NORTH HERTFORDSHIRE DISTRICT SURFACE WATER MANAGEMENT PLAN FINAL REPORT REPORT NO 70006808 - NH_SWMP_01 2017-03-14 NORTH HERTFORDSHIRE DISTRICT SURFACE WATER MANAGEMENT PLAN FINAL REPORT Hertfordshire County Council Final Project no: 70006808 Date: March 2017 – WSP | Parsons Brinckerhoff Unit 9 The Chase John Tate Road Hertford, SG13 7NN Tel: +44 01992 526000 Fax: +44 01992 526001 www.wsp-pb.co.uk QUALITY MANAGEMENT ISSUE/REVISION FIRST ISSUE REVISION 1 REVISION 2 REVISION 3 Remarks DRAFT Version 2 Version 3 Version 4 Date 22/06/16 28/09/16 23/12/16 14/03/17 Prepared by Sarah Foreman Sarah Foreman Rachel Bird Sam Curtis Signature Checked by James Berryman James Berryman James Berryman James Berryman Signature Authorised by Andy Smith Andy Smith Andy Smith Signature Project number 70006808 70006808 70006808 70006808 Report number V01 V02 V03 V04 File reference ii TABLE OF CONTENTS KEY DEFINITIONS ......................................................................................1 1 EXECUTIVE SUMMARY ..............................................................2 1.1 Background ...........................................................................................2 1.2 Identification of Hotspots ......................................................................2 1.3 Detailed Phase of SWMP .....................................................................3 1.4 Action plan ............................................................................................7 2 INTRODUCTION ...........................................................................8 2.1 Background ...........................................................................................8 2.2 Study Area ............................................................................................8 2.3 Surface Water Management Plans (SWMP) ..................................... 12 2.4 Stages of a SWMP ............................................................................ 12 3 WIDER POLICY AND LEGISLATIVE CONTEXT ....................... 15 3.1 Policy and Legislative History ............................................................ 15 3.2 Lead Local Flood Authority (LLFA) .................................................... 16 3.3 Other Planning Policies ..................................................................... 17 4 PREPARATION .......................................................................... 24 4.1 Identify the Need for a SWMP ........................................................... 24 4.2 Establish Partnership ......................................................................... 24 4.3 Scoping the SWMP Study ................................................................. 24 4.4 Policy Documents Reviewed ............................................................. 25 4.5 Collating Available Information .......................................................... 26 4.6 Quality, Limitations and Restrictions ................................................. 26 5 STRATEGIC AND INTERMEDIATE RISK ASSESSMENT ........ 27 5.1 Introduction ........................................................................................ 27 5.2 Flooding History ................................................................................. 28 5.3 Available Data .................................................................................... 30 5.4 Areas Identified at Significant Risk of Flooding ................................. 32 6 DETAILED RISK ASSESSMENT - APPROACH ....................... 34 6.1 Introduction ........................................................................................ 34 6.2 Data Collection .................................................................................. 34 6.3 Model Approach................................................................................. 36 North Hertfordshire District Surface Water Management Plan WSP | Parsons Brinckerhoff Hertfordshire County Council Project No 70006808 March 2017 iii 6.4 Mitigation Optioneering ...................................................................... 37 6.5 Economic Assessment ...................................................................... 38 7 DETAILED RISK ASSESSMENT - INVESTIGATION ................ 40 7.1 Definitions .......................................................................................... 40 7.2 Hotspot 6 - Hitchin ............................................................................. 40 7.4 Hotspot 7 – Oakfield .......................................................................... 46 7.5 Hotspot 12 - Baldock ......................................................................... 50 7.6 Hotspot 13 - Clothall Common, Baldock ........................................... 54 7.7 Hotspot 17 - Knebworth ..................................................................... 58 7.8 Hotspot 30 - Cambridge Road – A505 Hitchin .................................. 63 8 VIABILITY SUMMARY ............................................................... 67 9 FUNDING .................................................................................... 69 9.1 National Funding................................................................................ 69 9.2 Regional Funding............................................................................... 70 9.3 Local Funding .................................................................................... 70 9.4 Combination of Funding Sources ...................................................... 71 9.5 Funding Conclusions ......................................................................... 71 10 IMPLEMENTATION AND REVIEW ............................................ 72 10.1 Action Plan ......................................................................................... 72 10.2 Emergency Planning ......................................................................... 73 10.3 Next Steps ......................................................................................... 73 11 CONCLUSIONS .......................................................................... 75 T A B L E S TABLE 1: RECOMMENDATIONS AND ACTIONS FOR THE DISTRICT OF NORTH HERTFORDSHIRE – SWMP NON-MODELLED HOTSPOTS ........................ 33 TABLE 2: KEY COMPONENTS OF DETAILED ASSESSMENT (BASED ON TABLE 6-1 IN THE DEFRA GUIDANCE) ........................................................................... 34 TABLE 3: OPTION CRITERIA .......................................................................................... 37 TABLE 4: KEY FINDINGS – HOTSPOT 6 - HITCHIN - UPSTREAM ............................... 42 TABLE 5: KEY FINDINGS – HOTSPOT 6 - HITCHIN - CENTRAL .................................. 43 TABLE 6: KEY FINDINGS – HOTSPOT 6 - HITCHIN - DOWNSTREAM ........................ 44 TABLE 7: KEY FINDINGS – HOTSPOT 7 - OAKFIELD ................................................... 47 TABLE 8: KEY FINDINGS – HOTSPOT 12 - BALDOCK ................................................. 51 North Hertfordshire District Surface Water Management Plan WSP | Parsons Brinckerhoff Hertfordshire County Council Project No 70006808 March 2017 iv TABLE 9: KEY FINDINGS – HOTSPOT 13 - CLOTHALL COMMON .............................. 56 TABLE 10: KEY FINDINGS – HOTSPOT 17 - KNEBWORTH ......................................... 60 TABLE 11: SENSITIVITY FINDINGS – HOTSPOT 17 - KNEBWORTH .......................... 61 TABLE 12: KEY FINDINGS – HOTSPOT 30 - CAMBRIDGE ROAD, HITCHIN ............... 65 TABLE 13: NUMBER OF PROPERTIES AT RISK OF FLOODING ................................. 67 TABLE 14: BASELINE AND MITIGATION OPTIONS ECONOMIC DAMAGES .............. 67 TABLE 15: FURTHER ASSESSMENT PHASES ............................................................. 72 F I G U R E S FIGURE 1: A DIAGRAMMATIC SUMMARY OF THE KEY DEFINITIONS ........................ 1 FIGURE 2: DISTRICT OF NORTH HERTFORDSHIRE LOCATION PLAN ....................... 9 FIGURE 3: RIVER CATCHMENTS WITHIN THE DISTRICT OF NORTH HERTFORDSHIRE ........................................................................................ 10 FIGURE 4: BRITISH GEOLOGICAL SURVEY MAP OF NORTH HERTFORDSHIRE DISTRICT – BEDROCK DEPOSITS ............................................................. 11 FIGURE 5: BRITISH GEOLOGICAL SURVEY MAP OF NORTH HERTFORDSHIRE DISTRICT – SUPERFICIAL DEPOSITS ....................................................... 12 FIGURE 6: DIFFERENT STAGES OF A SWMP STUDY ................................................. 14 FIGURE 7: LINK BETWEEN SURFACE WATER MANAGEMENT PLANS AND OTHER STRATEGIES, PLANS AND POLICIES ........................................................ 17 FIGURE 8: ENVIRONMENT AGENCY CFMP AREAS AND RBMP AREAS COVERED WITHIN THE DISTRICT OF NORTH HERTFORDSHIRE ............................ 19 FIGURE 9: RYE MEADS WATER CYCLE STRATEGY STUDY AREA. (RYE MEADS WATER CYCLE STRATEGY, DETAILED STUDY REPORT, OCTOBER 2009, PAGE 11). ........................................................................................... 21 FIGURE 10: EXAMPLE OF UNCERTAINTIES IN THE DTM ........................................... 35 FIGURE 11: HOTSPOT 6 - HITCHIN– EXTENTS AND BASELINE INFORMATION ...... 41 FIGURE 12: HOTSPOT 7 - OAKFIELD– EXTENTS AND BASELINE INFORMATION ... 46 FIGURE 13: HOTSPOT 12 - BALDOCK– EXTENTS AND BASELINE INFORMATION . 50 FIGURE 14: HOTSPOT 13 - CLOTHALL COMMON – EXTENTS AND BASELINE INFORMATION ............................................................................................. 54 FIGURE 15: HOTSPOT