St Austell to A30 Link Road

Flood Risk Assessment with Surface

Water Management Strategy

EDG0718-CSL-HDG-00MZ-RP-CD 0001

CORMAC Consultancy CORMAC Western Region, Radnor Road, Scorrier, Redruth, Cornwall, TR16 5EH.

EDG0718 Flood Risk Assessment & Surface Water Management Strategy St Austell to A30 Link Road

Issue & Revision Record

Revision Date Originator Checked Authorised Purpose of Issue Nature of Change

01.02 31-10-18 BR DP Aecom review

01.03 01-10-18 BR EM Approval check Approver comments

01 05-10-18 BR DP/EM EM First issue

02.01 06-12-18 BR DP/EM Check Updated Executive 02 07-12-18 BR DP/EM EM P02 Summary

If you would like this report in another format, please contact

CORMAC Solutions Ltd Head Office Higher Trenant Road Wadebridge Cornwall PL27 6TW

Tel: 01872 323 313 Prepared by Email: [email protected] Engineering Design Group www.cormacltd.co.uk/

This document has been prepared for the titled project or named part thereof and should not be relied upon or used for any other project without an independent check being carried out as to its suitability and prior written authority of Cormac Solutions Ltd being obtained. Cormac Solutions Ltd accepts no responsibility or liability for the consequences of this document being used for a purpose other than the purposes for which it was commissioned. Any person using or relying on the document for such other purposes agrees, and will by such use or reliance be taken to confirm his agreement to indemnify Cormac Solutions Ltd for all loss or damage resulting therefrom. Cormac Solutions Ltd accepts no responsibility or liability for this document to any party other than the person by whom it was commissioned.

CORMAC Solutions Ltd Head Office, Higher Trenant Road, Wadebridge, Cornwall PL27 6TW

EDG0718 Flood Risk Assessment & Surface Water Management Strategy St Austell to A30 Link Road

Contents

1 EXECUTIVE SUMMARY 1 2 INTRODUCTION 6 2.1 Background 6 2.2 Purpose of this report - FRA Requirements 6 3 THE DEVELOPMENT SITE 8 3.1 Site Location 8 3.2 Proposed Development Overview 8 4 HYDROLOGY OVERVIEW 15 4.1 River Networks 15 4.2 The St Austell River 15 4.3 The River Par 16 4.4 The River Fal 18 4.5 Lower River Ruthern 20 4.6 Critical Drainage Areas 21 4.7 Water abstraction 21 4.8 Source Protection Zones 22 4.9 Enclosed Water Bodies 22 5 FLOOD MECHANISMS REVIEW 23 5.1 Overview 23 5.2 Coastal / Tidal Flooding 23 5.3 Fluvial Flooding 23 5.4 Groundwater Flooding 24 5.5 Surface Water Flooding 26 5.6 Flooding from enclosed water bodies 27 5.7 Existing Utilities Flooding 28 6 FLUVIAL FLOOD RISK 29 6.2 Upper River Fal (A) Catchment Review 29 6.3 Upper River Fal (A) - Peak flow assessment 31 6.4 Upper River Fal (A) – Culvert requirements 31 6.5 Upper River Fal (A) – Footpath crossing 32 6.6 Upper River Fal (A) – Field access accommodation works 32 6.7 Upper River Fal (A) – Flood Risk Summary 33 6.8 Upper River Fal (C) Catchment Review 34 6.9 Upper River Fal (C) - Peak flow assessment 35 6.10 Upper River Fal (C) – Culvert requirements 35 6.11 Upper River Fal (C) – Footpath crossing 36 6.12 Upper River Fal (C) – Flood Risk Summary 37 6.13 Upper River Fal (D) Catchment Review 37 6.14 Upper River Fal (D) - Peak flow assessment 38 6.15 Upper River Fal (D) – Culvert requirements 38 6.16 Upper River Fal (D) – Footpath crossing 39 6.17 Upper River Fal (D) – Flood Risk Summary 40 6.18 Upper River Fal (E) Description 40 6.19 Upper River Fal (E) Catchment 42 6.20 Upper River Fal (E) - Peak flow assessment 42 6.21 Upper River Fal (E) – Culvert requirements 43 6.22 Upper Carbis Stream 45 6.23 Upper Carbis stream - Peak flow assessment 46

EDG0718 Flood Risk Assessment & Surface Water Management Strategy St Austell to A30 Link Road

7 SURFACE WATER MANAGEMENT GUIDANCE 47 7.1 Drainage Guidance for Cornwall (DGfC) 2010 Version 2 47 8 HIGHWAY DRAINAGE SURFACE WATER MANAGEMENT 49 8.2 Infiltration Assessments 50 8.3 Network and Attenuation Basin modelling 50 8.4 SuDS Treatment Train 51 8.5 Drainage Zone D1 53 8.6 Drainage Zone D2- Description 54 8.7 Drainage Zone D2- Existing Drainage System 55 8.8 Drainage Zone D2- Proposed Drainage System 58 8.9 Drainage Zone D3 60 8.10 Drainage Zone D4_5 62 8.11 Drainage Zone D6 64 8.12 Drainage Zone D7 66 8.13 Drainage Zone D8 68 8.14 Drainage Zone D9 72 8.15 Offsite Interceptor Drainage 74 8.16 Water Management Strategy Conclusion 76

EDG0718 Flood Risk Assessment & Surface Water Management Strategy St Austell to A30 Link Road

Appendices

Appendix A Scheme Drawings Appendix B CCTV Report

Drawings

Drawing Ref: Drawing Name

0718_CSL_HDG_00MZ_DR_CD_0001 Surface Water Drainage Key Plan

0718_CSL_HDG_01R1_DR_CD_0002 Surface Water Drainage - Sheet 1 of 12

0718_CSL_HDG_02MZ_DR_CD_0003 Surface Water Drainage - Sheet 2 of 12

0718_CSL_HDG_03M1_DR_CD_0004 Surface Water Drainage - Sheet 3 of 12

0718_CSL_HDG_04M1_DR_CD_0005 Surface Water Drainage - Sheet 4 of 12

0718_CSL_HDG_05M1_DR_CD_0006 Surface Water Drainage - Sheet 5 of 12

0718_CSL_HDG_06MZ_DR_CD_0007 Surface Water Drainage - Sheet 6 of 12

0718_CSL_HDG_07R2_DR_CD_0008 Surface Water Drainage - Sheet 7 of 12

0718_CSL_HDG_08M2_DR_CD_0009 Surface Water Drainage - Sheet 8 of 12

0718_CSL_HDG_09M2_DR_CD_0010 Surface Water Drainage - Sheet 9 of 12

0718_CSL_HDG_10R3_DR_CD_0011 Surface Water Drainage - Sheet 10 of 12

0718_CSL_HDG_11MZ_DR_CD_0012 Surface Water Drainage - Sheet 11 of 12

0718_CSL_HDG_12R4_DR_CD_0013 Surface Water Drainage - Sheet 12 of 12

0718_CSL_HDG_00MZ_DR_CD_0014 Watercourse Catchment and Indicative Flow Rate Estimates

0718_CSL_HDG_00MZ_DR_CD_0015 Indicative Flood Risk Zones

0718_CSL_HDG_00MZ_DR_CD_0016 Rural Runoff Catchments Intersected by Design Sheet 1 of 2

0718_CSL_HDG_00MZ_DR_CD_0017 Highway Drainage Catchment Overview

0718_CSL_HDG_00MZ_DR_CD_0018 Indicative Sections of Watercourse Culverts A-E

0718_CSL_HDG_00MZ_DR_CD_0019 Indicative Detail of Key Attenuation Basin Features

0718_CSL_HDG_00MZ_DR_CD_0029 Rural Runoff Catchments Intersected by Design Sheet 1 of 2

0718_CSL_GEN_00MZ_EX_CH_3000 Scheme Overview Key plan

0718_CSL_GEN_01MZ_EX_CH_3001 Scheme Overview - Sheet 1 of 4

0718_CSL_GEN_02MZ_EX_CH_3002 Scheme Overview - Sheet 2 of 4

0718_CSL_GEN_03MZ_EX_CH_3003 Scheme Overview - Sheet 3 of 4

0718_CSL_GEN_04MZ_EX_CH_3004 Scheme Overview - Sheet 4 of 4

EDG0718 Flood Risk Assessment & Surface Water Management Strategy St Austell to A30 Link Road

1 EXECUTIVE SUMMARY

1.1.1 Cornwall Council has long aspired to create a strategic link road between St Austell and the A30. Drawings showing the proposed scheme intended to achieve this are included in Appendix A.

1.1.2 The scheme is approximately 6.5km in length and includes 4 roundabouts, and will provide connectivity from Stenalees in the south to Victoria in the north with a bypass route alleviating traffic flows through Roche, Bugle and Stenalees.

1.1.3 This report aims to fulfil the requirements for a Flood Risk Assessment (FRA) as outlined by the National Planning Policy Framework (NPPF) and its associated technical guidance. It will also outline the Surface Water Management Strategy proposed for the scheme in order to demonstrate that the proposals can be implemented without increasing flood risk to downstream receptors.

1.1.4 Initial assessment of the Environment Agency and Cornwall Council Flood Risk mapping, reviewed in Sections 5 & 6 of this report, shows the development to be located in Flood Zone 1 (low risk), beyond the 1 in 1,000 year flood extent. As such the existing site is considered to be at low risk of Fluvial or Tidal flooding. The development site is in excess of 1Ha in area, which constitutes the requirement for an FRA to be submitted with the planning application.

1.1.5 The development is classed as essential infrastructure according to Table 2 of the NPPF technical guidance and is therefore appropriate in higher risk flood risk zones subject to an exception test. As the development is indicated to be in Flood Zone 1 an exception test has not been formally applied.

1.1.6 The proposed highway alignment is located centrally within Cornwall and is situated on the upper most boundaries of four significant river catchments.

• The St Austell River (also known as the White River) • The Upper River Par • The Upper River Fal • Tributary to the Lower River Ruthern (River Camel)

1.1.7 The design is understood to cross four small watercourses associated with the Upper River Fal and one watercourse associated with the Upper River Par. The scheme includes culvert proposals which have been influenced by the ecological requirements for mammal ledges and maintenance access relating to safe clearance heights. The proposals are not considered to have a negative impact on flood risk as the culvert capacity is well in excess of the 100 year+40% flows estimated using the ReFH 2 method.

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1.1.8 A range of potential flood mechanisms have been reviewed. Given the scale and nature of the scheme it is considered that any flood risks associated with the scheme can be appropriately managed or mitigated through good engineering practice.

1.1.9 A Surface Water Management Strategy has been developed based on eight drainage zones serving the scheme. Infiltration rates are generally poor across the site and therefore proposals are typically based upon attenuation based drainage systems. A summary of the drainage zone catchments is shown in Table 1.1.

Drainage Zones D3, D4_5, D6, D7 and D8

1.1.10 Drainage Zones D3, D4_5, D6, D7 and D8 are proposed to discharge to existing watercourses. Flows from the carriageway will be collected via road edge drainage systems (filter drains, surface water channels, gullies etc) and conveyed to an attenuation storage basin at the lower end of the network, near to the receiving watercourse.

1.1.11 The basins have been provisionally sized to accept flows for the 1 in 100 year event with a 40% uplift in rainfall intensities to allow for climate change effects.

1.1.12 Greenfield runoff rates for the drained catchment areas have been estimated using the ICP SUDS method. The peak design discharge rates from the attenuation basins to the watercourses have been set to mimic Greenfield runoff rates for the drained catchment. Where possible betterment has been proposed by storing and discharging the 100 year +40% rainfall event at the QBar Greenfield runoff rate. (approx. 2year event). The basin storage requirements reflect the reduced discharge rate. This is summarised in table 1.1.

1.1.13 Attenuation basins are proposed to have a 300mm Freeboard above the 100 Year+40%CC predicted water level. In the event of design exceedance or blockage of the outfalls. The locations of basins allow for Exceedance flows to enter the receiving water course, with no vulnerable receptors along the exceedance flow route to the watercourse. Overflow weirs and Overland flow channels are proposed to direct exceedance flows.

Drainage Zones D2 and D3

1.1.14 Drainage zones D1 and D2 are proposed to discharge into the existing highway drainage network at Stenalees. The larger system Zone D2 includes Singlerose roundabout, proposed and existing carriageway. D2 is proposed to discharge to the existing drainage network that flows west to from Single Rose towards Carthew.

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1.1.15 Drainage zone D1 provides drainage for the proposed Roche access road and also serves to intercept flows from the existing Roche road before they reach drainage zone D2.

1.1.16 Where possible drainage zones D1 and drainage zone D3 have intercepted catchment areas to reduce flows leading into the Singlerose Roundabout drainage system. This was to limit discharge flow rates and attenuation storage requirements from the system in drainage zone D2, to enable betterment of pre-development discharge rates to be achieved. Approximately 300m of the existing Roche road is managed by drainage zone D1, with a further 220m length being managed by zone D3.

1.1.17 To accommodate drainage Zone D1 (draining the Roche access road). Two basins are proposed to limit flows to the lowest practicable attenuated peak discharge rate of 5lps. The system will connect to the existing carriageway drainage network which is believed to discharge to the watercourse south of the A391 in Stenalees, East of Singlerose roundabout.

1.1.18 The basins have been provisionally sized to accept flows for the 1 in 100 year event with a 40% uplift in rainfall intensities to allow for climate change effects.

1.1.19 Drainage zone D2 has limited options for locating attenuation infrastructure. Adjacent to the main alignment approximately 150m north of Singlerose roundabout the design earthworks transition between cut and fill over an existing benched earthworks area (associated with historic Imerys activities). A relatively level area between the earthworks benches allows for positioning of an intermediate attenuation basin which serves approximately 600m of the proposed design. The transition between cut and fill in the design allows for flows generated north of the basin to be directed into the basin. The flows can be attenuated and discharged back into the proposed drainage network which continues south to Singlerose.

1.1.20 The basin D2- Basin 2 is proposed to be limited to a peak discharge rate of 5.0lps to maximise reduction of flow rates further down the network. The modelling indicates D2-Basin 2 will require ~1,250m3 of attenuation storage to achieve the 5lps discharge rate for the 100 year +40% event.

1.1.21 A wide verge between the east and south arms of the Single Rose Roundabout will be enlarged due to repositioning of the roundabout. A smaller attenuation basin is proposed to be situated in the verge as it is closer to the downstream extent of the network and able to accept more of the downstream catchment area.

1.1.22 D2-Basin 1 is proposed to be limit peak discharge rate to 20lps. The modelling indicates D2-Basin 1 will require ~280m3 of attenuation storage to achieve the 20lps discharge rate. The specified storage/discharge rate utilises the practicable space available in the verge for attenuation storage.

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1.1.23 The available space in the verge is limited therefore a small modular storage tank is proposed to be located at the base of the Roche Road between the north and east arms of the roundabout on a non-trafficked area. Flows from the Roche Road will enter the tank and be attenuated to a peak of 60lps before crossing the road into D2- Basin 1. The additional storage of 120m3 provided is necessary for higher order rainfall events to prevent D2- Basin 1 from surcharging. The staged reduction in flows allow for a peak discharge of 20lps from D2- Basin 1.

1.1.24 Due to lack of suitable locations for attenuation storage on the south-west arm of Singlerose, part of the system will not be intercepted by the attenuation system. This causes predicted flow rates to rise to a peak of ~340lps for the 100 year+40% CC (15min duration) intense storm. This rate occurs at the downstream scheme extents. When compared with the existing catchment 100 year flow rate estimate, discussed in paragraph 8.7.6 (without a 40% uplift for CC), this is a 210lps reduction, whilst increasing the input by 40% for climate change allowance.

1.1.25 The existing drainage network downstream of the scheme extent heading towards Carthew and the St Austell river appears to be in a poor state of repair.

1.1.26 In general terms the CCTV report indicates the existing drainage to be in a poor state of repair with level 5 defects. The CCTV survey had to be abandoned on several pipe runs due to blockage and damaged infrastructure, leaving assessment gaps within the surveyed network. Further review and investigation works will be required in the detailed stage of the project to specify any required remediation and upgrade works to the existing downstream system.

1.1.27 Although the scheme proposals aim to lower discharge rates flowing towards Carthew, it is expected that the downstream drainage system from the scheme extent to the outfall at the St Austell river will need improvement works, in advance of the scheme proposals. This is expected to be progressed under highways permitted development rights and as such is not included within the current planning application. It is expected that in order to discharge the schemes associated planning conditions, details of the proposed upgrades and system capacity will need to be provided.

Conclusion

1.1.28 In conclusion, following review of the potential flood risks associated with the scheme and the provisions considered within the surface water management strategy. It is believed that the detailed design of the scheme can be progressed sufficiently within the proposed extents to ensure flood risk is not increased to downstream receptors. It is understood that planning conditions will require the submission of finalised detailed designs, construction phase drainage plans and a drainage maintenance plan.

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Greenfield runoff for drained area ( ICP SUDS method (lps))

for Gross area to define

discharge rate to watercourse

Gross Equivalent Drained Impermeable Flow Attenuation Attenuation Drainage Area for area with control Attenuation Attenuation Attenuation 100 yr 30 yr 1 yr Qbar design Storage Zone highway Green area discharge Storage Storage depth (m) design Storage drainage runoff factored rates (lps) (Ha) at 0.3 x A (Ha)

Entering existing highway discharge 100 year 1=5.0lps 3 Basin 1= Zone D1 0.628 0.628 1.2 Basin 2 =150m 3 reduced to max of 5lps +40CC 2=5.0lps 400m Entering existing highway drainage 1= 5.0lps Modular system reduced existing discharge 100 year Basin 2 Basin 1 = Zone D2 4.94 2.879 1.2 2= 60 lps storage tank rate by 210 lps for 100yr +40CC +40CC =1,250m3 280m3 = 120m3 event 3= 20 lps

100 year Zone D3 1.375 0.848 37.9 29.9 12.2 15.7 1.2 15.7 (Qbar) 625m3 +40CC 100 year Zone D4 _5 5.748 3.071 53.6 42.2 17.3 22.2 1.2 42.2 (30yr) 1,900m3 +40CC 100 year Zone D6 6.057 3.701 58.2 45.9 18.8 24.1 1.5 58.2 (100yr) 2,200m3 +40CC 100 year Zone D7 1.748 1.162 15.9 12.5 5.1 6.6 1.2 10 800m3 +40CC Zone D8 100 year 5.822 2.596 51.8 40.8 16.7 21.4 1.2 21.4 (Qbar) 1,800m3 Network A +40CC Zone D8 100 year 2.327 0.991 20.7 16.3 6.7 8.6 1.2 8.6 (Qbar 675m3 Network D +40CC 100 year Zone D9 2.61 1.749 24.2 19 7.8 10 1.2 10 (Qbar) 1,400m3 +40CC Table 1.1 Drainage Zone Summary

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2 INTRODUCTION

2.1 Background

2.1.1 Cornwall Council has long aspired to create a strategic link road between St Austell and the A30. The town, currently accessed by the variable standard A391 and B3274, is the largest settlement in Cornwall not currently served by a high-quality route from the A30.

2.1.2 The lack of a high-quality continuous route from the A30 to St Austell is a barrier to further economic growth in the St Austell area. Increasing pressure on the existing routes through residential areas is likely to reduce the attractiveness of the area and limit further investment, as the area is unable to support associated volumes of traffic.

2.1.3 The government recognises the importance of the provision of a strategic route between the A30 and St Austell, and the Department for Transport has allocated £79m from the Large Local Majors investment programme. Cornwall Council will also contribute a further £6m towards the estimated £85m scheme cost.

2.1.4 The proposed scheme will create improved transport links and promote opportunities for economic development within St Austell and the surrounding areas. It will continue the improved network connectivity created by the A391 Carluddon road improvement scheme (1.6km), completed in 2015 and the St Austell North-Eastern Distributor Road constructed in 1997/98.

2.1.5 CORMAC Solutions Ltd (CSL) on behalf of Cornwall Council were commissioned to develop preliminary designs for the St Austell to A30 Link Road scheme in order to facilitate the submission of a planning application. The current programme aims for the scheme design to be site ready by the end of 2019 with construction to be completed by the end of 2022.

2.1.6 Drawings showing the proposed scheme route are included in Appendix A. The scheme is approximately 6.5km in length and includes 4 roundabouts, and will provide connectivity from Stenalees in the south to Victoria in the north with a bypass route alleviating traffic flows through Roche, Bugle and Stenalees

2.2 Purpose of this report - FRA Requirements

2.2.1 This report aims to fulfil the requirements for a Flood Risk Assessment (FRA). The National Planning Policy Framework (NPPF) technical guidance Table 1 states an FRA is required if the development is within Flood Zone 2 or 3, or is greater than 1Ha in area.

2.2.2 If a development site is located within a Critical Drainage Area (CDA), as identified within Drainage Guidance for Cornwall (DGfC), this would further constitute the requirement for a Flood Risk Assessment. 6 EDG0718-CSL-HDG-00MZ-RP-CD 0001 EDG0718 Flood Risk Assessment & Surface Water Management Strategy St Austell to A30 Link Road

2.2.3 Initial assessment of the Environment Agency and Cornwall Council Flood Risk mapping, reviewed in Section 5 of this report, shows the development to be predominantly located in Flood Zone 1 (low risk), beyond the 1 in 1,000 year flood extent. As such the existing site is considered to be at low risk of Fluvial or Tidal flooding. However the development site is in excess of 1Ha in area, which constitutes the requirement for an FRA to be submitted with the planning application.

2.2.4 The 1Ha area requirement, relates to the effective management of surface water runoff. Potentially, replacing permeable areas with impermeable surfaces can increase runoff rates, which in turn, may increase flood risk to downstream receptors.

2.2.5 Runoff rates are typically managed by the inclusion of Sustainable Drainage Systems (SuDS). These are developed in the early design stages to ensure they can be accommodated on the site and within the development proposals.

2.2.6 The following report investigates the potential flood mechanisms that can affect the development site and proposes mitigation measures where necessary. It continues by assessing the pre-development and post- development runoff rates to allow suitable design of a drainage system incorporating SuDS features.

2.2.7 A Preliminary SuDS strategy has been prepared to show a suitable drainage design can be incorporated within the development proposals, which can adequately mitigate flood risk and is in keeping with design guidance found within NPPF, DGfC, DMRB and the SuDS manual (CIRIA report 753).

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3 THE DEVELOPMENT SITE

3.1 Site Location

3.1.1 The 6.5km route is located roughly on a north-south alignment, approximately 1km west of Roche in Cornwall. It connects Stenalees in the south to Tregross in the north (adjacent to the A30).

3.1.2 In the wider context, the town centre of St Austell is located 2.6 miles south of the scheme. Bodmin is located 7 miles to the north-east while Newquay is located 10 miles to the west. Detailed drawings showing the proposed route alignment are enclosed in Appendix A with the route indicatively shown in Figure 3.1 below.

Proposed Road

Figure 3.1: Site location

3.2 Proposed Development Overview

3.2.1 The proposed A30 to St Austell link road extends between the old A30, west of Victoria village to the Single Rose roundabout at Stenalees as shown in Figure 3.2. It consists largely of a new 60mph single carriageway road, with associated junctions, climbing lanes, and laybys. The following section briefly describes the scheme proposals.

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3.2.2 The alignment intersects the B3274 Newquay Road between Tregoss and Roche before passing to the west of Roche. It then intersects Harmony Road close to the existing side road junction to Tregoss.

3.2.3 South of Harmony Road the alignment curves south-east, passing west of Trezaise, crossing Reeshill Road (U6114), which will be stopped up either side of the new road. A new roundabout to the south of Trezaise will link the villages of Roches and Trezaise to New Road (C0298) and Hensbarrow Hill (C0298) to the south and west.

Figure 3.2: Proposed Design

3.2.4 To the south-east of Trezaise the new road largely follows the existing Roche Road (B3274) corridor on an improved alignment, diverting to the west of Stenalees before joining the B3274 at an improved roundabout junction at Single Rose.

3.2.5 Figure 3.2 shows the proposed road alignment and the junctions with the existing highway network. Figures 3.3 to 3.6 show the proposed design in further detail. Figures 3.2 to 3.6 are extracts from drawings included in Appendix A.

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3.2.6 The proposed carriageway has a design speed of 60mph, the principal junctions include 4 No. four arm roundabouts, identified as (from north to south respectively):

• Tregoss Roundabout

• Harmony Roundabout

• Trezaise Roundabout

• Single Rose Roundabout

3.2.7 The design includes 2 No. Underbridges. The northern most underbridge is located approximately 200m south of Harmony Roundabout. The southernmost underbridge being located approximately 900m south-east of Trezaise Roundabout.

3.2.8 A direct access is currently proposed at the Blockworks junction located approximately 1,500m north-west of Single Rose Roundabout.

3.2.9 The Roche Road access is approximately 280m of new road. It is separate from the main alignment and connects Stannary Road in Stenalees to the existing Roche Road.

Figure 3.3: Proposed Design - Tregoss Roundabout to Harmony Roundabout

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Tregoss Roundabout to Harmony Roundabout

3.2.10 Figure 3.3 shows the proposed design from Tregoss Roundabout to Harmony Roundabout. The road is largely situated on a greenfield site consisting of agricultural grazing land.

3.2.11 Heading north from Harmony Roundabout the road will be situated within a cut embankment, with the lowest point being approximately 9m below existing ground level. As the road heads further north it returns closer to existing ground levels. A lay-by is situated adjacent to the southbound carriageway south of Tregoss Roundabout.

3.2.12 The head of a watercourse is located to the north of the Old A30 Road, which subsequently flows to the north. An attenuation basin serving the new road is located north of the alignment in the adjacent field and discharges into the watercourse, the details of which are discussed in subsequent sections of this report.

Harmony Roundabout to Trezaise Roundabout

3.2.13 Figure 3.4 shows the proposed design from Harmony Roundabout to Trezaise Roundabout. The road is largely situated on a greenfield site consisting of agricultural grazing land.

Figure 3.4: Proposed Design - Harmony Roundabout to Trezaise Roundabout

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3.2.14 The design within this region is largely on fill embankment above the existing ground levels. The underbridge is located directly north of a region dedicated for three attenuation basins which discharge into a watercourse which flows to the west. An existing pond is located near the head of the watercourse and a culvert is proposed to allow flows to pass from the upstream catchment in line with the existing watercourse/pond.

3.2.15 Heading south another attenuation basin located on the south-west boundary of the design serves the main alignment catchment from the northern boundary of Trezaise roundabout to the basin. This basin discharges to a watercourse that also flows to the west.

3.2.16 The road section between Trezaise and Harmony junctions in addition to the single carriageway road also contains an additional climbing lane on the southbound route and layby facility on the northbound.

3.2.17 Footpaths, bridleways and additional farm access tracks are included in the design within this region, running adjacent to the main road alignment.

Trezaise Roundabout to the Blockworks Junction

3.2.18 Figure 3.5 shows the proposed design from Trezaise Roundabout to the Blockworks junction. Nearer Trezaise the design is largely situated on a greenfield site consisting of agricultural grazing land. Within the vicinity of the southern underbridge the design crosses an area of Heathland before moving largely onto the existing Roche Road alignment, as it continues towards the Blockworks.

3.2.19 Between Trezaise and the southern Hensbarrow underpass the design is predominantly on fill embankment above existing levels. As the road heads south from the underpass it enters a region of cut, before returning to close to existing levels near to the Blockworks junction.

3.2.20 The road section between Trezaise and the Blockworks junction, in addition to the single carriageway road, contains an additional climbing lane on the southbound route. An escape lane facility is also provided on the northbound route on the approach to Trezaise roundabout.

3.2.21 The southern underbridge at Hensbarrow allows a conveyance route for the existing watercourse, as well as providing a crossing point for Pedestrians, equestrians and wildlife. The watercourse will be in open channel for the majority, however where the non-motorised user paths cross the watercourse, short culverts are proposed.

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Figure 3.5: Proposed Design - Trezaise Roundabout to Blockworks Junction 3.2.22 An attenuation basin is located on the northern side of design alignment close to the underbridge. It will serve to attenuate flows generated from the proposed road from further south of the underbridge and will discharge into the watercourse that flows to the north.

3.2.23 The attenuation basin located between the roundabout arms at Trezaise roundabout serves three of the four arms leading into the roundabout, excluding the north-western arm which drains to the north. The main design alignment from the Hensbarrow underpass to Trezaise drains to the Trezaise basin. The basin discharges to the watercourse located adjacent to the south-west arm, which flows towards the north-west.

Blockworks junction to Single Rose Roundabout

3.2.24 Figure 3.6 shows the proposed design from the Blockworks junction to Single Rose Roundabout. The design alignment is largely situated on land associated with the China Clay Mining activities, prevalent within the area. It is typically densely vegetated scrubland with historic earthworks movements resulting in a largely artificial landscape.

3.2.25 This region of the design is almost entirely in cutting to enable geometric design standards to be maintained whilst tying into the existing Single Rose Roundabout levels.

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3.2.26 The design mimics the existing Roche Road horizontal alignment from the Blockworks for around 600m before moving further south to create a more direct alignment into the Single Rose junction.

3.2.27 The road section between Single Rose and the Blockworks junction, in addition to the single carriageway road, contains an additional climbing lane on the northbound route. An escape lane facility is also provided on the southbound route on the approach to Single Rose Roundabout.

3.2.28 Five attenuation basins are proposed in this region. The most northern basin serves a section of the main alignment and the existing Roche Road. It discharges to a watercourse that flows to the east.

3.2.29 Two further basins are proposed to reduce flow rates from the main alignment which discharges into the existing highways drainage network, which flows to the west from Single Rose Roundabout, eventually discharging at a watercourse at Carthew.

3.2.30 Two smaller basins serve the Roche access road and a section of the existing Roche Road. The network discharges to the existing highways drainage network, which discharges to a watercourse which flows to the south east of Stenalees

3.2.31 The details relating to the proposed drainage systems are discussed in subsequent sections of this report.

Figure 3.6: Proposed Design - Blockworks Junction to Single Rose Roundabout

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4 HYDROLOGY OVERVIEW

4.1 River Networks

4.1.1 The proposed highway alignment is located centrally within Cornwall and is situated on the upper most boundaries of four significant river catchments. These being:

• The St Austell River (also known as the White River) • The Upper River Par • The Upper River Fal • Tributary to the Lower River Ruthern

4.2 The St Austell River

4.2.1 The St Austell River has a catchment area of approximately 37Km2 as shown in Figure 4.1. The catchment covers an area to the south of the scheme, and eventually discharges to the sea at Pentewen approximately 6 miles south of Stenalees.

Proposed Road

Figure 4.1: St Austell River (White River) Catchment Area

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4.2.2 The proposed scheme does not cross any known tributary of the St Austell River. However the southern end of the proposed scheme including Drainage Zone 2, discussed in Section 6 of this report, falls within the upper catchment boundary of the St Austell River. Drainage Zone D2 discharges to this watercourse. This is discussed further in Sections 5 & 6 of this report.

4.2.3 Single Rose Roundabout and the proposed road alignment falling towards Single Rose, lie within the St Austell River Catchment. The catchment boundary directly runs adjacent to the proposed road alignment with little or no catchment east of the proposals.

4.2.4 The river heads south from Carthew, passing through the western extent of St Austell, before continuing south through London Apprentice and Nansladron before discharging at Pentewan.

4.2.5 The St Austell River catchment includes the western extent of the St Austell Bay Critical Drainage Area (CDA). The CDA extent boundary is located approximately 1.5 miles south of Stenalees. The CDA area is shown in Section 5 within Figure 5.1.

4.2.6 The St Austell River is designated as a main river until it reaches the developed outskirts of St Austell near the Trenance viaduct. Upstream the status changes to an Ordinary Watercourse and is subject to LLFA risk management.

4.3 The River Par

4.3.1 The River Par has a catchment area of approximately 60Km2 as shown in Figure 4.2. The catchment covers an area to the east of the scheme, and eventually discharges to the sea at Par approximately 4.75 miles south of Stenalees.

4.3.2 The catchment includes the majority of land found to the east of the proposed road alignment. Its northern border is roughly aligned with the A30 trunk road. The catchment is found as far east as Redmoor and Lanlivery. It also includes the towns of Roche, Bugle, Luxulyan and St Blazey.

4.3.3 The proposed road design runs adjacent to the western boundary of the catchment. A tributary to the Upper River Par, identified as the Carbis Stream, is crossed by the proposed design, south of Trezaise at Hensbarrow. The stream flows in an easterly direction away from the scheme. Drainage Zone 4_5 discharges into the stream. This is discussed further in Sections 5 & 6 of this report.

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4.3.4 The head of the Rosevean Stream, a tributary to the Upper River Par is located in Stenalees, south of the A391. Watercourses north of Stenalees adjacent to the Goonbarrow pit site also drain to this tributary. These watercourses are not directly crossed by the scheme although they are proposed to accept attenuated discharge flows from Drainage Zone D3 and Drainage Zone D1. This is discussed further in Sections 5 & 6 of this report.

4.3.5 The Par & St Blazey Stream Critical Drainage Area (CDA) is found within the Par catchment. The northern CDA catchment boundary being located approximately 3.5 miles south east of Stenalees as shown in section 5, figure 5.1.

Proposed Road

Figure 4.2: Upper & Lower River Par Catchment Area

4.3.6 The River Par is designated as Main River until it reaches the sewage works at Luxulyan. Upstream from Luxulyan the status changes to Ordinary Watercourse and is subject to LLFA risk management.

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4.4 The River Fal

4.4.1 The River Fal has a catchment area of approximately 110Km2 as shown in Figure 4.3. This catchment covers an area to the west of the scheme, and eventually discharges to the sea via the tidal waters of the Carrick Roads, located approximately 16 miles to the south-west.

4.4.2 The Carrick Roads is an inland water body situated between Falmouth and St Mawes which accepts outfalls numerous water courses including the Truro River, Tresillian River and the Fal. The River Fal joins to the west of the Carrick roads near Ruan Lanihorne.

4.4.3 The Fal is designated as Main River until it reaches the village of Grampound. Upstream of Grampound the status changes to Ordinary Watercourse and is subject to LLFA risk management.

4.4.4 Figure 4.3 shows that the design intersects with the uppermost boundary of the Fal catchment. From a review of the Cornish River network (CC intranet mapping) and the proposed design, it is understood that the design would cross four unnamed minor tributaries of the Fal. Two of the four watercourses are at the top of the catchment and could be better defined as edge of field drainage ditches which are dry during low rainfall periods.

4.4.5 Drawing 0718_CSL_HDG_00MZ_DR_CD_0014 located in Appendix A shows an assessment of the existing catchment areas upstream of the scheme proposals. The catchments were typically too small to be assessed using the online FEH web-service as less than 0.5km2. An AutoCAD Civil 3D ground model was created using topographic survey data and supplemented by Lidar data to assess the upstream catchment areas of the watercourses.

4.4.6 The catchment areas were used to assess potential flow rates within the watercourses which are discussed in Section 6 of this report.

4.4.7 The drainage strategy discussed in Section 8 of this report utilises three of the tributaries for attenuated surface water drainage outfalls. The tributaries are referenced on Drawing 0718_CSL_HDG_00MZ_ DR_CD_0014 and Table 4.1 below

Drainage Zone Outfall (Receiving watercourses)

Zone D6 Tributary (E) to Upper River Fal

Zone D7 Tributary (C) to Upper River Fal

Zone D8 Tributary (B) to Upper River Fal

Table 4.1

4.4.8 The tributaries head to the north-west slowly converging before passing through Goss Moor area, following Goss Moor the watercourse swings to the south heading towards St Stephan and onto Grampound.

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Proposed Road

Figure 4.3: Upper & Lower River Fal Catchment Area

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4.5 Lower River Ruthern

4.5.1 The Lower River Ruthern is a tributary to the river Camel which has a catchment area of approximately 216Km2. This catchment covers a large area to the north of the scheme as shown in Figure 4.4 and eventually discharges to the sea via the tidal waters of the Camel Estuary, located approximately 7 miles to the north of the scheme.

4.5.2 The head of the River Ruthern tributary starts just south of the A30. It flows north for 2.7 miles before heading east to join the Camel a further 2.5 miles downstream.

4.5.3 The River Ruthern is designated as an Ordinary Watercourse and is subject to LLFA risk management. The Camel is designated as Main River and subject to EA management.

Proposed Road

Figure 4.4: Lower River Ruthern Catchment Area

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4.5.4 The proposed scheme does not cross any known tributary of the Ruthern and the catchment south of the A30 is negligible. Drawing 0718_CSL_HDG_00MZ_DR_CD_0014 located in Appendix A shows an assessment of the existing catchment areas upstream of the scheme proposals.

4.5.5 The drainage strategy discussed in Section 8 of this report utilises the tributary as a discharge point for the attenuated surface water drainage system (Drainage Zone D9).

4.6 Critical Drainage Areas

4.6.1 The scheme is entirely beyond the extent of any Critical Drainage Areas as defined by the DGFC and the EA.

4.7 Water abstraction

4.7.1 The available records indicate there are a number of private water abstraction points within the vicinity of the proposed scheme. These are listed in Table 4.2 below with the extent area shown in Figure 4.5.

TYPE X Y ADDRESS_1

1 Borehole 198384 59460 Trerank Farm Higher Trerank Lane Roche St Austell Cornwall PL26 8HA

2 Well 199265 59381 Upper Hendra Trezaise Roche St Austell Cornwall PL26 8HE

3 Well 199511 59605 Rock House Carbis Roche St Austell Cornwall PL26 8JZ

4 Spring 199341 59736 Burney Farm Carbis Roche St Austell Cornwall PL26 8JZ

5 Spring 199417 59330 Hendra Carbis Roche St Austell Cornwall PL26 8JZ

6 Borehole 198459 59001 Lower Trerank Farm Lower Trerank Roche St Austell Cornwall PL26 8LR

7 Borehole 198479 58798 Lower Coldvreath Farm Reeshill Roche St Austell Cornwall PL26 8LR

8 Spring 198602 58900 Falbrook Cottage Reeshill Road Reeshill Roche St Austell Cornwall PL26 8LR

9 Borehole 198619 58741 Hazeldene Reeshill Road Reeshill Roche St Austell Cornwall PL26 8LR

10 Borehole 198584 58832 Gwel An Praze Reeshill Road Reeshill Roche St Austell Cornwall PL26 8LR

11 Borehole 198611 58769 Moorlands Reeshill Road Reeshill Roche St Austell Cornwall PL26 8LR

12 Spring 198578 58881 Nans Avallen Reeshill Road Reeshill Roche St Austell Cornwall PL26 8LR

13 Borehole 198555 58852 Riverside Cottage Reeshill Roche St Austell Cornwall PL26 8LR

14 Borehole 198583 58534 Coldvreath Cottage Coldvreath Roche St Austell Cornwall PL26 8LS

15 Borehole 198538 58520 Coldvreath Farm Coldvreath Roche St Austell Cornwall PL26 8LS

16 Spring 197865 59398 Trerank Moor Cleers Roche St Austell Cornwall PL26 8NJ

17 Well 199558 59396 Hendra Farm Carbis Roche St Austell Cornwall PL26 8JZ

Table 4.2 Water Abstraction schedule

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Figure 4.5: Water Abstraction table area extent

4.8 Source Protection Zones

4.8.1 According to the Cornwall Council Intranet mapping system there are no Source Protection Zones within the vicinity of the scheme.

4.9 Enclosed Water Bodies

4.9.1 There are several enclosed water bodies in the vicinity of the scheme. These are typically associated with pits associated with the china clay mining activates and are discussed in Section 5 of this report.

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5 FLOOD MECHANISMS REVIEW

5.1 Overview

5.1.1 There are several mechanisms of flooding that need to be considered to appropriately evaluate Flood Risks associated with the scheme proposals.

5.1.2 The evaluation process is to consider:

. The effects of implementing the scheme upon existing flood risk levels

. The residual flood risks upon implementation of the scheme

. The flood risks that affect the scheme proposals, and their impact on ongoing operational safety and maintenance

5.1.3 The following section reviews a variety of potential flood mechanisms, identifying which mechanisms can be discounted or require further investigation and evaluation

5.2 Coastal / Tidal Flooding

5.2.1 The development area is high within the catchment well beyond any tidal flood plain extent. The risk of tidal flooding is not present and as such tidal influences will not be considered further within this report.

5.3 Fluvial Flooding

5.3.1 Initial review of Cornwall Council indicative mapping shows the development area is located within a Flood Zone 1 (low risk), well beyond the 1:1,000 year flood extent. This is expected as the proposed scheme is situated close the uppermost catchment boundaries of four catchments within the area.

5.3.2 Drawing 0718_CSL_HDG_00MZ_DR_CD_0015 located in Appendix A shows the CC indicative flood zones in relation to the scheme proposals. Flood zones are also indicatively shown in Figure 5.1.

5.3.3 Although indicative mapping indicates there is a low fluvial flood risk, the scheme design crosses four minor watercourses which are tributaries to the River Fal. Further south the Carbis Stream on the Upper River Par is also crossed by the proposed design. Fluvial flood risks and provisions to accommodate the watercourses are therefore further discussed in Section 6 of this report.

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Ordinary watercourse

Flood zone 3

Proposed route

Main River

Critical Drainage Area (CDA)

Figure 5.1: Indicative Flood Zone Maps

5.4 Groundwater Flooding

5.4.1 The Cornwall Level 1 Strategic Flood Risk Assessment (SFRA) states:

“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.” 24 EDG0718-CSL-HDG-00MZ-RP-CD 0001 EDG0718 Flood Risk Assessment & Surface Water Management Strategy St Austell to A30 Link Road

5.4.2 Phase one ground investigation works have been undertaken which are reported in: “A391 to A30 link Road Main alignment: Ground Investigation Report Project (GIR) No: 64307”

5.4.3 Groundwater strikes encountered during the ground investigation are detailed in Table 4.2 of the GIR report No: 64307. Of the one hundred and fifty-six trial pits/boreholes planned, just over a hundred trial pits were undertaken due to site access constraints. Only ten of the trial pits and five of the boreholes encountered groundwater. Typically where ground water was encountered it was in the region of 1.5m to 2.7m below existing ground level (EGL). The groundwater was typically encountered on test locations near to the tributaries of the River Fal identified as A and E on the drawings in Appendix A.

5.4.4 Groundwater monitoring standpipes were installed in sixteen boreholes, details of which are presented in Table 4.3 of the GIR report No: 64307. Where deemed appropriate groundwater monitoring installations were put into boreholes where water was intersected. These will be subject to a twelve month monitoring programme commencing in June 2018.

5.4.5 Given the site location being situated at the upstream boundary of four catchments and the 6.5km length of the scheme, there are a number of spring issue points in the general vicinity of the proposed works which typically discharge or are the sources of local watercourses.

5.4.6 The scale and nature of the scheme along with the first phase ground investigation suggests ground water management/mitigation measures at some locations may be required, as would be expected on a significant infrastructure scheme. The detailed design will need to appropriately consider and incorporate information gained from existing and ongoing ground investigation works. The forthcoming geotechnical design report (GDR) will be influential in design development and the management of groundwater flood risk.

5.4.7 At this stage of the project it is not possible to fully identify what measures will be necessary to mitigate ground water flood risk. As more ground investigation works occur the appropriate measure will become apparent for inclusion within the design. High levels objectives would include:-

. Review of design solutions, design levels, material types to assess the impact of ground water on the design functionality, durability and usability

. Measures to assess and ensure no significant changes to the ground water regime which could negatively affect the design or downstream receptors

. Measures to maintain existing flow conveyance routes across the design area

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. Measures to manage unidentified historic features such as unrecorded mining adits or leats.

5.4.8 The scale and nature of this significant civil engineering project along with its high profile visibility suggests that as the detailed design is developed any ground water risks will be appropriately identified and suitably mitigated. The phase 1 GIR report has begun the appropriate investigation and the forthcoming GDR report at detailed design will provide further design input.

5.4.9 Given the current available information it appears the scheme proposals can comfortably incorporate measures to manage groundwater flood risks to an appropriate level. The project management structure set in place to progress the scheme has the appropriate skills and resources to ensure the appropriate actions are taken within the detailed design and construction process to manage flood risk.

5.5 Surface Water Flooding

5.5.1 The CC SFRA has produced a surface water flood risk map. Figure 5.2 shows the areas identified at risk for the 1:30 and 1:1,000 year events. Within the scheme area the surface water flood risk mapping zones are typically associated with the presence of existing watercourses. The undeveloped nature of the existing site leads to naturalised flow routes.

Figure 5.2: SFRA indicative map for surface water flood risk

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5.5.2 The development has the potential to increase the rate of surface water runoff to downstream receptors, due to covering of greenfield surfaces with impermeable surfaces. It also has the potential to affect natural conveyance routes within the catchments.

5.5.3 A Surface Water Management Strategy has been prepared to demonstrate the scheme can be implemented with suitable consideration to the management of surface water. Section 8 of this report discusses the management of surface water in respect to design, mitigation and policy requirements.

5.6 Flooding from enclosed water bodies

5.6.1 A number of enclosed water bodies are within the vicinity of the development and are discussed below.

. A covered reservoir (SWW) is located at 200910, 057174, 30m west of the existing road (B3274).

. A covered reservoir (SWW) is located at 199186, 058356, adjacent to the C0298

5.6.2 The risk of reservoir flooding is typically considered low risk due to the inspection and maintenance regimes required for bodies classified as registered reservoirs.

5.6.3 Several Imerys Mining excavation pits are located in the vicinity of the scheme located at:

o 201027 E, 057621 N o 200718 E, 058183 N o 201023 E, 057621 N o 200112 E, 056695 N

5.6.4 The pits listed above generally appear at a lower level and/or downstream of the development area and do not constitute a significant flood risk to the scheme.

5.6.5 The current design proposal interfaces with the disused Dimension Stone Quarry located at 200431 E, 057731 N. The excavation is approx.. 150m x 150 and has filled with surface water runoff to create a pond. Due to other design constraints affecting the design alignment the earthworks footprint of the current design proposal passes through the east face of this pond bank.

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5.6.6 The current pond is elevated above the proposed road alignment as the road design is in cut at this location. The GIR report Risk Register has highlighted the risk of destabilisation of the bank and future seepage, resulting in the proposals to drain the pond and develop a design for reinstatement which could include lining and retaining works.

5.6.7 The current proposal is to drain the pool for the construction period. A design is being prepared to undertake works to the pond so it can be reinstated as a water body following construction of the main road alignment. The proposals will be subject to flood risk, geotechnical and Road Safety Audit (RSA) safety reviews as part of the process to obtain technical approval.

5.6.8 Potentially the pond could be removed in its entirety but it is not proposed at present as ecological impact investigations are ongoing.

5.6.9 The pond is understood to accept surface water runoff from the adjacent Imerys haul road. It is currently understood that the pond provides limited attenuation as incoming flows are directly displaced into a deep “V” channel which extends approximately 200m north of the pond. An outfall for this channel has not as yet been identified and potentially flows overtop from the pond into the channel and infiltrates into the ground. Difficult site access has made confirmation/investigation in this area problematic and a full topographic survey of this area is not yet available. Any amendments to the pond will need to consider appropriate management of surface water from the Imerys site haul road to appropriately manage flood risk.

5.6.10 The majority of the surrounding water bodies have provided no or a low risk in terms of the development proposals. Detailed design work is expected to appropriately manage the Dimension Stone Quarry in order that the risks are suitably avoided or mitigated. It is expected these details will be submitted with the documentation to discharge the planning conditions associated with the planning approval.

5.7 Existing Utilities Flooding

5.7.1 Existing utilities such as existing potable water, sewage, surface water and Imerys clay supply pipes have associated flood risks. The risks can be increased when excavation works are to progress within their vicinity. Although much of the scheme is beyond more vulnerable urban extents. There is potential that breaks to utilities and resulting flows could be directed via the road network to more vulnerable locations.

5.7.2 To mitigate this risk in the first instance desktop research and site based investigation works (first phase approximately seventy five trial pits) are ongoing to identify utilities and confirm their locations and depths within the site extents. Accurate knowledge of the services will significantly reduce the associated risk. If appropriate RAMS are employed by a competent contactor, as would be expected this scheme, the risk of utilities flooding is considered low. 28 EDG0718-CSL-HDG-00MZ-RP-CD 0001 EDG0718 Flood Risk Assessment & Surface Water Management Strategy St Austell to A30 Link Road

6 FLUVIAL FLOOD RISK

6.1.1 Initial review of Cornwall Council indicative mapping shows the development area is located within Flood Zone 1 (low risk), well beyond the 1:1,000 year fluvial flood extent. This is expected as the proposed scheme is situated close to the uppermost catchment boundaries of the four catchments within the area.

6.1.2 Although indicative mapping indicates there is a low fluvial flood risk, the scheme design crosses four minor watercourses which are tributaries to the Upper River Fal. The watercourses resemble edge of field drainage ditches as they are high within the catchment, but become more defined as flowing watercourses as they head west away from the scheme.

6.1.3 Further south the Carbis Stream on the Upper River Par is also crossed by the proposed design. This watercourse is more clearly defined.

6.1.4 Drawing 0718_CSL_HDG_00MZ_DR_CD_0014, located in Appendix A, shows the existing watercourse catchments in relation to the scheme proposals. The catchments have be extrapolated from contours generated from a Civil 3d ground model which was developed from available topographic and lidar data.

6.1.5 An initial assessment of peak flow rates within the watercourses has been undertaken using ReFH 2 software and catchment descriptors purchased from the CEH FEH web service. The node points identified on drawing 0718_CSL_HDG_00MZ_DR_CD_0014 identify the flow assessment locations which have typically been located on the downstream side of the main design proposal.

6.2 Upper River Fal (A) Catchment Review

6.2.1 The Upper River Fal (A) is the most northern watercourse crossed by the scheme proposals. The existing upstream catchment is estimated at approximately 0.48Km2. The catchment is largely agricultural fields although potentially it could accept some urbanised runoff from areas on the west of Roche that are not intercepted by existing surface water drainage systems.

6.2.2 CC intranet mapping indicates the watercourse as crossing the scheme proposals. However a site walkover inspection located the existing pond on the watercourse south-west of the scheme, but did not identify an obvious channel/conveyance route from the pond located further upstream to the east of the proposed road alignment. Potentially the western pond is the head of the defined watercourse, being fed by overland runoff from upstream in the catchment. The defined watercourse heads west from the pond along a densely vegetated field boundary. An inlet pipe, approx. 225mmØ has been observed at the pond and could potentially convey flows from the upstream pond but this is not confirmed.

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Pond 2

Figure 6.1 – Upper river Fal (A) Catchment area

Pond 1

No Defined watercourse channel

Pond 2

Defined watercourse

Figure 6.2 – Upper river Fal (A) Watercourse features

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6.3 Upper River Fal (A) - Peak flow assessment

6.3.1 ReFH 2 Peak flow rate estimates have been assessed for the Upper river Fal (A) catchment and are provided in Table 6.1 below

ReFH2 Peak Fluvial Flow Rates (m3/s) by Return Period event

Catchment Area 100 year Watercourse 2 1 year 5 year 30 year 100 year (km ) + 40% CC

Upper river Fal (A) 0.483 0.347 0.571 0.943 1.30 1.82

Table 6.1

6.4 Upper River Fal (A) – Culvert requirements

6.4.1 It is proposed to install a box culvert to convey watercourse flows under the proposed road design. Drawing 0718_CSL_HDG_00MZ_DR_CD_0018 shows an indicative long section of the proposed culvert. An indicative cross section is shown on drawing 0718_CSL_SGN_XXMZ_DE_CB_0018. These drawings are located in Appendix A.

6.4.2 The scheme culvert designs have been influenced by the ecological requirements for mammal ledges and maintenance access head clearance heights.

6.4.3 The 2.5m x 2.0m box culverts allow for 1.7m head clearance above 300mm of stream bed material. The proposed culvert at Upper River Fal (A) will be in the region of 57m in length. The 500mm wide mammal ledges are required to be set above the 5 year event flood level. A low flow channel is also specified within the proposed design.

6.4.4 Hydraflow Express was utilised to check the capacity of the culverts in relation to the predicted ReFH 2 flow rates. Culvert sizing requirements are typically to ensure capacity for the predicted 1in 100 year+ 40% flow rate, with a minimum 600mm of freeboard to crown.

6.4.5 The Hydraflow calculations indicate the proposed culvert will have in excess of 1.2m freeboard above the design flow rate. This is depicted in Figure 2 and based upon the watercourse culvert levels & bed slope remaining similar to existing levels.

6.4.6 The excess capacity indicates if design levels change due site conditions and detailed design, it is unlikely upsizing of the culvert will be required to accommodate the resulting flow/depth changes.

6.4.7 The excess capacity also would allow for exceedance events beyond the 100yr + 40% peak flow design condition.

6.4.8 Given the culvert provisions provided within the design it is considered unlikely that a negative impact on flood risk will occur on this watercourse as a result of the proposed works . 31 EDG0718-CSL-HDG-00MZ-RP-CD 0001 EDG0718 Flood Risk Assessment & Surface Water Management Strategy St Austell to A30 Link Road

Figure 6.3 – Upper river Fal (A) Hydraflow capacity assessment Main culvert

6.5 Upper River Fal (A) – Footpath crossing

6.5.1 Further downstream on Upper river Fal (A) a footpath crossing is proposed to cross the watercourse. It is understood that the footpath in this region will be a timber boardwalk design; elevated a minimum of 600mm above existing top bank levels of the existing watercourse and the surrounding soft ground.

6.5.2 Given the elevation of the boardwalk and the ability for exceedance flows to pass through the structure it is not considered likely that the structure will have a negative effect on local flood flows/levels.

6.5.3 Should the watercourse exceeded its banks, flood levels should remain relative shallow given the local topography as flood volumes can spread into the surrounding fields. The elevated boardwalk would potentially provide a safer access/egress route from the area.

6.6 Upper River Fal (A) – Field access accommodation works

6.6.1 Field access accommodation works include provision of a 5m wide unbound access track, which provides connectivity for fields located to the west of the main design. The access track is proposed to cross the Upper river Fal (A) approximately 25m west of the proposed footpath.

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6.6.2 A short 8m section of the watercourse will need to be culverted under the access track. The culvert has a number of options that can be investigated at detailed design. The current proposal is for twin 750mm diameter pipes, which should fit between the watercourse bed and the proposed track levels. There is no requirement for animal ledges at this location and exceedance flows can overtop the access track and return directly to the watercourse.

6.6.3 Hydraflow calculations indicate the 100yr +40% CC peak flow rate could be accommodated by the twin 750mm pipes without surcharging flows across the access track.

Figure 6.4 – Upper River Fal (A) Hydraflow capacity assessment - Access Track

6.7 Upper River Fal (A) – Flood Risk Summary

6.7.1 The provisions for the proposed crossings of Upper River Fal (A) have been proposed to allow unimpeded flows of the watercourse, thus in keeping with the existing situation as far as reasonably practicable. It is not considered that the implementation of the scheme will have a negative effect on any downstream receptors associated with the Upper River Fal (A) tributary. No increase in flood risk to third parties is foreseen relating to installation of the culvert crossings.

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6.7.2 The culvert crossings are to be designed in detail and constructed in accordance with the current guidance. The main road alignment is elevated well above the watercourse. With appropriately sized culverts the proposed design and its subsequent end users should be afforded a suitable level of flood risk protection.

6.8 Upper River Fal (C) Catchment Review

6.8.1 The Upper River Fal (C) crosses the design approximately 650m north of Trezaise Roundabout The existing upstream catchment is estimated at approximately 0.20Km2. The catchment is largely agricultural fields although potentially it could accept some urbanised runoff from areas on the west of Roche that are not intercepted by existing surface water drainage systems.

Figure 6.5 – Upper river Fal (C) Catchment 6.8.2 CC intranet mapping indicates the watercourse as crossing the scheme proposals. The topographic survey investigation did not identify a defined stream channel, however shallow edge of field drainage ditches were observed, which would be in keeping with the predicted flow rates from this relatively small catchment area.

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Flow Estimate Location

Figure 6.6 – Upper river Fal (C) Catchment Aerial image

6.9 Upper River Fal (C) - Peak flow assessment

6.9.1 ReFH 2 Peak flow rates have been assessed for the Upper river Fal (C) catchment and are provided in Table 6.2 below

ReFH2 Peak Fluvial Flow Rates (m3/s) by Return Period event

Fluvial 100 year Watercourse Catchment 1 year 5 year 30 year 100 year Area (km2) + 40% CC

Upper river Fal 0.204 0.150 0.247 0.407 0.565 0.791 (C)

Table 6.2

6.10 Upper River Fal (C) – Culvert requirements

6.10.1 It is proposed to install a box culvert to convey watercourse flows under the proposed road design. Drawing 0718_CSL_HDG_00MZ_DR_CD_0018 shows an indicative long section of the proposed culvert. An indicative cross section is shown on drawing 0718_CSL_SGN_XXMZ_DE_CB_0018. These drawings are located in Appendix A.

6.10.2 The scheme culvert designs have been influenced by the ecological requirements for mammal ledges and maintenance access head clearance heights.

6.10.3 The 2.5m x 2.0m box culverts allow for 1.7m head clearance above 300mm of stream bed material. The proposed culvert at Upper River Fal (C) will be in the region of 44m in length. The 500mm wide mammal ledges are required to be set above the 5 year event flood level. A low flow channel is also specified within the proposed design. 35 EDG0718-CSL-HDG-00MZ-RP-CD 0001 EDG0718 Flood Risk Assessment & Surface Water Management Strategy St Austell to A30 Link Road

6.10.4 Hydraflow Express was utilised to check the capacity of the culverts in relation to the predicted ReFH 2 flow rates. Culvert sizing requirements are typically to ensure capacity for the predicted 1 in 100 year+ 40% CC flow rate, with a minimum 600mm of freeboard to crown.

6.10.5 The Hydraflow calculations indicate the proposed culvert will have in excess of 1.2m freeboard above the design flow rate.

6.10.6 The excess capacity indicates if design levels change due to site conditions and detailed design, it is unlikely upsizing of the culvert will be required to accommodate the resulting flow/depth changes.

6.10.7 It is understood that following the EIA ecological assessment the requirements for mammal crossings will be further defined and potentially unrequired at this location. Potentially this culvert could be downsized at the detailed design stage. It is understood that any change would be subject to satisfying any conditions attached to the planning approval. Ignoring other design factors and considering flow capacity alone, a 900mm pipe would be sufficient to convey the predicted flow rates from the upstream catchment.

6.10.8 The excess capacity of the proposed culvert also would allow for exceedance events beyond the 100yr + 40% peak flow design condition.

6.11 Upper River Fal (C) – Footpath crossing

6.11.1 On the upstream side of the main alignment on Upper River Fal (C) a footpath crossing is proposed to cross the watercourse. It is understood that the footpath in this region will be a timber boardwalk design; elevated a minimum of 600mm above existing top bank levels of the existing watercourse.

6.11.2 Given the elevation of the boardwalk and the ability for exceedance flows to pass through the structure it is not considered likely that the structure will have a negative effect on local flood flows/levels.

6.11.3 Should the watercourse exceed its banks flood levels should remain relative shallow given the local topography as flood volumes can spread into the surrounding fields. Flow can then re-enter the watercourse and enter the Culvert. The elevated boardwalk would potentially provide a safer access/egress route from the area.

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6.12 Upper River Fal (C) – Flood Risk Summary

6.12.1 The provisions for the proposed crossings of Upper River Fal (C) have been proposed to allow unimpeded flows of the watercourse, thus in keeping with the existing situation as far as reasonably practicable. It is not considered that the implementation of the scheme will have a negative effect on any downstream receptors associated with the Upper River Fal (C) tributary. No increase in flood risk to third parties is foreseen relating to installation of the culvert crossings.

6.12.2 The culvert crossings are to be designed in detail and constructed in accordance with the current guidance. The main road alignment is elevated well above the watercourse. With appropriately sized culverts the proposed design and its subsequent end users should be afforded a suitable level of flood risk protection.

6.13 Upper River Fal (D) Catchment Review

6.13.1 The Upper River Fal (D) crosses the design approximately 480m north of Trezaise Roundabout. The existing upstream catchment is estimated at approximately 0.069Km2. The catchment is relatively small and is largely agricultural fields but potentially could accept some urbanised runoff from Trezaise that are not intercepted by existing surface water drainage systems.

Figure 6.7 – Upper river Fal (D) Catchment

6.13.2 CC intranet mapping indicates the watercourse as crossing the scheme proposals. The topographic survey investigation did not identify a defined stream channel, however shallow edge of field drainage ditches were observed, which would be in keeping with the predicted flow rates from this relatively small catchment area.

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Figure 6.7 – Upper River Fal (D) Ariel Image

6.14 Upper River Fal (D) - Peak flow assessment

6.14.1 ReFH 2 Peak flow rates have been assessed for the Upper River Fal (D) catchment and are provided in the Table 6.3 below

ReFH2 Peak Fluvial Flow Rates (m3/s) by Return Period event

Fluvial 100 year Watercourse Catchment 1 year 5 year 30 year 100 year Area (km2) + 40% CC

Upper river Fal 0.069 0.051 0.084 0.139 0.193 0.270 (D)

Table 6.3

6.15 Upper River Fal (D) – Culvert requirements

6.15.1 It is proposed to install a box culvert to convey watercourse flows under the proposed road design. Drawing 0718_CSL_HDG_00MZ_DR_CD_0018 shows an indicative long section of the proposed culvert. An indicative cross section is shown on drawing 0718_CSL_SGN_XXMZ_DE_CB_0018. These drawings are located in Appendix A.

6.15.2 The scheme culvert designs have been influenced by the ecological requirements1 for mammal ledges and maintenance access head clearance heights.

1 At the time of writing, ecological assessments are ongoing and mitigation proposals relating to locations for mammal crossings are yet to be confirmed.

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6.15.3 The 2.5m x 2.0m box culverts allow for 1.7m head clearance above 300mm of stream bed material. The proposed culvert at Upper River Fal (D) will be in the region of 44m in length. The 500mm wide mammal ledges are required to be set above the 5 year event flood level. A low flow channel is also specified within the proposed design.

6.15.4 Hydraflow Express was utilised to check the capacity of the culverts in relation to the predicted ReFH 2 flow rates. Culvert sizing requirements are typically to ensure capacity for the predicted 1 in 100 year+ 40% CC flow rate, with a minimum 600mm of freeboard to crown.

6.15.5 The Hydraflow calculations indicate the proposed culvert will have in excess of 1.2m freeboard above the design flow rate.

6.15.6 The excess capacity indicates if design levels change due to site conditions and detailed design, it is unlikely upsizing of the culvert will be required to accommodate the resulting flow/depth changes.

6.15.7 It is understood that following the EIA ecological assessment the requirements for mammal crossings will be further defined and potentially unrequired at this location. Potentially this culvert could be downsized at detailed design during value engineering. It is understood that any change would be subject to satisfying any conditions attached to the planning approval. Ignoring other design factors and considering flow capacity alone, a 900mm pipe would be sufficient to convey the predicted flow rates from the upstream catchment.

6.15.8 The excess capacity also would allow for exceedance events beyond the 100yr + 40% peak flow design condition.

6.16 Upper River Fal (D) – Footpath crossing

6.16.1 On the upstream side of the main alignment on Upper River Fal (D) a footpath crossing is proposed to cross the watercourse. It is understood that the footpath in this region will be a timber boardwalk design; elevated a minimum of 600mm above existing top bank levels of the existing watercourse.

6.16.2 Given the elevation of the boardwalk and the ability for exceedance flows to pass through the structure it is not considered likely that the structure will have a negative effect on local flood flows/levels.

6.16.3 Should the watercourse exceed its banks flood levels should remain relatively shallow given the local topography as flood volumes can spread into the surrounding fields. Flow can then re-enter the watercourse and enter the Culvert. The elevated boardwalk would potentially provide a safer access/egress route from the area.

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6.17 Upper River Fal (D) – Flood Risk Summary

6.17.1 The provisions for the proposed crossings of Upper River Fal (D) have been proposed to allow unimpeded flows of the watercourse, thus in keeping with the existing situation as far as reasonably practicable. It is not considered that the implementation of the scheme will have a negative effect on any downstream receptors associated with the Upper River Fal (D) tributary. No increase in flood risk to third parties is foreseen relating to installation of the culvert crossings.

6.17.2 The culvert crossings are to be designed in detail and constructed in accordance with the current guidance. The main road alignment is elevated well above the watercourse. With appropriately sized culverts the proposed design and its subsequent end users should be afforded a suitable level of flood risk protection.

6.18 Upper River Fal (E) Description

6.18.1 The Upper River Fal (E) runs adjacent to the western extent of the proposed Trezaise Roundabout. The system of watercourses at this location is moderately complex and is discussed below.

6.18.2 The main flow channel originates near the base of the china clay benched spoil tip south of Coldvreath Farm. It flows north for 440m, where it then passes under the existing culvert under the CO298 that links Trezaise with Cleers Hill. The culvert is known as the Reesehill culvert and is 1.8m wide by 1.32m high.

6.18.3 Heading downstream just north of the culvert, a watercourse joins the main flow channel. This watercourse runs adjacent to the C0298 and originates approximately 270m to the south east, flowing to the northwest. A spring is noted on the OS map at this location.

6.18.4 After the confluence, the Upper River Fal (E) continues north for approximately 100m adjacent to the road which provides access to the dwellings located at the base of ReesHill. Drainage ditches serving the fields to the east of the access road have outfalls into the watercourse along this section.

6.18.5 The watercourse moves away from the road briefly near the next junction to allow it to cross the junction at the base of Reese Hill adjacent to Nans Avallen via a stone culvert. A drainage channel from south of Gwel an Praze is shown to discharge into the watercourse around 25m downstream of the stone culvert.

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6.18.6 The watercourse continues in open channel adjacent to the access road to Lower Trerank farm. Approximately half way along the track a second watercourse joins the main flow route. This watercourse also originates near the base of the china clay benched spoil tip south of Coldvreath Farm. It passes through Coldvreath and Lower Coldvreath Farm on a route parallel and to the west of the main flow route. The watercourse then continues to the west.

Lower Trerank

Stone Culvert

Field drainage

Rees Hill Culvert

C0298

China clay benched spoil tip

Figure 6.8 – Upper river Fal (E) Aerial Image

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6.19 Upper River Fal (E) Catchment

6.20 Upper River Fal (E) - Peak flow assessment

6.20.1 ReFH 2 peak flow rate estimates have been assessed for the Upper River Fal (E) catchment and are provided in the Table 6.4 below assuming a contributing catchment of 0.432km2:

ReFH2 Peak Fluvial Flow Rates (m3/s) by Return Period event

Fluvial 100 year Watercourse Catchment 1 year 5 year 30 year 100 year Area (km2) + 40% CC

Upper river Fal 0.432 0.236 0.387 0.639 0.888 1.243 (E)

Table 6.4

Figure 6.9 – Upper river Fal (E) Catchment

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6.21 Upper River Fal (E) – Culvert requirements

6.21.1 The south-west arm of the proposed Trezaise Roundabout requires two construction operations relating to culverts. The existing Rees Hill Culvert will require extending to the north by approximately 18.9m to a total length of approximately 37.2m to accommodate the wider earthworks footprint of the junction. It should be noted earthworks designs currently conservatively assume 1:3 side slopes. There is potential the side slopes will be steepened during detailed design reducing the length of the culverts.

6.21.2 The short watercourse flowing adjacent to the C0298 is proposed to be culverted south across the C0298 shortly before the junction onto the south-west arm of Trezaise Roundabout. It will allow the watercourse to join the main reach just upstream of the Rees Hill Culvert. This moves the confluence of the watercourses approximately 30m upstream from the existing location but significantly reduces the length of the required culvert and other construction complications.

6.21.3 Drawing 0718_CSL_HDG_00MZ_DR_CD_0018 shows indicative long sections of the proposed culverts. Culvert section D showing the extension works, culvert section E showing the proposed new culvert.

6.21.4 An indicative cross section is shown on drawing 0718_CSL_SGN _XXMZ_DE_CB_0018. These drawings are located in Appendix A. It should be noted the existing culvert has internal dimensions of 1.8m W x 1.32m H, the extension is proposed to match the existing dimensions.

6.21.5 The scheme culvert proposals for culvert E has been influenced by the ecological requirements for mammal ledges and maintenance access head clearance heights.

6.21.6 The new 2.5m x 2.0m box culvert (Culvert E) allows for 1.7m head clearance above 300mm of stream bed material. The proposed culvert will be in the region of 30m in length. The 500mm wide mammal ledges are required to be set above the 5 year event flood level. A low flow channel is also specified within the proposed design.

6.21.7 Hydraflow Express was utilised to check the capacity of the culverts in relation to the predicted ReFH 2 flow rates. Culvert sizing requirements are typically to ensure capacity for the predicted 1in 100 year+ 40% CC flow rate, with a minimum 600mm of freeboard to crown.

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6.21.8 The Hydraflow calculations indicate the proposed culvert extension (Culvert D) will have approximately 0.7m of freeboard above the peak water level, assuming flow of 1.25m3/s, 300mm of bed material and a 7% slope. This check is conservative as part of the catchment will enter the watercourse downstream of the culvert.

Figure 6.10 – Upper river Fal (E) - Culvert (D)

6.21.9 The proposed culvert (Culvert section E) also has sufficient capacity for the potential design flow rates generate by the upstream catchment.

6.21.10 The excess capacity indicates if design levels change due to site conditions and detailed design, it is unlikely upsizing of the culvert will be required to accommodate the resulting flow/depth changes.

6.21.11 It is understood that following the EIA ecological assessment the requirements for mammal crossings will be further defined and potentially unrequired at this location. Potentially this culvert could be downsized at detailed design. It is understood that any change would be subject to satisfying any conditions attached to the planning approval.

6.21.12 The excess capacity also would allow for exceedance events beyond the 100yr + 40% peak flow design condition.

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6.22 Upper Carbis Stream

6.22.1 The Upper Carbis stream is estimated to have an upstream catchment of 0.518 and crosses the proposed design via the Hensbarrow underbridge as shown in Figure 6.11 and 6.12 below.

Figure 6.11– Hensbarrow Under bridge

Figure 6.12– Upper Carbis stream catchment 45 EDG0718-CSL-HDG-00MZ-RP-CD 0001 EDG0718 Flood Risk Assessment & Surface Water Management Strategy St Austell to A30 Link Road

6.23 Upper Carbis stream - Peak flow assessment

6.23.1 ReFH 2 peak flow rate estimates have been assessed for the Upper Carbis Stream catchment and are provided in Table 6.5 below assuming a contributing catchment of 0.528km2:

ReFH2 Peak Fluvial Flow Rates (m3/s) by Return Period event

Fluvial 100 year Watercourse Catchment 1 year 5 year 30 year 100 year Area (km2) + 40% CC

Upper river Fal 0.518 0.301 0.488 0.800 1.110 1.554 (D)

Table 6.5

6.23.2 As can be seen from Figure 6.11 the watercourse will remain in open channel as it passes through the under bridge.

6.23.3 The channel is proposed to be culverted under the 2 Non motorised user (NMU) paths located at each end of the under bridge. There is no requirement for mammal bridges within the two culverts. The existing watercourse is approximately 2m below existing ground levels in this region leaving sufficient clearance for installation of the two box culverts with sufficient freeboard to soffit level. It is currently proposed to install 2.4m W x 1.8m H box culverts

6.23.4 Given the short nature of the culverts if blockage occurs exceedance flows will be able to pass over the footpaths and re-enter the open channel for the watercourse.

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7 SURFACE WATER MANAGEMENT GUIDANCE

7.1 Drainage Guidance for Cornwall (DGfC) 2010 Version 2

** it is noted the DGfC is currently withdrawn and is being updated for re- issue. Section 7 is a direct extract from DGfC**

7.1.1 The following guidance is taken from DGfC and is applicable for sites outside a Critical Drainage Areas, for developments greater than or equal to 1 hectare, on Greenfield development sites.

• 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 an 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.

7.1.2 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 related to coastal waters. In these cases the impact on the receiving environment in terms of habitat, erosion and water quality should be assessed.

7.1.3 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.

7.1.4 Underground attenuation and piped sections should be designed for a minimum of the 30-year storm. However total discharge rates from the site must still be controlled for the 100-year storm. Attenuation of events exceeding the piped system may be achieved by temporary flooding of open spaces or car parks. If surface flooding of open areas is not appropriate, the formal drainage system should be designed for the 100 year storm.

7.1.5 Where infiltration is not used, long-term storage must be provided to store the additional volume of run-off caused by any increase in impermeable area. This is in addition to the attenuation storage required to address flow rates, see Appendix F. Alternatively rainwater harvesting can be used to offset this volume.

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7.1.6 The long-term storage should discharge at a rate not exceeding 2 litres/second/hectare, as per Preliminary rainfall run-off management for developments DEFRA /Environment Agency guidance W5-074 Revision D.

7.1.7 Safe and appropriate flow routes from blockage and exceedance of the drainage system must be evaluated. This must demonstrate no property flooding or increase in flood risk, either offsite or to third parties.

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8 HIGHWAY DRAINAGE SURFACE WATER MANAGEMENT

8.1.1 The surface water drainage proposals for the scheme have been divided into 8 separate Drainage Zones. The scheme drainage zones were divided in relation to their main outfall location as listed in Table 8.1 below.

Drainage Zone Outfall (Receiving watercourses)

Zone D1 Highway SW drainage network (subsequently, Upper River Par)

Zone D2 Highway SW drainage network (subsequently, St Austell River)

Zone D3 Tributary to Rosevean Stream (Upper River Par)

Zone D4_5 Tributary to Carbis Stream (Upper River Par)

Zone D6 Tributary (E) to Upper River Fal

Zone D7 Tributary (C) to Upper River Fal

Zone D8 Tributary (B) to Upper River Fal

Zone D9 Tributary to Lower River Ruthern

Table 8.1 8.1.2 The following sections from 8.5 to 8.12 discuss each of the drainage networks within the proposed drainage strategy. Details relating to all of the 8 networks are outlined below.

8.1.3 The drawings related to the surface water drainage proposals are listed in Table 8.2 below.

Drawing Ref: Drawing Name

0718_CSL_HDG_00MZ_DR_CD_0001 Surface Water Drainage Key Plan

0718_CSL_HDG_01R1_DR_CD_0002 Surface Water Drainage - Sheet 1 of 12

0718_CSL_HDG_02MZ_DR_CD_0003 Surface Water Drainage - Sheet 2 of 12

0718_CSL_HDG_03M1_DR_CD_0004 Surface Water Drainage - Sheet 3 of 12

0718_CSL_HDG_04M1_DR_CD_0005 Surface Water Drainage - Sheet 4 of 12

0718_CSL_HDG_05M1_DR_CD_0006 Surface Water Drainage - Sheet 5 of 12

0718_CSL_HDG_06MZ_DR_CD_0007 Surface Water Drainage - Sheet 6 of 12

0718_CSL_HDG_07R2_DR_CD_0008 Surface Water Drainage - Sheet 7 of 12

0718_CSL_HDG_08M2_DR_CD_0009 Surface Water Drainage - Sheet 8 of 12

0718_CSL_HDG_09M2_DR_CD_0010 Surface Water Drainage - Sheet 9 of 12

0718_CSL_HDG_10R3_DR_CD_0011 Surface Water Drainage - Sheet 10 of 12

0718_CSL_HDG_11MZ_DR_CD_0012 Surface Water Drainage - Sheet 11 of 12

0718_CSL_HDG_12R4_DR_CD_0013 Surface Water Drainage - Sheet 12 of 12

0718_CSL_HDG_00MZ_DR_CD_0014 Watercourse Catchment and Indicative Flow Rate Estimates

0718_CSL_HDG_00MZ_DR_CD_0015 Indicative Flood Risk Zones

0718_CSL_HDG_00MZ_DR_CD_0016 Rural Runoff Catchments Intersected by Design Sheet 1 of 12

0718_CSL_HDG_00MZ_DR_CD_0017 Highway Drainage Catchment Overview

0718_CSL_HDG_00MZ_DR_CD_0018 Indicative Sections of Watercourse Culverts A-E

0718_CSL_HDG_00MZ_DR_CD_0019 Indicative Detail of Key Attenuation Basin Features

0718_CSL_HDG_00MZ_DR_CD_0029 Rural Runoff Catchments Intersected by Design Sheet 1 of 12 Table 8.2

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8.2 Infiltration Assessments

8.2.1 Infiltration testing has been undertaken within twenty three trial pits across the site. This is documented within the attached GIR report. The limited testing was largely due to poor access and ecological constraints. In addition, the design level difference between existing and proposed levels made testing at some locations inappropriate until further earthworks are progressed, to enable testing at the appropriate design depths. Ground water levels at some location also made testing infiltration unviable.

8.2.2 From the testing undertaken, six of the tests locations failed to complete the first test cycle. A further four test locations failed to produce measurable rates to complete the second test cycle. A further two trial pit locations failed to produce measurable rates to complete the third test cycle.

8.2.3 The test locations that conformed to produce BRE approved infiltration rates produced slow to moderate rates in the region of 10-4 and 10-5 m/s.

8.2.4 Given the number of watercourse outfalls available and the poor infiltration rates obtained onsite, the drainage strategy predominantly utilises attenuation based systems. This ensures an appropriate drainage system can be installed onsite within the site constraints. Any inherent infiltration within the system; via basins, swales or filter drains, is considered as a secondary benefit and not included within the preliminary sizing of the system.

8.2.5 It is understood that as detailed design and production of the GDR progress there will be potential to incorporate more infiltration systems to optimise the design. However there is not sufficient evidence/confidence as yet in the available information to progress on that basis.

8.3 Network and Attenuation Basin modelling

8.3.1 Preliminary drainage models for each drainage network have been constructed using Microdrainage software. The systems have been simulated for storm scenarios up to the 1:100 year event with a 40% allowance for climate change.

8.3.2 The preliminary design of the pipe & swale network is sized to accommodate the 1:100 year event with 40% allowance for climate change, to allow no flooding from the system and is beyond the 1:30 year event required by DMRB.

8.3.3 Attenuation basins have been provisionally sized for the 1:100 year event with 40% allowance for climate change. A minimum of 0.3m Freeboard from the peak water level has been included within the designs. Basin side slopes are typically set to 1:4 unless otherwise stated.

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8.3.4 The attenuation and flow control modelling has been based on vortex flow control devices, with peak flows set to mimic greenfield rates from the drained area. The design head/attenuation storage depth is typically set at 1.2m unless otherwise stated.

8.3.5 The vortex flow control devices are to be located in chambers downstream of the basins. An engineered overflow weir to allow bypass of blockage; or provide overflow for a design exceedance events, is expected to be in the detailed design of the system.

8.3.6 Attenuation Basins are typically situated near to the receiving watercourse. In these situations the exceedance flows will be directed to the receiving watercourse by a defined over bank weir and overland flow route, unless otherwise stated below.

8.3.7 Inlet and outlet headwalls are proposed to be concrete bagwork headwalls in accordance with SfHW Cl.519. Detailed design may require vortex flow controls to be mounted directly to head walls. In these instances they would be likely be specified as precast concrete headwalls.

8.4 SuDS Treatment Train

8.4.1 The following design features have been included in the strategy where possible to add stages of filtration and water quality treatment to the SuDS treatment train.

• Non-concentrated flows directed over grass embankments which are hoped to act similar to grass filter strips. It is noted these would not be as effective as traditional filter strips defined within the SuDS manual, which are typically set at shallower gradients, but they should provide some degree of filtration and flow attenuation.

• High capacity Swales located off verges, at the base of embankments can have higher degrees of vegetation density for added filtration and flow attenuation and would typically be downstream of the over bank filter strips.

• In verge, grass V channels for flow collection and conveyance directly off the carriageway, typically provide the first stage of the SuDS treatment train within the scheme. These are to be in accordance with HA 119/06. Typically they are 150mm in depth with 1:5 side slopes.

• When the design is in cut, the collecting surface V channels will be able to discharge to Filter Drains as detailed in HCD F series drawings which will also provide sub-base drainage to the carriageway as shown in HCD B series drawings.

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• The attenuation basins provide the final stage in the SuDS treatment train. An indicative detail showing the key features intended to be included within the basins is shown on drawing 0718_CSL_HDG_00MZ_DR _CD_0019. The basin details include provision for a forbay/bund, low flow channels, permanently wet areas (soggy bottom), planting and downstream flow control. Furthermore additional GI in detailed design may demonstrate some basins can have dual discharge (infiltration and attenuation) if some degree of infiltration is possible at the basin locations. This could also reduce storage volumes required.

• In-chamber vortex separators have been identified as an additional stage in the treatment train on the indicative basin detail, located in a chamber directly upstream of the basins. Although they give an additional stage in the treatment train, the separators are a hard engineered costly addition, with added complications in respect to installation and future maintenance. Given the other provisions for soft SuDS within the scheme, these devices are expected to be further reviewed in detailed design (during the value engineering stage of the project) to assess if they add justifiable value to the scheme.

V channel with filter drain

Over bank edge to swale

V channel to carrier drain

Figure 8.1 – Indicative sections SUDS collection options

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8.5 Drainage Zone D1

8.5.1 Drainage Zone D1 includes and serves the Roche Access Road link, an access road separate to the main design alignment which falls from the Roche road down to the Stannary road in Stenalees.

Zone D1

Figure 8.1 – Highway Drainage Zone D1

8.5.2 The drainage system is proposed to drain ~0.628Ha which includes the Roche Road access and ~0.261Ha of the existing Roche Road.

8.5.3 Where possible drainage zone D1 and drainage zone D3 have intercepted catchment areas to reduce flows leading into the Single Rose Roundabout drainage system. This was to limit discharge flow rates and attenuation storage requirements from the system in drainage zone D2, to enable pre- development discharge rates to be achieved.

8.5.4 Due to limited access relating to ecological and topographical constraints infiltration testing has not been undertaken along this section of proposed road. However TP05 the nearest test location (approximately 200m east on the main alignment) failed to produce a viable infiltration rate as rates were too slow to complete the required tests. As such an attenuated drainage system has been proposed. Although there could be scope at detailed design to optimise the design utilising infiltration methods following further site investigation works.

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8.5.5 The D1 drainage system is proposed to discharge at the minimum practicable attenuated discharge rate of 5lps, as it is required to discharge into the existing highway network.

8.5.6 The Roche access road is to be kerbed along its entire length as such flow collection is intended via traditional road gullys. Road gully details to be in accordance with Cornwall Council HCD Drawing 5/7. Road Gully spacing in accordance with DMRB CD 526.

8.5.7 The existing Roche road north of the access road is intended to be downgraded for NMU access. The required NMU path width is 3m, narrowing the existing carriageway for NMU allows for the inclusion of swales, at the existing road edge within the existing highway curtilage.

8.5.8 Indicative sections showing likely basin levels for Zone D1 drainage basins (D1–basin 1 and D1-basin 2) are shown on 0718-CSL-HDG-XXRR-DR-CD- 0021.

8.5.9 The system allows for a small inline basin to intercept and attenuate flows from the existing Roche Road north of the proposed access road. The basin (D1-basin 2) located at the northern end of the access road is proposed to have a peak discharge of 5lps into the proposed drainage system.

8.5.10 The second lower basin (D1-basin 1) will then attenuate the entire D1 system flows before it enters the existing drainage system on Stannary road, to a peak flow rate of 5lps. Two basins were proposed to minimise the size of D1-basin 1, so it could be accommodated within the site constraints.

8.5.11 D1–basin 1 will provide 400m3 of attenuation storage. D1–basin 2 will provide 150m3 of attenuation storage.

8.5.12 Design information provided above is to demonstrate that a suitable system in accordance with current guidance could be installed within the carriageway design. Details are subject to change following design optimisation during detailed design. The finalised strategy will be subject to technical approval when discharging any associated surface water management planning condition.

8.6 Drainage Zone D2- Description

8.6.1 Drainage Zone D2 includes:

. The main design alignment M1 between CH000 and CH760m

. Single Rose roundabout including tie-ins on the NE, SE and SW arms.

. Approximately 300m of the existing Roche Road.

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8.6.2 Drainage Zone D2 will discharge into the highway existing drainage system on the south-west arm of Single Rose Roundabout. The drainage system is understood to discharge to the St Austell River at Carthew approximately 700m south-west of the proposed design.

Zone D2

Figure 8.2 – Highway Drainage Zone D2

8.7 Drainage Zone D2- Existing Drainage System

8.7.1 The proposed drainage system is required to connect into the existing drainage network as available infiltration tests and site characteristics indicate infiltration systems are not viable in this area.

8.7.2 It is necessary to review the existing drainage system to determine if a connection is viable and what discharge rates are appropriate to ensure flood risk is not increased downstream of the development.

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8.7.3 The existing highway catchment is understood to pick up the carriageways leading into Single Rose Roundabout before being directed along the south- west arm towards Carthew. The proposed design scheme extent is approximately 120m west of Single Rose on the B3274 near Carbean cottages. The drainage system from Zone D2 would ideally connect to the existing system near to that location.

8.7.4 From an initial review of the existing highway catchment upstream from Carbean cottages, the expected upstream catchment area is 1.56Ha. This area does not take account of any potential offsite rural runoff entering the highway network.

Catchment area

Assumed Drainage Network Layout

Carbean Cottages

Figure 8.3 – Existing catchment Area

8.7.5 In advance of receiving the full drainage survey an interpretive drainage model was created to review the expected flows generated by the catchment. It was also to identify what pipe size requirements could accommodate the 100 year flow for the existing catchment. The system assumed a 1.2m cover depth below cover levels and used the Microdrainage optimise function to identify appropriate pipe sizes.

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8.7.6 The interpretative model indicated that adjacent to Carbean cottages the 100 year flow could peak at around 550lps for the 15 min duration, 100 year storm. The model indicated a 450mm diameter pipe would be close to full bore capacity for that scenario (0.98 flow capacity ratio).

8.7.7 A CCTV condition survey of the existing drainage system from Single Rose roundabout to Carthew has been undertaken. The report is located in Appendix B. The CCTV report will form the basis for specifying any upgrades to the existing drainage network from the design extent to the outfall at Carthew.

8.7.8 The CCTV report identifies the pipes from Single Rose to Carbean Cottage to be 300mm diameter. In general terms the CCTV report indicates the existing drainage to be in a poor state of repair with level 5 defects. The CCTV survey had to be abandoned on several pipe runs due to blockage and damaged infrastructure, leaving assessment gaps with the surveyed network.

8.7.9 When considering the existing hydraulic capacity, the 300mm pipes are considered undersized in respect to current design standards and the state of repair further reduces the functionality of the system, causing any assessment of capacity to have a low confidence level in terms of accuracy.

8.7.10 Blockages and defects in addition to the undersized pipes could potentially surcharge significant flows onto the existing carriageway with flow rates being determined by the carriageway gradient until they re-enter a drainage system or the watercourse at Carthew (this is in keeping with anecdotal remarks relating to surface flows on the road).

8.7.11 In conclusion the existing flow rates generated by the catchment at the scheme extent/connection location are likely to peak at 550lps for the 100 year event, with no allowance for climate change, if they are contained within a suitable pipe system. In the current situation flows are expected to surcharge on the carriageway with velocities determined by carriageway gradients.

8.7.12 It is proposed that the system downstream of the design extent be upgraded to meet current design standards. To ensure the scheme does not increase flow rates downstream towards Carthew and subsequently increase flood risk levels.

8.7.13 The proposed drainage system, which will include the existing Roche Road catchment and the proposed road design, will be attenuated as far a reasonable practicable below the 100 year existing catchment flow rate, for the 100 year+40% event. The proposals are discussed below.

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8.8 Drainage Zone D2- Proposed Drainage System

8.8.1 Where possible drainage zones D1 and drainage zone D3 have intercepted catchment areas to reduce flows leading into the Single Rose Roundabout drainage system. This was to limit discharge flow rates and attenuation storage requirements from the system in drainage zone D2, to enable pre- development discharge rates to be achieved. Approximately 300m of the existing Roche road is managed by drainage zone D1, with a further 220m length being managed by zone D3.

8.8.2 The existing Roche Road below the northern junction of the proposed Roche access road will continue to drain to Single Rose Roundabout.

8.8.3 The D2 drainage system is proposed to drain an area of ~4.940Ha. With permeable area runoff factored for 30% SPR (verges etc.) the equivalent impermeable area drained is taken as 2.879Ha.

8.8.4 Drainage zone D2 has limited options for locating attenuation infrastructure. Adjacent to the main alignment approximately 150m north of Single Rose Roundabout the design earthworks transition between cut and fill over an existing benched earthworks area (associated with historic Imerys activities). A relatively level area between the earthworks benches allows for positioning of an intermediate attenuation basin which serves approximately 600m of the proposed design. The transition between cut and fill in the design allows for flows generated north of the basin to be directed into the basin. The flows can be attenuated and discharged back into the proposed drainage network which continues south to Single Rose.

8.8.5 The basin D2- Basin 2 is proposed to be limited to a peak discharge rate of 5.0lps to maximise reduction of flow rates further down the network. The modelling indicates D2-Basin 2 will require ~1,250m3 of attenuation storage to achieve the 5lps discharge rate for the 100 year +40% event.

8.8.6 A wide verge between the east and south arms of the Single Rose Roundabout will be enlarged due to repositioning of the roundabout. A smaller attenuation basin is proposed to be situated in the verge as it is closer to the downstream extent of the network and able to accept more of the catchment area.

8.8.7 D2-Basin 1 is proposed to be limit peak discharge rate to 20lps. The modelling indicates D2-Basin 1 will require ~280m3 of attenuation storage to achieve the 20lps discharge rate. The specified storage/discharge rate utilises the practicable space available in the verge for attenuation storage.

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8.8.8 The available space in the verge is limited therefore a small modular storage tank is proposed to be located at the base of the Roche Road between the north and east arms of the roundabout on a non-trafficked area. Flows from the Roche Road will enter the tank and be attenuated to a peak of 60lps before crossing the road into D2- Basin 1. The additional storage of 120m3 provided is necessary for higher order rainfall events to prevent D2- Basin 1 from surcharging. The staged reduction in flows allow for a peak discharge of 20lps from D2- Basin 1.

8.8.9 Due to lack of suitable locations for attenuation storage on the south-west arm of Single Rose, part of the system will not be intercepted by the attenuation system. This causes predicted flow rates to rise to a peak of ~340lps for the 100 year+40% CC (15min duration) intense storm. This rate occurs at the downstream scheme extents. When compared with the existing catchment 100 year flow rate estimate, discussed in paragraph 8.7.6(without a 40% uplift for CC), this is a 210lps reduction, whilst increasing the input by 40% for climate change allowance.

8.8.10 Design information provided above is to demonstrate that a suitable system providing betterment could be installed within the carriageway design. Details are subject to change following design optimisation during detailed design. The finalised strategy will be subject to technical approval when discharging any associated surface water management planning condition.

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8.9 Drainage Zone D3

8.9.1 Drainage Zone D3 serves ~330m of the main alignment to the north of Drainage zone D2. The drainage network takes flows from the main alignment down the existing Roche Road to an attenuation basin which discharges to a tributary of the Rosevean Stream (Upper River Par).

8.9.2 The drainage system is proposed to drain an area of ~1.375Ha. With permeable area runoff factored for 30% SPR (verges etc) the equivalent impermeable area drained is taken as 0.848Ha.

Zone D3

Figure 8.4 – Highway Drainage Zone D3

8.9.3 The Greenfield runoff for the drained area was assessed using the ICP SuDS method (lps) for the gross drained area to define appropriate discharge rates. This is shown in Table 8.3 below.

Greenfield runoff for drained area ( ICP SUDS method (lps))

Equivalent Gross Drained Impermeable area Drainage Area for with Green area runoff 100 yr 30 yr 1 yr Qbar Zone highway factored at 0.3 x A drainage (Ha) (Ha)

Zone D3 1.375 0.848 37.9 29.9 12.2 15.7

Table 8.3

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8.9.4 The D3 drainage system is proposed to discharge at the Greenfield Qbar rate of 15.7lps.

8.9.5 The modelling indicates D3–basin 1 will require to 625m3 of attenuation storage to achieve the Qbar discharge rate.

8.9.6 Where possible drainage zone D1 and drainage zone D3 have intercepted catchment areas to reduce flows leading into the Single Rose Roundabout drainage system. This was to limit discharge flow rates and attenuation storage requirements from the system in drainage zone D2.

8.9.7 In Zone D3 the carriageway is to be kerbed along much of its eastern (southbound) extent due to the presence of the footway. Carriageway crossfalls are to be super-elevated to the east. Flow collection is intended via traditional road gullys. Road gully details to be in accordance with Cornwall Council HCD Drawing 5/7. Road Gully spacing in accordance with DMRB CD 526. However detailed design may conclude liner kerb drainage is a preferable collection solution due to required spacing of road gullies.

8.9.8 The existing Roche Road north of the D3 basin is intended to be downgraded for NMU access. The required NMU path width is 3m, narrowing the existing carriageway for NMU allows for the inclusion of Swales, at the existing road edge.

8.9.9 Design information provided above is to demonstrate that a suitable system in accordance with current guidance could be installed within the carriageway design. Details are subject to change following design optimisation during detailed design. The finalised strategy will be subject to technical approval when discharging any associated surface water management planning condition.

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8.10 Drainage Zone D4_5

8.10.1 Drainage Zone D4_5 serves ~1,450m of the main alignment to the north of drainage zone D3. The drainage network takes flows from the main alignment down to an attenuation basin located just to the north of the Hensbarrow Underbridge and south of the existing carriageway. The basin discharges to a tributary of the Carbis Stream (Upper River Par).

8.10.2 The drainage system is proposed to drain an area of ~5.748Ha. With permeable area runoff factored for 30% SPR (verges, cuttings etc) the equivalent impermeable area drained is taken as 3.071Ha.

Zone D4_5

Figure 8.5 – Highway Drainage Zone D4_5

8.10.3 The Greenfield runoff for the drained area was assessed using the ICP SUDS method (lps) for the gross drained area to define appropriate discharge rates. This is shown in Table 8.4 below.

Greenfield runoff for drained area ( ICP SUDS method (lps))

Equivalent Gross Drained Impermeable area Drainage Area for with Green area runoff 100 yr 30 yr 1 yr Qbar Zone highway factored at 0.3 x A drainage (Ha) (Ha)

Zone D4_5 5.748 3.071 53.6 42.2 17.3 22.2

Table 8.4

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8.10.4 The D4_5 drainage system is proposed to discharge at the Greenfield 30 year rate of 42.2lps. The 30 year discharge rate was proposed to assist keeping the basin extents within the preferred design area relating to existing site features. The discharge rate is noted to give betterment upon the required design requirements as the 100year +40% event will be attenuated to the 30 year greenfield discharge rate. At this location discharging at the Qbar Greenfield rate would lead to unacceptable land take and earthworks requirements.

8.10.5 The modelling indicates the D4_5 basin will require ~1,900m3 of attenuation storage to achieve the 30 year Greenfield discharge rate, for the 100 year+40% storm event.

8.10.6 In Zone D4_5 the dominant method of surface water collection will be via ‘over the edge’ drainage to a verge situated grass lined V channel with 1:5 side slopes. The collection V channel having regular grated outfalls to the filter drain (or carrier depending on cut/fill location) situated below the channel. The edge of carriageway details being in accordance with guidance provided in HD33/06.

8.10.7 Linear kerb drainage is to be utilised across the Hensbarrow underpass to convey flows back toward the main drainage flow route.

8.10.8 Design information provided above is to demonstrate that a suitable system in accordance with current guidance could be installed within the carriageway design. Details are subject to change following detailed design. The finalised strategy will be subject to technical approval when discharging any associated surface water management planning condition.

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8.11 Drainage Zone D6

8.11.1 Drainage Zone 6 serves ~800m of the main alignment to the north of Drainage zone D4_5. It includes the proposed NE and SW arms of the Trezaise roundabout. The drainage network takes flows from the main alignment down to an attenuation basin located between on the internal radius of the south-west roundabout arm. The basin discharges to a tributary of the Upper River Fal via an outfall pipe which is culverted under the proposed carriageway to a headwall at the watercourse.

8.11.2 The drainage system is proposed to drain an area of ~6.057Ha. With permeable area runoff factored for 30% SPR (verges, cuttings etc) the equivalent impermeable area drained is taken as 3.701Ha.

Zone D6

Figure 8.6 – Highway Drainage Zone D6

8.11.3 The Greenfield runoff for the drained area was assessed using the ICP SUDS method (lps) for the gross drained area to define appropriate discharge rates. This is shown in Table 8.5 below. 64 EDG0718-CSL-HDG-00MZ-RP-CD 0001 EDG0718 Flood Risk Assessment & Surface Water Management Strategy St Austell to A30 Link Road

Greenfield runoff for drained area ( ICP SUDS method (lps))

Equivalent Gross Drained Impermeable area Drainage Area for with Green area runoff 100 yr 30 yr 1 yr Qbar Zone highway factored at 0.3 x A drainage (Ha) (Ha)

Zone D6 6.057 3.701 58.2 45.9 18.8 24.1

Table 8.5

8.11.4 The D6 drainage system is proposed to have a peak discharge at the Greenfield 100 year Greenfield rate of 58.2lps. The 100 year rate was chosen to keep the basin with the preferred design area constraints.

8.11.5 The modelling indicates D4_5–basin 1 will require 2,200m3 of attenuation storage to achieve the 100 year Greenfield discharge rate, for the 100 year+40% storm event. At this location discharging at the Qbar Greenfield rate would lead to unacceptable land take and earthworks requirements. The discharge rate is noted to give betterment upon the required design requirements as the 100year +40% CC event will be attenuated to the 100 year greenfield discharge rate.

8.11.6 In zone D6 the dominant method of surface water collection on the main alignment will be via ‘over the edge’ drainage to a verge situated grass lined V channel with 1:5 side slopes. The collection V channel having regular grated outfalls to the filter drain (or carrier depending on cut/fill location) situated below the channel. The edge of carriageway details being in accordance with guidance provided in HD33/06.

8.11.7 The region around Trezaise roundabout is to be kerbed due to the presence of the footways. Flow collection is intended via traditional road gullys. Road gully details to be in accordance with Cornwall Council HCD Drawing 5/7. Road Gully spacing in accordance with DMRB CD 526. However detailed design may conclude liner kerb drainage is a preferable collection solution in this area.

8.11.8 Design information provided above is to demonstrate that a suitable system in accordance with current guidance could be installed within the carriageway design. Details are subject to change following design optimisation during detailed design. The finalised strategy will be subject to technical approval when discharging any associated surface water management planning condition.

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8.12 Drainage Zone D7

8.12.1 Drainage Zone 7 serves ~600m of the main alignment to the north of Trezaise Roundabout. The drainage network takes flows from the main alignment down to an attenuation southwest of the main alignment near to a proposed culvert. The basin discharges to a tributary of the Upper River Fal (Tributary C).

8.12.2 The drainage system is proposed to drain an area of ~1.748Ha. With permeable area runoff factored for 30% SPR (verges, cuttings etc.) the equivalent impermeable area drained is taken as 1.162Ha.

Zone D7

Figure 8.7 – Highway Drainage Zone D7

8.12.3 The zone D7 drainage system is proposed to have a peak discharge at the rate of 10lps. The rate was chosen to keep the basin within the preferred design area constraints (existing field boundary) and is between the Qbar and the 30 year equivalent greenfield runoff rate. This is shown in Table 8.6 below.

Greenfield runoff for drained area ( ICP SUDS method (lps))

Equivalent Gross Drained Impermeable area Drainage Area for with Green area runoff 100 yr 30 yr 1 yr Qbar Zone highway factored at 0.3 x A drainage (Ha) (Ha)

Zone D7 1.748 1.162 15.9 12.5 5.1 6.6

Table 8.6

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8.12.4 The modelling indicates D7 basin 1 will require 800m3 of attenuation storage to achieve the design discharge rate of 10lps, for the 100 year+40% storm event. The discharge rate is noted to give betterment upon the required design requirements as the 100year +40% event will be attenuated to 10lps, less than the 30 year greenfield rate. At this location it was deemed prudent given the small range within the greenfield discharge rates to size a basin that could fit within the existing field constraints. This limits land take requirements and the need to remove existing hedgelines which could have a negative impact when considering environment and ecology.

8.12.5 In Zone D7 the dominant method of surface water collection on the main alignment will be via ‘over the edge’ drainage to a verge situated grass lined V channel with 1:5 side slopes. The collection V channel having regular grated outfalls to the filter drain (or carrier depending on cut/fill location) situated below the channel. The edge of carriageway details being in accordance with guidance provided in HD33/06.

8.12.6 Design information provided above is to demonstrate that a suitable system in accordance with current guidance could be installed within the carriageway design. Details are subject to change following design optimisation during detailed design. The finalised strategy will be subject to technical approval when discharging any associated surface water management planning condition.

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8.13 Drainage Zone D8

8.13.1 The strategy for Drainage Zone 8 is comprised of 4 drainage networks.

• Network A drains a length of approximately 1100m of the design, which flows south via two attenuation basins before discharging into the Upper River Fal (A). • Network B drains 160m of the east arm of harmony JCT before discharging to an infiltration system east of harmony on the south side of the carriageway • Network C drains 110m of the west arm of harmony JCT before discharging to an infiltration system west of harmony on the south side of the carriageway • Network D drains a length of approx 490m of the design, which flows north via an attenuation basin before discharging into the upper river Fal ( A)

Zone D8 Network B

Zone D8 Network A

Zone D8 Network C

Zone D8 Network D

Figure 8.8 – Highway Drainage Zone D8

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8.13.2 The drainage system for D8 network A is proposed to drain an area of ~5.822Ha. With permeable area runoff factored for 30% SPR (verges, cuttings etc) the equivalent impermeable area drained is taken as 2.596Ha.

8.13.3 The drainage system for D8 network B is proposed to drain an area of ~0.264Ha. With permeable area runoff factored for 30% SPR (verges, cuttings etc) the equivalent impermeable area drained is taken as 0.163Ha.

8.13.4 The drainage system for D8 network C is proposed to drain an area of ~0.199Ha. With permeable area runoff factored for 30% SPR (verges, cuttings etc) the equivalent impermeable area drained is taken as 0.170Ha.

8.13.5 The drainage system for D8 network D is proposed to drain an area of ~2.327Ha. With permeable area runoff factored for 30% SPR (verges, cuttings etc) the equivalent impermeable area drained is taken as 0.991Ha.

8.13.6 The Zone 8 Network A attenuated drainage system is proposed to have a peak discharge at the equivalent greenfield Qbar rate of 21.4lps. The Zone 8 Network D Attenuation drainage system is proposed to have a peak discharge at the equivalent greenfield Qbar rate of 8.6lps. The combined peak discharge into the river Fal A being 30lps.

Greenfield runoff for drained area ( ICP SUDS method (lps))

Equivalent Gross Drained Impermeable area Drainage Area for with Green area runoff 100 yr 30 yr 1 yr Qbar Zone highway factored at 0.3 x A drainage (Ha) (Ha)

Zone D8-A 5.822 2.596 51.8 40.8 16.7 21.4

Zone D8-D 2.327 0.991 20.7 16.3 6.7 8.6

A&D 8.149 3.587 72.5 57.1 23.4 30

Table 8.7

8.13.7 The modelling indicates the D8 network A basin will require a total of 1,800m3 of attenuation storage to achieve the design discharge rate, for the 100 year+40% storm event. It is proposed to achieve this storage using two basins with approximately 900m3 of storage each to better fit existing site levels.

8.13.8 The modelling indicates the D8 network D basin will require a total of 675m3 of attenuation storage to achieve the design discharge rate, for the 100 year+40% storm event.

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8.13.9 Network A is predominantly in cut embankment the dominant method of surface water collection on the main alignment will be via ‘over the edge’ drainage to a verge situated grass lined V channel with 1:5 side slopes. The collection V channel having regular grated outfalls to the filter drain (or carrier depending on cut/fill location) situated below the channel. The edge of carriageway details being in accordance with guidance provided in HD33/06.

8.13.10 Network D will is typically upon a fill embankment. Again it is proposed that ‘over the edge’ could be utilised to drain the carriageway but in this instance it is proposed that like HCD Drawing B13 the flows go directly overbank. This adds an extra step to the SuDS treatment train as the bank is hoped to act similar to a filter strip. Swales located at the base of the embankments will convey flows onto the attenuation basin. Typically these swales will have a 0.3m depth, 1:4 side slopes and a 0.5m base width. This overbank drainage solution is subject to the forthcoming GDR confirming the 1:3 embankment slope will have suitable soil characteristics to accept overbank flow as outlined in HD33/06.

8.13.11 Networks B and C include relatively short arms of the Harmony Roundabout junction. An infiltration test on the main alignment (TP115) suggests an infiltration solution is possible at these locations as it provided an infiltration rate of 1.84x10-4 m/s. Detailed design will need to confirm the rate at the location of the finalised infiltration system. Modular infiltration systems have been sized for each network as shown in the table below. It is understood detailed design can review other options for the infiltration design solutions however a modular soakaway is currently proposed due to its limited land take requirement.

Equivalent Gross Drained Impermeable Cellular Closest Drainage Area for area with Green Design Trial pit infiltration FOS Trial Zone highway area runoff storm rate m/s tank pit drainage (Ha) factored at 0.3 x A (Ha)

Zone D8 15m x 5m x 0.264 0.163 100yr +40CC 10 TP115 1.84X10-4 Network B 1.2m

Zone D8 15m x 5m x 0.199 0.17 100yr +40CC 10 TP115 1.84X10-4 Network C 1.2m

Table 8.8

8.13.12 Farm access tracks are included within the accommodation works within the scheme to ensure access to fields to the west and east of the northern underbridge. It is intended that these tracks will be of permeable/porous construction.

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8.13.13 Surfaces shall consist of either unbound material or porous concretes where necessary, with a permeable graded subbase. Allowing flows to enter at source. In addition the tracks are typically 4m in width and will be crossfalled to induce a short flow path to encourage any over the edge drainage into enter a liner filter channel at/near to source. The filter channel will be adjacent to the downstream side of the track and will provides storage, with a secondary benefit of additional limited infiltration.

8.13.14 The porous surface and linear channel are intended to ensure runoff rates due to installation of the tracks are not increased as rainfall will be managed at source mimicking the existing situation.

8.13.15 Design information provided above is to demonstrate that a suitable system in accordance with current guidance could be installed within the carriageway design. Details are subject to change following design detailed design. The finalised strategy will be subject to technical approval when discharging any associated surface water management planning condition.

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8.14 Drainage Zone D9

8.14.1 Drainage Zone 9 serves ~650m of the main alignment to the north of Drainage zone 8. It includes the proposed NW and SE arms of the Tregoss roundabout. The drainage network takes flows from the main alignment in a northerly direction to an attenuation basin located in a field located just to the north of the Old A30 road. The basin is intended to discharge to a minor tributary of the Upper river Ruthern located on the fields eastern boundary .

8.14.2 The drainage system is proposed to drain an area of ~2.610Ha. With permeable area runoff factored for 30% SPR (verges, cuttings etc) the equivalent impermeable area drained is taken as 1.749Ha.

Zone D9

Figure 8.9 – Highway Drainage Zone D9

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8.14.3 The Greenfield runoff for the drained area was assessed using the ICP SUDS method (lps) for the gross drained area to define appropriate discharge rates.

Greenfield runoff for drained area ( ICP SUDS method (lps)) Equivalent Gross Drained Impermeable area Drainage Area for with Green area 100 yr 30 yr 1 yr Qbar Zone highway runoff factored at 0.3 drainage (Ha) x A (Ha) Zone D9 2.61 1.749 24.2 19 7.8 10 Table 8.9

8.14.4 The zone D9 drainage system is proposed to have a peak discharge at the Equivalent Greenfield Qbar rate of 10lps.

8.14.5 The modelling indicates the D9 network attenuation basin will require a ~1400m3 of attenuation storage to achieve the design discharge rate, for the 100 year+40% storm event. The field proposed has natural levels that assist in creating a basin that can discharge into the shallow depth watercourse.

8.14.6 Zone D9 will is typically upon a fill embankment. It is proposed that ‘over the edge’ could be utilised to drain the carriageway but in this instance it is proposed that like HCD Drawing B13 the flows go directly overbank. This adds an extra step to the SUDS treatment train as the bank is hoped to act similar to a filter strip. Swales located at the base of the embankments will convey flows onto the attenuation basin.

8.14.7 Typically these swales will have a 0.3m depth, 1:4 side slopes and a 0.5m base width. This overbank drainage solution is subject to the forthcoming GDR confirming the 1:3 embankment slope will have suitable soil characteristics to accept overbank flow as outlined in HD33/06. If soil characteristics are deemed not suitable it is intended flows will be intercepted at the carriageway edge and discharge via piped outfall to the swales.

8.14.8 The southern side of the main alignment south of Tregoss is close to grade in cut. The surface water collection will be via ‘over the edge’ drainage to a verge situated grass lined V channel with 1:5 side slopes. The collection V channel having regular grated outfalls to the filter drain situated below the channel. The edge of carriageway details being in accordance with guidance provided in HD33/06. This system is able to discharge to the swale system north of the Tregoss roundbout.

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8.14.9 A basin was originally proposed to the south of Tregoss roundabout to serve the Highway drainage system. Through design development this basin was deemed not required to achieve the design attenuation rates. However offsite greenfield runoff has the potential to be enclosed in the area. The basin has therefore been repurposed as an additional measure to manage greenfield offsite greenfield runoff rates, providing additional storage, habitat creation and some degree of infiltration. It is intended to be able to discharge to the adjacent fields drainage systems.

8.14.10 Design information provided above is to demonstrate that a suitable system in accordance with current guidance could be installed within the carriageway design. Details are subject to change following design optimisation during detailed design. The finalised strategy will be subject to technical approval when discharging any associated Surface water management planning condition.

8.15 Offsite Interceptor Drainage

8.15.1 The development has the potential to affect off site drainage routes by intersecting existing catchment areas. Given the development site is high within the catchment close to catchment boundary’s the impact is considered to be manageable utilising interceptor drainage ditches as the upstream catchments are relatively small.

8.15.2 A conceptual layout for interceptor drainage ditches has been considered which will be further developed at detailed design when offsite field boundaries and landscaping layouts are fully developed.

8.15.3 The principle is that edge of field drainage ditches will convey flows to the nearest watercourse or culvert crossing. It is understood that new hedging in the region of 10,000m in length will be installed. Typically at upstream locations field drainage will also be installed adjacent to the hedge line to manage greenfield runoff and direct it to the appropriate watercourse. The runoff is at greenfield rate so additional attenuation is not considered required.

8.15.4 Imerys sites to the south of the scheme have significant surface water drainage management systems in place. Further development of the design will need to ensure the existing drainage systems are not negatively affected by development of the scheme. This appears entirely manageable at preset. A detailed strategy for offsite drainage provisions is expected to be submitted when proposals are finalised for discharge of the expected surface water planning condition.

8.15.5 Offsite greenfield runoff rates have been provisionally assessed using the ICP SUDS method. Drawing 0718_CSL_HDG_00MZ_DR_CD_0016 shows 13 areas upstream of the design and the corresponding Greenfield estimates which are also detailed in the table 8.10 below.

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8.15.6 Drawing 0718_CSL_HDG_00MZ_DR_CD_0016 also indicates indicative locations and flow directions for the interceptor drainage network. The drawing shows locations where the interceptor drains can join the existing watercourse network

Runoff Assessment for Intercepted Catchments using ICP SUDS method (lps)) SAARm Rural runoff Ref: 100 yr 30yr 1yr Qbar Area m2 ST m Area 1 58.3 45.9 18.8 24.1 64099 0.3 1300 Area2 103.3 81.3 33.3 42.7 113481 0.3 1300 Area 3 218.1 171.8 70.3 90.1 239712 0.3 1300 Area 4 199.1 156.8 64.2 82.3 218804 0.3 1300 Area 5 133.9 105.5 43.2 55.3 147147 0.3 1300 Area 6 112.8 88.9 36.4 46.6 127955 0.3 1267 Area 7 71.7 56.5 23.1 29.6 81165 0.3 1267 Area 8 33.1 26.1 10.7 13.7 36423 0.3 1300 Area 9 75.4 59.4 24.3 31.2 81151 0.3 1324 Area 10 478.2 376.7 154.1 197.6 516196 0.3 1324 Area 11 218.2 171.9 70.3 90.2 241290 0.3 1293 Area 12 544.5 386.9 145.9 170.7 149840 0.5 1300 Area 13 225.5 177.6 72.7 93.2 81786 0.5 1300 Table 8.9 Greenfield runoff for off site upstream catchments .

8.15.7 The interceptor drainage is expected to be largely edge of field drainage ditches. CC HCD Drawing 5/1 provides a typical ditch detail. Greenfield flows will occasionally need to be culverted under side roads tracks or main carriageway.

8.15.8 Referring to pipe capacity tables a 0.5m Ø pipe at 1:100 slope has a capacity of 426 lps. Referring to table 8.9, indicates a 0.5m pipe will typically be more than sufficient to convey the expected Greenfield flows at intermediate crossing points.

8.15.9 The higher greenfield rates for Area 10, are related to the fluvial catchment for the existing watercourse (carbis stream) which utilises the Hensbarrow crossing previously discussed in section 6.

8.15.10 The higher greenfield rates in Area 12 fall within an area on an Imerys site which is served by the Imerys site drainage system. It was noted upon a site walkover that haul roads and their associated drainage channels intercepted much of this catchment and directed it into a significant pit excavation/waterbody and as such remains onsite.

8.15.11 Offsite Interceptor drainage will need to be further considered at detailed design. At this stage of the project the management of offset runoff appears entirely manageable within the scope of the scheme.

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8.16 Water Management Strategy Conclusion

8.16.1 The Drainage strategy outlined above demonstrates that the surface water runoff from the proposed development can be suitably managed within the scheme extents without increasing flood risk elsewhere.

8.16.2 Where possible the strategy has proposed measures beyond the minimum requirements outlined by policy and the current design guidance. Soft SUDS features have been included where possible to enhance water quality standards.

8.16.3 Secondary benefits from partial infiltration have not been taken into account during sizing of the scheme drainage proposals, but are likely to provide added attenuation and water quality benefits.

8.16.4 Landscape designers and ecologists are reviewing the basin locations to soften features such as the basin outlines and include biodiversity features, so they sympathetically blend in with the surroundings whilst maintaining there primary function for water storage. The landscaping details will be developed in further in detailed design.

8.16.5 The details within this report are to define strategy only, to demonstrate a suitable drainage scheme can be implemented to manage surface water runoff within the scheme proposals. The calculations and analysis undertaken should not be relied upon for detailed design proposals without further more detailed analysis including audit of assumptions and baseline design data. It is expected that detailed design will provide betterment to the current proposals

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