Proposed Wind Turbine Development on Land at Longstone, St Mewan, St Austell, Cornwall

PROPOSED WIND TURBINE DEVELOPMENT ON LAND AT LONGSTONE, ST MEWAN, ST AUSTELL, CORNWALL

FLOOD RISK ASSESSMENT

J-1520-Rev.01

www.eadsolutions.co.uk

PROPOSED WIND TURBINE DEVELOPMENT ON LAND AT LONGSTONE, ST MEWAN, ST AUSTELL, CORNWALL

FLOOD RISK ASSESSMENT

Report No. Issue Detail Originator Date Checked by Date

J-1520 01 AW 27/08/2020 TPS 27/08/2020

For: Clean Earth Energy Job No: J-1520 Unit 2A Bess Park Road Date: August 2020 Trenant Industrial Estate Edition: 01 Wadebridge Cornwall PL27 6HB

www.eadsolutions.co.uk

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 7

4.1 Fluvial and Tidal Flooding 7 4.2 Groundwater 7 4.3 Overland Flow 8 4.4 Flooding from Sewers 8 4.5 Flooding from Reservoirs, Canals and Other Artificial Sources 8 4.6 Flooding as a Result of Development 8

5.0 Design Standards 10

5.1 The CIRIA SuDS Manual (C753) 10 5.2 Building Regulations Part H 10 5.3 The Wallingford Procedure 10 5.4 National Planning Policy Framework 10 5.5 Drainage Guidance for Cornwall 10

6.0 Proposed Sustainable Drainage System (SuDS) 12

6.1 Drainage Design 12 6.2 Exceedance Events 12 6.3 Maintenance 13 6.4 Residual Risks After Development 13 6.5 Construction Stage Drainage 13

7.0 Summary and Conclusions 15

APPENDICES

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

Appendix B Calculations

www.eadsolutions.co.uk ______

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

1.0 INTRODUCTION

Clean Earth Energy are proposing to develop a site on land at Longstone, St Mewan, St Austell, to erect a single wind turbine with associated infrastructure. The site lies to the north of an un- named access road between Karslake and Greensplat, as shown in Figures 1 and 2 below. An aerial photo of the site location is included as Figure 3.

Site Location

Figure 1 - Geographical Area & Location

Indicative Site Boundary

Figure 2 - Indicative Site Boundary

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 1 ______

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

Site Boundary

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 is for over 1 hectares in size, 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 Devon, and Planning Practice Guidance (PPG). This report details the findings of the study.

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

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 4km from St Austell town centre. The site covers an area of around 1.6 hectares. The Ordnance Survey Grid Reference for the turbine location is SW 98316 55310.

The site is located between two china clay extraction pits on an area of land to the north of the public road linking Karslake to the west and Greensplat to the east.

In terms of existing topography, the site generally falls from north west to south east from an elevation of 255m AOD to 228m AOD. The typical site gradient is 1:9. A topographic survey is included in Appendix A.

In a wider context, land to the north rises towards Longstone Downs before reducing in elevation into the Longstone China Clay Works site at an elevation of approximately 240m AOD. A local high point is located to the north west at the site of a disused china clay waste tip at an elevation of approximately 318m AOD. Land to the south of the site drops in elevation over a tarmac access road and into the Blackpool China Clay Works pit at an elevation of 168m AOD. To the west of the site, the land rises slightly before dropping again towards Old Pound Road and agricultural fields beyond. Several properties and development features associated with the china clay works are located to the west of the site beyond greenfield land.

2.2 Existing Usage

The site currently comprises greenfield land between two china clay quarries.

2.3 Proposed Usage

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

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

3.0 HYDROLOGICAL AND HYDROGEOLOGICAL CONTEXT

3.1 Hydrology

The local hydrology is primarily defined by a local ditch which runs in a west to east direction along the southern boundary of the site (Figure 4). This ditch flows east where it meets a tributary and then flows under the access road towards Goonmarth Farm and then south and east until it meets another tributary and becomes the Gover Stream.

The Gover Stream is a tributary of the St Austell River with its confluence in the west of St Austell town which then flows in a southerly direction towards the coast and out to sea at Pentewan.

The ditch catchment in the vicinity of the site is small, with an area of approximately 0.64km2; as such, there is limited upslope area drained above the site. The approximate catchment area can be seen in Figure 5 below. The catchment has been modified by human intervention with the presence of china clay quarrying and processing activities to the north.

Site Location Ditch becomes Gover Stream to the south Runoff Route

Local Ditch Assumed Culvert

Figure 4 - Plan Showing Local Hydrology

Consideration of land falls and contours indicates that surface water runoff from the site would generally flow in a south east direction over the site and would be intercepted by the ditch along the south boundary and conveyed as described.

The presence of the china clay works to the north and the south appears to have a limited interaction with the surface water catchment due to topography. The Longstone China Clay Works to the north is located at an elevation slightly lower than the highest point on the site with an access track to the north of the site rising in elevation to 274m AOD, therefore, flows in relation to this pit would not be conveyed into the ditch catchment.

Blackpool China Clay Works, to the south of the site, are located at lower elevation than the site and local ditch. The mapping in Figures 2 and 4 indicates that the local ditch does not flow into these works. Any runoff originating from levels lower than the ditch, and therefore off site, would likely flow into this clay pit.

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

Site Location

Catchment Area 0.64km2

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.”

There are no superficial deposits shown to exist on the site, through the local china clay material is likely to be residual weathered granite.

Site Location

St Austell Intrusion shown Pink

Figure 6 - Plan Showing Bedrock Types in Vicinity of Site

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

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.

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 is a map of the available boreholes. As shown, there are no nearby borehole logs appropriate to assess likely groundwater levels on site.

Site Location

Figure 6 - 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 Longstone China Clay Works to the north of the site sits at an elevation of approximately 240m AOD. The water surface in the Blackpool China Clay Works to the south sits at approximately 168m AOD. As such, it is anticipated the groundwater levels on the site will be at an elevation of about 205m AOD and therefore well below ground level.

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

4.0 ASSESSMENT OF FLOOD RISKS

4.1 Fluvial and Tidal Flooding

The Environment Agency indicative flood map for planning (Figure 7, 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.

Site Location

Figure 7 - 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.” Inspection of local mapping does not indicate the presence of any mines or disused addits in the area.

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 pits to the north and south of the site provide an indication of possible groundwater levels in the area. Water levels in the pit to the south are approximately 40m below the surrounding ground level and in the pit to the north are around 30m below ground level. As such, the construction and siting of the proposed wind turbine is unlikely to interact with groundwater flows.

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

Therefore, the risk of groundwater flooding is low and is not considered any further in this report.

4.3 Overland Flow

The site is located towards the upper reaches of a local ditch catchment which is very limited in size. Runoff has been considered in the hydrology section of this report (Section 3.1).

In addition, the EA map extract, Figure 8 below, shows the risk of flooding from surface water for the site. It indicates that the site is at very low risk of flooding from surface water.

Site Location

Figure 8 - EA Flood Risk from Surface Water Map Extract

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.

With reference to 1:25,000 Ordnance Survey mapping and aerial imagery, there are two waterbodies located in the vicinity of the site and its catchment. The Blackpool China Clay works to the south, which is situated at a lower elevation than the site; and the Longstone China Clay Works located to the north of the site. The Langstone waterbody is separated by higher ground, as seen in the topographic survey data, which means the site is located in a different watershed than the clay pit and will therefore not impact on the site.

Therefore, flooding of the site from these sources can be discounted.

4.6 Flooding as a Result of Development

The development of the site will alter the nature of the surface permeability throughout the site through the implementation of an access road, hardstanding, a sub-station, and the wind turbine base.

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

The development will increase the overall impermeable area due to the introduction of impermeable areas. It is important that surface water runoff from the development is understood and managed by means of a sustainable surface water drainage system, 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.

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

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.

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

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.”

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

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 on site, which is already located in the upstream reaches of the catchment, and other ditches located nearby indicates the 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 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 50% permeable.

The sub-station and turbine base will impermeable and calculated as 100% impermeable.

The following points detail the proposed impermeable area:

• Access and Hardstanding = 4,352 x 50% = 2,176m2

• Sub-station and Turbine Base = 350m2

• Total Impermeable Area = 2,526m2

It is proposed to drain the above impermeable areas using a series of shallow swales across the site to convey flows into an above ground 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 to be 2.1 litres per second for the 1 in 100-year storm event.

The flow control device will limit discharge to a maximum of 2.1 l/s. A detention basin of 176m3 volume with 300mm freeboard 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 south of the site where it would be intercepted by the existing ditch as per the pre-developed scenario.

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

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 Cut grass and verges surrounding basin/swales to allow for access Clear upstream drainage features of Monthly or as required Regular maintenance debris (based on inspections) Inspect flow control device for blockages and remove any sediment in chamber Remove sediment and debris from As required, based on Occasional maintenance inlet and outlet to basin and swales inspections Inspect swales and detention basin Monthly in the first year and note rate of sediment then annually Monitoring accumulation Check detention basin to ensure Annually emptying is occurring 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.

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

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.

J-1520 Flood Risk Assessment – Wind Turbine, Longstone, St Mewan, St Austell, Cornwall ______

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 Longstone, St Mewan, 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.

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

· Flood Risk Assessment · Highway Design · SuDS and Surface Water · Civil Engineering · Foul and Sewage Treatment · Statutory Approvals

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

Email: [email protected] www.eadsolutions.co.uk

APPENDIX B CALCULATIONS

EDS Ltd Page 1 Unit 10, Penstraze Business C... Truro Cornwall Date 14/08/2020 10:19 Designed by AndyWoolley File J-1520 SW Detention Basi... Checked by Innovyze Source Control 2018.1.1

ICP SUDS Mean Annual Flood

Input

Return Period (years) 100SAAR (mm) 1200 Urban 0.000 Area (ha)0.253 Soil 0.300Region NumberRegion 8

Results l/s

QBAR Rural0.9 QBAR Urban0.9

Q100 years2.1

Q1 year 0.7 Q30 years 1.7 Q100 years2.1

©1982-2018 Innovyze EDS Ltd Page 1 Unit 10, Penstraze Business C... Truro Cornwall Date 14/08/2020 10:24 Designed by AndyWoolley File J-1520 SW Detention Basi... Checked by Innovyze Source Control 2018.1.1

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

Storm Max Max Max Max Max Max Status Event Level Depth Control Overflow Σ Outflow Volume (m) (m) (l/s) (l/s) (l/s) (m³)

15 min Summer98.861 0.661 1.6 0.0 1.6 48.0 O K 30 min Summer99.032 0.832 1.7 0.0 1.7 67.6 O K 60 min Summer99.201 1.001 1.8 0.0 1.8 90.2 O K 120 min Summer99.356 1.156 1.9 0.0 1.9 114.2 O K 180 min Summer99.431 1.231 1.9 0.0 1.9 127.0 O K 240 min Summer99.472 1.272 2.0 0.0 2.0 134.3 O K 360 min Summer99.520 1.320 2.0 0.0 2.0 143.3 O K 480 min Summer99.541 1.341 2.0 0.0 2.0 147.2 O K 600 min Summer99.546 1.346 2.0 0.0 2.0 148.2 O K 720 min Summer99.545 1.345 2.0 0.0 2.0 148.0 O K 960 min Summer99.536 1.336 2.0 0.0 2.0 146.3 O K 1440 min Summer 99.518 1.318 2.0 0.0 2.0 142.8 O K 2160 min Summer 99.483 1.283 2.0 0.0 2.0 136.4 O K 2880 min Summer 99.442 1.242 1.9 0.0 1.9 129.0 O K 4320 min Summer 99.357 1.157 1.9 0.0 1.9 114.4 O K 5760 min Summer 99.270 1.070 1.8 0.0 1.8 100.5 O K 7200 min Summer 99.184 0.984 1.8 0.0 1.8 87.7 O K 8640 min Summer 99.100 0.900 1.7 0.0 1.7 76.2 O K

Storm Rain Flooded Discharge Overflow Time-Peak Event (mm/hr) Volume Volume Volume (mins) (m³) (m³) (m³)

15 min Summer106.146 0.0 50.3 0.0 26 30 min Summer 75.217 0.0 71.4 0.0 40 60 min Summer 51.110 0.0 97.0 0.0 70 120 min Summer 33.480 0.0 127.0 0.0 128 180 min Summer 25.634 0.0 145.9 0.0 186 240 min Summer 21.010 0.0 159.4 0.0 246 360 min Summer 15.888 0.0 180.8 0.0 364 480 min Summer 13.001 0.0 197.4 0.0 482 600 min Summer 11.113 0.0 210.8 0.0 578 720 min Summer 9.769 0.0 222.4 0.0 628 960 min Summer 7.959 0.0 241.7 0.0 760 1440 min Summer 5.944 0.0 270.7 0.0 1024 2160 min Summer 4.423 0.0 302.0 0.0 1436 2880 min Summer 3.579 0.0 326.1 0.0 1852 4320 min Summer 2.663 0.0 363.9 0.0 2676 5760 min Summer 2.160 0.0 393.6 0.0 3464 7200 min Summer 1.838 0.0 418.6 0.0 4248 8640 min Summer 1.612 0.0 440.5 0.0 5016

©1982-2018 Innovyze EDS Ltd Page 2 Unit 10, Penstraze Business C... Truro Cornwall Date 14/08/2020 10:24 Designed by AndyWoolley File J-1520 SW Detention Basi... Checked by Innovyze Source Control 2018.1.1

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

Storm Max Max Max Max Max Max Status Event Level Depth Control Overflow Σ Outflow Volume (m) (m) (l/s) (l/s) (l/s) (m³)

10080 min Summer 99.020 0.820 1.7 0.0 1.7 66.1 O K 15 min Winter 98.917 0.717 1.6 0.0 1.6 54.1 O K 30 min Winter 99.099 0.899 1.7 0.0 1.7 76.2 O K 60 min Winter 99.279 1.079 1.8 0.0 1.8 101.9 O K 120 min Winter 99.446 1.246 2.0 0.0 2.0 129.7 O K 180 min Winter 99.528 1.328 2.0 0.0 2.0 144.8 O K 240 min Winter 99.575 1.375 2.0 0.0 2.0 153.8 O K 360 min Winter 99.633 1.433 2.1 0.0 2.1 165.5 O K 480 min Winter 99.663 1.463 2.1 0.0 2.1 171.6 O K 600 min Winter 99.676 1.476 2.1 0.0 2.1 174.5 O K 720 min Winter 99.680 1.480 2.1 0.0 2.1 175.4 O K 960 min Winter 99.671 1.471 2.1 0.0 2.1 173.5 O K 1440 min Winter 99.641 1.441 2.1 0.0 2.1 167.1 O K 2160 min Winter 99.591 1.391 2.0 0.0 2.0 157.0 O K 2880 min Winter 99.529 1.329 2.0 0.0 2.0 144.9 O K 4320 min Winter 99.398 1.198 1.9 0.0 1.9 121.3 O K 5760 min Winter 99.266 1.066 1.8 0.0 1.8 99.9 O K 7200 min Winter 99.137 0.937 1.8 0.0 1.8 81.2 O K

Storm Rain Flooded Discharge Overflow Time-Peak Event (mm/hr) Volume Volume Volume (mins) (m³) (m³) (m³)

10080 min Summer 1.444 0.0 460.2 0.0 5752 15 min Winter 106.146 0.0 56.4 0.0 26 30 min Winter 75.217 0.0 79.9 0.0 40 60 min Winter 51.110 0.0 108.5 0.0 68 120 min Winter 33.480 0.0 142.3 0.0 126 180 min Winter 25.634 0.0 163.5 0.0 184 240 min Winter 21.010 0.0 178.5 0.0 242 360 min Winter 15.888 0.0 202.5 0.0 356 480 min Winter 13.001 0.0 221.0 0.0 468 600 min Winter 11.113 0.0 236.2 0.0 580 720 min Winter 9.769 0.0 249.2 0.0 686 960 min Winter 7.959 0.0 270.6 0.0 864 1440 min Winter 5.944 0.0 294.2 0.0 1100 2160 min Winter 4.423 0.0 338.4 0.0 1560 2880 min Winter 3.579 0.0 365.1 0.0 2016 4320 min Winter 2.663 0.0 407.4 0.0 2860 5760 min Winter 2.160 0.0 440.4 0.0 3688 7200 min Winter 1.838 0.0 468.7 0.0 4472

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

Storm Max Max Max Max Max Max Status Event Level Depth Control Overflow Σ Outflow Volume (m) (m) (l/s) (l/s) (l/s) (m³)

8640 min Winter 99.013 0.813 1.7 0.0 1.7 65.2 O K 10080 min Winter 98.893 0.693 1.6 0.0 1.6 51.4 O K

Storm Rain Flooded Discharge Overflow Time-Peak Event (mm/hr) Volume Volume Volume (mins) (m³) (m³) (m³)

8640 min Winter 1.612 0.0 493.1 0.0 5272 10080 min Winter 1.444 0.0 515.6 0.0 6056

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Rainfall Details

Rainfall Model FSR Winter Storms Yes Return Period (years) 100 Cv (Summer) 0.750 Region England and Wales Cv (Winter) 0.840 M5-60 (mm) 18.100 Shortest Storm (mins) 15 Ratio R 0.258 Longest Storm (mins) 10080 Summer Storms Yes Climate Change % +40

Time Area Diagram

Total Area (ha) 0.253

Time (mins) Area Time (mins) Area Time (mins) Area From: To: (ha) From: To: (ha) From: To: (ha)

0 4 0.085 4 8 0.084 8 12 0.084

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Model Details

Storage is Online Cover Level (m) 100.000

Tank or Pond Structure

Invert Level (m) 98.200

Depth (m) Area (m²) Depth (m) Area (m²) Depth (m) Area (m²)

0.000 45.0 1.800 265.0 1.801 265.2

Hydro-Brake® Optimum Outflow Control

Unit Reference MD-SCL-0058-2100-1700-2100 Design Head (m) 1.700 Design Flow (l/s) 2.1 Flush-Flo™ Calculated Objective Minimise blockage risk Application Surface Sump Available Yes Diameter (mm) 58 Invert Level (m) 98.000 Minimum Outlet Pipe Diameter (mm) 75 Suggested Manhole Diameter (mm) 1200

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

Design Point (Calculated) 1.700 2.1 Kick-Flo® 0.517 1.2 Flush-Flo™ 0.237 1.6 Mean Flow over Head Range - 1.6

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

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

0.100 1.4 1.200 1.8 3.000 2.7 7.000 4.0 0.200 1.6 1.400 1.9 3.500 2.9 7.500 4.2 0.300 1.6 1.600 2.0 4.000 3.1 8.000 4.3 0.400 1.5 1.800 2.2 4.500 3.3 8.500 4.4 0.500 1.3 2.000 2.3 5.000 3.4 9.000 4.5 0.600 1.3 2.200 2.4 5.500 3.6 9.500 4.6 0.800 1.5 2.400 2.5 6.000 3.7 1.000 1.7 2.600 2.5 6.500 3.9

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Pipe Overflow Control

Diameter (m) 0.100 Entry Loss Coefficient 0.500 Slope (1:X) 100.0 Coefficient of Contraction 0.600 Length (m) 10.000 Upstream Invert Level (m) 99.700 Roughness k (mm) 0.600

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