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