Fearn Wind Energy Project Geology, Superficial Deposits and Hydrology Orkney Sustainable Energy Report OSE/2871 Geology Orkney Sustainable Energy Document OSE/2871 Geology March 2010 Report OSE/2871: Section 6 March 2010 Client: David Sutherland Tullich Farm Fearn Assessment: Richard Gauld IEng MInstMC BSc(Hons) Dip.GeoSci Dip.DesInn Orkney Sustainable Energy Ltd 6 North End Road Stromness Orkney KW16 3AG CONTENTS 1 SUMMARY 3 2 LANDSCAPE AND GEOMORPHOLOGY 4 3 GENERAL GEOLOGY 5 4 SUPERFICIAL DEPOSITS 6 5 HYDROLOGY AND HYDROGEOLOGICAL FEATURES 7 6 ICE AND SNOW EFFECTS 8 7 HYDROLOGICAL MANAGEMENT AND POLLUTION PREVENTION 9 REFERENCES AND BIBLIOGRAPHY 10 2 Orkney Sustainable Energy Document OSE/2871 Geology March 2010 1 Summary 1.1 A wind energy project is proposed for a derelict aerodrome on the Tarbat Peninsula, east of Tain in Easter Ross. The project consists of three large scale wind turbines, an upgraded access and a switchgear building, all to be owned by a local company. 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. 1.2 The site has been surveyed over four years 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, a walkover survey was completed, identifying any surface water features, including sources, drains and burns. The project has taken into account ground conditions, and has located the turbines on or adjacent to the derelict runways of the aerodrome; particular care has been taken to avoid areas of natural vegetation or bog. 1.3 This is a geologically interesting area, and the large flat plain around Fearn Aerodrome is the consequence of post-glacial isostatic uplift and relative sea level fall. The underlying bedrock is the Raddery Sandstone Formation, a pebbly red sandstone of Mid Devonian age. The main superficial deposits are gravel shingle, sand and silt forming a raised marine deposit that stretches from Nigg in the south to Cadboll in the east, dated from the current Holocene interglacial. These are very young deposits, and represent the sea bed during the warm period immediately after the Scottish ice sheets retreated, between 6,000 and 10,000 years ago. The extent of these raised marine deposits suggests that the Cromarty Firth had outfalls near Cadboll and Loch Eye. Isostatic effects and relative sea level fall over the last 5,000 years have raised the land, creating the flat plain extending from the intertidal zone at Nigg to Fearn. Without anthropogenic influences this would be a marshy area, but has seen extensive drainage to establish the fields around the aerodrome. 1.4 Assessment of the risk of possible ice and snow effects has been completed, with a 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 Fearn Wind Energy Project. 3 Orkney Sustainable Energy Document OSE/2871 Geology March 2010 2 Landscape and Geomorphology 2.1 The project is located on Tarbat Peninsula, a largely flat plain extending from Nigg in the south, to Portmahomack in the north and Balintore in the east. Nigg Hill is locally prominent, providing a backdrop to the well managed agricultural fields. To the south of Nigg Hill are the Sutors of Cromarty, two prominent headlands that define the entrance to the Cromarty Firth. The turbine site is the Fearn Aerodrome, a derelict Royal Navy air base, known as HMS Owl, with the turbines located on or adjacent to the disused runaways. Access to the site follows an existing track off the B9166 that runs between Hill of Fearn and Balintore, with a switchgear house located on the site. The project connects to the National Grid at a substation near the A9, Figure 1. 2.2 The general area is defined as Open Farmed Slopes in the Inner Moray Firth Landscape Character Assessment (SNH 1998), and consists of a largely flat plain gradually rising from Nigg Bay to Tarbat Ness. This landscape type is characterised by extensive drainage and land reclamation resulting in a strong geometric pattern of small and moderate scale fields, giving a diverse but well-ordered landscape. The location of the project could also be considered to be an area of Intensive Farming, similar to the landscape between Hill of Fearn and Tain, but in any event is a brown-field development site that requires significant renovation. 2.3 The geomorphology of the site has been largely controlled by the underlying geology. The bedrock is made up of hard sandstone sediments, which have been cut by glaciers to create a fjord-type river estuary. Throughout this area, the routes of glaciers, sea level changes, along with significant glacial sand and gravel deposits have created a series of large open firths. On the Fearn site itself, marine deposits cover much of the area, draping the bedrock with varying depths of gravel single, sand and silt due to falls in relative sea level from climatic changes and isostatic effects. 2.4 Anthropogenic influences in recent years have significantly altered the landscape. The intertidal zone around Nigg Bay and the boggy floor of the north shore of the Cromarty Firth around Tarbat and Fearn have been subject to drainage and land reclaimage, eliminating the need to move cattle to high ground during the summer months. As a result the character of the area has changed, with older traditional landuse disappearing, replaced by a more regular pattern of arable and improved grasslands, and the creation of large farmhouses surrounded by meadows. 4 Orkney Sustainable Energy Document OSE/2871 Geology March 2010 3 General Geology 3.1 This is a relatively homogenous area from a geological perspective, dominated by sedimentary bedrock that was originally an area of sand and mud deposited in the Mid Devonian period, around 390M years ago. The great glacial events over the last 2M years have worked this coastline, creating the varied coastal forms, cliffs, bays, firths and estuaries. The Great Glen fault extends northwards to the east of the Cromarty Firth, with Nigg Hill and Tarbat Ness defining the western side of the fault; the Great Glen is a major strike/slip fault that defines the boundary between the older Northern Highlands to the west, and the Grampian Highlands to the east. 3.2 An assessment of the geology and the hydrogeological features of the area was conducted with reference to maps produced by the British Geological Survey [2,3,4] .. These maps identified the extent of superficial deposits in the area, mainly marine gravel shingle, sand and silt, along with glacial till over the more elevated areas. The bedrock geology in this part of the Highlands is the Raddery Sandstone Formation, a pebbly red sandstone of Mid Devonian age (395 – 385 Ma), Figure 2. Around 3km to the south, Nigg Hill is an inlier of much older Neoproterozoic metamorphic Psammite and Pelite bedrock (1000 – 540 Ma), likely exposed due to glaciation. It can be concluded that land along the eastern side of the Tarbat peninsula is Neoproterzoic, overlaid with younger Devonian beds, which in turn have been subject to the effects of glaciation, isostatic depression and subsequent marine inundation. 3.3 The large open straths and firths present throughout this part of the Highlands are a consequence of the extensive glaciation that has occurred in recent times, with major ice sheets developing around 850,000 years ago. The last ice sheet disappeared around 15,000 years ago, and the passage of these ice sheets resulted in the heavily glaciated valleys cut into the underlying bedrock, along with significant (10-100m) post-glacial sedimentary deposits. 3.4 The turbines are to be positioned on an area of hard strata on the Tarbat peninsula, in an area of raised seabed. The depth of topsoil varies across the site with topography and aspect, with the fields between the site and Nigg Bay dominated by large depths of gravel, shingle and silt, with 10m of superficial deposits likely under Fearn aerodrome. As a consequence, it can be expected that the groundworks for the turbine foundations will require excavation to a depth of around 3 to 4 metres, creating a stable platform by backfilling with a layer of stone before the concrete structures can be completed. 5 Orkney Sustainable Energy Document OSE/2871 Geology March 2010 4 Superficial deposits 4.1 Peat is not present at the site proposed for the Fearn turbines, and consequently there is no requirement for assessment of peat slide in this area. 4.2 The main superficial deposits are gravel shingle, sand and silt forming raised marine deposits that stretch from Nigg in the south to Cadboll in the east, figure 2, dated as Devensian and Flandrian, or Holocene Interglacial. These are very young deposits, and represent the sea bed during the last ice age and in the warm period immediately after the ice sheets retreated, between 6,000 and 10,000 years ago. The extent of these raised marine deposits suggests that the Cromarty Firth had outfalls near Cadboll and Loch Eye in this period. 4.3 Isostatic uplift and relative sea level fall over the last 5,000 years raised the land, creating the flat plain extending from the intertidal zone at Nigg to Fearn. A similar raised sea bed and sedimentary deposits occurs on Tain shore of the Tarbat Peninsula.