REPORT NO 3671-FRA-001 BROOK HOUSE, FLEET PRELIMINARY FLOOD RISK ASSESSMENT
JUNE 2017 BROOK HOUSE, FLEET PRELIMINARY FLOOD RISK ASSESSMENT Thakeham Homes
Project no: 70033671 Date: June 2017
– WSP | Parsons Brinckerhoff Mountbatten House Basing View Basingstoke Hampshire RG21 4HJ
Tel: +44 (0) 1256 318 800 Fax: +44 (0) 1256 318 700 www.wsp-pb.com QUALITY MANAGEMENT
ISSUE/REVISION FIRST ISSUE REVISION 1 REVISION 2 REVISION 3
Red Line Boundary Remarks Draft for Comment First Issue Updated
Date May 2017 June 2017 June 2017
Prepared by Simon King Simon King Simon King
Signature
Checked by Steven Brown Steven Brown Steven Brown
Signature
Authorised by Martin Wheeler Martin Wheeler Martin Wheeler
Signature
Project number 70033671 70033671 70033671
Report number 3671-FRA-001 3671-FRA-001 3671-FRA-001
\\uk.wspgroup.com\Central Data\Projects\700336xx\70033671 - Brook House, Fleet- File reference Hampshire\02 WIP\DE Development\03 Document\Reports\Water\Preliminary Flood Risk Assessment\170601-SEK-Preliminary Flood Risk Assessment v3.docx 1
TABLE OF CONTENTS
EXECUTIVE SUMMARY ...... 3
1 INTRODUCTION ...... 4
2 POLICY ...... 6
3 THE SITE...... 11
4 OVERVIEW OF FLOOD RISK ...... 14
5 SURFACE WATER DRAINAGE ...... 20
6 CONCLUSIONS ...... 22
TABLES
TABLE 4-1 SUMMARY OF FLOOD RISK TO AND FROM THE SITE / PROPOSED DEVELOPMENT ...... 19 TABLE 5-1 PEAK GREENFIELD RUNOFF RATES ...... 20 TABLE 5-2 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 SUMMARY
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GLOSSARY
AOD Above Ordnance Datum AEP Annual Exceedance Probability BGS British Geological Survey FRA Flood Risk Assessment GWMP Groundwater Management Plan HCC Hampshire County Council HDC Hart District Council LLFA Lead Local Flood Authority LiDAR Light Detection and Ranging LFRMS Local Flood Risk Management Strategy LPA Local Planning Authority NPPF National Planning Policy Framework PPG Planning Practice Guidance PFRA Preliminary Flood Risk Assessment SWMP Surface Water Management Plans SuDS Sustainable Drainage Systems
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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 Accessible 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 18 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.
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.
The Proposed Development adopts the sequential approach to spatial planning to locate all the residential and educational development within areas identified as having Low or Very Low risk of flooding from these sources.
The Proposed Development would therefore have an annual probability of flooding of less than one percent and is considered to be suitable 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.
A summary of the findings of this assessment is provided in Table 1-1 below. Definitions of the grading criteria used are given in Paragraph 4.1.2 in the main body of this report.
Table 1-1 Summary of Flood Risk
FUTURE RISK TO THE FUTURE RISK FROM THE SOURCES OF FLOOD RISK CURRENT RISK TO THE SITE PROPOSED DEVELOPMENT PROPOSED DEVELOPMENT
Coastal and Tidal Negligible Negligible Negligible
Fluvial High Low Very Low
Pluvial and Overland Flow High Low Very Low
Groundwater Low Low Very Low
Sewer and Drainage Infrastructure Very Low Very Low Very Low
Artificial Sources Negligible Negligible Negligible
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.
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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 Accessible 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 18 consultation of the Draft Hart Local Plan1 (the ‘Local Plan’).
1.2 AIM
1.2.1 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
1.3.1 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 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;
1 Hart District Council (April 2017) Draft Hart Local Plan: Strategy and Sites 2011 – 2032, Regulation 18 Consultation; accessed May 2017 from https://www.hart.gov.uk/draft-local-plan 2 Department for Communities and Local Government (2012) National Planning Policy Framework; accessed May 2017 from https://www.gov.uk/guidance/national-planning-policy-framework
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∆ 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.
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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 the Site is located within Flood Zones 1, 2 and 3 the Sequential Test should be applied in order to demonstrate that there are no reasonably available alternative sites located entirely within Flood Zone 1 in Hart District.
2.1.7 As part of the strategic land allocation process it is anticipated that LPAs will undertake the sequential testing of proposed development sites.
2.1.8 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.
3 Environment Agency (2017) Flood Map for Planning; accessed May 2017 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/
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CLIMATE CHANGE ALLOWANCES
2.1.9 The Flood Risk and Coastal Change PPG states that the lifespan of a residential development should be considered for a minimum of 100 years.
2.1.10 In February 2016 the Environment Agency released updated climate change guidance 5 to support the NPPF. 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.
2.1.11 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 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.
2.1.12 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
2.1.13 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.
2.1.14 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.
2.1.15 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.
5 Environment Agency (February 2016) Flood risk assessments: climate change allowances; accessed October 2016 from https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances 6 Department for Food and Rural Affairs (2015) Sustainable Drainage Systems: Non-statutory technical standards for sustainable drainage systems; accessed May 2017 from https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/415773/sustainable- drainage-technical-standards.pdf
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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 91 st 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.
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. An update to the strategy is programmed to be released in 2017.
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.
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 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.
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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.
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.
10 Hampshire County Council (2013) Groundwater Management Plan; accessed May 2017 from https://www.hants.gov.uk/landplanningandenvironment/environment/flooding/strategies/Groundwater- Management-Plan 11 Hart District Council (2016) Strategic Flood Risk Assessment; accessed May 2017 from https://www.hart.gov.uk/Evidence-base
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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. 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 The Local Plan identifies that areas of the district are potentially at a high risk of flooding, with Fleet identified within the top four urban areas at risk of surface water and fluvial flooding. Northern Fleet is given as the worst affected area with regards to external sewer flooding.
2.2.24 Policy MG3 of The Local Plan states that ‘a holistic approach to green infrastructure is encouraged whereby public open space, sports and recreational facilities, Sites of Accessible Natural Green Space (SANGs), green networks, landscaping, biodiversity, flood risk and sustainable drainage systems are all considered in the round, exploiting synergies between these different elements.’
2.2.25 Policy NE4 describes that developments in areas identified as being at risk of flooding will be supported where the sequential and exception tests can be satisfied, development will be safe, the proposals will not increase flood risk elsewhere, safe access and egress can be provided, and the development will incorporate sustainable drainage systems (SuDS) with a view to providing additional amenity, biodiversity and water quality benefits.
2.2.26 Policy NE5 states that ‘development will be required to protect the quality of the District’s water environment, and where possible contribute towards improvements’. It describes that development will be supported provided that it incorporates SuDS where appropriate, will not prevent waterbodies from achieving ‘good’ ecological status and will protect or enhance the quality of surface water or groundwater features.
2.2.27 The supporting description included with Policy 12 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.’
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3 THE SITE 3.1 SITE LOCATION
3.1.1 The Site is located immediately to the 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 13.6 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 Interrogation of the Environment Agency’s Light Detection and Ranging (LiDAR) topographical data12 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 by the LiDAR data 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 Mapping 13 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.
12 Environment Agency (2017) LiDAR Composite DTM – 1m; obtained May 2017 from https://data.gov.uk/dataset/lidar-composite-dtm-1m1 13 British Geological Survey (2016) DiGMapGB-50 - WMS; accessed May 2017 from http://www.bgs.ac.uk/data/services/digmap50wms.html
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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.
3.4.4 BGS Geology maps are given in Appendix A-2 for reference.
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-A 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.
3.6.3 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.
14 British Geological Survey (2016) 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 October 2016 from http://maps.environment-agency.gov.uk/wiyby/wiybyController?ep=maptopics&lang=_e 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
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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-A 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.
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.
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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 possible 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 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
4.2.2 The Environment Agency’s Flood Map for Planning, reproduced on Drawing 3671-EA-001-A in Appendix C-1, shows the Site to lie within Flood Zones 1, 2 and 3.
4.2.3 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%.
4.2.4 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.
4.2.5 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, nor is planned, for the watercourses in question.
4.2.6 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.
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4.2.7 The Flood Map for Planning shows three overland flow paths across the M3 which connect the floodplain of Fleet Brook with the Site. Interrogation 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.
4.2.8 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 indicated on the Flood Map for Planning shown on the Flood Map for Planning are unlikely to be realistic.
4.2.9 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.
4.2.10 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 has been produced using InfoWorks ICM17. The model incorporates key hydraulic structures based on observations made during a site visit and published information available at the time of writing.
4.2.11 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.
4.2.12 The model has been produced using a Digital Terrain Model (DTM) 18 with a 1 m spatial resolution produced from LiDAR data. Inflows for Minley Brook and Fleet Brook along with rainfall profiles have been estimated using ReFH219 using the latest (2013) catchment descriptors and runoff model.
4.2.13 The results of the hydraulic modelling exercise are provided on Drawing 3671-HMR-001-A in Appendix C-4 for reference.
4.2.14 The peak flood extents at the Site given by the hydraulic model are broadly in accordance with those provided on the Risk of Flooding from Surface Water mapping given on Drawing 3671-EA- 002-A. This confirms that the flood risk at the Site is governed by a combination of surface water and fluvial flooding from Minley Brook and 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.
4.2.15 The level of fluvial flood risk posed to the Proposed Development is considered to be High with areas of the Site shown to be subject to a risk of fluvial flooding greater than 3.3%.
4.2.16 The hydraulic model will be amended to incorporate one-dimensional (1D) channel survey data as the project progresses to refine the peak flood extents at the Site in sufficient detail to support a planning application for the Proposed Development.
17 Innovyze (2017) InfoWorks Integrated Catchment Modelling software 18 A Digital Terrain Model contains ‘bare-earth’ ground levels with existing buildings and foliage removed 19 Wallingford HydroSolutions (2015) ReFH2 software
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FLOODING FROM PLUVIAL SOURCES
4.2.17 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.
4.2.18 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.
4.2.19 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 principal of the development of the Masterplan.
4.2.20 The mitigation measures incorporated within the Proposed Development to manage the fluvial and pluvial flood risk are described in Paragraphs 4.3.4 to 4.3.16 along with an assessment of the resulting risk posed to, and arising from, the Proposed Development.
FLOODING FROM GROUNDWATER SOURCES
4.2.21 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).
4.2.22 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.
4.2.23 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’.
4.2.24 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.
4.2.25 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.
4.2.26 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.
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FLOODING FROM SEWER AND DRAINAGE INFRASTRUCTURE
4.2.27 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.
4.2.28 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.
4.2.29 Thames Water has confirmed that it holds no records of historic sewer flooding in the vicinity of the Site.
4.2.30 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
4.2.31 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.
4.2.32 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.
4.2.33 The Environment Agency’s Flood Risk from Reservoirs mapping 20 does not show the Site to be at risk of flooding from the failure of reservoirs.
4.2.34 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.
4.3 RISK OF FLOODING TO AND ARISING FROM THE PROPOSED DEVELOPMENT
4.3.1 The NPPF requires any development proposals to demonstrate that 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.
20 Environment Agency (2017) Flood Risk from Reservoirs; accessed May 2017 from https://flood-warning- information.service.gov.uk/long-term-flood-risk/
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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 both fluvial and pluvial flood risk posed 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 either a Low or a Very Low risk of flooding from both of these sources (i.e. within the equivalent of Flood Zone 2 or Flood Zone 1).
4.3.6 The Proposed Development would therefore have an annual probability of flooding from Minley Brook and from surface water of less than one percent.
4.3.7 This is considered by the NPPF to be an acceptable use of land within Flood Zone 2 and Flood Zone 1. Consequently, the risk of fluvial flooding posed to the Site should not prohibit the Proposed Development from being successfully delivered.
4.3.8 A holistic approach has been adopted when developing the Masterplan in accordance with Policy 12 and Policy MG3 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.
4.3.9 In addition, the existing on-site drainage regime has been carefully 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.
4.3.10 The small isolated areas currently shown to be subject to a ‘Medium’ risk of flooding to the south of the main floodplain will be accommodated within the detailed design of the Proposed Development. As the peak flood extents include the impact of surface water generated by the Application Site itself in addition to fluvial flows generated off-site, some of these areas are likely to be produced entirely by surface water runoff generated by the Application Site. Consequently, these flows will be positively drained by the proposed SuDS system described in Section 5.
4.3.11 Drawings 3671-HMR-001-A 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.
4.3.12 The risk of flooding posed to the Proposed Development from both pluvial and fluvial sources following the implementation of appropriate spatial planning is Low.
4.3.13 The impact of the Proposed Development on pluvial flood risk is considered in Section 5.3.
4.3.14 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.
4.3.15 Finished floor levels should be set to a minimum of 150 mm above both surrounding ground levels and potential surface water flow levels.
4.3.16 In consideration of the mitigation measures outlined above the risk of the Proposed Development exacerbating flood risks from pluvial/overland sources is considered to be Very Low.
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FLOODING FROM GROUNDWATER SOURCES
4.3.17 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.
4.3.18 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
4.3.19 Additional surface water generated by the Proposed Development would be discharged directly to Minley Brook and consequently would not introduce additional flows into the public sewer network.
4.3.20 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
4.3.21 The risk of the Proposed Development exacerbating flooding from artificial 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.
SUMMARY OF FLOOD RISK
4.3.22 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 4-1.
Table 4-1 Summary of Flood Risk to and from the Site / Proposed Development
FUTURE RISK TO THE FUTURE RISK FROM THE SOURCES OF FLOOD RISK CURRENT RISK TO THE SITE PROPOSED DEVELOPMENT PROPOSED DEVELOPMENT
Coastal and Tidal Negligible Negligible Negligible
Fluvial High Low Very Low
Pluvial and Overland Flow High Low Very Low
Groundwater Low Low Very Low
Sewer and Drainage Very Low Very Low Very Low Infrastructure
Artificial Sources Very Low Very Low Negligible
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5 SURFACE WATER DRAINAGE 5.1 EXISTING DRAINAGE REGIME
5.1.1 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.
5.1.2 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 guidance 22.
5.1.3 Peak greenfield runoff rates currently generated by the Site are given in Table 5-1 with calculations provided for reference in Appendix D-1.
Table 5-1 Peak greenfield runoff rates UNIT RUNOFF RATE TOTAL SITE AREA TOTAL RUNOFF RATE STORM EVENT (L/S/HA) (HA) (L/S) QBAR 2.8 127.7 100% AEP 2.4 109.4 45.6 3.3% AEP 6.4 291.8 1.0% AEP 9.0 410.4
5.2 SURFACE WATER MANAGEMENT STRATEGY REQUIREMENTS
5.2.1 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.
5.2.2 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.
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
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5.3 INDICATIVE SURFACE WATER DRAINAGE STRATEGY
5.3.1 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.
5.3.2 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 5-1 above prior to discharge to Minley Brook.
5.3.3 The Proposed Development should incorporate a combination 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 guidance 25.
5.3.4 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 MicroDrainage 26 and are provided in Table 5-2.
Table 5-2 Required Attenuation Volumes DEVELOPABLE IMPERMEABLE REQUIRED ATTENUATION STORM EVENT 3 AREA (HA) AREA (HA) VOLUME (M ) 100% AEP 1,465 3.3% AEP 2,820 13.6 7.5 1.0% AEP + 20% CC 4,440 1.0% AEP + 40% CC 5,370
5.3.5 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 13.6 ha (i.e. 7.5 ha) in accordance with Table 11.2 of Urban Drainage Third Edition 27.
5.3.6 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.
5.3.7 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 5.2 above.
5.3.8 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.
5.3.9 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.
24 Building Research Establishment (2016) Digest 365: Soakaway Design 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
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6 CONCLUSIONS 6.1.1 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’).
6.1.2 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’).
6.1.3 The Proposed Development is being submitted to Hart District Council for Regulation 18 consultation of the Draft Hart Local Plan (the ‘Local Plan’).
6.1.4 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.
6.1.5 The Environment Agency’s Flood Map for Planning shows the Site to lie within Flood Zones 1, 2 and 3.
6.1.6 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.
6.1.7 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.
6.1.8 In order to refine the current estimates of the peak fluvial flood extents at the Site a two- dimensional (2D) hydraulic model has been produced using InfoWorks ICM. The model incorporates key hydraulic structures based on observations made during a site visit and published information available at the time of writing.
6.1.9 The peak flood extents at the Site given by the hydraulic model confirm that the flood risk at the Site is governed by a combination of surface water and fluvial flooding from Minley Brook. 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.
6.1.10 In order to manage both fluvial and pluvial flood risk posed 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 either a Low or a Very Low risk of flooding from both of these sources.
6.1.11 The Proposed Development would therefore have an annual probability of flooding of less than one percent and would be considered appropriate development by the NPPF.
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6.1.12 A holistic approach has been adopted to the development of the Masterplan in accordance with Policy 12 and Policy MG3 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.
6.1.13 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.
6.1.14 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.
6.1.15 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.
6.1.16 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.
6.1.17 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 MG3, NE4, NE5 and 12 along with all other relevant local and national flood risk legislation as detailed in the NPPF.
Brook House, Fleet WSP | Parsons Brinckerhoff Thakeham Homes Project No 70033671 June 2017 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 &ΤΧΨΚΠΙ ςΘ ∆Γ ΩΥΓΦ ΗΘΤ ΚΝΝΩΥςΤΧςΚΞΓ ΡΩΤΡΘΥΓΥ ΘΠΝ[ 5Ω∆ΛΓΕς ςΘ ΗΩΤςϑΓΤ ΦΓςΧΚΝΓΦ ΧΠΦ ςΓΕϑΠΚΕΧΝ ΥςΩΦΚΓΥ ΚΠΕΝΩΦΚΠΙ ∆Ως ΠΘς ΓΖϑΧΩΥςΚΞΓΝ[ ∗ΓΤΚςΧΙΓ #ΥΥΓΥΥΟΓΠς .ΧΠΦ 4ΓΙΚΥςΤ[ 5ΓΧΤΕϑ 7ςΚΝΚςΚΓΥ 5ΩΤΞΓ[ ∗ΚΙϑΨΧ[ 6ΤΧΠΥΡΘΤς 5ςΩΦΚΓΥ #Τ∆ΘΤΚΕΩΝςΩΤΧΝ 5ΩΤΞΓ[ (ΝΘΘΦ 4ΚΥΜ #ΥΥΓΥΥΟΓΠς ∋ΕΘΝΘΙΚΕΧΝ 5ΩΤΞΓ[Υ &ΤΧΚΠΧΙΓ ∗[ΦΤΘΝΘΙ[ 5ςΩΦΚΓΥ 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
Appendix D
SURFACE WATER DRAINAGE CALCULATIONS APPENDIX D-1
GREENFIELD RUNOFF RATES 952 )ΤΘΩΡ .ςΦ 2ΧΙΓ ∃ΤΘΘΜ ∗ΘΩΥΓ (ΝΓΓς 2ΤΓΝΚΟΚΠΧΤ[ 4ΩΠΘΗΗ %ΧΝΕΩΝΧςΚΘΠ &ΧςΓ &ΓΥΚΙΠΓΦ ∆[ 5∋− (ΚΝΓ %ϑΓΕΜΓΦ ∆[ :2 5ΘΝΩςΚΘΠΥ 5ΘΩΤΕΓ %ΘΠςΤΘΝ
+%2 57&5 /ΓΧΠ #ΠΠΩΧΝ (ΝΘΘΦ
+ΠΡΩς
4ΓςΩΤΠ 2ΓΤΚΘΦ [ΓΧΤΥ 5ΘΚΝ #ΤΓΧ ϑΧ 7Τ∆ΧΠ 5##4 ΟΟ 4ΓΙΚΘΠ 0ΩΟ∆ΓΤ 4ΓΙΚΘΠ
4ΓΥΩΝςΥ Ν Υ
3∃#4 4ΩΤΧΝ 3∃#4 7Τ∆ΧΠ
3 [ΓΧΤΥ
3 [ΓΧΤ 3 [ΓΧΤΥ 3 [ΓΧΤΥ
:2 5ΘΝΩςΚΘΠΥ APPENDIX D-2
ATTENUATION VOLUMES
Appendix E
HYDRAULIC MODELLING SUMMARY 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.
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