Hertsmere Borough Council Strategic Flood Risk Assessment Level I Hertsmere Borough Council May 2018 Level 1 Strategic Flood Risk Assessment Update Hertsmere Borough Council Quality information Prepared by Checked by Approved by Luis Martin Ipek Kaymak / Gordon Boote Fida Choudhury Engineer Senior Engineer Project Director Revision History Revision Revision date Details Authorized Name Position 1 31.08.2017 Draft FC Fida Choudhury Project Director 2 29.01.2018 Final for review FC Fida Choudhury Project Director 3 26.02.2018 Final FC Fida Choudhury Project Director 4 12.04.2018 Comments included FC Fida Choudhury Project Director from EA, HCC and HBC 5 23.05.2018 Comments included FC Fida Choudhury Project Director from HBC 6 04.06.2018 Minor edits to Table FC Fida Choudhury Project Director 1.2 Distribution List # Hard Copies PDF Required Association / Company Name - 1 Hertsmere Borough Council, Hertfordshire County Council, Environment Agency Prepared for: Hertsmere Borough Council AECOM Level 1 Strategic Flood Risk Assessment Update Hertsmere Borough Council Prepared for: Hertsmere Borough Council Civic Offices, Elstree Way, Borehamwood WD6 1WA Prepared by: Luis Martin Engineer AECOM Limited AECOM House 63-77 Victoria Street St Albans Hertfordshire AL1 3ER UK T: +44(0)1727 535000 aecom.com © 2018 AECOM Limited. 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Prepared for: Hertsmere Borough Council AECOM Level 1 Strategic Flood Risk Assessment Update Hertsmere Borough Council Contents 1 Introduction and Background................................................................................................................. 1 1.1 Terms of Reference ..................................................................................................................... 1 1.2 Project Background .................................................................................................................... 1 1.3 Approach to Flood Risk Management .......................................................................................... 1 1.4 Partner Organisations ................................................................................................................. 2 1.5 Level 1 SFRA Approach .............................................................................................................. 4 2 Legislative and Planning Policy Context ................................................................................................. 5 2.1 Introduction ................................................................................................................................ 5 2.2 Flood and Water Management Act .............................................................................................. 5 2.3 Flood Risk Regulations ................................................................................................................ 6 2.4 Thames Catchment Flood Management Plan .............................................................................. 7 2.5 National Planning Policy Framework ............................................................................................ 8 2.6 Local Planning Policy .................................................................................................................. 8 2.7 Water Cycle Strategy ................................................................................................................ 10 2.8 National Receptor Dataset ........................................................................................................ 10 2.9 Summary .................................................................................................................................. 10 3 Assessing Flood Risk ........................................................................................................................... 12 3.1 Introduction .............................................................................................................................. 12 3.2 Study Area ................................................................................................................................ 12 3.3 Summary of Flood Sources ....................................................................................................... 13 3.4 Flooding from Rivers ................................................................................................................. 13 3.5 Flooding from Surface Water .................................................................................................... 18 3.6 Flooding from Groundwater ...................................................................................................... 21 3.7 Flooding from Sewers ............................................................................................................... 22 3.8 Reservoirs, Canals and Other Artificial Sources ......................................................................... 23 4 Avoiding Flood Risk – Applying the Sequential Test .............................................................................. 24 4.1 Sequential Approach ................................................................................................................ 24 4.2 Applying the Sequential Test – Plan-Making .............................................................................. 24 4.3 Applying the Sequential Test – Individual Applications ............................................................... 28 4.4 Exception Test .......................................................................................................................... 29 5 Managing and Mitigating Flood Risk ..................................................................................................... 30 5.1 Overview .................................................................................................................................. 30 5.2 Development Layout and Sequential Approach ......................................................................... 30 5.3 Riverside Development (Main Rivers and Ordinary Watercourses) ............................................. 30 5.4 Floodplain Compensation Storage ............................................................................................ 31 5.5 Finished Floor Levels ................................................................................................................ 32 5.6 Flood Resistance ‘Water Exclusion Strategy’ ............................................................................. 32 5.7 Flood Resilience ‘Water Entry Strategy’ ..................................................................................... 34 5.8 Structures ................................................................................................................................ 35 5.9 Safe Access and Egress............................................................................................................ 35 5.10 Safe Refuge .............................................................................................................................. 36 5.11 Car Parks .................................................................................................................................. 36 5.12 Flood Routing ........................................................................................................................... 36 5.13 Flood Warning and Evacuation Plans ......................................................................................... 37 6 Guidance for the Application of Sustainable Drainage Systems (SuDS) ................................................ 38 6.1 What are SuDS? ........................................................................................................................ 38 6.2 Management Train .................................................................................................................... 40 6.3 SuDS Costs .............................................................................................................................. 40 6.4 Infiltration SuDS Specific to Hertsmere ..................................................................................... 43 6.5 What is the role of the HCC? ..................................................................................................... 44 Prepared for: Hertsmere Borough Council AECOM Level 1 Strategic Flood Risk Assessment Update Hertsmere Borough Council 7 Guidance for preparing site-specific FRAs ........................................................................................... 47 7.1 What is a Flood Risk Assessment? ............................................................................................ 47 7.2 When is a Flood Risk Assessment required? .............................................................................. 47 7.3 How detailed should a FRA be? ................................................................................................. 47 7.4 What needs to be addressed in a Flood