Land at King Edward Street, Ashbourne Flood Risk Assessment
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Land at King Edward Street, Ashbourne Carter Varley Limited Flood Risk Assessment & Drainage Strategy Flood Risk Assessment & Drainage Strategy Land at King Edward Street, Ashbourne Flood Risk Assessment & Drainage Strategy Job Title Land at King Edward Street, Ashbourne Project Number 0927 Date 25th November 2019 Revision A Client Carter Varley Limited Prepared by T Gilbert Checked by C Yalden Authorised by I Awcock P:\0927 Land at King Edward Street, Ashbourne, File Reference Derbyshire\C Documents\Reports\0927 - Ashbourne, Derbyshire - Flood Risk Assessment.docx Awcock Ward Partnership Kensington Court Woodwater Park Pynes Hill Exeter EX2 5TY Tel: 01392 409007 www.awpexeter.com Flood Risk Assessment Contents 1 Introduction ................................................................................. 1 2 Existing Conditions ....................................................................... 4 3 Development Proposals ........................................................... 10 4 Surface Water Management Plan .......................................... 14 5 Miscellaneous Issues .................................................................. 18 6 Mitigation, Conclusions and Recommendations ................... 19 Appendices Appendix A Existing Site Plan Appendix B Severn Trent Sewer Records Appendix C Proposed Site Layout Appendix D Greenfield Runoff Rates Appendix E Preliminary Drainage Layout Appendix F MicroDrainage Calculations Awcock Ward Partnership Kensington Court Woodwater Park Pynes Hill Exeter EX2 5TY Tel: 01392 409007 www.awpexeter.com 0927 Land at King Edward Street Flood Risk Assessment & Drainage Strategy 1 Introduction 1.1 Awcock Ward Partnership (AWP) have been commissioned by Carter Varley Limited to undertake a Flood Risk Assessment (FRA) and drainage strategy in support of an outline planning application for nine dwellings at land to the south of King Edward Street, Ashbourne, Derbyshire. 1.2 The location of the proposed development in relation to the wider area is shown as Figure 1.1 below. Figure 1.1 - Site Location – Wide Area National Planning Policy Framework 1.3 The National Planning Policy Framework (NPPF) and the Planning Practice Guidance were most recently published by the Ministry of Housing, Communities and Local Government in June 2019 and October 2019, respectively. 1.4 The NPPF states that “A site-specific flood risk assessment should be provided for all development in Flood Zones 2 and 3. In Flood Zone 1, an assessment should accompany all proposals involving: sites of 1 hectare or more; land which has been identified by the Environment Agency as having critical drainage problems; land identified in a strategic flood risk assessment as being at increased 0927 Land at King Edward Street Page 1 of 21 Flood Risk Assessment & Drainage Strategy flood risk in future; or land that may be subject to other sources of flooding, where its development would introduce a more vulnerable use”. 1.5 The aim of a site-specific flood risk assessment is to demonstrate that “the development should be made safe for its lifetime without increasing flood risk elsewhere”. Structure and limitations of this FRA 1.6 This site-specific FRA has been written in accordance with the guidance set by the NPPF and Planning Practice Guidance, using the information that is currently available. 1.7 The report has been structured to describe the existing site parameters, the proposed development and to offer a surface water management plan (SWMP), indicating how surface water runoff can be managed so that it does not increase flood risk within the downstream catchment. 1.8 It is important to note that this FRA does not attempt to present a final design of the surface water drainage system. This will be left until the reserved matters and detailed design stages when further site investigation work can be undertaken, and other systems can be evaluated. This evaluation will also need to include assessments due to health and safety, CDM etc. Consultation 1.9 To identify any site specific or catchment specific flood risk or drainage requirements we have liaised with Derbyshire County Council’s (DCC) Flood Risk Management team and have sought information from both the Environment Agency (EA) and Severn Trent Water. 0927 Land at King Edward Street Page 2 of 21 Flood Risk Assessment & Drainage Strategy Reference 1.10 This FRA has been prepared with reference to the following documents: • National Planning Policy Framework (June 2019); • Planning Practice Guidance (PPG) (October 2019); • Environment Agency (EA) Flood Warning Information Service ‘Flood Risk from Rivers or the Sea’, ‘Flood Risk from Surface Water’(online); • CIRIA Guides 522 Sustainable Drainage Systems, 609 Surface Water Management and the Interim Code of Practice for SuDS (ICoP), 753 The SuDS Manual; • Severn Trent Water Asset Records (2016); • JBA Consulting on behalf of Derbyshire County Council (DCC) Strategic Flood Risk Assessment (SFRA) (June 2016); • Peter Brett Associates (PBA) Flood Risk Assessment for adjacent Churchill retirement development (ref. 17/00374/FUL); and, • Derbyshire Dales District Council (DDDC) pre-app response (ref. 19/00973/PREAPP) 0927 Land at King Edward Street Page 3 of 21 Flood Risk Assessment & Drainage Strategy 2 Existing Conditions Context 2.1 The existing site is located in the town of Ashbourne, west of the A515 and south of the Henmore Brook, at National Grid Reference SX 17953 46458, as shown by Figure 2.1 below. Figure 2.1 – Site Location – Local Area Existing land uses 2.2 The site was previously hard surfaced and utilised as a storage compound. Since this time the site was been returned to greenfield use so that it could serve as a bowling green. Surrounding land use 2.3 The site is surrounded by the following land uses: • Directly to the north of the site lies King Edward Street, with a green corridor and the Henmore Brook beyond; • To the east of the site lies the approved Churchill Retirement Living development which is currently under construction; 0927 Land at King Edward Street Page 4 of 21 Flood Risk Assessment & Drainage Strategy • To the west of the site lies King Edward Street, with the Empire Club beyond; and, • To the south of the site lies the Sainsbury’s retail store with associated car parking. Topographic survey 2.4 The 2011 Subscan topographic survey produced for the adjacent Churchill development includes the land occupied by this application. 2.5 The survey included the output of ground penetrating radar (GPR) which detected the location of existing drainage and utilities within the site. The survey confirms the site is relatively level, with a minor high point at its approximate centre. 2.6 An ‘Existing Site Plan’ has been prepared to set the context of the site and can be found as drawing 0927-XS-101, within Appendix A of this report. Existing Flood Risk 2.7 The Planning Practice Guidance requires planning applications for areas at risk of flooding, or sites of 1 hectare or more, to be accompanied by a site-specific flood risk assessment which assesses “flood risk”. 2.8 In accordance with Para. 002 of the Planning Practice Guidance, it is required that new developments consider flood risk as a ‘combination of the probability and the potential consequences of flooding from all sources’ including rivers and the sea, rainfall, rising groundwater, infrastructure and artificial sources. 2.9 Each potential source of flooding has been assessed below; Fluvial sources (River flooding) 2.10 The EA’s ‘Flood Warning Information Service’ provides flood risk information and mapping throughout England. 2.11 An extract of the ‘Flood Map for Planning’ has been reproduced as Figure 2.2 and shows the site as being entirely within ‘Flood Zone 3’ (high risk), benefitting from flood defences in the form of the Ashbourne Flood Alleviation Scheme (2010). 0927 Land at King Edward Street Page 5 of 21 Flood Risk Assessment & Drainage Strategy 2.12 The existing alleviation scheme protects the area from fluvial flooding in up to the 1 in 100 year storm (1% annual probability) with 20% allowance for climate change. Figure 2.2 – Flood Map for Planning 2.13 Additional flood defences include: • Raising of Henmore Brook bank between Compton Street and School Lane to prevent overtopping • Reinforced concrete flood wall stretching 89.9m at the upstream section of the Henmore Brook. • Raised earth embankment running along the section of river north of the development, with defence crest level 119.76m AOD. Pluvial sources (surface water flooding) 2.14 An extract of the EA’s ‘Flooding from Surface Water’ map has been reproduced as Figure 2.3. The mapping is based on LIDAR data and indicates the typical conveyance routes of surface water runoff in up to the 100 year return period (medium to high risk). 0927 Land at King Edward Street Page 6 of 21 Flood Risk Assessment & Drainage Strategy 2.15 Figure 2.3 – Flood Risk from Surface Water 2.16 The mapping shows a small area of localised shallow flooding (<300mm) inside the eastern edge of the site. This will need to be considered by any future development to ensure it does not adversely impact properties. Groundwater sources 2.17 With reference to PBAs Flood Risk Assessment and the available British Geological Survey (BGS) borehole records in the area, groundwater levels are expected to sit around 2.5m below ground level (mBGL). 2.18 The likelihood of moderately elevated of groundwater is not expected to impact on the development but will need to be considered by the proposed SWMP. Artificial