Flood Risk Assessment Report
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Consulng Civil, Structural & Geo-Environmental Engineers Flood Risk Assessment Report Site Address: Burnley Road Rawtenstall Lancashire Paul Waite Associates Ltd Summit House Riparian Way Project Ref: 13161/I/01A The Crossing Cross Hills February 2014 BD20 7BW [email protected] www.pwaite.co.uk Report No.13161/1/01 Project Details. FRA – Site off Burnley Road, Rawtenstall, Lancashire Date. November 13 Flood Risk Assessment Paul Waite Associates have been appointed by the RTB Partnership, to undertake a Flood Risk Assessment in support of a planning application for residential development at a site off Burnley Road in Rawtenstall, Lancashire. Clients Details RTB Partnership The Business Centre Futures Park Bacup Lancashire OL13 0BB Documents Revision Status ISSUE: DATE COMMENTS ‐ November 4, 2013 FINAL A February 10, 2014 FINAL ‐ Revised following updated development plans Report No.13161/1/01 Project Details. FRA – Site off Burnley Road, Rawtenstall, Lancashire Date. November 13 Contents Executive Summary 1 1.0 Introduction 3 2.0 Approach to the Flood Risk Assessment 4 2.1 Approach 4 2.2 Application of the Sequential and Exceptions Test 4 3.0 Site Details 6 3.1 Location 6 3.2 Former/Current Use 6 3.3 Proposals 6 3.4 Boundaries 7 3.5 Topography 7 3.6 Existing Drainage 8 3.7 History of Flooding 9 3.7.1 British Hydrological Society – Hydrological Events 9 3.7.2 Internet Search for Historical Flooding 9 3.7.3 Lancashire Area PFRA Document (2011) 9 3.7.4 Rossendale Borough Council SFRA (May 2009) 11 3.7.5 Environment Agency Data – Historic Flooding 11 4.0 Flooding Mechanisms 12 4.1 Fluvial: Limy Water 12 4.1.1 General 12 4.1.2 Modeled Flood Level Data 13 4.1.3 Overtopping 14 4.1.4 Climate Change 15 4.1.5 Infrastructure Failure: Blockage 16 4.1.6 Conclusion 17 4.2 Artificial Water Sources: Reservoirs 17 4.3 Pluvial Sources 18 4.3.1 Sewer Flooding 18 4.3.2 Increase in Surface Water Runoff 19 Report No.13161/1/01 Project Details. FRA – Site off Burnley Road, Rawtenstall, Lancashire Date. November 13 4.4 Groundwater 25 4.5 Overland Flow 25 4.6 Ponding 26 5.0 Material Consideration In Respect of the National Planning Policy Framework 27 5.1 Environment Agency Flood Map 27 5.2 Finished Development Levels 28 5.3 Flood Resistance/Resilience Measures 28 5.4 Flood Protection Equipment 29 5.5 Access and Egress 30 5.6 Flood Warning 35 5.7 Flood Evacuation Plan 37 5.8 Safe Refuge 38 5.9 Flood Conveyance Routes 39 5.9 Compensatory Flood Storage 39 5.10 Easements 40 6.0 Conclusions and Recommendations 41 Tables Table 1: Exceptions and Sequential Test 4 Table 2: Sources of Flooding 12 Table 3: Limy Water Model Results Node Ref. ea0140LIMY01_1115 14 Table 4: SUDs Planner 21 Table 5: Existing Greenfield Runoff Rates (Site Area = 0.25Ha) 24 Table 6: Indicative Attenuation Volumes 24 Table 7: Typical Flood Proofing Measures 29 Table 8: Guidance on Debris Factors for Different Flood Depths, Velocities and Dominant Land Uses (Table 3.1 FD2321/TR1) 32 Table 9: Velocity, Depth and Flood Hazard Matrix 32 Table 10: Flood Hazard Rating for Safe Access/Egress Route 33 Table 11: Environment Agency Flood Warning Codes 36 Table 12: Volume Displacement Due to Building Footprint (Steps and Lobby Area) 40 Report No. 12156/I/01 Revision A Project Details. FRA – Site off Burnley Road, Rawtenstall, Lancashire Date. February 14 Figures Figure 1: Location Plan – Existing Site off Burnley Road, Rawtenstall, Lancashire 6 Figure 2: Existing Site Viewed North East along Burnley Road 7 Figure 3: Position of Existing Gully Viewed South West towards the Site Access 8 Figure 4: Lancashire PFRA – Past Flood Event Locations 10 Figure 5: Limy Water Taken from Over the North East Boundary 13 Figure 6: Limy Water Viewed at the Head of the Culvert Section (BP Garage) 14 Figure 7: Constable Lee Road Bridge 15 Figure 8: Risk of Flooding from Reservoirs 18 Figure 8: Environment Agency Flood Map 27 Figure 9: Typical Flood Resilience/Resistance Measures 30 Figure 11: Pedestrian Route & Levels from the New Building 34 Figure 11: Environment Agency Flood Warning Coverage Map 35 Figure 12: Suggested Evacuation Route to St James the Less School in Flood Zone 1 38 Appendices APPENDIX A Aerial Photographs APPENDIX B Topographical Survey APPENDIX C Proposed Development Plans APPENDIX D Environment Agency Data APPENDIX E United Utilities Sewer Records APPENDIX F Local Borehole Logs & Soilscape Map APPENDIX G SUD’s Planner APPENDIX H Greenfield Runoff Rates APPENDIX I Indicative Attenuation Volumes APPENDIX J Flood Extent Plans Report No. 12156/I/01 Revision A Project Details. FRA – Site off Burnley Road, Rawtenstall, Lancashire Date. February 14 Executive Summary The proposed development comprises 16No new apartment units with associated car parking located off Burnley Road, within the Lancashire Town of Rawtenstall. The development covers an area approximating 0.25 Hectares; and is located within Flood Zone 3 of the Environment Agency Flood Map; and therefore has a risk of flooding from fluvial sources. The primary source of flood risk to the development is identified to be from local watercourse Limy Water, which is situated directly adjacent to the east boundary of the site. From a topographical survey of the site, levels within the site range from 178.7mAOD up to 179.87mAOD; with an average ground level at the location of the new building of 179.15mAOD. The 1 in 75 year; 100 year and 1 in 1000 year flood levels for estimated as 179.23mAOD, 179.34mAOD and 179.45mAOD respectively; and undertaking a direct comparison indicates that the site is likely to become inundated during the 1 in 75 year flood event. In conclusion it is confirmed that Limy Water presents a flood risk at the site. As such the following measures have been; or are recommended to be included within the design of the site: Lobby area set at a level of 180.00mAOD i.e. elevated above the 1 in 1000 year flood level Ground floor accommodation set at a level of 181.00mAOD which again is elevated above the 1 in 1000 year flood level Open undercroft area provided to prevent impedance of flood conveyance routes and the displacement of flood storage volumes; hence ensuring that flooding is not increased elsewhere as a result of the development. Flood storage compensation is to be provided through ground re‐profiling and careful design of levels within the proposed car park area. Suitable flood resistance/resilience measures are to be incorporated into the building design i.e. non‐return valves; elevated electricity sockets etc…, where appropriate. Residents/Housing organisation advised to sign up to receive flood warnings from the EA’s Flood Warning’s Direct Service. Evacuation plan/procedure to be devised and implemented; undertaking assistance/consultation with Rossendale Emergency Planning Team. Surface water runoff rates and volumes will be increased as a result of the development. 1 Report No. 12156/I/01 Revision A Project Details. FRA – Site off Burnley Road, Rawtenstall, Lancashire Date. February 14 On‐site investigation indicates that surface water from previous development was directed via an outfall into Limy Water. Approved Document H of the current Building Regulations sets out a hierarchy for disposal of surface water from new development, as listed below in order of preference: Infiltration via soakaway or other infiltration device Watercourse Sewer A desk‐top study suggests that local ground conditions are unsuitable for disposal of surface water via infiltration methods such as soakaways; and as such it is recommended that the existing outfall into Limy Water is re‐utilised. Greenfield run off rates for the proposed development have been estimated at 2.43 l/s; 4.75 l/s; and 5.82 l/s for the 1 in 1 year; 1 in 30 year; and 1 in 100 year plus climate change events respectively. Discharge into the watercourse, should be restricted to greenfield runoff rates or a minimum of 5l/s whichever is the highest; to prevent an increase in surface water flooding occurring as a result of blockage. Flows in excess of these values should be attenuated on‐site up to the 1 in 100 year plus climate change event. Some flooding is permitted during this event; however flooding must be retained within the site; and not be allowed to migrate beyond the site boundary. An evaluation of SUDS indicates that source control methods such as permeable paving, green roof technology, or rainwater harvesting; with online/offline attenuation volume provided by large diameter pipes or geo‐cellular storage crates; are suitable for inclusion with a drainage strategy for the proposed development. 2 Report No. 12156/I/01 Revision A Project Details. FRA – Site off Burnley Road, Rawtenstall, Lancashire Date. February 14 1.0 Introduction Paul Waite Associates have been appointed to undertake a Flood Risk Assessment in support of a planning application for proposed residential development located on land off Burnley Road in Rawtenstall, Lancashire The development covers an area approximating 0.25 Hectares and is shown to be situated within Flood Zone 3 of the Environment Agency Flood Map, which is defined as land assessed as having a 1 in 100 or greater annual probability of river flooding (>1%) or a 1 in 200 or greater annual probability of flooding from the sea. The primary source of flood risk to the proposed development site has been identified as Limy Water, however other sources of flooding have been considered within this assessment. It is usual for the Environment Agency to raise an objection to development applications within the functional floodplain or Zone 2 or 3 of the flood map until the question of flood risk has been properly evaluated.