Proposed Strategic Site Allocations Wooburn and Bourne End Flood

Proposed Strategic Site Allocations Wooburn and Bourne End

Flood Risk Appraisal

for

Keep Bourne End Green

Document Control Sheet Flood Risk Appraisal Proposed Strategic Site Allocations, Wooburn and Bourne End Keep Bourne End Green

This document has been issued and amended as follows:

Date Issue Prepared by Approved by

18/09/2017 1st Draft SJO

20/09/2017 2nd Draft SJO

23/11/2017 Final SJO NJ

Motion 84 North Street Guildford Surrey GU1 4AU T 01483 531300 F 01483 531333 E [email protected] W www.motion.co.uk Proposed Strategic Site Allocations, Wooburn and Bourne End

Contents

1.0 Introduction ...... 2 2.0 Policy ...... 3 3.0 Emerging New Local Plan for Wycombe District Council ...... 5 4.0 Strategic Flood Risk Assessment ...... 6 5.0 Environment Agency and Development Plan Flood Data ...... 8 6.0 Design Considerations ...... 10 7.0 Conclusion ...... 11

Appendices

A Wycombe District Council Sequential Test Report Maps B Wycombe District Council Sequential Test Report Site Appraisals C Strategic Flood Risk Assessment Flood Maps D Environment Agency Flood Maps

1 Proposed Strategic Site Allocations, Wooburn and Bourne End

1.0 Introduction

1.1 Wycombe District Council (WDC) has identified a need for areas of land for development to deliver the circa 13,200 new homes required within their Local Plan for the period up to 2033. The Plan has identified Bourne End as providing suitable land at Hollands Farm and Slate Meadow.

1.2 Hollands Farm, WDC Policy Site Referenced BE2, encompasses an area in the order of 24 hectares with the potential to accommodate 467 dwellings. This site is also identified as having capacity for a single form entry school. Slate Meadow, WDC Policy Site Referenced BE1, encompasses an area of 10 hectares with the potential to accommodate 150 dwellings.

1.3 Previous reviews of the available sites has shown that there may be flood issues which would impact on the delivery of the quantity of new homes required within the emerging New Local Plan.

1.4 Motion has been appointed by Keep Bourne End Green (KBEG) to review the flood issues related to sites in Hollands Farm and Slate Meadow, and has prepared this report summarising the effect of flooding from all sources including rivers, watercourses, groundwater and surface water flooding.

Draft Local Plan Bourne End and Wooburn Map

2 Proposed Strategic Site Allocations, Wooburn and Bourne End

2.0 Policy

Overview

2.1 Planning guidance documents provide information relevant to flood risk. The reference documents for policy guidance and the extent of any flood risk associated with the sites are as follows:

► National Planning Policy Framework – March 2012;

► National Planning Policy Guidance – March 2014;

► Wycombe Development Core Strategy – July 2008;

► Draft New Local Plan for Wycombe District Local Plan – June 2016;

► Draft Infrastructure Delivery Plan – June 2016;

► Jacobs Level 2 Strategic Flood Risk Assessment – September 2017;

► Wycombe District Local Plan, Flood Risk Sequential Test Report – September 2017; and

► Draft Wycombe District Council Development Brief – August 2017

National Planning Policy Framework

2.2 The National Planning Policy Framework (NPPF) aims to ensure that flood risk is taken into account at all stages in the planning process to avoid inappropriate development in areas at risk of flooding and to direct development away from areas at high risk.

2.3 Paragraph 100 of the NPPF discusses the importance of directing development away from areas at highest risk of flooding, but does acknowledge that where development is necessary, making it safe without increasing the risk of flooding elsewhere.

2.4 All land in England and Wales is classified into three main Flood Zones which refer to the probability of river or sea flooding (ignoring the existence of defences). The three Flood Zones are:

► Flood Zone 1 (Low Probability) – less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1%)

► Flood Zone 2 (Medium Probability) – having between a 1 in 100 and 1 in 1000 annual probability of river flooding (1% - 0.1%) in any year or having between a 1 in 200 and 1 in 1000 annual probability of sea flooding in any year (0.5% - 0.1%)

► Flood Zone 3a (High Probability) – having a 1 in 100 or greater annual probability of river flooding (≥1%) or having a 1 in 200 or greater annual probability of sea flooding (≥0.5%)

► Flood Zone 3b (the Functional Floodplain) – having a 1 in 20 or greater annual probability of river flooding. The zone comprises land where water has to flow or be stored in times of flood.

Planning Practice Guidance

2.5 In March 2014, the Department for Communities and Local Government (DCGL) launched the Planning Policy Guidance (PPG) that supports the overarching aims of the National Planning Policy Framework (NPPF). Section ID07, Flood Risk and Coastal Change advises on how to take account of and address the risks associated with flooding and coastal change in the planning process.

2.6 ID07 considers all aspects of flood risk to any new development including: Assessing flood risk, Avoiding flood risk, Manage and mitigate flood risk, Flood risk in the preparation of Local Plans, Sequential risk based approach to the location of the development and the aim of the sequential test.

3 Proposed Strategic Site Allocations, Wooburn and Bourne End

Sequential Test

2.7 The PPG states that; “The Sequential Test ensures that a sequential approach is followed to steer new development to areas with the lowest probability of flooding. The flood zones as refined in the Strategic Flood Risk Assessment for the area provide the basis for applying the Test. The aim is to steer new development to Flood Zone 1 (areas with a low probability of river or sea flooding). Where there are no reasonably available sites in Flood Zone 1, local planning authorities in their decision making should take into account the flood risk vulnerability of land uses and consider reasonably available sites in Flood Zone 2 (areas with a medium probability of river or sea flooding), applying the Exception Test if required.

2.8 WDC prepared a Sequential Test report and prepared the “WDC Local Plan Flood Risk Sequential Test Report, September 2017” (LPFRSTR). This report states that WDC seek to allocate 58 new sites, 12 of which are partially within Flood Zone 2.

2.9 The Sequential Testing of sites is undertaken as part of the Strategic Flood Risk Assessment Levels 1 and 2. The Bourne End sites have been assessed in the Strategic Flood Risk Assessment undertaken by Jacobs, September 2017, as detailed in Section 4 of this report.

2.10 The Fluvial Flood Risk section within the Sequential Test report provides Map 1 showing the allocated sites within the Local Plan as susceptible to flood risk. The LPFRSTR also provides a map of sites susceptible to surface water flooding. These maps show the Slate Meadow site is susceptible to both flood risk and surface water flooding, with Holland Farm showing some flooding at the most southerly tip of the site. A copy of Map 1 and Map 2 is provided in Appendix A.

2.11 The LPFRSTR includes a sequential assessment of Holland Farm, and in respect of flooding, is deemed to have passed the Exception Test with the exception of the aforementioned southern area in Flood Zones 2 and 3 where development should not be considered. A copy of Table 4, Holland Farm Sequential Test report is provided in Appendix B.

2.12 The LPFRSTR also includes a sequential assessment of Slate Meadow and in respect of flooding, is deemed to have passed the Exception Test. However, the report recommends that development within the site is steered toward the area designated as Flood Zone 1. The LPFRSTR indicates that development could proceed within Flood Zone 2 by raising floor levels in areas where it can be shown that flood levels do not exceed 150mm in depth. The report advises:

“The risk to the site can be managed by developing it in such a way that areas of fluvial flood risk are avoided for the location of housing. By avoiding the 1 in 100 AEP+ 35% climate change allowance and designing out the risk of the 1 in 100+70% climate change allowance by raising floor levels, the applicant will be able to develop the site safely in terms of fluvial flood risk, for the lifetime of the development”

2.13 As with all development within Flood Zone 2, there is a requirement to show a safe, dry ingress and egress within areas of flooding and there are construction and usage restrictions in the form of no basements and no sleeping quarters on the ground flood. A copy of Table 6, Slate Meadow Sequential Test report is provided in Appendix B.

4 Proposed Strategic Site Allocations, Wooburn and Bourne End

3.0 Emerging New Local Plan for Wycombe District Council

3.1 Wycombe District Council has prepared a draft New Local Plan, June 2016, due for consultation October to November 2017, which sets out the planning objectives and strategy for the period 2016 to 2031 and has identified the need to deliver circa 13,200 new homes for this period up to 2033.

3.2 The Local Plan has provided supporting evidence on all cause and effect and sustainability of future development including flood risk. The supporting evidence includes a Level 1 and Level 2 Strategic Flood Risk Assessment incorporating flood maps.

3.3 The draft plan states; “… Most of the homes – 85% of the 11,000 homes to be accommodated in the District – are in the main towns: the High Wycombe area (including the surrounding villages such as Hazlemere), Princes Risborough, Marlow and Bourne End and Wooburn.”

3.4 The Policy CP3 - Settlement Strategy, Tier 2 sites identified in Bourne End are Slate Meadow and Hollands Farm. Slate Meadow is one of the sites released for development in 2017 under the previous Local Plan with Holland Farm being a new site as a result of the Green Belt review. Hollands Farm will also provide a primary school for the local area.

3.5 Policy CP4 - Delivering Homes, states that 3,200 homes will be distributed through the Tier 2 site with Bourne End and Wooburn accommodating 800 homes. Paragraph 4.23 provides a reason for the classification within the settlement hierarchy, which states;

“Whilst the River Thames floodplain restricts the scope for development to the south/south west of the village, the village is not surrounded by the Area of Outstanding Natural Beauty. The former reserve site together with opportunity(s) arising from the Green Belt assessment means that there is scope for further development in this location. “

3.6 There are changes to policies from the Delivery and Site Allocations Plan which includes DM17 Planning for Flood Risk Management, which will be replaced by DM39 Managing Flood Risk and Sustainable Drainage Systems.

3.7 Policy CP7 - Delivering Infrastructure to Support Growth, sets out the key infrastructure requirements to support growth, with item 4c stating: “Flood management measures including sustainable drainage systems, including provision for their long term management and maintenance”.

3.8 Paragraph 4.83 acknowledges that whilst the plan directs development away from flood risk, some provision will be required to mitigate against flood risk with reference to the aforementioned Policy DM39.

3.9 Policy CP10, Green Infrastructure and the Natural Environment, refers to working in partnership with the EA, Natural England and the water companies to protect water quality.

3.10 Any new development would not only include flood mitigation measures but also Sustainable Drainage Systems to prevent any future contamination of the local watercourses and the River Thames or Wye.

3.11 Climate change is considered within Policy CP12 with reference to the aforementioned Level 2 Strategic Flood Risk Assessment: “Ensuring allocations in this plan have taken into account climate change allowances using the information provided by the Strategic Flood Risk Assessment Level 1 and 2 and through the sequential testing of sites, and ensuring through detailed development management policy that applications fully factor in climate change in their flood risk assessments”. Design of new development is to include the use of Sustainable Drainage Systems.

3.12 Paragraph 4.126 advises that flood risk and climate change has been factored into the site allocation. The sites at Bourne End have had these factors considered and have been defined as allocated sites.

5 Proposed Strategic Site Allocations, Wooburn and Bourne End

4.0 Strategic Flood Risk Assessment

Strategic Flood Risk

4.1 Further to the release of the Level 1 Strategic Flood Risk Assessment (SFRA) in 2014, Wycombe District Council appointed Jacobs’ to prepare a Level 2 Strategic Flood Risk Assessment that was published September 2017-B127F005-03/03, to accompany the emerging New Local Plan. This SFRA provided updated modelling of the River Thames and River Wye/Hughenden Stream, to determine the extents of the Flood Zones 2 and 3 associated with the allocated sites. Copies of the Strategic Flood Maps and groundwater information are provided in Appendix C.

4.2 The SFRA provides evidence of the flood risk to Hollands Farm, Bourne End which indicates the south western extents of the site, an area of 0.1%, is within Flood Zone 2, with 0.4% within Flood Zone 3 associated with the confluence of the River Thames and the River Wye. The depth of the flooding is assessed to be in the order of 0.25m.

4.3 The SFRA Hydraulic Modelling Technical Note schedule which accompanies the SFRA shows that only 7% of the site is located within Flood Zone 2 with 1% at the southernmost point within Flood Zone 3.

4.4 The surface water mapping shows the northern and western area of Hollands Farm to be at risk of surface water flooding. These are believed to be due to rainfall collecting in localised depressions. However these have been classified as low risk with the developer designing this risk mitigation into future developments.

4.5 The groundwater mapping shows the site to be at risk from groundwater flooding which would need to be taken into consideration in the development proposals. The mapping shows the eastern section of the site at low risk with the western side of the site at a greater risk of up to 75% higher from groundwater flooding. However it should be noted that site specific seasonal groundwater monitoring will need to be undertaken to accurately determine the groundwater levels.

4.6 The high groundwater could be accommodated within the development by the raising of site levels and utilising a shallow Sustainable Drainage System (SuDS) across the site.

4.7 The SFRA also provides flood data on the Slate Meadow, Bourne End site. The SFRA mapping shows the southern and central area of the site to be located within Flood Zone 2, an area of approximately 47%. However, the EA outline of the historic flooding does not reflect the same area, with the historic flooding only encroaching on the southwestern corner of the site. The SFRA indicates the historic flooding was as a result of surcharging of the sewer network and not as a result of breach of the river bank.

4.8 The mapping shows the south west of the site to be flooded during a 1 in 100 year storm event plus a 35% allowance for climate change. The surface water for any new development would be designed to accommodate a 1 in 100 year storm event plus a 40% allowance for climate change in compliance with current recommendations.

4.9 The SFRA mapping also shows the southern 10% of the site is susceptible to surface water flooding although the overall risk is low.

4.10 The mapping does show the site to be susceptible to groundwater flooding however site specific seasonal groundwater monitoring should be undertaken to accurately determine the extents of the groundwater. As advised for Holland Farm, the site levels and drainage would take into consideration the groundwater levels.

4.11 The SFRA Hydraulic Modelling Technical Note schedule shows that 46% of the site is located within Flood Zone 2.

Slate Meadow HR Wallingford Flood Risk/Drainage

4.12 In support of the Local Plan, a site appraisal and river model was undertaken by HR Wallingford, Oct 2016.

6 Proposed Strategic Site Allocations, Wooburn and Bourne End

4.13 A detailed survey was undertaken of the River Wye including the width of the channel and heights of the embankments. The survey also surveyed the bridges and weirs to ensure an accurate assessment of the capacity of the channels. Catchment areas were also updated from the 2013 EA flood model.

4.14 Taking the updated parameters into consideration, the remodelling of the River Wye undertaken by HR Wallingford shows the flooding to the site is predominantly caused by a 9m section of bank believed to be made low by livestock in the field. The modelling from the more accurate survey data shows site to be at less risk of flooding and re-classifies the central section of the site from Flood Zone 2 into Flood Zone 1.

4.15 The model has been reviewed and accepted by the EA, although they suggest flood extents are variable depending on flow and bank levels but depths are less susceptible. This should be taken into consideration when setting floor levels, containing exceedance flows and risk to properties.

4.16 The south of the site remains within Flood Zone 2. This flooding could readily be accommodated within the south of the site by the provision of a designated area of floodplain. The floodplain would allow free ingress and egress of exceedance flow from the River Wye by replicating the existing floodplain levels. Providing that there is sufficient area allocation for the floodplain the remainder of the site can be utilised for future development.

4.17 The site can accommodate residential development, although the development footprint may be restricted to the northern and central areas of the site.

7 Proposed Strategic Site Allocations, Wooburn and Bourne End

5.0 Environment Agency and Development Plan Flood Data

Environment Agency Data

5.1 An application was made to the Environment Agency (EA) to obtain the current flood data for the site within Bourne End. The data received shows the flood zones for both Hollands Farm and Slate Meadow. A copy of the EA mapping for both sites is provided in Appendix D.

5.2 The EA flood map for Holland Farm shows the southernmost point of the site lies within Flood Zone 2 with a small section along the northern edge of Hedsor Road lies within Flood Zone 3.

5.3 The EA advise the River Wye modelling is taken from the River Wye (West Wye to Thames Confluence) 2002 Detailed Mapping Project and therefore earlier than the modelled results from the Jacobs’ modelled data. The data was collected from six nodes along approximately 550m of the River Wye between the junctions of Hedsor Road/Wessex Road, to the north of the weir, to the north of the recreation ground. The model levels have an accuracy of +/- 250mm.

5.4 During flood events Hedsor Road will flood and will be closed to pedestrian and vehicular traffic. The development proposals show a secondary access to the north of the proposed development outside the floodplain allowing pedestrian and vehicular ingress and egress during flood events.

5.5 The EA advise there are currently no defences associated with the River Thames or River Wye within this stretch, therefore the extents of the floodplain will not be affected by defences.

5.6 The historic flood mapping shows no record of historic flooding to the site although Hedsor Road and a small area at the southernmost tip of the site were flooded in the 1947 flood event. The flooding was recorded as a result of river channel exceedance and not as a result of other causes such as blockages.

5.7 The EA mapping for the Slate Meadow site shows the site to be located within Flood Zone 1 to the north and within Flood Zone 2 to the south and centre. The EA mapping does reflect the floodplain shown within the Jacobs SFRA. The proportion of the site in each flood zone is approximately 50% reflecting the 47% advised within the Jacobs SFRA.

EA Flood Mapping for Slate Meadow

5.8 In accordance with the NPPF, it is acceptable to provide residential properties within Flood Zone 2, however, it is recommended that proposed residential development lies within Flood Zone 1.

8 Proposed Strategic Site Allocations, Wooburn and Bourne End

5.9 The area designated as Flood Zone 3 is an existing situation. Mitigation measures undertaken within Slate Meadow would not exacerbate the flooding within Core End Road and the surrounding area.

5.10 As with Holland Farm, this site is not currently protected by formal flood defences and therefore the extents of the floodplain will not be affected by defences.

5.11 The EA historic mapping does not reflect the flood probability map and shows historic flooding to the southernmost corner during the 1981 flood event. The site did not flood during the 1947 period which caused flooding at the southernmost point of the Holland Farm site.

5.12 Development Plan Information

5.13 The WDC Development Brief also shows the areas susceptible to flooding within Slate Meadow. The document highlights a strip of land along the western boundary designated as an ecological corridor with the southern section designated as an ecological floodplain as shown in the following plan:

5.14 It can be seen form the plan that the majority of the southern section of the site is flooded to a depth in excess of 0.101 to 0.200m. This depth is in excess of the standard 0.15mm height of the damp proof course constructed within a residential property. To achieve build within this area, properties would require flood resilient construction with no sleeping quarters on the ground floor and safe, dry access and egress from the flood area.

5.15 The document recommends this ecological corridor is retained as part of any new development proposals for Slate Meadow, thereby retaining the floodplain and the associated ecology. Whilst it is acknowledged that this would reduce the potential number of dwellings, it would alleviate the risk of flooding to the site and surrounding area.

5.16 It should also be noted that there is no dry route from the site to the west along Core End Road as this route is potentially in flood during a 1 in 100 to 1 in 1000 annual probability flood event.

9 Proposed Strategic Site Allocations, Wooburn and Bourne End

6.0 Design Considerations

Surface Water

6.1 The aforementioned information has shown there are flood issues with the proposed site at Slate Meadow in respect of the River Wye. The site is partially within the floodplain and as such, any development within the floodplain could increase flood risk by diverting the flood water to the surrounding areas.

6.2 Local information on historic flood data has shown that the residential development to the west of Slate Meadow has a history of flooding along Trees Road and Willows Road which ultimately floods into Cores End Road before overflowing into the River Wye.

6.3 The historic data shows the flooding is from surface water run-off from the north. This area comprises open greenfield, having a steep gradient falling to the residential development. The steeper gradient of the greenfield land increases the flow of surface water run-off and reduces the infiltration of surface water.

6.4 It should be noted that the greenfield expanse to the north of Slate Meadow falls to the River Wye thereby increasing the risk of flooding to the proposed development.

Foul Water

6.5 Historic data has also shown that there have been issues with TW infrastructure leading to flooding of the sewers with discharge of raw sewage into the River Wye and the River Thames.

6.6 There is local historic information to show that flooding has overwhelmed the Little Marlow Sewage Treatment Works resulting in raw sewage discharging to the local area.

6.7 Due consideration should be given to the future capacity of not just the local foul sewers, but also the Marlow Treatment Works.

6.8 TW issued a report ‘Little Marlow, (Hughenen Valley) Drainage Strategy, Stage 1 – Initialise/Prepare’ which outlines the flooding issues within the TW sewer system and the TW proposals to reduce the risk of flooding in the future.

6.9 The report is the Stage 1 initial preparing undertaken in 2015 with Stage 2, Risk Assessment and Stage 3 Options appraisal being completed by 2017. Stage 4, Intervention, is from 2018 onwards.

6.10 The document states: the strategies will develop throughout the 4-stage framework to define how we intend to alleviate sewer flooding or to address growth related issues in each area sustainably, and economically, over the next few years.’

6.11 The report does indicate that sewer flooding can be associated with high groundwater. However, as this is only the initial appraisal, the outcome shows that TW intend to maintain and clean sewers until the final assessment has been undertaken in the latter stages.

10 Proposed Strategic Site Allocations, Wooburn and Bourne End

7.0 Conclusion

7.1 This statement has been based on current information available from the WDC Local Plan, Strategic Flood Risk Assessment, EA flood data and the WDC Flood Risk Sequential Test Report.

7.2 All new development would provide flood mitigation to ensure no increased risk of flood to or from any future development by way of a Sustainable Drainage Management Train. The Sustainable Drainage offered would also protect the water quality of the local watercourses and main rivers, the River Thames and River Wye.

7.3 The Local Plan policies advise that flood risk and climate change has been factored into site allocation. The sites at Bourne End have had these factors considered and have been defined as allocated sites.

Holland Farm

7.4 The allocated site at Holland Farm is shown to be predominantly in Flood Zone 1, with a small area in the southern point to be within Flood Zones 2 and 3, and is suitable for residential development. However, any proposed development should be contained within Flood Zone 1 and not encroach within the southern area designated Flood Zone 2 and 3.

7.5 The site is shown to be susceptible to groundwater flooding, and therefore the developer would be required to undertake seasonal groundwater monitoring across the extent of the site to accurately determine any affect the groundwater may have on the detail design. Any groundwater flooding may be overcome by a slight localised raising of the ground levels.

7.6 The ground formation would allow for a Sustainable Drainage System (SuDS) with a shallow SuDS infiltration system in compliance with the NPPF.

Slate Meadow

7.7 The Jacobs and EA flood data show the allocated site at Slate Meadow within Flood Zone 1 and Flood Zone 2. However the HR Wallingford model undertaken October 2016, incorporating the updated flow data and survey data, show a considerable reduction in the flood plain designated as Flood Zone 2.

7.8 In compliance with Table 3 of the PPG ‘Flood Risk Vulnerability and Flood Zone Compatibility’ residential development is acceptable within Flood Zone 2 and therefore this site can accommodate the new homes required within the WDC Local Plan.

7.9 However, the development must incorporate flood resilient design and prove the development does not cause flood elsewhere, to or from the site.

7.10 The recommendation would be the introduction of an assigned area for floodplain, located within the southern section of the site, thereby reducing the flooding risk to the remainder of the site and proposed development.

7.11 The development proposals would include a safe, dry ingress and egress route to and from the site mitigating risk to residents of the development.s

7.12 Although it is accepted that there would be a reduction in the number of new homes allocated to the Slate Meadow site, the site could be developed with the above mentioned mitigation measures in the form of an assignment floodplain, raised site levels, flood resilient construction and safe, dry access to the site.

11 Appendix A

WDC Sequential Test Report Maps Flood Risk Sequential Test Report – September 2017

Map 1 Wycombe District Local Plan proposed allocations and Areas of Fluvial Flood Risk

Flood Risk Sequential Test Report – September 2017

Map 2. Wycombe District Local Plan allocations and Areas of Surface Water Flood Risk

Appendix B

WDC Sequential Test Report Site Appraisals Flood Risk Sequential Test Report – September 2017

Table 4 – Hollands Farm Site detail Hollands Farm (north) Bourne End Allocation Residential-led mixed use (residential (467 dwellings), education)

Highest More vulnerable Vulnerability of proposed use Flood zone and Fluvial Flood Risk other sources of -Current flood zone(s): Majority of the site in flood zone 1, a flooding small area in zone 2 and a very small area zone 3a. -With climate change allowance: the extent of flood zone 3a remains very small in the 70%+CC scenario

Other risks: -Surface water flood risk (1 in 10, 1 in 100, 1 in 1000); risk could increase with climate change. -Groundwater emergence: the site is partly at risk of groundwater flooding; the level of risk differs across the site. -Historic records of sewer flooding in the area Reasonably None - Wycombe District Council is a very constrained area available sites in due to the Green Belt and the AONB. The first parts of the same or lower Sequential Test above show that there are not enough sites

flood zone in flood zone 1 or 2 to meet the Objectively Assessed Need. Exception Test Required considering the vulnerability of users and fluvial flood zone classification with climate change. It must be The site will make a significant contribution to the Council’s demonstrated that housing land supply across the plan period. The site will also the development provide a new primary school for the local area, and may be provides wider also accommodating a new GP surgery. The site will provide sustainability a substantial amount of open space on site and will also benefits to the contribute to improvements of access to the Little Marlow

Flood Risk Sequential Test Report – September 2017

community that Lakes Country Park. A new link road will run through the outweigh flood site, which will make the development more permeable and risk, informed by a provide some relief to the local road network (although it is Strategic Flood not meant to be strategic in nature) Risk assessment where one has been prepared A site-specific The site is currently mostly located within Flood zone 1 and flood risk 2, with 0.1% at the most southern end in the 1 in 100 AEP assessment must event. The proposed development of 467 residential demonstrate that properties and a school would be classified as ‘More the development Vulnerable’ 15 and can be located within flood zones 1 and 2 will be safe for its and would therefore be acceptable and would not need to lifetime taking pass the Exception Test 16 . The area within the south- account of the western corner of the site is designated as Flood zone 3 and vulnerability of its therefore the applicant should seek to not develop this area uses, without as sufficient space is available elsewhere within the site for increasing flood the proposed residential development. The level of risk is risk elsewhere, not predicted to change when the impact of climate change and, where is considered and therefore the site could be developed possible, will safely. reduce flood risk The proposed access from the south-west of the site from overall. Hedsor Road does cross flood zone 3 and as such the development would have to pass the Exception Test. The final alignment of the proposed access would affect the planning requirements to be met [1] . If the highway is developed within Flood zone 3a, the development as a whole should be designed and constructed to remain operational and safe in times of flood in order to pass the Exception Test. In this case the development would include

15 NPPG Table 2 / Paragraph 066 16 NPPG Table 3 / Paragraph 067 [1] NPPG Table 3 / Paragraph 067 37

Flood Risk Sequential Test Report – September 2017

multiple access points, including in flood zone 1, consequently this particular access could be closed during a flood event. If located within flood zone 3b, additional regulations must be followed to pass the Exception Test: the road should be designed and constructed to: Remain operational and safe for users in times of flood; Result in no net loss of floodplain storage; and Not impede water flows and not increase flood risk elsewhere. To meet these requirements the access would not change existing ground levels and the development would include facilities to close this access route during a flood event. The site would remain operational and safe and the access would not change existing flood risk. Therefore the site can pass the second part of the Exception Test. Risk Management The risk to the site can be managed by developing it in such a way that areas of fluvial flood risk are avoided for the location of housing or the school. To minimise the risk of surface and groundwater flooding, the site should be developed in such a manner as to avoid encroachment into the WCDA wherever practicable; If the development does encroach into the WCDA finished floor levels should be above the High and Medium uFMfSW risk peak water level (plus an allowance for climate change); applicants will need to demonstrate that the flow path can be maintained, or modifications to it as a result of the development will not detrimentally affect third parties No habitable below-ground rooms should be constructed due to the groundwater flood risk. Source control SuDS and drainage improvements can be implemented. If a 1 in 100 AEP flood event occurs, the southern access from Hedsor Road can be closed and alternative access and egress be provided at two other locations to the north of the site.

Flood Risk Sequential Test Report – September 2017

Summary Subject to site layout, appropriate finished floor levels, SuDS implementation, and closure of the southern access in an extreme event, the site can be developed safely for the proposed use. The site passes the Sequential Test and Exception Test. Planning recommendation Allocate for development in the WDLP

Flood Risk Sequential Test Report – September 2017

Table 6 – Slate Meadow Site detail Slate Meadow, Bourne End Allocation Residential use (150 dwellings) Highest More Vulnerable Vulnerability of proposed use Flood zone and Fluvial flood risk: other sources of -Current flood zone(s): The site is mostly in flood zones 1 flooding and 2. The 1 in 100 AEP event 17 shows a small area of zone 3a to the south. -With climate change allowance: the extent of flood zone 3a increases (up to 20% of the site in the 70% CC scenario).

Other risks: -Surface water flood risk areas (1 in 10, 1 in 100, 1 in 1000) -Groundwater emergence: the site is at high risk of groundwater flooding -Culvert blockage at Cores End road -Historic records of sewer flooding in the area Reasonably None - Wycombe District Council is a very constrained available sites in area due to the Green Belt and the AONB. The first parts same or lower flood of the Sequential Test above show that there are not zone enough sites in flood zone 1 or 2 to meet the Objectively Assessed Need. . Exception Test Required considering the vulnerability of users and fluvial flood zone classification with climate change. It must be The development will make an important contribution to demonstrated that the supply of housing in Bourne End across the plan

17 The 1 in 100 event used in the SFRA level 2 for Slate Meadow is from the applicant’s own modelling for the site, which was accepted by the Environment Agency in August 2016. 43

Flood Risk Sequential Test Report – September 2017

the development period. The site was designated as Safeguarded Land in provides wider the 2004 Local Plan and a Reserve Site reserved for sustainability development in the 2008 Core Strategy. The site was benefits to the released for development in 2014 and considerable work community that has been undertaken to produce a development brief for outweigh flood risk, the site with the local community (through a liaison group) informed by a which will guide how this site comes forward. Strategic Flood Risk assessment where one has been prepared A site-specific flood The site is mainly located within Flood zones 1 and 2, risk assessment with 5% within the 1 in 100 AEP event. The proposed must demonstrate development of 150 residential properties would be that the classified as ‘More Vulnerable’ 18 . A sequential approach development will be to the site layout should be followed to place ‘More safe for its lifetime Vulnerable’ development within the lowest areas of flood taking account of risk on the site. the vulnerability of Hydraulic modelling of the predicted impact of climate its uses, without change on fluvial flooding indicates that part of the site increasing flood could be at a relatively extensive risk from the 1% (1 in risk elsewhere, and, 100 AEP) +70% 19 event (20% of the site) to the south where possible, will west of the site. The applicant would need to consider the reduce flood risk consequences of such an event on the development and overall. should avoid encroachment into the flood extent where possible. However if the 70% scenario is indicated as covering part of the site, the flood depths across most of that 1 in 100 AEP+ 70% extent are considered to be low. Generally flood depths are less than 150 mm for such an event so it should be possible to avoid flooding of

18 NPPG Table 2 / Paragraph 066 19 Climate Change allowance for more vulnerable in flood zone 3 needs to consider a range between the 35% CC and 70% CC scenario in Wycombe District. 44

Flood Risk Sequential Test Report – September 2017

properties at such a depth. There are areas (approximately 0.6ha / 6%) of greater depth of up to 600mm which it should be possible to avoid. Assuming the development can progress without encroaching into the +35% and ideally the +70% extent, or that the +70% extent is designed out by raising floor levels, then the development can pass the second part of the Exception Test. Risk Management The risk to the site can be managed by developing it in such a way that areas of fluvial flood risk are avoided for the location of housing. By avoiding the 1 in 100 AEP+ 35% climate change allowance and designing out the risk of the 1 in 100+70% climate change allowance by raising floor levels, the applicant will be able to develop the site safely in terms of fluvial flood risk, for the lifetime of the development. The applicant will need to demonstrate how the risk of surface water and groundwater is mitigated. To minimise the risk of surface and groundwater flooding, the site should be developed in such a manner as to avoid encroachment into the WCDA wherever practicable; If the development does encroach into the WCDA finished floor levels should be above the High and Medium uFMfSW risk peak water level (plus an allowance for climate change); applicants will need to demonstrate that the flow path can be maintained, or modifications to it as a result of the development will not detrimentally affect third parties No habitable below-ground rooms should be constructed due to the groundwater flood risk;. Source control SuDS and drainage improvements can be implemented. Preventative measures to reduce exacerbation of risk of blockage to the culvert immediately downstream at Cores End Road can be included as part of the development. A development brief is currently being prepared for the site, which reflects the SFRA level 2 findings and recommendations. Summary

Flood Risk Sequential Test Report – September 2017

Subject to site layout (avoiding area at higher risk of fluvial flooding), appropriate finished floor levels and SuDS implementation, the site can be developed safely for the proposed use. The site passes the Sequential Test and Exception Test.

Planning recommendation Allocate for development in the Wycombe District Local Plan

Appendix C

Strategic Flood Risk Assessment Maps Level 2 Strategic Flood Risk Assessment

classified as WCDA. Such a risk would need to be quantified by the applicant in order to demonstrate that the site could be developed safely for future users, taking into account the planning implications of such a result.

3.5.3.2 Groundwater

For the two sources of groundwater risk mapping, the AStGWF depicts the likelihood of groundwater emergence; the JBA mapping inidcates depth to groundwater level beneath ground.

A review of EA AStGWF mapping indicates that the eastern side of the site is at low risk of flooding (within a grid square where less than 25% is at risk of groundwater emergence). The western side of the site is at high risk of flooding (within a grid square where greater than 75% of the area is at risk of groundwater emergence). However further modelling places the eastern half of the site in a zone of elevated risk of groundwater emergency which would need to be considered in the site-specific FRA; see Figure 3-34.

Figure 3-34: Site 15 Risk of Groundwater Flooding

Figure 3-35 depicts groundwater mapping completed by JBA, showing a high risk of groundwater flooding and that the groundwater levels are either at or very near to (within 0.025m) the ground surface within the central sections of this site. The eastern side of the site is shown to have groundwater depths between 0.025-5m; the western is shown to have groundwater depths of greater than 5m.

B127F005-03 55

Level 2 Strategic Flood Risk Assessment

Figure 3-35 : Site 15 JBA Groundwater Mapping

3.5.3.3 Sewers

The Hollands Farm (north) allocation site is entirely located within post code area SL8, as identified in Figure 7 of the Level 1 SFRA. The figure indicates between one and five properties have been reported to flood internally within the post code boundary; between 16 and 20 instances of external flooding have been reported within SL8 in the 20 years preceding 2014.

3.5.3.4 Reservoir Failure

A review of the predicted flood extent as a result of reservoir failure available online indicates that the site is not at risk of flooding from such an event. See Figure 3-36.

B127F005-03 56

Appendix D

Environment Agency Flood Maps Product 4 (Detailed Flood Risk) for Holland Farm, Bourne End Our Ref: THM_54430

Product 4 is designed for developers where Flood Risk Standing Advice FRA (Flood Risk Assessment) Guidance Note 3 Applies. This is: i) "all applications in Flood Zone 3, other than non-domestic extensions less than 250 sq metres; and all domestic extensions", and ii) "all applications with a site area greater than 1 ha" in Flood Zone 2.

Product 4 includes the following information: Please note:

Ordnance Survey 1:25k colour raster base mapping; If you will be carrying out computer modelling as part of your Flood Risk Flood Zone 2 and Flood Zone 3; Assessment, please read the enclosed guidance which sets out our Relevant model node locations and unique identifiers (for cross referencing to the water requirements and best practice for computer river modelling. levels, depths and flows table); Model extents showing defended scenarios; This information is based on that currently available as of the date of this FRA site boundary (where a suitable GIS layer is supplied); letter. You may feel it is appropriate to contact our office at regular intervals, Flood defence locations (where available/relevant) and unique identifiers; (supplied to check whether any amendments/ improvements have been made. Should separately) you re-contact us after a period of time, please quote the above reference in Flood Map areas benefiting from defences (where available/relevant); order to help us deal with your query. Flood Map flood storage areas (where available/relevant); Historic flood events outlines (where available/relevant, not the Historic Flood Map) and This information is provided subject to the enclosed notice which you should unique identifiers; read. Statutory (Sealed) Main River (where available within map extents); This letter is not a Flood Risk Assessment. The information supplied can be used to form part of your Flood Risk Assessment. Further advice and guidance regarding Flood Risk Assessments can be found on our website at A table showing: i) Model node X/Y coordinate locations, unique identifiers, and levels and flows for http://www.environment-agency.gov.uk/research/planning/82584.aspx defended scenarios. ii) Flood defence locations unique identifiers and attributes; (supplied seperately) If you would like advice from us regarding your development proposals you iii) Historic flood events outlines unique identifiers and attributes; and can complete our pre application enquiry form which can be found at iv) Local flood history data (where available/relevant). http://www.environment-agency.gov.uk/research/planning/33580.aspx

Red Kite House, Howbery Park, Wallingford, Oxon OX10 8BD Customer services line: 08708 506 506 Email: [email protected] www.environment-agency.gov.uk Flood Map for Planning centred on: Holland Farm, Bourne End Created on 04/08/2017 REF: THM_54430

Kilometres 0 0.09 0.18

Legend

Main River Flooding from rivers or sea (FZ3) Extent of extreme flood (FZ2)

Flooding from rivers or sea without defences (Flood Zone 3) shows the area that could be affected by flooding: - from the sea with a 1 in 200 or greater chance of happening each year - or from a river with a 1 in 100 or greater chance of happening each year.

The Extent of an extreme flood (Flood Zone 2) shows the extent of an extreme flood from rivers or the sea with up to a 1 in 1000 chance of occurring each year.

© Environment Agency copyright and / or database rights 2015. All rights reserved. © Crown Copyright and database right. All rights reserved. Environment Agency, 100024198, 2015. Contact Us: National Customer Contact Centre, PO Box 544, Rotherham, S60 1BY. Tel: 08708 506 506 (Mon-Fri 8-6). Email: [email protected] Defence information THM_54430

Defence Location: No defences on Main River

Description: This location is not currently protected by any formal defences and we do not currently have any flood alleviation works planned for the area. However we continue to maintain certain watercourses and the schedule of these can be found on our internet pages.

Model: Wye (West Wycombe to Thames Confluence) 2002

Description: The information provided is taken from the Wye (West Wycombe to Thames Confluence) 2002 Detailed Mapping project. The study was carried out using ISIS 1D modelling software.

Model design runs: 1 in 5 / 20% AEP; 1 in 20 / 5% AEP; 1 in 100 / 1% AEP

Mapped Outputs: 1 in 5 / 20% AEP; 1 in 20 / 5% AEP; 1 in 100 / 1% AEP

Model accuracy: Levels ± 250mm

06125_MN_WY1016 06125_MN_WY1017U Kilometres 06125_MN_WY1014 (! 0 0.09 0.18 (! 06125_MN_WY1013

(! 06125_MN_WY1012 Legend (! (! Wye Model Node Data (! Main River 20% AEP flood extent 5% AEP flood extent

06125_MN_WY1011 1% AEP flood extent

AEP = Annual Exceedance Probability The probability of a flood of a particular magnitude, or greater, occuring in any given year

1%CC = 1% Climate Change extent This is the 1% AEP event with an allowance for climate change (+20% on river flows)

The modelled flood levels and flows for the closest most appropriate model node points for your site that are within the river channel are provided below:

Flood Levels (mAOD)

1% AEP (+20% on Node label Model Easting Northing 20% AEP 5% AEP 1% AEP 0.1% AEP river flows)

06125_MN_WY1017U Wye (West Wycombe to Thames confluence) 2002 489770 187191 29.27 29.32 29.34 0.00 0.00 06125_MN_WY1016 Wye (West Wycombe to Thames confluence) 2002 489786 187115 28.29 28.34 28.36 0.00 0.00 06125_MN_WY1014 Wye (West Wycombe to Thames confluence) 2002 489699 186949 27.78 27.83 27.84 0.00 0.00 06125_MN_WY1013 Wye (West Wycombe to Thames confluence) 2002 489649 186865 27.50 27.56 27.58 0.00 0.00 06125_MN_WY1012 Wye (West Wycombe to Thames confluence) 2002 489621 186775 27.40 27.45 27.48 0.00 0.00 06125_MN_WY1011 Wye (West Wycombe to Thames confluence) 2002 489590 186693 27.26 27.29 27.31 0.00 0.00

Flood Flows (m3/s)

1% AEP (+20% on Node label Model Easting Northing 20% AEP 5% AEP 1% AEP 0.1% AEP river flows)

06125_MN_WY1017U Wye (West Wycombe to Thames confluence) 2002 489770 187191 4.66 5.29 5.55 0.00 0.00 06125_MN_WY1016 Wye (West Wycombe to Thames confluence) 2002 489786 187115 4.66 5.29 5.55 0.00 0.00 06125_MN_WY1014 Wye (West Wycombe to Thames confluence) 2002 489699 186949 4.66 5.29 5.55 0.00 0.00 06125_MN_WY1013 Wye (West Wycombe to Thames confluence) 2002 489649 186865 4.66 5.29 5.55 0.00 0.00 06125_MN_WY1012 Wye (West Wycombe to Thames confluence) 2002 489621 186775 4.66 5.29 5.55 0.00 0.00 06125_MN_WY1011 Wye (West Wycombe to Thames confluence) 2002 489590 186693 4.66 5.29 5.55 0.00 0.00

Note: Due to recent changes in guidance on the allowances for climate change, the 20% increase in river flows should no longer to be used for development design purposes. The data included in this Product can be used for interpolation of levels as part of an Intermediate level assessment.

For further advice on the new allowances please visit https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances

Kilometres 0 0.09 0.18

Legend

Main River Flood Event Outlines year 1947 1974 2002 2014

The Extent of an extreme flood (Flood Zone 2) shows the extent of an extreme flood from rivers or the sea with up to a 1 in 1000 chance of occurring each year.

Our records show that the area of your site has been affected by flooding. Information on the floods that have affected your site is provided in the table below:

Flood Event Code Flood Event Name Start Date End Date Source of Flooding Cause of Flooding

EA0619470300510 06MarchSpring1947 01/01/1947 12/12/1947 main river channel capacity exceeded (no raised defences) EA0619741100173 06NovemberAutumn1974 01/01/1974 12/12/1974 main river channel capacity exceeded (no raised defences) EA0620030101570 06JanuaryNewYear2003 23/12/2002 12/01/2003 main river channel capacity exceeded (no raised defences) EA061002014009 Winter 2013/2014 23/11/2013 28/02/2014 main river channel capacity exceeded (no raised defences)

Please note the Environment Agency maps flooding to land not individual properties. Floodplain extents are an indication of the geographical extent of a historic flood. They do not provide information regarding levels of individual properties, nor do they imply that a property has flooded internally.

Start and End Dates shown above may represent a wider range where the exact dates are not available.

Product 4 includes the following information: Please note:

Red Kite House, Howbery Park, Wallingford, Oxon OX10 8BD Customer services line: 08708 506 506 Email: [email protected] www.environment-agency.gov.uk Flood Map for Planning centred on: Slate Meadow, Wooburn Town Created on 04/08/2017 REF: THM_54436

Kilometres 0 0.09 0.18

Legend

Main River Flooding from rivers or sea (FZ3) Extent of extreme flood (FZ2)

The Extent of an extreme flood (Flood Zone 2) shows the extent of an extreme flood from rivers or the sea with up to a 1 in 1000 chance of occurring each year.

Defence Location: No defences on Main River

Model: Wye (West Wycombe to Thames Confluence) 2002

Description: The information provided is taken from the Wye (West Wycombe to Thames Confluence) 2002 Detailed Mapping project. The study was carried out using ISIS 1D modelling software.

Model design runs: 1 in 5 / 20% AEP; 1 in 20 / 5% AEP; 1 in 100 / 1% AEP

Mapped Outputs: 1 in 5 / 20% AEP; 1 in 20 / 5% AEP; 1 in 100 / 1% AEP

Model accuracy: Levels ± 250mm

Kilometres 0 0.09 0.18

Legend

(! Wye Model Node Data Main River 20% AEP flood extent 5% AEP flood extent 06125_MN_WY1017U 1% AEP flood extent

06125_MN_WY1016 (! AEP = Annual Exceedance Probability The probability of a flood of a particular (! magnitude, or greater, occuring in any given year 06125_MN_WY1014 06125_MN_WY1013 1%CC = 1% Climate Change extent (! This is the 1% AEP event with an allowance for climate change (+20% on river flows) (! 06125_MN_WY1012

(! © Environment Agency copyright and / or database rights 2015. All rights reserved. © Crown Copyright and database right. All rights reserved. Environment Agency, 100024198, 2015. Contact U(!s: National Customer Contact Centre, PO Box 544, Rotherham, S60 1BY. Tel: 08708 506 506 (Mon-Fri 8-6). Email: [email protected] Modelled in-channel flood flows and levels THM_54436

The modelled flood levels and flows for the closest most appropriate model node points for your site that are within the river channel are provided below:

Flood Levels (mAOD)

1% AEP (+20% on Node label Model Easting Northing 20% AEP 5% AEP 1% AEP 0.1% AEP river flows)

Flood Flows (m3/s)

1% AEP (+20% on Node label Model Easting Northing 20% AEP 5% AEP 1% AEP 0.1% AEP river flows)

For further advice on the new allowances please visit https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances

Kilometres 0 0.09 0.18

Legend

Main River Flood Event Outlines year 1947 1981

The Extent of an extreme flood (Flood Zone 2) shows the extent of an extreme flood from rivers or the sea with up to a 1 in 1000 chance of occurring each year.

Our records show that the area of your site has been affected by flooding. Information on the floods that have affected your site is provided in the table below:

Flood Event Code Flood Event Name Start Date End Date Source of Flooding Cause of Flooding

EA0619470300510 06MarchSpring1947 01/01/1947 12/12/1947 main river channel capacity exceeded (no raised defences) EA0619811200005 06DecemberWinter 1981 01/01/1981 12/12/1981 main river channel capacity exceeded (no raised defences)

Start and End Dates shown above may represent a wider range where the exact dates are not available.