chapter 3: scheme description & design

P730-1 Allt Garaidh Ghualaich Hydro SchemeKMMarch 2017

P730-1 Scheme Description & Design r0

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

One intake on the main Allt Garaidh Ghualaich BurnOne intake on a tributary of the Allt Garaidh Ghualaich Burn, the Allt Lon GlasBheinnA powerhouse located on the right bank of the Allt Garaidh Ghualaich Burn, approximately 500m upstream of an existing older hydro scheme intake. Approximately 4.7km of buried pipelineA tailrace at the powerhouse, to take water from the turbine back into the AlltGaraidh Ghualaich BurnApproximately 4.7km of new, permanent forest road extending from the existingforestry commission road to the intakes. A vehicle and pipe bridge to provide access to Intake 2.

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.



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 Allt Garaidh Ghualaich burnwas selected for 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 main pipeline was originally located on the right bank of the river ( looking downstream) but this was revised following the habitat survey, which identified some high quality blanketbog on the pipeline route. A different route on the right bank was considered but this wouldhave involved removing some trees and increased the effect on GWDTE habitat.

3.3.3 Rationale for Scheme Layout

The layout is such that it minimises the pipeline and access track lengths whilst maximisingthe output from the scheme. Existing forest roads are used in preference to creating newroads wherever possible.

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. The intake weircrests will be 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 Garaidh Gualaich NN 1528 9676 355. 90

2 Allt Lon Glas Bheinn NN 1297 9626 360. 00

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.

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 4.7km 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

One bridge will be required to grid reference NN 1535 9817, to carry a pipeline and vehiclesacross the Allt Garaidh Ghualaich Burn towards Intakes 1 and 2.

3.3.8 Powerhouse

The powerhouse and the associated equipment will be built at grid reference NN 15579873, at an elevation of 215mAOD. The location has been selected to minimise thepotential landscape and visual effects and yet serve the requirements of the pipeline andgrid connection.

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 Allt Garaidh Ghualaich burn through an adjacenttailrace structure.

The powerhouse building will measure approximately 10m by 8m in plan and will extend toa height of 7.7m at the roof ridgeline. The building will comprise a portal frame structurebolted to a concrete floor slab. The exterior will have vertical timber cladding on the walls, with a profiled sheet metal roof. Top and subsoil removed from the building and hardstanding areas will be used for restoration where appropriate. The transformers are likely tobe of an oil-filled design and will be located outside the buildings.



The tailrace, in the form of a partially covered open channel, will be located on the northernside of the powerhouse building. This will transfer water from the powerhouse to the AlltGaraidh Ghualaich burn. Where the water passes from the tailrace into the burn, a screenwith 20mm 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 intakeand powerhouse positions. The compounds will contain provision for fuel storage, storageof materials and welfare facilities for site staff. They are chosen at sufficient stand-offdistances from the watercourses. The area of the compounds will be restored oncompletion 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- 1 10106). 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

The existing forestry commission roads reach approximately 500m beyond the proposedpowerhouse location, in the direction of the proposed pipeline and intakes. From this point, the scheme would require the construction of around 4.2km of additional vehicle accessroad, reaching as far as the 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).

3.4.4 Road Specification

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



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:

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 scheme



would involve the removal of the hydro turbine and all above ground structures and 12months would be set aside for this process.

chapter 3: scheme description & design

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