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PROPOSED WIND TURBINE DEVELOPMENT ON LAND AT WHEAL MARTYN CHINA CLAY WORKS. ST. AUSTELL, CORNWALL

FLOOD RISK ASSESSMENT

J-2012-Rev.02


PROPOSED WIND TURBINE DEVELOPMENT ON LAND AT WHEAL MARTYN CHINA CLAY WORKS,

ST. AUSTELL, CORNWALL

FLOOD RISK ASSESSMENT

For: Clean Earth Energy Job No: J-2012

Unit 2A Bess Park Road Date: May 2021 Trenant Industrial Estate Edition: 02

Wadebridge Cornwall PL27 6HB

Report No.

Issue Detail Originator Date Checked by Date

J-2012

02

TPS

26/05/2021

JM

26/05/2021


CONTENTS

Item Content Page No.

1.0 Introduction 1

2.0 Site Location & Description 3

2.1 Site Location 3 2.2 Existing Usage 3 2.3 Proposed Usage 3

3.0 Hydrological and Hydrogeological context 4

3.1 Hydrology 4 3.2 Hydrogeology 5

4.0 Assessment of Flood Risks 8

4.1 Fluvial and Tidal Flooding 8 4.2 Groundwater 8 4.3 Overland Flow 9 4.4 Flooding from Sewers 10 4.5 Flooding from Reservoirs, Canals and Other Artificial Sources 10 4.6 Flooding as a Result of Development 10

5.0 Design Standards 11

5.1 The CIRIA SuDS Manual (C753) 11 5.2 Building Regulations Part H 11 5.3 The Wallingford Procedure 11 5.4 National Planning Policy Framework 11 5.5 Drainage Guidance for Cornwall 11

6.0 Proposed Sustainable Drainage System (SuDS) 13

6.1 Drainage Design 13 6.2 Exceedance Events 13 6.3 Maintenance 14 6.4 Residual Risks After Development 14 6.5 Construction Stage Drainage 14

7.0 Summary and Conclusions 16

APPENDICES Appendix A Proposed Site Layout Including Topographic Survey and Conceptual Drainage Plan Appendix B Calculations

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J-2012 Hydrological Assessment and FRA– Wind Turbine, Wheal Martyn, St Austell

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Engineering and Development Solutions Ltd

Registered Office: Engineering and Development Solutions, Unit 10 Penstraze Business Centre, Truro, Cornwall, TR4 8PN Registered in England and Wales No. 10467487

Phone 01872 306311 Mobile 07973816457

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1.0 INTRODUCTION

Clean Earth Energy are proposing to develop a site within the Wheal Martyn China Clay Works, St Austell to provide a single wind turbine with associated infrastructure. The site lies between the Greensplat China Clay Extraction Pit and the C0298 public highway, as shown in Figures 1 and 2 below. An aerial photo of the site location is included as Figure 3.

Figure 1 - Geographical Area & Location

Figure 2 - Indicative Site Boundary

Site Location

Indicative Site Boundary

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Figure 3 – Aerial View of the Site

Reference to the Environment Agency (EA) flood map for planning shows the site to be within Flood Zone 1 (Low Risk). However, as the proposal site is over 1 hectares in area, the development will require a Flood Risk Assessment (FRA) in accordance with the National Planning Policy Framework (NPPF) on Planning and Flood Risk.

As the site is within Flood Zone 1 (Low Probability), the primary aim of the FRA will be to ensure that the development does not increase flood risk elsewhere. This can be achieved by providing a suitable sustainable drainage scheme (SuDS) that manages surface water runoff from the development.

To address this requirement, Engineering & Development Solutions (EDS) have been commissioned to prepare an FRA including a surface water drainage strategy for the proposed development, in accordance with the best practice principles of SuDS, the National Planning Policy Framework (NPPF), Sustainable Drainage Systems (SuDS) Guidance for Cornwall and Planning Practice Guidance (PPG). This report details the findings of the study.

Site Boundary

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2.0 SITE LOCATION & DESCRIPTION

2.1 Site Location

The proposed development site is located within the china clay mining area to the north west of the town of St Austell, Cornwall. The site is approximately 4.2km from St Austell town centre. The site covers an area of approximately 3 hectares. The Ordnance Survey Grid Reference for the turbine location is SW 99935 56269.

The site is positioned on a small plateau area of high ground located between two china clay extraction pits with Greensplat Pit being situated to the south of the site and Gunheath Pit being situated to the north. An array of open china clay settlement tanks are located to the immediate east of the site whilst a disused china clay waste tip is situated close to the west of the site.

The site is accessed by means of a private road which has a junction with the C0298 public highway about 190m to the north of the site. The C0298 highway runs between the road to Hensbarrow Hill (Greensplat Road) and the B3274.

In terms of topography, the site sits on a locally raised area of ground set at an elevation of approximately 250m AOD. The surrounding ground profile falls to the north and south into the Gunheath and Greensplat Pits respectively. To the east, the land falls to the valley base associated with the St. Austell River around Carthew, down to an elevation of about 183m AOD. To the west, the topography falls away for a short distance but then rises generally towards the Greensplat Road at an elevation of about 278m AOD, though the landscape has been modified in this direction by the waste material tip that exists close to the west boundary of the site.

2.2 Existing Usage

The site currently comprises an area of unused land set within the general china clay extraction operational area between two china clay pits.

2.3 Proposed Usage

The development proposal is for a single wind turbine with associated base, foundation, access track and hardstanding. The proposed layout which includes topographic information is shown in Appendix A.

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3.0 HYDROLOGICAL AND HYDROGEOLOGICAL CONTEXT

3.1 Hydrology

Being set on an effective plateau, a minor component of surface water runoff from the site could run in a range of directions, however, the predominant land fall is to the north and east. Consequently, it is considered that the majority of the runoff from the site would run to the north and east and be intercepted by the existing drainage ditch which runs close to the north east boundary of the site. This ditch conveys flow to the upper reaches of the St. Austell River which runs to the north of the site and thence into the St. Austell River; see (Figure 4) below.

Figure 4 - Plan Showing Local Hydrology and Flow Routes

The catchment area upslope of the site is minimal as it is set on a local high point in the topography; as such, the catchment area contributing flow into the ditch at this point is likely to be limited to the site area itself which is about 3.0 ha.

Reference to the Flood Estimation Handbook website for the site location confirms that it lays within the catchment of the St Austell River. The catchment extends to the north and west of the site as far as Hensbarrow Farm to the north and just past the alignment of the Greensplat Road to the west; see (Figure 5) below. The area of the catchment of the river is about 1.9km2

at the point where the site drainage ditch flows into the river.

Runoff Route

St Austell River

Local drainage ditch

Upper reach

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Figure 5 - Plan Showing Catchment of Local Ditch at Site

3.2 Hydrogeology

Reference to information published by the British Geological Society (BGS) indicates that the site is underlain by the St Austell Intrusion of granite bedrock, an igneous rock (Figure 6). The BGS Geology of Britain mapping describes the bedrock as follows: “Igneous Bedrock formed approximately 252 to 359 million years ago in the Permian and Carboniferous Periods. Local environment previously dominated by intrusions of silica-rich magma.”

Geological mapping also shows the potential presence of superficial deposits just to the north of the site comprising alluvial clays to gravels, though these deposits are likely to have been significantly disturbed by man-made intervention arising from clay extraction activities.

The area is designated as a “Secondary A” Aquifer type, which is the general designation for most of Cornwall. This is described as permeable strata capable of supporting water supplies at a local rather than strategic level and in some cases forming an important source of base flow to rivers.

With respect to Groundwater Vulnerability, the area is classified as ‘High’. This is a measure of the vulnerability of groundwater to a pollutant discharged at ground level based upon hydrological, geological, hydrogeological and soil properties within the area.

St Austell River

The Site

Catchment to the St Austell River 1.9km2

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Figure 6 - Plan Showing Bedrock Types in Vicinity of Site

A search has been undertaken with respect to borehole information available on the BGS database to determine groundwater depths in the vicinity of the site; Figure 7 provides a map of the available boreholes. There are two borehole logs available within 1km of the site which may be appropriate in assessing likely groundwater levels at the site. Reference to the logs reveals that the boreholes were only taken down to a depth of 3m below ground level and no groundwater was reported within the logs. Ground conditions were shown to comprise grey, brown sand of quartz and granite (Kaolinized granite).

Site Location

St Austell Intrusion shown Pink

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Figure 7 - Plan Showing Borehole Records for the Site

Groundwater levels on the site are likely to be linked with the china clay pits to the north and south with these acting as local sumps to drain down groundwater levels.

The water surface level in the Gunheath Pit China Clay Works to the north of the site sits at an elevation of approximately 160m AOD. As such, it is anticipated the groundwater levels on the site will be at a similar elevation and therefore well below ground level. The direction of groundwater flow is likely to follow the overall surface topography; as such groundwater flow is likely to move in a west to east at direction at depth beneath the site and will act to recharge base flow in the St. Austell River down in the valley to the east.

Site Location

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4.0 ASSESSMENT OF FLOOD RISKS

4.1 Fluvial and Tidal Flooding

The Environment Agency indicative flood map for planning (Figure 8, below) shows that the entire site is in Flood Zone 1 (less than 1 in 1,000 annual probability of river or sea flooding) and is therefore not at significant risk from either fluvial or tidal flooding.

Figure 8 - Environment Agency Flood Map for Planning (Rivers & Sea) Extract

4.2 Groundwater

Groundwater flooding is linked to the ability of the ground to hold water. The Cornwall Level 1 Strategic Flood Risk Assessment (SFRA) notes the following about groundwater flooding in Cornwall:

“Groundwater flooding is linked to the ability of the ground to hold water. Due to its geology Cornwall has only minor aquifers (2) and generally does not experience much groundwater type flooding. The exception to this is found in areas that have extensive mine drainage systems, where blockages within drainage tunnels can lead to unexpected breakout of groundwater at the surface.”

In addition, inspection of available information suggests that the groundwater table is depressed at considerable depth beneath the site.

The proposal is to install a wind turbine with foundation with a radius of 20m and depth of 3m below ground level. The presence of water in the clay pit to the north of the site provides an indication of possible groundwater levels in the area. The standing water levels in the pit to the north is approximately 90m below the ground level at the site. As such, construction of the foundation for the proposed wind turbine is likely to be well above the phreatic surface and is unlikely to interact with groundwater flows.

Therefore, the risk of groundwater flooding or impact of the proposed works on the groundwater regime is considered to be low and is not considered any further in this report.

Site Location

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4.3 Overland Flow

The site is located on a locally elevated area of land at the head of a drainage ditch which runs close to the north east boundary of the site. The topography provides little potential for any sizeable catchment area to exist upslope of the site for the generation of significant runoff flows. Though the general land profile does rise to the west up to Greensplat Road, the existing quarry access road which runs to the north west of the site is set in cutting at an elevation some 6m+ lower than the ground level at the proposed turbine. The road would act as an effective feature to intercept any overland flow approaching from a westerly direction and convey it to the north down to the C0298 without flow encroaching onto the site.

On the basis of an initial review, the site is not considered to be at any meaningful risk of flooding from overland flow sources.

The EA map extract, Figure 8 below provides further assessment of the risk of flooding from surface water for the site. It shows the majority of surface flows routed down the access track to the north of the site as predicted above, however, there is a minor flow route shown around the south and east boundary of the site (medium risk) and a small area of standing water is shown to exist on the northern boundary.

Figure 8 - EA Flood Risk from Surface Water Map Extract

With regard to the standing water shown on the north boundary, review of site levels shows that this area is in fact set at a moderate gradient falling away from the site and it is unlikely that water would pond in this area as shown on the mapping. As such the risk identified within the mapping in this area may not exist in reality.

The flow route along the southern boundary appears to be feasible, however, this does not impinge upon any sensitive areas of the development such as the turbine or hardstanding. The flow route does cross the alignment of the proposed access into the site and as such it is important to ensure that construction of the access road does not impede the mapped flow

Site Location

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route. This can be achieved by keeping the surface of the access track at ground level or by providing a culvert under the track if levels are to be raised.

Subject to maintaining this theoretical overland flow route, flooding from surface water is not considered to represent a significant risk to the development.

4.4 Flooding from Sewers

There are no mains sewers in the area and no residential dwellings lie upstream of the site. As such, likelihood of flooding from sewers is negligible.

4.5 Flooding from Reservoirs, Canals and Other Artificial Sources

Interrogation of the online EA flood risk mapping service does not indicate the site is at risk of flooding from reservoirs.

Considering local bodies of water associated with the china clay extraction activities it is apparent that the water level within Gunheath Pit is at significantly lower elevation than the site and escape of water from this pit would not represent a flood risk to the site.

There is a relatively small flooded pit situated at Cockbarrow some 800m to the north west of the site which is at slightly higher elevation that the site. However, it is evident by inspection of the topography that should this pit overtop for any reason, then the route of overflow would be along the short sub valley directly into Gunheath Pit and flow would not approach towards the site.

Therefore, flooding of the site from reservoirs and other artificial sources may be discounted.

4.6 Flooding as a Result of Development

The development of the site will alter the nature of the surface permeability across the site through the implementation of the access road and hardstanding .

It is important that surface water runoff from the development is understood and managed by means of a sustainable surface water drainage system in order to prevent an increase in the risk of flooding to areas downstream of the site.

By designing the site’s surface water drainage infrastructure in accordance with the advice reproduced in Section 5, the proposed development will not increase flood risk to third parties downslope. In consideration of the above, the proposed sustainable drainage system to be installed within the development is described in more detail in Section 6 of this report.

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5.0 DESIGN STANDARDS

Design of the site drainage infrastructure and Sustainable Drainage System (SuDS) is to be carried out in line with best practice, and to industry standard design procedures. Several publications, including design guidance and best practice guidance will be applied to different components of the final SuDS infrastructure. The sections below provide an overview of the design standards to be used on this project for various aspects of the SuDs infrastructure design.

5.1 The CIRIA SuDS Manual (C753)

This document is a comprehensive publication covering design, construction, operation, and maintenance of SuDS. The advice and best practice outlined in this document has been utilised in the design of the site SuDs features which have been detailed in this report.

5.2 Building Regulations Part H

Building Regulations Part H ‘Drainage and Waste Disposal’ covers the design and installation of surface water and foul water systems. All private drainage including pipes, manholes, down pipes, and other drainage infrastructure on the site should be designed and installed in accordance with this document.

5.3 The Wallingford Procedure

Developed by HR Wallingford, this publication covers the design of urban drainage systems. In addition, the document includes regional rainfall data for use in design for varying return period events. Basic sizing calculations for the proposed SuDS system and the estimation of the runoff volumes have been made using this method.

5.4 National Planning Policy Framework

The National Planning Policy Framework (NPPF) contains the policy relating to the appropriate assessment of flood risk within the UK. The associated technical guidance provides further details on the definitions, classifications and constraints used to apply national policy to new developments.

It contains details on flood zone definition, site specific FRAs, vulnerability classifications, appropriate development, climate change allowances, residual risk management, flood resilience, the sequential test and the exception test.

5.5 Drainage Guidance for Cornwall

This document provides advice for Cornwall Council as the Local Planning Authority and those involved in developing the built environment on:

The location of Critical Drainage Areas, where the flood risks from surface water runoff are likely to be most significant.

Standards to be achieved by surface water drainage. The content of a FRA considering surface water drainage. Sustainable Drainage techniques (SuDS) Sources of further information

The Drainage Guidance for Cornwall (DGfC) document is currently under review though until an updated version is published, advice appropriate to the proposed development considered within this report is reproduced below for ease of reference.

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The development site is not within a Critical Drainage Area though is over 1 hectare, therefore using DGfC it is considered that the site may be classified as ‘E4 – Developments greater than or equal to 1 hectare’ and the following guidance would apply:

“Outside Critical Drainage Areas – Greenfield Sites

E4 – Developments greater than or equal to 1 hectare

Following the Building Regulations Drainage hierarchy, surface water should:-

i. Drain to a soakaway or infiltration system designed in accordance with the SUDS Manual - CIRIA C697, using a minimum of a 30-year return period storm.

Where a FRA demonstrates that infiltration is not possible:-

ii. A sustainable drainage system shall be provided ensuring flow attenuation, no adverse impact on water quality and where possible habitat creation.

The total discharge from the site should aim to mimic greenfield rates. These shall be no more than the theoretical greenfield run-off rates from each of the corresponding 1, 10, 30 and 100 year storms. When these values are less than 5 litres/second, a rate of 5 litres/second can be used. Attenuation may not be necessary if the discharge is directly to coastal waters. In these cases the impact on the receiving environment in terms of habitat, erosion and water quality should be assessed

The design must take into account the appropriate allowance for increased rainfall from climate change. This should be based on the lifetime of the development, the guidance in Annex B of PPS25 and the PPS25 Practice Guide.”

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6.0 PROPOSED SUSTAINABLE DRAINAGE SYSTEM (SUDS)

The preferable drainage solution would be to drain all surface water runoff from the development using infiltration, in line with best practice guidance to deal with runoff as close to source as possible. Due to the site location in the china clay mining area, it is unlikely that infiltration would work effectively. In addition, the presence of a ditch close to the site and other ditches located nearby indicates that ground permeability is likely to be low. Therefore, an attenuation-based drainage system is proposed for the development.

6.1 Drainage Design

The introduction of an access road and hardstanding area will introduce partially impermeable areas. This infrastructure is proposed to be installed as imported hardcore capped with Type 1 material. This will result in a partially permeable road and hardstanding area. As such, the impermeable area will be calculated as being 50% permeable.

The foundation base to the turbine will be buried and will allow infiltration into the finished surfacing over the base. The footprint of the base is within the coverage of the hardstanding area, so it will be treated as being 50% permeable by default.

The following items detail the proposed impermeable area:

Access and Hardstanding = 5,700 x 50% = 2,850m2

Total Impermeable Area = 2,850m2

It is proposed to drain the above impermeable areas using a series of shallow swales across the site to convey flows into a surface detention basin with flow control discharging into the local ditch on site.

This system will provide initial filtering of the site runoff during the construction phase and settlement in the detention basin to reduce the impacts of silt laden runoff on the watercourses downstream of the site.

The greenfield runoff rate for the site has been calculated using the ICP SuDS method in MicroDrainage based on the gross footprint of the access track and hardstanding area to be 4.8 litres per second for the 1 in 100-year storm event.

The flow control device will therefore limit discharge to a maximum of 4.8 l/s. A detention basin of 158m3 volume with water depth of 700mm and 1:3 side slopes would provide sufficient storage for the 1 in 100 year plus 40% allowance for climate change storm event.

A conceptual surface water drainage layout is included as drawing 3001 in Appendix A.

The detention basin has been sized using MicroDrainage; calculations are included in Appendix B.

6.2 Exceedance Events

In the unlikely event of a storm in excess of the 1 in 100-year return period rainfall event (including climate change allowance) or if the proposed drainage systems were to become blocked, water may flood the system. In this case it is considered that the overflowing water would run over ground towards the north east of the site where it would be intercepted by the existing ditch as per the pre-developed scenario.

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Due to the storage provided in the proposed drainage systems, and design standard used (1 in 100 year storm with an additional 40% allowance for the effects of climate change), any exceedance flows would be lower than would flow off the site in the pre-development scenario for a similar storm event.

6.3 Maintenance

The proposed surface water drainage systems will remain private and will not be offered for adoption. Management and maintenance responsibility for the infrastructure will be the responsibility of the site owner/operator.

Maintenance activities for the systems will broadly comprise regular maintenance, occasional tasks, and remedial work where necessary, as per the guidance in the CIRIA SuDS Manual C753 which is summarised in Table 1 below. Inspection of the surface water drainage systems can generally be undertaken during routine site visits e.g. for grass cutting, leaf collection and/or litter collection.

DETENTION BASISN AND SWALES

Maintenance Activity Required Action Typical Frequency

Regular maintenance

Cut grass and verges surrounding basin/swales to allow for access

Monthly or as required (based on inspections)

Clear upstream drainage features of debris

Inspect flow control device for blockages and remove any sediment in chamber

Occasional maintenance Remove sediment and debris from inlet and outlet to basin and swales

As required, based on inspections

Monitoring

Inspect swales and detention basin and note rate of sediment accumulation

Monthly in the first year then annually

Check detention basin to ensure emptying is occurring

Annually

Table 1 – Detention Basin and Swales Typical Maintenance Activity Schedule

6.4 Residual Risks After Development

Rainfall over and above the design event could cause the sustainable drainage system to flood, however, any exceedance flows would be dealt with as outlined above.

The sustainable surface water drainage systems proposed in this report have been designed for the volume of surface runoff resulting from the proposed development, thus any unauthorised future connections into the proposed networks could potentially overload the system. Any future development on the site, beyond the current proposal, should be suitably planned and considered.

6.5 Construction Stage Drainage

In order to limit the potential for silt discoloured water to run off the site during the construction stage, it is proposed that the attenuation basin and swale collection system be constructed at the front end of the works. In this way any runoff from the subsequent construction of the access road and turbine foundation may be intercepted by the SUDS system and provided with filtration and settlement within the conveyance swales and the attenuation basin.

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A line of silt fencing should also be installed between the works area and swale system during the construction phase. Moveable straw bales should be provided at the lower end of the access track to allow interception and filtration of any runoff bypassing the SUDS system along the roadway.

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7.0 SUMMARY AND CONCLUSIONS

This study has investigated mechanisms of flooding and the potential for Sustainable Drainage (SuDS) to be installed as part of the development of a wind turbine and associated infrastructure at Wheal Martyn China Clay Works, St Austell, Cornwall.

Environment Agency (EA) indicative flood mapping shows that the development site is located entirely within Flood Zone 1; at little or no risk from tidal or fluvial flooding and is therefore suitable for all types of development. The development proposal is for an area over 1 hectare in size, therefore further consideration of surface water drainage has been undertaken.

Additional investigation of the existing hydrology and hydrogeology has been undertaken at the request of Clean Earth Energy for completeness.

The study has investigated alternative mechanisms for flooding at the site and has concluded that the site is not at risk of flooding and will not cause any increase in flood risk elsewhere once the proposed sustainable drainage system is operational.

It is necessary to ensure that the theoretical overland flow routes shown to exist on EA Surface Water Maps are not impeded by the works especially where flow crosses the proposed access road. This may be achieved by setting the access road no higher than existing ground levels or by providing a culvert under the road if levels are to be raised in this area.

Due to the location of the site within the china clay quarrying area to the north west of St Austell, and the presence of a local ditch on site and other ditches in the vicinity, the use of infiltration for disposal of surface water has been ruled out. As such, a conceptual attenuation-based drainage system has been proposed and outlined for the site.

The attenuation system has been designed to the 100-year standard with a 40% allowance for climate change.

Proposals have been provided to mitigate against the escape of silty runoff water during the construction stage.

Provided the recommendations outlined in this report are adopted in the development proposal then there is the capacity to manage the surface water runoff from the development onsite. The proposed drainage infrastructure has been designed in accordance with guidance outlined in the NPPF, PPG, and Drainage Guidance for Cornwall and therefore the development is entirely appropriate on this site from a flood risk perspective.

APPENDIX A PROPOSED LAYOUT

INCLUDING TOPOGRAPHIC SURVEY AND CONCEPTUAL SUDS PLAN

Clean Earth EnergyUnit 2, Bess Park RdTrenant Industrial

EstatePL27 6HB

Tel 01208 455028

Client

Project Name

Drawing Details

Drawing Number

Scale Drawn Checked

Rev DateDescriptionDrawing notes

Sheet

Rev.

GPR3756-IFL-OP-G

1VKRB1:2500

Wheal Martyn

Imerys

https://cleanearthenergy.com/












































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Option Area

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Access track and crane pad

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Existing access road

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33kV Cable

Sub StaEl

222.8m

Sks

Issues 231.6m

Track

Pond

Ponds

Track

Track

Track

204.5m

Email: [email protected]

Flood Risk AssessmentSuDS and Surface WaterFoul and Sewage Treatment

Highway DesignCivil EngineeringStatutory Approvals

EDS, Unit 10, Penstraze Business Centre, Truro, Cornwall TR4 8PN(01872) 306311 (Mob) 07973816457

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DISCHARGE TO EXISTING DITCH

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EXISTING DITCH

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PROPOSED ACCESS TRACK ROUTE

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HARDSTANDING / BUILD AREA

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TURBINE BASE (BURIED)

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CONVEYANCE SWALE

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ASSUMED DIRECTION OF CROSSFALL. IF ROAD IS CAMBERED PROVIDE SWALE ON BOTH SIDES OF TRACK

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TRANSVERSE INTERCEPTION CHANNELS TO PICK UP LONGITUDINAL FLOW AS NECESSARY

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PROVIDE 300mm CULVERT TOMAINTAIN OVERLAND FLOW ROUTES IF GROUND LEVELS ARE TO BE RAISED IN THIS AREA

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PROPOSED DETENTION BASIN WITH STORAGE VOLUME OF 158m3, BASE AREA 169m2 1m DEEP OVERALL WITH WATER DEPTH OF 0.7m 1:3 SIDE SLOPES. TO PROVIDE STORAGE FOR STORM EVENTS UP TO AND INCLUDING THE 1 IN 100-YEAR PLUS 40% ALLOWANCE FOR CLIMATE CHANGE.

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STRAW BALES WRAPPED IN PERMEABLE GEOTEXTILE TO PROVIDE SILT FILTRATION BARRIER. FILTER TO BE PROVIDE SILT FILTRATION BARRIER. FILTER TO BE PROVIDE SILT FILTRATION BARRIER. FILTER TO BE SILT FILTRATION BARRIER. FILTER TO BE SILT FILTRATION BARRIER. FILTER TO BE FILTRATION BARRIER. FILTER TO BE FILTRATION BARRIER. FILTER TO BE BARRIER. FILTER TO BE BARRIER. FILTER TO BE FILTER TO BE FILTER TO BE TO BE TO BE BE BE STAKED/ANCHORED TO GROUND. STAKE DETAILS TO TO GROUND. STAKE DETAILS TO TO GROUND. STAKE DETAILS TO GROUND. STAKE DETAILS TO GROUND. STAKE DETAILS TO STAKE DETAILS TO STAKE DETAILS TO STAKE DETAILS TO DETAILS TO DETAILS TO TO TO SUITE GROUND CONDITIONS.

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PERMIABLE GEOTEXTILE, TENSILE TENSILE STRENGTH WARP 27 WEFT 16 MPA PUNCTURE RESISTANCE 2000N APPARENT OPENING SIZE 380 MICRONS WEIGHT 120G/SQ.M OR SIMILAR

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CONSTRUCTION AREA

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SUPPORTING POSTS AT 1.5m CENTRES

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BACKFILL MATERIAL

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EXISTING GROUND

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CONSTRUCTION PHASE SILT FENCE 1:20

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PERMIABLE GEOTEXTILE, TENSILE TENSILE STRENGTH WARP 27 WEFT 16 MPA PUNCTURE RESISTANCE 2000N APPARENT OPENING SIZE 380 MICRONS WEIGHT 120G/SQ.M OR SIMILAR

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CONSTRUCTION AREA

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EXISTING GROUND

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CONSTRUCTION PHASE SILT FENCE ('NO DIG' AREA - IF REQUIRED) 1:20

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NOTES

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Copyright - This drawing and any ancillary drawings or data are copyright of EDS and may not be used, copied or amended for any and may not be used, copied or amended for any purpose whatsoever without written approval.

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PRELIMINARY

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B

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REV.

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3001

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DRAWING No.

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J-2012

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PROJECT No.

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CLIENT

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PROJECT

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DRAWING TITLE

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CONCEPTUAL DRAINAGE LAYOUT

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SCALE & SHEET SIZE:-

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DRAWN DATE:-

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PROJECT MANAGER:-

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1:750 @ A1

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MAY 2021

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TIM SULLIVAN

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DATE.

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NOTES.

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DRWN.

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REV.

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PRELIMINARY ISSUE

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A

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TPS

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13:08:20

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WIND TURBINE DEVELOPMENT, WHEAL MARTYN CHINA CLAY WORKS ST AUSTELL

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CHKD.

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JM

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1. This drawing is copyright. Refer to details above. 2. This drawing is only to be used for the purposes described in the status box below. Work to figured dimensions only, do not scale. 3. This drawing is to be read in conjunction with all other drawings, details and specifications pertaining to the work described. It should only be used for the purpose marked in the status box below, and shall not be used for construction unless clearly marked CONSTRUCTION. 4. Materials and workmanship shall comply to the appropriate British Standards and Codes of Practice unless otherwise stated. 5. The activities required to construct the work, shown on drawings clearly marked CONSTRUCTION, may be subject to the provisions of the Construction (Design & Management) Regulations 2015. The Contractor and Client must ensure that they are adequately conversant with these regulations and that the appropriate procedures required under the regulations are observed at all times. 6. Design Risk Assessment Design Risk Assessment A risk assessment relating to potential hazards associated with the works described within this drawing, in so far as they have been designed by EDS Ltd, has been undertaken. Risks identified have been eliminated by design wherever practicable. The status with regard to residual risks is as follows: The work is at an early planning stage and is not sufficiently advanced to allow a meaningful assessment of risks to be undertaken at this time. Designer - EDS Drawing revision - A Date - 21:05:21

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CLEAN EARTH ENERGY

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PROJECT ENGINEER:-

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TIM SULLIVAN

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KEY

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CHANNEL/ACO TYPE DRAIN

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DETENTION BASIN TO PROVIDE SILT SETTLEMENT AND ATTENUATION

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SURFACE WATER DRAINAGE THE SURFACE WATER DRAINAGE SYSTEM HAS BEEN DESIGNED TO ACCEPT RUNOFF FROM THE PROPOSED HARDCORE ACCESS TRACK, AND HARDSTANDING AREAS . THE HARDCORE ACCESS TRACK AND HARDSTANDING AREAS ARE ASSUMED TO BE SEMI-PERMEABLE (50% PERMEABLE) CONVEYANCE SWALES WILL DRAIN RUNOFF FROM THE SITE INTO A DETENTION BASIN TO ALLOW FOR SETTLEMENT OF SILT LADEN RUNOFF. THE DETENTION BASIN HAS BEEN DESIGNED TO ACCEPT FLOWS FROM EVENTS UP TO AND INCLUDING THE 1 IN 100 YEAR STORM EVENT PLUS AN ALLOWANCE OF 40% FOR CLIMATE CHANGE. THE DISCHARGE FROM THE BASIN WILL BE RELEASED AT A CONTROLLED RATE OF 4.8 l/s MAXIMUM TO THE EXISTING DITCH ON SITE A HYDRO INTERNATIONAL HYDROBRAKE OR SIMILAR WILL PROVIDE THE NECESSARY FLOW CONTROL. NOTE ALL INFRASTRUCTURE LOCATIONS ARE INDICATIVE ONLY. CONSTRUCTION PHASE DRAINAGE A FILTER STRIP OF MINIMUM 1m SHOULD BE PROVIDED BETWEEN THE EDGE OF ANY CONSTRUCTION AREAS TO PROVIDE INITIAL FILTRATION OF CONSTRUCTION RUNOFF. SILT FENCES SHOULD BE INSTALLED PARALLEL WITH CONTOURS WHERE POSSIBLE TO PREVENT SILT LADEN RUNOFF DISCHARGING DIRECTLY TO THE SWALES. SEQUENCE OF CONSTRUCTION THE DETENTION BASIN WITH FLOW CONTROL SHOULD BE CONSTRUCTED FIRST THEN THE SWALES AND FINALLY THE SILT FENCING. ALL DRAINAGE SHOULD BE CONSTRUCTED PRIOR TO ANY OTHER SITE ENABLING WORKS.

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FLOW CONTROL MANHOLE 1200 PCC CHAMBERWITH HYDROBRAKE FLOW CONTROL DEVICE

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CONVEYANCE SWALE CONSTRUCTED PARALLEL TO CONTOURS WHERE POSSIBLE TO PROVIDE SETTLEMENT OF SILT LADEN RUNOFF

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OVERLAND FLOW ROUTES

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CONSTRUCTION PHASE SILT FENCE

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SURFACE WATER SEWER 100mm

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SITE BOUNDARY

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PLAN SHOWING CONCEPTUAL DRAINAGE LAYOUT

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SUBSTATION REMOVED DETENTION POND MOVED

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B

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TPS

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25:05:20

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JM

APPENDIX B CALCULATIONS

EDS Ltd Page 1

Unit 10, Penstraze Business ... Wind Turbine Wheal Martyn

Truro

Cornwall

Date 13/05/2021 Designed by TPS

File Checked by AW

Innovyze Source Control 2020.1

ICP SUDS Mean Annual Flood

©1982-2020 Innovyze

Input

Return Period (years) 100 Soil 0.300Area (ha) 0.574 Urban 0.000SAAR (mm) 1210 Region Number Region 8

Results l/s

QBAR Rural 2.0QBAR Urban 2.0

Q100 years 4.8

Q1 year 1.5Q30 years 3.8Q100 years 4.8

EDS Ltd Page 1


Truro Attenuation Basin

Cornwall


File J-2012 Attenuation basi... Checked by


Summary of Results for 100 year Return Period (+40%)


Half Drain Time : 284 minutes.

Storm

Event

Max

Level

(m)

Max

Depth

(m)

Max

Infiltration

(l/s)

Max

Control

(l/s)

Max

Σ Outflow

(l/s)

Max

Volume

(m³)

Status

15 min Summer 238.736 0.236 0.0 4.8 4.8 53.2 O K30 min Summer 238.831 0.331 0.0 4.8 4.8 74.5 O K60 min Summer 238.938 0.438 0.0 4.8 4.8 98.5 O K120 min Summer 239.044 0.544 0.0 4.8 4.8 122.3 O K180 min Summer 239.086 0.586 0.0 4.8 4.8 131.9 O K240 min Summer 239.102 0.602 0.0 4.8 4.8 135.5 O K360 min Summer 239.116 0.616 0.0 4.8 4.8 138.7 O K480 min Summer 239.118 0.618 0.0 4.8 4.8 139.1 O K600 min Summer 239.114 0.614 0.0 4.8 4.8 138.1 O K720 min Summer 239.105 0.605 0.0 4.8 4.8 136.2 O K960 min Summer 239.082 0.582 0.0 4.8 4.8 131.0 O K1440 min Summer 239.023 0.523 0.0 4.8 4.8 117.7 O K2160 min Summer 238.914 0.414 0.0 4.8 4.8 93.2 O K2880 min Summer 238.825 0.325 0.0 4.8 4.8 73.1 O K4320 min Summer 238.704 0.204 0.0 4.8 4.8 45.8 O K5760 min Summer 238.642 0.142 0.0 4.6 4.6 31.9 O K7200 min Summer 238.617 0.117 0.0 4.3 4.3 26.4 O K8640 min Summer 238.605 0.105 0.0 3.8 3.8 23.5 O K10080 min Summer 238.596 0.096 0.0 3.5 3.5 21.5 O K

15 min Winter 238.766 0.266 0.0 4.8 4.8 59.9 O K

Storm

Event

Rain

(mm/hr)

Flooded

Volume

(m³)

Discharge

Volume

(m³)

Time-Peak

(mins)

15 min Summer 105.591 0.0 55.8 2130 min Summer 75.240 0.0 79.8 3660 min Summer 51.407 0.0 110.1 64120 min Summer 33.834 0.0 145.1 124180 min Summer 25.951 0.0 167.0 182240 min Summer 21.313 0.0 182.9 234360 min Summer 16.157 0.0 208.0 292480 min Summer 13.243 0.0 227.3 358600 min Summer 11.335 0.0 243.2 426720 min Summer 9.973 0.0 256.8 496960 min Summer 8.137 0.0 279.4 6381440 min Summer 6.089 0.0 313.5 9142160 min Summer 4.539 0.0 351.3 12962880 min Summer 3.681 0.0 379.8 16484320 min Summer 2.746 0.0 424.7 23365760 min Summer 2.233 0.0 461.1 30007200 min Summer 1.903 0.0 491.4 36728640 min Summer 1.673 0.0 518.1 440810080 min Summer 1.501 0.0 542.1 5136

15 min Winter 105.591 0.0 62.6 21

EDS Ltd Page 2



Cornwall




Summary of Results for 100 year Return Period (+40%)


Storm

Event

Max

Level

(m)

Max

Depth

(m)

Max

Infiltration

(l/s)

Max

Control

(l/s)

Max

Σ Outflow

(l/s)

Max

Volume

(m³)

Status

30 min Winter 238.874 0.374 0.0 4.8 4.8 84.1 O K60 min Winter 238.997 0.497 0.0 4.8 4.8 111.8 O K120 min Winter 239.117 0.617 0.0 4.8 4.8 138.9 O K180 min Winter 239.170 0.670 0.0 4.8 4.8 150.6 O K240 min Winter 239.192 0.692 0.0 4.8 4.8 155.6 O K360 min Winter 239.205 0.705 0.0 4.8 4.8 158.7 O K480 min Winter 239.205 0.705 0.0 4.8 4.8 158.7 O K600 min Winter 239.197 0.697 0.0 4.8 4.8 156.8 O K720 min Winter 239.182 0.682 0.0 4.8 4.8 153.6 O K960 min Winter 239.144 0.644 0.0 4.8 4.8 144.9 O K1440 min Winter 239.049 0.549 0.0 4.8 4.8 123.6 O K2160 min Winter 238.874 0.374 0.0 4.8 4.8 84.2 O K2880 min Winter 238.745 0.245 0.0 4.8 4.8 55.1 O K4320 min Winter 238.625 0.125 0.0 4.5 4.5 28.0 O K5760 min Winter 238.602 0.102 0.0 3.7 3.7 23.0 O K7200 min Winter 238.589 0.089 0.0 3.2 3.2 20.1 O K8640 min Winter 238.582 0.082 0.0 2.8 2.8 18.3 O K10080 min Winter 238.576 0.076 0.0 2.5 2.5 17.0 O K

Storm

Event

Rain

(mm/hr)

Flooded

Volume

(m³)

Discharge

Volume

(m³)

Time-Peak

(mins)

30 min Winter 75.240 0.0 89.5 3560 min Winter 51.407 0.0 123.4 64120 min Winter 33.834 0.0 162.5 120180 min Winter 25.951 0.0 187.1 178240 min Winter 21.313 0.0 204.9 232360 min Winter 16.157 0.0 233.0 332480 min Winter 13.243 0.0 254.7 378600 min Winter 11.335 0.0 272.5 456720 min Winter 9.973 0.0 287.7 534960 min Winter 8.137 0.0 313.0 6881440 min Winter 6.089 0.0 351.2 9942160 min Winter 4.539 0.0 393.6 13642880 min Winter 3.681 0.0 425.5 17004320 min Winter 2.746 0.0 475.8 22525760 min Winter 2.233 0.0 516.5 29767200 min Winter 1.903 0.0 550.4 36808640 min Winter 1.673 0.0 580.3 440810080 min Winter 1.501 0.0 607.3 5136

EDS Ltd Page 3



Cornwall




Rainfall Details


Rainfall Model FSR Winter Storms YesReturn Period (years) 100 Cv (Summer) 0.750

Region England and Wales Cv (Winter) 0.840M5-60 (mm) 18.200 Shortest Storm (mins) 15

Ratio R 0.250 Longest Storm (mins) 10080Summer Storms Yes Climate Change % +40

Time Area Diagram

Total Area (ha) 0.287

Time

From:

(mins)

To:

Area

(ha)

Time

From:

(mins)

To:

Area

(ha)

0 4 0.143 4 8 0.144

EDS Ltd Page 4



Cornwall




Model Details


Storage is Online Cover Level (m) 240.000

Cellular Storage Structure

Invert Level (m) 238.500 Safety Factor 2.0Infiltration Coefficient Base (m/hr) 0.00000 Porosity 1.00Infiltration Coefficient Side (m/hr) 0.00000

Depth (m) Area (m²) Inf. Area (m²) Depth (m) Area (m²) Inf. Area (m²)

0.000 225.0 225.0 1.100 0.0 285.01.000 225.0 285.0

Hydro-Brake® Optimum Outflow Control

Unit Reference MD-SHE-0107-4800-0750-4800Design Head (m) 0.750

Design Flow (l/s) 4.8Flush-Flo™ CalculatedObjective Minimise upstream storage

Application SurfaceSump Available YesDiameter (mm) 107

Invert Level (m) 238.500Minimum Outlet Pipe Diameter (mm) 150Suggested Manhole Diameter (mm) 1200

Control Points Head (m) Flow (l/s)

Design Point (Calculated) 0.750 4.8Flush-Flo™ 0.225 4.8Kick-Flo® 0.506 4.0

Mean Flow over Head Range - 4.1

The hydrological calculations have been based on the Head/Discharge relationship for theHydro-Brake® Optimum as specified. Should another type of control device other than aHydro-Brake Optimum® be utilised then these storage routing calculations will beinvalidated

Depth (m) Flow (l/s) Depth (m) Flow (l/s) Depth (m) Flow (l/s) Depth (m) Flow (l/s)

0.100 3.7 1.200 6.0 3.000 9.2 7.000 13.70.200 4.8 1.400 6.4 3.500 9.9 7.500 14.20.300 4.7 1.600 6.8 4.000 10.5 8.000 14.60.400 4.6 1.800 7.2 4.500 11.1 8.500 15.10.500 4.1 2.000 7.6 5.000 11.7 9.000 15.50.600 4.3 2.200 7.9 5.500 12.3 9.500 15.90.800 4.9 2.400 8.3 6.000 12.81.000 5.5 2.600 8.6 6.500 13.3

Engineering and Development Solutions Ltd Registered Office: Unit 10 | Penstraze Business Centre | Truro | Cornwall | TR4 8PN Registered in England No. 10467487

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