9713 Planning document covers 22/10/08 14:01 Page 1

24 and Local25 Development Framework SeptemberTechnical Background Report Strategic Flood Risk Assessment 2008 10am – 4pm

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www.hillingdon.gov.uk London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

Revision Schedule

Strategic Flood Risk Assessment Level 1 November 2008

Rev Date Details Prepared by Reviewed by Approved by

00 August Draft Heather Rich Jon Robinson 2007 Strategic Flood Risk Specialist Associate Director Flood Risk Assessment 01 November Final Level 1 Heather Rich Liz Williams Jon Robinson 2007 Strategic Flood Risk Specialist Senior Consultant Associate Director Flood Risk Assessment 02 January Final Level 1 Liz Williams Heather Rich Jon Robinson 2008 Report Rev 1 Senior Consultant Flood Risk Specialist Associate Director

03 February Final Level 1 Heather Rich Jon Robinson 2008 Report Rev 2 Flood Risk Specialist Associate Director

04 April 2008 Final Level 1 Heather Rich Liz Williams Jon Robinson Report Rev 3 Flood Risk Specialist Senior Consultant Associate Director

04 November Final Level 1 Eleanor Cole Liz Williams Jon Robinson 2008 Report Rev Assistant Senior Consultant Associate Director 4-Hillingdon Hydrologist Comments

Scott Wilson Ltd., 6-8 Greencoat Place London SW1P 1PL

Tel: +44 (0)20 7798 5200 Fax: +44 (0)20 7798 5001

www.scottwilson.com London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

Abbreviations

Acronym Definition ABDs Areas Benefiting from Defences AONB Area of Outstanding Natural Beauty CFMP Catchment Flood Management Plan CLG Communities and Local Government DEM Digital Elevation Model DPD Development Plan Documents DTLR UK Department of Transport, Local Government and Regions EA Environment Agency for England and Wales FRA Flood Risk Assessment FRIS Flood Reconnaissance Information System GIS Geographical Information Systems IDB Internal Drainage Board LBH The London Borough of Hillingdon LDDs Local Development Documents LDF Local Development Framework LDS Local Development Scheme LiDAR Light Detection and Ranging LPA Local Planning Authority LPD Local Planning Documents mOD Meters Ordnance Datum MoD Ministry of Defence ODPM Office of the Deputy Prime Minister PBA Peter Brett Associates PCPA 2004 Planning and Compulsory Purchase Act 2004 PPG25 Planning Policy Guidance Note 25: Development and Flood Risk PPS25 Planning Policy Statement 25: Development and Flood Risk RFRA Regional Flood Risk Appraisal RPG Regional Planning Guidance London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

RSS Regional Spatial Strategy SAR Synthetic Aperture Radar SAC Special Area of Conservation SA Sustainability Appraisal SFRA Strategic Flood Risk Assessment SPA Special Protection Area SPG Supplementary Planning Guidance SSSI Site of Special Scientific Interest SuDS Sustainable Drainage Systems

London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

Glossary

Term Definition A source of groundwater comprising water-bearing rock, sand or Aquifer gravel capable of yielding significant quantities of water. Catchment A high-level planning strategy through which the Environment Flood Agency works with their key decision makers within a river catchment Management to identify and agree policies to secure the long-term sustainable Plan management of flood risk. Culvert A channel or pipe that carries water below the level of the ground. Infrastructure used to protect an area against floods as floodwalls Flood defence and embankments; they are designed to a specific standard of protection (design standard). Area adjacent to river, coast or estuary that is naturally susceptible to Flood plain flooding. A temporary area that stores excess runoff or river flow often ponds Flood storage or reservoirs. Fluvial flooding Flooding by a river or a watercourse. Height of flood defence crest level (or building level) above designed Freeboard water level. Water that is in the ground, this is usually referring to water in the Groundwater saturated zone below the water table. Inundation Flooding. The core of the updated planning system (introduced by the Planning Local and Compulsory Purchase Act 2004). The LDF comprises the Local Development Development Documents, including the development plan Framework documents that expand on policies and provide greater detail. The (LDF) development plan includes a core strategy, site allocations and a proposals map. Local Planning Body that is responsible for controlling planning and development Authority through the planning system. Mitigation An element of development design which may be used to manage measure flood risk or avoid an increase in flood risk elsewhere. Flooding caused when intense rainfall exceeds the capacity of the Overland Flow drainage systems or when, during prolonged periods of wet weather, the soil is so saturated such that it cannot accept any more water. Pluvial Flooding associated with rainfall Risk The probability or likelihood of an event occurring.

Sewer flooding Flooding caused by a blockage or overflowing in a sewer or urban drainage system. London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

Sustainable Methods of management practices and control structures that are drainage designed to drain surface water in a more sustainable manner than system some conventional techniques. Development that meets the needs of the present without Sustainable compromising the ability of future generations meeting their own development needs. Event that on average will occur once every 100 years. Also 1 in 100 year expressed as an event, which has a 1% probability of occurring in event any one year. 1 in 100 year Flood defence that is designed for an event, which has an annual design probability of 1%. In events more severe than this the defence would standard be expected to fail or to allow flooding. London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

Table of Contents

Abbreviations...... 2

Glossary ...... 4

1 Introduction...... 1 1.1 Hillingdon’s Profile...... 3 1.2 Aim ...... 7 1.3 The Strategic Flood Risk Assessment Objectives...... 8 1.4 The SFRA Structure ...... 8

2 Policy Context ...... 11 2.1 National Policies...... 11 2.2 Regional Policies...... 12 2.3 Local Policies ...... 18 2.4 Environment Agency Policies...... 20 2.5 Neighbouring Strategic Flood Risk Assessment Status ...... 22 2.6 Other Relevant Policies...... 23

3 Data Collection and Methodology...... 24 3.1 Review of Data Collection Process ...... 24 3.2 Data Collection ...... 25 3.3 Fluvial Flooding ...... 26 3.4 Sewer Flooding ...... 35 3.5 Overland Flow ...... 36 3.6 Groundwater Flooding...... 37 3.7 Artificial Sources...... 38 3.8 Additional Mapping...... 38

4 Flood Risk in Hillingdon...... 40 4.1 Fluvial Flooding ...... 40 4.2 Sewer Flooding ...... 46 4.3 Overland Flow ...... 46 4.4 Groundwater Flooding...... 47 4.5 Artificial Sources...... 47 4.6 Residual Risks...... 48

London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

5 Guidance on Applying the PPS25 Sequential Test ...... 50 5.1 What is the Sequential Test? ...... 50 5.2 How should the SFRA be used to apply the Sequential Test?...... 53 5.3 Additional Guidance ...... 56

6 Guidance on Applying the PPS25 Exception Test ...... 59 6.1 What is the Exception Test?...... 59 6.2 What is Required to Pass the Exception Test? ...... 59

7 Flood Risk Management...... 60 7.1 Flood Defences ...... 60 7.2 Flood Warning Areas...... 60

8 Sustainable Drainage Systems (SuDS)...... 62 8.1 Principles...... 62 8.2 SuDS Policies...... 62 8.3 SuDS Recommendations ...... 65 8.4 Where can SuDS be utilised in Hillingdon?...... 67

9 Suggested Policy Considerations...... 71 9.1 Flood Risk ...... 71 9.2 Flood Risk Management ...... 73 9.3 Sustainable Drainage Systems ...... 74 9.4 Flood Risk & Environment...... 76 9.5 Heathrow Airport Development ...... 76

10 Site Specific FRA Guidance ...... 78 10.1 When is a Flood Risk Assessment Required? ...... 78 10.2 FRA Requirements...... 79 10.3 Flood Risk Assessment Guidance Table ...... 79

11 Emergency Planning ...... 85 11.1 Existing Flood Warnings - Details of the Environment Agency’s Flood coverage...... 85 11.2 Flood Warnings and procedures in the London Borough of Hillingdon ...... 86 11.3 Emergency Planning Considerations ...... 87 11.4 Emergency Planning Recommendations ...... 88

London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

12 Suggested Actions for Hillingdon...... 90

13 References ...... 91

14 Figures ...... 93

Appendix A: Data Register ...... 103

Appendix B: Sewer Flooding Data ...... 105

Appendix C: Sequential & Exception Test & CD of GIS layers ...... 107

Appendix D: SuDS Tables...... 112

Appendix E: Coarse Assessment...... 120

London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

Table of Figures

Figure 1: Study Area Location Map...... 94 Figure 2 Main Rivers ...... 95 Figure 3: Fluvial Sources of Flood Risk...... 96 Figure 4: Other Sources of Flood Risk ...... 97 Figure 5: NFCDD Map...... 98 Figure 6: Flood Warning Area Map ...... 99 Figure 7: Vulnerability Institutions Location Map...... 100 Figure 8: Solid Geology...... 101 Figure 9: Drift Geology ...... 102

London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

1 Introduction

The Planning and Compulsory Purchase Act 2004 (PCPA) (HMSO, 2004) requires Local Planning Authorities to produce Local Development Frameworks (LDFs) that will replace the system of Local and Unitary Development Plans. LDFs are a portfolio of documents (Local Development Documents (LDDs) that collectively deliver the spatial planning strategy for the local authority area.

The PCPA 2004 requires LDDs to undergo a Sustainability Appraisal (SA), which assists Planning Authorities in ensuring that their policies fulfil the principles of sustainability.

Strategic Flood Risk Assessments (SFRAs) constitute a component of the SA process and should be used in the review of LDDs or in their production.

The release of Planning Policy Guidance Note 25: Development and Flood Risk in July 2001 (PPG25) (DTLR, 2001) introduced a new emphasis on flood risk. This increased the responsibility of Local Authorities to ensure that flood risk is understood and managed effectively using a risk-based approach as an integral part of the planning process.

On the 7th December 2006 PPG25 (first published in July 2001) was superseded by Planning Policy Statement 25: Development and Flood Risk (PPS25) (CLG, 2006). This re-emphasises the active role Local Authorities should have in ensuring that flood risk is considered in strategic land use planning. PPS25 requires Local Planning Authorities to undertake SFRAs and to use the findings to inform land use planning. In February 2007, a ‘living draft’ of the Practice Guide Companion to PPS25 was released for consultation. Although this is a consultation document, it provides a suggested approach to the production of SFRAs that should be considered.

To assist local Authorities in their strategic land use-planning, SFRAs should present sufficient information to enable Local Authorities to apply the Sequential Test to their proposed development sites. The SFRA should have regard to catchment wide flood issues and also involve a: - ’process which allows the Local Planning Authority to determine the variations in flood risk across and from their area as the basis for preparing appropriate policies for flood risk management for these areas’.

In addition, where development sites cannot be located in accordance with the Sequential Test as set out in PPS25 (i.e. to steer development towards areas of lowest risk): ’The scope of the SFRA should be increased to provide the information necessary for the application of the Exception Test’.

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The spatial planning of these new developments must be considered with regard to the current and future risk of flooding from a number of sources. These flood sources include fluvial, stormwater management systems and groundwater. It is therefore vitally important that flood risk is considered at a strategic scale to inform land allocations and future developments within the emerging LDFs. To ensure the above targets are accommodated and that the emerging policies for the area are compatible, the London Borough of Hillingdon has commissioned a Strategic Flood Risk Assessment.

In accordance with the most recent Practice Guide Companion to PPS25, (June 2008), Strategic Flood Risk Assessments may be completed in two consecutive stages. The Level 1 SFRA should present sufficient information to enable the Local Planning Authority to apply the Sequential Test (Appendix C) to potential development areas and to assist in identifying whether application of the Exception Test (Appendix C) will be necessary through a Level 2 SFRA. In addition, the Level 1 SFRA provides background information and a preliminary review of available data, sufficient to scope the type of assessment necessary should a Level 2 SFRA1 be required.

The Level 1 SFRA should be used by the London Borough of Hillingdon to undertake the Sequential Test to identify where potential development sites can be located within Flood Zone 1. Development that cannot be located within Flood Zone 1 should be assessed through a Level 2 SFRA where required. This report presents information generated during the Level 1 SFRA and provides recommendations for a Level 2 SFRA should this be required.

The London Borough of Hillingdon commissioned Scott Wilson to undertake a Level 1 Strategic Flood Risk Assessment. The SFRA identifies flood risk issues relevant to both existing and proposed developments within the Borough in order to allow a direct input into the strategic planning of Hillingdon through Local Development Frameworks.

The Local Development Framework (LDF) for the London Borough of Hillingdon will replace the Unitary Development Plan (UDP) 1998 Saved Policies (Sep 2007). The LDF is being prepared under the provisions of the

1 A Level 2 SFRA includes an increased scope. This ‘increased scope’ should provide necessary information for the application of the Exception Test where decision makers are unable to allocate all proposed development and infrastructure in accordance with the Sequential Test , taking account of the flood vulnerability category of the intended use. More detailed information is necessary in this case, where development pressure is evident in areas at medium or high risk of flooding. The Level 2 SFRA should consider the detailed nature of flood hazard, taking account of flood risk management measures, e.g. defences. This should allow a more sequential approach to site allocation within a flood zone. For more information see PPS25: Development and Flood Risk Practice Guide (C&LG June 2008) p48

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Planning and Compulsory Purchase Act 2004, in which Local Planning Authorities prepare and adopt a LDF with the objective of delivering sustainable development in their areas.

1.1 Hillingdon’s Profile The LDF contains a spatial portrait of Hillingdon, identifies issues, and sets out a vision and objectives covering the next 10 to 15 years. The core strategy and supporting development plans contain spatial policies and proposals as the means of delivering the spatial vision for Hillingdon.

The study area is defined by the administrative boundary of the London Borough of Hillingdon (Figure 1, page 94).

Hillingdon is London’s second largest borough, by area, covering 42 sq. miles (109 sq. kilometres) with a population of almost 250,000 people. The borough has a distinctive character with its combination of suburban streets and shopping centre’s, industrial land, major office developments and large areas of open land, historic woodland and inland waterways including 4,960 hectares of Green Belt.

Current Census estimates suggest that the population of Hillingdon will grow to around 260,000 people around the year 2015. As the population of Hillingdon grows, it is anticipated that the diversity of the borough will increase. At present around 21% of Hillingdon's population is made up of black and ethnic minority communities, this figure is projected to rise by a further 15-20% over the next 10 years. Compared to many other London Borough's, which have ageing populations, Hillingdon has a population with a high proportion of young residents, with around 41% being under 30 years of age and 19% being under 15 years of age.

Despite remaining one of London’s greenest boroughs, Hillingdon has a pivotal role in the economic success of the capital being the home of Heathrow Airport, the busiest international airport in the world, and London's gateway to the world. Heathrow Airport, attracts over 67 million passengers each year, and employs 68,400 people on site. In addition, a large number of international corporations have their headquarters in the borough. There are around 7,500 registered businesses in Hillingdon, ranging from large multinational corporations to local family firms.

The majority of dwellings in Hillingdon are houses (82.6%) compared to flats (17.4%). The most common house type in Hillingdon is semi- detached (40.3%) followed by terraced (27.9%)

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From: Revised Core Strategy Preferred Options (Feb 2007), Section 2: Hillingdon Characteristics

There are a number of watercourses present within the London Borough of Hillingdon. These are discussed below (Section 1.1.2). The River Colne is the dominant river in the Hillingdon area and is large and complex in this area (discussed further below 1.1.2 a)).

1.1.1 Geology The dominant solid geology within Hillingdon is the London Clay Formation. To the north of the Borough, within the vicinity of Ruislip and Northwood, outcrops of the Lambeth group (formerly known as the Woolwich and Reading beds) occur within river valleys. Along the western boundary of the district some areas of Chalk are located within the vicinity of the River Colne.

Drift deposits overlying the solid geology in the southern area of the district consist of pockets of Langley Silt (sandy clay and silt ’brick earth‘) overlying the River Terraced Deposits (mainly gravels), which have been locally excavated, creating lakes and reservoirs where they have not been backfilled and areas of infilled ground where they have. Within the vicinity of West Drayton, Langley silt is found to extensively overlie the gravels.

To the north of the A40 drift deposits are limited to pockets of Glacial Sand and Gravel, which includes undifferentiated head (the glacial deposits will consist mainly of sands and gravels and the head deposits of sandy clay and silt).

Along the line of river channels, alluvial deposits are located and in some areas the underlying solid formation has been exposed.

1.1.2 Local Watercourses The London Borough of Hillingdon is a land locked Borough with local watercourses affected by fluvial flood risk. The local watercourses within the Borough are:- a) River Colne (including the tributaries of the Frays River and New Years Greene Bourne); b) River Crane (including the Yeading Brook); c) River Pinn; d) Duke of Northumberland River & Longford River; e) Grand Union Canal;

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The main rivers areas are illustrated in Figure 2, page 95.

a) River Colne

The River Colne is the dominant river in the Hillingdon area. The River Colne enters Hillingdon at the northern perimeter before flowing in a southerly direction, meandering inside and outside the administrative border to form the western border in the north of the Borough. The Colne is often referred to in two sections; the Upper Colne and the Lower Colne system differentiated by Denham Weir. The Upper Colne is predominantly rural land use and the Lower Colne is mainly urban. The Colne is a very complex river system with large reservoirs used to store potable supply water for Greater London. These can restrict historical floodplain area. Frays River and the River Pinn form some of the major tributaries into the River Colne.

Frays River

Fray’s River forms the western border of Hillingdon splitting from the New Years Greene Bourne and the River Colne at Denham Weir (National Grid Reference 505310, 186270). From Denham Weir the Fray’s River flows in a southerly direction for approximately 9km through the urban areas of Uxbridge, Cowley, Yiewsley and West Drayton before its confluence with the River Colne upstream of Cricketfield Road (National Grid Reference 505270, 178190).

New Years Greene Bourne

The New Years Greene Bourne is predominantly a rural watercourse 6.2km in length with little or no known flood risk associated with it. The Bourne sources at National Grid Reference 506820, 190940 approximately 250m north of Deathman’s Grove before flowing in a south-westerly direction through green open space and woodland into the Gravel Pit Lakes system and under the railway. The Bourne then flows due south in close parallel to the Grand Union Canal before its confluence with the River Colne near Denham Lock (National Grid Reference 505130, 190940).

Shire Ditch

The Shire Ditch emanates from the River Colne at National Grid Reference 5805269, 185964 where it passes underground and re-emerges to the east of the Grand Union Canal at National Grid Reference 505347, 185930. The Shire Ditch then flows in a south-easterly direction before passing under the A40 on the western side of the dismantled railway before flowing down to its confluence with the River Colne and Grand Union Canal. There are two points at which the Shire Ditch interacts with the Frays River which are discussed further in Section 5.1.

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b) Yeading Brook

The Yeading Brook flows into the Borough of Hillingdon from the east through two principal branches the East arm and West arm.

The Yeading Brook East arm enters Hillingdon through a long culvert at Field End Road (National Grid Reference 512340, 185690) before surfacing again to the southwest of Victoria Retail Park (National Grid Reference 511721, 185382). The Eastern arm flows in a south-westerly direction for 3.6km through South Ruislip and then west along the southern boundary of Northolt Aerodrome before its confluence with the West arm at National Grid Reference 549950, 184190.

The Yeading Brook West arm enters Hillingdon through Ruislip recreation ground and flows in parallel with the East arm in a south-westerly direction for 5km, curving round the northern and western boundary of Northolt Aerodrome, before its confluence with the Ickenham Stream to the south. The West arm then flows for 1.7km through rural pasture before its confluence with the East arm at National Grid Reference 509950, 184190.

The Yeading Brook main branch then flows south for 7.6km passing through green open space to the southeast of Yeading and the easterly edge of Hayes. Of this 7.6km length the Yeading Brook travels in parallel with the Grand Union Canal for 2.5km before flowing under an Aqueduct becoming the River Crane at Craneford Park.

River Crane

The Yeading Brook changes its name to River Crane at Hayes, north of the M4 to the south of the Grand Union Canal and before flowing under the M4 into Cranford Park. Here the Crane is bordered by parkland and green open space providing some natural flood plain area. The River Crane continues to flow through green open space in a southerly direction for 2km before it is joined by the small tributary Frog’s Ditch. The Crane then flows for 1.7km into the Heathrow Airport grounds before flowing out into the neighbouring Borough of Hounslow.

c) River Pinn

The River Pinn is 20km long and rises in the neighbouring Borough of Harrow before passing into Hillingdon where it flows through Ruislip, Eastcote, Uxbridge and Yiewsley. River Pinn joins the Frays River at Yiewsley to form a tributary of the River Colne. The Pinn is very a fast responding catchment that requires maintenance by the Environment Agency as vegetation has a strong affect on flows and levels. The River Pinn is also vulnerable to erosion along its banks.

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The Lower Pinn catchment is steep, highly urbanised and prone to short duration flooding caused by intense rainfall events between 1 in 5 and 1 in 30 years at various locations along the watercourse (Lower Pinn Flood Risk Management Review, Inception Report, EA, 2006).

d) Duke of Northumberland River & Longford River

The Duke of Northumberland River and Longford River are two channels that split from the River Colne at Harmondsworth (National Grid Reference 505350, 178160). The Duke of Northumberland is an artificial channel and is one of the main tributaries of the River Crane. It consists of two sections; the Harmondsworth or Western Section and the Mogden or Eastern Section. The Duke of Northumberland River flows in a southerly direction before turning in an easterly direction to form the southern border of the Borough along the perimeter of Heathrow airport before flowing into the Borough of Hounslow, running in parallel to the Longford River. As part of the Terminal 5 development this watercourse has been slightly diverted.

e) Grand Union Canal

The Grand Union Canal has two branches within the Borough of Hillingdon to the east and west. The westerly branch of the Grand Union Canal runs from the north of the Borough in parallel with the Upper Colne, through the Borough to Yiewsley, before turning east and travelling across the Borough through Hayes and on to Bull’s Bridge. Here it joins with the easterly branch, known as the Paddington Branch. There is no known interaction between the eastern branch and associated watercourses, however, there is known interaction between the Colne and the Grand Union Canal west branch which will be discussed further in Section 5.1.

1.2 Aim The aim of an SFRA is to identify land at risk of flooding from fluvial and other sources and thereafter to be actively used to guide land allocations to areas of least flood risk.

The SFRA should present sufficient information to enable the Local Planning Authority to apply the PPS25 Sequential Test to potential development sites and, where there are no ‘more reasonably’ available sites, to assist in identifying if application of the PPS25 Exception Test will be necessary and can be satisfied. The Exception Test should only be required when there are no available sites in areas not at risk of flooding.

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1.3 The Strategic Flood Risk Assessment Objectives The objectives of the Hillingdon SFRA, as set out in the revised Scott Wilson proposal of February 2007, are:

• To provide an SFRA in line with the policies and guidance presented in PPS25 and other associated documents and to inform the Sequential Test on flood risk, which should, in turn, inform land allocations and appropriate polices for flood risk management of these areas; • To determine and map land at risk of flooding (including specific potential land allocations) and the degree of risk from all relevant sources through identification of Flood Zones taking into account local flood defences and other sources of flooding, and the potential effects of climate change; • To identify residual flood risk taking climate change into consideration and safe access and egress to allocated sites; • To provide a tool to the Council that will inform the development control process about the potential flood risk associated with future development and the requirements for site-specific flood risk assessments and guidance on suitable flood risk policies for inclusion in the Local Planning Authority’s Local Development Framework.

1.4 The SFRA Structure Since this study was commissioned, the Department of Communities and Local Government (CLG) has released the Practice Guide Companion to accompany PPS25. The Practice Guide Companion to PPS25 recommends SFRAs are completed in two consecutive stages, this provides Local Planning Authorities with tools throughout the LDF and SFRA process sufficient to inform decisions regarding development sites. The two stages are:

• Level 1 SFRA – Study Area Flood Source Review & Data Review • Level 2 SFRA – Main SFRA and development sites assessments.

The results of the Level 1 SFRA will enable a prompt start to the commencement of Level 2 (where required). The data review element of Level 1 also enables a robust specification and programme to be developed for Level 2.

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In addition, this Level 1 SFRA provides background information and a preliminary review of available data, sufficient to scope the type of assessments necessary should a Level 2 SFRA be required.

1.4.1 Level 1 SFRA – Study Area, Flood Source Review and Data Review The objective of the Level 1 SFRA is to collate and review available information on flood risk for the study area. Information has been sought from a variety of stakeholders including the Environment Agency, London Borough of Hillingdon and Thames Water.

The deliverables from Level 1 should be used by the Local Planning Authority to complete the Sequential Test. Where the Sequential Test identifies the potential need to apply the Exception Test, further data collection and/or analysis will need to be carried out in a Level 2 report. This report presents the findings of a Level 1 SFRA study.

In addition to the review of data and consultation with local stakeholders, Level 1 also reviews the available data to meet the requirements of a Level 2 SFRA where required. Where necessary the report also identifies works beyond the critical scope that may benefit the assessment.

The flood risk information presented in this Level 1 SFRA should not be considered as an exhaustive list of all available flood related data for the study area. Any additional local knowledge of flood risk to an area should also be considered. The Level 1 SFRA report is a presentation of flood sources and risk based on data collected following consultation with and input from the partner Local Authorities and agencies within the timeframe available. It is hoped that should a Level 2 SFRA be required, the contacts and relationships developed in Level 1 will continue to assist in providing data and information for the SFRA.

The Level 1 SFRA addresses Objective 1 (above) and forms part of the evidence base (Objective 2) for the study area.

1.4.2 Level 2 SFRA - Main SFRA and Development Sites Assessments The purpose of a Level 2 SFRA is to facilitate the application of the Exception Test.

The Level 2 SFRA will use information obtained in the Level 1 SFRA (and additional works where necessary) to generate information sufficient for the application of the Exception Test to those sites that cannot be located in lower flood risk zones for other, wider sustainability, reasons.

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The Exception Test is the application of a three part test, as set out in PPS25. The test considered the wider sustainability benefits of the development, whether the site is where possible located on previously developed land, and the flood risks to the development to ensure it is safe and doesn’t increase flood risk elsewhere (see Section 7).

This information will supplement the Level 1 SFRA to provide the London Borough of Hillingdon with an evidence base sufficient to inform the strategic planning of the areas administered by the council.

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2 Policy Context

2.1 National Policies

2.1.1 Making Space for Water (2004) In 2004 the Government’s Making Space for Water strategy set out a new national direction for flood risk management planning in England over the next 20 years. The report recognised the requirement for a holistic approach between the various responsible bodies, including flood defence operating authorities, sewerage undertakers and highways authorities, to achieve sustainable development. The report also highlighted the need for a more integrated approach to urban drainage. The protection of the functional floodplain forms an integral aspiration of the strategy.

In January 2007 details of 15 new pilot studies were released that will aim to identify the causes and consider the most suitable ways to manage urban drainage and reduce future flooding, improving the resilience of towns and cites across the country to climate change from this particular source. The outcome of these studies will culminate into guidance upon urban flood risk and integrated drainage, which will be released in autumn 2008.

Amongst several other key drivers2, the Making Space for Water document intended to improve the manner in which land use planning was undertaken. Since 2004 the particular goals alluded to in this document have been achieved. The Environment Agency’s role as a statutory consultee has been extended in areas that are at risk of flooding. In essence, an objection made by the Environment Agency has the same weighting as that of the LPA. An integral part of this new direction for flood risk management planning in England was the production of a new Planning Policy Statement (PPS). As discussed within the Making Space for Water document itself, the intention was ‘to replace and improve the operational effectiveness of’, Planning Policy Guidance Note (PPG) 25. The overriding document PPS25 was released in December 2006 and is discussed below.

2.1.2 Planning Policy Statement 25: Development & Flood Risk (2006) This policy document establishes the national policy for development and flood risk. The overarching aim of PPS25 is to support the Government’s objectives for sustainable development. ‘The aims of planning policy on development and flood risk are to ensure that flood risk is taken into account

2 Including coastal erosion, management of water in a rural setting, improved provision of data and research and an improved incorporation of the three pillars of sustainable development (i.e. economic, social and environmental) in risk management activities.

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at all stages in the planning process to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas at high risk’. Therefore, PPS25 seeks towards steering of new development to sites of lower flood risk.

1. Allocate all sites in accordance with the Sequential Test with respect to flood risk and ensure that the vulnerability classification of the proposed development is where possible appropriate to the Flood Zone classification; 2. Flood Risk Assessments (FRAs) should be undertaken for all developments within Flood Zones 2 and 3 and sites with identified flood sources to assess the risk of flooding to, and from the development and identify options to mitigate the flood risk to the development, site users and surrounding area; 3. Flood Risk Assessments are required for all major developments in Flood Zone 1. These are residential developments consisting of sites greater than 0.5 ha or greater than 10 dwellings and commercial developments that are greater than 1 ha or have a floor area greater than 1000 m2. 4. Flood Risk to development should be assessed for all forms of flooding; 5. Where floodplain storage is removed, the development should provide compensatory storage on a level for level and volume for volume basis to ensure that there is no loss in flood storage capacity.

2.2 Regional Policies

2.2.1 London Plan Consolidated with Alterations since 2004 (February 2008) The Mayor of London published a revised London Plan Consolidated with Alternations since 2004 (February 2008). The published London Plan has been approved by the Secretary of State and is a statutory development planning document, and covers a period up to 2025/6. One of the important roles of the London Plan is to ‘translate strategy into proposals for the provision of new homes’. The plan sets out a strategy for distributing housing numbers amongst the boroughs and for realising and monitoring that development. Hillingdon provision for additional ‘homes’ targets 2007/8 to 2016/17 is 3650 (10 year target) with an annual monitoring target of 365.

Furthermore the London Plan sets out policies pertaining to flood risk that should be considered as part of the development process:-

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Flooding Policy 4A.12 Flooding: In reviewing their Development Plan Documents (DPDs), boroughs should carry out strategic flood risk assessments to identify locations suitable for development and those required for flood risk management. Within areas at risk from flooding (flood zones) the assessment of flood risk for development proposals should be carried out in line with PPS25. Policy 4A.13 Flood risk management: Where development in areas at risk from flooding is permitted, (taking into account the provisions of PPS25), the Mayor will, and boroughs and other agencies should, manage the existing risk of flooding, and the future increased risk and consequences of flooding as a result of climate change, by: • protecting the integrity of existing flood defences • setting permanent built development back from existing flood defences to allow for the management, maintenance and upgrading of those defences to be undertaken in a sustainable and cost effective way • incorporating flood resilient design • establishing flood warning and emergency procedures. Opportunities should also be taken to identify and utilise areas for flood risk management, including the creation of new floodplain or the restoration of all or part of the natural floodplain to its original function, as well as using open space in the flood plain for the attenuation of flood water.

The Mayor will, and boroughs and other agencies should, take fully into account the emerging findings of the Thames Estuary 2100 Study, the Regional Flood Risk Appraisal and the Thames Catchment Flood Management Plan.

Sustainable Drainage

Policy 4A.14 Sustainable drainage: The Mayor will, and boroughs should, seek to ensure that surface water run-off is managed as close to its source as possible in line with the following drainage hierarchy: store rainwater for later use use infiltration techniques, such as porous surfaces in non-clay areas attenuate rainwater in ponds or open water features for gradual release to a watercourse

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attenuate rainwater by storing in tanks or sealed water features for gradual release to a watercourse discharge rainwater direct to a watercourse discharge rainwater to a surface water drain discharge rainwater to the combined sewer.

The use of sustainable urban drainage systems should be promoted for development 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 run-off as possible on site and explore sustainable methods of managing the remainder as close as possible to the site.

The Mayor will encourage multi agency collaboration (GLA Group, Environment Agency, Thames Water) to identify sustainable solutions to strategic surface water and combined sewer drainage flooding/overflows.

Developers should aim to achieve greenfield run off from their site through incorporating rainwater harvesting and sustainable drainage. Boroughs should encourage the retention of soft landscaping in front gardens and other means of reducing, or at least not increasing, the amount of hard standing associated with existing homes.

Policy 4A.15 Rising groundwater: In considering major planning applications in areas where rising groundwater is an existing or potential problem, the Mayor will, and 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 development or by a water supply company.

2.2.2 The Mayor’s Draft Water Strategy – Mayor of London, March 2007 The Mayor of London’s Draft Water Strategy details these proposals further. Proposal 7 states that: The Mayor will, and the Boroughs should, require new developments (larger than 1,000 m2 or more than 10 dwellings) to manage their surface water runoff so that there is a 50 per cent reduction in the volume and rate of surface water drainage when compared to that of the undeveloped site at peak times. Proposal 8 states that: The Mayor will, and the Boroughs should, require new developments (larger than 3,000 m2 or more than 100 dwellings) to establish separate foul sewer and surface water drains

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and not to discharge excess surface water into the combined sewer system.

2.2.3 Additional Development Considerations In addition, development should integrate successfully with the water space in terms of use, appearance and physical impact and should in particular:

• Include a mix of uses appropriate to the water space, including public uses and open spaces, to ensure an inclusive accessible and active waterside and ground level frontage; • Integrate into the public realm, especially in relation to walking and cycling routes and borough open space strategies. Public art will often be appropriate in such locations as well as clear signage, information and lighting to promote the use of waterside spaces by all; • Incorporate built form that has a human scale of interaction with the street, public spaces and waterside and integrates with existing communities and places; • Recognise the opportunity to provide landmarks that are of cultural and social significance along the waterways, providing orientation points and pleasing views without causing undue harm to the cohesiveness of the water’s edge; • Relate successfully in terms of scale, materials, colour and richness of detail, not only to direct neighbours but also to buildings on the opposite bank and those seen in the same context with the River Prospects (see Policy 4B.15) or other locally identified views. Such juxtaposition of buildings should take into account river meanders and the impact these can have on how buildings may be seen together; • Incorporate sustainable design and construction techniques, in particular a precautionary approach to flood risk.

2.2.4 Sub Regional Development Framework West London (2006) In May 2006 the GLA published the Sub-Regional Development Framework (SRDF) covering the West London sub-region. The West London sub- region is made up of the six boroughs of Brent, Ealing, Hammersmith and Fulham, Harrow, Hillingdon and Hounslow. The purpose of this strategy in line with PPS 12, is to provide guidance on the implementation of policies in the London Plan in order to help deliver a sustainable and prosperous future for the sub-region in two parts:-

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• Part One providing an overall direction for the sub-region which has been broadly endorsed through the consultation process; • Part Two looks at implementation, in particular it quantifies the various impacts of the growth that is projected to take place in the sub-region and proposes how it can be accommodated in the right place, at the right time and in a sustainable way.

Projections for the London Plan suggested the sub-regional economy might create 86,000 new jobs by 2016 – a very significant addition to an already densely developed area. Alongside this, the London Plan anticipates minimum growth of 59,400 additional homes between 1997 and 2016 (averaging 2,970 per year).

The sub-regional plan focuses on revitalising areas of deprivation and poor environment and recognises this as a clear opportunity to direct growth into the town centres and the areas with redevelopment or regeneration potential. Alongside this there will need to be a substantial increase in densities in appropriate locations. The most recent demographic projections (DMAG 2005/40) suggest that the west London population will grow to 1,504,100 by 2016, an increase of 6% or 5,750 a year.

The Sub-Regional Strategy recognises flood risk as an important issue and notes in particular the Rivers Colne, Crane and their associated tributaries have an identified flood risk. For developments adjacent to flood defences, good practice and London Plan Policy 4C.7 dictates that the development should be set back from the defences to allow for their sustainable maintenance and enhancement. In all development cases surface water run off should be managed as close as possible to its source. In line with London Plan Policy 4C.8 the use of Sustainable Drainage Systems (SuDS) should be promoted unless there are practical reasons for not doing so.

Following the ‘Consolidation with Alterations since 2004’ London Plan document (2008) there are now 7 boroughs incorporated within the Sub Regional Development Strategy West London. An updated sub regional implementation framework is being produced to replace the current SRDF’s. The updated documents are anticipated in 2009. It is expected that policy development will be carried forwards within this document with more emphasis on implementation.

2.2.5 The London Regional Flood Risk Appraisal (RFRA) 2007 The GLA as the regional planning body produce a RFRA to accompany the London Plan. The draft RFRA was issued in June 2007 for informal consultation and is drawn from in this SFRA. The purpose of the RFRA is to provide a broad regional understanding of the risk that flooding creates in

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Greater London, and was undertaken following the stipulation within PPS25. The RFRA is a descriptive document, intended to feed into the Strategic Sustainability Assessment (SSA) and the RSS in order to help determine broad regionally significant locations for development. The regional appraisal of flood risk concludes that there are five major flood sources- tidal, fluvial, groundwater, surface water and sewer that influence London; tidal flooding does not affect the London Borough of Hillingdon.

The RFRA identifies three of the main rivers within Hillingdon and describes the flood risk associated, as follows:-

River Crane: This river has a history of flooding in its lower reaches. The upper reaches, known as the Yeading Brook, flow through considerable lengths of parks and open spaces providing less developed areas for floodwater to be accommodated. Nevertheless a strategic examination of options for sustainable surface water management, taking into account climate change predictions, should be used to influence future development decisions and considerations of the management of the riverside open spaces. Rivers Colne and Pinn: The River Colne is a large tributary which in places forms London’s western boundary. It has suffered extensive flooding in the past, although mostly on undeveloped land. Flood alleviation works have been undertaken. The River Pinn has had several recorded localised floods over recent years, a strategic examination of options for sustainable surface water management and flood storage should be investigated, taking into account climate change, and should be used to influence future development decisions and considerations of the management of the riverside open spaces.

The RFRA provided twenty three strategy recommendations pertaining to the Local Authorities. More specifically the regional policies that should be considered as part of this SFRA in the context of the Borough of Hillingdon are:-

Recommendation 6: Developments all across London should implement the Drainage Hierarchy set out in Policy 4A.14 of the London Plan (LP).

Recommendation 7: Regeneration and redevelopment of London’s fluvial river corridors offer a crucial opportunity to reduce flood risk. Strategic Flood Risk Assessments and policies should focus on making the most of this opportunity through appropriate location, layout and design of development as set out in PPS25 and the Thames Catchment Flood Management Plan. In particular opportunities should be sought to:

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i. Set back of development from the river edge to enable sustainable and cost effective flood risk management options (LP Policy 4A.13).

ii. Ensure that the buildings with residual flood risk are designed to be flood compatible or flood resilient (LP Policy 4A.13).

iii. Use open spaces within developments which have a residual flood risk to act as flood storage areas

Recommendation 8: The GLA will work with the Environment Agency and adjoining regions/local authorities to identify longer term options for strategic flood storage on the major tributaries of the Rivers Lee, Roding, Colne and the fluvial River Thames. It is recognised that there are technical difficulties in some catchments but a full understanding of these issues is needed;

Recommendation 10: Organisations responsible for development with large roof areas should investigate providing additional surface water run-off storage (such as BAA at Heathrow) should investigate providing additional surface water run-off storage.;

Recommendation 20: All of London’s major hospitals, including those not on the above table (i.e. outside floodplains) have large roof, parking and other hard surfaced areas which will generate high volumes of surface water run-off. Opportunities should be taken during refurbishment or extension works to introduce sustainable drainage techniques. This is particularly viable in those hospitals which are set in large grounds.

2.3 Local Policies

2.3.1 London Borough of Hillingdon Unitary Development Plan (adopted 1998) Saved Policies 27th September 2007 The following policies, OE7-OE10, are taken from the above document and are also available online at: http://www.hillingdon.gov.uk/media/pdf/3/7/saved_policies_udp_sep07.pdf (accessed on 22/09/08).

OE7 IN THE AREAS LIABLE TO FLOODING PERMISSION WILL NOT BE GRANTED FOR NEW DEVELOPMENT OR THE INTENSIFICATION OF EXISTING DEVELOPMENT UNLESS A DEVELOPER IS PREPARED TO IMPLEMENT FLOOD PROTECTION MEASURES AS PART OF THE PROPOSED DEVELOPMENT TO A STANDARD SATISFACTORY TO THE COUNCIL, IN CONSULTATION WITH THE ENVIRONMENT

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AGENCY AND WHERE APPROPRIATE, OTHER DRAINAGE BODIES.

OE8 PLANNING PERMISSION WILL NOT BE GRANTED FOR NEW DEVELOPMENT OR REDEVELOPMENT OF EXISTING URBAN AREAS WHICH WOULD RESULT IN AN INCREASED FLOOD RISK DUE TO ADDITIONAL SURFACE WATER RUN- OFF, UNLESS THE PROPOSED DEVELOPMENT INCLUDES APPROPRIATE ATTENUATION MEASURES TO A STANDARD SATISFACTORY TO THE COUNCIL, IN CONSULTATION WITH THE ENVIRONMENT AGENCY AND, WHERE APPROPRIATE, OTHER DRAINAGE BODIES. 6.18 The Council acts as agents for Thames Water Utilities for the sewerage functions of the Borough. Under the agency arrangement the Council has recently completed the Harrow Branch Relief Foul Water Sewer to alleviate the foul flooding problem in the Ruislip area and other schemes are being investigated. The most significant is likely to be a storm relief sewer for the South Ruislip area. Surface water run-off generated by the development of a green field site or redevelopment of an existing built-up area can exacerbate an existing flooding problem or may overload the existing surface water sewerage system. The Council should therefore be consulted at an early stage about the drainage requirements for development proposals.

OE9 IN CATCHMENTS WHICH HAVE A POTENTIAL FOR SEWERAGE FLOODING PERMISSION WILL NOT NORMALLY BE GRANTED FOR DEVELOPMENT WHICH MAY PRECIPITATE SUCH FLOODING OR FURTHER FLOODING, UNTIL SUFFICIENT IMPROVEMENT HAS BEEN IMPLEMENTED TO ALLEVIATE THE PROBLEM.

OE10 IN AREAS OF POTENTIAL FLOODING OR INADEQUATE CAPACITY OF THE FOUL AND SURFACE WATER SEWERAGE SYSTEM TO RECEIVE THE ADDITIONAL DISCHARGE FROM THE DEVELOPMENT, A PHASED DEVELOPMENT MAY BE REQUIRED TO MATCH THE PROVISION OF INFRASTRUCTURE SO THAT THE EXITING SYSTEM IS NOT OVERLOADED TO THE DETRIMENT OF THE COMMUNITY AS A WHOLE.

Hazardous Substances and Contaminated Land 6.19 The Local Planning Authority must take account of potential safety hazards arising through the use and storage of materials

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which may constitute such a potential safety hazard. Also of concern is the extent of unused land which is polluted from either industrial processes or the tipping of waste. Contamination may give rise to hazards which put at risk people working on the site, the occupiers and users of the buildings and land. Contaminants may also escape from the site to cause air and water pollution and pollution of nearby land. Under the Environment Act, a new regime for the identification and management of contaminated land is being implemented. Although land contamination is subject to controls by the Environment Agency and the Environmental Protection Unit of the Council, it can also be a material planning consideration. The responsibility for providing information on whether the land allocated for development is contaminated rests primarily with the developer. When determining a planning application for land which might be contaminated, the Local Planning Authority will need to consider whether the proposal takes proper account of the contamination. It may be necessary for the developer to carry out site investigations and in certain circumstances, to make detailed investigation or to put forward specific remedial measures before a planning application is determined. Contaminated land which remains untreated can prevent the introduction of uses which involve public access (for example open air recreation in the Green Belt) or restrict development potential in the developed area of the Borough. The Local Planning Authority will apply the precepts of PPG23, ‘Planning and Pollution Control’ 1994. The LPA must take account of potential safety hazards arising through the use and storage of materials which may constitute such a potential safety hazard.

2.4 Environment Agency Policies

2.4.1 Catchment Flood Management Plans (CFMP) Catchment Flood Management Plans are high level strategic planning documents that provide a catchment overview of the main sources of flood risk and how these can be managed in a sustainable way in the next 50 to 100 years. The Environment Agency engages stakeholders within the catchment to produce policies in terms of sustainable flood management solutions whilst also considering the land use changes and effects of climate change.

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2.4.2 Thames Regional Catchment Flood Management Plan (CFMP) (Summary document January 2007) The Thames Regional CFMP covers the London Borough of Hillingdon. The Thames Regional CFMP has summarised the future approach to flood risk management into four key messages:

1. Flood defences cannot be built to protect everything Current flood defences will be maintained but it is unrealistic to continue to build defences to protect all of the peoples and properties at risk of flooding. The focus should be placed on the consequences of flooding rather than the likelihood of flooding.

2. Climate change will be the major cause of increased flood risk in the future The predicted change in weather patterns due to climate change suggests that winter floods will happen more frequently.

3. The floodplain is our most important asset in managing flood risk Many floodplains in the Thames region have no flood defences and can serve their natural function of storing water during times of flood. The value of this floodplain needs to be recognised in reducing the impacts of flooding. Improving the effectiveness of the floodplain can reduce flooding to properties both locally and further downstream.

4. Development and urban regeneration provide a crucial opportunity to manage the risk The consequences of flooding can be managed through forward planning. By considering the location, layout and the design of the development, flood risk to properties can be reduced. For example, locate more vulnerable properties out of the floodplain, open up urban river corridors to provide more floodplain and make new buildings flood resilient in combination with the use of SuDS.

The Thames region CFMP highlights two catchments which fall within the London Borough of Hillingdon:-

• River Crane catchment as narrow floodplain and mixed land use with significant stretched of the river network flow through open spaces; • River Colne and River Pinn: the Colne being described as undeveloped natural floodplain and the Pinn as narrow floodplain with mixed land use.

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The Environment Agency provide the following policies to manage flood risk on the River Pinn and River Crane:

• In many places flood defences provide flood protection and should continue to be maintained; • Minimise fluvial flooding by transferring the water in modified channels and conveying it out of the catchments; • Maximise the remaining life of the conveyance system to provide some management of the probability of fluvial events; • Promote flood resilience at the individual property level; • The Environment Agency are progressing some schemes on the Lower Colne, but on other streams they are unlikely to be take further action to reduce the probability of flooding within these catchments in the foreseeable future; • Continue to maintain those assets that are effective in managing current and future flood risk; for example the Hayes Flood Storage Area; • Take opportunities to reduce the dependency on assets that do not contribute to effective flood risk management.

2.5 Neighbouring Strategic Flood Risk Assessment Status Local Authority areas do not follow river catchment boundaries and therefore share neighbouring river catchments. On this basis a list of neighbouring SFRAs is provided below, their status at the time of writing is provided to allow users to cross reference catchments where necessary:-

• The London Borough of Hounslow: Final SFRA issued September 2007; • Spelthorne SFRA: SFRA issued in December 2006, Supplementary update to PPS25 standard Part II issued in January 2007; and • Three Rivers District Council: Final SFRA issued August 2007; • South Buckinghamshire: Level 1 (Feb 2008) & Level 2 (August 2008); • Chiltern: Level 1 (Feb 2008) & Level 2 (June 2008); and • Harrow; Drafting.

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2.6 Other Relevant Policies

2.6.1 Sewers for Adoption (A Design and Construction Guide for Developers) The Sewers for Adoption Guide is to be used by developers undertaking new development when planning, designing and constructing conventional foul and surface water gravity sewers, lateral drains and pumping stations intended for adoption under an Agreement made in accordance with Section 104 of the Water Industry Act 1991. The developer should consult the sewage undertaker and all other relevant bodies at the earliest opportunity before a planning application has been made, so that drainage arrangements can be agreed.

No policies have been presented by the Sewer Authorities. However, there is national guidance on new sewer infrastructure which should be adhered to as part of any new development.

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3 Data Collection and Methodology

The objective of this Level 1 SFRA is to collect, collate and review all currently available information relating to flooding in the study area. This information is presented in a format which will enable the London Borough of Hillingdon to apply the Sequential Test. This Test must be applied to determine potential sites for development and, where necessary, identify sites that will require the Exception Test. It is useful to review the data collection process in order to identify gaps in the data/information. Additional requirements that may be needed to meet the objectives of a Level 2 SFRA can then be identified.

As a result of the data collection process two sets of large-scale flood risk maps have been produced as part of this SFRA for the London Borough of Hillingdon. Figure 3, page 96, shows the fluvial flood risk to the district based on the collated data (there are no sources of tidal flood risk within the study area). Figure 4 is a collective representation of ‘Other Flood Risk Sources Maps’, which summarises the main sources of sewer, pluvial, groundwater overland and artificial flooding. The methodology of how these Figures were produced is detailed in the following section in addition to the source of data used.

3.1 Review of Data Collection Process

3.1.1 Environment Agency The study area falls within the Environment Agency’s Thames Region and is administered by the Hatfield office, the initial point of contact for the provision of data for the catchments within the study area.

3.1.2 Drainage Management of storm water/foul water for the study area is the responsibility of Thames Water. There are no internal drainage boards situated within the study area. Private individuals may be responsible for drainage systems on private land prior to discharge either into a watercourse or public sewer.

3.1.3 Stakeholders Stakeholders contacted to provide data/information used for this SFRA were: -

• London Borough of Hillingdon;

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• Environment Agency (Hatfield Office); • Thames Water.

3.1.4 Stakeholder Meetings An inception meeting was held with the London Borough of Hillingdon on 23rd February 2007 outlining the requirements of the SFRA, likely inputs and deliverables.

A meeting was held with the Environment Agency on the 21st May 2007. This meeting was a round table discussion concerning SFRAs and reviewed the data/information held by the Environment Agency in addition to the flood risk issues relating to potential allocation sites provided by the London Borough of Hillingdon.

3.2 Data Collection Information and data was requested from the stakeholders identified above. The data collected was integrated within a GIS system where possible, and reviewed to establish a baseline data set and identify missing data.

3.2.1 London Borough of Hillingdon • Terrain Information- LiDAR in OS contours Arc View format • OS Tiles: 10k raster images; • London Borough of Hillingdon Local Development Framework Core Strategy; Revised Core Strategy Preferred Options; • London Borough of Hillingdon Sustainability Appraisal (October 2005); • London Borough of Hillingdon Sustainability Appraisal of the Core Strategy Preferred Options Addendum (October 2007).

3.2.2 Thames Water • List of postcode areas flooded by foul, surface water or combined sewers;

3.2.3 Environment Agency • Terrain Information e.g. LiDAR (in ASCII grid format) and river channel survey; • Hydrology e.g. the main and ordinary watercourses, location and information;

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• Environment Agency detailed hydraulic models for the River Pinn, River Crane and Yeading Brook, Upper and Lower Colne; • NFCCD data- Flood Defence e.g. flood storage areas, banks and structures; • Environment Agency Flood Reconnaissance Information e.g. historic flooding, water levels, photos; • Environment Agency Flood Zone Maps and detailed modelled outlines; • Thames Region Catchment Flood Management Plan Summary, July 2007; • Lower Pinn Flood Risk Management Review Inception Report; • Lower Colne Flood Study Modelling Report, June 2004. • River Pinn Revised Mapping (Pinn Mapping Study, Mott MacDonald, 2008) • River Crane Revised Mapping (Crane Mapping Study, Halcrow 2008).

3.3 Fluvial Flooding

3.3.1 Requirements For the purpose of the Level 1 SFRA to assist in the completion of the Sequential Test, definitions of the extent of Flood Zones 1, 2, 3a and 3b are required.

The fluvial flood extent map (Figure 3, page 96) defines the geographical extent of Flood Zones 1, 2, 3a and 3b for the administrative area of Hillingdon. The definitions of Flood Zones are provided in Table 3.1 below:

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Table 3.1: Fluvial Flood Zone Definitions (as defined in PPS25, Table D.1)

Flood Probability of Definition Zone Flooding

At risk from flood event greater than the Flood Zone 1 in 1000 year event (greater than 0.1% Low Probability 1 annual probability).

At risk from flood event between the 1 in Flood Zone Medium 100 and 1 in 1000 year event (between 2 Probability 1% and 0.1% annual probability).

Flood Zone At risk from a flood event less than or High Probability 3a equal to the 1 in 100 year event.

Land which would flood with an annual probability of 1 in 20 (5%) or greater in any year or is designed to flood in an Flood Zone extreme (0.1%) flood or otherwise Functional 3b defined by the Local Planning Authority. Floodplain This zone also comprises land where water has to flow or be stored in times of a flood.

3.3.2 Data Sources The Environment Agency provided detailed hydraulic model extents for the fluvial catchments within the study area, as specified within Table 3.2.

Environment Agency Flood Extents Indicative floodplain maps are produced by the Environment Agency. These flood extent maps ignore the presence of defences, such as embankments and walls from the NFCDD database, which are however identified on the maps.

The Environment Agency has provided hydraulic models for the catchments within Hillingdon. These have been used to produce fluvial flood extent maps showing the estimated extents of Flood Zones 2, 3a and 3b for all main rivers and/or watercourses within the study area (Figure 3).

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The flood extent map (Figure 3) does not provide information on flood depth, speed or volume of flow. Other sources of flooding, such as groundwater, direct runoff from fields, or overflowing sewers are identified in Figure 4, page 97.

The following watercourses within the study area have been mapped using the Environment Agency Flood Zones (see Figure 2, page 95, for locations of main rivers). For a detailed breakdown on the data used and assumptions made in the mapping see Table 3.3:

• River Pinn • River Colne • New Years Greene Bourne • Holme Lodge Ditch and Stanwell More Ditch (located along Spout Lane at the approximate grid reference TQ043751 (504380, 175100) and to the south of Holme Lodge Ditch at the approximate grid reference TQ045749 (504551, 174941) respectively. Both watercourses drain into Duke of Northumberlands.) • Cannon Brook • Madbess Brook • Yeading Brook • Ickenham Stream/Canal Feeder • Joel Stream Farm Ditch • Frogs Ditch • Duke of Northumberlands

Hydraulic Modelling Table 3.2 outlines existing hydraulic models held by the Environment Agency. The model structures and results have been reviewed to assess their suitability for application within this Level 1 SFRA flood extent mapping. The model reliability and outputs have also been considered in relation to a potential Level 2 SFRA. This provides more detailed information on the rate of onset, and depth of flood water, should this be necessary for sites that require the Exception Test as part of a Level 2 SFRA report.

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Table 3.2 Detailed hydraulic modelling undertaken in the London Borough of Hillingdon

Location Model Return Period Limitation

This is a strategic model and not designed for looking at specific sites. To perform this ISIS function accurately the model Lower (Includes 5, 20,100 year may need extra cross sections Colne areas and 100 year + added in the location of the (Peter Brett benefiting climate change development. Associates) from The terrain modelling that was defences) used in some areas is quite old, so this may need to be updated in the future.

Upper This is a strategic model and Colne ISIS 5, 20,100 year not designed for looking at Model and 200 year specific sites. To perform this (Halcrow, function accurately the model 2004) may need extra cross sections added in the location of the development and a possible model of different sections of the Upper Colne to increase the confidence in the results along the full length of the model.

20, 100year The River Crane modelling has River defended and recently been revised by the ISIS Crane undefended, Environment Agency and Revised (Halcrow 100year + 20%, completed in April 2008. The model 2008) 1000year new flood outlines have been undefended included in this SFRA. 20, 100 year undefended, The Pinn modelling has River Pinn 1000 year recently been revised by the ISIS (Mott undefended and Environment Agency and the Revised MacDonald 100 year + 20%, new flood outlines have been model, 2008) and areas included in this SFRA. These benefiting from were completed in April 2008. defences.

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Flood Defences The Environment Agency has provided a GIS layer of the National Flood and Coastal Defence Database (NFCDD), listing details of structures and flood defences. This database aims to provide the following information:

• The location, composition and condition of fluvial defences and watercourses referenced to identified risk areas; • The types of asset (i.e. property, infrastructure, environmental) at risk within identified risk areas, including those protected by fluvial, tidal and coastal defences;

The NFCDD details the asset reference, the defence location, level of protection provided by the structure and the geographic extent of the structure/defence. The NFCDD is used to identify where structures may cause increased risk of flooding during a blockage scenario and/or could benefit from replacement or removal.

Whilst PPS25 ignores the presence of defences in Flood Zone 3a and 2, it is still important to recognise and acknowledge the locations of flood defences and the residual risk associated with defence failure. Flood defences, (identified from the NFCDD), have been mapped as a GIS layer to aid the assessment of flood risk within the Hillingdon area.

Topographic Data LiDAR Light Detection And Ranging (LiDAR) is an airborne mapping technique utilising a laser to measure the distance between the aircraft and the ground. The Environment Agency has provided LiDAR data coverage for Hillingdon. This has been used to create a terrain model of the catchment. LiDAR data varies in accuracy depending on the nature of the terrain, such as woodlands, complex urban areas and lake margins, due to the limitations of this technique. LiDAR is however generally recognised to be accurate to within +/- 100mm when compared to actual vertical levels. A GIS layer providing terrain information for the area has been created for this SFRA to assist the assessment of flood risk in the Hillingdon area.

Channel survey Channel surveys have been provided for each of the river systems where detailed hydraulic modelling has been undertaken as part of an Environment Agency National Flood Risk Mapping Framework.

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3.3.3 Functional Floodplain PPS25 stipulates that the functional floodplain is defined as any land that:

• would flood with an annual probability of 1 in 20 (5 per cent) or greater in any year, or at another probability to be agreed between the LPA and the Environment Agency (EA), or: • is designed to flood in an extreme (0.1 per cent) flood, or at another probability to be agreed between the LPA and the EA.

The functional floodplain is classified as Flood Zone 3b and in accordance with PPS25 residential development in this area would not be permitted. Therefore it is important to consider the location of the functional floodplain in future spatial planning and emerging LDF documents.

• PPS25 states that functional floodplain should be determined considering the effects of defences and other flood risk management infrastructure. The presence of a workable and robust flood defence removes the definition of functional floodplain. The functional floodplain relates only to river flooding, it does not include areas at risk of flooding from other sources (e.g, surface water, sewers).

Data Source The delineation of Flood Zone 3b has been produced from the Upper and Lower Colne 1 in 20 year detailed hydraulic outlines provided for this study by the Environment Agency.

Flood outlines representing the Flood Zone 3b have also be been provided by the EA for the River Pinn and Crane, following recent completion of additional hydraulic modeling work of the two rivers.

Due to the absence of further information, and considering the rural nature of New Year Greene Bourne watercourse, an 8 meter buffer zone has been defined for the functional floodplain of this watercourse.

Where functional floodplain outlines are unavailable, Flood Zone 3 cannot be delineated into 3a and 3b. Therefore, the entire Flood Zone 3 is considered as functional floodplain as a precautionary approach to unsure that inappropriate development is not situated in areas of high risk. The emphasis is then on the developer to undertake modelling to redefine the floodplain surrounding the area of interest and then present this to the Environment Agency for consideration.

Table 3.3 details all data sources that have been used to map the flood extents for the various watercourses within the study area.

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3.3.4 Climate change Climate change must be considered for the lifetime of the proposed development, defined in PPS25 to be 100 years for residential development and 60 years for commercial development.

PPS25 requires that allowances for climate change be made when considering the extent of flooding on a yearly increment scale for the lifetime of proposed developments. One of the main aims of the Level 1 SFRA is to assist in the Sequential Test and development of local planning policy including the development of the Core Strategy and the Site Allocation plans.

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Table 3.3 Mapping Data Sources and Assumptions

Watercourse Flood Zone and outlines Mapping Source Flood 3b: 20year Halcrow - Detailed Mapping Halcrow - Detailed Mapping plus Flood Zone 3a: 100year EA 100yr JFlow Flood Zone 3a + CC: 200- Halcrow - Detailed Mapping year Upper Colne EA 1000yr JFlow combined with historic flood outlines plus Halcrow Flood Zone 2: 1000yr 100yr detailed mapping to ensure larger than Flood Zone 3 ABDs N/A

Flood 3b: 20year PBA - Detailed Mapping

Flood Zone 3a: 100year PBA - Detailed Mapping Flood Zone 3a + CC: 100 + PBA - Detailed Mapping Lower Colne 20% EA 1000 year Jflow plus PBA 100 Flood Zone 2: 1000yr year undefended and 100 year + 20% outlines ABDs PBA - Detailed Mapping MottMacdonald - Detailed Flood 3b: 20year Mapping MottMacdonald - Detailed Flood Zone 3a: 100year Mapping Flood Zone 3a + CC: 100 + MottMacdonald - Detailed River Pinn 20% Mapping MottMacdonald - Detailed Flood Zone 2: 1000yr Mapping 1000yr combined with historic flood outlines MottMacdonald - Detailed ABDs Mapping Flood Zone 3b: 20year 8 metre watercourse buffer zone Flood Zone 3a: 100year EA 100yr JFlow modelling New Year Flood Zone 3a + CC: 100 + NOT AVAILABLE Green Bourne 20% Flood Zone 2: 1000yr EA 1000yr JFlow modelling ABDs NOT AVAILABLE

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Watercourse Flood Zone and outlines Mapping Source Duke of Controlled River, therefore no No Flood Zones Northumberland associated flood zones Controlled River, therefore no Longford River No Flood Zones associated flood zone Flood Zone 3b: 20year Halcrow - Detailed Mapping Halcrow - Detailed Mapping Flood Zone 3a: 100year modified to included FSA Flood Zone 3a + CC: 100 + Halcrow - Detailed Mapping 20% River Crane & Yeading Brook Halcrow - Detailed Mapping Flood Zone 2: 1000yr 1000yr combined with historic flood outlines Mapped as difference between ABDs 100yr defended outline and 100yr undefended outline Unmapped Minor Flood Zone 3b 20 metre watercourse buffer zone watercourses (Various)

Data Source A Technical Note is included in Appendix A detailing the methods used to derive the climate change outlines for the Lower River Colne, Yeading Brook and River Crane.

For watercourses where climate change model extents are unavailable, the outline of the 200yr event may be regarded as equivalent to the 1 in 100year event inclusive of climate change. Flood zone 3a + climate change for the Upper River Colne has therefore been mapped as the 1 in 200-year event as agreed with the Environment Agency.

3.3.5 Unmapped Watercourses Flood extents provided by the Environment Agency are not defined for all watercourses. Typically watercourses with a catchment area less than 3km2 are omitted from Environment Agency mapping, unless there is a history of flooding affecting a population in the area. Consequently there will be some locations adjacent to watercourses that do not have an associated flood risk on first inspection of the flood maps.

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As part of this SFRA, watercourses where modelling information was not available have been mapped with a 20 metre buffer (these watercourses are identified in Table 3.3) either side of the bank to represent Flood Zone 3b in line with the precautionary principle adopted by PPS25 (refer to Figure 3, page 96). This should be fully investigated and the floodplain extents refined as part of any future site specific Flood Risk Assessments. Localised hydraulic modelling may be required in these areas to fully assess the flood risk implications where no previous modelling is available.

3.3.6 Mapping The Flood Zone maps, detailed hydraulic model extents and ordinary watercourse layers have been overlaid on an Ordnance Survey base in order to assess the geographic extent of flood risk as seen in Figure 3.

3.4 Sewer Flooding

3.4.1 Requirements The majority of sewers are built to guidelines set within ‘sewers for adoption’ (WRC, 2006). As a minimum, these sewers are designed to manage runoff from storm events of a size and intensity of 1 in 30 years. It is therefore likely that the majority of the existing sewer systems will surcharge to some degree during rainstorm events with a return period greater than 1 in 30 years (e.g. 100 years). This can cause flooding to property and land both from surface water and foul water wastewater sewers. It is important to understand where the existing system is restricted in capacity by considering where historical sewer flooding has occurred and as such, where sewer flooding is considered to be at highest risk.

3.4.2 Data Source The London Borough of Hillingdon does not keep records of flooding occurrences from any sources whether it is pluvial (rainwater), fluvial, surface water or groundwater. The Environment Agency recommends that for future revisions of this report the London Borough of Hillingdon keep records of flooding occurrences to identify in greater detail areas at risk from flooding and in particular critical drainage problems.

Thames Water is the statutory water undertaker for the London Borough of Hillingdon and is responsible for public sewer systems. Thames Water will only release a four figure alphanumeric postcode. This information is of limited use in terms of mapping as it covers a large area that itself may not have been affected by sewer flooding.

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3.4.3 Mapping The data provided by Thames Water is not suitable to be mapped Borough wide, but is provided in Appendix A. A GIS layer providing information on the locations of pluvial, sewer, and groundwater flooding has been provided to aid the assessment of flood risk in the Hillingdon area.

3.5 Overland Flow

3.5.1 Requirements Overland flow results from rainfall that fails to infiltrate the surface and travels over the ground surface. This is exacerbated by low permeable urban development or low permeability soils and geology (such as clayey soils). Overland flow is likely to occur at the base of an escarpment and low points in terrain.

Local topography and built form can have a strong influence on the direction and depth of flow. The design of development down to a micro-level can influence or exacerbate this. Overland flow paths should be taken into account in spatial planning for urban developments. In addition, surface water flooding can be exacerbated if development increases the percentage of impervious area. An assessment of overflow must be undertaken and the risks assessed as part of a site specific FRA.

3.5.2 Data Source Intense rainfall that is unable to soak into the ground or enter drainage systems can quickly run overland and result in local flooding. In developed areas, this flood water can be polluted with domestic sewage where foul sewers surcharge and overflow. Overland flow paths should be taken into account in spatial planning for urban developments.

Using a combination of LiDAR data a topographic map has been produced (Figure 4, page 97) to identify high and low points in the study area terrain. Overland flow is likely to occur at the base of escarpments. These have been identified using OS maps. The low points highlighted in the terrain have been used to identify areas that maybe subject to surface water ponding during periods of intensive rainfall. Whilst surface water flooding is difficult to quantify, in the absence of historical data this local topography mapping within the catchment provides the basis for identification of potential significant overland flow paths.

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3.5.3 Mapping A GIS layer has been produced to assess the level of risk of overland flow to an area based on the escarpment mapping technique. Where an area is identified as being at risk from overland flow, site specific flood risk assessments should consider localised flow paths to establish the risks to the site. Areas at risk from overland flow have been provided on the collective maps in Figure 4.

3.6 Groundwater Flooding

3.6.1 Requirements Groundwater flooding occurs when water levels in the ground rise above surface elevations. Groundwater flooding may take weeks or months to dissipate as groundwater flow is much slower than surface water flow and water levels thus take much longer to fall.

An assessment of the risk of groundwater flooding needs to be considered; however, a quantified assessment of risk from groundwater flooding is difficult to undertake, especially on a strategic scale. This is due to lack of groundwater level records, the variability in geological conditions and the lack of predictive tools (such as modelling), which can make assessments of the risk of groundwater flow and flooding following rainfall events.

3.6.2 Data Source A small number of recorded groundwater flooding occurrences in the study area has been provided by the Environment Agency. The identification of local geology within the catchment provides a strategic basis for flood risk estimation, which should be used in conjunction with the Source Protection Zones layers in Appendix C as an indication of groundwater flooding risk.

3.6.3 Mapping Anecdotal records documented by the Environment Agency have been digitised to produce a GIS layer of incidences.

The groundwater incidence GIS layer has been overlaid on the collective flood risk maps and mapping of the incident locations provided by Thames Region (Figure 4 - after the main text).

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3.7 Artificial Sources

3.7.1 Requirements Artificial flood sources include raised channels such as canals or storage features e.g. ponds and reservoirs, where water is retained above natural ground level. Artificial sources of flooding within a 1km radius of any of the sites should be considered in order to assess the residual flood risk. This could be in the form of a water body overtopping or the failure of a retaining wall. Potential flood water flow paths should then be investigated.

The potential effects of flood risk management infrastructure and other structures also need to be considered. Reservoir or canal flooding may occur as a result of the structure being overwhelmed and/or as a result of dam or bank failure. The latter can happen suddenly resulting in rapidly flowing; deep water that can cause significant threat to life and major property damage. Flooding can also occur when infrastructure such as pumps fail and surface water or groundwater exceeds the systems capacity.

3.7.2 Data Source Ordnance survey data has been used to identify artificial water bodies such as raised channels, canals, reservoirs and storage features. For the purpose of this SFRA, this technique satisfactorily highlights perched watercourses and water bodies.

3.7.3 Mapping A GIS layer of the artificial sources has been produced and overlain on an OS base with other sources of flood risk data (Figure 4, page 97). In line with PPS25, the functional floodplain is defined as an area of land where water has to flow or be stored in times of flood. Therefore ponds and other water bodies that will store water during a time of flood have been mapped. No assessment of overtopping of structures or the failure of retaining walls or flood routes has been undertaken as part of this study but should be considered at a site specific level.

3.8 Additional Mapping In addition to the mapping associated with the numerous sources of flooding identified above, the following information has been collated into a series of GIS layers:

• Historic Flood Outlines; • Flood Warning Areas;

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• Locations of Vulnerable Institutes (e.g. hospitals); • London Borough of Hillingdon Boundary; • Emergency Refuge Centres (e.g. village halls, schools…).

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4 Flood Risk in Hillingdon

The data/information collected from the various stakeholders, including previous reports relating to flooding within the catchment, has been reviewed. The following sections summarises the flood risk within the borough from all forms of flooding (fluvial, groundwater, surface water and sewer flooding). The information has been presented in a series of maps to aid the Sequential Test process for the allocation of future sites as part of the LDF process.

The flood risk sources within the London Borough of Hillingdon are summarised by a series of maps listed below (see Table of Figures for page numbers):-

• Figure 1: Study Area District Map; • Figure 2: Main Rivers; • Figure 3: Fluvial Flood Risk Map; • Figure 4: Other Flood Risk Sources Map.

4.1 Fluvial Flooding The Thames Catchment Flood Management Plan (CFMP) was published in July 2007. Further to this the Environment Agency has provided an outline of key characteristics of the river catchments within Hillingdon which are included as part of the main CFMP:

• Extensive, heavily developed floodplain with residential and industrial development built right up to the edge of river channels; • Tributaries are flashy with a rapid response to rainfall; • Existing flood defences are maintained by the Environment Agency and private owners; • The Lower Colne river system is a complex heavily managed river system comprising of a network of interconnecting rivers and flood control structures; • The Pinn and Crane are less engineered than the Colne, though the channels have been straightened and managed in the past including modifications such as toe boarding; • On the Rivers Crane and Pinn a moderate proportion of the properties at risk have less than 3 hours flood warning time;

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• There are many in-channel flood control structures, which may need replacing over the next 50 to 60 years; • Multiple sources of flooding are prevalent, climate change and growth pressure could all result in greater frequency and severity of flooding.

4.1.1 River Colne The River Colne flows along the western boundary of the Borough and is sub divided into the Upper and Lower Colne systems. The River Colne system is a complex and diverse system with a mix of permeable and impermeable geology. The Upper Colne catchment is predominantly rural and the Lower Colne catchment is a mixture of urban and rural, flowing through the settlements of Uxbridge, Cowley and West Drayton.

The Lower Colne Improvement Scheme started in 1985 and a Flood Alleviation Scheme using gravel winnings as storage areas was implemented in the Upper Colne area in the mid 1990s. Detailed hydraulic modelling has been undertaken for the Lower and Upper Colne including an assessment of areas benefiting from defences (see Figure 3, page 96).

Many areas of the Lower Colne are defended to a high level, however there is a residual risk associated with a potential failure or overtopping of these defences. In accordance with Annex E of PPS25 site specific flood risk assessments must include an assessment of residual risk after risk reduction measures have been taken into account to demonstrate that the level of remaining risk is acceptable for a particular development of land use.

The Environment Agency has provided the following information regarding the flow mechanisms and flood risks on the Lower Colne:

• Trout Lane area is known to flood. Flood waters could connect across the lane between reservoirs; • Palmers Moor Lane is at risk of flooding; • Cowley Mill is not beneficially operated and increases flood risk downstream; • Access to the Colne Brook flood defence alongside the M25 is poor; • The River Colne and Colne Brook run in parallel. The poor maintenance of the channel vegetation defines the direction of overspill channel flow; • Fassnidge Park and Rockingham Recreation Ground; flood walls keep flood water in-channel;

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• Water spills into the Grand Union Canal and then back down into River Colne downstream of the Colne Brook confluence; • At Little Britain, Cowley, water comes out of Frays River and overtops into a lake. In this area there has been some flood defence work completed previously; • The Lower Colne takes excess water from the River Fray and River Pinn. There is an inverted Siphon on the Pinn under the Canal, and in this area water can flood the High Road, near Eastcote Village, either due to insufficient channel capacity or through siphoning; • The Willowbank Flood defence scheme has been designed to a 1 in 100 year design standard; • The Bigley Ditch (approximate Grid Reference X505334 Y179483) downstream of the Railway Bridge is at risk from flooding from the River Colne. There has been a pre feasibility study to look at raising banks in this area.

Interaction between the River Colne and Grand Union Canal The interaction between the River Colne and Grand Union Canal is at the following locations:-

• Denham Weir (National Grid Reference 505288, 186279) at the split between the River Colne, Grand Union Canal and Frays River; • Willowbank, the River Colne occupies the Grand Union Canal for a short reach at National Grid Reference 505273, 185221; and • Confluence between the Shire Ditch, River Colne and Grand Union Canal at National Grid Reference 505234, 184541;

The Willowbank area is a very complex area of the River Colne with the interaction of the Shire Ditch, Frays River, Grand Union Canal and River Colne. Any development within the area would require a detailed analysis of the associated watercourses.

Interaction between the Shire Ditch and Frays River There are two points at which the Frays River and Shire Ditch interact and the Shire Ditch received overspill from the Frays River:

Downstream of A40 between the culvert under the dismantled railway between the Frays and Shire Ditch (National Grid Reference 505730, 185580); and The culvert which passes under Braybourne Close at National Grid Reference 505347, 185930.

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4.1.2 River Pinn The Lower Pinn catchment area is approximately 4500ha in size, and runs from Harrow to its confluence with the River Colne. The River Pinn catchment is steep, highly urbanised and prone to short duration flooding caused by intense rainfall events between 1 in 5 and 1 in 30 years at various locations along the watercourse (Lower Pinn Flood Risk Management Review, Inception Report, EA, 2006). The main tributaries’ to the River Pinn within the study area are the Joel Street Ditch in Eastcote and the Cannon Brook in Ruislip, draining the Ruislip Lido and surrounding areas.

Following major flooding across the catchment in 1977 the River Pinn Flood Alleviation Scheme (FAS) was implemented. During the severe flooding of 1977, 718 residential and commercial properties were affected in Hillingdon. Further flooding events occurred after the implementation of the FAS in 1988 when 160 properties experienced flooding. Records indicate that additional flooding was experienced in 1984, 1987, 2000 and 2001. The areas at particular flood risk are Kings College playing fields, Ruislip Gardens, Ruislip and the Zodiac Business Park in Yiewsley.

The Flood Alleviation Scheme was implemented in four phases between 1980- 1989, Phases I and IV where undertaken in Hillingdon:

• Phase I: Channel re-grading at Swakeleys Road, Ickenham; bypass channel and bund at Hercies Road, Hillingdon; • Phase IV: Channel regarding and bypass, channel between Breakspear Road, Ruislip and Grassmere Avenue Ruislip; channel re-grading and lowering of bridge inverts between Elmbridge Drive, Ruislip and High Road, Eastcote.

King’s College Road to Bury Street in Ruislip is considered to be at high risk of flooding in the vicinity of the most recently flooded area (during the winter 2002/2003 where the River Pinn flowed out of bank). The floodwaters in this area reactivated a previously dormant spring. The Environment Agency has worked with Hillingdon Council to install a flood relief flapped drainage system. There have also been incidences of flooding in the hospital grounds near Apple Tree Avenue.

The Lower Pinn Flood Risk Management Study highlights the following areas as being at flood risk:-

• Flood Risk Area (Eastcote Village & Ruislip) – Eastcote Road, Canon Lane, Lyncroft Avenue, Marsh Road and Lloyd Court;

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• Flood Risk Area (Ickenham) - Hoylake Crescent. Wallasey Crescent, The Avenue, Thornhill Road, Melbourne Close and Bellamy Close; • Flood Risk Area (Uxbridge) - Sweetcroft Lane, Brookside, Tedder Close and Portal Close; • Flood Risk Area (Uxbridge) - Dawes Road, Turnpike Lane and Amber Way; • Flood Risk Area (Yiewsley) - Zodiac Business Park, High Road and nearby houses. The River Pinn is a very rapidly responding catchment with very short flood warning times and requires a high level of maintenance. The overgrown channel of the River Pinn and associated bank vegetation is actively maintained as such to offer some flood management.

4.1.3 The Canal Feeder/Ickenham Stream The Canal Feeder or Ickenham Stream is an excavated channel which formed the historic water supply route from the Ruislip Lido to the Grand Union Canal Paddington Branch. There is limited information on the history of the Canal Feeder however information provided from the River Crane Flood Study and a local website3 form the basis of the following information.

The Canal Feeder is now redundant as the River Colne and Gade are the main water supply sources to the Grand Union Canal. The Ruislip Lido now discharges all its water into the River Pinn.

The Canal Feeders route exits the Ruislip Lido through a culvert to the ‘west’ of the channel under Bury Street, Ladygate Lane and on towards Tile Kiln Lane, whereupon the route is less clear until it surfaces again in Austins Lane, Ickenham. From here it runs into the fields adjacent to Northolt aerodrome and then under the A40 almost in parallel with the Yeading Brook. The Canal Feeder passes underneath the River Pinn but its location at this point is uncertain.

The original watercourse has been retained and now collects surface water before feeding into the Yeading Brook. The Ickenham Stream converges with the Yeading Brook west arm just downstream of the A40, before meeting the east arm to form the main branch of the Yeading Brook just to the west of Northolt Aerodrome.

3 http://www.ruislip.co.uk/lido/ last accessed 22/11/07

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4.1.4 Yeading Brook The Yeading Brook flows into the Borough from the east through two principal branches, the east arm and a west arm, before forming the main branch at National Grid Reference 549950, 184190. The Yeading Brook then flows in a southerly direction into the neighbouring London Borough of Hounslow.

The main historic flooding event that affected the Yeading Brook as listed in the River Crane Flood History Database and form Environment Agency sources are:-

• September 1958: Yeading Brook overtop in the Delamere Road Area in Hayes; • August 1977: severe flooding inundated and damaged approximately 1200 homes in north-west and south seat London. Within the Yeading Brook area Ruislip Gardens were seriously affected. • Summer 2007: Flooding at Field End Road and Brookside Close. Field End Trash Screen had become blocked by debris and caused the Roxbourne to overtop with flooding affecting 20 properties, 6 of which were flooded internally.

At the A312 road the Hayes Bypass Channel splits off from the Yeading Brook flowing down a manmade concrete channel in a southerly direction for 2km before entering a 0.5km culvert and then rejoining the Yeading Brook. The Hayes Bypass Channel provides flood relief to properties on the western floodplain area.

There are two large flood storage reservoirs on the Yeading Brook which provide storage capacity up to the 1 in 50 year standard. The first of these is located just off the A40 opposite RAF Northolt at NGR 509420, 184410. The second of these, Charville Lane Flood Storage Area, is located at the end of Charville Lane at NGR 509740, 183900.

Interaction between Yeading Brook and Hayes Bypass Channel The Yeading Brook and Hayes Bypass channel interact at the A312 where flow from Yeading Brook spills into Hayes Bypass channel, this is modelled as part of the Crane modelling. There is a wall on the bank of the Yeading Brook and once water levels rise above this wall level it spills into the Bypass channel.

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Interaction between Yeading Brook and Grand Union Canal South of Beaconsfield Road there is a weir between the Grand Union Canal and Yeading Brook where the two watercourses interact. And there is also a weir on the Grand Union Canal Paddington branch. The Environment Agency’s Flood Defence Inspector has not witnessed interaction between these two watercourses. There are broad crested weirs on:

• Southall weir at National Grid Reference 511511/179882 (length 52.7m, max discharge 2.7m3/s); and • Bulls bridge weir at National Grid Reference 510513/179130 (length 28.5m, max discharge 1.5m3/s).

4.2 Sewer Flooding

4.2.1 Thames Water Thames Water has provided a database of surface water incidences. Thames Water report that 164 properties have previously flooded from overloaded sewers in the last ten years. 93 of these have been flooded by surface water from overloaded sewers, 63 from foul sewers and 8 properties from combined sewers. A summary of the postcode areas has been broken down by Thames Water and can be seen in Appendix B (rear of document).

4.2.2 Climate Change Thames Water have not provided any further information in respect managing the existing infrastructure to accommodate climate change or how Thames Water plan to reduce the number of sewage flooding incidents.

4.3 Overland Flow Figure 4 (page 97) shows the mapped low points and should be used as a reference point to further investigate flood sources as part of site specific Flood Risk Assessments proposed in these areas.

4.3.1 Climate Change It is anticipated that climate change trends and patterns indicate that more frequent short-duration, high-intensity rainfall and more frequent periods of long-duration rainfall are to be expected. These high intensity rainfall events are likely to lead to overwhelmed drains on a more frequent basis.

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4.4 Groundwater Flooding Information has been provided by the Environment Agency regarding groundwater flooding incidents within the Borough. Some properties are susceptible to groundwater flooding due to high water table levels. Problems encountered may include ingress of water into cellars and basements, or waterlogged ground.

The majority of the study area underlying geology is formed of London Clay and the Lambeth Formation.

Very large water bearing gravel deposits lie within the southern area of Hillingdon's boundary. These can pose a risk of groundwater flooding, especially where property is close to main rivers such as the Frays River, River Colne, River Crane, River Pinn, and the Yeading Brook where shallow groundwater levels may be high.

Within the London Borough of Hillingdon the Environment Agency has recorded four incidences of groundwater flooding and these have been plotted on the collative ‘other flood sources maps’, Figure 4 (page 97).

4.5 Artificial Sources Artificial sources of flooding within a 1km radius of any site should be considered in order to assess the residual risk of a water body overtopping and potential flow paths. Figure 4 provides the location of all perched water bodies within the catchment.

4.5.1 Ruislip Lido The Ruislip Lido is located towards the north-east of Ruislip in Park wood. The upstream extent of the reservoir is located at National Grid Reference 509070, 189730 and the downstream extent is National Grid Reference 508670, 18880. Ruislip Lido was built in 1811 as Ruislip reservoir in order to feed the Grand Junction Canal (Paddington Branch) which lies some distance away in the south of the Borough. The full length of the feeder no longer exists and parts of it were developed in the 1970s.

As the Ruislip Lido is a perched water body an assessment of topping and or failure and likely flood hazard paths should be considered as part of any flood risk assessment.

4.5.2 Duke of Northumberland & Longford River The Duke of Northumberland River is an artificial channel that flows in two sections, the Harmondsworth Section or the Western Section flows in

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parallel to the Longford River into Hillingdon. The Duke of Northumberland river channel is generally straight but with bare earth and vegetated banks.

The Duke of Northumberland and Longford River are outside the Flood Zone 2 & 3 of the Environment Agency’s floodplain maps. The Duke of Northumberland River’s off-take gates restrict flows from the River Colne to limit the water level immediately downstream of the offtake to a maximum level of 23.52 metres Ordnance Datum (mOD). This level is reached by flows ranging from about 1.60 m³/s (summer) to 2.10 m³/s (winter).

The Longford River off-take gate restricts flows from the River Colne to limit the water level immediately downstream of the gate to a maximum level of 22.31 mOD. This level is reached by flows ranging from about 0.90 m³/s (summer) to 1.20 m³/s (winter).

These flows have been modelled throughout the length of the Longford River, and for the whole length of the upstream Duke of Northumberland River (i.e. to the River Crane), and are the flows just below which flooding commences.

Therefore with the off-take gates operating according to these rules no flooding from either of these artificial rivers is anticipated and as such no floodplains are associated with either of these stretches of river. It is not possible to assign a flood extent as the flow is within bank throughout the entire length of the Longford River and the upstream stretch of the Duke of Northumberland River. There is no interaction between the River Crane and Longford River.

4.6 Residual Risks Residual risk of flooding arises from the following scenarios:

• Failure or overtopping of flood defence structures; • Blockage of key structures such as culverts or bridges; • Blockage of surface water drainage system or a pumped water system; • Overtopping or failure of flood storage areas; • Exceedence of the design standard of flood defence infrastructure due to the severity of the flood event; • Lack of maintenance of flood defence structures, channel and floodplain.

Failure of flood defence infrastructure can lead to severe consequences due to often high velocity flows and volumes of water which can lead to

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substantial building damage or loss of life. There are several areas within Hillingdon that benefit from flood defence schemes. Figure 3 (page 96) represents the mapped fluvial flood sources and incorporates the areas benefiting from defences and identifies the flood storage areas. Furthermore, the residual risk of overtopping from artificial water bodies such as the Ruislip Lido should be considered in any flood risk assessment.

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5 Guidance on Applying the PPS25 Sequential Test

5.1 What is the Sequential Test? PPS25 requires application of the Sequential Test at all stages of the planning process to encourage Local Planning Authorities to guide new development towards areas of the lowest flood probability. Only where there are no reasonably available sites in Flood Zone 1 or 2 should allocations be proposed in Flood Zone 3.

In applying the Sequential Test planners should also bear in mind the vulnerability classification of their proposed development. Table D2 of PPS25 describes types of development according to their flood vulnerability. For example, PPS25 classifies police stations, ambulance stations, fire stations and command centres as Highly Vulnerable buildings. Hospitals and care homes are classified as More Vulnerable establishments. In the event of an emergency, to ensure that services vital to the rescue operation do not become victims of the floodwaters, it is essential that all establishments related to these services are located in the lowest flood risk zones. In addition future development control polices should seek to locate more vulnerable institutes such as schools and care homes in areas of the lowest risk to minimise the potential for flood casualties.

By using this information and the information supplied in PPS25, in tandem with the Sequential Test planners should guide developments to those areas where the flooding probability is appropriate to the vulnerability of the proposed development as presented in Table 5-1.

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Table 5-1 : Flood Risk Vulnerability Classification (from PPS25, Appendix D, Table D2)

Essential transport infrastructure (including mass evacuation routes), which has to cross the area at risk, Essential and strategic utility infrastructure, including electricity Infrastructure generating power stations and grid and primary substations. Police stations, Ambulance stations and Fire stations and Command Centres and telecommunications installations required to be operational during flooding. Highly Emergency dispersal points. Vulnerable Basement dwellings. Caravans, mobile homes and park homes intended for permanent residential use. Installations requiring hazardous substances consent. Hospitals. Residential institutions such as residential care homes, children’s homes, social services homes, prisons and hostels. Buildings used for: dwelling houses; student halls of residence; drinking establishments; nightclubs; and More hotels. Vulnerable 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. Buildings used for: shops; financial, professional and other services; 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. Less Waste treatment (except landfill and hazardous waste Vulnerable facilities). Minerals working and processing (except for sand and gravel working). Water treatment plants. Sewage treatment plants (if adequate pollution control measures are in place).

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Flood control infrastructure. Water transmission infrastructure and pumping stations. Sewage transmission infrastructure and pumping stations. Sand and gravel workings. Docks, marinas and wharves. Navigation facilities. MOD defence installations. Ship building, repairing and dismantling, dockside fish Water- processing and refrigeration and compatible activities compatible requiring a waterside location. Development Water-based recreation (excluding sleeping accommodation). Lifeguard and coastguard stations. Amenity open space, nature conservation and biodiversity, outdoor sports and recreation and essential facilities such as changing rooms. Essential ancillary sleeping or residential accommodation for staff required by uses in this category, subject to a specific warning and evacuation plan.

PPS25 acknowledges that some areas will be at risk of flooding from flood sources other than fluvial or tidal systems. Consequently all sources of flooding must be considered when looking to locate development in one of the Flood Zones described above. The other sources of flooding requiring consideration when situating new development allocations include:

• Overland Flow; • Groundwater; • Sewers; and • Artificial Sources.

These sources (as sources of flooding) are typically less well understood than tidal and fluvial sources. Consequently data often only exists as point source data or through interpretation of local conditions. In addition there is conflicting guidance on suitable return periods to associate with floods arising from these sources. For example modern storm water drainage systems are constructed to a 1 in 30 year standard. Any storm event in excess of the 30 year return period storm would be expected to cause flooding. Consequently when assessing these sources through the Sequential Test, if a location is recorded as having experienced repeated flooding from the same source this should be investigated further in a site specific flood risk assessment.

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5.2 How should the SFRA be used to apply the Sequential Test? The Sequential Test should be undertaken by the LPA and accurately documented to ensure decision processes can be transparently communicated and reviewed where necessary. The Sequential Test should be carried out on all development sites, seeking to balance the flood probability and development vulnerability of sites throughout a planning authority area.

The recommended steps required in undertaking the Sequential Test are detailed below. This is based on the various constraints placed on the types of vulnerable development presented in Table D3 of PPS25, reproduced below (Table 5-2).

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Table 5-2 :PPS25 Table D3 Flood Risk Vulnerability and Flood Zone 'Compatibility' (CLG, 2006)

FLOOD RISK ESSENTIAL WATER HIGHLY MORE LESS VULNERABILITY INFRASTRUCTURE COMPATIBLE VULNERABLE VULNERABLE VULNERABLE CLASSIFICATION 1

Exception Test

ONE 2

Z Required Exception Exception Test LOOD 3A Test

F Required Required Exception Test 3B Required

- Development is appropriate - Development should not be permitted

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Notes 1 Flood Zone 1 for fluvial and tidal flooding and with a low risk of flooding from other sources 2 Flood Zone 2 for fluvial and tidal flooding and with a medium risk of flooding from other sources 3 As defined by the Sequential Test 4 Development to be safe and to not increase flood risk elsewhere. Required to pass part c) of the Exception Test, where applicable. 5 Including susceptibility to future climate change and residual flood risk

Figure 6.1: Application of the Sequential Test (from Figure 3.1 of PPS25: Practice Guide, A ‘Living Draft’ (Feb 2007))

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5.3 Additional Guidance In tandem with Figure 6.1, the sequence of steps outlined in Appendix C (pages 107 to 112) is designed to guide the LPA and developers through the Sequential Test. The steps are designed to ensure land allocations are primarily allocated in line with the principles of the Sequential Test and where necessary the application of the Exception Test is clearly identified.

Appendix C provides a series of GIS layers which should be used in tandem by the LPA when used taking the Sequential Test. This information can also be used by developers to further their understanding of flood risks on a particular site. The GIS layers provided are:-

• Flood Zone 2 and 3; • Worst historic outlines; • Pluvial flood risk zones; • Groundwater incidences; • Artificial flood sources; and • Source Protection Zones.

5.3.1 Recommended stages for LPA application of the Sequential Test

Stage 1. The developments (i.e. housing, hospitals, industrial etc) that need to be accommodated by the LPA should be assigned a vulnerability classification in accordance with Table D.2 ‘Flood Risk Vulnerability Classification’ in PPS25; Stage 2. The Flood Zone classification of all development sites should be determined based on a review of the Environment Agency Flood Zones for fluvial sources. This should consider the effects of climate change on Flood Zone definition for the design life of any development that the site may be suitable for i.e: • 60- years – up to 2070 for commercial / industrial developments; and • 100 years – up to 2110 for residential developments Stage 3. In the first instance the ‘highly vulnerable’ developments should be located in potential sites it has identified as being within Flood Zone 1. If the ‘highly vulnerable developments’ cannot be located in Flood Zone 1, because the identified sites are unsuitable or there are insufficient sites in Flood Zone 1 then sites in Flood Zone 2 can be considered. In accordance with PPS25 ‘highly vulnerable’ uses would not be permitted in Flood Zone 3.

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Stage 4. Once all ‘highly vulnerable’ developments have been allocated to a development site, the LPA can consider those development types defined as ‘more vulnerable’. In the first instance ‘more vulnerable’ development should be located in any unallocated sites in Flood Zone 1. Where these sites are unsuitable or there are insufficient sites, sites in Flood Zone 2 can be considered. If there are insufficient sites in Flood Zone 1 or 2 to accommodate the ‘more vulnerable’ development types, sites in Flood Zone 3a can be considered. However, any ‘more vulnerable’ developments in Flood Zone 3a will require the application of the Exception Test. Responses to parts ‘a’ and ‘b’ of the Exception Test should be prepared and agreed through consultation with the Environment Agency before ‘part c’ is tackled. Stage 5. Once all ‘more vulnerable’ developments have been allocated to a development site, the LPA can consider those development types defined as ‘less vulnerable’. In the first instance ‘less vulnerable’ development should be located in any remaining unallocated sites in Flood Zone 1, 2 or 3a. Less vulnerable development types are not appropriate in Flood Zone 3b – Functional Floodplain. Stage 6. ‘Essential infrastructure’ developments should also be preferentially located in the lowest flood risk zones. However this type of development can be located in Flood Zones 3a and 3b, where necessary, through application of the Exception Test. Where these types of development are located in Flood Zone 3a or 3b responses to parts ‘a’ and ‘b’ of the Exception Test will be required before ‘part c’ is tackled. Stage 7. Water compatible development typically has the least flood risk constraints and it is therefore suggested to consider them last when allocating development sites. Stage 8. For decisions made through stages 4 to 8 it will also be necessary to consider the risks posed to the site from other flood sources and where comparable development sites in the same Flood Zone may be more suitable due to: • flood risk management measures, • the rate of flooding, • flood water depth, or, • flood water velocity. Stage 9. It is recommended that London Borough of Hillingdon complete the Proforma in Table 1 (Appendix C) during the site specific investigation into flood risk stage of the Sequential Test, to assist in

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completion of the Sequential Test to provide a transparent framework and justification of sites that may need to be Exception Tested. The potential allocation sites identified as part of the Core Strategy are presented in Figure 10-32 in Appendix E in relation to flood risk. This provides a starting point for the Local Authority to consider the Sequential Test.

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6 Guidance on Applying the PPS25 Exception Test

6.1 What is the Exception Test? The Exception Test is only appropriate for use when there are large areas of development in Flood Zones 2 and 3, where the Sequential Test alone cannot deliver acceptable sites, but where continuing development is necessary for wider sustainable development reasons. There must be evidence to prove that the Sequential Test has been applied to a particular area to support the outcome.

6.2 What is Required to Pass the Exception Test? For the Exception Test to be passed:

a) It must be demonstrated that the development provides wider sustainability benefits4 to the community that outweigh flood risk, informed by a SFRA; b) The development should be on developable/previously developed land or, if not, it must be demonstrated there is no such alternative land available; and c) A FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, reducing flood risk overall. All three parts of this test must be satisfied in order for the development to be considered appropriate in terms of flood risk. There must be robust evidence in support of every part of the test. Should the Sequential Test identify the need for allocations to undergo the Exception Test this will be addressed in a Level 2 SFRA, which corresponds to a more in-depth study of flood risk required to facilitate the application of the Exception Test. It also allows a sequential approach to site allocation within a Flood Zone i.e. preferentially developing those sites situated in an area of lower hazard within a Flood Zone.

4 Sustainable development should meet ‘the needs of the present without compromising the ability of future generations to meet their own needs’ (from the World Commission on Environment and Development, 1987). Sustainable development should aim to provide: social progress which recognises the needs of everyone; effective protection of the environment; the prudent use of natural resources; and, the maintenance of high and stable levels of economic growth and employment. This is taken from Planning Policy Statement 1: Delivering Sustainable Development (2005) and should be referred to for more detail.

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7 Flood Risk Management

7.1 Flood Defences The National Flood and Coastal Defence Database (NFCDD) compiled by the Environment Agency holds information on natural and man-made defences. The standard of these flood defences is only available for man- made defences. Figure 5 (page 98) displays the location of NFCDD defences throughout the Borough of Hillingdon.

The Environment Agency flood maps define the extent of flooding ignoring the presence of defences. The reason for this approach is to make an allowance for residual flood risk in the event of a failure or breach/blockage/overtopping of the flood defences. This conservative approach raises the awareness of flood risk in defended areas and helps to ensure that is it not discounted as part of development but is managed appropriately.

7.2 Flood Warning Areas The Environment Agency operates a flood warning service in areas at risk of flooding. It consists of an initial Flood Warning, notifying houses and businesses within the flood warning area. This is the signal to undertake measures in preparing for a flood. When a flood event is imminent a Severe Flood Warning is issued which means there is extreme danger to properties or businesses.

The flood warnings are disseminated through a variety of mediums that include TV, radio, Automated Voice Messaging service direct to recipients’ phone/fax/pager, internet and/or loudhailer. There is also an emergency Floodline number (0845 988 1188) and a quickdial number for individual rivers.

Figure 6 (page 99) demonstrates the flood warning areas within the district. The main settlements on the flood warning system are:-

• Ruislip; • Yeading; and • Yiewsley.

The Flood warning system helps residents in areas of flood risk prepare for floods by obtaining sand bags, moving valuables upstairs and where

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necessary evacuating the property to minimise the potential consequences of flooding.

The Hillingdon Emergency Response team should respond during a flooding event. It is possible that two or more emergency departments will be called upon in the event of a flood. Hillingdon Council ‘major incident arrangements’ are likely to be necessary in this case.

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8 Sustainable Drainage Systems (SuDS)

8.1 Principles Traditionally, built developments have utilised piped drainage systems to manage surface water and convey surface water run-off away from developed areas as quickly as possible. Typically these systems connect to the public sewer system for treatment and/or disposal to local watercourses. Whilst this approach rapidly transfers storm water from developed areas, the alteration of natural drainage processes can potentially impact on downstream areas by increasing flood risk and reducing water quality. Receiving watercourses are therefore much more sensitive to rainfall intensity, volume and catchment land uses after a catchment or areas of a catchment have been developed.

Due to the difficulties associated with up rating sewer systems it is uncommon for sewer and drainage systems to keep pace with the rate of development/re-development and there are increasingly stringent controls placed on discharges to watercourses. As development progresses and/or urban areas expand these systems become inadequate for the volumes and rates of storm water they receive, resulting in increased flood risk and/or pollution of watercourses. Allied to this are the implications of climate change on rainfall intensities, leading to flashier catchment/site responses and surcharging of piped systems.

In addition, as flood risk has increased in importance within planning policy, a disparity has emerged between the design standard of conventional sewer systems (1 in 30 year) and the typical design standard flood (1 in 100 year). This results in drainage inadequacies for the flood return period developments need to consider, often resulting in potential flood risk from surface water/combined sewer systems.

A sustainable solution to these issues is to reduce the volume and/or rate of water entering the sewer system and watercourses.

8.2 SuDS Policies There are a number of policies and planning documents that promote the implementation of SuDS in new developments.

8.2.1 PPS25 In accordance with PPS25 the local authority and Environment Agency would require any greenfield development to retain the existing greenfield

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runoff rate post-development and should allow for climate change. SuDS should be included in new developments where possible to manage storm water.

PPS25 requires the use of SuDS as an opportunity of managing flood risk, improving water quality and increasing amenity and biodiversity. SuDS should be located in accordance with the restrictions set out in Policy and Practice for the Protection of Groundwater.

The detailed ground conditions of the potential growth sites have not been assessed as part of this SFRA and would be subject to the results of ground investigations for the site determining their suitability.

The Environment Agency Thames Region SUDS guidance states that for sites greater than 1 Ha a surface water flood risk assessment should be submitted, detailing the drainage proposals for a site which would be designed to reduce runoff rates by achieving the following criteria: i. Greenfield discharge rates are achieved on greenfield sites, and on brownfield sites (where possible). The Environment Agency request the methodology detailed in R&D Technical Report W5-074/A/TR1 is used for calculations. ii. Demonstrate that opportunities to implement sustainable drainage techniques at the site have been maximised. iii. Demonstrate that the surface water drainage system can accommodate any storm event up to the critical duration 1 in 100 year storm event for the site without the flow balancing system being bypassed, whilst also taking into account PPS25’s climate change requirements. Sufficient information must be provided to demonstrate that the critical duration storm event has been used. iv. Demonstrate that surface water discharges to watercourses do not exceed a velocity of 1m/s.

It is recognised that the Environment Agency’s “Thames Region SUDS Guide” that some SuDS mechanisms are more sustainable than others as outlined through the ranking system in the table below ().

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Table 8-1 Preferred SUDS Features (adapted from Thames Region SUDS Guide) SUDS FEATURE RANK (where 1 is the most desirable)

Living/Green Roofs 1

Constructed Wetlands/Retention 2 Pond

Detention Basins 3

Filter strips and Swales 4

Soakaways 5

Infiltration Trenches 6

Gravelled Areas 7

Porous Paving 8

Over-sized pipes 9

Storage Tanks/Cells 10

Surface water runoff should be controlled as near to its source as possible through the use of a sustainable approach to drainage. For further details on flood risk assessment requirements regarding surface water refer to www.pipernetworking.com.

8.2.2 Communities for Sustainable Homes The Code for Sustainable Homes identifies the proactive reduction of surface water run off as a mandatory element worth two credits towards the 57 required for the Code’s level 3 rating. Through incorporating suitably designed systems into a development SuDS can also contribute to several other assessment criteria under Code for Sustainable Homes, such as those relating to ecology and potable water consumption, which offer a further 9 and 5 points respectively towards the Level 3 rating.

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8.3 SuDS Recommendations PPS25 indicates that Regional Planning Bodies and Local Authorities should promote the use of SuDS for the management of storm water runoff generated by development. PPS25 also recognises that flood risk and other environmental damage, that can be caused by traditional drainage systems, can be managed through the use of SuDS. Flood risk can be reduced by systems that minimise the changes in the volume and rate of surface runoff from development sites. This is complementary to the control of surface water within developments inside the floodplain. Where possible SuDS should be incorporated into all new developments. However, if for instance infiltration techniques were proposed, this must first be assessed subject to the appropriate geology and ground conditions to accommodate this technique.

The PPS25 Practice Guide (Communities and Local Government, 2007) outlines the potential use of surface water management plans by LPAs to manage the surface water at both a generic policy level and through local surface water management plans. These plans will provide a method of:

• Safeguarding existing features of the water environment; • Improving flood risk management; • Managing surface water; • Improving environmental quality; • Setting a strategic template for SUDS and protecting the areas needed for these features; and • Securing the ongoing management of a sustainable surface water drainage system. In accordance with the London Plan Policy 4A.14 (detailed in section 2.2.1of this report) developers should aim to achieve greenfield run off from their site through incorporating rainwater harvesting and sustainable drainage. Boroughs should encourage the retention of soft landscaping in front gardens and other means of reducing, or at least not increasing, the amount of hard standing associated with existing homes. In addition, drainage of rainwater from roofs and paved areas around buildings should comply with the 2002 Amendment of Building Regulations Part H (3). The requirements are as follows:

• Adequate provision shall be made for rainwater to be carried from the roof of the building. • Paved areas around the building shall be so constructed as to be adequately drained.

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• Rainwater from a system provided pursuant to sub-paragraphs (1) or (2) shall discharge to one of the following in order of priority: i. An adequate soakaway or some other adequate infiltration system; or where that is not reasonably practicable; ii. A watercourse; or where that is not reasonably practicable a sewer.

SuDS seek to manage storm water as close to its source as possible, mimicking storm water flows arising from the site, prior to the proposed development. Typically this approach involves a move away from piped systems to softer engineering solutions inspired by natural drainage processes.

SuDS should be designed to take into account the surface run-off quantity, rates and also water quality ensuring their effective operation up to and including the 1 in 100 year design standard flood including an increase in peak rainfall of up to 30% to account for climate change.

Wherever possible, a SuDS technique should seek to contribute to each of the three goals identified below with the favoured system contributing significantly to each objective. Where possible SuDS solutions for a site should seek to:

• Reduce flood risk (to the site and neighbouring areas), • Reduce pollution, and, • Provide landscape and wildlife benefits.

These goals can be achieved by utilising a management plan incorporating a chain of techniques, (as outlined in Interim Code of Practice for Sustainable Drainage Systems 2004), where each component adds to the performance of the whole system:

• Prevention: good site design and upkeep to prevent runoff and pollution (e.g. limited paved areas, regular pavement sweeping) • Source control: runoff control at/near to source (e.g. rainwater harvesting, green roofs, pervious pavements) • Site control: water management from a multitude of catchments (e.g. route water from roofs, impermeable paved areas to one infiltration/holding site) • Regional control: integrate runoff manage from a number of sites (e.g. into a detention pond)

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The application of SuDS is not limited to a single technique per site. Often a successful SuDS solution will utilise a combination of techniques, providing flood risk, pollution and landscape/wildlife benefits. In addition, SuDS can be employed on a strategic scale, for example with a number of sites contributing to large scale jointly funded and managed SuDS. It should be noted, each development site must offset its own increase in runoff and attenuation cannot be ‘traded’ between developments.

8.4 Where can SuDS be utilised in Hillingdon? The underlying ground conditions of a development site will often determine the type of SuDS approach to be used. This will need to be determined through ground investigations carried out on-site. However an initial assessment of site suitability to the use of SuDS can be obtained from a review of the available soils/geological survey of the area.

Based on a review of the following geological maps, together with the general description of the geology in the area, one can recommend suitable SuDS techniques that would be compatible with the underlying geology based on information provided in The Geological Survey of Great Britain (England and Wales) 1:50,000 Series Solid and Drift Edition:

• Beaconsfield 255, Drift 1968 • North London, 256, Solid and drift 1994 • Windsor, 269 Solid and Drift 1981 • South London, 270 Solid and drift 1981

The dominant solid geology of the London Borough of Hillingdon is the London Clay Formation. To the north of the district within the vicinity of Ruislip and Northwood outcrops of the Lambeth group (formerly known as the Woolwich and Reading beds) occur within river valleys. Along the western boundary of the district some areas of Chalk are located within the vicinity of the River Colne.

Drift deposits overlying the solid geology in the southern area of the district consist of pockets of Langley Silt (sandy clay and silt ‘brick earth’) overlying the River Terraced Deposits (mainly gravels), which have been locally excavated creating lakes and reservoirs where they have not been backfilled and areas of in-filled ground where they have. Within the vicinity of West Drayton, Langley silt is found to extensively overlie the gravels.

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In the northern part of the district (Hillingdon and further north) drift deposits are limited to pockets of Glacial Sand and Gravel, which includes undifferentiated head (the glacial deposits will consist mainly of sands and gravels and the head deposits of sandy clay and silt).

Along the line of river channels, alluvial deposits are located and in some areas the underlying solid formation has been exposed. The solid and drift geology map for Hillingdon can be seen in Figures 8 and 9 (pages 101 & 102).

In the design of any drainage system and SuDS approach, consideration should be given to site-specific characteristics and where possible be based on primary data from site investigations. The information presented is provided as a guide and should not be used to accept or refuse SuDS techniques.

The description of the geology of Hillingdon in Appendix D Tables D2 and D3 provide a general summary of the solid and drift geology. A site specific ground investigation should be undertaken on a site by site basis.

If, after geotechnical analysis of the geology and associated permeability of the strata underlying the allocation site, infiltration is considered appropriate, the allocation site must also be categorised in terms of proximity to a groundwater abstraction source.

Source protection zones are defined for public water supply, other private potable supply, (including mineral and bottled water) or in commercial food and drink production and depending on the geology in the area of the abstraction point this will effect the transmission time it takes for pollutants to reach the abstraction point, leading to the development of three source protection zones:

• Zone I (Inner Source Protection) • Zone II (Outer Source Protection) • Zone III (Source Catchment)

The type of development proposed within the allocation site and the Source Protection Zones in which the allocation is situated will determine the restriction imposed on infiltration as illustrated in Table D-4.

The most convenient vehicle for agreeing long-term management responsibilities is through Section 106 of the Town and Country Planning Act. Under this, agreement for SuDS maintenance can be a requirement of the planning application, forcing the issue to be addressed.

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It is recommended that the London Borough of Hillingdon completes Table D5 to assist in identifying where various types of SuDS are most suitable and enable developers to account for SuDS when developing masterplans for development sites.

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 London Borough of Hillingdon 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:

i. Land use characteristics; ii. Site characteristics; iii. Catchment characteristics; iv. Quantity and quality performance requirements; v. Amenity and environmental requirements. Each technique is scored on each of the five criteria as shown in Table 8-2 below. A weighting score has been given to each criteria, scored from 1 to 3, with one having least weighting/importance. Therefore the higher the resultant score, the more suitable the technique. The results have been put into three bands as follows:

Red 0 – 45 not suitable for use within the London Borough of Hillingdon Orange 45 – 55 may be possible to design into schemes Green 55 – 65 a good option for SuDS within the London Borough of Hillingdon LBH London Borough of Hillingdon

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Table 8-2 - SUDS Score Techniques for Site Specific Constraints

Criteria Assessment onds Weighting Pervious Pavements Green roofs Bioretention Filtration techniques strips Grassed filter Swales Infiltration devices Filter drains Infiltration basin Extended detention p Wet ponds Storm water wetlands On-/off-line storage Is pollutant Most instances LBH removal a will be roof runoff, 5 5 5 4 2 4 3 3 3 3 4 5 1 priority? therefore pollutant 1 removal not a priority. Is water Yes, attenuation quantity required in LBH to 10 8 4 4 4 6 10 6 10 8 10 6 10 control a prevent downstream 2 priority? flooding. Is flow rate If flow can be control a reduced it would 3 8 8 4 4 4 8 8 8 8 6 6 6 6 Hydrological priority? have a positive impact on flood risk. Is No, London has groundwater historically had high 3 5 3 5 3 3 1 5 1 2 2 2 5 recharge Groundwater levels. 1 required? Suitability to It has been assumed

d type of that all development 1 5 5 3 4 1 2 3 3 1 1 1 1 5 an

use developmen will be Dense urban L t developments Catchment Assumed that due to Area nature of 5 5 5 5 5 5 5 5 5 1 1 1 5 development, all 1 catchments will be <2ha Site slope A site slope of 0 to 10% has been 5 5 5 5 5 5 5 5 5 5 5 5 5 assumed. 1 Space Limited space for required SUDs across LBH 10 10 6 10 4 4 10 6 2 2 2 2 10 (depends on specific 3 site) Physical site features Soil The majority of LBH infiltration is located on clay 5 5 5 5 5 5 1 5 1 5 5 5 5 rate soils with a low 1 infiltration rate Total Score 61 61 45 51 38 47 51 51 41 38 41 37 57

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9 Suggested Policy Considerations

National and local policies have been reviewed against the local flood risk issues and objectives identified by the Environment Agency. From these policies the following catchment wide and specific area strategies have been developed under the headings Flood Risk, SuDS, Flood Mitigation and the Water Environment. Integration of these suggested policy considerations into LDF / LDD should ensure that the objectives and aspirations of the Environment Agency and national policy are met whilst strengthening the position of the Local Planning Authority with regard to flood management.

9.1 Flood Risk

9.1.1 Catchment Wide Strategies 1. Ensure the Sequential Test is undertaken for all land allocations to reduce the flood risk to the allocation and ensure that the vulnerability classification of the proposed development is appropriate to the Flood Zone classification;

2. Flood Risk Assessments should be undertaken for all developments within Flood Zones 2 and 3 to assess the risk of flooding to the development and identify options to mitigate the flood risk to the development, site users and surrounding area;

3. Flood Risk Assessments are required for all developments greater than 1.0 ha or over 10 dwellings. Furthermore, for a development in Flood Zone 1 which is less than 1.0ha and has a critical drainage problem, a Flood Risk Assessment is also required;

4. Flood Risk to development should be assessed for all forms of flooding;

5. Where floodplain storage is removed, the development should provide compensatory storage on a level for level and volume for volume basis to ensure that there is no loss in flood storage capacity;

6. Define the functional floodplain; protect Greenfield functional floodplain from future development. Develop appropriate flood risk management policies for the Brownfield functional floodplain, focusing on risk reduction (see section 10 for further criteria for brownfield functional floodplain);

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7. Remaining greenfield floodplain is the greatest flood risk management asset; protect remaining greenfield floodplain from future development;

8. Look at opportunities to make space for water to accommodate climate change in order to assist in managing future flood risk;

9. An 8 metre buffer strip must be maintained to along the river corridor to ensure that maintenance of the channel can be undertaken; 10. Flood defences provide flood protection and should continue to be maintained; 11. Minimise fluvial flooding by transferring the water in modified channels and conveying it out of the catchments; 12. Maximise the remaining life of the conveyance system to provide some management of the probability of fluvial events; 13. Promote flood resilience at the individual property level and identify the need to preserve sites for future flood storage and flood alleviation from future development; 14. Be aware that the Environment Agency are progressing some schemes on the Lower Colne, but on other streams they are not likely to be taking any action to further reduce the probability of flooding within these catchments in the foreseeable future; 15. Continue to maintain those assets that are effective in managing current and future flood risk; for example the Hayes Flood Storage area; 16. Take opportunities to reduce the dependency on assets that do not contribute to effective flood risk management;

17. Developers should also consider the London Plan Policies 4A.12, 4A.13, 4A.14 and 4A.15 with respect to Flooding, Flood Risk Management, Sustainable Drainage and Rising Groundwater;

Compliance with PPS25 would be assured with the integration of these suggestions into the emerging LDF and the policies of the core strategy.

9.1.2 Area Specific The River Pinn needs to be maintained at a high standard as the vegetation impacts on flows and levels.

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9.2 Flood Risk Management

9.2.1 Catchment Wide Strategies The Environment Agency recommends the following flood risk management policies:-

1. Where an allocation borders an area benefiting from flood defence, opportunities should be sought for the maintenance and upgrading of these flood defences to be partly funded by the development for its lifetime;

2. Opportunities should be sought to open culverted watercourses, where possible, to return them to a natural system, reducing back up of flows and under capacity where this does not exacerbate the flooding elsewhere. Figure 5 (page 98) should be used as a reference point for this; in addition further culverting should be discouraged. Where opening up the channel is not possible, development set-back from the culvert should be encouraged (with a minimum of 4 metres).

3. River channel restoration should be undertaken where possible to return the river to its natural state and restore floodplain to reduce the impact of flooding downstream;

4. Build resilience into a site’s design (e.g. flood-proofing, raised floor levels);

5. Ensure that redevelopment behind defenses reduces residual flood risk;

6. Ensure development is ‘Safe’. For ‘more’ and ‘highly’ vulnerable developments ‘safe’ is dry access;

7. Dry escape for residential dwellings should be up to the 1 in 100 year event taking into account climate change.

8. ‘Safe’ should preferably be dry for educational establishments and ‘less vulnerable’ land use classifications;

9. Assess condition of existing assets (e.g. bridges, culverts, river walls) and renew so that its lifetime is commensurate with lifetime of the development. Enhancement opportunities should be sought when renewing assets e.g. de-culverting, bioengineered river walls. More sustainable solutions to flood risk management will be easier and less costly to maintain and ultimately less expensive to replace;

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10. Remove permitted development rights in areas where side extensions and outhouses pose a flood risk management problem;

11. Developers should also consider the London Plan Policy 4A.12 and 4A.13 Flooding and Flood Risk Management.

9.2.2 Area Specific Strategies 1. The River Pinn needs to be retained at a high standard as vegetation growth impacts flows and levels;

2. It is essential an 8 metre buffer strip along river corridors are maintained to ensure that maintenance of the channel can be undertaken, as well as for ecology values;

3. The Environment Agency specifically name Fountains Mill (long culvert at Fassnidge Park) as benefiting long-term from de-culverting;

4. Specific areas of proposed river restoration should be undertaken in accordance with ‘Bringing your Rivers Back to Life’ (Environment Agency 2006), which proposes full river restoration including de-silting and habitat enhancement in the borough.

The London Plan identifies a series of areas for ‘regeneration’ and ‘opportunity’, some of which may be suitable locations for river restoration. In Hillingdon these areas include:

• Hayes/West Drayton- including the Grand Union Canal, Hayes Bypass Channel and Yeading Brook • Heathrow/Feltham/Bedfont Lakes- including the River Crane, River Colne, Longford River and Duke of Northumberland River. Through integration of these suggestions, the emerging LDF will comply with PPS25.

9.3 Sustainable Drainage Systems

9.3.1 Catchment Wide Strategies 1. Sustainable Drainage Systems must be included in new developments where possible as a way to manage surface water;

2. PPS25 requires the use of SuDS as an opportunity of managing flood risk, improving water quality and increasing amenity and biodiversity;

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3. Flood risk assessments should be undertaken for all developments in Flood Zone 1 consisting of sites greater than 1 ha to ensure that flood risk is not increased to other properties due to increased site runoff; 4. The London Plan Policy 4A.14 states that developers should aim to achieve greenfield run off from their site through incorporating rainwater harvesting and sustainable drainage. Boroughs should encourage the retention of soft landscaping in front gardens and other means of reducing, or at least not increasing, the amount of hard standing associated with existing homes.

5. Runoff rates should be restricted to greenfield runoff rates in areas known to have a history of sewer flooding; 6. Sustainable Drainage Systems should be located in accordance with the restrictions set out in Policy and Practice for the Protection of Groundwater, Appendix D, pages 112-120 (Table D-4);

7. The Environment Agency recommends that all sites greater than 1 ha in size require the following: • Greenfield discharge rates on greenfield and brownfield sites; • 1 in 100 year attenuation including a climate change allowance; • SUDS techniques.

9.3.2 Area Specific Strategies 1. The implementation of strategic flood storage areas operated by a single authority on the Upper and Lower Colne reduce flood risk to towns and villages; 2. The Environment Agency anticipate that the storage capacity of the surface water attenuation reservoirs used for Heathrow is now approaching capacity and a detailed surface water drainage assessment should be submitted as part of any proposed development; 3. Ensure that new development reduces the risk of foul flooding and achieves the goals of the Making Space for Water foul flooding project. 4. Developers should also consider the London Plan Policy 4A.14 with regards to Sustainable Drainage;

Compliance with PPS25 would be assured with the integration of these suggestions into the emerging LDF, and the policies of the core strategy. A specific policy should be integrated to incorporate Sustainable Drainage systems.

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9.4 Flood Risk & Environment

9.4.1 Catchment Wide Strategy 1. Development should not have a detrimental impact on the water environment through changes to water chemistry or resource. 2. Developments in areas of stressed water resources should look to incorporate water reuse and minimisation technology; 3. Any development should not be located within 8 metres of the riverbank to ensure access for maintenance but also to ensure a riparian corridor for improvement of the reverie environment. This will make space for water and additional capacity to accommodate climate change. 4. Preserve existing undeveloped river corridors from further development to help attenuate floodwaters. 5. Look at opportunities for river restoration/enhancement as part of a development to make space for water and to reduce the legacy costs associated with the maintenance of hard engineering structure. Compliance with PPS25 would be assured with the integration of these suggestions into the emerging LDF, and the policies of the core strategy.

9.5 Heathrow Airport Development The Environment Agency has remarked that all developments within Heathrow Airport should be treated in the same way as other developments and require a Flood Risk Assessment if the development is of the following criteria:

• Flood Risk Assessments are required for all developments in Flood Zone 1 consisting of sites greater than 1 ha, further details can be obtained on www.pipernetworking.com/floodrisk; • Flood Risk to development should be assessed for all forms of flooding; • The proposed development should be classed according to the vulnerability classification appropriate to the Flood Zone classification; • Flood Risk Assessments should be undertaken for all developments within Flood Zones 2 and 3 to assess the risk of flooding to the development and identify options to mitigate the flood risk to the development, site users and surrounding area;

The Environment Agency recommends that all future development at Heathrow needs to achieve:-

76 London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

• Greenfield discharge rates. • 1 in 100 year attenuation taking into account climate change. • SUDS systems

It is noted by the Environment Agency that there have been many drainage studies undertaken for Heathrow. These identify the capacity of storage reservoirs and have demonstrated that there has been some on site surface water flooding of the airport.

The Environment Agency anticipate that the surface water capacity of the reservoirs is now approaching capacity and therefore a detailed surface water drainage assessment should be submitted as part of any proposed development.

77 London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

10 Site Specific FRA Guidance

Site specific flood risk assessments are required to assess the flood risk posed to proposed developments and to ensure that where necessary and appropriate, suitable mitigation measures are included in the development.

This section presents the recommendations for site specific flood risk assessments prepared for submission with planning applications in the Hillingdon administrative area.

The site specific flood risk assessment guidance presented in the following sections has been developed based on:

• the recommendations presented in Planning Policy Statement 25 and the consultation draft of the Practice Guide companion to PPS25; • a review of the policies contained within the existing Unitary Development Plan for the London Borough of Hillingdon; and • the information gathered through and findings of the Level 1 SFRA process.

10.1 When is a Flood Risk Assessment Required? When informing developers of the requirements of a flood risk assessment for a development site, consideration should be given to the position of the development relative to flood sources, the vulnerability of the proposed development and its scale.

In the following situations a Flood Risk Assessment should always be provided with a planning application:

1 The development site is located in Flood Zone 2 or 3; 2 The proposed development site area is greater than 1 hectare (even if the site is located in Flood Zone 1). This is to ensure storm water generated by the site is managed in a sustainable manner and does not increase the burden on existing infrastructure and/or flood risk to neighbouring property); 3 The floor space of proposed non-residential development is greater than 1000m2 or the site areas is greater than 1 hectare; 4 The development site is located in an areas known to have experienced flooding problems from any flood source; and,

78 London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

5 The development is located within 20m of top of bank of a main river watercourse regardless of Flood Zone classification.

To identify when the Environment Agency should be consulted on planning applications please refer to www.pipernetworking.com/floodrisk which presents a matrix detailing the Environment Agency’s standing advice on flood risk assessment requirements for different types of development.

10.2 FRA Requirements Annex E of PPS25 presents the minimum requirements for flood risk assessment. The Practice Guide Companion to PPS25 (consultation document) advocates a staged approach to site specific flood risk assessment with the findings from each stage informing the next iteratively throughout the development process. These documents describe when an FRA is required and advice on what should be contained within a site specific assessment.

10.3 Flood Risk Assessment Guidance Table The Flood Risk Assessment Guidance Table (Table 10-1) provides guidance to developers and Local Authorities on the requirements of a FRA. In addition guidance can be obtained from www.pipernetworking.com/floodrisk and Annex E of PPS25 with respect to site specific Flood Risk Assessments.

Table 10.1 (end of section 10) provides a framework with which Local Authorities and Developers will be able to assess the requirements of each individual development with regard to flood risk.

10.3.1 Risks of Developing in Flood Risk Areas Developing in flood risk areas can result in significant risk to a development and site users. Table 10.1 should identify the main flood risks posed to the site. Additional issues to consider include:

i. Failure to consider wider plans prepared by the Environment Agency or other operating authorities may result in a proposed scheme being objected to; ii. Failure to identify flood risk issues early in a development project could result in failure of a development proposal, requiring redesign of the site to mitigate flood risk; iii. Failure to adequately assess all flood risk sources and construct a development that is safe over its lifetime could increase the number

79 London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

of people at risk from flooding and/or increase the risk to existing populations; iv. Failure to mitigate the risk arising from development may lead to claims against the developer if an adverse effect can be demonstrated (i.e. flooding didn’t occur prior to development) by neighbouring properties/residents; v. Failure to assess residual risk of failure of flood defence infrastructure, or failure of other artificial sources; vi. Properties may be un-insurable and therefore un-sellable if flood risk management is not adequately provided for the lifetime of the development; vii. By installing SuDS without arranging for their adoption or maintenance the SuDS will eventually cease to operate as designed and may present a flood risk to the development and/or neighbouring property; viii. The restoration of river corridors and natural floodplains can significantly enhance the quality of the built environment whilst reducing flood risk. Such an approach can significantly reduce the developable area of sites or lead to fragmented developments, however positive planning and integration throughout the master planning process should resolve these.

Advice from the Environment Agency’s National Development Control Policy team on brownfield functional floodplain is that the Environment Agency will consider existing building footprints to be part of the functional floodplain, unless it can be proven that they exclude flood waters. If these buildings do exclude flood waters, then solely the area around these buildings will be deemed functional. When undertaking an FRA this matter should be clarified and ideally pre-agreed with the Environment Agency.

10.3.2 ‘Safe’ Development The following items should be addressed as part of a Flood Risk Assessment in order to demonstrate that proposed developments are ‘safe’ in line with PPS25:

• ‘Safe’ access/egress for ‘more vulnerable’ and ‘highly vulnerable’ uses is dry. Dry escape for residential dwellings should be up to the 1 in 100 year event taking into account climate change.

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• The Environment Agency have specifically suggested that 'Safe' should preferably be dry for other uses such as educational establishments and 'less vulnerable' land use classifications. • For ‘More Vulnerable’ and ‘Highly Vulnerable’ finished floor levels should be set 300mm above the 1 in 100 year plus climate change level. • Where floodplain compensation is undertaken the Environment Agency requires this is on a ‘Level for Level, Volume for Volume Basis’. • Flood flow routes should be preserved. • Flood resilient constructions measures should be incorporated into new developments.

Figure 5 demonstrates the location structures and defences using the National Flood Coastal Defence Database (NFCDD), this should be used as a reference point to identify key structures which may become blocked during times of flood and cause increased flood risk as part of a site specific FRA.

10.3.3 Groundwater Flood Risk Areas In areas at risk of ground water flooding a site specific flood risk assessment should assess the level of risk to the site. Local groundwater monitoring should be identified and where possible analysed to assess ground water levels as part of a Flood Risk Assessment.

10.3.4 Surface Water Flood Risk Areas In areas at risk of surface water flooding, development should seek to reduce surface water runoff rates as a result of development. Furthermore, the appropriate application of sustainable drainage systems (where possible) to reduce the overall level of flood risk in the area through the outlay and form of the development would be required. Figure 4 (page 97) shows the areas assessed to be at risk of flooding from overland flow and this should be investigated further as part of a site specific FRA.

10.3.5 Artificial Source/Infrastructure Failure Flood Risk Areas Artificial sources of flooding within a 1km radius of the any site should be considered in order to assess the residual risk of a water body overtopping and potential flow paths. Reservoir or canal flooding may occur as a result of the facility being overwhelmed and/or as a result of dam or bank failure and therefore a crude assessment of flood flow routes should be assessed.

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If a perched waterbody is in close proximity, where possible a cross section should be taken in relation to the proposed site, showing level of the waterbody and its water level. More specifically areas in proximity to the Ruislip Lido and Colne Lakes system should consider the residual risks further as part of a site specific FRA.

10.3.6 Riverside Development i. Main River (for Main Rivers in Hillingdon see section 1.1.2) Under the terms of the Water Resources Act 1991 and the Land Drainage Byelaws 1981, the prior written consent of the Environment Agency is required for any proposed works or structures in, under, over or within 8 meters of the brink of a Main River. Furthermore the Environment Agency would seek an 8 metre wide undeveloped buffer strip alongside main rivers and would also ask to developers to explore opportunities for river restoration as part of the development.

ii. Ordinary watercourses5/canals The Environment Agency requires a 5 metre undeveloped buffer strip alongside such watercourses. This is to allow access for maintenance and encourage conservation and wildlife habitat.

iii. De-culverting The Environment Agency would seek de-culverting as part of any development next to a culverted river. If de-culverting was not technically feasible the Environment Agency would require a minimum of a 4 metre setback from the culvert for access/maintenance. Any proposals of further culverting or the placing of buildings over a culvert should be strongly discouraged.

Figure 5 (page 98) of the NFCDD shows existing structures. Figure 5 should be used in conjunction with the large scale flood risk maps (Figures 3- 4) as a reference point to identify any potential structures that could result in increased flood risk within the area and could benefit from de-culverting in future.

iv. Brownfield Functional Floodplain The following criteria apply to development in the brownfield functional floodplain:

5 Ordinary watercourses are statutory watercourses that are not classified as main rivers.

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• Buildings, unless permeable to floodwaters, are not considered to be part of the functional floodplain. Land/infrastructure around these buildings is considered to be functional. • The Environment Agency states that the following measures should be considered to reduce risk on these sites: • Removal of buildings and restoration of natural floodplain; • Change of use to a less vulnerable classification; • Reduced building footprints; • Preservation of flow routes; • Improved conveyance/storage, replacing solid building with building on stilts; • Meeting the objectives of the Thames Catchment Flood Management Plan (Summary Document Jan 2007, available at www.environment-agency.gov.uk); • Adopting a sequential approach to design of the site using the principles of ‘Making Space for Water’ (DEFRA 2004 and ongoing, available at http://www.defra.gov.uk/environ/fcd/policy/strategy.htm)

83 Table 10.1 – Guidance to developers and Local Authorities on the requirements of a Flood Risk Assessment

Development Development Includes culverting or Within Flood Zone 3b Within Flood Zone 3a Within Flood Zone 2 Within Flood Zone 1 Sewer flooding Groundwater flooding Overland flow Artificial sources Category (including boundary control of flow of any walls etc.) within 20 river or stream. metres of the top of a bank of a main river. Householder Ref: http://www.pipernetworking.com Ref: http://www.pipernetworking.com Rainwater to be discharged to one of following in order of priority: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: Home owner to ensure that they are registered with Consult EA • Consult EA with FRA Residential development • Planning application must include details of flood mitigation measures, either: • Planning application must include details of flood mitigation measures, either: • Adequate soakaway or infiltration system • Assessment of risk from GW flooding the LPA’s flood warning service. development and showing design details of should not be permitted. • Raising finished floor levels – to be considered on site by • Assessment of risk posed by overland flow, • Set floor levels no lower than existing levels AND include flood proofing where • Set floor levels no lower than existing levels AND, include flood proofing • a watercourse; or, where not reasonably practicable, • Flood resilient and flood resistant measures should be considered in alterations any culvert or flow control This statement requires appropriate, OR, set floor levels 300mm above the 1 in 100 year flood level plus where appropriate. • a sewer site basis. line with Building Regulations. Home owner to be aware of emergency plans. structure proposed. justification through policy. climate change. • Or, set floor levels 300mm above the 1 in 100 year flood level plus climate • Construct buildings with solid floors Possible mitigation may include: • Details of Flood Resilience or Resistant techniques to be included. change. Alterations to include adequate provision for rainwater to be carried • Flood resilient and flood resistant measures should be Possible mitigation may include: • Raising finished floor levels – to be considered on LPA emergency planning service must obtain flood • EA Flood Risk Consent • If the site is considered defended, residual risk must be identified in the FRA in the • If the site is considered defended, residual risk must be identified in the FRA from roof of building and adequate drainage from paved areas. considered in line with Building Regulations. • Raising finished floor levels – to be considered on site by site basis site by site basis plans for the residential area and incorporate them. required. event of a failure of the defences, either through overtopping or breach (depending on in the event of a failure of the defences, either through overtopping or breach • Constructing buildings with solid floors • Construct buildings with solid floors the nature of the defences). Flow paths to the site should be identified to determine the (depending on the nature of the defences). Flow paths to the site should be Ref: Code for sustainable Homes • Providing raised walkways to ensure safe access and egress during a • Orientate buildings to prevent impediment of More specifically areas in proximity to the Ruislip level of residual flood risk to the site and potential egress/access routes. identified to determine the level of residual flood risk to the site and potential Ref: Building regulations 2002 Ref: DCLG flood proof construction flood event. overland flow routes. Lido and Colne Lakes system should consider the • If the site is considered within the undefended floodplain, any increases in building egress/access routes. Ref: Building regulations 2002 • Flood resilient and flood resistant measures should residual risks further as part of a site specific FRA footprint may require flood compensation storage on a level for level and volume for • Details of Flood Resilience or Resistant techniques to be included. Ensure that building foundations and piling do not disrupt groundwater flow be considered in line with Building Regulations. volume basis. • If the site is considered within the undefended floodplain, any increases in routes. • Exception test may be required depending on PPS25 vulnerability classification building footprint may require flood compensation storage on a level for level Ref: Building regulations 2002 and volume for volume basis. Ref: Building regulations 2002 Ref: Ref: Code for sustainable Homes 2006 http://www.pipernetworking.c Ref: Building Regulations 2002 Ref: Code for sustainable Homes om Ref: Building Regulations 2002

Non- residential Ref: http://www.pipernetworking.com Ref: http://www.pipernetworking.com Rainwater to be discharged to one of following in order of priority: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: Consult EA Extension to non-residential LPA to set up flood warning service for extensions with a • Consult EA with FRA • Planning application must include details of flood mitigation measures, either: • Planning application must include details of flood mitigation measures, either: • Adequate soakaway or infiltration system • Stormwater runoff calculations in relation to the capacity of • Assessment of risk from GW flooding • Assessment of risk posed by overland flow, showing design details of developments in Flood Zone • Set floor levels no lower than existing levels AND, include flood proofing where • Set floor levels no lower than existing levels AND, include flood proofing • a watercourse; or, where not reasonably practicable, the sewer system • GW monitoring should be considered to assess GW levels in relation to including velocities, pathways, flood depths, development at risk. footprint of less any culvert or flow control 3b should not be permitted. ponding etc. 2 appropriate. Details of Flood Resilience or Resistant techniques to be included. where appropriate. • a sewer • Analysis of surcharged flood levels topographic levels of the site. than 250m structure proposed. This statement requires • If the site is considered defended, residual risk must be identified in the FRA in the • Or, set floor levels 300mm above the 1 in 100 year flood level plus climate • Drainage strategy to ensure sewer flooding will not be • Flood resilient and flood resistant measures should be considered in • Flood evacuation plan Owner/user to have a flood evacuation plan in justification through policy. A event of a failure of the defences, either through overtopping or breach (depending on change. Alterations to include adequate provision for rainwater to be carried exacerbated. line with Building Regulations. place • EA Flood Risk Consent flood evacuation plan should the nature of the defences). Flow paths to the site should be identified to determine the • Details of Flood Resilience or Resistant techniques to be included. from roof of building and adequate drainage from paved areas. • Flood evacuation plan Possible mitigation may include: required. be in place. level of residual flood risk to the site and potential egress/access routes. • Flood Evacuation plan required • Flood resilient and flood resistant measures should be Possible mitigation may include: • Raising finished floor levels More specifically areas in proximity to the Ruislip • Flood Evacuation plan required • Exception test may be required depending on PPS25 vulnerability considered in line with Building Regulations. • Raising finished floor levels • Construct buildings with solid floors Lido and Colne Lakes system should consider the • Access to higher floors is desirable (essential for higher vulnerability classifications classification Ref: Code for sustainable Homes • Constructing buildings with solid floors • Incorporate Suds to limit runoff residual risks further as part of a site specific FRA such as schools). • If the site is considered within the undefended floodplain, any increases in Ref: Building regulations 2002 Possible mitigation may include: • Providing raised walkways to ensure safe access and egress during a • Flood resilient and flood resistant measures should • If the site is considered within the undefended floodplain, any increases in building building footprint may require flood compensation storage on a level for level • Raising finished floor levels flood event. be considered in line with Building Regulations. footprint may require flood compensation storage on a level for level and volume for and volume for volume basis. • Construct buildings with solid floors

volume basis. • Provide raised walkways to ensure safe access and egress Ensure that building foundations and piling do not disrupt groundwater flow Orientate buildings to prevent impediment of overland

• Exception test may be required depending on PPS25 vulnerability classification Ref: Code for sustainable Homes during a flood event routes. flow routes. Ref: Ref: Building Regulations 2002 • Incorporate SuDS to limit runoff http://www.pipernetworking.c Ref: Building regulations 2002 Ref: Building regulations 2002 om Ref: Code for sustainable Homes2006 Ref: DCLG flood proof construction Ref: Building regulations 2002 Ref: Building regulations 2002

Change of use • Water treatment plants and waste treatment plants need to include flood Rainwater to be discharged to one of following in order of priority: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: Consult EA Less vulnerable uses are not Consult EA with FRA Possible mitigation measures may include: FROM ‘water • Consult EA under part IVV, resistant measures. • Adequate soakaway or infiltration system • Stormwater runoff calculations in relation to the capacity of • Assessment of risk from GW flooding • Assessment of risk posed by overland flow, Section 109 (1) of the Water permitted in Flood Zone 3b. • All other uses to include flood resilient measures. • a watercourse; or, where not reasonably practicable, the sewer system • GW monitoring should be considered to assess GW levels in relation to including velocities, pathways, flood depths, • Raising finished floor levels compatible’ TO ‘less Resources Act 1991 to gain • Flood evacuation plan required. • a sewer • Analysis of surcharged flood levels topographic levels of the site. ponding etc. • Construct buildings with solid floors vulnerable’ consent when plans include Alterations to include adequate provision for rainwater to be carried • Drainage strategy to ensure sewer flooding will not be • Flood resilient and flood resistant measures should be considered in • Drainage strategy to ensure overland flow will not development erection of any structure in, from roof of building and adequate drainage from paved areas. exacerbated. line with Building Regulations. be exacerbated. LPA to set up flood warning service for over or under a watercourse • Flood evacuation plan Possible mitigation may include: • Flood evacuation plan development at risk. which is part of a main river. Ref: Code for sustainable Homes • Flood resilient and flood resistant measures should be • Raising finished floor levels Ref: Building regulations 2002 considered in line with Building Regulations. • Constructing buildings with solid floors Possible mitigation may include: Owner/user to have a flood evacuation plan in • Consult the LA for all other Possible mitigation may include: • Providing raised walkways to ensure safe access and egress during a • Raising finished floor levels place watercourses. • Raising finished floor levels flood event. • The use of roads as flood channels • Construct buildings with solid floors • Construct buildings with solid floors More specifically areas in proximity to the Ruislip Ref: • EA Flood Risk Consent • Provide raised walkways to ensure safe access and egress Ensure that building foundations and piling do not disrupt groundwater flow • Incorporate Suds to limit runoff Lido and Colne Lakes system should consider the http://www.pipernetworking.c required. during a flood event routes. • Flood resilient and flood resistant measures should residual risks further as part of a site specific FRA om • Incorporate SuDS to limit runoff Ref: Building regulations 2002 be considered in line with Building Regulations. Ref: Building regulations 2002 Ref: DCLG flood proof construction Ref: Building regulations 2002 Change of use Rainwater to be discharged to one of following in order of priority: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: Consult EA Highly and more vulnerable Highly vulnerable uses are not permitted in Flood Zone 3a Consult EA with FRA Possible mitigation measures may include: RESULTING IN • Consult EA under part IVV, • Adequate soakaway or infiltration system • Stormwater runoff calculations in relation to the capacity of • Assessment of risk from GW flooding • Assessment of risk posed by overland flow, Section 109 (1) of the Water uses are not permitted in • a watercourse; or, where not reasonably practicable, the sewer system • GW monitoring should be considered to assess GW levels in relation to including velocities, pathways, flood depths, • Raising finished floor levels ‘highly vulnerable’ Resources Act 1991 to gain Flood Zone 3b. Consult EA with FRA • a sewer • Analysis of surcharged flood levels topographic levels of the site. ponding etc. • Construct buildings with solid floors or ‘more vulnerable’ consent when plans include Alterations to include adequate provision for rainwater to be carried • Drainage strategy to ensure sewer flooding will not be • Flood evacuation plan • Drainage strategy to ensure overland flow will not development erection of any structure in, from roof of building and adequate drainage from paved areas. exacerbated. • Flood resilient and flood resistant measures should be considered in be exacerbated. LPA to set up flood warning service for over or under a watercourse • Flood evacuation plan line with Building Regulations. • Flood evacuation plan development at risk. which is part of a main river. Ref: Code for sustainable Homes • Flood resilient and flood resistant measures should be Ref: Building regulations 2002 considered in line with Building Regulations. Possible mitigation may include: Possible mitigation may include: Owner/user to have a flood evacuation plan in • Consult the LA for all other • Raising finished floor levels • Raising finished floor levels place watercourses. Possible mitigation may include: • Constructing buildings with solid floors • The use of roads as flood channels • Raising finished floor levels • Providing raised walkways to ensure safe access and egress during a • Construct buildings with solid floors More specifically areas in proximity to the Ruislip • EA Flood Risk Consent • Construct buildings with solid floors flood event. • Incorporate Suds to limit runoff Lido and Colne Lakes system should consider the

required. • Provide raised walkways to ensure safe access and egress • Flood resilient and flood resistant measures should residual risks further as part of a site specific FRA

during a flood event Ensure that building foundations and piling do not disrupt groundwater flow be considered in line with Building Regulations. accordance with LPA Ref: • Incorporate SuDS to limit runoff routes. http://www.pipernetworking.c Ref: DCLG flood proof construction Ref: Building regulations 2002 om Ref: Building regulations 2002 Ref: Building regulations 2002

Operational Ref: http://www.pipernetworking.com In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: Consult EA Consult EA with FRA Consult EA with FRA and Consult EA with FRA and Sequential Test Evidence (and where required confirm Consult EA with FRA and Sequential Test Evidence (and where required Possible mitigation measures may include: development of less • Stormwater runoff calculations in relation to the capacity of • Using a sequential approach, areas at high risk of GW flooding should • Using a sequential approach, development should showing design details of Sequential Test Evidence Exception Test has been applied) confirm Exception Test has been applied) Rainwater to be discharged to one of following in order of priority: the sewer system be avoided when allocating development GW monitoring should be be avoided within overland flow routes. • Raising finished floor levels than 1 hectare any culvert or flow control (and where required confirm • Adequate soakaway or infiltration system • Analysis of surcharged flood levels considered to assess GW levels in relation to topographic levels of the • Drainage strategy to ensure overland flow will not • Construct buildings with solid floors structure proposed. Exception Test has been • a watercourse; or, where not reasonably practicable, • Drainage strategy to ensure sewer flooding will not be site. be exacerbated. applied). Only water • a sewer exacerbated. • Flood evacuation plan • Flood evacuation plan LPA to set up flood warning service for EA Flood Risk Consent compatible uses are Alterations to include adequate provision for rainwater to be carried • Flood evacuation plan • Flood resilient and flood resistant measures should be considered in development at risk. required. permitted in this flood zone from roof of building and adequate drainage from paved areas. • Flood resilient and flood resistant measures should be line with Building Regulations. Possible mitigation may include: and ‘essential infrastructure’ considered in line with Building Regulations. • Raising finished floor levels Owner/user to have a flood evacuation plan in through the process of the Ref: Code for sustainable Homes Possible mitigation may include: • The use of roads as flood channels place Exceptions Test. Ref: Building regulations 2002 Possible mitigation may include: • Raising finished floor levels • Construct buildings with solid floors • Raising finished floor levels • Constructing buildings with solid floors • Incorporate Suds to limit runoff More specifically areas in proximity to the Ruislip

• Construct buildings with solid floors • Providing raised walkways to ensure safe access and egress during a • Flood resilient and flood resistant measures should Lido and Colne Lakes system should consider the

• Provide raised walkways to ensure safe access and egress flood event. be considered in line with Building Regulations. residual risks further as part of a site specific FRA

during a flood event Ref: • Incorporate SuDS to limit runoff Ensure that building foundations and piling do not disrupt groundwater flow Orientate buildings to prevent impediment of overland http://www.pipernetworking.c routes. flow routes. om Ref: DCLG flood proof construction Ref: Building regulations 2002 Ref: Building regulations 2002 Ref: Building regulations 2002 Operational Consult EA with FRA. In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: In addition to PPS25 requirements FRA to include: Consult EA Consult EA with FRA Consult EA with FRA and Consult EA with FRA and Sequential Test Evidence (and where required confirm Consult EA with FRA and Sequential Test Evidence (and where required Possible mitigation measures may include: development of 1ha • Stormwater runoff calculations in relation to the capacity of • Using a sequential approach, areas at high risk of GW flooding should • Using a sequential approach, development should showing design details of Sequential Test Evidence Exception Test has been applied) confirm Exception Test has been applied) A surface water flood risk assessment would need to be undertaken the sewer system be avoided when allocating development GW monitoring should be be avoided within overland flow routes. Drainage • Raising finished floor levels or greater A surface water FRA is any culvert or flow control (and where required confirm on all sites greater than 1Ha in size in compliance with PPS25. • Analysis of surcharged flood levels considered to assess GW levels in relation to topographic levels of the strategy to ensure overland flow will not be • Construct buildings with solid floors required for all sites over structure proposed. Exception Test has been • Drainage strategy to ensure sewer flooding will not be site. exacerbated. 1ha. applied). Only water exacerbated. • Flood evacuation plan • Flood evacuation plan LPA to set up flood warning service for EA Flood Risk Consent compatible uses are • Flood evacuation plan if non residential development • Flood resilient and flood resistant measures should be considered in development at risk. required. permitted in this flood zone • Flood resilient and flood resistant measures should be line with Building Regulations. Possible mitigation may include: and ‘essential infrastructure’ considered in line with Building Regulations. Possible mitigation may include: • Raising finished floor levels Owner/user to have a flood evacuation plan in through the process of the • Raising finished floor levels • The use of roads as flood channels place Exceptions Test. Possible mitigation may include: • Constructing buildings with solid floors • Construct buildings with solid floors • Raising finished floor levels • Providing raised walkways to ensure safe access and egress during a • Incorporate Suds to limit runoff More specifically areas in proximity to the Ruislip • Construct buildings with solid floors flood event. • Flood resilient and flood resistant measures should Lido and Colne Lakes system should consider the Ref: • Provide raised walkways to ensure safe access and egress be considered in line with Building Regulations. residual risks further as part of a site specific FRA http://www.pipernetworking.c during a flood event Ensure that building foundations and piling do not disrupt groundwater flow om • Incorporate SuDS to limit runoff routes. Orientate buildings to prevent impediment of overland Ref: DCLG flood proof construction Ref: Building regulations 2002 flow routes. Ref: Building regulations 2002 Ref: Building regulations 2002 In Addition: Glossary: Development on Brownfield Sites – SuDS Policies Safe access and egress • Developments on brownfield sites will not result in any worsening in site discharge rates and should be designed to decrease the load on the existing drainage system. Any attenuation is to be achieved through the use of • Fluvial/undefended floodplain – ‘safe’ access is dry for ‘more’ and ‘highly’ vulnerable uses. All new residential units within the floodplain (more vulnerable), DRY pedestrian access and egress should be provided from each residential unit to an area wholly outside the floodplain. New properties within a ‘dry island’ of the fluvial floodplain will also require dry access due to the disruption to essential services(gas, water, SuDS where practicable. etc) that would be experienced during a flood event. • The London Plan Supplementary Planning Guidance outlines that in accordance with Policy 4A.14 the Mayor requires that developers should aim to achieve greenfield run off from their site through incorporating rainwater Finished flood levels harvesting and sustainable drainage. • Fluvial/undefended floodplain – for all new buildings within the floodplain, finished floor levels should be set at or above the 1 in 100 year + 20% climate change flood level, with a 300mm freeboard to allow for modelling uncertainties. Where this can not be achieved due to practicality reasons, flood proofing measures should be utilised up to the 1 in 100 year + 20% climate change flood level. • In areas with a history of sewer and surface water flooding, Greenfield discharge rates should be achieved for proposed development. Level for level compensatory storage - offset any loss of flood storage capacity through development with an area of compensatory storage. This storage can take the form of a depression that would fill during a flood event, however this is required to be on a level for level basis i.e. that the area will flood at the same time during the flood event as the original flood plain would have done before redevelopment and should Riverside development be freely filling and emptying without human intervention • Where the site lies close to a water body, but is not shown in the SFRA as being within an area liable to flood risk, the FRA should perform an analysis to confirm that the site is not at risk from any of the possible flood Preservation of flood flow routes – Ensuring on redevelopment that known flood routes are not blocked by buildings. This could be preventing through opening up green corridors adjacent to the river or orientating buildings in such away not to obstruct flood flows. pathways.; Flood resilience – (wet proofing), acceptance that flood waters will enter buildings and designing to ensure minimal damage to the property when this occurs. For example raising electrical sockets above the flood level. • Additionally, the FRA should identify any issues with flood defence maintenance and provide information sufficient to support an application for Flood Defence Consent (required by the EA for works under, over or within Flood resistance – (dry proofing), prevention of flood waters from entering a building. For example this can be achieved through raising finished floor levels or using flood barriers ion doorways. Flood resistance can be achieved in new builds or by retrofitting into existing buildings. 8m of the bank of any main river or for works affecting flow in any watercourse). Life time of a development – Expected duration the development is expected to exist for: • Developers should model culvert blockage if a site is close to a structure in the river to assess the impact of blockage on flood risk to the site. • 100 years for residential properties • An 8 meter wide undeveloped buffer strip should be maintained alongside culverted main rivers and non-culverted rivers. • 60 years for non residential properties De-culverting • Seek to de-culvert the river in the location of the development, whilst not increasing the flood risk to the development or surrounding areas; • Through the de-culverting of a watercourse, the policies and aspirations presented in the Environment Agency’s Catchment Flood Management Plan and Flood Risk Management Strategies will be adhered to.

London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report

11 Emergency Planning

11.1 Existing Flood Warnings - Details of the Environment Agency’s Flood coverage The Environment Agency provides a flood warning service online or via Floodline 24 hours a day. This information is updated every 15 minutes. Flood Warnings Direct can be subscribed to by calling Floodline.

Flood Warnings Direct is a service offered by the Environment Agency to provide flood warning information directly to telephones, mobile phones, fax or pager. Individuals, response teams, emergency services etc can be informed of a predicted flood event simultaneously, speeding up emergency planning procedures. In addition, this service provides advice on how to prepare for a flood, what to do in the event of a flood, and what to do after a flood event.

Before a flood includes: • Making a list of useful numbers

• Understanding the flood warning codes

• Checking insurance

• Knowing where to turn off gas, electricity and water

• Preparing a flood kit

• Storing valuable and sentimental items upstairs or in a high place

Further details under each of these headings can be found in the Environment Agency’s advice on flooding booklet or online.

Flood Warning Codes: • Flood Warning – Flooding of homes and businesses expected. Act Now

• Severe Flood Warning – Severe flooding expected. Extreme danger to life and property. Act Now

• All Clear – Flood warnings no longer in force in this area

After a flood includes: • Contacting gas, electricity and water companies

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• Seeking alternative accommodation

• Contacting your insurance company

• Ventilating the property adequately

For additional information see the Environment Agency’s ‘advice on flooding’ booklet or online (available at http://www.environment- agency.gov.uk/commondata/acrobat/flood_leaflet_1829467.pdf). In addition, health advice following a flood can be found online on the Environment Agency’s Flooding pages (http://www.environment- agency.gov.uk/subjects/flood/).

11.2 Flood Warnings and procedures in the London Borough of Hillingdon There are details of the London Borough of Hillingdon’s Emergency Plan on the Hillingdon website (http://www.hillingdon.gov.uk/index.jsp?articleid =8850). The following is a summary of the information available at the above.

Flooding incidents are classified as major incidents on the Emergency Planning pages of Hillingdon’s website. Hillingdon Emergency Planners work with the Environment Agency, Fire Service, Metropolitan Police and the Ambulance Service. Any responding organization can implement special arrangements for managing an emergency incident. The councils Incident Management Team (IMT) will co-ordinate the response from the Incident Management Suite. The Local Authority Liaison Officer (LALO) will travel to the site of the emergency to work with the Emergency Services. The Council will be involved in both working with and assisting the emergency services and ensuring the welfare of the public. The Council will also be involved in the aftermath of the emergency event; controlling the recovery of the area and population until normality has returned.

Hillingdon Civil Protection Service has provided the following information in relation to their Emergency planning in relation to flooding. The Civil Protection Service has generic emergency plans in place to respond to any incident that occurs with in the Borough. The Borough’s response is informed by its highways team and the emergency plan would only become operational if requested by either Council services, or the emergency services.

During a flood event the Council does not provide sand bags to private residents unless the water is coming directly off the highway, and there are sufficient resources to safely distribute them to these residents. Before any

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period of predicted heavy rain the Council strategically work to reduce the impact of the excess water. Methods include clearing gullies and emptying storm drains and soakaways.

During a flood the main function of the Council would be to provide temporary accommodation to any displaced people until such time that they are in a position to return to their homes or their insurance companies can arrange temporary accommodation for them. These shelters is provided in the form of rest centres, and provide a warm dry place to sleep and basic facilities including shower, food, etc.

11.3 Emergency Planning Considerations PPS25 classifies police stations, ambulance stations, fire stations and command centres as Highly Vulnerable buildings. Hospitals and care homes are classified as More Vulnerable establishments. In the event of an emergency, to ensure that services vital to the rescue operation do not become victims of the floodwaters, it is essential that all establishments related to these services are located in the lowest flood risk zones. In addition future development control polices should seek to locate more vulnerable institutes such as schools and care homes in areas of the lowest risk to minimise the potential for flood casualties.

Allied to this, nominated rest and reception centres should also be identified within the study area and compared with the outputs of this SFRA to ensure allocated centres are not at high risk of flooding, so that evacuees will be safe during a flood event, see Figure 7 (page 100). Developments suitable for such uses would include:

• Leisure centres; • Churches; • Schools; and • Community Centres.

Table D.2 of PPS25 (Department for Communities and Local Government, 2006) classifies ‘Highly Vulnerable’ developments. Those that should be taken into consideration in the event of an emergency are:

• Hospitals; Residential institutions such as residential care homes, children’s homes, social services homes, prisons and hostels; • Student halls of residence; and, • Non-residential uses for health service, nurseries and educational establishments.

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Situations may arise in an emergency where the occupants of the above institutions cannot be evacuated (such as prisons). Therefore particular significance must be given to these development types when looking to allocate them. These allocations should be assessed against the outputs of the SFRA to develop robust emergency plans.

The emergency planning information can then also be used as a planning tool. Any development of institutions that could be used as places of rest and reception during times of flood or are involved in the emergency response system during times of flood should not be located within high flood risk areas. These developments include: Police stations, Ambulance stations, Fire stations, Command Centres, Leisure centres, Churches, Community Centres, etc. In addition, Highly Vulnerable and More Vulnerable developments, where the mobilisation of residents/users may be problematic, should not be located in areas classified as subject to high risk of flooding. These developments include: Hospitals, care homes, children’s homes, prisons, hospitals etc.

11.4 Emergency Planning Recommendations In light of the findings of this SFRA, it is recommended that the Hillingdon Civil Protection Service include an emergency response plan specifically relating to flooding. This could provide a specific response plan for the Borough in the event of a flood. Emergency evacuation routes should not direct people though areas associated with flood risk. The SFRA should be used to inform these plans.

A GIS layer has been produced of all the institutes such as nursing homes, police stations, hospitals and schools and could be used by Hillingdon to overlay on the Flood Zones. This could then be provided to the emergency planning team in order to identify priority institutions that may need evacuating during times of flood. The GIS layer can be found on the CD found in Appendix C and Figure 7 (page 100) shows flood zones and the location of emergency services, refuge centres and some vulnerable developments.

There are a number of areas and transport networks that are known to be affected by flood water. These are summarised below and mapped in Figure 7:

• The A40 may be affected by flooding from the River Colne and Frays River to the west and Yeading Brook to the east.

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• All train lines located within the floodplains of the River Colne and Frays River to the west and Yeading Brook to the east may be affected by flood water from these rivers.

• The M4 and the A4 may be affected by floodwater originating from the River Colne and Frays River to the west and the Grand Union Canal to the east.

• Areas of South Ruislip may be affected by flooding originating from the Yeading Brook, including some schools, care homes and places of refuge6.

• Areas of West Drayton and Yiewsley within close proximity to Frays River and the River Colne may be affected by flooding originating from these watercourses, again including some schools, care homes and places of refuge.

Particular attention should be paid to these areas during the development of an emergency plan. Methods for evacuating vulnerable people from areas at high risk of flooding should be developed and refuge centres within flood risk areas should be avoided. Transport routes susceptible to flooding should be avoided when developing an emergency evacuation plan. Figure 7 should be consulted when developing an emergency evacuation plan in response to a flood event in the London Borough of Hillingdon.

6 Places of refuge must be above the flood level and could be, for example, town halls, schools etc.

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12 Suggested Actions for Hillingdon

The following ‘suggested actions for Hillingdon’ are a collection of the recommendations made throughout the report.

• The Environment Agency recommends that for future revisions of this report the London Borough of Hillingdon keep records of flooding occurrences to identify in greater detail areas at risk from flooding and in particular critical drainage problems.

• The potential effects of flood risk management infrastructure and other structures also need to be considered.

• No assessment of overtopping of structures or the failure of retaining walls or flood routes has been undertaken as part of this study but should be considered at a site specific level.

• Multiple sources of flooding are prevalent, climate change and growth pressure could all result in greater frequency and severity of flooding.

• the residual risk of overtopping from artificial water bodies such as the Ruislip Lido should be considered in any flood risk assessment.

• It is recommended that the London Borough of Hillingdon completes Table D5 to assist in identifying where various types of SuDS are most suitable and enable developers to account for SuDS when developing masterplans for development sites.

• The Environment Agency anticipate that the surface water capacity of the reservoirs is now approaching capacity and therefore a detailed surface water drainage assessment should be submitted as part of any proposed development.

• In light of the findings of this SFRA, it is recommended that the Hillingdon Civil Protection Service include an emergency response plan specifically relating to flooding.

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13 References

Environment Agency, Thames Region NE Area. February 2006. Lower Pinn Flood Risk Management Strategy- Inception Study

The Environment Agency Flood Zone Map online, http://www.environment- agency.gov.uk/subjects/flood/?lang=_e

Environment Agency- Thames Catchment Flood Management Plan, Summary Document, Consultation 2007, available at www.environment- agency.gov.uk

Atkins, Lower Pinn Flood Risk Management Strategy Scoping Report 2005.

Department for Communities and Local Government, 2006. Planning Policy Statement 25: Development and Flood Risk. TSO publications, Norwich

Department for Communities and Local Government, 2007. Development and Flood Risk: A Practice Guide Companion to PPS25 ‘Living Draft’, A Consultation Paper.

Department for Communities and Local Government, 2008. Planning Policy Statement 25: Development and Flood Risk Practice Guide

DEFRA 2005. Flood Risk Assessment Guidance for New Development - Phase 2. Framework and Guidance for Assessing and Managing Flood Risk for New Development – Full Documentation and Tools. DEFRA and Environment Agency R&D Technical Report FD2320/TR2

DEFRA 2004 Making Space for Water, available at: http://www.defra.gov.uk/environ/fcd/policy/strategy.htm

Greater London Authority, May 2006, The London Plan Sub-Regional Development Framework – West London, Mayor of London

Greater London Authority, Feb 2007, Water Matters, The Mayor’s Draft Water Strategy – Draft for consultation with the London Assembly and functional bodies

HMSO. 2004. Planning and Compulsory Purchase Act 2004. Published by the Queen's Printer of Acts of Parliament. ISBN: 0 10 540504 3

Peter Brett Associates/Environment Agency: Lower Colne Flood Study Modelling Report.

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London Borough of Hillingdon, Local Development Framework Core Strategy. Revised Core Strategy referred options Consultation period: 7 February to 26 March 2007

London Borough of Hillingdon; Local Development preferred options; Sustainability Appraisal for the London Borough of Hillingdon’s Core Strategy Development Plan Document and Site Allocation DPD. October 2005

London Borough of Hillingdon; Local Development preferred options; Sustainability Appraisal for the London Borough of Hillingdon’s Core Strategy Development Plan Document and Site Allocation DPD. Technical Appendices. October 2005

London Borough of Hillingdon; Local Development Framework; Development Documents: Draft Proposed Changes for Submission. Appendix B Site Allocations DPD Cabinet 9th March 2006

London Borough Of Hillingdon; Revised Core Strategy, Preferred Options February 2007 available at:

http://consultation.limehouse.co.uk/hillingdon/drafts/17/chapter_548.html

http://www.ruislip.co.uk/lido/

Office of the Deputy Prime Minister (2005) Planning Policy Statement 1: Delivering Sustainable Development

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14 Figures

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