East Hampshire District Council Strategic Flood Risk Assessment for Local Development Framework
Recommended national precautionary sensitivity ranges for peak rainfall intensities, peak river flows, offshore wind speeds and wave heights (from Table B.2 PPS25) Parameter 1990 2025 2055 2085 to to to to 2025 2055 2085 2115 Peak rainfall intensity +5% +10% +20% +30% Peak river flow +10% +20% Offshore wind speed +5% +10% Extreme wave height +5% +10%
East Hampshire District Council Strategic Flood Risk Assessment for Local Development Framework
Final April 2008
Halcrow Group Limited
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Halcrow Group Limited has prepared this report in accordance with the instructions of their client, Hampshire County Council, 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
East Hampshire District Council Strategic Flood Risk Assessment for Local Development Framework
Contents Amendment Record This report has been issued and amended as follows:
Issue Revision Description Date Signed
1 1 Draft June 07 AS/BV
2 1 Draft July 07 AS/BV
3 1 Draft April 08 AS/BV
4 1 Final April 08 AS/BV
Contents
1 Executive Summary 1
2 Background Information 3 2.1 Terms of Reference 3 2.2 The Study Area 3 2.3 Constraints on Development 3
3 Planning Context 5 3.1 Introduction 5 3.2 The EU Water Framework Directive 5 3.3 National Planning Policy 6 3.4 Regional Planning Policy 7 3.5 Local Planning Policy 7
4 PPS25 Flood Zones, Environment Agency Flood Zones and SFRA Flood Maps 9 4.1 Introduction 9 4.2 The PPS25 Flood Zones 9 4.3 Environment Agency Flood Zones 10 4.4 SFRA Flood Maps 11
5 PPS25 and its Practice Guide Companion 12 5.1 PPS25 - Key Aims 12 5.2 Outcomes of the SFRA Process 13 5.3 SFRA Levels 1 and 2 13 5.4 The Sequential Test of PPS25 14 5.5 The Exception Test of PPS25 15 5.6 The Practice Guide Companion to PPS25 16
6 Study Methodology 18 6.1 Specific Project Outputs 18 6.2 Approach to Data Gathering 18 6.3 Forms of Flooding and Data Limitations 19 6.4 Production of Flood Maps 25
7 Flood Risk in the Study Area 29 7.1 Introduction 29
7.2 Geology and Hydrology 29 7.3 Historical Flooding 31 7.4 Planned Development Areas 35 7.5 Effect of Development on Flood Risk Elsewhere 38
8 Catchment Flood Management Plans, Flood Management Measures and Flood Warning Systems 40 8.1 Introduction 40 8.2 Catchment Flood Management Plans 40 8.3 Flood Warning Systems and Future Flood Risk Management Schemes 45 8.4 Defences and Future Flood Risk Management Schemes 46
9 Guidance on the Application of the Sequential Test 48 9.1 Introduction 48 9.2 First Step – Strategic Overview of Flood Risk for all Potential Areas 50 9.3 Second Step – Analysis of Flood Risk Issues 50 9.4 Third Step – Apply the PPS25 Flood Risk Management Measures 51 9.5 Fourth Step – Apply the Sequential Test in Flood Zone 2 51 9.6 Fifth Step – Apply the Sequential Test in Flood Zone 3 51
10 Guidance for the Preparation of Flood Risk Assessments 52 10.1 Introduction 52 10.2 Proposed Developments within Flood Zone 3a and 2 52 10.3 Proposed Development within Flood Zone 1 53 10.4 Proposed Development within Groundwater Emergence Zones 55 10.5 Risk Management by Design 55 10.6 Raised Floor Levels 55 10.7 The effect of development on floodwater flow conveyance and loss of floodplain storage 56 10.8 Basements 57 10.9 Safe Access and Egress 57
11 Guidance for the Application of Sustainable Drainage Systems 59 11.1 Introduction 59 11.2 Types of SUDS 59 11.3 Effective Application of SUDS Techniques 62
Appendices Appendix A – Details of the Environment Agency Flood Zones Appendix B – Audit Trail Database Appendix C – Groundwater Emergence maps from Groundwater Flooding Scoping Study LDS 23 (Jacobs, 2004) Appendix D – Rural Wey CFMP Policy Unit Appendix E – Rainfall Runoff Management for Developments – Interim National Procedure
Tables Table 1 – Overview of the flood risk management hierarchy Table 2 – Historical Flooding - Southern Water Table 3 – Historical Flooding - Thames Water Table 4 – Localised Flooding Areas Table 5 – Groundwater Flooding Incidents (Southern Region) Table 6 – Groundwater Flooding Incidents (Thames Region) Table 7 – Other Sources of Flooding Table 8 – Recommended National Precautionary Sensitivity Ranges Table 9 – Flood Risk Within Potential Development Areas Table 10 – Flood Defences
Figures Figure 1 – Study Area Figure 2 – Schematic of the PPS25 Flood Zones Figure 3 – Distribution of Soil Permeability Figure 4 – Application of the Sequential Test Figure 5 – SUDS Management Train Tiles A to E – Flood Maps: Zones 2, 3a, 3b and Localised Flooding Areas
Glossary of Terms
Annual Monitoring Report (AMR) - Assesses the implementation of the Local Development Scheme and the extent to which policies in Local Development Documents are being successfully implemented. Area action plans – Development Plan Documents that provide a planning framework for areas of change and areas of conservation. Areas of Outstanding Natural Beauty (AONB) - Were brought into being by the same legislation as National Parks - the National Parks and Access to the Countryside Act of 1949. They are fine landscapes, of great variety in character and extent. The criteria for designation is their outstanding natural beauty. Many AONBs also fulfil a recreational role but, unlike national parks, this is not a designation criterion. The Countryside Agency and the Countryside Council for Wales are responsible for designating AONBs and advising Government on policies for their protection. Catchment Flood Management Plan (CFMP) – 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. Core Strategy - The Development Plan Document which sets the long-term spatial planning vision and objectives for the area. It contains a set of strategic policies that are required to deliver the vision including the broad approach to development. 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. DEFRA - Department of Environment, Food and Rural Affairs Development. 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 Wales. 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 the 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 assesses 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.
Habitable Room - A room used as living accommodation within a dwelling but excludes bathrooms, toilets, halls, landings or rooms that are only capable of being used for storage. All other rooms, such as kitchens, living rooms, bedrooms, utility rooms and studies are counted. 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. ‘Making Space for Water’ (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 to: a) reduce the threat to people and their property; b) deliver the greatest environmental, social and economic benefit, consistent with the Government's sustainable development principles, and c) secure efficient and reliable funding mechanisms that deliver the levels of investment required. 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 Flood Risk Assessment (RFRAs) – A strategic analysis of flood risk undertaken by Regional Planning Bodies to inform their Regional Spatial Strategies and Sustainability Appraisals. The RFRA identifies broad locations of flood risk at a regionally significant level to highlight flooding issues that local planning authorities should address through their SFRAs. 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 Management Plan (RBMP) – A strategic tool introduced by the Water 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. Sites of Special Scientific Interest (SSSI) – A national designation that provides statutory protection for the best sites for wildlife and geology in the UK. Natural England has responsibility for identifying and protecting the SSSIs in England and these sites underpin the selection of sites under other national and international nature conservation designations.
Special Areas of Conservation (SAC) – Sites protected under the EC Habitats Directive to contribute towards conserving the species and habitat types identified in Annexes I and II of the directive. The species and habitats listed are those considered to be most in need of conservation at a European level (excluding birds). Special Protection Areas (SPA) – Sites protected under the EC Directive on the Conservation of Wild Birds (79/409/EEC) to contribute towards conserving rare and vulnerable birds as identified in Annex I of the directive, and for regularly occurring migratory species. Statement of Community Involvement (SCI) - Sets out the standards which authorities will achieve with regard to involving local communities in the preparation of local development documents and development control decisions. It is subject to independent examination. 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 Level 1 SFRA involves the compilation of available data from organisations such as the Environment Agency, Water Companies and Local Authorities and is 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. A Level 2 SFRA is applied to developments proposed in areas of flood risk to aid in the application of the Exception Test, and is a more detailed analysis, including features such as flood hazard mapping which identifies the level of risk to life and property. 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. The Exception Test - If, following application of the Sequential Test, it is not possible (consistent with wider sustainability objectives) to demonstrate that there are no reasonably available sites in areas with less risk of flooding that would be appropriate to the type of development or land use proposed, the Exception Test may apply. PPS25 sets out strict requirements for the application of the Test. The Sequential Test - Informed by a Strategic Flood Risk Assessment, a planning authority applies the Sequential Test to demonstrate that there are no reasonably available sites in areas with less risk of flooding that would be appropriate to the type of development or land use proposed. World Heritage Site (WHS) – Specific sites of outstanding cultural or natural importance, listed by the international UNESCO World Heritage Committee.
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1 Executive Summary
In March 2007, East Hampshire District Council commissioned Halcrow to produce a Strategic Flood Risk Assessment in accordance with Planning Policy Statement 25 (PPS25): Development and Flood Risk and the recently published document Development and Flood Risk, a Practice Guide Companion to PPS25 (February 2007). This Strategic Flood Risk Assessment will inform the plan-making process of the Local Development Framework. It will in particular inform the Core Strategy, the Development Control Policies and the Development Provision and Allocations documents. In addition, the SFRA will allow the District Council to: • prepare appropriate policies for the management of flood risk within the Local Development Documents, • inform the sustainability appraisal so that flood risk is taken account of when considering options and in the preparation of strategic land use policies, • identify the level of detail required for site-specific Flood Risk Assessments (FRAs) in particular locations, and • enable them to determine the acceptability of flood risk in relation to emergency planning capability.
This report provides an overview of the methodology, assumptions, uncertainties, tasks undertaken and the links to the wider sustainability appraisal process. It provides policy recommendations and guidance for the application of the Sequential Test, the preparation of flood risk assessments and the use of sustainable drainage systems, within the District Council’s administrative boundary.
East Hampshire District is covered by the South East Hampshire, the Thames, and the Arun and Western Streams Catchment Flood Management Plans (CFMPs), the first two of which are now available in draft form. The proposed main message and implementations for this SFRA and consequently the Local Development Framework are:
Main Messages The floodplain is one of the most important measures against flood risk, and should be protected and, where possible, increased. Collaboration between key stakeholders should be encouraged to ensure integrated urban drainage and adequate maintenance of watercourses and drainage systems. Public awareness of groundwater flooding needs to be raised, and uptake of flood warning services encouraged.
SFRA Local authorities should be encouraged to follow PPS25 principles to ensure that Implementation flood risk is taken into account at all stages in the planning process to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas at highest risk. There may be exceptional circumstances where new
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development is necessary. In such areas, policy aims to make it safe without increasing the risk elsewhere and, where possible, to reduce the overall flood risk. Structural works may be required to reduce flood hazard to within acceptable limits at Level 2 SFRA locations.
This report provides the information for allocating development sites following the Sequential Test. After having undertaken the Sequential Test, should there be any development which needs to be located in an area at risk of flooding, a Level 2 SFRA will be required to aid in the application of the Exception Test.
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2 Background Information
2.1 Terms of Reference In March 2007, East Hampshire District Council commissioned Halcrow to produce a Strategic Flood Risk Assessment in accordance with Planning Policy Statement 25 (PPS25): Development and Flood Risk. Advice on flood risk within the administrative boundary of the planning authority is required to support the preparation of their Local Development Framework (LDF), in accordance with government guidance and advice from the Environment Agency.
This report complies with the proposal dated 12 March 2007.
2.2 The Study Area East Hampshire (see Figure 1) is a rural district within the county of Hampshire, at the borders with West Sussex and Surrey. The district comprises an area of some 51,480 hectares with a population of 111,300. The four main towns are Alton (population 17,000), Petersfield (14,000), Whitehill/Bordon (13,600) and Horndean (12,500). On the eastern side of the district are the larger villages of Liphook (8,300) and Liss (6,200). Other villages with a population of over 3,000 are Clanfield, Four Marks and Headley. The western side of East Hampshire District is predominantly underlain by chalk, although with scattered clay/flint deposits on top, which then gives way to bands of Upper Greensand, Gault Clay and Lower Greensand along the eastern border.
2.3 Constraints on Development East Hampshire is justifiably renowned for its attractive countryside, much of which is of an exceptionally high quality. Approximately 40% of the district lies within the East Hampshire Area of Outstanding Natural Beauty (AONB) and about 53% could lie within the proposed South Downs National Park. Petersfield and Liss lie within the AONB and proposed National Park.
East Hampshire is one of the most diverse areas of the County for wildlife and habitats. Examples include the chalk downland of the South Downs, chalk and greensand woodland hangers, lowland heath, and the catchments of the Rivers Rother, Wey and Meon. Some of these habitats are extremely rare and large areas of the downland, heathland and woodland of the district are covered by Sites of
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Special Scientific Interest (SSSIs), Special Areas of Conservation (SACs) and a Special Protection Area (SPAs). A further 6,000 hectares of the district are designated as having local importance for nature conservation.
Maintaining the separate identity of the towns and villages is also important. 15 gap designations protect the land between those settlements that are under threat of coalescence.
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3 Planning Context
3.1 Introduction This Strategic Flood Risk Assessment (SFRA) conforms with National and Regional Planning Policy. A SFRA is a living document which is used as a tool by a planning authority to assess flood risk for spatial planning, producing development briefs, setting constraints, informing sustainability appraisals, identifying locations of emergency planning measures and indicating requirements for flood risk assessments.
The success of the Strategic Flood Risk Assessment is dependent upon the Planning Authority’s ability to implement the recommendations put forward for future sustainable flood risk management in conjunction with the Environment Agency. It is their responsibility to establish policies to ensure future sustainability with respect to flood risk.
Emerging planning policies normally cover about 20 years in advance. Planning for flood management is a longer-term practice and SFRAs consider implications for spatial planning about 100 years ahead.
3.2 The EU Water Framework Directive An integrated approach to the management of water is a key aim of the EU Water Framework Directive (Water Framework (England and Wales) Regulations 2003), which aims to integrate sustainable water planning and management. The Water Framework Directive applies to all surface and ground water bodies with significant effects for spatial and development management planning. A new system of river basin management plans (RBMPs) will be statutory plans that set out the actions required to meet the Water Framework Directive with the overall aim of achieving good water status. RBMPs are strategic plans, and will be subject to strategic environmental assessment and appropriate assessment under the Habitats Directive. All these processes are based on multi-criteria analysis to enable correlation between the objectives. RBMPs will need to take into account existing studies and reports such as this Level 1 SFRA and the CFMPs being prepared.
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3.3 National Planning Policy The Government has updated its planning advice contained within Planning Policy Guidance Notes (PPGs) with the publication of new style Planning Policy Statements (PPSs). As they are policy documents PPSs carry more weight than their predecessors.
PPS3: Housing (December 2005) specifically mentions the need to have regard to strategic flood risk assessments when local authorities are producing development plan documents relating to housing.
In December 2006 the Government published PPS25: Development and Flood Risk (a restatement of PPG25). It reflected the general direction set out in ‘Making Space for Water’ (DEFRA, 2004), the evolving new strategy to shape flood and coastal erosion risk over the next 10-20 years (see Glossary of Terms).
PPS25 advises that regional planning bodies in preparing regional spatial strategies should include a broad consideration of flood risk from all sources and set out a strategy for managing it in accordance with policies and plans prepared under the Water Framework Directive. Local planning authorities should prepare local development documents in their LDFs that set out policies for the allocation of sites and the control of development which avoid flood risk to people and property where possible and manage it elsewhere. The guidance also advises that flood risk should be considered alongside other spatial planning issues such as transport, housing, economic growth, natural resources etc and that the findings of the SFRA should inform the sustainability appraisal of the LDF.
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3.4 Regional Planning Policy 3.4.1 Regional Planning Guidance for the South East (RPG9) Regional planning policies provide the overarching framework for the preparation of a LDF. Regional Planning Guidance for the South East (RPG9) covers the period up to 2016 and sets out the housing requirement for each county within the region.
3.4.2 The South East Plan Under the Planning and Compulsory Purchase Act 2004, RPG9 is to be replaced by a new Regional Spatial Strategy, entitled the South East Plan. The South East Plan has been prepared by the South East England Regional Assembly (SEERA) and was submitted to the Government in March 2006. It sets out the vision for the region through to 2026. The examination into the South East Plan ran from November 2006 to March 2007, and the panel report was published in August 2007. The adoption of the final plan is anticipated in 2008.
The submitted South East Plan sets new requirements for housing and other developments in each district or borough. The target for housing in East Hampshire is to build an additional 5,200 dwellings between 2006 and 2026. It is a requirement that the Local Authority’s Core Strategy is in general conformity with regional planning policy.
Policy NRM3: Sustainable Flood Risk Management indicates that the sequential approach to development in flood risk areas will be followed. In addition, the policy states that local authorities and developers, with advice from the Environment Agency, should undertake a Strategic Flood Risk Assessment. This should have regard for climate change.
3.5 Local Planning Policy 3.5.1 Local Development Framework The District Council is producing a Local Development Framework (LDF) which is a portfolio of documents. The LDF will be the “spatial expression” of the priorities identified in the Sustainable Community Strategy and the Council’s own
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priorities to create successful communities, protect the environment and improve people’s lives.
The LDF will replace the East Hampshire District Local Plan: Second Review which was adopted in March 2006. The Second Review Local Plan will be “saved” for three years from its adoption and will be used during the transitional period as the move from the existing system to the new system is made. During this period the various policies and proposals will be replaced by new documents within the LDF.
3.5.2 Emerging Core Strategy Development Plan Document The District Council is working on a Core Strategy Development Plan Document (DPD). This sets out the spatial vision, objectives and strategy for the development of the district; including site allocations at Whitehill/Bordon to create a more sustainable community. It will provide the overall framework for development control. The Preferred Options for the Core Strategy are to be published in late 2008; followed by Submission in 2009, and Examination should commence in early 2010.
The Authority will also prepare an Allocations Development Plan Document (DPD), work on which is to commence this year, with Issues and Options being debated in 2009, Preferred Options in late 2009, and Submission in 2010.
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4 PPS25 Flood Zones, Environment Agency Flood Zones and SFRA Flood Maps
4.1 Introduction Flood Maps are a key element in a SFRA as they provide a visual understanding of flood risk at strategic level. A good understanding of the PPS25 Flood Zones, the Environment Agency Flood Zones and the SFRA Flood Maps, and of the relationships between them, is of fundamental importance for SFRAs.
4.2 The PPS25 Flood Zones PPS25 describes a system for indicating flood risk from fluvial and tidal sources using ‘Flood Zones’. The PPS25 Flood Zones subdivide the land, according to its spatial variation of flood probability, into four classifications; the low (Flood Zone 1), medium (Flood Zone 2) and high probability (Flood Zone 3a) flood zones and the functional floodplain (Flood Zone 3b) (see Figure 2).
PPS25 defines the flood zones as follows: Figure 2: Schematic of the PPS25 Flood Zones
Flood Zone 1 - Low Probability This flood zone comprises land assessed as having a less than 0.1% (1 in 1000) annual probability of river or sea flooding in any year.
Flood Zone 2 - Medium Probability This flood zone comprises land assessed as having between a 1% (1 in 100) and 0.1% (1 in 1000) annual probability of river flooding or between a 0.5% (1 in 200) and 0.1% (1 in 1000) annual probability of sea flooding in any year.
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Flood Zone 3a - High Probability This flood zone comprises land assessed as having a 1% (1 in 100) or greater annual probability of river flooding or a 0.5% (1 in 200) or greater annual probability of flooding from the sea in any year.
Flood Zone 3b - The Functional Floodplain This flood zone comprises land where water has to flow or be stored in times of flood. SFRAs should identify this flood zone (land which would flood with an annual probability of 5% (1 in 20) or greater in any year or is designed to flood in an extreme (0.1%) flood, or at another probability to be agreed between the Local Planning Authority (LPA) and the Environment Agency, including water conveyance routes).
The range of probabilities covered by each flood zone is defined in PPS25 in terms of annual average probability of flooding from rivers and the sea. The term ‘average’ means that, for example, a flood that has a 100% (or 1 in 1) probability of occurrence will have occurred once a year on average, although in any given year it may have occurred more than once or not flooded at all.
There are many methods that estimate the probability of occurrence of a flood, based on historical events, measurements of flows, modelling studies, etc. In the case of the functional floodplain, it may be possible for this to be drawn on a map by combining the flood extents of many frequent historical flood events (up to the 5% probability event). For more extreme flood events (lower probability events), however, it will be increasingly necessary to rely on modelling to determine the extents, as there are not many sufficiently accurate records available.
4.3 Environment Agency Flood Zones Historically the Environment Agency and its predecessors have kept formal maps of tidal and fluvial flooding to the standards required by legislation. Originally this mapping simply recorded flood events, but in 2001 PPG25 (the predecessor of PPS25 – see Section 3.3) imposed a duty on the Environment Agency to produce maps which showed the predicted extent of tidal and fluvial flood zones in England and Wales. The Environment Agency flood zones are published on their website at http://www.environment-agency.gov.uk/subjects/flood/?lang=_e, and are updated on a quarterly basis as improved modelling and recent events provide data for refining flood extents. The Environment Agency flood zones differ from the PPS25 definitions quoted in Section 4.2 in that there are only three zones; Flood Zones 1 and 2 are defined as above but the higher probability flood zone includes both the Flood Zone 3a and 3b areas in one zone called Flood Zone 3 (Flood Zone 3a + Flood Zone 3b = Flood Zone 3). The division of Zone 3 into
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3a and 3b was introduced only recently, in PPS25. Furthermore, Environment Agency policy in relation to mapping fluvial flooding is to only include flooding caused by ‘main rivers’ (where main rivers are large or locally significant watercourses as designated by DEFRA or the Welsh Assembly Government) although when producing the outlines from historical records they may also include other types of flooding or flooding along ordinary watercourses where the source of flooding has been influenced by main river. Therefore, whilst PPS25 flood zones are not intended to include flooding from groundwater or other sources, in practice they may be included if zones have been defined based on historical records where there was a combination of sources (see Appendix A for further details about the Environment Agency flood zones).
The flood zones produced by the Environment Agency are based on current environmental conditions, so do not make any allowance for climate change.
The flood zones (both the Environment Agency and PPS25 definitions) do not rely on the presence of defences (formal or informal, see Glossary of Terms) as there remains a risk that these defences can fail through overtopping or structural failure.
4.4 SFRA Flood Maps SFRA flood maps in general reproduce the Environment Agency high, medium and low probability flood zones (see Section 6.4.2) where no other more detailed or up-to-date information is available. SFRA flood maps also include assessments of the functional floodplain and the effect of climate change on the flood zones.
However, SFRA flood maps do not only show fluvial/tidal flood zones, they also show localised flooding areas (see Tiles A to E). The localised flooding areas relate to historical flooding at individual locations, and may arise from any source of flooding, including groundwater, surface runoff or insufficient drainage as well as historical incidents of fluvial or tidal flooding. In many cases, the flooding may have been caused by a combination of factors, or the source of flooding may not have been identified.
The flood risk at localised flooding areas that fall in Flood Zones 1 and 2 may in some cases be significant, with deep and frequent flooding. It may therefore be more appropriate to view these areas as if they were high risk areas (equivalent to the fluvial/tidal Flood Zone 3a) when applying the Sequential Test (see Chapter 9).
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5 PPS25 and its Practice Guide Companion
5.1 PPS25 - Key Aims The key aims of PPS25 are reproduced below:
‘The aims of planning policy on development and flood risk are to ensure that flood risk is taken into account at all stages in the planning process to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas at highest risk. Where new development is, exceptionally, necessary in such areas, policy aims to make it safe without increasing flood risk elsewhere and where possible, reducing flood risk overall. Regional planning bodies and local planning authorities (LPAs) should prepare and implement planning strategies that help to deliver sustainable development by: Appraising risk • identifying land at risk and the degree of risk of flooding from river, sea and other sources in their areas; • preparing Regional Flood Risk Appraisals (RFRAs) or Strategic Flood Risk Assessments (SFRAs) as appropriate, as freestanding assessments that contribute to the Sustainability Appraisal of their plans; Managing risk • framing policies for the location of development which avoid flood risk to people and property where possible, and manage any residual risk, taking account of the impacts of climate change; • only permitting development in areas of flood risk when there are no reasonably available sites in areas of lower flood risk and benefits of the development outweigh the risks from flooding; Reducing risk • safeguarding land from development that is required for current and future flood management e.g. conveyance and storage of floodwater, and flood defences; • reducing flood risk to and from new development through location, layout and design, incorporating sustainable drainage systems (SUDS); • using opportunities offered by new development to reduce the causes and impacts of flooding e.g. surface water management plans; making the most of the benefits of green infrastructure for flood storage, conveyance and SUDS; recreating functional floodplain; and setting back defences;
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A partnership approach • working effectively with the Environment Agency, other operating authorities and other stakeholders to ensure that best use is made of their expertise and information so that plans are effective and decisions on planning applications can be delivered expeditiously (this is currently being implemented by a series of pilot projects for DEFRA) and • ensuring spatial planning supports flood risk management policies and plans, River Basin Management Plans and emergency planning.’
5.2 Outcomes of the SFRA Process The broad planning objectives of PPS25 described in Section 5.1, effectively set the scope for the specific outcomes of the SFRA process. The SFRA, in turn, then informs forward planning and development control decisions that ensure the objectives set out above can be achieved.
It is important to reiterate that PPS25 is not applied in isolation but as part of the wider planning process. The formulation of flood risk policy and the allocation of land for future development must also meet the requirements of other planning policy, while seeking an overall reduction of flood risk. Clearly, a careful balance must be sought in these instances, and the SFRA aims to assist in this process through the provision of a clear and robust evidence base upon which informed decisions can be made.
5.3 SFRA Levels 1 and 2 There are two categories of SFRAs; a ‘Level 1’ SFRA, which analyses flood risk at a strategic level across the Local Authority area, and a ‘Level 2’ SFRA, which provides a more detailed analysis of flood risk for a specified development site.
The Level 1 SFRA provides the necessary information for use as a tool for allocating development sites in the application of the Sequential Test. Local authorities should apply the Sequential Test when allocating sites in their Local Developments Framework. The Sequential Test should also be applied to Windfall sites; since these sites will not have been included in the LDF, the test should be applied at the planning application stage.
However, the Sequential Test may not be able to provide a sufficient number of suitably available sites for development within zones of lower flood risk. A Level 2 SFRA is required in the event that a development is proposed in an area of flood risk, and the vulnerability of the land use requires an Exception Test. Flood risk vulnerability (or level of resilience to damages from flooding) reflects the land
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uses/property types within a site, and PPS25 provides a classification system for these, ranging from highly vulnerable to water-compatible, which can be cross- matched with the flood zones to determine suitable land uses for a site (see PPS25 Annex D). For a Level 2 SFRA, the scope of the SFRA is widened to a more detailed investigation of flooding on the site, using more detailed modelling and considering factors such as flood depth, velocity, rate of inundation and presence of defences in order to gain a greater understanding of flood hazard within the area. The Level 2 SFRA does not negate the need for a site specific flood risk assessment which is normally required for sites within the floodplain and some outside, and for which responsibility would fall upon the potential developer to submit with their planning application.
The scope of this report is a Level 1 SFRA to inform the plan-making process of the Core Strategy and other development plan documents as required (see Section 3.5). This information will be used by the planning authority to undertake sequential testing in identifying general locations for development and to formulate strategic policies, and may in some cases assist in informing the District Council’s emergency plan. The study covers the area within the administrative boundary of East Hampshire District Council.
As well as identifying the level of flood risk across the district an important strategic policy is to identify opportunities that reduce overall flood risk (as opposed to a non-increase in flood risk policy) as part of the LDF process. This approach is consistent with the CFMP aims (see Section 8.2).
5.4 The Sequential Test of PPS25 In seeking to allocate a specific type of development or land use, or determining a windfall application, planning authorities should apply the Sequential Test to demonstrate that there are no reasonably available, appropriate sites in areas of less risk of flooding.
Preference should be given to locating new development in Flood Zone 1 because this zone has the lowest risk of flooding (see Section 4.2). However, localised flooding should also be taken into account since if this is deep or frequent enough, it may be more appropriate to treat the area as if it were a higher risk flood zone (see Section 4.4). If there is no reasonably available site in Flood Zone 1, the flood vulnerability of the proposed development can be taken into account in locating development in Flood Zone 2 and then, if still no appropriate sites are available,
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Flood Zone 3. For example ambulance stations, being in the highly vulnerable category, may be placed in Flood Zone 1. If no reasonably available site can be found in Flood Zone 1, it may be possible to locate it in Flood Zone 2 if it passes the Exception Test. However, it would not be acceptable in Flood Zone 3.
Within each flood zone new development should be directed to sites with lower flood risk (towards the adjacent zone of lower probability of flooding) from all sources as indicated by the SFRA.
5.5 The Exception Test of PPS25 If, following application of the Sequential Test, it is not possible for the development to be located in flood zones of lower probability of flooding consistent with wider sustainability objectives, the development vulnerability should be checked for ‘compatibility’ with the flood zone using Table D.3 of PPS25. A copy of this table can be found on Flood Map Tiles A to E. Where indicated by this table the Exception Test needs to be applied. This Test provides a method of managing flood risk while still allowing necessary development to occur. The level of detail required for an Exception Test is beyond the scope of this Level 1 SFRA; in the event that an Exception Test is needed it will be necessary to carry out a Level 2 SFRA to further define the hazards within the flood zones. This includes analysis of a number of factors such as flood probability, flood depth, flood velocity and rate of onset of flooding, and takes into account the presence of flood defences, including future policies for the defences.
The Exception Test is only appropriate for use when the Sequential Test alone cannot deliver acceptable sites, but where some continuing development is necessary for wider sustainable development reasons, taking into account the need to avoid social or economic blight and the need for essential civil infrastructure to remain operational during floods. It may also be appropriate to use it where restrictive national designations such as landscape, heritage and nature conservation designations, e.g. Areas of Outstanding Natural Beauty (AONBs), Sites of Special Scientific Interest (SSSIs) and World Heritage Sites (WHS), prevent the availability of unconstrained sites in lower risk areas.
For the Exception Test to be passed: 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 Development Plan Document has
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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, there are no reasonable alternative sites on developable previously-developed land; and c) a flood risk assessment (FRA) must demonstrate that the development will be safe, without increasing flood risk elsewhere, and where possible, will reduce flood risk overall.
5.6 The Practice Guide Companion to PPS25 A practice guide companion to PPS25 was published in February 2007. It is a ‘living draft’ web-based consultation paper (see http://www.communities.gov.uk/index.asp?id=1504639). It is comprehensive and incorporates many recommendations from previous Guidance documents.
The guide reaffirms the adoption of a risk-based approach to flooding by following stepped hierarchical measures at all stages in the planning process. Avoidance/prevention of inappropriate development in areas of flood risk is always the first measure, followed by substitution of different development types, control of flood risk and finally mitigation. This is summarised in Table 1, which is reproduced from Table 1.2 of the Practice Guide Companion to PPS25.
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Table 1 – Overview of the flood risk management hierarchy (from the Practice Guide Companion to PPS25, table 1.2).
Following the sequential approach of PPS25 is the most important flood risk management tool for spatial planning, as this ensures the high level measures of avoidance/prevention and substitution are implemented.
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6 Study Methodology
6.1 Specific Project Outputs The specific outputs are based on the required outputs for a Level 1 SFRA, as follows: i. Map existing Flood Zones 1, 2 and 3 across the plan area (see Section 6.4.2, Tables 2 to 6 and Tiles A to E). ii. Map Flood Zones 1, 2 and 3 for the future climate change scenarios of 2070 and 2115 as set out in PPS25 Annex B, taking account of recommended national precautionary sensitivity ranges for peak rainfall intensities, peak river flows and wave heights (see Section 6.4.2 and 6.4.3). iii. Identify areas at risk of flooding from sources other than rivers and the sea (see Sections 6.3, Chapter 7, Tables 2 to 6, and Tiles A to E). iv. Identify and take into account flood risk management measures including flood defences and emergency warning systems (see Chapter 8). v. Guidance on the Application of the Sequential Test (see Chapter 9). vi. Guidance for the preparation of Flood Risk Assessments (see Chapter 10). vii. Guidance on possible mitigation measures, including the likely applicability of different sustainable drainage systems (SUDS) techniques for managing surface water runoff at key Level 1 SFRA development sites (see Chapter 11). viii. Identify locations where development would significantly increase the risk of flooding elsewhere (see Chapter 7 and 11).
6.2 Approach to Data Gathering The main source of data for this study has been the Environment Agency, previous Halcrow projects, a number of relevant websites, the Local Planning Authorities, Hampshire County Council, Southern Water and Thames Water (see Audit Trail Database in Appendix B).
Priority has been given to the collection of geo-referenced information in electronic format, to ensure the effective management of the data within a GIS environment. All incoming data has been recorded on a project data register by a specialist document controller/GIS data manager, specifically designated for this project.
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The quality review of the information has been carried out by an experienced core team. The team has been able to review the collected data, assess its significance and quality, and advise on which part of the collected data needed to be used for the SFRA.
The main approach has been to build on the large number of strategic studies and relevant available data. East Hampshire District is covered by the South East Hampshire, the Thames, and the Arun and Western Streams Catchment Flood Management Plans (CFMPs), the first two of which are now available in draft form and have provided a good foundation for catchment understanding and flood risk assessment.
Valuable flood risk information for localised flooding areas (as opposed to the ‘non-localised’ flood zones) was obtained from the drainage specialists working for East Hampshire District Council and Hampshire County Council. The collected information complemented information provided by the Environment Agency, Southern Water and Thames Water.
6.3 Forms of Flooding and Data Limitations 6.3.1 Introduction For the purpose of this assessment, forms of flooding (also defined as sources of flooding) are divided into four categories: a) river floods; b) flooding from impounded water bodies such as canals and reservoirs; c) groundwater flooding; d) flooding from other sources.
The reason for adopting this classification is to provide an understanding of data limitations and assumptions as there are different standards for the collection of each of these types of data.
The various sources of flooding within the study area are described and shown in Tables 2 to 6 and Tiles A to E.
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6.3.2 Tidal and Fluvial Flooding Fluvial flooding (flood zones) is described in Sections 4.2 and 4.3, with further details, including assumptions and limitations, in Appendix A. As East Hampshire District is inland, it does not experience tidal flooding.
6.3.3 Records of Flooding from Impounded Water Bodies Records of flooding from reservoirs and canals are erratic as there is no requirement for the Environment Agency to provide information on historic flooding from canals and raised reservoirs on plans. In particular, PPS25 does not require flood risk from canals and raised reservoirs to be shown on the Environment Agency flood zones.
Overflows from canals can be common as they are often fed by land drainage, and often do not have controlled overflow spillways. Occasionally, major bank breaches also occur, leading to rapid and deep flooding of adjacent land.
Reservoirs with an impounded volume in excess of 25,000 cubic metres (measured above natural ground level) are governed by the Reservoirs Act and are listed on a register held by the Environment Agency. There is one listed reservoir in East Hampshire, the Wylds Lake, which is just northeast of Liss. Due to high standards of inspection and maintenance required by legislation, flood risk from registered reservoirs is normally moderately low.
6.3.4 Records of Groundwater Flooding Both the Environment Agency and the District Council keep records of individual groundwater flooding events. However, as yet there is no government requirement for groundwater flooding to be recorded and thus records tend to be on an adhoc basis and simply list those incidents reported to the Agency or Council.
Areas underlain by chalk geology are typically subject to groundwater flooding, but many areas of East Hampshire are separated from this by clay and flint deposits which provide some measure of protection, so there have been relatively few reported episodes of groundwater flooding in this district. It should be noted, however, that Environment Agency records only include those incidents which were reported to them so it is likely to be only a sample of affected properties.
Several of the rivers in East Hampshire are sourced in the chalk area (such as the River Meon, River Wey and Lavant Stream) and thus are groundwater fed.
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Flooding caused by storm events during periods of high groundwater levels may be exacerbated by the high baseflow contribution.
6.3.5 Records of Flooding from Other Sources Until 2006 methodologies for recording flooding from sources other than tidal and fluvial were not standardised, so records held of such flooding are neither complete nor to a uniform standard. As part of DEFRA’s Making Space for Water study, a report was published by the Environment Agency titled “Flooding from other sources”. The report recommended a classification for such flooding and methods for recording other sources of flooding.
The sources of flooding from the Environment Agency (Source report, JBA 2006) have been merged and are reproduced in Table 7.
Sources of information on flooding from other sources can be obtained from local government, highway authorities, the Environment Agency, sewerage undertakers, businesses, individuals and archives such as libraries.
The recording of flood instances by the authorities has often led to improvements intended to prevent recurrence, and hence historical flooding is not necessarily evidence of propensity for future flooding.
Currently few records of flooding from other sources contain sufficient detail to enable them to be classified in accordance with the Environment Agency classification of “flooding from other sources’’ and, indeed, many historical flood incidents have more than one cause.
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Type Flooding Phenomenon Sources Pathways Receptors Hazard
1 Direct runoff Intense All surfaces People, vehicles, Deep fast water, with high rate of rainfall including road properties, inundation network commercial, environment Deep water / debris / cellar flooding
Fast water – erosion
2 Sewerage and drainage system Heavy Surcharging People, vehicles, Cellar and ground floor flooding with flooding from pipe capacity rainfall over from manholes properties, water quality issues exceedance a long and openings in commercial, duration or the drainage environment. intense system. rainfall Surcharging
3 Sewerage and drainage system Long Manholes and People, vehicles, Deep ponded water. Cellar and ground flooding from ‘other causes’ duration or overflows in properties, floor flooding with water quality issues (blockage and collapse) intense drainage and commercial, rainfall sewerage environment network
4 Restricted outlets from drainage Heavy All surfaces and People, vehicles, Deep ponded water, and water diverted systems due to high flood levels in rainfall over drainage properties, along unexpected routes the receiving watercourse long network commercial duration
5 Surcharge from small (ordinary) Heavy or All surfaces and People, vehicles, Deep ponded water and ‘lost’ watercourses intense drainage properties, rainfall network commercial
6 Floodplain flooding from ordinary Heavy Ordinary People, vehicles, Deep ponded water and fast flowing watercourses not covered by the rainfall watercourse properties, floodplain flows flood map (catchment area>3km2) embankments commercial and floodplains
7 Intense rainfall leading to Intense Land, field People, vehicles, Fast water erosion of soil for high grade overland flow including rainfall or drainage, river agricultural land. Rapid rates of mud/debris flow and flow along long and inundation affect road users. Runoff old drainage lines, roads and duration watercourse properties, from land on urban fringe to flood railways. heavy network commercial, properties rainfall environment
8 Heavy, long duration rainfall Long Rural surfaces People, vehicles, Deep water, runoff from fields onto leading to ponding on for example duration and field properties, rural roads can cause serious hazard to roads or fields heavy drainage commercial, drivers rainfall environment
9 Changes to drainage or land Loss of Field drains, People, properties, Reduction in capacity of land to drain management. Reduction in pumping / drift geology, environment water away – leading to ponding and or agricultural pumping / land use irrigation watercourses more surface runoff and erosion. Await management / drainage leading to and land findings of FD2120 (DEFRA increased risk of flooding surface document).
Table 7: Other Sources of Flooding (Source report, JBA 2006)
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6.4 Production of Flood Maps
6.4.1 Introduction For a Level 1 SFRA and in accordance with the Practice Guide Companion to PPS25, the current and climate change flood maps assume that the defences are not in place. This is a reasonable precautionary option for the application of the Sequential Test, as it gives priority to development areas that do not require the presence and maintenance of defences.
The scale of the Flood Maps is 1:25 000 to give both a strategic overview and reasonable clarity of general features.
6.4.2 Current Flood Maps (without climate change allowance) The December 2007 Environment Agency flood zones have been used for the production of the SFRA Flood Maps (see Tiles A to E) which means that they are based on the most up to date information available at the time of writing this report.
However, further updates are likely to occur as more detailed studies are carried out on river catchments. Once the Environment Agency approves these studies and incorporates the new flood outlines within their flood zone maps, the SFRA flood maps can then be updated in turn. SFRAs are living documents and, in order to ensure consistency, at least the digital flood maps should be updated in conjunction with Environment Agency Flood Zone updates.
As discussed in Section 4.2, the functional floodplain covers flooding that occurs frequently, so it may in some cases be possible to estimate its extent based upon historical data. However, there is insufficient data to determine this for the whole East Hampshire District, and as this method is complex and time-consuming it is not generally appropriate for a Level 1 SFRA.
In the absence of sufficient historical data or modelling work, it was not possible to identify which part of the Environment Agency Flood Zone 3 corresponded to Flood Zone 3a and which part to Flood Zone 3b (Flood Zone 3a + Flood Zone 3b = Flood Zone 3). Therefore a precautionary principle was adopted where it was assumed that Flood Zone 3b covers all of Flood Zone 3. In this
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case Flood Zone 3a is represented in the flood maps merely as an outline since it is subsumed completely by the functional floodplain.
The exception is the River Wey area, for which modelling outlines are available for the 5% (1 in 20) annual probability event from a detailed study of the River Wey. This has been used to replace the Environment Agency outline for Flood Zone 3b for the main river reaches, but where tributaries had not all been included in the modelling these were left as the Environment Agency flood zone outlines.
PPS25 permits only water compatible and essential infrastructure land uses in Flood Zone 3b. In the event that the Sequential Test leads to a ‘more vulnerable’ or ‘less vulnerable’ development being considered for a Flood Zone 3 area where Flood Zone 3b has not been distinguished from Flood Zone 3a, a more detailed Level 2 assessment will be required.
As mentioned in Section 6.2, valuable advice on the main areas subject to localised flooding (see Tiles A to E) was obtained from the drainage specialists working for East Hampshire District Council and Hampshire County Council. Information was provided by Thames Water (illustrated using postcode boundaries) and Southern Water (indicated as separate points) in relation to flooding from drainage systems. Incidents of groundwater flooding have been obtained from the Environment Agency. The Environment Agency also maintains records of fluvial flooding but these have not been included on the map to minimise duplication with the Environment Agency flood zones. The Environment Agency should, however, be consulted in relation to the flood incident records during the application of the Sequential Test, as explained in Section 9.3.
6.4.3 Effects of Climate Change In October 2006, DEFRA published the document Flood and Coastal Defence Appraisal Guidance FCDPAG3 Economic Appraisal (Supplementary Note to Operating Authorities – Climate Change Impacts) identifying the climate change impacts to be considered in undertaking flood risk appraisals in the United Kingdom. In addition to sea level rise of approximately 1m in southeast England over the next 100 years, the document also sets out how short duration rainfall could increase by 30% and flows by 20%, and suggests winters will become generally wetter. These effects will tend to increase both the size of flood zones associated with the sea and rivers, and the amount of flooding experienced from “other sources”.
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During the lifespan of new commercial and residential developments, it is expected that peak river flows will first increase for a short period by 10% (2007-2025) and for the remaining period by 20%. Table 8 (reproduced from PPS25) indicates that an increase of 20% in peak river flow is estimated between years 2025 and 2115.
Parameter 1990 to 2025 2025 to 2055 2055 to 2085 2085 to 2115
Peak rainfall intensity +5% +10% +20% +30%
Peak river flow +10% +20%
Offshore wind speed +5% +10%
Extreme wave height +5% +10% Table 8 - Recommended national precautionary sensitivity ranges (from PPS25, table B.2).
A lifespan of 60 years is used for a commercial development, and 100 years for residential development, e.g. a commercial building built in 2008 could be assumed to reach the end of its design life in 2068 – so an appropriate allowance for an increase in the peak rainfall intensity would be 20%. It should however, be noted that, where possible, a managed adaptive approach to the effects of climate change should be adopted, which will allow for further adaptation in the future as understanding of the effects of climate change improves (further guidance on the ‘managed adaptive approach’ can be found in the DEFRA FCDPAG3 supplementary note on climate change).
6.4.4 Climate Change Flood Maps As recommended by the Environment Agency, it has been assumed that commercial and residential developments planned in the LDFs will reach the end of their life in 2070 and 2115 respectively (based on projecting the lifespans quoted in Section 6.4.3). Based on an estimated increase of 20% in peak river flow (which is expected to occur during these lifespans) the following precautionary rules have been adopted for this Level 1 SFRA:
C1) ‘Climate Change’ functional floodplain Zone 3b = Current Flood Zones 3b + 3a = Current Flood Zone 3
C2) ‘Climate Change’ Flood Zone 3a = Current Flood Zone 2
Many previous flood mapping studies by Halcrow confirmed that increases in
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flows by 20% to Flood Zone 3, result in flood extents which are in general smaller than Flood Zone 2, so this is a conservative rule.
C3) ‘Climate Change’ Flood Zone 2 = Current Flood Zone 2
There is little certainty about the effect that climate change will have on very extreme fluvial events, but it is currently assumed that ‘Climate Change’ Flood Zone 2 is slightly larger than Current Flood Zone 2. However, at the scale of these SFRA flood maps, it is reasonable to assume that the difference is indistinguishable.
The current flood maps (Tiles A to E) can therefore be used to take account of climate change predictions along the fluvial reaches by applying rules C1, C2 and C3. In applying the Sequential Test, the climate change scenarios should be used in order to take into account the expected lifetime of the development.
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7 Flood Risk in the Study Area
7.1 Introduction All forms of historical flooding have been considered, both in isolation and in combination. For references to locations within the study area see the set of maps, Tiles A to E.
7.2 Geology and Hydrology 7.2.1 Geology Approximately three quarters of the district is Upper Chalk downland. This covers the northwest, west and southern areas. To the north of the South Downs and to the east of Froxfield, Hawkley, Selbourne, Binsted and Bentley the surface rocks are older. Travelling eastwards from the bottom edge of the chalk, a succession of marls, clays and sandstones of increasing age are encountered. South of Petersfield, at Froxfield, Hawkley and Selbourne the transition from chalk to greensand and Gault clay is marked by high and steep escarpments.
At the southern tip of the district the chalk dips under younger rocks comprising Lambeth group members, London clay and Bognor Sand.
Much of East Hampshire is underlain by chalk, and recent research tends to suggest that chalk aquifers are more prone to groundwater flooding than other aquifers. During the winter of 2000/2001, extreme rainfall conditions prevailed across much of the UK, and Hampshire was one of the areas which was severely affected. Numerous flooding incidents occurred, many of which related to elevated groundwater levels and abnormally high spring flows. Subsequent to the incidents of groundwater flooding in 2000/2001, and similar (though less severe) events in 2002/2003, reports were commissioned by the EA, Southern Region relating to groundwater flooding in Hampshire (Groundwater and Fluvial Flooding in Hampshire, Halcrow, June 2002 and August 2005, respectively) and the EA, Thames Region (Groundwater Flooding in the Thames Region – Winter 2000/2001, V Robinson, J Solomon and S Morris, Environment Agency Thames Region Water Resources, October 2001). DEFRA has commissioned research into the most appropriate way to monitor, record and collate data from groundwater flooding and how this information may be used in groundwater flood risk assessments, reported in an initial Scoping Report (Jacobs 2004), and this has led to a further programme of research under the ‘Making Space for Water’ programme. This research is intended
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to produce recommendations for the effective monitoring and collation of groundwater flooding information; to identify any organisational and funding changes required to implement this; and to influence the direction of the strategic overview role of the Environment Agency with respect to groundwater flooding.
Prior to the introduction of these measures, any development within the study area must take into account the potential for groundwater flooding. An evaluation of historic incidents may provide some indication but are unlikely to give a complete account due to the typical sparcity of records. The Jacobs Scoping Report includes a set of maps indicating groundwater emergence zones (defined as the appearance of groundwater at the land surface or below the surface in sub-surface structure such as excavations, cellars, communication conduits and tunnels) for each of the Environment Agency administrative areas in England. The maps covering East Hampshire are provided in Appendix C. However, it should be noted that these maps are produced at a national scale with the aim of suggesting where groundwater flooding is likely. Detailed analysis of the site may indicate a different geology or topography at the local scale. In East Hampshire several areas are covered by local deposits of clay, which may provide protection from groundwater flooding.
Once appropriate procedures have been developed, groundwater flood risk assessment measures should be applied.
7.2.2 Hydrology The northern half of East Hampshire District drains to the Thames river system via branches of the Wey, and southern half to four coastal rivers, which are the Arun (via the Western Rother), the Lavant Stream, the Meon and the Itchen. The catchment divide follows an approximate line from Greatham (between Petersfield and Bordon) via Four Marks to Lasham.
The southern tip of the district also is drained by the Hermitage Stream and River Wallington, though the permanent parts of these watercourses are entirely outside the district boundary.
Except for the eastern branch of the Wey, which rises to the east of Haslemere, all the local rivers rise within East Hampshire District.
All the principal rivers are shown and named on Tiles A to E.
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In the Upper Chalk areas permanent watercourses are absent in all except the deepest valleys. Upper Chalk is a major aquifer capable of absorbing large amounts of rainfall and releasing it slowly over a long period. This buffering effect together with the mainly rural nature of the Upper Chalk area means that the Itchen, Meon and (Havant) Lavant river systems, which are mainly spring fed by the chalk aquifers, have relatively narrow ranges of flows in a normal year and generally do not flood in response to short to medium duration heavy rainfall. The part of the (Havant) Lavant within the district is ephemeral.
After prolonged rainfall the water table in the Upper Chalk aquifer can rise to the ground surface causing springs to erupt in the valley floors and the creation of ephemeral watercourses. These effects can lead to ‘groundwater flooding’ which can last for several months in very wet winters. Public supply and agricultural water abstraction from the Upper Chalk tends to increase the Upper Chalk’s buffering effect, thereby suppressing the frequency at which ephemeral watercourses and springs occur. However, when the water table is sufficiently high for the aquifer to flow freely into the valleys, the runoff from the Upper Chalk can be similar to that from a generally impermeable catchment. Snow melt and rainfall on a frozen Upper Chalk catchment also can lead to rapid surface water runoff to the river system and widespread valley flooding.
The Western Rother rises on the Gault Formation, but passes though sandstone aquifers, and drains the urban area of Petersfield. Two of the branches of the Wey rise in the Upper Chalk, but most of the catchment of the Wey comprises clays and sandstones. These rivers therefore have a mixed character.
Culvert blockage can be a factor which exacerbates flooding situations. Site specific flood risk assessments have identified that the depth and extents of flood zones can be significantly different when considering blockage scenarios. In Alton there is a problem with potential culvert blockages, and at Arford the culverts are at increased risk of blockage due to debris where the Hanger Stream passes through a steep-sided wooded area.
7.3 Historical Flooding 7.3.1 Introduction This Section is to be read in conjunction with Tiles A to E and Tables 2 to 6.
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7.3.2 Notable Flood Events December 1927 / January 1928 – Flooding along Oakhanger Stream in Selbourne and Shortheath.
Winter 1947 – Extensive fluvial flooding.
November 1951 and November 1953 – Flooding at East Meon.
1954 – Flooding along Wey northern branch and Oakhanger Stream.
1960 – Flooding along Wey northern branch.
1968 – Flooding in and around Petersfield, and in the Wey river system.
23 November 1974 – Flooding in and around Petersfield.
13 October 1982 – Flooding at Sheet, near Petersfield.
1990 – Flooding in the Wey river system.
2 October 1993 – Fluvial flooding at East Meon.
2 December 1996 – Flooding at Horndean.
Winter of 2000-2001. Very high rainfall caused widespread flooding in southern England. Chalton, Finchdean, Rowlands Castle, Clanfield, East Meon and the River Wey were particularly affected.
Winter of 2002-2003. More prolonged rainfall caused widespread flooding, though generally not as severe as in 2000-2001. The southern part of the district, in particular Chalton, Finchdean and East Meon, was badly affected.
7.3.3 Flooding shown on the Environment Agency’s Formal Flood Map The flood zones shown on the Environment Agency’s formal flood map are reproduced on Tiles A to E. Flood Zone 2 represents the 1 in 1000 year event and Flood Zone 3 represents the 1 in 100 year event.
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7.3.4 Fluvial Flood Events within Tile A The 1947 event caused extensive flooding along the Caker Stream and River Wey Northern Branch, although within Alton flooding only extended as far upstream as Hangers Way. The flood extents were within the boundaries of Flood Zone 2.
7.3.5 Fluvial Flood Events within Tile B The historical flood map shows extensive fluvial flooding in 1947 and 1968, covering most of the Flood Zone 2 area.
7.3.6 Fluvial Flood Events within Tile C Flooding occurred at Petersfield Road, Ropley in 1995. The watercourse at this location is ephemeral.
General fluvial flooding of East Meon and Frogmore occurred in 1951, 1953 and 1993. Flooding in the winter of 2000-2001 at East Meon is described in the Hampshire groundwater flooding series of reports prepared by Halcrow. Further flooding occurred in 2002-3.
Main watercourses at Petersfield flood regularly, but following improvements of the watercourses, damage to property generally is low.
7.3.7 Fluvial Flood Events within Tile D The Rother and its tributaries flood regularly. There are a large number of flood records within Flood Zone 3.
7.3.8 Fluvial and Tidal Flood Events within Tile E Extensive flooding of Chalton (including Localised Flooding Area EH003) Finchdean and Rowland’s Castle in 2000-2001 is recorded in the “Groundwater flooding” series of reports prepared by Halcrow Group. Flooding at these places occurred also in 2002-2003.
7.3.9 Flooding from Impounded Water Bodies No records of flooding from impounded bodies have been discovered. However, an incident is known to have occurred at the Wylds Lake Reservoir in November 2000 where excessive flood flows and blockage of a downstream culvert caused the collapse of a weir. This resulted in water scouring beneath the high pressure water main running under Warren Road which then fractured due to the loss of foundation. Whilst the floodwater was able to dissipate into the downstream
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system of braided streams, thus avoiding flooding of the road, the fracturing of the pipe undermined and caused damage to the road.
7.3.10 Records of Groundwater Flooding within Tile A Lower Farringdon and Chawton Localised Flooding Areas EH001 and EH002 mainly fall within Flood Zone 3 of an ephemeral watercourse and associated springs. Here there was extensive flooding in the winter of 2000-2001.
7.3.11 Records of Groundwater Flooding within Tile B Incidents of groundwater flooding have occurred in Bordon, Whitehill and Lindford.
7.3.12 Records of Groundwater Flooding within Tile C Localised Flooding Area EH006 at Langrish affects approximately 2 ha of mainly agricultural land in the chalk valley floor.
7.3.13 Records of Groundwater Flooding within Tile D Localised Flooding Areas EH007 and EH008 at West Liss are partially associated with groundwater flooding.
7.3.14 Records of Groundwater Flooding within Tile E Springs are the cause of flooding at Localised Flooding Areas EH003 and EH004 at Clanfield.
Groundwater flooding occurs outside Flood Zones 2 and 3 at Finchdean and Rowlands Castle.
7.3.15 Flooding from Other Sources Although extensive records of flooding from other sources have been obtained as part of the research for this report, these records (summarised in Tables 2 to 4) should not be considered a complete record of such flooding.
7.3.16 Flooding from Other Sources within Tile A No records of flooding from other sources have been discovered relating to Tile A.
7.3.17 Flooding from Other Sources within Tile B Thames Water provided a summary of property sewerage flooding in the last ten years within postcode areas GU10 4 (Rowledge), GU10 5 (Crondall) and GU27 1
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(Shottermill). There was a total of five such incidents in these areas. As most of the postcode areas and sewerage are outside East Hampshire District, it is unclear if any of the flood incidents are relevant to the study area.
7.3.18 Flooding from Other Sources within Tile C Most Environment Agency flood records are for areas within Flood Zones 2 and 3 are for groundwater flooding, but within Tile C they show also flood incidents at Hawkley, Steep and to the south of Petersfield.
7.3.19 Flooding from Other Sources within Tile D Thames Water advised that 18 foul sewerage related flooding incidents and 7 combined sewerage flooding related incidents occurred in the GU 30 7 (Liphook) post code area in the last ten years. The actual locations were not advised by Thames Water.
Environment Agency records show six flooding incidents in East Liss and in the steep lanes below Rake.
7.3.20 Flooding from Other Sources within Tile E Flash flooding occurs under saturated conditions at Clanfield (EH025).
7.4 Planned Development Areas As noted in Section 3.5, East Hampshire District Council is currently working on a Core Strategy Development Plan Document, for which the issues and options stage is out for public consultation until 6th May 2008; it is currently available at http://www.easthants.gov.uk/ehdc/localplanweb.nsf/webpages/Core+Strategy. This includes the options the council is considering in relation to housing development; as yet the locations have not been decided but those under consideration are Alton, Petersfield, Whitehill/Bordon, Liphook, Four Marks/Medstead, Liss, Horndean, Clanfield and Rowlands Castle, along with the possibility of some development in smaller villages. As well as the houses themselves, it may be necessary for development to include other infrastructure, such as premises for business, retail, healthcare, education and leisure, in order to maintain sustainable communities.
As the exact locations for new developments are not yet known, Table 9 summarises the main flood related issues for each of the settlements mentioned above. More detailed information can be obtained from Tiles A to E once the site locations are known.
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Town Tile Types of flood risk*
Alton A The floodplain of the River Wey and its tributaries the Caker Stream and Lavant Stream run through the town, and historical flooding has occurred at the northern end. Flooding is known to have occurred along the Lavant Stream as a result of groundwater and overtopping of the river (EH002).
Defences are present for the downstream reach of the River Wey but the estimated standard of protection is only 5 years.
As noted in Section 7.2.2 there are potential problems of worsened flooding due to culvert blockage. To the north of Alton there are steep hills with dry valleys which suggest erosion due to water (see Tile A). These could potentially be ephemeral stream lines which would constitute a potential localised flooding risk requiring further detailed site specific investigation.
Petersfield C, D The floodplain of the River Rother and tributaries run through several areas of the town, and several incidents are recorded within these areas. Further sewer-related incidents have occurred outside the floodplain suggesting there may be difficulties with the capacity of the sewer system. The groundwater emergence maps suggest a risk of groundwater flooding, which may also have been an influence in the incidents recorded.
Whitehill/Bordon B Fluvial floodplains run along the west and east borders of Bordon and Whitehill – development so far has largely allowed space for this so further development should avoid encroaching on the floodplain. There have been some incidents of groundwater flooding (GW03, GW04 and GW06) and the groundwater emergence maps indicate a risk of groundwater flooding.
Liphook D The northeast border of Liphook is bounded by the floodplain of the River Wey and the floodplain of the Deadwater approaches quite close to the town at points. There may have been incidents of flooding from the sewer system and the groundwater emergence maps suggest a risk of groundwater flooding.
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Four Marks/ A No flooding issues are known for Four Marks or Meadstead; however, as Meadstead they are in a chalk area there is a risk of groundwater flooding.
Liss D The floodplain of the River Rother runs through the town, near the railway line, which may constrain development to the west of the town. Furthermore, incidents of localised flooding have occurred in the West Liss area relating to poor surface drainage when ground conditions are saturated (EH007, EH008 and EH027). An incident of sewer-related flooding has also occurred in the east of the town (SOW656). The groundwater emergence maps indicate a risk of groundwater flooding in Liss.
Horndean E Floodplains of tributaries of the Lavant Stream and River Wallington run through the town, although the latter is indicated as only Flood Zone 2. There are numerous examples of sewer-related flooding. The groundwater emergence maps suggest there might be a possibility of groundwater flooding although the zone is not as extensive as elsewhere, possibly because this is the edge of the chalk region and other geology may be intruding.
Clanfield E The floodplain of the Lavant Stream tributary extends through Clanfield and that of the River Wallington tributary stops just short of the town. However, flooding is known to have occurred even further north along the roads through which the floodplains run, as a result of rainfall when ground conditions are saturated. Sewer-related incidents have also occurred (SOW692 and SOW687) within the localised flooding area, so their causes are likely to be related.
Rowlands Castle E The floodplain of the Lavant Stream runs through the town and historical incidents have occurred (EH016, EH017, EH019, EH020 and EH021), both fluvially and groundwater related, along with one sewer- related incident (SOW680).
Table 9: Flood Risk Within Potential Development Areas. *see Chapter 7 and Tables 2 to 6 for further details.
The development will have a further effect on neighbouring watercourses as the additional properties will cause an increase in foul water, after treatment, being
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discharged from the water treatment plants into the rivers. Discharge from Rowlands Castle is likely to be into the Lavant Stream where it runs over low permeability soil and thus major development work could result in significant increase in flow downstream of the discharge. Most of the other settlements are likely to discharge into rivers where they run over relatively permeable geology (see Figure 3) so the affect of the additional houses may, apart from immediately after discharge, be less significant if water is able to infiltrate into the underlying chalk as it travels downstream. In rivers with low flow the additional discharge may be beneficial in maintaining water levels, particularly in drought periods, although if the percentage of effluent is too high then water quality may be adversely affected. In rivers where flow is close to channel capacity, the additional water may pose a flood risk.
7.5 Effect of Development on Flood Risk Elsewhere In addition to consideration of flood risk within the development site, it is important to consider the effect on the surrounding area. Where development takes place on an area subject to flooding, it may change the pattern of flood risk. In particular additional built-up area may reduce the space for floodwater causing it to be diverted elsewhere. Development planning should include measures to avoid this, using techniques such as providing alternative floodplain storage or use of SUDS (see Chapter 11).
Furthermore, it is not sufficient to assume that locating development away from Flood Zones 2 and 3 and localised flooding areas, and using sustainable drainage systems (SUDS) will automatically render flood risk to third parties adequately low irrespective of location. A situation may arise in which there is no spare capacity at an outfall (for example a surface water drainage system located a few kilometres downstream from the proposed allocation). The approach could be to produce a specific policy in which development will not take place until the downstream surface water drainage system is upgraded (unless an alternative outfall is identified and subject to approval by the planning authority and the Environment Agency). It is important to note that a local upgrade in channel or pipe capacity may increase flood risk downstream which may result in the need for storage or wetland areas to attenuate flows. All surface water drainage systems need to consider the requirements in the document ‘Rainfall runoff management for developments – Interim national procedure’ including controlling the peak rates, the additional volume, and exceedance (see Appendix E).
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The study of flood risk in this chapter leads to the conclusion that careful investigation of local flood risk (with a detailed investigation of flood incident records, management and maintenance issues) is required at most locations in the SFRA area before development is allocated. A general policy for localised flooding issues could be as follows: ‘No development will be allowed unless it is demonstrated that: a) dry access and egress is provided (see Section 10.9), b) the receiving watercourse has sufficient capacity and c) flood risk is reduced where possible, or at least not increased, in the development and in surrounding areas’.
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8 Catchment Flood Management Plans, Flood Management Measures and Flood Warning Systems
8.1 Introduction SFRA reports are ‘living documents’ which should be updated when Environment Agency flood zones and other relevant documents (for example Catchment Flood Management Plans (CFMPs), Strategies, Flood Warning Systems) are updated. It ensures a consistent and integrated approach to flood risk management.
8.2 Catchment Flood Management Plans A Catchment Flood Management Plan (CFMP) is a high-level strategic planning tool used to identify key factors contributing to flood risk within a river catchment, such as how the land is used, and recommend the best ways of managing flooding over the next 50 to 100 years to ensure sustainable policies and an integrated approach to land use planning and management. There are four main stages in preparing a CFMP and a report is prepared for each stage. The first stage leads to the Inception Report, where a summary of relevant flood risk issues and readily available catchment data is produced. The Scoping Report builds on this data to present a fuller understanding of the catchment, and sets out possible future scenarios to be assessed at the next stage. The Draft CFMP extends this to identify policy options and develop a plan for actions on the preferred policy and monitoring of its effect. The Final CFMP updates the draft version based on the final consultation responses. Both the Scoping and Draft Report are issued for full public consultation, and following completion, the CFMP is formally reviewed on a periodic basis - generally every six years.
East Hampshire District falls within three CFMP catchments; the Thames (for the River Wey), the South East Hampshire (Meon River and Lavant Stream) and the Arun & Western Streams (River Rother) catchments. The first two have currently reached the Draft CFMP stage and are available for consultation at http://www.environment- agency.gov.uk/regions/thames/323150/335688/1687000/1687502/?version=1&l ang=_e (Thames) and http://www.environment- agency.gov.uk/regions/southern/290158/954666/1005511/?version=1&lang=_e (South East Hampshire). The Arun & Western Streams has yet to produce a
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Scoping Report, so in the absence of policies for the River Rother, it is recommended that those for the River Wey be applied.
Within the South East Hampshire Draft CFMP, the relevant policies are the Chalk Catchment Policy Unit for the Upper Meon and the Havant Policy Unit for the Lavant Stream, the latter of which is an example of a ‘Developed floodplain with typically concrete river channels’.
For Developed Floodplain with Typically Concrete River Channels, the policy unit states that: • Where possible, a natural floodplain should be created to allow natural flooding. • Planning for development and infill needs to consider measures to reduce flood risk and reduce the impacts of residual flood risk such as making building resilient to flooding. • For the drainage network to work properly blockages need to be prevented if possible and cleared regularly. • Organisations need to work together to manage flooding from all sources; fluvial, surface water and sewer flooding.
The following actions are recommended: • Develop a collaborative Integrated Urban Drainage Plan to address current and future pressures of the drainage network. • Encourage local planning authorities to apply PPS25, avoiding inappropriate development in the floodplain, and influence Local Development Frameworks to effectively manage flood risk. • Raise awareness of the impacts of blocked drainage pathways from vandalism and build up of obstructions in the watercourses. • Improve flood warning on the Hermitage and Lavant Streams by seeking to expand the service, reducing lead-in times and developing better predictive tools. • Decommission Warren Dam. • Hermitage and Lavant Streams River Restoration Project. Also seek out funding and partnership opportunities to consider options to open up river corridors through Havant. The focus should be on creating natural flow paths and floodplain storage that will offset the flood risk impacts from climate change.
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The actions from this policy will serve to improve the current situation and address the known locations where surface water flooding has occurred in the past. Measures will be taken to ensure the increased storminess due to climate change does not significantly increase flood risk in the future. New development in the unit will need to demonstrate that it will not increase the risk of flooding. Some surface drainage flooding is still likely to occur due to the unpredictable nature of the causes and locations of flooding.
For Areas Underlain by Chalk, the policy unit states that: • Flooding does not necessarily occur in connection to the river or fluvial floodplain. Responses are required for specific locations and actions may be required by individual property owners. • Understanding of the pathway of groundwater flooding, and keeping the pathway clear is a key aspect of flood risk management in areas at risk from groundwater flooding.
The following actions are recommended: • Raise awareness of groundwater flooding and promote flood-proofing schemes where appropriate. To include advice concerning development control. • Awareness needs to be improved through mapping of potential groundwater emergence zones and to better understand the effects of climate change on this type of flooding and the hydrology of permeable catchments. • Develop a collaborative Integrated Urban Drainage Plan to address current and future pressures of the drainage network in key villages, such as Hambledon, Wickham, Exton, Finchdean, Rowlands Castle. This should include maintaining clear drainage pathways. • Develop a Groundwater Flood Warning Plan to improve the levels of service across the Rural Chalk Policy Unit by expanding coverage, improving predictive tools, and enhancing the existing network of observation boreholes from which ‘trigger’ levels can be monitored. As part of the Flood Warning Plan consider how present and future liaison with local authorities/ parish councils will operate to ensure continued use of flood wardens and Village Action Plans, plus possible additional methods of disseminating flood warnings in the future. • Encourage local planning authorities to apply PPS25, avoiding inappropriate development in the floodplain, and influence Local Development
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Frameworks to effectively manage flood risk. This should include preventing development in known groundwater flooding areas that may not be covered by the flood zone maps. • Develop Land Management Plan to explore the potential for changes in land use and land management practices within the policy unit. • Improve data mapping information by undertaking S105 modelling, concentrating on Wickham, Corhampton, Meonstoke, Warnford, Exton, West Meon, East Meon, and Frogmore. • Implement high priority Flood Alleviation Schemes such as Hambledon and Wickham. • Implement medium priority Flood Alleviation Schemes such as Exton, Finchdean and Rowlands Castle.
The actions from this policy will help ensure that, through a proactive awareness raising campaign property owners will understand the mechanisms by which groundwater flooding occurs and will be able to take a role in reducing flood risk from groundwater flooding. Measures will be taken to ensure that emergent groundwater is able to pass downstream with minimal obstruction. Individual property owners will be able to take a role in reducing flood risk to their property.
The portion of the River Wey within East Hampshire is covered by the Rural Wey Policy Unit (see Appendix D) from the Thames CFMP, which is characterised by extensive and predominantly undeveloped floodplain. The actions proposed in this policy unit are to: • Maintain the capacity and function of the undeveloped natural floodplain to retain water so that it can continue to reduce the impact of low order flood events to people and property. • Seek to enhance the capacity of the undeveloped natural floodplain. Recognising that this will require structural measures, this is more likely to be achievable upstream of sizeable communities at risk from flooding where the social, economic benefits are more clear-cut. • Align the objective of maintaining or enhancing floodplain capacity with expansion and enhancement of floodplain environments, particularly BAP habitat. • Continue to reduce the impact of low order flooding in urban areas (up to a 10% to 20% AEP flood – 1 in 10 to 1 in 5 year return period) by maintaining conveyance where it is both effective and sustainable to do so.
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• Reduce the consequences of flooding through continued action to raise public awareness of flooding, tailoring the advice and approach (e.g. community based) to ensure those ‘at risk’ take appropriate action to respond to flooding. • Safeguard the existing undeveloped natural floodplain through the appropriate application of the Sequential Test within PPS25. • Maintain, or in some cases re-establish, river corridors so that urban areas can better accommodate flooding (location and layout) and the buildings are more resilient to flooding (design). In the long-term this should be achievable through redevelopment. It must be recognised that this is a long-term objective. • Develop and establish a ‘water exclusion strategy’ – where emphasis is placed on minimising water entry whilst maintaining structural integrity, and on using materials and construction techniques to facilitate drying and cleaning. (This strategy is favoured when low floodwater depths are involved (not more than 0.3m) and occupation of the floodplain is deemed necessary and sustainable. • In those locations where the level of flood risk merits direct intervention, progress those options that are the most effective and sustainable long-term. This could be to manage the probability of flooding (for example through defences), or more likely to manage the consequences (for example through resilience). Option selection should be based on what is most effective and sustainable and not short-term factors (for example, the ease of capital funding streams).
These actions are in accordance with the ‘Making Space for Water’ principles, and the policy highlights the floodplain as the most important asset in managing floodplain. The purpose of the actions for this policy are to maximise the capacity of the floodplain to retain water in these areas in order to deliver benefits locally or elsewhere for people and the natural environment. As well as serving to create an overall reduction of flood risk, managed flooding of some areas of the natural floodplain can have additional benefits such as habitat inundation.
The main messages from the policy units can be summarised as follows:
Main Messages The floodplain is one of the most important measures against flood risk, and should be protected and, where possible, increased. Collaboration between key stakeholders should be encouraged to ensure integrated urban drainage and adequate maintenance of watercourses and drainage systems. Public
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awareness of groundwater flooding needs to be raised, and uptake of flood warning services encouraged.
SFRA Local authorities should be encouraged to follow PPS25 Implementation principles to ensure that flood risk is taken into account at all stages in the planning process to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas at highest risk. There may be exceptional circumstances where new development is necessary. In such areas, policy aims to make it safe without increasing the risk elsewhere and, where possible, to reduce the overall flood risk. Structural works may be required to reduce flood hazard to within acceptable limits at Level 2 SFRA locations.
It is important to note that CFMP policies consider a 100 year horizon and SFRAs should consider how to implement these in the short, medium and long term.
8.3 Flood Warning Systems and Future Flood Risk Management Schemes In England and Wales the Environment Agency operates a flood warning service in areas at risk of flooding from rivers or the sea. Using the latest available technology, Agency staff monitor rainfall, river levels and sea conditions 24 hours a day and use this information to forecast the possibility of flooding. If flooding is forecast, warnings are issued using a set of four easily recognisable codes. (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).
Each of the four codes indicates the level of danger associated with the warning. The codes are not always used in sequence; for example in the case of a flash flood a Severe Flood Warning may be issued immediately, with no other warning preceding it. • A Flood Watch would be issued when water levels along the river are forecast to overtop the banks. Flood watches are generally issued for the whole of the river catchment. • A Flood Warning is issued when the Environment Agency anticipates flooding to property. Flood Warnings are issued for a specific flood warning area within a catchment.
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• The trigger for issue of a Severe Flood Warning 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 warning area. • The All Clear is issued once Flood Watches or Warnings are no longer in force for the area.
The Environment Agency aims to give a two-hour warning in advance of any flooding taking place. However in certain cases this may not always be possible.
All warnings are highly dependent on the Environment Agency’s ability to forecast. They have a comprehensive raingauge network and have direct access to Met Office radar products which show rainfall intensities and amounts.
As part of an improvements programme, the Environment Agency will be improving the flood warning service within the study area through the development of community based Flood Warning Areas. These have recently been prepared for the Upper Wey area. The Environment Agency is also monitoring an observation borehole in Farringdon which may in the future help to provide advanced warning of rising groundwater.
8.4 Defences and Future Flood Risk Management Schemes The Environment Agency maintains a database of informal and formal defences (see Table 10). This records information such as the standard of protection (which is a measure of the magnitude of flooding the defence can offer protection against), type of ownership and the condition of the defences. An understanding of these factors is important for understanding the potential residual risk (overtopping for large events, breaching or blockage) when considering locating sites in areas behind defences. Level 2 SFRAs and individual flood risk assessments need to consider this and use the SFRA information as a starting point. An important point to note in regard to the ‘design standard’ (Flooding return period, in years) quoted for the defences in Table 10, is that it is calculated based on current climate conditions. As more severe flooding becomes more frequent, with increased river flows and rising sea levels under the impacts of climate change, the design standard will fall.
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Flood Risk Management Schemes encompass a variety of approaches to defending a settlement, or part of a settlement, from flooding and range from works such as flood walls and embankments, channel diversions and enlargements, addition or removal of structures such as culverts or weirs, hard and soft sea defences, flood storage areas, pumping stations, channel maintenance such as dredging or vegetation clearance, environmental improvements such as wetland restoration, and changes in land management practices to reduce storm runoff. In the case of schemes such as flood storage, wetland areas or land management restrictions, since this land has been specifically designated to aid flood defence, it should be considered as a spatial constraint to development proposals. Structures, such as culverts, may also act as a constraint to future development since access will need to be maintained.
Currently there are no areas within East Hampshire that are potentially allocated for Flood Risk Management Schemes by the Environment Agency or others.
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9 Guidance on the Application of the Sequential Test
9.1 Introduction As described in Chapter 5, the application of the Sequential Test can be summarised as follows: • to look for sites in areas at least risk from flooding (Flood Zone 1), only making allocations in Flood Zones 2 or 3 if there are no alternatives, • within higher risk flood zones 2 and 3, give preference in the order Flood Zone 2, then Flood Zone 3a, and finally Flood Zone 3b only as a last resort, and • for sites in Flood Zones 2 and 3, unless the vulnerability category of the land use within the development (see Section 5.3) is suitable, the Exception Test should be applied as set out in PPS25.
At all stages of the planning process, the impacts of climate change should be taken into account.
Figure 3.1 from the Practice Guide Companion to PPS25 provides a flow chart describing the Sequential Test and is reproduced in Figure 4.
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Figure 4: Application of the Sequential Test (from the Practice Guide Companion to PPS25, figure 3.1).
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The following steps provide additional guidance to that from PPS25 and its Practice Guide, and have been produced in consultation with the Environment Agency. They provide details on how to take account of other sources of flooding (and not just the flood zones) during the application of the Sequential Test, and as part of a Sustainability Appraisal.
9.2 First Step – Strategic Overview of Flood Risk for all Potential Areas The recommended initial step is to determine broad extents of potential land allocations in large scale maps showing the most up-to-date flood zones (for example the SFRA Flood Maps -Tiles A to E). Summary tables of flood risk issues are then prepared for each location, indicating if the potential areas overlap Flood Zones 2, 3 or localised flooding areas, or if there are records of previous flood incidents shown in the maps. As mentioned in Section 7.5, particular care should be taken by identifying allocations that could increase flood risk elsewhere (for example upstream of areas with limited drainage capacity) and sites which, whilst not subject to flooding themselves would suffer a lack of dry access due to surrounding flooding.
9.3 Second Step – Analysis of Flood Risk Issues The next step begins the Sequential Test by analysing all potential sites within Flood Zone 1. Whilst locations in Flood Zone 1 are at low risk of flooding from rivers or the sea, they may still be subject to other sources of flooding. Sites with flood risk issues, including those that do not have dry access routes during flood events, should be identified and for these sites an assessment of likely significance of flood risk is then carried out in terms of likely probability of flooding and potential consequences/flood damages. (Advice from a drainage specialist may be required, such as the SFRA consultant, the Environment Agency, a highways drainage engineer and/or the planning authority drainage specialist.) The purpose is to identify sites with significant flood risk including a high probability of flooding, with deep water and high velocities causing significant flood damages which could result in loss of property and potentially loss of life.
Ideally the land uses most vulnerable to flood risk should be located in Flood Zone 1. However if a site within Flood Zone 1 is identified as having significant risk of localised flooding then it would now be considered as if it was in the High Probability Zone 3a for further application of the Sequential Test (see Section 9.6). Therefore if a more vulnerable land use is required for the site, it will have to pass the Exception Test (see PPS25 Flood Risk Vulnerability and Flood Zone Compatibility table in the tiles).
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9.4 Third Step – Apply the PPS25 Flood Risk Management Measures Where sites in Flood Zone 1 overlap into a higher risk flood zone, it is recommended that where possible, the following actions take place prior to the application of the Sequential Test in Flood Zones 2 and 3: a) Apply the measure of avoidance/prevention (see Section 5.6) by moving the boundaries of the potential site allocations away from Flood Zones 2, 3a and 3b, for those cases where the loss of part of the site area is acceptable. b) Within the portions inside Flood Zones 2 and 3, provisionally adopt land uses that are fully compatible with the vulnerability classification of PPS25, to try to avoid the need to apply the Exception Test. This may involve rearrangement of the site layout or substitution of different land uses as appropriate. c) The new development may be an opportunity to carry out measures to reduce flood risk by including work to control flood frequency, for example, open up culverts, provide additional storage, or facilitate habitat improvement. This will not negate the need for the Exception Test but can assist in demonstrating that the development will be safe, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall. d) Mitigation measures, such as flood resilient building design, may also be applied.
It should be noted that the data in the SFRA should be regarded as initial guidance for the Sequential Test. However, as the flood data may be based on relatively course assessment (e.g. JFlow modelling or historical flooding where only approximate extents are known) then a detailed site assessment, such as a Level 2 SFRA or FRA, may indicate different flood extents.
9.5 Fourth Step – Apply the Sequential Test in Flood Zone 2 Having exhausted the options in Flood Zone 1, the fourth step is to repeat the procedures in steps 2 and 3 for sites in Flood Zone 2. If any of the sites are required for more vulnerable land uses, the Exception Test will need to be applied to determine if the need for the development outweighs the flood risk.
9.6 Fifth Step – Apply the Sequential Test in Flood Zone 3 The fifth step is to apply the Sequential Test in Flood Zone 3, and where required the Exception Test in accordance with PPS25. This applies to all potential sites that fall within Flood Zone 3 as well as those that encroach or are located within a 100m radius from a high risk ‘localised flooding area’ in Flood Zones 1 and 2.
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10 Guidance for the Preparation of Flood Risk Assessments
10.1 Introduction A SFRA is a strategic document that provides an overview of flood risk throughout a study area. Flood Risk Assessments will be required for most proposed developments and the level of detail will depend on the existing level of flood risk in the site (see general FRA requirements for each flood zone in PPS25 Annex D tables and Annex E. Further guidance can be found in the Practice Guide Companion to PPS25).
For those sites within localised flooding areas or with flood incident records where flood risk issues are not significant (for example shallow flooding and non- frequent blockages, etc), development should still be acceptable provided that adequate policies are in place for mitigating the risk. Options range from using on site water balancing and other SUDS solutions, and may include contributions from the developer for the upgrade of the surface water system, if feasible.
It is imperative that site-based Flood Risk Assessments (FRAs) should be discussed early in the planning process and submitted as an integral part of the planning application. It is now a government directive that planning applications seeking approval for development within flood affected areas can be regarded as invalid if not supported by a Flood Risk Assessment. The following section reflects best practice on what should be addressed within a FRA:
10.2 Proposed Developments within Flood Zone 3a and 2 All FRAs supporting proposed development within High Probability Flood Zone 3a and 2 (as the existing Flood Zone 2 could become a high risk zone in the future due to the effects of climate change) should include an assessment of the following: • The vulnerability of the development to flooding from other sources (for example surface water drainage, groundwater, etc) as well as from river/tidal flooding. This will involve discussion with the planning authority and the Environment Agency to confirm whether a localised risk of flooding exists at the proposed site. The localised sources of flooding identified within this SFRA should provide a good starting point for discussion.
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• The vulnerability of the development to flooding over the lifetime of the development (including the potential impacts of climate change), for example maximum water levels, flow paths and flood extents within the property and surrounding area. The Environment Agency may have carried out detailed flood risk mapping within localised areas that could be used to underpin this assessment. Where available, this will be provided at a cost to the developer. Where detailed modelling is not available, hydraulic modelling by suitably qualified specialists will be required to determine the risk of flooding to the site. • The potential of the development to increase flood risk elsewhere through the addition of hard surfaces, the effect of the new development on surface water runoff, and the effect of the new development on depth and speed of flooding to adjacent and surrounding property. This will require a detailed assessment, to be carried out by suitably qualified specialists. The use of SUDS techniques can help mitigate the risks posed by the new development. • A demonstration that residual risks of flooding (after existing and proposed flood management and mitigation measures are taken into account) are acceptable. Measures may include flood defences, flood resistant and resilient design, escape/evacuation, effective flood warning and emergency planning. • Details of existing site levels, proposed site levels and proposed ground floor levels. All levels should be stated relevant to Ordnance Datum.
It is stressed that all forms of flooding need to be considered and not just that from fluvial sources. Localised flooding is typically associated with local catchment runoff following intense rainfall and must be considered as an integral part of the detailed Flood Risk Assessment.
It is essential that developers thoroughly review the existing and future structural integrity of formal and informal defences, if present, upon which the development will rely (over the lifetime of the development), and ensure that emergency planning measures are in place to minimise risk to life in the unlikely event of overtopping or defence failure.
10.3 Proposed Development within Flood Zone 1 For all sites within low probability Flood Zone 1, unless the planning authority and the Environment Agency suggest otherwise, a FRA should be prepared, proportionate to the nature and scale of the development, based upon readily available existing flooding information (sourced from the Environment Agency,
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the local planning authority and information contained in this SFRA). PPS25 recommends that an FRA is carried out in Flood Zone 1 for development areas of 1 hectare or more. The PPS25 recommendation has been extended in this SFRA to development areas less than 1 hectare, at least within the urban areas of the borough, due to the extensive flood risk issues within the study area. However, it should be noted that the Environment Agency will not have the resource to provide detailed comments on surface water FRAs in Flood Zone 1 for sites under 1 hectare, although comments are provided in the form of standing advice.
In addition to flooding from the river and the sea, the FRA should consider the vulnerability to flooding from other sources. For those sites with localised flooding issues such as shallow flooding and non-frequent blockages etc, development should still be acceptable provided that adequate policies are in place for mitigating the risk. Options range from using on site water balancing and other SUDS solutions, and may include contributions from the developer for the upgrade of the surface water system, if feasible.
An assessment should be made of the potential of the development to increase flood risk elsewhere through the addition of hard surfaces, the effect of the new development on surface water runoff, and the effect of the new development on depth and speed of flooding to adjacent and surrounding property. This should be considered for the lifetime of the development, including an allowance for the effects of climate change, as well as suitable provisions for future likely minor development such as paving of front gardens or minor extensions.
It is recommended that sustainable drainage systems are employed even where localised flooding does not occur, to ensure no worsening of flood risk on site or elsewhere within the area. The FRA should demonstrate no increase in: • The peak rate of stormwater runoff leaving the site; • The volume of runoff leaving the site; • The pollution load to receiving waters from stormwater runoff by following the SUDS ‘management train’.
As assessment of overland flows and temporary flood storage across the site will also need to be undertaken. Further guidance can be found in Chapter 11 of this SFRA.
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10.4 Proposed Development within Groundwater Emergence Zones Appendix D contains maps showing predicted Groundwater Emergence Zones, and these should be consulted in regard to risk of groundwater flooding. In some cases, the SFRA Flood maps may show evidence of historical events of groundwater flooding, but this may not be the case for all groundwater emergence zones. This may be due the topography of the area such as a local impermeable layer protecting the surface from the underlying chalk. However, it may simply be due to a lack of recording of groundwater incidents, as people can be reluctant to acknowledge this due to the effect on insurance or home value, or it may be an undeveloped area where groundwater flooding has occurred but not been noticed. Furthermore, even if groundwater flooding was prevented by an impermeable layer, the excavation work during development may breach this layer, especially if basements are included in the properties. If groundwater flooding seems likely to be an issue, then there is a need for further site specific assessment proportionate to the nature and scale of development.
10.5 Risk Management by Design Risk management by design should only be used after having followed a sequential approach to the location of development.
10.6 Raised Floor Levels To ensure damage to property is minimised, floor levels should be raised above flood level by a specified amount known as the ‘freeboard’. The current recommended flood level is the 1% annual probability peak flood level, including an appropriate allowance for the predicted effects of climate change over the lifetime of the development (see Table 8). Wherever possible, floor levels should be situated a minimum of 300 mm above the 1% with climate change flood level, determined as an outcome of the site-based FRA. If no climate change data is available then a precautionary freeboard of 600mm above the 1% annual probability floodwater level should be used. In the case of there being no 1% annual probability floodwater level available then for minor development where it is not reasonable to undertake detailed flood modelling, a conservative assumption should be made to determine the appropriate floodwater level and a suitably conservative freeboard should also be applied.
For some situations it may not be possible to raise floor levels due to other constraints, and in some instances therefore, it may be appropriate to consider flood resilient construction. Further guidance can be found in the Practice Guide
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Companion to PPS25 and the Communities and Local Government document ‘Improving the flood performance of new buildings’.
It is highlighted that many of those areas currently situated within Medium Probability Flood Zone 2 could, under the impacts of climate change, become part of the High Probability Flood Zone 3. This is important as it means that properties that are today at relatively low risk are likely to be, in 20 to 100 years, within High Probability Flood Zone 3a (see precautionary assumption C2 in Section 6.4.4). It is imperative therefore that planning and development control decisions take due consideration of the potential risk of flooding in future years. For this reason development in Flood Zone 2 should apply the same approach to finished floor levels and flood resilience as mentioned above.
10.7 The effect of development on floodwater flow conveyance and loss of floodplain storage Any additional built development footprint within a flood risk area will need to ensure it does not impact upon the ability of the floodplain to store water or impact upon floodwater flow conveyance. The potential impact on floodwater storage and flow should be considered as part of the sequential approach within a site. If after having followed this approach, the development will still impact upon floodplain storage, then ‘level for level’ floodplain storage compensation will need to be provided to balance the loss. The use of under-floor stilts or voids alone (without floodplain storage compensation) to justify additional built development in a flood risk area is not an acceptable solution. These design solutions are methods of flood resistant construction. Experience indicates that problems do occur from the inability to ensure that the voids beneath the building are not obstructed by domestic effects, flood debris etc, therefore reducing or removing any mitigatory effect they provide.
Once the above mentioned issues have been addressed, where feasible, buildings located in flood risk areas should also incorporate an under-floor void space and openings. These will help provide flood risk reduction whilst they remain operational (i.e. following the CFMP approach in improving the ability of the floodplain to behave in a natural manner).
A condition should be included in any granted application to ensure that the void spaces and openings should remain clear for the lifetime of the development. It is also recommended that a Section 106 agreement or unilateral agreement is
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undertaken to further increase confidence in the voids and openings remaining clear.
10.8 Basements The construction of basements within flood affected areas (fluvial and other types of flooding) should be discouraged. Basement dwellings are highly vulnerable and should not be permitted within Flood Zone 3 and have to pass the Exception Test, based on a level 2 SFRA, within Flood Zone 2. Where basement use is considered acceptable, however, it is necessary to ensure that the basement access points are situated 300 mm above the 1% probability plus climate change flood level. The basement must have unimpeded access and be of waterproof construction to avoid seepage during flooding conditions. Habitable uses of basements constitute a ‘highly vulnerable’ land use and therefore should not be permitted within Flood Zones 3a and 3b, and will be subject to the Exception Test in Flood Zone 2 (see tables in PPS25 Annex D).
Groundwater should also be considered when constructing basements, since they will be particularly susceptible to flooding from this source. If the water table is near to the surface, the basement may intercept flow resulting in groundwater damming up outside the walls, where the build-up in pressure can force water through cracks into the building causing seepage, and may undermine the structural integrity of the building. Where basements are constructed, they should have adequate lining, drainage and ventilation. It should be noted that similar problems can be caused by surface water runoff as water can be trapped by the building and cause excessive infiltration into the ground nearby; this is most particularly a problem for buildings located where the land slopes down towards them, as significant amounts of water may be collected. Where buildings are located in the flow path of surface water runoff, measures should be taken to ensure that water can be routed around the building, and it may be advisable to cut a swale into the hillslope to intercept the water and prevent it from collecting at the building walls.
10.9 Safe Access and Egress As the areas affected by flooding in East Hampshire are not generally expansive in size, the Environment Agency recommendations in regard to safe access are: • Safe access and egress should be from the individual buildings to an area wholly outside of the floodplain using public rights of way. Vehicular access should also be provided for emergency services. • ‘Safe’ access is dry for ‘more’ and ‘highly vulnerable’ uses.
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• Dry escape for residential dwellings should be up to the 1 in 100 year event taking climate change into account. • ‘Safe’ should preferably be dry for other uses such as educational establishments, hotels and ‘less vulnerable’ land use classifications.
Where SUDS devices involving open water, such as ponds and swales, are constructed, these should be placed so that they do not impede safe access.
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11 Guidance for the Application of Sustainable Drainage Systems
11.1 Introduction PPS1: Delivering Sustainable Development and PPS25 require that LPAs should promote Sustainable Drainage Systems (SUDS). LPAs should ensure policies encourage sustainable drainage practices in their Local Development Documents.
SUDS is a term which encompasses the use of a variety of drainage elements for managing surface water in a way which is more sympathetic to the natural and human environment than conventional below-ground drainage systems. Management of surface water is an essential element for reducing flood risk and SUDS techniques are often designed to achieve this in a way that mimics the natural environment. Indeed, reducing the rate of discharge from urban sites to near-Greenfield (ideally a minimum of 5% below the pre-development) runoff rates is one of the most effective ways of reducing and managing flood risk and accords with the approach adopted by the Environment Agency. For further information on methods used for the estimation of green and impermeable/paved area runoff, and the management of rainfall runoff in developments, see DEFRA/Environment Agency R&D Technical Report W5-074/A/TR/1 Revision D “Preliminary rainfall runoff management for developments” (July 2007). This also offers guidance on calculating the amount of rainfall expected at a site, since this will determine how much storage is needed.
11.2 Types of SUDS SUDS may improve the sustainable management of water for a site by: • controlling or reducing peak flows to watercourses or sewers and potentially reducing the risk of flooding downstream; • reducing volumes of water flowing directly to watercourses or sewers from developed sites; • improving water quality, compared with conventional surface water sewers, by removing pollutants from diffuse pollutant sources; • reducing potable water demand through rainwater harvesting; • improving amenity through the provision of aesthetic elements and varied habitat within the public open space;
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• replicating natural drainage patterns, including the recharge of groundwater so that base flows are maintained.
To meet the requirements of PPS25 to reduce flood risk: • development should comply with the document ‘Rainfall runoff management for developments – interim national procedure’ included in Appendix E. • there needs to be a reduction in the current instantaneous runoff rates. Development on Greenfield sites will be expected to mimic the existing Greenfield drainage conditions. Development on Brownfield sites will be expected to make a positive reduction to revert the site back to Greenfield conditions unless for exceptional reasons agreed with East Hampshire District Council and the Environment Agency it is not possible or reasonable to achieve this. In such instances a positive reduction will still be required however the amount will need to be agreed based upon what is possible and reasonable; • there should preferably be a reduction in the volume of runoff by the use of infiltration in accordance with Building Regulations Part H3.
If this is applied across a catchment, the cumulative effect, particularly in the future as the impacts of climate change increase, from a number of sites could be significant.
The most commonly found surface elements of a SUDS system are described below:
Pervious surfaces:- Surfaces that allow infiltration of rainwater into the underlying soil or construction, such as porous surfacing (e.g. gravel), permeable hard surfacing, permeable block paving, porous tarmac and porous concrete. The storage can be created within the sub-base of these surfaces given careful selection of the stone fill or use of plastic box systems.
Green roofs:- Vegetated roofs that reduce the volume and rate of runoff and remove pollution. These also create habitat in urban areas and can improve air quality.
Filter drains:- Linear drains consisting of trenches filled with a permeable material, often with a perforated pipe in the base of the trench to assist drainage, to store and conduct water. They may also permit infiltration.
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Filter strips:- Vegetated areas of gently sloping ground designed to drain water evenly off impermeable areas and to filter out silt and other particulates.
Swales:- Shallow vegetated channels that conduct and can retain water in larger storm events. The vegetation filters out particulate matter in the flow thus providing treatment and improving water quality. They may also permit infiltration.
Basins:- Ponds and Wetlands Areas that may be utilised for surface runoff storage.
Infiltration Devices:- Structures to promote the infiltration of surface water into the ground. They can be trenches, basins or soakaways.
Bio-retention areas:- Vegetated areas designed to collect and treat water before discharge via a piped system or infiltration to the ground.
The most commonly found sub-surface elements of a SUDS system are described below. These are particularly useful where there is limited open space on the site. Some are the only options where sections of the drainage network are to be adopted. Plastic Box Storage:- Sub-surface storage structure that has a very high void ratio (proportion of empty space to solid fill) and thus occupies a reduced space compared to other options, e.g. stone-filled trenches. It is particularly useful where there is limited open space in the site, and it can also be used as a very effective infiltration device due to the very large areas in contact with the ground.
Large diameter pipes, culverts or tanks:- Provide a volume of below ground storage with a high void ratio and good man entry provision to allow for future maintenance and cleaning. These would generally be suitable for adoption in most circumstances.
Pipes and accessories:- A series of conduits and their accessories, normally laid underground, that convey surface water to a suitable location for treatment and/or disposal (these drainage elements should generally only be considered where at- surface SUDS techniques are not practicable, e.g. under a road crossing).
For more guidance on SUDS, the following documents and websites are recommended as a starting point: • C697 The SUDS Manual, Woods Ballard B; Kellagher R et al, 2007 – available from CIRIA bookshop www.ciria.org.uk
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• Interim Code of Practice for Sustainable Drainage Systems, National SUDS Working Group, 2004 – available from CIRIA bookshop www.ciria.org.uk or Environment Agency website www.environment-agency.gov.uk • Preliminary rainfall runoff management for developments, DEFRA/Environment Agency R&D Technical Report W5-074/A/TR/1 Revision D, July 2007, - Free download from Environment Agency website www.environment- agency.gov.uk • C625 Model Agreements for Sustainable Water Management Systems, Shaffer et al, 2004, – available from CIRIA bookshop www.ciria.org.uk • C539 Rainwater and greywater use in buildings – best practice guide, Leggett et al, 2001, – available from CIRIA bookshop www.ciria.org.uk • C582 Source control using constructed pervious surface: hydraulic, structural and water quality performance issues, Pratt et al, 2002, – available from CIRIA bookshop www.ciria.org.uk • C635 Designing for exceedance in urban drainage – good practice, Digman et al, 2006, – available from CIRIA bookshop www.ciria.org.uk • Report 156 Infiltration drainage – manual of good practice, Betess R, 1996, – available from CIRIA bookshop www.ciria.org.uk • Harvesting rainwater for domestic uses: an information guide, Environment Agency, 2003, - Free download from Environment Agency website www.environment-agency.gov.uk • Planning Policy Statement 25 (PPS25) Development and flood risk, Department for Communities and Local Government, 2006, - Free download from CLG web site http://www.communities.com • Development and flood risk: A Practice Guide Companion to PPS25, - Department for Communities and Local Government, 2006, - Free download from CLG web site http://www.communities.com
11.3 Effective Application of SUDS Techniques
A hierarchical approach is recommended for selection of SUDS techniques to dispose of surface runoff. This is illustrated by the SUDS Management Train (see Figure 5).
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Figure 5: SUDS Management Train (from the Environment Agency website).
The first stage, ‘Prevention’ stresses the benefit of avoiding runoff in the first place, and also refers to the need to prevent pollution. Prevention of runoff can be achieved by measures such as avoiding paving a surface or using acceptable alternatives such as gravel, which allows rainfall to soak directly into the ground. It may be possible to allow roof water to discharge straight onto a lawn in order to soak into the ground. These methods must consider the surfaces the rain falls onto in order to avoid pollution of the soil and groundwater. This includes ensuring minimal use of herbicides on lawns, secure storage of oils and chemicals to avoid leakage, dog litter policies etc. Green roofs are another useful technique for reducing runoff from roofs and also offer improvement of water quality.
If prevention methods are not sufficient to avoid runoff, the next preferred option is to store and dispose of it on site. This includes measures such as permeable paving, where water is stored below the surface in the storage space provided until it can soak into the ground. Rainwater harvesting is another valuable technique, where rainfall is collected and then used as a substitute for mains water for activities such as watering lawns or flushing toilets. This has the added benefit of reducing demand on public water supply, and reduces costs for the user of the rainwater (if they purchase water using a water meter). Where water cannot be directly infiltrated into the ground, it may be conveyed some distance before infiltration or, alternatively discharged into a watercourse. As the runoff is conveyed further, it moves from source control to site control and then regional control.
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Infiltration is preferred over disposal to a watercourse or the public sewer system as this more commonly deals with runoff nearer to source and serves to replenish groundwater. This recommendation is reinforced by the requirements of the Building Regulations Part H3. If infiltration is not viable (due to a high water table, local impermeable soils, contamination issues including source protection zones etc), then the next option of preference is for the runoff to be discharged into a nearby watercourse. Only if neither of these options is possible should the water be discharged into the public sewer system.
The main variables which affect the choice of a SUDS technique are:
a) catchment permeability; b) source protection zones; c) general slope of the ground and ground levels; d) current land use (Greenfield vs Brownfield sites); e) location of the development relative to other urban areas; f) type of land use proposed and layout of the development; g) presence of contaminated land; h) depth to permeable layer beneath a more impermeable layer; i) normal and maximum height of groundwater table; j) amount of rainfall expected; k) willingness of the water company to adopt part of the drainage network.
Figure 3: Distribution of Soil Permeability and Source Protection Zones gives an overview of infiltration potential across East Hampshire District. An indication of soil hydrological properties is illustrated as colour coded shading based on the estimate of the Standard Percentage Runoff from the Flood Estimation Handbook. With the exception of the soil area highlighted in green (which has a SPR of 49.6%) the soil areas are in principle sufficiently permeable to allow the infiltration of surface runoff.
However, whilst this map gives an indication of infiltration potential, a further site specific assessment should always be undertaken for a detailed determination of the potential for infiltration at a proposed development site. The soil properties shown may not be representative of the situation for a particular site at the local scale, for example it may have an atypical geology, or a local waterbody, not
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necessarily an aquifer, which requires protection.
Large increases in impermeable areas contribute to significant increases in surface runoff volumes and peak flows and could increase flood risk unless adequate SUDS techniques are implemented. This may even apply for developments which are at low risk of flooding themselves, such as sites in Flood Zone 1 without localised flooding, since they may still increase the risk of flooding elsewhere, particularly downstream of discharge.
A critical situation could be that of building a new large development just upstream of an existing development which already suffers from frequent flooding. In this case an effective SUDS technique could be to have large areas of pervious surfaces (pervious paving etc, where natural surfaces are not possible) combined with infiltration and rainfall harvesting techniques. The use of large attenuation areas may not be the appropriate SUDS technique, as whilst these reduce peak flows they do not affect flood volumes.
The protection of groundwater quality is a factor in determining how SUDS are implemented. It should be noted that where soakaways are proposed, their use will only be acceptable to the Environment Agency subject to the following conditions: • Soakaways shall not be constructed through contaminated material. • The depth of any soakaway should normally not exceed 2.0 meters and under no circumstances shall be permitted to intersect the water table. • A minimum of a 1.0 metre unsaturated zone shall be maintained between the base of any soakaway and the maximum seasonal water table for that site. A groundwater contour map is available from the EA. • Soakaways intended to drain highway or parking areas will usually require additional safeguards such as seal-trapped gullies or a suitably sized oil/grit separator. • Soakaways designed to receive clean roof water should be kept separate from those receiving surface water runoff from highway or parking areas. • The use of borehole soakaways will only be acceptable subject to written agreement from the EA.
It is important to note that it is an offence to discharge list 1 substances (petroleum, hydrocarbons etc) to groundwater. Of greatest concern would be if a
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list 1 substance was to be discharged directly into groundwater. This may affect the suitability of individual SUDS designs. In particular this may restrict the use of infiltration from swales for the drainage of car parks, in some areas where groundwater is likely to occur at shallow depth.
Other factors are also important in selecting a specific SUDS device. For example the amount of space available for the development will affect the choice of technique as some, such as swales and ponds, take up more space than others. A couple of important points to bear in mind with respect to infiltration is that the water may affect ground stability and thus pose a risk to nearby buildings, and in periods of prolonged rainfall the soil may become saturated so that the infiltration devices will cease to work. The land use in the site and surrounding area will affect the type of pollution that occurs, and different SUDS techniques are better at dealing with different types of pollution. A further factor to consider is that SUDS techniques often require maintenance in order to remain effective so it is necessary to ensure that a suitable authority agrees to take this on.
Whilst the above chapter gives an overview of different types of SUDS, there are a wide variety of SUDS available, which use different materials, designs or combinations. SUDS methods enable a very flexible approach to drainage with benefits to managing water quantity and quality, and enhancing the environment.
SUDS techniques will be required for most, if not all, proposed land allocations. Attenuation to below existing (pre-development) discharge rates, whilst allowing for the effects of climate change, should be the norm, in particular at Brownfield sites. There are numerous different ways that SUDS can be incorporated into a development, and many of the devices can be applied to both new and existing developments. The appropriate application of a SUDS scheme to a specific development is heavily dependent upon the individual site and the surrounding areas, and careful consideration of the site characteristics is necessary to ensure the future sustainability of the adopted drainage system; often a combination of techniques will provide the best solution. Developers should consult with the Environment Agency at an early stage about their SUDS proposals, to ensure that they are adopting the most affective methods for their site.
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Appendix A Appendix A
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Appendix A Details of the Environment Agency Flood Zone
A.1 Introduction A more detailed understanding of the Environment Agency flood zones and their limitations is important, as these are often used (unless more accurate flood outlines are available) for the production of SFRA flood maps.
A.1.1 Environment Agency Tidal Maps Mapped tidal Flood Zones 2 and 3 generally comprise land that is lower than the estimated height of the extreme surge tide in the relevant event. Where detailed studies have been undertaken, tidal Flood Zones 2 and 3 have been modified to take into account wave height, the gradient of the land and the relatively short duration of the high tide. In appropriate circumstances, the build up of tidal water trapped behind tidal defences over several high tides is mapped.
A.1.2 Environment Agency Fluvial Maps Data for fluvial Flood Zones 2 and 3 is derived from a number of sources. Flood Zone 2, although not Flood Zone 3, may be updated based upon historic events, for instance the extent of the severe 1947 floods as recorded by the Environment Agency’s predecessors. Most fluvial flood outlines are derived from the “JFlow” generalised computer modelling, which is a ‘coarse’ modelling approach (Ref. 31 and 32).
Caution must be exercised in interpreting JFlow derived flood outlines due to the large number of assumptions incorporated into the JFlow model. For instance, at some locations the river centreline incorporated into the model was found to be erroneous with the result that the associated flood plains deviate from the natural valleys.
A.1.3 Updates of the Environment Agency Flood Maps from Modelling
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Output from a 2D TuFlow model
In many places the results of flood mapping studies have superseded the JFlow model. Generally these studies included high quality hydrological research, surveyed river cross sections, and more precise digital modelling such as ISIS, TuFlow and HecRas.
ISIS Software Graphic Interface
Although fluvial flooding is dependent on the standard of maintenance of watercourses and structures, the degree of maintenance allowed for tends to vary from model to model, with the result that flood maps based on modelling do not offer a consistent approach in this respect. As a consequence, serious blockages occurring during a flood might produce much more flooding than shown on previous modelling for a similar hydrological event.
A.1.4 Updates of the Environment Agency Flood Maps from Recent Events Records of recent flood events, of appropriate return period, have been used to modify the flood map (Flood Zone 2). Often the flood extents are determined from aerial photographs.
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When flood extents are based on historical data, there is often uncertainty about a) whether the flooding has emanated from the river or is the result of other land drainage, b) the precise flood return period and c) whether the flooding was the result of blockage or some other maintenance factor. Occasionally therefore, flood zone modifications based on observed flooding are unreliable.
A.1.5 Other Forms of Flooding in the Environment Agency Flood Maps Although PPG25 and PPS25 advise that the flood zone maps, which are primarily intended as a planning tool rather than a definitive record, should only show tidal and fluvial flooding, in practice they may show other sources of flooding. The historical flood events used for parts of Flood Zone 2 are all either main river flooding or flooding which has been influenced by main river; this latter category may therefore include other types of flooding, which may exacerbate the extent of flooding. In Hampshire for instance, much flooding derived from groundwater sources is included, both in dry valleys and isolated ponds. These groundwater flooding outlines are derived from both JFlow and observations.
A.1.6 Non Main River flooding in the Environment Agency Flood Maps Inland flood zone maps show some non main river watercourse flooding as well as main river watercourse flooding. “Main rivers” are principal watercourses defined by Section 93 of the Water Resources Act, 1991 and shown on a formal map held by the Environment Agency – the Environment Agency flood zones. Larger ordinary watercourses are shown on the background Ordnance Survey mapping.
There is no precise definition of how much non main river-related flooding is included. If no flood plain is shown for a catchment that is less than 3 square kilometres in area, it should be presumed that the area has not been modelled and/or it has not been recorded (as opposed to assuming that flooding has not occurred or would not occur).
A.2 Areas Benefiting from Defences The current flood maps, although they are based on the “undefended situation”, show selected raised formal flood defences, and selected “areas benefiting from defences” (ABDs). This is land where flooding is prevented by defences, although it is assumed that the defences are robust, leak free and maintained, which is not always the case. Improved channels are not regarded as defences for the purposes of flood zone mapping.
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In the absence of better information, the current fluvial Flood Zone 2 can be considered an estimate of the extent of fluvial Flood Zone 3 within 100 years. This principle does not hold for tidal floods, however, as the level of difference between a 0.5% and a 0.1% annual probability tide is only about 200 mm, whereas the forecast increase in tide levels over the next 100 years is in excess of 1m.
Current Environment Agency formal flood maps do not take into account the effect of climate change on winter rainfall and tide levels, or the effect of changes in the levels of tectonic plates on tide levels.
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file:///K|/Planning%20Policy/Strategic%20Flood%20Risk%20Assessment%201/App%20B.htm [20/05/2008 10:15:05] Hampshire County Council SFRA – Audit Trail Database Appendix B: Audit Trail Database
Ref. No Subject & Type of data Medium Date Source Summary Description Used for 1 in 10,000 and 1 in 25,000 OS maps Hampshire County Hampshire County HSFR/003 and layers for Hampshire Boundary Dec 2006 GIS files showing geographical features related to Hampshire. Production of Figures in reports and planning tool. Superceded Council CD Council and Districts T&I, HA and NF CFMP data – DTMs based on SAR and LiDAR data, geo-referenced results and Halcrow internal Production of some of the Flood Zones 3a, 3b, 2 and 3+CC HSFR/004 Boundaries, DTMs, geo-referenced Jan 2007 Environment Agencyflood extents from detailed and broad based models using MDSF transfer (where applicable). Superceded results and layers of flood extents (includes model outputs outside CFMP – South West Region) Wey Flood Mapping (layers and Environment Agency All deliverables under SFRM Flood Mapping Contract (flood Production of some of the Flood Zones 3a, 3b, 2 and 3+CC HSFR/006 May 2006 Atkins reports) DVD extents for a range of return periods, etc) (where applicable) GIS data - Flood events database, flood zones, historical flood maps, main rivers layer (1 in 10,000). Folders of GIS files, reports and Environment Environment Agency Pdf and Word documents - Flood Reports 00-01 and 02-03. Production of some of the Flood Zones 3a, 3b, 2 and 3+CC - HSFR/007 databases, with copies of 20 Dec 06 Agency Winchester CD Spreadsheets - Environment Agency catalogues of reports, CDs superceded. Flood risk from reservoirs to describe in report accompanying emails. Office and Microfiche in the Area Office, and database of Reservoirs under Act. Southern Water Flood Risk Points Records of flooding in the last 10 years for events more frequent HSFR/011 Southern Water email 7 Feb 07 Southern Water Show sewage flooding with X, Y and Postcode (spreadsheets) than 20 years Spreadsheet listing surface water, foul and combined flooding, HSFR/012 Thames Water Spreadsheet Thames Water email 7 Mar 07 Thames Water Show sewage flooding classified by postcode. Environment Agency Environment Agency Guidelines with questions & answers. It also HSFR/018 SFRA questions & Answers. Text file. Oct 06 Environment Agency To ensure compliance with Guidelines memory stick includes comments from James about Climate change, etc. East Hampshire District Council – East Hampshire East Hampshire Spreadsheet of surface water flood events at locations in East HSFR/032 23 Apr 07 Localised flooding in floodmaps Surface Water Flooding Locations District Council email District Council Hampshire Environment Environment Agency HSFR/035 2000-01 flood event photos 20 Dec 06 Agency Winchester Jpg photos of flooding in Hampshire during 2000/2001. Photos on cover. CD Office Environment Environment Agency Jpg photos of flooding in Hampshire during November 2005, plus HSFR/037 2005 flood event photos 20 Dec 06 Agency Winchester Photos on cover. CD some accompanying documents. Office Shapefiles of floodzones 2 and 3 (Mar 07), modelled/historical Environment flooding in Tadburn Lake, Wallington River and River Avon, and Updating SFRA flood maps – MapInfo files of floodzones 2 and Environment Agency HSFR/040 EA updated floodmaps and documents 29 June 07 Agency Winchester accompanying pdf documents. 3 were created and replaced the files used in all tiles. Floodzones CD Office NB; missing flood areas (River Avon, rivers in Rushmoor, Hart, now superceded. East Hampshire, Basingstoke and Deane) Environment Shapefiles of floodzones 2 and 3 for south-east part of Thames Updating SFRA flood maps – MapInfo files of floodzones 2 and Environment Agency HSFR/041 EA updated floodmaps and documents 11 July 07 Agency Thames region. 3 were created and replaced the files used in all tiles. Floodzones CD Region NB; missing river in Basingstoke and Deane borough. now superceded. Environment Updating SFRA flood maps – MapInfo files of floodzones 2 and Environment Agency Shapefiles of floodzones 2 and 3 for eastern side of South West HSFR/042 EA updated floodmaps and documents 11 July 07 Agency Blandford 3 were created and replaced the files used in all tiles. Floodzones CD region (i.e. especially River Avon) Office now superceded. Environment Agency HSFR/048 Rural_Wey_CFMP.doc 4 Oct 07 Environment AgencyWord document outlining the flood risk policy for the River Wey email GIS files showing formal & informal defences in East Hampshire, Photos and GIS files for East Environment Agency HSFR/049 4 Oct 07 Environment Agencyand groundwater flooding along with licence and disclaimer. East Hampshire flood map tiles Hampshire email Photos of defence assets. Shape files of Hampshire fz2 (in 2 parts; north and south) and fz3. Updated EA floodzones 2 and 3 Environment Agency HSFR/055 20 Feb 08 Environment AgencyData licences for Hampshire County, East Hampshire District, Update Flood map tiles (December 2007) email Rushmoor Borough. London LPA & Developer Drainage Guidance.doc, and Drainage Halcrow, South Standards & Submissions v3 - 9 May 2007.pdf (= calculations for Leeds and South Gloucestershire Gloucestershire SUDS), South Glos SUDS L8.pdf (= a description of types of HSFR/056 Halcrow email 28 Feb 08 Reference for SUDS and drainage report Drainage Strategy documents Council & SUDS), SUDS EA Thames Region A Practical Guide.doc (= a Environment Agencydescription of types of SUDS and their application in the Thames region) Word document outlining the flood risk policy for the River Wey. Environment Agency Environment HSFR/057 Rural_Wey.doc 17 Mar 08 (Check to ensure updated version. – HM confirmed that the CFMP chapter of SFRA report email Agency (Thames) Thames CFMP content won’t change now)
file:///K|/Planning%20Policy/Strategic%20Flood%20Risk%20Assessment%201/Appendix%20B.htm (1 of 2) [20/05/2008 10:15:06] Hampshire County Council SFRA – Audit Trail Database
Excel sheet “list.xls” and GIS layers of groundwater flooding in East Hampshire Environment Hampshire for EA Southern Region: “06_11_2000_FEO”, HSFR/061 East Hampshire Groundwater flooding 23 Apr 08 Groundwater flooding in SFRA maps. District Council email Agency (Southern) “Win1995 FEO.DAT”, “Win2000_01gw_FEO.DAT” and “Win2000_01gw_FEO.DAT”. Plus letter and license.
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Maps of Areas Potentially Vulnerable to Groundwater Emergence
The following text has been extracted from the Jacobs 2004 Scoping Report as a first point of reference to accompany the above maps, but for
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The Groundwater Emergence Maps (GEMs) identify those parts of England where, in exceptionally wet winters, groundwater levels could be expected to be at or close to the ground surface. Where possible these maps have been calibrated on observations made in the winter of 2000-01. Where no flooding was reported, or information was not made available, the maps indicated estimated areas based on anticipated groundwater levels using relevant aquifer properties or river baseflow indexes.
The GEMs do not imply flooding per se, only that groundwater could be sufficiently close to the surface to cause flooding, pending local conditions such as local geology, drainage and developments. The impact of groundwater being at or close to the surface is largely dependent on its frequency of occurrence. Those areas that have groundwater near the surface on an annual basis, would normally have drainage systems adapted to the annual flow range experienced. It is in those areas, normally at the upper reaches of the catchment that experience high groundwater only rarely, where problems of flooding are most likely to be experienced. Typically, drainage paths in these areas will be poorly defined and unable to cope with the flow, and cellars and basements may flood from seepage. Within the zones identified the following impacts are possible:
Emergence of new or rarely experienced springs; • Migration of stream sources high into the headwaters; • Emergence of water into underground structures; DEFRA Final_rev.doc/May-04 2-12 • Emergence of water at the surface; • Flooding of properties; • Local drainage network overwhelmed by rate of flow; • Large areas of standing water; • Surcharging sewer network; • Failure of electricity supplies; • Inundation of roads, restricting vehicle movements and deterioration of the surface cover; and • Damage to crops.
The GEMs are composite maps constructed using information from a variety of sources. Inevitably, the quality and applicability of the data varied, for example the available data on groundwater levels typically covered the 1970s rather than 2000-01. Consequently, the confidence that can be placed on the maps will vary from area to area. In order to provide an indication as to the reliability of the maps, they have been ranked from 1 to 3 where 1 represents a high degree of confidence in the mapped area.
There are some locations, mainly consisting of minor aquifers, where groundwater level data are unavailable or, in many cases, groundwater levels are not monitored. To ensure comprehensive coverage of all areas with the potential to cause groundwater flooding the Base Flow Index derived from the Hydrology Of Soil Types (BFIHOST) classification has been applied to the rivers network from the Centre for Ecology and Hydrology (CEH). The Base Flow Index is an indication of the proportion of water in the river that is derived from groundwater and other sources of stored or slowly released water. Simply, the greater the proportion of baseflow, the greater the propensity to flood from this stored water. The GEMs all show BFIHOST classified according to four categories. Those with a base flow in excess of 0.9 (90% flow) being heavily dominated by baseflow and typically being Chalk fed. The next two categories are less dominated by baseflow with proportions of 80% and 70%. All other rivers are shown as blue and indicate those rivers in which baseflow does not play a major part in the flow regime. Those sections of rivers coloured grey are unclassified and arise from a poor fit between the rivers network and the BFIHOST network. As the proportion of baseflow drops, the more flashy the flow regime becomes and it would be expected that surface runoff floods become dominant. Whilst the coloured rivers do indicate the proportion of baseflow on the established rivers, it is the minor rivers upstream that are the most vulnerable. However, displaying the more detailed drainage path network is inappropriate at this scale of mapping.
In addition to the groundwater emergence zones and the river networks, the GEMs show locations of historical flooding in 2000/01 for comparison. In Hampshire there are some outlying observations beyond the defined zones suggesting local influences on groundwater emergence.
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file:///K|/Planning%20Policy/Strategic%20Flood%20Risk%20Assessment%201/App%20D.htm [20/05/2008 10:18:53] Wey and Mole Rural Wey Policy Unit
The Rural Wey policy unit is characterised by extensive and predominantly undeveloped floodplain. Our flood risk management approach for this type of catchment is outlined below.
Undeveloped natural floodplain
• The floodplain is our most important asset in managing flood risk.
• Maintaining (and in some places enhancing) the capacity of the natural floodplain to retain water, combined with maintaining conveyance of watercourses in urban areas reduces the risk of flooding and has benefits for the natural environment.
• We want to safeguard the natural floodplain from inappropriate development.
• Managing the consequences of flooding will become increasingly important, particularly by buildings and communities becoming more resilient to flooding and those who are at risk taking effective action at times of flooding.
This approach will deliver our policy for the Rural Wey: Take action to increase the frequency of flooding to deliver benefits locally or elsewhere (which may constitute an overall flood risk reduction, e.g. for habitat inundation)
Rural Wey - What we want to achieve
Ø Maintain the capacity and function of the undeveloped natural floodplain to retain water so that it can continue to reduce the impact of low order flood events to people and property.
Ø Seek to enhance the capacity of the undeveloped natural floodplain. Recognising that this will require structural measures, this is more likely to be achievable upstream of sizeable communities at risk from flooding where the social, economic benefits are more clear-cut.
Ø Align the objective of maintaining or enhancing floodplain capacity with expansion and enhancement of floodplain environments, particularly BAP habitat.
Ø Continue to reduce the impact of low order flooding in urban areas (up to a 10% to 20% AEP flood – 1in 10 to 1 in 5 year return period) by maintaining conveyance where it both effective and sustainable to do so.
Ø Reduce the consequences of flooding through continued action to raise public awareness of flooding, tailoring the advice and approach (e.g. community based) to ensure those ‘at risk’ take appropriate action to respond to flooding.
Ø Safeguard the existing undeveloped natural floodplain through the appropriate application of the sequential test within PPS25.
Ø Maintain, or in some cases re-establish, river corridors so that urban areas can better accommodate flooding (location file:///K|/Planning%20Policy/Strategic%20Flood%20Risk%20Assessment%201/Appendix%20D%20-%20Rural%20Wey%20Policy%20Unit.htm (1 of 10) [20/05/2008 10:18:56] Wey and Mole and layout) and the buildings are more resilient to flooding (design). In the long-term this should be achievable through re-development. It must be recognised that this is a long-term objective.
Ø In those locations where the level of flood risk merits direct intervention, progress those options that are the most effective and sustainable long-term. This could be to manage the probability of flooding (for example through defences), or more likely to manage the consequences (for example through resilience). Option selection should be based on what is most effective and sustainable and not short-term factors (for example, the ease of capital funding streams). This could include a ‘water exclusion strategy’ – where emphasis is placed on minimising water entry whilst maintaining structural integrity, and on using materials and construction techniques to facilitate drying and cleaning. (This strategy is favoured when low flood water depths are involved (not more than 0.3m) and occupation of the floodplain is deemed necessary and sustainable.
Rural Wey: SEA, Key Approaches and Regional Priority
Summary of the Preferred Approach Policy Unit Rural Wey
1.3% of the economic consequences of fluvial flooding in Thames region
0.7% of the social consequences of fluvial flooding in Thames region
2.7% of the floodplain, channel and designated environmental assets in Thames region
• 2990 properties at risk from a 1% AEP flood event • 4410 properties at risk from a 0.1% AEP flood event • Low levels of social deprivation • Approximately 3km2 of floodplain BAP habitat (wet woodland, floodplain grazing marsh and reedbed) Problem / Risk • 38km2 of floodplain (90% undeveloped and 10% urban) • 195km of natural channel • 5km of modified channel (at Alton, Farnham and Cranleigh)
People and property at risk of flooding is widely dispersed, with some clusters of property at risk (for example in Farnham, Cranleigh and Godalming)
Within the Rural Wey there are numerous internationally designated sites. The majority of these sites are remote from the floodplain and support woodland, grassland, heath and bog habitats. Several of the sites; Thursley & Ockley Bogs and Thursley, Hankley and Fensham Common depend on groundwater levels and the principle risks arise from abstraction. Shortheath Common SSSI is a water dependant SAC, located in the floodplain of the Wey. The current condition of the site is acceptable.
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P6: Take action to increase the frequency of flooding to deliver benefits locally or Policy elsewhere (which may constitute an overall flood risk reduction, e.g. for habitat inundation).
• Approx £750 k pa on maintaining channel conveyance to reduce the impact of high frequency, low order events (up to 5% AEP). Existing Approaches • and Impacts Approximately 15% of properties are signed up to the direct flood warning service. • Application of PPS25
In a regional context, the potential to make large scale reductions in the level of flood risk to people and property is limited at present. The focus of this policy is to bring about long-term change to reduce the consequences of flooding (through land use planning and flood warning / flood awareness) and at the same time enhance or expand the environmental assets (the main opportunities are BAP Justification habitat and river restoration in specific locations such as Cranleigh). There is an (Balancing Objectives) aspiration to increase the resilience of the existing properties at risk.
The selected policy and the regional priority point towards an evolution of flood risk management in the Rural Wey policy unit and long-term changes in the consequences of flooding.
Maintaining or enhancing floodplain capacity to store water to provide direct environmental benefit and small, localised economic and social benefits. 90% of Policy unit-wide the floodplain in this policy unit is undeveloped natural floodplain. Opportunities & Constraints Small to moderate scale redevelopment of towns provides an opportunity to gradually reduce the consequences of flooding.
Long-term river restoration opportunities (perhaps alongside redevelopment); for example in Cranleigh. P1 assumes 500 properties flood annually. This scenario increases AAD from £1,800k to £14,000k. For P3, damages, properties and people at risk reflect the impacts of climate change in this policy unit.
In P4, removal of restrictions to flow can only have a limited impact in this policy unit. It is unlikely to maintain flood damages at current levels against a background of climate change. Widespread strategic application of PPS25 can reduce the consequences of climate change in the long-term. The majority of the Wey is already natural channel, therefore there are only some limited opportunities (in the urban areas) for further river restoration in this policy unit.
Assessment of Under P5, resilience could be a very effective approach in this policy unit. proposed approach (Against Economic, However its adoption is uncertain. Adoption for 25% of properties at risk has been Social and assumed. Environmental Indicators) Under P6, Making space for water with a focus on attenuation can be applied in combination with P4 or P5. P4 is largely a question of resource levels and P5 is largely a question of resilience becoming a more standard flood risk management file:///K|/Planning%20Policy/Strategic%20Flood%20Risk%20Assessment%201/Appendix%20D%20-%20Rural%20Wey%20Policy%20Unit.htm (3 of 10) [20/05/2008 10:18:56] Wey and Mole tool.
The middle section of the Wey floodplain is one of the areas with the highest potential for wetland BAP creation across Thames region, due to current soil type, geology and environmental conditions. The Wey already has over 2km² of wetland BAP habitat (mainly consisting of Fen habitat) which is advantageous when looking for suitable locations for new sites. These BAP creation opportunities can be realised through P6 as they will also provide flood risk benefits through flood storage and enhancing the capacity of the floodplain. Dependent on the application of Making Space for Water principles (floodplain Risks, Uncertainties & management, resilience and resistance measures) for a for a significant change. Dependencies Dependent upon successful application of the sequential test, community engagement and acceptance of flood risk for an evolutionary change. Regional Priority (0- Low. Subject to any changes in the main dependencies identified, in the short- 5yrs) term changes in approach within the Rural Wey policy unit will be evolutionary. Policy key and Selected Policy policy approaches Sustainable Policy (not selected because of constraints) Most likely short-term outcome (where this differs from the selected policy) P1 Do Nothing P2 Flood Warning Emergency maintenance to remove blockages P3 P2 + Maintain conveyance in urban locations Application of PPS25 P4 P3 + Removal of restrictions to flow in urban locations BAP creation Strategic application of PPS25 to achieve optimal location, layout and design of redevelopment P5 P4 + Resilience (for example in Godalming) P6 P3, P4 or P5 + Maintaining or enhancing the capacity of the natural floodplain Restoring river channels in urban areas (eg Cranleigh) Resource Resource levels will determine the pace of implementation rather than what is Implications implemented. Taking account of the level of risk, change will be gradual rather than fast. A small and temporary increase in resource (£50k for a few years) should secure the approaches to spatial planning are secured. Moderate increases in resources over a longer time period (£100k for a defined period) would be needed to secure expansion of environmental assets (mainly floodplain BAP habitat). Key Actions Maintaining urban conveyance (Developed in Action Flood warning and flood awareness Plan) River restoration Resilience
Policy Unit Commentary file:///K|/Planning%20Policy/Strategic%20Flood%20Risk%20Assessment%201/Appendix%20D%20-%20Rural%20Wey%20Policy%20Unit.htm (4 of 10) [20/05/2008 10:18:56] Wey and Mole The Rural Wey policy unit covers a very large area of the Wey catchment. This includes all of the catchment upstream of Guildford, and the majority of the floodplain between Guildford and Byfleet. Our general message that the floodplain is the most important asset in managing flood risk is extremely important here.
The Rural Wey has the following characteristics;
• An extensive and predominantly undeveloped floodplain. Flood defences contribute to reducing the risk of flooding to some communities; for example at Farnham. • Flooding tends to occur between October and April following prolonged periods of heavy rain. • Approximately 3,500 properties at 1% AEP risk from flooding. Most are widely dispersed throughout the policy unit, but there are clusters of properties at risk (more than 100 properties) in Woking, Godalming, Cranleigh Farnham and Alton. • The main flow carrying tributaries are the Cranleigh Waters, the Tillingbourne and the Hoe Stream. • The channel contains a large number of mill structures, side channels, bifurcations and weirs throughout its entire length, including the rural reaches. • A range of floodplain environmental assets including extensive areas of BAP habitat (mainly grazing marsh). • The river is navigable from the River Thames to Godalming and includes a number of navigation reaches separated from the main river with water levels regulated by structures such as lock gates and weirs.
Our proposed approach to managing the flood risk is based on maintaining the capacity and function of the floodplain to retain water. The floodplains of the Rural Wey provide natural storage of water during times of flood. This reduces the impact of flooding to property locally within this flood risk area and downstream. We are adopting a policy of accommodating more flooding in the undeveloped; this recognises the current value of the Wey floodplain and how with further intervention its value can be maintained or enhanced. This approach may, for instance, have a role in our adaptation to the possible impacts of climate change.
This entire approach is dependent upon Making Space for Water principles being put into practice. We recognise the constraints and uncertainty that exist in making a transition to this kind of flood risk management. The uncertainty in our ability to deliver this is based on;
• At present we can only implement a few of these measures in a few locations. There are no UK precedents for these types of actions on this type of scale. • For some of these interventions we can only demonstrate a local benefit. To justify them we need to demonstrate the cumulative benefit. • Many of these interventions are not ‘tried and tested’. They appear more risky than other types of intervention. • There will need to be significant changes to policy and operational practice with some wide ranging implications.
The focus of the future policy, which seeks to maximise the natural characteristics of the catchment to manage flood risk, will require wide spread and long term interventions across the catchment. A considerable amount of collaboration between land and water management will be required. Even a significant scale of intervention may only generate relatively file:///K|/Planning%20Policy/Strategic%20Flood%20Risk%20Assessment%201/Appendix%20D%20-%20Rural%20Wey%20Policy%20Unit.htm (5 of 10) [20/05/2008 10:18:56] Wey and Mole localised benefits. Adopting an approach that values the floodplain provides new opportunities for flood management. This includes the potential to implement this approach on a scale that contributes to the management of flood risk in the other Wey catchment policy units, particularly in the long term.
This large area of floodplain does contain some clusters of properties at risk from fluvial flooding, notably Woking, Godalming, Cranleigh Farnham and Alton. In these places our flood risk management activities will need to compliment our overall aim of maintaining the capacity of the floodplain. Any flood management intervention is likely to be localised and will not conflict with our overall objective. Our current activities can only continue to maintain the current level of flood risk for a limited time. Asset management and maintenance is likely to become increasingly expensive but with decreasing benefits. The likelihood and consequences of flooding are expected to increase in the future. This highlights how critical future planning decisions will be, not just in ensuring flood compatible use in the floodplain, but also delivering greater flood resilience through redevelopment.
Flood defences could have a role in reducing flood risk to parts of some of the larger communities that are at risk from flooding. At present no flood defence schemes are planned and this situation will need to be kept under review as funding and prioritisation evolve. However, flood defences will not provide a complete solution to the existing and future flood risk at any of the major localities and may not therefore be the most sustainable option. In most locations, notably where there are clusters of properties at risk, adaptation and resilience of the existing buildings (in combination with the appropriate level of flood warning and flood awareness) may offer a more comprehensive option.
Our message that regeneration and redevelopment offer a crucial opportunity to reduce the flood risk is important, but will have a smaller impact here than in other parts of the region, reflecting the low level of regeneration. Providing the appropriate Strategic Flood Risk Assessments (SFRAs) are carried out and we have policies based on PPS25 within Local Development Frameworks (LDFs), then a Spatial Planning delivery plan will not be needed across the whole policy unit.
With properties at flood risk dispersed widely throughout the policy unit, flood warning is an important approach to managing the risk. However this needs to be complimented with an awareness and action plan if the consequences of flooding are to be significantly reduced. This may include enhancing the telemetry network and improving forecasting potential and techniques, along with raising general flood risk awareness and increasing the number of people registered to receive direct flood warnings. Where the characteristics of fluvial flooding make it applicable a ‘water exclusion strategy’ should be developed – where emphasis is placed on minimising water entry whilst maintaining structural integrity.
During the next five years there will be scrutiny of watercourse maintenance in rural areas and there may be some reduction in the scale of watercourse maintenance in parts of the Rural Wey. Some flood protection is provided by the capacity of the river channels and storage within the natural floodplain. The level of maintenance that we carry out to river channels and riverbanks is prioritised according to the level of flood risk and the effectiveness of the maintenance in reducing the likelihood of flooding in a particular place. Maintenance is generally effective in reducing the impacts of flooding that would be expected to occur most winters so the objective of our maintenance is to reduce the impact of low order flooding up to a 20% to 10% Annual Exceedance Event (AEP) flood (1 in 5 year to 1 in 10 year return period). In flood events more extreme than this, channel and bank maintenance (including dredging) has no impact and is therefore ineffective in managing the file:///K|/Planning%20Policy/Strategic%20Flood%20Risk%20Assessment%201/Appendix%20D%20-%20Rural%20Wey%20Policy%20Unit.htm (6 of 10) [20/05/2008 10:18:56] Wey and Mole flood risk in these situations.
Rural Wey - Policy Delivery
What we want to achieve Action and Mechanism Indicator Partners Timescale Maintain the capacity and function of the Through the Wey strategy, SAMPS and Performance Specifications: Environment By 2010 undeveloped natural floodplain to retain Agency, water so that it can continue to reduce the Align maintenance regimes defined in these plans to meet our aim of impact of low order flood events to people maintaining the capacity of the natural floodplain (recognising that this is and property. largely passive)
Seek to enhance the capacity of the Identify areas of floodplain where the capacity to retain water could be undeveloped natural floodplain. Recognising enhanced. that this will require structural measures, this is more likely to be achievable upstream of sizeable communities at risk from flooding Maintain existing defences that compliment the overall policy (for example at where the social, economic benefits are Farnham) more clear-cut.
Align the objective of maintaining or The level of maintenance that we carry out to river channels and riverbanks enhancing floodplain capacity with is prioritised according to the level of flood risk and the effectiveness of the expansion and enhancement of floodplain maintenance in reducing the likelihood of flooding in a particular place. environments, particularly BAP habitat Maintenace is generally effective in reducing the impacts of flooding that would be expected to occur most winters so the objective of our Continue to reduce the impact of low order maintenance is to reduce the impact of low order flooding up to a 20% to flooding in urban areas (up to a 10% to 20% 10% AEP flood (1 in 5 year to 1 in 10 year return period). In flood events AEP flood – 1in 10 to 1 in 5 year return period) by maintaining conveyance where it more extreme than this, channel and bank maintenance (including dredging) both effective and sustainable to do so has no impact and is therefore ineffective in managing the flood risk in these situations.
Reduce the consequences of flooding Through the Wey strategy plan: through continued action to raise public awareness of flooding, tailoring the advice Improvements to the flood warning telemetry infrastructure will be needed. and approach (e.g. community based) to ensure those ‘at risk’ take appropriate action to respond to flooding. Activity for raising flood awareness using a locally tailored approach combined with providing targeted advice on what action to take. Safeguard the existing undeveloped natural Existing mechanisms (through future iterations of SFRA’s and LDF policy floodplain through the appropriate reviews) are likely to be sufficient to take forward these aims. application of the sequential test within PPS25. There are locations where more action is needed to restore river corridors (for example Cranleigh Waters). In these locations it may be necessary to Maintain, or in some cases re-establish, river establish and agree (Local Planning Authority and Environment Agency) a corridors so that urban areas can better accommodate flooding (location and layout) vision for the enhancement of the watercourses linked to the redevelopment and the buildings are more resilient to of the towns. This will need to recognise both flood risk management flooding (design). In the long-term this objectives and the wider objectives of the Planning Authority and seek a net should be achievable through re- reduction in flood risk. Not all objectives will necessarily be met at all sites. development. It must be recognised that this is a long-term objective In those locations where the level of flood Through the Wey strategy: risk merits direct intervention, progress those options that are the most effective and Define those locations where resilience options (sometimes in combination file:///K|/Planning%20Policy/Strategic%20Flood%20Risk%20Assessment%201/Appendix%20D%20-%20Rural%20Wey%20Policy%20Unit.htm (7 of 10) [20/05/2008 10:18:56] Wey and Mole sustainable long-term. This could be to with flood warning arrangements) will be the most effective and sustainable. manage the probability of flooding (for This will include Godalming. example through defences), or more likely to manage the consequences (for example through resilience). Option selection should be based on what is most effective and sustainable and not short-term factors (for example, the ease of capital funding streams).
Figure 6.3.15a Properties at risk from a 0.1% AEP flood event in the Rural Wey policy unit
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Figure 6.3.15b Wetland BAP habitat in the Wey catchment
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file:///K|/Planning%20Policy/Strategic%20Flood%20Risk%20Assessment%201/App%20E.htm [20/05/2008 10:20:47] RAINFALL RUNOFF MANAGEMENT FOR
RAINFALL RUNOFF MANAGEMENT FOR DEVELOPMENTS – INTERIM NATIONAL PROCEDURE
1. Procedure status. This procedure is an interim method, which is expected to be revised as improved tools are developed. It utilises well recognised existing methods, but revision is anticipated to provide a more consistent approach as and when FEH procedures can be extended to catchments at development scale.
2. Compliance to national guidance. The objective of this procedure is to assist developers and their designers to conform to PPS25.
3. Application of the procedure. This procedure applies to both greenfield and brownfield sites. In the case of brownfield sites, drainage proposals will be measured against the existing performance of the site (although it is preferable for solutions to provide runoff characteristics which are similar to greenfield behaviour). Therefore where greenfield performance is referred to in this document, this should be considered as meaning the existing site conditions for brownfield redevelopment sites. Sites with polluted land will have particular consent requirements and affect the drainage techniques that can be used.
4. Use of infiltration. Part H of the Building Regulations requires that the first choice of surface water disposal should be to discharge to infiltration systems where practicable. Infiltration techniques should therefore be applied wherever they are appropriate.
5. Sewers for Adoption. Drainage calculations and criteria, where appropriate, should comply with the 6th edition of Sewers for Adoption.
6. Need for this procedure. It is recognised that the impact of urban development on greenfield areas increases both the rate of run-off and the volume of run-off in response to rainfall and that the water quality impact on the receiving watercourse is likely to be detrimental.
7. Procedure philosophy. The objectives of this procedure are to: • stormwater runoff discharged from urban developments to replicate or achieve a reduction from the greenfield response of the site over an extended range of storm probabilities (return periods) • manage runoff on site for extreme events.
This requires: • the peak rate of stormwater run-off to be controlled • the volume of run-off to be reduced • the pollution load to receiving waters from stormwater runoff to be minimised • the assessment of overland flows and temporary flood storage across the site.
8. Discharge rate criteria. The Environment Agency will normally require that, for the range of
file:///K|/Planning%20Policy/Strategic%20Flood%20Ris...%20-%20Interim%20Procedure%20Rev%20D%20protected.htm (1 of 5) [20/05/2008 10:20:48] RAINFALL RUNOFF MANAGEMENT FOR annual flow rate probabilities, up to and including the 1% annual probability (1 in 100 year event) the developed rate of runoff into a watercourse should be no greater than the undeveloped rate of runoff for the same event. Exceptions only apply where it is not practical to achieve this due to either constraints on the size of the hydraulic control unit (see point 17), or excessive storage volumes. The purpose of this is to retain a natural flow regime in the receiving watercourse and not increase peak rates of flow for events of an annual probability greater than 1%. Three annual probabilities merit specific consideration; 100%, 3.33% and 1%. (Note that in many places elsewhere in this Guide return periods are used instead of annual probabilities, as much historic nomenclature and many formulae use return periods).
8.1 The 100% annual probability (1 in 1 year event) is the highest probability event to be specifically considered to ensure that flows to the watercourse are tightly controlled for these more frequent events.
8.2 The 3.33% annual probability (1 in 30 years event) is of importance because of its linkage with the level of service requirement of Sewers for Adoption 6th edition (SfA6). SfA6 requires that surface water sewers should be capable of carrying the 3.33% annual probability event within the system without causing flooding to any part of the site.
8.3 The 1% annual probability (1 in 100 years event) has been selected since it represents the boundary between high and medium risks of fluvial flooding defined by PPS25 and also recognises it is not practicable to fully limit flows for the most extreme events. Also SfA6 recognises that, during extreme wet weather, the capacity of surface water sewers may be inadequate. SfA6 requires that the site layout should be such that internal property flooding does not result, by demonstrating safe above ground flow paths. The return period for this analysis is not specified, but it is recommended that 1% annual probability event (i.e. an event with a return period of 100 years) is used.
8.4 Flood flows. up to the 1% annual probability event should preferably be contained within the site at designated temporary storage locations unless it can be shown to have no material impact in terms of nuisance or damage, or increase river flows during periods of river flooding. Analysis for overland flood flows within the site will need to use short high intensity rainfall events of between 15 minutes and 1 hour duration.
9. The calculation of greenfield runoff rate. The calculation of peak rates of runoff from a greenfield site is related to its size. The values derived should be regarded as indicative due to the limitations of the existing tools. Table 9.1 summarises the techniques to be used.
Table 9.1 Tools to be used for calculation of greenfield run-off criteria Development size Method
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0 – 50 ha The Institute of Hydrology Report 124 Flood Estimation for Small Catchments (1994) is to be used to determine peak green field runoff rates.
Where developments are smaller than 50 ha, the analysis for determining the peak greenfield discharge rate should use 50 ha in the formula and linearly interpolate the flow rate value based on the ratio of the development to 50 ha.
FSSR 2 and 14 regional growth curve factors are to be used to calculate the greenfield peak flow rates for 1, 30 and 100 year return periods. 50 ha – 200 ha IH Report 124 will be used to calculate greenfield peak flow rates. Regional growth factors to be applied. Above 200 ha IH Report 124 can be used for catchments that are much larger than 200 ha. However, for schemes of this size it is recommended that the Flood Estimation Handbook (FEH) should be applied. Both the statistical approach and the unit hydrograph approach should be used to calculate peak flow rates. The unit hydrograph method will also provide the volume of greenfield run-off. However, where FEH is not considered appropriate for the calculation of greenfield run-off for the development site, for whatever reasons, IH 124 should be used.
10. Volumetric criteria. The stormwater runoff volume from a site should be limited to the greenfield runoff volume wherever possible. The additional runoff volume caused by urbanisation should be controlled using two criteria.
10.1 Interception. Where possible, infiltration or other techniques are to be used to ensure minimal discharge to receiving waters for rainfall depths up to 5mm.
10.2 Additional runoff due to development. The difference in runoff volume pre and post- development for the 100 year 6 hour event, (the additional runoff generated) should be disposed of by way of infiltration, or if this is not feasible due to soil type, discharged from the site at flow rates below 2l/s/ha.
10.3 Where compliance to 100 year volumetric criterion, as defined in section 10.2, is not provided, the limiting discharge for the 30 and 100 year return periods will be constrained to the mean annual peak rate of runoff for the greenfield site (Referred to as QBAR in IH Report 124).
11. Percentage runoff from greenfield sites. The percentage runoff of the rainfall on a greenfield site can be assumed to be approximately equal to the SPR value of the soil type of
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12. Percentage runoff from developments. Calculation of the run-off volume from the developed site for preliminary assessment and design of drainage facilities will assume 100% run-off from paved areas and 0% run-off from pervious areas. Runoff from impermeable surfaces served by effective infiltration systems can be assumed to contribute no runoff for storage volumes assessment.
13. Detailed design of stormwater runoff. All network design for stormwater runoff and proof of compliance in meeting peak flow rate discharge criteria, using computer simulation, should use the standard Wallingford Procedure variable UK runoff model using appropriate parameters.
14. SUDS for water quality. SUDS units should be used to achieve water quality improvements and amenity benefits as well as achieving compliance to these hydraulic criteria. Best practice in achieving water quality protection should be used.
15. Reliability of SUDS. At present certain SUDS units are considered to have some degree of risk of medium term hydraulic failure, due to either maintenance or possible change of status. In these situations, to ensure compliance with pipe capacity criteria, they will be deemed not to be effective when calculating pipe sizes and storage requirements. For pipe sizing the current view of the Water Undertakers should apply (see the National SUDS Framework document). For storage sizing of all structures which are not to be adopted by Water Undertakers, the view of the Environment Agency should normally apply.
16 Climate change factor. Climate change will be taken into account in hydrological regions by increasing the rainfall depth by the recommended allowances in PPS25 for computing storage volumes. No allowance for climate change should be applied to calculated greenfield peak rates of runoff from the site for any hydrological region. It should be recognised that although climate change is acknowledged as taking place, certainty regarding the hydrological changes, particularly of extreme short duration events, is very low.
17. Minimum limit of discharge rate. A practicable minimum limit on the discharge rate from a flow attenuation device is often a compromise between attenuating to a satisfactorily low flow rate while keeping the risk of blockage to an acceptable level. It is suggested that this is 5 litres per second, using an appropriate vortex flow control device or other technically acceptable flow control device. The minimum size of pipe discharging from a flow attenuation device should be 150mm laid at a gradient not flatter than 1 in 150, which meets the requirements of Sewers for Adoption 6th Edition.
18. Catchment Flood Management Plans. CFMPs (Catchment Flood Management Plans), consider the impact of development on flood risk in the catchment based on existing land use plans contained in the local plan published by the Local Planning Authority and projections of development beyond the periods covered by the land use plans. Strategy Plans identified in the
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CFMPs each cover part of the catchment and may consider the local impact of these developments in more detail. Where these exist for an area proposed for development, their findings must be taken into account in the development proposal.
Further information can be found in the books:
Preliminary rainfall runoff management for developments (R & D Technical Report W5-074/ A Revision D (Environment Agency and Kellagher R, 2004 - Free download from the Environment Agency web site www.environment-agency.gov.uk).
CIRIA C697 The SUDS manual (Woods Ballard B; Kellagher R et al, 2007 – available from CIRIA bookshop www.ciria.org)
Interim code of practice for sustainable drainage (National SUDS Working Group, 2004) - Free download from CIRIA web site www.ciria.org or Environment Agency web site www. environment-agency.gov.uk)
file:///K|/Planning%20Policy/Strategic%20Flood%20Ris...%20-%20Interim%20Procedure%20Rev%20D%20protected.htm (5 of 5) [20/05/2008 10:20:48] Table 2: Historical Flooding - Southern Water Source: Southern Water
Date of most Sewer Problem Sewer Type Internal Curtilage Highway or Reference Tile recent incident Post town Locality Street Description Description Flooding Flooding Open Space SOW656 D 19960205 Liss Highfield Gardens Hydraulic Foul/Combined Yes SOW657 D 20031126 Liss Greatham Petersfield Road Hydraulic Foul/Combined Yes Yes SOW658 D 20030102 Petersfield Barnfield Road Hydraulic Foul/Combined Yes SOW659 C 19990116 Petersfield Borough Road Hydraulic Foul/Combined Yes SOW660 C 20000508 Petersfield Petersfield Charles Street Hydraulic Foul/Combined Yes SOW661 C 20040203 Petersfield East Meon Church Street Hydraulic Foul/Combined Yes Yes SOW662 D 20040202 Petersfield Crundles Hydraulic Foul/Combined Yes Yes SOW663 D 20061117 Petersfield Head Down Hydraulic Foul/Combined Yes SOW664 C 20000916 Petersfield East Meon High Street Hydraulic Foul/Combined Yes SOW665 C 20030102 Petersfield Highfield Road Hydraulic Foul/Combined Yes SOW666 D 20050905 Petersfield Lower Mead Hydraulic Surface Water Yes SOW667 C 20000510 Petersfield Lyndum Close Hydraulic Surface Water Yes SOW668 D 20030102 Petersfield Mill Lane Hydraulic Foul/Combined Yes SOW669 D 20001213 Petersfield Petersfield Moggs Mead Hydraulic Foul/Combined Yes SOW670 D 20051025 Petersfield Old Mill Lane Hydraulic Foul/Combined Yes SOW671 C 20061001 Petersfield Rushes Road Hydraulic Foul/Combined Yes SOW672 C 20000512 Petersfield Petersfield Stafford Road Hydraulic Foul/Combined Yes SOW673 C 19960218 Petersfield Station Road Hydraulic Foul/Combined Yes SOW674 C 20031201 Petersfield Sussex Road Hydraulic Foul/Combined Yes SOW675 C 20061207 Petersfield East Meon The Cross Hydraulic Foul/Combined Yes Yes SOW676 C 19981230 Petersfield Tilmore Gardens Hydraulic Foul/Combined Yes Yes SOW677 C 20051028 Petersfield Woodbury Avenue Hydraulic Surface Water Yes SOW678 C 20010124 Petersfield East Meon Workhouse Lane Hydraulic Foul/Combined Yes SOW679 C 20000420 Petersfield East Meon Workhouse Lane Hydraulic Foul/Combined Yes SOW680 E 20001214 Rowland's Castle Glen Dale Hydraulic Foul/Combined Yes Yes SOW681 E 20051102 Waterlooville Applegate Place, Frogmore Lane Hydraulic Foul/Combined Yes SOW682 E 20031201 Waterlooville Birdlip Close Hydraulic Foul/Combined Yes Yes SOW683 E 20040109 Waterlooville Catherington Lane Hydraulic Surface Water Yes SOW684 E 20010101 Waterlooville Cross Lane Hydraulic Foul/Combined Yes Yes SOW685 E 20001107 Waterlooville Deep Dell Hydraulic Surface Water Yes SOW686 E 20061212 Waterlooville Dorset Close Hydraulic Foul/Combined Yes SOW687 E 20061002 Waterlooville Clanfield Green Lane Hydraulic Foul/Combined Yes SOW688 E 20050904 Waterlooville Horndean Havant Road Hydraulic Foul/Combined Yes SOW689 E 20001010 Waterlooville Hill View Hydraulic Foul/Combined Yes SOW690 E 20061022 Waterlooville Keydell Avenue Hydraulic Foul/Combined Yes SOW691 E 20040108 Waterlooville Horndean Portsmouth Road Hydraulic Foul/Combined Yes SOW692 E 20010808 Waterlooville South Lane Hydraulic Foul/Combined Yes Yes SOW693 E 20041128 Waterlooville St. Vincent Crescent Hydraulic Foul/Combined Yes SOW656 D 19960205 Liss Highfield Gardens Hydraulic Foul/Combined Yes
Note: This table is to be read in conjunction with Tiles A to E
Table 3: Historical Flooding – Thames Water Source: Thames Water
Postcode Tile Total number of properties Total number of properties Total number of properties Total number of properties flooded from overloaded flooded by surface water flooded by foul water from flooded by combined sewer in the last ten years from overloaded sewers in overloaded sewers in the overloaded sewers in the the last ten years last ten years last ten years GU10 4 B 3 0 3 0 GU10 5 B 1 0 1 0 GU27 1 B/D 1 0 1 0 GU30 7 B/D 25 0 18 7
Note: This table is to be read in conjunction with Tiles A to E
Table 4: Localised Flooding Areas Source: East Hampshire District Council
Frequency of ID Tile Source Of Flooding Source of Outline Date Received Flooding EH001 A Groundwater and poor surface drainage 1 in 20 years Workshop with council 18/01/2007 EH002 A Groundwater and overtopping of river flows 1 in 20 years Workshop with council 18/01/2007 EH003 E Groundwater flooding 1 in 20 to 1 in 50 years Workshop with council 18/01/2007 EH004 E Groundwater and surface 1 in 100 year plus Workshop with council 18/01/2007 EH005 E Flash flooding 1 in 100 year plus Workshop with council 18/01/2007 EH006 C Groundwater 1 in 100 year plus Workshop with council 18/01/2007 EH007 D Groundwater / surface drainage 1 in 50 years Workshop with council 18/01/2007 EH008 D Groundwater / surface water drainage 1 in 20 years Workshop with council 18/01/2007 groundwater flooding - springs issue adj Annetts 23/04/2007 EH009 A Farm, A32 - generally flows follow valley bottom Spreadsheet from council EH010 A junction with Caker Stream Spreadsheet from council 23/04/2007 groundwater flooding - springs issue north Woodcroft Farm - generally flows follow line of 23/04/2007 EH011 E railway/highway Spreadsheet from council EH012 E junction with flows at Finchdean Spreadsheet from council 23/04/2007 groundwater flooding - springs issue from lower slopes Windmill Hill - generally flows follow line 23/04/2007 EH013 E highway Spreadsheet from council junction with flows at Finchdean & groundwater flows/defined watercourse generally flows follow line 23/04/2007 EH014 E railway Spreadsheet from council junction with Deane Lane End & groundwater 23/04/2007 EH015 E flows/defined watercourse Spreadsheet from council end of defined watercourse Stansted Park & junction 23/04/2007 EH016 E with main flow Stansted Park Spreadsheet from council EH017 E flows across Woodbury Lane, Rowlands Castle Spreadsheet from council 23/04/2007 Frequency of ID Tile Source Of Flooding Source of Outline Date Received Flooding defined watercourse under railway/B2148 flows into 23/04/2007 EH018 E Havant BC area Spreadsheet from council groundwater flooding - springs issue adj railway line 23/04/2007 EH019 E generally flows follow line Finchdean Road Spreadsheet from council Castle Road ditch discharges to swallow hole Redhill Road, at times of high water table/prolonged rainfall 23/04/2007 EH020 E - Spreadsheet from council flooding occurs Redhill Road/The Green village 23/04/2007 EH021 E centre Spreadsheet from council flash flooding - saturated ground conditions/extreme rainfall event southern slope Butser Hill - affects 23/04/2007 EH022 E Green Lane/White Dirt Lane Spreadsheet from council flash flooding - saturated ground conditions/extreme rainfall event southern slope Butser Hill - affects 23/04/2007 EH023 E Green Lane/White Dirt Lane Spreadsheet from council flash flooding - saturated ground conditions/extreme rainfall event southern slope Butser Hill - affects 23/04/2007 EH024 E South Lane/Downhouse Road Spreadsheet from council flash flooding - saturated ground conditions/extreme rainfall event southern slope Butser Hill - affects 23/04/2007 EH025 E South Lane/Downhouse Road Spreadsheet from council EH026 C groundwater flooding affecting A272 and adj church Spreadsheet from council 23/04/2007 flash flooding - saturated ground conditions/prolonged rainfall affects junction 23/04/2007 Farnham Rd/Station Rd and some properties in St EH027 D Mary's Road Spreadsheet from council
Note: This table is to be read in conjunction with Tiles A to E
Table 5: Groundwater Flooding Incidents Source: Environment Agency (Southern Region)
ID Tile Start Date End Date Source of Flooding Cause of Flooding Residential Commercial Properties Properties Flooded Flooded GWS25 E 30/01/1995 12/03/1995 Ephermeral Watercourse Groundwater/High Water Table 0 0 GWS26 E 30/01/1995 12/03/1995 Ephermeral Watercourse Groundwater/High Water Table 0 0 GWS27 E 30/01/1995 12/03/1995 Ephermeral Watercourse Groundwater/High Water Table 0 0 GWS28 E 30/01/1995 12/03/1995 Ephermeral Watercourse Groundwater/High Water Table 0 0 GWS72 E 01/12/2000 01/01/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS91 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS92 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS93 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS112 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS113 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS114 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS115 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS116 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS117 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS118 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS119 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS120 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS121 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS122 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS123 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS124 E 01/11/2000 01/03/2001 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS536 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS537 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS538 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS539 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS540 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS541 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS542 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS543 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS544 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS545 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS546 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS547 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS548 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS549 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS550 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS551 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS552 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS553 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS554 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS555 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS556 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS557 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS558 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS559 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS560 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS561 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS562 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS563 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS564 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS565 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS566 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS567 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS568 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS569 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS570 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS571 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS572 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS573 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS574 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS575 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS576 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS577 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS578 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS579 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS580 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS581 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS582 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS583 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS584 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS585 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS586 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS587 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS588 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS589 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS590 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS591 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS592 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS593 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS594 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS595 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS596 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS597 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS598 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS599 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS600 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS601 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS602 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS603 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS604 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS605 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS606 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS607 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS608 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS798 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0 GWS799 E 01/12/2002 01/03/2003 Ephemeral Watercourse Groundwater/High Water Table 0 0
Note: This table is to be read in conjunction with Tiles A to E
Table 6: Groundwater Flooding Incidents Source: Environment Agency (Thames Region)
ID Tile Date Location Incident Grid Reference Aquifer Depth GW01 A 12/12/2000 Farringdon Water in basement SU7135 GW02 A 18/12/2000 Farringdon GW flooding -evacuated! SU704349 GW03 B 16/01/2001 Bordon Water in Garden SU800360 Spring from London Clay and Woolwich and GW04 B 26/03/2001 Whitehill Water in cellar SU7934933999 Reading Beds. Underfloor flooding GW05 A 18/04/2001 Farringdon through winter SU7052235194 6 inches of water at bottom of garden, renewed by heavy GW06 B 09/07/2001 Bordon Standing water in garden SU7868734427 Folkestone Beds rain, usually well drained soil.
Note: This table is to be read in conjunction with Tiles A to E
Table 10: Flood Defences Source: Environment Agency
Defence Design Grid ID Asset Type Maintainer Asset Comments Asset Description Location Length Height Type Standard Bank Reference non-flood defence D1 structure private Embankment grown up with young trees Road and rail embankments A31 flyover B3004 266.6 6 minor 5 left SU7284038537 non-flood defence Old Railway Line. D2 structure private Road and rail embankments Kingsley 606.5 0 minor 5 left SU7864038764 non-flood defence Old Railway Line. D3 structure private Road and rail embankments Kingsley 268.3 0 minor 5 right SU7819338000 non-flood defence Old Railway Line. D4 structure private Embankment vegetated with shrubs and trees. Road and rail embankments Kingsley 129.3 4 minor 5 left SU7815137819 non-flood defence Disused railway D5 structure private Embankment vegetated with shrubs and trees. Embankment (upstream of bridge) 22.2 4 major 5 left SU7814037697 non-flood defence Disused railway ( D6 structure private Embankment vegetated with shrubs and trees. Embankment downstream of bridge) 25 4 major 5 right SU7815837670 non-flood defence Disused railway D7 structure private Embankment vegetated with shrubs and trees. embankment (upstream of bridge) 23.5 4 minor 5 right SU7814637668 non-flood defence Such an asset not found at the plotted location. LUB=AB north east of D8 structure private No such asset found at the plotted location. Embankment Trottsford Farm 60.7 0 minor 5 left SU8107938759 non-flood defence Such an asset not found at the plotted location. LUB=AB north east of D9 structure private No such asset found at the plotted location. Embankment Trottsford Farm 118.4 0 minor 5 left SU8103638907 non-flood defence Such an asset not found at the plotted location. LUB=AB north west of D10 structure private No such asset found at the plotted location. Embankment Trottsford Farm 143.8 0 minor 5 right SU8057838699 non-flood defence Concrete bagwork built into road to protect road from U/S of Brockford D11 structure local authority erosion in high flow LUB=CD Photo: U/S R/B Bridge 4.3 0.5 major 5 right SU8181138522 non-flood defence Road embankment with brick upstand walls at carriage D12 structure private way edges. 3m wide c'way. road and rail embankments Stream Manor 35.5 2 minor 5 left SU8132135550 non-flood defence D13 structure private Lake Standford Mill 165.1 0 minor 5 right SU8140434929 non-flood defence U/S from Passfield D14 structure private Raised earth embankment between road and lake Lake Embankment. Road. 56 1 major 5 left SU8230634092 non-flood defence Concrete flyover supported by 10 large concrete pillars. U/S of Heronslake, D15 structure local authority Bridge is approx 30m in height and 70m in width. A3 flyover. Liphook. 174.8 30 minor 5 right SU8403632462 non-flood defence Lake that is located and maintained by the owners of the U/S of Tunbridge D16 structure private private property. lake Lane, Liphook 167.7 0 major 5 right SU8396832735 non-flood defence A31 road embankment. Gentle rise to 2m above A31. south of D17 structure private floodplain LUB=AB PHOTO:VIEW D/S A31 road embankment Doctor's Copse 92.8 0 minor 5 left SU8037844362 non-flood defence Cotton's Copse - D18 structure private Road and rail embankments Willey Mill 1658.5 0 minor 5 right SU8043343965 non-flood defence Brick wall leading down to weirs LUB=CD Photo: D/S D19 structure private R/B Bentley Mill 23.8 -999 major 5 left SU8020744256 A31 Road embankment. Gentle gradient from between 2 - non-flood defence 6 meters above floodplain. LUB=AB PHOTO:VIEW A31. south of D20 structure private D/S A31 road embankment Doctor's Copse 239.6 0 minor 5 left SU8028844323 Brick and stone bank protection. stone set in concrete with brick capping. Approx 40m in length D/S of sluice non-flood defence structure. There is concrete/mortar loss at the water line D21 structure private which needs attention. LUB=CD Photo: U/S R/B Isington Mill 49.6 -999 major 5 left SU7744442794 non-flood defence Haw Bridge - D22 structure private Road and rail embankments River/Rail crossing 1685.2 0 minor 5 left SU7430641231 D23 non-flood defence private Stone retaining wall approx 50m in length and 1.5m in Fulling Mill Cottage 66 -999 major 5 left SU7549241722 Defence Design Grid ID Asset Type Maintainer Asset Comments Asset Description Location Length Height Type Standard Bank Reference structure height. Extensive mortar loss at water line and above - any more loss will cause the stability of the wall to be compromised. LUB=CD Photo: D/S non-flood defence Brick retaining wall protecting garden from undercutting D24 structure private LUB=CD Photo: D/S Fulling Mill Cottage 80.3 -999 major 5 right SU7549641718 non-flood defence Upper Needham D25 structure private Well vegetated trees and scrub. Road and rail embankments Farm 82.1 8 minor 5 left SU7322540368 non-flood defence Neatham Mill - Haw D26 structure private Road and rail embankments Bridge 897.7 0 minor 5 left SU7369740549 non-flood defence Upper Needham D27 structure private Vegetated with trees Outside of Flood Plain. Road and rail embankments Farm 614.5 30 minor 5 right SU7330639820 non-flood defence Earth Embankment. vegetated with grass and trees NOT Upper Needham D28 structure private WITHIN FLOOD PLAIN Road and rail embankments Farm 57.7 10 minor 5 right SU7365540529 non-flood defence Upper Needham D29 structure private Road and rail embankments Farm 305.2 0 minor 5 right SU7359840231 non-flood defence Upper Needham D30 structure private earth embankment vegetated with trees and shrubs. Road and rail embankments Farm 117.2 10 minor 5 left SU7321140509 raised defence Nr Frenchmans Road D31 (man-made) private Asset owner: Private Masonry Bank Culvert. Petersfield 26.7 0.5 major 75 right SU7419423519 raised defence Adjacent to railway D32 (man-made) private Owner = private Open Channel embankment 109.5 -999 major 10 left SU7381322856 raised defence Adj to Railway D33 (man-made) private Owner = private Open Channel Embankment 85.1 -999 major 10 left SU7389823031 raised defence D34 (man-made) private Owner = private Open Channel Adj to Railway Line 22.4 -999 major 10 left SU7391723068 raised defence D35 (man-made) private Owner = private Open Channel To A3 Road Culvert 25.8 -999 major 10 left SU7361023299 raised defence D36 (man-made) private Owner = private Lined channel side U/S of footbridge 31.4 -999 major 10 right SU7375723178 raised defence D37 (man-made) private Owner = private Open Channel To A3 Road Culvert 26.8 -999 major 10 right SU7359123315 raised defence Open channel with masonry D38 (man-made) private Private owner wall Back of North Road 52.7 -999 major 2 right SU7493023663 non-flood defence D39 structure private Embankment grown up with young trees Road and rail embankments A31 flyover B3004 266.6 6 minor 5 left SU7284038537
Note: This table is to be read in conjunction with Tiles A to E
Note 2: The Design Standard (Return Period, in years) is for current climate conditions, but will be expected to decrease under the impact of climate change.