Strategic Flood Risk Assessment 2007 - City of London · 2014-05-01 · Strategic Flood Risk Assessment 9 City of London Final - August 2007 1 Introduction 1.1 Background Planning

Strategic Flood Risk Assessment August 2007

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Strategic Flood Risk Assessment

10 August 2007 Produced for City of London Corporation Prepared by Ian Bakewell Knights House 2 Parade Sutton Coldfield West Midlands B72 1PH T 0121 355 8949 F 0121 355 8901 E [email protected]

Strategic Flood Risk Assessment 1 City of London

Final - August 2007

Document Control Sheet

Report Title Strategic Flood Risk Assessment

Revision 01

Status Final

Control Date 10 August 2007

Record of Issue

Issue Status Authors Date Check Date Authorised Date

01 Final Ian Bakewell

Emily Blanco 10/8/07 Faruk Pekbeken 10/8/07 Faruk Pekbeken 10/8/07

Distribution

Organisation Contact Copies

City of London Corporation Janet Laban 3


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Contents

Document Control Sheet ............................................................................................1 Contents ......................................................................................................................1 Table of figures ...........................................................................................................1 Tables...........................................................................................................................1 1 Introduction ........................................................................................................1 1.1 Background..........................................................................................................1 1.2 Scope and Objectives ..........................................................................................1 1.3 Setting the Scene.................................................................................................1 2 Planning Policy and Flood Risk ........................................................................1 2.1 National Planning Policy and Guidance................................................................1

2.1.1 Planning and Compulsory Purchase Act 2004 1 2.1.2 Planning Policy Statement 1 1 2.1.3 Planning Policy Statement 25 1

2.2 The Sequential Test and Exception Test ..............................................................1 2.2.1 The Sequential Test 1 2.2.2 The Exception Test 1

2.3 Planning Responsibilities .....................................................................................1 2.4 Other Planning Statements and Policies ..............................................................1 2.5 Making Space For Water......................................................................................1 2.6 Key Stakeholders in the Planning Process ...........................................................1

2.6.1 Environment Agency 1 2.6.2 Thames Water 1

3 Review of Development Framework .................................................................1 3.1 Regional Planning Policy – The London Plan.......................................................1 3.2 Local Planning Policy – The UDP and LDF Core Strategy ...................................1 4 Data Collection and Validation..........................................................................1 4.1 Mapping and Topographic Data ...........................................................................1 4.2 Flood Defences....................................................................................................1 4.3 Sewer Data ..........................................................................................................1 4.4 River Thames Data ..............................................................................................1

4.4.1 Historic Flood Records 1 4.4.2 Flood Modelling Results 1


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4.5 Data Gaps............................................................................................................1 5 Sources of Flooding...........................................................................................1 5.1 Introduction ..........................................................................................................1 5.2 Primary Sources of Flooding ................................................................................1

5.2.1 River Thames 1 5.2.2 The 1928 Floods 1 5.2.3 Overtopping 1 5.2.4 Breach 1 5.2.5 River Fleet and River Walbrook 1

5.3 Secondary Sources of Flooding ...........................................................................1 5.3.1 Sewer Flooding 1 5.3.2 Sewer Flooding with Fluvial Interaction 1 5.3.3 Groundwater 1 5.3.4 London Fire Brigade 1 5.3.5 London Underground Flooding 1 5.3.6 Infrastructure Failure 1

5.4 Source Pathway Receptor Model .........................................................................1 5.4.1 Scale of Consequences 1

6 Flood Risk Assessment.....................................................................................1 6.1 Sustainability and Climate Change.......................................................................1 6.2 Fluvial Flooding ....................................................................................................1

6.2.1 Flood Risk Mapping 1 6.2.2 Flood Defence Assets 1 6.2.3 Flood Defence Levels 1 6.2.4 Thames Barrier 1 6.2.5 Extreme Water Level Assessment 1 6.2.6 Overtopping Assessment 1 6.2.7 Breach Analysis 1 6.2.8 Combined Overtopping and Breach Assessment 1 6.2.9 Mapping of Flood and Hazard Zones 1

7 Application of Sequential Flood Risk Test .......................................................1 7.1 Flood Risk Zone Categories .................................................................................1 7.2 Application of sequential test................................................................................1 7.3 Assessment of Risk within Zone 3........................................................................1

7.3.1 Application of the Exception Test 1


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8 Local Policy Guidance .......................................................................................1 8.1 Emergency Planning ............................................................................................1 8.2 Future use of the SFRA........................................................................................1 8.3 Monitoring and Review of Policies........................................................................1 9 Guidance for Developers...................................................................................1 9.1 Development and the management of residual flood risk .....................................1 9.2 Drainage Assessments ........................................................................................1 9.3 Sustainable Urban Drainage Systems..................................................................1

9.3.1 SUDS Selection 1 9.3.2 Pervious Pavements 1 9.3.3 Green Roofs 1 9.3.4 On/Off-line storage 1 9.3.5 Adoption and Maintenance of SUDS 1

10 Conclusions .......................................................................................................1 10.1 Further Studies.....................................................................................................1 10.2 Recommendations for Further Work.....................................................................1


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Table of figures

Figure 1 - City of London Study Area ........................................................................1

Figure 2 - Planning Hierachy and Flood Risk.............................................................1

Figure 3 - Plan of London Sewers .............................................................................1

Figure 4 - Leith House Borehole Level ......................................................................1

Figure 5 - ISIS Flood Hydrograph (2002 levels).........................................................1

Figure 6 - Flood Hydrograph With Against Flood Defence Level ...............................1

Figure 7 - Illustration of a Pervious Pavement using Block Paving ............................1

Figure 8 - Illustration of an extensive green roof........................................................1

Figure 9 - Illustration of an Intensive Green Roof ......................................................1

Figure 10 - Illustration of a Simple Intensive Green Roof...........................................1


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Tables

Table 1 – Project Objective Reference Table ............................................................1

Table 2 - Definition of Flood Zones............................................................................1

Table 3 - Flood Risk Vulnerability Classification ........................................................1

Table 4 - Flood Risk Vulnerability and Flood Zone Compatibility ...............................1

Table 5 - Summary of SFRA Data .............................................................................1

Table 6 - Source Pathway Receptor Model ...............................................................1

Table 7 - Scale of Consequences..............................................................................1

Table 8 - Flood Defence Locations............................................................................1

Table 9 - Flood Levels from River Thames One Dimensional ISIS Model..................1

Table 10 - Danger to people from overtopping relative to distance from defence ......1

Table 11 - Key to Overtopping Danger Classifications...............................................1

Table 12 - Flood Risk to People Behind Defences ....................................................1

Table 13 - Danger to people from breaching relative to distance from defence .........1

Table 14 - Flood Risk Zones and Development Restrictions .....................................1

Table 15 - SUDS Score Techniques for Site Specific Constraints .............................1


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Executive Summary The City of London Corporation is in the process of compiling its Local Development Framework (LDF) to guide future development needs of the City. In order to ensure that flood risk is considered as part of the spatial planning process, The City of London Corporation invited Mouchel Parkman to undertake a Strategic Flood Risk Assessment on their behalf.

The objectives of the SFRA were predominantly informed by the requirements of Planning Policy Statement 25, which requires decision makers involved in the planning process to consider regional and local flood risk issues when planning development.

The Primary aims of the SFRA were:

• To Identify the areas within the City of London that are at risk of flooding for all Flood Zones identified in table D1 in PPS 25, and within Flood Zone 3, the variations in the actual flood risk including the effect of any defences.

• Identify the risk of flooding due to surface water either in the form of flash flooding due to surface water run-off, rising groundwater, inadequate drain/sewer capacity or inadequate drain/sewer maintenance

• Identify the likely effects of climate change on flood risk

• Provide the basis for allocating sites in the Local Development Framework (LDF) including, if necessary, applying the sequential test approach to site allocation within the indicative flood plain.

• Provide a clear rationale for assessing the merits of potential development allocations based on a sequential flood risk assessment, taking into account the flood risk vulnerability of proposed uses (table D2, PPS25)

• Recommend policy options for dealing with the range of flood risks

• Recommend appropriate monitoring and review methods

All forms of flooding were investigated, primarily by compiling and reviewing relevant information provided by a wide variety of sources, but primarily the Environment Agency, the City of London Corporation and Thames Water.

A source pathway receptor model was used to assess those flood sources which had the greatest consequences. The primary source of flood risk was unsurprisingly determined to be the risks posed by the River Thames. The risk of flooding from


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secondary sources was in general found to be low, also the information required to make detailed assessments of the secondary sources was generally unavailable.

The delineation of Flood Zones was undertaken by analysing the residual risk to people behind flood defences through simplified breach and overtopping assessments. This assessment of residual risk was then combined with the projection of Flood Zones to produce a map which can be used to allocate development by applying the sequential and exception tests as set out in PPS25.

Guidance on applying the sequential test to developments in the City of London has also been provided.

The findings of the SFRA were used to advise on local planning policy issues and provide guidance to developers on the management of residual flood risk and surface water drainage through the use of Sustainable Urban Drainage Systems.


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1 Introduction

1.1 Background Planning Policy Statement 25: Development and Flood Risk (PPS25) issued in December 2006 by the Department for Communities and Local Government; sets out the national policy for land use planning and flood risk management in England. The policy highlights the requirement for each Local Planning Authority (LPA) to consider flood risk and flood risk management within local development documents and ensure that informed decisions on the flood risk attributed to new developments are made by those involved in the planning process. Key to this risk based approach is the production of a Strategic Flood Risk Assessment (SFRA) report to consider catchment wide flood risk issues. SFRAs are required to be produced by individual or groups of LPAs in conjunction with their Local Development Framework (LDF).

The City of London Corporation appointed Mouchel Parkman on 25th January 2007 to undertake a SFRA on their behalf. The relative size of the City of London and surrounding Boroughs would ordinarily lend themselves to a combined approach to compiling a SFRA; however, the City of London Corporation’s timescale for producing their LDF required them to undertake an independent study ahead of their neighbouring Boroughs.

The City of London square mile incorporates the world’s leading financial and business centre which is a major driver for both the London and national economies. Approximately 300,0001 people work within the commercial heart of the City and business activities are supported by an appropriate scale of infrastructure services. Commercial floorspace is the predominant land use, although there are approximately 8,6001 residents within the study area. The City has a significant historic value as the core from which the surrounding London area developed. The City is also a major public transport hub, delivering the substantial numbers of commuters in and out of the city. Figure 1 - City of London Study Area, shows the City of London boundary and key features. A more detailed map (Map No. 1) is also included in Appendix B for reference.

1 City of London Local Development Framework Core Strategy, Issue and Options, May 2006


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Figure 1 - City of London Study Area

1.2 Scope and Objectives

The objectives of this SFRA are based on the brief issued by the City of London Corporation which was predominantly informed by the requirements of PPS25. The primary objective of any SFRA is to provide decision makers involved in the planning process with a better understanding of local flood risk issues and enable the LPA to consider flood risk at the earliest stages of the planning process. The SFRA process is aimed at identifying areas most suitable for sustainable development through the application of the sequential test as set out in PPS25. A summary of the objectives of this study are summarised below:

• Identify the areas within the City of London that are at risk of flooding for all Flood Zones identified in table D1 in PPS 25, and within Flood Zone 3, the variations in the actual flood risk including the effect of any defences.


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• Identify the risk of flooding due to surface water either in the form of flash flooding due to surface water run-off, rising groundwater, inadequate drain/sewer capacity or inadequate drain/sewer maintenance

• Identify the likely effects of climate change on flood risk

• Identify potential for City development to affect flood risk in areas beyond the City’s boundaries

• Provide the basis for allocating sites in the Local Development Framework (LDF) including, if necessary, applying the sequential test approach to site allocation within the indicative flood plain.

• Provide a clear rationale for assessing the merits of potential development allocations based on a sequential flood risk assessment, taking into account the flood risk vulnerability of proposed uses (table D2, PPS25)

• Recommend policy options for dealing with the range of flood risks

• Recommend appropriate monitoring and review methods

Table 1 – Project Objective Reference Table

Project Objective Chapter(s) Where

Objective is achieved

1 Identify the areas within the City of London that are at risk of flooding for all flood zones identified in

table D1 in PPS 25, and within Flood Zone 3, the variations in the actual flood risk including the effect

of any defences.

5, 5.2, 5.4, 6.3

2 Identify the risk of flooding due to surface water either in the form of flash flooding due to surface water

run-off, rising groundwater, inadequate drain/sewer capacity or inadequate drain/sewer maintenance 5, 5.3, 5.4

3 Identify the likely effects of climate change on flood risk 6.2

4 Identify potential for City development to affect flood risk in areas beyond the City’s boundaries 5.3

5 Provide the basis for allocating sites in the Local Development Framework (LDF) including, if

necessary, applying the sequential test approach to site allocation within the indicative flood plain. 7

6 Provide a clear rationale for assessing the merits of potential development allocations based on a

sequential flood risk assessment, taking into account the flood risk vulnerability of proposed uses

(table D2 in PPS 25)

7

7 Recommend policy options for dealing with the range of flood risks 8, 9

8 Recommend appropriate monitoring and review methods 8.3


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In addition to informing the LDF the aims of the SFRA are to provide advice on flood risk management policies, inform the local sustainability appraisal and provide advice on the requirements for local development flood risk assessments. Although the objectives of the SFRA are clearly defined in PPS25, the scope of the assessment has been tailored to suit the unique requirements of the City of London. The square mile area of the City of London is largely located outside of the fluvial floodplain, and it was determined early on in the scheme that a high level assessment of flood risk would be appropriate for the majority of the study area. However, the high rate of change of development required to provide the City’s leading businesses with appropriate commercial floorspace necessitates planners to maximise development opportunities. In order to ensure that the SFRA informs the planning process the scope of the assessment has been widened to provide sufficient level of detail to enable the application of the exception test within flood risk zones by determining the residual risk within Flood Zones 2 and 3.

It is inevitable the River Thames will be viewed as the primary source of flooding and therefore the greatest flood risk facing the City of London. However, it is important that other sources of flooding are not overlooked during the planning process and this SFRA ensures that secondary sources of flooding are considered at scale appropriate to the risk which they pose to development and infrastructure within the City.

1.3 Setting the Scene Geographically the City of London Borough lies on the north bank of the River Thames at the heart of the Greater London area and is the historic core from which the rest of the London grew. Historically The City was founded on the higher ground along the banks of the River Thames, now known as Ludgate Hill. The location of the City on Ludgate Hill provides a large proportion of the Borough with natural protection from fluvial flooding, especially in comparison with neighbouring Boroughs and the south bank of the Thames. A plan showing the topography of the City is included in Appendix B, (Map No. 2).

The City of London has borders with five of the surrounding Boroughs, The City of Westminster to the west, where the border runs east along Strand and Fleet Street before turning northwards along Chancery Lane. Part way along Chancery Lane the neighbouring borough becomes the London Borough of Camden, continuing north to Holborn where it keeps going in an easterly direction before joining Charterhouse Lane. As the border intersects Farringdon Road it becomes the border with the Borough of Islington. To the east of Aldersgate the city borders the London Borough of Hackney and to the East the Borough of Tower Hamlets. A map (No. 3) showing


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the location of the City of London and its relationship to the other boroughs is included in Appendix B.

Two natural watercourses, The River Fleet and River Walbrook flow through the City and confluence with the River Thames within the City’s boundary. Both rivers are entirely canalised and have long since been incorporated into the extensive sewer network beneath London.

Historically there is little evidence of flooding impacting on the City of London. However, there are records to show that part of the City flooded during the 1928 floods. This flood is the worst recorded incident to affect London with an upstream flood level of 5.55m Above Ordnance Datum (AOD). The floods of January 1928 are widely considered to be attributed to a combination of high spring tide, storm surge off the east coast of England and snow melt within the upper Thames catchment. Although this was the worst recorded flood, Map 4 in Appendix B shows that only a relatively small area of City was affected. The actual cause of flooding is unknown, however, local knowledge suggests that the flooding may have been attributed to inundation of the sewer system from flooding elsewhere in London. Alternative mechanisms may have been attributed to a local embankment failure or the local flood defences may have been lower in height than their current state.

The economic, commercial and historic importance of the City and surrounding London Boroughs ensures that a substantial amount of information and data relevant to the SFRA is already available to inform this study. As such, the assessment largely relies on the collection, analysis and representation of this information. A review of all the data made available for the study from a wide variety of sources is presented in Section 4, along with the identification of data gaps and their relevance to the study.


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2 Planning Policy and Flood Risk

Major changes to the national, regional and local planning policies have been instigated by the enactment of the Planning and Compulsory Purchase Act 2004. Regional Planning Guidance has been replaced with Regional Spatial Strategies (RSSs), with the London Plan being published in February 2004. Following on from the RSSs, Local Planning Authorities (LPAs) are required to replace existing Unitary Development Plans with Local Development Frameworks (LDFs), a suite of documents providing guidance on the use and development of land, which must conform with the overarching RSS.

This City of London SFRA is a freestanding document that will support the LDF and compliments the overall City of London Sustainability Appraisal. The assessment is undertaken in the context of Planning Policy Statement 25: Development and Flood Risk (PPS 25) published in December 2006. This statement replaces Planning Policy Guidance note 25: Development and Flood Risk (PPG 25), published in 2001, and is part of the Government’s ongoing strategy of replacing Planning Policy Guidance with Planning Policy Statement that provide “statements of government policy on nationally important land use and other planning matters, supported where appropriate by locational framework”2.

2.1 National Planning Policy and Guidance 2.1.1 Planning and Compulsory Purchase Act 2004

The Planning and Compulsory Purchase Act 2004 received Royal Assent in May 2004. Its primary goal is to reform and speed up the planning process and make planning decisions more predictable, while delivering a more sustainable approach to planning. The Act affects the planning process at both regional and local levels, with the requirement for development of RSSs and LDFs.

Regional planning bodies and LPAs will now have a statutory duty to ensure that development documents promote sustainable development and Sustainability Appraisals must be carried out as part of the LDF process. Local Planning Authorities will also be required to report annually on the achievement of locally set policies and targets. This puts the emphasis on LPAs to promote and implement good quality development rather than just facilitating the planning process.

2 Department of Communities and Local Government Planning Policy Guidance 1: General Policy and Principles, 1997


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2.1.2 Planning Policy Statement 1 Planning Policy Statement 1 Delivering Sustainable Development, published in 2005, sets out the overarching planning policies and provides the guidance from which the other PPSs will follow. PPS1 clearly identifies the requirement for future development to take account of sustainability and flood risk. The following extracts are taken from PPS1 and identify the need for regional and local plans to take account of flood risk and flood management.

“Regional planning bodies and local planning authorities should ensure that development plans contribute to global sustainability by addressing the causes and potential impacts of climate change.”

“Development plan policies should take account of environmental issues such as: the potential impact of the environment on proposed developments by avoiding new development in areas at risk of flooding and sea-level rise, and as far as possible, by accommodating natural hazards and the impact of climate change.”

“Key objectives should include ensuring that developments are sustainable, durable and adaptable (including taking account of natural hazards such as flooding)…”

The introduction of PPS1 and subsequently PPS25 (see chapter 2.1.3) is a clear policy change towards the management of flood risk through the planning process. In order to effectively action this policy change, those involved in the planning process must be provided with clear guidance.

2.1.3 Planning Policy Statement 25 The introduction of Planning Policy Statement 25 (PPS 25) in December 2006 outlines the responsibility of Regional Planning Bodies and Local Planning Authorities to prepare and implement planning strategies which help to deliver sustainable development by ensuring that flood risk is understood and managed effectively as an integral part of planning process.

Regional and Local Planning Authorities are required to prepare and implement planning strategies that help deliver sustainable development by preparing Regional Flood Risk Appraisals (RFRAs) or Strategic Flood Risk Assessments (SRFAs) as appropriate. These look at catchment wide flooding issues with the aim to appraise, manage and reduce risk by:

“Identifying land at risk and the degree of risk of flooding from river, sea and other sources in their areas”


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“Only permit development in areas of flood risk when there are no reasonably available sites in areas of lower flood risk and benefits of the development outweigh the risks from flooding”

“Reduce flood risk to and from new development through location, layout and design, incorporating sustainable drainage systems (SUDS)”

Local Planning Authorities must prepare SFRAs in consultation with the Environment Agency, emergency response teams and the local drainage authority. The SFRA should build upon existing flood maps by taking into account other sources of flooding in order that it can provide a basis from which to apply the Sequential Test and Exception Test in development allocation.

2.2 The Sequential Test and Exception Test 2.2.1 The Sequential Test

A sequential risk based approach should be applied at all levels of the planning process to determine the suitability of land for development in areas at risk of flooding. The aim of the Sequential Test is to steer new developments to areas with the lowest probability of flooding.

Flood zones are the basis of the sequential approach. Zones 2 and 3 are shown on Environment Agency Indicative Floodplain Maps with Flood Zone 1 being all land falling outside Zones 2 and 3.

In areas at risk of river or sea flooding, preference should be given to locating development in Flood Zone 1. If there is no reasonably available site in Flood Zone 1, the flood vulnerability of the proposed development should be taken into account in locating the development in Flood Zone 2 and then Flood Zone 3.

Within each Flood Zone, new development should be located at sites with the lowest probability of flooding from all sources as indicated by the SFRA.

Table 2 below defines each Flood Zone referring to the probability of sea and river flooding only, ignoring the presence of all existing flood defences (including the Thames Barrier). Table 4 defines the vulnerability classification of proposed developments.


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Table 2 - Definition of Flood Zones

Flood Zone Definition Appropriate Uses

Zone 1

Low Probability

< 1 in 100 annual probability of river or sea flooding in any year (<0.1%)

All uses of land are appropriate in this zone.

Zone 2 Medium Probability

1 in 100 to 1 in 1000 annual probability of river flooding (1% - 0.1%) or between a 1 in 200 and 1 in 1000 annual probability of sea flooding (0.5% - 0.1%) in any year.

“Water-compatible”, “less vulnerable” and “more vulnerable” uses of land and essential infrastructure are appropriate in this zone.

Subject to the Sequential Test being applied, the “highly vulnerable” uses of land are only appropriate in this zone if the Exception Test is passed.

Zone 3a High Probability

1 in 100 or greater annual probability of river flooding (>1% ) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year.

“Water-compatible” and “less vulnerable” uses of land are in this zone.

“Highly vulnerable” development should not be permitted in this zone.

“More vulnerable” uses of land and “essential infrastructure” should only be permitted in this zone if the Exception Test is passed.

Essential infrastructure permitted in this zone should be designed and constructed to remain operational and safe for users in times of flood.

Zone 3b The Functional Floodplain

Land where water has to flow or be stored in times of flood.

SFRAs should identify this Flood Zone (land which would flood either an annual probability of 1 in 20 (5%) or greater in any year or is designed to flood in an extreme (0.1%) flood, or at another probability to be agreed between the LPA and the Environment Agency, including water conveyance routes).

Only the water-compatible uses and the essential infrastructure 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 flows: and

• not increase flood risk elsewhere

Essential infrastructure in this zone should pass the Exception Test.


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Table 3 - Flood Risk Vulnerability Classification

Essential Infrastructure

• Essential transport infrastructure (including mass evacuation routes) which has to cross the area at risk, strategic utility infrastructure, including electricity generating power stations and grid and primary substations.

Highly Vulnerable • Police stations, Ambulance stations and fire stations and Command Centres and telecommunications installations required to be operational during flooding.

• Emergency dispersal points • Basement dwellings • Caravans, mobile homes and park homes intended for

permanent residential use. • Installations requiring hazardous substances consent

More Vulnerable • Hospitals • Residential institutions such as residential care homes,

children’s homes, social services homes, prisons and hostels • Buildings used for : dwellings houses, student halls of

residence, drinking establishments, nightclubs and hotels • Non-residential uses for health services, nurseries and

educational establishments • Landfill and sites used for waste management facilities for

hazardous waste • Sites used for holiday or short-let caravans and camping,

subject to a specific warning and evacuation plan. Less Vulnerable • Buildings used for: shops, financial, professional and other

service, restaurants and cafes; hot food takeaways, offices, general industry, storage and distribution, non-residential institutions not included in ‘more vulnerable’ and assembly and leisure.

• Land and buildings used for agriculture and forestry. • Waste treatment (except landfill and hazardous waste

facilities). • Minerals working and processing (except for sand and gravel

working). • Water treatment plants (if adequate pollution control

measures are in place).

Explanation of Flood Risk Probability

Flood events and flood risk zones are commonly referred to in terms of the probability that a particular flood event will occur. A flood event with a probability of 1% is defined as an event that has a 1 in 100-year or greater chance of occurring in any one year.


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2.2.2 The Exception Test If following the application of the Sequential Test it is not possible for development to be located in zones of lower probability of flooding, the Exception Test can be applied as a method of managing flood risk while still allowing necessary development to occur.

The Exception Test should be applied in the circumstances as shown in Table 4 below when “more vulnerable” development and “essential infrastructure” cannot be located in Zones 1 or 2 and “highly vulnerable” development can not be located in Zone 1.

Table 4 - Flood Risk Vulnerability and Flood Zone Compatibility

Flood Risk Vulnerability classification

Essential Infrastructure

Water Compatible

Highly Vulnerable

More Vulnerable

Less Vulnerable

Zone 1 � � � � �

Zone 2 � �

Exception Test required � �

Zone 3a Exception

Test required � �

Exception Test

required �

Zone 3b Exception

Test required � � � �

Source Annex D PPS 25

Further guidance on the exception test is provided in section 7.3.1.

2.3 Planning Responsibilities DEFRA has overall policy responsibility for flood risk in the England and the Environment Agency has statutory responsibility for flood management and defence.

The Regional Planning body should prepare a Regional Flood Risk Assessment (RFRA) in consultation with the Environment Agency to inform their Regional Spatial Strategy.


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The Local Planning Authority is responsible for the planning system which aims to direct development away from areas at highest flood risk. They are required under PPS 25 to produce a SFRA to inform their sustainability appraisals, land allocations and development control policies, in consultation with the Environment Agency.

At a site level developers should consult the Environment Agency, sewerage undertakers, highways authorities and any other relevant bodies to supply information for a Flood Risk Assessment of the site. This will provide information to the Local Planning Authority from which they can reach a decision on the development application. Planning applications for development proposals of 1 hectare or greater in Flood Zone 1 and all proposals for new development located in flood zones 2 and 3 should be accompanied by a FRA.

Figure 2 - Planning Hierachy and Flood Risk

NATIONAL

Legislation, Planning Policy Statements

REGIONAL

Regional Spatial Strategies (RSS)

REGIONAL

Regional Flood Risk Assessments (RFRA)

Catchment Flood Management Plans (CMFP)

LOCAL

Local Development Framework (LDF)

LOCAL

Strategic Flood Risk Assessments (SFRA)

SITE

Site Specific Plans

SITE

Site Flood Risk Assessments


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2.4 Other Planning Statements and Policies PPS3 and PPS6 sets out the Government’s national policy framework for delivering Housing Objectives and national policy on Planning for Town Centres and Retail Developments. While these are not directly related to the SFRA they should be read together with other relevant statements of national planning policy including PPS 25.

PPS3 states that physical restrictions such as flood risk should be taken into account when identifying locations for development but emphasis is placed upon the priority for “development to be located on previously developed land, in particular vacant and derelict sites and buildings.” The national annual target is that at least 60 percent of new housing should be provided on previously developed land.

PPS6 requires that a sequential approach be applied to retail developments in identifying suitable locations with growth largely encouraged in existing centres. While this has obvious benefits, it may also direct future development towards areas of high residual flood risk.

These two planning documents highlight that in exceptional circumstances, with sufficient justification, development within areas of higher residual flood risk may be permitted.

2.5 Making Space For Water Recent flooding incidents such as those experienced in 1998 and 2000 highlighted the need for the Government to develop an integrated strategy for managing future flood risks in England. In response to this in 2004 DEFRA carried out a consultation exercise with stakeholders to debate the future direction of UK flooding strategy. This consultation exercise was known as “Making Space for Water”.

Policies outlined in Making Space for Water follow the governments ideal of sustainable development with an overall strategy aim of managing risks from flooding whilst working to deliver the greatest environmental, social and economic benefit.

The Making space for Water strategy has highlighted the importance of a holistic approach when dealing with all forms of flooding. This is especially important in urban areas where complex interaction of drainage systems can exist leading to difficulty in identifying responsibility for dealing with floods. The government propose that different authorities responsible for different parts of the drainage system work together to manage flood risk and take a long term strategic approach.

The strategy also outlines the Governments commitment to the use of Sustainable Urban Drainage Systems (SUDS) through the support of the National SUDS working


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group working to address key issues such as ensuring adoption, ownership and operation responsibility is clearly defined.

Section 7 of this report deals with measures to reduce flood risk through land use planning. Although the City of London has not been sighted as a specific growth area in England, consideration needs to be taken with respect to flood risk and new development.

It is highlighted that approximately 10 per cent of England is located within mapped areas of flood risk with a significant proportion of previously developed land sighted for redevelopment also being within areas of higher flood risk. The report advises planning authorities when considering developments located within flood plains to ensure that the minimum standard of protection be provided for the lifetime of the development in line with PPS25.

Making Space for water also refers to the need for Regional Spatial Strategies and Local Development Frameworks to take full account of current and future flood risks and incorporate the sequential approach in PPS25 and integrated approach with catchment flood management plans.

Although the EA does not have the power to prevent development on areas at risk of flooding, as a key stakeholder they can advise against such developments. If this is the case and the development proceeds against EA advice the Association of British Insurers have made it clear that it is highly unlikely that insurance will be available for such developments.

2.6 Key Stakeholders in the Planning Process 2.6.1 Environment Agency

The Town and County Planning Order was amended in 2005 to make the Environment Agency a statutory consultee for development where flood risk is an issue under any of the following criteria:

• Development within 20m of the bank top of a Main River

• Any culverting operation or development which controls the flow of any river or stream

• Development other than minor development in Flood Zones 2 & 3

• Development in Flood Zone 1 where there are critical drainage problems

• Any development exceeding one hectare in extent


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Major development is defined as residential development with 10 or more dwellings or site area of 0.5 hectare or more, or a non-residential development of 1000 square metres or more.

2.6.2 Thames Water Thames Water as the Sewerage Undertaker in the City of London Borough are responsible for surface and foul drainage discharge from developments, where disposal is to the adopted sewer network. SFRAs are required to take account of any specific capacity problems associated with artificial drainage networks. Developers are responsible for consulting sewerage undertakers directly for surface and foul water disposal issues.

The City is the only London Authority which still has a maintenance responsibility for the sewerage system within its boundary. Thames Water employs the City as its sewer management contractor with responsibility for the day to day maintenance of the network and looking after its interest in any associated planning issues.


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3 Review of Development Framework

3.1 Regional Planning Policy – The London Plan The London Plan is the Regional Spatial Strategy for London produced by the Mayor. The plan covers a wide range of issues from housing and transport to employment and resources.

Chapter 2 of The London Plan outlines the key components of the spatial development strategy. Policy 2A.2 highlights “Opportunity Areas” where as part of producing the Sub-Regional Development Frameworks a sustainable development programme will be created for each Opportunity Area. The London Plan highlights Whitechapel and Aldgate to the East of the City of London as opportunity areas, as an area capable of accommodating substantial new jobs or homes. These areas generally include major brownfield sites with capacity for new development and increases in density.

Policy 2A.3 highlights area for “intensification” where development should exploit public transport and accessibility and potential for increased in residential, employment and other uses through higher densities and more mixed and intensive use. Farringdon, Smithfield and Holborn have been highlighted as intensification areas.

Chapter 4C of The London Plan concentrates on the river and water network and the inter-relationships of all of London’s waterways, referred to as the “Blue Ribbon Network”. It highlights the importance that development and use of water and waterside land should respect natural forces in order to ensure that future development and uses are sustainable and safe.

The London Plan outlines 34 policies referring to the blue ribbon network, a number of which refer directly to flooding and flood plains.

Policy 4C.6 Flood Plains states that “in reviewing their Unitary Development Plan boroughs should identify areas at risk from flooding (Flood Zones). Within these areas the assessment of development proposals should be carried out in line with PPG25. In particular, boroughs should avoid permitting development in functional flood plains”.

Policy 4C.7 refers to flood defences. “For locations adjacent to flood defences, permanent built development should be set back from defences to allow for the replacement/repair of the defences and any future raising to be done in a sustainable and cost effective way. Boroughs should ensure that development does not breach or undermine flood defences in any way”.


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Policy 4C.8 “Boroughs should seek to ensure that surface water run-off is managed as close to its source as possible. The use of SUDS should be promoted for developments unless there are practical reasons for not doing so. Such reasons may include the local ground conditions or density of development. In such cases the developer should seek to manage as much runoff as possible on site and explore sustainable methods of managing the remainder as close as possible to the site.”

Policy 4C.9 The issue of rising groundwater is also covered stating that “in areas where rising groundwater is a existing or potential problem, boroughs should expect reasonable steps to be taken to abstract and use that groundwater, the water may be used for cooling or watering purposes or may be suitable for use within the developments or by a water supply company”.

The London Plan promotes the use of SUDS and highlights the importance that developers and local planning authorities to work together with water supply and sewerage companies to enable the inspection, repair or replacement of water supply and sewerage infrastructure.

Supplementary Planning Guidance (SPG) has been produced to provide additional information to support the implementation of the London Plan.3

The SPG outlines the Mayors “essential” and “preferred standards” to include the importance of the use of SUDS wherever practical and the need to “achieve 50% attenuation of the undeveloped site’s surface water runoff at peak times” as an essential standard and “achieve 100% attenuation of the undeveloped sites surface water runoff as peak times” as a preferred standard.

3.2 Local Planning Policy – The UDP and LDF Core Strategy The Unitary Development Plan (UDP) is a ”land use” plan for the City and is the development plan used for the purpose of Section 54A of the Town and Country Planning Act 1990. The UDP was adopted in 2002 to set out the City’s policies for planning and transportation.

The Local Development Framework is the spatial planning strategy introduced in England and Wales by the Planning and Compulsory Purchase Act 2004. The LDF replaces the UDP. The LDF is to be made up of a series of planning documents that

3 Supplementary planning guidance, Sustainable design and construction Mayor of London, May 2006


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together will set out the overall planning vision for the City, looking forward 10 to 15 years, in general conformity with the London Plan. The LDF for the City is currently undergoing consultation. The most important planning document contained in the LDF is the “Core Strategy”.

The City of London has identified 24 issues relating to planning within the City in the Core Strategy. These were published by the City in May 2006 with consultation on the strategy during July 2006 with the view for publication in 2007.

Other planning documents will flow from the Core Strategy and will contain details on policies including their implementation.

The 24 issues outlined in the Core Strategy include issues relating to land use which may in turn have issues regarding flood risk.

The dominant land use in the City is commercial office floor space occupied by the financial and business sector. In order to accommodate the anticipated growth in demand for this commercial space and not impact on the physical environment in a negative way, the City needs to become more compact. In turn this policy has led to planning permissions being granted for a series of large and tall buildings. Since 2000 there have been 9 tall buildings (over 100m) and 13 large buildings (over 25,000 sqm) permitted in the City.

Currently there are five main retail centres within the City located in and around Cheapside, Leadenhall Market, Liverpool Street, Moorgate and Fleet Street. The UDP approach is to encourage the provision of more shops and the demand for new retail uses in the City is expected to rise with increasing numbers or workers, residents and visitors. Planning permission has been granted for a number of large retail schemes in the Cheapside area following the increase in demand.

The Core Strategy highlights that while the City is predominantly a centre for business, there is still demand for new housing. Due to the nature of the area there are sites that are not suitable for residential use due to traffic disturbance or noise from local amenities. The City’s policy is to encourage new residential development in the Cities conservation areas including Smithfield, Ludgate Hill, Carter Lane and Fleet Street near to existing residential areas and the riverside.

While not all development sites highlighted in the above sections are within the City of London boundary, they may have the potential to increase flood risk within the City.


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4 Data Collection and Validation

A large amount of data has been made available for this SFRA study, obtained from a variety of sources. All incoming data has been investigated to determine its authenticity and validated where appropriate to determine accuracy. Not all data was deemed suitable for inclusion in the study. Table 5 - Summary of SFRA Data, presented at the end of this chapter lists all data and reports acquired for the study including the source, its relevance to the scheme and any licensing restrictions.

The majority of the data for this study has been provided by the Environment Agency and City of London Corporation.

4.1 Mapping and Topographic Data Mapping data for this study has been provided entirely by the City of London Corporation and is licensed for use in this study only. The primary mapping type used is Ordnance Survey’s MasterMap topography layer, which can be viewed at up to 1:1250 scale. Ordnance Survey Landline mapping is also used to represent the wider catchment area as MasterMap data is only provided to cover the City of London boundary.

A Digital Terrain Model (DTM) has been provided by the EA for use in the study. The DTM is supplied in an Arcview compatible format and consists of LiDAR data provided in 500m x 500m files at a 0.5m grid cell. The DTM is a bare earth terrain model with building and vegetation removed by filtering the original Digital Surface Model using the supervised classification technique. The data has been briefly reviewed to ensure that buildings or vegetation have been correctly identified and filtered. LiDAR data covering the entire City of London boundary has been provided and no supplementary or infilling of data has been required for the study.

4.2 Flood Defences Location and standard of protection of flood defences has been provided from the National Flood and Coastal Defence Database (NFCDD) in GIS format. The condition grade of each flood defence has also been supplied.

4.3 Sewer Data Thames Water keep records of flooding arising from surcharging of adopted drains and sewers within its responsibility. Thames have provided an extract from this DG5 register, however, the data is referenced by post code only and does not identify the exact location or source of flooding.

In general only limited data pertaining to sewer flooding has been obtained by the study team. The data obtained so far is insufficient to enable a thorough investigation


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of sewer flooding in the City within the time constraints of the project. GIS extracts of almost all of the sewers in the City were made available but no modelling data or results of Drainage Area studies have been obtained. The Corporation’s own drainage engineer, Martin Coulthard, has provided useful supplementary information pertaining to flooding incidents and flood risk issues associated with the sewerage network, however, without the requisite information from Thames Water no verification or quantification of these flood risks could be undertaken. Further discussion on the missing data and the subsequent limitations of the study are included in section 4.5.

4.4 River Thames Data 4.4.1 Historic Flood Records

The Environment Agency maintains records of past flood events which are used in the preparation of flood risk maps which are available to the public via the internet. The EA have limited historic flood data for the City of London. They have however sighted the 1928 floods with records taken from The Times newspaper printed shortly after the event. These details have been transferred into a flood outline shown in map 4, in Appendix B. The only other major flood event to have affected the tidal Thames in the 20th Century was the 1953 surge tide, however, no flooding resulted in the City of London from this flood event.

4.4.2 Flood Modelling Results

The EA have commissioned numerous flood studies for the River Thames including the ‘Calculation of Defence Levels’ report (2002) undertaken jointly by Halcrow Group Limited and the Centre for Ecology and Hydrology (CEH). This report built upon work already undertaken in the 1988 ‘Tidal Thames Defence Levels’ report for Thames Water. More recently the EA have commissioned Halcrow to make minor revisions to the previous studies which resulted in the ‘Tidal Thames Extreme Water Levels – Joint Probability Analysis’ report issued in August 2005. The latest study updated return period estimates, extended climate change predictions to include 2050 and 2100 and investigated four new closure scenarios for the Thames Barrier. The flood level data used in this study are all based on this report.

4.5 Data Gaps Data required to enable assessment of the primary source of flooding has been made available for the study, predominantly by the EA. However, the information required to facilitate a thorough assessment of secondary sources of flood risk such as sewer flooding and infrastructure failure is not widely available. Without the requisite background information on these flood risk sources a comprehensive assessment has not been possible.


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Information pertaining to sewer flooding is particularly limited, which has resulted in a low confidence on the potential risks and impacts of such flooding included in the study. While Thames Water have provided extracts from their flooding database, the sensitivity of the data restricts them from identifying individual flooding problems. As such it is not possible to attribute these flooding problems to specific areas of the drainage network. No hydraulic models or results of hydraulic studies of the sewer network were made available, which prevented a thorough assessment of the potential limitations of the network. In particular it would have been beneficial to use computational modelling to determine the impact that high River levels in the Thames could have on sewer flood risk and also assess the potential for a breach or overtopping event elsewhere in London to result in significant sewer flooding within the City of London. Both of these flood mechanisms could have severe consequences albeit with a low probability of occurrence. Sewer data has been made available in GIS format which could be used to create a hydraulic sewer model, however, the timescale and complexity of producing such a model from scratch means it is outside the scope of this study. Further discussion on these flood mechanisms and the limitations of the assessment of sewer flooding is provided in section 5.3.1, along with a discussion on the potential for further work.


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Table 5 - Summary of SFRA Data

Data Category Format Source Contribution to SFRA Licensing

Climate Change Scenarios for the United Kingdom: The UKCIP02 Scientific Report Climate Report UK Climate Impact Partnership

Assessment of impact of climate change on flood risk

Climate Change and London Transport Systems Climate Report Transport for London


London’s Warming: A Climate Change Impacts in London Evaluation Study Climate Report London Climate Change Partnership


Acclimatise, Actions to ‘climate-proof’ the City of London Corporation. The City of London Corporation’s climate adaptation stategy Climate Report City of London

Corporation (CoL) Assessment of impact of climate change on flood risk

DG5 Register (Extract) Drainage Database Thames Water Used in assessing sewer flood risk

Licensed for this study only

Sewer Records Drainage Infoworks / GIS Format

Thames Water Used in assessing sewer flood risk


National Flood and Coastal Defence Database (NFCDD) Extract Flood Defence Database / GIS

Environment Agency (EA)

Used to identify protected areas and identify potential for risk of breach

Historic Flooding Data Flood History GIS EA Used to assess potential flood risk


1953 Floods, Timetable of Events Flood History Report EA Used to assess potential flood risk

Thames Region Catchment Flood Management Plan (CFMP) Summary Document Flood Risk Report EA General Information

Thames Flood Levels Flood Risk Data EA Used in assessment of fluvial/tidal flood risk


Flood Risk Zone Maps Flood Risk GIS EA Used in initial assessment of flood risk

Existing Local Flood Risk Assessments Flood Risk Report CoL Used to steer developer guidance information


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East London SFRA Flood Risk Report PDF EA General Information

ISIS Flood Hydrograph (2002 levels) Flood Risk Report EA / Halcrow Used in assessment of fluvial/tidal flood risk

Flood Risk Assessment Guidance for New Development Phase 2 Flood Risk Report DEFRA / EA Guidance on breach and overtopping assessment

Thames Tidal Contributions Policy, Thames Region Flood Defence Committee, 2003 Flood Risk Report EA General Information

Flood risks to people in defended areas, presented at the London CIWEM conference, January 2005. Flood Risk Report HR Wallingford Guidance on breach and

overtopping assessment

Development and Flood Risk – guidance for the construction industry (C624) Flood Risk Report CIRIA Developer Guidance

Thames Region Catchment Flood Management Plan, Summary Document, Consultation, January 2007. Flood Risk Report EA

Flood Resilient Homes Flood Risk Report Association of British Insurers (ABI)

Developer Guidance

Locations of Transport Infrastructure Flood Risk GIS CoL Used to determine potential flood risk from groundwater


Halcrow Group Limited (2005) Tidal Thames Extreme Water Levels – Reassessment of Joint Probability Analysis Flood Risk Report EA Providing Flood Levels

Study Area Information General GIS CoL Used to determine potential flood risk from groundwater

Design Guidance on Flood Damage to Buildings General Report Building Reseath Establishment (BRE)

Developer Guidance

Location of deep basements - (Locations at potential risk of flooding from groundwater) Groundwater GIS CoL

Used to investigate historic groundwater flood risk inconjunction with records from fire service

Groundwater Levels Groundwater GIS EA Used to investigate groundwater flood risk



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London Catchment Abstraction Management Plan Groundwater Report EA Used to investigate groundwater flood risk

Groundwater levels in the Chalk-Basal Sands Aquifer of the London Basin Groundwater Report EA Used to investigate groundwater flood risk

Fire Brigade Basement Flooding Records Groundwater Database London Fire Brigade

Used to investigate historic groundwater flood risk inconjunction with records from fire service


GARDIT – General Aquifer, Research, Development and Investigation Team Factsheets Groundwater GARDIT Used to investigate groundwater flood risk

London Catchment Abstraction Management Plan Groundwater Report EA Used to investigate groundwater flood risk

Groundwater levels in the Chalk-Basal Sands Aquifer of the London Basin Groundwater Report EA Used to investigate groundwater flood risk

Catchments of River Fleet Hydrometric GIS FEH Used to determine potential flood risk from groundwater

Catchments of River Walbrook Hydrometric GIS FEH

Critical Water Mains Infrastructure Failure Plans Thames Water Licensed for this

study only

Boundary of Study Area Mapping GIS CoL

OS Mapping - Mastermap Mapping GIS CoL Mapping Base Licensed for this study only

OS Mapping - 10k Mapping GIS CoL Mapping Base Licensed for this study only

London Underground Infrastructure Mapping GIS CoL Licensed for this study only

Historic Maps Mapping Plans CoL


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City of Londoon Local Development Framework - Core Strategy Issues and options Planning Report CoL Used to review planning policies

City of London Unitary Development Plan 2002 Planning Report CoL Used to review planning policies

PPS 1: Delivering Sustainable Development Planning Report Her Majesty's Stationary Office (HMSO)

Planning Guidance

PPS 25: Development and Flood Risk Planning Report HMSO Planning Guidance

PPG 1: General Policy and Principles, 1997 Planning Report HMSO Planning Guidance

Development and Flood Risk: Consultation on a Practice Guide Companion to PPS25 Planning Report Department of Communities and Local Government

Planning Guidance

Making Space for Water Planning Report DEFRA Planning Guidance

Planning and Compulsory Purchase Act 2004 Planning Report ODPM Planning Guidance

The London Plan Planning Report GLA Used to review planning policies

Sustainable Urban Communities, Building for the Future Planning Report ODPM General Information

Strategic Planning for Flood Risk in Growth Area – Insurance Considerations Planning Report ABI General Information

Preparing for Floods Planning Report ODPM General Information

LiDAR data Topographic GIS EA Used to assess flood risks from groundwater, fluvial and flood depths



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5 Sources of Flooding

5.1 Introduction Flood Risk throughout the City of London Borough has been assessed principally through the review of existing data sources. The assessment focuses on fluvial flooding as the primary source of flood risk and to a lesser extent on secondary sources, including groundwater and sewer flooding. The degree of assessment attributed to secondary sources has been dictated by the approximate risk posed by each source, partly assessed through the source pathway receptor model approach and by the limited data available pertaining to these flood sources.

The main sources of information used for the assessment have been provided by the EA and include River Thames flood levels, historic flood records and a digital terrain model.

5.2 Primary Sources of Flooding The following section provides details of the specific flood sources which pose a risk to the City of London. The sources have been identified through the review of the various data supplied in the initial phase of the study. This initial assessment is used to inform the source pathway receptor model presented at the end of this chapter, which in turn focuses the needs of this study into further areas of investigation.

5.2.1 River Thames The River Thames poses the most significant flood risk to the City of London and although the City is currently well defended against such a risk through a combination of local defences and the Thames Barrier, a significant residual risk still remains since the consequences of flooding are high. The River Thames not only poses a direct risk from overtopping or breach of flood defences but also influences the risk of flooding from secondary sources such as sewer flooding.

Extreme flood levels in the River Thames estuary are primarily influenced by tidal levels and storm surges, with secondary influence from fluvial flows. The tidal reach of the River Thames extends right through the centre of London and all the way to Teddington Weir in West London.

The primary sources of flooding from the River Thames would arise from breach or overtopping of flood defences or operational failure of the Thames Barrier. The Thames Barrier currently provides London with a high level of protection from fluvial and tidal flooding in combination with the local riverside ‘hard’ flood defences. Further information on the Thames Barrier is provided in chapter 6.2.2.

Tide levels are affected by a combination of astronomic and storm surges, with the later providing potentially the most dangerous short term rises. The most severe risk to London is caused by storm surges originating from low pressure conditions in the


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Atlantic moving into the North Sea and travelling down the east coast of England. The resultant rise in sea level associated with the low pressure event can turn into a surge event when passing from the deep water to shallow water in the southern part of the North Sea. When combined with Spring tides they can result in dangerously high flood levels of the sort that caused widespread flooding in the east of England in 1953. The height of storm surges can be further increased by strong winds.

5.2.2 The 1928 Floods The City of London did not suffer any flooding during the 1953 floods, however, it did experience some flooding during the 1928 flood events which affected a large swathe of central London. The EA have provided historic flood maps showing the extent to which the City of London flooded during that event. Map No. 4 is included for information. The area flooded was immediately east of Blackfriars Station and extended westwards to Trig Lane and northwards as far as Queen Victoria Street, today it would have affected the area occupied by the City of London School and Mermaid Conference and Events Centre.

The exact cause of the 1928 flooding is unknown but local knowledge suggests that it may have been attributed to inundation of the sewer system from flooding elsewhere in London. Alternatively it may have been caused by a localised breach of defences or localised defences with a lower standard of protection than present day. Comparison of the 1928 flood extent and EA flood risk maps suggests that the area in question still remains within the 1 in 200 year undefended floodplain. It is certain that the area is now afforded a much higher level of protection by the Thames Barrier and local flood defences. Given the same tidal levels in the Thames the area would be protected from flooding providing no breach occurred.

5.2.3 Overtopping The City of London is heavily defended against flooding from the River Thames, with the standard of protection exceeding 0.1% (1 in 1000 annual probability). However, overtopping of flood defences is a residual risk within areas defended from flooding. Although the protection against flooding is high the consequences of an overtopping event could be high if insufficient warning is provided. The Thames warning system is relatively good, as in order to function effectively, the Thames Barrier is required to be closed prior to high tides occurring.

Overtopping of flood defences occurs when water levels exceed the protection level of the defence. The worst case occurs when the fluvial or tidal levels exceed the defence level as this can lead to rapid inundation and prolonged flooding. Less severe overtopping can occur when flood levels are below defence levels, but wave action causes cyclic overtopping. Flood defences are commonly designed with a freeboard to provide protection against overtopping from waves. The City of London flood defences upstream of London Bridge have an approximate freeboard of 200mm, and downstream of London Bridge the freeboard is approximately 100mm. No assessment of potential wave heights has been undertaken for this study, but it is


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more than likely than these freeboards would be exceeded and some minor overtopping would occur during a 1 in 1000 year event.

The risk from overtopping due to exceedance of the flood defence level is much more significant than the risk posed by wave overtopping. Exceedance of the flood defence level can lead to prolonged and rapid flooding with properties immediately behind the defences at highest risk. Further assessment of the risks posed by overtopping are provided in section 6.2.6.

5.2.4 Breach Breaching of flood defences can cause rapid inundation of areas behind flood defences as flow in the river channel discharges through the breach. A breach can occur with little or no warning, although they are much more likely to concur with extreme tides or river levels when the stresses on flood defences are highest. Flood water flowing through a breach will normally discharge at a high velocity, rapidly filling up the areas behind the defences, resulting in significant damage to buildings and a high risk of loss of life.

Breaches are most likely to occur in soft defences such as earth embankments although poorly maintained hard defences can also be a potential source of breach. The grading of the City of London riverside defences has been obtained from the National Fluvial and Coastal Defence Database (NFCDD) and this data will be used to identify potential breach risks. This is covered in more detail in section 6.2.7.

5.2.5 River Fleet and River Walbrook The River Fleet and River Walbrook were two natural watercourses that historically flowed through the City of London and confluenced with the River Thames within the City’s boundary. Both rivers have been canalised for at least 200 years and the culverted sections of watercourse are now incorporated into the sewer network. For the purpose of this study the rivers are considered as sewers since they are so inextricably linked with the drainage network and maintained as part of Thames Water’s adopted sewerage system. In terms of assessing the potential flood risk from upstream development it is useful to consider the Fleet and Walbrook briefly in their historic context as watercourses as the natural catchments will be somewhat representative of the areas served by the present day sewer network. Map No. 5 showing the catchments as extracted from the Flood Estimation Handbook (FEH) is included in Appendix B. The catchments have not been verified for accuracy and are intended as a guide only.

The Fleet was the larger of the two watercourses, historically originating from springs on Hamstead Heath and draining to the Thames approximately via Kentish Town, Camden Town and Holborn. The present day sewer outfall is located immediately beneath Blackfriars Bridge and the topography of the catchment confirms that this is also approximately the natural River Fleet outfall, as the sewer exactly traces the line


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of the fleet valley. A topographic representation of the City is included in Appendix B which clearly identifies the natural fleet valley.

Near to the confluence with the River Thames the Fleet sewer is approximately 3.7m in width as shown in the photograph below.

The catchment served by the sewer network indicates that development in the upstream Borough of Camden could have a significant impact on flood risk within the City of London if surface water run-off were not adequately managed. See section 5.3.1 for further discussion on sewer flooding sources.

The Walbrook catchment is much smaller than the Fleet and historically drained a marsh area mainly contained within the City Boundary.

5.3 Secondary Sources of Flooding 5.3.1 Sewer Flooding

Sewer flooding generally results in localised short term flooding caused by intense rainfall events overloading the capacity of sewers. Flooding can also occur as a result of blockage, poor maintenance or structural failure. Sewer systems in London are commonly very old and can be designed to convey storms of relatively low return periods such as a 1 in 10 year rainfall event. New surface water systems are designed to a minimum standard of 1 in 30 years, much less than the 1 in 100 year standard of protection expected from fluvial flooding. As a result sewer flooding events where they occur can often be frequent, although the scale of consequence is generally small. Much of the London sewer network is a combined system with storm


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and foul drainage served by a single sewer. This makes flash flood events particularly inconvenient and unpleasant as floodwaters will often be contaminated with sewage.

In future climate change will increase the potential risk from sewer flooding as summer storms become more intense and winter storms more prolonged. Over time the standard of protection of existing sewers will reduce leading to an increase in localised flooding incidents.

It is essential that any new development takes account of known sewer flooding problems to ensure that the development is not put at risk and that the development does not worsen an existing problem. Future development if not adequately planned can increase the flood risk from sewer flooding and in some cases cause new flood problems to occur. Potential increases in surface water or sewage discharge from new development must be adequately managed and mitigation measures introduced where required.

Thames Water have provided an extract of their flood register for use in this study, however, the data is referenced by truncated postcode only and therefore cannot specifically identify a particular flooding problem. This is not a unique situation as Thames Water have in general been reluctant to release flooding data for use in other SFRAs due to the sensitive nature of the information. In this instance Thames Water only have one flood record in the City of London, occurring in postcode EC1 as a result of overloaded combined sewer flooding. Map No. 6 (Appendix B) shows the extent of overlap between the City of London and EC1 Postcode boundary. The small overlap enables the flood event to be pinpointed to the St Bartholomew Hospital area of the City of London, although the scale of the flooding problem is unknown. For the purpose of this study the area has been defined as a critical drainage area and any future development in this zone must take into account the potential flood risk from surface water flooding and take steps to manage surface water discharge.

With only one recorded sewer flooding problem, the City of London would appear to be well protected against this particular source of flooding. However, it should be noted that historically Water Authorities were only required to maintain records of flood events which occurred more frequently than once in ten years. This is primarily down to targets set by OFWAT, the regulating body of the water industry. This inconsistent approach in recording sewer flooding compared with other sources of flooding can make it difficult to assess the risks and consequences of sewer flooding.

It is possible that a less frequent but substantial flood risk from sewers exists in the City of London, but without the requisite information these potential risks are difficult to quantify and address. The EA support a pro-active approach to surface and foul water management including reduction in runoff and the use of SUDS wherever


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practical. Section 9.2 of this report discusses development requirements with regard to surface water management in greater detail.

It should be noted that the flood records provided by Thames Water may not be a complete and accurate record of flood events in the city over the last 10 years. Some minor flooding incidents may go unreported, particularly if no property is affected by such flooding.

Sewer flooding does not always respect the topography of a catchment and flooding can just as easily occur at the head of a network as it can near to the outfall. However, flood events occurring at the downstream end of a drainage system are more likely to have consequences commensurate with those of fluvial or tidal flooding. London’s sewer network is generally protected from such large scale flooding by storm overflows which discharge high storm flows from the sewer system into the River Thames, thus preventing flooding from the sewer network. In the event that an extreme tidal event coincided with heavy rainfall within the River Fleet catchment, storm water would be unable to discharge via storm overflows and would therefore surcharge within the sewer network. This could result in significant sewer flooding problems within the catchment. Without access to hydraulic sewer models it is not possible to determine the extent of this risk or the areas most likely to be affected. However, such a risk is likely to be rare due to the requirement for a high tide and storm surge combined with heavy rainfall. Naturally low lying areas such as the River Fleet valley and the areas immediately behind the local flood defences could be potential receptors of such a flood event. Map 7 in Appendix B shows the locations of the trunk sewers including the former River Fleet and River Walbrook locations.

At the time of writing this report the Government had granted permission to Thames Water to construct the Tideway Tunnel which will intercept sewer overflows on the north bank of the Thames and carry them to Beckton Sewage Treatment works for treatment. The project is intended to intercept storm overflows prior to reaching the River Thames and its primary driver is to prevent unintentional discharges and protect the water quality in Thames. This scheme if instigated could have a secondary benefit of helping to mitigate the risk of extreme tide levels causing flooding from the sewer network.

As well as flooding information from Thames Water, this study has also received useful local knowledge on the sewerage system from Martin Coulthard, the City of London’s drainage engineer. In particular Martin was able to contribute details of an additional flooding incident and also highlighted a potentially substantial flood risk associated with inundation of the sewer network from flood water from the Thames. This flood risk is discussed further in section 5.3.2.

The additional flooding problem identified by Martin Coulthard occurred at St Brides’ house in the Fleet Valley. The property is known to have flooded twice in


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approximately the last 20-years, which may explain why it isn’t included on the Thames Water flood database. Alternatively the flood event may not have been reported to Thames Water. The flooding problem is believed to be attributed to surcharge levels in the sewer network and highlights the potential for sewer flooding problems in the Fleet Valley.

5.3.2 Sewer Flooding with Fluvial Interaction Much of the sewerage network in London is served by large interceptor sewers which capture flows from the old lateral sewers which would previously have discharged directly to the River Thames. On the Northern bank of the Thames there are 4 main interceptor sewers, The Middle Level, Middle Level No.2, Low Level and Low Level No.2 sewers. These sewers all outfall to the Becton Treatment works in East london via the Abbey Mills pumping station. The 2 Low Level Relief sewers originate in Hammersmith in West London and pass through the heart of London approximately following a route parallel to the Thames. Figure 3 - Plan of London Sewers shows the approximate route of the sewers through London4. Both low relief sewers take a route through the City of London and intercept flows from the lateral sewers which serve the City catchment and beyond.

The relief sewers provide a hydraulic connection across the whole of the northern bank of the Thames. This connection introduces a potential flood mechanism where a breach or overtopping scenario in one flood cell in London could result in flooding in another area. If an area defended by local flood defences becomes inundated flood water can discharge into the relief sewer from the ground level and discharge through the gravity relief sewers towards east London. The volumes of flood water associated with a flood defence breach could significantly overload the capacity of the sewer and result in flooding elsewhere in London. This particular flood mechanism may be the cause of the 1928 flooding which affected a small area of the City of London. During the 1928 floods large areas of Chelsea suffered flooding during breaches of the flood defences. This flooding may have inundated the low level relief sewers, causing flooding within the low lying river frontage south of Queen Victoria Street. This potential flood risk is intrinsically linked to flooding from the Thames and as such shares a similar or lower probability of occurrence. The areas at greatest risk from this mechanism are likely to be the lower lying areas consistent with the areas at risk from fluvial flooding. This mechanism of flooding highlights the importance of maintaining flood defences to a high standard throughout London. However, it must be noted that flood defences maintained to a high standard are still at risk from breaching either from land side or riverside. These risks include unauthorised works by riparian owners cutting through flood defences or carrying out activities which may destabilise flood defences.

4 London County Council (1930) Main Drainage of London, Descriptive Account


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Verification and quantification of this potential flood mechanism is not possible without a hydraulic model of the sewer network. The assessment is complex and would require collation of large amounts of sewer records, assessment of breach risks for large lengths of flood defences and assessment of numerous local sewer connections to the relief sewers. The complexity of such a study is not feasible within the scope and timescale of this report. The potentially high consequences of such a flood risk would appear to warrant further investigation, which could identify the areas of London most at risk from this source of flooding. The scale of such a study may lend itself to a wider cross-borough approach rather than a study by individual boroughs. Any such study would require considerable assistance from Thames Water through the provision of sewer records and previous hydraulic studies.


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Figure 3 - Plan of London Sewers


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5.3.3 Groundwater During the 19th and 20th centuries significant volumes of groundwater were abstracted from the deep aquifer below London to support the industries prevalent at the time. This intensive abstraction of groundwater had a dramatic impact on the groundwater levels in the aquifer. As these industries began to decline the volume of water extracted correspondingly reduced and groundwater levels began to rise back to their natural levels. The decline in abstraction began in the 1940’s and from around the 1970’s onwards groundwater levels began to rise dramatically, at more than 1m / year5 in many areas. Building foundations and infrastructure which had previously been designed with little regard for groundwater began to be at risk as the water table began returning to its natural level. It is estimated that during the period of heavy abstraction groundwater levels may have lowered by as much as 90m 6, and a corresponding rise would cause a substantial flood risk to critical infrastructure and basements, while also causing instability to building foundations.

In 1992 GARDIT (General Aquifer Research, Development and Investigation Team), an umbrella organisation consisting of Thames Water, London Underground and the EA, was formed to address the issue of rising groundwater. The GARDIT group established a five stage solution to maintain groundwater levels at an acceptably agreed level on an area by area basis. The solution involved reusing existing boreholes and creating a series of new boreholes which would extract a total of 70Ml/day across London. The project was due to be fully implemented by 2005 and as such groundwater levels are expected to stabilise to a manageable level.

The EAs groundwater monitoring team have provided historic levels of a borehole located within the City Boundary at Leith House, Gresham Street. The results of the boreholes are presented below. A plan showing the location of the borehole as well as groundwater levels is included in Appendix B, (Map No. 8).

5 CIRIA Special Publication 69 (1989) The Engineering Implications of rising groundwater levels in the deep aquifer beneath London

6 Thames Water Central London Rising Groundwater


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Figure 4 - Leith House Borehole Level

Leith House Borehole level (mAOD)

-55

-50

-45

-40

-35

-30

11/08/1987 07/05/1990 31/01/1993 28/10/1995 24/07/1998 19/04/2001 14/01/2004 10/10/2006 06/07/2009

Date

Leve

l (m

AO

D)

The borehole record shows the trend of rising in groundwater during the 1990’s with an approximate rise of 2m/year occurring from 1990 to 1998. Since then levels have stabilised below -35m AOD (Above Ordnance Datum) as a result of abstraction of groundwater across London.

The actions of the GARDIT team will significantly reduce the future risk of groundwater flooding in the City of London providing the current abstraction rates are maintained indefinitely. However, a residual risk of groundwater flooding could still remain to some new developments or below ground infrastructure. For the purposes of this study it is assumed that groundwater will be maintained at a level of approximately -35m AOD. Considerably lower than ground level even in low lying areas such as the Fleet Valley.

The impacts of groundwater levels on proposed development and the planning process are not considered further in this report as the overall risk and consequences of groundwater flooding is considered to be low given the current stability in groundwater levels.

5.3.4 London Fire Brigade The London Fire Brigade have provided records of responses to flooding incidents within the City of London over the past 2 years. The call out locations are plotted on Map 7 included in Appendix B. The call out locations identified 6 flooding incidents which affected approximately 10 non-domestic properties. The flooding incidents are not referred to by building name, however they do identify areas within the City which may be at risk from Sewer flooding. One of the flooding incidents is located within the fleet valley in the vicinity of St Brides House, which reiterates the sewer flooding problem which exists in this area of the fleet valley.


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Another two flooding incidents were reported along Cannon Street between the junctions of Queen Street and College Hill. These flooding incidents occurred at the same time and would have been attributed to the same flooding mechanism.

These events could be attributed to sewer flooding or infrastructure failure such as sewer collapse, blockage or a water main burst. In order to determine whether the flooding incidents are related to sewer flooding or run-off, rainfall data for the dates in question have been requested from the Environment Agency. This dated is still awaited at the time of writing.

5.3.5 London Underground Flooding Mouchel Parkman are currently undertaking a 12 month study to assess the potential sources of flood risk affecting the London Underground assets across London. The study includes monitoring of rainfall data and groundwater levels over a 12 month period to determine the sources of flooding. While the information obtained from this study have not been made available to inform this SFRA, it may be beneficial to review this document in light of this information once the study is complete. The completion date of this study is currently unknown.

5.3.6 Infrastructure Failure Infrastructure failure is not generally considered as part of a Strategic Flood Risk Assessment as the identification of the sources of flooding is hampered by limited information. In particular the City of London Corporation requested that this report consider the potential risk of flooding from failure of clean water assets and primarily pipe bursts. Thames Water are currently undertaking a project to look at critical water mains across London, however, at this stage no information has been made available to inform this study. It is unlikely that Thames Water will be prepared to supply the information in the future. The risks of a pipe burst can vary considerably from main to main depending on a number of factors including age, condition and operating pressure. Much of London’s water mains are old and in need of replacement, indeed Thames Water are currently investing heavily in a Victorian Water mains replacement scheme. The presence of these old mains increases likelihood of a burst, however, a burst in itself will not result in flooding. The likelihood of a flood event will also be a function of the diameter of the main, the pressure of the main, the topography around the burst and the ability of the artificial drainage to cope with the outflow. All of these unknown parameters means that assessment of risk posed by infrastructure failure is complex cannot be undertaken as part of this study due to both limited information and timescale. The consequence of such flooding is likely to be far lower than those associated with flooding from the Thames or sewer flooding and as such does not require substantial consideration at the planning level.


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5.4 Source Pathway Receptor Model In strategic terms the City of London Borough is small area, however, it is potentially at risk from a wide range of flood sources. The strategic level at which this report is targeted permits only a broad level of assessment of flood risk and the timescales required by the City of London do not allow a detailed assessment of all flood risks. In order to focus the SFRA it is necessary to identify sources of flooding with the largest potential consequences. To this end a source pathway receptor model has been created. The model identifies sources of flooding while assessing the possible scale of consequences of such flooding.


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Table 6 - Source Pathway Receptor Model

Source Pathway Receptor Potential Scale of

Consequence Assessment in

SFRA Comment

Tidal Flooding from the River Thames

Overtopping of flood defences / breach of flood defences / Conveyance in Sewers

Flood Cells Fronting River Thames

Very Large

Assessment of tidal flood levels, flood risk zoning and assessment of breach and overtopping risks.

Potentially large consequence and risk of loss of life

Surface Water / Combined Sewer Flooding

Exceedance of sewer capacity

Blockage of Pipework

Tidelocking of outfalls

Properties in locality of Pathway

Small Limited flooding predicted. Broad assessment of flood risk required only.

Thames water have provided limited information on flood history

Flooding from the Rivers Fleet and Walbrook are included in this model

Groundwater Flooding Rising Groundwater

Development in low lying areas or developments with low basements.

Infrastructure, Tube Stations

Medium

Groundwater levels obtained from EA. Comparison with ground levels and research into future trends in groundwater levels and comparison deep basement levels

Limited information on which to base assessment. Potential for flood risk to infrastructure.

Mechanical Error / Operation of Thames Barrier or future flood defences

Overtopping of flood defences or breach of flood defences

Flood Cells Fronting River Thames

Very Large

No analysis required. City of London is well defended against extreme flood levels and risk of failure to close or mechanical failure is small. General assessment of tidal flooding will suffice

Potentially large consequence and risk of loss of life


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5.4.1 Scale of Consequences The scale of consequences used in the source pathway receptor model are presented in Table 7 - Scale of Consequences. The assessment of the consequences are approximate only and intended to guide the study towards the pertinent flooding issues and allow further assessment of the critical flood sources affecting planning decisions.

Table 7 - Scale of Consequences

Scale Consequence

Very Large > 50 Buidlings

Large 10 – 50 Building

Medium 5 – 10 Buildings

Small 1 – 5 Buildings


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6 Flood Risk Assessment

6.1 Sustainability and Climate Change It is now universally accepted that climate change will continue to have a detrimental impact on flood risk on a global scale. Sea levels are predicted to rise and changes in weather patterns are predicted to lead to longer winter storms and increasingly intense summer storms. The resultant shift in sea levels and weather patterns may lead to an increase in the frequency and scale of flooding, with predicted rises in fluvial flows, rising groundwater and increases to peak rainfall run-off and volumes all increasing flood risk. PPS 1 and 25 explicitly require future development to take into account climate change when considering flood risk as part of the overall aim of sustainable development.

A number of climate change studies exist which use global climate predictions to model potential impacts on sea level rise and rainfall. Despite these various studies there is still uncertainty about the extent of global warming and therefore the potential impact on flood risk. The UK Climate Impacts Program (UKCIP)7 has published a series of scenarios based on varying global emission levels which form the basis for the London’s Warming Technical Report8. The scenarios presented in UKCIP02 use 4 different scales of emissions to model the potential impact on rainfall and sea levels. The output of the models provides a range of results which could be applied to this strategic flood risk assessment through sensitivity testing of computational modelling, overtopping and breach analysis. However, the EA study which supplied predicted flood levels for this study used the precautionary allowances stated in PPS25 as the basis for climate change consideration.

PPS25 predicts a steadily increasing rate of sea level rise varying from 4mm per year from 1990 to 2025 and 15mm per year between 2085 and 2115. In addition to this is a predicted increase in peak fluvial flows of +10% from 1990 to 2025 and +20% between 2085 and 2115.

The River Thames flood levels provided by the EA are based on the previous estimates of sea level rise of 6mm per year. Applying the 6mm year-on-year increase results in a 300mm rise in the astronomical tide level by 2050 and a 600mm rise in astronomical tide level by 210010. The EA study applies an adjustment of 20% to fluvial flows for all climate change scenarios, consistent with PPS 25, which

7 UK Climate Impact Partnership (2002) Climate Change Scenarios for the United Kingdom: The UKCIP02 Scientific Report

8 London Climate Change Partnership (2002) London’s Warming: A Climate Change Impacts in London Evaluation Study


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recommends this increase up to 2115. While the climate change allowances for sea level rise included in the EA study are not consistent with current climate change predictions, the actual impact on flood levels upstream of the Thames Barrier will remain largely unaffected. The joint probability analysis report confirms that the prediction for climate change impact in the River Thames reach is a lowering of extreme flood levels as a result of the increased operation of the Thames Barrier. The assessment made by the joint probability analysis is only confirmed for a sea level rise of 600mm, which, under PPS25 would approximately occur in 2075, at which point it is likely that a replacement or supplementary defence would be provided. The joint probability analysis report and the impact of the Thames Barrier and future flood defence options for the Thames Gateway are discussed briefly in section 6.2.5 and section 6.2.4 respectively.

On the basis that the joint probability analysis report predicts a lowering of flood levels upstream of the Thames Barrier due to an increase in closure events, the climate change allowance and predicted levels are considered satisfactory for use within this Strategic Flood Risk Assessment and associated uses.

Climate change forecasts are likely to be periodically revised in years to come. UKCIP are expected to release further climate change scenarios in 2008. Site based flood risk assessments for future development would be expected to make use of the most up to date guidance on climate change allowances.

6.2 Fluvial Flooding

6.2.1 Flood Risk Mapping Section 105 of the Water Resources Act, 1991 required the Environment Agency to produce a series of maps showing the extent of areas at risk of flooding from tidal or fluvial sources. These maps have subsequently been modified and released under various guises. The latest incarnation of these maps is the flood zone mapping which have been provided to Local Planning Authorities in electronic format for use at the planning stage. The zones defined by the flood risk maps are those areas considered to be at risk from annual fluvial flooding with a probability of 1% or more, and areas considered to be at risk from annual tidal flooding with a probability of 0.5% or more. The flood risk maps generally provide a broad high level assessment of flooding and the level of accuracy varies considerably from catchment to catchment. It is important to note that the flood maps do not account for structures within the flood plains such as bridges and culverts, nor do they distinguish between defended or undefended areas or take into account climate change. The flood risk maps only provide an indication of flood risk from fluvial or tidal sources. The zones are consistent with the approach to flood risk assessment set out in PPG25. These maps form the basis of the initial flood risk assessment for the City of London study,


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however for the purposes of applying the sequential test the flood zones taken from these maps are refined and verified.

6.2.2 Flood Defence Assets The City of London study area is protected by the Thames tidal flood defences including the Thames Barrier which is maintained and operated by the EA. There are 49 tidal walls in the City of London which are privately owned and maintained. The EA regularly inspects them to ensure that they remain fit for purpose and takes enforcement action where required. The EA have supplied information about the location and grade of these defences within the study area. Map No. 9 in Appendix B shows the location of the defences along with the condition grade. Of the 49 flood defences 4 are categorised as fair condition, 1 is defined as poor condition and 43 are defined as either good or excellent condition. Of the 5 fair or poor conditioned flood defences, 2 are scheduled for replacement during the forthcoming year.

Table 8 - Flood Defence Locations

Asset Reference Grade Location

06304TH000810L02 3 (Fair) Exploration House

06304TH000810L04/05 3 (Fair) Seal House / Old Swan Stairs

06304TH000810L08-L11 3 (Fair) Riverside Walk / Mondial House / All Hallows Lane

06304TH000810L21 4 (Poor) Queenhithe Wharf

06304TH000810L30 3 (Fair) Sunlight Wharf

In general the river side flood defences protecting the City of London are in an acceptable condition.

6.2.3 Flood Defence Levels The City of London is protected from flooding by the Thames tidal defences to a standard of 0.1% annual probability. The standard of protection will continue until 2030 at which point the forecasted climate change scenarios will begin to degrade the standard of protection until such time that defences are improved.

The statutory flood defence level in the study area is 5.41m AOD upstream of London Bridge and 5.28m AOD downstream of London Bridge.


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6.2.4 Thames Barrier The Thames Barrier was opened in 1984 as a somewhat delayed reaction to the result of the 1953 floods which severely affected the Thames Estuary. The Barrier was constructed in conjunction with raising of the riverside defences in London and will provide protection to up to the 1 in 1000 year flood until approximately 2030. By this point it is considered likely that a replacement or supplementary flood barrier will be required and the EA led Thames Estuary 2100 (TE2100)9 project is currently looking at potential solutions, which also include catchment level management of flood risk. While it is inconceivable to consider that London would in future be left with a reduced flood defence standard, the complex nature of constructing a replacement barrier or alternative flood defence would suggest that the Thames Barrier will continue to be London’s primary source of flood defence in excess of 2030, where the standard of protection will begin to reduce.

6.2.5 Extreme Water Level Assessment The EA have commissioned numerous flood studies for the River Thames including the ‘Calculation of Defence Levels’ report (2002) undertaken jointly by Halcrow Group Limited and the Centre for Ecology and Hydrology (CEH). This report built upon work already undertaken in the 1988 ‘Tidal Thames Defence Levels’ report for Thames Water. More recently the EA have commissioned Halcrow to make minor revisions to the previous studies which resulted in the ‘Tidal Thames Extreme Water Levels – Joint Probability Analysis’ report issued in August 2005. The latest study updated return period estimates, extended climate change predictions to include 2050 and 2100 and investigated four new closure scenarios for the Thames Barrier. The flood level data used in this report are based on the Tidal Thames Extreme Water Levels – Joint Probability Analysis.

All of the above studies use an ISIS one dimensional model of the River Thames as the basis for the study. The model extends upstream as far as Molesey, the tidal limit of the River Thames and downstream to Southend. Flood levels extrapolated from the model have been provided by the EA for use in this study and are detailed in Table 9 - Flood Levels from River Thames One Dimensional ISIS Model.

The ISIS Model uses a boundary condition of 6 repeating tides, with 4 tides being allowed before a Thames Barrier closure. For extreme water level assessment the tide is a combination of astronomical and surge. The 1 in 1000 year flood level hydrograph derived from the Tower Pier node of the ISIS model is presented in Figure 5 - ISIS Flood Hydrograph (2002 levels), which clearly shows the 6 tide peaks with the forth higher peak resulting from the fluvial flow in the River Thames.

9 http://te2100.dialoguebydesign.net/dbyd.asp


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The effect of closing the barrier either earlier or later than 2.5 hours after low tide has a marked effect on the resulting peak levels that build up behind as stored fluvial flow. The level behind the barrier will initially be the tidal level at the point of closure, the area of estuary behind the barrier then acts as a reservoir, storing fluvial flows until the high tide has passed. For the assessment of extreme water levels required for this report, the EA study concludes that the exact timing of the Barrier closure is likely to have very little effect on return period levels upstream of the Barrier (exceptions may occur at more extreme return periods (i.e. >1000 yr return periods)”10, when compared with levels occurring during a ‘near closure event’.

For the more extreme events the levels when allowing for climate change can be lower than the 2002 levels. This is due to an anticipated greater use of the Thames Barrier (or any replacement structure). For a conservative approach the present day levels will be used through this study.

It should be noted that the ISIS model when verified against recorded high water events at Tower Pier suggested an over prediction of flood levels by more than 200mm. This provides a certain level of conservatism to the study, however, this may be offset by the uncertainty regarding the application of climate change results and the overall uncertainty of the modelling results caused by the underlying assumptions, particularly with the barrier closure scenarios10.

10 Halcrow Group Limited (2005) Tidal Thames Extreme Water Levels – Reassessment of Joint Probability Analysis


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Table 9 - Flood Levels from River Thames One Dimensional ISIS Model

Return Period 10 year 20 year 50 year 100 year 200 year 1000 year

Climate Change Prediction

Node Ref.

Node Location

2002 2052 2102 2002 2052 2102 2002 2052 2102 2002 2052 2102 2002 2052 2102 2002 2052 2102

2.34 Blackfriars Bridge 5.014 5.089 5.058 5.065 5.11 5.068 5.117 5.129 5.08 5.145 5.14 5.09 5.17 5.151 5.098 5.207 5.172 5.114

2.35 London Bridge 5.002 5.078 5.047 5.052 5.099 5.057 5.104 5.117 5.069 5.132 5.128 5.079 5.157 5.138 5.088 5.194 5.159 5.103

2.36 Tower Pier 4.989 5.066 5.037 5.039 5.087 5.046 5.091 5.105 5.058 5.119 5.115 5.068 5.144 5.125 5.077 5.18 5.146 5.092


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ISIS Model Hydrograph for Node 2.34

-3.000

-2.000

-1.000

0.000

1.000

2.000

3.000

4.000

5.000

6.000

0 3 6 8 11 14 17 19 22 25 28 31 33 36 39 42 44 47 50 53 56 58 61 64 67 69 72 75 78

Time (hours)

Leve

l (m

AO

DN

)

Figure 5 - ISIS Flood Hydrograph (2002 levels)


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6.2.6 Overtopping Assessment Areas behind defences are still at risk of flooding from fluvial or tidal sources from breach or overtopping. Flood risk behind defences is a function of the probability and consequences of such flooding. Consequences of flooding are generally measured against risk to people and damage to property. For the purposes of this assessment only the risks to people are considered, as “….the safety of the public is the single most influential consideration for decision-makers.”11

In order to determine the residual flood risk behind defences it is necessary to undertake breach and overtopping assessment. The methodology which can be applied to such an assessment can vary from a basic assessment, which determines risk based on the distance from the defence, to a complex approach using 2D modelling of flows through the embayment. The flood cells in the City of London generally extend no more than 200m from the defence in a 0.1% flood event. The Topography of area behind the defence is generally consistent, rising relatively steeply up from the natural floodplain. Due to the distance from the flood defence and consistent topography a complex approach is considered unnecessary for this strategic assessment.

The approach adopted for both the overtopping and breach assessments is the intermediate approach as outline in the EA / DEFRA report, Flood Risk Assessment Guidance for New Development. This method uses simple look-up tables to determine the consequence of flooding based on the distance from defences and the head above crest level of the defence.

Table 10 - Danger to people from overtopping relative to distance from defence

Head above crest level (m) Distance from Defence (m) 0.5 1.0 2.0 3.0

100 250 500 1000 1500 2000 2500 3000

Reproduced from EA / DEFRA report FD2320 Flood Risk Assessment Guidance for New Development

11 EA / DEFRA (2005) Flood Risk Assessment Guidance for New Development Phase 2


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Where the following key represents the groups of people included in the danger classifications

Table 11 - Key to Overtopping Danger Classifications

Key

Danger for Some Includes children, the elderly and the infirm

Danger for Most Includes the general public

Danger for All Includes emergency services

The flood level used to assess the risk of overtopping was taken from the Tidal Thames Extreme Water Levels – Reassessment of Joint Probability Analysis report. The flood levels are based on an extreme event of the 1 in 10000 year flood with the Thames Barrier open and a corresponding tide level at Southend of 5.52m AOD. The predicted flood level for this event at Blackfriars Bridge is 6.56m AOD. For the overtopping assessment a head over crest level in excess of 1.0m will be assumed. Given that flooding is predicted to occur no more than 500m beyond the flood defences the residual risk is shown to be “Danger for Some” throughout. The 1 in 10000 year flood level with no barrier in place is intended to show the potential impact of flooding given mechanical failure of the Thames Barrier.

In order to determine the residual risk the potential danger must be assessed alongside the probability of an event occurring. The following look-up table considers the probability of event of an overtopping event or breach occurring and assigns an overall residual risk depending on the danger to people criteria. Map No. 10 in Appendix B presents the residual risk from overtopping.

Table 12 - Flood Risk to People Behind Defences

Annual Probability of Inundation Danger to people

Prob � 1% 1% < Prob. � 0.5% 0.5% > Prob. � 0.1% Prob. � 0.1%

Danger for all High High High Medium

Danger for most High Medium Medium Low

Danger for some Medium Medium Low Low



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Figure 6 - Flood Hydrograph With Against Flood Defence Level

1 in 1000 Year Hydrograph (Node 2.34)

4.000

4.200

4.400

4.600

4.800

5.000

5.200

5.400

5.600

3 3.5 4 4.5 5 5.5 6

Duration (Hours)

Leve

l (m

AO

D)

Flood Level Statutory Defence Level (5.41m AOD) Flood Risk Level (500mm below defence)

Figure 6 above presents the peak of the 1 in 1000 hydrograph at Blackfriars Bridge as predicted by the Thames ISIS model. The graph shows the statutory defence level and an approximation of the flood risk period assuming that overtopping from waves and wind action would only occur when flood levels are less than 500mm below the crest of the defences. Although this should only be used as a guide it indicates the period of time over which overtopping may occur to be approximately 1 hour.

6.2.7 Breach Analysis The danger to people from breaching is assessed in a similar manor to overtopping by using the relative head above floodplain to determine the risk in terms of distance from the defence. For assessment of breach the 1 in 1000 year flood level has been used, which results in a head over the floodplain in excess of 1m. Rather than analyse individual breaches, the risk of breach across the whole defence line has been plotted.

Table 13 - Danger to people from breaching relative to distance from defence

Head above floodplain (m) Distance from Defence (m) 0.5 1.0 2.0 3.0

100 250 500 1000 1500 2000



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The base level of the breach is set to the lowest ground level behind the defence determined by interrogation of the DTM. The look-up table above is based on a relatively flat flood plain and assumes a breach width of 100m to provide an indication the danger classification (in accordance with Table 11 - Key to Overtopping Danger Classifications).

In order to determine the residual risk the potential danger must be assessed alongside the probability of the breach occurring, which is dependant on a number of factors, including the condition grade of the flood defence, the risk of failure due to interference such as removing bricks, cutting holes or impact from vessels and the relative height of tidal level event.

Given that the standard of protection of the flood defences exceeds the 0.1% probability, and given the ‘hard’ nature of the flood defences, it follows that the probability of a breach occurring will have a lower than 0.1% chance of occurring in any one year.

The residual flood risk to people behind defences can then be assessed by applying the annual probability of a breach event occurring to the danger classification using the look-up table (Table 12 - Flood Risk to People Behind Defences). A map showing this residual risk from breach is presented in Appendix B. A breach could take place at any location along flood defences protecting the City of London and the risk should therefore be considered as such when compiling the overall risk plan, see section 6.2.8.

6.2.8 Combined Overtopping and Breach Assessment In order to determine the overall residual risk to people behind defences a composite plan of the breach and overtopping scenarios is produced. The composite breach and overtopping map (No. 12) is presented in Appendix B. The overall risk to people is shown to be low, primarily due to the high standard of protection provided by the flood defences. However, at all stages of planning it should be bourn in mind that the consequences of a breach or overtopping event can be high.

Overall the residual risk is shown to be low predominantly down to the standard of protection provided by the defences and the structural condition. It is assumed that the structural condition of the defences will continue to be monitored by the EA and maintenance enforced where required.

The overall residual risk obtained through the combined breach and overtopping analysis should be viewed with caution due to the simplicity of the assessment method. Rather than identifying areas where development should not take place the plans should be viewed as a basic tool for determining areas of least risk as part of the overall sequential test. The identified residual risks should be used to determining suitable types of development and invoke discussion on potential


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mitigation measures that might be employed to mitigate the impact of breach or overtopping scenarios at a development scale.

6.2.9 Mapping of Flood and Hazard Zones The bare earth Digital Terrain Model supplied by the EA has been used to refine the flood risk zones by extrapolating the flood levels sourced from the Thames ISIS model study undertaken by Halcrow. Flood zones have been plotted based on three sets of levels, those upstream of Tower Bridge, upstream of London Bridge and upstream of Blackfriars Bridge as presented in Table 9 - Flood Levels from River Thames One Dimensional ISIS Model. The revised flood maps include an allowance for climate change and will therefore show an increase in flood risk compared with the EA flood risk maps. Flood Zone 3 is further delineated on the maps by provision of Zone 3b, the functional flood plain, which represents the area which would flood with an annual probability of 1 in 20 or (5%). The flood zones defined on the maps are shown below in Table 14, Section 7.1.

The Flood Zone Map (No. 14) is presented in Appendix A.


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7 Application of Sequential Flood Risk Test

PPS25 encourages LPAs to undertake the sequential flood risk test to help gain an understanding of the potential flood risk associated with future development. The key aspect of the sequential flood risk test is the assessment of the flood risk zones in accordance with PPS25. This is done by delineation of the flood risk zones using topographic data and flood modelling results. Following the application of the sequential test, further separation of development sites can be assessed using the exception test. This requires consideration of individual development risks such as the risk to vulnerable users and also further appraisal of area in Flood Zone 3 by determining the risk from overtopping and breach. In practice the risk from overtopping and breach has been identified as low risk primarily because of the high standard of protection of flood defences, although a risk of failure due to interference is still possible. The sequential and exception tests are risk based assessment tools intended to ensure that preference is given to development of sites of little or no risk of flooding over areas of higher risk. The approach uses flood risk zones as defined in section 2.2.

7.1 Flood Risk Zone Categories The following Flood Zone risk categories are highlighted on the flood zone map, which should be employed when applying the sequential test to development proposals.

Table 14 - Flood Risk Zones and Development Restrictions

Flood Zone Description and Development Restrictions

Zone 1 All Development types acceptable

Zone 2 Exception Test Required for Highly Vulnerable Development

Zone 3a

More Vulnerable development and essential infrastructure requires Exception Test,

Highly Vulnerable Development not permitted. In practice, more vulnerable

development types such as ‘residential’ sited on previously developed land is likely to

pass the exception test. However, it must be noted that all three sections of the

exception test must be met having first satisfied the Sequential Test in order for this to

occur. Flood resilience measure should be taken into account for this type of

development.

Zone 3b Functional Floodplain or medium to high risk from breach or overtopping. Only water

compatible and essential infrastructure is permitted. Essential infrastructure must

pass the Exception Test.

For vulnerability of development classes refer to section 2.2.


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In addition to these flood risk zones a critical drainage zone is also highlighted on map no. 6. This zone does should not restrict development types, but does highlight the need for new development in this area to take account of the surface water flood risk and manage surface water discharges accordingly.

7.2 Application of sequential test PPG3 encourages LPAs to give preference to previously developed site and disused properties prior to allocating greenfield sites for development. These brownfield sites characterise much of the potential development land within the City of London. In some instances those brownfield sites within 100m of the River Thames flood defences will be at the highest risk of flooding, thus contradicting the Sequential Test of PPS25, which is based on the principle that all other factors are equal. PPS2512 advises that ‘a balanced flexible approach is required’ to such decisions, and the following guidance is provided from the Defra / EA as part of their Flood and Coastal Defence R & D Programme.

A site located within a flood risk zone that has been previously developed does not automatically warrant planning approval for a replacement development. The flood risk assessment and management of the new application should be dealt with in the same manner as a greenfield site.

In practice some of the PPS25 flood zonings are difficult to apply to London since development already lies within Zones 2 and 3 which would not now pass the sequential test. However, when considering re-development of these areas the sequential test must be considered in full and sites should not be allocated within flood zone 3b.

12 Department of Communities and Local Government (2006) Planning Policy Statement 25: Development and Flood Risk

Brownfield Development Advice

If there are two site with the same probability of flooding, the brownfield site should be developed in preference to the greenfield site

If the brownfield site has a higher probability of flooding than the greenfield site, then a trade-off must be made between benefits and disadvantages of the two sites. Should the LPA consider that developing on the brownfield site is beneficial then appropriate mitigation measures need to be included in the design and guidance provided in PPS25 regarding elderly and vulnerable occupants still applies


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7.3 Assessment of Risk within Zone 3 7.3.1 Application of the Exception Test

The Exception test is applied after the Sequential test and is intended to ensure that vulnerable type of property are not developed in areas of high flood risk. Three conditions must be met in order to pass the exception test:

Criteria for Passing the Exception Test

1) “it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared. If the Development Plan Document has reached the submission stage, (as defined in Planning Policy 12: Local Development Frameworks), the benefits of the development should contribute to the Core Strategy’s Sustainability Appraisal.”

2) “the development should be on developable land (as defined in Planning Policy Statement 3: Housing), previously-developed land (commonly known as brownfield land) or if it is not on previously developed land, that there are no reasonable alternative sites on developable previously-developed land.

3) a site specific flood risk assessment must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall.


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8 Local Policy Guidance

The current evolution from Unitary Development Plans to Local Development Frameworks provides an opportunity for the City of London Corporation to introduce planning policies which are consistence with both national and regional policy.

The findings of this SFRA have been used to produce a series of draft policies which the Corporation may choose to adopt at a local level, where applicable these are written to reflect the national and regional policies outlined in the London Plan. The wording and approach of these policies would need to be reviewed in the context of other local policies.

Spatial planning shall be carried out in line with PPS25 by applying the sequential test in accordance with paragraphs 16 and 17. The sequential and exception tests shall be applied in allocating sites and reviewing applications for permission.

Development in Zones 3b, the functional floodplain, shall not be permitted. Applications for development in Zones 2 and 3 must be accompanied by a local flood risk assessment, which should address flood mitigation measures and flood resilience issues where appropriate.

Development permissions in flood Zones 2 and 3 will favour proposals which adopt have been subjected to a sustainability appraisal and adopt flood resilience methods to manage residual flood risk.

Development adjacent to flood defences should be set back from defences to allow for the replacement/repair of the defences and any future raising to be done in a sustainable and cost effective way. Boroughs should ensure that development does not breach or undermine flood defences in any way.

Development should seek to ensure that surface water run-off is managed as close to its source as possible. The use of SUDS should be promoted for developments unless there are practical reasons for not doing so. The developer should seek to restrict peak run-off from the site to the undeveloped greenfield run-off rate.

8.1 Emergency Planning The London Resilience Forum are currently undertaking a strategic flood plan which will address emergency planning. As part of the stakeholder workshop organised for the City of London SFRA, the London Resilience Team asked if implications of flood risk on existing utilities could be considered as part of this SFRA. To date the utilities information required to undertake such an exercise has not been made available by the majority of the utility companies. It is understood that the data has been supplied to the London Resilience Team and many of the utilities companies were unwilling to


Final - August 2007

supply the data for a second time. This demonstrates a wider reluctance of many organisations other than the Environment Agency to engage in the SFRA process.

8.2 Future use of the SFRA It is proposed that the findings of this SFRA are reviewed in 12 months time and on an ongoing basis thereafter in line with reviews of the local development framework and associated documents. This will ensure that studies such as the Thames Estuary 2100 project and its potential implications on flood protection of the City of London are taken into account.

8.3 Monitoring and Review of Policies Regional planning bodies and LPAs will now have a statutory duty to ensure that development documents promote sustainable development and Sustainability Appraisals must be carried out as part of the LDF process. Local Planning Authorities will be required to report annually on the achievement of locally set policies and targets.

The City of London Corporation must policy based target on which to monitor and report. Targets must be specific and measurable and most importantly, realistic. Suggested policies on which to monitor are as follows:

1) Monitor allocation of development sites which successfully pass the sequential test.

2) Monitor the number of successful planning applications which include use of Sustainable Urban Drainage Systems.

3) Monitor new developments which include the management of residual risk if located within flood zones 2 or 3.


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9 Guidance for Developers

9.1 Development and the management of residual flood risk At a site level developers should consult the Environment Agency, sewerage undertakers, highways authorities and any other relevant bodies to supply information for a Flood Risk Assessment of the site. This will provide information to the Local Planning Authority from which they can reach a decision on the development application.

Planning applications for development proposals of 1 hectare or greater in Flood Zone 1 and all proposals for new development located in Flood Zones 2 and 3 should be accompanied by a local FRA. For major developments in Flood Zones 2 and 3, the FRA should identify opportunities to reduce the probability and consequence of flooding.

With reference to PPS 25, the objectives of an FRA are to establish:

• Whether a proposed development is likely to be affected by current or future flooding from any source.

• Whether it will increase flood risk elsewhere

• Whether the measures proposed to deal with these effects and risks are appropriate

• Whether the developer will be able to demonstrate that the site will be safe, without increasing flood risk elsewhere and where possible will reduce flood risk overall (part c of the exception test).

The scope of each FRA should be agreed with the LPA, EA and any other relevant consultees but it may include some or all of the following outputs:

• Development description and location

• Definition and assessment of all potential sources of flooding including: fluvial, coastal and tidal, estuarial, groundwater, artificial drainage systems, infrastructure failure.

• Assessment of the probability of flooding (existing and post development)

• Assessment of the sequence of flooding across the site, rate of rise of water level, flow velocities, depths and the duration of flood (existing and post development and including consideration of climate change)


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• Detailed development proposals including estimation of the volume of runoff likely to be generated by the development and assessment of the hydraulic performance of the artificial drainage system for both storm and foul whether existing or proposed.

• Flood risk management measures and assessment of long term sustainability

• Assessment of the change in flood risk to the surrounding area caused by the development site both upstream and downstream including volumes of displaced water and flood levels

• Residual risks to both the development site and the surrounding area after inclusion of any mitigation measures. Where new or modified structural measures are provided, an assessment of their behaviour in extreme events should be provided.

Developments within medium or high risk flood zones should be further classified to ensure that flood resilience measures are considered to manage the residual risk of flooding. These measures should be designed to last as long as the development and may include:

• The location of most vulnerable elements of a development within the areas of lowest risk.

• Maintenance of access/egress routes

• Location of critical infrastructure.

• The use of flood resilient and securely fastened infrastructure, seats etc

• Planning of escape routes. In areas of high and medium risk it may not be possible to ensure dry egress routes in time of flood. In these cases consideration should be given to the likely occupancy during flooding, the availability of safe refuge, potential for provision of key services and the expected duration of inundation.

Reference should be made to “C624 Development and flood risk - guidance for the construction industry CIRIA 2004” for details of best practice.


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9.2 Drainage Assessments All developments should require a surface and foul water management strategy, which must comply with the policies outlined in the LDD unless otherwise agreed with the LPA.

All drainage design should comply with the latest national and regional design guidance. At present these are Sewers for Adoption 6th Edition and, Document H – Drainage and Waste Disposal of the Building Regulations (BR part H). The integrity of the existing drainage should be maintained in compliance with the Building Regulations Part C.

Responsibility for drainage is fragmented which makes management more complex. Sewerage undertakers are responsible for the public sewerage system that serves most urban areas. Within the City of London Thames Water is the responsible sewerage undertaker. Thames Water is responsible for the drainage of flow arising from the boundaries of developments. Property owners are responsible for drainage within the boundary of their development.

Where new developments are proposed the planners role is to ensure that the development can be effectively drained from above and below ground without any detrimental effects downstream.

Guidance from Sewers for Adoption 6th Edition and PPS 25 is as follows:

• PPS 25 states that “In general terms, sewers should be designed to ensure that no flooding occurs above ground level for events with a return period in the range of 30 to 50 years, depending on the development type”. Sewers for Adoption 6th edition states that “all systems should be designed not to flood any part of the site in a 1 in 30 year return period design storm”

• “any underground storage to be constructed to attenuate the 1 in 30 year event should be sited within the system being offered for adoption. Storage over and above the 1 in 30 year event should not be sited within the adoptable system” (Sewers for Adoption 6th Edition)

• Sewers for Adoption 6th Edition states that “During extremely wet weather, the capacity of the surface water sewers may be inadequate. Under such conditions surface water may escape from the system. Checks should be made to ensure that an adequate level of protection against flooding of properties is achieved”. Following PPS 25 guidance, for return periods greater than 1 in 30 years, surface flooding of open spaces such as car parks or landscaping is acceptable for short periods of time.

• “In designing the site sewage and layout, developers should demonstrate flow paths and potential effects of flooding resulting from storm events


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exceeding the design criteria.” Design of the drainage system should ensure that water is routed away from any vulnerable property and no flooding of property should occur as a result of a 1 in 100 year event including allowance for climate change.

• PPS 25 states that “The development rate of runoff into a watercourse or other receiving water body should be no greater than the existing rate of runoff for the same event.” This should also be applied to discharges to sewer system as per the majority of the City of London.

Developers should be encouraged to reduce runoff rates from previously developed sites as far as possible and supplementary planning guidance of the London Plan outlines the Mayors “essential” and “preferred standards” to include the importance of the use of SUDS wherever practical and the need to achieve:

• “50% attenuation of the undeveloped site’s surface water runoff at peak times” (essential standard)

• “100% attenuation of the undeveloped sites surface water runoff as peak times” (preferred standard).

All drainage design should be undertaken in close liaison with Thames Water and where the scale of development permits, in liaison with the Environment Agency.

Further guidance is provided in Designing for exceedance in urban drainage, good practice (CIRIA C 635) 2006.

9.3 Sustainable Urban Drainage Systems Both PPS1 and PPS25 require that Regional Planning Bodies and LPAs should promote SUDS. To comply with PPS25 Regional Spatial Strategies should include specific policies to encourage the use of SUDS. In response to this The London Plan highlights the importance of using SUDS in new developments wherever possible:

Ploicy 4C.8 “Boroughs should seek to ensure that surface water run-off is managed as close to its source as possible. The use of SUDS should be promoted for developments unless there are practical reasons for not doing so. Such reasons may include the local ground conditions or density of development. In such cases the developer should seek to manage as much runoff as possible on site and explore sustainable methods of managing the remainder as close as possible to the site.”

With concerns surrounding the impacts of climate change a more sustainable approach to drainage is required to reduce flood risk and manage water quality.

PPS 25 frequently refers to the use of SUDS to cover the whole range of sustainable approaches to surface water management including:


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• Source control measures including rainwater recycling and drainage;

• Infiltration devices to allow water to soak into the ground;

• Filter strips and swales, which are vegetated features that hold and drain water downhill mimicking natural drainage patterns;

• Filter drains and porous pavements to allow rainwater to run-off to infiltrate into permeable material below ground and provide storage if needed;

• Basins and ponds to hold excess water after rain and allow controlled discharge that avoids flooding.

There are many different SUDS techniques that can be used within a development site, however, not all techniques will be suitable for all locations, therefore, it is important that site constraints are identified in order that the preferred SUDS technique can be used. Despite the relatively high density of development in the City of London, a number of suitable SUDS techniques exist which can be incorporated into the new developments to help manage flood risk.

9.3.1 SUDS Selection The following method for assessing the suitability of SUDS techniques based on site characteristics has been adapted from (CIRIA Sustainable Drainage Systems, Hydraulic, structural and water quality advice 2004”) and applied to the City of London to in order that the most suitable SUDS techniques can be identified.

The selection of SUDS techniques is divided into five sections based on the following criteria:

• Land use characteristics

• Site characteristics

• Catchment characteristics

• Quantity and quality performance requirements

• Amenity and environmental requirements

Each technique is scored on each of the five criteria as shown in Table 15 below:

A weighting score has been given to each criteria, scored from 1 to 3, with one having least weighting.

The results have been put into three bands as follows:


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Red = 0 – 45 Not suitable for use within the City of London

Orange = 45 – 55 May be possible to design into schemes

Green = 55 – 65 A good option for SUD within the City of London

SUDs techniques that score the highest based on a suitable and realistic set of design criteria for the City of London are Pervious Pavements, Green roofs and On-/ Off-line storage. The following sections provide a brief overview of these SUDS techniques.

It should be noted that although the above three criteria scored highest for suitability in the City of London, each SUDS selection should be undertaken by developers on a site by site basis.


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Table 15 - SUDS Score Techniques for Site Specific Constraints

Criteria Assessment

Wei

ghtin

g

Per

viou

s P

avem

ents

Gre

en r

oofs

Bio

rete

ntio

n

Filtr

atio

n te

chni

ques

Gra

ssed

filte

r st

rips

Sw

ales

Infil

trat

ion

devi

ces

Filte

r dr

ains

Infil

trat

ion

basi

n

Ext

ende

d de

tent

ion

pond

s

Wet

pon

ds

Sto

rm w

ater

wet

land

s

On-

/off

-line

sto

rage

Is pollutant

removal a

priority?

Most instances CoL will

be roof runoff, therefore

pollutant removal not a

priority.

1 5 5 5 4 2 4 3 3 3 3 4 5 1

Is water

quantity control

a priority?

Yes, attenuation required

in CoL to prevent

downstream flooding.

2 10 8 4 4 4 6 10 6 10 8 10 6 10

Is flow rate

control a

priority?

If flow can be reduced it

would have a positive

impact on flood risk.

3 8 8 4 4 4 8 8 8 8 6 6 6 6

Hyd

rolo

gica

l

Is groundwater

recharge

required?

No, high Groundwater

levels have been a

problem in CoL.

1 3 5 3 5 3 3 1 5 1 2 2 2 5

Land

use

Suitability to

type of

development

It has been assumed that

all development will be

Dense urban

developments

1 5 5 3 4 1 2 3 3 1 1 1 1 5

Catchment

Area

Assumed that due to

nature of development, all

catchments will be <2ha

1 5 5 5 5 5 5 5 5 5 1 1 1 5

Site slope A site slope of 0 to 10%

has been assumed.

1 5 5 5 5 5 5 5 5 5 5 5 5 5

Space required Limited space for SUDs

across CoL

3 10 10 6 10 4 4 10 6 2 2 2 2 10

Soil infiltration

rate

The CoL is on clay soils

with a low infiltration rate

1 5 5 5 5 5 5 1 5 1 5 5 5 5

Phy

sica

l sut

e fe

atur

es

Water table

depth

Greater than 1m depth to

water table across CoL

1 5 5 5 5 5 5 5 5 5 5 5 4 5

Total Score 61 61 45 51 38 47 51 51 41 38 41 37 57


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9.3.2 Pervious Pavements Pervious pavements allow rainwater to infiltrate through the surface into underlying construction layers where water is stored prior to infiltration to the ground, reused or released to a surface water drainage system or watercourse at an attenuated rate.

While pervious pavements are a good choice of SUDS for use within the City of London, consideration of the proximity of basements and foundations must be made. Where pervious pavements are located within 5m of foundations or basements, an impermeable membrane liner is required to prevent infiltration.

Pervious pavements can either be made from porous materials which allow infiltration across their entire surface e.g. gravels, grass and porous concretes, or permeable surfaces which are made from impermeable materials with voids to allow infiltration e.g. brick paving.

Figure 7 - Illustration of a Pervious Pavement using Block Paving

(Source CIRIA website)

Pervious pavements can be used for both infiltration and attenuation collecting water from paved areas and roof catchments. They have been shown to reduce both the peak flow rate and total runoff volume from developments. They can be designed to temporarily store runoff from storm events with a return period of 1 in 100 years and are suitable for incorporation into rainwater utilisation projects.

Pervious surfaces can be incorporated into soft landscaping and oil interceptors can be added to improve pollutant retention and removal. In urban areas where there is a high percentage of hard cover the use of pervious surfaces for car parks and hard areas is a valuable technique that should be used wherever possible.

9.3.3 Green Roofs “Green roofs comprise a multi layered system that covers the roof of a building with vegetation cover/landscaping/permeable car parking, over a drainage layer. They are designed to intercept and retain precipitation, reducing the volume of runoff and


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attenuating peak flows.” (CIRIA Sustainable Drainage Systems, Hydraulic, structural and water quality advice 2004”)

Rooftops form a major part of the cityscape, but have been vastly under utilised. The use of green roofs can reduce the size of downstream SUDS and drainage infrastructure that is required.

There are two main types of green roof as described below:

Extensive green roof This covers the whole roof area with low growing, low maintenance plants. They usually comprise of 25 – 125mm thick soil layer in which a variety of hardy, drought tolerant, low level plants are grown. Extensive green roofs are designed to be self sustaining and cost effective and can be used in a wide variety of locations.

Figure 8 - Illustration of an extensive green roof

(Source The Greater Vancouver Regional District (GVRD) Web site) Intensive green roof Intensive green roofs are landscaped areas which include planters or trees and are usually publicly accessible. They may include irrigation and storage for rainwater. They often require more maintenance and impose a greater load on the roof structure than extensive green roofs.


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Figure 9 - Illustration of an Intensive Green Roof

(Source Trelleborg website information green roofs)

Intensive roofs can be adapted to be simple intensive green roofs. These are vegetated with lawns and still require maintenance, however, they impose a reduced load on the roof structure and are less expensive.

Figure 10 - Illustration of a Simple Intensive Green Roof

(Source Rehbein Environmental Solutions website)

9.3.4 On/Off-line storage On-line or off-line storage refers to tanks or other underground storage structures. Tanked storage collects and stores runoff to be released at the required rate into the receiving watercourse or sewer. On/Off-line storage can take the form of oversized pipes, concrete storage and cellular storage systems. They can be applied where space is a constraint and above ground SUDS features such as ponds are not suitable. Tank storage systems do not provide any pollutant removal potential and should, where possible be used in conjunction with other SUDS features. The


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primary benefit of On/Off-line storage is their ability to attenuate run-off to the greenfield run-off rate.

9.3.5 Adoption and Maintenance of SUDS There is currently no specific provision for the adoption of SUDS techniques by Water Authorities. However, within the city of London much of the development drainage will fall within private development ownership e.g. green roofs, meaning that the adoption scenario should not be a constraint to the use of SUDS. Owners of developments using SUDS should be provided with an owners manual to include details of the location of SUDS, a maintenance plan, brief summary of how they work and identification of areas where activities are prohibited e.g. stockpiling materials on pervious surfaces. Maintenance inspections can generally be carried out monthly to include activities such as grass cutting, plant control and debris removal. If SUDS systems are properly monitored and maintained, any deterioration in performance can be managed out.


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10 Conclusions

The main objective of this study was to enable the City of London Corporation to apply the sequential flood risk test when allocating development sites, thus ensuring compliance with PPS25, but more importantly, providing sustainable development throughout the Borough.

A source pathway receptor model was produced which assessed the risks and consequences posed by the different sources of flooding. The information required to enable a thorough assessment of secondary flooding sources was in general unavailable or not received in time to inform this study.

A broad assessment of risks from sewer flooding was undertaken which indicated that the risk of flooding from sewers is low, and where flooding may occur the consequences are likely to be very small such that they are unlikely to restrict development. Groundwater flooding was found to be a relatively low risk due to the ongoing abstraction scheme operated across London by GARDIT.

The primary source of flood risk to the City of London was found to be from Tidal Flooding in the River Thames. The standard of protection provided by the Thames Barrier and Riverside protections is high and will continue to protect The City up to the 1 in 1000 year flood event (that is the event which has a 1 in 1000 year annual probability). However the residual risk of breaching or overtopping of the defences must still be considered when planning development because the consequences of these two sources can be high, with the potential for loss of life within the development areas immediately behind the defences.

The Zoning required to undertake the sequential test has been assessed and presented in map form to inform planning process. The residual risk of flooding behind defences has been taken into account when assessing these Zones.

10.1 Further Studies In the course of this study a number of investigations were identified which may be able to provide further advice to this study in the future. These studies are:

• River Thames Tidal Catchment Flood Management Plan

• Thames Estuary 2100 Project

• London Underground Flooding Investigation

• Thames Water Critical Water Main Investigation

A review of future flood defences in London is currently ongoing through the Thames Estuary 2100 project which will review protection of The City of London beyond 2030. The Corporation should be minded to keep themselves informed of the


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outcomes and take part in any consultation on both this study and the River Thames Tidal Catchment Flood Management Plan.

10.2 Recommendations for Further Work The changing nature of flood defences, climate change, the groundwater abstraction scheme and the London Underground flooding investigation all highlight the changing nature of flood risk assessment. It is important that this SFRA is periodically reviewed to determine whether additional information is available which may change the flood risk picture in The City.


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References 1) City of London Local Development Framework Core Strategy, Issues and

Options, May 2006 2) Department of Communities and Local Government Planning Policy

Guidance 1: General Policy and Principles, 1997 3) Supplementary Planning Guidance, Sustainable design and construction.

Mayor of London, May 2006 4) London County Council, Main Drainage of London Descriptive Account, 1930 5) CIRIA Special Publication 69 The Engineering Implications of Rising

Groundwater levels in the deep aquifer beneath London, 1989 6) Thames Water Central London Rising Groundwater 7) UK Climate Impact Partnership, Climate Change Scenarios for the United

Kingdom: The UKCIP02 Scientific Report, 2002 8) London Climate Change Partnership, London’s Warming: A Climate Change

Impacts in London Evaluation Study, 2002 9) http://te2100.dialoguebydesign.net/dbyd.asp 10) Halcrow Group Limited, Tidal Thames Extreme Water Levels –

Reassessment of Joint Probability Analysis, 2005 11) EA / DEFRA, Flood Risk Assessment Guidance for New Development Phase

2, 2005 12) Department of Communities and Local Government, Planning Policy

Statement 25: Development and Flood Risk, 2005 13) City of London Unitary Development Plan 2002 14) Sustainable Urban Communities, Building for the Future, ODPM, 2003 15) Strategic Planning for Flood Risk in Growth Area – Insurance Considerations,

ABI, July 2004 16) Preparing for Floods, ODPM October 2003 17) Design Guidance on Flood Damage to Buildings, BRE, 1996 18) Thames Tidal Contributions Policy, Environment Agency, Thames Region

Flood Defence Committee, 2003 19) Hother, J., Applying Reliability Risk Analysis to Flood Risk Management, 39th

DEFRA Flood and Coastal Management Conference, 2004 20) Steven Wade, Mohammed Hassan & Valerie Bain, Flood risks to people in

defended areas, (HR Wallingford Ltd) presented at the London CIWEM conference, January 2005.

21) Flood Resilient Homes, Association of British Insurers, April 2004 22) ABI, 2005; Statement of principles on the provision of Flood Insurance –

Updated Version, Association of British Insurers, November 2005 23) DEFRA, 2004; Making Space for Water-Developing a New Government

Strategy for Flood and Coastal Erosion Risk Management in England. 24) Environment Agency 2003; Your Environment – 1953 Floods, Timetable of

Events 25) Lancaster, J.W., Preene, M. and Marshall, C.T. 2004; Development and

Flood Risk – guidance for the construction industry (C624). CIRIA Publications. ISBN 0-86017-624-X

26) Development and Flood Risk: A Practical Guide Companion to PPS25 ‘Living Draft’ a consultation paper.


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27) Environment Agency 2007. Managine Flood Risk, Thames Region Catchment Flood Management Plan, Summary Document, Consultation, January 2007.

28) Acclimatise, Actions to ‘climate-proof’ the City of London Corporation. The City of London Corporation’s climate adaptation strategy. January 2007.

29) LCCP Report – “Climate Change and London Transport Systems” 30) London Catchment Abstraction Management Plan 31) Groundwater levels in the Chalk-Basal Sands Aquifer of the London Basin

Strategic Flood Risk Assessment City of London

Final - August 2007

Appendix A Flood Risk Zone Maps

Contents

Map 13: Flood Risk Zone Map

Map 14: Combined Flood Risk Map

Strategic Flood Risk Assessment City of London

Final - August 2007

Appendix B Maps

Contents

Map 1: Study Area

Map 2: Topography Map

Map 3: Borough Boundary and Neighbouring Boroughs

Map 4: Historical Flood Map

Map 5: River Fleet and Walbrook Catchments

Map 6: Critical Sewer Flooding

Map 7: Sewer Locations

Map 8: Groundwater Map

Map 9: Flood Defence Gradings

Map 10: Residual Risk of Overtopping with no Thames Barrier

Map 11: Residual Risk of Breach with no Thames Barrier

Map 12: Combined Residual Risk with no Thames Barrier

Documents

Strategic Flood Risk Assessment 2007 - City of London · 2014-05-01 · Strategic Flood Risk Assessment 9 City of London Final - August 2007 1 Introduction 1.1 Background Planning