strategic flood risk assessment (level 2 sfra) technical ... · the level 1 sfra comprises three...

London Borough of Harrow Council

Strategic Flood Risk Assessment (Level 2 SFRA)

Technical Report

June 2011

Client: London Borough of Harrow Council

Project: Strategic Flood Risk Assessment Level 2

Document Title: Technical Report

MWH Project Code: 41517712 Document No: RT/EWI/NE11/0020/

Version Date Description/Amendment Prepared by (Author)

Checked by Reviewed by

01 27.09. 2010 Draft for Comment Gaye McKay Vicky Boorman

Steve Dickie

02 13.12.2010 Draft for comment Gaye McKay Steve Dickie Steve Dickie

03 03.02.2011 Final Draft Gaye McKay Steve Dickie Steve Dickie

04 17.03.11 Final Draft incorporating Environment Agency Comments Gaye McKay Steve Dickie Steve Dickie

05 28.03.11 Final Draft incorporating LB Harrow Comments Gaye McKay Steve Dickie Steve Dickie

06 01.04.11 1st Issue Gaye McKay Steve Dickie Steve Dickie

07 19.05.11 Final Issue Gaye McKay Steve Dickie Steve Dickie

08 07.06.11 Amended Final Issue Gaye McKay Steve Dickie Steve Dickie

09 16.06.11 Amended Final Issue Gaye McKay Steve Dickie Steve Dickie

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London Borough of Harrow Council – Level 2 Strategic Flood Risk Assessment Technical Report

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EXECUTIVE SUMMARY

The London Borough of Harrow is subject to fluvial 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’, MWH prepared a Level 1 Strategic Flood Risk Assessment (SFRA) to provide 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 Level 1 SFRA comprises 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.

The Level 2 SFRA provides a detailed understanding of the flood hazard, taking into account flood management measures to allow the local authority to put forward sites which answer Part C of the Exception test.

“development will be safe, without increasing flood risk elsewhere, and where possible will reduce flood risk”.

It comprises this document providing an over view of all the sites and a number of Appendices covering each site put forward by the local authority providing in more detail the flood risk constraints.


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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.

It should be remembered that this SFRA is a living document and should be reviewed annually, or as new flood information becomes available. This Level 2 SFRA has identified nine sites located partially in Flood Zone 3; six were sequentially tested, three were disregarded as they were already under construction and the Exception Test was carried out on three of those sites which passed the Sequential Test

Guidance has been provided on site specific flood risk assessments, by flood zone and flood risk. Guidance is also included on the application of sustainable drainage (SUDS) techniques to reduce runoff from redevelopment within the Borough.


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Contents

EXECUTIVE SUMMARY I

1. INTRODUCTION 4

1.1. Background 4

1.2. Objectives of this Study 5

1.3. SFRA as a Living Document 6

2. METHODOLOGY 7

2.1. Application of the Sequential Test 8

2.2. Application of Exception Test 11

3. FLOOD RISK VULNERABILITY OF DEVELOPMENTS 13

3.1. Development Suitability in Flood Zone 1 – Low Probability 13

3.2. Development Suitability in Flood Zone 2 – Medium Probability 13

3.3. Flood Zone 3a - High Probability 13

3.4. Flood Zone 3b – Functional Floodplain 14

3.5. Other Sources of Flooding 14

4. RESULTS OF THE SEQUENTIAL TEST 16

5. DEFINING FLOOD HAZARDS IN AREAS OF DEVELOPMENT PRESSURE 21

5.1. Flood defence infrastructure 23

6. RESULTS OF THE EXCEPTION TEST 24

7. SITE SPECIFIC FLOOD RISK ASSESSMENTS 30

7.1.1. Flood Zone 1 – Low Probability 32

7.1.2. Flood Zone 2 – Medium Probability 33

7.1.3. Flood Zone 3a – High Probability 33

7.1.4. Flood Zone 3b – The Functional Floodplain 34

7.2. Requirements for FRAs in Areas of Known Non-Fluvial Flood Risks36

7.2.1. Surface Water Flooding 36

7.2.2. Sewer Flooding 36


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7.2.3. Groundwater Flooding 36

8. SUDS GUIDANCE 37

9. RECOMMENDATIONS 40

REFERENCES 43

Appendix A

Sites Identified for Development and Redevelopment

Appendix B

Site Specific Information Sheets

Appendix C

Summary of Sequential Test Findings

Appendix D

Wealdstone Brook Modelled Flood Extents


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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

Flood and Water Management Act 2010

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

An Act to make provision about water, including provision of the management of risks in connection with flooding and coastal erosion. A lead local flood authority is considered to be the unitary authority for an area or the county council if no unitary authority exists.

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.


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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).

Or some system, defined by the Environment Agency, where there may be a significant consequence if there is a failure.

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.


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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. With this come responsibilities for managing the watercourse appropriately.

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.


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1. INTRODUCTION

1.1. Background

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

2 and is divided into 21 wards 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.


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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:

MWH prepared a Level 1 SFRA to enable the Local Planning Authority to review the current site allocations for preparation of the LDF and to inform the Sustainability Appraisal of the Core Strategy. The findings of a Level 1 assessment have been used to develop the scope of this Level 2 SFRA.

1.2. Objectives of this Study

This document provides supplementary information to support the Level 1 SFRA. The Level 1 SFRA provides information that is a requirement of PPS25, but also includes more detailed information than is required. This report should therefore be read in parallel with the three volumes of the Level 1 SFRA. This Level 2 builds on the findings of the Level 1 SFRA, specifically providing:

• Definition and mapping of the functional floodplain in locations where this is required (identified in Level 1 SFRA);

• Nature of the flood hazard within a flood zone including flood probability, flood depth, flood velocity and rate of onset of flooding;

• Guidance on appropriate policies for sites which could satisfy parts 1 and 2 of the Exception Test, and the requirements that would be necessary for a flood risk assessment supporting a planning application;

• Guidance on the preparation of flood risk assessments for sites of varying risk across the flood zones, including information about the use of sustainable drainage techniques;

• Meaningful recommendations to inform policy, development control and technical issues;

Other elements which are identified in PPS25 Practice Guide as being requirements of a Level 2 SFRA are covered in the Level 1 SFRA.

Strategic Flood Risk

Assessment


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1.3. 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.

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


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2. METHODOLOGY

The Level 1 and 2 SFRA will be used in the preparation of the Local Development Framework (including the Sustainability Appraisal) which will ultimately direct how development will be planned for and managed.

The Level 2 SFRA covers the whole of the London Borough of Harrow. However, more detailed assessments were carried out in proposed intensification/growth areas in Wealdstone and Harrow, namely.

• Wealdstone District Centre;

• Wealdstone preferred industrial location;

• Harrow town centre;

• Station Road corridor.

The Harrow and Wealdstone Area Action Plan (AAP) will set out the specific policies relating to development and land use within the Harrow and Wealdstone area of intensification.

It is not the role of the Harrow’s Core Strategy to allocate specific development sites1 but,

instead, to provide the spatial strategy for the management of growth within the Borough. However a Housing Trajectory published annually in Harrow’s AMR identifies a pipeline of development sites comprising those with planning permission and under construction, sites with planning permission but not under construction, and other potential deliverable sites (such as existing UDP allocations or where the principle has otherwise been accepted). The Core Strategy Preferred Option (2009/10) document included a schedule of strategic development sites (with and without planning permission) taken from the (then) current Housing Trajectory, and UDP Proposal Sites assessed as having housing capacity (as well as other uses in some cases).

For the purposes of this Level 2 SFRA, 62 sites have been identified from the Core Strategy Preferred Option for assessment, and are listed at Appendix C. This includes sites which at the time of the Preferred Option document had planning permission but were not under construction and in a small number of cases development has subsequently commenced. For sites without planning permission, it should be noted that the land uses and scale of development shown is indicative only and cannot prejudice any future planning application.

1 Site specific allocations will be made through the Area Action Plan, for Harrow & Wealdstone, and a

Site Allocations DPD for the rest of the Borough.

This section outlines:

• The application of the sequential test

• The application of the exception test


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2.1. Application of the Sequential Test

PPS25 outlines the requirement for a sequential risk based approach to be adopted for determining the suitability of land for development in flood risk areas (either through fluvial, coastal, sewer, groundwater and/or surface water flooding mechanisms). Central to this approach is the application of a Sequential Test to prioritise sites in order of flood risk probability and suitability for development. A flow chart illustrating the process of the Sequential Test is shown in Figure 2.

The test is used to assess what land is available for development and direct development to areas of lowest risk in the first instance. Where development is proposed within either medium (Zone 2) or high (Zone 3) flood risk zones, PPS25 requires the Local Planning Authority to undertake the Sequential Test to demonstrate that there are no reasonable alternative development sites in areas of lower risk. However development of sites within flood risk areas must take account of how development impacts can be mitigated as well as the probability of the flood risk. These factors are considered through the application of the Exception Test.

Figure 2: Application of the Sequential Test (Source: Figure 4.1 in Communities and Local Government: Planning Policy Statement 25: Development and Flood Risk Practice Guide, December 2009)


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A Local Planning Authority allocating land for development must demonstrate that it has considered the range of possible site options in conjunction with the flood risk information contained within this SFRA and the vulnerability of proposed development, and applied the Sequential Test (and where necessary the Exception Test) in the site allocation process. Evidence should be provided through the sustainability appraisal of the Area Action Plan and Site Allocations DPDs.

A Sequential Test was carried out in accordance with the requirements of Planning Policy Statement 25, for the 62 sites listed at Appendix C and taken from the Core Strategy Preferred Option (2009/10). It should be noted that the sites which were included for sequential testing were provided solely for the purposes of determining flood risk at these locations and no final decisions have been made on the location, land use and size of these developments (other than those with planning permission).

In assessing the suitability of the development proposals, the SFRA has adopted the PPS25 classification of different types of development into five (5) flood risk vulnerability categories. These categories are described in Figure 3.

Figure 3 – Flood risk vulnerability categories

The compatibility between the flood zones and different development categories is shown in Table 1 This table defines the scenarios in which development is considered appropriate, subject to passing the Sequential and Exception Tests, or is not permitted based on the degree of flood risk and the vulnerability of the use(s) to flooding, and the requirements of PPS25.

A ‘traffic light’ system has been adopted by this SFRA to mirror the decision matrix provided within PPS25. This traffic light assessment is provided in Table 1 and highlights areas where the:

• development type is permissible under PPS25;

• development type is permissible subject to passing an Exception Test; and

• development type is not permitted by PPS25.


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Flood Risk Vulnerability Classification

Essential Infrastructure

2

Water Compatible

Development3

Highly Vulnerable Development

4

More Vulnerable

Development5

Less Vulnerable

Development6

Zone 1 � � � � �

Zone 2 � � Exception test required

� �

Zone 3a Exception test required

� � Exception test required

�

Zone 3b Exception test required

� � � �

Key: �- Development is appropriate � - Development is not permitted

Table 1 - Compatibility between Flood Vulnerability of Development and Flood Zones (Source: Adapted from Table D.3 in Communities and Local Government Planning Policy Statement 25: development and Flood Risk, March 2010).

Further information on the definitions of different flood zones can be found in Section 5, Volume 1 – Planning and Policy report of the Level 1 SFRA.

When undertaking the Sequential Test, PPS25 specifies the Local Planning Authority should focus on:

• Development Vulnerability -

The Local Planning Authority should firstly consider whether the flood risk vulnerability classification of the proposal is appropriate for the flood zone (refer Figure 4).

If the development is identified as being appropriate, the next step is to identify reasonable available sites within a lower flood risk zone and identify the geographic area of search over which the test is to be applied.

• Defining the Evidence Base –

At the local level, the geographical area of search over which the test is to be applied will typically be the whole Local Planning Authority area. However, this area may be reduced, where justified, by the functional requirements of the development (e.g. catchment area if for a school, hospital or doctor’s surgery) or relevant objectives in the DPD. Alternatively, the areas of search may be increased to take in other LPAs if appropriate (e.g. if a site is close to a local authority boundary).

2 Includes essential transport infrastructure

3 Includes flood control infrastructure, water based recreation, amenity open space

4 Includes emergency services infrastructure, basement dwellings

5 Includes housing, residential institutions, hotels, hospitals, landfill sites used for waste management

facilities for hazardous waste 6 Includes shops, offices, general industry


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• Comparing Sites –

This SFRA provides information that should be used by the Local Planning Authority to compare the flood risk between reasonably available sites. Other planning considerations, such as development plan status, constraints to the delivery of the development and other potential impacts of the development should also be considered.

• Application of the Exception Test (where applicable) –

The final issue to be considered by the Local Planning Authority is whether alternative sites are less suitable, taking into account other planning issues. If the alternative sites are deemed less appropriate, the original site should be considered with the application of the Exception Test.

2.2. Application of Exception Test

The Level 1 SFRA identified where a Level 2 SFRA may be required and the process of site allocation has highlighted the need for a Level 2 SFRA in accordance with PPS25 guidelines.

“Where decision makers have been unable to allocate all proposed development and infrastructure in accordance with the Sequential Test.... it will be necessary to increase the scope of the SFRA to provide the information necessary for application of the Exception test”

Where the Sequential Test has not identified purely sites in low flood risk areas and some continuing development is necessary to meet the needs of the community and/or provides wider sustainability benefits, the Exception Test is used to assess the safety of that potential development in Flood Zones 2 and 3, A flow chart illustrating the process of the Exception Test is shown in Figure 4.

Figure 4: Application of the Exception Test (Source: Figure 4.2 in Communities and Local Government Planning Policy Statement 25: Development and Flood Risk Practice Guide, December 2009)


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The Exception Test can only be applied in certain circumstances.

For the Exception Test to be passed, a development must satisfy all of the following three criteria:

1. The development provides wider sustainability benefits to the community that outweigh the flood risk, informed by a SFRA where one has been prepared. If the DPD has reached the ‘submission stage’, the benefits of the development should contribute to the Core Strategy Sustainability Appraisal.

2. The development should be on developable previously-developed land (commonly known as ‘Brownfield land’) or, if it is not on previously-developed land, it should be shown that there are no reasonable alternative sites on developable previously-developed land.

3. A Flood Risk Assessment (FRA) must demonstrate the development will be safe, without increasing flood risk elsewhere and, where possible, will reduce the flood risk overall.

The Exception Test was carried out on three sites, namely

• 47-49 High Street, Edgware

• Harrow Arts Centres, Uxbridge Road, Hatch End

• The Case is Altered and Land Fronting Canning Road, Wealdstone

Details of the Exception Test are described in Chapter 6.


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3. FLOOD RISK VULNERABILITY OF DEVELOPMENTS

Describe

PPS25 defines three basic flood zones:

Please refer to the Level 1 SFRA, Volume 1 Planning and Policy report for detailed descriptions of each flood zone.

3.1. Development Suitability in Flood Zone 1 – Low Probability

The flood risk in Flood Zone 1 is considered to be low, but this should not be interpreted as there being no constraints to development of land within this zone. Development proposals on sites greater than, or equal to, one hectare in area are considered to be at risk of flooding from other sources (e.g. surface water flooding) and have the potential to increase flood risk in adjoining areas through the addition of hard surfaces and the effect of the development on overland flow paths. Accordingly, a site-specific Flood Risk Assessment (FRA) must be prepared for these proposals to consider these factors and other local considerations, such as sustainable drainage techniques.

3.2. Development Suitability in Flood Zone 2 – Medium Probability

Flood Zone 2 is considered suitable for most development except highly vulnerable uses such as police and fire stations. Subject to the Sequential Test be applied, highly vulnerable development in Zone 2 is only permitted through the application of the Exception Test.

3.3. Flood Zone 3a - High Probability

Water-compatible and less vulnerable land uses are appropriate in this zone, however highly vulnerable land uses should not be permitted. More vulnerable uses and essential infrastructure should only be permitted in the zone if the development passes the Sequential and Exception Tests. If permitted, essential infrastructure should be designed and constructed in order to remain operational during major flood events.

This section explains:

• Development suitability in Flood Zones 1, 2, 3a and 3b

• Development suitability where other sources of flood have been identified


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All development proposals in this zone should be accompanied by a Flood Risk Assessment (FRA).

The policy aims for this zone, as set out in PPS25, are to seek opportunities to:

• Reduce the overall flood risk to the area through layout/design and sustainable drainage techniques

• Relocate existing development to land of lower flood risk

• Restore the functional floodplain in order to restore flood water pathways and space for flood water storage. (Communities and Local Government, Planning Policy Statement 25: Development and Flood Risk Practice Guide, December 2009).

3.4. Flood Zone 3b – Functional Floodplain

Flood Zone 3b is defined as comprising land where water has to flow or be stored in times of flood. In LB Harrow this is classified as land that will flood for the 5% annual probability or 1 in 200 chance in any year.

Only water-compatible uses and essential infrastructure should be permitted in this zone, if it passes the Exception Test. It should be designed and constructed to:

• Remain operational and safe for users in times of flood;

• Result in no net loss of floodplain storage;

• Not impede water flows; and

• Not increase flood risk elsewhere across the floodplain.

All development proposals within this zone that are deemed appropriate based on the flood zone and development vulnerability compatibility should be accompanied by a FRA. The policy aims for this zone, as set out in PPS25, are to seek opportunities to:

• Reduce the overall flood risk to the area through layout/design and sustainable drainage techniques; and

• Relocate existing development to land of lower flood risk.

3.5. Other Sources of Flooding

When deciding whether a development type is suitable, it is essential that all sources of flooding are considered, including:

• pluvial/surface water runoff;

• inadequate drainage systems; and

• sewer networks (surface water and foul);

• groundwater; and

• flooding from manmade structures e.g. canals and reservoirs.

Flooding mechanisms in Harrow and the implications for development must be taken into account at early stages in the development. These issues are discussed in further detail in Section 3.4 Level 1 SFRA Volume II – Technical Report and in Section 5 Level 1 SFRA Volume I – Policy Report.


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Flooding from surface water sources has been mapped in Figure 9 in Level 1 SFRA Volume III – Mapping

Flooding from surface water sewers has been mapped in Figure 9 in Level 1 SFRA Volume III - Mapping.

Known locations of groundwater flooding are shown in Figure 10 in Level 1 SFRA Volume III – Mapping (groundwater flooding should be able to be addressed through suitable site design). .

Reservoirs are discussed in Section 5.3.3 in Level 1 SFRA Volume 1 – Policy Report.


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4. RESULTS OF THE SEQUENTIAL TEST

Identification of sites at risk

An assessment was carried out to identify development areas, based on the 62 sites taken from the Core Strategy Preferred Options document, which are located within medium and high risk areas.

The J flow modelling, which forms the basis for the Flood Map on the Environment Agency website, was obtained. This modelling was used to undertake the initial analysis of sites at medium and high risk.

Further ISIS modelling was also obtained where the Environment Agency had undertaken more detailed modelling. This allowed the identification of areas where further more detailed modelling was required to understand the nature of the flood hazard on the required sites and answer Part C of the Exception Test.

A review of the sites at medium/high risk from fluvial flooding, identified 9 sites included at Appendix A of the Core Strategy Preferred Option, as being at risk from flooding.

THIS SECTION:

• Identifies core strategy sites at risk from flooding

• Explains the findings of the sequential test


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Table 2 describes the zones in which each of these sites are located:

SITE DEVELOPMENT NAME FLOOD ZONE

Planning

Permission

Granted

Action

1 T3 Land West of High St,

Wealdstone 3b

No Sequentially test this

site

2 T3 Harrow Arts Centre,

Uxbridge Rd, Hatch End 3b

No Sequentially test this

site

3

T2 Case is altered and

Land fronting Canning

Rd, Wealdstone 3b

Yes Sequentially Test in

case existing

planning permission

lapses

4

T1 Raebarn House 86-

100 Northolt Rd, South

Harrow 3b

Yes Site already under

construction

therefore not

appropriate to

sequentially test

5

T1 Government

Buildings, Honeypot

Lane, Stanmore 3b


construction

therefore not

appropriate to

sequentially test

6

T1 Edgware Town

Football Club, White

Lion Ground, Edgware 3a

Yes Sequentially Test in

case existing

planning permission

lapses

7 T2 Dellers Palmerston

Road, Wealdstone 3b

No Sequentially test

8 T2 47-49 High Street,

Edgware 3a

No Sequentially test

9

T1 14-20 High Street,

Wealdstone 3b


construction

therefore not

appropriate to

sequentially test

Table 2 – Sites to be sequentially tested

Planning permission was granted at 5 of the sites described in Table 2, prior to this Level 2 SFRA being carried out. Three of these sites were under construction as this Level 2 was being prepared and were therefore considered inappropriate for Sequential Testing. The remainder (6 sites) were carried forward for sequential testing, described in Table 3.

A traffic lights system was developed, which is similar to that provided in PPS25, describing the flood zone within which each of the core strategy sites fall and the restrictions that this places on any future development at that location (Appendix C). It should be noted that in some instances sites are only partially affected by flooding, or lie within multiple flood zones. In these cases, the ‘traffic light’ within Table 3 reflects the most severe risk of flooding within the site. More detailed assessments of the flood risk within the medium to high risk sites have also been carried out to determine their development potential adopting a sequential approach to development within the site, complying with the vulnerability of development permitted in each zone and Part C of the Exception Test. This information for each of the sites is contained in Appendix B.

Seq

ue

ntia

lly T

est S

ites –

Go

to T

ab

le 3


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It should be noted that some strategic development sites, identified in the Level 1 SFRA which were identified as being located in the floodplain have not been considered for sequential testing. More detailed modelling was carried out for the purposes of the Level 2 SFRA which suggests these sites lie outside the flood floodplain and therefore are not subject to a sequential test.

Initial assessments of the sites using previous modelling results also predicted flooding at Buckingham Road Chandos Parade and at the Rayners PH sites. Detailed modelling carried out by MWH has confirmed these sites lie outside of the fluvial floodplain.

Planning land uses described in Table 3 are classified as:

Planning Use

A1 Shops

A2 Financial and professional services

A3 Restaurants and cafes

A4 Drinking establishments

A5 Hot food takeaway

B1 Business

B2 General industry

B8 Storage and distribution

C1 Hotels

C2 Residential institutions

C3 Dwelling houses

D1 Non residential institutions

D2 Assembly and leisure.


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Of the six sites requiring Sequential Testing, suitable alternative sites were identified (at lower risk of flooding) at three, namely:

• Edgware Town FC – an alternative site was identified at the Royal National Orthopaedic Hospital Site;

• Land West of High Street – an alternative site was identified to accommodate the development at Land at Stanmore Station and adjacent land;

• Dellers, Palmerston Road – an alternative site was identified at 1-10 Whitefriars Industrial Estate.

It should be noted that the sites which were included for sequential testing were provided solely for the purposes of determining flood risk at these locations and no final decisions have been made on the location, land use and size of these developments (other than those with planning permission). Other planning considerations should be allowed for when assessing the applicability of a site for development. Other planning considerations should be allowed for when assessing the applicability of a site for development.

Further details on these sites and the Sequential Tests carried out for these sites are included as Appendix B.

Legend for Table 3:

Development type is permissible under PPS25. A site-specific FRA is required.

Development type is permissible under PPS25, only if the Exception Test

is passed. A site specific FRA is required.

Development type is not permitted by PPS25.


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Development name

Development name

Planning Land Use

Mixed

Planning

Permission

Flood

Zone

% of site

within

highest

risk zone Vulnerability

The suggested permissible land use under PPS25

A1 to

A5 B1 C3

Essential

Infrastructure

Water

Compatible

Development

Highly

vulnerable

development

More

vulnerable

development

Less vulnerable

development

T3 Land West of High St, Wealdstone � � Remaining 3b 25%

More vulnerable

EXCEPTION TEST REQUIRED

T3 Harrow Arts Centre, Uxbridge Rd, Hatch End � � Remaining 3b 2.8%

More vulnerable


T2 Case is altered and Land fronting Canning Rd, Wealdstone � � � � No 3b 59%

More vulnerable


T1 Edgware town Football Club, White Lion Ground, Edgware � Yes 3a 21%

More vulnerable



T2 Dellers Palmerston Road, Wealdstone � � � No 3b 39%

More vulnerable


T2 47-49 High Street, Edgware � � � No 3a 5%

More vulnerable



Table 3 – Permissible development types in Flood Zone 3


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5. DEFINING FLOOD HAZARDS IN AREAS OF DEVELOPMENT PRESSURE

In order to address Part C of the Exception Test, the nature of the flood hazard within flood zones must be understood, this was achieved by determining:

• Flood probability;

• Flood depth;

• Flood velocity; and

• Rate of onset of flooding.

Modelled flood levels, flood outline mapping and/or flood zone mapping was available for a number of watercourses in the Borough. Accordingly, there was a large volume of existing data that could be used to define the flood risk in the Borough.

Different methodologies have been adopted for waterways where Environment Agency flood levels and flood outline mapping are available (i.e. ‘mapped’ watercourses), and for those areas where no detailed flood information is available (i.e. ‘unmapped’ watercourses).

It has been necessary to undertake more detailed modelling investigations to define the flood behaviour and associated flood risk along coarsely mapped (simple J flow modelling, which forms the basis of the flood map ) or unmapped watercourses. Where EA have not undertaken detailed mapping MWH have refined existing broad scale models to allow detailed assessment of the flood hazard. in order to establish the risk at these sites sufficiently for a Level 2 SFRA

To provide a robust evidence base for the Level 2 SFRA, sections of the existing ISIS river models were run to simulate the 5%, 1%, 1% plus climate change and 0.1% events. 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, for open channels, and onto the DSM for the culverted sections. These mapped levels were then converted into a water surface and ‘mapped’ on to the DTM for the study area to create flood mapping for the modelled watercourses.

To determine flows in culverts, the modelling of the Wealdstone Brook culvert has been completed using InfoWorks CS. Three conditions have been applied to the model, these are:

a. The urban runoff inputs based on the double linear reservoir model using the Wallingford procedure runoff parameters;

THIS SECTION:

• Describes the hydraulic modelling which has been carried out for both the Level 1 and 2 SFRAs

• Describes the flood defence infrastructure


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b. FEH derived inflows for the watercourse element flowing into the head of the culvert; and

c. ISIS river model results applied to the outfall of the culvert to act as a downstream boundary condition.

The 5%, 1%, 1% plus climate change and 0.1% events were simulated and the flood map was determined using the InfoWorks CS 2D overland flow route module.

Models were also constructed of the culverted watercourses in InfoWorks to simulate the predicted flooding associated with the same events. The hydraulic modelling of the culverted watercourses was undertaken using estimated inflows not recorded data. MWH has carried out no model calibration, or comparison against reported flooding incidents. The percentage impermeable area within the catchment was provided by LB Harrow; MWH has not carried out any validation of this data. Existing buildings and overground structures are included in the digital surface model and so preclude any associated flooding, which may result in an over estimation of flood depths. The model is considered conservative in its ability to predict flooding, this is because there is a time lag applied to the rainfall so that the peak rainfall coincides with the peak inflow to the head of the culvert. The modelled flood extents are shown in Appendix D.


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5.1. Flood defence infrastructure

The flood defence within the catchment are discussed in Section 3.7 of Level 1 SFRA Volume II – Technical report, including details of:

• Current condition of infrastructure;

• Probability and consequences of overtopping or failure;

• Potential impact of climate change;

• Future flood management policy – maintenance and upgrade.

The defences which were identified are listed in Table 4.

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 Flood storage area Hatch End Arts Centre Deculverting Montesole Park Bunded storage

Crane Headstone Manor Storage

Newton Farm Ecology Park Attenuation ponds

Table 4 – Flood defences in the catchment


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6. RESULTS OF THE EXCEPTION TEST

Whilst the Sequential Test should ensure that more vulnerable property types are not located in areas at high risk of flooding, in exceptional circumstances there may be a valid reason for a development type which is not compatible with the level of flood risk to be considered. The Exception Test has three parts. As discussed in Section 2.2, a development must satisfy all of the three criteria outlined in PPS25, to pass the Exception Test and there must be robust evidence in support of every part of the test:

• a) It must be demonstrated that development provides wider sustainability benefits to the community that outweigh flood risk;

• b) The development must be on developable previously developed land or, if it is not on previously developed land, that there are no reasonable alternative sites on developable previously developed land;

• c) A FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, will reduce flood risk.

• After carrying out Sequential Tests on six sites, only three sites were identified to be taken forward and therefore requiring an Exception Test. The three sites which were not taken forward from the Sequential Test (Dellers on Palmerston Road, Land West of the High Street in Wealdstone, and Edgware Town FC, Edgware), had similar sites available to accommodate the proposed developments which were located in areas at less risk of flooding. However, it should be noted that the sites which were included for sequential testing were provided solely for the purposes of determining flood risk at these locations and no final decisions have been made on the location, land use and size of these developments (other than those with planning permission). Other planning considerations should be allowed for when assessing the applicability of a site for development. The sites identified are described in Table 5

THIS SECTION:

• Explains the findings of the exception tests


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SITES TAKEN FORWARD FOR SEQUENTIAL TESTING FROM TABLE 2

SITE

IS

EXCEPTION

TEST

REQUIRED?

(TABLE D3

IN PPS25)

OTHER

SUITABLE

SITES

IDENTIFIED

IN LOWER

RISK FLOOD

ZONE?

WHY IS AN EXCEPTION TEST

REQUIRED?

EDGWARE

TOWN FC NO YES

Another site has been identified which could

accommodate this development, therefore no Exception Test is required

47-49

HIGH

STREET

YES NO

As described in Table 3, this site has been identified for a

more vulnerable development land use and is located within Flood Zone 3a, therefore an Exception Test is required

HARROW

ARTS

CENTRE

YES NO

This site is located partially within Flood Zone 3b. , Parts of the site may be suitable for

development (lowest risk areas first), therefore an Exception

Test is required

LAND

WEST OF

HIGH

STREET

NO YES


accommodate this development therefore no Exception Test is required

DELLERS,

PALMERST

ON ROAD

NO YES


accommodate this development therefore no Exception Test is required

THE CASE

IS

ALTERED

AND LAND

FRONTING

CANNING

ROAD

YES NO


accommodate this development. However, the

town centre uses would not be directly accessible to the

population it is meant to serve, therefore an alternative site

would not be feasible

Table 5 – Sites where an Exception Test is required Table 5describes why Exception Tests were required, following on from the findings of the Sequential Test.

Excep

tion

Te

st - G

o to

Tab

le 6


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It was considered appropriate to go ahead with Exception Tests at:

• Harrow Arts Centre, Hatch End;

• 47-49 High Street, Edgware; and

• The Case is altered land Fronting Canning Road, Wealdstone.

Harrow Arts Centre, Hatch End

Flood Risk Constraint

4.4% in high risk flood zone

Part a)

Wider Sustainability Benefits

The extent of high flood risk represents a very small part of the site and the area proposed to be developed for residential concerns only the previously developed component (the car park and ancillary buildings to the rear of Elliot Hall) within Flood Zone 1. Therefore it will be possible to develop the more vulnerable uses without any conflict with the sequential test. In terms of sustainability benefits, the development represents the opportunity through design & layout and financial contributions to enhance the setting and appearance of Elliot Hall (listed) and any other cartilage buildings to be retained.

Part b)

Previously Developed Land

Located on previously developed land and greenfield land

Part c):

Safe Development

Existing listed building, therefore there are opportunities to design the development to improve the existing situation and reduce the flood risk on the site. All development will be directed away from high risk flood zones. Almost 90.7% of the site lies within Flood Zone 1, which is considered to be at low risk from flooding. As described in Appendix B, flooding is only predicted to occur to the south part of the site, which is currently used as a car park. Safe access and egress is readily available to the surrounding streets. In order to minimise potential damage to property from floodwater, it is recommended that finished floor levels are at least 300mm above the predicted flood level for the 0.1% annual probability or 1 in 1000 chance in any year plus an allowance for climate change).


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It is recommended that all residents receive an information pack, identifying the areas at risk from flooding, how this flood risk is being managed and explaining actions which should be taken in the event of a flood. All residents should also be encouraged to sign up to Floodline. If considered necessary, signposts should be provided showing safe access routes.

Surface water should be managed through the use of SUDS. These should be designed to ensure as a minimum that the runoff from the site is no greater than greenfield runoff rates. Developers should also be encouraged to adopt a treatment train approach to managing surface water

47 -49 High Street, Edgware


5% in high risk flood zone

Highway flooding has occurred to the front of the properties – at the High Street

Part a) Wider Sustainability Benefits

The extent of high flood risk represents a very small part of the site and the whole site is previously developed. Therefore it will be possible to develop the more vulnerable uses without any conflict with the sequential test. In terms of the sustainability benefits, the redevelopment of this site will contribute to the more efficient use of this previously-developed land that is well-served by public transport and in close proximity to Edgware major centre,

Part b)


Located on previously developed land

Part c):

Safe Development

This is an existing building in a highly urbanised location. The floodplain extends to the southern end of the building, hydraulic modelling predicts that only 5% of the site lies in Flood Zone 3a and therefore development should be directed to lower risk areas within the site, if possible. Safe access should be possible to the north, east and west of the site, however, access routes should be confirmed in a site specific flood risk assessment. It is recommended that all residents receive an information pack, identifying the areas at risk from flooding, how this flood risk is being managed and explaining actions which should be taken in the event of a flood. All


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residents should also be encouraged to sign up to Floodline. If considered necessary, signposts should be provided showing safe access routes.

To protect the property from flooding, it is recommended that finished floor levels are set at least 300mm above the predicted flood level for the 0.1% annual probability or 1 in 1000 chance in any year plus an allowance for climate change. No basement dwellings should be permitted.

Opportunities should be sought to include SUDS wherever possible. However, due to the highly urbanised nature of this development, opportunities may be limited.

Case is Altered, Land fronting Canning Road, Wealdstone


59% is in high risk flood zone

Part a) Wider Sustainability Benefits

The site is located within Wealdstone district centre, itself a part of the Harrow & Wealdstone Intensification Area to deliver at least 2,500 new homes and at least 3,000 new full time jobs. Detailed objectives for the Area include urban renewal, the regeneration of Wealdstone, and increased retail (and other) provision in the town centres. The subject site occupies a primary location within Wealdstone district centre and its redevelopment will contribute to an uplift in residential capacity and a renewal of floor space for town centre uses, as well as the urban renewal of this partially derelict site which affects the setting of the neighbouring listed Metropolitan Police Station building. The redevelopment would also provide the opportunity to reduce surface water run off rates by implementing sustainable drainage systems.

Part b)


Located on previously developed land


MWH UK LTD Page 29

Part c):

Safe Development

The existing building in a highly urbanised location. The, existing building will already impede flood flows and impact on flood storage, therefore opportunities should be through appropriate design to reduce this impact.

Ideally this redevelopment should be moved to a location at less risk from flooding, however that will not be possible, as there were no other sites identified to accommodate town centre uses, which would be directly accessible to the population it was meant to serve.

A detailed flood risk assessment is required to determine whether safe access/ egress can be provided. Signage should be provided detailing theses access routes.

Any development must be either resistant or resilient to flooding. It is recommended that commercial properties are likely be located on ground floor are designed to be resilient (allowing flood water to enter buildings, but minimising disruption), typical solutions include the use of tiled surfaces that can easily be cleaned after a flood and the siting of electrical sockets and circuits above the predicted flood level . Any residential property have floor levels at least 300mm above the predicted flood level for the 1% annual probability or 1 in 100 chance in any year plus an allowance for climate change

Opportunities should be sought to include SUDS wherever possible. However, due to the highly urbanised nature of this development, opportunities may be limited.

It should be noted that planning permission has already been granted for this development, therefore these recommendations will only be valid if this permission was to lapse or if the applicant was to reapply for permission.

• Table 6–Findings of Exception Test

* based on Section D.9 of PPS25

The results of the Exception test suggest that the sites described in Table 5 may be suitable for development subject to a site specific flood risk assessments demonstrating that the proposed developments will be safe, without increasing flood risk elsewhere.


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7. SITE SPECIFIC FLOOD RISK ASSESSMENTS

Development is permitted within areas at risk of flooding if the proposed land use is acceptable for the degree of flood risk to which the land is subjected. The specific requirements for a development proposal will depend on the scale and vulnerability of the development and the level of flood risk at the site.

The requirements for site-specific flood risk assessments are detailed in PPS25. In general, it will be necessary for a developer to prepare a site-specific FRA to support a planning application if the:

• development is located in Flood Zone 2, 3a or 3b;

• proposed development site area is greater than 1 hectare (even in Flood Zone 1);

• floor space of proposed non-residential development is greater than 1,000m2 or the

site area is greater than 1 hectare;

• development site is located in an area known to have experienced flooding problems from any flood source; and

• development is located within 20m of the top-of-bank of a main river watercourse regardless of zone classification.

The level of information and detail in a FRA should be proportionate to the degree of flood risk and the scale, nature and location of the proposed development. Specifically, the three levels of Flood Risk Assessment can be described as:

1. Screening study - to identify whether there are any flooding issues related to a development site which may warrant further consideration.

2. Scoping study - to be undertaken if the Level 1 study indicates that the site may lie within an area which is at risk of flooding or that the site may increase flood risk due to increased runoff, to confirm the possible sources of flooding which may affect the site. The study should include the following objectives:

- assessment of the availability and adequacy of existing information;

- qualitative assessment of the flood risk to the site, and the impact of the site on flood risk elsewhere; and

- assessment of the possible scope for appropriate development design and to scope additional work required.

3. Detailed study - to be undertaken if the Level 2 study concludes that quantitative analysis is required to assess flood risk issues related to the development site. The study should include a:

- quantitative assessment of the potential flood risk to the development;

- quantitative assessment of the potential impact of development site on flood risk elsewhere; and

- quantitative demonstration of the effectiveness of any proposed mitigation measures (CIRIA, 2004).

THIS SECTION PROVIDES:

• Guidance on the requirements of site specific FRAs


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Where developers promote development outside of the allocated areas identified within the LDD, that is, windfall sites, and within flood risk areas defined by the SFRA, they are responsible for demonstrating that the Sequential Test has been applied and the Exception Test, where required has been passed. A site-specific FRA will be needed.

A site specific FRA must demonstrate that the proposed land use is acceptable and that the development can be designed to be safe and reduce flood risk. FRAs must consider flooding from all relevant sources:

• Fluvial flooding;

• Flooding from the sea;

• Flooding from land;

• Flooding from groundwater;

• Flooding from sewers;

• Flooding from reservoirs, canals and artificial sources.

Guidance on the preparation of FRAs can be found in Section 7 of Level 1 SFRA, Volume I – Planning and Policy report.

The level of detail included in FRA will depend on the nature and scale of the proposed development. Paragraphs 22 and 23 of PPS25 clarify the responsibilities of developers to consider flood risk issues at a site as early as possible including:

• consulting with the Environment Agency as early as possible;

• the responsibility of landowners for safeguarding land and other property against hazards;

• the responsibility of property owners and users to manage the drainage of their land, as far as possible to prevent adverse impacts on neighbouring land;

• advice that developers undertake independent checks regarding flood risk before purchasing a site. Where a site is allocated within the LDD that has been sequentially tested for the type of development proposed and is supported by a SFRA, the Sequential Test does not have to be applied. However the developer should apply a sequential approach to determine the appropriate land uses across the site with respect to any flood risk within the site;

• the requirement that the scope of any FRA should be agreed with the Local Planning Authority, and if necessary the Environment Agency), and it should be agreed who the developer needs to consult. For example the developer may need to consult Sewerage undertakers, Highways Authorities, Reservoir Undertakers, British Waterways, etc;

• the developer must demonstrate that the development is consistent with the policies in PPS25 and those on flood risk in the LDDs; and

• FRAs should consider all sources of flooding and demonstrate how flood risk will be managed taking into account climate change.

The following sections outline the specific requirements for FRAs for development within each flood zone.


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7.1.1. Flood Zone 1 – Low Probability

In Flood Zone 1, developers and local authorities should seek opportunities to reduce the overall level of flood risk in the area and beyond through the layout and form of the development, and the appropriate application of sustainable drainage techniques.

Operational development less than 1 hectare

Developers for all sites within Flood Zone 1 should undertake a basic ‘screening’ FRA to identify whether there are any flooding issues related to a development site which may warrant further consideration. It is not necessary to submit a FRA to the Local Planning Authority for this type of development, however a FRA may be required it is identified that there is a known drainage problem that may affect the site and the Local Planning Authority requires assurance that flood risk has been addressed.

The main flood risk issue to consider for these sites will often be managing the surface water runoff.

Development greater than 1 hectare

Although the flood risk in Zone 1 is considered to be low, a Flood Risk Assessment is required that is focused on the management of surface water runoff. The assessment should focus on:

• alternative sources of flooding (for example, groundwater or surface water flooding); and

• the potential for the development to increase flood risk elsewhere through the removal of permeable surfaces such as grass and the addition of hard surfaces and the associated runoff. Details of the proposed mitigation measures and potential SUDS must be provided, in conjunction with hydraulic calculations to demonstrate the robustness of the methodology adopted. A reduction in surface water flows should be sought for the lifetime of the development and Greenfield rates should be achieved on all sites.

The type of information that should be included in a FRA in Flood Zone 1 is:

• location plan;

• site plan showing existing features and proposed development;

• existing and proposed site levels related to Ordnance Datum;

• information about the surface water disposal method;

• assessment of the existing and proposed volume and rates of runoff from the development should be undertaken to demonstrate that any proposals reduce the impact of development;

• proposals for surface water management according to sustainable drainage principles with the aim of reducing the rate of runoff from the site;

• allowance in design for climate change effects;

• information on other potential sources of flooding and how they will be managed within the development proposal;

• consideration of the proposal relative to this SFRA; and

• confirmation whether the Environment Agency consent is necessary for any aspect of the work and whether or not this consent has been applied for.


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7.1.2. Flood Zone 2 – Medium Probability

In Flood Zone 2, developers and local authorities should seek opportunities to reduce the overall level of flood risk in the area through the layout and form of the development, and the appropriate application of sustainable drainage techniques.

All development proposals within this zone should be accompanied by a FRA that addresses of the following issues:

• details of the existing site levels, proposed site levels and proposed ground floor levels (must be 300mm above the 1% annual probability or 1 in 100 chance in any year plus an allowance for climate change);

• vulnerability of the development to flooding from other sources (e.g. groundwater, sewer and surface water drainage), as well as river flooding. This would involve discussion with the Environment Agency, Thames Water and Local Planning Authority to confirm whether a risk of flooding exists at the proposed site;

• vulnerability of the development to flooding over the lifetime of the development, including the future impact that climate change may have on flooding;

• volumes of flood storage displaced as a result the development proposals. Details of compensatory flood storage must be provided within the site on a level basis up to and including the 1% annual probability flood level plus climate change;

• demonstration that the residual risk is managed appropriately (e.g. raising floor levels or the provision of an evacuation route);

• preservation of flood flow routes;

• safe access/egress to and from the site;

• raised floor levels (i.e. raised 300mm above the 1% annual probability or 1 in 100 chance in any year plus an allowance for climate change); and

• potential for the development to increase flood risk elsewhere through the removal of permeable surfaces such as grass and the addition of hard surfaces and the associated runoff. Details of the proposed mitigation measures and potential SUDS must be provided, in conjunction with hydraulic calculations to demonstrate the robustness of the methodology adopted. A reduction in surface water flows should be sought for the lifetime of the development and Greenfield rates should be achieved on all sites.

7.1.3. Flood Zone 3a – High Probability

In this zone, developers and local authorities should seek opportunities to:

1. reduce the overall level of flood risk in the area through the layout and form of the development and the appropriate application of sustainable drainage techniques;

2. relocate existing development to land in zones with a lower probability of flooding; and

3. create space for flooding to occur by restoring functional floodplain and flood flow pathways and by identifying, allocating and safeguarding open space for flood storage.

A FRA is required to support all proposed developments within Flood Zone 3a. The FRA should include an assessment of the following areas:



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• vulnerability of the development to flooding from other sources (e.g. groundwater, sewer and surface water drainage), as well as river flooding. This would involve discussion with the Environment Agency, Thames Water, District Council and County Council to confirm whether a risk of flooding exists at the proposed site;

• vulnerability of the development to flooding over the lifetime of the development (including the potential impact of climate change), for example, maximum water levels, flow paths and flood extents. The Environment Agency may have carried out detailed flood risk mapping within some localised areas, which may be made available to the developer at cost. Where this kind of detailed modelling is not available, hydraulic modelling by a competent person will be required in order to determine the risk of flooding to the site;

• effect of the new development on the depth and velocity of floodwater. This will require a detailed modelling assessment by a competent person;

• volumes of flood storage displaced as a result the development proposals. Details of compensatory flood storage must be provided within the site on a level basis up to and including the 1% annual probability or 1 in 100 chance in any year plus an allowance for climate change;



• raised floor levels (i.e. raised 300mm above the 1% annual probability or 1 in 100 chance in any year plus an allowance for climate change);

• potential for the development to increase flood risk elsewhere through the removal of permeable surfaces such as grass and the addition of hard surfaces and the associated runoff. Details of the proposed mitigation measures and potential SUDS must be provided, in conjunction with hydraulic calculations to demonstrate the robustness of the methodology adopted. A reduction in surface water flows should be sought for the lifetime of the development and Greenfield rates should be achieved on all sites;

• for areas that are deemed to already be protected by flood defences or alleviation schemes, the risk of failure or exceedance of design criteria should be outlined. This will require a detailed assessment, carried out by a competent person; and

• demonstration that residual risks of flooding (following existing and planned mitigation techniques are carried out) are an acceptable risk. Further measures may include, for example, flood resistant and resilient design, evacuation planning and effective flood warning systems.

7.1.4. Flood Zone 3b – The Functional Floodplain

Flood Zone 3b is defined as comprising land where water has to flow or be stored in times of flood. In LB Harrow this is classified as land that will flood for the 5% annual probability or 1 in 200 chance in any year.

Inappropriate development should be resisted within Flood Zone 3b. Only the water-compatible uses and the essential infrastructure that has to be there should be permitted in this zone. Essential infrastructure in this zone should pass the Exception Test. It should be designed and constructed to:

• remain operational and safe for users in times of flood;

• result in no net loss of floodplain storage;

• not impede water flows; and

• not increase flood risk elsewhere.


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In this zone, developers and local authorities should seek opportunities to:

1. reduce the overall level of flood risk in the area through the layout and form of the development and the appropriate application of sustainable drainage techniques; and

2. relocate existing development to land with a lower probability of flooding.

All development proposals within Flood Zone 3b require the preparation of a FRA supporting the proposed development. The FRA should provide the following information in relation to the development:


• vulnerability of the development to flooding from other sources (e.g. groundwater, sewer and surface water drainage), as well as river flooding. This would involve discussion with the Environment Agency, Thames Water, District Council and County Council to confirm whether a risk of flooding exists at the proposed site;

• vulnerability of the development to flooding over the lifetime of the development (including the potential impact of climate change), for example, maximum water levels, flow paths and flood extents. The Environment Agency may have carried out detailed flood risk mapping within some localised areas, which may be made available to the developer at cost. Where this kind of detailed modelling is not available, hydraulic modelling by a competent person will be required in order to determine the risk of flooding to the site;

• effect of the new development on the depth and velocity of floodwater. This will require a detailed modelling assessment by a competent person;

• volumes of flood storage displaced as a result the development proposals. Details of compensatory flood storage must be provided within the site on a level basis up to and including the 1% annual probability or 1 in 100 chance in any year plus an allowance for climate change flood level plus climate change;



• raised floor levels (i.e. raised 300mm above the 1% annual probability or 1 in 100 chance in any year plus an allowance for climate change);

• potential for the development to increase flood risk elsewhere through the removal of permeable surfaces such as grass and the addition of hard surfaces and the associated runoff. Details of the proposed mitigation measures and potential SUDS must be provided, in conjunction with hydraulic calculations to demonstrate the robustness of the methodology adopted. A reduction in surface water flows should be sought for the lifetime of the development and greenfield rates should be achieved on all sites;

• for areas that are deemed to already be protected by flood defences or alleviation schemes, the risk of failure or exceedance of design criteria should be outlined. This will require a detailed assessment, carried out by a competent person; and

• demonstration that residual risks of flooding (following existing and planned mitigation techniques are carried out) are an acceptable risk. Further measures may include, for example, flood resistant and resilient design, evacuation planning and effective flood warning systems.


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7.2. Requirements for FRAs in Areas of Known Non-Fluvial Flood Risks

In addition to addressing the fluvial flood risks within the London Borough of Harrow, it is also necessary to address non-fluvial flood risk to developments in areas of known flood threats from surface water, and sewer and groundwater flood risk.

7.2.1. Surface Water Flooding

In areas at risk of surface water flooding, development should seek to reduce surface water runoff rates as a result of development. Moreover, the appropriate SUDS measures should be applied to reduce the overall level of risk in the areas. Figure 10 in Level 1 Volume III – Mapping shows the areas assessed to be at risk of flooding from overland flow and this flood risk should be investigated further as part of a FRA.

7.2.2. Sewer Flooding

Figure 11 in Level 1 Volume III – Mapping shows the areas assessed to be at risk of sewer flooding. The potential sewer flood risk should be investigated further as part of a FRA, particularly in terms of where the development will connect to the network, the potential outflow to the network, the current capacity of the sewer network and its ability to cope with the proposed additional flows.

7.2.3. Groundwater Flooding

In areas at risk of groundwater flooding (refer Figure 12 in Level 1 Volume III – Mapping), a FRA should establish the level of risk at the site by identifying local groundwater monitoring (where available) and assessing local groundwater levels.


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8. SUDS GUIDANCE

The application of SUDS techniques should be encouraged through the adoption of appropriate policy for new development and redevelopment within the Borough. Surface water management is a material planning consideration and will need to be considered at the earliest possible stage in the planning and design process, in consultation with the LPA, sewerage undertakers, Environment Agency and other relevant bodies. There may be opportunities to alleviate surface water flooding in adjacent and downstream areas, as well as in the development site. It is recommended that developments with a size greater than one hectare (i.e. >1ha) be required to comply with the following in order to encourage more sustainable development practices:

• allocate suitable land and incorporate SUDS techniques within the drainage design for the development;

• ensure that discharge rates from the development do not exceed greenfield rates (see definition below); and

• provide on-site attenuation for the 1% annual probability or 1 in 100 chance in any year plus an allowance for climate change.

The greenfield discharge rate is the surface water runoff regime from a site prior to development. To maintain the natural equilibrium of a site, the surface water discharge from a developed site should not exceed the natural greenfield run-off rate.

Typically, the application of SUDS techniques should not limited to one technique per site. A successful SUDS solution will often involve the use a number of methods. SUDS can also be employed on a strategic scale, for example with a number of sites contributing to large scale jointly coordinated and managed SUDS.

Where development proposals are on brownfield sites, there are real benefits to be gained by making substantial reductions in the amount of surface water runoff generated through the redevelopment of a site. Due to the highly urbanised nature of the catchment and the large number of small scale redevelopments, it is likely that in some circumstances the most suitable types of SUDS will be source control methods such as permeable paving, green roofs, swales and filter drains. The use of water butts and rainwater harvesting should also be considered for all new development. There may be limited opportunities for larger SUDS measures in highly urbanised areas. However, larger sites which are located in less densely developed areas, more specifically

• RAF Bentley;

• Royal National Orthopaedic Hospital

THIS SECTION PROVIDES:

• Guidance on the application of SUDS


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may provide opportunities to significantly improve flood risk and water quality issues associated with urban runoff through implementing regional controls. It is likely there will be opportunities to incorporate site and regional controls on these sites.

The use of site controls should also be considered for the following sites (which are large enough to accommodate site controls, but not regional controls):

• T2 Civic Centre, Wealdstone;

• T3 Land at Stanmore Station;

• T3 Driving Centre, Wealdstone;

• T3 Harrow on the Hill station and adjacent land;

• Government Buildings, Honeypot Lane, Stanmore.

(The above sites are greater than 4ha in size, providing the ideal conditions for the use of site control measures)

As described in Part H of the Building Regulations, the first option to consider is the use of infiltration devices, where this is not possible due to ground conditions the use of SUDS should not be precluded. Measures such as permeable paving can be used, as long as they incorporate high level overflows in their design.

Particular care should be taken on potentially contaminated land when selecting measures, reference should be made to the CIRIA C697 SUDS Manual guidance for use of SUDS in specific circumstances.

All SUDS should be designed in accordance with CIRIA C697 SUDS Manual (and the new national design standards which were being prepared when this report was issued). Water quality and amenity benefits should also be maximised wherever possible, in accordance with Annex F of PPS25.

SUDS retrofitting should be encouraged wherever possible, further guidance is available in the CIRA document CIRIA RP922 Retrofitting Surface Water Management Measures Guidance (in prep).

STEP STRATEGIC LEVEL

SITE LEVEL

BOTH

Opportunities should be sought to retrofit Sustainable Drainage Systems (SUDS) in known problem areas to help reduce surface water flood risk. Seek developer contributions to fund this retrofitted SUDS scheme.

�

In those areas identified as having a surface water flood risk problem seek opportunities to undertake a Surface Water Management Plan.

�

SUDS systems must be included in all new development where technically possible. These must be appropriate to the local soil and geology are utilized, adopted and maintained.

�

The Environment Agency’s SUDS hierarchy shall be used to ensure that the most sustainable SUDS solutions are utilised (refer SUDS - A Practical Guide

�


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STEP STRATEGIC LEVEL

SITE LEVEL

BOTH

(October 2006)).

A Surface Water Flood Risk Assessment must be undertaken for all developments greater than 1ha in size.

�

Reduce surface water runoff from all new development and redevelopment to provide no greater than greenfield discharge rates and the 1% annual probability or 1 in 100 chance in any year plus an allowance for climate change attenuation taking into account climate change shall be achieved on all developments greater than 1ha in size.

�

Introduce a consistent approach towards paving over front gardens in existing and new developments.

�

It is essential that the Local Planning Authority ensure adequate land is set aside to accommodate SUDS in land allocations within the Borough.

Further detail on SUDS requirements is provided in Section 8 of Level 1 SFRA, Volume I – Planning and Policy Report.


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9. RECOMMENDATIONS

Planning Policy recommendations

Greenfield runoff rates

EA have recommended that Policy 9 is reworded so that ‘all’ new development and redevelopment within the London Borough of Harrow aim to achieve Greenfield runoff rates. This in line with the recommendations in Section 5.115 of the Strategic Flood Risk Assessment for Harrow (2009) which states: ‘Redevelopment sites should use a variety of sustainable drainage systems to achieve a Greenfield runoff rate.

Sustainable Drainage

Policy P9 also supports the use of sustainable drainage systems for new development within the Harrow and Wealdstone Intensification Area and for all major developments within Borough. The EA stated that this should be changed so that all new development and redevelopment within the Borough, regardless of size, provides sustainable drainage systems where appropriate and seek opportunities to retrofit SUDS. A Surface Water Management Plan is currently being undertaken in conjunction with the LBs of Barnet and Brent. This will identify areas at risk from surface water flooding and where the use of SUDS (including retrofitting) will be more beneficial.

River Corridors

The EA have expressed concern that both P6 and P9 fail to highlight the importance of protecting river corridors and maintaining a development free buffer strip next to watercourses. They would like to see a commitment to retaining and providing a 8m wide undeveloped zone for all ordinary watercourses and designated main rivers.

Reinstatement of watercourses

P9 states that opportunities will be sought to reinstate natural watercourses and drainage channels. The EA have suggested that a paragraph is included within P6 or in P9 which states that: Where sites are located within close proximity to river, the council will, in appropriate circumstances, seek river restoration and /or financial; contributions towards the restoration of rivers’. In particular it suggests that where appropriate the council seeks the following:

• Improvements to the Edgware Brook, including restoration of banks, the removal of toeboarding, deculverting and in channel enhancement;

• Improvements to the River Pinn including full river restoration;

• Improvements to the Yeading Brook and Yeading Brook (east arm), including river channel restoration, in channel habitat enhancement and restoration of river banks.

The EA also suggested that the above recommendations are included within Strategic Objective 6.

Sequential and Exception tests

P9 states that ‘more vulnerable uses will not be provided within the area of higher flood risk except for dwellings above ground floor level and with secure, dry means of escape to land not at high risk’ The EA have stated that this needs to be reworded, as they do not consider it acceptable to place additional burden on the Emergency Services. MWH would suggest that this is reworded:


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‘a sequential approach should be adopted to demonstrate that there are no reasonably available sites in areas with a lower probability of flooding that would be appropriate to the type of development or land use proposed, in line with Table D.3 of PPS25. All new development should be steered in ascending order from Flood Zone 1 to 3. If this is not possible, consistent with wider sustainability objectives, the Exception Test should be applied. A FRA must also demonstrate the development would be safe and not increase flood risk elsewhere’.

Where development must be located in the floodplain, flood resilient and resistant design and construction principles should be applied to ensure that new development remains operational as long as practicable and can return to use with minimal remedial requirements. Harrow should implement a policy to promote sustainable flood management through resilient design and construction.

Strategic Objective 4 should state that the development type is compatible with the flood zone in line with Tables D1, D2 and D3 of PPS25.

Strategic Flood Risk Assessment

Flood risk has been assessed strategically for the proposed developments within Harrow and the impacts on existing flooding, development restrictions and planning policy implications. This Level 2 SFRA will classify the existing and future flood risk of all land within the Borough and to provide the evidence necessary to direct new developments to 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.

Local Recommendations

Flood Risk Assessments

The EA have specified that the Core Strategy must refer to flood risk assessment requirements. Paragraph 12 of PPS25 states: Policies in Local Development Documents should set out the requirements for site specific flood risk assessments to be carried out by developers and submitted with planning applications in areas of flood risk. They have suggested that the following wording is included:

‘ A Flood Risk Assessment (FRA) must be submitted with planning applications for proposals in flood risk zones 2 and 3, appropriate to the scale and nature of the development. This must identify and assess the risk from all forms of flooding to and from the development and demonstrate how the flood risks will be managed over the lifetime of the development’.

‘A Surface Water Flood Risk Assessment must be submitted with planning applications for all developments over 1 hectare in size’.

Flood Storage

The EA have stated that Policy 9 should include a statement on flood storage and its protection and referred to the Brent and Pinn Policy Units of the Thames Catchment Flood Management Plan which ‘Encourage safeguarding of existing areas of open space in the floodplain’.

Flood Warning

‘Provision of flood warning systems and promoting flood awareness within the community’ should be included within Strategic Objective 4.


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Biodiversity

There is a need to mention specific mention of watercourses and their corridors in the strategy. The EA suggest as an example, an additional statement to be included in Policy 6 ‘Where development is proposed adjacent to a watercourse, enhancement of the river corridor should be sought as part of the development.

Site Assessments

A Sequential Test was carried out in accordance with the requirements of Planning Policy Statement 25, for 62 sites and taken from the Core Strategy Preferred Option (2009/10). Sites included for sequential testing were provided solely for the purposes of determining flood risk at these locations and no final decisions have been made on the location, land use and size of these developments (other than those with planning permission).

Land West of High Street

The site is currently developed, with 40% located within Flood Zone 1. However, there is an alternative site available to accommodate this development in an area at less risk from flooding and development should be directed there in the first instance. If development is permitted at this location, it should be suitable in terms of flood risk. It is also recommended that property owner should sign up to the Environment Agency’s Floodline service. Flood resistance and resilience measures should be incorporated into any development

Edgware Town FC

A possible alternative site has been identified in a location at less risk from flooding

The Case is Altered

This site is located partially within Flood Zone 3b, therefore is not suitable for the suggested land uses. Ideally development should be directed to an alternative site which is at lower risk.

Dellers

This ‘more vulnerable’ development is located partially within Flood Zone 3b, therefore is not considered suitable for the suggested land uses. Development should be directed to an alternative site, which is at lower risk from flooding. Development at this location should be appropriate to the flood risk. In this case, water compatible uses are considered acceptable and essential infrastructure may be permitted subject to an Exception Test being carried out and the findings of a site specific flood risk assessment.

Arts Centre

There are opportunities to allow the redevelopment of many of the existing buildings for residential and the Arts and Leisure uses, as most of the area to the north of the site lies outside the floodplain. Development in high risk flood zones should be avoided.

47 - 49 High Street

There are opportunities to allow redevelopment at this location. However, this must be supported by a flood risk assessment which demonstrates the proposed development will be safe without increasing flood risk elsewhere.


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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).

CIRIA RP922 Retrofitting Surface Water Management Measures Guidance (in prep).

Communities and Local Government. 2010 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. 2009 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).


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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

Sites Identified for Development and Redevelopment


MWH UK LTD

APPENDIX B

Site Specific Information Sheets


MWH UK LTD

APPENDIX C

Summary of Sequential Test Findings


MWH UK LTD

Development Name

Area

(ha) Possible Use

Essential

Infrastructure

Water

Compatible

Development

Highly

vulnerable

Development

More

vulnerable

Development

Less

vulnerable

development

Flood Zone

T1 Phase D2 Rayners Lane

Estate Goldsmith Close,

Rayners Lane 1.09 Residential

1

T1 Former Clinic and Scout Hut

Tenby Road, Edgware 0.06 Residential

1

T1 5 Manor Road, Harrow 0.1

Residential - houses +

flats

1

T1 Buckingham Rd Chandos

Parade, Edgware 0.11 Residential

1

T1 287-293 Whitchurch Lane,

Edgware 0.15 Residential - flats

1

T1 19 and 21 and Rear of 11 to

29 Alexandra Avenue, Rayners

Lane 0.33 Residential

1

T1 Wood Farm, Stanmore 1.09

Residential and

public open space

1

T1 Westfield Park land at 1-4

Eaton Court, Hatch End 0.15 Residential - flats

1

T1 90-100 Pinner Road, North

Harrow 0.11 Residential - flats

1

T2 Rayners Lane Station Car

Park, Rayners Lane 0.68

Residential & town

centre uses

1

T3 Eastern Electricity and

Brember Day Centre, South

Harrow 1.28 Industrial

1


MWH UK LTD

Development Name

Area

(ha) Possible Use

Essential

Infrastructure

Water

Compatible

Development

Highly

vulnerable

Development

More

vulnerable

Development

Less

vulnerable

development

Flood Zone

T1 Former Council Allotments

Kenmore Road, Queensbury 0.24 Residential

1


Wealdstone 0.08 Residential - flats

1

T1 186-194 Pinner Road, North


1

T1 2 Walton Avenue,

Wealdstone 0.11 Residential - flats

1

T2 Land Fronting and

Palmerston Road Car park,

Wealdstone 0.56

Residential & town

centre uses

1

T1 19 Northolt Road Timber

Carriage PH, South Harrow 0.1 Residential - flats

1

T3 Vaughan Centre, West

Harrow 0.29 Community Use

1

T2 Rayners PH Village Way

East, Rayners Lane 0.49 Mixed

1

T1 Richards Close, Harrow 0.52

Residential - Extra

Care Scheme

1

T2 96-116 Greenford Road,

Sudbury Hill 0.42 Residential

1

T1 50-54 Northolt Road, South


1

T2 Goodwill to All PH,

Wealdstone 0.25 Residential

1


MWH UK LTD

Development Name

Area

(ha) Possible Use

Essential

Infrastructure

Water

Compatible

Development

Highly

vulnerable

Development

More

vulnerable

Development

Less

vulnerable

development

Flood Zone

T2 Bacon Lane, Edgware 0.25 Residential

1

T2 Dellers Palmerston Road,

Wealdstone 0.37 Mixed

EXCEPTION

TEST

REQUIRED

3b

T1 Land to rear of 71 Bridge

Street, Pinner 0.03 Residential- flats

1

T1 24-33 Pinner Road, Harrow 0.1 Residential - flats

1

T1 Jubilee House Merrion

Avenue, Stanmore 0.57 Residential - flats

1

T2 Telephone Exchange

Harrow View, Harrow 0.28 Residential

1

T2 Masters House Sandridge

Close, Wealdstone 0.33 Residential

1

T2 Case is altered and Land

fronting Canning Rd,


EXCEPTION

TEST

REQUIRED

3b

T2 Telephone Exchange

Osmond Close, South Harrow 0.37 Residential

1

T2 Brent House Kenton Road,

Kenton 0.3

Residential & town

centre uses

1

T3 Land West of High St,


EXCEPTION

TEST

REQUIRED

3b


MWH UK LTD

Development Name

Area

(ha) Possible Use

Essential

Infrastructure

Water

Compatible

Development

Highly

vulnerable

Development

More

vulnerable

Development

Less

vulnerable

development

Flood Zone

T1 Parks Depot Site and

former Mortuary, Wealdstone 0.34

Residential (flats &

houses)

1

T2 Mill Farm Close, Pinner 2.12 Residential

1

T2 Teachers Centre Cecil Road,


1

T2 1-10 Whitefriars Industrial

Estate, Wealdstone 1.06

Residential +

replacement B1

business/light

industrial use

1



EXCEPTION

TEST

REQUIRED

3b

T2 Harrow Postal Delivery

Office, Harrow 0.44 Residential

1

T3 Harrow Arts Centre,

Uxbridge Rd, Hatch End 3.3 Mixed

EXCEPTION

TEST

REQUIRED

3b

T3 Land at Stanmore Station,

Stanmore 6.62

Residential or B1 use

with retention of

station use and car

parking

1

T3 St Anns Service Yard,

Harrow 0.92 Mixed use

1

T2 47-49 High Street, Edgware 0.26 Mixed

EXCEPTION

TEST

EXCEPTION

TEST

3a


MWH UK LTD

Development Name

Area

(ha) Possible Use

Essential

Infrastructure

Water

Compatible

Development

Highly

vulnerable

Development

More

vulnerable

Development

Less

vulnerable

development

Flood Zone

REQUIRED REQUIRED

T1 1 Strongbridge Close,

Rayners Lane 2.76

Residential - flats &

houses

1

T1 RAF Bentley Priory,

Stanmore 9.99 Residential

1

T2 Harrow College Weald

Campus, Harrow Weald 2.42 Residential

1

T3 Territorial Army Centre,

Kingsbury 1.18

Retail Housing or

community use

1

T2 76 -132 College Road

College Road, Harrow 0.65

residential +

hotel/leisure + B1

office

1

T3 Amner Lodge, Stanmore 0.66

Housing or

community use

1

T1 Bradstowe House

Headstone Road, Harrow 0.35 Residential - flats

1

T1 Travis Perkins & 19 Pinner

Road, Harrow 0.68 Residential

1

T1 Raebarn House 86-100

Northolt Rd, South Harrow 0.41 Residential

EXCEPTION

TEST

REQUIRED

3b

T2 Kodak Sports Ground,

Wealdstone 3.2

Residential & open

space/leisure

1


MWH UK LTD

Development Name

Area

(ha) Possible Use

Essential

Infrastructure

Water

Compatible

Development

Highly

vulnerable

Development

More

vulnerable

Development

Less

vulnerable

development

Flood Zone

T1 Edgware Town Football

Club, White Lion Ground,

Edgware 1.25 Residential

EXCEPTION

TEST

REQUIRED

EXCEPTION

TEST

REQUIRED

3a

T1 Royal National Orthopaedic

Hospital, Stanmore 9.99

Hospital, Residential

& Car Park

1

T3 Harrow on the Hill Station

and Adjacent Land, Harrow 5.9

Public transport

improvements and

mixed use

development

1

T3 Driving Centre, Wealdstone 1.29

Mixed use scheme -

could involve

education, leisure or

community use

1

T2 Civic Centre, Wealdstone 4.58 Residential

1

T1 Gayton Road Car Park,

Harrow 1.19

Mixed Use - possibly

commercial/residenti

al

1

T3 Land South of Greenhill

Way, Harrow 0.75 Mixed use

1

T1 Government Buildings

Honeypot Lane, Stanmore 6.54 Residential

3b


MWH UK LTD

APPENDIX D

Wealdstone Brook Modelled Flood Extents

REVISION

A

DRAWN BY

MCCHECKED BY APPROVED BY SCALE

1:125000DATE

June 2011

THIS DRAWING IS ISSUED BY MWH. SUBJECT TO THE CONDITIONSTHAT IT IS NOT COPIED EITHER IN WHOLE OR IN PART OR DISCLOSED TOTHIRD PARTIES UNLESS PRIOR WRITTEN AUTHORISATION IS GIVEN BY MWH.

Wealdstone BrookeModelled Flood Extents

London Borough of HarrowLevel 2 Surface Water Management Plan

Based upon the Ordnance Survey's digital mapswith the permission of the Controller ofHer Majesty's Stationery Office (c) Crown Copyright

ORIGINAL DRAWING SIZE A3

1000yr (FZ 2) Modelled Flood Outline

100yr (FZ 3) Modelled Flood Outline

20yr (FZ 3a) Modelled Flood Outline

Main Rivers

Core Strategy Development Sites

Insert Plan


MWH UK LTD Page C1