Thakeham Homes BROOK HOUSE, FLEET Initial Flood Risk Assessment
33671-FRA-001 MARCH 2018 PUBLIC Thakeham Homes BROOK HOUSE, FLEET Initial Flood Risk Assessment
TYPE OF DOCUMENT (VERSION) PUBLIC
PROJECT NO. 70033671 OUR REF. NO. 33671-FRA-001
DATE: MARCH 2018
WSP QUALITY CONTROL
Issue/revision First issue Revision 1 Revision 2 Revision 3 Revision 4 Remarks Draft for First Issue Red Line Draft Update Revision Comment Boundary for Comment Incorporating Updated Client Comments Date May 2017 June 2017 June 2017 March 2018 March 2018
Prepared by Simon King Simon King Simon King Simon King Simon King
Signature
Checked by Steven Brown Steven Brown Steven Brown Steven Brown Steven Brown
Signature
Authorised by Martin Martin Wheeler Martin Wheeler Martin Wheeler Martin Wheeler Wheeler
Signature
Project number 70033671
Report number 3671-FRA-001
File reference \\uk.wspgroup.com\central data\Projects\700336xx\70033671 - Brook House, Fleet- Hampshire\02 WIP\DE Development\03 Document\Reports\Water\Initial Flood Risk Assessment\180314-SEK-Initial Flood Risk Assessment v5.docx
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes CONTENTS
1 INTRODUCTION 4
1.1 APPOINTMENT AND BRIEF 4 1.2 AIM 4 1.3 OBJECTIVES 4
2 POLICY 5
2.1 NATIONAL POLICY 5 2.2 LOCAL POLICY 6
3 THE SITE 10
3.1 SITE LOCATION 10 3.2 SITE DESCRIPTION 10 3.3 TOPOGRAPHY 10 3.4 GEOLOGY AND HYDROGEOLOGY 10 3.5 EXISTING WATERCOURSES 11 3.6 FLOOD DEFENCES 11 3.7 EXISTING SEWER AND DRAINAGE INFRASTRUCTURE 11 3.8 HISTORIC FLOOD RECORDS 11
4 OVERVIEW OF FLOOD RISK 12
4.1 INTRODUCTION 12 4.2 EXISTING RISK OF FLOODING TO THE SITE 12 Flooding from Coastal and Tidal Sources 12 Flooding from Fluvial Sources 12 Flooding from Pluvial Sources 14 Flooding from Groundwater Sources 14 Flooding from Sewer and Drainage Infrastructure 15 Flooding from Artificial Sources 15
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes Page 1 of 24 4.3 RISK OF FLOODING TO AND ARISING FROM THE PROPOSED DEVELOPMENT 16 Flooding from Coastal and Tidal Sources 16 Flooding from Fluvial and Pluvial Sources 16 Flooding from Groundwater Sources 17 Flooding from Sewers and Drainage Infrastructure 17 Flooding from Artificial Sources 17 4.4 SUMMARY OF FLOOD RISK 18
5 CONSULTATION 19
5.1 ENVIRONMENT AGENCY 19
6 SURFACE WATER DRAINAGE 20
6.1 EXISTING DRAINAGE REGIME 20 6.2 SURFACE WATER MANAGEMENT STRATEGY REQUIREMENTS 20 6.3 INDICATIVE SURFACE WATER DRAINAGE STRATEGY 20
7 CONCLUSIONS 22
TABLES Table 1 – Summary of Flood Risk 3 Table 2 - Summary of Flood Extents 14 Table 3 – Summary of Flood Risk to and from the Site/Proposed Development 18 Table 4 – Peak Greenfield Runoff Rates 20 Table 5 – Required Attenuation Volumes 21
APPENDICES Appendix A - Site Plans Appendix B - Proposed Development Plans Appendix C - Flood Risk Mapping Appendix D - Surface Water Drainage Calculations Appendix E - Hydraulic Modelling Study
WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Page 2 of 24 Thakeham Homes EXECUTIVE SUMMARY
This report has been prepared in support of a proposed residential-led development of circa 500 dwellings, community facilities including a primary school, and 15 ha of Suitable Alternative Natural Greenspace (the ‘Proposed Development’) at a 45.6 ha agricultural site to the north of the M3 in Fleet, Hampshire (the ‘Site’) The Site is being submitted by Thakeham Homes to Hart District Council for Regulation 19 consultation of the Draft Hart Local Plan.
This assessment has been undertaken in accordance with the criteria specified in the National Planning Policy Framework, the accompanying Flood Risk and Coastal Change Planning Practice Guidance and Local Planning Guidance. Existing flood risk posed to the Site and the Proposed Development from all potential sources has been assessed along with the potential of the Proposed Development to exacerbate these risks. Where risks have been identified suitable mitigation measures have been proposed. Part of the Site is considered to be subject to a High risk of fluvial and pluvial (i.e. surface water) flooding associated with the Ordinary Watercourse which runs through the Site. All other sources of flooding were assessed and found to range from Low to Negligible. Definitions of the grading criteria used are given in Paragraph 4.1.2 in the main body of this report. The Proposed Development adopts the sequential approach to spatial planning to locate all the residential and educational development within areas identified as having Very Low risk of fluvial flooding. The proposed strategy for the management of surface water will reduce the risk of flooding from surface water to the Proposed Development to Low. The Proposed Development would therefore have an annual probability of fluvial flooding of less than 0.1% and is consistent with the requirements by the NPPF with regards to flood risk. Consequently, the risk of flooding currently posed to the Site would not prohibit the Proposed Development from being successfully delivered. This report demonstrates that the Proposed Development can be delivered sustainably with regards to flood risk and surface water management and would be in accordance with both local and national policy.
Table 1 – Summary of Flood Risk Sources of Flood Risk Current Risk to the Site Future Risk to the Future Risk from the Proposed Proposed Development Development Coastal and Tidal Negligible Negligible Negligible Fluvial High (4.7%), Moderate (4.4%) Very Low Very Low Low (6.5 %), Very Low (84.4%) Pluvial and Overland High to Low Low Very Low Flow Groundwater Low Low Very Low Drainage Infrastructure Very Low Very Low Very Low Artificial Sources Negligible Negligible Negligible
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes Page 3 of 24 1 INTRODUCTION
1.1 APPOINTMENT AND BRIEF 1.1.1. WSP was appointed by Thakeham Homes to produce a Preliminary Flood Risk Assessment in support of a proposed residential-led development of circa 500 dwellings, community facilities including a primary school, and 15 hectares (ha) of Suitable Alternative Natural Greenspace (the ‘Proposed Development’). An indicative plan of the Proposed Development is provided in Appendix B.1 (the ‘Masterplan’). 1.1.2. The Proposed Development is to be located at a 45.6 ha agricultural site to the north of the M3 in Fleet, Hampshire (the ‘Site’). 1.1.3. The Proposed Development is being submitted to Hart District Council (HDC) as the Local Planning Authority (LPA) for consideration as part of the Regulation 19 consultation of the Draft Hart Local Plan1 (the ‘Local Plan’). 1.2 AIM The aim of this document is to identify flood risks currently posed to the Site in accordance with the National Planning Policy Framework (NPPF)2 and those which may arise as a result of the Proposed Development in order to inform HDC’s consideration of the Site for inclusion within the Local Plan. 1.3 OBJECTIVES To achieve this aim the following objectives have been met: ¡ A desk study and data research including review of relevant local and national flood risk policies; ¡ A qualitative assessment of flood risk posed to the Site and the Proposed Development from all possible sources, including fluvial, pluvial, groundwater, sewers and man-made infrastructure; ¡ A hydrological assessment to predict river flows and rainfall intensities accounting for the anticipated effects of climate change; ¡ Development of a 1D-2D hydraulic model to enable a preliminary quantitative assessment of the flood risk posed to the Site; ¡ A walkover of the Site to visually verify critical flood flow routes, and to understand the local hydrological setting; ¡ An assessment of the impact of the Proposed Development on local flood risks; ¡ Estimation of surface water flows based on the current conditions at the Site and existing drainage systems; ¡ Estimation of pre and post development surface water flows based on the development proposals; ¡ Specification of mitigation measures to alleviate any unacceptable flood risks to and/or arising from the proposed Development; ¡ An assessment of the residual flood risk to the Proposed Development and neighbouring property from local flood sources; and, ¡ Production of a standalone Preliminary Flood Risk Assessment in accordance with the requirements of the NPPF and local policy.
1 Hart District Council (April 2017) Draft Hart Local Plan: Strategy and Sites 2016 – 2032, Proposed Submission Version; accessed March 2018 from https://www.hart.gov.uk/Local-Plan-Publication-2018 2 Department for Communities and Local Government (2012) National Planning Policy Framework; accessed March 2018 from https://www.gov.uk/guidance/national-planning-policy-framework
WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Page 4 of 24 Thakeham Homes 2 POLICY
2.1 NATIONAL POLICY National Planning Policy Framework 2.1.1. The NPPF states that ‘a site specific flood risk assessment is required for proposals of 1 hectare (ha) or greater in Flood Zone 1; all proposals for new development (including minor development and change of use) in Flood Zones 2 and 3, or in an area within Flood Zone 1 which has critical drainage problems (as notified to the local planning authority by the Environment Agency), and where Proposed Development or a change of use to a more vulnerable class may be subject to other sources of flooding’. 2.1.2. The Environment Agency’s Flood Map for Planning3 indicates that the Site is located within Flood Zones 1, 2 and 3. The Site covers an area of approximately 45.6 ha. 2.1.3. Consequently, a Flood Risk Assessment (FRA) would be required to support a planning application for the Proposed Development in order to ‘demonstrate that the Proposed Development will be safe for its lifetime taking account of the vulnerability of its users, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall’. 2.1.4. Within Table 2 (Flood Risk Vulnerability Classification) of the NPPF Flood Risk and Coastal Change Planning Practice Guidance4 (PPG), the Proposed Development is classified as ‘More Vulnerable’ (residential and educational). 2.1.5. Table 3 (Flood Risk Vulnerability and Flood Zone Compatibility) of the PPG, states that More Vulnerable development is appropriate in Flood Zones 1 and 2 but that development within Flood Zone 3 would be subject to the successful application of the Exception Test. 2.1.6. The Sequential Test aims to steer new development to areas with the lowest probability of flooding (i.e. Flood Zone 1). As part of the strategic land allocation process it is anticipated that LPAs will undertake the sequential testing of proposed development sites. 2.1.7. In order to mitigate the risk of fluvial flooding to the Proposed Development the sequential approach has been adopted when developing the proposals with all land-uses located in appropriate flood risk zones in accordance with the NPPF. Climate Change Allowances 2.1.8. The Flood Risk and Coastal Change PPG states that the lifespan of a residential development should be considered for a minimum of 100 years. 2.1.9. In February 2016 the Environment Agency released updated climate change guidance5 to support the NPPF, this guidance was last updated in February 2017. The guidance contains recommended allowances for increases in peak river flow, peak rainfall intensity, sea levels, offshore wind speeds and extreme wave heights anticipated over a range of epochs to enable an assessment of the potential impacts of climate change over the lifespan of proposed developments. 2.1.10. The guidance states that for More Vulnerable developments (e.g. residential and educational) in the Thames River Basin District, the assessment of fluvial flood risk should consider the new ‘Central’ allowance of a 25% increase in peak river flows when located entirely within Flood Zone 1. An assessment of the Proposed Development should consider the new ‘Central’ and ‘Higher Central’ allowances of 25% and 35% respectively
3 Environment Agency (2018) Flood Map for Planning; accessed March 2018 from https://flood-map-for- planning.service.gov.uk/ 4 Department for Communities and Local Government (2014) Flood Risk and Coastal Change Planning Practice Guidance to the National Planning Policy Framework; accessed May 2017 from http://planningguidance.planningportal.gov.uk/blog/guidance/flood-risk-and-coastal-change/ 5 Environment Agency (February 2017) Flood risk assessments: climate change allowances; accessed March 2018 from https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes Page 5 of 24 when located within Flood Zone 2. The ‘Higher Central’ and ‘Upper End’ allowances of 35% and 70% should be used when located within Flood Zone 3a. 2.1.11. The guidance states that the appropriate allowances for the anticipated increase in peak rainfall intensity over the lifetime of the Proposed Development are the ‘Central’ and ‘Upper End’ allowances of 20% and 40% respectively. Non-Statutory Technical Standards for Sustainable Drainage systems 2.1.12. The Non-Statutory Technical Standards for Sustainable Drainage Systems6 state that for greenfield developments the peak runoff rate from the Proposed Development to any highway drain, sewer or surface water body for the 100% Annual Exceedance Probability (AEP) and 1.0% AEP events should never exceed the peak greenfield runoff rate for the same event. 2.1.13. Where it is reasonably practicable, the runoff volume from the Proposed Development to any highway drain, sewer or surface water body in the 1.0% AEP, 6 hour rainfall event should never exceed the existing runoff volume for the same event. If this is not practicable the runoff volume must be discharged at a rate that does not adversely affect flood risk. 2.1.14. The drainage system must be designed so that unless an area is designated to hold and / or convey water as part of the design, flooding does not occur on any part of the Proposed Development for a 3.3% AEP event, and flooding does not occur during a 1.0% AEP event in any part of a building or utility plant susceptible to water. The design of the Proposed Development must ensure that where reasonably practicable flows resulting from a rainfall in excess of a 1.0% AEP event are managed in exceedance routes that minimise risks to people and property. 2.2 LOCAL POLICY Preliminary Flood Risk Assessment 2.2.1. Hampshire County Council (HCC) is required under the Flood Risk Regulations 2009 to undertake a high level screening of flood risk within the county. This is recorded in the form of a Preliminary Flood Risk Assessment (PFRA)7. 2.2.2. One of the primary objectives of the PFRA is to identify which areas should be prioritised for Surface Water Management Plans (SWMP), see Paragraphs 2.2.14 to 2.2.15, and to inform the development of the Local Flood Risk Management Strategy (LFRMS)8, see Paragraphs 2.2.5 to 2.2.11. 2.2.3. The PFRA identifies that Fleet, in which the Site is located, is ranked 91st on the Environment Agency’s national assessment of flood risk, with 3,980 people potentially at risk. 2.2.4. Fleet forms one of eight areas within Hampshire initially considered to be subject to a substantial risk of flooding. The PFRA concludes that these areas would undergo further investigation as part of the development of the LFRMS to further identify problems, develop mitigation measures and to determine whether a SWMP is required.
6 Department for Food and Rural Affairs (2015) Sustainable Drainage Systems: Non-statutory technical standards for sustainable drainage systems; accessed March 2018 from https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/415773/sustainable-drainage-technical- standards.pdf 7 Hampshire County Council (2011) Preliminary Flood Risk Assessment; accessed May 2017 from https://www.hants.gov.uk/landplanningandenvironment/environment/flooding/strategies/Preliminary-Flood-Risk- Assessment 8 Hampshire County Council (2013) Local Flood Risk Management Strategy; accessed May 2017 from https://www.hants.gov.uk/landplanningandenvironment/environment/flooding/strategies/Local-Flood-Risk-Management- Strategy
WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Page 6 of 24 Thakeham Homes LOCAL FLOOD RISK MANAGEMENT STRATEGY 2.2.5. As part of the implementation of the Flood and Water Management Act 2010, HCC were designated a Lead Local Flood Authority (LLFA). This provided them with additional powers and responsibilities for coordinating local flood risk management. As part of their role as LLFA, HCC are required to ‘develop, maintain, apply and monitor a Strategy for local flood risk management’ within Hampshire. 2.2.6. In order to meet this obligation HCC have developed and maintain a LFRMS. This strategy covers a timeframe of fifteen years in order to enable short, medium and long-term objectives to be set for the improvement of flood risk management within Hampshire. 2.2.7. The LFRMS focuses on local flooding from surface water (pluvial flooding), groundwater, Ordinary Watercourses9 and drainage ditches. It was developed with a view to providing opportunities to deliver additional benefits beyond a reduction in flood risk, such as improving water quality or creating green space, to the county. 2.2.8. The findings of the risk assessment undertaken as part of the development of the LFRMS are used to inform the assessment of the flood risk posed to the Site and the Proposed Development contained within Section 4 of this report. 2.2.9. As part of the development of the LFRMS, action plans were produced for those wards shown by the strategy to be at the highest risk of flooding. Tables 4.4 to 4.7 of the main strategy document identify those wards with the highest risk from each of the identified flood risk sources. 2.2.10. Fleet is not ranked within the top five wards for any source of flooding nor the combined risk of flooding from multiple sources. Consequently, no action plans were developed as part of the LFRMS that cover the Site. 2.2.11. The LFRMS identified that a SWMP is required to be undertaken by HDC to investigate surface water flood risk posed to Fleet. Groundwater Management Plan 2.2.12. As part of their role as LLFA, HCC are responsible for managing the risk of flooding posed by groundwater. The Groundwater Management Plan (GWMP)10 forms the basis of HCC’s strategy for understanding and managing this risk. 2.2.13. The GWMP evaluated flood risk information from a number of sources to identify those areas considered to be subject to a high risk of groundwater flooding. Fleet is not shown to be within the top ten risk areas; consequently, no actions are proposed as part of the GWMP for managing groundwater flood risk in the vicinity of the Site. Surface Water Management Plan 2.2.14. The PFRA identified Fleet as one of eight areas within Hampshire understood to be subject to a significant risk of flooding. The LFRMS identified that a SWMP is required to be produced by HDC to evaluate the existing risk of surface water flooding at a district level in order to develop a greater understanding of flooding within Fleet. 2.2.15. The PFRA identifies that a SWMP is being prepared by HDC; however, at the time of writing the report is not available.
9 Ordinary Watercourses in England are those watercourses not designated by the Department for Environment, Food and Rural Affairs (DEFRA) as Main Rivers (see 16), with the responsibility for management and maintenance falling to the Lead Local Flood Authority. 10 Hampshire County Council (2013) Groundwater Management Plan; accessed May 2017 from https://www.hants.gov.uk/landplanningandenvironment/environment/flooding/strategies/Groundwater-Management- Plan
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes Page 7 of 24 Strategic Flood Risk Assessment 2.2.16. The NPPF requires LPAs to produce a Strategic Flood Risk Assessment (SFRA) to investigate flood risk within their administrative boundaries. The findings of the SFRA should be used to inform the LPAs strategic land use planning. 2.2.17. The conclusions of HDC’s SFRA11 evaluation of flood risk within Hart are used to inform the assessment of the flood risk posed to the Site and the Proposed Development contained within Section 4 of this report. 2.2.18. The SFRA states that More Vulnerable land-uses, such as the Proposed Development, located within Flood Zone 2 or 3, or within a fluvial dry island, are required to assess how hazardous proposed access and egress routes are to end users of the development. Developers are required to demonstrate that the maximum flood hazard during the 1 in 100 year plus climate change fluvial flood event will be no greater than ‘Very Low’ with routes extending from buildings to a location wholly outside of the floodplain. 2.2.19. The report identifies that on occasions where it is appropriate to locate More Vulnerable or Highly Vulnerable development within flood risk areas for which safe access and egress cannot be demonstrated the use of an emergency flood plan may be sufficient to manage the residual flood risk. 2.2.20. The SFRA states that in exceptional cases where land allocation within flood risk areas is unavoidable, new developments should be designed so that flood waning complements other measures to control and minimise the residual risk posed to end users of the development. However, the report identifies that the smaller tributaries of the River Hart, such as those in the vicinity in the Site, are not covered by the Environment Agency’s Flood Warning system due to the flashy nature of the flooding associated with them. 2.2.21. The SFRA concludes that the outputs from the study should be used as the evidence base from which to steer new development to Flood Zone 1, and this approach has been adopted in developing the masterplan. Where sufficient development cannot be located in Flood Zone 1 the council should apply the Sequential Test to their remaining land use allocations to ensure that flood risk across Hart is minimised whilst still enabling strategic housing demand to be satisfied. Local Plan 2.2.22. HDC’s Local Plan provides a framework for how development will be planned and delivered across the district until 2032. 2.2.23. Policy NBE6 (Managing Flood Risk) states that ‘Development will be permitted provided: a) Over its lifetime it would not increase the risk of flooding elsewhere and will be safe from flooding; b) Sustainable drainage systems (SuDS) are used in major developments unless demonstrated to be inappropriate; c) Within Causal Areas (as defined in the SFRA) all development takes opportunities to reduce the causes and impacts of flooding; d) If located within an area at risk from any source of flooding now and in the future, it is supported by a site-specific flood risk assessment and complies fully with national policy; e) It would not compromise the integrity and function of a reservoir or canal embankment. 2.2.24. The Site is not shown by the SFRA to be located within a Causal Area. 2.2.25. Paragraph 316 states that ‘Sustainable Drainage Systems (SuDS) that mimic natural systems are required where feasible on all major developments [and] should seek to enhance water quality, amenity and biodiversity’.
11 Hart District Council (2016) Strategic Flood Risk Assessment; accessed May 2017 from https://www.hart.gov.uk/Evidence-base
WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Page 8 of 24 Thakeham Homes 2.2.26. Paragraph 319 states that ‘Sites should look to discharge their surface water to as sustainable location as possible. Generally, the aim should be to discharge surface run off as high up the following hierarchy of drainage options as reasonably practicable; 1) into the ground (infiltration); 2) to a surface water body; 3) to a surface water sewer, highway drain, or another drainage system; (sic.) 2.2.27. Due to the anticipated impermeability of the underlying geology at the Site, surface water generated by the Proposed Development would be discharged to Minley Brook (i.e. a surface water body) in accordance with the proposed hierarchy above. 2.2.28. Policy NBE7 (Water Quality) states that ‘development will be required to protect the quality of the District’s water environment, and where possible contribute towards improvements that are necessary to meet statutory requirements for water quality. Development will be supported provided that: a) it would not deteriorate the individual element band status of the District’s waterbodies or prevent the waterbodies from achieving ‘good’ ecological status; b) it would protect and, where possible, enhance the quality of groundwater and surface water features; c) any adverse impacts on local water bodies would be fully mitigated; d) it incorporates sustainable drainage systems where appropriate. 2.2.29. Policy I1 states that for ‘[a]ll development that requires planning permission […] developers will be required to demonstrate that there is adequate waste water capacity and surface water drainage both on and off the site to serve the development’. 2.2.30. The supporting description included with Policy I2 outlines that ‘Green infrastructure provided on site should be designed to be multi-functional and planned holistically alongside other policy requirements for open space recreation, biodiversity, landscaping, flood risk and sustainable drainage.’
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes Page 9 of 24 3 THE SITE
3.1 SITE LOCATION 3.1.1. The Site is located north of Fleet and can be located by Ordnance Survey National Grid reference 481700, 156500. The nearest postcode is GU51 2RF. A site location plan is provided in Appendix A.1 for reference. 3.1.2. The Site is bound by the M3 to the south, woodland to the north, greenfield land to the east and Minley Road to the west. 3.2 SITE DESCRIPTION 3.2.1. The Site has a gross area of 45.6 ha, of which the Proposed Development would cover 12.6 ha, and is predominantly let for pasture. The existing development comprises Brook House and accompanying out- buildings along with hardstanding access and parking areas. 3.3 TOPOGRAPHY 3.3.1. A review of the Environment Agency’s Light Detection and Ranging (LiDAR) topographical data12 and a detailed topographical site survey shows existing ground levels to generally fall towards Minley Brook which runs from east to west through the northern half of the Site. 3.3.2. The maximum and minimum ground levels within the Site are shown to be approximately 67.5 m Above Ordnance Datum (AOD) in the north-western corner of the Site and 60.5 m AOD along the route of Minley Brook respectively. 3.3.3. The LiDAR data shows that the M3 and Minley Road, to the south and west of the Site respectively, are significantly embanked. The M3 is shown to have a maximum height of approximately 2 m above surrounding ground levels and Minley Road is shown to be a maximum of 7 m higher than the Site. 3.4 GEOLOGY AND HYDROGEOLOGY 3.4.1. British Geological Survey (BGS) 1:50,000 scale On-shore Digital Mapping13 (see Appendix A.2) indicates that the Site is underlain by a combination of the Windlesham Formation (Sand, Silt and Clay) and the Camberley Sand Formation (Sand). Isolated superficial deposits of Head (Clay, Silt, Sand and Gravel) are shown to be located along Minley Brook at the eastern Site boundary and in the far north-western corner of the Site. 3.4.2. BGS 1:625,000 scale GeoIndex Hydrogeology mapping14 and the Environment Agency Groundwater mapping15 shows that the underlying bedrock is classified as a Secondary A aquifer. The Environment Agency mapping shows that the isolated superficial deposits described above are designated as a Secondary A or Secondary (undifferentiated) aquifer. Secondary A aquifers are defined as deposits capable of supporting groundwater transmission on a local rather than strategic scale. 3.4.3. The Environment Agency Groundwater mapping shows that the site is not located within a Source Protection Zone.
12 Environment Agency (2017) LiDAR Composite DTM – 1m; obtained November 2018 from https://data.gov.uk/dataset/lidar-composite-dtm-1m1 13 British Geological Survey (2018) DiGMapGB-50 - WMS; accessed May 2017 from http://www.bgs.ac.uk/data/services/digmap50wms.html 14 British Geological Survey (2018) GeoIndex Hydrogeology – WMS; obtained from http://www.bgs.ac.uk/geoindex/wms.htm 15 Environment Agency (2016) What’s in your backyard? Interactive Maps – Groundwater; accessed March 2018 from http://maps.environment-agency.gov.uk/wiyby/wiybyController?ep=maptopics&lang=_e
WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Page 10 of 24 Thakeham Homes 3.5 EXISTING WATERCOURSES 3.5.1. There are two Main Rivers16 within the vicinity of the Site, Minley Brook and Fleet Brook. Both watercourses are tributaries of the River Hart which lies approximately 2.8 km to the west of the Site and are shown on Drawing 3671-EA-001-B in Appendix C.1. 3.5.2. Minley Brook flows from east to west through the northern half of the Site. Fleet Brook flows from south to north from its origin at Fleet Pond, approximately 800 m to the south of the Site, through the northern part of Fleet and under the M3 before running along the western boundary of the Site to its confluence with Minley Brook. 3.5.3. There is a comprehensive network of drainage ditches within the Site which intercept surface water runoff generated by the Site prior to discharging to Minley Brook. A site visit, undertaken on 25 April 2017, found the existing drainage network to be very well maintained. 3.5.4. According to previous flood investigation work undertaken on behalf of the owner of the Site, prior to the construction of the M3 and the subsequent industrial development immediately to the south of the motorway Fleet Brook is understood to have run from south to north through the centre of the Site before discharging into Minley Brook in the vicinity of Brook House. 3.6 FLOOD DEFENCES 3.6.1. The Environment Agency’s Flood Map for Planning does not show the site to benefit from existing formal flood defences. 3.6.2. As described in Paragraph 3.3.3, both the M3 and Minley Road, which form the southern and western boundaries of the Site respectively, are significantly embanked in the south western corner of the Site. Both highways would therefore act as informal defences against fluvial flooding from Fleet Brook which runs from south to north on the far side of Minley Road to the Site. 3.7 EXISTING SEWER AND DRAINAGE INFRASTRUCTURE 3.7.1. Thames Water asset location plans, acquired as part of previous flood investigation work on behalf of the owner of the Site, show the presence of extensive foul water sewerage and surface water drainage infrastructure within Fleet immediately to the south of the M3. 3.7.2. The asset location plans also show two existing attenuation basins, serving Ancells Business Park to the south of the M3, that discharge into separate culverts beneath the M3. The westernmost culvert provides a connection to the existing pond within the Site, whilst the easternmost culvert connects into a drainage ditch which runs from south to north along the eastern boundary of the Site. 3.7.3. The asset location plans show a foul rising main crossing the southern part of the Site from east to west. 3.8 HISTORIC FLOOD RECORDS 3.8.1. The Environment Agency’s Historic Flood Map is included on Drawing 3671-EA-003-B in Appendix C.3 and does not show any records of a historic flood event in the vicinity of the Site. The nearest recorded historic flood event is shown along the River Hart approximately 2.8 km to the west of the Site. The current owner of the Site is not aware on any significant flood events at the Site. 3.8.2. As part of a previous study commissioned by the owner of Brook House, Thames Water confirmed that they hold no records of a historic flood event associated with the surcharging or failure of their sewerage assets in the vicinity of the Site.
16 Main Rivers in England are designated by the Department for Environment, Food and Rural Affairs (DEFRA) with the responsibility for management and maintenance falling to the Environment Agency
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes Page 11 of 24 4 OVERVIEW OF FLOOD RISK
4.1 INTRODUCTION 4.1.1. The following section provides an overview of flood risk to the Site, along with risk arising to and from the Proposed Development. An assessment has been undertaken for each flood source in accordance with the national planning policy detailed in Section 2.1. 4.1.2. Flood risks have been qualitatively assessed on the following basis: ¡ Negligible risk (e.g. coastal flood risk posed to inland areas); ¡ Very Low risk (e.g. Flood Zone 1 or <0.1% annual probability of flooding from surface water); ¡ Low risk (e.g. Flood Zone 2 or between 0.1% and 1.0% annual probability of flooding from surface water); ¡ Moderate risk (e.g. Flood Zone 3a or between 1.0% and 3.3% annual probability of flooding from surface water); and, ¡ High risk (e.g. Flood Zone 3b or >3.3% annual probability of flooding from surface water). 4.2 EXISTING RISK OF FLOODING TO THE SITE FLOODING FROM COASTAL AND TIDAL SOURCES 4.2.1. Tidal flooding occurs when sea levels rise above the level of the land or beyond the operational level of flood defences. 4.2.2. The Site is not located within the vicinity of the coast or a tidal watercourse; consequently, the risk of coastal and tidal flooding to the Site is Negligible. FLOODING FROM FLUVIAL SOURCES 4.2.3. Fluvial flooding occurs when flows within watercourses exceed the capacity of the channel causing out of bank flows. 4.2.4. The Environment Agency’s Flood Map for Planning, reproduced on Drawing 3671-EA-001-B in Appendix C.1, shows the Site to lie within Flood Zones 1, 2 and 3. 4.2.5. Flood Zone 1 is defined as being subject to an annual probability of flooding from Main Rivers of less than 0.1% (i.e. 1 in 1000 year return period event). Flood Zone 2 is classified as having an annual probability of flooding of between 0.1% and 1.0% (i.e. 1 in 100 year return period event), whilst Flood Zone 3 is defined as having a fluvial flood risk of greater than 1.0%. 4.2.6. The Environment Agency have confirmed as part of previous flood investigation work undertaken on behalf of the owner of the Site that the flood extents in the vicinity of the Site given on the Flood Map for Planning were produced using the JFlow hydraulic modelling package in 2006. The flood extents were produced as part of a broad scale national modelling exercise with the objective of providing coarse estimates of peak flood extents across the country. 4.2.7. The estimates of the peak flood extents associated with the vast majority of Main Rivers in England and Wales have subsequently been refined using detailed 1D-2D hydraulic modelling on a local or regional scale. The Environment Agency have confirmed that a detailed modelling study has not been undertaken but is planned for 2018. 4.2.8. The Environment Agency have stated that the outputs from the national scale JFlow modelling exercise are not sufficiently accurate to enable the assessment of flood risk on an individual site basis. 4.2.9. The Flood Map for Planning shows three overland flow paths across the M3 which connect the floodplain of Fleet Brook with the Site. Review of the available LiDAR data, corroborated with observations made during a site visit, shows that both the M3 and Minley Road are significantly embanked in the areas in which the floodplain is shown to extend across the local road network onto the Site. 4.2.10. The M3 is shown by the LiDAR data to have a maximum height of approximately 2 m above surrounding ground levels whilst Minley Road, which crosses the M3, has a maximum height of approximately 7 m. This indicates that the three major flow paths crossing the M3 onto the Site shown on the Flood Map for Planning are unrealistic.
WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Page 12 of 24 Thakeham Homes 4.2.11. From inspection of the Flood Map for Planning it does not appear to provide an estimate of the fluvial flood risk associated with Minley Brook. The flood extents shown are not consistent along the length of Minley Brook within the Site and there are no flood extents shown to the east of the Site. 4.2.12. The Environment Agency confirmed, as part of the pre-application consultation process that the Flood Map for Planning may not be accurate in the vicinity of the Site. They agreed that undertaking a site specific hydraulic model would improve the definition of fluvial flood risk information for the Site. 4.2.13. The Flood Map for Planning shows peak flood extents produced by a model which is not considered appropriate for development scale flood risk appraisal. Consequently, a two-dimensional (2D) hydraulic model (i.e. with no channel data) was produced to inform the previous Regulation 18 consultation representations using InfoWorks ICM17. The model incorporated key hydraulic structures based on observations made during a site visit and published information available at the time of writing and is detailed in Revision 2 of this report 4.2.14. InfoWorks ICM is an industry standard hydraulic modelling software package that has been benchmarked and approved by the Environment Agency for the use of fluvial flood risk estimation. A summary of the development of the 2D hydraulic model used to assess flood risk at the Site is included in Appendix E.1. 4.2.15. The flood extents at the Site given by the preliminary hydraulic model are given in Appendix C.4 and confirm that the flood risk at the Site is governed by a combination of surface water and fluvial flooding from Minley Brook rather than fluvial flooding from Fleet Brook as suggested by the Flood Map for Planning. It shows that there is minimal fluvial flood risk posed to the Site from Fleet Brook as the embanked M3 and Minley Road isolate the Site from the watercourse. 4.2.16. However, the 2D only model is limited in its ability to represent the influence of Minley Brook, the associated internal drainage ditches, and Fleet Brook. Consideration of the influence of these features is necessary to provide a comprehensive assessment of flood risk at the Site. 4.2.17. Consequently, as part of our assessment we have incorporated one-dimensional (1D) channel survey of Minley Brook, the associated internal drainage ditches, and Fleet Brook, in order to improve confidence in model results and to refine the peak flood extents at the Site. This data was procured from CD Surveys Ltd with EACSD18 validation certificates provided for each reach surveyed. Incorporation of this data enables an assessment of flood risk at the Site in sufficient detail to support planning application. 4.2.18. The proposed methodology was submitted to the Environment Agency for approval in advance of the the hydraulic modelling process, as part of the pre-application consultation process. The Environment Agency’s review recommended two minor amendments to the proposed methodology which have been incorporated into the development of the model. 4.2.19. The development of the detailed 1D-2D hydraulic model for the Site is described in our standalone Hydraulic Modelling Report19 and is summarised in Appendix E.2 of this document. 4.2.20. The results of the detailed 1D-2D hydraulic model are presented in Appendix C.5 and show that the primary flood risk to the site is presented by fluvial flooding generated by Minley Brook, the capacity of which is exceeded during large storm events resulting in water spilling out of bank on to the Site. 4.2.21. The detailed 1D-2D model shows areas of the Site to be at risk of fluvial flooding in general conformity with the preliminary 2D model. However, the peak fluvial flood extents at the Site are significantly reduced as a result of the inclusion of the detailed channel and structure survey data, obtained since the development of the 2D model. The update included the extensive and well maintained drainage network found at the Site along with the separation of fluvial and pluvial inflows (the preliminary 2D model showed the combined flood risk from both sources). 4.2.22. The results of the detailed 1D-2D model shows the Site to lie within the equivalent of Flood Zones 1, 2 and 3. Flood Zone 3 comprises 9.1 % of the total area of the Site whilst Flood Zone 2 covers a further 6.5 %. The remaining 84.4 % of the Site is located entirely within the equivalent of Flood Zone 1.
17 Innovyze (2017) InfoWorks Integrated Catchment Modelling software 18 Environment Agency Channel Survey Data 19 WSP (2018) Brook House, Fleet - Hydraulic Modelling Report
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes Page 13 of 24 4.2.23. Table 2 below summarises the alteration in the anticipated peak flood extents at the Site as a result of the development of the detailed hydraulic model. This indicates that the Flood Map for Planning over-predicts the extent of Flood Zones 2 and 3.
Table 2 - Summary of Flood Extents Flood Extent (ha / %) Source Flood Zone 3 Flood Zone 2 Flood Zone 1 Flood Map for Planning 7.2 ha (15.5% ) 8.9 ha (19.2%) 30.3 ha (65.3%) (Environment Agency) Detailed Hydraulic Model 4.2 ha (9.1%) 3.0 ha (6.5%) 39.2 ha (84.4%) (WSP)
4.2.24. The detailed 1D-2D model and the accompanying Hydraulic Modelling Report have been reviewed by the Environment Agency, and they have made minor suggestions, which have been included in the modelling. Consequently, the outputs of the detailed 1D-2D model will provide the best available representation of flood risk at the Site, superseding the Environment Agency’s current published fluvial and pluvial mapping described above. FLOODING FROM PLUVIAL SOURCES 4.2.25. Surface water flooding may arise during extreme storms, when the ground may become saturated, and the drains and sewers which carry away the surface water may not be able to accommodate these flows. 4.2.26. The Environment Agency’s Risk of Flooding from Surface Water mapping, reproduced on Drawing 3671-EA- 002-A in Appendix C.2, shows that the majority (approximately 74%) of the Site is not subject to a risk of surface water (i.e. pluvial) flooding. These areas have an annual probability of surface water flooding of less than 0.1% and are considered to be at a Very Low risk. The remainder of the Site (approximately 26%) is shown to lie in areas subject to between a Low and High risk of flooding from surface water, with these areas concentrated along Minley Brook. 4.2.27. The surface water flood risk at the Site is shown to originate from three primary sources; flows from Minley Brook to the east, runoff generated off-site by Ancells Business Park to the south and the greenfield land to the north of the Site, and surface water runoff generated by the Site itself. 4.2.28. The peak surface water flood extents produced by the detailed 1D-2D model are given on Drawing 33671- HMR-101-B in Appendix C.5 and, following successful completion of the Environment Agency’s review, will represent the best available estimation of pluvial flood risk at the Site. The model shows that isolated areas of the Site are at risk of flooding from surface water. 4.2.29. The majority of the areas of the Site shown to be at risk of flooding from this source are concentrated along Minley Brook and generally lie within the fluvial flood extents discussed in paragraph 4.2.22. 4.2.30. Smaller isolated areas of flooding are shown outside of the fluvial floodplain; however, these areas are likely to be predominantly at risk from surface water runoff generated by rainfall falling on the Site itself and as such would be managed by the proposed drainage strategy described in Section 6 of this report. 4.2.31. As the catchment of Minley Brook is largely surface water fed and the flows entering the Site from Ancells Farm are controlled by an existing attenuation basin, the fluvial flood risk and pluvial flood risk posed to the Site are intrinsically linked. Consequently, the management of both of these sources of risk forms a key principle of the development of the Masterplan. 4.2.32. The mitigation measures incorporated within the Proposed Development to manage the fluvial and pluvial flood risk are described in Section 4.3 along with an assessment of the resulting risk posed to, and arising from, the Proposed Development. FLOODING FROM GROUNDWATER SOURCES 4.2.33. Groundwater flooding occurs when the water table rises above the level of the ground. 4.2.34. HDC’s SFRA identifies that groundwater flooding is particularly difficult to identify as incidents tend to occur in combination with other forms of flooding (for example pluvial flooding and sewer flooding).
WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Page 14 of 24 Thakeham Homes 4.2.35. Historic groundwater flood events were reported within Hart during the winters of 2000/2001 and 2013/2014. There were no reported incidents of groundwater flooding within Fleet during the winter of 2000/2001, however there were three reported incidents of groundwater and combined groundwater/surface water flooding in Fleet in 2013/2014. The location of the reported incidents was not confirmed by the SFRA. 4.2.36. The SFRA contains an extract of the BGS’s Areas Susceptible to Groundwater Flooding mapping which provides a broad estimate of those areas which may be vulnerable to groundwater flooding based on their underlying geology. This mapping shows the Site to lie within an area considered to have ‘Limited potential for groundwater flooding to occur’. 4.2.37. HCC’s GWMP does not identify Fleet to be subject to a substantial risk of flooding from groundwater and no action plans were developed to manage groundwater flood risk in the vicinity of the Site. 4.2.38. The bedrock geology expected to underlie the Site as described in Section 3.4 is not anticipated to be highly permeable. It is therefore unlikely to facilitate a rapid groundwater response and consequently limits the risk of potential groundwater flooding posed to the Site. 4.2.39. Based on the information available at the time of writing it is considered that then Site is at a Low risk of groundwater flooding. It is recommended that a targeted site investigation be undertaken as the project progresses to confirm the seasonal groundwater levels at the Site. FLOODING FROM SEWER AND DRAINAGE INFRASTRUCTURE 4.2.40. Sewer flooding occurs when the capacity of the local sewer or drainage network is exceeded. 4.2.41. Thames Water asset location plans show a foul rising main crossing the southern half of the Site. However, there are no other public sewers serving, or within the immediate vicinity of, the Site. The nearest surface and foul water sewer networks are located to the south of the M3. 4.2.42. The Local Plan identifies that Fleet is subject to the highest risk of flooding from sewer and drainage infrastructure within the district, although the magnitude of this risk is not stated. 4.2.43. Thames Water has confirmed that it holds no records of historic sewer flooding in the vicinity of the Site. 4.2.44. Any flooding resulting from the exceedance or blockage of sewerage assets would likely be localised and confined to the road network to the south of the M3. Consequently, the risk of flooding to the Site from sewer and drainage infrastructure is considered to be Very Low. FLOODING FROM ARTIFICIAL SOURCES 4.2.45. Flooding from artificial sources occurs when artificial waterbodies, such as canals or large ponds, experience full or partial failure. 4.2.46. The Site is located to the north of Fleet Pond, a large freshwater lake and nature reserve. Fleet Pond provides significant attenuation of flows generated by the Fleet Brook catchment to the south of the Site. 4.2.47. Fleet Pond lies below surrounding ground levels with water levels controlled by a weir at the upstream end Fleet Brook. As the Site is largely isolated from the floodplain of Fleet Brook by the M3 and Minley Road a controlled draw-down of the Fleet Pond is unlikely to significantly impact the Site. 4.2.48. The Environment Agency’s Flood Risk from Reservoirs mapping20 does not show the Site to be at risk of flooding from the failure of reservoirs. 4.2.49. The probability of failure of existing reservoirs is extremely remote and, therefore, the overall risk of flooding from reservoirs to the Site is considered to be Very Low.
20 Environment Agency (2017) Flood Risk from Reservoirs; accessed May 2017 from https://flood-warning- information.service.gov.uk/long-term-flood-risk/
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes Page 15 of 24 4.3 RISK OF FLOODING TO AND ARISING FROM THE PROPOSED DEVELOPMENT 4.3.1. The NPPF requires any development proposals to demonstrate that flood risk can be managed over the lifetime of said development, taking into account the anticipated effects of climate change. FLOODING FROM COASTAL AND TIDAL SOURCES 4.3.2. As the Site is not in the vicinity of the coast or a tidal estuary the risk of Proposed Development exacerbating the risk of coastal and tidal flooding both to and in the vicinity of the Proposed Development is considered to be Negligible. 4.3.3. Consequently, no mitigation measures are necessary for inclusion within the Proposed Development to address the risks presented by these sources. FLOODING FROM FLUVIAL AND PLUVIAL SOURCES 4.3.4. As fluvial and pluvial flood risk is intrinsically linked at the Site the mitigation of these risks to the Proposed Development are considered in combination. 4.3.5. In order to manage fluvial flood risk to the Proposed Development, a sequential approach has been adopted for the development of the Masterplan with all More Vulnerable land-uses located in areas shown to be subject to Very Low risk of flooding from this source (i.e. within the equivalent of Flood Zone 1). 4.3.6. The built development would therefore have an annual probability of flooding from Minley Brook and from surface water of less than 0.1% as it would be located entirely outside of the floodplain and within Flood Zone 1. 4.3.7. This is considered by the NPPF to be an acceptable use of land within Flood Zone 1. Consequently, the risk of fluvial flooding posed to the Site will not prohibit the Proposed Development from being successfully delivered. 4.3.8. Drawings 3671-HMR-001-B shows that safe access and egress to and from the Proposed Development would be provided by Minley Road during the design flood event (i.e. the 1.0 % AEP event) which is not shown to be at risk of fluvial or pluvial flooding during an event of this magnitude. 4.3.9. A holistic approach has been adopted when developing the Masterplan in accordance with Policy I2 of the Local Plan with natural parkland and recreational facilities provided in those areas currently shown to be at risk of flooding. This provides the desired synergy between green infrastructure provision, ecological enhancement, flood risk management, and amenity value stated in the Local Plan. 4.3.10. In addition, the existing on-site drainage regime has been incorporated within the development proposals with all major existing overland flow routes retained. This will ensure that exceedance flows are managed using existing flows paths, minimising the risk to the Proposed Development during an extreme storm event. 4.3.11. The majority of the areas within the Site subject to pluvial flood risk lie within the fluvial flood extents and consequently this risk will be mitigated by the adoption of the sequential approach as described above. The small isolated areas of surface water flooding outside of the fluvial floodplain are likely to be predominantly at risk from surface water runoff generated by rainfall falling on the Site itself and as such would be managed by the proposed drainage strategy described in Section 6 of this report. 4.3.12. The management of surface water within the Proposed Development in accordance with national and local policy along with current best practice guidance will reduce the risk of pluvial flooding both to and from the Proposed Development to Low. 4.3.13. Finished site levels, where practicable, should be engineered to prevent ponding. Gradients of external areas should be designed to fall away from buildings, such that any overland flow route from extreme events would follow the path of least resistance, follow natural topography and be channelled away from proposed and existing properties. The accumulation of standing water would therefore be minimised and thus not pose a significant risk. 4.3.14. In consideration of the mitigation measures outlined above, the risk to the Proposed Development and of the Proposed Development exacerbating flood risks from fluvial and pluvial sources is considered to be Very Low and Low respectively.
WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Page 16 of 24 Thakeham Homes FLOODING FROM GROUNDWATER SOURCES 4.3.15. As the Site is currently considered to be subject to a Low risk of groundwater flooding no mitigation measures are necessary to mitigate the risk posed to the Proposed Development from this source. This conclusion should be reassessed once seasonal groundwater levels have been established at the Site as part of a targeted site investigation. 4.3.16. No basements or underground car parks are currently proposed as part of the Proposed Development. The detailed design will be informed by geotechnical investigations however it is considered unlikely that the construction of the Proposed Development will impact existing groundwater flow routes. The risk of the Proposed Development exacerbating groundwater flooding to neighbouring property therefore considered to be Very Low. FLOODING FROM SEWERS AND DRAINAGE INFRASTRUCTURE 4.3.17. Additional surface water generated by the Proposed Development would be discharged directly to Minley Brook via a controlled outfall and consequently would not introduce additional flows into the public sewer network. 4.3.18. The Proposed Development would have no impact on flood risk from this source to the Site, or surrounding area. Consequently, the risk of flooding from this source both posed to, and resulting from, the Proposed Development is Very Low and no mitigation measures are necessary for inclusion within the Proposed Development to manage this risk. FLOODING FROM ARTIFICIAL SOURCES 4.3.19. As the Site is subject to a Very Low risk of flooding from artificial sources, as described in paragraph 4.2.49, the risk of the Proposed Development exacerbating flooding from these sources to neighbouring property is Negligible. Consequently, no mitigation measures are considered necessary for inclusion within the Proposed Development to address the risks presented by these sources.
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes Page 17 of 24 4.4 SUMMARY OF FLOOD RISK 4.4.1. A summary of flood risk currently posed to the Site along with the potential risk both to and from the Proposed Development is shown in Table 2. 4.4.2. The following mitigation measures will be adopted to reduce the risk to the Proposed Development: ¡ Adoption of the sequential approach to locate all built development within the equivalent of Flood Zone 1 (i.e. a Very Low risk of fluvial flooding) will reduce the risk of both fluvial and pluvial flooding; and, ¡ Management of surface water within the Proposed Development in accordance with national and local policy along with current best practice guidance will reduce the risk of pluvial flooding both to and from the Proposed Development to Low.
Table 3 – Summary of Flood Risk to and from the Site/Proposed Development Sources of Flood Risk Current Risk to the Site Future Risk to the Future Risk from Proposed the Proposed Development Development Coastal and Tidal Negligible Negligible Negligible Fluvial High (4.7%), Moderate (4.4%) Very Low Very Low Low (6.5 %), Very Low (84.4%) Pluvial and Overland Flow High to Low Low Low Groundwater Low Low Very Low Drainage Infrastructure Very Low Very Low Very Low Artificial Sources Negligible Negligible Negligible
WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Page 18 of 24 Thakeham Homes 5 CONSULTATION
5.1 ENVIRONMENT AGENCY 5.1.1. WSP and Thakeham Homes attended a meeting with the Environment Agency in January 2018 to discuss flood risk in relation to development at the Site. 5.1.2. WSP presented the Flood Map for Planning and discussed the likely inaccuracies associated with the modelling approach and flow-paths shown across the M3 onto the Site as described in paragraph 4.2.11. 5.1.3. The Environment Agency acknowledged that, due to the coarse nature of the modelling on which some areas of the Flood Map for Planning are based, there are occasionally local inaccuracies which require further investigation. It also acknowledged that, from a first look at the Flood Map for Planning in this area and the explanation of the validation exercise undertaken by WSP, the published information is unlikely to be fully representative of fluvial flood risk at the site. 5.1.4. The Environment Agency confirmed that its review of the Hydraulic Model Brief submitted by WSP, which forms the basis on which the detailed 1D-2D model was developed, had been completed and that two changes to the proposed methodology were recommended. WSP confirmed that the recommend changes to the model methodology, namely undertaking additional hydrological analysis using a separate recognised method and separating out the pluvial and fluvial inflows, had already been incorporated into the model following an internal review. 5.1.5. The Environment Agency acknowledged that the modelled flood extents presented by WSP appear to provide a more accurate representation of flood risk at the Site than the Flood Map for Planning. The detailed 1D-2D model and the accompanying Hydraulic Modelling Report have been reviewed by the Environment Agency, and they have made minor suggestions, which have been included in the modelling. Consequently, the outputs of the detailed 1D-2D model will provide the best available representation of flood risk at the Site, superseding the Environment Agency’s current published fluvial and pluvial mapping described above. 5.1.6. The peak flood extents given by the detailed hydraulic model shows approximately 85% of the Site to lie within the equivalent of Flood Zone 1, with a resulting risk of fluvial flooding of less than 0.1%. Consequently, the entirety of the built development can be located within Flood Zone 1 with safe access and egress provided to and from the Proposed Development via Minley Road.
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes Page 19 of 24 6 SURFACE WATER DRAINAGE
6.1 EXISTING DRAINAGE REGIME 6.1.1. The Site currently drains through a combination of infiltration and overland flow towards Minley Brook or the existing network of drainage ditches which discharge into Minley Brook. 6.1.2. The Institute of Hydrology Report 12421 method has been used to determine the existing, greenfield, runoff rates for a range of storm events in accordance with current national guidance22. 6.1.3. Peak greenfield runoff rates currently generated by the Site are given in Table 4 with calculations provided for reference in Appendix D.1.
Table 4 – Peak Greenfield Runoff Rates Storm Event Unit runoff rate (l/s/ha) Total Site Area (Ha) Total Runoff Rate (l/s) QBAR 2.8 45.6 127.7 100% AEP 2.4 109.4 3.3% AEP 6.4 291.8 1.0% AEP 9.0 410.4
6.2 SURFACE WATER MANAGEMENT STRATEGY REQUIREMENTS 6.2.1. The NPPF requires that flood risk to land and property is not increased as a result of development upstream. The Non-Statutory Technical Standards for Sustainable Drainage Systems state that peak runoff rates should be limited to the greenfield runoff rates, and the volume of runoff from the site for the 1 in 100 year, 6 hour rainfall event should not exceed the greenfield runoff volume for the same event. Exceedance routes should be shown to not impact the safety of property. 6.2.2. Building Regulations Approved Document H23 establishes a hierarchy for surface water disposal, which encourages an approach based on the use of Sustainable Drainage Systems (SuDS). This hierarchy stipulates that surface water runoff not collected for reuse must be discharged to one or more of the following in order of priority: 1. Discharge into ground (infiltration); or, where not reasonably practicable, 2. Discharge to a surface water body; or, where not reasonably practicable, 3. Discharge to a surface water sewer, highway drain, or another drainage system; or, where not reasonably practicable, 4. Discharge to a combined sewer.
6.3 INDICATIVE SURFACE WATER DRAINAGE STRATEGY 6.3.1. As a result of the underlying superficial geology described in Section 3.4 above it is anticipated that discharge to ground is unlikely to be a feasible option for the disposal of surface water generated by the Proposed Development. This assumption should be confirmed with infiltration testing using the methodology described in BRE 36524 prior to the detailed design of the Proposed Development.
21 Institute of Hydrology (1994) Flood Estimation for Small Catchments 22 Environment Agency (2013) Report – SC030219: Rainfall Runoff Management for Developments 23 Communities and Local Government (2010) Building Regulations, Approved Document H: Drainage and Waste Disposal 24 Building Research Establishment (2016) Digest 365: Soakaway Design
WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Page 20 of 24 Thakeham Homes 6.3.2. Should, as anticipated, an infiltration based drainage solution not prove to be practical, discharge from the Proposed Development should be limited to the greenfield runoff rates provided in Table 5 prior to discharge to Minley Brook. 6.3.3. The Proposed Development should incorporate a range of SuDS features to ensure satisfactory levels of treatment of collected runoff with a view to protecting the quality of the water environment in accordance with the requirements of current best practice guidance25. 6.3.4. The volumes of attenuation required to enable surface water at the Proposed Development to be sustainably managed in accordance with the principles above has been determined for a range of storm events using the Source Control module of MicroDrainage26 and are provided in Table 5.
Table 5 – Required Attenuation Volumes Storm Event Developable Area Impermeable Area Required Attenuation (Ha) (Ha) Volume (m3) 100% AEP 12.6 6.9 1,306 3.3% AEP 2,532 1.0% AEP + 20% CC 3,995 1.0% AEP + 40% CC 4,848
6.3.5. These volumes have been calculated on the assumption that the contributing impermeable area of the Proposed Development would be 55% of the proposed developable area of 12.6 ha (i.e. 6.9 ha) in accordance with Table 11.2 of Urban Drainage Third Edition27. 6.3.6. A combination of source control and site control SuDS techniques should be provided to attenuate the surface water generated by the Proposed Development prior to discharge Minley Brook such as permeable paving, swales, and/or attenuation basins. 6.3.7. These techniques could be readily accommodated within the development parcels and the green open space currently allocated for SuDS. They would provide sufficient attenuation to enable the Proposed Development to be drained in accordance with the requirements outlined in 6.2 above. 6.3.8. Once the indicative drainage strategy outlined above has been developed to a level suitable to accompany a planning application for the Proposed Development, the future ownership of the proposed SuDS features should be identified along with the associated maintenance obligations to ensure the performance of the proposed drainage system in perpetuity. 6.3.9. It should be noted that the required attenuation volumes above are approximate and the proposed SuDS techniques presented are indicative only with a view to demonstrating the viability of the Proposed Development with regard to the sustainable management of surface water, in accordance with the requirements of the NPPF and local policies.
25 CIRIA (2016) C753 - The SuDS Manual 26 XP Solutions (2016) MicroDrainage v. 2015.1 27 David Butler and John W. Davies; Spon Press (2011) Urban Drainage, 3rd Edition
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes Page 21 of 24 7 CONCLUSIONS
7.1.1. WSP was appointed by Thakeham Homes to produce a Preliminary Flood Risk Assessment in support of a proposed residential-led development of circa 500 dwellings, community facilities including a primary school, and 15 ha of Suitable Accessible Natural Greenspace (the ‘Proposed Development’). 7.1.2. The Proposed Development is to be located at a 45.6 ha agricultural site to the north of the M3 in Fleet, Hampshire (the ‘Site’). 7.1.3. The Proposed Development is being submitted to Hart District Council for Regulation 19 consultation of the Draft Hart Local Plan (the ‘Local Plan’). 7.1.4. There are two Main Rivers within the vicinity of the Site, Minley Brook and Fleet Brook. Minley Brook flows from east to west through the northern half of the Site. Fleet Brook flows from south to north along the western boundary of the Site prior to its confluence with Minley Brook. 7.1.5. The Environment Agency’s Flood Map for Planning shows the Site to lie within Flood Zones 1, 2 and 3. 7.1.6. The Environment Agency have confirmed that the flood extents at, and in the vicinity of, the Site given on the Flood Map for Planning were produced using the JFlow hydraulic modelling package in 2006 and have stated that the outputs from this national scale modelling exercise are not sufficiently accurate to enable the assessment of flood risk on an individual site basis. 7.1.7. The M3 is shown by the LiDAR data to have a maximum height of approximately 2 m above surrounding ground levels whilst Minley Road, which crosses the M3, is shown to have a maximum height of approximately 7 m. This indicates that the three major flow paths crossing the M3 onto the Site shown on the Flood Map for Planning are unlikely to be realistic. The Flood Map for Planning does not provide an estimate of flood risk posed by Minley Brook. 7.1.8. The results of the detailed 1D-2D hydraulic model show that the primary flood risk to the site is presented by fluvial flooding generated by Minley Brook, the capacity of which is exceeded during large storm events resulting in water spilling out of bank on to the Site. Flood Zone 3 comprises 9.1 % of the total area of the Site whilst Flood Zone 2 covers a further 6.5 %. The remaining 84.4 % of the Site is located entirely within the equivalent of Flood Zone 1 (i.e. subject to a Very Low risk of fluvial flooding). 7.1.9. The detailed 1D-2D model and the accompanying Hydraulic Modelling Report have been reviewed by the Environment Agency, and they have provided minor comments. . Consequently, the outputs of the detailed 1D-2D model will provide the best available representation of flood risk at the Site, superseding the Environment Agency’s current published fluvial and pluvial mapping described above. 7.1.10. In order to manage fluvial flood risk to the Proposed Development, a sequential approach has been adopted for the development of the Masterplan with all More Vulnerable land-uses (i.e. residential and educational) located in areas shown to be subject to a Very Low risk of flooding from both of these sources. 7.1.11. The Proposed Development would therefore have an annual probability of fluvial flooding of less than 0.1% (i.e. located entirely within the equivalent of Flood Zone 1) and would be considered appropriate development by the NPPF. 7.1.12. Safe access and egress to and from the Proposed Development would be provided by Minley Road during the design flood event (i.e. the 1.0 % AEP event) which is not shown to be at risk of fluvial or pluvial flooding during an event of this magnitude. 7.1.13. A holistic approach has been adopted to the development of the Masterplan in accordance with Policy I2 of the Local Plan with natural parkland and recreational facilities provided in those areas currently shown to be at risk of flooding. This provides the desired synergy between green infrastructure provision, ecological enhancement, flood risk management, and amenity value. Furthermore, the existing on-site drainage regime has been carefully incorporated within the development proposals with all major existing overland flow routes retained. 7.1.14. The management of surface water within the Proposed Development in accordance with national and local policy along with current best practice guidance will reduce the risk of pluvial flooding both to and from the Proposed Development to Low.
WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Page 22 of 24 Thakeham Homes 7.1.15. The Site currently drains through mainly via overland flow towards Minley Brook or the existing network of drainage ditches which discharge into Minley Brook with a limited contribution from infiltration. Should, as anticipated, an infiltration based drainage solution not prove to be practical, discharge from the Proposed Development should be limited to the greenfield runoff rates prior to discharge to Minley Brook. 7.1.16. A combination of source control and site control SuDS techniques should be provided to attenuate the surface water generated by the Proposed Development prior to discharge to Minley Brook such as permeable paving, swales, and/or attenuation basins. 7.1.17. These techniques could be readily accommodated within the development parcels and the green open space currently allocated for SuDS and would provide sufficient attenuation to ensure that surface water generated by the Proposed Development can be managed sustainably. 7.1.18. This report demonstrates that the Proposed Development can be delivered sustainably with regards to flood risk and would be in accordance with the requirements of the Local Plan policies NBE6 (Flood Risk), NBE7 (Water Quality) and I2 (Green Infrastructure) along with all other relevant local and national flood risk legislation as detailed in the NPPF.
BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes Page 23 of 24 Appendix A
SITE PLANS Appendix A.1
SITE LOCATION PLAN
Appendix A.2
GEOLOGY MAPS
Appendix B
PROPOSED DEVELOPMENT PLANS Appendix B.1
INDICATIVE MASTERPLAN Reproduced from the Ordnance Survey Map with the permission of the Controller of H.M. Stationery Office Crown copyright licence number 100024244 Savills (UK) Ltd. Published for the purposes of identification only and although believed to be correct accuracy is not guaranteed. Savills does not act as Principal Designer and this drawing is not intended to inform Construction Design Management procedures. \\southampton03\Data\URBAN DESIGN\JOBS\SNUD 386233 - Brook House\B) Drawings\INDD\Illustrative masterplan - land use plan March 2018 15/03/18 Illustrative Masterplan (showing Flod Risk Areas)
SANG trails form circular loops 01 Brook House retained and converted as apartments through the existing woodland areas 02 Brook House garden becomes the neighbourhood park
03 1 form entry primary school with land for expansion to a 2 form entry Attenuation ponds are located 1 form entry primary school throughout the site in close 04 Western crescent - town houses overlook the village park with land for expansion to proximity to the existing drainage a 2 form entry ditches 05 Eastern crescent - town houses overlook the mere
06 Attenuation ponds
07 Barns converted for community use
09 08 Allotments & community orchard
SuDS streets permeate this part 09 Playing fields of the neighbourhood 10 New acoustic fencing and bunding
11 11 Small gravel surfaced car park
12 Pedestrian link to M3 bridges LEAP 13 Neighbourhood park NEAP 07 08 14 Apartments 13
06 06 SANG walks
LEAP 14
05 Accessible woodland
01 02 Primary school 06
04 06 14 Housing
Attenuation ponds 03 14 LEAP 12 Playing fields 09
10 Noise attenuation bund
Minley Road Ped. Link NEAP Neighbourhood equipped play area
M3 LEAP Locally equipped play area Cycle/pedestrian route between the site 12 The focal point of the whole entrance and Ancells scheme will be the refurbished New sports pitches could be roundabout older buildings with the large accomodated within the buffer Vehicle junction 10 minute walk to Fleet gardens becoming a park zone next to the M3 railway station Pedestrian access P.R.O.W. 5 minute walk to Ancells business park
title Illustrative Masterplan job no SNUD 386233 drawn by CO/SM *Drawing to be used for illustrative purposes only. Subject to further detailed and technical studies including (but not exhaustively): urban design studio drawing no MP 002 checked by CO »» Heritage Assessment »» Land Registry Search »» Utilities Survey »» Highway & Transport Studies project Brook House, Fleet revision C (15 Mar 2018) »» Arboricultural Survey »» Flood Risk Assessment »» Ecological Surveys »» Drainage & Hydrology Studies
client Thakeham date 12 Mar 2017 N Scale NTS @ A2 savills.com/urbandesign Appendix C
FLOOD RISK MAPPING Appendix C.1
FLOOD MAP FOR PLANNING
Appendix C.2
RISK OF FLOODING FROM SURFACE
WATER
Appendix C.3
HISTORIC FLOOD MAP
Appendix C.4
HYDRAULIC MODELLING RESULTS –
PRELIMINARY 2D MODEL
Appendix C.5
HYDRAULIC MODELLING RESULTS -
DETAILED 1D-2D MODEL
Appendix D
SURFACE WATER DRAINAGE
CALCULATIONS Appendix D.1
GREENFIELD RUNOFF REPORT WSP Group Ltd Page 1 . Brook House . Fleet . Preliminary Runoff Calculation Date 26/05/2017 Designed by SEK File Checked by XP Solutions Source Control 2016.1.1
ICP SUDS Mean Annual Flood
Input
Return Period (years) 100 Soil 0.370 Area (ha) 1.000 Urban 0.000 SAAR (mm) 690 Region Number Region 6
Results l/s
QBAR Rural 2.8 QBAR Urban 2.8
Q100 years 9.0
Q1 year 2.4 Q30 years 6.4 Q100 years 9.0
©1982-2016 XP Solutions Appendix D.2
ATTENUATION VOLUMES BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Thakeham Homes BROOK HOUSE, FLEET WSP Project No.: 70033671 | Our Ref No.: 33671-FRA-001 March 2018 Thakeham Homes WSP BROOK HOUSE, FLEET March 2018 Project No.: 70033671 | Our Ref No.: 33671-FRA-001 Thakeham Homes Appendix E
HYDRAULIC MODELLING SUMMARY Appendix E.1
PRELIMINARY 2D MODEL TECHNICAL NOTE TO: n/a FROM: Simon King SUBJECT: Brook House, Fleet – Hydraulic Modelling Summary DATE: 26 May 2017
INTRODUCTION
This Technical Note has been produced to summarise the development of the preliminary two- dimensional (2D) hydraulic model (the ‘Model’) for Brook House, Fleet (the ‘Application Site’).
OBJECTIVES
The Flood Map for Planning shows peak flood extents produced by an outdated JFlow model which is not considered appropriate for development scale flood risk appraisal.
Consequently, the Model was developed in order to refine the understanding of the flood risk posed to the Application Site from two Main Rivers, Fleet Brook and Minley Brook. The outputs of the Model are to be used to inform the development proposals and support the inclusion of the Application Site within the Hart District Council Local Plan.
MODELLING APPROACH
The Model is predominantly two-dimensional as no survey data was available at the time of production. It combines inflows for the two watercourses with rainfall for the remainder of the catchment to provide an estimate of the overall flood risk posed to the Application Site.
The Model incorporates key hydraulic structures based on observations made during a site visit and published information available at the time of writing.
SOFTWARE
InfoWorks ICM1 has been used as an industry standard hydraulic modelling software package that has been benchmarked and approved by the Environment Agency for the use of flood risk estimation.
HYDROLOGY
Inflows for Minley Brook and Fleet Brook along with rainfall profiles for the remainder of the catchment have been estimated using ReFH22 using the latest (2013) catchment descriptors and runoff model.
TERRAIN DATA
The Model has been produced using a Digital Terrain Model (DTM) with a 1 m spatial resolution derived from LiDAR data obtained from the Environment Agency.
1 Innovyze (2017) InfoWorks Integrated Catchment Modelling software 2 Wallingford HydroSolutions (2015) ReFH2 software CALIBRATION AND VERIFICATION
The peak flood extents at the Application Site given by the Model are broadly in accordance with those provided on the Risk of Flooding from Surface Water mapping.
MODEL PRODUCTION AND AUDITING
The Model was produced by Simon King and has been internally reviewed by Mouludul Islam.
FURTHER WORK
The Model will be updated to incorporate one-dimensional (1D) channel survey data as the project progresses to refine the peak flood extents at the Application Site in sufficient detail to support a planning application.
Page 2 of 2 Appendix E.2
DETAILED 1D-2D MODEL TECHNICAL NOTE
TO n/a FROM Simon King
DATE 08 March 2018 CONFIDENTIALITY Public
SUBJECT Brook House, Fleet – Hydraulic Modelling Summary
1. INTRODUCTION This Technical Note has been produced to summarise the development of the detailed (1D-2D) hydraulic model (the ‘Model’) for Brook House, Fleet (the ‘Application Site’). 2. OBJECTIVES The Flood Map for Planning shows peak flood extents produced by an outdated JFlow model which is not considered appropriate for development scale flood risk appraisal. Consequently, the Model was developed in order to refine the understanding of the flood risk posed to the Application Site from two Main Rivers, Fleet Brook and Minley Brook. The outputs of the Model are to be used to inform the development proposals and support the inclusion of the Application Site within the Hart District Council Local Plan. 3. MODELLING APPROACH The hydraulic model is a combined 1-Dimensional (1D) & 2-Dimensional (2D) model. Fleet Brook, Minley Brook and numerous tributary watercourses were represented in the 1D domain based on data obtained through a detailed channel survey. Structures along the watercourses have been represented in the hydraulic model using details obtained from the channel survey and supplemented with details obtained from Highways England data for the existing culverts underneath the M3. The model has been constructed using a flexible mesh to allow for accurate delineation of floodplains and flowpaths around the small watercourses and buildings within the model domain, whilst maintaining a suitable resolution for results and without adversely impacting model run time. 4. SOFTWARE InfoWorks ICM1 has been used as an industry standard hydraulic modelling software package that has been benchmarked and approved by the Environment Agency for the use of flood risk estimation. 5. HYDROLOGY The hydrological assessment undertaken in order to generate model inflows and rainfall data utilised the ReFH2 method, which represents the latest version of the Revitalised Flood Hydrograph (ReFH) model. The ReFH2 methodology is the current approach for calculating model inflows for use in these types of studies, it is therefore considered to be an appropriate method for this study. The previous version of the ReFH methodology was used to generate additional flows for comparison with those generated by the ReFH2 methodology. However, these were not considered appropriate for generating inflows due to the catchments highly permeable nature.
1 Innovyze (2017) InfoWorks Integrated Catchment Modelling software
www.wsp.com 6. TERRAIN DATA Environment Agency LiDAR 1 m Digital Terrain Model (DTM) was used to define the topography of the overland flow routes within the 2D domain of the hydraulic model. Topographical edits have been used in areas where the ground model needs reinforcing/amending as the DTM has been post processed and ‘carved through’ roads where rivers flow through culverts underneath. 7. CALIBRATION AND VERIFICATION The mass balance error at the end of each model run was within the limits suggested by the Environment Agency of within +/-2%. All other checks and warnings prior to the simulation were as a result of geometry alterations and channel/structure sections and were reviewed prior to producing results. None of these have an effect on the results of the hydraulic modelling. Comparison of the modelled results to the Environment Agency Flood Map for Planning shows a high degree of similarity in the fluvial flood extents associated with the banks of the Minley Brook. There are differences in the overland flow routes from the Fleet Brook towards the Minley Brook. These differences are largely due to the accuracy of the ground model for the respective Environment Agency Models. The existing fluvial flood maps show significant flow routes over the embanked M3, and the pluvial flood maps do not have any connectivity of the Minley Brook underneath/through Minley Road which is the only place for water to exit the Site. Sensitivity testing was undertaken on inflows, hydraulic roughness and downstream boundary conditions. The impact of the blockage of four culverts was also undertaken. 8. MODEL PRODUCTION AND AUDITING The Model was produced by Christopher Goode (Senior Engineer) and has been internally reviewed by Michele Zornitta (Principal Engineer). The Model is currently being reviewed by the Environment Agency. Once the review has been completed the outputs from the Model will represent the best available estimation of flood risk at the Application Site.