Bristol City Council South Gloucestershire Council Lower Severn Drainage Board Strategic Flood Risk Assessment Avonmouth / Severnside TECHNICAL REPORT FINAL February 2011 successful people successful projects successful performance Bristol City Council Strategic Flood Risk Assessment South Gloucestershire Council Technical Report FINAL Lower Severn Drainage Board March 2011 Quays Office Park Conference Avenue Portishead Bristol BS20 7LZ Tel: 01275 840 840 Fax: 01275 840 830 Project manager: Claire Storer www.capitasymonds.co.uk Capita Symonds Management System Job Number: CS043163 PD: RG PM: CS Report status: FINAL v4 Date of issue: March 2011 Main author(s): CS, LA, JT, AM Checked: CS Approved: CS i Bristol City Council Strategic Flood Risk Assessment South Gloucestershire Council Technical Report FINAL Lower Severn Drainage Board March 2011 Contents ABOUT THIS DOCUMENT......................................................................................... XI GLOSSARY AND NOTATION................................................................................... XII SECTION 1. INTRODUCTION ...............................................................................1-1 1.1. Background...................................................................................................1-1 1.2. Scope of Work ..............................................................................................1-2 1.3. Overall Methodology.....................................................................................1-2 Section 1 Appendices......................................................................................................1-11 A1.1 Specification............................................................................................1-13 SECTION 2. DATA COLLECTION, SITE VISITS AND REVIEW ..........................2-1 2.1. Revision Status.............................................................................................2-1 2.2. Data Collection .............................................................................................2-1 2.3. Site Visits......................................................................................................2-2 2.4. Literature Review..........................................................................................2-2 Mapping and Figures.........................................................................................................2-5 Section 2 Appendices........................................................................................................2-9 A2.1 Data Collection........................................................................................2-11 A2.2 Literature Review ....................................................................................2-19 SECTION 3. HYDROLOGY, DRAINAGE AND GROUNDWATER........................3-1 3.1. Revision Status.............................................................................................3-1 3.2. Hydrological Assessment .............................................................................3-1 3.2.1 Introduction ...............................................................................................3-1 3.2.2 Catchment Boundaries and Flow Nodes...................................................3-1 3.2.3 Catchment Description..............................................................................3-3 3.2.4 Hydrometric Monitoring.............................................................................3-4 3.2.5 Description of the Hydrological Model.......................................................3-5 3.2.6 QMED Derivation ......................................................................................3-6 3.2.7 Pooling Group Analysis and Growth Curve Derivation .............................3-8 3.2.8 Final Statistical Method Flows...................................................................3-9 3.2.9 Limitations...............................................................................................3-10 3.2.10 The Effects of Climate Change ...............................................................3-10 3.2.11 Comparison with BTP Flows Routed Through Hydraulic Model .............3-12 3.3. Drainage Assessment.................................................................................3-13 3.3.1 Introduction .............................................................................................3-13 3.3.2 BTP InfoWorks Drainage Model Data Transfer for TUFLOW .................3-13 3.4. Groundwater Assessment ..........................................................................3-14 Mapping and Figures.......................................................................................................3-15 Section 3 Appendices......................................................................................................3-19 A3.1 Hydrology Study......................................................................................3-21 A3.2 Review of Bettridge Turner and Partners (BTP) Modelling Reports .......3-23 SECTION 4. FLOOD DEFENCE ASSESSMENT ..................................................4-1 4.1. Revision Status.............................................................................................4-1 4.2. Introduction to Flood Defence Assessment..................................................4-1 4.3. Site Description in regard to Flood Defence Assessment ............................4-2 ii Bristol City Council Strategic Flood Risk Assessment South Gloucestershire Council Technical Report FINAL Lower Severn Drainage Board March 2011 4.3.2 Geology & Hydrogeology ..........................................................................4-2 Flood Defence Inspections ................................................................................................4-3 4.4. Summary of Tidal Defence Structures..........................................................4-4 4.4.2 Types of Structure.....................................................................................4-5 4.5. Flood Defence Assessment Results.............................................................4-7 4.5.1 Proposed Approach to Flood Defence Assessment .................................4-7 4.5.2 Breach Assessments ................................................................................4-8 4.5.3 Summary of Findings of Flood Defence Assessment .............................4-12 4.6. Flood Defence Engineering Assessment....................................................4-15 4.6.1 Scope and Objectives .............................................................................4-15 4.6.2 Hydraulic Modelling.................................................................................4-16 4.6.3 Conceptual Engineering Design Options and Cost Estimates................4-17 4.7. Funding Assessment ....................................................................................4-6 4.7.1 Potential for Developer Contributions .......................................................4-6 4.7.2 Potential Funding Regimes.....................................................................4-11 Implementation of the SMP / CFMP policies:...............................................................4-11 Grant in Aid: 4-12 Local Levies 4-12 Community Infrastructure Levy ....................................................................................4-12 4.7.3 Quick wins...............................................................................................4-13 Conclusions of Flood Defence Assessment ....................................................................4-15 4.7.4 Flood Defence Assessment ....................................................................4-15 4.7.5 Flood Defence Engineering Assessment................................................4-18 4.7.6 Areas for Further Investigation................................................................4-18 Mapping and Figures.......................................................................................................4-20 Section 4 Appendices......................................................................................................4-24 A4.1 Site Photography ....................................................................................4-26 A4.2 Cross Sections........................................................................................4-28 A4.3 Flood Defence Review 27 March 2005 Meeting Minutes........................4-30 SECTION 5. HYDRAULIC MODELLING...............................................................5-1 5.1. Revision Status.............................................................................................5-1 5.2. Previous Modelling Studies ..........................................................................5-1 5.3. Current Flood Mapping and Flood History....................................................5-1 5.4. Hydraulic Modelling of Study Area................................................................5-2 5.4.1 Introduction ...............................................................................................5-2 5.4.2 Type of Model Selection............................................................................5-2 5.4.3 Modelling Software Selection....................................................................5-2 5.5. Hydraulic Modelling of Study Area................................................................5-3 5.5.1 Development of 2D Model Domain ...........................................................5-3