Dobbs Weir Road, Essex
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DOBBS WEIR ROAD, ESSEX FLOOD RISK ASSESSMENT Final Report v3.2 May 201 3 Weetwood Suite 1, Park House Broncoed Business Park Wrexham Road Mold CH7 1HP T: 01352 700045 E:[email protected] www.weetwood.net L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex Report Title: Dobbs Weir Road, Essex Flood Risk Assessment Final Report v3.2 Client: L.W. Developments Ltd Date of Issue: 24 May 2013 Prepared by: Adam Edgerley BSc (Hons) Technical Engineer Checked by: Rebecca Ellis BSc (Hons) Associate Director Approved by: Andrew Grime BEng MBA CEng C.WEM MICE FCIWEM Managing Director This document has been prepared solely as a Flood Risk Assessment for L.W. Developments. Weetwood Services Ltd accepts no responsibility or liability for any use that is made of this document other than by L.W. Developments Ltd for the purposes for which it was originally commissioned and prepared. ©Weetwood i 1282/FRA_v3.2 www.weetwood.net 24 May 2013 L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex CONTENTS Page Signature Sheet i Contents ii & iii List of Tables, Figures & Appendices iv 1 INTRODUCTION ............................................................... 1 1.1 background ............................................................................................... 1 1.2 Site Location ............................................................................................. 1 1.3 Existing and Proposed Development ............................................................. 2 1.4 Site Levels ................................................................................................ 2 1.5 Access and Egress ...................................................................................... 2 2 NATIONAL PLANNING POLICY FRAMEWORK (NPPF) ............... 4 2.1 Flood Zone Designation .............................................................................. 4 2.2 Sequential Test .......................................................................................... 6 2.3 Exception Test ........................................................................................... 6 3 FLOOD RISK ................................................................... 7 3.1 Fluvial – River Lee System .......................................................................... 7 3.2 FLuvial - River Lynch ................................................................................ 12 3.3 Nazeing Drain .......................................................................................... 13 3.4 Reservoirs, Canals and Other Artificial Sources ............................................ 13 3.5 Groundwater ........................................................................................... 14 3.6 Surface Water ......................................................................................... 15 3.7 application of the sequential and exception tests .......................................... 16 4 MITIGATION MEASURES ................................................. 17 4.1 Flood Mitigation ....................................................................................... 17 4.2 Compensatory Storage / Flood Risk Elsewhere ............................................. 18 4.3 Access and Egress .................................................................................... 19 4.4 Flood Warning ......................................................................................... 22 5 SURFACE WATER ........................................................... 23 5.1 Requirements for Surface Water Drainage ................................................... 23 5.2 Site Areas ............................................................................................... 23 5.3 Surface Water Runoff from the Existing Site ................................................ 23 5.4 Surface Water Runoff from Developed Site .................................................. 24 6 SUMMARY ..................................................................... 28 ©Weetwood ii 1282/FRA_v3.2 www.weetwood.net 24 May 2013 L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex 7 RECOMMENDATIONS ...................................................... 29 ©Weetwood iii 1282/FRA_v3.2 www.weetwood.net 24 May 2013 L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex LIST OF TABLES Table 1: Modelled Flood Levels (m AOD) for the River Lee and RLN ......................... 8 Table 2: Modelled Flood Levels (m AOD) for the RLFRC ........................................ 10 Table 3: Modelled Flood Levels (m AOD) for Reservoir Units near Site ................... 10 Table 4: Modelled Flood Levels (m AOD) for the Nazeing Mead and Reservoir Unit .. 11 Table 5: Modelled Flood Levels (m AOD) for the River Lynch ................................ 12 Table 6: Flow Across Dobbs Weir Road (Velocity Calculation) ................................ 20 Table 7: Access & Egress Routes - Hazard Rating ................................................ 21 Table 8: Site Areas ......................................................................................... 23 Table 9: Total Peak Runoff Rate – Developed Site ............................................... 25 LIST OF FIGURES Figure 1: Site Location ..................................................................................... 2 Figure 2: Environment Agency Flood Map ........................................................... 5 Figure 3: SFRA Flood Map ................................................................................ 6 Figure 4: Flood Routes at Dobbs Weir Road ........................................................ 7 Figure 5: River Lee Modelled Node Locations ...................................................... 8 Figure 6: Richard White Sluices (RLFRC) ............................................................ 9 Figure 7: Node Locations – River Lynch ........................................................... 13 Figure 8: Environment Agency Risk of Flooding from Reservoirs Map ................... 14 Figure 9: BGS Groundwater Flooding Hazard Map ............................................. 15 Figure 10: Diagram Illustrating Flooding Mechanism at Site ................................. 18 Figure 11: Dobbs Weir Road Flow Depths ........................................................... 20 Figure 12: Dobbs Weir Road Flow Area .............................................................. 20 Figure 13: Environment Agency Flood Warning Map ............................................ 22 LIST OF APPENDICES Appendix A: Development Proposals Appendix B: Topographic Survey Appendix C: Sequential Test Appendix D: Micro Drainage Outputs for Greenfield Runoff Appendix E: MicroD rainage Storage Volume Calculation ©Weetwood iv 1282/FRA_v3.2 www.weetwood.net 24 May 2013 L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex 1 INTRODUCTION Weetwood has been instructed 1 by L.W. Developments to undertake a Flood Risk Assessment (FRA) for the proposed redevelopment of a site located off Dobbs Weir Road, Hoddesdon, in accordance with the requirements of the National Planning Policy Framework (NPPF) and its supporting Technical Guidance. 1.1 BACKGROUND Weetwood has previously undertaken two FRAs for the proposed development site. The first FRA 2 was submitted to the Environment Agency (EA) on 22 January 2009. The EA's response to this was received in their letter dated 20 February 2013 3 and indicated the information available was insufficient to determine the level of flood risk to the site from the River Lee. At this time the EA were undertaking further modelling to resolve inaccuracies in the floodplain representation in the River Lee model. Once the revised model had been completed, a second FRA 4 was prepared in support of a planning application for the development of residential houses. The EA generally agreed 5 with the content of the FRA, stating that all the relevant sources of flood risk had been correctly identified and that the proposed finished floor level was acceptable. However, the EA requested further information concerning the availability of safe access and egress, and the provision of compensatory storage. This updated FRA has been produced in support of a planning application to provide the additional information requested. 1.2 SITE LOCATION The site is located at Ordnance Survey National Grid Reference TL 388 079, as shown in Figure 1. The site is approximately 0.18 hectares (ha) in area. 1 Instructed to proceed received 3 April 2013 2 Weetwood report entitled “Flood Risk Assessment: Dobbs Weir Road, Essex” dated 19 January 2009, ref 1282/R01 3 Letter from Sandeep Shankar (EA) to Sarah Cowie (Weetwood) dated 20 February 2009, ref NE/2009/107713/01- L01 4 Weetwood report entitled “Flood Risk Assessment: Dobbs Weir Road, Essex” dated 12 May 2010, ref 1282/R02 submitted 16 September 2010 5 Letter from Joe Martyn (EA) to Sarah Cowie (Weetwood) dated 21 September 2010, ref NE/2010/110326/01-L01 ©Weetwood 1 1282/FRA_v3.2 www.weetwood.net 24 May 2013 L.W. Developments Ltd Flood Risk Assessment – Dobbs Weir Road, Essex Essex Road River Lee Flood River Lynch Relief Channel Dobbs Weir Road River Lee Proposed development site Sedge River Lee Green Navigation Eldon Road Nazeing Crown Copyright. All rights reserved. Mead Lake Image reproduced with permission of Ordnance Survey and Ordnance Survey of Northern Ireland Figure 1: Site Location 1.3 EXISTING AND PROPOSED DEVELOPMENT The site comprises an area of previously developed