Hart District Council Strategic Flood Risk Assessment December 2016
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Hart District Council Strategic Flood Risk Assessment December 2016 Hart District Council Strategic Flood Risk Assessment December 2016 Contents 1. Introduction 7 2. Study Area 9 2.1 Main Urban Areas 10 2.2 Infrastructure 10 2.3 Hydrology 11 2.4 Regional Geology 11 2.5 Topography 12 3. Policy and Local Context 14 3.1 National Policy 14 3.2 Local Context 15 4. SFRA Methodology 18 4.1 Description 18 4.2 Data Collection/processing 18 4.3 Stakeholders 18 4.4 Need for a Level 2 SFRA 20 5. Flooding From Rivers 21 5.1 Description 21 5.2 Data Collection 21 5.3 Historical Fluvial Flood Events 21 5.4 Assessing Flooding From Rivers 25 5.5 Discussion of Fluvial Flooding in Hart 27 5.6 Management of Fluvial Flooding in Hart 32 5.7 Planning Considerations 32 6. Flooding From Surface Water 33 6.1 Description 33 6.2 Data Collection 36 6.3 Historical Surface Water Flood Events 36 6.4 Assessing Flooding From Surface Water 38 6.5 Discussion of Surface Water Flooding in Hart 39 6.6 Planning Considerations 40 7. Flooding From Sewers 41 7.1 Description 41 7.2 Data Collection 43 7.3 Historic Sewer Flooding 43 7.4 Discussion of Sewer Flooding in Hart 49 8. Flooding From Groundwater 50 8.1 Description 50 8.2 Causes of high groundwater levels 50 Page intentionally blank 52 8.3 Impacts of groundwater flooding 53 8.4 Topography, geology and groundwater flooding 54 8.5 Data Collection 54 8.6 Assessing Flooding From Groundwater 55 8.7 Discussion of Groundwater Flooding in Hart 56 8.8 Climate Change 58 8.9 Management of Groundwater Flooding in Hart 58 8.10 Planning Considerations 58 9. Flooding from Artificial Sources 60 9.1 Description 60 9.2 Discussion of Flooding From Reservoirs in Hart 60 9.3 Discussion of Flooding From Canals in Hart 62 9.4 Management of Flooding From Artificial Sources in Hart 64 9.5 Planning Considerations 64 Hart District Council Strategic Flood Risk Assessment December 2016 10. Indicative Flood Problem Areas 66 10.1 Description 66 10.2 Data Collection 66 11. Causal Areas 73 11.1 Description of Causal Areas 73 11.2 Data Collection 73 11.3 Planning Considerations 77 12. Climate Change Allowances 78 12.1 Description 78 12.2 Environment Agency Best Practice Guidance 78 12.3 Applying Appropriate Climate Change Allowances in Hart 79 12.4 Determining Climate Change Allowances 79 12.5 Assessing the Impact of Climate Change Allowances in Hart 82 12.6 The Impact of Climate Change on Existing Development 86 13. Guidance on application of the Sequential and Exception Tests 88 13.1 Sequential Test 88 13.2 Exception Test 93 14. Site Specific FRA Guidance 96 14.1 Managing Flooding In New Development 96 14.2 Managing surface water runoff from new developments 97 14.3 The Sustainable Drainage System (SuDS) Approach 99 14.4 Flood Resistant and Resilient Design 107 14.5 Flood Hazard 108 14.6 Dry Islands 109 15. Emergency Planning & Flood Warning 111 15.1 Introduction 111 15.2 Emergency Planning 111 15.3 Flood Warning 111 15.4 Flood Warning Areas 113 15.5 Flood Alert Areas 113 15.6 Using Emergency Flood Plans in Planning Applications 115 16. Defences and Asset Management 116 16.1 Introduction 116 16.2 Defences 116 16.3 Maintenance 117 16.4 Works in or near a watercourse 118 17. Summary and Recommendations 119 17.1 Site Allocation Process 119 17.2 Council Policy 119 17.3 Emergency Planning 121 17.4 Future Updates of the SFRA 121 18. Glossary 123 19. Appendix 1 125 Tables Table 0.1 Responsibilities managing flood risk within Hart 4 Table 0.2 Statutory and/or non-statutory planning consultees for Flood Risk Issues (Source: The Town & Country Planning (Development Management Procedure) (England) Order 2015 – Schedule 4) 5 Table 3.1 National Policies and guidance relevant to Hart and SFRAs 14 Table 3.2 Local level policy and guidance relevant to the SFRA 15 Table 3.3 Responsibilities for managing flood risk in Hart 16 Table 4.1 Key datasets collated for analysis 19 Table 5.1 Historic Flood events in Hart as captured in the previous SFRA 24 Table 5.2 Definitions of Flood Zones (Table 1, NPPG) 25 Table 5.3 SFRA fluvial flood zone mapping sources 30 Hart District Council Strategic Flood Risk Assessment December 2016 Table 5.4 Factors that increase fluvial flood risk in Hart 31 Table 6.1 General factors that increase surface water flood risk 34 Table 7.1 Causes of flooding from sewers 42 Table 8.1 Causes of high groundwater levels 51 Table 8.2 BGS susceptibility to groundwater flooding classifications 56 Table 9.1 Properties at risk from reservoir failure 60 Table 10.1 Performance of the uFMfSW against historic data 67 Table 10.2 Performance of the FMfSW against historic data 67 Table 11.1 Surface water flood risk in Hart (based on FMfSW) 73 Table 11.2 Groundwater flood risk in Hart (based on BGS susceptibility to groundwater flooding) 75 Table 11.3 Fluvial risk based on the Environment Agency’s Flood Map for Planning 76 Table 12.1 Climate change allowances as per development vulnerability and flood zone 80 Table 12.2 Peak river flow allowances by river basin district (1961-1990 baseline) 80 Table 12.3 Peak rainfall intensities in small and urban catchments (1961- 1990 baseline) 81 Table 12.4 High ++ for peak river flows 81 Table 12.5 Indicative guides to an assessment approach for climate change 85 Table 12.6 Comparison of properties within ‘current’ and ‘future’ Flood Zone 3 86 Table 13.1 Flood Risk Vulnerability Classifications (NPPG, 2014) 90 Table 13.2 Flood Risk Vulnerability Classifications and Flood Zone ‘Compatibility’ (NPPG, 2014) 91 Table 14.1 Summary of SuDS Techniques and their Suitability to meet the three goals of Sustainable Drainage 101 Table 14.2 Drainage summary map classifications 105 Table 14.3 Flood Hazard (source Table 13.1 of FD2320/TR2- extended version) 109 Table 15.1 Environment Agency Flood Warnings 112 Table 15.2 River monitoring gauges in Hart 113 Table 16.1 Medium Term Plan 117 Table 17.1 Key policy recommendations 120 Figures Figure 1.1 Taking flood risk into account in Local Plan preparation (NPPG, 2014) 8 Figure 2.1 Study Area 9 Figure 2.2 Topography of Hart 13 Figure 5.1 Historic fluvial flood records across Hart 22 Figure 5.2 Fluvial flooding across Hart by flood event 23 Figure 5.3 Properties across Hart that fall within Flood Zone 3 28 Figure 5.4 Environment Agency Flood Maps for Planning 29 Figure 6.1 Factors that influence surface water flooding as per the source- pathway-receptor model 33 Figure 6.2 Occurrences of surface water flooding across Hart 37 Figure 6.3 Surface water flooding across Hart by flood event 38 Figure 6.4 Properties at risk of surface water flooding according to the Flood Map for Surface Water 39 Figure 7.1 Internal sewer flooding incidents in Hart 44 Figure 7.2 External sewer flooding incidents in Hart 45 Figure 7.3 Internal sewer flooding by postcode area 46 Figure 7.4 External sewer flooding by postcode area 47 Hart District Council Strategic Flood Risk Assessment December 2016 Figure 7.5 Foul flooding incidents reported to Hart District Council 48 Figure 8.1 Winter 2013/2014 reported groundwater flooding incidents 55 Figure 8.2 Areas susceptible to groundwater flooding 57 Figure 9.1 Key features of Fleet Pond Reservoir 62 Figure 9.2 Raised embankments along the Basingstoke Canal 63 Figure 10.1 Properties located in the uFMfSW and FMfSW 1 in 30 flood extents 68 Figure 10.2 Properties in the FMfSW 1 in 200 extent vs. the uFMfSW 1 in 1000 extent 69 Figure 10.3 Mapped historic flood records against the FMfSW 70 Figure 10.4 Mapped historic flood records against the uFMfSW 70 Figure 12.1 Comparison of the 1 in 100 peak flow adjusted for each climate change allowance with the existing 1 in 1000 modelled flow 83 Figure 13.1 Sequential Test Flow Chart 89 Figure 13.2 Application of the Exception Test for Local Plan Preparation (NPPG Flood Risk and Coastal Change) 95 Figure 14.1 Infiltration SuDS suitability 106 Figure 14.2 Rationale for design strategies, improving flood performance of new buildings: Flood Resilient Construction, CLG 2007 108 Figure 14.3 A dry island in Fleet 110 Figure 15.1 Direct river alarm and groundwater alerts at Crondall 114 Prepared By: Susanna Hope (Flood Risk Infrastructure Engineer, Hart District Council) Amanda Dunn Principal Policy Planner (East Hampshire District Council on behalf of Hart District Council) Hart District Council Strategic Flood Risk Assessment December 2016 Executive Summary Introduction This report is a Level 1 Strategic Flood Risk Assessment (SFRA) carried out by Hart District Council (HDC). The National Planning Policy Framework (NPPF) and associated National Planning Practice Guidance (NPPG) for Flood Risk and Coastal Change emphasise the active role Local Planning Authorities such as HDC should take to ensure that flood risk is understood and managed effectively and sustainably throughout all stages of the planning process. The NPPF outlines that Local Plans should be supported by a SFRA and Local Planning Authorities should use the findings to inform strategic land use planning. The purpose of a Level 1 SFRA is to collate and analyse the most up to date flood risk information for use by HDC to inform the preparation of robust planning documents as part of the upcoming HDC Local Plan.