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P730- 2 Greenfield Burn Hydro Scheme KM March 2017 P730- 2 Scheme Description & Design r0 Page 1 CHAPTER 3: Scheme Description & Design 3.1 Introduction The principal dimensions and locations of the development components will be described in this document and associated drawings. In addition, the detailed design and construction contracts will ensure that the mitigation measures identified in this Environmental Statement ES) ( or measures achieving equivalent performance) are incorporated into the scheme. However, the detailed design of the development ( post consent) may necessitate minor changes to the layout. 3.2 Overview of Project Components Drawings of the scheme layout and components, including the powerhouse layout and elevations, intake general arrangements and pipeline routes, are available in Appendix J – Drawings. The main components of the proposed scheme are: One intake on the main on the main tributary of the Greenfield burn, Allt Coire nan Saobhaidh. One intake on a minor tributary, the Allt Doir an Daimh. A powerhouse located on the right bank of the Greenfield Burn, approximately 800m upstream of where it enters loch Garry. Approximately 5.1km of buried pipeline A tailrace at the powerhouse, to take water from the turbine back into the Greenfield Burn. Approximately 2.5km of new, permanent forest roads extending from the existing forestry commission road to the intakes. Approximately 3km of temporary track to be built alongside and enable construction of sections of the pipeline, which will be reinstated on completion of the works. Water will be transferred from the intake weirs to the powerhouses to drive water turbines, which in turn, will drive generators that generate the electricity. The plant will be of run- of- river status and the electricity generated will be exported to the National Grid via a new 33kV overhead line. The operation of the scheme will be dependent on the water levels and flows within the watercourse and its tributaries. The turbine will only operate when there is sufficient water for both abstraction and compensation flow. The compensation flows, released downstream of the intakes, are required to maintain the integrity of the aquatic environment and the species that it supports.

CHAPTER 3: Scheme Description & Design€¦ · forestry commission road to the intakes. Approximately 3km of temporary track to be built alongside and enable construction of sections

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Page 1: CHAPTER 3: Scheme Description & Design€¦ · forestry commission road to the intakes. Approximately 3km of temporary track to be built alongside and enable construction of sections

P730-2 Greenfield Burn Hydro SchemeKMMarch 2017

P730-2 Scheme Description & Design r0 Page 1

CHAPTER 3: Scheme Description & Design

3.1 Introduction

The principal dimensions and locations of the development components will be described inthis document and associated drawings. In addition, the detailed design and constructioncontracts will ensure that the mitigation measures identified in this Environmental StatementES) ( or measures achieving equivalent performance) are incorporated into the scheme.

However, the detailed design of the development ( post consent) may necessitate minorchanges to the layout.

3.2 Overview of Project Components

Drawings of the scheme layout and components, including the powerhouse layout andelevations, intake general arrangements and pipeline routes, are available in Appendix J – Drawings. The main components of the proposed scheme are:

One intake on the main on the main tributary of the Greenfield burn, Allt Coire nanSaobhaidh. One intake on a minor tributary, the Allt Doir an Daimh. A powerhouse located on the right bank of the Greenfield Burn, approximately 800mupstream of where it enters loch Garry. Approximately 5.1km of buried pipelineA tailrace at the powerhouse, to take water from the turbine back into the GreenfieldBurn. Approximately 2.5km of new, permanent forest roads extending from the existingforestry commission road to the intakes. Approximately 3km of temporary track to be built alongside and enable constructionof sections of the pipeline, which will be reinstated on completion of the works.

Water will be transferred from the intake weirs to the powerhouses to drive water turbines, which in turn, will drive generators that generate the electricity. The plant will be of run-of-river status and the electricity generated will be exported to the National Grid via a new33kV overhead line.

The operation of the scheme will be dependent on the water levels and flows within thewatercourse and its tributaries. The turbine will only operate when there is sufficient waterfor both abstraction and compensation flow. The compensation flows, released downstreamof the intakes, are required to maintain the integrity of the aquatic environment and thespecies that it supports.

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3.3 Scheme Design

3.3.1 Site Selection

The HwLDP provides support for renewable energy developments and is designed to guidedevelopments towards appropriate locations. The site at the Greenfield burn was selectedfor a number of reasons:

It has a good water resourceThere is already good access to the development area for construction traffic andturbine deliveryThere are no international/ national statutory natural heritage or landscapedesignations within the development area.

3.3.2 Evolution of Scheme Design

The pipeline route was originally on the left bank of the river ( looking downstream) but thiswas revised in order to avoid the rare Caledonian Pinewood habitat and to minimise treefelling. Several routes were considered on the right bank but eventually the most directroute was taken. The powerhouse position was also considered by the loch edge but thishas been moved upstream to return water to the river above the spawning habitat formigratory fish.

3.3.3 Rationale for Scheme Layout

The chosen route minimises the lengths of pipeline and access road required and makesuse of existing forest roads where possible. It also minimises the potential effect onmigratory fish.

3.3.4 Intakes

The intakes will be weir structures that span the width of the burn. General arrangementviews of the proposed intake designs are shown in Appendix J – Drawings P730-2 20104Intake GF1 Area Plan, and P730-2 20204 Intake GF2 Area Plan. The intake weir crests willbe located as shown in Table 3.1.

Table 3.1 Intake locations and crest levels

Intake Watercourse Grid reference Crest level (m AOD)

1 Allt Coire nan Saobhaidh NN 1831 9778 261. 40

2 Allt Doir an Daimh NN 1705 9797 267. 50

Integrated into each of these structures will be a screen, collection chamber and stillingchamber. The final design of these elements will be developed at the detailed designstage. A valve will be incorporated within or close to the intake chamber in order to isolatethe pipeline as required. A drain valve will also be fitted within each intake chamber formaintenance purposes.

During use, abstraction would begin with water flowing through the screens to the intakecollection and stilling chambers below, from where it will enter the pipelines and betransferred down to each powerhouse. A compensation flow will be passed over the weirsand allowed to continue its natural path downstream. There will be periods when the naturalriver flow will exceed the capacity of the intake chambers ( a flood) and in thesecircumstances this excess would flow unimpeded over the weirs, to join the natural riverflow.

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3.3.5 Fish Protection

The screen bar spacing on the intakes will be sized ( 10mm spacing) to prevent fish fromdropping into the collection chambers and hence into the pipelines.

Boulder- free plunge pools will also be constructed as part of the structures to facilitate thedownstream passage of fish.

3.3.6 Pipeline

The scheme will require approximately 5.1km of pipeline. The proposed route of thepipeline is shown on the Scheme Layout drawing in Appendix J.

The pressure rating of the pipeline will increase as it descends from the intakes to thepowerhouse. The pipelines will consist of a combination of High Performance PolyethyleneHPPE) at the low-pressure sections near the intakes, and high-pressure Glass-Reinforced

Plastic ( GRP) pipe for the higher pressure sections near the powerhouse.

The selection of the pipeline routes takes into account the following technical andenvironmental considerations:

The location of the intakes and the powerhouseThe route of the existing forest roadsThe aim to minimise the length of pipeline and use of land that is required forconstructionThe aim to route the pipeline away from sensitive ecological and cultural heritageareas, including blanket bog and deep peat, wherever possibleThe topography of the siteThe requirement to minimise the visual impact

The construction corridor for each pipeline will be restricted to a width of approximately 18-30m, depending on the type of pipe used. Along the route, the pipeline will be buried ormounded over. Air release valves will be installed on the pipeline at approximate spacing of400-500m and at high points along the pipe. These will consist of a flanged tee section ofpipe to allow the connection of the air valve to the main pipe. A simple chamber will beconstructed, probably using pre-cast concrete rings with a pre-cast concrete lid, to providemanhole access and ventilation.

3.3.7 Water Crossings

There are several minor water crossings on the pipe route, which will be crossed usingclosed culverts.

3.3.8 Powerhouse

The powerhouse and the associated equipment will be built at grid reference NN 19940057, at an elevation of 95mAOD. The location has been selected to minimise the potentiallandscape and visual effects and yet serve the requirements of the pipeline and gridconnection.

The powerhouse has been designed to accommodate one turbine and associatedgeneration and control equipment. The incoming pipeline will be buried and from thepowerhouse the water will return to the Greenfield Burn through an adjacent tailracestructure.

The powerhouse building will measure approximately 8m by 10m and will extend to amaximum height of 7.7m. The building will take the form of a portal frame structure bolted toa concrete floor slab. The walls will be wood clad, and the roof will be formed by profiled

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sheet metal. Top and subsoil removed from the building and hard standing areas will beused for restoration where appropriate. The transformers are likely to be of an oil-filleddesign and will be located outside the buildings.

The tailrace, in the form of a partially covered open channel, will be located on the northeastern side of the powerhouse building. This will transfer water from the powerhouse tothe Greenfield burn. Where the water passes from the tailrace into the burn, a screen with20mm openings will be fitted to prevent entry of otters and fish. At this point the tailracestructure will widen and incorporate a stilling basin or steps. This arrangement will avoiderosion of the main riverbed and banks.

Powerhouse plan, section and elevation views are available in Appendix J – Drawings.

3.3.9 Temporary Construction Compound / Laydown Areas

Temporary construction compounds of approximately 50m x 50m will be located at thepowerhouse, midway along the main pipeline, and at the two intake locations. Thecompounds will contain provision for fuel storage, storage of materials and welfare facilitiesfor site staff. They are chosen at sufficient stand-off distances from the watercourses. Thearea of the compounds will be restored on completion of the scheme.

3.3.10 Quarries

There is a requirement for stone aggregate for formation of permanent forest roads andalso for bedding the pipeline. Potential new quarries have been identified ( as shown inDrawing No. P730- 2 10206). Any additional material required will be taken from an existingForestry Commission Quarry within the local area.

3.4 Access & Traffic

3.4.1 Site Access from Public Road Network

The construction access will be taken from an existing forestry commission road to thesouth of Loch Garry. This road joins the public road network at a bridge across loch Garry, near the west end of the loch. From there, the access will be via 5km of single track road onthe north side of Loch Garry, to a junction on the A87 Fort William to Skye trunk road, justwest of Invergarry.

3.4.2 Onsite Forest Roads

Access for the scheme construction and operation would require around 10km of existingforestry commission roads, and the construction of around 2km of additional road, to reachthe two intake locations.

3.4.3 Road Design

In designing the access layout within the confines of the site area, the followingconsiderations have been made with respect to the forest road requirements:

Minimise requirement for additional lengths of roadEnable operational access for maintenance to weir and powerhouse, andFollow existing ground topography as much as possible and accommodate sensitiveecological features, cultural heritage interest and minimise visual impacts. Minimise requirement for watercourse crossings where possible.

The exact details of road construction methodologies will vary depending on localtopographic and ground conditions ( Please refer to the Construction Method Statement).

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3.4.4 Road Specification

The new permanent roads will be of minimum running surface width of 3.5m, and of stonesurface fill, to provide vehicle access during construction, and for future operation andmaintenance.

3.5 Grid Connection

A new overhead line will be erected from the nearby 33kV overhead line network to theproposed powerhouse location.

3.6 Operation and Maintenance

3.6.1 General Description and Operational Activities

It is intended that these generating stations are fully automated and only manned on a part-time basis. Once commissioned, the basic control philosophy will be as follows:

Plant is switched to ‘auto’ on panel. The control will sense if all systems are satisfactory ( e.g. grid healthy etc.). The control will sense when the level in the collection chamber is high enough toallow start up and running of the plant. The inlet valve will partially open which will cause the turbine to accelerate tosynchronous speed. A load bank may be employed to provide some inertia to thesystem. At synchronous speed, the circuit breaker will close, connecting the generator to thegrid. The turbine then opens up to the appropriate flow setting, being controlled by a levelsensor at the intake. The level sensor will constantly monitor the flow through the turbine. This maintainsthe minimum residual river flow between the intake and the tailrace discharge, at theturbine house. As the level rises and falls, the level control will open or close theturbine flow as necessary. In a fault condition, the control will disconnect the generator from the grid and shutthe turbine down.

There is normally a manual facility included in this type of controller. This is mainly forcommissioning purposes and maintenance procedures.

3.6.2 Operational Maintenance

The proposed development would be unmanned, with operations being controlled via aremote modem link. The maintenance for this type of plant is minimal. A typicalmaintenance routine would be as follows:

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Weekly • Test shut down facility on controlInspect powerhouse for maintenance purposes;

Monthly • Inspect intake screens and tailraces for blockages or damageCheck and lubricate relevant parts of equipmentClean screens, during low flow conditions, to remove accumulated algal and other growth

Annually • Drain system and intakes. Exercise gates. Inspect conveyance, intake chamber and tailracechannel. Clean the inside surface of the pipeline with a foam swab to remove particulate built up. Thiswould be undertaken during periods of flow suitable to disperse the discharged residue andis not expected to be more frequent than once every three years; andAs-required maintenance of the access tracks as noted during routine visits to the site

3.7 Decommissioning

The operational life of the development is expected to be up to 50 years although somemaintenance work may be required to ensure longevity. Decommissioning of the schemewould involve the removal of the hydro turbine and all above ground structures and 12months would be set aside for this process.