B R O M L E Y T O W N C E N T R E A R E A A C T IO N P L A N

STRATEGIC FLOOD RISK ASSESSMENT

London Borough of Bromley Bromley Town Centre Area Action Plan Strategic Flood Risk Assessment November 2008

Halcrow Group Limited

Halcrow Group Limited Griffin House 135 High Street Crawley West Sussex RH10 1DQ Tel +44 (0)1293 434500 Fax +44 (0)1293 434599 www.halcrow.com

Halcrow Group Limited has prepared this report in accordance with the instructions of their client, London Borough of Bromley, for their sole and specific use. Any other persons who use any information contained herein do so at their own risk.

© Halcrow Group Limited 2008

London Borough of Bromley Bromley Town Centre Area Action Plan Strategic Flood Risk Assessment

Contents Amendment Record This report has been issued and amended as follows:

Issue Revision Description Date Signed

1 1 Draft Jan 08 RA/BV

2 1 Draft Apr 08 RA/BV

3 1 Draft Sept 08 RA/BV

4 1 Final Nov 08 RA/BV

Contents

1 Executive Summary 1

2 Background Information 2 2.1 Area Action Plan, Sustainability Appraisal and Preferred Options 2 2.2 Review of the Preferred Options 2 2.3 Proposal Sites 2 2.4 Potential Impacts of Revised Preferred Options 4

3 Requirement for a Level 2 Strategic Flood Risk Assessment 2

4 Relevant Documents 4 4.1 ‘Making Space for W ater’ 4 4.2 Catchment Flood Management Plans 4 4.3 Ravensbourne Policy Unit 6 4.4 Managing flood risk through development 6 4.5 Planning Policy Statement 25 (PPS25) 7

5 Sequential Test and Exception Test 9 5.1 Flood Zones and Flood Maps 9 5.2 The Sequential Test - Overview 10 5.3 Sequential Test for Bromley Town Centre 11 5.4 The Exception Test 13 5.5 Exception Test for Bromley Town Centre 13 5.6 Existing Development and Long-term Future Redevelopment 14

6 Flood Risk and Proposal Sites 17 6.1 Proposal Sites 17 6.2 Groundwater Flooding 17 6.3 Fluvial flooding 18 6.4 Surface W ater Flooding 19 6.5 Residual Fluvial Flood Risk 19 6.6 Sewerage Flooding 19

7 Drainage Strategy 20 7.1 Introduction 20 7.2 The existing drainage system 20

7.3 General Design Requirements 21 7.4 Constraints on drainage design 22 7.5 Source Protection Zones 23 7.6 Underlying geology in the Bromley Town Centre 23 7.7 Contaminated Land 24 7.8 Other constraints on drainage design and SUDS 24 7.9 Drainage design for Bromley Town Centre - Strategic Approach 24 7.10 Drainage design for Bromley Town Centre - Site specific drainage criteria 26 7.11 Outline estimate of SUDS control measures for each Proposed Site 26

8 Flood Warning 29 8.1 Introduction 29 8.2 Flood W arning Codes 29 8.3 Efficiency of Flood W arning Service 30

9 Recommended Policies 31 9.1 Interim Policies 31 9.2 Flooding 31 9.3 River Ravensbourne 31 9.4 Recreational use of the River Ravensbourne 32 9.5 Drainage Design 32 9.6 Development Layout 33

Appendices

Appendix A Run-off calculations for drainage design

Appendix B Audit Trail Database

Figures

Figure 1 – Town Centre Opportunity Sites

Figure 2 – Thames Catchment: Types of CFMP Flood Plain

Figure 3 – Schematic of the PPS25 Flood Zones

Figure 4 – Proposed area to be reserved for channel improvements within site K

Figure 5 – Study area geology (reproduced from the Environment Agency)

Figure 6 - Approximate location of culverts in the study area

Figure 7 – Proposed Exception Test measure and off-site attenuation of surface runoff

M aps

M ap 1 - Flood risk vulnerability: Proposed Town Centre Development Sites

M ap 2 – Flood zones and defences

M ap 3 – Source Protection Zones

Glossary of Terms

Area Action Plans – Development Plan Documents that provide a planning framework for areas of change and areas of conservation. Catchment Flood M anagement Plan (CFM P) – A strategic planning tool through which the Environment Agency seeks to work with other key decision-makers within a river catchment, to identify and agree policies for sustainable flood risk management. Development Plan - As set out in Section 38(6) of the Planning and Compulsory Purchase Act (2004), an authority’s development plan consists of the relevant Regional Spatial Strategy (or the Spatial Development Strategy in London) and the Development Plan Documents contained within its Local Development Framework. Development Plan Documents (DPDs) - Spatial planning documents within the Council’s Local Development Framework which set out policies for development and the use of land. Together with the Regional Spatial Strategy they form the development plan for the area. They are subject to independent examination. They are required to include a core strategy and a site allocations document, and may include area action plans if required; other DPDs may also be included, e.g. development control policies. Emergency Planning – Planning for and response to emergencies such as flooding, including consideration of the resilience of emergency infrastructure that will need to operate during flooding. Environment Agency - The leading public body for protecting and improving the environment in England and W ales. Flood management and defence are a statutory responsibility of the Environment Agency; it is consulted by local planning authorities on applications for development in flood risk areas, and also provides advice and support to those proposing developments and undertaking Flood Risk Assessments. The Environment Agency reports to DEFRA. Environment Agency Flood Zones - Nationally consistent delineation of ‘high’ and ‘medium’ flood risk, published on a quarterly basis by the Environment Agency. Flood Estimation Handbook - The latest hydrological approach for the estimate of flood flows in UK. Flood Risk Assessment – A site specific investigation usually carried out by the site developers to be submitted as part of their planning applications. It assess both current flood risk to the site and the impact of development of the site to flood risk in the area. Flood Risk Vulnerability - PPS25 provides a vulnerability classification to assess which uses of land may be appropriate in each flood risk zone. Formal Flood Defence - A structure built and maintained specifically for flood defence purposes.

Informal Flood Defence - A structure that provides a flood defence function, but has not been built or maintained for this specific purpose (e.g. boundary wall). JFlow - A computer river model based on routeing a flood calculated by Flood Estimation Handbook methodology along a river corridor the levels of which are derived from a Side Aperture Radar (SAR) remote sensed Digital Terrain Model. LiDAR – ‘Light Detection and Ranging’ is an airborne terrain mapping technique which uses a laser to measure the distance between the aircraft and the ground. It therefore provides accurate topographical/contour mapping. Local Development Documents (LDDs) – the collective term for Development Plan Documents and Supplementary Planning Documents. Local Development Framework (LDF) - The name for the portfolio of Local Development Documents. It consists of the Local Development Scheme, a Statement of Community Involvement, Development Plan Documents, Supplementary Planning Documents, and the Annual Monitoring Report. Local Development Scheme (LDS) - Sets out the programme for preparing Local Development Documents. All authorities must submit a Scheme to the Secretary of State for approval within six months of commencement of the 2004 Act (thus all authorities should now have submitted an LDS). LDSs are subject to review. ‘M aking Space for W ater’ (DEFRA 2004) - The Government’s new evolving strategy to manage the risks from flooding and coastal erosion by employing an integrated portfolio of approaches, so as: a) to reduce the threat to people and their property; b) to deliver the greatest environmental, social and economic benefit, consistent with the Government's sustainable development principles, c) to secure efficient and reliable funding mechanisms that deliver the levels of investment required. National Flood Risk Assessment (NaFRA) - Flood risk assessment that uses a risk-based approach to factor in the location, type, condition and effects of flood defences. It calculates the actual likelihood of flooding to areas of land within the floodplain of an extreme flood (0.1% or 1 in 1000 chance in any year), considering the distance from the river or the sea, and factors in the probability that the flood defences will overtop or breach. A variety of scenarios are analysed in order to determine properties at low, moderate and significant flood risk. Planning Policy Statements - The Government has updated its planning advice contained within Planning Policy Guidance Notes (PPGs) with the publication of new style Planning Policy Statements (PPSs), which set out its policy for a range of topics. Previously Developed (Brownfield) Land - Land which is or was occupied by a building (excluding those used for agriculture and forestry). It also includes land within the curtilage of the building, for example a house and its garden would be considered to be previously developed land. Land used for mineral working and not subject to restoration proposals can also be regarded as brownfield land.

Regional Spatial Strategy - Sets out the region’s policies in relation to the development and use of land and forms part of the development plan for local planning authorities. Residual Risk - The risk which remains after all risk avoidance, reduction and mitigation measures have been implemented. River Basin M anagement Plan (RBM P) – A strategic tool introduced by the W ater Framework Directive (2000/60/EC) which integrates the management of land and water within a river basin (river catchment or group of catchments). The river basin may cover several political areas. Strategic Environmental Assessment (SEA) - A generic term used to describe environmental assessment as applied to policies, plans and programmes. The European ‘SEA Directive’ (2001/42/EC) requires a formal ‘environmental assessment of certain plans and programmes, including those in the field of planning and land use’. Strategic Flood Risk Assessment (SFRA) – a district-wide assessment of flood risk, usually carried out by a local authority to inform the preparation of its Local Development Documents (LDDs) and to provide the information necessary for applying the Sequential Test in planning developments. Supplementary Planning Documents (SPDs) - Provide supplementary information in respect of the policies in Development Plan Documents. They do not form part of the Development Plan and are not subject to independent statutory examination, but are normally subject to public consultation. Sustainability Appraisal (SA) - Tool for appraising policies to ensure they reflect sustainable development objectives (i.e. social, environmental and economic factors) and required in the 2004 Act to be undertaken for all local development documents. It incorporates Strategic Environmental Assessment. 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). Sustainable Drainage Systems (SUDs) – Surface water drainage systems which manage runoff in a more sustainable way than conventional drainage, through improved methods of managing flow rates, protecting or enhancing water quality and encouraging groundwater recharge. A variety of types are available and can be chosen as appropriate for the location and needs of the development, and many have added benefits such as enhancement of the environmental setting, provision of habitat for wildlife and amenity value for the community.

Bromley Town Centre – Level 2 Strategic Flood Risk Assessment

1 Executive Summary

In December 2007, the London Borough of Bromley commissioned Halcrow to produce a Level 2 Strategic Flood Risk Assessment for Bromley Town Centre in accordance with Planning Policy Statement 25 (PPS 25).

This Level 2 Strategic Flood Risk Assessment (Level 2 SFRA) has been prepared to inform the LDF planning process and in particular the Area Action Plan (AAP). The Bromley Town Centre AAP, Revised Preferred Options Sustainability Appraisal Report (October 2007) will use the information from this Level 2 SFRA as evidence base information, to confirm and if necessary review its findings. It will in particular use the necessary flood risk information for the application of the Sequential and Exception Tests of PPS25.

Detailed modelling has been undertaken on the River Ravensbourne in order to obtain more accurate estimates of Flood Zones 3a revise flood zones 3a and 3b. Flood Zone 2 remains unchanged and 2 extents were obtained from the Environment Agency Flood Maps. The majority of the proposal sites are within flood zone 1. Parts of sites J and L are within flood zone 2 and therefore the Exception Test is required if ‘Highly Vulnerable’ land uses are to be permitted in these zones. Part of site K is in flood zone 3a and 3b therefore the Exception Test is required if ‘More Vulnerable’ land uses are to be permitted in this zone.

Guidance on using well-designed drainage systems to mitigate increased flood risk arising from development is given. This includes values of run-off volumes for each site, guidance on the use of SUDS and proposed policies and schemes pertaining to drainage system design at a strategic and site-specific scale. A scheme to open up some of the culverts to the west of the town centre into a naturalised channel is proposed. This would provide regional drainage attenuation and help to reduce flood risk as well as meet wider objectives which could be used to support the Exception Test.

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2 Background Information

2.1 Area Action Plan, Sustainability Appraisal and Preferred Options 2.1.1 As part of the Local Development Framework (LDF) for the London Borough of Bromley, an Area Action Plan (AAP) for Bromley Town Centre is being prepared. This will establish the policy framework and guide future development in the centre. The Bromley Town Centre AAP will form one of the LDF Development Documents (DPDs).

2.1.2 Under the Planning and Compulsory Purchase Act 2004, preparation of Development Plan Documents must be informed by a Sustainability Appraisal (SA). A preliminary SA was prepared in October 2005 to accompany the Preferred Options Report which was the subject of public consultation between January and February 2006. The preferred options were based on two out of three scenarios (Scenarios 2 and 3).

2.2 Review of the Preferred Options 2.2.1 The Preferred Options were revised in the light of representations received. The SA report was also reviewed in the light of these changes and due to preliminary market activity which helped to establish what can be implemented. The changes that have resulted from the three scenarios are listed in the October 2007 Revised Preferred Options Sustainability Appraisal report (Table 4.3 - Proposed Changes to Preferred Options).

2.3 Proposal Sites 2.3.1 As part of the Spatial Strategy and based on the proposed changes to the Preferred Options, fourteen Proposal Sites have been identified (Sites A to P).

2.3.2 Development options and broad development principles for each of the Proposal Sites are set out in Table 5.1 and Figure 5.2 of the October 2007 Revised Preferred Options SA report. The figure in the SA report has been reproduced in this document as Figure 1 (Town Centre Opportunity Sites).

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Figure 1 – Town Centre Opportunity Sites

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2.4 Potential Impacts of Revised Preferred Options 2.4.1 Table 7.2 (Potential Impacts of Revised Preferred Options) of the October 2007 Revised Preferred Options SA report, shows the impacts of the Proposal Sites against the SA objectives. Although there are many positive and significant positive effects, it is clear that this is not the case for Sustainability Objectives 8, 9, 10, 13 and 14. These are:

‹ SA Objective 8 To reduce the effect of traffic and travel on the environment ‹ SA Objective 9 To improve air quality and reduce noise and light pollution ‹ SA Objective 10 To maintain and enhance the quality of landscapes and townscapes ‹ SA Objective 13 To reduce contributions to climate change (and to make appropriate adaptations) ‹ SA Objective 14 To improve the quality of the water environment, the drinking water supply in the long terms and reduce flood risk

2.4.2 W ithout mitigation, the impacts of the Proposal Sites against SA objectives 8, 9, 10, 13 and 14 will result in general, in negative or significantly negative effects. This Level 2 Strategic Flood Risk Assessment proposes mitigation measures and opportunities which will eliminate many negative and significant negative effects (see Sections 5.5 and 7.9).

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3 Requirement for a Level 2 Strategic Flood Risk Assessment

3.1.1 This Level 2 Strategic Flood Risk Assessment has been prepared in accordance with Planning Policy 25 (PPS25, see Section 4.5) to inform the LDF planning process and in particular the Bromley Town Centre AAP. The Bromley Town Centre AAP, Revised Preferred Options Sustainability Appraisal Report (October 2007) will use the information from this Level 2 Strategic Flood Risk Assessment (SFRA) as evidence base information, to confirm and if necessary review its findings.

3.1.2 Based on the Environment Agency flood zones, parts of some of the proposal sites J, K and L fall within Zones 2 and 3 - see definitions in Section 5.1.2. This study investigates proposal sites J, K and L in further detail however it does not remove the need to undertake specific flood risk assessments (FRAs) by developers, when submitting planning applications for these sites. In practice most future planning applications on sites within the Area Action Plan will require specific flood risk assessments, to ensure that the development proposals are consistent with the policies on flooding informed by this SFRA and developed within the AAP.

3.1.3 This SFRA provides information which will allow the Council to address the following issues (letter dated 10 December 2007) raised by the Environment Agency:

• Parts of some of the proposed development sites are within flood zone 3 and comprise land uses which may not be suitable in areas with this level of flood risk.

• No evidence has been provided within the AAP to indicate that the Sequential Test and Exception Test have been carried out when designating the locations of the proposed developments, and therefore the requirements of PPS25 have not been met.

• The effect of the proposed development on fluvial and surface water flood risk has not been considered and reported on.

• The effect of climate change on fluvial and surface water flood risk has not been considered and reported on.

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• A management strategy for the flood risk associated with the proposed developments has not been put forward.

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4 Relevant Documents

4.1 ‘M aking Space for W ater’ 4.1.1 ‘Making Space for W ater’ (DEFRA 2004) is the evolving new strategy to shape flood and coastal erosion risk over the next 10-20 years. Part of the vision of the strategy is to ensure:

‘a holistic approach to the assessment of options through a strong and continuing commitment to Catchment Flood Management Plans and Shoreline Management Plans, within a broader planning matrix which will include River Basin Management Plans prepared under the Water Framework Directive and Integrated Coastal Zone Management.’

4.2 Catchment Flood M anagement Plans 4.2.1 Catchment Flood Management Plans (CFMPs) are strategic planning tools through which the Environment Agency seeks to work with planning authorities and other key decision-makers within a river catchment, to identify and agree policies for sustainable flood risk management. The main aims of CFMPs are to: a) understand the factors that contribute to flood risk within a catchment, such as how the land is used, and b) recommend the best ways to manage the risk of flooding within the catchment over the next 50 to 100 years.

4.2.2 The Thames CFMP identifies the Ravensbourne catchment (which includes the Bromley Town Centre) as a catchment that has developed flood plain with typically concrete river channels. It confirms that the risk in these areas is relatively high and it is likely that this will increase in the future.

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4.2.3 The Thames CFMP messages for this type of catchment are listed below (see also figure showing types of CFMP flood plain):

• W e need to re-create a river corridor so there is more space for the river

to flow and flood naturally.

• Flood risk management planning needs to be linked closely with re generation and redevelopment so that the location and layout of d evelopment can help to reduce flood risk.

• T here is a large and increasing residual flood risk in these flood plain ar eas. PPS25 sets out a range of measures that can reduce the impacts of residual flood risk, such as making buildings resilient to flooding.

• Organisations need to work together to manage all flood sources: fluvial,

tidal, surface water and sewer flooding.

Figure 2 – Thames Catchment: Types of CFM P Flood Plain

Ravensbourne Catchment

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4.3 Ravensbourne Policy Unit 4.3.1 The Ravensbourne Policy Unit document develops specific policies from the Thames CFMP for the Ravensbourne catchment.

4.3.2 The overall approach in the Ravensbourne catchment is to take further action to sustain the current level of flood risk into the future by responding to the potential increase in risk from urban development, land use change and climate change.

4.3.3 The flood risk management objectives/options for this type of catchment are outlined in the box below:

Developed floodplain with little open space and often with concrete river channels • Options to reduce the probability of flooding are highly constrained in these catchments. There is significant flood risk from a variety of sources and the rivers have been significantly altered. • It is necessary to change the character of the urban footprint through re-development so that the consequences of flooding are reduced by better layout out greater resilience to flooding. • In most areas, it is necessary to change the character of the urban area through re-development before measures can be introduced to reduce the probability of flooding that will be sustainable. For example, re-creating river corridors so that there is space for the river to flow and flood more naturally.

4.4 M anaging flood risk through development 4.4.1 The role of development planning in reducing flood risk is driven by ‘Making Space for W ater’, PPS25 and the messages and objectives of the Thames CFMP and Ravensbourne Policy Unit (see Sections 4.2.3 and 4.3.3).

4.4.2 The AAP can facilitate meeting the aims of the Thames CFMP and Ravensbourne Policy Unit through the location, layout and nature of development. The table below outlines in more detail what is sought to be achieved on the Ravensbourne (as stated in the Ravensbourne Delivery Unit) and how the AAP can contribute to achieving these aims:

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Aim of CFM P and Ravensbourne Policy How the AAP can contribute to achieving Unit these aims

Prevent further development at locations adjacent to the channel and encourage development at Change the character of the urban locations set back from the river. floodplain through re-development. The consequences of flooding can be reduced by Ensure new development buildings are more re-establishing river corridors so that urban resilient to flooding through their design areas can better accommodate flooding Where feasible, re-establish a more natural river corridor by opening up culverts

Naturalise the river where practical. Remove culverts, trash screens, artificial channel and bank lining where possible and replace with a Reduce the maintenance burden in the naturalized channel. future

Safeguard existing open space so that the Prevent or restrict the type of development on sites opportunity to attenuate water will still be that have been identified as being potential storage there in the future areas.

Implement a drainage strategy which uses sustainable techniques and the right balance of conveyance and attenuation to reduce flood risk Continue to reduce the impact of low order flooding in urban areas (up to a 10% to 20% Ensure all redevelopment is flood resistant in AEP flood – 1in 10 to 1 in 5 year return design period) Use the Sequential Test and Exception Test to steer vulnerable land-uses away from zones of higher flood risk

The river is seen more as a community asset, reducing the level of fly-tipping but Encourage opening up and naturalization of the continuing to enforce where it is effective watercourse where feasible. and necessary

4.5 Planning Policy Statement 25 (PPS25) 4.5.1 PPS25 confirms that flooding, in all its forms, is a material planning consideration in the determination of planning applications and in the formulation of planning policy. PPS25 states that flood risk should be considered alongside other spatial planning concerns such as transport, housing, economic growth, natural resources, regeneration and the management of other hazards.

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4.5.2 PPS25 reaffirms the adoption of a risk-based approach to flooding, including the avoidance of new sources of flooding; managing flood pathways and avoiding inappropriate development in areas at risk from flooding.

4.5.3 PPS25 introduces and explains the Sequential Test and Exception Test. These Tests are the tools which ensure that development aids Flood Risk Management through reducing, or as a minimum not increasing, flood risk. Section 5 gives further details on the Sequential Test and Exception Test and how they should be applied.

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5 Sequential Test and Exception Test

5.1 Flood Zones and Flood M aps 5.1.1 All land within the Environment Agency’s remit is classified into a flood zone, based on its probability of flooding. The extent of the flood zones for different flood events is predicted mainly using modeling and shown on flood maps. These are accessible from the Environment Agency website. A schematic of the flood zones is shown below.

(Z o n e 3 b )

Figure 3 – Schematic of the PPS25 Flood Zones

5.1.2 The flood zones shown on the Environment Agency flood maps have been mainly derived from the “JFlow” generalised computer modelling. This is a ‘coarse’ modelling approach which incorporates a large number of assumptions.

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5.1.3 Halcrow is currently undertaking computer modeling work on the River Ravensbourne. This modelling is based on a more detailed approach, using 2D modelling techniques and based on the ISIS- TUFLOW software. Although this work is complete in the area surrounding the Bromley Town Centre, this is still in draft version throughout the catchment.

5.1.4 The Environment Agency has confirmed that approval of the model will require resolving catchment-wide issues that affect the Bromley Town centre (such as the catchment hydrology). It is recommended in the interim to use the Environment Agency flood zones.

5.2 The Sequential Test - Overview 5.2.1 The Sequential Test is an essential component of the spatial planning process as it applies the principle of avoidance when allocating land uses with different flood risk vulnerability.

5.2.2 All types of development land use are categorized based on their vulnerability to flooding (see table below - from PPS25 Table D.2) as follows:

PPS25 Land Uses Example Essential Infrastructure Essential transport infrastructure Highly Vulnerable Such as ambulance stations More Vulnerable Such as dwelling houses Less Vulnerable Such as commercial units W ater Compatible W ater based recreation

5.2.3 The Sequential Test is based on the premise that only certain categories of land use should be allowed on certain flood zones. ‘Highly vulnerable’ land uses, for example, are not ‘compatible’ with high risk flood zones. The table below (from PPS25 Table D.3) shows those combinations of land-use type and flood zones that are acceptable according to PPS25 and those which are not. In some situations a land use may only be allowed in a flood zone if the Exception Test of PPS25 is undertaken (see Section 5.4).

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5.3 Sequential Test for Bromley Town Centre 5.3.1 As mentioned in Section 3.1.1, this Level 2 SFRA has been prepared to inform the AAP. It will in particular provide the necessary information for the application of the Sequential Test as part of the Sustainability Appraisal (which will be updated by using evidence base, flood risk information from this study). It will also assist in the assessment of the three scenarios of the SA (see Section 2.1.2) based on flood risk issues and other factors.

5.3.2 As part of the Sequential Test approach it will be important to demonstrate that:

(a) Consideration has been given to any other ‘reasonably available’ alternative sites at a lower flood risk outside of the AAP area, but within reasonable distance from the Town Centre.

(b) The feasibility/sustainability of meeting housing targets by intensifying residential development on any of the other allocated sites within flood zone 1 (or outside of the AAP area) has been investigated.

5.3.3 The following information will facilitate the application of the Sequential Test (see Figure 2 – Flood risk vulnerability: Proposed Town Centre Development sites based on the flood zones as proposed in Section 5.1.4):

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a) Proposal Sites A to H, M, N and P are within Zone 1. The land use types within these sites are a mixture of more vulnerable, less vulnerable, water compatible and essential infrastructure. No Exception Test would be required for these sites as they are in Zone 1. b) Sites J, K and L each contain three different flood zones. They also contain land uses of different levels of vulnerability. The table below summarizes the approximate extents of each flood zone within each of the site areas J, K and L and the different land uses proposed for each site:

% of total site area Vulnerability Proposal covered by flood Proposed land uses classification Site zone within site of land use 1 2 3 Residential (500 More units); community vulnerable and health facilities New station and forecourt; improved Essential

public transport infrastructure interchange facilities Retail (3,000 sq m); café/restaurants J 50 25 25 (2,000 sq m); car Less parking; vulnerable offices/business (2,000 sq m) Improved public W ater space (1,000 sq m) compatible Residential (100 More units) vulnerable

Cinema and leisure K 25 10 65 (4,000 sq m); food Less and drink (1,000 sq vulnerable m) Residential (75 units); More L 5 80 15 hotel (100 beds) vulnerable

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The Sequential Test aims at allocating the most vulnerable proposal sites in the lowest flood risk areas. However, when planning design and location of development within each site, a sequential approach should be used to ensure that the most vulnerable land uses are located in lower flood risk areas. W ithin Site K for example, it will be important to place the more vulnerable uses in the areas at less risk (such as Zone 1 and then Zone 2, if not sufficient Zone 1 area is available; and then followed by Zone 3a If not sufficient Zone 2 area is available).

c) Provided that the SA demonstrates that sites J, K and L cannot be located in Zone 1 (within the Town Centre boundary or elsewhere within the administrative boundary of the planning authority) it will be necessary to apply the Exception Test.

5.4 The Exception Test 5.4.1 Departures from the sequential approach can only be justified in exceptional circumstances where it is necessary to deliver sustainable development. This justification is known as the Exception Test.

5.4.2 For the Exception Test to be passed (copied from PPS25):

a) it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared. If the DPD has reached the ‘submission’ stage – see Figure 4 of PPS12: Local Development Frameworks – the benefits of the development should contribute to the Core Strategy’s Sustainability Appraisal; b) the development should 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; and c) a FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall. 5.5 Exception Test for Bromley Town Centre 5.5.1 The Exception Test will be required if the Sequential Test indicates that it will be necessary to allocate Highly Vulnerable land uses within sites J and L and a More Vulnerable land use within site K.

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5.5.2 The preferred mitigation measure is to replace the culverts downstream of the Bromley South railway line with larger capacity natural open channels and nearby downstream storage at Queen’s Mead (see Figure 7). Such a scheme would meet the requirements of the Exception Test because:

• the benefits will contribute to the Core Strategy’s Sustainability Appraisal (a number of negative and significant negative effects will be removed from Table 7.2 of the October 2007 Revised Preferred Options SA report),

• it is consistent with the aims of the CFMP Ravensbourne Policy Unit document,

• the development will be safe, without increasing flood risk elsewhere, and will reduce flood risk overall.

5.5.3 The replacement of culverts with open channels could potentially contribute to alleviating upstream localized surface water flooding (the event that occurred in July 2007), provided that additional storage is built within the open channels.

5.5.4 Another potential mitigation option is to reduce the flood risk at the sites by providing upstream storage attenuation. This is likely to be a more expensive solution however it could be more viable (higher benefit/costs) if it also reduces flood risk outside the Bromley Town Centre. Potential attenuation storage locations are in W hitehall Recreation Ground or upstream of Magpie Hall Lane. These areas are outside of the AAP area boundary however their impact on flood risk would be felt within the AAP region.

5.6 Existing Development and Long-term Future Redevelopment 5.6.1 Consideration should be given to how longer-term redevelopment could aid flood risk management. For example consideration could be given to the possibility of re-locating more vulnerable land use types that have been built in higher flood risk zones in the past into more compatible flood risk zones.

5.6.2 A culverted reach of the Ravensbourne currently cuts through the proposal site K. The FRA for this site needs to ensure that no building is constructed above the culvert and that an adequate ‘buffer zone’ is in place from the nearest building. Alternatively, the culvert could be re-aligned, subject to approval from the Environment Agency.

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5.6.3 There may be scope at some point in the future for opening up the watercourse within site K. It is envisaged that this measure could be implemented as an alternative to the mitigation proposal in section 5.5.2 to open up the culverts downstream of the site. It is advised that an 11m wide strip running along the western edge of site K is protected from development in order that the space can be utilised in the future to accommodate an open river channel to replace the existing culverted sections of the watercourse in this area (see policies in Section 4.4 and Figure 4 below). Development could be allowed to occur up to the boundary of the new channel bank. However, leaving a buffer alongside this boundary line should be encouraged in order to allow maintenance access to the channel.

Area available for Building development boundary line

Area potentially River reserved for future Ravensbourne channel improvements

Figure 4 – Proposed area to be reserved for channel improvements within site K

It is anticipated that the new flood mapping modelling outputs from the Environment Agency will indicate that Zone 3 will not extend beyond the 11m wide strip highlighted in blue in Figure 4. Based on this assumption the remaining developable area could be considered as Zone 2 when undertaking the Sequential Test.

Any development that is built in site K will be subject to design standards in order to ensure flood risk is acceptably low:

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• The finished floor levels of the buildings will need to be above the 100 year plus climate change surface water flood risk levels (refer to Halcrow surface water maps - more detailed maps will be required for the area as part of the FRA). This will result in having the buildings finished floor levels about 0.3m above surrounding ground levels.

• Flood resilient/resistant design will be required for 0.3m above the finished floor levels to allow for a potential blockage of the culvert during a fluvial flood event.

• Rainfall harvesting in the form of green roofs and water re- cycling for flushing toilets, etc is recommended to ensure that surface water flood risk is reduced.

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6 Flood Risk and Proposal Sites

6.1 Proposal Sites 6.1.1 Figure 1 shows the location of the 14 proposal sites within the town centre. The sites vary in size from 0.3 to 4.27 hectares and their total area is 19.18 hectares. The current land use for each of the sites varies, but typically includes infrastructure, retail, public buildings and open public space.

6.1.2 This section outlines the current nature of different sources of flooding within the Bromley Town Centre. The likely future effect of the development on flooding is also investigated.

6.2 Groundwater Flooding 6.2.1 Historic records of groundwater flooding were made available from the Environment Agency. Although this dataset contains no records of historical incidents within the Town Centre boundary, it cannot be confidently assumed from this that groundwater poses no flood risk in the Town Centre as the records may be incomplete or non existent.

6.2.2 The geology of the area (shown in Figure 5 below) suggests where groundwater flooding may be a risk. The sandy Harwich Formation holds a water table which could cause flooding. Emergence of the water table would be most likely near the edge of the formation outcrop and where valleys cut into the side of the hill. Development in these areas could thus be at risk from groundwater flooding. A site-specific FRA would be needed for developments at these areas, especially those with basements or underground services, in order to ascertain the exact nature of this risk. The Environment Agency has a report of groundwater flooding at Farnaby Road in March 2003.

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Figure 5 – Study area geology (reproduced from the Environment Agency)

6.3 Fluvial flooding 6.3.1 The main watercourse that runs along the south and south west area of the Bromley Town Centre is the River Ravensbourne (see Figure 3). Some areas in the southern part of the Town Centre AAP boundary are at risk of fluvial flooding from the Ravensbourne and its tributaries, as shown by the flood risk zones on Figure 3. Historical records show that the river last flooded in 1968, and in the 1970s many reaches were culverted or straightened with concrete channels. Since then flooding has not occurred however there is still a residual fluvial risk of fluvial flooding due to potential failure of the culverts (see Section 6.5).

6.3.2 Increased urbanisation and development is associated with a decrease in permeable area and therefore increased run-off and

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reduced lag times. These changes in response of the catchment to rainfall have contributed over many years, to increased fluvial flood risk.

6.4 Surface W ater Flooding 6.4.1 There have been a number of minor isolated past incidents of surface water flooding within the Bromley Town Centre. It is possible that the causes of these incidents have since been resolved.

6.4.2 The proposal sites will increase the impermeable areas and will result in increased runoff unless adequate mitigation measures are in place.

6.5 Residual Fluvial Flood Risk 6.5.1 The current 2D model that is being developed for the entire Ravensbourne catchment has been run for the scenario of failure/blockage of the Ravensbourne culverts (in the vicinity of the Bromley Town Centre) during a flood event. This condition has been tested at a location downstream of Proposal Sites J, K, and L.

6.5.2 The model indicates that significant flooding will occur with flood depths exceeding 0.5m. Although this event is unlikely, the damage to property and commercial property will be very high. The likelihood of this event will become more significant over the next 100 years (a timeline of 100 years is being considered this is the expected design life of a residential developments). The reason for this is because the remaining design life some of these culverts is less than 30 years.

6.5.3 Mitigation of this residual fluvial flood risk would be achieved through implementing the scheme to open up the culverts into naturalized channels, as described in Section 5.5.2.

6.6 Sewerage Flooding 6.6.1 The foul drainage system is separate from the surface water system. Foul flooding is however still possible as a result of surface water flooding entering the sewer system.

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7 Drainage Strategy

7.1 Introduction 7.1.1 The Exception Test of PPS25 requires that “the FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall”. Potential increases in flood risk arising as a result of development must be mitigated by adopting an adequate drainage strategy.

7.1.2 There is potential for development to increase flood risk through increasing surface water run-off by increasing the impermeable area at the site. This increased run-off could increase surface water flooding and fluvial flood risk downstream. This chapter first outlines the existing drainage system before proposing the drainage design requirements for new developments. Guidance is also provided on the design of a drainage system for the proposal sites in the Bromley Town Centre AAP.

7.2 The existing drainage system 7.2.1 Currently, surface water runoff in Bromley Town Centre is collected into a pipe drainage system that discharges into the Ravensbourne culverts, at a number of locations (the approximate alignment of the Ravensbourne culverts is highlighted in red in Figure 6 below):

Figure 6 – Approximate location of culverts in the study area

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7.2.2 It is possible that the existing pipe network system that collects surface water runoff from Bromley Town Centre is not at full capacity (as there are no records of recent flooding), however it is unlikely that this will be sufficient over the next 100 years. The current PPS25 estimates indicate that rainfall intensities will increase by 30% over the next 100 years.

7.3 General Design Requirements 7.3.1 The following sections outline the design requirements that drainage systems should meet. It is recommended that the design requirements in this Section apply to all developer’s Flood Risk Assessments for the Proposal sites.

7.3.2 The London Plan Supplementary Planning Guidance on Sustainable Planning and Construction states that development should: Use SUDS wherever practical (Essential Standard). Achieve 50% attenuation of the undeveloped site’s surface water runoff at peak times (Essential Standard). Achieve 100% attenuation of the undeveloped site’s surface water runoff at peak times (Mayor’s Preferred Standard).

7.3.3 Drainage must be designed to cope appropriately with the peak runoff rates and volumes for the following design criteria:

• 1 to 2 year event to protect the watercourse from erosion if discharging into a watercourse,

• 1 in 30 year event to meet Sewers for Adoption (Thames W ater) requirements,

• 1 in 100 year events to ensure the site does not increase flooding.

7.3.4 The layout of a development should take into account the management of extreme flood flows, by showing any flow paths in extreme events (i.e. 1 in 1000 year event).

7.3.5 Runoff from the sites, where possible, should be limited to the Greenfield rates for the above design events, if this is not possible the London Plan Standards should be used as minimum requirements for site runoff rates.

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7.3.6 All drainage design must be undertaken with an allowance for climate change in accordance with the increases in rainfall predicted and outlined in PPS25. As a result of climate change, rainfall intensity and peak river flows are set to increase and drainage must be designed to cope with these changes.

7.3.7 Preference should be given to those techniques located higher up the SUDS management hierarchy. These are the techniques that attenuate water at source rather than further downstream. This hierarchy (from the CIRIA Interim Code of Practice for Sustainable Drainage Systems) is reproduced in the box below (reproduced from CIRIA Interim Code of Practice for Sustainable Drainage Systems). Attenuation prior to discharge into the Ravensbourne is preferable where feasible, as it will minimise the increase in volume reaching the watercourse from the development sites and in turn flood risk downstream. 1. Prevention – the use of good site design and housekeeping measures on individual sites to prevent runoff and pollution (examples include minimising paved areas and the use of sweeping to remove surface dust from car parks) 2. Source control – control of runoff at or very near its source (such as the use of rainwater harvesting, pervious pavements, green roofs or soakaways for individual houses). 3. Site control – management of water from several sub- catchments (including routeing water from roofs and car parks to one large soakaway or infiltration basin for the whole site). 4. Regional control – management of runoff from several sites, typically in a detention pond or wetland.

7.3.8 Further guidance on SUDS is available from CIRIA (at http://www.ciria.org/suds/index.html). H R W aliingford has published a guide: “Use of SUDS in High Density Developments. Guidance Manual” Ref: SR666 by R Kellagher, 2005.

7.4 Constraints on drainage design 7.4.1 The following sections highlight potential constraints on the type and location of drainage techniques. The design requirements outlined in the previous section may not be fully achievable due to these constraints. The final detailed drainage design should be informed by the findings of in-depth investigation into the nature of each of these constraining factors within the town centre.

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7.5 Source Protection Zones 7.5.1 The Source Protection Zones within the area (as shown in Figure 5) place constraints on the drainage methods that can be utilized. The table below summarizes the Source Protection Zones covering each of the sites:

SPZ Sites in that zone W hole of sites: D, G, H, K, N 1 ( inner protection zone) Part of sites: J, L, P W hole of sites: E, M 2 ( outer protection zone) Part of sites: B, F, J, L, P W hole of sites : A, C 3 Part of sites: B, F

7.5.2 According to the Environment Agency’s ‘Policy and Practice for the Protection of Groundwater’, only roof water is permitted to discharge to ground at sites on inner Source Protection Zones. Pollution prevention methods must be undertaken at these sites to ensure that there is no pathway for polluted water (e.g. surface water from roads) to reach soil in Source Protection Zones. The pollution control methods are further described under the heading Groundwater and Land Contamination in the letter from the Environment Agency dated 10 December 2007 (ref. SL/2006/100000/AP-01/PO1-L01).

7.6 Underlying geology in the Bromley Town Centre 7.6.1 The local geology will affect the type of SUDS that can be utilized, for example only certain strata types are suitable for use as soakage zones for soakaways. W ithin the Bromley Town Centre it is likely that suitable strata does not exist at shallow depths, therefore deep- bore soakaways would be necessary if this methodology were to be employed. Deep-bore soakaways may not be feasible however within the Town Centre, as voids in the underlying chalk have been known to cause collapse of overlying strata (deep-bore soakaways may cause similar voids).

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7.7 Contaminated Land 7.7.1 W hen designing the drainage system, consideration needs to be given to contaminated land. The design should ensure the drainage system does not increase risk posed by contaminated land by, for example, mobilizing contaminants within the land. It is recommended that an investigation into the nature of any contaminating material in the land is undertaken and the findings used to inform an appropriate drainage design.

7.8 Other constraints on drainage design and SUDS 7.8.1 Slope and area of available space will determine the type and location of drainage techniques. As the town centre is a built-up area, there is unlikely to be sufficient space to accommodate some SUDS techniques (for example on-site wetlands or large retention ponds). However, SUDS can be incorporated in dense urban sites; examples and guidance can be found in HR Wallingford’s “Use of SUDS in High Density Developments. Guidance Manual” Ref: SR666 by R Kellagher, 2005.

7.9 Drainage design for Bromley Town Centre - Strategic Approach 7.9.1 A strategic approach is recommended in order to effectively mitigate against the impact of climate change on surface runoff as well as that from new developments (increased impermeable areas). Consideration needs to be given to how additional volumes of surface water coming from a site as a result of future further intensification on the site will be dealt with. The following strategic measures are recommended for all Proposal sites:

1) Apply the SUDS hierarchy as described in Section 7.3.7.

2) In relation to roof runoff:

• Consider discharging into the ground only if it is proved that there is no risk of collapse of the overlying strata (see Section 7.6.1). Particular care needs to be taken in avoiding mixing with surface runoff from car parks, etc.

• Use rainfall harvesting of roof runoff in residential units for its use as grey water for washing machines and flushing toilets.

• Retain part of the rainfall by using green roofs in residential units

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3) Permeable paving could be used for the purpose of attenuating flows in open spaces/car parks. If this method is proposed, the ground beneath the permeable paving needs to be sealed to avoid contact with the source protection zone. Consideration needs to be given to the maintenance and replacement requirements of permeable paving.

4) Building design and layout should ensure that dry egress is available and, where possible, residential use is not on basement or ground floor level.

5) If there is no sufficient space availability for source control and site control measures (see Section 7.3.7) a regional control will be required, to attenuate elsewhere the runoff that cannot be attenuated on site or on nearby sites. This will be achieved by discharging flows through the Thames W ater drainage system and into off-site wetlands/ponds, prior to discharge into the River Ravensbourne (ideally the opened up sections of the watercourse – see Figure 7 below).

Queen’s M ead – Potential storage area to balance increased capacity from upstream opened up watercourse (as part of the Exception Test measure) and alternative location for attenuating surface runoff

Culverts opened up into larger-capacity natural watercourse (Exception Test measure) and linear wetlands/ponds for the attenuation of surface runoff

Figure 7 – Proposed Exception Test measure and off-site attenuation of surface runoff

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An alternative location is Queen’s Mead however there are some disadvantages about this, such as its relatively long distance from the proposal sites or a possible high water table at Queen’ Park.

7.10 Drainage design for Bromley Town Centre - Site specific drainage criteria 7.10.1 The developer for each site must produce detailed drainage designs which show how the required volume of attenuation will be achieved. The design should demonstrate that:

• The hierarchal approach to SUDS drainage design (site control, source control and then regional control – see Section 7.3.7) has been followed when formulating the drainage design (see Section 7.3).

• Opportunities to reduce surface water flood risk have been identified and incorporated.

• The principles of the overall strategic approach (Section 7.9) have been adhered to.

7.11 Outline estimate of SUDS control measures for each Proposed Site 7.11.1 Based on the indicative green areas, built areas and car park areas estimated for each proposed site, it is anticipated that source control measures could be achieved for a large proportion of the design rainfall. It is likely however that there will be local constraints on each proposed site and that a proportion of the surface runoff will require site control or regional control (see the table of required volumes and available volumes for each proposal site in next page and calculations in Appendix A). Site control is likely to involve discharge into soakaways however there are potential constraints (see Section 7.6.1).

7.11.2 The volume estimates for each proposal site are based on the following design criteria and assumptions: a) a 100 year plus climate change, 8hr storm duration rainfall, b) 100% impervious surfaces for built and car park areas, c) no surface runoff originated from the green areas, d) permeable paving in car parks, e) underground storage beneath 60% of the green areas and f) 2l/s per ha of permitted greenfield runoff.

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AAP Site Built Paving Green Required Estimated Required M easure Proposal Area Area Area Area Attenuation Attenuation Off-Site Site (Ha) (%) (%) (%) Volume on site Volume (cubic m) (cubic m) (cubic m) Source A 60 30 10 2425 2672 -247 ** 2.62 Control

Source B 0.36 50 20 30 259 850 -591 ** Control

Mainly Source C 0.64 70 20 10 592 589 3 Control

Mainly Source D 1.95 70 20 10 1805 1794 11 Control

Mainly site

control or E 90 10 0 874 85 789 0.85 regional

control

Source F 6.11 30 20 50 3142 23218 -20076 ** Control

Mainly Source G 2.14 70 20 10 1981 1969 12 Control

Mainly Source H 0.44 70 20 10 407 405 2 Control

Source J 2.31 60 30 10 2138 2356 -218 ** control

Source K 0.95 60 30 10 879 969 -90 Control

Mainly Source L 0.4 70 10 10 370 368 2 Control

Source M 1.19 5 90 90 122 7771 -7649 ** Control

Source N 0.3 0 70 30 216 858 -642 ** Control

Mainly Source P 1.34 70 10 10 1240 1233 7 Control

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** Negative number indicates spare capacity within the site.

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8 Flood Warning

8.1 Introduction 8.1.1 Providing a flood warning service is advocated as an important component in the overall approach to reducing flood risk by both the Ravensbourne Delivery Plan and Ravensbourne Unit Policy document. The Town Centre is located within the River Ravensbourne East Branch at Bromley flood warning area. (for further details about this service see the Environment Agency website at: http://www.environment- agency.gov.uk/subjects/flood/826674/829803/946278/?lang=_e).

8.2 Flood W arning Codes 8.2.1 The flood warning system consists of a set of codes with the following meanings:

• Flood W atch - Flooding of low lying land and roads is expected. Be aware, be prepared, watch out!

A Flood W atch would be issued when water levels along the river are forecast to overtop the banks. Flood watches are issued for the whole of the River Ravensbourne Catchment.

• Flood Warning - Flooding of homes and businesses is expected. Act now!

A Flood W arning is issued when the Environment Agency anticipates flooding to property. Flood Warnings are issued for a specific flood warning area within a catchment e.g the River Ravensbourne East Branch at Bromley

• Severe Flood W arning - Severe flooding is expected. There is extreme danger to life and property. Act now!

The trigger for issue of a Severe Flood W arning is dependent on a number of factors, but is essentially used when there is thought to be extreme danger to life. This is a decision that would be made on the basis of river levels, large numbers of properties affected, response required by emergency services and local authorities, likely impact on major infrastructure etc. Severe Flood Warnings are issued for a specific flood warning area.

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• All Clear - Flood W atches or W arnings are no longer in force for this area.

8.3 Efficiency of Flood W arning Service 8.3.1 All warnings are highly dependent on our ability to forecast. There are two basic meteorological systems that concern Thames Region; these are frontal rain bringing heavy and prolonged rainfall over a catchment and convective storms which produce very high intensity rainfall for shorter periods directly over part of a catchment. Both are regularly experienced in Thames Region. Both types of rainfall event brings risk of severe flooding, however, it is the unpredictable nature of the convective storm cells which present the most significant risk to delivery of an effective warning service.

8.3.2 Although nationally the Environment Agency aims to give a two- hour warning in advance of any flooding taking place such a warning lead time is impractical in many of the urban areas and upper catchments within Thames Region (including the Ravensbourne) as low-to-peak flow can occur over just half an hour. In these instances, the provision of warnings based on Met Office forecasts is often required. The difficulty in detection and unpredictable nature of convective storms make providing reliable meteorological forecasts the most difficult. In these cases the Environment Agency are dependent on meteorological forecasts from the Met Office and not flood forecasts to issue warnings.

.

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9 Recommended Policies

9.1 Interim Policies 9.1.1 It is recommended to initially adopt the following list of recommended policies, which includes those proposed in the Environment Agency letter dated 10 December 2007 (ref. SL/2006/100000/AP-01/PO1-L01). These policies should be revised once the strategy for the Ravensbourne catchment is completed (the Ravensbourne Delivery Plan which is due to be completed by the end of June 2008) in consultation with the London Borough of Bromley and other stakeholders.

9.2 Flooding 9.2.1 It is proposed to reduce flood risk and its adverse effects on people and property in Bromley Town Centre by:

a) Appropriate comprehensive flood risk management measures within or affecting the Town Centre in consultation with the Environment Agency.

b) Reducing the risk of flooding from surface water and its contribution to fluvial flooding by requiring all developments of one or more dwellings, and all other development over 500m2 of floor space in the Town Centre, to have appropriate drainage schemes. Floor levels for the buildings to be set at a minimum level of the 1 in 100 year flood level plus climate change allowance with an additional 300mm ‘freeboard’ added to that level.

9.3 River Ravensbourne 9.3.1 As part of the establishment of a sustainable community in the Bromley Town Centre, it is proposed to maintain and look for opportunities to enhance the setting of and increase space for the River Ravensbourne and its tributaries. The culverts downstream/to the north of the Bromley South railway line could be replaced for example, with larger capacity natural open channels and downstream storage at Queen’s Mead. In considering development proposals it will:

a) Ensure the protection of landscape features that contribute to the setting of the rivers.

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b) Seek to protect and enhance existing views of the rivers.

c) Pay special attention to the design of development located in riverside settings to ensure that it respects and makes a positive contribution to the setting of the rivers.

d) Ensure that the quality of the water environment is maintained.

e) Seek opportunities to improve public access to and alongside the rivers and ensure that existing public access is maintained.

9.4 Recreational use of the River Ravensbourne 9.4.1 Facilities which support the recreational use of the River Ravensbourne will be safeguarded and promoted by:

a) Refusing development which involves the loss of facilities unless it can be demonstrated they are no longer required,

b) Supporting the maintenance and provision of visitor facilities, including those for access to the water.

9.5 Drainage Design 9.5.1 Drainage strategies included within Site Specific FRAs for town centre developments will:

a) Follow the design requirements set out in Chapter 7 of the Level 2 SFRA for Bromley Town Centre.

b) Seek to incorporate SUDS techniques where possible.

c) Design drainage to cope appropriately with the peak runoff rates and volumes for the following design criteria:

• 1 to 2 year event to protect the watercourse from erosion if discharging into a watercourse,

• 1 in 30 year event to meet Sewers for Adoption (Thames W ater) requirements,

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• 1 in 100 year events to ensure the site does not increase flooding.

d) Ensure runoff from development sites, where possible, is limited to the Greenfield rates for the above design events, if this is not possible the London Plan Standards should be used as minimum requirements for site runoff rates.

e) Ensure drainage design is undertaken with an allowance for climate change in accordance with the increases in rainfall predicted and outlined in PPS25.

f) Ensure discharges to Source Protection Zones reflect EA guidelines; only roof water will be permitted to discharge to ground at sites on Inner SPZ’s.

g) Ensure pollution protection methods are used on sites identified within the Bromley Town Centre Level 2 SFRA to ensure there is no pathway for polluted water.

h) Ensure roof runoff does not discharge into the existing surface water pipe network system.

9.6 Development Layout 9.6.1 The layout of development will take into account the management of extreme flood flows, by showing any flow paths in extreme events (i.e. 1 in 1000 year event).

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Appendix A 1km grid point location 8hr 100 yr rainfall 541000 170000 83.6 540000 690000 83.8 541000 169000 82.4 540000 168000 84.4 average 83.55 Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 2.62 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 26200 sq m Green areas (not discharging into system) 10 2620.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 60 15720.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 30 7860.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 2358.0 cubic m Water storage volume = 1/3 of permeable pavement volume 786.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 1572.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 1886.4 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 2.358 ha Permitted discharge from non-green area 4.7 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley -see sheet 'FEH data' 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 2845.7 cubic m Volume of rainfall falling on non-green areas 2561.1 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 135.8 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 2425.3 cubic m Volume of storage that can be accomodated by porous paving 786.0 cubic m Volume of water for which can be accomodated in underground storage 1886.4 cubic m total volume of water attenuated on-site 2672.4 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) -247.1 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 0.36 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 3600 sq m Green areas (not discharging into system) 30 1080.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 50 1800.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 20 720.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 216.0 cubic m Water storage volume = 1/3 of permeable pavement volume 72.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 648.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 777.6 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 0.252 ha Permitted discharge from non-green area 0.5 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 391.0 cubic m Volume of rainfall falling on non-green areas 273.7 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 14.5 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 259.2 cubic m Volume of storage that can be accomodated by porous paving 72.0 cubic m Volume of water for which can be accomodated in underground storage 777.6 cubic m total volume of water attenuated on-site 849.6 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) -590.4 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 0.64 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 6400 sq m Green areas (not discharging into system) 10 640.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 70 4480.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 20 1280.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 384.0 cubic m Water storage volume = 1/3 of permeable pavement volume 128.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 384.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 460.8 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 0.576 ha Permitted discharge from non-green area 1.2 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 695.1 cubic m Volume of rainfall falling on non-green areas 625.6 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 33.2 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 592.4 cubic m Volume of storage that can be accomodated by porous paving 128.0 cubic m Volume of water for which can be accomodated in underground storage 460.8 cubic m total volume of water attenuated on-site 588.8 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) 3.6 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 1.95 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 19500 sq m Green areas (not discharging into system) 10 1950.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 70 13650.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 20 3900.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 1170.0 cubic m Water storage volume = 1/3 of permeable pavement volume 390.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 1170.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 1404.0 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 1.755 ha Permitted discharge from non-green area 3.5 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 2118.0 cubic m Volume of rainfall falling on non-green areas 1906.2 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 101.1 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 1805.1 cubic m Volume of storage that can be accomodated by porous paving 390.0 cubic m Volume of water for which can be accomodated in underground storage 1404.0 cubic m total volume of water attenuated on-site 1794.0 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) 11.1 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 0.85 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 8500 sq m Green areas (not discharging into system) 0 0.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 90 7650.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 10 850.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 255.0 cubic m Water storage volume = 1/3 of permeable pavement volume 85.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 0.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 0.0 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 0.85 ha Permitted discharge from non-green area 1.7 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 923.2 cubic m Volume of rainfall falling on non-green areas 923.2 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 49.0 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 874.3 cubic m Volume of storage that can be accomodated by porous paving 85.0 cubic m Volume of water for which can be accomodated in underground storage 0.0 cubic m total volume of water attenuated on-site 85.0 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) 789.3 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 6.11 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 61100 sq m Green areas (not discharging into system) 50 30550.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 30 18330.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 20 12220.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 3666.0 cubic m Water storage volume = 1/3 of permeable pavement volume 1222.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 18330.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 21996.0 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 3.055 ha Permitted discharge from non-green area 6.1 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 6636.4 cubic m Volume of rainfall falling on non-green areas 3318.2 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 176.0 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 3142.2 cubic m Volume of storage that can be accomodated by porous paving 1222.0 cubic m Volume of water for which can be accomodated in underground storage 21996.0 cubic m total volume of water attenuated on-site 23218.0 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) -20075.8 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 2.14 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 21400 sq m Green areas (not discharging into system) 10 2140.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 70 14980.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 20 4280.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 1284.0 cubic m Water storage volume = 1/3 of permeable pavement volume 428.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 1284.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 1540.8 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 1.926 ha Permitted discharge from non-green area 3.9 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 2324.4 cubic m Volume of rainfall falling on non-green areas 2091.9 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 110.9 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 1981.0 cubic m Volume of storage that can be accomodated by porous paving 428.0 cubic m Volume of water for which can be accomodated in underground storage 1540.8 cubic m total volume of water attenuated on-site 1968.8 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) 12.2 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 0.44 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 4400 sq m Green areas (not discharging into system) 10 440.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 70 3080.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 20 880.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 264.0 cubic m Water storage volume = 1/3 of permeable pavement volume 88.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 264.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 316.8 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 0.396 ha Permitted discharge from non-green area 0.8 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 477.9 cubic m Volume of rainfall falling on non-green areas 430.1 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 22.8 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 407.3 cubic m Volume of storage that can be accomodated by porous paving 88.0 cubic m Volume of water for which can be accomodated in underground storage 316.8 cubic m total volume of water attenuated on-site 404.8 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) 2.5 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 2.31 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 23100 sq m Green areas (not discharging into system) 10 2310.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 60 13860.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 30 6930.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 2079.0 cubic m Water storage volume = 1/3 of permeable pavement volume 693.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 1386.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 1663.2 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 2.079 ha Permitted discharge from non-green area 4.2 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 2509.0 cubic m Volume of rainfall falling on non-green areas 2258.1 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 119.8 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 2138.4 cubic m Volume of storage that can be accomodated by porous paving 693.0 cubic m Volume of water for which can be accomodated in underground storage 1663.2 cubic m total volume of water attenuated on-site 2356.2 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) -217.8 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 0.95 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 9500 sq m Green areas (not discharging into system) 10 950.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 60 5700.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 30 2850.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 855.0 cubic m Water storage volume = 1/3 of permeable pavement volume 285.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 570.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 684.0 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 0.855 ha Permitted discharge from non-green area 1.7 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 1031.8 cubic m Volume of rainfall falling on non-green areas 928.7 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 49.2 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 879.4 cubic m Volume of storage that can be accomodated by porous paving 285.0 cubic m Volume of water for which can be accomodated in underground storage 684.0 cubic m total volume of water attenuated on-site 969.0 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) -89.6 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 0.4 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 4000 sq m Green areas (not discharging into system) 10 400.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 70 2800.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 20 800.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 240.0 cubic m Water storage volume = 1/3 of permeable pavement volume 80.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 240.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 288.0 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 0.36 ha Permitted discharge from non-green area 0.7 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 434.5 cubic m Volume of rainfall falling on non-green areas 391.0 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 20.7 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 370.3 cubic m Volume of storage that can be accomodated by porous paving 80.0 cubic m Volume of water for which can be accomodated in underground storage 288.0 cubic m total volume of water attenuated on-site 368.0 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) 2.3 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 1.19 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 11900 sq m Green areas (not discharging into system) 90 10710.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 5 595.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 5 595.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 178.5 cubic m Water storage volume = 1/3 of permeable pavement volume 59.5 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 6426.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 7711.2 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 0.119 ha Permitted discharge from non-green area 0.2 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 1292.5 cubic m Volume of rainfall falling on non-green areas 129.3 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 6.9 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 122.4 cubic m Volume of storage that can be accomodated by porous paving 59.5 cubic m Volume of water for which can be accomodated in underground storage 7711.2 cubic m total volume of water attenuated on-site 7770.7 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) -7648.3 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 0.3 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 3000 sq m Green areas (not discharging into system) 30 900.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 0 0.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 70 2100.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 630.0 cubic m Water storage volume = 1/3 of permeable pavement volume 210.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 540.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 648.0 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 0.21 ha Permitted discharge from non-green area 0.4 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 325.8 cubic m Volume of rainfall falling on non-green areas 228.1 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 12.1 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 216.0 cubic m Volume of storage that can be accomodated by porous paving 210.0 cubic m Volume of water for which can be accomodated in underground storage 648.0 cubic m total volume of water attenuated on-site 858.0 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) -642.0 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls) Landuse areas Percentage Area as given in Bromley AAP Total Site area = Roof area + Green area + Permeable pavement area 100 1.34 Ha Total Site area = Roof area + Green area + Permeable pavement area 100 13400 sq m Green areas (not discharging into system) 10 1340.0 sq m rainfall falling on green areas infiltrates and does not contribute to runoff Area where landuse = buildings 70 9380.0 sq m total built area on site comprising of buildings Potential Permeable pavement area 20 2680.0 sq m total paving area on site

Permeable pavement characteristics Depth 0.3 m Permeable pavement volume 804.0 cubic m Water storage volume = 1/3 of permeable pavement volume 268.0 cubic m 1/3 of the permeable paving depth is void space - this is where the water is stored

Underground storage tanks characteristics Depth of tanks 1.2 m Area available for underground tanks 804.0 sq m assume 60% of green spaces can be underlain with storage tanks Water storage volume 964.8 cubic m

Outflow EA permitted discharge rate from non-green areas 2 l/s per ha Non-green area, in hectares (1ha = 10,000 sq m) 1.206 ha Permitted discharge from non-green area 2.4 l/s

Rainfall Storm duration 8 hours 100 year storm at 1km point 83.55 mm this is the average value from the 4 corners of the 1km grid square covering Bromley 100 year storm at 1km point in 2115 108.615 mm based on prediction of 30% increase due to climate change

Results (using climate change rainfall) Volume of rainfall falling on whole site 1455.4 cubic m Volume of rainfall falling on non-green areas 1309.9 cubic m non-greenn areas are areas where land use is paving or buildings Volume discharged into drainage pipes 69.5 cubic m based on the allowable discharge of 2l/s/ha Volume of rainfall not discharged, hence requiring storage 1240.4 cubic m Volume of storage that can be accomodated by porous paving 268.0 cubic m Volume of water for which can be accomodated in underground storage 964.8 cubic m total volume of water attenuated on-site 1232.8 cubic m attenuated in underground storage and permeable paving Storage still required (negative numbers indicate spare capacity) 7.6 cubic m if number is positive, this volume will need to be attenuated off-site (eg regional controls)

Appendix B Bromley Borough Council Level 2 SFRA for Town Centre AAP: Audit Trail Database

Ref. No Subject & Type of data Source Date Author Summary Description Used for Data Location CD entitled Bromley W:\Projects\WB\Bromley_SFRA\Cal “London Borough cs\GIS\Mapping\Bromley OS Data Borough of OS map tiles (1 in 10,000 and 1 in 50,000) amd Background mapping for Bromley 8 Jan 2008 Council Creating Flood Map Tiles And Bromley Borough Boundary. (Graham W:\Projects\WB\Bromley_SFRA\Cal Ordnance Survey Bull) cs\GIS\Tables\Boundaries Data 7/01/08” CD entitled Modelling Data for Bromley including shapefiles of Data Provided by Environment Creating Flood Map Tiles. Photos W:\Projects\WB\Bromley_SFRA\Da “Bromley SFRA Dec 2007 EA Floodzones 2 and 3, historic flooding outlines, Agency for use in Bromley SFRA used on report covers. ta\EA_Bromley_SFRA_Data Data” groundwater flooding, defences and photographs \\Craw-fs- Thames Water Flooding Data - Excel spreadsheet 01\water\Projects\WB\WBBRSF\Da Showing ‘postcode areas within which Email from Steve Thames summarising number of properties flooded from ta\Thames_Water\LB Bromley Thames Water Historical Records 4 Jan 2008 flood incidents have been recorded’ on Dummer Water different sources in last 10 years within 4 digit SFRA.xls and \\Craw-fs- the Flood Map Tiles postcode areas in Bromley 01\water\Projects\WB\WBBRSF\Ca

lcs\GIS\Tables\Postcodes Email with \\Craw-fs- NaFRA stats_Bromley SFRA 13 Feb Data included in Bromley Borough attachment from EA NaFRA stats on risk to property from flooding 01\water\Projects\WB\WBBRSF\Da word doc 2008 SFRA report Hayley Mizen ta\NaFRA_stats CD entitled \\Craw-fs-

“Thames CFMP 26 Nov Shapefiles of spatial pattern of environmental Creating flood map/SUDS map tiles 01\water\Projects\WB\WBBRSF\Ca Thames CFMP Environmental EA Environmental 2004 variables within Thames catchment and informing SFRA report lcs\GIS\Tables\Environmental_basel Baseline Data GIS shapefiles Baseline Data” ine_data and CD in WBBRSF project Meeting with Bromley drainage \\Craw-fs- Borough MapInfo TAB file containing info on the location, specialists Showing zones of localised flooding 01\water\Projects\WB\WBBRSF\Ca Localised flooding zones Council sources and other attribute information of localised working for on the Flood Map Tiles lcs\GIS\Tables\Floodzones\localised drainage flooding Bromley _flooding.TAB specialists Borough Council \\Craw-fs- Table 7.1-Town Centre Preferred e-mail from 2 April Table showing landuses within the AAP town centre Informing run-off and storage 01\water\Projects\WB\WBBRSF\Da Options- Indicative Development Edaw Joanna Chambers 2008 proposal sites calculations for the drainage strategy ta\Table Areas and Development 7_Edaw_site_landuse_data.doc Principles

Maps

This map is reproduced from Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationery Office © Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. London Borough of Bromley. Licence No 100017661 2009