swindon sfra level 1 sfra report 28aug08 · and identifies the need for 35,000 new dwellings in...

Swindon Borough Council Strategic Flood Risk Assessment Level 1 SFRA - Final Report (Volume I) August 2008 Halcrow Group Limited

Halcrow Group Limited Burderop Park Swindon Wiltshire SN4 0QD Tel +44 (0)1793 812479 Fax +44 (0)1793 812089 www.halcrow.com © Halcrow Group Limited 2008

Halcrow Group Limited Burderop Park Swindon Wiltshire SN4 0QD Tel +44 (0)1793 812479 Fax +44 (0)1793 812089 www.halcrow.com © Halcrow Group Limited 2008

Swindon Borough Council Strategic Flood Risk Assessment Level 1 SFRA - Final Report (Volume I) August 2008 Halcrow Group Limited

Halcrow Group Limited Burderop Park Swindon Wiltshire SN4 0QD Tel +44 (0)1793 812479 Fax +44 (0)1793 812089 www.halcrow.com

Swindon Borough Council Strategic Flood Risk Assessment Level 1 SFRA - Final Report (Volume I)

Contents Amendment Record This report has been issued and amended as follows: Issue Revision Description Date Signed 1 0 First draft 12 September 2007 2 1 Second draft 7 February 2008 3 2 Third draft 14 March 2008 4 3 Fourth draft 8 May 2008 5 4 Final 28 August 2008

T Houghton CJ Weeks D Kensett A Corner A McConkey M Barker P Crozier D Wilson PS Rayner

Swindon Borough Council Strategic Flood Risk Assessment

Table of Contents

Executive Summary

Volume I - Final Report (contents list below)

Volume II - SFRA Flood Maps

1 Introduction.................................................................................................... 1

2 The Swindon SFRA Area .............................................................................. 2 2.1 Overview ..................................................................................................................... 2 2.2 Planning Context......................................................................................................... 2 2.3 Thames Corridor Catchment....................................................................................... 3 2.4 Swindon Rivers ........................................................................................................... 3 2.5 Geology....................................................................................................................... 3 3 SFRA Approach & Methodology .................................................................. 5 3.1 Overview ..................................................................................................................... 5 3.2 Outcomes of the SFRA Process ................................................................................. 6 3.3 The Sequential Test .................................................................................................... 6 3.4 The Exception Test ..................................................................................................... 8 3.5 Level 1 SFRA methodology ........................................................................................ 8 3.6 The need for Level 2 SFRA......................................................................................... 9 4 Policy framework......................................................................................... 11 4.1 Overview ................................................................................................................... 11 4.2 Planning policy framework ........................................................................................ 11 4.3 National policy........................................................................................................... 11 4.4 Regional planning policy ........................................................................................... 13 5 Data sources ................................................................................................ 16 5.1 Overview ................................................................................................................... 16 5.2 Consultation process................................................................................................. 16 5.3 Environment agency Flood Zone maps .................................................................... 17 5.4 Detailed hydraulic modelling ..................................................................................... 17 5.5 Localised flooding ..................................................................................................... 17 6 Assessment of potential causes of flooding............................................. 19 6.1 Overview ................................................................................................................... 19 6.2 SFRA flood maps ...................................................................................................... 19 6.3 Localised flooding ..................................................................................................... 19 6.4 PPS25 Flood zones .................................................................................................. 28 6.5 Existing services/infrastructure within PPS25 Flood zones....................................... 30 7 Assessment of flood risk management practices .................................... 32 7.1 Overview ................................................................................................................... 32 7.2 Flood risk managment responsibilities ...................................................................... 32 7.3 Existing flood defences ............................................................................................. 32 7.4 Flood warning areas.................................................................................................. 34 7.5 Variation in actual flood risk ...................................................................................... 35


8 Assessment of the capacity for the use of SUDS..................................... 36 8.1 Overview ................................................................................................................... 36 8.2 Types of SUDS systems ........................................................................................... 36 8.3 SUDS at the planning stage...................................................................................... 36 8.4 Application of SUDS within Swindon......................................................................... 37 9 Flood risk & climate change....................................................................... 40 9.1 Overview ................................................................................................................... 40 9.2 Flood risk & climate change ...................................................................................... 40 9.3 Integrated urban drainage......................................................................................... 41 9.4 Potential increase in flood risk caused by future development ................................. 42 9.5 Potential effect of flood defence failure (residual risk) .............................................. 42 9.6 Extent and costs required to raise flood defence standard to 1%............................. 43 9.7 Sustainability of land use in medium and high risk flood areas................................. 44 10 Appropriate planning responses for low, medium & high risk areas ..... 45 10.1 Overview ................................................................................................................... 45 10.2 Strategic flood risk management studies – Environment Agency ............................. 45 10.3 Indicative Sites .......................................................................................................... 46 10.4 Policy Recommendations.......................................................................................... 47 10.5 Planning Response Matrix ........................................................................................ 49 10.6 Development within areas affected by surface water and sewer flooding................. 50 11 Measures to make development permissible in higher risk flood zones51 11.1 Overview ................................................................................................................... 51 11.2 Potential measures ................................................................................................... 51 11.3 Recommendations for reducing existing flood risks.................................................. 52 11.4 Need for flood risk assessment................................................................................. 52 11.5 Monitoring post development runoff .......................................................................... 55 12 Conclusions & Recommendations ............................................................ 56

References.............................................................................................................. 59

APPENDIX A Audit trail database

APPENDIX B Geology & soils

APPENDIX C Model reviews

APPENDIX D Locations with blockage potential APPENDIX E July 2007 flood study of Whitehill Stream, Swindon

APPENDIX F Localised flooding database

APPENDIX G Sewer flooding

APPENDIX H Site visit APPENDIX I Main defences

APPENDIX J PPS25 flood risk vulnerability classification

APPENDIX K Using computer river modelling as part of a FRA Best practice guidance

APPENDIX L Swindon’s Flood Review: July 2007 An investigation into the causes and flood risk management options


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1 Introduction

In June 2007 Swindon Borough Council commissioned Halcrow Group Ltd to produce this Strategic Flood Risk Assessment in accordance with national Planning Guidance and the Environment Agency’s guidance, which includes:

Planning Policy Statement 25: Development and Flood Risk (2006)

Development and Flood Risk, a Practice Guide Companion to PPS25 (2007).

The draft Regional Spatial Strategy (RSS) identifies Swindon as a key regional growth area and identifies the need for 35,000 new dwellings in Swindon and a component of the North Wiltshire District by 2026. The Examination in Public Panel Report into the draft RSS has recommended increasing the numbers to 37,200 at Swindon and in the remainder of the Borough, including 3,000 units in North Wiltshire at Swindon. The draft RSS also identifies an area of search to accommodate an urban expansion of 12,000 dwellings to the east of Swindon, informed through the Swindon Joint Study (2005).

The Joint Study concluded that, in order to ensure that this scale of development can be accommodated without impacting on flood risk, a Strategic Flood Risk Assessment (SFRA) is required to explore different options to ensure that informed decisions can be taken on allocating sites using the sequential test and addressing flood risk issues at the strategic plan scale. This SFRA will be used to accommodate growth whilst protecting areas at risk or that are likely to be at risk from flooding in the future.

The Level 1 SFRA approach taken has been agreed with Swindon Borough Council and the Environment Agency. This assessment level is defined in the Practice Guide Companion to PPS25, as a desk-based study using existing information to inform the planning process - for application of the Sequential Test as per PPS25 Table D1 and to determine if the Exception Test is likely to be necessary.

Outside the specific role of the SFRA in relation to appraising, managing and reducing flood risk related to development, PPS25 sets out important "efficiency" issues linked to the sourcing and assembly of data, models and information that enable a strategic approach to be taken to flood and surface water management at the local level.

The SFRA output is relevant not only to planning and development control, but also site specific flood risk assessments and mapping for emergency planning, alleviation of flood risk within existing urban development and surface water management plans.

The exceptional flooding in July 2007 and other past flood events has informed this SFRA. The 2007 event is notable as it caused widespread disruption across Swindon and emphasised the flooding problems of some local rivers and the urban drainage systems.

This SFRA report is a ‘living’ document in that as new information becomes available updates will be made to ensure that the best information is used to guide the site selection process for future developments.

Flooding in West Swindon July 2007


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2 The Swindon SFRA Area

2.1 Overview

The Swindon SFRA area is 26.3km2 and encompasses the whole of Swindon Borough and a component of North Wiltshire District (Tile A1 in Volume II).

Swindon Borough is located to the north-east of the UK South West region, and adjoins the counties of Wiltshire, Gloucestershire, Oxfordshire and the former county of Berkshire.

Swindon is situated within the M4 corridor and is on the mainline railway between London and Bristol. With a population of approximately 185,000, the borough is predominantly urban in character.

2.2 Planning Context

As with many towns, Swindon is under pressure to provide a contribution towards increasing housing stock over the next decade and beyond, and the draft South West Regional Spatial Strategy (RSS) identifies Swindon as a key regional growth area.

An additional 35,000 new dwellings are proposed for Swindon to 2026 in the draft RSS and the public examination of the draft RSS has recommended that this be increased to 37,200 at Swindon and within the Borough, including 3,000 in North Wiltshire at Swindon. The same document also identifies an area of search to accommodate an urban extension of 12,000 dwellings to the east of Swindon

The scale of the proposed developments has prompted a Water Cycle Strategy to be undertaken, a study in which Halcrow has been actively involved. Phase 1 of this study was completed in 2007, with one of the six key messages recommending a SFRA be completed to further the understanding of flood risk in the Borough.

As well as being an important consideration in the Regional Spatial Strategy, issues regarding flood risk are crucial in supporting the wider Local Development Framework. In particular, the SFRA is needed to inform Swindon Borough Council’s Core Strategy Development Plan Document (DPD), and will also be used to inform the Site Allocations DPD.


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2.3 Thames Corridor Catchment

The Swindon Borough area forms part of the Thames Corridor catchment, and there are two main rivers that drain the Swindon area into the River Thames, the River Cole and the River Ray (Tile A1 in Volume II). Watercourses, drainage ditches and catchment areas within the allocated development sites and areas of search for development have been identified on the map for reference.

The underlying geology dictates how the natural drainage catchment responds to rainfall, which in turn dictates the natural course of rivers. Urbanisation over time can alter the runoff characteristics of the catchment making the natural drainage system incapable of adequately coping with rainfall runoff, and increasing the risk of flooding. Development over the last century in Swindon will have already altered the natural runoff and new development may further alter the natural drainage patterns further increasing the risk of flooding.

2.4 Swindon Rivers

The River Ray flows in a northerly direction from its source near Wroughton towards the urban area of Swindon.

The Ray drains a relatively flat, mainly impervious catchment. The catchment is largely agricultural, except where it passes through the village of Wroughton to the south and in Swindon to the west.

The River Cole flows from its source in Swindon in a north-easterly direction, being joined by several tributaries along its course.

The Cole catchment is low-lying, with its headwaters draining a chalk escarpment, but the majority of the catchments geology is impervious clay. The catchment is largely rural, except for the urban area of Swindon which is situated in the west of the catchment.

2.5 Geology

The geological and hydrogeological setting provides an indication of the potential for groundwater flooding and for an understanding of the role of infiltration drainage either within the overall natural water cycle, or as part of sustainable drainage systems.

The surface geology includes drift deposits which primarily comprise the alluvium associated with the River Ray and River Cole. These deposits tend to be relatively thin and limited in extent. Further to the north, there are more significant sand and gravel drift deposits associated with the River Thames.

River Ray

River Cole

River Thames

SFRA boundary


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The drift deposits tend to have a shallow water table, in hydraulic continuity with, and drained by adjacent rivers and streams. When levels in these watercourses are high less groundwater will be able to drain away. Furthermore, when in flood there is potential for leakage out of the watercourse into the sand/gravel aquifer which may cause localised ‘groundwater’ flooding/ waterlogging.

Clay strata underlie much of the northern (West Walton & Oxford Clay) and central (Ampthill, Kimmeridge and Gault Clay) areas. A mix of inter-bedded sandstones, limestones and clays (Corallian deposits) are present in an east west band running from north Swindon to Highworth. This strata can form a relatively thin aquifer and groundwater may be close to the surface, such that springs may readily emerge in valleys and other low lying areas.

Source: Derived from 1:50,000 BGS Digital Data under Licence 2006/017A British Geological Survey. ©NERC.

Highly permeable chalk deposits are present across the southern extent of the SFRA area, south of a line approximately between Wroughton, Chisledon, Wanborough and Bishopstone, i.e. to the extreme south of the SFRA area. The chalk strata dip gently southward. At the foot of the chalk scarp, groundwater emerges from the chalk in a series of northward flowing springs. These form the headwaters of the River Cole and the River Ray.

The Chalk is classified by the Environment Agency as a major aquifer and is the most significant aquifer in southern England. The Environment Agency undertake monitoring of groundwater levels throughout the Chalk – there are records available for about 20 observation boreholes in the area.

Further detail on the geological strata present around the Swindon area, and a summary of both their hydrogeological properties and their potential for infiltration drainage are summarised in Appendix B and in Section 8.4.

SFRA boundary


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3 SFRA Approach & Methodology

3.1 Overview

This SFRA will feed directly into the preparation of Local Development Documents, including the Core Strategy and Site Allocation DPDs (see Figure 3.1). In addition, the SFRA allows Swindon Borough Council to:

Prepare appropriate policies for the management of flood risk;

Inform the sustainability appraisal so that flood risk is taken account of, when considering options and in the preparation of strategic land use policies;

Identify the level of detail required for site-specific Flood Risk Assessments (FRAs);

Determine the acceptability of flood risk in relation to emergency planning capability.

Figure 3.1 How the SFRA is used to inform site allocations Source: Development and Flood Risk: A Practice Guide Companion to PPS25 (2007)


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The SFRA is consistent with the aims of PPS25 planning policy on development and flood risk, namely, to ensure that flood risk is taken into account at all stages of the planning process to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas at highest risk.

Where new development is necessary in areas at highest risk, the policy aims to make it safe without increasing flood risk elsewhere and where possible, reducing flood risk overall. Safe in the context of this study means that dry pedestrian egress is possible through the floodplain and emergency vehicles can gain access.

Where development cannot be located in Flood Zone 1 Swindon Borough Council will need to apply the Sequential Test to land use allocations and, where necessary, the Exception Test.

3.2 Outcomes of the SFRA Process

This SFRA provides sufficient data and information to enable Swindon Borough Council to apply the Sequential Test to land use allocations and, where necessary, the Exception Test (see Sections 3.3 and 3.4).

PPS25 also indicates that Sustainability Appraisals (SAs) should be informed by the SFRA for their area. Sustainability Appraisals are required for all LDFs under the Town and Country Planning (Local Development - England) Regulations 2004. The purpose is to promote sustainable development through better integration of sustainability considerations in the preparation and adoption of plans.

The Town and Country Planning Regulations stipulate that SAs for LDFs should meet the requirements of the Strategic Environmental Assessment (SEA) Directive, which ensures that the environmental consequences of certain plans and programmes are identified and assessed during their preparation and before their adoption.

This SFRA will be used as a tool by Swindon Borough Council for the production of development briefs, setting constraints, identifying locations of emergency planning measures and specifying the requirements of Flood Risk Assessments.

It is important to reiterate that PPS25 is not applied in isolation as part of the planning process. In formulating policy and allocating land for future development, Swindon Borough Council must also meet the requirements of other planning policy. Clearly a careful balance must be sought in these instances, and the SFRA aims to assist in this process through the provision of a clear and robust evidence base upon which informed decisions can be made.

3.3 The Sequential Test

Swindon Borough Council will apply the Sequential Test to demonstrate that there are no reasonably available sites in areas with less risk of flooding that would be appropriate to the type of development or land use proposed. Figure 3.2 shows the Sequential Test process as advocated in PPS25.


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Preference should be given to locating new development in Flood Zone 1. If there is no reasonably available site in Flood Zone 1, the flood vulnerability (see PPS25 Table D.2) of the proposed development can be taken into account in locating development in Flood Zone 2 (Medium Probability) and then Flood Zone 3 (High Probability).

Within each Flood Zone new development should be directed to sites with lower flood risk (towards the adjacent zone of lower probability of flooding) from all sources as indicated by the SFRA.

Figure 3.2 Application of the Sequential Test Source: Development and Flood Risk: A Practice Guide Companion ‘Living Draft’ (2007)


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3.4 The Exception Test

The Exception Test can be applied if following the Sequential Test it is not possible, or consistent with wider sustainability objectives, for the development to be located in zones of lower probability of flooding. This second test provides a method of managing flood risk while still allowing necessary development to occur.

The Exception Test is only appropriate for use when there are large areas in Flood Zones 2 and 3, where the Sequential Test alone cannot deliver acceptable sites, but where some continuing development is necessary for wider sustainable development reasons (the need to avoid social or economic blight and the need for essential civil infrastructure to remain operational during floods).

The Exception Test may also be appropriate to use where restrictive national designations such as landscape, heritage and nature conservation designations, e.g. Areas of Outstanding Natural Beauty (AONBs), Sites of Special Scientific Interest (SSSIs) and World Heritage Sites (WHS), prevent the availability of unconstrained sites in lower risk areas.

For the Exception Test to be passed:

a) It must be demonstrated that the development provides wider sustainability benefits to the community which outweigh flood risk, informed by SFRA where prepared.

If the Development Plan Document 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 flood risk elsewhere, and, where possible, will reduce flood risk overall.

It is possible, subject to the sequential test that the exception test may need to be applied for sites within the areas already allocated for development and the areas of search for development as part of these areas fall within Flood Zone 3 (see Section 10.3).

3.5 Level 1 SFRA methodology

Level 1 SFRA is defined in the Practice Guide Companion to PPS25 as a desk-based study using existing information to allow application of the Sequential Test (see Figure 3.1) and to identify if application of the Exception Test is likely to be necessary.

The main tasks listed below were undertaken in the preparation of this SFRA:

Understanding the planning context (see Section 4) Reviewed the Local Development Framework process and Local Policy to get a clear picture of the challenges faced by the planning team, and the various opportunities and constraints guiding the site allocation process.


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Data collection (Section 5) Reviewed and collated the available data regarding flood risk within the SFRA area.

Assessment of potential causes of flooding (Section 6) Produced a series of GIS maps using the data gathered. The main outputs are PPS25 Flood Maps for the entire study area taking into account flooding from all sources, including climate change impacts up to the year 2115. Other maps contain information on flood defences, flood storage and flood warning areas.

Hardcopy maps are provided in Volume II of the SFRA report.

Assessment of flood risk management practices (Section 7) Reviewed the existing flood defences, flood warning areas and emergency planning procedures, together with the Environment Agency’s planned future improvements to their flood warning service.

Assessment of the capacity for the use of SUDS (Section 8) Reviewed the types of Sustainable Drainage Systems (SUDS) available and their applicability within the Swindon SFRA area.

Flood risk and climate change (Section 9) Reviewed the potential impact of climate change on the current flood zones and other sources of flooding, together with the potential increase in flood risk caused by future development and the failure of flood defences. Sustainable land uses for medium and high risk flood areas are detailed.

Appropriate planning responses for low, medium and high risk areas (Section 10) Advised planning recommendations to enable appropriate planning responses with regards to flood risk.

Measures to make development permissible in higher risk flood zones (Section 11) Development within higher flood risk areas should only be considered if lower risk sites cannot be identified through application of the sequential and exception tests. Should development be deemed appropriate for higher risk sites measures that may be adopted to minimise the risk of flooding were reviewed.

3.6 The need for Level 2 SFRA

Where the Exception Test is necessary, due to there being an insufficient number of suitably available sites for development within zones of lower flood risk or due to possible increases in flood risk arising from climate change, the scope of the SFRA may need to be widened to a Level 2 assessment.

This increased scope involves a more detailed review of flood hazard (flood probability, flood depth, flood velocity, rate of onset of flooding) taking into account the presence of flood risk management measures such as flood defences.


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Level 2 SFRA often includes 2D modelling and breach/overtopping analysis for certain locations. Only one of the main defences (on National Flood and Coastal Defence Database (NFCDD); see Appendix H) within the study area provides flood protection up to the 1 in 100-year return period flood in their current state (see Section 7.1).

As a result, Level 2 SFRA may be required if infill development is proposed behind this defence or downstream of Coate Water or Stanton Park Lake (see Section 6.3.6). However, this cannot be fully determined until the Sequential Test has been undertaken on all possible site allocations.

Level 2 SFRA, where required, involves the following:

Appraisal of any likely future policy for flood risk management;

Appraisal of the probability and consequence of breach or overtopping of defences and water retaining structures;

Preparation of maps showing the distribution of flood risk across Flood Zones;

Provision of guidance on appropriate policies for making sites which satisfy parts a) and b) of the Exception Test safe; and the requirements for satisfying part c) of the Exception Test;

Guidance on preparation of FRAs for sites with varying flood risk across Flood Zones.


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Coate Water, Swindon

4 Policy framework

4.1 Overview

This chapter outlines the planning policy framework relevant to this Strategic Flood Risk Assessment (SFRA). Information contained in the SFRA on flooding and flood risk will provide evidence to facilitate the preparation of robust policies for flood risk management.

The SFRA will inform the Sustainability Appraisal of Local Development Documents and will enable informed decisions to be made relating to land use and development allocation within the respective Development Plan Documents.

4.2 Planning policy framework

The UK planning system has a comprehensive hierarchy of policies and plans, beginning with national guidance which provides a broad framework for regional plans through to development plans at the local level.

Development plans are intended to provide clear guidance for prospective developers. They are prepared following public and stakeholder involvement and are intended to reconcile conflicts between the need for development and the need to protect the wider built and natural environment.

The Government is currently implementing reforms to the planning system with:

Planning Policy Statements (PPS) replacing Planning Policy Guidance (PPG);

Regional Spatial Strategies (RSS) replacing Regional Planning Guidance (RPG);

Local Development Frameworks (LDF) replacing Structure and Local Plans and Unitary Development Plans.

The relevant policy documents and their significance for the SFRA are described below.

4.3 National policy

4.3.1 Planning policy statement 1: Creating sustainable communities (2005) PPS1 sets out the Government’s objectives for the planning system. It confirms that good planning should deliver the right development in the right place and time, and protect the environment. It identifies sustainable development as the core principle underpinning planning and requires that development plans ensure it is pursued in an integrated manner.

4.3.2 Planning policy Statement 3: Housing (2006) PPS3 has been developed in response to recommendations in the Barker Review of Housing Supply (March 2004). Its principal aim is to underpin the necessary step change in housing delivery, improving the supply and affordability of housing in all communities including rural areas.


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PPS3 states that the Government’s key housing policy goal is to ensure that everyone has the opportunity of living in a decent home, which they can afford, in a community where they want to live.

The specific outcomes that the planning system should deliver are:

• well designed, high quality housing that is built to a high standard; • a mix of market and affordable housing for all households in all areas; • a sufficient quantity of housing, taking into account need and demand and seeking

to improve choice; • housing developments in suitable locations offering a good range of community

facilities and with good access to jobs, key services and infrastructure; • a flexible, responsive supply of land; which is used efficiently and effectively,

including the use of previously developed land.

Housing policies should help to deliver sustainable development objectives, in particular seeking to minimise environmental impact taking account of climate change and flood risk, and take into account market information, in particular housing need and demand.

4.3.3 Planning policy statement 9: biodiversity and geological conservation (2005) PPS9 sets out policies on protection of biodiversity and geological conservation through the planning system. The broad aim is that development should have minimal impacts on biodiversity and geological conservation interests and enhance them where possible. Appropriate weight should be attached to the need to protect international and national designated sites.

4.3.4 Planning policy guidance 15: planning and the historic environment (1994) PPG15 sets out policies on the protection of the historic environment and recognises that planning plays an important role in preserving built and natural heritage.

4.3.5 Planning policy guidance 17: planning for open space and recreation (2002) PPG17 recognises the importance that public open spaces, green areas and recreational rights of way can play in supporting regeneration and contributing to local quality of life.

4.3.6 Planning policy statement 25: development and flood risk (2006) PPS25 sets out a plan led approach to flood risk. It confirms that all forms of flooding and their impact on the natural and built environment are material planning considerations. It clarifies the sequential test that matches types of development to degrees of flood risk and strengthens the requirement to include flood risk assessments at all levels of the planning process.

Regional planning bodies and local planning authorities (LPA) should, inter alia, reduce flood risk by safeguarding land from development that is required for current and future flood management e.g. conveyance and storage of flood water and flood defences.

4.3.7 Town and country planning amendments Amendments to the Town and Country Planning (General Development Procedure) Order 1995 came into force on 1 October 2006 introducing further requirements for LPA to consult the Environment Agency before determining applications for development in flood risk areas.


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The Town and Country Planning (Flooding) (England) Direction 2007 was published in December 2006. To safeguard against inappropriate development in flood risk areas, it introduces a requirement for LPA to notify the Secretary of State of any application for major development (e.g. 10 or more dwellings) in a flood risk area which it proposes to approve against Environment Agency advice. The Direction came into force on 1 January 2007.

4.4 Regional planning policy

Regional Planning Guidance for the South West (RPG10) covers the period up to 2016 and sets the regional planning policy framework for the area.

RPG10 acknowledges that climate change is likely to exacerbate the risk of flooding and requires that development should be guided away from areas at risk or likely to be at risk in future from flooding (Policy RE2; Flood Risk).

RPG10 is now being taken forward as the Regional Spatial Strategy for the South West (RSS) and the Draft RSS was the subject of an Examination in Public between April and July 2007. The Examination in Public Panel Report was published in January 2008 and Proposed Changes will be published for consultation (expected in May 2008) before issue of the approved RSS, expected in late 2008.

The Draft RSS sets out the dwelling requirements for Swindon Borough Council (Policies SR7 and SR8) over the period 2006-2026. This equates to an average of about 1,750 dwellings per annum (DPA) within and adjoining Swindon’s urban area, distributed as follows:

• average of about 950 DPA within the Swindon Urban Area; • about 12,000 dwellings, at a strategic urban extension to the east of Swindon; • average of 100 DPA at smaller scale urban extensions within Swindon Borough; • average of 50 DPA at the urban extension to Swindon in the North Wiltshire District • average of 50 DPA in the remainder of Swindon Borough.

The Panel Report into the Draft RSS has recommended changes to Policies SR7 and SR8 to be replaced by SR2.2 and SR2.4. This equates to an average of about 1,860 dwellings per annum (DPA) within and adjoining Swindon’s urban area, distributed as follows:

• average of about 950 DPA within the Swindon Urban Area; • about 12,000 dwellings, at a strategic urban extension to the east of Swindon; • average of 100 DPA at smaller scale urban extensions within Swindon Borough; • average of 150 DPA at the urban extension to Swindon in the North Wiltshire District • average of 60 DPA in the remainder of Swindon Borough.

The SFRA should take account of whichever policy is carried and finally adopted in the Regional Spatial Strategy.


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The Draft RSS makes a provision for job growth within the Swindon ‘Travel to Work Area’ (TTWA)1 of 32,000 jobs over the same period, with the primary focus for this development being towards Swindon.

Policy F1 (Flood Risk) prioritises the defence of existing properties from flooding and the location of new development in areas that have little or no risk from flooding. In taking into account the risk of climate change and the increasing risk of flooding, Policy F1 seeks to:

• defend existing properties and, where possible, locate new development in places with little or no risk of flooding;

• protect flood plains and land liable to tidal or coastal flooding from development; • follow a sequential approach to development in flood risk areas; • use development to reduce flood risk through location, layout and design; and • identify areas of opportunity for managed realignment to reduce flood risk and

create new wildlife areas. The Panel Report adds that “in preparing LDDs local authorities should have regard to the Regional Flood Risk Assessment”. Policy SD2 (Climate Change) seeks to prepare the region for the effects of global warming by avoiding the need for development in flood risk areas and incorporating measures in design and construction to reduce the effects of flooding.

4.4.1 Wiltshire and Swindon Structure Plan - 2016 (2006) The Structure Plan provides a strategic policy framework for land use planning, development and transport across the administrative areas of Wiltshire County and Swindon Borough up to 2016. The Structure Plan was formally adopted on 1 April 2006, but as noted above, it will be formally superseded by the RSS and the respective District and Borough Core Strategies.

Policy C5 (The Water Environment) seeks to protect the role of flood plains and to ensure that development does not increase the risk of flooding. In addition, it seeks to inform sites/proposals for new development through a risk-based sequential approach. It states:

“The water environment, including surface waters, floodplains and groundwater resources, should be protected by the control of development. The strategic planning authorities will support initiatives which seek to protect, restore or enhance the natural elements of the river or waterway environment, and which improve the quality and efficient use of water”.

The Structure Plan sets out the proposed housing allocations in the period to 2016. This includes 26,000 additional dwellings between 1996-2016, and equates to a development rate of 1,300 DPA, within the Swindon Borough.

1 TTWA is defined as “Zones in which the bulk of the resident population also work”.


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4.4.2 Swindon Borough Council Local Plan 2011

The Swindon Borough Local Plan (2011) was adopted in July 2006. As required by the Planning and Compulsory Purchase Order 2004, this will be replaced by a Local Development Framework which is currently at an early stage of preparation.

Within the adopted Local Plan there are three particularly relevant policies:

Policy ENV23 (Protection of Floodplains) requires that the management and reduction of flood risk through a precautionary approach is crucial in minimising potential threats in addition to seeking to reduce the effect of development elsewhere.

This Policy ENV23 states: “In areas at potential risk from flooding, as defined by the Environment Agency, development shall only be permitted where appropriate flood protection and full compensation works for losses in floodplain storage and conveyance are determined and provided as part of the development, including sustainable drainage systems.”

Policy ENV24 (Riparian and Other Buffer Zones) seeks to ensure that biodiversity is safeguarded within areas near to, or proposed for development and that development proposals would not be detrimental or materially affect water courses and river banks.

This Policy ENV24 states: “Development that includes or adjoins rivers, watercourses and other water bodies shall only be permitted where development free buffers extending outwards from the feature are included in the proposal, their scale and width reflecting the intrinsic natural value of each specific zone”.

Policy H1 (Housing Land Provision) seeks to ensure that sufficient land is identified to meet the Wiltshire Structure Plan 2011 requirement for approximately 23,000 dwellings to be provided between 1991 and 2011.

In moving towards producing its Local Development Framework, Swindon B.C. published its Core Strategy and Generic Development Control Policies: Issues and Options Paper for consultation in April 2007. This draft document outlines three options that could accommodate the growth of existing urban areas:

Option 1: Maximise development within Swindon’s built-up area by increasing the potential of previously developed and undeveloped land;

Option 2: Proposed new growth within Swindon, outside existing locations. The early development of greenfield extensions include the Northern Development Area, Wichelstowe, the Eastern Development area and Commonhead;

Option 3: Contain growth within existing allocations and through proposals within the RSS. This would involve increasing the capacity of one or more of the above sites, in order to make up for the loss of unidentified small strategic extensions.

This SFRA provides information on the implications of proceeding with the above options and has assisted informing the Core Strategy.


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5 Data sources

5.1 Overview

The SFRA makes the best use of the significant amount of information that exists with respect to flood risk - held by Swindon Borough Council, the Environment Agency, Thames Water, the Highways Agency and other key consultees. The key data sources are:

Environment Agency Flood Zone Maps and detailed flood risk mapping outputs.

Localised flooding information from Swindon Borough Council, the Environment Agency, North Wiltshire District Council, Thames Water and the Highways Agency.

Detailed information on the major flood defences and flow control structures - from the National Fluvial and Coastal Defence Database held by the Environment Agency.

Past flood risk assessments, detailed modelling reports and the Thames CFMP.

A full data register is provided in Appendix A.

5.2 Consultation process

Consultation focussed on data collection, with the following key stakeholders consulted:

Swindon Borough Council, North Wiltshire District Council, Wiltshire County Council - Planners from Swindon Borough Council advised on site allocations (Section

4.4.2). - Drainage Engineers advised on localised flooding (Section 6.3) - Emergency Planners advised on flood incidents and emergency procedures (Section 7.4.3).

Environment Agency - advised on data availability/suitability, historical fluvial and groundwater flooding,

modelling studies, flood risk assessments, flood defences and flood warning procedures (Sections 6 & 7).

Wiltshire & Berkshire Canal Trust - advised on potential flood risks posed by the canal restoration (Section 6.3.7).

Thames Water - advised on sewer flooding and planned schemes to alleviate flooding (Section

6.3.4).

Highways Agency - advised on past flooding of the M4 and A419 (Section 6.3.4).

The Environment Agency Development Control, and Flood Risk Mapping and Data Management teams from Thames West Area office (Wallingford) provided invaluable guidance and support for this SFRA. This has been essential given the role of the Environment Agency as a Statutory Consultee under PPS25, and the need for their agreement of the scope, key findings and recommendations of the SFRA.


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As part of the consultation process, key stakeholders within Swindon Borough Council and the Environment Agency attended a workshop (August 2007) to review the draft flood maps and provide feedback on the initial findings of the SFRA.

5.3 Environment agency Flood Zone maps

The Environment Agency Flood Zone maps show the areas at risk of flooding from rivers and the sea, ignoring the presence of flood defences. The original maps have been produced from a National generalised computer model (JFlow), and are continuously being improved as new studies are undertaken, such as detailed hydraulic modelling, as more flood data and information becomes available.

5.4 Detailed hydraulic modelling

The detailed hydraulic modelling studies within the SFRA area include:

Upper Cole study (2006) - flood risk mapping and modelling commissioned by the Environment Agency

Iffley to Purton Flood Study (2004) commissioned by Swindon Borough Council

Wichelstowe (formerly Southern Development Area) modelling study (2007), commissioned jointly by Swindon Borough Council and Taylor Woodrow Development Limited

Priory Vale, Hydraulic analysis of Haydon End Farm (2005) commissioned by the North Swindon Development Company

Eastern Development Area study (2007) commissioned by the East Swindon Development Group

The flood mapping from the Upper Cole study (upstream of Acorn Bridge), included Dorcan Brook and Liden Brook has been approved by the Environment Agency, and therefore can be used for the SFRA (i.e. updates the Environment Agency Flood Zone Maps).

Technical reviews of the other studies, as detailed in Appendix C, indicate the following:

Wichelstowe flood mapping - considered appropriate for the SFRA as recently improved modelling provides greater accuracy than the Environment Agency Flood Zone (3) maps.

Priory Vale flood mapping - extends outside the limits of the Environment Agency Flood Zones, and for the SFRA provided a first indication of flood risk in this area.

Iffley to Purton flood mapping - cannot be used for the SFRA due to concern over quality/accuracy that requires further study to resolve.

Eastern Development Area - cannot be used for the SFRA due to concern over quality/accuracy that requires further study to resolve.

Only results approved by the Environment Agency should be used in future SFRA updates.

5.5 Localised flooding

Evidence of flooding within the SFRA area derives from past studies and reports and through consultation with key stakeholders. In accordance with PPS25 guidance, this has included a


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review of flooding from all sources, i.e. fluvial, groundwater, surface water, drainage and sewerage infrastructure and other artificial water bodies such as canals and reservoirs (Section 6.3). Further evidence derives specifically from the flood event in July 2007.


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6 Assessment of potential causes of flooding

6.1 Overview

This chapter details the SFRA maps produced for the Swindon area in accordance with emerging best practice and PPS25 guidance. These maps provide the information required for Swindon Borough Council to carry out the Sequential Test on possible site allocations.

This chapter presents the assessment of potential causes of flooding based on historical incidents, including the flooding in July 2007, and by reference to the Flood Zone maps. Later chapters cover the assessment of the flood management infrastructure (Section 7) and the potential implications of climate change (Section 9).

6.2 SFRA flood maps

The following SFRA maps are included in Volume II:

Tile A1: Study area, watercourses, areas of search for development and catchments within the areas of search for development (as discussed in Section 2)

Tiles B1 to B5: Historical flooding - excludes flooding on 20 July 2007

Tiles C1 to C4: Flooding on 20 July 2007

Tiles D1 to D4: SFRA Flood Zone map, 2007

Tiles E1 to E4: Defences, flood warning and storage areas (Section 7)

Tiles F1 to F4: SFRA Flood Zone map, 2025-2115 (Section 9.2)

Tiles G1 to G4: Variations in actual flood depth for Flood Zone 3 (Section 7.5)

Tile A1 is presented at 1:45,000 scale and the other tiles at 1:25,000 scale.

Historical flooding includes both fluvial and other sources of flooding (Tiles B and C).

GIS (ArcView) data for the SFRA maps in Volume II of this report are also available.

6.3 Localised flooding

Flood risk from fluvial, groundwater and surface water are shown on the historical flood map Tiles B1 to B4. Flood risk from sewer flooding is shown on the separate historical flood map Tile B5, since Thames Water only provided this information on a postcode basis. Both of these maps are to be used together with the SFRA Flood Zone map Tiles D1 to D4 to guide the sequential test.

6.3.1 Fluvial flooding The Environment Agency hold records of historical flood events in the Swindon area in 1947, 1968, 1971, 1977, 1992 and 1993. The locations affected by these flood events are shown on Tiles B1 to B4 and listed in Table 6.1. An assessment of locations on the Rivers Ray and Cole with the potential for flooding due to blockages is detailed in Appendix D, and a Flood Risk Assessment (FRA) for the Whitehill Stream that has caused flooding of properties within West Swindon (Moresby Close and Bennett Close) is detailed in Appendix E.


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Table 6.1. Historical fluvial flood events Flood event Area affected March 1947 North of the study area, including the lower reaches of the Rivers Ray and

Cole, minor tributaries to the River Thames and the River Thames. Sept 1968 River Ray, Upper Cole and Coate Water June 1971 Upper reaches of the River Ray Aug 1977 River Thames, affected far north of study area Sept 1992 River Thames, affected far north of study area Oct 1993 River Cole to the east of Highworth

Source: Environment Agency

This assessment of historical fluvial flood events excludes the exceptional flooding in July 2007 as the extents are not yet available from the Environment Agency, and for this reason revisions to the SFRA flood maps are recommended in future updates. Other information on this recent event are detailed in Section 6.3.5 and shown on Tiles C.

6.3.2 Groundwater flooding PPS 25 states: “groundwater flooding occurs when water levels in the ground rise above surface elevations,” however groundwater may also cause harm in other ways, for example when it enters sub-surface structures (such as basements).

Recent research being carried out for Defra (in preparation), identifies the seven types of groundwater flooding event listed below. Of these, only (i), (iv) (v) and (vii) are likely to apply in the Swindon area, and even these are expected to be limited in extent.

(i) Rise of typically high groundwater levels to extreme levels in response to prolonged extreme rainfall;

(ii) Rise of groundwater levels in response to reduced groundwater abstraction in an urban area (termed groundwater rebound) or a mining area (termed minewater rebound);

(iii) Subsidence of the ground surface below the current groundwater level;

(iv) Rise of groundwater level in aquifers in hydraulic continuity with high in-bank river levels or extreme tidal conditions;

(v) Rise of groundwater levels due to leaking sewers, drains and water supply mains;

(vi) Faulty borehole headworks or casings causing upward leakage of groundwater through confining layers driven by artesian heads;

(vii) Increases in groundwater levels and changed flow paths due to artificial obstructions or pathways, and loss of natural storage and drainage paths.

The Defra research also identifies the following impacts observed as a direct result of excess groundwater at or close to surface:

• flooding of basements of buildings below ground level; • flooding of buried services or other assets below ground level; • inundation of farmland, roads, commercial, residential and amenity areas; • flooding of ground floors of buildings above ground level; and • overflowing (surcharging) of sewers and drains.


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Often, effects of groundwater flooding are indistinguishable from effects of fluvial flooding, or are not obviously attributable to groundwater (e.g. surcharge of sewers). As a result the recording of groundwater flooding is often inconsistent.

The Environment Agency’s record of groundwater flooding in the Swindon area is given in Table 6.2 below and shown on the historical flood map (Tiles B1 to B4). A Defra Study (2004), maps groundwater flooding recorded during the most severe recent groundwater flooding episodes (winter 2000/2001 and winter 2003) and any future updates to the SFRA should take on board any future work carried out in relation to this Defra study.

Table 6.2 Groundwater flooding in the Swindon Area GIS ref Name of area affected Date Geology GW_001 Old Town, Swindon 18/04/2001 Portland Beds/Kimmeridge Clay boundary GW_002 Caen View, Swindon 22/10/2001 Kimmeridge Clay GW_003 Riversdale Close, Swindon 02/04/2002 Kimmeridge Clay GW_004 The Weavers, Swindon 24/10/2002 Portland Beds/Kimmeridge Clay GW_005 Bowood Road, Swindon 05/09/2003 Portland/Purbeck Beds on Kimmeridge Clay GW_006 Kent Road, Swindon 27/01/2005 Portland beds overlying Kimmeridge Clay GW_007 Osprey Close, Swindon 18/12/2006 Clay, sand & gravel over Kimmeridge Clay Note: Above to be read in conjunction with Historical Flood Map Tiles B1 to B4

Source: Environment Agency Groundwater flooding of the types described above is generally unlikely to occur in strata defined as non aquifers by the Environment Agency (e.g. the clay formations: Oxford; Kimmeridge; Gault etc.) that dominate the Swindon area. However, interestingly of all the recorded “groundwater flooding” incidents (Table 6.2), most were associated with the Kimmeridge Clay. This may result from:

• groundwater within local features of the clay formation; or • groundwater emergence from overlying formations

(e.g. the Portland /Purbeck beds found in Swindon Old Town).

Groundwater flooding could occur in the Minor Aquifers of the Corallian, - indeed where groundwater is near the surface or perched, local groundwater flooding may occur. However with the depth and extent of the Corallian aquifer, it is not considered this would be extensive or widespread.

Some groundwater flooding may occur in the drift deposits (e.g. the alluvium of the River Ray) but this is likely to be related directly to surface (river) water and is unlikely to occur in isolation from such events.

Where drift deposits lie above clay strata, the clay may act as a barrier to the downward movement of groundwater. As a result, the drift strata may be poorly drained leading to water-logging and some surface flooding. In all such deposits, where groundwater may be near the surface, the impact of deep foundations on groundwater movement must be considered in any development proposals.


22 Surface water flooding - July 2007

Drift deposits can have a variable permeability. In the north of the study area, near the Cotswold Water Park especially, the gravel deposits (e.g. Northmoor member sand and gravel) may be permeable and not poor draining.

Recorded groundwater flooding events in the UK are most prevalent in the Chalk, and particularly during the winters of 2000/2001 and 2003, when groundwater flooding occurred extensively throughout the Chalk of southern England.

According to Environment Agency records, although the Swindon area remained largely unaffected by these flooding events, sections of Draycot Foliat flooded in 2000 and the dry valley of the River Albourne, at the very southern tip of the study area, was badly affected.

As a result of the 2000 event, the Environment Agency has set ‘trigger’ levels at a few key indicator boreholes in the vicinity. The levels were designed to provide enough time to prepare for similar events in the future. One of these indicator boreholes (SU 17/110) is located in Draycot Foliat.

6.3.3 Surface water (land drainage) flooding Known areas of surface water flooding, caused mainly by local drainage problems, were advised by Swindon Borough Council, North Wiltshire District Council, the Highways Agency, Wiltshire Fire Brigade and the Front Garden Action Group.

This historical flooding is shown on Tiles B1 to B4 and cross-referenced with the tables in Appendix F (the preceding reference code can be used to locate the flooding incident in Tile set B). GIS points and polygons have been used to delineate locations and areas where surface water and groundwater flooding occurs.

The assessment to date is not considered to be exhaustive and since data are based on historical events rather than predictive modelling (and therefore may not represent very rare events) the full extent of these flooding mechanisms may not have been captured. In future SFRA updates, additional reviews and consultations are recommended to ensure the best information is used.

6.3.4 Sewer flooding Urban sewer flooding occurs when flows entering the sewer network are in excess of those leaving the network at the associated treatment works or outfall. These events manifest due to a number of possible causes such as: general incapacity in the sewerage system, ground water infiltration, blockages and pipe failure, pumping station failures or incapacity, excess surface water connectivity, and overwhelming rainfall events.


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The problem has been exacerbated over the last decade, as a result of the EU Directive to reduce the number of consented overflows to watercourses and the increasing popularity to pave grassed areas.

During an incapacity flooding scenario, the volume of flow entering the sewerage network is in excess of the volume of sewage that is able to be conveyed through the pipe under gravity. The pipes and associated manholes then surcharge and flooding may be witnessed at manholes or property connections depending on the systems hydraulic grade line and local topography.

Thames Water provided data on historical sewer flooding (data for the July 2007 flood event assessed separately in Section 6.3.5). However, the data are restricted to postcode level and hence gives no street level information on flooding. The postcode areas are shown in the historical flooding Tile B5 (Volume II).

The total number of properties flooded from overloaded sewers in the last ten years (1997 to 2007) are recorded (Table 6.3), broken down into the number of properties flooded by surface water, foul water and combined sewers (full breakdown in Appendix G). Overloaded foul water sewers are found to be the dominant source of flooding.

Table 6.3 Thames Water sewer flooding records in last 10 years Number of properties flooded by sewers

Surface water Foul water Combined type Total 24 70 42 136

Source: Thames Water The majority of sewer flooding is located around the old town area (see Tile B5), and the following factors contribute to the sewer flooding problem in this area:

Topography that lends itself to surface water flooding with steep gradients present.

Age of the sewerage system means it is likely that surface and sewer flooding are combined in many places.

Additional properties connecting to the old system over past decades means it is likely that the system is now over capacity with foul and surface water flows utilising a system designed for lower flow rates, with flooding now seen during storm events.

In conclusion, it is evident there is no spare capacity in Swindon’s existing sewage system.

Storage, diversions, local upsizing of sewers and upgrading of pumping stations may alleviate current flooding, though alternative solutions should be sought for future developments.

New developments such as Wichelstowe are planned to pump flows directly to Swindon sewage treatment works (STW) via a new rising which will by-pass the old system, negating the possible exacerbating effect of connecting into it. Future developments (in all areas) should by-pass the older town systems in a similar fashion or be directed to a dedicated treatment works, as there is no spare capacity in the existing sewage system.

Some flooding around East Swindon is known to be caused by incapacity during storm events and blockage problems within local sewers caused by tree roots. Solutions in this


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dense residential area have been to divert flows to large buried storage tanks and culverts capable of holding the calculated storm peak flows. Weirs have been utilised to transfer excess flows to the tanks with a pumped return system to empty the tanks post event. The pipe networks are also cleaned to remove root blockages.

Any new development in the area should take into account current problems with capacity and be designed as not to exacerbate capacity problems. New flows should be directed to areas without capacity problems.

The construction of sewerage systems within private gardens has resulted in difficulties in promoting sewer upsizing and is often found to be the cause of blockages through root ingress, the lack of public open space and the installation of underground utilities which further hinders the design of new sewers and/or attenuation facilities.

The Purton area of West Swindon is shown to have a number of flooding problems. The area is an older rural village which is also seen to be subject to localised flooding. Flooding in this location may be due to any of the reasons mentioned above.

Some reports of foul flooding may be related to private sewers or surface water problems, whereby local surface drainage is insufficient to remove excess storm water to sewers. Problems related to such may not be the responsibility of the utility company but the local council or private residents. This information should be ascertained during the initial flooding study prior to detailed design of a flood alleviation scheme.

Further information about sewer flooding, flooding investigations and general engineering solutions is provided in Appendix G.

6.3.5 Flooding on the 20 July 2007 On the 20 July, heavy rain following a persistent rainfall for several weeks caused severe flooding in and around Swindon. Information on the areas affected by this flood event derives from the following sources, and is shown on Tiles C1 to C4 (except the data from Thames Water as postcode level only).

Swindon Borough Council

Thames Water (refer to Table 6.4 and Appendix G)

Bureau Veritas

Front Garden Action Group

Western Vale Villages Consortium of Parish Councils:

- Wanborough - Bishopstone & Hinton Parva - Ashbury - Compton Beauchamp & Knighton

- Watchfield - Shrivenham - Bourton

The July 2007 event caused unprecedented surface water flooding across the region, with the drainage systems in places almost irrelevant. Widespread disruption resulted and flooding directly affected homes in Haydon Wick and other areas (shown on Tile C).


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Table 6.4 Thames Water sewer flooding records for 20 July 2007 Number of properties flooded by sewers

Surface water Foul water Combined type Total 1 21 0 22

Source: Thames Water Traffic grid-lock due to flooding across many roads within Swindon and the surrounding area affected for example the A419, A420, Queens Drive, Pipers Way, Marlborough Road, Greenbridge roundabout, Stratton St Margaret, Cricklade Road. Flooding of one electricity sub-station (Drakes Way) temporarily cut power to 1,400 homes.

The Environment Agency prepared a review report on the July 2007 flood - see Appendix L. This report focuses on river flooding and presents an investigation into the causes and flood risk management options for Swindon. Flooding in the following places is reviewed:

River Ray - Wroughton, Westlea, Pembroke Park, Haydon Wick, Tadpole Brook area River Cole - Covingham and Nythe, Dorcan Brook and Honda site, South Marston

The Environment Agency report the following two key findings:

Return period for the flooding estimated between 50 and 100 years (2% and 1% annual event probability) in the Swindon area, except 30 years in Haydon Wick - this exceeds the carrying capacity of the watercourses and bridges in the Swindon area.

River bank surveys recommended in certain flood affected areas to identify where there are low points that would benefit from flood bunding to protect against lower order flood events (i.e. smaller in magnitude than the July 2007 event).

The Environment Agency report that they will continue to:

Carry out maintenance of the watercourses as programmed for 2008/09.

Raise public awareness to improve the uptake and effectiveness of Flood Warning Direct - free access to all, warning service.

Work with Swindon Borough Council to disapprove inappropriate development in the floodplain.

Work with developers to encourage improvements to the existing drainage network as part of their development programme.

Improve flood warnings by checking and improving monitoring flood levels. Raise awareness about increased flood resistance and resilience of properties by

using prevention and construction techniques. Encourage and support communities in producing flood plans.

A420 blocked by floodwater at Acorn Bridge, midday on 21 July 2007

Source: Western Vale Villages Consortium of Parish Councils


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Coate Water

6.3.6 Reservoirs The SFRA considers the risk of overtopping or breach of reservoirs. Swindon Borough Council were consulted to determine the flood risks associated with Coate Water and Stanton Park Lake, for which they are the undertaker.

Coate Water (grid reference SU 175 820) is situated approximately 4km south-east of Swindon town centre. The reservoir has a capacity of 527,000m3 and is retained by two embankments. Coate Water is designated as Category A under the Reservoirs Act 1975, given its proximity to property and potential for loss of life should a breach or overtopping occur (Eldene and Covingham are located immediately downstream of the dam).

Stanton Park Lake (grid reference: SU 175 900) is situated 2km north-east of Swindon near the village of Stanton Fitzwarren. This lake has a capacity of 42,000m3 and is retained by an earthfill dam. It is designated as Category B under the Reservoirs Act 1975, where a breach would endanger lives, but not in a community or could result in excessive damage.

Both reservoirs have undergone annual safety checks by a Supervising Engineer, ten year reviews by an Inspecting Engineer and Swindon Borough Council monitor water levels once a fortnight. Swindon Borough Council have provided assurance that these water supply reservoirs are actively managed and the required safety standards are met.

The most significant raised embankment with the potential for loss of life is Coate Water. The main dam at Coate water is approximately 8m high and 230m long. It is likely that should any major development be proposed in the area downstream of the reservoir that Level 2 SFRA would be required to determine the residual risk of overtopping or breach of the embankment and inform appropriate mitigation measures.

There are two other storage areas of similar capacity to Stanton Park Lake:

Blunsdon C service reservoir owned by Thames Water is 30, 000m3.

Whitehill Stream Flood Storage Area classified as a reservoir with 47, 412 m3. (Environment Agency advised of 2007 Section 10 Inspection on 23 January 2007).

There is also an ornamental lake at Lydiard Park, the dam of which was recently restored.


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Wiltshire & Berkshire Canal

6.3.7 Other artificial water retaining structures The SFRA considers the risk of overtopping or breach of canals. The Wiltshire & Berkshire Canal Trust have been consulted to determine the risk of flooding from the Wiltshire & Berkshire canal.

The canal was abandoned by an Act of Parliament in 1914 and the section through the centre of Swindon was subsequently infilled and built over.

Part of the canal to the southwest of Swindon has been restored and re-watered by the Canal Trust and there are plans in place to restore the canal and reintroduce boat traffic by 2014. However this plan proposes to take the canal in a new alignment around the south of Swindon and avoids the traditional route through the town centre.

Swindon Borough Council commissioned a study (ongoing) to assess the feasibility of constructing a new canal into Swindon town centre from the west. The route is approximately 1.6 km long and has been termed the Swindon Canal Central Route (Figure 6.1)

The ultimate plan is to link this arm up with the North Wilts Canal to reform the north south canal link through Swindon. There are also plans to extend the canal to the east of Swindon, and this route is safeguarded in the Local Plan (Figure 6.1).

Figure 6.1. Proposed route of the canal through Swindon Source: www.wbct.org.uk/swRoute.htm (2007)

The likely flood risk of the proposed canal route through the centre of Swindon is being assessed as part of the feasibility study. Preliminary findings suggest that the operating water level of the canal will be at least 300mm below ground level (based on minimum freeboard requirements) to reduce the risk of overtopping during a flood event. In the event of surface water flooding, the canal will intercept surface water runoff and reduce flooding.

Future North Wiltshire canal connection

Proposed central canal route


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If designed correctly the whole length of canal could provide approximately 3,600 cubic metres of flood storage before overtopping. If prior warning of a flood event was available the water levels in the canal could be lowered to provide additional storage.

Due to the anticipated ground conditions and need for vertical walls along the majority of the canal the canal will be watertight, which will reduce any potential impacts on the basements of any adjacent buildings. This will also help to conserve water.

The area alongside Faringdon Park will be the lowest point of the canal system in the town centre, therefore should overtopping occur in the town centre area, any water will gravitate to this location away from residential and commercial properties in the Faringdon Park area. From this area a waste pipe would need to be laid from this low point across to the River Ray to take excess water away from the canal.

Although the route of the canal does not directly impact on existing watercourses or flood plain a scheme-specific FRA will be requested by the Environment Agency during the detailed design and planning process to ensure fluvial flood risks associated with the River Ray are not increased by waste canal water being discharged into the river.

6.4 PPS25 Flood zones

The current SFRA Flood Zones are shown in Tiles D1 to D4 (Volume II), and defined below. As new river models become available, these Flood Zones will need updating to ensure they are based on the best information available.

Flood Zone 1 – All areas that are considered to be at low risk of fluvial flooding, however there remains the risk of flooding from other sources, such as surface water, groundwater, sewers and artificial sources (reservoirs).

Flood Zone 2 – Shows areas at risk of flooding in an extreme fluvial flood event. This zone shows those areas with a risk of flooding between a 0.1% and 1% Annual Exceedence Probability (AEP).

Flood Zone 3a – This represents the area that is part of Flood Zone 3, but outside Flood Zone 3b (Functional Floodplain). This zone identifies the areas at risk from a 1% AEP fluvial flood event (or a 0.5% AEP flood event caused by flooding from the sea).

Flood Zone 3b (Functional Floodplain) – The functional floodplain shows areas of land which are frequently flooded. For all areas except for the Upper Cole, it has been necessary to make conservative assumptions about the extent of the functional floodplain in the absence of historical flood outlines and detailed models. As such, for all areas except for the Upper Cole, the functional floodplain has been assumed to be equivalent to Flood Zone 3a.

Further discussion for these Flood Zone classifications is given below - in order of flood risk.


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6.4.1 Zone 3b – functional floodplain Functional Floodplain Zone 3b is defined as those areas in which water has to flow or be stored in times of flood. For this SFRA, functional floodplain has been defined by the following criteria:

Land subject to flooding in the 1 in 20-year flood event Land which provides a function of flood conveyance or flood storage, through natural

processes or by design (e.g. washlands, flood storage areas) Areas which would naturally flood with a 1 in 20-year return period or greater, but

which are prevented from doing so by existing buildings, flood defences and other flood risk management infrastructure

The PPS25 Companion Guide recommends that all areas within Zone 3 should be considered as Zone 3b Functional Floodplain unless, or until, an appropriate FRA shows to the satisfaction of the Environment Agency that it can be considered as falling within Zone 3a (High Probability).

In some areas, detailed models have been used to define the functional floodplain. In other areas it has been necessary to make conservative assumptions about the extent of the functional floodplain in the absence of historical flood outlines and detailed models.

The approaches used to map Zone 3b for each watercourse and their associated confidence levels are summarised in Table 6.5. It is recommended the extent of the functional floodplain is revised once concerns with the WSP Iffley to Purton and Bureau Veritas Eastern Development Area models are addressed (Appendix C). For other areas it is recommended that detailed modelling is undertaken as part of a FRA when seeking to allocate sites for development.

Table 6.5 Flood Zone 3b (Functional Floodplain) mapping Watercourse Zone 3b Data Source ConfidenceUpper Cole Upper Cole S105 study (2007)

Modelled 1 in 20-year flood outlines Medium to high

Lower Cole, River Ray & tributaries Zone 3b is assumed to be the same as Zone 3a Low

6.4.2 Zone 3a – High probability High Probability Zone 3a is defined as those areas situated within the undefended 1 in 100-year (or 1% AEP) flood extent. A number of approaches have been used to define the Zone 3a extent, including the use of detailed hydraulic modelling studies and two-dimensional modelling using J-Flow.

The approaches used to map Zone 3a for each watercourse and their associated confidence levels are summarised in Table 6.6. For areas of low to medium confidence the onus should be on developers to provide more refined information in site-specific FRAs (Section 11.4).


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Table 6.6 Flood Zone 3a (high probability) mapping Watercourse Zone 3a Data Source ConfidenceUpper Cole Upper Cole S105 study (2007) - modelled 1 in 100-year flood

outlines merged with Environment Agency Flood Map outlines based on coarse national computer model (using JFlow).

Medium to high

Headwaters of River Ray

Wichelstowe study (2007) modelled 1 in 100-year flood outlines High

Lower Cole, River Ray & tributaries

Environment Agency Flood Map outlines based on coarse national computer model (using JFlow).

Low to medium

6.4.3 Zone 2 – Medium probability The Medium Probability Zone 2 is defined as those areas within the study area which are situated between the undefended 1 in 1000-year (0.1% AEP) and 1 in 100-year (1% AEP) flood extents. The Environment Agency’s Flood Map includes a 1 in 1000-year flood outline and this has been adopted for all areas.

It is recommended that Flood Zone 2 is revised once concerns with the Iffley to Purton and Bureau Veritas Eastern Development Area models are addressed (see Appendix C). For other areas it is recommended that detailed modelling is undertaken as part of a FRA when seeking to allocate sites for development.

It should be noted that given its rarity it is very difficult to attach a high level of confidence in this delineation of this zone. The approach used to map Zone 2 and its associated confidence levels are summarised in Table 6.7.

Table 6.7 Flood Zone 2 (medium probability) mapping Watercourse Zone 2 Data Source ConfidenceAll watercourses

The Environment Agency Flood Map outlines based on coarse national computer model (using JFlow)

Low

6.4.4 Zone 1 – Low probability The Low Probability Zone 1 is defined as those areas within the study area which are situated outside of the undefended 1 in 1000-year flood extent. For the purpose of the SFRA maps, this includes all land that is outside of Zone 2 and Zone 3 flood risk areas. It is important to note however that for sites greater than one hectare it will still be necessary for a developer to produce a site-specific FRA which takes account of all sources of flooding (Section 11.4).

6.5 Existing services/infrastructure within PPS25 Flood zones

This section examines flood risk based on the SFRA Flood Zone map in relation to particular locations within Swindon, including hospitals, schools and important infrastructure such as major roads, rail, water treatment works, electricity stations, etc. The locations identified where intersected by the Flood Zone, are detailed in Table 6.8 and Figure 6.2 The assessment of potential development sites is considered later (Section 10).


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Figure 6.2 Services and infrastructure location Table 6.8 Services & infrastructure details Watercourse Ref Grid reference Type Description FZ3Elcombe Brook 1 SU1213482378 Motorway M4 Motorway Swin Bourne 2 SU1372683999 Road A4361 Croft Road

3 SU1515482529 Rail London – Bristol Mainline 4 SU1311184135 Road A3102 Wootton Bassett Rd 5 SU1313983791 Road A3102 Wootton Bassett Rd 6 SU1314685139 Road B4006 Great Western Way 7 SU1309385631 STW STW 8 SU1287286165 Rail Swindon – Kemble Railway Line

13 SU1212486734 Road B4534 Purton Road 10 SU1074985210 Road Tewkesbury Way

River Ray

11 SU1170286391 Road B4534 Mead Way Rod Bourne 12 SU1494786424 Road Pinehurst Road

16 SU1212187225 Road A4259, Coate Water R/bout 17 SU1349388432 Road Eldene Drive 18 SU1089385225 Road St. Paul’s Drive 19 SU1777882993 School Dorcan Secondary School 20 SU1800983802 Road A419

Dorcan Stream

21 SU1922384884 Road Wanborough Road 22 SU1879784642 School Colebrook Primary School 23 SU1950484986 Road B4006 Dorcan Road 24 SU1956885186 Road Covingham Drive

River Cole

25 SU1791485662 Road A420 9 SU1789085535 Rail Swindon – Kemble Railway Line

14 SU1841085642 Road B4534 unknown

15 SU1885285893 School Middleleaze School


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7 Assessment of flood risk management practices

7.1 Overview

This Chapter presents the assessment of flood risk management practices and reviews the riparian responsibilities, flood defences, flood warning areas and emergency planning procedures within the SFRA area.

7.2 Flood risk managment responsibilities 7.2.1 Environment Agency The Environment Agency has a duty to supervise all matters relating to flood defence, and aims to reduce the likelihood of flooding through flood risk management. The Environment Agency also seeks to reduce the risk of flooding by influencing land use planning and through improved flood warning and emergency responses.

7.2.2 Riparian responsibilities Anyone owning land next to a watercourse has responsibilities as a ‘riparian owner’. These responsibilities are explained in the Environment Agency document ‘Living on the edge, 2007’ (available from their website) together with the responsibilities of other organisations.

Land owners have the right to protect their property from flooding, but in most cases the Environment Agency must agree plans before any work commences. Land owners must also accept flood flows through their land even if flooding is caused by inadequate capacity down stream. There is no duty for a landowner to improve the drainage capacity of a watercourse, but the maintenance of any defences should be discussed with the Environment Agency.

7.2.3 Local Authority responsibilities During a flood a Local Authority provides emergency aid to householders and any works on a watercourse may require planning permission from the Local Authority.

7.3 Existing flood defences 7.3.1 Defence structures Flood defences are structures which affect flow in times of flooding. They generally fall into one of two categories: ‘formal’ or ‘defacto’. A ‘formal’ defence is a structure which has been specifically built to control floodwater. It is maintained by its owner (not necessarily the Environment Agency) so that it remains in the necessary condition to function.

A ‘defacto’ defence includes road and rail embankments and other linear infrastructure (buildings and boundary walls) which may act as water retaining structures or create enclosures to form flood storage areas in addition to their primary function. Other structures are identified on the Environment Agency database, but these have not necessarily been built to control floodwater and are not maintained for this purpose.

In accordance with the scope of a Level 1 SFRA, a high level review of formal flood defences has been carried out (Appendix I) using data from the National Flood and Coastal Defence Database (NFCDD) and a site visit (Appendix H). The NFCDD is a good starting point for identifying significant flood defences and potential areas benefiting from defence, but the quantity and quality of information provided differs considerably between structures.


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The NFCDD is intended to give a reasonable indication of the condition of an asset, though may not contain consistently detailed and accurate data (this would be undertaken as part of Level 2 SFRA where the need arises).

A total of 24 flood defences on the NFCDD database were identified as ‘formal’ or ‘defacto’ flood defences. These flood defences are mapped in Tiles E1 to E4 (Volume II) together with the 21 flood storage areas that reduce the frequency and extent of flooding.

The Standard of Protection (SoP) provided by these defences has been assessed. Of these, only one is found to provide protection against the 1 in 100-year flood as detailed below (also refer to Appendix I). The SoP quoted in the NFCDD for all other flood defences is either 1 in 5-year or 2-year.

Flood Defence - protecting properties on Shaftesbury Avenue (Environment Agency NFCDD asset reference: 0610707DO0201L02)

This defence is a raised earth embankment situated alongside the River Cole and is designed to protect houses on Shaftesbury Avenue (Figure 7.2). As this defence has been included in the Upper Cole model, the Environment Agency Flood Zone extents have been used to demark the Flood Zone extents behind this defence (since Flood Zones represent the without defences condition).

The model cross-sections and SAR data show this defence provides protection against a 1 in a 100-year flood event plus an allowance for climate change (20%). However, due to the inaccuracies in SAR data, it is recommended that the area benefiting from this defence is explored further in any future updates to the SFRA.

Figure 7.2 Raised embankment protecting properties on Shaftesbury Avenue

Shaftsbury Avenue Flood

Zone 3b

Flood Zone 3a

Flood Zone 2

Flood Defence


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7.3.2 Flood storage areas There are 15 Flood Storage Areas (FSAs) within the SFRA study area as identified on NFCDD. These FSAs, are mapped on Tiles E1 to E4 and detailed on Table 7.1

Table 7.1 Flood Storage Areas River Maintainer Grid reference NFCDD Asset

reference* Ray Environment Agency SU1174388065 0610303HA0101L02 Ray Environment Agency SU1202487938 0610303HA0101L03 Ray Environment Agency SU1210487909 0610303HA0101L04 Ray Private SU1242887222 0610303RO0101R03 Ray Local Authority SU1168086386 0610303LY0101R02 Ray Environment Agency SU1316183754 0610303EL0101L03 Ray Private SU1332085090 0610303RA0302R04 Ray Local Authority SU1365183620 0610303SW0102R07 Ray Local Authority SU1369183901 0610303SW0102L02 Cole Local Authority SU1789883739 0610707do02r01 Cole Environment Agency SU1815784707 0610707DO0103R06 Cole Private SU1941083437 0610707LI0103L02 Cole Environment Agency SU1952585209 0610707DO0101R04 Ray Local Authority SU1247785728 0610303WH0101R06 Cole Private SU1760385809 0610707CO0502R02

*Environment Agency reference

7.4 Flood warning areas 7.4.1 Existing flood warnings The Environment Agency operate the current flood warning service in the SFRA area. Across this area, they monitor rainfall and river levels 24 hours a day at a number of Flood Warning telemetry stations (Water Eaton, Rodbourne and Rushey Platt, Kestral Drive and Lower Stratton) and use this information to forecast the probability of flooding.

Flood warnings are issued using a set of four codes, each indicating the level of risk with respect to flooding. The warnings issued are Flood Watch, Flood Warning, Severe Flood Warning and All Clear.

Within the study area approximately 2000 properties are known to be at risk of flooding. A Flood Warning is issued if property is expected to flood and a Severe Flood Warning if there are over 100 properties expected to flood and/or major infrastructure could be affected. The ‘All Clear’ is issued when river levels are back within banks.

Four Flood Warning Areas in the SFRA area cover:

River Cole, Tuckermill Brook and their tributaries from Stratton St. Margaret, Swindon to Lechlade

River Ray and its tributaries from Mannington to Cricklade River Thames from Cirencester to St Johns Lock, Lechlade River Thames from St Johns Lock, Lechlade to Eynsham Lock, Eynsham


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The Environment Agency is only able to offer a flood warning service where they have flood warning capabilities on a Main River. Where a flood warning service is available, the Environment Agency encourages people at flood risk to register for the service.

Flood warnings are issued via Floodline Warnings Direct which enables individuals, emergency services, local authority emergency planners and response teams to be effectively warned by delivering warnings simultaneously via telephone, mobile, pager, fax, email, SMS text messaging, digital TV and radio.

7.4.2 Future improvements to flood warnings Since the July 2007 flood event the Environment Agency are drawing up a list of communities which can and cannot be offered a flood warning service. This is being shared with professional partners.

In areas where the Environment Agency cannot provide a flood warning service they are planning a 3-5 year priority programme of river level gauge installation. The existing river level gauges are under study to make the property flooding threshold levels more accurate at Rushey Platt, Kestral Drive and Lower Stratton on the River Cole.

The on-going National Flood Risk Area/Flood Warning Area Project being undertaken by the Environment Agency is working towards refining the flood risk areas in order to provide a more targeted flood warning service to local communities. The flood risk areas represent areas of similar land-use, floods from the same scenario and floods of similar return period.

The flood risk areas will form flood warning areas based on communities in the floodplain; a flood warning area will consist of one or more flood risk areas.

7.4.3 Emergency planning Swindon’s Local Incident Flood Plan is designed to enable Swindon Borough Council to deal with main river, surface and groundwater flooding. It details role and responsibilities before, during and after a flood event and provides local information useful to those responding to flooding.

South West Region of the Environment Agency have organised a meeting with all those involved in providing a flood response within Wiltshire. This is to improve multi-agency responses to flooding, and it is hoped that these meetings will continue.

7.5 Variation in actual flood risk

New development should be directed to sites with lower flood risk within each Flood Zone, which is generally towards the adjacent zone of lower probability of flooding. For the high probability Flood Zone an indication of the actual variation in flood depth is presented in Tiles G1 to G4 (Volume II).

The flood depths presented are preliminary only, obtained from the Environment Agency’s JFlow modelling.


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Examples of SUDS techniques

8 Assessment of the capacity for the use of SUDS

8.1 Overview

PPS1 “Delivering Sustainable Development” and PPS25 require that LPAs should promote Sustainable Drainage Systems (SUDS). LPAs should ensure policies encourage sustainable drainage practices in their Local Development Documents. SUDS is a term used to describe the various approaches that can be used to manage surface water drainage in a way that mimics the natural environment.

The management of rainfall (surface water) is considered an essential element for reducing future flood risk to both the site and its surroundings. The Environment Agency expect attenuation of runoff from any development site to greenfield rates, and SUDS provide an opportunity for achieving this.

8.2 Types of SUDS systems

SUDS may improve the sustainable management of water for a site by:

Reducing peak flows to watercourses or sewers and potentially reducing the risk of flooding downstream;

Reducing volumes of water flowing into watercourses or sewers from developed sites;

Improving water quality, compared with conventional surface water sewers, by removing pollutants from diffuse pollutant sources;

Reducing potable water demand through rainwater harvesting;

Improving amenity through the provision of public open space and wildlife habitat;

Replicating natural drainage patterns, including the recharge of groundwater so that base flows are maintained.

8.3 SUDS at the planning stage At the drainage design concept stage an assessment can be made of the superficial and underlying geology as this has a fundamental impact on the approach to be followed for the SUDS system.

Amenity/Green area over under croft parking

Plan

Green roofs for rainfall harvesting to reduce surface runoff in drains and for flushing


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The two main variations in SUDS systems (listed below) both balance the increase in runoff due to climate change and hence minimises the effect of any development work on the receiving watercourses.

Use of infiltration within the attenuation facilities to partly or fully dispose of runoff; Not using any infiltration techniques but providing attenuation facilities that maintain

the discharges at pre-development levels.

For any significant site the Environment Agency will be consulted by the Planning Authority during the outline planning process. The Environment Agency will want assurances that the requirements of PPS25 are being implemented and will be followed during the detailed planning stage and through to construction.

To achieve this a Zone 1 Flood Risk Assessment is required that demonstrates an achievable layout and details the methodology for the construction of SUDS within the boundary of the development site. The FRA must comply with PPS25 requirements and for the Swindon area should also accord with Defra/Environment Agency publication “Preliminary Rainfall Runoff for Developments Revision D”.

8.4 Application of SUDS within Swindon There are numerous different ways that SUDS can be incorporated into a development. The appropriate application of a SUDS scheme to a specific development is heavily dependent upon the topography and geology of the site and the surrounding areas. Careful consideration of the site characteristics is necessary to ensure the future sustainability of the adopted drainage system.

Within Swindon it is recommended that priority is given to discharging surface water to watercourses as opposed to the use of infiltration drainage techniques. This is due to the presumption that infiltration techniques are not viable (due to a high water table and local impermeable soils). However each site should confirm that the presumption is correct.

The key loss of benefit from not utilising infiltration is that these SUDS systems attenuate peak flows but do not significantly reduce flood volumes. Discharging attenuated site runoff directly to watercourses should be used instead of routing flows through the sewer network.

Large increases in impermeable areas contribute to significant increases in surface runoff volumes and peak flows and could increase flood risk elsewhere unless adequate SUDS techniques are implemented. It is relatively simple to avoid the increase in peak flows by providing attenuation or detention storage that temporarily store the required amounts of runoff within the site boundary. The use of water recycling and permeable paving can allow transevaporation of up to 20% of the water attenuated.

SUDS techniques will be required for all proposed land allocations unless suitable facilities can be provided at a suitable adjacent location. The Environment Agency expect attenuation to greenfield rates. The techniques employed will depend on the individual circumstances. Developers should consult with the Environment Agency at an early stage about their SUDS proposals, to ensure that they are adopting the most affective methods for their site.


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There are a number of SUDS elements that could be used within development sites in the Swindon area. Generally those that are specifically infiltration techniques have been omitted as the underlying geology is presumed to preclude large scale infiltration. However the Environment Agency would expect that the initial assumption of any drainage designer would be to include infiltration where possible.

Thus infiltration should be used unless ground investigation and in particular infiltration tests determine that it is not practicable. It should also be noted that the Building Regulation Part H state that preferred option for the disposal of property runoff should be via a soakaway.

Specific attenuation for the Swindon area could comprise of:

Swales that can be constructed alongside roads and within green areas to transfer runoff to storage facilities, and also provide limited storage. Infiltration swales are preferred as they keep dry between rainfall events and so avoid becoming marshy, and allow as much infiltration as the surrounding ground can accommodate.

Pond / dry basin to provide the majority of the volume required to attenuate the surface water runoff.

Permeable/porous paving may be used within development areas, subject to consideration of the adoption issues with the highway department, to attenuate runoff at source as it will collect the rainfall below the surface and discharge it after a significant delay.

8.4.1 Constraints on discharges to ground The nature of an aquifer body and the groundwater within it provide significant constraints when considering the potential of SUDS that rely on infiltration to the ground to provide the means of (storm water) drainage, storage and flow attenuation.

Constraints on discharges include:

Groundwater will be a receptor of man-made drainage – whether this be deliberately, e.g. through soakaways, infiltration drainage) or incidentally (e.g. through mains water pipe or sewer leakage.

In an urban/ semi urban environment groundwater is under considerable pressure with respect to quality, for example from contaminants on brownfield sites; from uncontrolled drainage; leachates from uncontrolled landfill; leakage from sewers, agro chemicals in field drainage; drainage from roads and other hard surfaces; and seepage from poor quality surface water bodies (channels, ditches, streams, rivers).

Even though locally groundwater may not have value as a major drinking water resource, it may have value in supporting local water and have a role in determining the water quality of these water bodies and any dependant ecosystems.

UK groundwater policy has just been revised and the EA have recently released their first report on the state of groundwater in England and Wales. These documents stress the need to protect groundwater.

It is an offence to discharge a List 1 substance (petroleum hydrocarbons etc.) to groundwater.


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A daughter directive of the European Water Framework Directive (Directive 2000/60/EC) which will replace current groundwater specific legislation provides for more stringent protection of groundwater.

8.4.2 The role of groundwater and aquifer bodies in sustainable drainage Drainage to groundwater is a significant component for the discharge of sustainable drainage systems. Aquifers provide for both storage and transmission of collected drainage water and provide the opportunity to attenuate flow from stormwater discharges. In addition, the unsaturated zone of aquifers may provide for the attenuation of contaminants introduced at the surface.

Other than as described above, constraints on groundwater as a receptor of drainage include

Hydrogeological – requires permeable “free draining” strata, providing means to store and transmit water.

Groundwater occurrence – near surface water tables limit potential drainage.

Potential to cause waterlogging or groundwater flooding downgradient or downslope (see also below).

Topographic setting - infiltration drainage at higher elevations may re-emerge downslope.

The benefits of using infiltration as part of a sustainable drainage system include:

Infiltration of (good quality) drainage discharges recharge the aquifer and may benefit local groundwater use (or groundwater dependent ecosystems)

In naturally permeable soil locations, infiltration may mimic the natural water cycle otherwise lost under the development process.

Significant flow attenuation may be provided.


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9 Flood risk & climate change

9.1 Overview

Defra guidance (November 2006) on the predicted effects of climate change describes how short duration rainfall could increase by 30% and flows by 20%. The guidance is based on research that suggests winters will become generally wetter whilst summers, although drier, will be characterised by more intense rainfall events.

The climate change effects will tend to increase both the size of flood zones associated with the sea and rivers, and the amount of flooding experienced from other sources. Current guidance on incorporating climate change effects into flood risk assessments is detailed in Table 9.1.

Table 9.1 Climate change guidance (from PPS25) Parameter 1990 to 2025 2025 to 2055 2055 to 2085 2085 to 2115

Peak rainfall intensity +5% +10% +20% +30%

Peak river flow +10% +20%

9.2 Flood risk & climate change

The following approaches have been used to map the potential impacts of climate change on Flood Zone 3b (Functional Floodplain) and Flood Zone 3a (High Probability) (Table 9.2). Climate Change Flood Zone 2 is assumed to equal the current Flood Zone 2 as there is very little certainty about the effect climate change will have on extreme events.

SFRA Flood Zone maps for climate change are shown on Tiles F1 to F4 (Volume II).

Table 9.2 Climate change mapping (Future = 2025 – 2115, Present = 2007) Watercourse Climate Change Assumptions Upper Cole Future Flood Zone 3b = Present Flood Zone 3a

Future Flood Zone 3a = Present 1 in 100-year flow +20% Future Flood Zone 2 = Present Flood Zone 2

River Ray - headwaters Future Flood Zone 3b = Present Flood Zone 3a Future Flood Zone 3a = Present 1 in 100-year flow +20% Future Flood Zone 2 = Present Flood Zone 2

Lower Cole, River Ray & tributaries

Future Flood Zone 3b = Present Flood Zone 3a Future Flood Zone 3a = Present Flood Zone 2 Future Flood Zone 2 = Present Flood Zone 2

A review of the available hydraulic models and Flood Zones using the assumptions detailed in Table 9.2 suggests that the changes in the aerial extent of inundation are likely to be negligible for much of the River Ray and Cole. This is because the floodplains in the study area are fairly well-defined. However, there are localised areas where climate change may have a larger effect.

These localised areas include three developed areas adjacent to the River Cole: Park North, Park South and north Covingham. Whereas to the north of the SFRA area, flatter areas along the Thames and lower reaches of both the Ray and Cole mean the climate change


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effects are more dramatic. However, even if flood extents show little change it is important to note that changes in the depth of flooding as a result of climate change will have a significant impact on flood hazard.

It is expected that flood risk from groundwater, sewer or surface water flooding will generally increase due to the expected wetter winters (causing more frequent and prolonged groundwater flooding) and incidence of short-duration high intensity rainfall events associated with summer convective storms (causing more frequent surface water and sewer flooding).

Further guidance on how planning should secure new development to the effects of climate change will soon be available in the new Planning Policy Statement: Planning and Climate Change (a supplement to PPS1). It is recommended that future updates to the SFRA take account of this and other emerging guidance (Section 10).

9.3 Integrated urban drainage

Defra commissioned a series of ‘Integrated Urban Drainage Pilot Studies’. The 15 projects will test new approaches to reduce the impact of urban drainage flooding, so that towns and cities across the country are better prepared for the impacts of climate change. The pilots were set up primarily to "plug the gap" between the well studied pathways of flooding, fluvial and coastal flooding; and the lesser understood 'other causes' of flooding.

The Government's Foresight and Making Space for Water projects identified that these other sources of flooding can make up a large proportion of the damage caused, and cost incurred, by flooding, and this proportion is likely to increase as the impacts of climate change start to become more frequent.

Swindon Borough Council expect to examine further the risk of integrated urban drainage flooding during the Water Cycle Strategy (WCS; Phase II - Halcrow) constraints assessment. The WCS proposes to:

Expand on this SFRA by looking at responsibilities for urban drainage flooding, and advising on high level strategic solutions for integrated urban drainage.

Advise on best practice based on the pilot studies, but will not involve detailed modelling of urban flood pathways because of the restricted data available from Thames Water regarding sewer flooding incidents (Section 6.3.4).

Consider the climate change impact on foul sewerage constraints (using the Thames Water Infoworks model).

The impact of climate change on integrated urban drainage flooding is not currently being assessed by any party. There is currently no model of the surface water sewer system and its links to the foul, highway and river system.

Development will not have an impact on the drainage system provided that it is well designed to current SUDS best practice, including PPS25 climate change allowances. However, there are, as recent flood events have identified, existing deficiencies in the system that lead to urban flooding from integrated pathways. Options for strategic flood risk management in new development areas will explore high level opportunities for reducing flood risk in existing urban areas.


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9.4 Potential increase in flood risk caused by future development

The Flood Zones and localised flood incidents (Tiles B, C, D) require careful consideration before sites for development are allocated, but once allocated a SUDS network can be constructed to ensure runoff from the site is the same after development is completed as it was before development started.

In an undeveloped area a percentage of the rainfall seeps or infiltrates into the soil and so does not contribute to runoff into watercourses, ditches or sewers. A SUDS network creates a series of opportunities for this same degree of infiltration to continue when a site is developed with properties, buildings and roads all of which cut off the natural path of the rainfall to the soil.

The SUDS system must take into account climate change that is predicted to occur. It should be noted that existing sites could discharge up to 30% greater runoff in the future even if there was no change at all made to the site, purely as a result of increased rainfall landing on the ground within the site.

To achieve this equivalency or slight betterment the site drainage and SUDS network should be designed to meet the requirements set out in PPS25. To determine the actual design one approach that can be used is set out in the Defra/Environment Agency publication “Preliminary Rainfall Runoff for Developments Revision D”.

A SUDS network will accommodate all rainfall, including the extra as a result of climate change, falling onto the site for all storm events up to and including 1 in 100-year return period events. This storm return period can be described as a storm that on average would occur only once in 100 years. However with all probabilities it is possible that this event could occur more frequently than once every 100 years.

For the storm events within the design parameter of up to 1 in 100-year the outflow from the site will be equal to or slightly better than the existing arrangement. The storage can be within formal elements, e.g. a pond, but during extreme events, informal storage areas such as car parks, playing fields and public open space can be used.

The drainage and SUDS networks on a site will collect and transfer all of the rainfall to attenuation or storage areas without any surface flooding affecting properties or key infrastructure. In extreme events sites should be designed to meet the requirements of “Designing for exceedance in urban drainage – good practice” by CIRIA reference C635.

The CIRIA document defines an approach that minimises damage caused when the flow carrying capacity of the piped drainage system or SUDS network is exceeded. The key outcome is that excess flow is managed. It can be designed to be carried as overland flow along road surfaces, cycleways or along depressions in public open space.

9.5 Potential effect of flood defence failure (residual risk)

There is one defence within Swindon (protecting properties at Shaftesbury Avenue; refer to Section 7.3) that currently provides localised protection against fluvial flooding. As with any flood defence there is the residual risk that this defence may fail, as a result of either overtopping and/or a breach. Should such an event occur it may result in rapid inundation of the local community behind the flood defence, and may pose a risk to life.


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If the Exception Test needs to be applied to specific site allocations behind this flood defence, then Level 2 SFRA would be appropriate at this location to provide information on the flood hazards (flood depth, velocity, speed of onset of flooding, etc).

9.6 Extent and costs required to raise flood defence standard to 1% There are no flood defences within the SFRA area where raising alone will increase the flood defence standard to 1% (Appendix I) - flood storage areas are excluded from this appraisal. However, there is one defence (near Haydon) as detailed below that could be both extended and raised to protect an area from flooding in a 1 in 100-year flood event.

Flood Defence, near Haydon (in the Northern Development area) (Environment Agency NFCDD asset reference: 0610303HA0101L06)

This defence is a raised earth bund designed to protect a housing estate, but needs to be both extended and raised to provide protection to properties within Clary Road, Parsley Close and Coriander Way (Figure 9.1). However, prior to any such works being undertaken it must be ensured that any ground level raising would not increase the flood risk to the surrounding area, and compensatory flood storage may be appropriate as mitigation.

Figure 9.1 Raised earth bund designed to protect housing

The Environment Agency on their NFCDD database estimate this defence needs extending by 100m to provide protection to properties. This reach within the Northern development area (tributary to River Ray) has not been modelled, but water levels extracted from downstream of the site suggest the water level of a 100-year flood event is approximately 86 mAOD near the defence, whereas the footpath level along the river is 85.6 mAOD (obtained from LiDAR).

An indicative cost to raise the Standard of Protection for these flood defences is £25-50k (capital cost), based on construction of a flood bund up to 1m high along the length of the existing defence (150m approximately) and extended over a similar distance. And there are additional costs, such as for any environmental works (say 5% of capital cost), regular maintenance (say annual 1% of capital cost), periodic major remedial works (say 25% of capital cost in 25 years).

Flood Zone 3b

Flood Zone 2

Flood Defence

The site visit photos (Appendix H) show there is only a small difference between the elevation of the footpath and the flood bund. This suggests the standard of protection currently provided by the height of the defence is less than 1 in 100-year.


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9.7 Sustainability of land use in medium and high risk flood areas The following types of land uses are suitable for medium and high risk flood areas:

Zone 1 – Low probability All uses of land are appropriate in this zone. In this zone, developers and local authorities should seek opportunities to reduce the overall level of flood risk in the area and beyond through the layout and form of the development, and the appropriate application of sustainable drainage techniques.

Zone 2 – Medium probability of flooding The highly vulnerable, water-compatible, less vulnerable and more vulnerable uses of land and essential infrastructure as detailed in Appendix J are appropriate for this Flood Zone. All land uses are subject to the Sequential Test (footnote 22 of PPS25). The highly vulnerable uses are only appropriate in this zone if the Exception Test (Section 3.4) is passed.

In this zone, developers and local authorities should seek opportunities to reduce the overall level of flood risk in the area through the layout and form of the development, and the appropriate application of sustainable drainage techniques.

Zone 3a High Probability The water-compatible and less vulnerable uses of land in Appendix J are appropriate in this zone. The highly vulnerable uses should not be permitted.

The application of the Sequential Test is described in Section 3.3 and the more vulnerable and essential infrastructure uses should only be permitted in this zone if the Exception Test (Section 3.4) is passed. Essential infrastructure permitted in this zone should be designed and constructed to remain operational and safe for users in times of flood.

In this zone, developers and local authorities should seek opportunities to: • reduce the overall level of flood risk in the area through the layout and form of the

development and the appropriate application of sustainable drainage techniques; • relocate existing development to land in zones with a lower probability of flooding; • create space for flooding to occur by restoring functional floodplain and flood flow

paths and by identifying, allocating and safeguarding open space for flood storage.

Zone 3b The Functional Floodplain Only the water-compatible uses and the essential infrastructure listed in Appendix J that has to be there should be permitted in this zone. It should be designed and constructed to:

• remain operational and safe for users in times of flood; • result in no net loss of floodplain storage; • not impede water (river) and groundwater* flows; • not increase flood risk elsewhere.

*large underground structures can impede groundwater flows.

The application of the Sequential Test is described in Section 3.3, and essential infrastructure in this zone should pass the Exception Test as described in Section 3.4.

In this zone, developers and local authorities should seek opportunities to: • reduce the overall level of flood risk in the area through the layout and form of the

development and the appropriate application of sustainable drainage techniques; • relocate existing development to land with a lower probability of flooding.


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10 Appropriate planning responses for low, medium & high risk areas

10.1 Overview This Section provides planning recommendations to enable appropriate planning responses for low, medium and high risk areas as regards flood risk. Appropriate policy set by Swindon Borough Council is essential to ensure that the recommended development control conditions can be imposed consistently at the planning application stage.

The policy recommendations provided in this Section are not exhaustive and it is therefore recommended that Swindon Borough Council refer to the following key flood risk management documents in order to fully inform their flood risk management policy.

Planning Policy Statement 25: Development and Flood Risk (PPS25) – sets out national policy for development and flood risk and supports the Government’s objectives for sustainable communities.

Thames Catchment Flood Management Plan - strategic planning document through which the Environment Agency will work with other stakeholders to identify and agree policies for long-term flood risk management over the next 50 to 100 years.

Making Space for Water - outlines the Government’s proposals for forward planning of flood management over the next 20 years advocating a holistic approach to achieve sustainable development. The protection of the functional floodplain and creation of blue corridors are central to the strategy.

Water Framework Directive - European Community (EC) water legislation which requires all inland and coastal waters to reach good ecological status by 2015.

10.2 Strategic flood risk management studies – Environment Agency The Environment Agency advocates a strategic approach to flood risk management on a ‘whole catchment’ basis. In line with this thinking, the Thames Catchment Flood Management Plan (CFMP, 2007) has been undertaken by the Environment Agency. A brief overview of the CFMP and its main recommendations for flood risk management follows.

10.2.1 Thames CFMP The Thames CFMP is a high-level strategic planning document through which the Environment Agency will work with other stakeholders to identify and agree policies for long-term flood risk management over the next 50 to 100 years. A summary consultation document was published in 2007 outlining proposed flood risk management policies for the Thames Region - this consultation has now closed and the final plan is due to be published later this year.

The Thames CFMP takes into account the likely impacts of climate change and future development across the region. The plan does not propose specific or detailed measures but identifies where further work is needed.

The four main messages from the Thames Region CFMP are: Flood defences are not sustainable and cannot be built to protect everything; Climate change will be the major cause of increased flood risk in the future; The floodplain is our most important asset in managing flood risk; and Development and regeneration provide a crucial opportunity to manage the risk.


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Key CFMP messages for the River Ray and Cole are: The location, layout and design of developments – in that order – are the most vital

factors in managing flood risk. Development should only be permitted in areas of flood risk when there are no reasonably available sites in areas of lower flood risk and the benefits of the development outweigh the risks from flooding.

Effective ways of managing the risk must be incorporated into planning and design to prevent the need for future intervention. This is dependent on the location and layout of development.

Development should avoid flood risk to people and property where possible. It should manage any residual risk, taking into account the impacts of climate change.

Flood risk, water resources and water quality need to be balanced through management of waste water, surface water and sewers.

The main action identified within the CFMP is for Swindon Borough Council and the Environment Agency to make sure that flood risk is managed appropriately within the River Ray and Cole catchments by applying PPS25.

10.3 Indicative Sites

A preliminary review of the proposed or allocated development sites and areas of search for development has been undertaken. Table 10.1 provides a summary of these areas according to the PPS25 Flood Zones taking into account climate change and other sources of flooding. It should be noted that the Sequential Test has not yet been undertaken, and these areas of search are themselves subject to review - for this reason the classifications detailed in Table 10.1 are likely to change.

Table 10.1 Flood Zone classification of the allocated development sites and the areas of search for development

Number of development sites/areas Total Intersecting with

climate change FZ 3b

Intersecting with climate change FZ 3a

Intersecting with climate change FZ 2

Intersecting with FZ 1 affected by other sources of flooding

19 14 14 14 7 From Table 10.1 it is apparent that some of the proposed or allocated development sites and areas of search for development intersect with Flood Zone 3b (functional floodplain) and 3a (High Probability) when the potential effects of climate change are taken into account.

In allocating sites for development Swindon Borough Council will be required to undertake the Sequential Test if promoting any areas that lie within Flood Zones 2, 3a or 3b at any point throughout the developments life. By applying the Sequential Test the more vulnerable uses of land can be allocated to the lowest risk sites.

Only where there are no reasonably available sites in Flood Zones 1 or 2 should the suitability of sites in Flood Zone 3 be considered, taking into account the flood risk vulnerability of land uses and applying the Exception Test if required.

As there are relatively low levels of confidence in the delineation of Flood Zone 3b (Section 5.3), those proposing development may wish to undertake a more detailed FRA to show, to the satisfaction of the Environment Agency whether it can be considered as falling within Zone 3a (high probability).


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10.4 Policy Recommendations For development control purposes, detailed policies will need to be set out to ensure that flood risk is taken account of appropriately for both allocated and non-allocated sites. The following reflects the minimum requirements under PPS25 (refer to PPS25 Tables D.1-D.3).

Future Development within Flood Zone 1 Developers and local authorities should seek opportunities to reduce the overall level of flood risk in the area and beyond through the layout and form of the development. All uses of land are appropriate in Flood Zone 1, although the vulnerability of a development from other sources of flooding should be considered as well as the effect of the new development on surface water runoff.

Typically, a Drainage Impact Assessment will be required to demonstrate that runoff from the site is not increased. This will involve the use of SUDS techniques which should take into account the local geological and groundwater conditions (Section 8 - SUDS guidance). For sites larger than one hectare, the post development runoff volumes and peak flow rates should be attenuated to the Greenfield (pre-development) condition.

The Environment Agency will object to any planning application for a site over 1 ha with no FRA. Developers are encouraged to contact the Environment Agency as early as possible to discuss the proposed drainage strategy.

Future Development within Flood Zone 2 Flood risk is a material planning consideration and for this reason a site-specific FRA must be carried out prior to granting any permission. The FRA is also necessary to check the proposal passes the Exception test prior to permission. The Flood Risk Assessment should not be dealt with by condition on planning permission.

Land use within Medium Probability Flood Zone 2 should be restricted to the ‘water compatible’, ‘less vulnerable’, ‘more vulnerable’ and ‘essential infrastructure’ categories. Where other planning pressures dictate that ‘highly vulnerable’ land uses should proceed, it will be necessary to ensure the requirements of the Exception Test are satisfied (Section 2.4).

The following should be submitted as part of the Flood Risk Assessment:

Finished floor levels should be above the 1 in 100-year (1%) climate change predicted maximum flood level plus a minimum freeboard of 300mm.

The development should be safe meaning that dry pedestrian egress should be possible above the 1 in 100-year (1%) climate change predicted maximum flood level and emergency vehicular access should be possible during times of flood.

300mm should be added to the 1 in 100- year level if there is no modelled climate change level (approved by the Environment Agency).

SUDS should be implemented to ensure that runoff from the site (post development) is not increased. For sites greater than one hectare the post development runoff volumes and peak flow rate should revert back to the greenfield rate or better (pre-development). Space should be set-aside for SUDS.

Other factors that may be relevant in the Flood Risk Assessment include reducing the footprint of the development, land swapping and the presumption against householder extensions.


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Future development within High Probability Flood Zone 3a Flood risk is a material planning consideration and for this reason a site-specific FRA must be carried out prior to granting any permission. The FRA is also necessary to check the proposal passes the Exception test prior to permission. The Flood Risk Assessment should not be dealt with by condition on planning permission.

Land-use within High Probability Flood Zone 3a should be restricted to the ‘water compatible’ and ‘less vulnerable’ uses to satisfy the requirements of the Sequential Test. For ‘more vulnerable’ and ‘essential infrastructure’ uses it is necessary to ensure that the requirements of the Exception Test are satisfied. ‘Highly vulnerable’ uses of land should not be permitted.

The following should be submitted as part of a flood risk assessment:

Properties situated within close proximity to formal defences or water retaining structures (reservoirs/canals) will require a detailed breach and overtopping assessment to ensure that the potential risk to life can be safely managed throughout the lifetime of the development. The nature of any breach failure analysis should be agreed with the Environment Agency.

The development should not increase flood risk elsewhere, and where possible, opportunities should be taken to decrease overall flood risk (such as use of SUDS and deculverting).

Finished floor levels should be above the 1 in 100-year (1%) climate change predicted maximum flood level plus a minimum freeboard of 300mm. Within defended areas the maximum water level should be assessed from a breach analysis.

The development should be safe meaning that dry pedestrian egress should be possible above the 1 in 100-year (1%) climate change predicted maximum flood level and emergency vehicular access should be possible during times of flood.

300mm should be added to the 1 in 100- year level if there is no modelled climate change level (approved by the Environment Agency)

An evacuation plan should be prepared. With respect to new developments, those proposing the development should take advice from the LPA’s emergency planning officer and for large-scale developments, the emergency services, when producing an evacuation plan as part of a FRA.

Basements should not be used for habitable purposes. Where basements are permitted for commercial use, it is necessary to ensure that the basement access points are situated 300 mm above the 1% probability flood level plus climate change.

SUDS should be implemented to ensure that runoff from the site (post development) is not increased. For sites greater than one hectare the post development runoff volumes and peak flow rates should revert back to the greenfield rate or better (pre-development). Space should be set-aside for SUDS.

The proposed development should be set-back from the watercourse to allow appropriate access for routine maintenance and emergency clearance, if necessary.

Other factors that may be relevant in the Flood Risk Assessment include reducing the footprint of the development, land swapping and the presumption against householder extensions.


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Future development within Functional Floodplain Zone 3b Development should be restricted to ‘water-compatible uses’ and ‘essential infrastructure’ that has to be there. ‘Essential infrastructure’ in this zone must pass the Exception Test and be designed/constructed to remain operational in times of flood and not impede water flow. Mitigation options to be considered include land swapping /reducing footprint for redevelopment applications. There will be the presumption against householder extensions.

10.5 Planning Response Matrix Table 10.2 details the planning responses appropriate for each of the flood zones identified as guided by PPS25. Appropriate planning responses are defined based on the vulnerability of the infrastructure to flood risk and are grouped on the basis of:

Essential infrastructure

Highly vulnerable

More vulnerable

Less vulnerable

Water-compatible development

Examples of the types of buildings/infrastructure which fall under these categories are detailed in Appendix J.

Table 10.2 does not show: the application of the Sequential test that guides development to Flood Zone 1 first, then Flood Zone 2, and then Food Zone 3; FRA requirements; or the policy aims for each Flood Zone.

Reference should also be made to the guidance provided by the Environment Agency - up to Summer 2008 this can be found on www.pipernetworking.com, and from Summer 2008 on the Environment Agency website www.environment-agency.gov.uk. This guidance also includes advice about the need to consult the Environment Agency regarding planning applications.

Table 10.2 Appropriate planning responses when seeking to develop within Flood Zones

Vulnerability of development

Essential infrastructure

Water compatible

Highly vulnerable

More vulnerable

Less vulnerable

Flood Zone 1 Yes Yes Yes Yes Yes

Flood Zone 2 Yes Yes Exception

Test required Yes Yes

Flood Zone 3a

Exception Test required Yes No Exception

Test required Yes

Zone 3b (functional floodplain)

Exception Test required Yes No No No

Yes = Development is appropriate, No = Development should not be permitted


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10.6 Development within areas affected by surface water and sewer flooding

Existing flooding locations should not be seen as areas to avoid development and the reverse is also true, that areas with no flooding should not always be viewed as areas best placed to accommodate development. What is essential is that all development locations are checked to ensure capacity exists within the network. Where capacity does not exist it is vital that upgrades are provided ahead of development. Failure to do so will increase the risk of internal / external flooding of properties and pollution of the wider environment.

The best way is to ensure capacity exists in the network is to carry out computer modelling of the existing network and then add in additional flows where development sizes and locations are known. An element of this was done in the Swindon Water Cycle Study (Phase I). Due to the complexities of the sewage and surface water networks and the infinite number of development opportunities at this point of the planning process, it is not possible to accurately assess areas which will be affected by flooding as a result of future development.

To ensure all future development is sustainable, detailed computer modelling of development sites will be carried out by Thames Water to identify infrastructure requirements once the exact location and scale of development is known. Development will not be allowed to precede the delivery of essential infrastructure, identified as part of this modelling work and Swindon Borough Council will work closely with Thames Water to reject unsustainable sites or attach planning conditions on sites where essential infrastructure is required.

At present, Thames Water has 14 potential flood relief schemes (Appendix G) that are currently under investigation to resolve existing flooding problems. All of these schemes will need to meet criteria agreed with OFWAT and receive financial approval before they go on site, but the aim is for the majority of these to be complete by 2010.


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11 Measures to make development permissible in higher risk flood zones

11.1 Overview This Chapter considers the measures to make development permissible within higher risk sites. Preference should always be given to locating development on sites with the lowest flood risk.

11.2 Potential measures Development within higher flood risk areas should only be considered if lower risk sites cannot be identified through application of the Sequential and Exception tests. Should development be deemed appropriate for higher risk sites this Section details measures that may be adopted to minimise the risk of flooding.

11.2.1 Flood defences There is only one flood defence within the study area that currently provides a 1 in 100-year standard of flood protection. Flood defences should not be used as an option to make development within higher flood risk areas permissible due to the risks of flood defence failure.

11.2.2 Flood storage areas If it is not possible to allocate sites in Flood Zones 1 and 2, and is deemed appropriate to site development within Flood Zone 3a or the Functional Floodplain (Flood Zone 3b) the only real mitigation option is flood storage areas along the route of a watercourse. This flood risk management approach is compatible with the Thames CFMP policy for enhancing the ability of the floodplain to retain water.

Potential sites for flood storage areas may include the Eastern, Northern and Wichelstowe Development areas and the area for search for development which is traversed by the lower reaches of the River Ray. If any of these sites are to be considered as potential flood storage areas, it is recommended that where practicable sites within these areas are safeguarded from development, until the feasibility of such a scheme has been investigated.

If this option is considered, the Environment Agency will need to be consulted about storage volumes and the feasibility of allocating development on the floodplain.

In some cases it may be reasonable for future developers to contribute (in full or in part) to the cost of such flood alleviation schemes which provide benefit to the wider community.

11.2.3 SUDS SUDS cannot be used to make development within high risk sites permissible but should be used to ensure that flood risk is not increased as a result of development. SUDS should be designed to limit the surface water runoff to greenfield rates.

11.2.4 Other measures Other measures may be appropriate such as flood resistant and resilient design, escape/evacuation, effective flood warning, emergency planning, etc. Further information can be found on the Environment Agency website.


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11.3 Recommendations for reducing existing flood risks

Where possible, identify long-term opportunities to remove development from the functional floodplain through land swapping.

Build resilience into design, e.g. flood resistant or resilient design, raised floor levels.

Enhancement opportunities should be sought when renewing assets, e.g. deculverting, the use of bioengineered river walls, raising bridge soffits to take into account climate change.

Avoid further culverting and building over of culverts - all new developments with culverts running through their site should seek to deculvert rivers for flood risk management and conservation benefit.

Seek to protect Greenfield functional floodplain from future development and where possible reinstate areas of functional floodplain which have been developed (e.g. reduce building footprints or relocate to lower flood risk zones).

Seek to improve the emergency planning process using the outputs from the SFRA.

Swindon Borough Council should work with the Environment Agency to promote awareness of flood risk to maximise the number of people signed up for the Flood Warning Direct service

Swindon Borough Council should work with the Environment Agency to encourage flood risk communities to develop a community flood plan to improve community awareness and resilience to flooding.

Encourage all those living in flood risk areas where a flood warning is available to sign up to Flood Warnings Direct service operated by the Environment Agency.

11.4 Need for flood risk assessment

This SFRA provides an overview of flood risk within the SFRA area. Further guidance is provided below on the need for FRAs and what should be addressed, which depends on the location of the site in relation to the SFRA flood zones on Tiles D1 to D4 (Volume II).

Site-specific Flood Risk Assessments (FRAs) will be required for most proposed developments to a level of detail that depends on the level of flood risk. The onus is on the developer to provide this information in support of a planning application, and obtain a ‘Letter of Compliance’ to be submitted with the planning application. It is therefore imperative that developers consult with Swindon Borough Council over the need for FRA early in the planning process.

Developers should be referred to the separate Developer Guidance Document prepared as part of the SFRA, and PPS25 Annex F that sets out a recommended process for undertaking FRA as part of an individual planning application (Figure 11.1).

Swindon Borough Council and the Environment Agency should continue to consult regularly to ensure that the Council’s policies on flood risk management are respected and taken account of, and that the scope of FRAs is commensurate with the level of flood risk.


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If Swindon Borough Council are minded to approve an application against the Environment Agency advice then the Government have been given the power to “call-in” the application and make the decision. This relates to ‘major sites’ as defined in the Flooding Direction/PPS25 (see also Section 4.3.7)

11.4.1 Proposed developments within Flood Zone 3b - functional floodplain In line with PPS25, development will not normally be allowed in the Functional Floodplain unless it is classified as a ‘water compatible’ or ‘essential infrastructure’ use. It is recognised however that the Functional Floodplain defined during this Level 1 SFRA study is conservative along the Lower Cole, River Ray and their tributaries, assuming that the whole of Flood Zone 3 is Functional Floodplain Zone 3b (Section 5.3).

Figure 11.1 Guidance for developers for individual planning applications


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It is therefore recommended that any FRA supporting a proposed development within Functional Floodplain 3b, as defined by the SFRA maps, will need to accurately define the Functional Floodplain (through detailed modelling) and show to the satisfaction of the Environment Agency that it can be considered as falling within Zone 3a (High Probability). Should this be the case then the assessment of flood risk should follow the recommendations in Section 11.4.2

11.4.2 Proposed developments within high probability Flood Zone 3a

This SFRA provides recommendations on the provision of sustainable flood risk mitigation opportunities within Flood Zones. These recommendations should inform the site-specific FRAs, and all FRAs supporting development within High Probability Zone 3a should assess:

Vulnerability of the development to flooding from other sources (e.g. surface water drainage, groundwater) as well as from river flooding. This will involve discussion with the Council and the Environment Agency to confirm whether a localised risk of flooding exists at the proposed site.

Vulnerability of the development to flooding over the lifetime of the development (including the potential impacts of climate change), i.e. maximum water levels, flow paths and flood extents within the property and surrounding area.

The Environment Agency may have carried out detailed flood risk mapping within localised areas that could be used to underpin this assessment. Where available, this will be provided at a cost to the developer. Where not available, hydraulic modelling by suitably qualified engineers will be required to determine the flood risk to the site (guidance on modelling of watercourses is available from the Environment Agency).

Potential of the development to increase flood risk elsewhere through the addition of hard surfaces, the effect of the new development on surface water runoff, and the effect of the new development on depth and speed of flooding to adjacent and surrounding property. This will require a detailed assessment to be carried out by a suitably qualified engineer.

All forms of flooding as localised flooding may also occur, typically associated with local catchment runoff following intense rainfall. This localised risk of flooding must also be considered as an integral part of the detailed Flood Risk Assessment.

Residual risks of flooding (after existing and proposed flood management and mitigation measures are taken into account) to determine if acceptable. Measures may include flood defences, flood resistant and resilient design, escape/evacuation, effective flood warning and emergency planning.

Levels across the site, including details of existing site levels, proposed site levels and proposed ground floor levels - all levels to be stated relative to Ordnance Datum.

Any flood compensatory storage, dry access requirements and surface water drainage systems should be designed to the design flood/storm event. The design flood/storm event is based on the 1 in 100-year flood/storm event plus an allowance for climate change - the Environment Agency advise details of what this allowance will be.


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Additionally, it is essential that developers thoroughly review the existing and future structural integrity of any defences upon which the development will rely (i.e. over the lifetime of the development), and ensure that emergency planning measures are in place to minimise risk to life in the event of a defence failure.

11.4.3 Proposed development within medium probability Flood Zone 2 For all sites within Medium Probability Zone 2, a scoping level FRA should be prepared based upon readily available existing flooding information, sourced from the Environment Agency.

The risk of other sources of flooding (e.g. surface water, sewage, and/or groundwater) must be considered, and sustainable urban drainage techniques must be employed to ensure no worsening of existing flooding problems elsewhere within the area.

If significant flood risk from other sources (e.g. groundwater or sewer flooding) is identified then a more detailed FRA should be prepared. It will be necessary to demonstrate that the residual risk of flooding to the property is effectively managed throughout, e.g. the provision of raised floor levels and the provision of planned evacuation routes or safe havens.

11.4.4 Proposed development within Flood Zone 1 The risk of other sources of flooding (e.g. surface water, sewage, and/or groundwater) must be considered, and sustainable urban drainage techniques must be employed to ensure no worsening of existing flooding problems elsewhere within the area.

If significant flood risk from other sources (e.g. groundwater or sewer flooding) is identified then a more detailed FRA should be prepared. It will be necessary to demonstrate that the residual risk of flooding to the property is effectively managed throughout, e.g. the provision of raised floor levels and the provision of planned evacuation routes or safe havens.

11.5 Monitoring post development runoff

The Environment Agency requires a Flood Risk Assessment (FRA) to be undertaken at the planning stage of a development, and that any development is built in accordance with this FRA. Monitoring post-development runoff could provide a ‘check’ that the proposed flood risk management measures have been implemented and perform effectively.

At present, there is no formal requirement (by the Environment Agency or others) for monitoring post-development runoff. Such monitoring by developers is deemed appropriate for larger development sites (say greater than one hectare), and the proposals for this monitoring would be approved by the Environment Agency.

It is possible that post-development runoff can be measured by installing sewer flow monitors and obtaining rainfall data from the Meteorological Office to calculate the proportion of rainfall that is flowing through the drainage system post-development. The feasibility of measuring flow will be depend on pipe sizes. This is because it is difficult to accurately measure flow in small pipes and as a result any flow measured tends to be unreliable. Whereas, for larger pipes (greater than 300mm in diameter) the flow measured tends to be more accurate.


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12 Conclusions & Recommendations

In June 2007, Swindon Borough Council commissioned Halcrow to produce this Level 1 SFRA in accordance with PPS25 and Environment Agency guidance. The SFRA output is relevant not only to planning and development control, but also other related assessment such as site-specific FRAs, mapping for emergency planning, flood alleviation studies and surface water management plans.

The flood risk within the SFRA area largely arises from river and surface water flooding, with a limited amount of sewer and groundwater flooding. Limited urban areas of Swindon are currently at risk of flooding from a 1 in 100-year flood event, with some risk to particular locations and key infrastructure identified (Table 6.8).

The SFRA Flood Zone map Tiles D1 to D1 (Volume II) and Table 10.1 show that 13 of the allocated development areas and areas of search for development lie within Flood Zone 3 (although in many cases the area affected is small). The Sequential Test should be applied to direct any development away from these higher flood risk areas, but where this is not possible the Exception Test must be passed.

Based on this SFRA the following recommendations are made.

12.1.1 Site allocation process It is recommended the SFRA outputs are used as an evidence base from which to

direct new development where possible to areas of low flood risk (Flood Zone 1).

Where development cannot be located in Flood Zone 1, the SFRA flood maps should be used to apply the Sequential Test to the remaining land use allocations.

Where the need to apply the Exception Test is identified, due to there being an insufficient number of suitable sites for development within zones of lower flood risk, the scope of the SFRA will need to be widened to Level 2 SFRA.

The need for a Level 2 SFRA cannot be fully determined until the Sequential Test has been applied.

As soon as the need for the Exception Test is established, Level 2 SFRA is undertaken by a suitably qualified engineer so as to provide timely input to the overall LDF process.

12.1.2 Possible Funding Mechanisms - Planning Obligations & Statutory Planning Changes

The funding mechanism to implement flood risk reduction measures and other facilities is likely to be an important policy consideration. Circular 05/2005 provides for S106 planning obligations to be sought where they meet the tests set out in the Circular.

Such obligations are intended to secure contributions from developers to address the impact of new development, without which such development should not be permitted. Such impacts can include flood water conveyance/storage and flood defences.


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There have been a number of recent initiatives to achieve enhanced contributions via S106 planning obligations. One of the most advanced schemes involves a tariff-based funding system covering development in the Expansion Areas in Milton Keynes.

The objective of the approach is to ensure that Expansion Area development is supported by appropriate facilities, amenities and infrastructure. Milton Keynes’ tariff includes flood risk management and drainage provision.

Swindon Borough Council may wish to consider the potential of S106 planning obligation contributions to fund (or part fund) strategic flood risk management facilities.

In some cases it may be reasonable for the developer to contribute to the up-grade or replacement of existing flood defences and surface water infrastructure, or to flood alleviation schemes which provide benefit to the wider community.

12.1.3 Swindon Borough Council policy Swindon Borough Council policy is essential to ensure that the recommended

development control is imposed consistently and ultimately leads to sustainability with respect to flood risk management.

Current Swindon Borough Council policy should be reviewed in light of PPS25 and this SFRA to ensure a consistent policy is being promoted with regard to flood risk, and the following key considerations are adhered.

Key considerations for Swindon Borough Council policy:

• Directing vulnerable development away from flood affected areas • Where possible, identify long-term opportunities to remove development from

the functional floodplain through land swapping. • Seeking to protect the greenfield Functional Floodplain from development • Where possible reinstate areas of Functional Floodplain previously developed

(e.g. reduce building footprints or relocate to lower flood risk zones). • Ensuring all new development is ‘Safe’, meaning that dry pedestrian egress

through the floodplain and emergency vehicular access is possible • Promote the application of SUDS for all new development • Support flood alleviation measures under consideration by the Environment

Agency by safeguarding possible sites for flood storage and channel works. • Seek developer contributions via S106 planning obligation to fund (or part

fund) strategic flood risk management facilities to benefit the wider community • Build resilience into design, eg. flood resistant or resilient design, raised floors • Seek enhancement opportunities when renewing assets. • Avoid further culverting and building over of culverts. • Seek to improve the emergency planning process using the SFRA outputs.


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12.1.4 Sewer flooding Storage, diversions, local upsizing of sewers and upgrading of pumping stations may

alleviate current flooding, though alternative solutions should be sought for future developments.

New developments such as Wichelstowe are planned to pump flows directly to Swindon sewage treatment works (STW) via a new rising which will by-pass the old system, negating the possible exacerbating effect of connecting into it.

Other future developments (in all areas) should by-pass the older town systems in a similar fashion or be directed to a dedicated treatment works, as there is no spare capacity in the existing sewage system.

12.1.5 Emergency planning Swindon Borough Council’s Flood Guide should be reviewed and updated in light of

the findings of the SFRA to ensure that safe evacuation and access for emergency services is possible during times of flood both for existing developments and those being promoted as possible sites within the LDF process.

Swindon Borough Council should work with the Environment Agency to promote awareness of flood risk to maximise the number of people signed up for the Flood Warning Direct service.

Swindon Borough Council should work with the Environment Agency to encourage flood risk communities to develop a community flood plan to improve community awareness and resilience to flooding.

Encourage all those living/in flood risk areas where a flood warning is available to sign up to Flood Warnings Direct service operated by the Environment Agency.

12.1.6 Future updates to the SFRA The SFRA should be retained as a ‘living’ document and reviewed on a regular basis

in light of better flood risk information and emerging policy guidance.

Flood mapping outputs from the following studies, once approved by the Environment Agency, should be used for future SFRA updates:

• Iffley to Purton Flood Study (Swindon Borough Council) – will provide refined PPS25 Flood Zones for the River Ray

• Eastern Development Area (East Swindon Development Group) - will provide refined PPS25 Flood Zones for the River Cole

Planning Policy Statement: Planning and Climate Change (supplement to PPS 1) (DCLG, December 2007) provides further guidance on how planning should secure new development to the effects of climate change.


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References

CIRIA; The SUDS Manual (C697)

CIRIA; SUDS –Design manual for England & Wales (C522)

CIRIA; SUDS – Best practice manual (C523)

CIRIA; SUDS – hydraulic, structural and water quality advice (C609)

Defra 2004; Strategy for Flood and Coastal Erosion Risk Management: Groundwater Flooding Scoping Study (LDS 23) Final Report

Defra (in preparation); Making Space for Water; Groundwater flooding records collation, monitoring and risk assessment (reference HA5); Draft Consolidated Report Main Summary Report

Communities and Local Government 2007, Development and Flood Risk, a Practice Guide Companion to PPS25

Environment Agency 2006; Groundwater Protection: policy and practice; Underground, under threat; The state of groundwater in England and Wales.

Environment Agency 2007; Thames Region Catchment Flood Management Plan; Consultation Document

Jacobs 2004; Strategy for Flood and Coastal Erosion Risk Management: Groundwater Flooding Scoping Study (LDS 23), Final Report

National SUDS Working Group 2004; Interim Code of Practice for Sustainable Drainage Systems

Planning Policy Statement: Planning and Climate Change (supplement to PPS 1), 2007

Planning Policy Statement 25: Development and Flood Risk, 2006

The Groundwater Regulations 1998; Statutory Instrument 1998; No 2746 Environmental Protection


APPENDIX A Audit trail database


APPENDIX B Geology & soils


APPENDIX C Model reviews


APPENDIX D Locations with blockage potential


APPENDIX E July 2007 flood study of Whitehill Stream, Swindon


APPENDIX F Localised flooding database


APPENDIX G Sewer flooding


APPENDIX H Site visit


APPENDIX I Main defences


APPENDIX J PPS25 flood risk vulnerability classification


APPENDIX K Using computer river modelling as part of a FRA Best practice guidance


APPENDIX L Swindon’s Flood Review: July 2007 An investigation into the causes and flood risk management options