BOROUGH OF STRATEGIC FLOOD RISK ASSESSMENT

For Ealing Council

March 2008

VOLUME 2:

DATA AND MAPPING

(TECHNICAL REPORT)

Capita Symonds © Capita Symonds Ltd 24-30 London EC1N 2LX

Tel: 02078709300 Fax: 02078709399 Email: [email protected] SFRA – Volume 2

London Borough of Ealing Strategic Flood Risk Assessment Volume 2: Data and Mapping

Final Report March 2008

This document is the property of Ealing Council. If you have received it in error, please return it immediately to Ealing Council.

Job Number: CS/025608

PD: RG

PM: LS

Report status: FINAL

Date of issue: 03/08

CSL Main author(s): LT/GW/LS

CSL Checked: LS

Client Approval: IW

This report has been prepared by Capita Symonds Limited with all reasonable skill, care and diligence within the terms of the Contract with the Client, incorporating our General Terms and Conditions of Business and taking account of the resources devoted to it by agreement with the client. We disclaim any responsibility to the client and others in respect of any matters outside the scope of the above. This report is confidential to the client and we accept no responsibility of whatsoever nature to third parties to whom this report, or any part thereof, is made known. Any such party relies on the report at their own risk

Ealing Council November 2007 London Borough of Ealing SFRA – Volume 2

CONTENTS

MAPPING...... IV

DOCUMENT STATUS ...... V

GLOSSARY ...... VI

LONG TERM MANAGEMENT...... IX

1. INTRODUCTION TO THE LONDON BOROUGH OF EALING SFRA...... 1

2. DATA REVIEW...... 3 Hydrological and Hydraulic Model...... 4

3. CATCHMENT DESCRIPTION ...... 6 The Study Area...... 6 Watercourses in the Study Area ...... 6 Regional Geology ...... 10 Groundwater...... 10

4. FLOOD WARNING AND EMERGENCY PLANNING...... 11 Emergency Planning ...... 11 Key Emergency Documents ...... 11 Flood Warning...... 12

5. ASSET AND STRUCTURE DATA...... 14

6. FLOODING OVERVIEW AND RECORD OF FLOODING...... 17

7. DRAINAGE AND FLOODING...... 18 Surface Water Sewer Flooding...... 19 Sustainable Drainage Systems (SUDS)...... 20 Foul Water Sewer Flooding ...... 23 Combined Sewer Flooding ...... 23 Summary and Recommendations...... 25

8. RIVER FLOODING ...... 27 Actual Risk Flood Hazard Mapping...... 32 Assessment of Residual Risk...... 35 Summary and Recommendations...... 38

9. TIDAL FLOODING ...... 39 Summary and Recommendations...... 41

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10. GROUNDWATER FLOODING...... 42 Summary and Recommendations...... 46

11. FLOODING FROM OTHER SOURCES...... 47 Summary and Recommendations...... 50

12. UNCERTAINTIES IN ASSESSING FLOOD RISK...... 53

13. CONCLUSIONS AND RECOMMENDATIONS...... 55

14. SUMMARY ...... 58

Ealing Council Page iii November 2007 London Borough of Ealing SFRA – Volume 2

MAPPING INCLUDED AS A SEPARATE DOCUMENT VOLUME 2: DATA AND MAPPING (APPENDICES) APPENDIX A: RISK BASED SEQUENTIAL APPROACH APPENDIX B: CATCHMENT INFORMATION APPENDIX C: FLOOD ZONES APPENDIX D: ACTUAL AND RESIDUAL RISK APPENDIX E: FLOOD WARNING AREAS APPENDIX F: GROUNDWATER FLOODING

Ealing Council Page iv November 2007 London Borough of Ealing SFRA – Volume 2

DOCUMENT STATUS This report has been produced as a working document as part of the London Borough of Ealing SFRA. The scope of each stage of the SFRA is described in subsequent sections.

Version Date Issued by Issued to

Volume 2: 05/12/2007 LS Ealing Council Report Environment Agency Draft REV 1.0

Draft REV Ealing Council 20/02/2008 LS 1.1 Environment Agency

Final Ealing Council 16/04/2008 LS Environment Agency

Ealing Council Page v November 2007 London Borough of Ealing SFRA – Volume 2

GLOSSARY

ABD Areas Benefiting from Defence

Actual Risk The risk from flooding from all sources based on best available information and representing the influence of flood defences, large infrastructure features.

BHS British Hydrological Society

CC Climate Change

DEFRA Department for Environment, Food and Rural Affairs

DTM Digital Terrain Model created using LiDAR, IfSAR or Photogrammetry data.

EA Environment Agency

EC Ealing Council

FEH The Flood Estimation Handbook (1999) gives guidance on rainfall and river flood frequency estimation in the UK and is the main method used for the calculation of peak flood flows. The Handbook is accompanied by the FEH CD-ROM containing catchment descriptors and gauging station details for catchments throughout the UK.

Flood Zones This refers to the Flood Zones in accordance with Table D1 of PPS 25 derived for this SFRA

Flood Zones (EA) This refers to the Environment Agency’s Flood Zones.

GIS Geographical Information System

km 2 Square kilometres

LBE London Borough of Ealing

LDD Local Development Documents

LiDAR Light Detection and Ranging survey method used to collect data for construction of a ground model.

M Metres

m/sec Metres per second

m AOD Metres Above Ordnance Datum

Ealing Council Page vi December 2007 London Borough of Ealing SFRA – Volume 2

Main River As Defined by the Environment Agency main rivers are usually larger streams and rivers, but also include smaller watercourses of strategic drainage importance. A main river is defined as a watercourse shown as such on a main river map, and can include any structure or appliance for controlling or regulating the flow of water in, into or out of the main river. The Agency’s powers to carry out flood defence works apply to main rivers only. Main rivers are designated by the Department for Environment, Food & Rural Affairs in and by the Welsh Assembly Government.

Mm Millimetres

Ordinary Watercourse As Defined by the Environment Agency an ordinary watercourse is every river, stream, ditch, drain, cut, dyke, sluice, sewer (other than public sewer) and passage through which water flows which does not form part of a main river. On ordinary watercourses, the local authority and, where relevant, IDBs have similar permissive powers as the Agency has on main rivers.

PPS 25 Planning Policy Statement 25. Development and Flood Risk Guidance replacing PPG 25 in December 2006 and outlining how flood risk should be considered at all stages of the development process.

Precautionary Principle “Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost effective measures to prevent environmental degradation ’’. The precautionary principle was stated in the Rio Declaration in 1992. Its application in dealing with the hazard of flooding acknowledges the uncertainty inherent in flood estimation.

Residual Risk Risk remaining due to the possibility of an event more severe than that for which particular flood defences have been designed or more severed than that which significant infrastructure features, such as motorway embankments are able to provide protection. A more severe event includes future flood events resulting from the impacts of climate change.

Return Period The average time until the next occurrence of a defined event.

Ealing Council Page vii December 2007 London Borough of Ealing SFRA – Volume 2

Sequential risk-based Priority in allocating or permitting sites for development, in assessment descending order to the Flood Zones set out in Table D1 of PPS 25, including the sub divisions in Zone 3. Those responsible for land development plans or deciding applications for development would be expected to demonstrate that there are no reasonable options available in a lower- risk category.

SFRA Strategic Flood Risk Assessment

Study Area Refers to the London Borough of Ealing Area.

1 in 20 year return period flood The flood event that is predicted to occur with an annual event probability of 5.0% (there is a 1 in 20 (5%) chance each year this event will be witnessed).

1 in 50 year return period flood The flood event that is predicted to occur with an annual event probability of 2% (there is 1 in 50 (2%) chance each year this event will be witnessed)

1 in 100 year return period The flood event that is predicted to occur with an annual flood event probability of 1.0% (there is a 1 in 100 (1%) chance each year this event will be witnessed)

1 in 1000 year return period The flood event that is predicted to occur with an annual flood event probability of 0.1% (there is a 1 in 1000 (0.1%) chance each year this event will be witnessed)

Ealing Council Page viii December 2007 London Borough of Ealing SFRA – Volume 2

LONG TERM MANAGEMENT The London Borough of Ealing SFRA is based on information that will inevitably be amended in the future, by better data, change in the baseline conditions due to development, and change in institutional and policy conditions. To be robust and able to withstand challenges in the planning process there is a need to ensure the London Borough of Ealing SFRA reflects conditions at the time particular evaluations are made. Failure to maintain the SFRA may reduce the effectiveness of the flood risk management measures; delay planning and development processes; and potentially lead to the neglect of flood risk considerations and the failure to capture strategic responses and interventions. The current London Borough of Ealing SFRA is based on the ‘best available’ information as on November 2007. The Planning Policy Teams at LBE have the prime responsibility for managing and maintaining this SFRA. The SFRA should be reviewed at least annually as part of the annual monitoring report. The Planning Policy Team may decide to identify a “Management Group” of appropriately selected parties with responsibility for monitoring, managing, and maintaining the London Borough of Ealing SFRA. This group should be led by the planning representatives from the London Borough of Ealing and may also include representatives from other influential organisations. The EA are likely to play a key role in providing technical and process guidance to this management group. The roles and terms of reference for a Management Group have not been identified in this report. The management group will ensure that the London Borough of Ealing SFRA is accessible and transparent to those responsible for land use decisions.

Ealing Council Page ix December 2007 London Borough of Ealing SFRA

1. INTRODUCTION TO THE LONDON BOROUGH OF EALING SFRA 1.1 This document is the SFRA Volume 2: Data and Mapping (Technical Report) and should be read in conjunction with Volume 1: Decision Support. The mapping for Volume 2 is included as a separate document, as appendices to this document. Volume 1 provides a summary of the background and methodology adopted for assessing strategic flood risk. 1.2 The London Borough of Ealing SFRA covers the area within the administrative boundaries of the London Borough of Ealing (Appendix B9). 1.3 This report provides a general overview of the Ealing Council study area. It provides information on the study area, its surrounding waterways, a brief history of flooding, existing flood defences and includes a discussion on the hydraulic and hydrological modelling on which the Strategic Flood Risk Assessment (SFRA) relies. 1.4 A Strategic Flood Risk Assessment is required to inform the Ealing Council Sustainability Appraisal, land allocations, and policies. Ealing Council have carried out the SFRA to inform the preparation LDD’s and to consider the Catchment wide flooding issues. The SFRA provides the information required to apply the Sequential Approach and Sequential Test. 1.5 The underlying objective of the SFRA is to initiate a strategy that provides a framework for the consistent consideration of flood risk in seeking to accommodate current practice and best available data for the lifetime of the planning process. 1.6 It should be noted that the London Borough of Ealing SFRA is a live document and is based on the best information available at the time. As new data is made available, the document should be updated to reflect this. 1.7 The assessment evaluates risk as the product of the probability and the consequence of a particular event. Probability is defined as the frequency and magnitude of floods that are generated by fluvial flows and intense rainfall activity. The consequence is defined as the impact of floodwater on receptors (people, property, land, etc). This approach is sympathetic to the concept of source, path and receptor now adopted for flood risk management. 1.8 PPS25 recommends a two-tiered approach in the assessment of flood risk in SFRAs, allowing flexibility in the level of assessment. The amount of detail required increases at each tier (level), with the risk assessment focussing more closely on high priority risks identified in the previous tier. 1.9 This Strategic Flood Risk Assessment (SFRA) is an assessment as described by the Practice Guide Companion to PPS25 1. The study has used the best available information to assess flood risk. This includes the Flood Zones (EA), and other information, which enables a broad assessment of the Actual Flood Risk and Residual Risk for the existing conditions within the study area. The Flood Zones are discussed in greater detail within Section 8.21 of this document. Actual Risk refers to the flood risk from all sources taking into consideration the influence of defences and large infrastructure such as railway and motorway embankments on flood extents. The Residual Risk is the risk remaining to proposed developments after taking mitigating actions, for example developments behind defences are at residual risk from overtopping or breaching of the defence. 1.10 The information used in the development of this SFRA is listed in Table 2.1.

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

Ealing Council Page 1 December 2007 London Borough of Ealing SFRA

1.11 This document does not replace, and should be read in conjunction with, national and regional policy including PPS25 and relevant regional policy. This SFRA does not dismiss responsibility at a broader level to consider catchment-wide flood risk management approaches and solutions whilst maintaining the need for site-specific FRAs.

Ealing Council Page 2 December 2007 London Borough of Ealing SFRA

2. DATA REVIEW 2.1 The following table details the key information received from various organisations in order to develop the Ealing SFRA. Table 2-1: Summary of Key Information DATE OWNER / DATA DESCRIPTION PROVIDED AUTHOR Study undertaken by Atkins/Jacobs/Peter Brett Associates consortium and completed by Flood Study Jacobs for the Environment Agency. 4 October 2007 EA/Jacobs Final Hydrology Report The report outlines the hydrology used to build the River Brent Flood Study. Study undertaken by Atkins/Jacobs/Peter Brett Associates River Brent Flood Study for the Environment Agency. The Atkins/Jacobs/Peter Final Hydraulic Modelling 4 October 2007 report outlines the hydraulic Brett Associates Report modelling used to generate floodplain maps. The iSIS model used in the Lower Lower Brent iSIS model Brent Model by PBA/Jacobs/Atkins 13 August 2007 Environment Agency and results as part of the River Brent Flood Mapping Study Presents the findings of scoping River Brent Flood Risk study undertaken as part of the Management Strategy Environment Agency’s Strategic Strategic Environmental 8 October 2007 Environment Agency Environment Assessment (SEA) Assessment Scoping process for the River Brent Flood Report Risk Management (FRM) Strategy. DTM of the London Borough of LiDAR 1 October 2007 Environment Agency Ealing GIS layers of Borough outline, Digital Catchment Aquifers, catchments, depth to 24 September 2007 Environment Agency Information Groundwater, Geology Solid and Drift, Ealing Flood Zones. List of wells, boreholes and Thames Region- Well and groundwater monitoring stations in 11 September 2007 Environment Agency Borehole Locations the Thames Region. Maps of potential groundwater Groundwater Emergence flooding areas based on historic DEFRA Zones events and recorded groundwater levels. Water Matters, The Mayor’s Draft Water Strategy – Draft for Flooding issues in London Internet consultation with the and functional bodies Regional Flood Risk Flooding issues in London Internet Appraisal (Draft) Flooding in London, A London Assembly Flooding issues in London Internet Scrutiny Report River Defence and Asset NFCDD GIS database information 24 September 2007 Environment Agency Information within the study area. Tidal defence data for the Tidal Defence data for the Thames 17 October 2007 Environment Agency Thames NE region GIS layers of the River River Brent Cross Section and reservoir Nodes used in 16 October 2007 Environment Agency Sections the River Brent Flood Mapping study Sites identified in Site Allocations London Borough of Ealing document that fall within River Brent 8 October 2007 Ealing Council Site allocations Flood Zones. Thames water records of surface History of flooding water flooding (foul and combined) 10 October 2007 Thames Water in the London Borough of Ealing. The survey used in the Brent Flood Survey Data 13 August 2007 Environment Agency Mapping Study

Ealing Council Page 3 December 2007 London Borough of Ealing SFRA

Information of incidences of previous Historical flood flood events within the London 22 August 2007 Ealing Library records/data Borough of Ealing. 1:10,000 mapping of Ealing Borough Mapping and postcodes 17 August 2007 Ealing Council and Postcode data Received from Ronnie Stevans information on the operational Welsh Harp Reservoir 24 September 2007 British Waterways aspects of the reservoir along with Emergency Planning. Information about the Canal received from Philip Adshed, Grand Union Canal 12 October 2007 British Waterways covering risk of breach and maintenance. Flood Zones GIS layer of EA flood zones 13 August 2007 Environment Agency GIS layer of historically flooded EA Flood warning zones 13 August 2007 Environment Agency areas Two of the key data sets for this SFRA are the EA Flood Zones and the Hydraulic Model.

EA Flood Zones 2.2 The EA Flood Zones are used to indicate broad areas that may be at risk of river and tidal flooding. The EA Flood Map is available online and mapped in Appendix C. 2.3 The maps indicated Flood Zones 1, 2, 3 and 3b, which are the starting point for applying the Sequential Test. These Flood Zones refer to the probability of river and sea flooding only, ignoring the presence of any existing defences and structures. 2.4 Flood Zone 1 comprises of land assessed as having a less than a 0.01% (1 in 1000 year return period) of river or sea flooding in any year. Flood Zone 2 is identified as being of medium probability in PPS25, and has between a 1% (1 in 100 year return period) and 0.01% (1 in 1000 year return period) annual probability of river flooding in any year. Flood Zone 3 is identified as an area of high probability flooding, and comprises land assessed as having a 0.01% (1 in 100 year return period) or greater annual probability of river flooding. 2.5 The EA Flood Zones only cover the risk of flooding from Main Rivers and Sea and therefore do not depict the potential flood risk posed by Ordinary Watercourses; therefore further investigation into these watercourses was undertaken to make sure all fluvial/tidal flooding was taken into consideration. 2.6 A more detailed description of how the EA flood Zones affect the study area is shown within Section 8.21. 2.7 The Flood Zones are the first port of call when applying the Sequential Test; however more detail is available if the river in question, the River Brent, has been hydraulically modelled.

Hydrological and Hydraulic Model 2.8 The Environment Agency has provided us, Capita Symonds, use of resources developed for the Strategic Flood Risk Management Framework: River Brent Flood Study, completed in September 2007 by Peter Brett Associates, Jacobs and Atkins 2.9 This Study includes hydrology, gauged data and hydraulic modelling to assess the risk of flooding from the Brent. The Study covers the River Brent from close to source up to the Grand Union Canal Thames Lock near the River Thames confluence at . The Study divided the Brent into two sections, the Upper and the Lower Brent. The Lower Brent model was used to inform this SFRA. The model includes Brook, which is beyond the London Borough of Ealing study area. The model runs from the outlet of the to the Grand Union Canal Thames Lock near the River

Ealing Council Page 4 December 2007 London Borough of Ealing SFRA

Thames confluence. The study area does not include the whole of the Lower Brent Model and study area boundary starts at the Grand Union Canal aqueduct in . 2.10 The EA Lower Brent model is a one-dimensional iSIS model. The watercourse was schematised using the 12D Model software package. Channel survey data was used to define the in-bank channel cross sections. Within the study area it appears that the majority of the river sections were extended into the floodplain using LiDAR and offline storage reservoirs were schematised where appropriate using the 12D Model. The ISIS model was developed for fluvial defended scenario events. A further GIS exercise was undertaken to produce certain return period’s undefended flood extents. Flood mapping provided within the River Brent Flooding Study, shows the London Borough of Ealing being influenced by flooding from the River Brent. 2.11 The hydrological modelling stage of the project was used as an input into the hydraulic model. The Brent Hydrological model was built in FRQSIM (Environment Agency, 1997) and six gauges throughout the catchment were used to develop the hydrological inputs. 2.12 Results from the hydrological and hydraulic modelling were used to inform the EA Flood Zones (Appendix C), in conjunction with historic flooding information for creation of EA Flood Zone 2. The results of the study have been used to form the latest EA Flood Zone Maps. The mapping study does not cover every watercourse in Ealing, therefore there are areas where the generalised JFLOW model outlines have been used (e.g. Park Stream).

Ealing Council Page 5 December 2007 London Borough of Ealing SFRA

3. CATCHMENT DESCRIPTION

The Study Area 3.1 The London Borough of Ealing covers an area of approximately 55.36km 2 in and includes a combination of land uses which include: • Residential areas of Ealing, , , and Acton; • ; • Thames Valley University Ealing campus; • Ealing Film Studios • Infrastructure including roads, 3 Underground Lines (Central, Piccadilly and District Lines) and National Rail Lines; and • Areas of designated parkland. 3.2 The River Brent, , River Thames (and its tributaries) and the Grand Union Canal pass through or adjacent to the Borough. Appendix B9 identifies the watercourses present within the study area.

Watercourses in the Study Area The River Brent 3.3 The River Brent, often termed the Brent, has an approximate catchment area of 168 km 2 which is a tributary of the River Thames. It runs for approximately 18 miles in a southerly direction towards its confluence with the River Thames in Brentford (NGR 518160 177370). 3.4 The River is formed by the confluence of the Silk Stream, Dollis Brook, Decoy Brook and a number of unnamed streams. Downstream of the Brent Reservoir, also known as the Welsh Harp Reservoir, the Brent flows through , past the junction of Harrow Road and North Circular Road, and behind factories on the north side of North Circular Road till it reaches the Grand Union Canal Aqueduct at the boundary to the London Borough of Ealing study area. 3.5 Within the Study Area the Brent flows parallel to Queensbury Road, before passing an impressive four-arch brick viaduct carrying the Piccadilly Line Underground. After the crossing by Ealing Road, the Brent flows into the industrial estate which occupies the site of the old sewage works. From , the river turns west under the A40 Western Avenue and through Brent River Park for two miles till it reaches Greenford. The Brent then flows through the Greenford Bridge to a mile away across the fields, and turns in an easterly direction around St. Mary’s Church before turning south and under the Great Western Railway at Warncliffe Viaduct. 3.6 A mile further downstream, the River Brent is joined from the west by the main line of the Grand Union Canal. From here, the Brent is canalised – the river and canal pass through Osterley and a further three locks before joining the tidal River Thames at Brentford Tidal Lock a mile upstream of Bridge.

Ealing Council Page 6 December 2007 London Borough of Ealing SFRA

Coston’s Brook 3.7 The Coston’s Brook is a tributary of the River Brent. 3.8 Its source can be found in two places, the western branch forming beyond Belvue Park in Northolt and the eastern branch in the Ealing Northern Sports Centre, which join together near the Greenford Road roundabout under the A40. The Brook then travels south meeting the Brent just before the Brent changes course from a westerly direction to southward. 3.9 The Coston’s Brook is culverted along the majority of its course as a result of development in the study area over the last century.

Dormers Wells 3.10 The Dormer’s Wells Stream is a tributary of the River Brent. 3.11 Its source lies within the Dormer’s Wells Golf Course where several smaller streams combine into a large pond to the west of Greenford Road. The stream then travels under Argyle Road before joining the Brent by Brent Lodge Park.

Perivale Park Brook 3.12 The Park Brook is a small tributary of the River Brent. 3.13 Its source lies somewhere within the Perivale Industrial Park. It then travels south, culverted much of the way until it joins the Brent to the north of Gurnell Swimming Pool.

Twyford Abbey Ditch 3.14 The Twyford Abbey Ditch is a small tributary of the River Brent. 3.15 The ditch/stream forms at Twyford Abbey Farm’s Pond (Mapping shows this now to be a Brewery). The stream then flows north joining the River Brent slightly downstream of the Grand Union Canal Aqueduct. 3.16 The stream is culverted for the majority of its course as it travels under a School and the North Circular (A406).

Osterley Park Boundary Stream 3.17 The Ostreley Park Boundary Stream is a tributary of the River Brent and is classified by the EA as a Main River. 3.18 Its source is located in . It forms the northern boundary of and the southern boundary of the study area. 3.19 The stream travels east forming ponds along the way and flowing under Windmill Lane before joining the River Brent at Wyncote Farm. 3.20 The stream forms a section of the study area’s southern boundary.

Yeading Brook 3.21 The Yeading Brook is tributary of the Crane and is approximately 25.8km in length. The Crane classified by the EA as a main river and is located to the west of the Study Area. 3.22 It runs along a short length of the study area’s western boundary. 3.23 There is currently a project underway to improve the flood mapping on the Yeading Brook as part of the SFRA. The SFRA is expected to be completed in March 2008, and thus the results should be included in the first update of the Ealing SFRA.

Ealing Council Page 7 December 2007 London Borough of Ealing SFRA

The Grand Union Canal 3.24 The Grand Union Canal came into being in January 1929 from an amalgamation of several different canals and was later extended in 1932. It was formed from the amalgamation of 11 different canals connecting London to Birmingham, stretching 135 miles and has 160 locks. 3.25 The London end of the Grand Union Canal begins in Brentford on the River Thames from where the canal follows the engineered course of the River Brent. The double Thames Lock at Brentford acts as a demarcation point between the Thames and the Grand Union Canal/River Brent. From the Thames Lock, the canal and River Brent are one and the same, and the waterway is semi-tidal until the double Gauging Lock at Brentford. Just upstream of this lock is a large canal basin, now known as Brentford Lock, from which the canal continues to follow the course of the River Brent through two more locks before entering the London Borough of Ealing. 3.26 The river and canal part company at the base of the Hanwell flight of locks (514710, 179770), before two more locks take the canal to Norwood Green. It then heads west through Southall. At Bulls Bridge (three miles to the west of Norwood and on the Borough’s border) the arm branches off to the north whilst the western branch continues into Hayes, leaving the London Borough of Ealing. 3.27 The Paddington branch forms the boundary between London Borough of Ealing and the London Borough of Hillingdon until Greenford where the boundary moves west whilst the canal continues north. The canal begins to turn eastward in Northolt before leaving the study area just to the east of .

Lost Watercourses

3.28 Within the London Borough of Ealing study area, as within the rest of London, there are a number of watercourses that have been culverted. Many of the smaller watercourses/open drains were culverted during the early 1900s to provide space for development to occur. 3.29 These watercourses would previously have had other streams feeding into them, which unfortunately would be unable to do so post ‘culverting’. In instances when the study area is exposed to large rainfall events, this obstruction in the natural flood path might contribute to localised flooding. In many instances, there are buildings above the culverts and both the public and landowners are unaware of their presence. 3.30 A review was undertaken of historical maps of the London Borough of Ealing to determine location of the culverted watercourses. A number of small watercourses were found to have been developed upon over the years within the study area, but it should be noted that this is unlikely to be a complete list of all the culverted watercourses throughout the study area. Most of the watercourses within the study area were found to be partially culverted, however the water courses listed below have been found to be culverted in their entirety and are not apparent either on mapping or on the ground: o Stamford Brook (Western Branch) – Flowed from the west under Stamford Brook Underground Station and was culverted in 1900 having been used as a sewer over the previous century. It is the confluence of three smaller streams (Bollo Brook, The Warple and another unnamed stream) and flows into the Creek (now also culverted) which then eventually discharges into the Thames.

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o Bollo Brook – rose near Fordhook () and flowed southward under Lane at Bollo Bridge before flowing alongside Bollo Lane before entering . The Brook appears to have been culverted between 1890 and 1910. o The Warple – rose at the top of Horn Lane and flowed through Springfield (now Springfield Gardens) on its way to The Steyne (an old Mill), continuing on to Acton Vale and Warple Way. The majority of this stream was culverted in the early 20th Century but a stretch of The Warple was still open along Horn Lane up until the mid 1960s. It is believed that this stream might still link into the Surface Water Pumping Station on Warple Way. o Unnamed Stream – rose close to and flowed down Old Oak Common Road and Old Oak Road. This appears to have been culverted between 1890 and 1910. o Other smaller watercourses were seen on historical maps as connecting the various Fish Ponds which seemed to be scattered around the Borough. An example of this was along the eastern edge of Lammas Park which then travelled south to Little Ealing where it met a Fish Pond, now Birkbeck Road and Radbourne Avenue. Another stream was identified at the eastern side of the Cemetery next to , forming part of the southern boundary of the study area.

The River Thames 3.31 The River Thames is not included in this study area but is included in this study due to its proximity and influence on other watercourses within the study area. The River Thames is widely referred to as the Thames. The natural floodplain of the river extends to the southeastern corner of the study area. From the river’s source to the sea, the Thames travels a distance of over 320 km. At (about 90 km from the sea) the river begins to exhibit influence from the North Sea. This tidal stretch of the river is known as The Tideway. Topography 3.32 Light Detection and Ranging Data (LiDAR) was supplied by the Environment Agency for the London Borough of Ealing, depicting the topography of the Borough to a 0.15cm RMSE (Root Mean Squared Error) vertical resolution. The LiDAR highlighted an obvious depression in the landscape where the River Brent flows through the Study Area. The river and its surrounding low areas are approximately 20m below the surrounding high ground with a gentle slope down towards the banks of the river. 3.33 The highest point within Study Area is located at Horsenden Hill (Approximate NGR - 516167, 184358) at approximately 85m AOD. Other high areas include Northolt (approximately 55m AOD), Greenford (approximately 40m AOD), West Acton (approximately 60m AOD) and Ealing Broadway (approximately 45m AOD). 3.34 The low lying areas are situated adjacent to the River Brent with the exception being South- (South Acton, Acton Green, East Acton and Bedford Park area). This area, at approximately 4m AOD, is significantly lower than its surrounding areas to the north and west. The LiDAR also shows that parts of the Central, Piccadilly and National Rail Lines in the East of the Study Area lie approximately 5m below the adjacent land.

Ealing Council Page 9 December 2007 London Borough of Ealing SFRA

Regional Geology 3.35 The study area geology was determined using 1:50,000 geological maps, sheets 256 and 270 of North and South London, from the Geological Survey of Great Britain (England and Wales) 3.36 Central Ealing and Acton are shown to sit upon a thin layer of Lyndon Hill Gravel and Langley Silt. A thick layer of London Clay (up to 75m deep in places) underlies the whole study area, beneath which is a 20m thick band of Group Clay followed by a bedrock of Chalk. 3.37 A mixture of Kempton Park, Taplow Gravels and Alluvium deposits underlie the immediate area surrounding the River Brent.

Groundwater 3.38 Groundwater datasets were supplied by the Environment Agency for the London Borough of Ealing SFRA. 3.39 The datasets also show that a majority of groundwater in London falls within the chalk formation and a rise in Groundwater within the chalk aquifer presents a potential problem with the foundation of structures and buildings across London.

Ealing Council Page 10 December 2007 London Borough of Ealing SFRA

4. FLOOD WARNING AND EMERGENCY PLANNING

Emergency Planning 4.1 Discussions with the Emergency Planner for Ealing Borough Council have been undertaken to inform this SFRA. Whilst Ealing Council are not responsible for addressing the causes of flooding, they do play an active role in addressing the consequences of flooding and response to an event. 4.2 Currently Ealing does not have a Flood Plan for the Borough, however an emergency response is advised by the London Wide Strategic Plan (see section 4.8) as flooding in Ealing is not considered a significant problem in terms of Emergency Planning. This position is currently under review and in future Ealing may develop a catchment specific Flood Plan. 4.3 Ealing is looking to develop an “inform and warn” network. This involves two different responses, a slow time response and fast time response. The slow time response is a home flood plan-available on the Council Website, intended to inform residents. The fast time response is what the council intend to do during a flood event. During a flood event Ealing Council will instruct the Ealing Voluntary Community Service (EVCS) to distribute the information to relevant groups. It is expected that within the next 12 months this network should be operational. At the moment research is being carried out to identify relevant at risk/vulnerable community groups that might have difficulty receiving information. This SFRA will be used to inform on the development of the network. 4.4 Brent Council store and distribute sand bags under a “mutual aid scheme” for all surrounding boroughs including Brent, Harrow, Hillingdon, , and Ealing. Unfortunately, these sandbags need replacing and replacement of the bags is under review as it is unknown if the current stock is sufficient. 4.5 Key messages from the Thames Catchment Flood Management Plan (CFMP), state the need to use the SFRA to inform emergency planning process and also to educate the local people to improve flood awareness.

Key Emergency Documents 4.6 The documents described below provide Emergency Planning Guidance for London and thus the study area. These documents are consulted by relevant key responders in an emergency situation.

Civil Contingencies Act 4.7 The Civil Contingencies Act states that Category 1 are those organisations at the core of the response to most emergencies (e.g. emergency services, local authorities, NHS bodies). Category 2 responders are less likely to be involved in the heart of the planning work but will be heavily involved in incidents that affect their sector. Examples include Health and Safety Executive, transport and utility companies). Category 2 responders have a lesser set of duties - co-operating and sharing relevant information with other Category 1 and 2 responders

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The London Flood Response Strategic Plan 4.8 This plan is produced by the London Resilience Forum and the current report is dated March 2007. The purpose of the plan is to set out the principles that govern the multi- agency response to a significant flood in London; specifically the area of the London regional government which equates to that of the Authority. The Plan sits alongside the Strategic Emergency Plan (SEP) for London and the relevant emergency plans of Category 1 responders and other organisations concerned with supporting the response of the community to a flood. The main objective of the plan is to ensure a coordinated response to a flood, which will protect life and well-being with the mitigation of property and environmental damage as a strong supporting objective.

West London Local Resilience Forum Community Risk Register 4.9 The Civil Contingencies Act 2004 places a Legal duty on Category 1 responders to produce a Community Risk Register. The Risk Register assesses the risk of serious emergencies and it is the first step in the emergency planning process to identify risks applicable to West London and therefore Ealing and then plan accordingly. The West London Local Resilience Forum, includes representatives of Category 1 and Category 2 responders within the West London region. It also includes representatives from agencies not included in the Civil Contingencies Act 2004 who are considered essential local planning partners such as the military and the voluntary sector.

Strategic Emergency Plan 4.10 The Strategic Emergency Plan aims to give a comprehensive overview of London’s co- coordinated responses to emergencies by summarising six key plans and procedures produced by the London Resilience Partnership. The plans summarised are, the London Command and Control Protocol, Media/Public Information Protocol, Mass Fatality Plan, Large Scale Evacuation, Site Clearance, Disaster Fund. The aim is to provide an overview of the response to ensure understanding within the London Resilience Partnership and to outline roles and responsibilities of agencies under the different plans.

Flood Warning 4.11 The Environment Agencies main flood warning responsibilities are to provide information to the relevant authorities, and public, and to carry out duties along the river channels to remove blockages, maintain defences etc. 4.12 The Environment Agency operate a formal flood warning alert system, which is used to indicate the imminence and severity of a potential flood. 4.13 There are 4 levels of Flood Warning- Flood Watch, Flood Warning, Severe Flooding Warnings and the All Clear. A flood watch is issued when river levels are expected to overtop river banks resulting in flooding of low-lying land. Flood watches can be issued on predicted heavy rainfall where the prediction has a high confidence associated with it. Flood Watches are issued over a catchment wide area. 4.14 All professional partners receive Flood Watches, but members of the public do not receive Flood Watches unless they have specifically requested them. Anyone registered to receive warnings will receive Flood Warnings, Severe Flood Warnings and the relevant All Clear. A Flood Watch is issued where flooding of low-lying land and maybe some garden flooding is expected. If property flooding is expected the Environment Agency would issue a Flood Warning, to a smaller more specific Flood

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Warnings Area. The Council receive advance notification for of a possible significant event from the MET Office in the form of severe weather warnings. The Council is also notified when the Sluices in Brent Reservoir have been opened. Ealing Study area 4.15 Flood warnings are issued directly to Ealing Council by fax to the contact details supplied by the local authority. Flood warnings can also be issued by telephone (landline and mobile), SMS, email and pager. All warnings are issued automatically via the Environment Agency Flood Warnings Direct System. The target is to issue warnings 2 hours before the flooding starts; however this is often unachievable in urban catchments owing to the “flashy” nature of the rivers. There is no specific first contact, as the system delivers the warning message simultaneously to all registered. 4.16 There are currently approximately 470 properties registered to receive warnings within the Ealing Borough Councils boundary. Appendix E identifies areas within the EA Flood Warning Register. The mapping also identifies the location of vulnerable institutions and emergency services and historically flooded outlines. 4.17 During a flood event, the Police and Fire Services notify the council. The Ealing Council Emergency Planners are on call 24 hours a day in case of an incident.

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5. ASSET AND STRUCTURE DATA Flood Defences Protecting the Study Area 5.1 This section describes the defences located within the study area, with reference to the information supplied by the National Flood and Coastal Defence Database (NFCDD) and other information supplied by the EA. Appendix B2 gives further details of these defences including Asset Reference Numbers. 5.2 A list of the datasets relevant to the assets and structures within the study area collated during the production of the SFRA are summarised in Table 5-1.

Table 5-1: Sources of Asset and Structure data DATE OWNER / DATA DESCRIPTION PROVIDED AUTHOR Study undertaken by Atkins/Jacobs/Peter Brett Associates River Brent Flood Study for the Environment Agency. The Atkins/Jacobs/Peter Final Hydraulic Modelling 4 October 2007 report outlines the hydraulic Brett Associates Report modelling used to generate floodplain maps. The iSIS model used in the Lower Lower Brent iSIS model Brent Model by PBA/Jacobs/Atkins 13 August 2007 Environment Agency and results as part of the River Brent Flood Mapping Study River Defence and Asset NFCDD GIS database information 24 September 2007 Environment Agency Information within the study area.

Fluvial 5.3 By the time the River Brent enters the London Borough of Ealing it runs within a concrete channel. This formalised section runs for approximately 2km until the River Brent passes under the A40. There are 7 identified defences along this stretch that, according to the NFCDD, vary in height from 1.2m to 3.5m above channel bed level. The database also indicates that these defences are privately maintained, with the exception of the left bank wall upstream of the bridge under the A40 which is maintained by the EA. The NFCDD states that all the flood walls within the Study Area are designed to a 1 in 50 year standard with an allowance for freeboard. 5.4 The culvert under the A40 is registered within the NFCDD as a defence. It is a 0.45m diameter 50.4m long, precast concrete pipe with concrete headwalls on both the upstream and downstream ends. This culvert is maintained by the Environment Agency. 5.5 Past the A40, the Brent meanders naturally through Brent River Park for 3km until it reaches Greenford. This part of the river, as it passes through the southern boundary of Greenford Golf Course, was dredged deeper in the 1960s and a control weir built, to reduce the risk of flooding to the area around Costons Lane. 5.6 The Brent turns south at Costons Lane, where a 450mm thick brick flood wall has been erected between two earth embankments. The wall is privately maintained and was built where the road cuts through the natural embankment. The wall is recorded on the NFCDD as being 1.1m tall with a crest level of between15.23-15.25mAOD and runs for 56.1m along the right hand bank of the Brent. 5.7 The next recognised flood defence lies at the intersection with the Grand Union Canal (GUC) in the form of a privately maintained bank protection for the left bank of the Brent. The bank consists of a brick wall which begins slightly upstream of the intersection protecting property gardens on Green Lane and continues along the

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Brent/GUC until it reaches the Osterley Lock. The wall is approximately 3.5m tall, with a recorded defended height of 1.2m and a crest level of 10.82mAOD. 5.8 The last recognised protection is on the right bank of the Brent/GUC and consists of a privately maintained concrete piled bank protection that runs from the intersection of the Brent and the GUC to the Osterley Lock. The average height is recorded within the NFCDD as being 0.6m above water level. Tidal 5.9 Despite the River Thames not flowing through the Study Area, the defences along the River Thames at Chiswick defend parts of the study area against the risk of tidal flooding from the Thames. The EA Flood Zone Maps (FZM) show (Appendix C) that part of the Study Area, namely the southeast corner including Acton Green and Bedford Park, are located within Flood Zones 2 and 3 if the presence of flood defences is ignored. 5.10 The defences along the tidal Thames in this area are raised, man-made and privately owned. The defence levels are 5.94m AOD upstream of the and 5.54m AOD downstream of the study area. These defences are regularly inspected by the Environment Agency to ensure that they remain fit for purpose. 5.11 The Thames is defended along this section to a standard of the 1 in 1000 year return period. The defences protect against a tidal flooding event of this magnitude occurring up to the year 2030. This means that the ACTUAL risk of flooding to the area is negligible and the flood risk posed by tidal inundation is considered to be RESIDUAL. 5.12 After 2030 the standard of protection can be expected to decrease over time. However, the EA’s Thames Estuary 2100 project is currently considering options to manage flood risk in the Thames estuary up to 2100. Areas Benefiting from Defences (ABDs) 5.13 Appendix G of the SFRM River Brent Flood Study’s Hydraulic Modelling Report (Jacobs et al 2007) identifies ABDs for the Brent during both 1 in 100 year and 1 in 1000 year return period flood events along the Brent. The Areas Benefiting from Defences for the Brent Mapping Study were created from a GIS exercise spreading the 100 year and 1000 year water levels behind the flood defences. The defences were not removed from the hydraulic models. There are no areas within the London Borough of Ealing which benefit from any of the River Brent Flood Defences during a 1 in 1000 year event as all the defences (NFCDD) are designed to a 1 in 50 year standard or lower. 5.14 The model identified the area around Green Lane as being an ABD during a 1 in 100 year event stating that the Green Lane Flood Wall protects housing behind the southern corner of the allotments. However subsequent consultation between the EA and Jacobs has resulted in this area no longer being counted as an area benefiting from defences for the 100 year return period event and above. The model had taken the wall along the southeastern edge of the allotments, which is not the Green Lane Flood Wall, as defending the properties along Green Lane, but due to this being a private wall with an unknown maintenance programme. It has been agreed with the EA that this wall should not be included as a flood defence. 5.15 There is a flood wall on Costons Lane which protects the properties behind up to a 50 year return period storm event, with a crest level of between 15.23-15.25mAOD. The SFRM River Brent Flood Study Report identifies the housing beyond the southern corner of the allotments at the end of Green Lane as an ABD but it appears that the flood wall is not the same structure as the one identified within the NFCDD and is in fact the structure described within section 5.13. It does not defend Costons Lane

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beyond the 50 year event and therefore is not classified as an ABD according to the EA definition. Flooding due to overtopping of the defence is likely to occur in events greater than the 1 in 50 year return period storm, therefore any redevelopment within this area will need to meet the requirements of PPS25 as far as appropriate proposed use (Flood Zone 3a), refer PPS25 Table D1. 5.16 The area of Bedford Park and South Acton are also ABDs, as they are defended by tidal defences along the Thames. The tidal defences along this stretch of the Thames are designed to withstand a 1 in 1000 year event. Key Structures 5.17 There are a number of structures spanning the watercourse including vehicular bridges, pedestrian bridges, pipe bridges, aqueducts and railways which are not formal flood defences but which effect the hydraulic capacity of the river channels. Not all the structures spanning the watercourse were included within the Hydraulic Model within the River Brent Flood Study (Jacobs et al, 2007). Structures that were excluded from the model were generally ones that were assessed as having no affect on the watercourse due to their size. However the hydraulically significant structures were included as were structures located in areas prone to flooding. 5.18 Structures that appear to restrict the flow of the Brent have been identified as key structures that may cause flooding problems. These structures are identified and further details about them are given within Appendix B9. 5.19 Structures were identified as restricting the flow if they were submerged, i.e. the flood level was higher than the structure’s soffit level and was considered to be overtopped during events where the flood level rose above the crest level the structure.

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6. FLOODING OVERVIEW AND RECORD OF FLOODING Sources of Flooding 6.1 Flooding is a catchment specific issue and a result of a number of different, or a combination of flood mechanisms. Flooding mechanisms are influenced by catchment geology, dominant land use and can range from groundwater dominated flooding in an area with an upper chalk layer to flash flooding caused by overwhelming of urban drainage system by event based runoff. 6.2 Flooding is also dependent on the catchment response time due to soil saturation and is likely to be different for a single large event in comparison to a series of successive smaller storms. The different forms of flooding likely to affect the study area are discussed in detail in subsequent sections of this report and are:  Drainage Flooding (Surface Water, Foul Water and Combined Sewer Flooding);  River/Fluvial Flooding;  Tidal Flooding;  Groundwater Flooding  Flooding from other sources (GUC, Brent Reservoir) Also discussed within these sections are any documented records of historical flooding. 6.3 Planning Policy Statement 25 states that flood risk should be taken into account at all stages in the planning process to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas at highest risk. 6.4 The joint DEFRA and Environment Agency flood and costal Erosion Risk Management R&D programme report on the impact of flooding on urban and rural communities (December 2005) identified that the costs associated with flooding are likely to be economic, social and community based. 6.5 The economic costs associated with flooding depends on the type of flooding, for example failure of a drainage system is likely to have highly localised costs whist river and tidal flooding may have a lower impact but effect a larger area of the catchment. PPS25 identifies that the consequences of river and tidal flooding varies with the level of risk but also infrastructure affected. 6.6 The social and community costs of flooding are often less tangible than the economic losses but are likely to have a significant impact on an effected area. The social impacts of flooding cause distress within the community as it poses a risk to community health, potentially damages property and personal possessions.

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7. DRAINAGE AND FLOODING 7.1 The London Borough of Ealing is an established catchment with five key urban areas, Northolt, Greenford, Southall, Ealing and Acton that are all heavily developed with a mixture of residential and light industry. The floodplain along the River Brent and the Grand Union Canal has been largely undeveloped. 7.2 Discussions with Ealing Council and research into their supplementary planning guidance (2004) illustrate that prior to development of sewerage treatment plants such as Mogend, rivers and streams were used as outfalls for both surface water and foul sewerage. 7.3 The study area is currently serviced by surface water, foul and combined sewer drainage managed by Thames Water. Drainage flooding within the study area can be attributed to either a lack of capacity or due to infrastructure failure. 7.4 As mentioned earlier, flooding within the study area is likely to have economic, social and community impacts. In particular, flooding from combined surface water and foul water sewers can be expected to be particularly damaging due to the additional costs associated with public health impacts due to contaminated water. 7.5 Appendix B4 illustrates the total number of properties flooded from overloaded sewers from 1997 to 2007 within the catchment area. It shows that areas in the centre of Acton, extending north from the High Street, have recorded 231 properties that have flooded from overloaded sewers. Thus is most susceptible to flooding from sewers. There is a stormwater pumping station on Canham Road in Acton that is owned and operated by Thames Water. The facility also receives surface water runoff from other areas including Hammersmith and Chiswick. The pumping station was previously a mini sewerage works and upto the 1970s and was known as the Acton Strategic Sewage Pumping Station. The pumping station has six storm water storage tanks that provide additional storage for surface water during storm events. 7.6 Should capacity be reached, there is a storm outlet channel that discharges into the Thames. The last time capacity was reached was on 20th July 2007, when most of west and south London experienced heavy rainfall. Thames Water is aware that capacity is reached often and thus the system discharges into the Thames however, as it does not cause any immediate problems for the area, this practice is acceptable. 7.7 The topography (Appendix B8) shows the ground levels slope in a southerly direction providing a clear flow path for water to be conveyed down through Acton and flood properties in the area. Also, South Acton has significantly lower ground levels than other parts of the study area, resulting in an increased likelihood of flooding. 7.8 The flooding information has been provided by Thames Water and is postcode based. It is important to note that the pattern of flooding is likely to be influenced by the formal drainage systems servicing the area.

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Previous Flood Risk Assessments (FRA’s) 7.9 The EA provided a list of site specific FRA’s completed for the study area and are listed in Table 12-1 below The FRA’s were used to assess any specific issues that might arise.

Table 7-0 Previous Flood Risk Assessments

FRA DATE

Rockware Avenue, Greenford, Greater London 2005 112 St Margaret's Road, Hanwell, W7 2PL 2004 Brewery, Twyford Abbey Road 2004 West World, Hanger Lane 2004 St Benedict's School, Ealing, London 2004 , Ealing, London 2004 Proposed High School, 2005 Trumpers Way, Hanwell, London, W7 2006 Atlas Road, Park Royal 2006 Cambridge Road, Hanwell 2006 Atlip Road, Alperton Village 2006 , Origin, Former Guinness Brewery Site 2007

7.10 Analysis of the FRA’s showed that on average the Greenfield runoff rate was calculated as being approximately 6/l/s/ha and that SUDS techniques used, if any, were compatible with the site geology of London Clay.

Surface Water Sewer Flooding

7.11 Discussions with Ealing Council highlighted that whilst it is important to understand impacts of flooding from other sources, flooding from surface water is considered to be a significant issue for the study area. 7.12 A list of the datasets relevant to surface water flooding covering the study area collated during the production of the SFRA are summarised in Table 7-1.

Table 7-1: Sources of Surface Water flooding data DATE OWNER / DATA DESCRIPTION PROVIDED AUTHOR Presents the findings of scoping River Brent Flood Risk study undertaken as part of the Management Strategy Environment Agency’s Strategic Strategic Environmental 8 October 2007 Environment Agency Environment Assessment (SEA) Assessment Scoping process for the River Brent Flood Report Risk Management (FRM) Strategy. DTM of the London Borough of LiDAR 1 October 2007 Environment Agency Ealing GIS layers of Borough outline, Digital Catchment Aquifers, catchments, depth to 24 September 2007 Environment Agency Information Groundwater, Geology Solid and Drift, Ealing Flood Zones. Thames water records of surface History of flooding water flooding (foul and combined) 10 October 2007 Thames Water in the London Borough of Ealing.

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Information of incidences of previous Historical flood flood events within the London 22 August 2007 Ealing Library records/data Borough of Ealing. 1:10,000 mapping of Ealing Borough Mapping and postcodes 17 August 2007 Ealing Council and Postcode data Received from Ronnie Stevans information on the operational Welsh Harp Reservoir 24 September 2007 British Waterways aspects of the reservoir along with Emergency Planning. Information about the Canal received from Philip Adshed, Grand Union Canal 12 October 2007 British Waterways covering risk of breach and maintenance. Flood Zones GIS layer of EA flood zones 13 August 2007 Environment Agency 7.13 Surface water flooding occurs when runoff from high intensity storms (often with a short duration) is unable to be infiltrated into the ground or be drained by drainage systems. The pathway for surface water flooding is dictated by the blockage or overflows of the drainage system and potential failure of sluice outfalls and pump systems. 7.14 Surface water flooding can potentially occur anywhere throughout the study area. It is more likely to be severe and of longer duration in low lying areas but local problems may result in all areas as a result of very heavy rain or infrastructure failure. 7.15 A percentage of the surface water in the study area does not drain directly into the surface water sewer but instead drains directly into the Grand Union Canal and the remaining runoff is likely to drain into the Brent and then ultimately the Thames. 7.16 An increase in discharge to the Brent may increase the water levels as it is generally accepted that a positive drainage system associated with development increases the peak flow rate from the development area into the receiving watercourse and thus impact the water levels. 7.17 The total numbers of properties flooded by overloaded surface water sewers in the last ten years are mapped in Appendix B5. 7.18 The records show that majority of flooding from surface water is in South Acton, Acton Green and . The ground levels in these areas range from approximately 5m AOD to 30m AOD with a majority of the flooded areas lying between 10m AOD and 15m AOD. 7.19 Within Ealing, a total of approximately 30 properties have been flooded as a result of surface water sewers in the past ten years.

Sustainable Drainage Systems (SUDS)

7.20 Sustainable drainage systems endeavour to mimic the natural movement of water over the land and aim to control runoff at source. SUDS have a number of benefits including reducing flood risk, improving water quality and often provide attractive features, enhancing development quality. The European Water Quality Framework Directive requires sustainable management of water resources and protection of water quality. SUDS offer an integrated approach that could play a part in delivering these requirements. 7.21 As land is developed, natural drainage patterns are disrupted. In the majority of cases development will result in an increase in the proportion of impermeable cover. Traditionally drainage systems have removed rainfall from developments as quickly as possible. This causes higher flow rates in receiving watercourses .Through this and the reduction in the time it takes for rainfall to reach rivers, flooding further downstream can result.

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7.22 SUDS fall into three broad groups based on their primary function: (a) Reduce the quantity of runoff from the site (source control techniques) (b) Slow the velocity of runoff to allow settlement, filtering and infiltration (permeable conveyance systems); and (c) Provide passive treatment to collected surface water before discharge into groundwater or to a watercourse (end of pipe systems) 7.23 Although many SUDS techniques can provide all three elements, the advantages and disadvantages of different surface water management techniques should be considered for each development site. When doing this consideration should be given to the particular setting and especially the ground conditions. Some of the benefits that may be offered by SUDS include: (a) Protection and enhancement of water quality and biodiversity; (b) Maintenance or restoration of natural flow regimes in streams (c) Protection of people and property from flooding, now and in the future (d) They can allow natural groundwater recharge where this is considered appropriate. 7.24 SUDS can be designed in a way that is sympathetic to their environmental setting and the needs of the community. SUDS include a wide range of techniques including permeable pavements, green roofing, infiltration basins, filter drains, swales, filter strips, detention basins, retention ponds, and wetlands. 7.25 The geology within the London Borough of Ealing is dominated by London Clay upto 75m deep in places. In places such as Acton and Ealing there are gravel formations to a maximum depth of 15m The clay strata prevents any infiltration to the chalk aquifer beneath, as a result SUDS infiltration techniques may not be achievable within these areas without further investigation. Further investigation would take the form of site specific borehole logs or trial pits that would identify depths and rock types that could be suitable for infiltration SUDS any infiltration potential should then be verified by soakage tests before infiltration based SUDS are implemented. It is important to note that there are a number of SUDS options that do not involve infiltration techniques and these can be seen Table 7-2. 7.26 The following table, taken from the SUDS Manual, 2007 2 , illustrates site characteristics that may restrict or prelude the use of a particular SUDS technique. This table is site specific and should be informed by borehole testing.

2 CIRIA, 2007, The SUDS Manual, London. UK.

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Table 7-2: Site Characteristics decision matrix Technique SUDS Group Soils Area Draining to a singleSUDS component Minimum depth to groundwater Available Head Available Space SiteSlope

Impermeable Permeable 0-2ha >2ha 0-1m >1m 0-5% >5% 0-1m 1-2m Low High Retention Retention pond Y Y1 Y Y5 Y Y Y Y Y Y N Y Subsurface Y Y Y Y5 Y Y Y Y Y Y Y Y storage Wetland Shallow Y2 Y4 Y4 Y6 Y2 Y2 Y N Y Y N Y wetland Extended Y2 Y4 Y4 Y6 Y2 Y2 Y N Y Y N Y detention wetland 2 4 6 2 2 Pond/wetland Y Y4 Y Y Y Y Y N Y Y N Y Pocket Y2 Y4 Y4 N Y2 Y2 Y N Y Y Y Y Wetland Submerged Y2 Y4 Y4 Y6 Y2 Y2 Y N Y Y N Y gravel wetland

Wetland Y2 Y4 Y4 Y6 Y2 Y2 Y N Y Y N Y Channel Infiltration Infiltration N Y Y N N Y Y Y Y N Y Y trench Infiltration N Y Y Y5 N Y Y Y Y N N Y Basin Soakaway N Y Y N N Y Y Y Y N Y Y Filtration Surface sand Y Y Y Y5 N Y Y N N Y N Y filter Sub-surface Y Y Y N N Y Y N N Y Y Y sand filter Perimeter sand Y Y Y N N Y Y N Y Y Y Y filter Bioretention/filt Y Y Y N N Y Y N Y Y N Y er strips Filter trench Y Y1 Y N N Y Y N Y Y Y Y Detention Detention Y Y1 Y Y5 N Y Y Y N Y N Y basin Open Conveyance Y Y Y N N Y Y N3 Y N N Y Channels swale Enhanced dry Y Y Y N N Y Y N3 Y N N Y swale Enhanced wet Y2 Y4 Y N Y Y Y N3 Y N N Y swale Source Green roof Y Y Y N Y Y Y Y Y Y Y Y Control Rain water Y Y Y N Y Y Y Y Y harvesting Permeable Y Y Y Y N Y Y N Y Y Y Y pavement

=Yes N=No

1 with linear

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2 with surface baseflow 3 unless follows contours 4 with linear and constant surface baseflow, or high ground water table 5 possible, but not recommended (implies appropriate management train not in place) 6 where high flows are diverted around SUDS component 7.27 Much of the London Borough of Ealing is already significantly developed. Opportunities to implement SUDS should not be restricted to new development but should be considered when redeveloping existing sites. 7.28 Developers should consult the Ealing Council, the Environment Agency, and the sewerage undertakers at the earliest stage of the development process to establish the best solution for the particular development. The Environment Agency advise on the widespread adoption of sustainable drainage systems techniques that would see a long-term improvement in the quality of rivers and the reduction in flood risk. 7.29 Relevant documents that should be consulted for further information include; • CIRIA 697 (The SUDS Design Manual) • CIRIA 53 (SUDS Best Practice Manual) • CIRIA 609 (SUDS-hydraulic, structural and water quality advice) • Environment Agency Thames Region-DRAFT Sustainable Drainage Systems, A Practical Guide, October 2006. 7.30 Source Protection Zones (SPZ) show the risk of contamination that might cause pollution in the area. The closer the activity the greater the risk. Inspection of the SZP maps of the London Borough of Ealing show that the study area does not have any source protection zones. This can largely be attributed to underlying London Clay geology.

Foul Water Sewer Flooding 7.31 Appendix B6 illustrates the total number of properties flooded from foul water sewers in the past ten years. Areas most at risk appear to be Acton, Southall and Northolt. 7.32 Analysis of the topography indicates that ground levels are quite varied between the three areas. The ground levels for areas that flood within Acton range from approximately 5m AOD to 30m AOD, 30m AOD in Southall and 45m AOD in Northolt. Flooding for foul water sewer is likely to be a result of either lack of capacity or blockage. 7.33 A total of approximately 140 properties have flooded as a result of foul water drainage systems.

Combined Sewer Flooding 7.34 A proportion of the drainage network is serviced by a combined network that accounts for foul and surface water runoff. 7.35 Appendix B7 illustrates that 135 properties, primarily in Acton, are affected by flooding from combined sewers. Historical records of drainage flooding 7.36 Historical flooding records from Ealing Council were used to identify specific areas that have experienced flooding. The flooding records dated back to the past three years prior to which the records were not detailed. In addition to identifying the sites, this section also discusses the probable cause of flooding in the study area. Specific areas affected were:

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(a) Victoria Road in has reported surface water flooding in August 2004 and minor flooding in May 2006. Victoria Road is 30mAOD whilst the surrounding ground levels are approximately 35-36mAOD. This would provide a flow route for surface water to flow towards Victoria Road where the water will then flow south. It is likely that the water will pond slightly before the bridge over the railway line by Acton Mainline Station. (b) Church Road/Ragley Close located in the centre of Acton also suffered from surface water flooding in July 2007 with three other unconfirmed reports in June and August 2005, as with Victoria Road, the ground levels for Church Road/Ragley Close (approximately 13.5m AOD) are lower than the surrounding ground levels (approximately 14.5m AOD) therefore water tends to pond in Ragley Close. (c) Ealing Council flooding records also indicated that flooding occurred at the intersection between Lower Boston Road and Road, Hanwell, in July 2007. Inspection of the LiDAR in this area suggests that Lower Boston Road slopes down towards this section and therefore in the event of surcharging, the road will provide a conveyance route towards the intersection. It is believed that should surface water ponding occur at the intersection, the majority of the surface water will eventually find its way into the Brent as the ground gently slopes down into the watercourse near the bridge, approximately 50m to the west of the intersection.

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Summary and Recommendations

General

The London Borough of Ealing is an established catchment with five key urban areas, Northolt, Greenford, Southall, Ealing and Acton that are all heavily developed with a mixture of residential and light industry. The floodplain along the River Brent and the Grand Union a) Canal has been largely undeveloped Sewer flooding information has been provided by Thames Water and is postcode based. It is important to note that the local pattern of flooding is likely to be influenced by the formal b) drainage systems servicing the area. Appendix B4 illustrates the total number of properties flooded from overloaded sewers from 1997 to 2007 within the catchment area. The southeast corner of the study area is most prone to flooding and has recorded 231 properties that have been flooded from overloaded c) sewers. Urban drainage flooding, though prevalent within the study area is not currently included in any systematic risk assessment and flood warning policy. Drainage flooding should be given serious considerations as the current drainage system is unlikely to be able to cope with an increase in development and also future storm events that may be a result of climate d) change. Developers should consult the London Borough of Ealing Council, the Environment Agency, and the sewerage undertakers at the earliest stage of the development process to establish the best solution for the particular site. The Environment Agency can advise on the widespread adoption of sustainable drainage systems techniques that would see a long- e) term improvement in the quality of rivers and the reduction in flood risk.

It is also recommended that opportunities for a joint study be explored between the London Borough of Ealing and Thames Water to monitor drainage systems in relation to flood risk f) and to identify key areas for future improvement Surface Water Sewer Flooding

Surface water flooding can potentially occur anywhere throughout the study area. It is more likely to be severe and of longer duration in low lying areas but local problems may result in g) all areas as a result of very heavy rain or infrastructure failure. A percentage of the surface water in the study area does not drain directly into the surface water sewer but instead drains directly into the Grand Union Canal and the remaining runoff h) is likely to drain into the Brent and then ultimately the Thames Within Ealing, a total of approximately 30 properties have been flooded as a result of surface water sewers in the past ten years.The total numbers of properties flooded by i) overloaded surface water sewers in the last ten years are mapped in Appendix B5. The records show that majority of flooding from surface water is in South Acton, Acton Green and West Ealing. The ground levels in these areas range from approximately 5m AOD to 30m AOD with a majority of the flooded areas lying between 10m AOD and 15m j) AOD. The geology within the London Borough of Ealing is dominated by London Clay with areas k) such as Acton and Ealing overlain by gravel formations. The clay strata prevents any infiltration to the chalk aquifer beneath, as a result SUDS infiltration techniques should be based on site specific borehole investigation and informed l) by the CIRIA SUDS Manual, 2007.

The London Borough of Ealing does not have any source protection zones and this can m) largely be attributed to underlying London Clay geology

Ealing Council Page 25 December 2007 London Borough of Ealing SFRA

Foul Water Sewer Flooding A total of approximately 140 properties have flooded as a result of foul water drainage systems and can be seen in Appendix B6 that illustrates the total number of properties flooded from foul water sewers in the past ten years. Areas most at risk appear to be Acton, n) Southall and the Northolt. Analysis of the topography indicates that ground levels are quite varied between the three areas. The ground levels for areas that flood within Acton range from approximately 5m AOD to 30m AOD, 30m AOD in Southall and 45m AOD in Northolt. Flooding for foul water o) sewer is likely to be a result of either lack of capacity or blockage. Combined Sewer Flooding A proportion of the drainage network is serviced by a combined network that accounts for p) foul and surface water runoff. Appendix B7 illustrates that 135 properties, primarily in Acton, are affected by flooding from q) combined sewers.

Ealing Council Page 26 December 2007 London Borough of Ealing SFRA

8. RIVER FLOODING 8.1 A list of the datasets relevant to river flooding covering the study area collated during the production of the SFRA are summarised in Table 8-1.

Table 8-1 Sources of River Flooding data DATE OWNER / DATA DESCRIPTION PROVIDED AUTHOR Study undertaken by Atkins/Jacobs/Peter Brett Associates River Brent Flood Study Atkins/Jacobs/Peter for the Environment Agency. The 4 October 2007 Final Hydrology Report Brett Associates report outlines the hydrology used to build the River Brent Flood Study. Study undertaken by Atkins/Jacobs/Peter Brett Associates River Brent Flood Study for the Environment Agency. The Atkins/Jacobs/Peter Final Hydraulic Modelling 4 October 2007 report outlines the hydraulic Brett Associates Report modelling used to generate floodplain maps. The iSIS model used in the Lower Lower Brent iSIS model Brent Model by PBA/Jacobs/Atkins 13 August 2007 Environment Agency and results as part of the River Brent Flood Mapping Study Presents the findings of scoping River Brent Flood Risk study undertaken as part of the Management Strategy Environment Agency’s Strategic Strategic Environmental 8 October 2007 Environment Agency Environment Assessment (SEA) Assessment Scoping process for the River Brent Flood Report Risk Management (FRM) Strategy. DTM of the London Borough of LiDAR 1 October 2007 Environment Agency Ealing GIS layers of Borough outline, Digital Catchment Aquifers, catchments, depth to 24 September 2007 Environment Agency Information Groundwater, Geology Solid and Drift, Ealing Flood Zones. River Defence and Asset NFCDD GIS database information 24 September 2007 Environment Agency Information within the study area. Tidal defence data for the Tidal Defence data for the Thames 17 October 2007 Environment Agency Thames NE region GIS layers of the River Brent Cross River Brent Cross Section and reservoir Nodes used in 16 October 2007 Environment Agency Sections the River Brent Flood Mapping study The survey used in the Brent Flood Survey Data 13 August 2007 Environment Agency Mapping Study Information of incidences of previous Historical flood flood events within the London 22 August 2007 Ealing Library records/data Borough of Ealing. Hydrochronology search database search for Ealing. Flood Zones GIS layer of EA flood zones 13 August 2007 Environment Agency 8.2 River flooding occurs when the amount of water within the banks of a river exceeds the channels capacity. Most rivers have a natural floodplain which stores the excess water when the channel is exceeded. 8.3 The causes of flooding from the surrounding rivers are likely to be; • Prolonged periods of rainfall that does not infiltrate into the ground and turn into overland runoff; • Channel/culvert blockage; • Rainfall from intense storm events; and more exceptionally • The rapid thaw of snow. 8.4 The above factors can lead to abnormally high river levels and lead to the bursting or overtopping of riverbanks.

Ealing Council Page 27 December 2007 London Borough of Ealing SFRA

8.5 Where as the majority of the River Brent floodplain outside of Ealing is heavily developed, much of the floodplain in the study area remains undeveloped. However, development in the catchment over the years, in places, encroached onto the Brent’s floodplain, and in some areas developments have taken place right up to the edge of the river channel. Historical mapping shows that the Brent’s course has been artificially altered over the years with extensive culverting of sections of the watercourse and the straightening of previously meandering sections to aid development in the area. This is particularly evident around the North Circular in the northeast corner of the Study Area and in the length of the Brent running from Costons Lane down to Dormer’s Wells, where the meanders have been removed. Hydraulically, removing a meander increases the gradient and promotes a faster flow. This action also removed valuable storage capacity but this was seen as an acceptable trade-off at that point in time. Records of Historical River Flooding Ealing Council Records of fluvial historical flooding for the study area have been used to inform this section of the SFRA. 8.6 The Brent is reported to flood regularly along its floodplain during periods of high rainfall or when the Welsh Harp Reservoir opens its flood gates to relieve pressure on the reservoir. Ealing Council Records indicate that areas previously affected by flooding from the Brent include: • Ealing Golf Course • Perivale Bridge Playing Fields • Perivale Park Golf Course • Brent Valley Golf Course Study of Flooding in the Borough (August 1977) by Rofe Kennard and Lapworth Consulting Engineers (1978) 8.7 This document reported the 16th and 17th August 1977 flood events and their effect on the London Borough of Ealing. The report highlighted that: • 68mm of rainfall fell over a period of 9hrs, although lasted 24 hrs. • There were 400 reported instances of flooding • 344 buildings flooded above air brick level • Worst affected areas were Greenford, Hanger Lane and Bridge Avenue

Table 8-2: Number of Reported instances of flooding in the London Borough of Ealing during the August 1977 severe floods

No.

Number of Houses flooded above airbrick level 344

Schools 7

Old Peoples Homes 2

Roads 59

Industrial Sites 3

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8.8 The report stated that of the houses flooded, 175 of the houses were located in Greenford. The primary source of flooding in this area was attributed to Costons Brook. 8.9 Historically the Costons Brook had been associated with flooding. Flooding from the brook reduced after the Costons Brook Improvement Scheme was completed in 1973. Areas previously at risk from flooding from the brook were properties to the north of the A40. 8.10 Also stated in this report was flooding from the Brent that resulted in inundation of 150 houses in areas including Hanger Lane, Greenford Bridge, Bridge Avenue and Green Lane. River Brent – Flood Alleviation Feasibility Study by The (1974) 8.11 This report refers to the “Serious Floods of 1927 and 1928” being the trigger behind the “first comprehensive study of the catchment by the County Engineer in 1931”. Unfortunately due to the outbreak of World War Two, recommendations from the study were never carried out. A County Engineer eventually undertook a new investigation, which was not submitted before the formation of the Greater London Council in 1965 and therefore never finished. Also, serious flooding in September 1968 in South London meant the focus was moved away from the Brent. 8.12 This report was written to finally fulfil the much needed study of the Brent and offer possible Flood Alleviation Solutions for the different Stages of the Brent. Stage A is described as running from the River Thames to Hanwell Bridge. The Study recommends: “Work on the Grand Union Canal comprises the construction of control structures with some minor bank raising in the form of earth banks or fixed defences to protect low lying areas. Unless the land constraints are removed the junction with the River Brent would have to be a 40ft. [12.2m] wide rigid channel and the remainder of the Brent up to Hanwell Bridge an improved and partly re-aligned 30ft. [9.1m] base width earth channel with 15ft. [4.6m] wide earth berms.” 8.13 For Stage B, which encompasses the stretch of the Brent between Hanwell Bridge and the Brent (Welsh Harp) Reservoir, the relevant recommendations made were: “From Hanwell Bridge to Western Avenue the required capacity could be provided by a new earth channel 30ft. [9.1m] base width with 15ft. [4.6m] earth berms. At the St. Benedict’s School playing fields, as there is insufficient space to use such a wide channel, a 49ft. [14.9m] wide rigid channel would need to be substituted. Also a 40ft. [12.2m] wide rigid channel would be needed where the area is restricted by the main line railway embankment and at the Ruislip Road and Western Avenue. The channel sizes have been restricted by retaining the open spaces as flood plains, albeit to a greatly reduced frequency than at present. “From Western Avenue to Brent Reservoir the river passes through a densely built up area necessitating the use of rigid or composite channels of 25ft. [7.6m] to 35ft. [10.7m] width to provide sufficient capacity but earth channels of 30ft. [9.1m] base width would be used where possible.” 8.14 The recommendation includes upstream stretches of the Brent outside of the Study Area. The total estimated cost of the recommendations in full was £15million in 1973. 8.15 Not all the recommendations were completed and unfortunately discussions with the EA highlighted that a comprehensive list of the recommendations was not available at present.

Ealing Council Page 29 December 2007 London Borough of Ealing SFRA

Hydrochronology Database compiled by the University of Dundee 8.16 The University of Dundee compiled a hydrochronological database of flooding from the River Brent as part of their study into flooding. 8.17 On January 16th 1841 it was reported that the swell in the River Brent caused “Inundation at Brentford: Immense Destruction of Property, and Loss of Life ” This was apparently due to the “ immense body of land water occasioned by the rapid thaw which poured down from the country, swelling the stream of the Brent, caused the catastrophe .” Majority of the damage was felt in Brentford at the confluence of the Brent and the Thames. Substantial flooding was also experienced within the Study Area by the Brent, which was reported as “ narrow stream of the Brent had swollen into a mighty river, and overflowing its banks, was pouring itself into the already increased waters of the canal. ” 8.18 On the 2nd April 1911 heavy rainfall resulted in the Brent overtopping its banks. According to the Times “ This was accelerated by the fact that the reservoir gates at the Welsh Harp at had been opened to relieve the extraordinary pressure of storm water. ” It goes on to mention that “ At Perivale the fields were flooded to the extent in one spot of over half a mile… The ancient church at Perivale was almost isolated… The Ealing Golf Links are under water, and considerable damage has been done to property on either side of the river. ” 8.19 Other reports suggest that in 1952, high waters in the Thames caused problems to other watercourses with an extract reporting that the “ Brent at Perivale [amongst other watercourses] all overflowed ”. Other reports describe immense thunderstorms occurring in 1920 across West London affecting the neighbourhoods of Hanwell and Ealing with “ wood pavements burst up…” Brent Magazine Extract – Hell and High Water September 2000 8.20 Extracts from this article commented on flooding occurring within the Study Area during the storm on the 16th August 1977 which resulted in the opening of the sluice gates of the Welsh Harp to “ release the vast amount of river water pouring into it .” The additional water from the Welsh Harp added to the “already swollen waters of the Lower Brent ” and resulted in reports the following morning of “ areas of [lying] under five feet of water when the river burst its banks .” It also references to severe floods of 1840 when the Welsh Harp’s dam burst resulting in several people drowning. Environment Agency Flood Zones 8.21 The EA Flood Zones are used to indicate broad areas that may be at risk of river and tidal flooding. The EA Flood Map is available online and mapped in Appendix C. The maps indicated Flood Zones 1, 2, 3 and 3b, which are the starting point for applying the Sequential Test. These Flood Zones refer to the probability of river and sea flooding only, ignoring the presence of any existing defences and structures. Flood Zone 1 comprises of land assessed as having a less than a 0.01% (1 in 1000 year return period) of river or sea flooding in any year. Flood Zone 2 is identified as being of medium probability in PPS25, and has between a 1% (1 in 100 year return period) and 0.01% (1 in 1000 year return period) annual probability of river flooding in any year. Flood Zone 3 is identified as an area of high probability flooding, and comprises land assessed as having a 0.01% (1 in 100 year return period) or greater annual probability of river flooding.

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Source: Capita Symonds Ltd Figure 8.1 Conceptual definition of Flood Zones as defined in Table D1 PPS 25

8.22 The Flood Zones on the EA website were originally based on broad scale modelling techniques but have recently been updated to reflect the detailed hydrological and hydraulic model results For Flood Zone 2 (1 in 1000 year) the undefended modelled results are overlaid with historic flooding maps for the London Borough of Ealing for the recent detail hydraulic modelled watercourses. For the watercourses that have not had detailed modelling carried out on them the broad scale model flood outlines are still utilised. 8.23 The risk based Sequential Test is described in PPS25 and should be applied at all stages of planning. Its aim is to steer new development to areas at the lowest probability of flooding (Zone 1) first. Therefore the Sequential Test is used to steer vulnerable development away from Flood Zones 2 and 3 wherever possible. 8.24 Flood Zone 3 is divided into Zone 3a and Zone 3b in PPS25. Flood Zone 3b (Functional Floodplain) is defined by PPS25 as ‘ land where water has to flow or be stored in times of flood ’ and provides a possible definition of the functional floodplain as ‘land which would flood with an annual probability of 1 in 20 or greater in any year ’. 8.25 PPS25 allows Local Authorities to define the Functional Floodplain using different criteria for their area if this is agreed with the EA. In Ealing the area defined by the 1 in 20 year return period flood extent has been accepted to define the Functional Floodplain. Impact of Development on Flood Risk 8.26 PPS25 explains that positive planning reduces and manages flood risk by taking full account of present and future flood risk, and by considering the wider implications for flood risk of development located outside areas identified to be at risk of flooding. 8.27 PPS25 states that “Planning applications for development proposals of 1 hectare or greater in Flood Zone 1 and all proposals for new development located in Flood Zones 2 and 3 should be accompanied by a FRA.” It is necessary to carry out a drainage design strategy on sites greater than 1 hectare in size at the outset of a proposed

Ealing Council Page 31 December 2007 London Borough of Ealing SFRA

development in order to identify the options for the design of the surface water drainage system and how it will affect the site layout. This strategy will then need to be submitted to the EA as part of a Surface Water Flood Risk Assessment for their approval prior to making a formal planning application. This applies to Brownfield and Greenfield sites and also sites that drain to surface water sewerage systems or combined sewers. 8.28 PPS25 states that “the surface water drainage arrangements for any development site should be such that the volumes and peak flow rates of surface water leaving a developed site are no greater than the rates prior to the proposed development, unless specific off-site arrangements are made and result in the same net effect.” The EA Thames Region SUDS guidance states that it is the EA’s aspiration that drainage proposals should “seek greenfield discharge rates on greenfield sites and on brownfield sites (where possible)”. It should always be the aspiration of developers of brownfield sites to reduce the surface water runoff rates as close as practicably possible to greenfield discharge rates, but it should be recognised that this is not always practical for all sites and therefore Ealing Council are advised to strive towards these rates but to bare in mind that they are not always possible. 8.29 The EA Thames Region guidance also states that it is the EA’s aspiration for all drainage proposals for sites should “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” and adds that climate change should be considered when designing the drainage system. 8.30 PPS25 encourages that SUDS be implemented with any new developments to reduce surface water flooding potential and increase water quality discharging from the site and should be considered on sites undergoing redevelopment.

Flood Hazard Mapping 8.31 The River Brent was the only watercourse in the study area to be included within the new hydraulic model ( Jacobs et al, 2007 ). However, the EA Flood Zone Maps also include flood extents for the other main river in the study area, the Osterley Park Boundary Stream, which was not included within the hydraulic model. Therefore the outlines for this watercourse are still the ones produced by the broad scale modelling techniques. 8.32 The Osterley Park Boundary Stream’s flood extents appear not to affect any properties within the study area. 8.33 The Yeading Brook runs along a short length of the study area’s western boundary and its flood zones spread into the study area. These flood zones are currently based on the generalised modelling but there is currently a mapping study on the Crane which will be used to update the flood zones in 2008. 8.34 Within the study area there is a canal and various other ordinary watercourses that have not been attributed Flood Zones within the EA mapping. This is because the EA only produce Flood Zones for main rivers. 8.35 It should be noted that the recent discovery of an error with the Flood Zones around the bridge under Argyle Road (B122) has resulted in the Flood Zone 3a changing slightly. The EA’s consultants (Jacobs) are in the process of changing the EA Flood Zones which will then be uploaded to the website. In the meantime a manual change has been made to the flood zones shown within the figures of this SFRA to represent what the flood zones will eventually look like. 8.36 The following sections identify the ACTUAL and RESIDUAL risk associated with the River Brent within the study area as identified by the new hydraulic model.

Ealing Council Page 32 December 2007 London Borough of Ealing SFRA

Assessment of Actual Risk 8.37 The actual level of risk affecting the study area is mapped in Appendix D. The mapping illustrates the updated Flood Zones 3a, 3b and the 1 in 100 year flood plain including climate change. These outlines are based on detailed hydraulic models of the Brent. 8.38 The following section identifies areas that lie within the flood extents of the new model for the various return periods. The areas are generally listed in the order in which the Brent flows downstream. 1 in 20 year return period (Functional Floodplain): EA Flood Zone 3b 8.39 In accordance with PPS25 consideration should be given to the Functional Floodplain. In line with PPS25 development is not generally appropriate within the Functional Floodplain, with the exception of certain ‘water compatible’ land uses (e.g. recreational and conservation uses), as well as essential transport/utilities infrastructure that have no viable alternative location. The Functional Floodplain comprises of land where water has to flow or be stored in times of flood. For the purpose of this SFRA the 1 in 20 year return period has been used as an indication of those areas which may be potentially classified as Functional Floodplain. 8.40 PPS25 suggests severe restrictions on development within the Functional Floodplain (refer PPS25 Table D1), with the emphasis on steering all development away from the Functional Floodplain wherever possible. Despite the restrictions on the type of development appropriate for this area, previous development has occurred within the Functional Floodplain. The 1 in 20 year outline in the Study Area includes areas, which are currently developed, including brownfield sites. However, the majority of flooding from the 1 in 20 year return period within the Study Area is limited to open space and rural or semi-rural areas’. The results of hydraulic modelling demonstrate the following areas (amongst others) may be at risk from a 1 in 20 year return period flood event, please refer though to Appendix C/D for completeness: • The North Circular (at the aqueduct) at the point at which the Brent travels under the Grand Union Canal; • The gardens of properties and allotments of Riverside Gardens, Hanger Lane and Cleveland Crescent; • Parts of Park (excluding the buildings); • Parts of Ealing Golf Course; • The playing fields NW of the golf course, including the corner of the Pavilion; • The Allotment Gardens to the east of Argyle Road; • Perivale Golf Course; • Sections of Ruislip Road East (B455); • The whole of the playing fields to the south of Ruislip Road East; • Areas of the Mayfield Primary School grounds; • Parts of High Lane and the open space between it and the Brent; • Part of the Brent Valley Municipal Golf Course and Sports Grounds, including one of the buildings; • Part of Brent Lodge Park; • Parts of the gardens to properties along Half Acre Road; • The allotment gardens opposite the hospital;

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• A number of properties along Betham Road, Wedmore Road and Costons Lane; • Corners of buildings on the corner of the A4020 and Walker Close; • The corner of the buildings by the intersection of the Brent and GUC; • Areas of the Waterside Trading Estate; • A small section of 2 buildings along St.Margaret’s Road. 8.41 Where development is already located within the Functional Floodplain there is no requirement to relocate or remove them. Despite this PPS25 seeks to achieve flood risk reduction wherever possible by considering the relocation of developments that are currently located within the Functional Floodplain outside when redevelopment of them is necessary. 8.42 Also, whilst PPS25 allows for the maintenance of pre-existing development in the functional floodplain, redevelopment of these sites will require the PPS25 principles to be addressed and for the new development to satisfy the requirements. When the opportunity arises to redevelop a site within the Functional Floodplain a less vulnerable future use for the site should be pursued. 1 in 100 year return period; EA Flood Zone 3a 8.43 The following areas fall within 1 in 100 year return period flood extents in addition to those areas previously mentioned to be within the functional floodplain, although this is not a complete list of areas located within Flood Zone 3a so please refer to Appendix C/D for completeness: • Properties on the corner of Hanger Lane and Cleveland Crescent; • Buildings along Queensbury Road (Northfields Industrial Estate); • Some of the properties on Elveden Place; • A larger section of the North Circular at the point at which the Brent travels under the Grand Union Canal; • A larger area of the allotment gardens directly south of the A40; • The corners of buildings to the north of the A40 near Hanger Lane; • The A40 at the point at which the Brent flows beneath; • The Sports Ground to the South of Pitshanger Park; • Slightly more of the playing fields NW of the golf course with the pavilion completely within the 1 in 100 year return period flood outline; • Part of Perivale Lane; • The Argyle Road bridge over the River Brent; • Properties within Peal Gardens; • The eastern part of the Gurnell Swimming Pool grounds; • Ruislip Road East before the railway underpass and a few of the properties in Gurnell Grove; • Several more properties along Betham Road, Wedmore Road, Costons Lane and now Lucarno Road; • Part of the Mayfield Primary School building and the fields in between the school and the Brent, but not the school entrance; • A larger area of Brent Valley Municipal Golf Course;

Ealing Council Page 34 December 2007 London Borough of Ealing SFRA

• Further properties by the intersection of the Brent and GUC; • A few of the properties within Billets Close; • The majority of Waterside Trading Estate. 8.44 PPS25 indicates that new development in the area shown as the 1 in 100 year return period flood outline (equating to Flood Zone 3) (Appendix D), should generally not include more vulnerable or highly vulnerable development types (refer Table D2 in Annex D of PPS25). Table D3 of PPS25 compares site vulnerability with Flood Zone ‘Compatibility’ with clear instructions as to whether or not the development is permitted and also if development is permitted after the Exception Test has been applied. The Exception Test requires that the development provide wider sustainability benefits to the community, the development is preferably on a brownfield site and that the development does not increase flood risk elsewhere. 1 in 100 year plus an allowance for climate change (+ 20% increase in magnitude) return event 8.45 Current predictions of climate change suggest river flows may increase by as much as 20% in extreme events over the next 50 years. It is therefore necessary to consider how flood risk may change and potentially increase in coming years. 8.46 A 20% increase in the 1 in 100 year return period flow results in a significant extension of the floodplain in some areas within Ealing. The following areas fall within the 100 year plus climate change flood extent, in addition to the areas included within the 100 year flood extent, additional areas at risk of flooding are listed below, although this is not a complete list of areas located within the climate change envelope so please refer to Appendix C/D for completeness: • More buildings to the north of the A40; • Greater area of sports ground and Ealing Golf Course; • The Gurnell Swimming Pool; • More properties along Gurnell Grove; • Almost the whole of Ruislip Road East and areas slightly further northwards along Betham Road; • Houses along Bridge Avenue and the gardens of properties along Brookbank Avenue; • Parts of Studland Road; • More of the Mayfield Primary School building and now the school entrance; • More of the allotments to the north of Ealing Hospital; • A few more properties within Billets Close and the gardens of properties in St.Margarets Road; • More of the Waterside Trading Estate.

Assessment of Residual Risk

8.47 In recognition that flood management and mitigation measures (including appropriate spatial planning in relation to Actual Risk), cannot eliminate flood-risk, there is a need to be aware of the Residual Risk generated by an event more severe than that for which particular flood management/planning and mitigation measures have been

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designed. Consideration of the Residual Risk is a key requirement for Flood Risk Assessments as defined in Annex E and Annex G of PPS25. 8.48 The assessment of Residual Risk provides information on the flood risk associated with more extreme events within the Study Area. The Residual Risk has been assessed for the same return period which Flood Zone 2 represents (1 in 1000 year return period) results from the detailed hydraulic model. The Residual Risk scenario is mapped in Appendix D. 1 in 1000 year return period; EA Flood Zone 2 8.49 As expected the Residual Risk floodplain is significantly larger than the Actual Risk floodplain in some areas as a result of increased flows, however generally flooding mechanisms within the study area remain the same. Due to the generally well defined river floodplain along the Brent within the study area, the increase in flows associated with the Residual Risk flood event has had only a minimal impact on flood extent in many areas within the Study Area. 8.50 Much of the flooding resulting from the Residual Risk flood event within the study area is limited to open space and rural or semi-rural areas although certain developed areas fall within the flood extents. 8.51 As discussed above, generally the Residual Risk scenario flooding mechanisms and extents for the London Borough of Ealing are similar to those for Actual Risk, (1 in 100 year outline) due to the generally well defined floodplain. The most notable exceptions to this, which impact on existing developments are discussed below, although this is not a complete list of areas located within Flood Zone 2 so please refer to Appendix C/D for completeness: • Properties along Iveagh Avenue; • Further buildings to the north of the North Circular; • Queensbury Road (10-15 properties); • A few more houses down Cleveley Crescent and Hanger Lane; • The Industrial Building off Quill Street and the building opposite it over the other side of the Brent; • More of the buildings to the north of the A40 near Hanger Lane; • Several buildings to the south of the A40 near Pitshanger Park; • More of Ealing Golf Course; • Larger area of Argyle Road; • A few properties along Stockdove Way; • The whole of the Gurnell Swimming Pool and its sports grounds by the railway; • Properties around Avalon Road/Ruislip Road East; • Most of Gurnell Road; • Ruislip Road at Rail Bridge crossing; • building off Greenford Road ; • Further properties along Betham Road, Wedmore Road, Costons Lane and Lucarno Road; • Property on Clifton Road; • Additional parts of Bridge Avenue;

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• The Mayfield Primary School entrance; • A number of properties on Holboyne Road and Studland Road; • Buildings within Brent Lodge Park; • The whole of the allotment gardens south of the viaduct ; • Part of the A4020 and down into the Ealing Hospital entrance road and grounds; • Several properties down St. Margaret’s Road and Green Lane; • Further amounts of Waterside Trading Estate; • The Business Park below Waterside (only a small part of the buildings affected). 8.52 In accordance with PPS25, highly vulnerable land uses should be avoided in areas potentially susceptible to Residual Flood Risk, unless the Exception Test is passed. 8.53 The flood extents and flood zones differ slightly for the 1000 year modelled flood extents and the EA Flood Zone 2. In addition to the modelled results, the EA Flood Zone 2 includes some historically flooded areas. 8.54 Historical flooded areas, that were not shown to flood in the model, were: • A further stretch of the North Circular (approximately an extra 80m to the south of the modelled outlines) • A greater area of Perivale Park Golf Course up to the Sports Arena there including the Pavilion located beside the railway line • A larger area around Costons Lane with Oldfireld Lane South included from the opposite side of the A40 to Croyde Avenue. This includes many of the roads to the east of Oldfield Lane South and includes the Schools to the west of it, at the end of Costons Lane • Several more properties are included down Bridge Avenue, with the whole of Mayfield Primary School now included.

Ealing Council Page 37 December 2007 London Borough of Ealing SFRA

Summary and Recommendations

General

The Brent’s course has been artificially altered over the years with extensive culverting of sections of the watercourse and the straightening of previously meandering sections to aid a) development in the area. Most of the River Brent floodplain within the London Borough of Ealing is undeveloped however development in the catchment over the years has encroached onto the Brent’s floodplain, and in some areas developments have taken place right up to the edge of the b) river channel.

The Brent is reported to flood regularly along its floodplain during periods of high rainfall or c) when the Welsh Harp Reservoir opens its flood gates to relieve pressure on the reservoir

The EA Flood Zones are used to indicate broad areas that may be at risk of river and tidal d) flooding and are mapped in Appendix C.

PPS25 explains that positive planning reduces and manages flood risk by taking full account of present and future flood risk, and by considering the wider implications for flood e) risk of development located outside areas identified to be at risk of flooding. Flood management and mitigation measures, cannot eliminate flood-risk, there is a need to be aware of the Residual Risk generated by an event more severe than that for which particular flood management/planning and mitigation measures have been designed. Consideration of the Residual Risk is a key requirement for Flood Risk Assessments as f) defined in Annex E and Annex G of PPS25.

It is recommended that the EA Flood Zones and maps of Actual and Residual Risk are used to inform any potential development. The Flood Maps should be used in conjunction with PPS25 for every development to decide if the development is acceptable and/or the g) need for the Exception Test to be carried out.

Actual Risk The Actual Risk affecting the study area is mapped in Appendix D. The mapping illustrates h) the updated Flood Zones 3a, 3b and the 1 in 100 year flood plain including climate change.

Residual Risk The Residual Risk has been assessed for the same return period for which Flood Zone 2 was assessed (1 in 1000 year return period) and the affected areas are mapped in i) Appendix D.

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9. TIDAL FLOODING 9.1 A list of the datasets relevant to tidal flooding covering the study area collated during the production of the SFRA are summarised in Table 9-1. Table 9-1: Sources of Tidal Flooding data DATE OWNER / DATA DESCRIPTION PROVIDED AUTHOR Presents the findings of scoping River Brent Flood Risk study undertaken as part of the Management Strategy Environment Agency’s Strategic Strategic Environmental 8 October 2007 Environment Agency Environment Assessment (SEA) Assessment Scoping process for the River Brent Flood Report Risk Management (FRM) Strategy. DTM of the London Borough of LiDAR 1 October 2007 Environment Agency Ealing Tidal defence data for the Tidal Defence data for the Thames 17 October 2007 Environment Agency Thames NE region Information of incidences of previous Historical flood flood events within the London 22 August 2007 Ealing Library records/data Borough of Ealing. Hydrochronology search database search for Ealing. Flood Zones GIS layer of EA flood zones 13 August 2007 Environment Agency 9.2 Tidal flooding could potentially affect Ealing when as a result of high river levels in the Thames the defences are overtopped and flooding occurs in low lying land. The defences along the stretch of the Thames close to the London Borough of Ealing are designed to defend the area behind them to a 1 in 1000 year return period standard. The defences along the tidal Thames in this area are raised, man-made and privately owned. The Environment Agency regularly inspects them to ensure that they remain fit for purpose. The Flood Defence Levels on this part of the Thames are 5.94m AOD upriver of the Chiswick Eyot and 5.54m AOD downriver of it. This is the crest level to which their owners must maintain these defences. 9.3 When an area is defended by walls/barriers the pathway for flooding to areas behind these defences occurs via overtopping or a breach in the defences. This can cause severe damage to properties and infrastructure as the flood event can occur fast and with little warning. The Study Area is well defended from tidal flooding when the defences function correctly, however the hazard resulting from a breach or overtopping event should be considered as a Residual Risk. 9.4 The only area within the study area potentially at risk of flooding from the Thames is the southeast corner, Bedford Park as can be seen in Appendix C. The area is located in Flood Zone 3. PPS25, Table D3 dictates that development is permitted in this zone, apart from water compatible development, provided it passes the Exception Test. 9.5 A detailed breach analysis has not been carried out for this SFRA however an assessment can be made about the impact of breach or overtopping of the defences using tide and ground level data. Also, more detailed work has been carried out in the neighbouring Borough (London Borough of Hammersmith and ) that shows the extent of flooding as a result of breach. 9.6 Should the defences be topped, the amount of water entering the area would be severely limited as a result of a weiring over effect in the case of overtopping or the size and geometry of any breach. Also as Bedford Park is approximately 1km from the tidal defences, there is a minor Residual Risk of tidal flooding. However the hazard associated with such an event is likely to be low given the restriction on water flow over or through a defence breach, the relative ground levels and the distance from the defence line.

Ealing Council Page 39 December 2007 London Borough of Ealing SFRA

9.7 A conservative attempt to determine the depth of flooding can be done by comparing the Thames peak tidal levels (Table 9-2) to the ground levels in the area, behind the defences.

Table 9-2: Modelled Thames Tidal Levels at Chiswick Eyot (Station Node Label 2.19) Return Periods Condition 10 20 50 100 200 1000 2002 5.218 5.278 5.337 5.371 5.398 5.442 2052 5.304 5.331 5.355 5.370 5.382 5.406 2102 5.267 5.280 5.294 5.301 5.307 5.322

9.8 The lowest point within this area currently benefiting from defences is located at NGR 521236, 179668 and is recorded as being at a height of 1.77m AOD. The area is a storm pumping station for the Acton area, consisting of 6 storm tanks. This pumping station acts as a ‘sump’ for surface water and so is located lower than surrounding land. Ignoring this pumping station, the area of Bedford Park is located on the lowest ground with much of it around 5m AOD and below. Blondford Road, which is within Bedford Park, is the location of the second lowest point in the study area at 2.82m AOD. 9.9 The average (median) height for the area currently benefiting from defences is 5.21m AOD, therefore an average potential flood depth of 230mm could occur should the defences breach/overtop during a 1 in 1000 year event. It should be noted that these depths would be unlikely to occur over the whole area and that the amount of water entering the area would be severely limited by either a weir effect in the case of overtopping or the size and geometry of any breach. In addition the area in question is approximately 1km from the defences with densely urbanised land in between. There is therefore a small Residual Risk of tidal flooding in the Bedford Park area of Ealing; however the hazard associated with such an event is likely to be low given the restriction on water flow over or through a defence breach, the relative ground levels and the distance from the defence line.

Ealing Council Page 40 December 2007 London Borough of Ealing SFRA

Summary and Recommendations

General Tidal flooding could occur when water enters the study area as a result of storm surge and high tide overtopping defences. The defences along the stretch of the Thames close to Ealing Borough are designed to defend the area behind them to a 1 in 1000 year return a) period standard.

The defences along the tidal Thames in the study area are raised, man-made and privately owned. The Environment Agency regularly inspects them to ensure that they remain fit for b) purpose. The Study Area is well defended from tidal flooding when the defences function correctly, however the hazard resulting from a breach or overtopping event should be considered as a c) residual risk. The southeast corner, Bedford Park, of the study area is at risk of flooding from the Thames d) (Appendix C)

A detailed breach analysis has not been carried out for this SFRA however an assessment can be made about the impact of breach or overtopping of the defences using tide and ground level data. Also, more detailed work has been carried out in the neighbouring Borough (London Borough of Hammersmith and Fulham) that shows the extent of flooding e) as a result of breach,

It is recommended that for any development in the southeast corner of the study area, developers liaise with the Environment Agency to determine measures required to manage f) the Residual Risk.

Ealing Council Page 41 December 2007 London Borough of Ealing SFRA

10. GROUNDWATER FLOODING 10.1 A list of the datasets relevant to groundwater flooding and covering the study area collated during the production of the SFRA are summarised in Table 10-1.

Table 10-1: Sources of groundwater data DATE OWNER / DATA DESCRIPTION PROVIDED AUTHOR DTM of the London Borough of LiDAR 1 October 2007 Environment Agency Ealing GIS layers of Borough outline, Digital Catchment Aquifers, catchments, depth to 24 September 2007 Environment Agency Information Groundwater, Geology Solid and Drift, Ealing Flood Zones. List of wells, boreholes and Thames Region- Well and groundwater monitoring stations in 11 September 2007 Environment Agency Borehole Locations the Thames Region. Maps of potential groundwater Groundwater Emergence flooding areas based on historic DEFRA Zones events and recorded groundwater levels. Water Matters, The Mayor’s Draft Water Strategy – Draft for Groundwater flooding issues in Internet consultation with the London London Assembly and functional bodies Regional Flood Risk Groundwater flooding issues in Internet Appraisal (Draft) London Flooding in London, A Groundwater flooding issues in London Assembly Internet London Scrutiny Report Information of incidences of previous Historical flood flood events within the London 22 August 2007 Ealing Library records/data Borough of Ealing. Hydrochronology search database search for Ealing. 10.2 Groundwater flooding is caused by the emergence of water originating from the sub- surface permeable strata (Jacobs 2006). A groundwater flood event results from a rise in groundwater level sufficient for the watertable to intersect the ground surface and inundate low lying land. Groundwater floods may emerge from either point or diffuse locations. They tend to be long in duration developing over weeks or months and prevailing for days or weeks. 10.3 Groundwater levels rise and fall in response to rainfall patterns and distribution, with a time scale of months rather than days. The significance of this rise and fall for flooding, depends largely on the type of rock it occurs in, i.e. how permeable to water the rock is, and whether the water level comes close or meets the ground surface. 10.4 The mechanisms associated with groundwater flooding, can be broadly classified as:

• Direct contribution to channel flow.

• Springs emerging at the surface.

• Inundation of drainage infrastructure.

• Inundation of low-lying property (basements).

Ealing Council Page 42 December 2007 London Borough of Ealing SFRA

10.5 Of the groundwater flooding mechanisms above rising groundwater levels in major aquifers as a result of long duration rainfall presents the most extensive level of risk. Impacts from other forms of groundwater flooding are considered as the hazard associated can be quite notable. 10.6 Groundwater flooding is often localised and difficult to identify on a strategic level. It also occurs more slowly than river flooding. Thus it does not often pose a risk to life however, can cause a change in the structural properties of rocks and soils, particularly clay overlying chalk aquifers and this may cause damage to foundations of existing structures. 10.7 The occurrence of groundwater flooding is very local and often results from the interaction of site specific factors, e.g. aquifer properties, topography, man made structures etc. Little consideration is given to groundwater as a source of flooding in the planning process and therefore both existing and new development may be at risk. The sparse frequency of groundwater flood events can contribute to poor decision-making. 10.8 The London Borough of Ealing study area geology consists primarily of clay overlying the chalk. However there are some areas generally to the south of the study area where drift deposits overlay the clay. These deposits consist of silts, sand and gravels, and form a number of aquifers. 10.9 Compared to other aquifer units, Chalk is more likely to contribute to groundwater flooding because of its geological formation. It contains many pores and fissures which can result in rapid rises in groundwater levels, which take a long time to recede. Historic Records of Groundwater Flooding 10.10 Groundwater flooding has not been a significant issue for the study area and the only historical record identified was during 2000/01 and is documented in the DEFRA Groundwater Flooding Scoping Study (DEFRA 2004) Impacts of groundwater flooding 10.11 The main likely impacts of groundwater flooding in the study area are:

• Flooding of basements of buildings below ground level – in the mildest case this may involve seepage of small volumes through walls, temporary loss of services etc. In more extreme cases larger volumes may lead to the catastrophic loss of stored items and failure of structural integrity.

• Overflowing of sewers and drains – surcharging of drainage networks can lead to overland flows causing significant but localised damage to property. Sewer surcharging can lead to inundation of property by polluted water. Note: it is complex to separate this flooding from other sources, such as surface water or sewer flooding.

• Flooding of buried services or other assets below ground level – prolonged inundation of buried services can lead to interruption and disruption of supply.

• Inundation of farmland, roads, commercial, residential and amenity areas – inundation of grassed areas can be inconvenient, however the inundation of hard-standing areas can lead to structural damage and the disruption of commercial activity. Inundation of agricultural land for long durations can have financial consequences.

• Flooding of ground floors of buildings above ground level – can be disruptive, and may result in structural damage the long duration of flooding can outweigh the lead time which would otherwise reduce the overall level of damages.

Ealing Council Page 43 December 2007 London Borough of Ealing SFRA

Assessment of Groundwater flooding risk 10.12 No single government body is responsible for monitoring or responding to groundwater flooding. DEFRA’s Making Space for Water Strategy (MSW) aims to provide greater clarity for the public and professional bodies impacted by and involved in the management of flooding. MSW recognises the need for an integrated understanding of flooding from all sources including groundwater. 10.13 As a consequence DEFRA have instigated a series of investigations into groundwater flooding such as:

• HA5 Groundwater Flooding Records Collation, Monitoring and Risk Assessment, March 2006 - aims to make recommendations for effective collation and monitoring of groundwater flooding information and identification organisational and funding arrangements. It has been identified that a national database for groundwater flooding is desirable and that scientific research into improving the understanding of groundwater flood processes is required.

• HA4a Flooding from Other Sources, October 2006 - aims to assess the feasibility of mapping flood risk from different types of flooding (including groundwater), together with the practicalities of implementing flood modelling methods considered for the significant types of flooding (including groundwater flooding). It has been identified that the greatest barrier to producing accurate flood risk maps of other sources of flooding is the availability of data for ground- truthing in consistent and useable formats. And that the modelling methods that would be required to capture all the observed processes are complex and may not be realistic in the immediate future

Groundwater Flooding Analysis 10.14 To determine the risk associated within the study area, GIS topographical and geological datasets were overlayed to produce a better understanding of groundwater emergence potential and therefore an indicative overview of potential groundwater flood risk. 10.15 The first stage of the spatial analysis was to identify drivers of groundwater flooding. Each driver was then assigned a weighting value based on the relative importance. The drivers were:

• Geology

• Aquifer

• Elevation

• Depth to Groundwater (used as a validation data set)

10.16 Datasets for each driver were collected and assembled in a GIS platform. Each driver dataset was divided into three categories (high, medium and low), based on likelihood of groundwater occurring. For example silt, sand and gravels were assigned a value of 2 or 3, whereas clay was assigned a 1. Where a major or minor aquifer was identified on the Aquifer vulnerability map the grid was assigned a value of 3, and where non- aquifer was classified a value of 1 was attributed to the grid.

Ealing Council Page 44 December 2007 London Borough of Ealing SFRA

10.17 The datasets were then interrogated for a 25m grid cell and the rankings summed to come up with a total value indicating where groundwater flooding should be investigated further as part of detailed site specific risk assessments. The cells were grouped into high, medium and low likelihood, high indicating areas most likely to be affected by groundwater. The results of the analyses were sensibility checked with known incidents of groundwater flooding and the depth to groundwater map. 10.18 There is currently no research specifically considering the impact of climate change on groundwater flooding. The mechanisms of flooding from aquifers are unlikely to be affected by climate change, however if winter rainfall becomes more frequent and heavier, groundwater levels may increase. Higher winter recharge may however be balanced by lower recharge during the predicted hotter and drier summers. 10.19 The results of the analyses are shown in Appendix F. These maps are indicative only areas shown at high risk indicate where further investigation and consideration of groundwater is required. The maps show that the eastern boundary of the study area is more likely to experience potential groundwater flooding. As expected, the maps also show the Brent floodplain to be at high risk. 10.20 Groundwater flooding is more likely to occur:

• after above average rainfall (and recharge), causing groundwater levels to rise;

• in areas where there is insufficient surface drainage;

10.21 Local controls which appear to affect the distribution of groundwater flooding include the:

• spatial and temporal distribution of rainfall;

• spatial distribution of aquifer properties (geological structures and drift deposits);

• recharge mechanisms;

• efficiency of the surface water and groundwater drainage network.

10.22 This broad scale analysis has identified areas where there is potential for groundwater emergence. And has therefore identified the areas where consideration should be given to groundwater flooding during detailed flood risk assessments.

Ealing Council Page 45 December 2007 London Borough of Ealing SFRA

Summary and Recommendations

General

Overall, based on the historic records of flooding, groundwater flooding has not been a) identified as a significant issue for the London Borough of Ealing.

Spatial analysis undertaken to determine the potential indicative areas at risk of b) groundwater flooding can be seen in the maps in Appendix F.

The eastern boundary of the study area and the Brent floodplain appears to be at a higher c) risk of potential groundwater flooding.

It is recommended that for development in areas identified as ‘high risk’, further analysis d) should be carried out to determine the presence of groundwater onsite.

Ealing Council Page 46 December 2007 London Borough of Ealing SFRA

11. FLOODING FROM OTHER SOURCES 11.1 In addition to the potential sources of flooding identified in the previous sections, the other sources of flooding that might affect the study area are discussed in this section. 11.2 A list of the datasets relevant to other sources of flooding covering the study area collated during the production of the SFRA are summarised in Table 11-1. Table 11-1 Other Sources of Flooding data DATE OWNER / DATA DESCRIPTION PROVIDED AUTHOR DTM of the London Borough of LiDAR 1 October 2007 Environment Agency Ealing GIS layers of Borough outline, Digital Catchment Aquifers, catchments, depth to 24 September 2007 Environment Agency Information Groundwater, Geology Solid and Drift, Ealing Flood Zones. Received from Ronnie Stevans information on the operational Welsh Harp Reservoir 24 September 2007 British Waterways aspects of the reservoir along with Emergency Planning. Information about the Canal received from Philip Adshed, Grand Union Canal 12 October 2007 British Waterways covering risk of breach and maintenance.

The Grand Union Canal (GUC) 11.3 The Grand Union Canal is a significant water course running through the study area. 11.4 The GUC connects two of the largest cities in England, London and Birmingham and stretches for 135km and has 160 locks. The canal enters the study area at Bull Bridge at the south west edge of Ealing Borough; here it splits into two branches the Main Grand Union Canal Branch and the Paddington Branch. The Paddington Branch flows in a north easterly direction across the borough of Ealing until it leaves the study area at Old Oak Lane in the east of the Borough to continue its journey to join the Regents Canal in Little Venice in West London. 11.5 The main arm of the canal flows across the southern boundary and the River Brent joins the canal to the east of Ealing Hospital at the end of Green Lane (NGR 515040 179580). The Main Branch continues its journey until joining the Thames at Brentford (NGR 518130 177360). In total 11 miles of the Grand Union Canal flow through Ealing, more than any other London Borough. 11.6 British Waterways are responsible for the maintenance and upkeep of the canal system. British Waterways have a strict maintenance program on the GUC that involves a monthly asset inspection procedure, this looks for structural defects along the canal. In addition an inspection boat travels along the canal once every three months; this is to ensure that both sides of the canal are inspected. An annual inspection is carried out by an engineer and asset inspector to provide a visual engineering inspection, which involves a detailed look at any previous inspection records that might have requested further inspection and recommends any further work for the following year. 11.7 On a more day to day basis, faults are reported by members of the public and land and water levels are monitored to monitor water seepage problems. There is a 24hr emergency line and the staff at British Waterways is on call 24 hours a day. Any potential embankment failures are expected to be picked up on the regular asset inspection.

Ealing Council Page 47 December 2007 London Borough of Ealing SFRA

11.8 There are a series of overflow weirs that control the water level between the ‘pounds’ (each individual section of canal between each of the locks). 11.9 British Waterways reported that currently there are no serious issues with the section of the GUC that flows though Ealing. 11.10 There are a number of locks upstream of the confluence with the River Brent, these include the Hanwell Flight, Norwood Top Lock and Osterley Lock. Perhaps the most impressive locks in the stretch are the “Hanwell Flight” which consists of six locks created in 1794, it raises the water level by about 53ft in half a mile. Most of the locks have ‘side ponds’ besides them, opened for use in 1816 as a means of saving water. The locks are subjected to the same maintenance regime as the canal itself. 11.11 The GUC acts as a receiver for some surface water drainage from study area. British Waterways are responsible for the issuing of discharge licences to the respective bodies. However, currently they are unsure of the capacity of the canal for any future drainage applications. 11.12 The River Brent meets the Grand Union Canal in the south of the borough (NGR: 519260 183540). Discussions held with British Waterways, and a site visit, confirm that there is no structure in place to control the amount of water entering the canal at this point. There have been no previous records of any localised flooding in the area around the confluence; as such this is not considered a significant area for flood risk. 11.13 As mentioned in section 11.11 there are a number of surface water drains that feed into the canal and also some that are culverted under the canal, an example of this is the drain between Empire Road and Jubilee Road that appears to flow under the canal and then continue into residents gardens where it then becomes culverted. It is believed that this may be part of the Ruislip Park Brook, travelling south in a culvert before joining the other branch of the Brook in Perivale Industrial Park. The maintenance of such drains should be monitored as it could provide a flood risk to the area. 11.14 The canal generally lies on lower or equivalent height land to its surroundings and therefore does not pose a substantial risk to its surroundings should a breach of its channel walls occur. However the canal does travel over the North Circular in the form of an aqueduct on the northeast border of the study area, which does pose a risk to the area below it. In the event of breach, the canal water is likely to follow the same path as the Brent does when it overtops the culvert upstream (see Section 5.17), flowing along the North Circular for 300m before the water finds its way into the Brent as the topography dictates. A breach is considered a residual risk, and as there is a strict maintenance program in place and any potential breach is likely to be picked up before any serious flooding occurs while the British Waterways regime continues. Historical flooding records 11.15 As per the Flood Risk Assessment carried out for 112 St Margaret’s Road, Hanwell carried out by STATS in 2004 a number of records of GUC flooding have been documented, namely: • The East Bank of the GUC overtopped in the 1977 floods and flooded into Green Lane, northern end of St Margaret’s Road and the Allotments adjacent to Greenlane. The ground level at the junction of Greenlane and St Margaret’s Road is 11.6m whilst the flood level was recorded at 11.7mAOD. • In the 1992 floods, the GUC flooded due to the failure of the Hanwell Locks. The British Waterways depot at Brentford was flooded as a result of the flooding.

Ealing Council Page 48 December 2007 London Borough of Ealing SFRA

• In 2000 floods, the East Bank of the GUC was overtopped causing flooding along the towpath and Green Lane. Overall the towpath is typically 0.9m above the GUC water level. Brent Reservoir (Welsh Harp Reservoir) 11.16 Although the Brent Reservoir is not strictly within the study area, it is considered to be hydraulically significant as it could potentially have a significant impact on flooding within the London Borough of Ealing. 11.17 The Brent Reservoir services the Grand Union Canal and is located (NGR: 205000, 870000), a canal feeder channel extends from the southern end of the reservoir under the Neasden Depot to join the canal near Waxlow Road. The River Brent and the Silk Stream feed the reservoir from the north (through Barnet). The water level is kept to a reduced level to prevent overtopping and the design height of the reservoir embankment is for a 1 in 10,000 year flood. The water level within the reservoir is controlled by an automated sluice control arrangement and the opening of and closing of these sluices depends on weather conditions and inflows to the reservoir. The discharged water flows along the River Brent. 11.18 If the level of water rises to the threshold point, the sluice gates at the southern end of the reservoir which allow flow to the River Brent will be opened to above 3 feet as an emergency measure. This will add significantly to flow and level in the River Brent. 11.19 Further opening of the sluice gates to 6 feet may occur. As a final emergency measure to avoid "overtopping" of the reservoir dam, there are high level siphons positioned at the dam, these will add at least an extra 50% of water into the River Brent and in this situation major flooding would be expected downstream. In extreme condition such as this then British Waterways would inform Ealing Town Hall, and the Environment Agency. 11.20 The reservoir falls within the provisions of the Reservoir Act 1975 and therefore inspections and works are carried out as required by The Act. Currently the dam and the sluice gates are inspected on a weekly basis. There are no records of breaches and presently there are only very minor seepages and leakages that are being monitored by British Waterways. 11.21 A failure (dam break) was experienced by the reservoir which caused significant flooding in the study area. According to Philip Adshead of British Waterways, “a failure was experienced in the 1840’s. The reservoir was subsequently reconstructed and has been modified on numerous occasions since. Most recently works were carried out to the dam during 2005/06 to ensure it could accommodate the Probable Maximum Flood .” No subsequent dam break analysis has been carried out to date.

Ealing Council Page 49 December 2007 London Borough of Ealing SFRA

Summary and Recommendations

Drainage Flooding The London Borough of Ealing is an established catchment with five key urban areas, Northolt, Greenford, Southall, Ealing and Acton that are all heavily developed with a mixture of residential and light industry. The floodplain along the River Brent and the Grand Union Canal has been largely undeveloped Sewer flooding information has been provided by Thames Water and is postcode based. It is important to note that the local pattern of flooding is likely to be influenced by the formal drainage systems servicing the area. Appendix B4 illustrates the total number of properties flooded from overloaded sewers from 1997 to 2007 within the catchment area. The southeast corner of the study area is most prone to flooding and has recorded 231 properties that have been flooded from overloaded sewers. Urban drainage flooding, though prevalent within the study area is not currently included in any systematic risk assessment and flood warning policy. Drainage flooding should be given serious considerations as the current drainage system is unlikely to be able to cope with an increase in development and also future storm events that may be a result of climate change. Developers should consult the London Borough of Ealing Council, the Environment Agency, and the sewerage undertakers at the earliest stage of the development process to establish the best solution for the particular site. The Environment Agency can advise on the widespread adoption of sustainable drainage systems techniques that would see a long-term improvement in the quality of rivers and the reduction in flood risk. It is also recommended that opportunities for a joint study be explored between the London Borough of Ealing and Thames Water to monitor drainage systems in relation to flood risk and to identify key areas for future improvement Surface Water Sewer Flooding

Surface water flooding can potentially occur anywhere throughout the study area. It is more likely to be severe and of longer duration in low lying areas but local problems may result in all areas as a result of very heavy rain or infrastructure failure.

A percentage of the surface water in the study area does not drain directly into the surface water sewer but instead drains directly into the Grand Union Canal and the remaining runoff is likely to drain into the Brent and then ultimately the Thames

Within Ealing, a total of approximately 30 properties have been flooded as a result of surface water sewers in the past ten years. The total numbers of properties flooded by overloaded surface water sewers in the last ten years are mapped in Appendix B5. The records show that majority of flooding from surface water is in South Acton, Acton Green and West Ealing. The ground levels in these areas range from approximately 5m AOD to 30m AOD with a majority of the flooded areas lying between 10m AOD and 15m AOD.

The geology within the London Borough of Ealing is dominated by London Clay with areas such as Acton and Ealing overlain by gravel formations. The clay strata prevents any infiltration to the chalk aquifer beneath, as a result SUDS infiltration techniques should be based on site specific borehole investigation and informed by the CIRIA SUDS Manual, 2007.

The London Borough of Ealing does not have any source protection zones and this can largely be attributed to underlying London Clay geology

Ealing Council Page 50 December 2007 London Borough of Ealing SFRA

Foul Water Sewer Flooding

A total of approximately 140 properties have flooded as a result of foul water drainage systems and can be seen in Appendix B6 that illustrates the total number of properties flooded from foul water sewers in the past ten years. Areas most at risk appear to be Acton, Southall and the Northolt.

Analysis of the topography indicates that ground levels are quite varied between the three areas. The ground levels for areas that flood within Acton range from approximately 5m AOD to 30m AOD, 30m AOD in Southall and 45m AOD in Northolt. Flooding for foul water sewer is likely to be a result of either lack of capacity or blockage. Combined Sewer Flooding A proportion of the drainage network is serviced by a combined network that accounts for foul and surface water runoff. Appendix B7 illustrates that 135 properties, primarily in Acton, are affected by flooding from combined sewers. River Flooding

The Brent’s course has been artificially altered over the years with extensive culverting of sections of the watercourse and the straightening of previously meandering sections to aid development in the area. Most of the River Brent floodplain within the London Borough of Ealing is undeveloped however development in the catchment over the years has encroached onto the Brent’s floodplain, and in some areas developments have taken place right up to the edge of the river channel.

The Brent is reported to flood regularly along its floodplain during periods of high rainfall or when the Welsh Harp Reservoir opens its flood gates to relieve pressure on the reservoir

The EA Flood Zones are used to indicate broad areas that may be at risk of river and tidal flooding and are mapped in Appendix C.

PPS25 explains that positive planning reduces and manages flood risk by taking full account of present and future flood risk, and by considering the wider implications for flood risk of development located outside areas identified to be at risk of flooding.

Flood management and mitigation measures, cannot eliminate flood-risk, there is a need to be aware of the Residual Risk generated by an event more severe than that for which particular flood management/planning and mitigation measures have been designed. Consideration of the Residual Risk is a key requirement for Flood Risk Assessments as defined in Annex E and Annex G of PPS25.

It is recommended that the EA Flood Zones and maps of Actual and Residual Risk are used to inform any potential development. The Flood Maps should be used in conjunction with PPS25 for every development to decide if the development is acceptable and/or the need for the Exception Test to be carried out.

Actual Risk

The Actual Risk affecting the study area is mapped in Appendix D. The mapping illustrates the updated Flood Zones 3a, 3b and the 1 in 100 year flood plain including climate change.

Ealing Council Page 51 December 2007 London Borough of Ealing SFRA

Residual Risk The Residual Risk has been assessed for the same return period for which Flood Zone 2 was assessed (1 in 1000 year return period) and the affected areas are mapped in Appendix D. Tidal Flooding

Tidal flooding could occur when water enters the study area as a result of storm surge and high tide overtopping defences. The defences along the stretch of the Thames close to Ealing Borough are designed to defend the area behind them to a 1 in 1000 year return period standard.

The defences along the tidal Thames in the study area are raised, man-made and privately owned. The Environment Agency regularly inspects them to ensure that they remain fit for purpose. The Study Area is well defended from tidal flooding when the defences function correctly, however the hazard resulting from a breach or overtopping event should be considered as a residual risk. The southeast corner, Bedford Park, of the study area is at risk of flooding from the Thames (Appendix C) A detailed breach analysis has not been carried out for this SFRA however an assessment can be made about the impact of breach or overtopping of the defences using tide and ground level data. Also, more detailed work has been carried out in the neighbouring Borough (London Borough of Hammersmith and Fulham) that shows the extent of flooding as a result of breach, It is recommended that for any development in the southeast corner of the study area, developers liaise with the Environment Agency to determine measures required to manage the Residual Risk. Groundwater Flooding

Overall, based on the historic records of flooding, groundwater flooding has not been identified as a significant issue for the London Borough of Ealing. Spatial analysis undertaken to determine the potential indicative areas at risk of groundwater flooding can be seen in the maps in Appendix F. The eastern boundary of the study area and the Brent floodplain appears to be at a higher risk of potential groundwater flooding. It is recommended that for development in areas identified as ‘high risk’, further analysis should be carried out to determine the presence of groundwater onsite. Other sources of Flooding

In addition to river, tidal and sewer flooding, other potential sources of flooding that affect the study area is flooding caused by the GUC and Brent Reservoir.

GUC The Grand Union Canal is a significant water course running through the study area and poses a residual flood risk. The GUC has flooded previously due to infrastructure failure (locks) and overtopping of the East Bank. Brent Reservoir The Brent Reservoir does not lie within the jurisdiction of the London Borough of Ealing however the potential flooding impact on the study area is large. The risks are residual however and related to potential dam break as was experienced in 1840's to devastating effect.

Ealing Council Page 52 December 2007 London Borough of Ealing SFRA

12. UNCERTAINTIES IN ASSESSING FLOOD RISK 12.1 When assessing risk, the impact of uncertainties associated with the predictions of the hazard and the consequences should be recognised and appreciated so informed decisions can be made. 12.2 Flood risk can be assessed using a number of techniques and also to various levels of detail. The level of detail in flood risk assigned to particular proposals is limited to the information available at the time of the submission of the planning applications. It should be noted that the SFRA is a live document and is based on the best information available in August 2007. As new data is made available the document will be updated thus it is the responsibility of the reader to be satisfied that they are using the latest information and that the SFRA accounts for this update. 12.3 Uncertainty in flood estimation arises from the: • complexity of the flooding;

• quality of the input data; and

• the potential impact of climate change

12.4 When using the SFRA to inform land use planning the following questions must be answered: • is the assessment suitable for the type of flooding and the scenarios being considered (fit for purpose)?

• is the study appropriate for the level of detail required for the flood risk assessment?

• are the limitations of the method clearly understood and reported?

• are the studies appropriately verified?

• are the key assumptions identified and stated?

• is the key input data justified and appropriate for the level of flood risk assessment (fit for purpose)?

• has a sensitivity analysis been carried out?

• have all relevant uncertainties (such as climate change) been identified and appropriately addressed?

12.5 There is a wide envelope of ‘fixed base’ uncertainty attached to the estimation of risk. It should be accepted that adopting a precautionary approach throughout the process could either result in the implementation of excessive defence proposals that envisage an event that is unlikely to be witnessed or the specification of defences at locations where the standard of protection is compromised as a consequence of provision of revised data. Different standards of risk may also be assigned to adjacent sites simply as a consequence of the timing of the application and the values obtained from the best available information at a particular time. To be consistent with current guidance a precautionary approach is adopted together with recognition of the need to review the results as circumstances change. It should be noted that if a non-precautionary

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approach was taken, could lead to inappropriate development and this can increase the level of flood risk onsite and potentially downstream. 12.6 It is possible that development proposals will be a focus for the collection of better data in the future and the catalyst for commissioning studies that lead to a reduction in the uncertainty in the magnitude or frequency of influential parameters, i.e. the improvement of hydrometric data, or completion of new hydraulic models on previously non-modelled reaches. A prudent response is to use the best available data at each stage of the planning process and prepare proposals that are respectively precautionary in accordance with the advice in PPS 25 and flexible with respect to uncertainty. The need to prepare stand alone Flood Risk Assessments in support of the submission of particular planning applications will serve to highlight new information that should be included in future updates of the London Borough of Ealing SFRA. 12.7 The Planning Policy Teams at the London Borough of Ealing have the prime responsibility for managing and maintaining this SFRA. The SFRA should be reviewed at least annually as part of the annual monitoring report. 12.8 The Planning Policy Team may decide to identify a “Management Group” of appropriately selected parties with responsibility for monitoring, managing, and maintaining the London Borough of Ealing SFRA. This group should be led by the planning representatives from the London Borough of Ealing and may also include representatives from other influential organisations. The EA are likely to play a key role in providing technical and process guidance to this management group.

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13. CONCLUSIONS AND RECOMMENDATIONS 13.1 This Strategic Flood Risk Assessment is prepared to inform the Ealing Council Sustainability Appraisal, land allocations, and policies. Ealing Council have carried out the SFRA to inform the preparation LDD’s and to consider the Catchment wide flooding issues. The SFRA provides the information required to apply the Sequential Approach and Sequential Test. 13.2 This SFRA defines the Flood Zones, the Actual Flood Risk, and Residual Risk gives information on all sources of flood risk in Ealing and gives guidance on the application of a risk-based sequential approach for implementation of development within the Study Area. The potential impacts of climate change have also been mapped. 13.3 The specific section conclusions and recommendations are listed below. Drainage Flooding 13.4 The London Borough of Ealing is an established catchment with five key urban areas, Northolt, Greenford, Southall, Ealing and Acton that are all heavily developed with a mixture of residential and light industry. The floodplain along the River Brent and the Grand Union Canal has been largely undeveloped 13.5 Sewer and drainage flooding information has been provided by Thames Water and is postcode based. It is important to note that the pattern of flooding is likely to be influenced by the formal drainage systems servicing the area. 13.6 Appendix B4 illustrates the total number of properties flooded from overloaded sewers from 1997 to 2007 within the catchment area. The southeast corner of the study area is most prone to flooding and has recorded 231 properties that have been flooded from overloaded sewers. 13.7 Much of the London Borough of Ealing is already significantly developed. Opportunities to implement SUDS should not be restricted to new development but should be considered when redeveloping existing sites. 13.8 Urban drainage flooding though prevalent within the study area is not currently included in any systematic risk assessment and flood warning policy. Drainage flooding should be given serious considerations as the current drainage system is unlikely to be able to cope with an increase in development and also future storm events that may be a result of climate change. 13.9 Developers should consult the London Borough of Ealing, the Environment Agency, and the sewerage undertakers at the earliest stage of the development process to establish the best solution for the particular site. Also recommended is working efficiently with the Environment Agency who advise on the widespread adoption of sustainable drainage systems techniques that would see a long-term improvement in the quality of rivers and the reduction in flood risk. 13.10 It is also recommended that a joint venture be explored between the London Borough of Ealing and Thames Water to improve permission on information and drainage flooding and to monitor drainage systems in relation to flood risk. River Flooding 13.11 The Brent’s course has been artificially altered over the years with extensive culverting of sections of the watercourse and the straightening of previously meandering sections to aid development in the area.

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13.12 Most of the River Brent floodplain is undeveloped however development in the catchment over the years has encroached onto the Brent’s floodplain, and in some areas developments have taken place right up to the edge of the river channel. 13.13 The Brent is reported to flood regularly along its floodplain during periods of high rainfall or when the Welsh Harp Reservoir opens its flood gates to relieve pressure on the reservoir 13.14 The EA Flood Zones are used to indicate broad areas that may be at risk of river and tidal flooding and are mapped in Appendix C. The Flood Zones are used to inform site specific PPS 25 implications. 13.15 PPS25 explains that positive planning reduces and manages flood risk by taking full account of present and future flood risk, and by considering the wider implications for flood risk of development located outside areas identified to be at risk of flooding. 13.16 The Actual Risk affecting the study area is mapped in Appendix D. The mapping illustrates the updated Flood Zones 3a, 3b and the 1 in 100 year flood plain including climate change. 13.17 Flood management and mitigation measures (including appropriate spatial planning in relation to Actual Risk), cannot eliminate flood-risk, there is a need to be aware of the Residual Risk generated by an event more severe than that for which particular flood management/planning and mitigation measures have been designed. Consideration of the Residual Risk is a key requirement for Flood Risk Assessments as defined in Annex E and Annex G of PPS25. 13.18 The Residual Risk has been assessed for the same return period for which Flood Zone 2 was assessed (1 in 1000 year return period) and the affected areas are mapped in Appendix D. 13.19 It is recommended that the EA flood zones are used to inform any potential development. The Flood Zones should be used in conjunction with PPS25 for every development to decide if the development is acceptable and/or the need for the sequential test or the exception test to be carried out. 13.20 The Flood Zones have been developed for the Brent, as the primary watercourse through Ealing. As part of the residual risk assessment, it is important in the future to identify areas that might flood as a result of the Grand Union Canal Tidal Flooding 13.21 Tidal flooding may occur in the study area when water levels in the River Thames exceed the river defence levels. The defences along the stretch of the Thames close to Ealing Borough are designed to defend the area behind them to a 1 in 1000 year return period standard. 13.22 The defences along the tidal Thames in the study area are raised, man-made and privately owned. The Environment Agency regularly inspects them to ensure that they remain fit for purpose. 13.23 The Study Area is well defended from tidal flooding when the defences function correctly, however the hazard resulting from a breach or overtopping event should be considered as a Residual Risk. 13.24 The southeast corner, Bedford Park, of the study area is at risk of flooding from the Thames if the defences fail or are overtopped and can be seen in Appendix C. 13.25 Should the defences be overtopped, the amount of water entering the area would be severely limited as a result of a weiring over effect, in the case of overtopping or the size and geometry of any breach. Also as Bedford Park is approximately 1km from the

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tidal defences, there is a minor Residual Risk of tidal flooding. However the hazard associated with such an event is likely to be low given the restriction on water flow over or through a defence breach, the relative ground levels and the distance from the defence line. 13.26 It is recommended that for any development in the southeast corner of the study area, the developers liaise with the Environment Agency to establish if any design or layout modification is necessary to respond to this Residual Risk. Groundwater Flooding 13.27 The spatial analysis undertaken to determine the groundwater flooding is highly qualitative. The maps, Appendix F, do not indicate specific areas that will flood, but instead indicate areas where further analysis is recommended. Local factors that cannot be assessed without more reliable quantitative data can affect groundwater and the potential for emergence. 13.28 The maps created are indicative only. Ideally historical records should be compared to gain a higher level of certainty in the groundwater model results. However, there is a lack of reliable measured datasets to undertake frequency analysis and even with datasets this analysis is complicated due to the non-independence of groundwater level data. Studies therefore tend to analyse historic flooding which means that it is difficult to assign a level of certainty. 13.29 The impact of climate change on groundwater levels is highly uncertain. More winter rainfall may increase the frequency of groundwater flooding incidents, but drier summers and lower recharge of aquifers may counteract this. 13.30 It is recommended that for development in areas identified as ‘high risk’ on the maps in Appendix F, further analysis should be carried out on a site specific basis (i.e. as part of any FRA) to determine the presence of groundwater onsite. Flooding from other sources 13.31 The Grand Union Canal flows across Ealing for 11 miles and is maintained by British Waterways. British Waterways have a strict maintenance program on the Grand Union Canal that involves a monthly and annual asset inspection. 13.32 Another potential source of flooding within the study area might be the Brent Reservoir. The reservoir is not strictly within the study area however failure of the valves or embankment could potentially have a significant impact on the London Borough of Ealing. 13.33 A failure (dam break) was experienced at the reservoir in the 1840’s which caused significant flooding in the study area. It is recommended that dam break analysis be carried out and relevant flood warning and emergency planning documentation be reviewed to ensure this risk is identified and planned for.

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14. SUMMARY 14.1 When assessing risk, the impact of uncertainties associated with the predictions of the hazard and the consequences should be recognised and appreciated so informed decisions can be made. 14.2 Flood risk to potential developments should be assessed using this SFRA which discusses risk based on: (a) Drainage Flooding (b) River Flooding (c) Tidal Flooding (d) Groundwater Flooding (e) Other potential sources of flooding The level of detail in flood risk assigned to particular flood mechanisms is limited to the information within this SFRA. The SFRA itself if a live document and should be amended in the future, by better data, change in the baseline conditions due to development, and change in institutional and policy conditions with 14.3 This volume (Volume 2: Data and Mapping) should be used to provide technical input to the main document (Volume 1: Decision Support).

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