5.6 Flood Risk Assessment Appendix N CAFRA Rlw Results 2019 Tidal.Xlsx”)
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Portishead Branch Line (MetroWest Phase 1) TR040011 Applicant: North Somerset District Council 5.6, Flood Risk Assessment, Part 1 of 17 The Infrastructure Planning (Applications: Prescribed Forms and Procedure) Regulations 2009, regulation 5(2)(e) Planning Act 2008 Author: CH2M Date: November 2019 1-1 The original submission version of this document can be found in Appendix 17.1 of the ES. The document contained within the ES will not be updated. However, this standalone version of this document may be updated and the latest version will be the final document for the purposes of the Order. 1-2 Notice © Copyright 2019 CH2M HILL United Kingdom. The concepts and information contained in this document are the property of CH2M HILL United Kingdom, a wholly owned subsidiary of Jacobs. Use or copying of this document in whole or in part without the written permission of Jacobs constitutes an infringement of copyright. Limitation: This document has been prepared on behalf of, and for the exclusive use of Jacobs’ client, and is subject to, and issued in accordance with, the provisions of the contract between Jacobs and the client. Jacobs accepts no liability or responsibility whatsoever for, or in respect of, any use of, or reliance upon, this document by any third party. Where any data supplied by the client or from other sources have been used, it has been assumed that the information is correct. No responsibility can be accepted by Jacobs for inaccuracies in the data supplied by any other party. The conclusions and recommendations in this report are based on the assumption that all relevant information has been supplied by those bodies from whom it was requested. Where field investigations have been carried out, these have been restricted to a level of detail required to achieve the stated objectives of the work. This work has been undertaken in accordance with the quality management system of Jacobs. Document history Project Portishead Branch Line (MetroWest Phase 1) Development Consent Order Scheme Planning Inspectorate Scheme TR040011 Reference Part and Application Document 5, 5.6 Reference Document title Flood Risk Assessment, Part 1 of 17 Regulation Number Regulation 5(2)(e) Applicant North Somerset District Council Lead Author RB at CH2M Version Date Status of Version Rev: 01 12/11/19 Application Issue i Table of Contents Section Page 1 Introduction ................................................................................................. 1-1 1.1 DCO Scheme Scope and Purpose .................................................... 1-1 1.2 The Need for a Flood Risk Assessment............................................. 1-1 1.3 Available Information ......................................................................... 1-2 1.4 Report Structure ................................................................................ 1-3 2 Background ................................................................................................. 2-1 2.1 Development location and description ............................................... 2-1 2.2 Regional overview of water features .................................................. 2-3 2.3 Surface Water Features and Drainage .............................................. 2-3 2.4 Other flood risk management plans, strategies and assessments ..... 2-3 3 Definition of flood hazard .......................................................................... 3-1 3.1 Sources of flooding and existing flood defences ................................ 3-1 3.2 Structures that may influence local hydraulics ................................... 3-6 3.3 Consequences of flooding on the site ................................................ 3-6 4 Probability of flooding ............................................................................... 4-1 4.1 Environment Agency flood maps ....................................................... 4-1 4.2 Hydraulic modelling results ................................................................ 4-3 4.3 Order, speed, and duration of flooding............................................. 4-24 5 Climate change ........................................................................................... 5-1 6 Consultation meetings ............................................................................... 6-1 6.2 Environment Agency meetings .......................................................... 6-1 6.3 North Somerset Levels Internal Drainage Board Meetings .............. 6-10 6.4 NSC, BCC and NSLIDB meeting on 24 October 2017 .................... 6-11 6.5 NSDC and NSLIDB meeting on 8 February 2018 ............................ 6-12 7 Development proposals ............................................................................. 7-1 7.1 The Nationally Significant Infrastructure Project ................................ 7-1 7.2 Associated Development ................................................................... 7-1 7.3 DCO Scheme drawings and description ............................................ 7-2 7.4 Flood Zone compatibility and Exception Test .................................... 7-2 8 Flood risk management ............................................................................. 8-1 8.1 Fluvial flood risk ................................................................................. 8-1 8.2 Coastal flood risk ............................................................................... 8-5 8.3 Surface water drainage ...................................................................... 8-6 8.4 Culverts and drainage ditches ........................................................... 8-8 8.5 Maintenance access to watercourses and structures ........................ 8-8 8.6 Portishead and Pill stations and carparks access and egress ........... 8-9 8.7 Outline Flood Plan (operational phase)............................................ 8-10 9 Off-site impacts .......................................................................................... 9-1 9.1 Impact of the DCO Scheme on flood risk elsewhere ......................... 9-1 9.2 Culverts .............................................................................................. 9-4 9.3 Surface water management ............................................................... 9-4 9.4 Displaced floodplain storage .............................................................. 9-5 10 Summary of mitigation measures ........................................................... 10-1 iii 11 Residual risks ........................................................................................... 11-1 12 Conclusions .............................................................................................. 12-1 13 References ................................................................................................ 13-1 Appendices A Not used B Environment Agency flood maps and modelled flood levels information C Bristol Avon Catchment Flood Management Plan and North and Mid Somerset Catchment Flood Management Plan summary reports D Environment Agency susceptibility to groundwater flood map E Environment Agency updated Flood Map for Surface Water Flooding (uFMSWF): Suitability map F Flood risk information provided by NSDC and BCC G Severn Estuary Shoreline Management Plan 2 Policy Units H Severn Estuary Flood Risk Management Strategy summary information I Not used J Environment Agency flood defences information provided as part of consultation K Hydraulic modelling of Drove Rhyne and Easton-in-Gordano Stream undertaken to inform this FRA L Flood Zone review M Further development of EA coastal modelling undertaken to inform this FRA N Further development of BCC CAFRA modelling undertaken to inform this FRA O Drainage design information P Environment Agency consultation response Q North Somerset Levels Internal Drainage Board consultation response R DCO application information S Design drawing cross sections with flood levels T Flood Plans List of Tables 2.1 CFMP Policy Units and Policies 2.2 NPPF Table 3 - Flood risk vulnerability and Flood Zone ‘compatibility’ 3.1 Details of existing flood defence data provided by the Environment Agency 4.1 Modelled maximum River Avon tidal flood depth relative to the lowest rail level of the DCO Scheme near Bower Ashton 4.2 Probabilities for number of flood events occurring in a 50 year period – assuming flooding occur once every 20 years on average iv 4.3 Probabilities for number of flood events occurring in a 50 year period – assuming flooding occur once every 50 years on average 4.4 Comparison of CFB 2018 and CFB 2011 EWLs, for 2017 (the CFB 2018 base year) 4.5 Modelled maximum River Avon flood depths relative to the lowest rail level at crossing of Longmoor and Colliter’s Brooks (fluvial events) 4.6 Modelled flood levels in Drove Rhyne tributary channels that are culverted through the railway embankment 4.7 Modelled fluvial flood levels in Easton-in-Gordano Stream 4.8 Modelled tidal flood levels in Easton-in-Gordano Stream 4.9 Review of Flood Zones – DCO Scheme proposed works within Flood Zone 3a and 3b 4.10 Lowest simulated return period with flooding of railway alignment 7.1 Summary of Works Required for the Portishead Branch Line (MetroWest Phase 1) DCO Scheme 7.2 Summary of the characteristics of the proposed construction compounds 8.2 Tidal River Avon - flood levels and volumes stored for the pre- and post- development cases near Bower Ashton 8.3 Drainage design criteria 8.4 Modelled coastal flood levels (mAOD) near Portishead station