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London Borough of Harrow Council

Strategic Flood Risk Assessment (Level 1 SFRA)

Volume II – Technical

June 2009

Client: London Borough of Harrow Council

Project: Strategic Flood Risk Assessment

Document Title: Volume II – Technical Report

MWH Project Code: 41515318.010101 Document No: FINAL

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A August 2008 Preliminary Draft Report Jacquie Hannan Gaye McKay Steve Dickie

B November 2008 Draft Report Jacquie Hannan Steve Dickie Steve Dickie

C April 2009 Final Draft Report Jacquie Hannan Steve Dickie Steve Dickie

D June 2009 Final Report Jacquie Hannan Steve Dickie Steve Dickie

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London Borough of Harrow Council - Strategic Flood Risk Assessment Volume II – Technical Report

MWH UK LTD Page i of 41

1. EXECUTIVE SUMMARY

The London Borough of Harrow is subject to riverine flooding from a number of watercourses, most notably the River Pinn and Wealdstone, Yeading and Edgware Brooks. The Borough also experiences pluvial, surface water and sewer flooding.

In accordance with ‘Planning Policy Statement 25 (PPS25): Development and Flood Risk’, this Strategic Flood Risk Assessment (SFRA) provides an assessment of all types of flood risk within the London Borough of Harrow to inform land use planning decisions. Specifically, it provides an overview of:

• main sources of flooding;

• potential flood risk in the Borough;

• the predicted influences of climate change;

• identification of options to mitigate the potential flood risk;

• the potential flood risk at allocated sites within the Borough; and

• relevant planning controls, appropriate sustainable design of developments and mitigation that will be sought in new development and in the retrofitting of existing buildings and extensions.

The outcomes of the SFRA will be used as guidance in influencing future Development Plan Document (DPD) policies within the Local Development Framework (LDF). The SFRA should be used by the London Borough of Harrow to inform the site allocations in the Core Strategy (which is currently being developed) and to provide evidence for the Sequential Test to be undertaken for potential development sites. It will also be used by the Council in the Development Control planning process to ensure inappropriate development does not take place in the floodplain.

In addition, the SFRA will be a useful tool for developers in determining the suitability of sites for certain types of development. It can also be used by developers and planners as a starting point for progressing site specific Flood Risk Assessments (FRAs), which are required to be submitted with planning applications in areas of flood risk/areas identified as at risk from flooding.

Throughout the data collection and review process it has been critical to make best use of the significant amount of information which already exists with respect to flood risk. This information was made available by the Environment Agency, Thames Water, Harrow Drainage team, and other key consultees, and includes:

• flood zone mapping;

• detailed flood outline mapping;

• flood level data;

• historical flood information;

• details of drainage structures;

• Council policy documents;

• GIS mapping;

• sewer flood risk areas;

• groundwater flood risk areas; and

• topographic data.


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Based on data from Council, the Environment Agency and Thames Water, and hydrologic and hydraulic modelling investigations completed as part of this study, the flood risk posed by fluvial, pluvial, sewer and groundwater sources have been mapped. For fluvial flooding, the mapping indicates the location and extent of areas considered to be at low, medium and high probability of fluvial flooding (i.e. PPS25 Flood Zones 1, 2 and 3a) and the Functional Floodplain (i.e. PPS25 Flood Zone 3b).

On the basis of flood risk mapping, the potential flood risks associated with each of Council’s Local Development Framework (LDF) allocation sites has been established and guidance has been provided to enable Council to apply the Sequential Test in the future.

It is recommended that an integrated approach be adopted within the Borough to address fluvial, surface water and sewer flooding. Based on an understanding of the characteristics of the area, potential flood risk sources and location and relevant local, regional, national and international policy, a number of policy directions have been developed for the London Borough of Harrow. These policy recommendations need to be considered by planners at the strategic level (e.g. when preparing the LDF and DC policy recommendations), site level (e.g. when preparing a FRA or the site layout plan) and/or both levels.

Central to these recommendations is the assertion that a planning solution to flood risk management should be sought wherever possible, steering vulnerable development away from areas affected by flooding in accordance with the PPS25 Sequential Test.

Where other planning considerations must guide the allocation of sites and the Sequential Test cannot be satisfied, specific recommendations have been provided to assist the Council and the developer to meet the Exception Test. In addition to passing the Exception Test, it would need to be demonstrated that the development is ‘safe’ and reduces flood risk overall. In these cases, a sequential approach should be applied in site design (e.g. siting buildings on higher ground and recreation areas on low-lying grounds).

Council should also seek to promote the following through a robust planning and policy framework:

• implementation of SUDS measures for new development.

• seeking opportunities to make space for water through such measures as ensuring an 8m wide buffer strip alongside watercourses within future development and ensuring preservation of flood flow routes;

• flood resilience and resistance measures at a property level;

• safeguarding the future operation and function of flood defences and flood-related infrastructure and continue their maintenance regime;

• safeguarding sites identified for flood storage and alleviation schemes and protect Greenfield Functional Floodplain;

• ensuring developers consult this SFRA prior to the formulation of development proposals and planning applications;

• flood awareness in the community and update flood plans in light of the SFRA findings.


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The preparation of a Surface Water Management Plan (SWMP) is also recommended. This would provide the Council with a greater understanding of the surface water flood risk within the Borough and assist in developing a coordinated approach to managing surface water drainage and flooding issues. This SWMP should refer to the findings and recommendations of the North Brent Integrated Urban Drainage Pilot Study for the Kenton area. It should identify opportunities to implement SUDS within the Borough and aim to provide cost effective solutions for the areas at greater risk of flooding.

The SFRA is comprises in three (3) separate volumes, as follows:

• Volume I - Planning & Policy Report - provides a summary of the catchment and flooding background, and details the planning policy framework relevant to this study, methodology used to define the flood risk in the Borough, potential flood risk at proposed site allocations, policy recommendations and guidance for planners and developers in preparing site-specific Flood Risk Assessments.

• Volume II - Technical Report - documents the data collection process, catchment and flood characteristics, and hydrologic and hydraulic analyses completed for the study.

• Volume III - Mapping - includes the mapping that has been produced for the SFRA.

It should be remembered that this SFRA is a living document and should be reviewed annually, or as new flood information becomes available.


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Contents

Page No

1. EXECUTIVE SUMMARY I

1. INTRODUCTION 4

1.1. Background 4

1.2. Objectives of this Study 5

1.3. Context of the Harrow SFRA 6

1.4. SFRA as a Living Document 6

1.5. Structure of the SFRA Report 7

2. DATA COLLECTION & CONSULTATION 8

2.1. Environment Agency Flood Zone and Flood Outline Mapping 9

2.2. Topographic Data 10

2.3. Historical Flood Data 10

2.4. Surface Water and Sewer Flooding Data 10

2.5. Groundwater Flooding Data 11

2.6. Details of Flood Defence Assets and Flood Structures 11

2.7. Background Documents 11

3. CATCHMENT DESCRIPTION 13

3.1. Overview of London Borough of Harrow 13

3.2. Topography of Borough 13

3.2.1. Geology 13

3.2.2. Drift Geology 14

3.3. History of Flooding in the Catchment 15

3.4. Flood Mechanisms within Harrow 17

3.4.1. River (Fluvial) Flooding 18

3.4.2. Groundwater Flooding 18

3.4.3. Surface Water Flooding 18

3.5. River Catchments in Harrow 19


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3.5.1. Brent River Catchment 20

3.5.2. Pinn River Catchment (part of the Colne Catchment) 21

3.5.3. Crane River Catchment 21

3.6. Reservoirs and Water Supplies (including lakes and ponds) 21

3.7. Flood Defences 23

3.7.1. Silk Stream Flood Alleviation Scheme (Brent Catchment) 24

3.7.2. Oxhey Lane Farm Flood Alleviation Scheme (Pinn Catchment) 25

3.7.3. Hatch End Flood Alleviation Scheme (Pinn Catchment) 25

4. HYDROLOGIC ANALYSIS 27

4.1. Choice of Methodology 27

4.2. Hydrological Modelling 28

4.2.1. Woodridings Stream 28

4.2.2. River Pinn 29

4.2.3. Yeading Brook West (including Elmshot Brook) 29

4.2.4. Smarts Brook 30

4.2.5. Yeading Brook East 30

4.2.6. The Roxbourne 30

4.2.7. Greenhill Stream 31

4.3. Hydrological Model Verification 31

5. HYDRAULIC ANALYSIS 33

5.1. Choice of Methodology 33

5.2. Hydraulic Model Development 33

5.2.1. Preparation of a Digital Terrain Model (DTM) 33

5.2.2. Network Development 34

5.3. Model Calibration 37

5.4. Sensitivity Analysis 38

5.4.1. Limitations of the model 38

5.4.2. Model Simulations, Results and Mapping 39


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6. REFERENCES 40

APPENDIX A

Historical Flood Information

APPENDIX B

Details of the Main Rivers within the Borough


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GLOSSARY

annual probability of occurrence

The probability that a given flood event will occur in any one year (for example, a flood with a 1% annual probability of occurrence occurs, on average, once in every 100 years). Also referred to as an “annual probability flood”.

catchment The catchment at a particular point is the area of land which drains to that point.

DCLG Department of Community and Local Government

DEFRA Department of Environment, Flood and Rural Affairs

development “The carrying out of building, mining, engineering, or other operations in, on, over, or under, land or the making of any material change in the use of any building or other land” (Town and Country Planning Act 1990).

discharge The rate of flow of water measured in terms of volume over time. Discharge and flow are interchangeable terms.

drain Defined under statute (Public Health Act 1875) as a conduit that drains a ‘single curtilage’; for most purposes this means single property. It includes both foul and surface water.

Environment Agency Statutory consultee for flood-related issues associated with the planning process and responsible for issuing flood defence consents.

flood behaviour The pattern/characteristics/nature of a flood. The flood behaviour is often presented in terms of the peak average velocity of floodwaters and the peak water level at a particular location.

flood defence A structure built and maintained to protect property from flooding (e.g. bunds, storage, etc).

floodplain Land adjacent to a watercourse which is periodically inundated due to floodwaters, that is ‘flood prone land’. Floodplains are a natural formation created by the deposition of sediment during floods.

flood storage Floodplain area which is important for the temporary storage of flood waters during a flood.

Flood Zone Map Map showing the delineation of flood risk zones (i.e. areas of ‘low’, ‘medium’ or ‘high’ flood risk) for England and Wales (i.e. Flood Zones 1, 2 and 3).

flow path Route taken by flowing water.

historic flood A flood which has previously occurred.

hydraulic The term given to the study of water flow in rivers, estuaries and coastal systems.

hydraulic model A computer simulation that routes runoff through a river channel to describe the behaviour of water within a river system, including depths and flows within the channel and floodplain areas.



hydraulic structure Constructed features that impact on the flood behaviour in a system (e.g. levee, weir, roadway, bridge).

hydrograph A graph showing how a watercourse’s depth or discharge changes with time.

LIDAR

(Light Detection and Ranging)

A method of detecting distant objects and determining their position, velocity, or other characteristics by analysis of pulsed laser light reflected from their surfaces. For this study, LIDAR refers to the collection of surface elevation data.

Local Development Framework (LDF)

Documents which collectively set out the spatial planning strategy for development and land use within a Local Planning Authority area.

Local Land Drainage Authority

Authority who implements powers and responsibilities as outlined in Land Drainage Act 1991 for Ordinary Watercourses. Responsibilities are, essentially, to ensure that "the proper flow of water" is not impeded; Permissive Powers are available for the LLDA to undertake flood relief or maintenance works.

London Borough of Harrow Act as Local Land Drainage Authority.

Local Planning Authority (LPA)

The local government body responsible for formulating planning policies (in a Local Development Framework), controlling development through determining planning applications and taking enforcement action when necessary.

Main River “A watercourse on a main river map and includes any structure or appliance for controlling or regulating the flow of water into, in or out of the channel which: (a) is a structure or appliance situated in the channel or in any part of the banks of the channel; and (b) is not a structure of appliance vested in or controlled by an internal drainage board” (S.113(1) Water Resources Act 1991 (WRA 1991), and see S.137(4) WRA 1991).

Main Rivers are managed by the Environment Agency.

Ordinary Watercourse All watercourses which are not designated as Main Rivers are identified as Ordinary Watercourses.

peak discharge, flood level, flow or velocity

The maximum discharge, flood level, flow or velocity occurring during a flood event.

Primary Land Drainage Authority

The Environment Agency is the Primary Land Drainage Authority with full responsibilities for Main Rivers. All Permissive Powers exercised by a Local Land Drainage Authority are subject to supervision by the Environment Agency.

public sewer A sewer may have been adopted by the Sewerage Authority or else vested in that authority by virtue of Public Health Act 1936. This Act declared that all sewers commissioned prior to the enactment (1 October 1937) were to become designated Public Sewers (known as S.24 sewers).

Thames Water Utilities Ltd. is the sewerage undertaker for this region.



riparian owner Under common law you are the riparian owner of any watercourse within or adjacent to the boundaries of your property.

runoff Water from rainfall, snow melt or irrigation that flows over the ground surface and returns to watercourses.

sewer Defined under statute as a conduit, which is not a drain, i.e. serves more than one curtilage (property).

SUDS Sustainable Drainage Systems

Sustainable Development “Development that meets the needs of the present without compromising the ability of future generations to meet their own needs” (The World Commission on Environment and Development, 1987).

Watercourse “Includes all rivers and streams and all ditches, drains, cuts, culverts, dikes, sluices, sewers (other than Public Sewers within the meaning of the Water Industry Act 1991) and passages, through which water flows.'” - Extract Land Drainage Act 1991 S.72. - For the purposes of this report the term watercourse is restricted to the historic system of channels which have formed naturally or constructed to drain land. It includes open channels that have subsequently been culverted.

windfall development Proposed development for a site that is not an allocated site in an adopted development plan.



1. INTRODUCTION

1.1. Background

The London Borough of Harrow is located in the north-west of London (refer Figure 1). It covers an area of 55km

2 and is divided into 21 wards (refer Figure 2). The

Borough lies within three river catchments: the Brent, Pinn and Crane Catchments.

Figure 1: London Borough of Harrow in North-western London

Local Planning Authorities are responsible for granting planning permission for development under the Town and Country Planning Act 1990. Before they make this decision, they are required to take into account all material considerations, such as planning policy statements, development frameworks and representations from all consultees.

In accordance with PPS25, Local Planning Authorities are required to demonstrate that due regard has been given to the issue of flood risk as part of the Local Development Framework (LDF) planning process. The London Borough of Harrow has also developed their own series of bye-laws to ensure that local issues are appropriately addressed.

Accordingly, it is necessary for a Strategic Flood Risk Assessment (SFRA) to be prepared to classify the existing and future flood risk of all land within the Borough and to provide the Local Planning Authority with the evidence to sequentially test all proposed land allocations and windfall sites with a view to locating new developments in lower flood risk areas, where possible. In the context of this document, flood risk refers to the likelihood of a particular event occurring and the impact or consequences that will result if the flood occurs. The Harrow SFRA will enable appropriate policies to be prepared to manage these risks and assist in making spatial planning decisions.



The Practice Guide Companion to PPS25 recommends that SFRAs are completed in two consecutive stages; this follows the iterative approach encouraged by PPS25 and provides Local Planning Authorities with sufficient tools throughout the LDF and SFRA process to inform and update decisions regarding development sites. The two stages are:

• Level 1 SFRA – enables application of the Sequential Test; and

• Level 2 SFRA – increases the scope of the SFRA for sites where the Exception Test is required.

The results of a Level 1 SFRA enable the Local Planning Authority to review the current site allocations for preparation of the LDF and to inform the scope of the Sustainability Appraisal. The findings of a Level 1 assessment will also enable the scope of a Level 2 SFRA to be defined.

In January 2008, MWH was commissioned to complete a Level 1 SFRA for Council. This has involved extensive data collection, mapping of the flood zones within the Borough, investigation of flood risk across site allocations and reviewing planning policy. This report comprises the Level 1 SFRA. It has been prepared in consultation with the Council’s Planning Policy and Drainage/Engineering Teams, and the Environment Agency (EA).

Once LDF site allocations have been finalised for the Borough, it will be necessary for the Council to apply the Sequential Test to the potential development sites and to

identify whether the application of the Exception Test will be required. A Level 2 SFRA will be necessary if it is identified that sites need to be assessed using the Exception Test.

1.2. Objectives of this Study

The Harrow SFRA will formalise the way the Council manage flood risk. In particular, the SFRA will identify and classify the existing and future flood risk, and define means to reduce and mitigate against the possible impact of flooding on existing and future development. The findings of the Harrow SFRA will inform the policies that direct development allocations in the core strategy.

Specifically, the key objectives of the study are to:

• identify possible flood risks from all potential sources of flooding;

• classify and map the Flood Risk Zones within the Borough;

• provide the Local Planning Authority with information to enable all proposed land allocations and windfall sites to be sequentially tested, with a view to siting new developments in areas of the lowest possible flood risk;

• consider the effects of climate change on flooding in the area;

• recommend policy inclusions to promote a proactive approach to reducing flood risk to new and existing development;

• set out design standards for SUDS; and

• identify suitable SUDS measures.



1.3. Context of the Harrow SFRA

The Harrow SFRA has been prepared in context with the degree of urbanisation, catchment characteristics and flood infrastructure within the Borough. These factors are discussed below.

The population of the Borough is 214,600 (ONS, 2004) and this is predicted to rise to 223,000 by 2025 (London Borough of Harrow, 2008). Development within the Borough is largely constrained by the greenbelt in the north and the current level of urbanisation. There is also a limited surplus of Brownfield land available for new housing development. Therefore, there is limited land available to accommodate the housing growth required for this predicted population increase.

A number of major rivers in the Brent, Pinn and Crane catchments rise within the Borough. Therefore, the Local Planning Authority must manage development to ensure that it does not impact on flood risk in downstream areas, both within the London Borough of Harrow, and in adjoining London Boroughs and Hertfordshire District Council areas.

The Borough has been subjected to flooding in the past. To address known flooding issues, a number of flood defences and flood alleviation schemes have been constructed within the Borough, most notably the Silk Stream Flood Alleviation Scheme, Oxhey Lane Farm Flood Alleviation Scheme (currently under construction) and the Hatch End Flood Alleviation Scheme. However, there is still the residual risk of flooding from failure of the flood defences or in the event of larger magnitude floods, and from flooding from other sources (e.g. surface water and sewer flooding).

In addition to fluvial flooding, parts of the Borough have experienced sewer and surface water flooding. For example, in July 2007 the Kenmore Avenue area was severely flooded as a result of surcharging from the surface water network. MWH recently completed the North Brent Integrated Urban Drainage Pilot Study for Thames Water, DEFRA and the North Brent Flood Working Group. This study investigates sewer, surface water and river flooding in the catchment served by the Wembley Branch Sewer and Wealdstone Brook, which covers a total area of about 30km

2 (including part of the Borough). The study identifies targets and strategies for

drainage planning that are complementary in providing managed protection against flood risk and environmental damage through a wide range of potential rainfall events. The effects of these strategies have been tested against real situations in the catchment. The findings of this study need to be integrated with this SFRA to ensure that an integrated approach to flooding is adopted.

1.4. SFRA as a Living Document

It is important to recognise that this SFRA is a ‘living’ document. This means that the document will be updated as new data becomes available, in order to ensure that the best available information is used to guide the site selection process. In particular, the Environment Agency Flood Zone Mapping is continuously being improved as new studies are undertaken, detailed hydraulic models are constructed and more flooding data becomes available.

Accordingly, it is proposed that the SFRA outputs should be revised annually in light of the latest available information.



1.5. Structure of the SFRA Report

The SFRA is comprises in three (3) separate volumes, as follows:

• Volume I - Planning & Policy Report

• Volume II - Technical Report; and

• Volume III - Mapping.

This document is Volume II: Technical Report of the London Borough of Harrow SFRA and should be read in conjunction with Volume I - Planning & Policy Report.

This report provides details of the data used in the preparation of this SFRA, specific characteristics of the study area, flooding mechanisms within the Borough and the history of flooding. It also outlines the hydrologic and hydraulic analyses that were undertaken to define the flood risk along a number of waterways within the Borough.



2. DATA COLLECTION & CONSULTATION

The data collection and review process has been critical to ensure that the best use is made of the significant amount of existing data on flood risk in the Borough. As part of this process, MWH has collected data from the following sources:

• Environment Agency;

• Thames Water;

• Harrow Drainage team; and

• flood researcher John Timms, MBE.

Available data for use in the strategic assessment of flood risk includes:

• Environment Agency Flood Zone Maps, flood data and detailed flood outline mapping (April 2008) (based on hydraulic modelling) (refer Section 2.1);

• LIDAR (Light Detection and Ranging) topographic data from the Environment Agency (refer Section 2.2);

• historical flooding information from Environment Agency flood reports (refer Section 2.3);

• local flooding knowledge and assistance from John Timms (refer Section 2.3);

• Thames Water high level property flooding information (refer Section 2.4);

• information relating to known local flooding issues (refer Section 2.4);

• details of properties subject to groundwater flooding from the Environment Agency (refer Section 2.5);

• details of existing flood defence assets from the Council and the Environment Agency’s National Flood and Coastal Defence Database (NFCDD) (refer Section 2.6);

• GIS data from the Council’s in-house GIS Team (including cadastre, ward boundaries, catchment boundaries and aerial photography); and

• background documents (refer Section 2.7).

This data is discussed in detail in the following sections.

This section outlines:

• the available data used for the study; and

• data sources.



2.1. Environment Agency Flood Zone and Flood Outline Mapping

The Environment Agency has developed a range of flood zone and flood outline maps which show the areas potentially at risk of flooding across England and Wales for planning purposes. These maps provide an indication of land at risk of flooding on a large scale.

Flood Zone Mapping was produced by the Environment Agency to meet the requirements of PPS25 (formerly PPG25). River flooding maps have been produced from a combination of sources, namely:

• National generalised computer flood modelling, combined with a Digital Terrain Model (DTM) of England and Wales that was developed from LIDAR topographic data. The computer modelling was undertaken using JFlow, a two-dimensional raster floodplain model used to route flood flows within channel and floodplain systems.

• Detailed flood data has also been taken from detailed hydraulic models (e.g. ISIS) that have been developed for regional or detailed studies. This data is summarised in Table 3 for the London Borough of Harrow.

• Historic flood outlines.

Table 3: Existing Detailed Hydraulic Modelling in the London Borough of Harrow

WATERCOURSE MODELLING SOFTWARE ANNUAL PROBABILITY

EVENTS MODELLED

River Crane (Halcrow, 2008)

ISIS

• Defended: 20%, 5%, 1%, 1% + climate change and 0.1%.

• Undefended: 1% and 0.1%.

River Pinn (Mott MacDonald,

2008) ISIS



Silk Stream (Halcrow, 2007)

ISIS

• Post-scheme: 20%, 10%, 4%, 2%, 1.3%, 1% and 1% + climate change.

• Pre-scheme: 20%, 10%, 4%, 2%, 1.3%, 1%, 1% + climate change and 0.1%.

River Brent (Jacobs, 2007)

ISIS





Environment Agency Flood Zone Mapping is a dynamic tool that is reviewed when actual flooding occurs by inputting real flow records and flood data into the predictive flood model and using the modelling outputs to update flood mapping. The mapping is updated quarterly.

It should be noted that Flood Zone Mapping is broad-scale and indicative only. Therefore, detailed modelling or topographic survey may provide a more reliable indication of flood risk in an area. In addition, the Flood Zone Mapping does not take into account protection afforded by flood defences.

Flood outline mapping is available for Harrow from the Environment Agency for a range of flood events (e.g. 0.1%, 1%, 1% + climate change and 5%), where detailed flood modelling has been completed (refer Table 3).

The Environment Agency team from the North East Thames Region were consulted in order to obtain available flood mapping for the study area.

2.2. Topographic Data

The Environment Agency has established a nationwide database of topographic spot level data derived from an airborne laser survey across parts of the UK. This data is collected through a process known as LIDAR and can be used to define the surface elevations within the study area. The LIDAR data was supplied at either 0.5, 1 or 2m resolution and to a typical vertical accuracy typically ranging between +/-0.2m.

This data was obtained from the Environment Agency team from the North East Thames Region. MWH has subsequently processed this data to create a Digital Terrain Model (DTM) that comprises a set of data points on the ground which define the topography of the Borough. Contours at 10m intervals are shown in Figure 3.

Detailed topographic data is available for the areas where hydraulic modelling has been undertaken as part of the studies listed in Table 3, however it has not been collected for use in this study.

2.3. Historical Flood Data

There have been numerous instances of flooding in the Borough in recent history, with major flood events occurring in 1977, 1987, 1988, 1992, 1993, 1997, 1999 and 2007. The Local Authority keeps records and mapping of where flooding has occurred and these were reviewed by MWH as part of the study.

Details and flood outline mapping for the historic floods that occurred in 1977, 1987, 1988 and 2007 were provided by the Environment Agency.

Flood researcher, John Timms, also provided details of historic flooding of Wealdstone Brook.

A discussion of historic flooding in the Borough is provided in Section 5.5 and displayed graphically in Figure 5 in Section 3.3.

2.4. Surface Water and Sewer Flooding Data

Details of the existing sewer network and past surface water sewer flooding were obtained through consultation with Thames Water. In particular, Thames Water provided MWH with data from the sewer flooding history database (SFHD). This data has been incorporated into the GIS mapping for the study and the locations of flooding from sewer sources are shown in Figure 9 in Volume III – Mapping.



Details of sewer and surface water flooding was also obtained through a review of the North Brent Integrated Urban Drainage Pilot Study, Final Report (Draft) (2008), prepared by MWH on behalf of Thames Water, DEFRA and the North Brent Flood Working Group. The study area covered by this report includes the catchment served by the Wembley Branch Sewer and Wealdstone Brook, which covers a total area of about 30km

2 (including parts of the Borough). The study involved the

merging and further development of existing foul sewerage and surface water drainage network models and river models into a single model in the InfoWorks CS (IWCS) v.7.5 software package.

2.5. Groundwater Flooding Data

Through consultation with the Environment Agency’s team in the North East Thames Region, MWH obtained GIS layers and records showing groundwater flooding in the Borough. This data has been incorporated into the GIS mapping for the study and the locations of flooding from groundwater sources are shown in Figure 10 in Volume III – Mapping.

2.6. Details of Flood Defence Assets and Flood Structures

Harrow Council provided mapping showing the locations of flood defence and reservoir structures within their area. Details of the flood defences were also provided in the following documents:

• Harrow Council Watercourse Maintenance Program (Harrow Drainage Engineers);

• Oxhey Lane Flood Defence Works, Oxhey Lane Farm Report (Harrow Council) and associated drawings; and

• Harrow Arts Centre - River Pinn Diversion Report (Harrow Council) and associated drawings.

The Council also provided the details of a number of pipe, culvert and bridge structures along watercourses within the Borough.

The Environment Agency also provided the National Flood Coastal Defence Database (NFCDD) data. The NFCDD was provided as a GIS layer that provides details of all structures and flood defences within England and Wales, including the location, composition and condition of the defences and the types of asset at risk within identified risk areas. This data has been incorporated into the GIS mapping for the study and is shown in Figure 6 in Section 3.7.

2.7. Background Documents

A number of documents have been prepared that are relevant to this SFRA. These include the following reports:

• London Borough of Harrow Local Development Framework (Draft) (London Borough of Harrow, 2008);

• North Brent Integrated Urban Drainage Pilot Study, Final Report (Draft) (MWH, 2008);

• River Brent Flood Risk Management Strategy: Inception Study Report (Jacobs, 2007)



• London Borough of Brent Strategic Flood Risk Assessment (SFRA) Level 1 (Jacobs, 2007);

• Thames Region Catchment Flood Management Plan Summary (Environment Agency, 2007);

• Draft Regional Flood Risk Appraisal (RFRA) (Greater London Authority, 2007);

• London Flood Response Strategic Plan (London Resilience Team, 2007)

• Sub-Regional Development Framework West London (West London Sub-regional Development Framework, 2006);

• London Plan, Housing Provision Targets, Waste and Minerals Alterations (Greater London Authority, 2006);

• Harrow Unitary Development Policy (HUDP) (London Borough of Harrow, 2004);

• Policy Statement for Flood Defence (London Borough of Harrow, 2001).

These documents provide relevant information for use in this study and have been reviewed in detail.



3. CATCHMENT DESCRIPTION

3.1. Overview of London Borough of Harrow

The London Borough of Harrow is located approximately 12 miles north-west of Central London. It covers an area of 19 square miles (or 55km

2), and has a

population of 214,600 (ONS, 2004). Harrow is situated between the London Boroughs of Hillingdon (to the west), Barnet (to the east), and Ealing and Brent (to the south) (refer Figure 1). To the north Harrow adjoins both Three Rivers and Hertsmere District councils, which are both located within Hertfordshire.

The Borough has 21 wards and is divided into three strategic areas - East, West and Central (refer Figure 2).

There are a number of watercourses within the Borough; these are the River Pinn and Woodridings Stream in the north-west; Yeading Brook and The Roxbourne in the south-west; Edgware Brook in the north-east; and Wealdstone and Kenton Brooks in the east. These watercourses are shown in Figure 3.

3.2. Topography of Borough

As shown in Figure 3, the ground surface across the Borough ranges in elevation from 50m AOD to 150m AOD. The central area of the Borough is gently sloping, with a typical elevation varying from 50 to 70m AOD.

However, the natural surface rises up to high ground in the north and south-east of the Borough. In particular, there is a ridgeline between Harrow Weald Common and Stanmore Hall in the north-east of the Borough (refer Figure 3) that divides the Brent and Pinn catchments. High ground is also situated at Pinner in the north-west of the Borough and at Harrow on the Hill in the south-west of the Borough,

The Borough is divided into three district river catchments (refer Figure 3). The Pinn catchment lies across the north-western portion of the Borough, the Brent catchment lies within the eastern half of the Borough and the Crane catchment is situated within the south-west of the Borough.

3.2.1. Geology

The Borough lies in the north-western part of the geological area known as the ‘London Basin’. The London Basin is a ‘V’ shaped geological formation approximately 250km (160 miles) long which underlies London and a large area of south east England and south eastern East Anglia.

This section provides:

• an overview of key characteristics of the study area;

• a discussion of each catchment;

• details of historic flooding in the Borough;

• a description of the flooding mechanisms relevant to the study area; and

• details of the existing flood defences within the Borough.



The basin is elongated and is constrained by the Chiltern Hills to the north, North Downs to the south and the Marlborough areas of Wiltshire to the west. The eastern end of the basin merges with the basin of the North Sea.

The London Basin is made up of solid rocks formed 425 million years ago during the Silurian period. The sediments upon which Harrow is built were laid down between approximately 65 million and 37 million years ago. These sediments are known as ‘London Clay Formation’, and consist of bagshot formation, claygate member, London clay and Lambeth group. The geology of the basin is shown in Figure 4.

Figure 4: Geology of the London Basin (Source: http://en.wikipedia.org/wiki/London_Basin)

The London Clay Formation underlies most of the Borough. This formation is predominantly bluish grey clay with minor amounts, usually in an isolated lense, of silt and fine-grained sand. There are also sporadic cementstone concretions known as septarian nodules. When exposed to surface weathering, the colour becomes brownish grey. London Clay is about 100 to 125m deep across much of the Harrow and was laid down in a deep-water marine environment.

Across the north-east and in the south of the Borough there are exposed outliers of the claygate member of the London Clay formation. Bagshot formation (also a member of the London Clay formation) is found only in the south of the Borough.

The predominantly clay geology within the Borough results in impermeable natural ground surfaces that create ideal conditions for runoff during storm events.

3.2.2. Drift Geology

Drift geology is the term used to describe transported rock debris (usually from river or glaciers) that overlay the solid bedrock. The oldest drift deposits within the Harrow are Stanmore Gravel, which is found across the northern half of the Borough. This deposit is composed predominantly of sand and gravel, but with occasional clay units and may be up to 4m thick in some areas.

The Black Park Gravel occurs as a small isolated patch on the south-western boundary of the Borough. It predominantly consists of sand and gravel, with occasional clay and silt lenses, and can be up to 3m thick. The most recent deposits are alluvial deposits, which are found in the Stanmore Marshes and the floodplains of the River Pinn, Yeading Brook and Wealdstone Brook. These deposits are of variable thickness, thickening both towards the middle of their valleys and



downstream along the floodplains of these watercourses. Predominantly silt and sand, these alluvium deposits also contain clay and peat horizons.

Both the Stanmore Gravels and Black Park Gravels form part of the major/minor aquifer system under Harrow. These sediments are considered to control the water flowing from the underground aquifer system to the river catchments in the Borough.

3.3. History of Flooding in the Catchment

The Council maintains records of all flooding events within the Borough. Historically, a number of large floods have occurred in Harrow, such as the 1977, 1987, 1988, 1992, 1993, 1997, 1999, 2004 and 2007 floods. Figure 5 provides an overview of when the largest flooding events occurred. This figure has been prepared based on Council’s data; details of historic flooding of Wealdstone Brook provided by local researcher, John Timms; and historic flood data from the Environment Agency.

Further details of historic flood events in the Borough are provided in Appendix A.

Historic flood extents have also been mapped based on the Environment Agency’s historic flood outline mapping and are shown in Figure A1 in Appendix A.



Figure 5: Timeline of Major Flood Events in the London Borough of Harrow (Source: John Timms MBE Principal Researcher and historic flood data from the London Borough of Harrow and the Environment Agency)

1977

August 1977 - Largest storm since

1928. Worst flooding in the Brent, Crane

and Pinn catchments in recent history.

1988 1993 1998

May 1988 – Two storms in one day

resulted in extensive fluvial flooding in the Borough. Surface

water, highway flooding and foul

network surcharging experienced.

Nov 2000 -Flooding at Hatch

End, Pinner, Stanmore and

Edgware. Sandbags and

pumps required.

2004

Aug 2004 - surcharging sewers in

Wealdstone and Harrow Weald

from surface and foul water flooding.

2000 1928

Oct 1987 - River Pinn and Wealstone Brook

flooding.

Oct 1993 – Heavy rainfall resulted in

flooding in the Brent and Crane

catchments.

2002 2003

Sept 1992 – Properties flooded in Stanmore,

Edgware and Harrow from flooding of Edgware and

Wealdstone Brooks.

Oct/Nov 1992 Major flooding in the River Pinn area. Worst hit areas are Hatch End and Yeading

Brook.

Nov 1998 - Flooding to properties

adjacent to the River Pinn.

Aug 2002 - Surface water

flooding to highways and properties in Stanmore.

Jan 2003 – EA Flood warnings

issued. Resulted in fluvial flooding at Edgware, Harrow, Kenton, Pinner, Stanmore and Wealdstone.

1999 2001

Aug 2001 - Flooding along Yeading Brook

corridor affecting Pinner South.

2005

July 2005 – flooding of Yeading Brook,

Wealdstone Brook and Edgware Brook.

1987 1992 2007

July 2007 – Surface water sewer flooding

occurred in Kenmore Avenue.

Yeading Brook East and

Weadstone Brook also experienced

flooding.

June & Aug 1999 - Edgware Bk, Yeading

Bk and Crane flooding.



3.4. Flood Mechanisms within Harrow

Based on an analysis of historic flood events, it can be surmised that following flood mechanisms may occur within Harrow:

a. overtopping of riverbanks, flood defences and damage to flood defence walls, particularly across Main River floodplains and where open channels enter culverts.

b. flooding from the surface water network – particularly in urban residential areas where the surface water network connects laterally to long, piped/culverted sections of Main Rivers.

c. pluvial flooding resulting from rapid surface water runoff from urban and rural areas, particularly in the northern part of the Borough where nature reserves and woodland, such as Bentley Priory, Harrow Weald Common and Stanmore Common, become very dry in summer months and/or areas of hard surfacing in urban areas have insufficient drainage or permeability.

d. flooding from culvert surcharge.

e. in-channel blockages and constrictions, however the Council believes that it has a very robust maintenance regime for its Main Rivers. This type of flooding is particularly a problem on culverted watercourses.

f. highway flooding – due to gully blockages and insufficient capacity of culvert crossings during major flood events.

g. sewer flooding – due to a variety of causes, including flows from Combined Storm Overflows (CSO’s) entering watercourses during peak rainfall of intense storm events. This type of flooding is a particular problem along the Wealdstone Brook. There are also potential issues regarding surcharging above ground level in some combined and surface water sewers.

h. possible groundwater flood risk occurring in the northern parts of the Borough where ephemeral streams become active and increase discharge levels into the local sewer networks following periods of heavy rainfall.

i. residual risk of floodwaters overtopping reservoirs or flood defences during heavy rainfall that exceeds the design standard for which the structure was designed.

Accordingly, it can be concluded that flooding within Harrow originates from the following sources:

• rivers (fluvial);

• pluvial/surface water runoff;

• inadequate drainage systems; and

• sewer networks (surface water and foul);

• groundwater; and

• residual risk from failure/overtopping of reservoir and flood defence structures.

Flooding within the Borough may occur as the result of these flooding mechanisms acting either in isolation or in combination. These mechanisms are detailed in the following sections.



3.4.1. River (Fluvial) Flooding

Fluvial flooding occurs when local catchment runoff exceeds the conveyance capacity of a channel, causing floodwaters to overtop the banks of the watercourse and inundate the adjoining floodplain. River flooding is known to be a problem within the Harrow area. In particular, the urbanised nature of much of the area results in significantly higher volumes and rates of runoff than would occur naturally. Therefore, there is a rapid runoff time from the onset of a storm event and watercourse channels have insufficient capacity to adequately convey the flow generated by the rainfall (i.e. ‘flash’ flooding).

In addition, the floodplains of the Borough’s watercourses have been heavily engineered in the past, resulting in rivers being straightened, concrete lined or culverted. There are many pipe and culvert structures at road crossings and often beneath substantial tracts of urbanised land. Such structures present flooding problems when they are of insufficient capacity, or are either damaged, blocked or surcharge.

To limit the impact of flooding, Harrow Council has installed flood defences, such as the dry impounding reservoir at George V Avenue, earth bunds, timber revetment along river corridors and hard defences such as concrete trash screens and bypass channels in each river catchment. The Silk Stream Flood Alleviation Scheme has also been constructed to reduce the risk of flooding to properties within the Borough that are situated within the floodplain of Edgware Brook. These flood defences are discussed in detail in Section 5.7.

However, even with these mitigation measures in place, fluvial flood events can still affect many areas.

3.4.2. Groundwater Flooding

Groundwater flooding may result from high levels of rainfall in winter months filling local aquifers and draining to ephemeral streams. It may also result in areas where the soil does not enable the water to permeate into the sub-strata. Harrow is not particularly susceptible to groundwater flooding; however according to the Environment Agency’s groundwater data (refer Section 4.5), there are some areas in Stanmore, Kenton, Headstone and Rayners Lane where ephemeral streams may become active.

3.4.3. Surface Water Flooding

The intensity of rainfall, particularly in urban areas, can create runoff which temporarily overwhelms the capacity of the local drainage systems. Under these conditions, localised ‘flash’ flooding can occur. Flood conditions can be exacerbated by temporary blockages or obstructions in a watercourse, especially one that has been culverted.

In addition, surface water sewers can flood into foul sewers and overload both the surface water and combined sewer networks. This type of flooding is especially problematic when these systems become overloaded simultaneously.

Specific surface water flooding mechanisms are detailed in the following sections.

Pluvial Flooding

Within Harrow, low-lying areas such as Pinner and Hatch End are susceptible to high levels of pluvial flooding, either due to the extensive urbanisation, problematic surface water drainage systems, the nature and condition of the local watercourses



or from overland flow from flooding of higher ground. Flooding at the top of the catchment may exacerbate the degree of flood risk downstream (both within and beyond the Borough boundary). Such areas are likely to be sensitive to increased development without sufficient investment (where possible) improving the drainage network.

Surface Water Sewer Flooding

Sewer flooding occurs when the network becomes blocked or overloaded, and floodwaters surcharge from the system, which can result in low lying properties being flooded. This is a particular a problem for properties that are connected to the sewer network below the hydraulic level of the sewer, such as basement flats or the ground floor of houses in low lying areas. While the flooding poses a risk in itself, it is also considered to be a serious health hazard.

However, upgrading of the sewer network undertaken by Thames Water over recent years has reduced the potential sewer flood risk in the Borough.

Failure of Reservoir Structures

There is a risk of flooding occurring from the failure of reservoir structures. This may either result from overtopping of reservoirs due to intensive rainfall increasing the water level within the storage structure, or from structural or mechanical failure of the reservoir. This type of flooding poses a flood risk to properties downstream of the reservoirs.

3.5. River Catchments in Harrow

There are three river catchments within the Borough of Harrow, all of which originate from springs. These catchments are:

• Brent catchment – in the east of the Borough;

• Pinn catchment – in the north-west of the Borough; and

• Crane catchment – in the south-west of the Borough.

The locations and extents of these catchments are shown in Figure 3. An overview of each catchment is provided later in this section.

Watercourses within these catchments are categorised as follows:

• Main River –the Environment Agency have permissive powers to carry out maintenance on Main River, which are defined as:

“a watercourse on a main river map and includes any structure or appliance for controlling or regulating the flow of water into, in or out of the channel which: (a) is a structure or appliance situated in the channel or in any part of the banks of the channel; and (b) is not a structure of appliance vested in or controlled by an internal drainage board” (S.113(1) Water Resources Act 1991 (WRA 1991), and see S.137(4) WRA 1991).

• Ordinary Watercourse - is a watercourse that does not form part of a Main River. Maintenance is the responsibility of the riparian owner with the Local Authorities having permissive powers to carry out maintenance. These are the



responsibility of the Council. The Environment Agency has permissive powers to carry out emergency works to prevent flooding of ordinary watercourses.

Table 4 identifies how the Main Rivers and Ordinary Watercourses in the Borough relate to the three river catchments.

Table 4: Main Rivers and Ordinary Watercourses

CATCHMENT MAIN RIVERS ORDINARY

WATERCOURSES

Brent

Edgware Brook

- Kenton Brook

Wealdstone Brook (also referred to as the Kenton Brook, the

Preston Brook and Fortie Brook)

Pinn

River Pinn River Pinn

Woodridings Stream Woodridings Stream

Woodhall Gate Ditch

Crane

Yeading Brook

Smarts Brook

Greenhill Stream

Elmshot Brook

Yeading Brook East Arm Roxbourne Brook

Yeading Brook (Victor Road Branch)

Yeading Brook (Headstone Park

Branch)

Roxbourne Brook Return Arm

The Roxbourne

3.5.1. Brent River Catchment

The Brent catchment is highly urbanised and covers an area of approximately 153km

2. It is located in the east of the Borough and covers 11 wards, including

Cannons, Stanmore Park, Harrow West, Wealdstone, Marlborough, Greenhill, Kenton West, Kenton East, Edgware, Queensway and Belmont.

The main tributaries within the Borough are Wealdstone Brook, Edgware Brook and Kenton Brook. Details of the watercourses within the catchment are included in Appendix B.

The Brent River catchment has historically experienced frequent flooding, with extensive floods being recorded in August 1977, June 1982, May 1988, September 1992 and October/November 2000. Flooding within the catchment can occur from multiple sources, including local sewer and fluvial flooding. River flooding is the most



common source, however the frequency of floods in this catchment can be exacerbated by the limited capacity of the sewer system, which is unable to cope with excessive runoff during moderate storm events. The result is that there is often flooding from the sewerage network when the rivers are only partially full.

The large urban areas and natural clay geology within this catchment make it prone to ‘flash flooding’ in watercourses and surface water runoff.

The Wealdstone Brook Flood Alleviation Scheme was undertaken in the 1970s and 1980s. As a result of this scheme, almost the whole length of Wealdstone Brook was canalised, replacing the natural earth bed and banks of the channel with concrete lined channels and upgrading of culvert crossings.

Major flooding in 1992 lead to the initiation of the Silk Stream Flood Alleviation Scheme with the aim of reducing the flood risk to properties along Edgware Brook and Silk Stream (downstream of the Borough). The scheme comprises flood storage at six sites, two trash screen renewals and a local flood defence. Three of these flood storage areas are located within the London Borough of Harrow. This scheme is discussed in detail Section 5.7.1.

3.5.2. Pinn River Catchment (part of the Colne Catchment)

The Pinn River catchment lies in the north-west of the Borough and feeds into the Colne River beyond the Harrow Borough boundary. The Pinn catchment covers a total area of 370km

2 and encompasses Harrow Weald, Hatch End, Pinner and

Pinner South.

The main watercourses within the Borough are the Pinn River, Woodridings Stream and Woodhall Gate Ditch. Details of these watercourses are included in Appendix B.

Flooding within the Pinn River catchment can occur from multiple sources, including sewerage network surcharging and fluvial sources. The worst affected areas include Hatch End, Pinner, Ruislip and Uxbridge. Recent flooding in the catchment occurred in 1998, 2000, 2005, 2006 and 2007.

3.5.3. Crane River Catchment

The Crane River catchment is predominantly urbanised and comprises an area of approximately 124km

2 and forms part of five London Boroughs (Harrow, Hillingdon,

Ealing, Hounslow and Richmond upon Thames).

The tributaries of the River Crane within the London Borough of Harrow include the Yeading Brook East, Yeading Brook West, the Roxbourne, Greenhill Stream, Elmshot Stream, and Smarts Brook. The principal tributary of the Crane River, Yeading Brook (formerly known as ‘The River Fishbourne’), originates in the high ground between the suburbs of Pinner and Harrow. Details of the main waterways in the Borough are enclosed in Appendix B.

Flooding within the Crane River catchment has occurred as a result of surface water, sewer or riverine flooding. Recent flood events in the catchment occurred in 2001, 2002, 2003, 2005 and 2007.

3.6. Reservoirs and Water Supplies (including lakes and ponds)

Three Valleys Water and Thames Water are the main suppliers of drinking water in the Borough.



There are five (5) statutory reservoirs in the London Borough of Harrow (refer locations in Figure 6). These are:

• Harrow on the Hill (drinking water reservoir);

• Summerhouse – Bentley Priory;

• Severn Acre – Edgware;

• Prince Edward Playing Fields; and

• George V – Pinner.

There is one drinking water reservoir situated at Harrow on the Hill. There are no known private water supplies for domestic purposes, although a number of boreholes exist. There are three private water supplies for commercial/industrial purposes at Kodak Ltd, which is located on Headstone Drive in Wealdstone.

Harrow is known to have one of the highest densities of ponds in greater London, with sixteen (16) ponds per km

2. The larger lakes in the area are Canons Lake

(Edgware), Summerhouse Lake (Bentley Priory), and Squirrels Lake and Serpentine Lake in the Harrow School grounds.

3.6.1.1. Harrow on the Hill

Harrow on the Hill reservoir is an earthfill dam with a maximum height of 6m and a capacity of 27Ml. The reservoir has an important strategic role in the local water network as it provides most of the immediate area’s water (it is at the highest point locally and the connecting water supply network is gravity-fed by this reservoir). It is also used as a short-term emergency water supply during interruptions to the normal supply in the Borough.

3.6.1.2. Seven Acre – Edgware

This is a raised structural retaining wall which provides additional capacity to regulate downstream flow into the Silk Stream. This reservoir forms part of the Silk Stream Flood Alleviation Scheme (refer Section 5.7.1). Works undertaken as part of the scheme raised the existing embankment around the east of the lake by 0.5m, thereby increasing its storage capacity from 28Ml to 53Ml. The upgraded embankment incorporates a weir and control structure, and the new embankment is designed to act as a spillway if the design 4% annual probability flood is exceeded.

3.6.1.3. Summerhouse – Bentley Priory

This lake is located within the Bentley Priory Nature Reserve and is formed by a raised structural retaining wall which dams Edgware Brook. The lake provides additional capacity to regulate downstream flow into the Silk Stream, reducing the flood risk in Stanmore and Edgware.

The capacity of the lake has been increased as part of the Silk Stream Flood Alleviation Scheme (refer Section 5.7.1). These works have involved raising the existing brick retaining wall and spillway along the eastern banks of the lake by approximately 0.6m to increase the storage volume to 34Ml. A 17m wide length of

spillway has been incorporated into the retaining wall and will operate if the design 4% annual probability event is exceeded (Halcrow, 2003).



3.6.1.4. Prince Edward Playing Fields

A new embankment dam has been constructed on the eastern side of the Prince Edward Playing Fields. This dam has a capacity of 48Ml and will provide flood storage to protect areas downstream along Edgware Brook against a 4% annual probability flood. The embankment incorporates a control structure to convey normal flows from Edgware Brook and two 41m wide spillways in the embankment will discharge flow if the design 4% annual probability storm is exceeded.

3.6.1.5. George V – Pinner

This is a statutory1 dry impounding

2 reservoir that regulates the flow of water to avoid

flooding in Pinner. It has a maximum height of 4m and a capacity of 62.4Ml.

It was constructed in 1989 and reduces the probability of flooding to properties alongside the River Pinn from the storage reservoir downstream to the confluence with the Woodridings Stream (Mott MacDonald, 2006).

3.7. Flood Defences

Flood defences can consists of man-made structures, such as embankments, bunds, sluice gates, reservoirs and flood barriers, which are designed to prevent flooding of areas adjacent to the defence. It is important to appreciate that flood defences can only prevent flooding up to their design standard; they cannot prevent all flooding. Table 5 provides a list of the existing flood defences in the Borough.

PPS25 considers defended areas to have some risk of flooding, and therefore sites within these areas must be assessed with respect to the adequacy of the defences. Accordingly, it should not be assumed that proposed new development in areas ‘protected’ by flood defences is acceptable.

The defences listed in Table 5 do not include underground controlled storage structures that have been installed by developers as part of requirements set by the local planning authority and/or major schemes by Thames Water Utilities Ltd. There are 51 of these additional storage structures that are inspected and maintained.

The locations of the flood defences within the Borough are shown in Figure 6.

Significant defences and flood alleviation schemes are discussed in more detail in the following sections.

1 Generally reservoirs with a capacity above 25Ml. 2 Traditional form of upland reservoir produced by damming a watercourse.



Table 5: List of Flood Defences

CATCHMENT LOCATION OF FLOOD

DEFENCE DEFENCE TYPE

Brent

Kenton Brook Concrete reinforced channel

Centenary Park Bunded storage

Dennis Lane Underground storage

Whitchurch Playing Fields Bunded storage

Prince Edward Playing Fields Bunded storage (maintained by Environment Agency)

RAF Stanmore Park Temple Pond

7 Acre Lake Reservoir Storage

Summerhouse Lake Reservoir

Storage (maintained by Environment Agency)

Pinn

George V Avenue Reservoir

George V Avenue Reservoir (maintained by Environment Agency)

Pinner Golf Course Attenuation ponds/controlled discharge

Oxhey Lane Farm Proposed storage area

Hatch End Arts Centre Deculverting

Montesole Park Bunded storage

Crane

Headstone Manor

Newton Farm Ecology Park Attenuation ponds

3.7.1. Silk Stream Flood Alleviation Scheme (Brent Catchment)

In 2004/2005, works were undertaken in the Borough as part of the Silk Stream Flood Alleviation Scheme at a total cost of £1.2M. The scheme comprises the formation of flood storage areas at six sites in the Brent catchment, two trash screen renewals and a local flood defence. The locations of the storage areas are:

• Stoney Wood Lake;

• Bury Farm;

• Edgwarebury Park;



• Seven Acre Lake (within the London Borough of Harrow) (refer Section 4.6.1.2);

• Summerhouse Lake (within the London Borough of Harrow) (refer Section 4.6.1.3); and

• Prince Edward Playing Fields (within the London Borough of Harrow) (refer Section 4.6.1.4).

The flood storage areas are designed to hold floodwaters in times of heavy rain to prevent the rivers from overtopping their banks and flooding properties and key infrastructure within the floodplain of Edgware Brook. The standard of flood protection is predicted to be improved to a minimum of a 4% annual probability of occurrence (Halcrow, 2003). The scheme aims to reduce the flood risk to 746 homes and businesses within Harrow and Brent.

3.7.2. Oxhey Lane Farm Flood Alleviation Scheme (Pinn Catchment)

In June 2008, the Council commenced work on the Oxhey Lane Farm Flood Alleviation Scheme to reduce the flood risk from Woodridings Stream in the Pinn catchment. The works are proposed to be sited within the farm which operates under an agricultural tenancy agreement with the Council.

It is proposed to construct a dry detention basin on Oxhey Lane Farm close to the boundary with The Avenue and to the rear of Royston Park Road. The works will include the construction of a 300m long earth embankment on the southern edge of Oxhey Lane Farm. The embankment will form a basin behind it, and will incorporate a pipe passing which will restrict outflow and thereby avoid the possibility of flooding downstream. This basin will temporarily store water upstream of the urban area by allowing farmland to flood for short periods at the peak of a storm, then gradually release this stored water once the peak has passed.

In order to maximise the use of this flood storage area, flows from the Oxhey Lane area, including runoff from Old Redding, will be diverted to the flood storage area by a new ditch close to the farm boundary with the gardens of Royston Park Road.

The maximum capacity of the flood storage area will be just less than 24 Ml. A storage area of this volume does not fall within the provisions of the Reservoirs Act 1975, which would require a formal inspection regime. Nevertheless, Harrow Council, as landowners and the Flood Defence Operating Authority, will carry out regular inspections to ensure the continuing safety of the embankment and its appurtenant structures. Should a more extreme flood occur than that predicted to arise once in every 100 years, a part of the embankment will overtop, discharging water to the same streams on the downstream side that would take the floodwater if no embankment had been constructed.

3.7.3. Hatch End Flood Alleviation Scheme (Pinn Catchment)

Work has been carried out in recent years to prevent the River Pinn from flooding surrounding areas. The Hatch End Flood Alleviation Scheme addresses the River Pinn catchment and the catchment of its tributary, known as Woodridings Stream. The Woodridings Stream catchment covers an area of approximately 850 hectares and drains the steep, highly urbanised areas of Hatch End, Harrow Weald and Pinner, where significant flooding damage has occurred in the floods of 1977, 1988, 1992 and 2000. It consists of both open channel sections and piped sections.

To reduce the flood risk in the Uxbridge Road, Rowlands Avenue and Headstone Lane areas, it is proposed to improve the outfalls to the River Pinn and increase the



capacity of the river downstream of Uxbridge Road. The scheme involves the diversion of the river into 550m length of open channel through the playing fields east of the Harrow Arts Centre that would replace the existing brick culvert carrying the River Pinn from Uxbridge Road to the railway line. These diversion works are now complete.

The scheme also involves the construction of a new flood storage area to the north of the arts centre, for which construction works recently commenced.



4. HYDROLOGIC ANALYSIS

As discussed in Section 3.1, modelled flood levels, flood outline mapping and/or flood zone mapping is available for a number of watercourses within the Borough. In those areas where it is available, the Environment Agency (EA) flood zone mapping has been adopted to provide an indication of the predicted extent and categorization of flood risk for the SFRA process.

However, it has been necessary to carry out specific detailed modelling investigations as part of this SFRA to define the flood behaviour and associated flood risk along unmapped watercourses. Broad-scale hydraulic models have been developed for the following watercourses:

• Top of River Pinn - upstream of the railway line;

• Top of Woodridings Stream - upstream of the railway line;

• Top of Yeading Brook West Arm - upstream of Pinner Road;

• Top of Yeading Brook East Arm - upstream of Malvern Avenue;

• The Roxbourne;

• Roxbourne Return Arm;

• Greenhill Stream;

• Smarts Brook; and

• Elmshot Stream.

Details of the hydraulic modelling are provided in Section 5.

In order to provide inputs to the hydraulic model, estimates of the 5%, 1%, 1% + 20% (i.e. taking into account estimates for climate change) and 0.1% annual probability flood flows are required. The hydrological modelling that has been completed for this SFRA is outlined in the following sections.

4.1. Choice of Methodology

Flow gauges on the unmapped watercourses are located at:

• Yeading Brook West at Gutteridge Wood (Hillingdon);

• Yeading Brook East at Northholt; and

• River Pinn at Uxbridge


• details of the methodology used to determine the hydrological conditions of unmapped watercourses; and

• results of the hydrological modelling.



Consequently, no gauged flows are available for the reaches of the watercourses within the study area and it is therefore necessary to use a hydrological modelling methodology suitable for ungauged catchments.

As discussed previously, the London Borough of Harrow is located at the headwaters of the unmapped watercourses for which hydraulic modelling was undertaken and, as a result, the catchments draining to the modelled watercourses are relatively small. Due to smaller catchment sizes, the Revitalised Flood Hydrograph Method (ReFH) has been adopted. This is the most up-to-date method available and is an improvement over the FSR/FEH model. It is a rainfall-runoff approach, deriving design rainfall from the FEH rainfall frequency statistics. A four-parameter model transforms the rainfall event into a flow hydrograph.

Version 2.0 of the FEH CD-Rom was used to obtain catchment descriptors for each of the contributing catchments.

Environment Agency guidance in Annex B of PPS25 states that an allowance of 20% should be made to peak river flows to allow for climate change (for the period 2055-2085). This guidance has been adopted to determine the flood discharges for the 1% annual probability flood + climate change effects.

4.2. Hydrological Modelling

Flow hydrographs were calculated using ReFH methodologies for the upstream boundary location of the hydraulic models. Flow hydrographs for contributing catchments within the extent of the hydraulic models have been derived by subtracting the hydrograph at the upstream boundary location from the hydrograph for the total catchment.

4.2.1. Woodridings Stream

The ReFH method has been adopted to calculate design hydrographs and peak flows based on a critical storm duration of 2.7 hours. The peak flows generated for Woodridings Stream at the location of inflows into the hydraulic model are listed in Table 6.

Please note that for Tables 6 to 12, the total catchment areas quoted are representative of the total contributing catchment area at each location. However, the peak inflows correspond to the flow hydrographs inputted into the hydraulic model rather than the total cumulative flow at the location specified.

Table 6: Peak Flows for Woodridings Stream

LOCATION

TOTAL CATCHMENT

AREA (km

2)

PEAK INFLOW (m

3/s)

5% 1% 1%+CC 0.1%

Oxley Lane 1.3 1.7 3.9 4.7 5.7

Railway Line 1.39 0.1 0.2 0.2 0.4



4.2.2. River Pinn

The ReFH method has been adopted to calculate design hydrographs and peak flows based on a critical storm duration of 3.5 hours. The peak flows generated for River Pinn at the location of inflows into the hydraulic model are listed in Table 7.

Table 7: Peak Flows for River Pinn

LOCATION

TOTAL CATCHMENT

AREA (km

2)

PEAK FLOW (m

3/s)

5% 1% 1%+CC 0.1%

470m upstream of Oxley Lane

0.52 0.8 1.2 1.5 2.5

Oxley Lane 0.7 0.3 0.4 0.5 0.9

Railway Line 1.11 0.7 1.0 1.2 2.2

4.2.3. Yeading Brook West (including Elmshot Brook)

The ReFH method has been adopted to calculate design hydrographs and peak flows based on a critical storm duration of 3.3 hours. The peak flows generated for Yeading Brook West at the location of inflows into the hydraulic model are listed in Table 8.

As there is no specific catchment delineated for Elmshot Stream on the FEHCD-Rom, the catchment area for the stream was estimated based on the topography of the area and an areal reduction factor was applied to the Yeading Brook West catchment at the junction of Elmshot Stream and Yeading Brook West in order to estimate the flood hydrograph for flood flows within the stream.

Table 8: Peak Flows for Yeading Brook West

LOCATION

TOTAL CATCHMENT

AREA (km

2)

PEAK FLOW (m

3/s)

5% 1% 1%+CC 0.1%

Yeading Brook West – Headstone Lane

1.28 2.9 4.8 5.8 12.3

Yeading Brook West – Pinner Road

3.69 1.5 2.5 3.0 6.3

Victor Road Branch – Headstone Park

1.52 2.5 4.2 5.0 10.7

Elmshot Stream at Parkside Way

0.52 1.7 2.7 3.2 6.9

Please note that the flows for Yeading Brook West that are quoted in Table 8 for Headstone Lane and Pinner Road may appear disproportionate to the relative



catchment sizes. However, this is due to the difference in characteristics (e.g. catchment size, slope, urban extent, runoff length, etc) for the catchments draining to these locations.

4.2.4. Smarts Brook

The ReFH method has been adopted to calculate design hydrographs and peak flows based on a critical storm duration of 1.3 hours. The peak flows generated for Smarts Brook at the location of inflows into the hydraulic model are listed in Table 9.

Table 9: Peak Flows for Smarts Brook

LOCATION

TOTAL CATCHMENT

AREA (km

2)

PEAK FLOW (m

3/s)

5% 1% 1%+CC 0.1%

Railway Line (near West Harrow

Station) 0.52 1.9 3.1 3.7 8.4

Imperial Drive 1.31 1.9 3.2 3.8 8.6

Railway Line (near Rayners Lane

Station) 2.04 0.4 2.3 2.8 6.0

Please note that the flows for Smarts Brook that are quoted in Table 9 may appear disproportionate to the relative catchment sizes. However, this is due to the difference in characteristics (e.g. catchment size, slope, urban extent, runoff length, etc) for the catchments draining to these locations.

4.2.5. Yeading Brook East

The ReFH method has been adopted to calculate design hydrographs and peak flows based on a critical storm duration of 1.3 hours. The peak flows generated for The Roxbourne at the location of inflows into the hydraulic model are listed in Table 10.

Table 10: Peak Flows for Yeading Brook East

LOCATION

TOTAL CATCHMENT

AREA (km

2)

PEAK FLOW (m

3/s)

5% 1% 1%+CC 0.1%

Alexandra Avenue 1.28 3.0 5.0 6.0 13.3

4.2.6. The Roxbourne

The ReFH method has been adopted to calculate design hydrographs and peak flows based on a critical storm duration of 0.5 hours. The peak flows generated for



The Roxbourne at the location of inflows into the hydraulic model are listed in Table 11.

Table 11: Peak Flows for The Roxbourne

LOCATION

TOTAL CATCHMENT

AREA (km

2)

PEAK FLOW (m

3/s)

5% 1% 1%+CC 0.1%

Northolt Road 0.62 2.2 3.7 4.4 10.2

Roxeth Green Avenue 0.83 0.6 1.0 1.2 2.7

Alexandra Avenue 0.94 0.4 0.6 0.7 1.7

4.2.7. Greenhill Stream

The ReFH method has been adopted to calculate design hydrographs and peak flows based on a critical storm duration of 3.3 hours. The peak flows generated for Greenhill Stream at the location of inflows into the hydraulic model are listed in Table 12.

Table 12: Peak Flows for Greenhill Stream

LOCATION

TOTAL CATCHMENT

AREA (km

2)

PEAK FLOW (m

3/s)

5% 1% 1%+CC 0.1%

Headstone Road 0.52 1.7 2.7 3.2 6.9

Station Road 1.05 1.4 2.2 2.7 5.8

4.3. Hydrological Model Verification

The peak flows generated using the ReFH methodology were verified against peak flows generated using the rational method. The rational method is a simple equation that can be applied to determine the peak flow for a catchment. It estimates the peak flow for a given location as a function of the drainage area, runoff coefficient and mean rainfall intensity for the duration equal to the critical storm duration. The equation for flow using this method is expressed as:

Qp= 0.278CiA

where:

Qp = Peak Flow (m3/s)

0.278 = Metric Conversion Factor

C = Coefficient of Runoff



I = Intensity of rainfall in time Tc (mm/hr) (where Tc = Time of concentration)

A = Catchment Area (km2)

The peak 1% annual probability flows for the upstream catchments of each modelled watercourse were calculated using the Rational Method. Runoff coefficient values were adopted based on catchment impermeable area and slope with typical coefficient values between 0.3 and 0.5 adopted for 1% annual probability event by using the depth-duration-frequency curves produced by the FEH CD-ROM for the catchments.

A comparison of the peak flows generated using the ReFH and Rational Methods is provided in Table 13. The results show that the ReFH peak flows are slightly higher for the majority of the watercourses, with the exception of The Roxbourne at Northholt Road. The assumption of uniform, steady rainfall in the Rational Method tends towards underestimation of the peak discharge.

Importantly, this verification demonstrates that the Rational Method generates a peak flow in the same order of magnitude as the ReFH method. Therefore, it is considered that the hydrographs generated for the modelled watercourses using the ReFH method are suitable for use for this study.

Table 13: Comparison of ReFH and Rational Method Flows

WATERCOURSE LOCATION

PEAK 1% ANNUAL PROBABILITY FLOWS

(m3/s)

ReFH METHOD RATIONAL METHOD

Woodridings Stream Oxley Lane 3.9 2.4

River Pinn Upstream of Oxley

Lane 1.2 0.8

Yeading Brook West Headstone Lane 4.8 2.7

Smarts Brook Railway Line 3.1 2.9

Yeading Brook East Alexandra Avenue 5.0 4.4

The Roxbourne Northolt Road 3.7 4.6

Greenhill Stream Headstone Road 2.7 1.4



5. HYDRAULIC ANALYSIS

Six (6) broad-scale hydraulic models have been developed as part of this SFRA to define the flood characteristics along Ordinary Watercourses for which EA flood zone mapping is not available (i.e. ‘unmapped watercourses’). The development and results of these models are discussed in the following sections.

5.1. Choice of Methodology

A one-dimensional (1D) modelling approach has been selected for the hydraulic modelling of unmapped watercourses. The use of 1D modelling approach is considered adequate for the level of detail required in this Level 2 SFRA. However, this approach implies a number of assumptions:

• water levels are assumed horizontal across the section;

• all flow is assumed to be perpendicular to the cross-sections; and

• no account is taken of attenuation within the channel or floodplains.

These assumptions are generally conservative for in-channel flow and simple flows within the floodplain.

The ISIS 1D modelling package was selected for use for the study. ISIS is a professional engineering software package for the simulation of flows and flow routing techniques. It is a dynamic, one-dimensional modelling tool capable of dynamic modelling of open channels, floodplains, embankments and hydraulic structures. It can also analyse over-bank and piped flow. ISIS Version 3.0.1 was used for this study.

5.2. Hydraulic Model Development

5.2.1. Preparation of a Digital Terrain Model (DTM)

As discussed in Section 2.2, LIDAR data was provided by the Environment Agency for the majority of the study area. This data comprises very large data sets that contain thousands of points defining the existing ground surface elevations within the study area.

MWH has subsequently processed this data to create a Digital Terrain Model (DTM) that comprises a set of data points on the ground which defines the topography of


• an outline of the methodology used to determine the flood conditions of unmapped watercourses

• details of the model development;

• results of hydraulic modelling;

• limitations of the use of the modelling results; and

• details of the mapping of flood extents and flood zones.



the Borough. The resulting DTM covers the floodplain within the study area and essentially forms a complete 3D representation of the terrain of the channels and floodplain within the London Borough of Harrow. This DTM is shown in Figure 3.

LIDAR procedures are unable to penetrate through water and do not typically include hydrographic features important for flood modelling, such as the bathymetry of watercourses that carry water under normal flow conditions. However, as a significant proportion of the watercourses are piped or culverted, and because this modelling is broad-scale only, it is considered that the DTM can be used to define both the channel and floodplain for unmapped watercourses.

5.2.2. Network Development

Six (6) 1D model networks were developed to extend along the unmapped watercourses within the Borough using the ISIS modelling software. The extent of each hydraulic model is summarised in Table 14 and shown in Figure 7. These extents are based on the extents of the watercourses and/or extent of available data required to undertake hydraulic modelling (e.g. structure details, LIDAR data, etc).

A series of river sections were used to model the open channel sections of the modelled watercourses. The elevations within the channel system and across the floodplain have been assigned based on the DTM developed for the study from the LIDAR data within the borough. It was necessary to manually adjust some of the model cross-sections to provide a better representation of the channels based on field observations and data from the Council.

A significant proportion of the watercourses that have been modelled using ISIS are piped or culverted. The details of these structures have been incorporated within the model based on specific data provided by the Council for the structures (e.g. pipe size, length, invert level, etc) (refer Section 2.6). For the purpose of this study, the model was simulated assuming the culverts were unblocked (i.e. 0% blockage). Spills were incorporated within the model to simulate overland flow resulting from insufficient pipe capacity and the resultant overtopping of structures.

Please note that the Yeading Brook West Headstone Park Branch has not been included in the model of Yeading Brook West as it is piped along its entire length to its confluence with the main branch of Yeading Brook West.



Table 14: Hydraulic Modelling Extents

WATERCOURSE UPSTREAM LIMIT DOWNSTREAM LIMIT MODEL LENGTH (km)

River Pinn 40m upstream of Oxley Lane 75m downstream of the

Railway Line 1.4

Woodridings Stream 900m upstream of Oxley Lane 40m downstream of the

Railway Line 0.95

Yeading Brook West Arm, Greenhill Stream and Elmshot

Brook

Yeading Brook West – Pinner Park (near Greystoke Avenue)

Yeading Brook West Victor Road branch – Headstone Park (near

Pinner Park Avenue)

Greenhill Stream – Headstone Road

Elmshot Stream – Walton Road

65m downstream of the Railway Line

3.3

Smarts Brook Railway Line

(Near West Harrow Station) Confluence with Yeading Brook

West Arm 2.45

Yeading Brook East Arm 140m upstream of Alexandra Avenue

25m downstream of Alexandra Avenue

0.3

The Roxbourne Northolt Road Yeading Brook -

25m downstream of Alexandra Avenue

1.4



5.2.2.1. Roughness Parameters

Values for the Manning’s ‘n’ roughness coefficient were estimated for the study area by comparing vegetation density and soil types with aerial photography, standard photographic records and descriptions of stream and floodplain conditions for which roughness values are documented. The adopted values are listed in Table 15.

Roughness values were delineated across each cross-section to ‘identify distinct variations in the hydraulic roughness across the river channel and floodplain. The adopted roughness values are all within the acceptable limits defined in relevant literature and from experience in the application of ISIS for flood modelling.

Table 15: Adopted Manning’s ‘n’ Values

LAND COVER MANNING’S ‘n’

River Channel

Lightly Vegetated Channel / Ditch 0.03

Moderately Vegetated Channel 0.05

Floodplain

Industrial 0.02

Residential 0.04

Short Grass / Golf Course 0.03

Pastureland 0.035

Pastureland with Scattered Trees 0.05

Heavy Vegetation 0.10

5.2.2.2. Model Boundaries

The upstream boundary conditions of the model were defined by flow discharges derived using the ReFH methodology (refer Sections 4.2.1 to 4.2.7). Lateral inflows were also incorporated into the model to account for contributing catchment areas lying within the extent of the hydraulic model. The peak flows corresponding to the hydrographs incorporated within the ISIS models are listed in Tables 6 to 12.

The downstream boundaries of the models were defined for 5 out of the 6 hydraulic models based on known design peak flood levels from previous modelling studies that were provided by the Environment Agency for the mapped watercourses. Accordingly, static tailwater levels were adopted for these models, as defined in Table 16.



Table 16: Adopted Tailwater Levels for the ISIS Models

LOCATION

TAILWATER LEVEL (mAOD)

5% 1% 1%+CC 0.1%

River Pinn 57.38 57.16 57.161 57.18

Woodridings Stream* - - - -

Yeading Brook West Arm, Greenhill Stream and

Elmshot Brook 46.618 46.722 46.795 47.246

Smarts Brook 40.894 41.034 41.134 42.336

Yeading Brook East Arm 40.17 40.307 40.406 41.187

The Roxbourne 40.17 40.307 40.406 41.187

* Refer explanation below.

For Woodridings Stream, the downstream boundary conditions for the ISIS model were assumed to be gradually varying water levels that change with increasing flood discharge. These tailwater conditions were determined by applying normal depth hydraulic formulae using the channel/floodplain conveyance at the downstream cross-section of the model and the peak design flows.

5.3. Model Calibration

Hydrological/hydraulic uncertainty may be broken down into three categories:

• natural/inherent uncertainty which arises from the random variability of hydrological/hydraulic phenomena.

• model uncertainty, which results from the inevitable approximations implicit in model equations.

• parameter uncertainty which stems from the unknown nature of the coefficients in the model equations and assumptions made by the modeller.

The ability of a hydraulic model to produce reliable estimations of design flood events is typically assessed based on calibration and verification of a model against historic flood events. That is, flow estimations from historic floods are routed through the model and the flood levels/flows produced by the model are compared against recorded levels/flows from the actual flood event. Model parameters are then adjusted to produce flood levels that are within a tolerable range of the recorded levels.

As there is no river gauge located on the reaches of the watercourses that were modelled and no recorded flood marks from historic floods, it has not been possible to calibrate the hydraulic models. However, consistency has been maintained between the model parameters listed in technical guidance and the conditions in the study area. Moreover, the adopted values of Manning’s n (refer



Table 14) are considered to be sufficiently conservative to be used for defining design flood levels.

In addition, a sensitivity analysis was carried out to assess the impact of a change in roughness values. That is, parameters within the model have been changed, the model simulated and the results compared to determine the effect on the model. The results of this analysis are discussed in the following section.

5.4. Sensitivity Analysis

The sensitivity of the models to changes in the Manning’s n roughness values varies for each watercourse. Changes in predicted peak flood level also vary within each model at different locations along a watercourse. The change in flood level for each model is listed in Table 17.

Table 17: Results of Sensitivity Analyses for the 1% Annual Probability Flood

LOCATION

MODEL SENSITIVITY (mm)

+10% MANNING’S n -10% MANNING’S n

River Pinn 0 to 65 -88 to 0

Woodridings Stream* 0 to 72 -80 to 0

Yeading Brook West Arm, Greenhill Stream and

Elmshot Brook 0 to 370 -160 to 0

Smarts Brook 0 to 20 -20 to 0

Yeading Brook East Arm 0 to 60 -80 to 0

The Roxbourne 0 to 280 -180 to 0

On the basis of this analysis, it can be seen that the ISIS models for Yeading Brook West and The Roxbourne are the most sensitive to changes to Manning’s n values. A 10% increase or decrease in Manning’s n value results in less than a 90mm change in predicted peak flood level for the other models.

5.4.1. Limitations of the model

Due to the absence of reliable historical data, the model has not been calibrated. However, the model has been built using the best available data and can be considered appropriate for flows up to bank full capacity and simple flow on the floodplain.

The model is less appropriate for complex out-of-bank flow routes that may not be represented in the schematisation. The model will also not represent flooding from the drainage system where backing up in the sewers may lead to flooding away from the watercourse.



It should also be noted that the broad-scale models developed for this SFRA assume ‘typical’ conditions within the respective river systems that are being analysed. The predicted flood water levels may change if the operating regimes of the rivers involved are altered (e.g. engineering works and/or mitigation works which may be implemented in the future), culverts are permitted to block, or the condition of the river channel is allowed to deteriorate.

5.4.2. Model Simulations, Results and Mapping

The ISIS models have been used to simulate the 5%, 1%, 1% + climate change and 0.1% annual probability floods. Outputs of flow rates, peak flood levels, floodwater depths and flow velocities were produced for each cross-section (or node) within the model.

The water levels predicted by the models were mapped at each of the cross-section locations. These mapped levels were then converted into a water surface and ‘mapped’ on to the DTM for the study area to create a flood zone mapping for the modelled watercourses. The mapping produced is shown in Figures 1 to 8 in Volume III – Mapping.



6. REFERENCES

CIRIA. 2000, Sustainable urban drainage systems - design manual for England and Wales (Report C522).

CIRIA. 2001, Sustainable urban drainage systems - best practice manual (Report C523).

CIRIA. 2003, Sustainable drainage systems. Hydraulic, structural and water quality advice (Report C609).

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

Communities and Local Government. 2007, Improving the Flood Performance of New Buildings, Flood Resilient Construction.

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

Defra. 2007, Future Water.

Defra. 2005, Making Space for Water.

Environment Agency. 2007, Thames Catchment Flood Management Plan.

Environment Agency. 2007, Preparing for a flood. Practical advice on what to do to protect you and your property.

Environment Agency. 2007, Living on the Edge.

Environment Agency. Thames Region Land Drainage Byelaws.

Environment Agency. 2006, Sustainable Drainage Systems – Draft Best Practice Guideline.

Greater London Authority. 2006, Draft Regional Flood Risk Appraisal (RFRA).

Greater London Authority. 2006, London Plan, Housing Provision Targets, Waste and Minerals Alterations.

Halcrow Group Limited. 2003, Silk Stream Flood Alleviation Scheme, Design Workshop Information, prepared for the Environment Agency.

Hulme, M., Jenkins, G.J., Lu, X., Turnpenny, J.R., Mitchell, T.D., Jones, R.G., Lowe, J., Murphy, J.M., Hassell, D., Boorman, P., McDonald, R. and Hill, S. 2002, Climate Change Scenarios for the United Kingdom: The UKCIP02 Scientific Report,Tyndall Centre for Climate Change Research, School of Environmental Sciences, University of East Anglia, Norwich, UK. 120pp.

Institute of Hydrology. 1994, Report No. 124 Flood Estimation for Small Catchments.

Jacobs. 2007, London Borough of Brent Strategic Flood Risk Assessment (SFRA) Level 1.

John Timms MBE Principal Researcher – Flood Information.

London Borough of Harrow. 2008, London Borough of Harrow Local Development Framework (Draft).



London Borough of Harrow. 2004, Harrow Unitary Development Policy (HUDP).

London Borough of Harrow. 2001, Policy Statement for Flood Defence.

London Development Agency. 2008, London Plan Spatial Development Strategy for Greater London (Consolidated with Alterations since 2004).

London Resilience Team. 2007, London Flood Response Strategic Plan.

Mott MacDonald. 2006, River Pinn Flood History Report, prepared for the Environment Agency.

MWH. 2008, North Brent Integrated Urban Drainage Pilot Study, Final Report (Draft).

National SuDS Working Group. 2004, Interim Code of Practice for Sustainable Drainage Systems.

Office for National Statistics. 2007, ONS Mid-year Population Estimates: 2001 to 2006.

PBA, Jacobs and Atkins. 2003, River Brent Flood Study, prepared for the Environment Agency.

PBA, Jacobs and Atkins. 2003, River Crane Flood Study, prepared for the Environment Agency.

Pitt, M. 2008, The Pitt Review: Learning lessons from the 2007 floods - Final Report.

Town and Country Planning Act 1990.

Town and Country Planning - Flooding (England) Direction 2007

Water Resources Act 1991 (WRA 1991).

West London Sub-regional Development Framework. 2006, Sub-Regional Development Framework West London.


MWH UK LTD

APPENDIX A

Historical Flood Information


MWH UK LTD

APPENDIX B

Details of the Main Rivers within the Borough