BSc(Hons) IEng MInstMC Dip.GeoSci
1 Summary
1.1 A small wind farm is proposed for a coastal pasture part of a farm at Lyness in Hoy. The project will consist of two medium sized wind turbines, an upgraded access track and a switchgear building, all to be owned by Orkney Wind Farms Ltd, a local development company.
1.2 As part of the planning process, studies relating to the possible effects of the wind turbine development on the geology, superficial deposits, hydrogeology and hydrology beneath and across the site have been undertaken, along with consideration of possible ice and snow effects.
1.3 The site and route of the access track have been surveyed as part of the project ecological assessment, with any hydrological features noted and mapped. The project layout has evolved over the assessment period, and as the project has been designed to have minimum ecological impact, care has been taken to avoid areas of sensitive habitats. In addition, two days were spent on walkover surveys, identifying any surface water features, including drains and burns.
1.4 The turbine site appears geologically stable, with the turbines located on fault-free Lower Eday sandstone, and there are no water crossings or other hydrological impacts to consider.
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2 Landscape and Geomorphology
2.1 The proposed location for the development is the largely flat pasture to the west of Ore Farm at Lyness on Hoy, 18m above sea level. On one side the site is on the margin of arable grazing land, where the land has been cultivated and improved from moorland over many years, and on the eastern margin are the remains of the World War 2 base. The turbine positions follow a linear pattern, running due north-south. The access to the site would follow the existing track to the Hoy Community wind turbine, with only short stretches of new track required.
2.2 Project analysis has considered the Orkney Landscape Character Assessment, and the wind turbines and switchgear building are positioned at the edge of Inclined Coastal
Pasture [1] . Ore Farm has a range of mainly small fields, clearly visible when viewed from the east. It should be noted that the western edge of Ore Farm marks the boundary between the managed farmland and the moorland hills that cover most of the island; the project has been located to be distant from any environmentally protected areas.
2.3 The geomorphology of the site is to a large degree controlled by the underlying geology, with the resultant landscape draped over the base geology and eroded through time. In the current period the landscape has been dominated by glaciation, which has moulded the underlying geology with superficial deposits of boulder clay, with peat
deposits draped over the moorland hills [2] . The landscape has seen post-glacial erosion over the past 10,000 to 15,000 years, and periods of climate change have altered the area; the poorer quality superficial deposits across the more elevated parts of the island are relatively recent, suggesting that much of this area has seen drier and warmer periods.
2.4 Anthropic influences in recent years have altered the landscape. This part of Hoy was the centre of a large naval base during World War 2, with many remains still present; there are extensive derelict concrete foundations over much of the farm. The fields are rough grazing, and are not particularly productive, although drainage and land management techniques have created the group of fields being used by the development. There are also various access tracks running between the fields, reducing the need for any additional infrastructure.
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3 General Geology
3.1 An assessment of the geology and the hydrogeological features of the area was conducted with reference to maps produced by the British Geological Survey, and with reference to the resources provided on the Orkney geology web site; http://www.landforms.eu/orkney/ . These assessments confirmed that this area is mainly underlain by Devonian Old Red Sandstone sedimentary rock.
3.2 Hoy is typical of Orkney, and is dominated by the Devonian Sandstone; Orkney is a raised part of Lake Orcadie, a former lake basin dating from around 380 million years ago, that has resulted from sediment laid down on the lake bed. These sediment rocks rest on much older basement granites and gneisses, which are exposed at Yesnaby and Stromness to the west of Mainland Orkney.
3.3 The basement rocks are a mixture of granite, gneiss and schists, which formed part of the Caledonian mountain chain 400 million years ago. These rocks originally enclosed Lake Orcades which became the focus of erosion and the deposition over time; this sediment formed Old Red Sandstone, which now largely obscures the basement rock complex.
3.4 Tectonic activity caused the sediments to become folded and faulted, creating a range of hills, which was then subjected to erosion and submergence; Hoy and South Walls consist of gently sloping Devonian rocks, known locally as Upper Stromness flagstone and Eday and Hoy Sandstone, Figures 1 and 2. Note that the Hoy and Eday Sandstones were laid down in dry desert conditions, whereas Stromness flagstone was a marine deposit and has a more laminate structure. The turbines are all sited on Lower Eday sandstone.
3.5 The rounded landforms present throughout Orkney are a result of the extensive glaciation that has occurred, with major ice sheets developing around 850,000 years ago. The last ice sheet disappeared around 15,000 years ago, leaving exposed striations in the more mountains parts of north Hoy. The passage of these ice sheets also resulted in the extraction of sea bed sediment, producing the layer of till and boulder clay that covers much of the southern parts of the island.
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3.6 Note that the Orkney Landscape Character Assessment distinguishes between Hoy and Eday Sandstone, however recent studies show that the two rocks are contemporaneous, with Hoy bedrock most likely to be upper Eday Devonian sandstone. Figure 1 also shows the location of the major faults across this part of Orkney; the Ore Farm project avoids all known faults. Devonian igneous rocks are not common in Orkney, however lava flows can be observed at the base of the Kame of Hoy at the northern edge of the island, volcanic ash deposits are prominent in the cliffs at the west end of Rackwick beach, and volcanic vents and pipes can be seen at the Round Hill in Hoy, and at Melsetter, to the south of the turbine site.
Figure 1 – Bedrock Geology (SNH LCA)
Lower Eday sandstone Upper Stromness flagstone Peat deposits Alluvial deposits Till
Figure 2 – Solid geology and superficial deposits (BGS)
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4 Hydrology and Hydrogeological Features
4.1 Annual rainfall in Orkney has been measured to be 1030mm at Kirkwall, averaged over
the last thirty years [5] . There is variation across Orkney, with Hoy seeing higher levels that the east, however rainfall is relatively low, largely due to the rainshadow effect of the mainland Scotland hills to the west and south.
4.2 Sedimentary sandstone is a good aquifer when fractured, however the flat land around the turbine site has resulted in water flowing quite slowly on the surface of the land, evident by the poorer quality moorland and peat deposits to the west and north of the turbine site.
4.3 There are no significant hydrological features in the area, and there are no water crossings to consider. The field boundary to the west of the site has a drain which feeds into Ore Burn, to the north of the site, ultimately draining into Scapa Flow on the eastern side of this part of Hoy, Figure 3.
Peat Deposits Ore Burn
Flow direction Hardstanding for on-site storage and fuel handling
Field drain Field drain
Burn
Figure 3 – Hydrological features
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5.1 The solid geology of Orkney was modified by glacial action during the ice ages, the last of which reached its maximum extent 18,000 years ago. The main effects of ice movement in this area were generally to smooth out the topography and to deposit glacial till boulder clay; peat deposits have subsequently become established over the hills to the west.
5.2 The till deposits have resulted in moderate quality rough grazing land, particularly over the Middle Eday bedrock. Where the soil is located to the west of the turbine fields, and where drainage has been poor, the superficial deposits have become peaty, resulting in the moorland evident over much of Hoy. Figure 2 shows that the whole area has been draped by a smooth layer of till and boulder clay topsoil, with peat over the western hills.
5.3 The turbines are to be located in an area of improved agricultural topsoil on boulder clay and sedimentary sandstone. The soil type on the site is defined by the Soil Survey
of Scotland as a poorly drained gley, derived from Stromness and Eday sandstone [3] . The Land Use Capability Survey indicates that the wind turbine area has Class 4 capability, or land with moderately severe limitations that restrict the choice of crops, and would require very careful management if it is to be suitable for arable agriculture
[4] .
5.4 The soil type, drainage conditions and topography have been important considerations for the design of the Ore Farm project. In contrast to moorland or hilltop developments, the turbine site is a flat plateau, and the access track is already in place, built on hard bedrock. Neither the track nor the hardstanding and turbine foundation areas have any of the characteristics of locations at risk of land or peat slide, and consequently there is no risk of any movement of peat or other superficial deposits on the site.
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6 Ice and Snow Effects
6.1 This part of Hoy has a relatively low average rainfall of around 1000mm per annum, similar to that of the east coast of mainland Scotland, and consequently snow volumes are low due to rainshadow effects.
6.2 The climate around Orkney is essentially maritime, with the sea and the gulf stream the main climatic influences. Consequently the climate is mild and wet, with heavy snow only very rarely occurring, although wind speeds on the hill tops are amongst the highest in the UK.
6.3 A European study has been completed into the likelihood of icing conditions occurring, and the areas most likely to have blade icing include Germany, Austria, Switzerland,
France and the Spanish border region [6] . The Wind Energy Production in Cold Climates Study (WECO) describes Scotland as a whole as having occasional to light icing conditions, however more relevant is that the Western Isles are shown as having no possibility of icing occurring at any time during the year. Given the similar maritime conditions, it is predicted that the Ore Farm turbines will never be subject to icing, due to the proximity of the sea on both sides of the island.
6.4 When considering the possibility of ice throw from a turbine, the same WECO project has the following statement:
“Ice throw is an essential problem when the site of the wind energy power plant is planned to be close to public roads, housing, power lines etc. According to the data and models developed and verified within the project it can be recommended for sites with high probability of icing to keep a distance between the turbines and nearest objects of about (hub height + diameter), or it can be recommended to stop the turbine automatically during the icing period and wind coming from unfavourable directions, if the public safety might be affected by ice throw.”
The Ore Farm turbines have an extremely low probability of icing, and in any event the turbine site is well away from housing, public roads and powerlines. Consequently ice throw is not predicted to be a significant issue for the Ore Farm Wind Energy Project.
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7 Pollution prevention
7.1 Minimal track construction is required, and will be achieved by excavating down to bedrock, then to mitigate any surface run-off problems the track will only be constructed in dry conditions, with a layer of stone immediately placed on the exposed bedrock as the construction progresses, with crushed rock aggregate sub-bottoming placed on top of the as-dug rock. Where topsoil has been removed, it is proposed to store vegetated turf and the underlying soil separately, and also to separate it from any underlying till deposits. The rock for the track will come from a quarry to the south of Melsetter; this quarry has a large volume of weathered overburden suitable for construction purposes.
7.2 Track construction will follow the general SUDS guidelines published by SEPA, with a
porous construction and a free-draining sub layer [11] . The access track will have a filter strip adjoining the peaty soils to ensure that any water is routed to the upper layers of the adjacent soil.
7.3 There is little risk of silt run-off, as the turbine area is generally flat and the project ground excavation works will be conducted in dry conditions. If necessary, any silt will be pumped on to free draining grassland around the site.
7.4 Fuel or oil pollution would have a significant affect on water quality should it enter any water courses, and accordingly fuels will be managed on the area of concrete hardstanding adjacent to the public road, Figure 3, well away from any drainage ditches, and good site management will be established:
• Fuel storage will be in double-skinned tanks
• No-refuelling will take place within 100m of a watercourse
• Plant will be regularly inspected for leaks and spillages
• All plant will carry sorbent pad spill kits
• Any static plant at the substation compound will be bunded or use a drip tray
• Self-contained portaloos and washing facilities will be located at hardstanding areas.
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References and bibliography
1 Orkney Landscape Character Assessment (1998). SNH Publications No. 100.
2 British Geological Survey. Geological Survey One-Inch Survey Map 119: Kirkwall
3 British Geological Survey. Soil Survey of Scotland, 1: 50000: Orkney Mainland
4 British Geological Survey. Land Use Capability, 1:50000: Orkney Mainland
5 Mean annual rainfall in Kirkwall, Orkney: 1971-2000 http://www.metoffice.gov.uk/climate/uk/averages/19712000/sites/kirkwall.html
6 Finnish Metrological Institute. Wind Energy Production in Cold Climates. http://www.fmi.fi/research_meteorology/meteorology_9.html#5
7 Tomlinson, R.W, Gardiner T, (1982). Seven bog slides in the Slieve-an-Orra Hills, County Antrim. Journal of Earth Science, Royal Dublin Society 5.
8 Warburton J., Holden J., Mills A.J. (2004). Hydrological controls of surficial mass movements in peat. Earth Science Review 67 139-156.
9 Bowes, D.R. (1960). A bog-burst in the Isle of Lewis. Scottish Geographical Journal 76, 21-23.
10 Hydrological Summary for the United Kingdom; October 2006: http://www.nwl.ac.uk/ih/nrfa/monthly_summaries/2006/10/summary.html#rainfall
11 Jefferies C. (2004) SUDS in Scotland – The Monitoring Programme , Urban Water Technology Centre, University of Abertay: http://sudsnet.abertay.ac.uk/SNIFFERReport0251.htm
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