SURFACE WATER MANAGEMENT PLAN

Transcription

1 SURFACE WATER MANAGEMENT PLAN DRAIN LONDON LONDON BOROUGH OF LEWISHAM

2 Document Information Document Information Title: Surface Water Management Plan for London Borough of Lewisham Owner: Bo Quatresols Version: 0.4 Status: Draft Project Number: WUDLSW File Name: SWMP Halcrow Report for Lewisham v0.4 REVISION HISTORY Summary of Changes Completed by Date of Issue Version Template Bruno Venturini 20/04/ Draft Bruno Venturini/Helen Winter 09/05/ AUTHOR Name Bruno Venturini Helen Winter Organisation and Role Halcrow Project Manager Halcrow GIS Package Manager APPROVALS Name Title Signature Date Imran Bukhari Bruno Venturini Elliot Gill Bob Quatresols DISTRIBUTION Project Director, Halcrow Project Manager, Halcrow Technical Director, Halcrow Borough Champion, London Borough of Lewisham Name Kevin Reid Michael Arthur Bob Quatresols Organisation and Role Principal Programme Manager, Greater London Authority Tier 1 Consultant, Senior Consultant, Capita Symonds Borough Champion, London Borough of Lewisham RELATED DOCUMENTS Doc Ref Document Title Author PFRA Preliminary Flood Risk Assessment for London Borough of Bexley Bruno Venturini/Helen Winter Date of Issue April 2011 Version Final Draft i

3 Executive Summary Executive Summary <Section to be completed when report finalised> ii

4 Glossary Glossary Term Definition AOD Aquifer AMP Asset Management Plan AStSWF Catchment Flood Management Plan CDA Critical Drainage Area CFMP CIRIA Civil Contingencies Act CLG Climate Change Culvert Defra DEM DG5 Register DTM EA Above ordnance datum A source of groundwater comprising water bearing rock, sand or gravel capable of yielding significant quantities of water. Asset Management Plan A plan for managing water and sewerage company (WaSC) infrastructure and other assets in order to deliver an agreed standard of service. Areas Susceptible to Surface Water Flooding A high-level planning strategy through which the Environment Agency works with their key decision makers within a river catchment to identify and agree policies to secure the long-term sustainable management of flood risk. Critical Drainage Area A discrete geographic area (usually a hydrological catchment) where multiple and interlinked sources of flood risk (surface water, groundwater, sewer, main river and/or tidal) cause flooding in one or more Local Flood Risk Zones during severe weather thereby affecting people, property or local infrastructure. Catchment Flood Management Plan Construction Industry Research and Information Association This Act delivers a single framework for civil protection in the UK. As part of the Act, Local Resilience Forums must put into place emergency plans for a range of circumstances including flooding. Government Department for Communities and Local Government Long term variations in global temperature and weather patterns caused by natural and human actions. A channel or pipe that carries water below the level of the ground. Department for Environment, Food and Rural Affairs Digital Elevation Model A water-company held register of properties which have experienced sewer flooding due to hydraulic overload, or properties which are 'at risk' of sewer flooding more frequently than once in 20 years. Digital Terrain Model Environment Agency iii

5 Glossary Term Definition Indicative Flood Risk Areas FMfSW Flood defence Flood Risk Area Flood Risk Regulations Floods and Water Management Act Fluvial Flooding FRR IDB IUD LB LDF LFRZ Local Flood Risk Zone Lead Local Flood Authority LiDAR LLFA Local Resilience Forum Areas determined by the Environment Agency as indicatively having a significant flood risk, based on guidance published by Defra and WAG and the use of certain national datasets. These indicative areas are intended to provide a starting point for the determination of Flood Risk Areas by LLFAs. Flood Map for Surface Water Infrastructure used to protect an area against floods as floodwalls and embankments; they are designed to a specific standard of protection (design standard). An area determined as having a significant risk of flooding in accordance with guidance published by Defra and WAG. Transposition of the EU Floods Directive into UK law. The EU Floods Directive is a piece of European Community (EC) legislation to specifically address flood risk by prescribing a common framework for its measurement and management. Part of the UK Government's response to Sir Michael Pitt's Report on the Summer 2007 floods, the aim of which is to clarify the legislative framework for managing surface water flood risk in England. Flooding resulting from water levels exceeding the bank level of a main river Flood Risk Regulations Internal Drainage Board Integrated Urban Drainage London Borough Local Development Framework Local Flood Risk Zone Local Flood Risk Zones are defined as discrete areas of flooding that do not exceed the national criteria for a Flood Risk Area but still affect houses, businesses or infrastructure. A LFRZ is defined as the actual spatial extent of predicted flooding in a single location Local Authority responsible for taking the lead on local flood risk management Light Detection and Ranging Lead Local Flood Authority A multi-agency forum, bringing together all the organisations that have a duty to cooperate under the Civil Contingencies Act, and those involved in responding to emergencies. They prepare emergency plans in a co-ordinated manner. iv

6 Glossary Term Definition LPA LRF Main River NRD Ordinary Watercourse Partner PFRA Pitt Review Pluvial Flooding PPS25 PA Policy Area Resilience Measures Resistance Measures Risk Risk Management Authority RMA Sewer flooding SFRA Local Planning Authority Local Resilience Forum A watercourse shown as such on the Main River Map, and for which the Environment Agency has responsibilities and powers National Receptor Dataset a collection of risk receptors produced by the Environment Agency All watercourses that are not designated Main River, and which are the responsibility of Local Authorities or, where they exist, IDBs A person or organisation with responsibility for the decision or actions that need to be taken. Preliminary Flood Risk Assessment Comprehensive independent review of the 2007 summer floods by Sir Michael Pitt, which provided recommendations to improve flood risk management in England. Flooding from water flowing over the surface of the ground; often occurs when the soil is saturated and natural drainage channels or artificial drainage systems have insufficient capacity to cope with additional flow. Planning and Policy Statement 25: Development and Flood Risk Policy Area One or more Critical Drainage Areas linked together to provide a planning policy tool for the end users. Primarily defined on a hydrological basis, but can also accommodate geological concerns where these significantly influence the implementation of SuDS Measures designed to reduce the impact of water that enters property and businesses; could include measures such as raising electrical appliances. Measures designed to keep flood water out of properties and businesses; could include flood guards for example. In flood risk management, risk is defined as a product of the probability or likelihood of a flood occurring, and the consequence of the flood. As defined by the Floods and Water Management Act Risk Management Authority Flooding caused by a blockage or overflowing in a sewer or urban drainage system. Strategic Flood Risk Assessment v

7 Glossary Term Definition Stakeholder SuDS Sustainable Drainage Systems Surface water SWMP TfL TWUL WaSC A person or organisation affected by the problem or solution, or interested in the problem or solution. They can be individuals or organisations, includes the public and communities. Sustainable Drainage Systems Methods of management practices and control structures that are designed to drain surface water in a more sustainable manner than some conventional techniques. Rainwater (including snow and other precipitation) which is on the surface of the ground (whether or not it is moving), and has not entered a watercourse, drainage system or public sewer. Surface Water Management Plan Transport for London Thames Water Utilities Ltd Water and Sewerage Company vi

8 Contents Contents Document Information... i Executive Summary... ii Glossary... iii 1.0 Introduction What is a Surface Water Management Plan? Background Objectives Study Area Flooding Interactions Linkages to other plans Existing Legislation Peer Review Phase 1 Preparation Partnership Data collection Data Review Asset Register Phase 2 Risk Assessment Intermediate Assessment Risk Overview Surface Water Ordinary Watercourse Flooding Groundwater Flooding Sewer Flooding Other influences Critical Drainage Areas Summary of Risk Phase 3 Options Objectives Measures Preferred Options Preferred Options Summary Option Prioritisation vii

9 Contents 5.0 Phase 4 Implementation and Review Action Plan Implementation Programme Review Timeframe and Responsibilities Ongoing Monitoring References Insert references using the Harvard system (author, date) Appendix A Data Review... A1 Appendix B Asset Register Recommendation... B1 Appendix C Risk Assessment: Technical Details... C1 Appendix D Maps... D1 Appendix E Option Assessment Details... E1 Appendix F Peer Review... F1 Appendix G Spatial Planner Information Pack... G1 Appendix H Resilience Forum and Emergency Planner Information Pack... H1 Appendix I Action Plan... I1 viii

10 1.0 Introduction 1.0 Introduction 1.1 What is a Surface Water Management Plan? A Surface Water Management Plan (SWMP) is a plan which outlines the preferred surface water management strategy in a given location. In this context surface water flooding describes flooding from sewers, drains, groundwater, and runoff from land, small water courses and ditches that occurs as a result of heavy rainfall. This SWMP study has been undertaken as part of the Drain London Project in consultation with key local partners who are responsible for surface water management and drainage in the London area including Thames Water, the Environment Agency and Transport for London. The Partners have worked together to understand the causes and effects of surface water flooding and agree the most cost effective way of managing surface water flood risk for the long term. This document also establishes a long-term action plan to manage surface water and will influence future capital investment, maintenance, public engagement and understanding, land-use planning, emergency planning and future developments. 1.2 Background In May 2007 the Mayor of London consulted on a draft Regional Flood Risk Appraisal (RFRA). One of the key conclusions was that the threat of surface water flooding in London was poorly understood. This was primarily because there were relatively few records of surface water flooding and those that did exist were neither comprehensive nor consistent. Furthermore the responsibility for managing flood risk is split between local planning authorities and other organisations such as Transport for London, London Underground, Network Rail and relationships with the Environment Agency and Thames Water and other sources of flood risk were unclear. To give the issue even greater urgency it is widely expected that heavy storms will increase in frequency with climate change. The Greater London Authority, London Councils, Environment Agency and Thames Water commissioned a scoping study to test these findings and found that this was an accurate reflection of the situation. The conclusions were brought into sharp focus later in the summer of 2007 when heavy rainfall resulted in extensive surface water flooding in parts of the UK such as Gloucestershire, Sheffield and Hull causing considerable damage and disruption. It was clear that a similar rainfall event in London would have resulted in major disruption. The Pitt Review examined the flooding of 2007 and made a range of recommendations for future flood management, most of these have been enacted through the Flood and Water Management Act 2010 (FWMA). DEFRA recognized the importance of addressing surface water flooding in London and fully funded the Drain London project to produce Surface Water Management Plans V0.4 13/06/ of 28

11 1.0 Introduction (SWMPs) for each London Borough. Through the subsequent enactment of the FWMA boroughs are also required to produce Preliminary Flood Risk Assessments (PFRA). The Drain London project has been adjusted to deliver both a PFRA and an SWMP for each London Borough. This will be a major step in meeting borough requirements as set out in the FWMA. Another key aspect of the Act is to ensure that boroughs work in partnership with other Local Risk Authorities. Drain London assists this by creating subregional partnerships as set out in Figure 1.1. Figure 1.1 Drain London Sub-regional Partnerships Objectives The objectives of the SWMP are to: Develop a robust understanding of surface water flood risk in and around the London borough of Lewisham, taking into account the challenges of climate change, population and demographic change and increasing urbanisation in London; Identify, define and prioritise Critical Drainage Areas, including further definition of existing local flood risk zones and mapping new areas of potential flood risk (see definitions in Section 3.8). Make holistic and multifunctional recommendations for surface water management which improve emergency and land use planning, and enable better flood risk and drainage infrastructure investments; V0.4 13/06/ of 28

12 1.0 Introduction Establish and consolidate partnerships between key drainage stakeholders to facilitate a collaborative culture of data, skills, resource and learning sharing and exchange, and closer coordination to utilise cross boundary working opportunities; Undertake engagement with stakeholders to raise awareness of surface water flooding, identify flood risks and assets, and agree mitigation measures and actions; Deliver outputs to enable a real change on the ground rather than just reports and models, whereby partners and stakeholders take ownership of their flood risk and commit to delivery and maintenance of the recommended measures and actions; Meet the London Borough of Lewisham s specific objectives as recorded during the development of the SWMP (see further details below); Facilitate discussions and report implications relating to wider issues falling outside the remit of this Tier 2 work, but deemed important by partners and stakeholders for effectively fulfilling their responsibilities and delivering future aspects of flood risk management. Specific aims and objectives were discussed at the various meetings held throughout the development of the SWMP. These are summarised below: Ensure where possible the SWMP is consistent with the Strategic Flood Risk Assessment (SFRA) undertaken by the London Borough of Lewisham. Develop an action plan that is specific to the London borough (in particular due to its major regeneration objectives) and which builds on Group 6 stakeholder workshops. Ensure the SWMP action plan promotes the integration and ownership of the relevant departments within the London borough (those departments that deal with highways, parks, development control, emergency planning, etc). Investigate how the SUDS Approval Authority role (as required by the FWMA next year, see Section 1.7) could be led at Group 6 level. 1.4 Study Area The study area covers the administrative boundary of the London Borough of Lewisham in South East London (see Figure 1.1). It however needs to take account of interactions with adjacent local planning authorities, as pipe network systems, catchment contributions and flood risk (see for example locations of past floods highlighted in green in Figure 1.2) are often not constrained by administrative boundaries. V0.4 13/06/ of 28

13 1.0 Introduction Figure Study Area and Locations of Past Floods The LiDAR Topographic Survey Map (Figure 1.4.1) shows that the general terrain gradients fall towards the River Thames in a south-north direction, where ground levels vary from around 111m AOD at the highest point, to 0 m AOD in the north of the Borough. The predominant land uses within the London Borough of Lewisham are shown in the Land Use Areas Map (Figure 1.4.2). Most of the Borough is developed ranging from sub-urban to heavily urbanised areas (for example Lewisham Town Centre). Significant infrastructure in the borough include: A water treatment works (Thames Water) Blackheath Hospital and the University Hospital Main line railway stations V0.4 13/06/ of 28

14 1.0 Introduction The Lewisham Core Strategy, which will be adopted in June 2011, identifies opportunity areas for Deptford Creek/Riverside (with LB Greenwich) and Lewisham/Catford/New Cross as a focus for new housing and jobs. Lewisham and Catford town centres are designated as major centres. Significant areas of the London Borough of Lewisham are identified as Metropolitan Open Land; notably Blackheath and Beckenham Place Park. 1.5 Flooding Interactions There are separate gravity surface water and foul drainage systems to the south of the borough however it is combined towards the River Thames. The main flooding interaction between the separate surface water drainage system and the fluvial network system occurs when water levels in the main river system are high enough to stop surface water discharging into them. This causes surface water to back up. Studies of major fluvial floods in the last fifty years in the borough indicate that the surface water component was significant. The main interaction of the combined sewer system and overland flows occurs at critical locations where the sewers overflow (out of gullies or blown out manhole covers) as a result of the storm event. 1.6 Linkages to other plans The increased focus on flood risk over recent years is an important element of adaptation to climate change. The clarification of the role of London boroughs as Lead Local Flood Authorities (LLFA) is welcomed. The creation of a number of new documents can at times be confusing. Drain London links into all of these: Regional Flood Risk Appraisal (RFRA) This is produced by the Greater London Authority and gives a regional overview of flooding from all sources. The RFRA will be updated in 2012 to reflect the additional information on local sources of flood risk (surface water, groundwater and ordinary watercourses) from Drain London. This may also generate new policies that would be incorporated into the London Plan when it is reviewed. Thames Catchment Flood Management Plan (CFMP) The Thames Catchment Flood Management Plan was published in 2008 by the Environment Agency and sets out policies for the sustainable management of flood risk across the whole of the Thames catchment over the long-term (50 to 100 years) taking climate change into account. More detailed flood risk management strategies for individual rivers or sections of river may sit under these. The Plan emphasises the role of the floodplain as an important asset for the management of flood risk, the crucial opportunities provided by new development and regeneration to V0.4 13/06/ of 28

15 1.0 Introduction manage risk, and the need to re-create river corridors so that rivers can flow and flood more naturally. This Plan will be periodically reviewed, approximately five years from when it was published, to ensure that it continues to reflect any changes in the catchment. There are links to Drain London where there are known interactions between surface water and fluvial flooding. Preliminary Flood Risk Assessment (PFRA) These are required as part of the Flood Risk Regulations (see details in Section 1.7), which implement the requirements of the European Floods Directive. Drain London is producing one of these for each London Borough (LLFA), to give an overview of all local sources of flood risk. In London PFRAs will benefit from an increased level of information relating to surface water from the Drain London SWMPs. Boroughs will need to review these PFRAs every 6 years. Surface Water Management Plans (SWMP) Drain London is producing one of these for each London Borough (this document). They provide much improved probabilistic 2-dimensional modelling and data on what has been made available at a national scale by the Environment Agency. In addition they contain an Action Plan that has been developed in conjunction with both the London borough of Lewisham and other risk management authorities. This data and actions and associated policy interventions will need to feed directly into the operational level of the London Borough across many departments, in particular into spatial and emergency planning policies and designations and into the management of local authority controlled land. Strategic Flood Risk Assessments (SFRA) Each local planning authority is required to produce a SFRA under Planning Policy Statement 25 (PPS25). This provides an important tool to guide planning policies and land use decisions. Current SFRAs have a strong emphasis on flooding from main rivers and the sea and are relatively weak in evaluating flooding from other local sources including surface water, groundwater and ordinary watercourses. The information from Drain London will assist in improving this understanding. Local Development Documents (LDD) LDDs including the Core Strategy (to be adopted June 2011) and relevant Area Action Plans (AAPs) will need to reflect the results from Drain London. This may include policies for the whole borough or for specific parts, for example Critical Drainage Areas. There may be a particular need to review Area Action Plans where surface water flood risk is a specific issue. A future SFRA update will assist with this as will the reviewed RFRA and any updated London Plan policies. In producing Opportunity Area Planning Frameworks, the GLA and boroughs will also examine surface water flood risk more closely. V0.4 13/06/ of 28

16 1.0 Introduction Local Flood Risk Management Strategies The Flood and Water Management Act 2010 (FWMA) requires each LLFA to produce one of these by December Whilst Drain London will not actually produce these, the SWMPs, PFRAs and their associated risk maps will provide the necessary evidence base to support the development of LFRMS. No new modelling is anticipated to produce these strategies. The schematic diagram in Figure 1.3 below illustrates how the CFMP, PFRA, SWMP and SFRA link to and underpin the development of a Local Flood Risk Management Strategy. Figure The inter-relationship between the current studies and the future Flood Risk Management Strategy LFRM Strategies CFMP PFRA SWMP SFRA Strategic Environmental Assessment The Strategic Environmental Assessment (SEA) Directive (2001/42/EC) is implemented in the UK by The Environmental Assessment of Plans and Programmes Regulations 2004 (Statutory Instrument No.1633). Its objective is to provide for a high level of protection of the environment and to contribute to the integration of environmental considerations into the preparation and adoption of plans and programmes with a view to promoting sustainable development. The flood risk management plans required under the Flood Risk Regulations 2009 fall under the scope of the SEA Directive. 1.7 Existing Legislation The Flood and Water Management Act 2010 (FWMA) presents a number of challenges for policy makers and the flood and coastal risk management authorities identified to coordinate and deliver local flood risk management (surface water, groundwater and flooding from ordinary water courses). Upper Tier local authorities (Lead Local Flood Authorities LLFA) have been empowered to manage local flood risk through new responsibilities for flooding from surface and groundwater. V0.4 13/06/ of 28

17 1.0 Introduction The FWMA reinforces the need to manage flooding holistically and in a sustainable manner. This has grown from the key principles within Making Space for Water (Defra, 2005) and was further reinforced by the summer 2007 floods and the Pitt Review (Cabinet Office, 2008). It implements several key recommendations of Sir Michael Pitt s Review of the Summer 2007 floods, whilst also protecting water supplies to consumers and protecting community groups from excessive charges for surface water drainage. The FWMA must also be considered in the context of the EU Floods Directive, which was transposed into law by the Flood Risk Regulations 2009 (the Regulations) on 10 December The Regulations requires three main types of assessment / plan: 1) Preliminary Flood Risk Assessments (maps and reports for Sea, Main River and Reservoirs flooding) to be completed by Lead Local Flood Authorities and the Environment Agency by the 22 December Flood Risk Areas, at potentially significant risk of flooding, will also be identified. Maps and management plans will be developed on the basis of these flood risk areas. 2) Flood Hazard Maps and Flood Risk Maps. The Environment Agency and Lead Local Flood Authorities are required to produce Hazard and Risk maps for Sea, Main River and Reservoir flooding as well as other relevant sources by 22 December ) Flood Risk Management Plans. The Environment Agency and Lead Local Flood Authorities are required to produce Flood Risk Management Plans for Sea, Main River and Reservoir flooding as well as other relevant sources by 22 December The diagram in Figure 1.4 illustrates how this SWMP fits into the delivery of local flood and coastal risk management, and where the responsibilities for this lie. 1.8 Peer Review It is essential for the Drain London Project that SWMPs are consistent and comparable across Greater London. This is to facilitate Fair, transparent and rapid allocation of funds to identified high priority flood risk areas within London Collaborative working practices between stakeholders Building of local capability (Council officers and consultants doing work in the future will be able to make use of outputs regardless of who produced them for each Borough) To ensure consistency and comparability between London Borough SWMPs produced, a Peer Review process has been used. The process involved the four consultant teams V0.4 13/06/ of 28

18 1.0 Introduction working on the Drain London SWMPs independently reviewing each others work. This has ensured that all outputs result from a consistent technical approach, are of a high technical quality and are communicated in the specified formats. The peer review report for this SWMP is included in Appendix F. Figure Delivery of local FCERM Overview Defra Flood and Coastal Erosion Risk Policy Environment Agency (National Strategy) Produce a National Strategy for FCERM as part of full strategic overview role for all FCERM (Main river, ordinary watercourse, sea water, surface run-off, groundwater, coastal erosion and flood risk from reservoirs). Support lead local authorities and others in FCERM by providing information and guidance on fulfilling their roles. Planning PFRAs SWMPs CFMPs SMPs Lead Local Flood Authorities Local Strategies surface water, groundwater, ordinary watercourses Delivery LLFAs - surface water and groundwater EA Main River and the Sea Water companies, reservoir owners, highways authorities Third Party assets V0.4 13/06/ of 28

19 2.0 Phase 1 - Preparation 2.0 Phase 1 Preparation 2.1 Partnership As Lead Local Flood Authority, it is the role of the London borough to forge effective partnerships with the adjacent LLFA and the Environment Agency (this is currently the case with the Drain London project) as well as other key stakeholders Thames Water, Network Rail, Transport for London and the Highways Agency. Some progress has been made toward establishing these partnerships already, although Network Rail and the Highways Agency have not yet fully engaged with the process. Ideally working arrangements should now be formalised by the LLFA to ensure clear lines of communication, mutual co-operation and management through the provision of Level of Service Agreements (LoSA) or Memorandums of Understanding (MoU). Figure 2.1 provides a schematic of the recommended partnership and stakeholder arrangements: Figure Partnership and Main Stakeholder Schematic Diagram Partners Key Stakeholders 2.2 Data collection The collection and collation of strategic level data was undertaken as part of the Tier 1 work and disseminated to Tier 2 consultants by the GLA. Data was collected from each of the following organisations: London Borough of Lewisham British Airports Authority V0.4 13/06/ of 28

20 2.0 Phase 1 - Preparation British Geological Survey British Waterways Environment Agency Greater London Authority Highways Agency London Underground Network Rail Thames Water Transport for London A comprehensive data set was passed onto Tier 2 consultants and in some cases additional supplementary data was provided by individual organisations. The key information that was obtained is listed in Table 2.1 below (a full list of the information is included in the Data Gap and Licensing Report issued by Tier 1 Consultants in October 2010): V0.4 13/06/ of 28

21 2.0 Phase 1 - Preparation Table 2.1 Summary of Key Drain London Tier 1 Data Source Environment Agency London Borough of Lewisham Thames Water Other (Highways Agency, Transport for London, Network Rail, Local flood groups, fire brigade, etc) Data/Studies Environment Agency Asset Data; Water Studies (including Thames Catchment Flood Management Plan, Thames Catchment Abstraction Management Strategy and Thames River Basin management Plan; Historic flood data (GIS flood event outlines extracted from NFCDD); Geostore data including Main River details, flood data for areas vulnerable to surface water flooding and Digital River Network (DRN) data for London; Numerous fluvial and surface water models located in the Greater London area; London hydrometric data including groundwater level data, rainfall data and river flow data; and Details of Flood Warning Areas in London SFRA report; Flooding records; GIS data for Critical Infrastructure; Gulley locations. Foul water and surface water sewer network models in GIS format; Sewer Flooding Incident Records by postcode; Pumping station and manhole locations. Various assets; Flood records; GIS layers for land use types; BGS Susceptibility to Groundwater Flooding Additional information has been obtained from the London Borough of Lewisham through an initial site visit followed by a more detailed virtual site visit/workshop of areas at risk of flooding. The virtual site visits/workshops have proved to be a highly valuable process which involved virtual walks by technical staff from the Environment Agency, Borough Council and Halcrow identifying many local flood risk areas, using a GIS environment and the use of Google Street View for 3D images. V0.4 13/06/ of 28

22 2.0 Phase 1 - Preparation 2.3 Data Review The key GIS datasets used for the main stages of the SWMP and the virtual site visits are: a) OS maps, b) the Thames Water pipe network system, c) the river networks, d) the flood zones and the historic flood map from the Environment Agency, e) flood incident records, f) local flood risk data from strategic data providers (for example the fire brigade), g) the Environment Agency national Flood Map for Surface Water (FMfSW), g) the Drain London surface water hazard and flood depth maps produced by Halcrow for the London Borough of Lewisham, h) a digital terrain model from LiDAR data to identify catchment boundaries and terrain gradients, i) Thames Water postcode records of flooding, j) the National Receptor Database and k) the potential for elevated groundwater maps. Appendix A provides further details of this data and their quality score (see further details in the Appendix). 2.4 Asset Register Section 21 of the FWMA 2010 sets a duty on each London Borough (LLFA) to maintain a register of structures or features, and a record of information about each of those structures or features, which, in the opinion of the authority, are likely to have a significant effect on flood risk in its area. From 6th of April 2011 all LLFAs have a duty to maintain a register. The legal characteristics of the register and record are outlined below: V0.4 13/06/ of 28

23 2.0 Phase 1 - Preparation Table 2.2 Main characteristics of the Asset Register Register Record (includes details of ownership and condition) a. Must be made available for inspection at all reasonable times. Up to the LLFA to decide if they wish to make it available for inspection b. Must contain a list of structures or features which in the opinion of the authority, are likely to have a significant effect on a local flood risk. For each structure or feature listed on the register, the record must contain information about its ownership and state of repair. c. s.21 (2) of the Act allows for further regulations to be made about the content of the register and record. There is currently no plan to provide such regulations therefore their content should be decided on by the LLFA depending on what information will be useful to them. d. There is no legal requirement to have a separate register and record although as indicated above, only the register needs to be made available for public inspection. Defra have provided each LLFA with templates to demonstrate what information should be contained in the asset register. Although these templates are not intended as a working tool, they provide a good example of how an asset register might be structured. Populating the asset register is outside the scope of the Drain London project and is the responsibility of each London Borough. The expectation from Defra is that LLFAs (London Boroughs) will utilise a risk-based approach to populate the register and record which of those structures or features are considered the most significant first. The London borough will be populating the asset register over this financial year focusing on the most significant assets to begin with. V0.4 13/06/ of 28

24 3.0 Phase 2 Risk Assessment 3.0 Phase 2 Risk Assessment 3.1 Intermediate Assessment The aim of the Phase 2 Intermediate Risk Assessment is to identify the sources and mechanisms of surface water flooding across the study area which are achieved through an intermediate assessment of pluvial flooding, sewer flooding, groundwater flooding and flooding from ordinary watercourses along with the interactions with main rivers and the sea. The modelling outputs are then mapped using GIS software. SWMPs can function at different geographical scales and therefore necessarily at differing scales of detail. Table 3.1 defines the potential levels of assessment within a SWMP. This SWMP has been prepared at a Borough scale, fulfilling the objectives of a second level Intermediate Assessment. Table 3.1: SWMP Study Levels of Assessment [Defra 2010] Level of Assessment 1. Strategic Assessment 2. Intermediate Assessment 3. Detailed Assessment Appropriate Scale Greater London Borough wide Known flooding hotspots Outputs Broad understanding of locations that are more vulnerable to surface water flooding. Prioritised list for further assessment. Outline maps to inform spatial and emergency planning. Identify flood hotspots which might require further analysis through detailed assessment. Identify immediate mitigation measures which can be implemented. Inform spatial and emergency planning. Detailed assessment of cause and consequences of flooding. Use to understand the mechanisms and test mitigation measures, through modelling of surface and sub-surface drainage systems. As shown in Table3.1 above, the intermediate assessment is applicable across a large town, city or borough. In the light of extensive and severe historical flooding and the results from the over-arching national pluvial modelling suggesting that there are 21,500 properties at risk across the London Borough of Lewisham, it has been considered appropriate to adopt this level of assessment to further quantify the risks. The purpose of this intermediate assessment is to further identify those parts of the borough that are likely to be at greater risk of surface water flooding and require more V0.4 13/06/ of 28

25 3.0 Phase 2 Risk Assessment detailed assessment. The methodology used for this SWMP is summarised below. Further detail of the methodology is provided in Appendix C. A Direct Rainfall approach using InfoWorks CS / TuFLOW software has been selected whereby rainfall events of known probability are applied directly to the ground surface and is routed overland to provide an indication of potential flow path directions and velocities and areas where surface water will pond. The outputs of the 2-dimensional pluvial modelling were reviewed at a large scale via the virtual site visits undertaken with key London Borough of Lewisham staff and the Environment Agency. In many cases the outputs from the pluvial modelling have been verified against historic surface water flood records. 3.2 Risk Overview Surface water flood risk The 1 in 100 year surface water Map (Figures 3.2.1a and b) is suitable to identify broad areas which are more likely to be vulnerable to surface water flooding. This allows the London Borough of Lewisham and its partners to undertake more detailed analysis in areas which are most vulnerable to surface water flooding. In addition, the map can also be used as evidence base to support spatial planning to ensure that surface water flooding is appropriately considered when allocating land for housing development. It can also be used to assist emergency planners in preparing or updating their Multi-Agency response plans. The surface water maps from this study show the predicted likelihood of surface water flooding for defined areas. They focus on overland flow paths and surface water flooding at local depressions however they also simulate (less accurately) flooding from sewers, drains, small watercourses and ditches). Due to the coarse nature of the source data used, these are not detailed enough to account for precise addresses. Individual properties therefore may not always face the same chance of flooding as the areas that surround them. There may also be particular occasions when flooding occurs and the observed pattern of flooding does not in reality match the predicted patterns shown on these maps. We have done all we can to ensure that the maps reflect all the data we possess and have applied our expert knowledge to create conclusions that are as reliable as possible. It is essential that anyone using these maps fully understands the complexity of the data utilised in production of the maps, is aware of the limitations (see Section 3.3 and Appendix C) and does not use the maps in isolation. V0.4 13/06/ of 28

26 3.0 Phase 2 Risk Assessment Halcrow Group will not be liable if the maps by their nature are not as accurate as might be desired or are misused or misunderstood despite the warnings. For this reason it is not possible to promise that the maps will always be completely accurate or up to date. Fluvial/Tidal flood risk The Environment Agency flood zones give a good representation of flooding from fluvial and/or tidal flood risk assuming that defences are not in place (see Figure 3.2.2). The undefended maps represent a conservative assumption that defences could deteriorate and therefore it may not always be possible to fully rely on them. Defended maps assuming that the defences are in place and perform as expected can be obtained from the Environment Agency (from flood mapping studies which in most cases involve 1d and 2d river modelling). The Ravensbourne catchment and its tributaries have been fully modelled by the Environment Agency and defended maps are available. Groundwater flood risk As part of the Drain London Project, Potential for Elevated Groundwater maps have been developed to identify where groundwater could be at, or near ground surface (see Figure and further details in Section 3.5). 3.3 Surface Water As mentioned in Section xxx, the borough has a separate sewer system to the north (drained by gravity) and a combined pumped sewer system to the north towards the River Thames. In the following sections, the term underground drainage system refers to both of these systems (separated or combined). V0.4 13/06/ of 28

27 3.0 Phase 2 Risk Assessment In urban developed areas of the borough surface runoff occurs when: a) intense rainfall in unable to soak sufficiently into the ground and b) the capacity of the underground drainage system is overwhelmed by the intensity of the storm (the capacity of the underground drainage system is limited as it can only cater for relatively small storms of the order of 1 in 10 year return period). In these conditions surface water builds up and locally floods if the ground terrain is flat or travels according to prevailing terrain gradients. Surface water flooding then occurs at locations where surface water flow paths converge, at local depressions/valleys in the ground or due to overland obstructions. This is the main mechanism of surface water flooding however it is exacerbated in many cases by partial or full blockages of gullies and/or within the underground drainage system (for example due to pipe collapse or siltation). The local situation for the London borough of Lewisham implies that for short duration and medium size storms (of the order of 30mm rainfall depth), only shallow flooding would occur. Deep flooding and widespread damages to property and infrastructure are however anticipated for short duration and large size storms (of the order of mm) as demonstrated by the Drain London surface water mapping. As the Environment Agency does with fluvial and tidal flooding, LLFAs also need also to be concerned with these large size storms (events similar to the Hull and Gloucester storms in 2007). The surface water modelling undertaken as part of the Drain London project aims at simulating the above overland flood mechanism by allowing for: a) the limited capacity of the combined drainage system (with no blockages) and b) ground infiltration. The outputs of the modelling provide an understanding of the main overland flow routes and how and where surface water flooding could occur. The modelling undertaken has however limitations which should be taken into account when interpreting potential surface water flooding (see further modelling details in Appendix C). The main limitations are described below: a) The combined sewers have not been modelled and therefore any variation in network capacity has not been taken into account (instead some of the rainfall has been removed at a constant rate of 6.5mm/hour everywhere). b) The modelled topography of the ground is based on a grid of points at a 5m distance between them and therefore any variations within these have not been modelled. c) Obstructions such as railway embankments have been modelled however culvert crossings beneath them (unless clearly seen on OS maps) have not been modelled. V0.4 13/06/ of 28

28 3.0 Phase 2 Risk Assessment d) the permeability of the ground has been modelled to a certain extent however only by allowing a limited number of soil categories. e) the capacity of watercourses has not been modelled and therefore there is a tendency of building up of surface water along the river floodplain. The LLFA is responsible for the management of surface water flooding and flooding from highway drainage. Flooding of highway drainage occurs due to the limited capacity of the road drains and can be exacerbated by gully blockages. Thames Water is responsible for flooding from internal blockages in the surface water drainage system, due to pipe collapse or siltation. The reason for this is that Thames Water is responsible for the maintenance of the underground drainage system. 3.4 Ordinary Watercourse Flooding None of the flooding from ordinary watercourses was considered sufficiently important to be included as past events in Table 4.1 of the PFRA. The reason behind this is that those ordinary watercourses that had significant flooding issues (named as critical ordinary watercourses at the time) have now become main rivers. It will be important nevertheless that the London borough identifies and digitises these assets in parallel to the task of populating the risk asset registers. The LLFA is responsible for ordinary watercourses. 3.5 Groundwater Flooding Groundwater has been a contributing factor to a number of Lewisham s past flooding. Namely, flooding events in Lewisham Town Centre, Catford, Deptford and Hither Green. These areas all lie within the river corridors of the Ravensbourne and Quaggy and as such are subject to high water tables. Groundwater may become elevated by a number of means: a) above average rainfall for a number of months in Chalk outcrop areas; b) shorter period of above average rainfall in permeable superficial deposits, c) permeable superficial deposits in hydraulic continuity with high water levels in the river, d) Interruption of groundwater flow paths; and e) cessation of groundwater abstraction causing groundwater rebound. As part of the Drain London project, modelling has been undertaken and has produced the Increased Potential for Elevated Groundwater Maps. Figure shows those areas within the London Borough of Lewisham where there is an increased potential for groundwater to rise sufficiently to interact with the ground surface or be within 2 m of the ground surface. The map shows those areas considered to have the greatest potential for elevated groundwater. It indicates that elevated groundwater from permeable superficial soils are V0.4 13/06/ of 28

29 3.0 Phase 2 Risk Assessment located in along the centre of the Borough, surrounding the flood plain of the River Ravensbourne. It also shows elevated ground water from consolidated aquifers in the north western corner of the borough around Deptford. Groundwater flooding is responsibility of the LLFA. 3.6 Sewer Flooding The SFRA indicates that there are no records of significant flooding from this source. Figure D5 in Appendix D shows the Thames Water sewer flooding records over the past decade for each postcode. Sewer flooding has different flood mechanisms in separate and combined drainage systems, as described below. Areas with separate drainage systems At locations where foul water is separate from the surface drainage system, flooding from this source is unlikely unless there is sewer blockage or due to surface water entering the foul sewers which then results in overflowing the system. Thames Water is responsible for foul flooding, due to internal blockages or surface water unexpectedly entering the sewers and consequent overflowing of the system. Thames water is responsible for the maintenance of the foul sewer system. Areas with combined sewers Sewer flooding from the combined sewers can occur even for relatively small return period events (of the order of 1 in 2 years return period) at those locations where sewers are relatively high when compared to their toilet connections at basements. This condition means that even partially full sewers could create backflow into toilets and flood basements. To stop this, Thames Water is investing heavily in providing FLIP devices (non-return valves and pumps) at critical locations throughout London. Overflows from the combined sewers through gullies and manholes (where their covers are blown due to the internal water pressure) into roads, footpaths, etc, can also occur during large storm events. This type of sewer flooding is partially alleviated via overflow pipes that currently take some of the excess flows from the combined system directly into the River Thames. The responsibilities for managing the above types of flood mechanisms as a result of the Act are summarised in Table 3.2 below: V0.4 13/06/ of 28

30 3.0 Phase 2 Risk Assessment Table 3.2 Responsibility for Sewer Overflows Type of flooding Overflows from combined sewer as a result of a rainfall event Overflows from combined sewer as a result of blockage within the combined sewers Responsibility LLFA * Thames Water * The PFRA guidance confirms that if flooding of a sewer is originated from a storm event then it falls within LLFA responsibility. 3.7 Other influences High water levels in watercourses can result in the backing up of surface water outfalls and potentially result in localised surface water flooding, until river levels recede. The above mechanism of flooding can be exacerbated by higher river levels as a result of partial blockages due to poor watercourse maintenance. Whilst the Environment Agency and the LLFA are responsible for the management of main rivers and ordinary watercourses respectively, the responsibility for maintaining these watercourses lies with the riparian owners. It will be therefore important to identify the riparian owners for critical assets those assets which could cause significant flood damages as a result of their failure/blockage during a flood event. The London Borough of Lewisham could be riparian owner of some of these critical assets (culverts, channels, structures, etc) in main rivers and/or ordinary watercourses. Flooding of basements can be originated from the following flood mechanisms (assuming there is an external low level door entrance to a basement): a) direct rainfall accumulating by the basement door entrance and air bricks, b) surface runoff which ends overtopping kerbs and/or steps (if any) into a basement entrance, c) overflows from the combined sewer system as a result of a rainfall event, which end overtopping kerbs and/or steps (if any) into a basement entrance, d) the combined sewer surcharges and backflows through its connection into a basement toilet, e) infiltration of water originated from a broken sewer affecting basement walls, a basement entrance and potentially internal flooding, f) infiltration of water originated from rainfall ending wetting the basement walls and potentially internal flooding, g) a high water table. V0.4 13/06/ of 28