Aylesbury Town Level 2 Strategic Flood Risk Assessment Final Report

Aylesbury Vale District Council

9 April 2009

9T5436

Strategic Flood Risk Assessment Level 2 for Aylesbury Town – August 2012 Update

This note has been prepared to advise the reader of the updated context for this evidence base in terms of the work on the Vale of Aylesbury Plan and planning applications around Aylesbury.

The SFRA Level 2 was prepared by Royal Haskoning and published by AVDC in 2009 following detailed watercourse modelling to inform the planning of potential major development site options around the town. The report identifies the locations of potential flood storage areas, the potential impact of breaches to flood defences, potential improvements to the existing Aylesbury Flood Alleviation Scheme at Broughton and the extent of the functional floodplain.

The evidence base is still valid for both assessing planning applications in Aylesbury and to inform the Vale of Aylesbury Plan or any Neighbourhood Planning work. However it should be noted that there have been significant legislative, regulatory and National planning policy changes since the publication of the SFRA Level 2 in April 2009. These include:-

 the Flood and Water Management Act 2010  Localism Act 2011  Town and Country Planning (Local Planning) (England) Regulations 2012  National Planning Policy Framework (2012) and  Technical Guidance to the National Planning Policy Framework 2012

The SFRA Level 1 for Aylesbury Vale (August 2012) provides a full coverage of current legislation, regulations and the impacts of the National Planning Policy Framework. Where the SFRA Level 2 for Aylesbury Town is inconsistent with the SFRA Level 1 on current legislation, regulations and the NPPF, those in the SFRA Level 1 are prevalent.

EXECUTIVE SUMMARY

Background The local development documents (LDDs), which make up the Local Development Framework (LDF) must be based on a robust and credible evidence base. This Level 2 Strategic Flood Risk Assessment (SFRA) informs the preparation of the Aylesbury Vale Core Strategy with regards to flood risk. It will assist the Aylesbury Vale District Council in applying the Sequential Test and the Exception Test, providing a method of managing flood risk while still allowing necessary development to occur.

It is recognised that this is a live document which should be updated accordingly with changes to flood risk management and planning policies.

In order to pass the Exception Test, a development must pass all the following aspects:

(a) it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared. If the Development Plan Documents (DPDs) have reached the ‘submission’ stage – see Figure 4 of PPS12: Local Development Frameworks – the benefits of the development should contribute to the Core Strategy’s Sustainability Appraisal;

(b) the development should be on developable previously-developed land or, if it is not on previously developed land, that there are no reasonable alternative sites on developable previously-developed land.

(c) a Flood Risk Assessment must demonstrate that the development will be safe, without increasing the flood risk elsewhere, and, where possible, will reduce flood risk overall. The assessment will refer to the joint Defra and Environment Agency document “Flood Risks to People Phase 2, Guidance Document, FD2321/TR2 (March 2006)” to demonstrate that the development is safe.

If the Growth Arcs can maintain the runoff at the Greenfield rates, reduce flood risk to the town centre and surrounding areas, such as at Watermead, and keep built development out of the present day 0.1% annual probability residual risk areas, then they would pass aspect (c).

This Final Level 2 SFRA satisfies the amplified guidance within the Living Draft Practice Guide Companion to PPS25 by: ƒ Defining the functional floodplain; ƒ Considering the nature of the flood hazard, taking account of the presence of flood management measures e.g. flood defences and defacto defences; ƒ Considering the nature of flood hazard if defences fail; ƒ Appraising the defence infrastructure condition and maintenance in light of flood risk; ƒ Informing Emergency Planning Procedures (access and egress to development sites); and ƒ Providing guidance on the preparation of Flood Risk Assessments and the consideration of managing surface water (Appendix A).

All the growth arc allocations include functional floodplain and Flood Zones 2 and 3 to ensure the Council has maximum control of these areas. The Council’s policy is that

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there should be no built development within any of these areas. This approach is in line with the selected policy from the Thames Catchment Flood Management Plan which is to “accept flood risk, but in the longer term take action to ensure that the risk does not increase from current level”.

The functional floodplain The functional floodplain has been defined using a flood with an annual probability of 5% (1:20 years) plus the areas within the existing flood storage reservoirs (Stocklake FSR and Bear Brook FSR), where water is stored in times of flooding. In this floodplain only water-compatible development (e.g. amenity open space) should be permitted, together with essential infrastructure if it passes the Exception Test.

The present and future 5% annual probability flood events affect small areas of each Growth Arc: ƒ Eastern Growth Arc –14.4 hectares (3.4%) and 15.4 hectares (3.6%) respectively; ƒ Southern Growth Arc – 1.4 hectares (0.3%) and 1.8 hectares (0.4%) respectively; and ƒ Combined East and Southern Growth Arc – 1.4 hectares (0.3%) and 1.8 hectares (0.4%) respectively.

Generally the flood hazard to people is low except in the north east Major Development Area (MDA) where it is moderate to significant to people.

Under the 5% annual probability flood event, the flood storage reservoirs and defences protect the urban areas through the town except small areas of Fairford Leys and Fowler Road. The River Thame functional floodplain extends to some properties at Quarrendon (north of Jackson Road) and also parts of the MDAs at Berryfields and Weedon Hill. The east part of Aston Clinton Road MDA is within the West End Ditch functional floodplain (next to Broughton Lane and below Old Manor Farm) and the Wendover Brook functional floodplain runs through the north east portion.

All of the proposed strategic link roads are within the present day and future functional floodplains and in accordance with PPS25 Table D.1 they should be designed to remain operational and safe for users in times of flood. However, the link roads provide the opportunity of multiple benefits, such as flood defence and increased biodiversity. Buckinghamshire County Council should take account of the level of flood risk and design the link roads in consultation with Aylesbury Vale District Council and the Environment Agency to make them sustainable and ensure they do not increase flood risk elsewhere.

Residual flood risk and flood hazard Small areas of all the Growth Arcs are within the defended present day and future 1% and 0.1% annual probability flood events. However, the Council is not proposing any built development in any of these areas. As there is enough land outside of the present day residual risk areas, only water-compatible development (e.g. amenity open space) should be permitted, together with essential infrastructure if it passes the Exception Test.

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For areas of previously developed land in the town and where there is no available land outside of the 0.1% residual flood risk area, the Flood Hazard maps should be used to steer water compatible and less vulnerable development to low hazard areas (shown as blue depth-velocity combinations).

Development in areas affected by an allowance for climate change should be adaptable e.g. where ground floor use can be changed to less vulnerable/ water compatible types to make space for water.

Key findings relative to areas at risk are: ƒ There is not much difference in the River Thame flood extent between the defended 5%, 1% and 0.1% annual probability events in the north east MDA of the Eastern Growth Arc as the valley sides are reasonably well defined. The flood hazard is moderate to significant; • The east MDA at Broughton Crossing (between the A418 Aylesbury Road and A41 Aston Clinton Road) is not within the future defended 0.1% annual probability flood event. Although the strategic link road from the A41 Aston Clinton Road crosses the floodplain, the flood hazard is generally low or moderate even considering climate change; • The Aston Clinton Road impedes floodwater causing flooding to occur in the south east MDA. Less Vulnerable land use (Table D.2 in PPS25) is appropriate in the affected area but More Vulnerable land use would have to pass the Exception Test. Also if the Aston Clinton Road is designated as a defacto defence, then the Environment Agency should be aware of the implications; ƒ At the Aston Clinton Road and Broughton Lane junction there is a pumping station that is partially flooded by the 0.1% annual probability event. It is recommended that the function of the pumping station is reviewed to assess if it should be considered as critical infrastructure during a flood event and consequently kept operational; and ƒ Only the northern edge of the sewage treatment works is within the present day defended 1% annual probability flood extent and the flood hazard is low to people. The northern and western edges are partially within the present day 0.1% flood extent. However, the future 0.1% flood event results in significant flood hazards in some of the areas. As none of the modelled flood storage reservoirs significantly reduce flood risk at the sewage treatment works, other mitigation measures should be considered. Raised flood defences are possible considering only a small amount of the floodplain would be displaced.

With regards to other sources of flooding, pluvial flooding has been assessed not to be a significant issue in Aylesbury in light of historical flooding records. However, this does not necessarily mean that surface water flooding could not be an issue in the future. It is recommended that the potentially critical drainage areas are investigated, potentially through the preparation of a Surface Water Management Plan in consultation with water companies. Also, the retrofitting of SUDS in the existing urban areas nearby should be investigated. It is recommended that until a Surface Water Management Plan has been produced, Growth Arc developers should demonstrate that they have a Surface Water Strategy, whereby they can show how they have identified ways to reduce surface water flood risk at the masterplanning stage.

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Thames Water has confirmed that sewer flooding has occurred in the last ten years. All the affected properties have been included in their AMP4 or AMP5 capital programmes covering 2005 to 2015 to address the issues. Nevertheless, the area near Manor Park could become a hotspot if development occurs nearby and surface water is not managed properly. Thames Water has expressed wishes to work closely with the Council to incorporate policies on water and sewerage infrastructure within the Local Development Framework Documents. It is recommended that the Council discusses how best to incorporate their proposed text into policy.

The risk of groundwater flooding in the Aylesbury area is low due to the geology of the area. There are only a few reported incidents or areas that may be affected by localised high groundwater table.

None of the recorded sewer flooding and groundwater incidents are within any of the Growth Arcs.

Failure of defences There are some defences that would have significant adverse impacts on flood risk if a breach occurred and so require regular inspections. They should be maintained to at least their current standard. These critical defences are: ƒ Bear Brook Flood Storage Reservoir (protecting downstream urban areas); ƒ Train station car park embankment along Bear Brook (protecting railway station and railway); ƒ Vale Park embankment along Bear Brook (protecting Vale Retail Park and Park Street); ƒ Holiday Inn embankment.

The banks of the Grand Union Canal should be maintained to a good condition as breaches in the northern banks can result in flood risk to the east MDA (leading to significant flood hazard) and increase flood risk to Stocklake Road, Pembroke Road Industrial Park and Vale Park.

There are some culverts that have noticeable adverse impacts on flood risk if they become significantly blocked and so need to be regularly inspected and kept clear of debris and silt. These critical culverts are located: ƒ On Southcourt Brook at Churchill Avenue (protecting nearby residential properties, a school and roads); ƒ Under the Grand Union Canal (affecting the Health Centre and properties along Park Street); ƒ On Bear Brook at the High Street (affecting Park Street properties and the Health Centre); and ƒ On Bedgrove Brook at Broughton Avenue (protecting a few residential properties and the road).

The owners of the defences and infrastructure should be made aware of the consequences of breaches/blockages and put in place appropriate maintenance plans to ensure that their integrity is continued.

The Aston Clinton Road holds some water back and so could be defined as a defacto defence. Similarly, the railway line crossing Stoke Brook (at Hall end) to the south holds flood water back and so could be defined as a defacto defence. The Council should be

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aware of the implications if there is a planning application for a change of use in these locations.

Using the breach scenarios and residual risk from the 1% annual probability event it has been possible to identify a “Rapid Inundation Zone”, where there is significant danger to people. The south western part of the East MDA is in the rapid inundation zone. It is recommended that only development that does not require rapid evacuation is permitted here.

Emergency planning In relation to emergency planning, the mapping should inform access routes, areas to evacuate first and where flooding would be a danger to people. Table S1 summarises the flood risk to the major ‘A’ roads, showing that the A413 Buckingham Road and A41 Aston Clinton Road are affected by all the modelled defended flood events. Nevertheless, the flood hazard is low to people on the A41 Aston Clinton Road and A413 Buckingham Road and so could be passable for emergency services. The A418 Aylesbury Road and A413 Wendover Road would be dry during all of the modelled flood events. The A418 Oxford Road could also be available up to the present day defended 0.1% annual probability flood event.

Table S1 – Summary of major roads affected by flood events

Present Future Present Future Present Future defended defended defended defended defended defended Major ‘A’ Road 5% event 5% event 1% event 1% event 0.1% event 0.1% event A41 Bicester Road No No No Yes Yes Yes A413 Buckingham Road Yes Yes Yes Yes Yes Yes A418 Aylesbury Road No No No No No No A41 Aston Clinton Road Yes Yes Yes Yes Yes Yes A413 Wendover Road No No No No No No A418 Oxford Road No No No No Yes Yes

Other findings related to emergency planning are: ƒ There are schools close to the defended 1% annual probability floodplain, such as near Coldharbour Way, but the hazard is low and there are dry access routes; ƒ Some of the less vulnerable sites (churches) are close to the future defended 0.1% floodplain but the flood hazard to people is low and there are dry access routes; ƒ No existing highly vulnerable sites identified in this study (Police, railway and bus station) are within the future defended 0.1% annual probability floodplain; and ƒ The Territorial Army centre is not affected by the present day 0.1% event but is susceptible to a low flood hazard from the future 0.1% event.

The Environment Agency generally aims to give a two-hour lead time for giving warning where it has a flood warning coverage within the District. However, in certain cases of severe or “flash flooding” this may not always be possible. The flood warning arrangements for this area should reviewed in light of the SFRA and WCS to ensure that adequate telemetry and flood warnings are provided. New developments should

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consider the role of flood warning and also develop evacuation plans for areas at flood risk in conjunction with the Aylesbury Vale District Council’s Emergency Planning Officer.

The Aylesbury Vale District Council is currently developing a flood management plan in response to the Pitt Review with specific actions on preparing for flooding, emergency response and recovery. The findings of the SFRA and modelling should be used to help finalise the flood management plan.

Flood risk management There are opportunities to alleviate flood risk in Aylesbury. The modelling shows that, in terms of reducing flood risk, the Bear Brook improvements would result in a significant reduction in the town centre. However, this option would increase the flood risk to existing properties and roads upstream. A flood storage option on the Burcott Brook would not increase flood risk upstream to existing properties or roads but would reduce flood risk to the A41 Aston Clinton Road. An option on the Wendover Brook would also alleviate flooding to the A41 Aston Clinton Road and some properties in the town centre, however, Brookfield farm would be affected by the increased water level upstream if left unmitigated.

An option on the West End Ditch could provide relief to the existing Bear Brook FSR and alleviate flood risk in the town centre. The Council have expressed that this additional modelling would be a desirable requirement at the next stage of detailed master planning for the chosen growth arc.

All the flood storage options present opportunities for ecological and amenity uses. While the main function would be flood protection to downstream existing properties, the reservoirs could be designed to have multiple benefits for the local community. Each option would require close consultation with the Environment Agency, land and asset owners as well as local stakeholders.

In light of the modelling results on the identified Flood Storage Reservoirs, the potential for flood risk improvement provided by additional storage areas on the River Thame and West End Ditch should be examined. In accordance with the requirements of PPS25, it is the responsibility of potential developers to investigate the flood risk reduction benefits of their schemes. The Council has identified the opportunity provided by joint working on the master plans for the chosen growth arc to carry out these investigations and to make provisions as appropriate.

There may also be an opportunity to provide further flood mitigation measures by constructing the strategic link roads to act as dams (and store water upstream), such as using the eastern and northern link roads. The benefits of building infrastructure on embankments are that less freeboard and maintenance are required. Although the Environment Agency is open to such opportunities, there are implications of ownership and maintenance which would require clear plans to be established. Contingency plans for failure would be required as well. Also they should not be built purely for development but also for multiple benefits, such as alleviating flood risk to existing properties and enhancing local amenity.

The breach modelling of the northern Grand Union Canal banks indicates that controlled flooding could present an opportunity for enhancing local biodiversity. This option would

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need to be discussed further with the Environment Agency, British Waterways and the land owners.

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CONTENTS

Page

EXECUTIVE SUMMARY I

1 INTRODUCTION 1 1.1 Background 1 1.2 The Study Area 3 1.3 The Growth Arcs 3 1.4 The Modelling Scope 5 1.5 Flood Hazard Mapping 6 1.6 Structure of Strategic Flood Risk Assessment Report 7

2 FLOOD DEFENCES 9

3 FUNCTIONAL FLOODPLAIN 13 3.1 Introduction 13 3.2 Eastern Growth Arc 14 3.3 Southern Growth Arc 14 3.4 Combined East and Southern Growth Arc 15 3.5 Residual Flood Risk to Existing Urban Areas 15

4 RESIDUAL RISK FROM 1% AND 0.1% ANNUAL PROBABILITY FLOODS 17 4.1 Introduction 17 4.2 Eastern Growth Arc 17 4.3 Southern Growth Arc 18 4.4 Combined East and Southern Growth Arc 18 4.5 Residual Flood Risk to Committed Development and Existing Urban Areas 18 4.6 Comparison of Growth Arcs 19

5 FAILURE OF DEFENCES 20 5.1 Introduction 20 5.2 Breaches of Defences 20 5.3 Breach Results and Implications 21 5.3.1 Breach results and implications for 1% annual probability 21 5.3.2 Breach results and implications for 0.1% annual probability 26 5.4 Culvert Blockages 32 5.5 Blockage Results and Implications 32 5.5.1 Blockage results and implications for 1% annual probability 32 5.5.2 Blockage results and implications for 0.1% annual probability 38 5.6 Rapid Inundation Zone 44 5.6.1 Method 44 5.6.2 Areas vulnerable to rapid inundation 44

6 OTHER SOURCES OF FLOODING 45 6.1 Surface Water Flooding 45 6.2 Groundwater Flooding 45

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6.3 Potential Drainage Problems Areas 46

7 MANAGING FLOOD RISK 47 7.1 Introduction 47 7.2 Flood Risk Due to Growth Arcs 47 7.3 Potential Flood Storage Reservoirs 48 7.3.1 Overview 48 7.3.2 Option on Stoke Brook 48 7.3.3 Option on Bear Brook 48 7.3.4 Option on Wendover Brook 49 7.3.5 Option on Burcott Brook 49 7.3.6 Combined options 49 7.3.7 Other options 50 7.3.8 Discussion on options 50 7.4 Application of Sustainable Drainage Systems 51 7.5 Flood Warning and Emergency Response 53 7.5.1 Environment Agency’s flood warning role 53 7.5.2 Aylesbury Vale District Council’s emergency role 54 7.5.3 Evacuation plans 54

8 CONCLUSIONS AND RECOMMENDATIONS 56 8.1 Overview 56 8.2 Defining the Functional Floodplain and Recommendations 57 8.3 Nature of Flood Hazard and Recommendations 57 8.4 Nature of Flood Hazard if Defences/Infrastructure Fails and the Implications 59 8.5 Informing Emergency Planning Procedures 60 8.6 Options to Mitigate Flooding and Enhance Local Environment 61 8.7 Limitations to Modelled Flood Extents 62 8.8 Updating the Strategic Flood Risk Assessment 63 8.9 Guidance for Developers 63

9 GLOSSARY 64

10 REFERENCES 69

FIGURES

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FIGURES

Figures 15 to 50 as well as Figure 62 can be found within the sections. The rest of the Figures are located at the back of the report.

1 Study area and watercourses 2 Option B – Eastern Growth Arc – with committed development areas 3 Option C – Southern Growth Arc – with committed development areas 4 Option D – Combined East and Southern Growth Arc – with committed development areas 5 Location of raised flood defences and defacto defences 6 5% annual probability flood outlines 7 Flood hazard from the present day 5% annual probability 8 Flood hazard from the future 5% annual probability with climate change 9 Defended 1% annual probability flood outlines 10 Flood hazard from the present day defended 1% annual probability 11 Flood hazard from the future defended 1% annual probability with climate change 12 Defended 0.1% annual probability flood outlines 13 Flood hazard from the present day defended 0.1% annual probability residual risk 14 Flood hazard from the future defended 0.1% annual probability with climate change 15 Flood hazard from a breach at Location 1 with the 1% annual probability 16 Flood hazard from a breach at Location 2 with the 1% annual probability 17 Flood hazard from a breach at Location 3 with the 1% annual probability 18 Flood hazard from a breach at Location 4 with the 1% annual probability 19 Flood hazard from a breach at Location 5 with the 1% annual probability 20 Flood hazard from a breach at Location 6 with the 1% annual probability 21 Flood hazard from a breach at Location 7 with the 1% annual probability 22 Flood hazard from a breach at Location 8 with the 1% annual probability 23 Flood hazard from a breach at Location 1 with the 0.1% annual probability 24 Flood hazard from a breach at Location 2 with the 0.1% annual probability 25 Flood hazard from a breach at Location 3 with the 0.1% annual probability 26 Flood hazard from a breach at Location 4 with the 0.1% annual probability 27 Flood hazard from a breach at Location 5 with the 0.1% annual probability 28 Flood hazard from a breach at Location 6 with the 0.1% annual probability 29 Flood hazard from a breach at Location 7 with the 0.1% annual probability 30 Flood hazard from a breach at Location 8 with the 0.1% annual probability 31 Flood extent from a blockage at Location 1 with the 1% annual probability 32 Flood extent from a blockage at Location 2 with the 1% annual probability 33 Flood extent from a blockage at Location 3 with the 1% annual probability 34 Flood extent from a blockage at Location 4 with the 1% annual probability 35 Flood extent from a blockage at Location 5 with the 1% annual probability 36 Flood extent from a blockage at Location 6 with the 1% annual probability 37 Flood extent from a blockage at Location 7 with the 1% annual probability 38 Flood extent from a blockage at Location 8 with the 1% annual probability 39 Flood extent from a blockage at Location 9 with the 1% annual probability 40 Flood extent from a blockage at Location 10 with the 1% annual probability

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41 Flood extent from a blockage at Location 1 with the 0.1% annual probability 42 Flood extent from a blockage at Location 2 with the 0.1% annual probability 43 Flood extent from a blockage at Location 3 with the 0.1% annual probability 44 Flood extent from a blockage at Location 4 with the 0.1% annual probability 45 Flood extent from a blockage at Location 5 with the 0.1% annual probability 46 Flood extent from a blockage at Location 6 with the 0.1% annual probability 47 Flood extent from a blockage at Location 7 with the 0.1% annual probability 48 Flood extent from a blockage at Location 8 with the 0.1% annual probability 49 Flood extent from a blockage at Location 9 with the 0.1% annual probability 50 Flood extent from a blockage at Location 10 with the 0.1% annual probability 51 Rapid Inundation Zone for present day 1% annual probability 52 Other sources of flooding in Aylesbury 53 Present day 1% annual probability residual flood risk with a storage reservoir at Bear Brook 54 Present day 1% annual probability residual flood risk with a storage reservoir at Wendover Brook 55 Present day 1% annual probability residual flood risk with a storage reservoir north of the Grand Union Canal 56 Present day 5% annual probability residual flood risk with all potential storage reservoirs 57 Future 5% annual probability residual flood risk with all potential storage reservoirs 58 Present day 1% annual probability residual flood risk with all potential storage reservoirs 59 Future 1% annual probability residual flood risk with all potential storage reservoirs 60 Present day 0.1% annual probability residual flood risk with all potential storage reservoirs 61 Future 0.1% annual probability residual flood risk with all potential storage reservoirs 62 Gradient of topography in Aylesbury

TABLES

Table 1 – Growth Arc details Table 2 – Flood hazard matrix (Defra, 2006) Table 3 – Flood Defences in Aylesbury Table 4 – Flood Risk Vulnerability Classification (DCLG, 2006) Table 5 – Areas of Growth Arcs affected by residual flood risk Table 6 – Modelled Breach Information Table 7 – Blockage Information Table 8 – Summary of major roads affected by flood events

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1 INTRODUCTION

1.1 Background

‘Making Space for Water’ is the Government’s new strategy for flood and coastal erosion risk management in England over the next 20 years. The aim is:

“To manage the risks from flooding and coastal erosion by employing an integrated portfolio of approaches which reflect both national and local priorities, so as: ƒ To reduce the threat to people and their property; and ƒ To deliver the greatest environmental, social and economic benefit, consistent with the Government’s sustainable development principles. ƒ To secure efficient and reliable funding mechanisms that deliver the levels of investment required to achieve the vision of this strategy.” (Defra, 2005)

Communities and Local Government (CLG) has the lead responsibility for development planning policy and discouraging inappropriate development which might increase flood risk. CLG's Planning Policy Statement (PPS) 25 was published in December 2006 and sets out Government policy on development and flood risk. The essence of ‘Making Space for Water’ is contained in PPS25. The aims of PPS25 are to: ƒ Ensure that flood risk is taken into account at all stages in the planning process; ƒ Avoid inappropriate development in areas at risk of flooding; and ƒ Direct development away from areas of highest risk.

Where new development is, exceptionally, necessary in flood risk areas, policy aims to make it safe, without increasing flood risk elsewhere, and, where possible, reducing flood risk overall.

The Thames Catchment Flood Management Plan (CFMP) was published by the Environment Agency in July 2008. The Thames CFMP will guide flood risk management from rivers for the next 50 to 100 years. The selected policy for Aylesbury Town is “Policy P4 – Accept the risk, but in the longer term take action to ensure that risk does not increase from current level’.

A Level 1 Strategic Flood Risk Assessment (SFRA) was completed in April 2007 for Aylesbury Vale District. Its purpose was fourfold: ƒ To ensure that the Aylesbury Vale District Council meets its obligations under Planning Policy Statement 25: Development and Flood Risk; ƒ A reference point to inform the preparation of Local Development Documents (LDDs), having regard for catchment-wide flooding issues; ƒ To provide information to apply the Sequential Test; and ƒ A reference point to inform and advise developers of their obligations in PPS25.

A key focus for the growth of Aylesbury is the planned renaissance of the town centre. Aylesbury Vale District Council (the Council) has also been investigating and consulting on various development options around the town as part of the emerging

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Core Strategy and determine the ‘Preferred Option’. Three options (Growth Arcs) now remain, which are being considered around the town: ƒ Option B (Eastern Growth Arc) ƒ Option C (Southern Growth Arc) ƒ Option D (Combined East and Southern)

As identified in the Level 1 SFRA, parts of the Growth Arcs are within Flood Zones 2 and 3. Due to the complex fluvial drainage system within Aylesbury, it was recognised by the Council that a Level 2 SFRA was required. Consequently the Council commissioned Royal Haskoning to undertake a Level 2 SFRA that satisfies the PPS25 requirements and the amplified guidance (Paragraphs 2.36 to 2.47) within the Living Draft Practice Guide Companion to PPS25 (February 2007).

The principal purpose of a Level 2 SFRA is to facilitate the application of the Sequential Test and, if necessary, the Exception Test, which provides a method of managing flood risk while still allowing necessary development to occur (paragraph 18 in PPS25, 2006). The other functions according to the Living Draft Practice Guide Companion to PPS25 are to: ƒ Consider the nature of the flood hazard, taking account of the presence of flood management measures e.g. flood defences and defacto defences; ƒ Define the functional floodplain; ƒ Consider the nature of flood hazard if defences fail; ƒ Appraise the defence infrastructure condition and maintenance in light of flood risk; ƒ Inform Emergency Planning Procedures (access and egress to development sites); ƒ Provide guidance on the preparation of Flood Risk Assessments; and ƒ Provide guidance to developers on how surface water should be managed.

In accordance with the proposal dated 18th October 2007, the Level 2 SFRA also addresses the Environment Agency’s comments received on 13th September 2007 for the Aylesbury Vale SFRA Level 1 Final Report. Many of the comments related to the functions above and shaped the scope of the Level 2 study for Aylesbury.

It should be noted that the Practice Guide was updated and published in June 2008. It offers further guidance on how to implement the policies of PPS25 by drawing on good practice. In preparing the report, the updated Practice Guide has been considered to enhance its soundness and also extend the longevity of the Level 2 SFRA. With regards to a Level 2 SFRA, the only additional output required by the updated Practice Guide should be the identification of the location of critical drainage areas and the identification of the need for Surface Water Management Plans (Paragraph 3.57). Potential critical drainage areas have been identified after receiving comments from the Environment Agency on the Level 1 SFRA in 2007.

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1.2 The Study Area

The study area is shown in Figure 1 along with the watercourses flowing through and around the town of Aylesbury. The River Thame joins with the Bear Brook at the western side of Aylesbury. The Bear Brook has several tributaries discharging both the upstream rural catchment and relatively quick urban runoff: ƒ Wendover Brook; ƒ West End Ditch; ƒ Bedgrove Brook; ƒ California Brook; ƒ Southcourt Brook; ƒ Sedrup Brook; ƒ Stoke Brook; ƒ Hartwell Ditch; ƒ Broughton Brook; ƒ Stocklake Brook; and ƒ Coldharbour Way Ditch.

The Aylesbury Arm of the Grand Union Canal is an artificial watercourse flowing down from Marsworth to Aylesbury and is used mainly for leisure purposes with several permanent house boats. It is approximately 9.6km long and it is regulated by means of several navigation locks. The canal ends at the town centre near Walton Street. West of Broughton, the canal is connected to the California Brook through an overflow weir. The water levels in the canal are maintained mainly by water from the Marsworth reservoir (in the east entering at Lock 1) and also fed by the Wilstone Reservoir (via the sluice on Wilstone (Gudgeon) Brook). Water also enters the canal from a pipe under the tow-path from the Draytonmead Brook. There is a side weir on the canal into Draytonmead Brook near Merrymead Farm.

1.3 The Growth Arcs

Figures 2, 3, and 4 illustrate all the Growth Arcs that are being considered. Table 1 gives a breakdown of each one. The Figures also show the Environment Agency’s Flood Zones 2 and 3 from November 2008. The Environment Agency’s knowledge of the floodplain is continuously being improved by a variety of studies, detailed models, data from river flow and level monitoring stations and actual flooding information. They have an ongoing programme of improvement, with updates being made on a quarterly basis. This report contains the best available information at the present time; however after the next update of the Flood Map, their website should be used for the best available information on the extents of Flood Zone 2 and 3.

Table 1 – Growth Arc details

Growth Arc and its Major Description Number of Dwellings Development Areas (MDA) Eastern Growth Arc (Option B) North East MDA Land between Watermead and 4,200 Bierton Eastern MDA Land to the north of the Grand 1,600 Union Canal near Broughton

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Growth Arc and its Major Description Number of Dwellings Development Areas (MDA) Crossing South East MDA Land to the north of Weston 3,500 Turville between A41 Aston Clinton Road and A413 Wendover Road Southern Growth Arc (Option C) South East MDA Land to the north of Weston 3,300 Turville between A41 Aston Clinton Road and A413 Wendover Road Southern MDA Land to the north of Stoke 2,700 Mandeville between A413 Wendover Road and B4443 Lower Road South West MDA Land to the north of 3,300 Bishopstone between B4443 Lower Road and A418 Oxford Road Combined East and Southern Growth Arc (Option D) Eastern MDA Land to the north of the Grand 1,600 Union Canal near Broughton Crossing South East MDA Land to the north of Weston 3,300 Turville between A41 Aston Clinton Road and A413 Wendover Road Southern MDA Land to the north of Stoke 1,100 Mandeville between A413 Wendover Road and B4443 Lower Road South West MDA Land to the north of 3,300 Bishopstone between B4443 Lower Road and A418 Oxford Road

The alignment of the proposed strategic link roads has not been finalised by Buckinghamshire County Council but their indicative positions are shown with the Growth Arcs.

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1.4 The Modelling Scope

Previously Peter Brett Associates (PBA) constructed an ISIS-TUFLOW model of the Bear Brook and Upper River Thame (on behalf of the Environment Agency). The approved and validated model was supplied by the Environment Agency for the purpose of the Level 2 SFRA.

In order to consider the residual risk to the Growth Arcs, further topographic survey was procured and incorporated into the model to include the entire length of Wendover Brook, extend Stoke Brook beyond the railway to the Southern Growth Arc and to extend the Grand Union Canal, as well as model culverts under the Grand Union Canal. Additional culverts and defacto defences have been added to the model in consultation with the Environment Agency. Since the link roads have not been designed yet, they have not been incorporated into the model but are shown in the Figures for illustration purposes.

The baseline scenarios below have been simulated using three different critical storm durations (3.25hr, 12.25hr and 30.25hr) in order to map the worst flood outline for each watercourse. The climate change allowance was a 20% increase in flows. All the scenarios include existing flood defence measures. ƒ Present day 5% annual probability (1:20 year) flood event; ƒ Future 5% annual probability (1:20 year) flood event with climate change; ƒ Present day 1% annual probability (1:100 year) flood event; ƒ Future 1% annual probability (1:100 year) flood event with climate change; ƒ Present day 0.1% annual probability (1:1,000 year) flood event; and ƒ Future 0.1% annual probability (1:1,000 year) flood event with climate change.

The modelling work also assessed the impact of the Growth Arcs (see Section 7.2). Other scenarios agreed with the Environment Agency included: ƒ The impact of partially blocking ten structures (culverts) individually on the present day 1% and 0.1% annual probability flood extents; ƒ The impact of individual breaches occurring in defences and raised embankments at eight locations on the present day 1% and 0.1% annual probability flood extents; ƒ The impact of individual strategic flood storage reservoirs at four locations on the present day 1% annual probability flood extent; and ƒ The impact of combining the strategic flood storage reservoirs on the present day and future (i.e. with climate change) 5%, 1% and 0.1% annual probability flood extents.

A separate modelling report has been produced for the Environment Agency that details all the changes made to the original model and also gives the resulting water levels and flows along each watercourse for each modelled scenario.

The Environment Agency has carried out a review of the changes made to the model. There were comments on the timing of breaches and the approach of blocking culverts. These have been resolved and the flood extents in the Final SFRA have been updated. The review also identified potential improvements to the core model that should be investigated after the SFRA. These relate to floodplain roughness, re-

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calibrating the model and re-visiting the accuracy of newly acquired topographic survey.

The modelling outputs in this SFRA represent the best available information on residual flood risk for the present day and with climate change and is sufficient for strategic decisions. However, for establishing the flood risk to a development site, a developer should not solely rely on the SFRA and overlay extents from the maps. Water level information should be requested from the Environment Agency’s Thames West Customer Contact and applied to a site specific topographic survey. Contacts can be found in Appendix A.

1.5 Flood Hazard Mapping

Using the 2D modelling software, it is possible to define the maximum flood extents and estimates of likely depths and speed of moving flood water. This information has been combined to create Flood Hazard maps using an agreed methodology with the Environment Agency to provide an overview of flood risk across the study area.

Flood Hazard Mapping brings together information on flood depth and speed (velocity) of floodwater to create a hazard rating to people within each area that experiences flooding. The hazard rating used is set out in the joint Defra and Environment Agency document “Flood Risks to People Phase 2, Guidance Document, FD2321/TR2 (March 2006).”

The hazard rating categorises flood risk in terms of Caution, Danger for Some, Danger for Most and Danger for All, with the hazard becoming dangerous to more people as depths and velocities increase. This is illustrated in Table 2.

Table 2 – Flood Hazard Matrix (Defra, 2006)

Velocity Depth (m) (m/s) 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 Description of Hazard Categories Degree of Flood Colour Code Description Hazard Low Caution

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Flood zone with shallow flowing water or deep standing water Moderate Dangerous for some (i.e. children) Danger: Flood zone with deep or fast flowing water Significant Dangerous for most people Danger: Flood zone with deep fast flowing water Extreme Dangerous for all Extreme danger: Flood Zone with deep fast flowing water * The blue colour code is not specified in FD2321/TR2 and has been employed within this SFRA in order to show maximum flood extent.

The guidance states that the maximum permissible hazard for safe access should correspond to the depth-velocity combinations shown in blue (low hazard) in Table 2. The safety of wading is affected by underwater hazards such as unevenness of the ground and presence of depressions or unsecured manhole covers.

With respect to requirements for safe access and exit from new developments in flood risk areas, the following should be provided (in decreasing order of preference): ƒ Safe dry route for people and vehicles; ƒ Safe dry route for people; ƒ A route where flood hazard is low and should not cause a risk to people; and ƒ A route for vehicles where the flood hazard is low to permit access for emergency vehicles.

1.6 Structure of Strategic Flood Risk Assessment Report

This Level 2 SFRA is structured into eight sections to best understand flood risk from all other sources in Aylesbury, both under present-day conditions and considering the impacts of climate change, so as to best inform planning decisions.

Section 1 – Sets the scene for this Level 2 SFRA, referring to the strategic flood risk management in the area and the Growth Arcs.

Section 2 – Depicts the flood defences in Aylesbury.

Section 3 – Defines the functional floodplain and discusses its implications for development.

Section 4 – Assesses the residual risk within Aylesbury.

Section 5 – Considers the risks associated with failure of flood defences, either from a breach or from blockage, and identifies its implications with regards to development.

Section 6 – Deals with sources of flooding other than fluvial and makes recommendations on the scope of further work to address the flood risk.

Section 7 – Discusses the different ways in which flood risk to development, people and infrastructure can be managed currently and in the longer run.

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Section 8 – Summarises the key findings of the work undertaken, draws conclusions on the nature of flood risk in Aylesbury and its implications on planning development.

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2 FLOOD DEFENCES

The Environment Agency’s National Flood and Coastal Defence Database (NFCDD) contains information on formal flood defences and defacto defences that protect the town of Aylesbury. Formal flood defences refer to structures that directly limit the spread of flooding and whose primary function is flood protection. Defacto defences are structures that perform the same basic function as formal defences, in that they impede the spread of floodwater, but for which flood defence is a secondary or indirect purpose (Environment Agency, 2005), such as road and railway embankments.

Figure 5 illustrates the defences that were included in the modelling and Table 3 summarises the raised flood defences and defacto defences. Along rivers the references to the left hand and right hand banks relate to the direction viewed when someone is facing downstream. Site investigations revealed that the following defences are bank protection and not raised, therefore have not been included in Table 3: 0611919BB0106L04, 0611919BG0101L02, 0611919BG0101L03, 0611919BG0101R03, 0611919BG0101R04, 0611919BG0101R05

The formal flood defences (i.e. designed for the purpose for reducing flood risk) include two flood storage reservoirs (FSRs) and associated clay embankments, which are maintained by the Environment Agency. One is present on Stocklake Brook with a capacity of 19,900m3 and the other on Bear Brook with a capacity of 90,000m3 (NFCDD). They were constructed as part of the Aylesbury Vale Flood Alleviation Scheme (AVFAS) in the mid-1990s and designed to provide a 1:100 year standard of protection for the urban areas of the Bear Brook through the town. These defences are deemed to be in good condition.

There are a variety of other flood defences owned by the Council and private owners. The majority of these are in good condition, except the embankment at Fowler Road which is in fair condition. Only the railway embankment, the A41 Bicester Road and the defence protecting the Health Centre were unchecked by the Environment Agency. As these are operating infrastructure, it is assumed that they are maintained to a good condition.

The Council inspects all of its watercourses, defences and structures at least annually to identify maintenance requirements. The Council’s land drainage function is carried out within the engineering team, which includes routine watercourse maintenance where the Council is the land owner.

Generally NFCDD only states the standard of defences for which the Environment Agency are responsible for. Where the standard was not included in NFCDD, it has been possible to indicate the standard of defence based on the results of the modelling – these are highlighted in blue. The majority give protection to at least the 1% annual probability flood event except the following: ƒ 0611919SO0101R02 (Fowler Road, right hand bank); ƒ 0611919SO0101R03 (Fowler Road, right hand bank); ƒ 0611919BB0106R04 (Vale Park, right hand bank); ƒ 0611919BB0106R05 (High Street, right hand bank); and ƒ 0611919TH1201R03 (Watermead, right hand bank).

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Table 3 Flood Defences in Aylesbury (defacto defences are shaded rows) Asset Reference NFCDD Asset Type (in Location and description Owner/ Condition* Defence ID NFCDD) Responsibility Standard (yr) 0611919BB0103R02 114 Road embankment West of A418. Road embankment as part of a new road Aylesbury Vale 2 >1000 (formal defence) (on Coldharbour Way). It extends into a flyover on Griffin DC Lane where it gets up to 10m high 0611919BB0104L02 115 Raised defence West of A418. Raised river bank with a public footpath. It Aylesbury Vale 2 >1000 (formal defence) runs alongside a railway line and there is some tree DC growth. There is some variation in height. 0611919BB0104R02 117 Raised river bank West of A418. Raised river bank adjacent to derelict Private 2 >100 (formal defence) industrial land. There is some variation in height. It is a but <1000 grassed bank with assorted trees (mature trees are adjacent to gauging station). 0611919BB0105L02 119 Earth embankment Railway station. 2.5m wide 1m higher than adjacent car Private 2 >1000 (formal defence) park. Uniform shape. Bank is in good condition 0611919BB0105L03 120 Earth embankment Downstream of High street. Soil river bank which is Private Unchecked >1000 (formal defence) landscaped and protects the grounds of "The Equitable Life Assurance Society" and "Clinic". It also protects Railtrack land. 0611919BB0106R02 130 Stone embankment Stone block bank channel side to ""Brookside terrace"" Private 2 100 (formal defence) row of houses. A brick wall is above 8M high and is a potential flood protection but is breached at the bridge. 0611919BB0106R04 132 Earth embankment The Vale Park. Approximately 0.5m above adjacent park Private 2 >20 (formal defence) land. but <100 0611919BB0106R05 133 Road embankment High Street. 1m high and in good condition. Aylesbury Vale 2 >20 (defacto defence) DC but <100 0611919BB0107L02 135 Earth embankment Part of Bear Brook Flood Storage Reservoir. Soil flood Environment 2 100 (formal defence) bank - clay filled and in good condition. Agency 0611919BB0107R02 137 Earth embankment Part of Bear Brook Flood Storage Reservoir. Clay filled Private 2 100 (formal defence) soil bank.

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Asset Reference NFCDD Asset Type (in Location and description Owner/ Condition* Defence ID NFCDD) Responsibility Standard (yr) 0611919BB0201L01 142 Earth embankment Part of Bear Brook Flood Storage Reservoir. Soil flood Environment 2 100 (formal defence) bank which offers 0.5m of protection to houses in Agency Richmond road. 0611919CA0101L02 150 Earth embankment Upstream of Oxford Rd. Built to protect the new (1995) Private 2 >1000 (formal defence) underpass to the south of the brook which has an invert level below the river level. 0611919CA0101L11 156 Earth embankment Thame Rd. 4m high old railway embankment. It is Private 2 >1000 (defacto defence) abandoned but on Railtrack land. 0611919CA0101R03 160 Earth embankment Oxford Rd. Steel sheet pile wall with a concrete capping Private 2 >1000 (defacto defence) beam. Upstream is stonework wall set into embankment. 0611919SL0101L02 167 Clay bund with spillway Part of Stocklake FSR. Clay bund with spillway and flood Environment 2 100 (formal defence) kerbs along crest. There is extensive plant growth on the Agency downstream bank. Uniform in shape. 0611919SL0101R01 169 Concrete embankment Concrete hunching - under cut, may start to undermine Private 2 >100 (formal defence) car park but <1000 0611919SL0101R07 171 Clay bund with spillway Part of Stocklake FSR. Clay bund with spillway and flood Environment 2 100 (formal defence) kerbs along crest. There is extensive plant growth on the Agency downstream bank. Uniform in shape. 0611919SO0101R02 174 Earth embankment Fowler Rd. Steel sheet piling. 2m high above river with Private 2 <20 (formal defence) soil on top (1.5m). 0611919SO0101R03 175 Earth embankment Fowler Rd. Private 3 <20 (formal defence) 0611919TH1101L02 179 Earth embankment Raised railway embankment covered in vegetation- no Private Unchecked >100 (defacto defence) obvious signs of decay but <1000 0611919TH1101R02 180 Earth embankment Raised railway embankment covered in thick vegetation Private Unchecked >100 (defacto defence) making survey difficult - no obvious signs of decay but <1000 0611919TH1102L02 181 Concrete embankment A41 Bicester Rd Private Unchecked >100 (defacto defence) but <1000

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Asset Reference NFCDD Asset Type (in Location and description Owner/ Condition* Defence ID NFCDD) Responsibility Standard (yr) 0611919TH1102R02 182 Earth embankment A41 Bicester Rd Private Unchecked >100 (defacto defence) but <1000 0611919TH1201L02 183 Road embankment Watermead Road acts as flood protection to the Holiday Aylesbury Vale 2 >100 (defacto defence) Inn. DC but <1000 0611919TH1201R02 184 Road embankment Watermead Road acts as flood protection to the Holiday Aylesbury Vale 2 >100 (defacto defence) Inn. DC but <1000 0611919TH1201R03 185 Earth Embankment Watermead. Around Holiday Inn. Private 2 <20 (formal defence) * Asset condition description: 1 = Very Good (in good condition, fully serviceable, no remedial work required. Maintenance to continue as present. No significant defect 2 = Good (Minor defects, non urgent. Minor routine maintenance work required. In reasonable condition, some increase in maintenance needed, probably no more than 5% affected with slight defect). 3 = Fair (Some cause for concern, requires careful monitoring. Significant maintenance works required. Average condition, some minor repairs needed and moderate 5% - 20% affected). 4 = Poor (Structurally unsound now or in the near future. Major remedial works required or replacement (1-5 years), Extensive repair required in short term. Extensive defect 20% - 50% affected). 5 = Very Poor (Completely failed or derelict requires complete reconstruction. Major urgent repairs or replacement needed without delay to avoid failure probably beyond repair. Extensive defect >50%).

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3 FUNCTIONAL FLOODPLAIN

3.1 Introduction

According to PPS25, the functional floodplain is the land where water has to flow or be stored in times of flood. This is normally defined using a flood with an annual probability of 5% (1:20 years) or greater. The 1D-2D hydrodynamic model was used to simulate the 5% annual probability flood event with defences and the results are presented in this section. However, where flood storage reservoirs are present the functional floodplain also includes the areas of the reservoirs (i.e. where water is stored in times of flooding).

Only Water-compatible Development and Essential Infrastructure should be permitted within the functional floodplain (see Table 4). However, in accordance with PPS25 Table D.1, such development or infrastructure is required to ensure the functional floodplain remains operational and safe for users in times of flood, not to impede water flows, not to increase flood risk elsewhere and to result in no net loss of floodplain storage.

Table 4 Flood Risk Vulnerability Classification (DCLG, 2006) Essential Infrastructure Water-compatible Development Essential transport infrastructure Flood control infrastructure (including mass evacuation routes) Water transmission infrastructure and pumping stations which has to cross the area at risk, and Sewage transmission infrastructure and pumping stations strategic utility infrastructure, including Sand and gravel workings electricity generating power stations Docks, marinas and wharves and grid and primary substations Navigation facilities Water-based recreation (excluding sleeping accommodation) Amenity open space, nature conservation and biodiversity, outdoor sports and recreation and essential facilities Essential ancillary sleeping or residential accommodation for staff required by uses in this category, subject to a specific warning and evacuation plan

Also Essential Infrastructure must pass all aspects of the Exception Test (paragraph D.9 of PPS25):

(a) it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared. If the DPD has reached the ‘submission’ stage – see Figure 4 of PPS12: Local Development Frameworks – the benefits of the development should contribute to the Core Strategy’s Sustainability Appraisal;

(b) the development should be on developable previously-developed land or, if it is not on previously-developed land, that there are no reasonable alternative sites on developable previously-developed land; and

(c) a Flood Risk Assessment must demonstrate that the development will be safe, without increasing the flood risk elsewhere, and, where possible, will reduce flood risk overall.

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With respect to the flood hazard maps, it is not advisable to locate essential infrastructure and water compatible development within the Moderate to Extreme flood hazard areas, unless the Exception Test can be passed. Although it could be permissible in the Low hazard areas, there should be measures to mitigate the flood risk such as emergency plans.

3.2 Eastern Growth Arc

Figure 6 illustrates the Eastern Growth Arc shown against the 5% annual probability flood under the present day and with climate change. With the inclusion of the Grand Union Canal and surveyed banks, water now flows out of the Grand Union Canal across the southern bank at low spots and ponds before flowing under the canal and along Burcott Brook. One of the low spots was identified while visiting the site with the Environment Agency in 2008. Although the East MDA (south of Bierton) is unaffected by the functional floodplain, the following areas are at risk: ƒ Up to 500m wide areas of the River Thame corridor of the north east MDA; ƒ A 50m wide corridor along the West End Ditch through the south east MDA; and ƒ A 20m wide corridor along Bedgrove Brook through the south east MDA.

In these areas only water-compatible development (e.g. amenity open space) and essential infrastructure should be permitted. Generally the flood hazard to people is low (Figures 7 and 8) except in the north east MDA where it is moderate to significant to people.

Many of the proposed strategic link roads are within the present day and future functional floodplains of the River Thame (Northern link road), Stocklake Brook, Broughton Brook, Bear Brook (Eastern link road), West End Ditch and Bedgrove Brook. The strategic roads should only be permitted if they comply with Table D.1 of PPS25 and pass the Exception Test.

In terms of evacuation routes for the Eastern Growth Arc, there are three major transport routes (A413 Buckingham Road, A418 Aylesbury Road and A413 Wendover Road) that are not at risk from the 5% annual probability flood. Part of the Aston Clinton Road holds some water back and so could be defined as a defacto defence. Considering climate change, the A413 Buckingham Road would be affected leaving two major transport routes.

The total area of the Eastern Growth Arc is 427 hectares and the areas affected by the present and future 5% annual probability floods are 14.4 hectares (3.4% of arc) and 15.4 hectares (3.6% of arc).

3.3 Southern Growth Arc

Figure 6 also depicts the Southern Growth Arc set against the 5% annual probability flood under the present day and with climate change. The majority of the MDAs are outside of the functional floodplain except for: ƒ A 50m wide corridor along the West End Ditch through the south east MDA; ƒ A 20m wide corridor along Bedgrove Brook through the south east MDA; and ƒ A 20m wide corridor along Stoke Brook through the south west MDA.

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In these areas only water-compatible development (e.g. amenity open space) and essential infrastructure should be permitted. Figures 7 and 8 show the flood hazard to people is low both under present day conditions and with climate change.

The Eastern and Southern link roads cross the functional floodplains of Wendover Brook, West End Ditch, Bedgrove Brook, Sedrup Brook and Stoke Brook. The strategic roads should only be permitted if they comply with Table D.1 of PPS25 and pass the Exception Test.

In terms of evacuation routes for the Southern Growth Arc, there are two major roads available that are not at risk from the 5% annual probability flood: ƒ A418 Oxford Road; and ƒ A413 Wendover Road.

Considering climate change, neither would be affected by the functional floodplain. Although the A41 Aston Clinton Road would be affected by the functional floodplain, it could be available for the emergency services as the flood hazard is low.

The total area of the Southern Growth Arc is 410 hectares and the areas affected by the present and future 5% annual probability floods are 1.4 hectares (0.3% of arc) and 1.8 hectares (0.4% of arc).

The railway line crossing Stoke Brook (at Hall end) to the south holds flood water back and so could be defined as a defacto defence. The indicative strategic master plan shows that the area upstream of the railway crossing would be left as an open space. The Council and Environment Agency should be aware of the implications of this structure if there is a planning application that could lead to a modification of the structure affecting its ability to act as a flood defence.

3.4 Combined East and Southern Growth Arc

The combined East and Southern Growth Arc (Figure 6) have the same issues as the Eastern Arc and Southern Arc except there are no issues with the River Thame functional floodplain affecting growth areas. However, the northern link road does cross the functional floodplain and should only be permitted if it complies with Table D.1 of PPS25 and passes the Exception Test.

The total area of the Combined East and Southern Growth Arc is 484 hectares and the areas affected by the present and future 5% annual probability floods are 1.4 hectares (0.3% of arc) and 1.8 hectares (0.4% of arc).

3.5 Residual Flood Risk to Existing Urban Areas

Under the 5% annual probability flood event, it can be seen that the flood storage reservoirs and defences protect the urban areas through the town except small areas of Fairford Leys and Fowler Road. It should be noted that the functional floodplain also includes the Stocklake and Bear Brook flood storage reservoirs (i.e. where water is stored in times of flooding). The defences (NFCDD 0611919TH1201R03) around the Holiday Inn are also overtopped during this event. The River Thame functional floodplain extends to some properties at Quarrendon (north of Jackson Road) and also

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parts of the MDAs at Berryfields and Weedon Hill. The eastern part of Aston Clinton Road MDA is within the West End Ditch functional floodplain (next to Broughton Lane and below Old Manor Farm) and the Wendover Brook functional floodplain runs through the north east portion. Considering climate change, the functional floodplain extends to Vale Park (Figure 5).

Only Water-compatible Development is appropriate in these areas affected by the functional floodplain, as stated in PPS25 Table D.1. Any proposed Essential Infrastructure will need to pass the Exception Test.

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4 RESIDUAL RISK FROM 1% AND 0.1% ANNUAL PROBABILITY FLOODS

4.1 Introduction

This section describes the residual risk from the 1% and 0.1% annual probability floods, essentially the flood risk remaining after all existing risk mitigation measures (formal and defacto defences) have been considered. The residual risk is different to the Environment Agency’s Flood Zones 2 and 3 (Figure 1) which do not consider the presence of formal defences and whose recent zones were produced using a less extensive model. Figures 9 to 14 illustrate the residual risk (outputs from the 1D-2D hydrodynamic models).

PPS25 recommends including a 20% increase in flows as contingency allowance for the effects of climate change. This allowance should be considered at the planning stage in that development located in an area affected by climate change should be made adaptable with its land use having the flexibility to be altered without large investment.

4.2 Eastern Growth Arc

Figures 9 and 12 reveal that the north east MDA (between the A413 Buckingham Road and the A418 Aylesbury Road) is affected by the defended 1% and 0.1% annual probability flood events next to the River Thame. There is not much difference in the flood extent between the two flood events or the 5% as the valley side here is reasonably well defined. Therefore only water-compatible development is appropriate in the areas affected and essential infrastructure (such as the strategic link road), and should only be permitted if the Exception Test is passed. Close to the River Thame, the flood hazard is significant (Figures 10 and 13) i.e. dangerous to most people. Moving away from the River Thame towards the east, the flood hazard changes to moderate and becomes low where the strategic link road is proposed (coming from Bierton).

The east MDA (between the A418 Aylesbury Road and A41 Aston Clinton Road) is neither affected by the defended 1% nor the 0.1% annual probability flood events. Although the strategic link road from the A41 Aston Clinton Road crosses the floodplain, the flood hazard is generally low even considering climate change (Figures 11 and 14).

The south east MDA is affected along corridors of Bedgrove Brook, West End Ditch and Wendover Brook. Also it can be seen that Aston Clinton Road continues to form a defacto defence and holds flood water back causing further flooding in the MDA. Only water-compatible development and less vulnerable uses of land are appropriate in the areas affected. More vulnerable and essential infrastructure (such as the strategic link road) should only be permitted if the Exception Test is passed in this area.

A flooding problem along the Watermead Lakes has also been identified during the preparation of this SFRA. The Lakes are connected to the River Thame and during times of heavy rainfall their level rises causing flooding to properties. Development on the River Thame catchment upstream of Watermead should therefore ensure flood risk downstream is reduced.

In terms of emergency evacuation and access routes, the A413 Wendover Road and A418 Aylesbury Road are the only major highways that are not within the defended 1% or 0.1% annual probability floodplain. Although the A41 Aston Clinton Road would be

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flooded, it could be available to the emergency services as the flood hazard is low to people (Figures 10, 11, 13 and 14).

4.3 Southern Growth Arc

Figures 9 and 12 depict the Southern Growth Arc set against the present day defended 1% and 0.1% annual probability floodplains. The majority of the MDAs are outside of the floodplain except small corridors of West End Ditch, Bedgrove Brook and Stoke Brook. As highlighted above, the south east MDA along Aston Clinton Road is also flooded. Although water backs up south of the A418 Oxford Road under the 0.1% annual probability flood event (Figure 12), the south west MDA is not affected. In the affected areas only water-compatible development (e.g. amenity open space) and essential infrastructure should be permitted.

The Eastern and Southern link roads cross the floodplains of Wendover Brook, West End Ditch, Bedgrove Brook, Sedrup Brook and Stoke Brook. It is important that the strategic roads should only be permitted if they can be designed not to impede the flow of water and be in accordance with Table D.1 of PPS25. The cost of construction of this road could be high in terms of keeping it operational and safe during times of floods as well as maintaining flows.

In terms of emergency evacuation and access routes, the A413 Wendover Road and the A418 Oxford Road are outside of the defended 1% annual probability floodplain (with and without climate change). However, considering the 0.1% annual probability floodplain (with and without climate change), all two major highways would be affected leaving no dry routes via major roads from the centre to the Southern Growth Arc. Nevertheless, the flood hazard is low to people across many of the roads and so could be available for the emergency services.

4.4 Combined East and Southern Growth Arc

The combined East and Southern Growth Arc (Figures 9 and 12) have the same issues as the Eastern Arc and Southern Arc except that it is unaffected by flood risk from the River Thame.

4.5 Residual Flood Risk to Committed Development and Existing Urban Areas

The present day residual flood risk (Figures 9 and 12) is much reduced in the centre of the town compared to the Flood Zones 2 and 31 (Figure 2) because of the inclusion of flood defences. The committed sites still affected by the defended 1% annual probability flood are Aston Clinton MDA (22), Walton Mill (0), south east part of Exchange Street “Waterside” (9) and small parts of Fairford Leys (19), Weedon Hill MDA (20) and Berryfields MDA (21). The flood hazard to people is low at these sites except at the MDAs where the flood hazard is moderate to significant (Figures 10 and 13).

Considering vulnerable uses of land, such as the Holiday Inn hotel, the defended 1% annual probability flood hazard to people is significant (Figure 10). Although the A413

1 It should be noted that the Flood Zones supplied by the Environment Agency in December 2008 were produced using the undefended PBA model that has not been extended with the changes made in this study.

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Buckingham Road is flooded, the hazard is low (even with climate change, Figures 11 and 14) and so emergency services could reach the hotel. Figure 10 also shows schools that are close to the floodplain, such as near Coldharbour Way, but fortunately the hazard is low and there are dry access routes. No existing highly vulnerable sites identified in this study (Police station, railway and bus station) are within the present day defended 1% annual probability floodplain.

Figures 13 and 14 depict the flood hazard from the present day and future defended 0.1% annual probability event. This shows that the Territorial Army centre (5) is not affected by the present day 0.1% event but is susceptible to a low flood hazard from the future 0.1% event. The northern and western edges of the “More Vulnerable” sewage treatment works are partially within the present day 0.1% flood extent, where the flood hazard is low to people. However, the future 0.1% flood event results in low to significant flood hazards in these areas. Although a couple of the less vulnerable sites (churches) are close to the defended floodplain, the flood hazard to people is low and there are dry access routes. At the Aston Clinton Road and Broughton Lane junction there is a pumping station that is partially flooded by the 0.1% annual probability event. Further committed development sites are within the present day and future defended 0.1% annual probability flood extent at Tring Road, Bear Brook House, Dayla, Brewery Close, Walton Street, and Schwarzkopf. There are also properties affected between Oxford Road and Griffin Lane next to Broughton Brook. However, the flood hazard to people is generally low at these sites.

4.6 Comparison of Growth Arcs

Table 5 shows the area of Growth Arcs affected by flooding. Although many of the developers’ plans show that built development is not proposed in the 0.1% annual probability residual risk areas, the area affected helps indicate the differences between the Growth Arcs.

Table 5 – Areas of Growth Arcs affected by residual flood risk

Eastern Growth Arc Southern Growth Arc Combined Growth Arc Approximate area of Growth Arc 427 410 484 (Ha) Area affected by present 1% 19.3 5.7 5.7 residual flood risk (Ha) (3.4) (1.4) (1.2) (% affected in brackets) Area affected by future 1% 24.3 9.6 9.6 residual flood risk (Ha) (5.7) (2.4) (2.0) (% affected in brackets) Area affected by present 0.1% 30.8 14.9 14.9 residual flood risk (Ha) (7.2) (3.6) (3.1) (% affected in brackets) Area affected by future 0.1% 36.8 20.0 20.0 residual flood risk (Ha) (8.6) (4.9) (4.1) (% affected in brackets)

The proposed strategic link roads in the north and east cross the floodplains. It is important that the link roads are designed to be in accordance with Table D.1 of PPS25, such as keeping it operational and safe during times of floods as well as maintaining

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flows. However, the link roads could provide the opportunity of multiple benefits, such as flood defence and recreation, as discussed in Section 7.1.

5 FAILURE OF DEFENCES

5.1 Introduction

This section gives detail of how and which failure scenarios have been modelled and the results in relation to the development areas. A total of eight defence failures (breaches) and ten structure blockage scenarios have been modelled for the 1% annual probability (1 in 100 year). Along rivers the references to the left hand and right hand banks relate to the direction viewed when someone is facing downstream.

The Council, as the Local Planning Authority, should use the modelling results to inform decisions on planning issues. In relation to emergency planning, the mapping should inform access routes, areas to evacuate first and where flooding would be a danger to people.

5.2 Breaches of Defences

The approach to the breaching scenarios (defence failure) was to simulate a breach of a raised defence or embankment (some of the locations are defacto defences) for a limited length of time based on a series of criteria to determine the possible flood risk due to a defence failure.

All of the breaches modelled have been 40m long and the duration of the breach has been 30 hours. Each breach has been modelled in isolation so that the effect can be assessed individually.

The criteria used for a breach to occur were either when the water level at the breach location to be assessed reaches its maximum water level during the model run or when the defence or embankment first begins to overtop. In order to determine when a breach would occur, it was necessary to look at the water level results of the present day scenario baseline model runs for the 1% annual probability flood (the flood outline for the 1% annual probability present scenario can be seen in Figure 9). Only the storm duration which gave the maximum water level for the present scenario model runs at the location of the breach has been used for the breach scenarios.

The location of the defence failures (breaches) are described in Table 6, as well as the details of the breaches. Although all the defences are deemed to be in a good condition, the likelihood of a breach is increased when there is a large head of water over the defence.

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Table 6 – Modelled Breach Information Breach Location and Number Storm Duration Defence Crest Breach Base Considered Elevation Level (hrs) (mAOD) (mAOD) 1 – Bear Brook Flood Storage Reservoir 30.25 81.55 80.00 2 – Grand Union Canal north bank near 30.25 79.95 79.00 Industrial Park (opposite Victoria Park) 3 – Grand Union Canal north bank near 30.25 82.80 81.50 Oak Farm 4 – Grand Union Canal north bank near 30.25 80.70 80.20 Bear Brook FSR 5 – Bear Brook left hand bank at the 30.25 77.50 76.50 Railway Station 6 – Stocklake Brook FSR 12.25 82.40 80.00 7 – Bear Brook in Vale Park upstream of 30.25 77.75 77.00 High Street 8 –Defence around Holiday Inn Hotel, 30.25 73.50 72.50 near Holmans Bridge

5.3 Breach Results and Implications

This section gives details of the increased flood risk due to the occurrence of a breach. It should be noted that a breach occurring can limit flood risk in other areas due to the drawdown of water levels in areas downstream of the breach; this report however presents the increase in risk compared to the baseline (present day defended 1% annual probability, Figure 9, and present day defended 0.1% annual probability, Figure 12).

5.3.1 Breach results and implications for 1% annual probability

A breach at the Bear Brook flood storage reservoir, Location 1 (Figure 15), will inundate more areas downstream than that seen in the baseline present day defended 1% annual probability (at the railway station, near the A413, north of Victoria Park and north of Stocklake road). The flood hazard to people increases from low to moderate close to the ‘Circus Field’ committed residential development site (4) close to the Broughton Crossing growth area, while the risk increases from moderate to significant in Vale Park further downstream on the right hand bank of the Bear Brook. On the left hand bank of the Grand Union Canal between High Street and the railway further to the south west, there is a significant increase in the area at risk, much of which is designated as a moderate hazard to people.

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Figure 15 – Flood hazard from a breach at Location 1 with the 1% annual probability event If a breach occurs in the north bank of the Grand Union Canal near the Industrial Park (Location 2), there is no increase to the flood extent and the flood hazard remains moderate at Pembroke Road (Figure 16). There is no increase in flood hazard at Vale Park either.

Figure 16 – Flood hazard from a breach at Location 2 with the 1% annual probability A breach along the north bank of the Grand Union Canal, Location 3 (Figure 17) would inundate the low-lying area to the north. It would affect part of the east MDA and

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increase the flood risk at Broughton Lane, Stocklake Road and a factory west of the A4157. The flood hazard varies from low to significant.

Figure 17 – Flood hazard from a breach at location 3 with the 1% annual probability A breach at Location 4 (Figure 18) would cause flooding to a factory west of the A4157 and increase flood risk to areas around Stocklake Road. However the east MDA (Broughton Crossing) would not be affected. Flood hazard would increase from moderate to significant to the east of the A4157.

Figure 18 – Flood hazard from a breach at Location 4 with the 1% annual probability

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Figure 19 depicts the flood risk if a breach occurs in the raised embankment on the left hand bank of Bear Brook, location 5. A breach could cause low to significant flood hazards to the railway station and railway track up to the California Brook. This may have serious implications for transport links due to disruptions to the rail network.

Figure 19 – Flood hazard from a breach at Location 5 with the 1% annual probability

A breach was modelled in the Stocklake FSR (Location 6) by lowering the bank crest by 2.4m. The flood hazard with the breach (Figure 20) would not inundate the downstream properties near Meadow Way and flood hazard would be low to moderate.

Figure 20 – Flood hazard from a breach at Location 6 with the 1% annual probability

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Considering a breach along Bear Brook at Vale Park; Location 7, the resultant flood risk (Figure 21) increases in the area close to the medical centre. The associated flood hazard to people increases to significant.

Figure 21 – Flood hazard from a breach at Location 7 with the 1% annual probability As the bank does not offer much protection to the site under the 1% annual probability flood event, the flood hazard is not significantly changed if a breach occurs along the right hand bank of the River Thame (Location 8, Figure 22). The flood hazard remains significant. There should be appropriate emergency evacuation procedures in place for the Holiday Inn and Weedon Hill MDA during extreme flood events.

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Figure 22 – Flood hazard from a breach at Location 8 with the 1% annual probability

5.3.2 Breach results and implications for 0.1% annual probability

A breach at the Bear Brook flood storage reservoir, Location 1 (Figure 23), will not significantly change the flood outline downstream of the breach due to a high inundation that occurs in the baseline condition (0.1% present day residual risk). The flood hazard remains similar to that of the baseline condition.

Figure 23 – Flood hazard from a breach at Location 1 with the 0.1% annual probability event

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Similar to breach Location 1, if a breach occurs in the north bank of the Grand Union Canal near the Industrial Park (location 2), the flood extent and the flood hazard remain similar to that of the baseline condition.

Figure 24 – Flood hazard from a breach at Location 2 with the 0.1% annual probability A breach along the north bank of the Grand Union Canal, Location 3 (Figure 25) would inundate the low-lying area to the north. It would affect a significant part of the east MDA and increase the flood risk at Broughton Lane, Stocklake Road and a factory west of the A4157. The flood hazard varies from low to significant.

Figure 25 – Flood hazard from a breach at Location 3 with the 0.1% annual probability

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A breach at location 4 (Figure 26) would cause flooding to a factory west of the A4157 and increase flood risk to areas around Stocklake road. However the east MDA (Broughton Crossing) would not be affected. Flood hazard will become significant to the east of the A4157.

Figure 26 – Flood hazard from a breach at Location 4 with the 0.1% annual probability Figure 27 depicts the flood risk if a breach occurs in the raised embankment on the left hand bank of Bear Brook, Location 5. A breach could cause significant flood hazard to the railway station and railway track up to the California Brook, having consequences on the transport system in Aylesbury. The college downstream of the breach will also be affected.

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Figure 27 – Flood hazard from a breach at Location 5 with the 0.1% annual probability

A breach was modelled in the dam of the Stocklake flood storage reservoir (Location 6) by lowering the bank crest by 2.4m. Figure 28 shows the flood hazard with the breach would not inundate the downstream properties near Meadow Way but it would cause inundation to parts of the floodplain which would not have been flooded in the baseline 0.1% model run.

Figure 28 – Flood hazard from a breach at Location 6 with the 0.1% annual probability

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Considering a breach along Bear Brook at Vale Park, Location 7, the resultant flood risk (Figure 29) remains similar to the baseline scenario. The flood hazard is significant in places.

Figure 29 – Flood hazard from a breach at Location 7 with the 0.1% annual probability

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Figure 30 shows the flood hazard to the Holiday Inn hotel if a breach occurred along the right hand bank of the River Thame, location 8, near Holman’s Bridge. As the bank does not offer protection to the site under the 0.1% annual probability flood event, the flood extent with a breach is identical. The flood hazard to people around the Inn remains significant. Therefore there should be appropriate emergency evacuation procedures in place for the Holiday Inn and downstream at Weedon Hill MDA during extreme flood events.

Figure 30 – Flood hazard from a breach at location 8 with the 0.1% annual probability

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5.4 Culvert Blockages

A total of 10 blockage scenarios have been simulated with the hydraulic model for the 1% annual probability flood event. Table 7 gives details of the structures that have been blocked in the assessment. The approach to the blockage assessment is a 70% reduction in the size of the opening of a structure. This reduction in area was agreed with the Environment Agency for the purpose of the Level 2 SFRA. Each blockage has been modelled in isolation so that the effect of each one of the ten blockages can be assessed individually.

Table 7 – Blockage Information

Blockage Model Node Location How Modelled No 1 BB.048 Bear Brook Railway Bridge Culvert in ISIS model 2 BB.086 Bear Brook Station Access Road Culvert with Trash Screen in ISIS model 3 BB.108 Bear Brook High Street Culvert Culvert in ISIS model 4 BD.005 Bedgrove Brook Broughton Avenue Culvert Culvert with Trash Screen in ISIS model 5 CB.025a Access Bridge across California Brook USBPR Bridge in ISIS model 6 SO.018 Southcourt Brook, Churchill Avenue Culvert Culvert with Trash Screen in ISIS model 7 ST.022 Stoke Brook Oxford Road Bridge USBPR Bridge in ISIS model 8 TH.050a River Thame A41 Bicester Road Arch bridge in ISIS model 9 WE.009 West End Ditch, Tring Road Culvert Culvert in ISIS model 10 TUFLOW Culverts under Grand Union Canal Culvert in TUFLOW model

5.5 Blockage Results and Implications

This section illustrates the results of the blockages. It should be noted that a blockage can also limit flood risk in areas downstream; this report presents the increase in risk in relation to the baseline (present day defended 1% annual probability, Figure 9, and present day defended 0.1% annual probability, Figure 12).

5.5.1 Blockage results and implications for 1% annual probability

Figure 31 shows the blockage of culvert BB.048, Location 1, would significantly change the flood outline upstream of the culvert. Many buildings on the right hand bank of the Bear Brook would be affected by the backing up of water upstream of the culvert.

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Figure 31 – Flood extent from a blockage at Location 1 with the 1% annual probability

The blockage of culvert BB.086, Location 2, does not change the flood outline upstream of the culvert (Figure 32).

Figure 32 – Flood extent from a blockage at Location 2 with the 1% annual probability The blockage of culvert BB.108, Location 3, creates implications on both the Bear Brook and Grand Union Canal. The extent of flooding both upstream and downstream of Park Street near Vale Retail Park (Figure 33) increases. The increase in water depth around the park allows more water to pass through the link between Bear Brook and the Grand

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Union Canal. This results in a larger flood area downstream of the culvert that encompasses the health centre and a significant portion of the California Brook floodplain.

Figure 33 – Flood extent from a blockage at Location 3 with the 1% annual probability event The blockage of culvert BD.005, Location 4, backs up water near Broughton Avenue, thus flooding a few properties on the left hand bank (Figure 34).

Figure 34 – Flood extent from a blockage at Location 4 with the 1% annual probability event

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If a blockage occurred at Bridge CB.025a, Location 5, flooding would increase (Figure 35) locally both upstream and downstream of the structure.

Figure 35 – Flood extent from a blockage at Location 5 with the 1% annual probability event A blockage of the culvert SO.018, Location 6, along Southcourt Brook at Churchill Avenue has a significant impact on flood risk (Figure 36). Several properties would be flooded either side of Churchill Avenue including a school as well as Churchill Avenue. Churchill Avenue is a highly utilised carriageway for access to several surrounding schools therefore blockage of the structure could lead to disruption to transport routes.

Figure 36 – Flood extent from a blockage at Location 6 with the 1% annual probability event

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The blockage of the culvert at Oxford Road bridge ST.022, Location 7, results in a significant increase in flood risk (Figure 37) which affects the housing estates either side of Ellen Road.

Figure 37 – Flood extent from a blockage at Location 7 with the 1% annual probability event Figure 38 shows that a blockage of Arch Bridge Th.050a, Location 8, would have an impact on the Berryfields MDA site with more of the floodplain becoming inundated.

Figure 38 – Flood extent from a blockage at Location 8 with the 1% annual probability event

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A blockage of culvert WE.009, Location 9, at Aston Clinton Road (Figure 39) does not significantly increase flood risk as flow is already restricted by the culvert.

Figure 39 – Flood extent from a blockage at Location 9 with the 1% annual probability event Figure 40 illustrates the impact of the blockage of five culverts under the Grand Union Canal, Location 10. The knock on effect is increased flood risk to urban areas of Aylesbury. Effectively water is prevented from flowing to the north of the Grand Union Canal, therefore allowing more water to flow towards the town centre. Consequently further flooding occurs at the Health Centre, Park Street and south of Broughton.

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Figure 40 – Flood extent from the blockage at Location 10 with the 1% annual probability event 5.5.2 Blockage results and implications for 0.1% annual probability

Figure 41 shows the blockage of culvert BB.048, Location 1, would significantly change the flood outline upstream of the culvert and cause flooding of more buildings on the right hand bank of the Bear Brook compared to the baseline case.

Figure 41 – Flood extent from a blockage at Location 1 with the 0.1% annual probability

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The blockage of culvert BB.086, Location 2, does not significantly change the flood outline upstream of the culvert (Figure 42).

Figure 42 – Flood extent from a blockage at Location 2 with the 0.1% annual probability The blockage of culvert BB.108, Location 3, increases the extent of flooding to Vale Retail Park (Figure 43) but the extent of flooding is not significantly increased as the floodplain is inundated to a similar extent in the baseline 0.1% probability baseline event. The college on the California Brook however is affected by the blockage further downstream due to an increased volume of water entering the Grand Union Canal .

Figure 43 – Flood extent from a blockage at Location 3 with the 0.1% annual probability event

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The blockage of culvert BD.005, Location 4, backs up water near Broughton Avenue, thus flooding a couple of properties on the left hand bank (Figure 44) and water spills onto and over the carriageway.

Figure 44 – Flood extent from a blockage at Location 4 with the 0.1% annual probability event If a blockage occurred at Bridge CB.025a, Location 5, flooding would not increase significantly (Figure 45).

Figure 45 – Flood extent from a blockage at Location 5 with the 0.1% annual probability event

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A blockage of the culvert SO.018, Location 6, along Southcourt Brook at Churchill Avenue has a little impact on flood risk (Figure 46). The extent of flooding at the school close to Prebendal Farm will increase although the school will be flooded in the baseline 0.1% scenario.

Figure 46 – Flood extent from a blockage at Location 6 with the 0.1% annual probability event The blockage of the culvert at Oxford Road bridge ST.022, Location 7, will result in an increase in flood risk (Figure 47) on the right hand bank. The flood extent will reach Edinburgh Sports Ground.

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Figure 47 – Flood extent from a blockage at Location 7 with the 0.1% annual probability event Figure 48 shows that a blockage of Arch Bridge Th.050a, Location 8, would have an impact on the Committed Residential Development at Berryfields as more of the floodplain will be inundated due to the blockage.

Figure 48 – Flood extent from a blockage at Location 8 with the 0.1% annual probability event Blockage of culvert WE.009, Location 9, at Aston Clinton Road (Figure 49) does not significantly increase flood risk due to the low capacity of the structure in combination with a high magnitude event.

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Figure 49 – Flood extent from a blockage at Location 9 with the 0.1% annual probability event Figure 50 illustrates the impact of the blockage of five culverts under the Grand Union Canal, location 10. The knock-on effect is increased flood risk to urban areas in Aylesbury. Effectively water is prevented from flowing to the north of the Grand Union Canal, therefore allowing more water to flow towards the town centre. Consequently further flooding occurs at the Health Centre, Park Street and south of Broughton.

Figure 50 – Flood extent from the blockage at location 10 with the 0.1% annual probability event

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5.6 Rapid Inundation Zone

5.6.1 Method

Zones of rapid inundation have been mapped (see Figure 51). Rapid inundation has been considered as areas where a breach of a defence will cause 300mm depth of flooding within half an hour of the breach occurring. As a limited number of breaches have been modelled from which to ascertain an area at risk of rapid inundation, it has been necessary to inspect the topography and bank levels along the rest of the embankments that have been breached in order to create contours to define the areas of rapid inundation. There is no risk of flooding from rapid inundation along reaches where defences or raised embankments do not exist as the rate of rise of the flood hydrographs are either not rapid enough to inundate the floodplain or the slope of the valley is sufficient to not allow 300mm to pond on the floodplain within half an hour. 5.6.2 Areas vulnerable to rapid inundation

Figure 51 depicts three areas vulnerable to rapid inundation within Aylesbury. The rapid inundation affects the south western edge of the East MDA. Firstly, the breach analysis along the Grand Union Canal (see Section 5.3, Breach Location 3) has demonstrated that a breach developing there would be sufficient to flood to 300mm within half an hour. Because a breach could develop anywhere along that stretch, the rapid inundation has been extended to cover the whole of it, on either bank.

Secondly, the breach analysis at Location 8 has demonstrated that the Holiday Inn is also at risk of rapid inundation.

Finally, the car park along the railway is at risk of rapid inundation based on the results of breach at Location 5.

It is also recommended that development is only allowed in areas outside those designated as a rapid inundation zone although a site specific FRA would be needed to better define the zones indicated in Figure 51.

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6 OTHER SOURCES OF FLOODING

6.1 Surface Water Flooding

The Level 1 SFRA for Aylesbury Vale stated that pluvial flooding is not a significant issue in Aylesbury in light of historical flooding records. However, this does not necessarily mean that surface water flooding could not be an issue in the future. Digital terrain data (LiDAR) has been used to identify potentially critical drainage areas in this SFRA. However, more detailed assessments are recommended along with sewer modelling to quantify the future issues.

Thames Water has supplied local data from their Sewer Flooding Database, which highlights places where sewer flooding (both surface and foul) is an issue. The properties affected by sewer flooding are reported to the Office of Water Services (Ofwat) as part of Director General Performance Measure 5 (known as DG5). DG5 is the performance measure that Ofwat judges water companies by for sewer flooding. It covers two measures: ƒ The number of properties at risk of internal flooding from sewers due to hydraulic overloading within last ten years; and ƒ Properties which are internally flooded. Sewer flooding can be caused by temporary problems, such as blockages or sewer collapses, or because of hydraulic overloading.

The locations of previously flooded properties are covered by the Data Protection Act. For this reason Thames Water is unable to supply a map indicating properties at risk of sewer flooding but they did supply this information in an alternative less detailed format using truncated post codes subject to a specified licensing agreement between Royal Haskoning and Thames Water. This made it possible to broadly identify where sewer flooding has occurred: ƒ HP20 1 (near Manor Park) – 7 properties flooded by foul water from overloaded sewers in the last ten years

The indicative locations of the properties within Aylesbury Town are illustrated in Figure 52, along with other sources of flooding. Thames Water has confirmed that all the properties are included in their AMP4 or AMP5 capital programmes (Asset Management Plans covering 2005 – 2010 and 2010 – 2015 respectively) to address the flooding issues. However, the area near Manor Park could become hotspots if development occurs nearby and surface water is not managed properly. Thames Water wishes to work closely with the Council to incorporate policies on water and sewerage infrastructure within Local Development Framework Documents.

6.2 Groundwater Flooding

The Environment Agency’s hydrogeology team and the Council have indicated some groundwater flooding issues but the risk of this form of flooding in the Aylesbury area is low due to the geology of the area. According to this information, there have been a few reported incidents of perceived groundwater flooding, which mainly occur in isolated properties on high ground where springs have occurred at the base of permeable chalk strata where they adjoin the impermeable clays beneath them.

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The Environment Agency has provided some reported groundwater flooding incidents in Aylesbury and the surrounding area although none of these are confirmed as they are reports from the public. The only one in the town is at Eastern Street (see Figure 52) in 2003 where springs from Portland Limestone outcrop over clay. The other is along Eythorpe Road in Stone (west of Aylesbury) in October 2001 from Portland and Purbeck beds over clay.

In addition, there are two more examples of areas that may have or possibly could be affected by the localised high groundwater table within Aylesbury urbanised area according to the Council, which include: ƒ The undeveloped area just to the east of Douglas Road (known as ‘three ponds field’; and ƒ The general area around Cleveland Road and the area surrounding the Roman Road.

The Aylesbury Arm of the Grand Union Canal has previously suffered from seepage problems at some locations which could lead to high groundwater table in the surrounding area, such as between Oakfield Road and Broughton Lane.

None of the above areas are within any of the Growth Arcs.

6.3 Potential Drainage Problems Areas

A preliminary assessment of potential drainage problem areas has been made based on a digital survey of the Aylesbury topography (LiDAR data) in order to identify locations in which water could pond if drainage is inadequate or if the stormwater system is surcharged.

This assessment is only preliminary and seeks to indicate areas for which further investigation with regards to the capacity of stormwater drains may be necessary. This investigation could be under the form of a Surface Water Management Plan (SWMP). The SWMP should be carried out in accordance with DEFRA guidance and would be developed by the Council in collaboration with the water companies. The SWMP will also deal with the issues identified in the Water Cycle Strategy (WCS) currently being prepared for the Council.

The potential problematic areas which have been identified are: ƒ East of Haydon Hill; ƒ North of Belgrave Road, to the back of Robinson Close and Adkins Close; ƒ The Broadfields trading estate and the Aylesbury Business Centre; ƒ Blenheim Place; ƒ Keats Close and Eleanor Gardens near to the railway embankment; ƒ Vale Park; ƒ Anglo Business Park; and ƒ The offices to the north of Fowler Road.

The soil types at these locations have been identified based on the information available at www.magic.gov.uk. While the site east of Haydon Hill is on free-draining soil, all the other sites are located on slowly permeable seasonally wet soils. The soil type will affect the amount of run off generated and will also influence the likelihood of water ponding in the area.

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7 MANAGING FLOOD RISK

7.1 Introduction

This section discusses the flood risk from the Growth Arcs, the opportunities for managing flood risk (using flood storage reservoirs and SUDS), the emergency services available for development and the evacuation requirements for the developers.

7.2 Flood Risk Due to Growth Arcs

The present flood risk to Aylesbury and surrounding area has been modelled using the hydrological investigation undertaken by Peter Brett Associates. An assessment has also been undertaken to determine the change to the present Greenfield runoff rates due to the Growth Arcs.

The modelling results of the East MDA (Broughton Crossing) have shown that it would be possible to not only match current runoff rates, but reduce the runoff rate from the sites by providing on site storage and attenuation (Odyssey, 2008). It is likely that the runoff volume will increase due to the increased impermeable surface coverage but the application of SUDS and other systems should be able to provide sufficient storage so that the peak runoff rate is reduced and the volume of water is released at a much slower rate than that of Greenfield runoff rates. It is advised that this approach should be adopted for the chosen Growth Arc.

The Environment Agency has stated that runoff rates from any of the developments should not exceed the baseline Greenfield runoff rate. There is sufficient gradient and area for all development options to implement SUDS and drainage measures so that the development option does not increase flood risk. Therefore it has not been necessary to undertake a series of ‘with development’ model simulations as implementing any of the developments have been assessed to not increase peak runoff rates and hence flood risk.

However, it should be noted that although the peak runoff rate can be reduced it is not certain that discharging the additional volume over an extended period would mitigate flood risk downstream. While masterplans are yet to be finalised for all the Growth Arcs, the total post development volumes could change and affect flood risk extents. Therefore it was decided that ‘with development’ modelling would not be carried out as this could later change and cause confusion. If all the surface water cannot be disposed of on site or to a local watercourse and existing surface water sewers have to be used, flooding could occur downstream as a result. Therefore the developers should demonstrate that flood risk will be reduced downstream. For example, new development upstream of the flooding issues at Waternead (stated in Section 4.2) provides an opportunity to improve the flooding situation.

There are a number of proposed road layouts illustrated within the figures in this report. These layouts often cross the floodplain and are shown to be at risk of flooding. The modelling undertaken does not include for these roads, the potential loss of floodplain and change in flood pathway that may result with their implementation. It will be necessary to design the roads in such a way that they do not have an adverse affect on flood risk once the preferred Growth Arc has been determined. Most importantly,

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infrastructure built in the floodplain should have a minimised footprint and not increase flood risk elsewhere.

There may be an opportunity to provide further flood mitigation measures by constructing the strategic roads to act as a dam (and store water upstream), such as using the eastern and northern link roads. The benefit of building infrastructure on embankments is that less freeboard and maintenance is required. Although the Environment Agency is open to such opportunities, there are implications of ownership and maintenance which would require clear plans to be established. Contingency plans for failure would be required as well. Also they should not be built purely for development but also for multiple benefits, such as alleviating flood risk to existing properties and enhancing local amenity.

7.3 Potential Flood Storage Reservoirs

7.3.1 Overview

A number of potential flood storage options have been modelled in order to assess the reduction of flood risk in Aylesbury. In consultation with the Environment Agency, the following options were considered: ƒ A new flood storage option on Stoke Brook; ƒ A new flood storage option on Wendover Brook; ƒ An increase to the dam of the current flood alleviation scheme on the Bear Brook; and ƒ A flood diversion scheme from the Bear Brook and associated storage option between the Grand Union Canal and the old disused railway embankment crossing Burcott Brook.

The principles applied for the flood storage options were that the option should be both technically and financially feasible to provide protection for the 1% annual probability flood event. The location of the flood storage options should, where possible, utilise land that is already at risk of flooding whilst not increasing flood risk to any properties.

7.3.2 Option on Stoke Brook

After simulating the baseline model (defended 1% annual probability without storage options), it was not considered necessary to investigate a Stoke Brook storage option further. The railway culvert close to Strandal’s Farm was not included in previous hydraulic modelling investigations. When this culvert was included in the current hydraulic model, it causes attenuation upstream of the railway. Consequently less water reaches the Walton Court area, which was previously shown to be at risk from the 1% annual probability flood event. Therefore it was deemed to be economically unviable to consider flood storage on the Stoke Brook.

7.3.3 Option on Bear Brook

Increasing the height of the current flood storage dam on the Bear Brook was investigated. The current FSR overtopped during the 1% annual probability flood event, therefore it was considered viable to increase the height of the dam to improve

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protection to properties downstream through the town centre. However, a consequence of this is that water levels would increase upstream.

Figure 53 shows the results of increasing the height of the Bear Brook FSR. It is evident that this option would decrease the flood risk to a number of properties particularly at Pembroke Road, Vale Retail Park, Old Brewery Close, the Health Centre, Gatehouse Way and Paterson Road. This option does, however, cause the flood embankment upstream (ID 142) to be overtopped and flood a few properties along Richmond Close. Also the flood risk to Broughton Lane increases. Therefore this option would require the flood embankment (ID 142) to be raised and also possibly an embankment built to protect Broughton Lane.

7.3.4 Option on Wendover Brook

A flood storage reservoir option was considered on the Wendover Brook upstream of Aston Clinton Road. This resulted in the reduction of flood risk in the south east MDA (in all Growth Arcs), Figure 54. However, the Environment Agency has expressed that flood storage reservoirs should not be built to purely benefit or allow development in the Growth Arcs. The floodplain between the A41 Aston Clinton Road and the Grand Union Canal would also reduce with this option. In the town centre, the flood risk could be slightly reduced particularly at Pembroke Road and the Health Centre.

7.3.5 Option on Burcott Brook

The final flood storage option investigated was on Burcott Brook which included the combination of increasing the number of openings under the Grand Union Canal and reducing the number of openings under the old disused railway embankment to the north, Figure 55. The baseline model had a rectangular culvert 590mm by 710mm opening under the Grand Union Canal; for the flood storage option a further two culverts with the same dimensions were added. The baseline model also had three culverts (each with 0.5m diameter) under the old disused railway embankment; two of these were removed from the model for the flood storage option to help limit the flow downstream and counter the increased flows flowing under the Grand Union Canal.

Figure 55 depicts the results of the modelling. By allowing more water to flow to the north, there is less water flowing through the town centre and so flood risk would noticeably reduce at Pembroke Road, Gatehouse Way and Paterson Road. Although more flood water is held back by the disused railway embankment, the flood risk has not increased to the east MDA (part of Eastern and Combined Growth Arcs). However, this option does not reduce flood risk to the north east MDA nor the south east MDA.

7.3.6 Combined options

If all flood storage options were implemented then the 1% annual probability flood risk to Aylesbury centre would be significantly reduced, Figure 56. However, there would be no reduction in flood risk to the north east MDA and only the south east MDA would experience a reduction in flood risk. Considering the 5% annual probability flood event with all the potential flood storage reservoirs, the functional floodplain does not significantly reduce in Aylesbury, Figure 57. The noticeable reduction occurs to the

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south of the Grand Union Canal due to the increased number of culverts under the canal.

The combined options were also modelled for the following scenarios: • Future 5% annual probability residual flood risk – Figure 57; • Present day 1% annual probability residual flood risk – Figure 58; • Future 1% annual probability residual flood risk – Figure 59; • Present day 1% annual probability residual flood risk – Figure 60; and • Future 1% annual probability residual flood risk – Figure 61.

In the 1% annual probability scenarios, Figures 58 and 59 demonstrate that yet again the most noticeable reduction is to the south of the Grand Union Canal. There is also a notable reduction in the flood extent at Vale Retail Park.

In the 0.1% annual probability scenarios, Figures 60 and 61 also provide evidence of a reduction in flood extents south of the Grand Union Canal and at Vale Retail Park. However, that reduction is less noticeable for such an extreme event which exceeds the design capacity of the FSRs. The proposed FSR at Wendover Brook is outflanked on the west.

7.3.7 Other options

A flood storage option on West End Ditch to the south of the A41 Aston Clinton Road has not been modelled after the Environment Agency advised that a reservoir would be unlikely to reduce flood risk significantly as the watercourse conveys small flows. However, after seeing the results of the improvements to the Bear Brook FSR affecting Broughton Lane and Richmond Close properties, an option on the West End Ditch could provide relief to the existing Bear Brook FSR and alleviate flood risk in the town centre. The Council have expressed that this additional modelling would be a desirable requirement at the next stage of detailed master planning for the chosen growth arc.

7.3.8 Discussion on options

All the flood storage options present opportunities for ecological and amenity uses. While the main function would be flood protection to downstream existing properties, the reservoirs could be designed to have multiple benefits for the local community. Each option will require close consultation with the Environment Agency, land/asset owners and local stakeholders.

In terms of reducing flood risk, the Bear Brook improvements would have the best outcome in the town centre particularly at Pembroke Road, Vale Retail Park, Old Brewery Close, the Health Centre and Gatehouse Way. The Burcott Brook option would give the next best reduction of flood risk to the town centre at Pembroke Road and Gatehouse Way. The Wendover option reduces flood risk to Aston Clinton Road and slightly reduces flood risk to Pembroke Road and the health centre. In addition, the potential for flood risk improvement on the River Thame and West End Ditch should be examined. Watermead is a specific area where flooding is a known issue and new development upstream can provide the opportunity to reduce that risk.

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The construction of flood storage reservoirs does also increase water levels upstream. The option that results in a larger flood risk upstream is the Bear Brook improvement, which causes overtopping of a flood embankment (NFCDD 0611919BB0201L01), flooding of properties along Richmond Close and increases flood risk to Broughton Lane. The Burcott Brook option results in the least impact upstream as no properties are affected nor the east MDA (Broughton Crossing), whereas the Wendover modelled option results in flooding of Brookfield Farm.

Although the Bear Brook improvement would offer the most flood alleviation to the town centre, it would require the upstream flood embankment to be raised and also possibly an embankment built to protect Broughton Lane. However, if the Wendover or the Burcott Brook FSR was constructed too, then the additional Bear Brook embankments might not be necessary.

7.4 Application of Sustainable Drainage Systems

Although Section 6 does not indicate that there is an existing problem in Aylesbury from surface water, this could change with climate change. Surface water can be managed through Sustainable Drainage Systems (SUDS) that can also enhance the visual aesthetics of urban areas and improve runoff water quality. In fact the EU Water Framework Directive states that all watercourses should have a ‘good’ ecological status. The status of the local watercourses is moderate according to the Environment Agency. Therefore there would be benefits in considering the retrofitting of SUDS in consultation with the Environment Agency, Thames Water and the Council. Possible systems include bio-retention swales and permeable pavements on gentle slopes (less than 5% gradient), as well as green roofs (where flat or gently sloping) and water butts. Figure 62 shows the gradient of land in and around Aylesbury, based on a digital survey of the topography (LiDAR). It shows that the majority of Aylesbury’s topography is gently sloping, such as St Andrews Way Industrial Estate and the Broughton area.

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BIERTON WITH BROUGHTON

AYLESBURY

WESTON TURVILLE

STOKE MANDEVILLE

Figure 62 – Gradient of topography in Aylesbury (Green areas have a gradient less than 5%, red areas have a gradient greater than 5%)

Only certain types of SUDS are suitable on steeper slopes (with a gradient more than 5%), namely retention, infiltration, detention and sources control (green roof and rainwater harvesting). Swales, however, can be used on steeper slopes if they follow the contours. Figure 62 shows that there is an area northwest of Bierton, within the North East MDA, where slopes are greater than a 5% gradient.

Presently the Council will adopt open watercourses and above ground SUDs features such as swales and wet or dry detention basins if they are in public open space and do not only serve highway drainage. This policy may be reconsidered in light of the Government's response to the Pitt Review Recommendation 20, to the effect that County and Unitary authorities should take responsibility. Also the Council are awaiting the details in the draft Floods and Water Bill due to be published in spring 2009.

Guidance on SUDS and further references can be found in Section 5 of the PPS25 Practice Guide (Communities and Local Government, 2008). A good local example of a SUDS strategy is at Berryfields MDA. Further information can be obtained from the Council and in Appendix A.

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7.5 Flood Warning and Emergency Response

7.5.1 Environment Agency’s flood warning role

The Environment Agency provides a Flood Warning service and encourages take up of the service to properties at risk of flooding in order to enable householders to protect life or take early action to manage the effect of flooding on property. New developments should consider the role of flood warning in managing residual risks although they should not rely solely on them.

The Environment Agency issues flood warnings using a set of four easily recognisable codes: ƒ Flood Watch, where flooding of low-lying land and roads is expected; ƒ Flood Warning, where flooding of homes, businesses and main roads is expected; ƒ Severe Flood Warning, where severe flooding is expected. Extreme danger to life and property; and ƒ All Clear, where flood watches or warnings are no longer in force.

Flood Warning areas are drawn to the extent of Flood Zone 2 and will cover all properties that fall within this boundary. The Environment Agency aims to issue a Flood Watch two hours prior to the start of flooding and Flood Warnings two hours prior to the start of property flooding although this is not always possible. Flood Warnings apply to flooding caused by rivers and streams, not to flooding from other sources, such as sewer and surface water flooding away from rivers, groundwater flooding and burst water mains. For fast responding catchments (particularly in urban areas) it may be necessary to issue Flood Warnings (or even Severe Flood Warnings) directly without issuing a Flood Watch first.

The flood warning arrangements for this area should be reviewed in light of the SFRA and WCS to ensure that adequate telemetry and flood warnings are provided.

Sir Michael Pitt’s review of the summer 2007 floods and ensuing recommendations have identified the need to improve flood warning tools to meet the needs of flood risk managers, emergency planners and responders. These new tools would allow visualisation of flood data and increase preparation lead time for emergency responders. Sir Michael Pitt also identified the need to increase awareness of flood risk.

Flood Warnings are disseminated by the Environment Agency via a system known as Floodline Warning’s Direct. The message details the level of warning issued, the area for which the warning is in force and advice on what action to take. As flood events develop the public is encouraged to phone Floodline for updates. This system requires residents of “at risk property” to register their telephone numbers with the Environment Agency. Concerned parties are able to obtain current flood warning information according to a particular river or Flood Warning Risk Area.

Other current methods of warning dissemination include: ƒ The media – warnings are issued through the media; they are broadcast on TV weather bulletins and on radio weather and travel reports. Flood warnings are

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also displayed on ITV Teletext regional weather pages (page 154) and on the BBC Ceefax (page 419); ƒ Floodline 0845 988 1188 – offers callers the option to listen to recorded flood warning information 24 hours a day and speak to a trained operator for more advice; and ƒ The Environment Agency’s website contains live warning information. www.environment-agency.gov.uk/homeandleisure/floods/31618.aspx

If anyone has not currently registered their phone number but is at risk of flooding, they should consider contacting the Environment Agency.

7.5.2 Aylesbury Vale District Council’s emergency role

The Aylesbury Vale District Council has no formal responsibility or system for providing flood warning, but have an emergency plan that is continuously updated. Although the Council does respond to requests for assistance in flooding situations, their capacity to assist through a standby contractor is very limited in delivering and placing gel flood bags or clearing trash screens. Therefore the Council encourage property owners to make their own preparations wherever possible.

In small scale events, emergency response is handled by the Engineering team. For larger scale events, the Council’s role is to provide rest centres and assist Buckinghamshire County Council in co-ordinating flood warning and informing (e.g. leaflet dropping and door-knocking) for evacuation of premises.

The Aylesbury Vale District Council is currently developing a flood management plan in response to the Pitt Review with specific actions on preparing for flooding, emergency response and recovery.

7.5.3 Evacuation plans

In exceptional circumstances, having undertaken the Sequential Test and the Exception Test, if development is allowed in areas known to be at risk of flooding then developers should prepare evacuation plans. Guidance is given in PPS25 Practice Guide and should make provision for:

i. How flood warning is to be provided; ƒ Availability of existing flood warning systems; ƒ Rate of onset of flooding and available flood warning time; and ƒ Method of dissemination of flood warning.

ii. What will be done to protect the infrastructure of the development and contents, such as: ƒ How more easily damaged items (including parked cars) will be relocated; ƒ The potential availability of staff/occupants/users to respond to a flood warning; and ƒ The potential time taken to respond to a flood warning.

iii. Ensuring safe occupancy and access to and from the development, such as:

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ƒ Occupant awareness of the potential frequency and duration of flood events; ƒ Provision of safe access to and from the development; ƒ Ability to maintain key services during an event; ƒ Vulnerability of occupants, and whether rescue by emergency services will be necessary and feasible; and ƒ Expected time taken to re-establish normal practices following a flood event (clean-up times, time to re-establish services etc.).

Where evacuation plans are required they need to consider the lifetime of the development by fully taking into account the prospective climate change impacts. As a minimum, where any such development takes place in flood risk areas it is important that there is adequate passive flood warning in place, with signs highlighting the susceptibility to flooding and clearly signed evacuation routes where necessary.

The Council is still considering its own criteria for flood hazards acceptable to people, therefore in the meantime developers should refer to R&D guidance FD2320 and FD2321 “FRA Guidance for New Development” and "Flood risks to people”, respectively. For example FD2321 suggests that the maximum permissible hazard for safe access should correspond to the depth-velocity combinations shown in blue (low hazard) in Table 2 (Section 1). The blue combinations are generally safe for pedestrians but the safety is affected by lack of visibility which could cause hazards due to the unevenness of the ground, the potential for damaged manhole covers and contamination of floodwater amongst other aspects.

With respect to requirements for safe access and exit from new developments in flood risk areas, the guidance states the following should be provided (in decreasing order of preference): ƒ Safe dry route for people and vehicles; ƒ Safe dry route for people; ƒ A route where flood hazard is low and should not cause a risk to people; and ƒ A route for vehicles where the flood hazard is low to permit access for emergency vehicles.

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8 CONCLUSIONS AND RECOMMENDATIONS

8.1 Overview

This Level 2 Strategic Flood Risk Assessment will assist the Aylesbury Vale District Council in applying the Sequential Test and the Exception Test, providing a method of managing flood risk while still allowing necessary development to occur (paragraph 18 in PPS25, 2006). In order to pass the Exception Test, the development must pass all the following aspects:

(a) it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared. If the DPDs have reached the ‘submission’ stage – see Figure 4 of PPS12: Local Development Frameworks – the benefits of the development should contribute to the Core Strategy’s Sustainability Appraisal;

(b) the development should be on developable previously-developed land or, if it is not on previously developed land, that there are no reasonable alternative sites on developable previously-developed land.

(c) a Flood Risk Assessment must demonstrate that the development will be safe, without increasing the flood risk elsewhere, and, where possible, will reduce flood risk overall. The assessment will refer to the joint Defra and Environment Agency document “Flood Risks to People Phase 2, Guidance Document, FD2321/TR2 (March 2006)” to demonstrate that the development is safe.

If the Growth Arcs can maintain the runoff at the Greenfield rates, reduce flood risk to the town centre and surrounding areas, such as at Watermead, and keep built development out of the present day 0.1% annual probability residual risk areas, then they would pass aspect (c).

This Final Level 2 SFRA satisfies the amplified guidance within the Living Draft Practice Guide Companion to PPS25 by: ƒ Defining the functional floodplain; ƒ Considering the nature of the flood hazard, taking account of the presence of flood management measures e.g. flood defences and defacto defences; ƒ Considering the nature of flood hazard if defences fail; ƒ Appraising the defence infrastructure condition and maintenance in light of flood risk; ƒ Informing Emergency Planning Procedures (access and egress to development sites); and ƒ Providing guidance on the preparation of Flood Risk Assessments and the consideration of managing surface water (Appendix A).

All the growth arc allocations include functional floodplain and Flood Zones 2 and 3 to ensure the Council has maximum control of these areas. The Council’s policy is that there should be no built development within any of these areas. This approach is in line with the selected policy from the Thames CFMP.

The following sub-sections summarise the findings of the SFRA and give recommendations in bold. Where the recommendations relate to policy, these are shown in red and bold.

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8.2 Defining the Functional Floodplain and Recommendations

The functional floodplain has been defined using a flood with an annual probability of 5% (1:20 years) plus the areas within the existing flood storage reservoirs (Stocklake FSR and Bear Brook FSR), where water is stored in times of flooding. In this floodplain only water-compatible development (e.g. amenity open space) should be permitted, together with essential infrastructure if it passes the Exception Test.

The present and future 5% annual probability flood events affect small areas of each Growth Arc: ƒ Eastern Growth Arc –14.4 hectares (3.4%) and 15.4 hectares (3.6%) respectively; ƒ Southern Growth Arc – 1.4 hectares (0.3%) and 1.8 hectares (0.4%) respectively; and ƒ Combined East and Southern Growth Arc – 1.4 hectares (0.3%) and 1.8 hectares (0.4%) respectively.

Generally the flood hazard to people is low except in the north east Major Development Area (MDA) where it is moderate to significant to people.

Under the 5% annual probability flood event, the flood storage reservoirs and defences protect the urban areas through the town except small areas of Fairford Leys and Fowler Road. The formal flood defence (NFCDD 0611919TH1201R03) around the Holiday Inn is overtopped during this event. The River Thame functional floodplain extends to some properties at Quarrendon (north of Jackson Road) and also parts of the MDAs at Berryfields and Weedon Hill. The east part of Aston Clinton Road MDA is within the West End Ditch functional floodplain (next to Broughton Lane and below Old Manor Farm) and the Wendover Brook functional floodplain runs through the north east portion. Considering climate change, the functional floodplain includes Vale Park.

All of the proposed strategic link roads are within the present day and future functional floodplains and in accordance with PPS25 Table D.1 they should be designed to remain operational and safe for users in times of flood: ƒ Northern link road crosses the River Thame; ƒ Eastern link road crosses Stocklake Brook, Broughton Brook, Bear Brook and Wendover Brook; and ƒ Southern link road crosses the West End Ditch, Bedgrove Brook, Sedrup Brook and Stoke Brook.

However, the link roads provide the opportunity of multiple benefits, such as flood defence and increased biodiversity. Buckinghamshire County Council should take account of the level of flood risk and design the link roads in consultation with Aylesbury Vale District Council and the Environment Agency to make them sustainable and ensure they do not increase flood risk elsewhere.

8.3 Nature of Flood Hazard and Recommendations

Small areas of all the Growth Arcs are within the defended present day and future 1% and 0.1% annual probability flood events. However, the Council is not proposing any

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built development in any of these areas. As there is enough land outside of the present day residual risk areas, only water-compatible development (e.g. amenity open space) should be permitted, together with essential infrastructure if it passes the Exception Test.

For areas of previously developed land in the town and where there is no available land outside of the 0.1% annual probability residual flood risk area, use the Flood Hazard maps to steer water compatible and less vulnerable development to low hazard areas (shown as blue depth-velocity combinations).

Development in areas affected by an allowance for climate change should be adaptable e.g. where ground floor use can be changed to less vulnerable/ water compatible types to make space for water.

Notable findings in the nature of the flood hazard and recommendations are: ƒ There is not much difference in the River Thame flood extent between the defended 5%, 1% and 0.1% annual probability events in the north east MDA of the Eastern Growth Arc as the valley sides are reasonably well defined. The flood hazard is moderate to significant.

ƒ The east MDA at Broughton Crossing (between the A418 Aylesbury Road and A41 Aston Clinton Road) is not within the future defended 0.1% annual probability flood event. Although the strategic link road from the A41 Aston Clinton Road crosses the floodplain, the flood hazard is generally low or moderate even considering climate change.

ƒ The Aston Clinton Road impedes floodwater causing flooding to occur in the south east MDA. Less Vulnerable land use (Table D.2 in PPS25) is appropriate in the affected area but More Vulnerable land use would have to pass the Exception Test. Also if the Aston Clinton Road is designated as a defacto defence, then the Environment Agency should be aware of the implications.

ƒ At the Aston Clinton Road and Broughton Lane junction there is a pumping station that is partially flooded by the 0.1% annual probability event. It is recommended that the function of the pumping station is reviewed to assess if it should be considered as critical infrastructure during a flood event and consequently kept operational.

ƒ Only the northern edge of the sewage treatment works is within the present day defended 1% annual probability flood extent and the flood hazard is low to people. The northern and western edges are partially within the present day 0.1% flood extent. However, the future 0.1% flood event results in significant flood hazards in some of the areas. As none of the modelled flood storage reservoirs significantly reduce flood risk at the sewage treatment works, other mitigation measures should be considered. Raised flood defences are possible considering only a small amount of the floodplain would be displaced.

ƒ Pluvial flooding has been assessed not to be a significant issue in Aylesbury in light of historical flooding records. However, this does not necessarily mean that

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surface water flooding could not be an issue in the future. It is recommended that the potentially critical drainage areas are investigated, potentially through the preparation of a Surface Water Management Plan in consultation with water companies. Also, the retrofitting of SUDS in the existing urban areas nearby should be investigated. It is recommended that until a Surface Water Management Plan has been produced, Growth Arc developers should demonstrate that they have a Surface Water Strategy, whereby they can show how they have identified ways to reduce surface water flood risk at the masterplanning stage.

ƒ Thames Water has confirmed that seven properties within HP20 1 (near Manor Park) flooded by foul water from overloaded sewers in the last ten years. All the properties are included in their AMP4 or AMP5 capital programmes covering 2005 to 2015 to address the issues. Nevertheless, the area near Manor Park could become a hotspot if development occurs nearby and surface water is not managed properly. Thames Water has expressed wishes to work closely with the Council to incorporate policies on water and sewerage infrastructure within the Local Development Framework Documents. It is recommended that the Council discusses how best to incorporate their proposed text into policy.

ƒ The risk of groundwater flooding in the Aylesbury area is low due to the geology of the area. There are only a few reported incidents or areas that may be affected by tlocalised high groundwater table: ƒ One at Eastern Street in 2003 reported from the public where springs from Portland Limestone outcrop over clay (Environment Agency); ƒ The undeveloped area just to the east of Douglas Road, known as ‘three ponds field’ (Aylesbury Vale District Council); ƒ The general area around Cleveland Road, and the area surrounding the Roman Road (Aylesbury Vale District Council); and ƒ The area between Oakfield Road and Broughton Lane along the Aylesbury Arm of the Grand Union Canal, where there are seepage problems. It is recommended that Flood Risk Assessments for development in these areas include groundwater investigations.

ƒ None of the recorded sewer flooding and groundwater incidents are within any of the Growth Arcs.

8.4 Nature of Flood Hazard if Defences/Infrastructure Fails and the Implications

There are some defences that would have significant adverse impacts on flood risk if a breach occurred and so require regular inspections. They should be maintained to at least their current standard. These critical defences are: ƒ Bear Brook Flood Storage Reservoir (protecting downstream urban areas); ƒ Train station car park embankment along Bear Brook (protecting railway station and railway); ƒ Vale Park embankment along Bear Brook (protecting Vale Retail Park and Park Street); ƒ Holiday Inn embankment.

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The banks of the Grand Union Canal should be maintained to a good condition as breaches in the northern banks can result in flood risk to the east MDA (leading to significant flood hazard) and increase flood risk to Stocklake Road, Pembroke Road Industrial Park and Vale Park. However, no built development of the MDA should be located inside the flood risk area.

There are some culverts that have noticeable adverse impacts on flood risk if they become significantly blocked and so need to be regularly inspected and kept clear of debris and silt. These critical culverts are located: ƒ On Southcourt Brook at Churchill Avenue (protecting nearby residential properties, a school and roads); ƒ Under the Grand Union Canal (affecting the Health Centre and properties along Park Street); ƒ On Bear Brook at the High Street (affecting Park Street properties and the Health Centre); and ƒ On Bedgrove Brook at Broughton Avenue (protecting a few residential properties and the road).

The owners of the defences and infrastructure should be made aware of the consequences of breaches/blockages and put in place appropriate maintenance plans to ensure that their integrity is continued.

The Aston Clinton Road holds some water back and so could be defined as a defacto defence. Similarly, the railway line crossing Stoke Brook (at Hall end) to the south holds flood water back and so could be defined as a defacto defence. The Council should be aware of the implications if there is a planning application for a change of use in these locations.

Using the breach scenarios and residual risk from the 1% annual probability event it has been possible to identify a “Rapid Inundation Zone”, where there is significant danger to people. The south western part of the East MDA is in the rapid inundation zone. It is recommended that only development that does not require rapid evacuation is permitted here.

8.5 Informing Emergency Planning Procedures

In relation to emergency planning, the mapping should inform access routes, areas to evacuate first and where flooding would be a danger to people. Table 8 summarises the flood risk to the major ‘A’ roads, showing that the A413 Buckingham Road and A41 Aston Clinton Road are affected by all the modelled defended flood events. Nevertheless, the flood hazard is low to people on the A41 Aston Clinton Road and A413 Buckingham Road and so could be passable for emergency services. The A418 Aylesbury Road and A413 Wendover Road would be dry during all of the modelled flood events. The A418 Oxford Road could also be available up to the present day defended 0.1% annual probability flood event.

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Table 8 – Summary of major roads affected by flood events

Present Future Present Future Present Future defended defended defended defended defended defended Major ‘A’ Road 5% event 5% event 1% event 1% event 0.1% event 0.1% event A41 Bicester Road No No No Yes Yes Yes A413 Buckingham Road Yes Yes Yes Yes Yes Yes A418 Aylesbury Road No No No No No No A41 Aston Clinton Road Yes Yes Yes Yes Yes Yes A413 Wendover Road No No No No No No A418 Oxford Road No No No No Yes Yes

Other findings related to emergency planning are: ƒ There are schools close to the defended 1% annual probability floodplain, such as near Coldharbour Way, but the hazard is low and there are dry access routes; ƒ Some of the less vulnerable sites (churches) are close to the future defended 0.1% floodplain but the flood hazard to people is low and there are dry access routes; ƒ No existing highly vulnerable sites identified in this study (Police, railway and bus station) are within the future defended 0.1% annual probability floodplain; and ƒ The Territorial Army centre is not affected by the present day 0.1% event but is susceptible to a low flood hazard from the future 0.1% event.

The Environment Agency generally aims to give a two-hour lead time for giving warning where it has a flood warning coverage within the District. However, in certain cases of severe or “flash flooding” this may not always be possible. The flood warning arrangements for this area should reviewed in light of the SFRA and WCS to ensure that adequate telemetry and flood warnings are provided. New developments should consider the role of flood warning and also develop evacuation plans for areas at flood risk in conjunction with the Aylesbury Vale District Council’s Emergency Planning Officer.

The Aylesbury Vale District Council is currently developing a flood management plan in response to the Pitt Review with specific actions on preparing for flooding, emergency response and recovery. The findings of the SFRA and modelling should be used to help finalise the flood management plan.

8.6 Options to Mitigate Flooding and Enhance Local Environment

There are opportunities to alleviate flood risk in Aylesbury. The modelling shows that, in terms of reducing flood risk, the Bear Brook improvements would result in a significant reduction in the town centre. However, this option would increase the flood risk to existing properties and roads upstream. A flood storage option on the Burcott Brook would not increase flood risk upstream to existing properties or roads but would reduce flood risk to the A41 Aston Clinton Road. An option on the Wendover Brook would also alleviate flooding to the A41 Aston Clinton Road and some properties in the town centre, however, Brookfield farm would be affected by the increased water level upstream if left unmitigated.

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An option on the West End Ditch could provide relief to the existing Bear Brook FSR and alleviate flood risk in the town centre. The Council have expressed that this additional modelling would be a desirable requirement at the next stage of detailed master planning for the chosen growth arc.

All the flood storage options present opportunities for ecological and amenity uses. While the main function would be flood protection to downstream existing properties, the reservoirs could be designed to have multiple benefits for the local community. Each option would require close consultation with the Environment Agency, land and asset owners as well as local stakeholders.

In light of the modelling results on the identified Flood Storage Reservoirs, the potential for flood risk improvement provided by additional storage areas on the River Thame and West End Ditch should be examined. In accordance with the requirements of PPS25, it is the responsibility of potential developers to investigate the flood risk reduction benefits of their schemes. The Council has identified the opportunity provided by joint working on the master plans for the chosen growth arc to carry out these investigations and to make provisions as appropriate.

There may also be an opportunity to provide further flood mitigation measures by constructing the strategic link roads to act as dams (and store water upstream), such as using the eastern and northern link roads. The benefits of building infrastructure on embankments are that less freeboard and maintenance are required. Although the Environment Agency is open to such opportunities, there are implications of ownership and maintenance which would require clear plans to be established. Contingency plans for failure would be required as well. Also they should not be built purely for development but also for multiple benefits, such as alleviating flood risk to existing properties and enhancing local amenity.

The breach modelling of the northern Grand Union Canal banks indicates that controlled flooding could present an opportunity for enhancing local biodiversity. This option would need to be discussed further with the Environment Agency, British Waterways and the land owners.

8.7 Limitations to Modelled Flood Extents

The flood extents were all based on a 1D-2D hydrodynamic model (ISIS-TUFLOW) that had previously been produced for the Environment Agency. After alterations had been made for the SFRA, Edenvale Young Associates Limited carried out a review of the model on behalf of the Environment Agency. This highlighted that although the modelling is sufficient for strategic planning decisions, there are some limitations. Consequently the modelling should be updated at some point in the future. For large development sites some changes may be required to ensure flood risk is established appropriately. Recommendations include: ƒ Review and re-schematise floodplain roughness based on OS MasterMap data within the extent of 0.1% annual probability event with climate change. ƒ Gauged water level data should be sought from the Environment Agency at local gauging stations and used for calibration and validation of the baseline models. ƒ Consider local flooding information from Watermead and check topography in model reflects reality.

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ƒ Carry out a comparison to assess accuracy of newly acquired topographic survey against overlapping topographic survey due to disparity with previous data. ƒ A review of the hydrology may be necessary if the model is further updated to allow routing of water into the Grand Union Canal rather than the River Thame. The review is also necessary as flood outlines along some of the tributaries appear confined to the channel even in extreme flood events.

8.8 Updating the Strategic Flood Risk Assessment

The SFRA is a live document and should be reviewed and updated in the future. Potential triggers for a review and update are: ƒ After a major flood event; ƒ When the hydrodynamic model has been updated; ƒ Following updates to PPS25; ƒ Following the publication of new climate change allowances; and ƒ Following the publication of the draft Floods and Water Bill.

8.9 Guidance for Developers

Throughout the SFRA there are aspects that developers should be aware of and take into consideration. Appendix A contains a useful reference for developers in relation to the development framework and flood risk, as well as local contacts and regional references.

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9 GLOSSARY

Annual exceedence The estimated probability of a flood of given magnitude probability occurring or being exceeded in any year. Expressed as, for example, 1-in-100 chance or 1 per cent.

Brownfield site Any land or site that has been previously developed.

Catchment The area contributing flow or runoff to a particular point on a watercourse.

Climate change Long-term variations in global temperature and weather patterns both natural and as a result of human activity.

Critical ordinary An Ordinary watercourse which the Environment Agency and watercourse other operating authorities agree is critical because it has the potential to put at risk from flooding large numbers of people and property.

Culvert Covered channel or pipe that forms a watercourse below ground level.

Defacto Defence Semi-permanent structures which act as a barrier to flow but are not formal defences (e.g. railway embankments, roads etc).

Design event An historic or notional flood event of a given annual flood probability, against which the suitability of a proposed development is assessed and mitigation measures, if any, are designed.

Design flood level The maximum estimated water level during the design event.

Development The carrying out of building, engineering, mining or other operations in, on, over or under land or the making of any material change in the use of any buildings or other land.

Exceedence flow Excess flow that emerges on the surface once the conveyance capacity of a drainage system is exceeded.

Enmained Watercourse designated as a Main River

Environment Agency Non-departmental public body responsible for the delivery of government policy relating to the environment and flood risk management in England and Wales.

Exception Test The final process of the PPS25 Sequential Test (Tiers 3 & 4). It is required when a development application is made for a

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site within Flood Zones 2 & 3 and no other site of lower flood risk is available.

Flood defence Flood defence infrastructure, such as flood walls and embankments, intended to protect an area against flooding, to a specified standard of protection.

Flood event A flooding incident characterised by its level or flow hydrograph.

Flood Hazard The potential risk to life and potential damage to property resulting from flooding

Flood Map A map produced by the Environment Agency providing an indication of the likelihood of flooding within all areas of England and Wales, assuming there are no flood defences. Only covers river and sea flooding.

Flood probability The estimated probability of a flood of given magnitude occurring or being exceeded in any specified time period. See also annual flood probability.

Flood risk An expression of the combination of the flood probability and the magnitude of the flood event and their consequences (such as loss, damage, harm, distress and disruption).

Flood risk A study to assess the risk of a site or area flooding, and to assessment assess the impact that any changes or development in the site or area will have on flood risk.

Flood risk Any measure which reduces flood risk such as flood management defences, flood resilience measures, flood warning. measure

Flood Zones Flood Zones have been produced by the Environment Agency in response to Planning Policy Guidance Note 25 (PPG25, which is now superseded by Planning Policy Statement 25). They provide planning authorities with quality assured flood risk data. The zones show the area at risk if there were no defences and are classified as follows:

Zone 1 (little or no risk) – the area that falls outside the extent of the flood with an annual probability of 0.1% (1 in 1000 year) fluvial and tidal.

Zone 2 (low to medium risk) – the area between the extent of the flood with an annual probability of 0.1% (1 in 1000 year) fluvial and tidal and the extent of the flood with an annual

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probability of 1% (1 in 100 year) fluvial/ 0.5% (1 in 200 year) tidal.

Zone 3 (high risk) – the extent of the flood with an annual probability of 1% (1 in 100 year) fluvial or 0.5% (1 in 200 year) tidal

Floodplain Area of land that borders a watercourse, an estuary or the sea, over which water flows in time of flood, or would flow but for the presence of flood defences where they exist.

Fluvial flooding Flooding from a river or other watercourse.

Freeboard Vertical distance from the normal water surface to the top of a flood defence or river/canal bank.

Functional floodplain Land where water has to flow or be stored in times of flood. It includes the land which would flood with an annual probability of 1 in 20 (5%) or greater in any year or is designed to flood in an extreme (0.1%) flood, or at another probability to be agreed between the Local Planning Authority and the Environment Agency, including water conveyance routes.

Greenfield land Land that has not been previously developed.

Greenfield runoff rate The rate of runoff that would occur from the site in its undeveloped (and therefore undisturbed) state.

Groundwater Water in the ground, usually referring to water in the saturated zone below the water table.

Groundwater flooding Flooding caused by groundwater escaping from the ground when the water table rises to or above ground level.

Highway authority A local authority with responsibility for the maintenance and drainage of highways maintainable at public expense.

Historic Flood Map This shows the mapped extents of known (and validated) historical flooding.

Hydrograph A graph that shows the variation with time of the level or discharge in a watercourse.

Local Development Documents that set out the spatial strategy for local planning Documents authorities which comprise development plan documents.

Local Development Framework which forms part of the statutory development Framework plan and supplementary planning documents which expand policies in a development plan document or provide additional

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detail. Local planning Body responsible for planning and controlling development, authority through the planning system.

Local Resilience A group required under the Civil Contingencies Act 2004 who Forum is responsible for the coordination of emergency planning within local areas.

Main River A watercourse designated on a statutory map of Main rivers, maintained by Department for Environment, Food and Rural Affairs (DEFRA).

Along rivers the references to the left hand and right hand banks relate to the direction viewed when someone is facing downstream.

Mitigation measure A generic term used in this guide to refer to an element of development design which may be used to manage flood risk to the development, or to avoid an increase in flood risk elsewhere.

Ordinary watercourse A watercourse which is not a private drain and is not designated a Main river.

Overland flow Flooding caused by surface water runoff when rainfall flooding intensity exceeds the infiltration capacity of the ground, or when the soil is so saturated that it cannot accept any more water.

Passive flood plain Areas that are within the “natural” floodplain but are not now subject to frequent flooding, because of the presence of flood alleviation measures.

Pluvial flooding Flooding caused by rain.

Precautionary An approach to risk management that can be applied in principle circumstances of scientific uncertainty, reflecting a perceived need to take action in the face of a potentially serious risk without waiting for results of scientific research.

Residual risk The risk which remains after all risk avoidance, reduction and mitigation measures have been implemented.

Return period A term sometimes used to express flood probability It is the long term average interval between events of a given magnitude which have the same annual exceedence probability of occurring e.g. a flow with a return period of 1 in 100 years will be equalled ore exceeded on average once in every 100 years). However, this does not imply regular

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occurrence, more correctly the 100 year flood should be expressed as the event that has a 1% annual probability of being met or exceeded in any one year.

Risk of occurrence The risk (expressed as a percentage) of a fluvial or tidal flood event being met or exceeded in any one year.

River flooding See fluvial flooding.

Runoff The flow of water from an area caused by rainfall. This occurs if the ground is impermeable or saturated, or if rainfall is particularly intense.

Sequential test A risk-based approach to flood risk assessment in accordance with Planning Policy Statement 25, applied through the use of flood risk zoning, where the type of development that is acceptable in a given zone is dependent on the assessed flood risk of that zone and flood vulnerability of the proposed development.

Standard of The design event or standard to which a building, asset or protection area is protected against flooding, generally expressed as an annual exceedence probability.

Strategic flood risk A study to examine flood risk issues on a sub-regional scale, assessment (SFRA) typically for a river catchment or local authority area during the preparation of a development plan.

Sustainable drainage A sequence of management practices and control structures, systems (SUDS) often referred to as SUDS, designed to drain surface water in a more sustainable manner. Typically, these techniques are used to attenuate rates of runoff from development sites.

Vulnerability Classes PPS25 provides a vulnerability classification to assess which uses of land maybe appropriate in each flood zone.

Watercourse Any natural or artificial channel that conveys surface water.

Water Cycle Strategy Provides a plan and programme of Water Services Infrastructure implementation.

Water table The level of groundwater in soil and rock, below which the ground is saturated.

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10 REFERENCES

Aylesbury Vale District Council, (2008), Direction of housing growth at Aylesbury, Consultation on options. Local Development Framework, October 2008.

Communities and Local Government, (2008), Planning Policy Statement 25: Development and Flood Risk - Practice Guide, June 2008.

Communities and Local Government, (2007), Development and Flood Risk: A Practice Guide Companion to PPS25 ‘Living Draft’, February 2007.

Communities and Local Government, (2006), Planning Policy Statement 25: Development and Flood Risk, December 2006.

Environment Agency, Thames Catchment Flood Management Plan, July 2008.

Department for Environment, Food and Rural Affairs (2005) Making space for water - Taking forward a new Government strategy for flood and coastal erosion risk management in England. First Government response to the autumn 2004 Making Space for Water consultation exercise, March 2005.

Environment Agency, (2005), Guidance for Identification of Areas Benefiting from Defences and Producing the Flood Map, July 2005.

Environment Agency, (2001), Bear Brook Flood Storage Reservoir, Report of an Inspection carried out under Section 10 of the Reservoirs Act 1975, Nov 2001.

Environment Agency, (1998), Grand Union Canal/Upper Thame Flood Study, April 1998.

Peter Bretts and Associates, (2008), Bear Brook & Upper Thame Flood Risk Mapping Study, Draft Final Modelling Report, January 2008.

Royal Haskoning, (2007), Aylesbury Vale Strategic Flood Risk Assessment - Level 1 Report, April 2007.

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FIGURES

Aylesbury Town Level 2 SFRA 9T5436/D1/R003/302072/PBor Final Report - 71 - 9 April 2009