HERTFORDSHIRE COUNTY COUNCIL LFRMS 2 2019 - 2029 A STRATEGY FOR THE MANAGEMENT OF LOCAL SOURCES OF FLOOD RISK Table of Contents Glossary ..................................................................................................................... 6 Overview .................................................................................................................. 12 1. Why is a strategy needed? ................................................................................ 14 2. Understanding Local Flood Risk ........................................................................ 18 2.1. Surface Water Flooding .............................................................................. 23 2.2. Fluvial Flooding ........................................................................................... 29 2.3. Groundwater Flooding ................................................................................. 35 2.4. Sewer Flooding ........................................................................................... 44 2.5. Flooding from other sources ....................................................................... 44 2.6. Climate change ........................................................................................... 45 3. Who’s Involved in Managing Flood Risk? .......................................................... 47 3.1. Risk Management Authorities ..................................................................... 47 3.2. Other Key Stakeholders .............................................................................. 48 3.2.1. Regional Flood and Coastal Committees (RFCC) .......................................... 49 3.2.2. Hertfordshire Resilience ................................................................................. 49 3.2.3. Hertfordshire Local Enterprise Partnership (LEP) .......................................... 49 4. Principles for Flood Risk Management in Hertfordshire ..................................... 52 4.1. Principle 1: Taking a risk-based approach to local flood risk management . 52 4.2. Principle 2 Working in partnership to manage flood risk in the county ........ 52 4.2.1. The needs and benefits of partnership working .............................................. 52 4.2.2. Review of LFRMS1 Partnership Working ....................................................... 54 4.2.3. Multi-benefit projects ...................................................................................... 54 4.2.4. Proposals for the period covered by LFRMS2 ................................................ 56 4.3. Principle 3: Improving our understanding of flood risk to better inform decision making .................................................................................................... 58 4.3.1. Risk of Flooding from Surface Water (RoFfSW) mapping .............................. 58 4.3.2. Groundwater .................................................................................................. 70 4.4. Principle 4: Supporting those at risk of flooding to manage that risk ........... 72 4.4.1. Impacts on people ......................................................................................... 72 4.4.2. Immediate risks to health. .............................................................................. 72 4.4.3. Secondary risks to health ............................................................................... 73 4.4.4. Longer-term risks ........................................................................................... 73 4.4.5. Groups vulnerable to the health risks posed by flooding ................................ 73 Adopted 18 February 2019 1 4.4.6. Mitigating health risks from floods .................................................................. 74 4.4.7. Resilience and Response .............................................................................. 74 4.4.8. Investigations ................................................................................................. 80 4.5. Principle 5: Working to reduce the likelihood of flooding where possible .... 86 4.5.1. Surface Water Management Plans ................................................................ 86 4.5.2. Ordinary Watercourses .................................................................................. 93 4.5.3. Asset register ................................................................................................. 97 4.5.4. Small projects fund ...................................................................................... 101 4.5.5. New flood risk management schemes ......................................................... 103 4.5.6. Prioritising investment .................................................................................. 107 4.6. Principle 6: Ensuring that flood risk arising from new development is managed appropriately ....................................................................................... 113 4.6.1. Run-off Destination (disposal hierarchy) ...................................................... 115 4.6.2. Peak Flow and Volume Control – Greenfield Sites ....................................... 116 4.6.3. Peak Flow and Volume Control - Previously developed sites ....................... 116 4.6.4. Flood Risk Within & Outside the Development ............................................. 117 4.6.5. Managing Overland Flow Routes ................................................................. 117 4.6.6. Maximise Resilience and Source Control ..................................................... 118 4.6.7. Management of drainage during construction period .................................... 118 4.6.8. Maintenance, Structural Integrity & Construction ......................................... 119 4.6.9. Sustainability and additional design criteria .................................................. 119 4.6.10. Development and Watercourses .............................................................. 120 5. List of Policies .................................................................................................. 122 6. List of Actions .................................................................................................. 122 7. Indicative Work Programme ............................................................................ 123 7.1. Local Flood Risk Management Strategy Indicative Work Programme 2019 - 2024 123 7.1.1. Ongoing and responsive activities ................................................................ 125 7.2. Resources associated with the work programme and strategy implementation ................................................................................................... 126 8. Monitoring and updating the strategy ............................................................... 130 Appendices ............................................................................................................ 131 A1. Appendix 1: Responsibilities of Risk Management Authorities ..................... 131 A1.1 Hertfordshire County Council ....................................................................... 131 A1.2 Environment Agency .................................................................................... 132 Adopted 18 February 2019 2 A1.3 District and Borough Councils ...................................................................... 133 A1.4 Internal Drainage Boards ............................................................................. 133 A1.5 Water Companies ........................................................................................ 133 A1.6 Highways England ....................................................................................... 134 A2. Appendix 2: Responsibilities of Other LFRMS Stakeholders ........................ 135 A2.1 Property owners and businesses ................................................................. 135 A2.2 Utility and Infrastructure Providers ............................................................... 135 A2.3 Parish Councils and Communities ............................................................... 136 A3. Appendix 3: Links to resilience information .................................................. 137 References ............................................................................................................. 139 Table of Figures Figure 1: Flood Risk Management Cycle ................................................................. 75 Figure 2: Rainfall Radar showing localised intense storm ........................................ 78 Figure 3: Section 19 Flood Investigation Flow Chart ................................................ 85 Figure 4: Illustrating completed Surface Water Management Plans in Hertfordshire 88 Figure 5: Remaining Surface Water Management Plans in Hertfordshire ................ 88 Figure 6: Prioritisation Decision Tree ..................................................................... 110 Table of Tables Table 1: Number of properties shown to be at risk in the RoFfSW map ................... 23 Table 2: Risk Management Authorities in Hertfordshire ........................................... 47 Table 3: Risk Management Authorities bordering Hertfordshire ............................... 48 Table 4: Key Local Flood Risk Management Strategy Infrastructure Stakeholders in Hertfordshire ............................................................................................................ 48 Table 5: Roles and Responsibilities in